Autoxidation of aromatics

Autoxidation of aromatics Autoxidation is a conversion pathway that has the potential to add value to multinuclear aromatic-rich coal liquids, heavy oils and bitumens, which are typically considered low-value liquids. In particular, autoxidation of these heavy materials could lead to products that may have petrochemical values, e.g., lubricity improvers and emulsifiers. Proper assessment of an oxidative transformation to ring-open the multinuclear aromatics present in heavy feeds relies on the understanding of the fundamentals of aromatic oxidation. This work reviews the selective oxidation chemistry of atoms that form part of an aromatic ring structure using oxygen (O ) as oxidant, i.e., the oxidation of aromatic carbons as well as heteroatoms contained in an aromatic ring. Examples of industrially relevant oxidations of aromatic and heterocyclic aromatic hydrocarbons are provided. The requirements to produce oxygenates involving the selective cleavage of the ring C–C bonds, as well as com- peting non-selective oxidation reactions are discussed. On the other hand, the Clar formalism, i.e., a rule that describes the stability of polycyclic systems, assists the interpretation of the reactivity of multinuclear aromatics towards oxidation. Two aspects are developed. First, since the interaction of oxygen with aromatic hydrocarbons depends on their structure, oxida- tion chemistries which are fundamentally different are possible, namely, transannular oxygen addition, oxygen addition to a carbon–carbon double bond, or free radical chemistry. Second, hydrogen abstraction is not necessary for the initiation of the oxidation of aromatics compared to that of aliphatics. Keywords Autoxidation · Catalytic oxidation · Multinuclear aromatics · PAHs · Heterocyclic aromatics · Clar formalism Introduction the petrochemical industry. The breadth of the industrial application of oxidation can be seen from books that discuss Processes for the oxidation of organic materials cover the industrially practiced oxidation processes [4, 5]. entire selectivity spectrum ranging from total or near total The scope of this review is much narrower. The review combustion to give carbon oxides, to mild partial oxida- is concerned only with oxidation of atoms that form part tion to functionalize organic molecules to produce chemical of an aromatic ring structure. This is a topic that received intermediates [1]. Selective oxidation is a direct and eco- relatively little attention in the literature since the report by nomic route to convert petroleum products into petrochemi- Tipson [6]. Oxidation of aromatic carbons as well as heter- cals [2]. This type of chemistry is present in the large-scale oatoms contained in aromatic rings will be considered. production of commodities, as well as in the synthesis of Heavy aromatic feed materials can in principle be con- small amounts of pharmaceuticals and fine chemicals [ 3]. verted into higher value petrochemicals that are also mix- Selective oxidation of alkanes (paraffins), alkenes (olefins) tures, for example, lubricity improvers, emulsifiers and and aromatics feedstocks produce oxygenated compounds, greases. Oxidized aromatic compounds may also be low-cost such as alcohols, aldehydes, ketones, carboxylic acids, anhy- intermediates that can be used as feedstocks for the produc- drides and epoxides, which are important commodities in tion of petrochemical products, or that are useful products in their own right. For example, autoxidation (oxidation using air) is commercially employed for asphalt hardening [7], * Natalia Montoya Sánchez which is an important process for producing asphalt for road nmontoya@ualberta.ca construction. Autoxidation is also a conversion pathway by which value can potentially be added to other predominantly Department of Chemical and Materials Engineering, aromatic feed materials, such as coal liquids, petroleum University of Alberta, 9211-116th Street, Edmonton, AB T6G 1H9, Canada Vol.:(0123456789) 1 3 56 Applied Petrochemical Research (2018) 8:55–78 vacuum residues, heavy aromatic oils and bitumens. Such complex mixtures are not normally considered for petro- chemical production. Petrochemicals are usually produced from feed materials that are purified compounds, or well- defined mixtures of compounds. However, due to the low value associated with heavy aromatic feedstocks, there is an economic incentive to increase their quality if possible. Oxidative transformation of heavy aromatic oils is likely challenging and one of the objectives of this review is to assess the potential of oxidation as pathway for the conver- sion of aromatics and in particular multinuclear aromatics. When selecting an oxidant there are some points to con- sider: price, ease of handling, nature of the by-products and percentage of oxygen available. Depending on the scale, pro- duction options change. Bulk industrial oxidation processes are largely limited to air, but fine chemistry can afford the luxury of using more expensive oxidants, as illustrated by Table 1 adapted from [8]. When converting low-value heavy aromatic oils, air and possibly purified oxygen, are likely the only oxidants that make economic sense. Without proving the point, the review was restricted to O as oxidant. It also 2 Fig. 1 Examples of aromatic oxidation involving a non-cleavage of touches on the catalytic selective oxidation of multinuclear the aromatic ring, b cleavage of the aromatic ring, and c oxidation of the side chains attached to the aromatic ring aromatic compounds. Oxidation of aromatic hydrocarbons (b) Formation of oxygenates accompanied by the cleav- age of the aromatic rings, e.g., the formation of maleic The selective oxidation of aromatic hydrocarbons involves anhydride from benzene. three general types of reactions [9], as illustrated by Fig. 1: (c) Formation of oxygenates by oxidation of the side chains attached to the aromatic rings, e.g., the formation of (a) Formation of oxygenates without the cleavage of the benzaldehyde or benzoic acid from toluene. aromatic rings, e.g., the formation of phenol or benzo- quinone from benzene. Table 1 Common oxidants for liquid phase oxidation. Adapted from [8] Oxidant Active oxygen By-product Comments (wt%) Air 23 N /Ar Oxidation forms hydroperoxides; lowest cost for low-pressure open-loop processes a a Purified O 100 None Air separation needed, but purified O becomes more efficient for higher pressure 2 2 and closed-loop processes H O 47 H O Usually available as aqueous solution 2 2 2 c a O 33 O Generated from O by corona discharge 3 2 2 NaClO 22 NaCl ClO can produce chlorocarbon by-products (CH ) CO H 18 (CH ) COH Usually available in solution 3 3 2 3 3 CH COO H 21 CH COOH Usually produced in situ by H O added to acetic acid 3 2 3 2 2 KHSO 11 KHSO Water compatible 5 4 C H IO 7 C H I High cost; metal-catalyzed oxidations often selective 6 5 6 5 Depends on purity, but in practice purified O from an air separation unit is ~ 99.5 wt% Depends on purity, but in practice the H O in water is 35 wt% or less 2 2 Depends on efficiency of corona discharge, typically ~ 2 wt% or less Depends on purity, typically available in solution 1 3 Applied Petrochemical Research (2018) 8:55–78 57 Of these reactions, only (a) and (b) are considered in this In industrial operation benzene is oxidized with 67–72% of review, since (c) does not involve the oxidation of an atom the theoretical yield of maleic anhydride [18]. that is part of the aromatic ring structure. The mechanism of benzene oxidation has been the subject Aromatic C–C bonds are strong and substitution reac- of considerable research work. Yet, the literature indicates tions, reactions of type (a), are more readily achieved. For some disagreements regarding the nature of the intermedi- example, oxidation to produce quinonoids is well-described ates. The generally adopted reaction network for benzene [10]. Reactions of type (a) may also lead to addition products oxidation is presented in Fig. 2. This scheme is the basis of that do not necessarily incorporate oxygen; these oxidative most kinetic studies. Benzene oxidizes in two independent aromatic coupling reactions can be both intra- and intermo- ways to produce either maleic anhydride or total combus- lecular [11]. tion products. Both reactions are the result of a number of Even though aromatic oxidation involving splitting of the short-lived intermediates [17]. Further oxidation of maleic ring C–C bonds represented a milestone in the heterogene- anhydride also leads to the production of carbon oxides and ous catalyzed oxidation technology [12], the literature on water. In the absence of a catalyst, thermal decomposition of the topic is somewhat limited. The literature on ring scission benzene is favored. Transition metal oxides have been suc- is mostly focused on the discussion of a few examples of cessfully used to control the selectivity of the reaction and industrially successful selective catalytic oxidations, e.g., promote the production of maleic anhydride. Industrially, the commercial production of phthalic anhydride from the metal oxides are of interest because the temperature window, vapor-phase oxidation of naphthalene. This type of chem- in which they catalyze the desired oxidation at appreciable istry is of particular interest for the present work, due to its rate, is wider than the one offered by most metal catalysts potential application for the conversion of heavy aromatic [16]. materials. As a result, the oxidation of some key aromatic For benzene oxidation, vanadium, molybdenum and tung- hydrocarbons such as benzene, naphthalene, anthracene and sten oxides have proved to be very selective [16, 18]. How- phenanthrene will be discussed in the following sections. ever, systems based on V O have received special attention. 2 5 There are important differences in the way oxidation of these According to the patent literature, the development of active four compounds proceed. and selective vanadium-based catalysts became an important research topic. A number of different catalyst compositions Benzene oxidation have been proposed and evaluated. Mixtures of vanadium and molybdenum oxides, in combination with one or more The vapor-phase oxidation of benzene to produce maleic oxides of other elements, e.g., P, W, Co, Fe, Ni or their mix- anhydride, using air as oxidant, is one of the first successful tures, or in combination with salts, e.g., sodium borate, have catalytic oxidations of aromatic hydrocarbons. Commer- been tested in the temperature range 340–450 °C [20–22]. cial production of maleic anhydride from benzene started Industrial catalysts for benzene oxidation are based on mix- around 1933. Currently, the industrial process for benzene tures of V O and MoO . The latter can be found in concentra- 2 5 3 to maleic anhydride autoxidation is only of historical and tions up to 30% mol [18]. Typical promoters used are oxides educational interest [12]. The selective vapor-phase oxida- or salts of elements such as W, Bi, Sn, P, Ag, Cu, Na, B, Ti tion of n-butane over vanadium phosphorous oxide (VPO) and Ni [23]. The special catalytic behavior of vanadium-based catalysts has been the dominant chemical route to produce catalysts is related to the valence changes of the metal during maleic anhydride since the mid-1970s [13]. Because of its reaction [16]. Some interesting discussion on the structure, the availability and lower cost, n-butane became an attractive redox behavior and the phases formed in the catalyst during raw material [14]. The industrial routes for phenol production from benzene all employ intermediate products and there are no processes to produce phenol by direct oxidation of benzene [15]. Even though benzene is the simplest aromatic hydro- carbon, its oxidation to maleic anhydride is a complicated chemical reaction, with many side-products. Produc- tion of phenol, quinone and hydroquinone from benzene autoxidation has been reported [16–18]. The presence of small amounts of other compounds such as formaldehyde, biphenyl, acrylic and formic acid has also been mentioned [17–19]. In this case, maleic anhydride is the only product obtained in yields high enough for commercial production. Fig. 2 Benzene oxidation reaction scheme 1 3 58 Applied Petrochemical Research (2018) 8:55–78 reaction as well as the interactions of oxygen and benzene on In steady state, the reaction rates of these two steps the catalysts surface can be found in [23, 24]. (reduction–oxidation cycle) are equal. Also, if for one aro- Mars and Van Krevelen studied the catalytic oxidation matic molecule, β molecules of oxygen are required, it can of some aromatic hydrocarbons, namely benzene, toluene, be written as naphthalene and anthracene [25]. Their work suggested that k p  = k p (1 − ), 1 Ar 2 o (5) the oxidation of these compounds occurred through a redox mechanism (appropriately known as the Mars–Van Krevelen k p mechanism), which involves two independent steps. First, (6) k p + k p . the reaction between the aromatic hydrocarbon and the metal 1 Ar 2 oxide catalyst (MO ) takes place (Eq. 1). Second, the partially reduced catalyst (MO ) is reoxidized with gas phase oxygen Combining Eq. 3 and 6, the overall reaction rate can be n−1 expressed as (Eq. 2), Aromatic hydrocarbon + MO → Oxidation products r = . Ar + MO +ΔQ , (7) (1) n−1 ox + k p k p 1 Ar 2 MO + 1∕2O → MO . (2) n−1 2 n The validity of the kinetic expression (Eq.  7) with Due to the generality of the kinetics following from this m = 1 for the oxidation of benzene was confirmed by mechanism, it is worthwhile providing a brief description. Mars and Van Krevelen [25]. Furthermore, this type of Assuming that the first step is proportional to the partial pres - equation was also valid for the oxidation of naphthalene sure of the aromatic hydrocarbon (p ), i.e., first-order reac- Ar and anthracene; the value of the rate constant k of the tion, and to the fraction of catalyst surface covered with active reoxidation process on a V O –Mo O/Al O catalyst was 2 5 3 2 3 oxygen (θ), the reaction rate can be expressed according to found to be the same for the catalytic oxidation of ben- Eq. 3. On the other hand, if the rate of reoxidation of the cata- zene, naphthalene and anthracene. lyst is assumed to be proportional to a certain power (m) of The reaction kinetics of benzene oxidation over a number oxygen partial pressure and to the catalyst surface area not of vanadium-based catalysts have been reported. A valu- covered with active oxygen (1 − θ), the reaction rate for the able compilation of kinetic studies for benzene oxidation second step can be expressed according to Eq. 4, can be found in [17, 24]. Results from selected studies are r = k p , (3) Ar 1 Ar presented in Table  2. At high concentrations of benzene, reactions (1) and (3) in Fig. 2 appear to be of pseudo-first r = k p (1 − ). o 2 o (4) order on benzene [23]. The effect of the mass transfer rate on the pseudo-first-order rate constants has been discussed Table 2 Catalytic oxidation of benzene: kinetics Reaction conditions Catalyst Kinetic expressionKinetic order Refer- ences T (°C) P (kPa) 375–400 p = 0.3–0.6 V O –MoO on Al O r = kp 1st on B [17] B 2 5 3 2 3 B p = 20 ~ 2nd on M 450–530 p ~ 1.3–2.6 AgO, V O, MoO, Al O on SiC – 1st on B [17] B 2 2 5 3 2 3 p ~ 22 325–450 p = 1–2 V O on Al O – 1st on B [17] B 2 5 2 3 p = 20 377 p = 0.1–4 V O –MoO on Al O 1∕r = 1∕k p + ∕k p p < 6, 1st on B [25] B 2 5 3 2 3 1 B 2 O B p = 10–101 p > 6, 0th on B O B p < 200, 1st on O O 2 p > 300, 0th on O O 2 −3 −3 2 −3 2 380–440 C = (0.1–1.4) × 10 V O –MoO on corundum C < 4 × 10 , r = [O ] / C < 4 × 10 , 2nd on O [18] B 2 5 3 O 2 O 2 2 0.74 0.78 0.71 −3 2 mol/L [M] (k [B] + k [B] ) C < 4 × 10 , 0th on O 1 3 O −3 −3 C = (1–3.2) × 10 C > 4 × 10 , O O 2 2 0.74 mol/L r = 1/[M] 0.78 0.71 (k [B] + k [B] ) 1 3 Kinetic order for the overall disappearance of benzene (B benzene, M maleic anhydride) 1 3 Applied Petrochemical Research (2018) 8:55–78 59 [17]. The dependence of reaction (2) on the concentration of maleic anhydride is less clear [23]. It has been accepted that kinetic models derived from the reaction scheme presented in Fig. 2 are a good approximation when the concentration of benzene is high and the formation of side-products, such as phenol and quinone, can be neglected. More comprehen- sive studies have taken into account the oxidation of benzene as well as the oxidation of maleic anhydride, quinone, and phenol [18]. Naphthalene oxidation The oxidation of naphthalene to phthalic anhydride is com- mercially one of the most important vapor-phase catalytic oxidations of aromatic compounds. The first commercial process started operation in 1916. Industrially, naphthalene oxidation is presently carried out in fixed-bed reactors operating in the temperature range 340–380 °C and using supported vanadium oxide catalysts [26, 27]. Current processes are flexible and allow for using dual feedstocks, i.e., mixtures of naphthalene and o-xylene [26]. In recent years, the use of o-xylene as the main raw material has spread due to its lower cost, good availability and ease of transportation [28]. Fig. 3 Naphthalene oxidation reaction scheme Compared to benzene, the presence of a second aromatic ring in the naphthalene molecule increases the complexity of the reaction network. Several parallel and consecutive be simplified to reactions ( 1)–(4) [27], as discussed later on in this section. reactions take place in this case. Even though naphthoqui- none is a key intermediate in the reaction sequence, phthalic It is believed that the stepwise oxidation of the naph- thalene ring to form phthalic anhydride proceeds by a anhydride is not formed exclusively through this compound. Production of maleic anhydride and carbon oxides as sec- similar mechanism to that of benzene oxidation to form maleic anhydride. The formation of the 1,4-naphthoquinone ondary products has been reported [28]. Figure 3 presents the generally adopted reaction scheme. (α-quinone), which is the equivalent of the quinone in ben- zene oxidation, supports this premise [16]. The formation Naphthalene is oxidized by three independent routes. Pro- duction of phthalic anhydride and 1,4-naphthoquinone rep- of the 1,4-naphthoquinone also points out the difficulty of breaking the aromatic structure. The oxidation of the naph- resents the most important primary reactions [17]. The dis- tribution of these products may vary according to the initial thalene molecule may be analyzed in three steps: addition of oxygen into the structure, breaking of one of the two rings point of attack in the naphthalene molecule [17]. It has been stated that for the sake of completeness, 1,2-naphthoqui- in naphthalene and further destruction of the remaining ben- zene ring [16]. none should be indicated as an intermediate in reaction (1); however, this compound is usually neglected because of its At ambient conditions, naphthalene is inert to air (oxy- gen) and cannot be partially oxidized. Therefore, the use rapid oxidation to phthalic anhydride [27]. Naphthalene is also directly oxidized, to a much lesser extent, to maleic of higher temperatures is required. In the absence of a catalyst, two scenarios are possible [16]. First, oxidation at anhydride and carbon oxides [17, 27]. Secondary reactions are also important in this network. Typically, a large frac- high temperature (i.e., at a temperature high enough for the reaction rate to be significant) favors total combustion over tion of the 1,4-naphthoquinone formed is further oxidized to give phthalic anhydride, while a small portion contributes partial oxidation. Second, oxidation at moderate conditions promotes addition reactions (condensation reactions) and to the production of maleic anhydride and carbon oxides. Similarly, part of the phthalic anhydride formed by reactions polymerization of the intermediates due to the longer reac- tion times needed to achieve significant conversion. The use (1) and (3) is oxidized to give maleic anhydride and carbon oxides. Production of aromatic acids, i.e., phthalic and ben- of an appropriate catalyst is essential to control the reaction selectivity. zoic acids has also been reported [16]. For reactor design purposes, the kinetic scheme of naphthalene oxidation may 1 3 60 Applied Petrochemical Research (2018) 8:55–78 Transition metal oxides of the fifth and sixth groups using vanadium-based catalysts, namely V O and K S O 2 5 2 2 7 of the periodic table have been the candidates of choice. or K SO supported on SiO , are in agreement with this 2 4 2 Vanadium and molybdenum oxides as well as tin and bis- behavior. Mars and Van Krevelen investigated the effect muth vanadates have been tested as catalysts for this reac- of naphthalene and oxygen pressure on the rate of naph- tion [16, 17]. Oxides of magnesium, aluminum, silicon, thalene oxidation [25]. According to their work, the rate titanium, zirconium, copper and cobalt have shown rather is approximately first-order with respect to naphthalene poor performance in naphthalene oxidation [16]. Cata- at p < 0.5 kPa, but it is almost independent of the naph- lysts based on vanadium oxides are quite active and can thalene concentration at p > 1 kPa. A similar trend was cause over-oxidation to maleic anhydride or carbon oxides. observed by Calderbank [30], who studied the reaction Strict control of the reaction residence time or addition of over the range of naphthalene concentrations of 2–10 kPa, less active oxides to the catalyst (e.g., manganese, cop- and by Ioffe and Sherman [31], who explored the pressure per or cobalt oxides) is common practices to avoid this range 0.7–1.5 kPa. On the other hand, the dependence of problem [16]. For vanadium-based catalysts, yields to reaction rate on the concentration of oxygen was found phthalic anhydride of more than 70% have been reported to vary almost linearly with the O concentration in the [17]. Similarly, selectivities towards phthalic anhydride region of oxygen partial pressures below 40 kPa [25]. of nearly 90% have been achieved using mixtures of V O Valuable compilations of kinetic studies for naphthalene 2 5 and K SO supported on SiO , i.e., commercial type of oxidation were prepared by Dixon and Longfield [17] and 2 4 2 catalysts [29]. A great deal of information on the cata- Wainwright and Foster [28]. In general, reactions (1)–(4) in lysts used for naphthalene oxidation is found in the pat- Fig. 3 appear to depend on the concentration of both oxy- ents while only some is found in the journal literature. gen and the organic reactant to an order between 0.5 and There is no recent review dealing with commercially used 1.0 [17]. The observed differences in individual studies are catalysts for naphthalene oxidation. A valuable discussion attributed to the variation in the reaction conditions rather on the parameters affecting the catalytic performance of than a real effect. There is little consistency in the reaction vanadium-based catalysts (e.g., V O concentration, active temperature, catalyst, reactor type and conversion achieved 2 5 sites, role of alkali metal sulfates, among others) can be in the reported studies; therefore, comparisons are not easy found in [28]. In addition, a compilation of the catalysts [28]. and conditions used in naphthalene oxidation can be found in [17]. Anthracene and phenanthrene oxidation The reaction mechanism and kinetics of naphthalene oxidation have been extensively studied. Yet, they are not The catalytic vapor-phase autoxidation of anthracene to completely elucidated. As mentioned in the previous sec- anthraquinone with yields of 85–90% has been described in tion on benzene oxidation, it has been found that naphtha- patents in the 1930s and has found some industrial applica- lene oxidation also proceeds through a redox mechanism tions [2, 17]. The vapor-phase oxidation of anthracene with [25]. In other words, the aromatic hydrocarbon reacts with air is the preferred synthesis method to produce 9,10-anth- a metal oxide catalyst to give the oxidized products and raquinone [32], and > 90% yield is possible. However, the a partially reduced catalyst; then, the catalyst is reoxi- most widespread use of oxidation in relation to anthracene is dized by gas phase oxygen. Hence, the kinetic expression the use of the autoxidation-and-reduction cycle of 9,10-dihy- derived before, Eq. 7, can be used to describe this reac- droxyanthracene and anthraquinone to produce hydrogen tion too. At constant oxygen partial pressure, Eq. 7 can be peroxide [2], as illustrated by Fig. 4. rearranged to Eq. 8. Vapor-phase catalytic oxidations of anthracene and phen- anthrene have been less extensively investigated compared k p 1 N r = , to benzene and naphthalene oxidation. The additional aro- (8) 1 + k Cp 1 N matic ring further increases the complexity of the reaction network. Thus, depending on the position of attack and the extent of the reaction, oxidation products from the inner and where C = . (9) outer rings are possible. As in other aromatic oxidations, k p achieving selectivity is a challenge and using an appropriate catalyst to promote the production of the desired products The latter expression shows that at low partial pressures is necessary. of naphthalene (p ), the oxidation rate should approach Almost complete conversion of anthracene with high first order in naphthalene concentration; whereas at high selectivity to 9,10-anthraquinone is possible and 9,10-anth- partial pressures of naphthalene, it should approach zero raquinone of 99% purity is industrially produced in this way order in this reactant [17]. A number of kinetic studies [32]. Secondary and complete combustion products are also 1 3 Applied Petrochemical Research (2018) 8:55–78 61 addition of oxygen causes the carbons at the 9,10-positions 2 3 to change from sp to sp hybridized carbons. In other words, these carbons are now benzylic aliphatic carbons with a much weaker C–H bond, which facilitates hydrogen abstrac- tion by further reaction with oxygen. It has been suggested that the catalytic conversion takes place in two steps, which are in agreement with the Mars–Van Krevelen mechanism [17, 25]. First, anthracene is oxidized by a metal oxide catalyst producing anthraquinone and water; during this step the oxide catalyst is reduced. Second, the catalyst is reoxidized by gas phase oxygen to return to its original state. Vanadium oxide, iron vanadate or vanadic acid doped with alkali or alkaline earth metal ions are suitable cata- Fig. 4 Catalytic hydrogen peroxide production through the anthraqui- lysts for anthracene oxidation [16, 33, 34]. Industrially, the none process reaction has been carried out at temperatures in the range 320–390 °C and using catalysts consisting of V O, K SO 2 5 2 4 obtained is small amounts. Carbons in the 9- and 10-posi- and Fe O on pumice; yields of 95–97 wt% of anthraqui- 2 3 tions are the most reactive centers of attack in the anthracene none were reported [17]. A reaction scheme derived from molecule [6]. The attack of oxygen on these carbons results the oxidation of anthracene using V O –MoO –P O and 2 5 3 2 5 in the production of 9,10-anthraquinone. On the other hand, V O –Fe O as catalysts can be found [33]. In this case, 2 5 2 3 the unselective oxidation of the carbons in the 1- to 4-posi- oxidation at the inner and outer rings of anthracene leads to tions, i.e., carbons in the outer ring, gives ketone (1,4-anth- 9,10-anthraquinone, 1,4-anthraquinone and 2,3-naphthalic raquinone) and anhydride (2,3-naphthalic anhydride and anhydride. At high conversion, the selectivities towards pyromellitic anhydride) species as by-products [33]. The these compounds decreases and production of phthalic anhy- production of small quantities of phthalic anhydride, maleic dride, pyromellitic anhydride and CO is favored. anhydride, CO and CO has also been reported [16, 33, The liquid phase oxidation of anthracene and 9,10-anth- 34]. The higher selectivity of the oxidation reaction can be raquinone has been investigated by thermal analysis [36]. In explained in terms of the Clar formalism [35], noting that this study, a number of metal oxides (V O, MoO, Fe O 2 5 3 2 3 in anthracene additional stabilization of the outermost rings and NiO) were evaluated as catalysts. Among them, V O 2 5 can be achieved if the π-electrons on the 9,10-positions are showed the highest catalytic contribution during reaction; more localized. it measurably accelerated the autoxidation of both aromatic The overall reaction for vapor-phase anthracene oxida- compounds. Experiments to determine whether this oxide tion is shown in Fig. 5. The oxidation process involves the just activated the O from air, or whether oxidation took interaction of one hydrocarbon molecule with three oxygen place by transfer of lattice oxygen were also performed. atoms. However, the actual reaction mechanism is more Interestingly, reaction under nitrogen atmosphere confirmed complex than that. With the localization of the π-electrons that V O was capable of using the lattice oxygen for oxi- 2 5 on the 9,10-positions, not only does the transannular addi- dation of anthracene and 9,10-anthraquinone in the liquid tion of oxygen take place, but two aromatic sextets are also phase. A Mars–Van Krevelen mechanism was likely. formed. Even more, the removal of hydrogen from an aro- Phenanthrene is used for industrial production of matic carbon is not required for oxidation to take place. The 9,10-phenanthrene quinone as well as diphenic acid by Fig. 5 Vapor-phase oxidation of anthracene 1 3 62 Applied Petrochemical Research (2018) 8:55–78 vapor-phase oxidation over vanadium-based catalysts [33]. routes, which are governed by the position of attack, i.e., Same as anthracene, the 9 and 10 carbons in the phenan- oxidation in the inner or outer rings. threne molecule are much more reactive than the other Compared to the benzene or naphthalene cases, there ones, and so the 9- and 10-positions are the preferred are a lot fewer publications dealing with the kinetics of the centers of attack [6]. As in the case of anthracene, this is vapor-phase anthracene and phenanthrene oxidation. Mars explained by the Clar formalism [35]. Selective oxidation and Van Krevelen found that at anthracene partial pres- on these carbons gives the 9,10-dione; whereas further sures > 0.1 kPa, the reaction rate was essentially independ- reaction produces anhydride (diphenic anhydride), lactone ent with respect to the concentration of the aromatic reactant (2-hydroxy-diphenyl-2′-carboxylic acid lactone) and ketone [25]. In addition, the reaction rate showed some dependence (9-fluorenone) species as side-products [33]. When oxygen on oxygen over the entire concentration range (oxygen par- attacks the carbons on the outer rings (not preferred path- tial pressures up to 100 kPa). Pyatnitskii derived a kinetic way), 1,2-naphthalic anhydride is produced [33]. On the expression for aromatic oxidations [38] based on the work other hand, a number of experimental studies taking place of Mars and Van Krevelen. In his work, the two independent in the temperature range 370–440 °C and using vanadium steps of the redox mechanism (Eqs. 1 and 2) were written as pentoxide, mixtures of vanadium and molybdenum or iron r = k p  , (10) Ar Ar Ar oxides and tin vanadates as oxidation catalysts resulted in the production of phthalic anhydride and phthalic acid [17]. m b r = k p (1 − ) , o o (11) The reaction network for phenanthrene oxidation is pre- 2 2 sented in Fig. 6. The scheme was derived from the study of where a and b are the orders with respect to adsorbed oxygen the reaction over V O supported on SiO with and without 2 5 2 and to oxygen vacancies on the catalyst surface, respectively. K SO [37]. Phenanthrene is oxidized by two independent 2 4 Fig. 6 Phenanthrene oxidation reaction scheme 1 3 Applied Petrochemical Research (2018) 8:55–78 63 These equations reduce to Eqs. 3 and 4 when a = 1 and b = 1. industrial processes. Reaction at near-isothermal conditions In steady state, is preferred to maintain optimal catalyst efficiency and prod- uct selectivity. Multitubular reactors and fluidized beds with r = r ∕v, Ar o (12) heat exchange internals have been widely used for this pur- where the number of oxygen molecules required to oxidize pose; after all, these configurations are suitable to minimize one aromatic molecule (Ar) to the products P , P , …, P , is 1 2 i the temperature gradients across the catalyst bed. represented by the stoichiometric coefficient (v ) as follows: Most of the processes for industrial production of maleic anhydride from benzene use multitubular reactors. A typical v = v S + v S + v S +… + v S , (13) 1 1 2 2 3 3 i i reactor contains a bundle of 10,000–15,000 vertical tubes, where S is the selectivity for P and v corresponds to the i i i ~ 25 mm in diameter, enclosed by a jacket through which number of oxygen molecules necessary to oxidize one aro- cooling medium circulates [23]. During operation, the reac- matic molecule to P . By replacing Eqs. 10 and 11 in Eq. 5, tion gas flows through the tubes and over the catalyst, while it can be seen that the surface concentration of oxygen is a the process pressure is kept between 0.15 and 0.25 MPa [19]. function of the ratio of the partial pressures of oxygen and Circulating heat-transfer fluids or agitated eutectic salt mix- the aromatic hydrocarbon ( P ∕P ): O Ar tures are commonly used to deal with the large duty [23]. Steam, which can be used in other operations, is generated a k p o o 2 2 from the heat removed. Nearly 27 MJ of heat are dissipated = . (14) k v p (1 − ) Ar Ar per each tonne of reacted benzene, which translates to the production of almost 10 tonnes of saturated steam per tonne For the simplest case a = b = 1, and combining Eqs. 11 of reacted benzene [19]. Considering that the reaction is and 14, the total rate of oxidation can be expressed as conducted at temperatures > 300 °C, production of high- pressure steam, typically 4–5 MPa steam, is possible. k k p p Ar o Ar o 2 2 r = . A typical flow diagram of a process using a multitubular (15) Ar (k p + vk p ) o o Ar Ar 2 2 reactor for benzene oxidation [23] is presented in Fig. 7. In this configuration, a mixture of benzene and preheated The kinetic expression (Eq. 15) derived in Pyatnitskii’s air is reacted in the multitubular reactor. The conversion of work describes the oxidation of anthracene on V O –K SO 2 5 2 4 benzene is not complete. The feed-product heat exchange with Fe O and S iO , and on C oMoO /SiO [38]. On the 2 3 2 4 2 decreases the duty of the feed preheating. On the other other hand, the kinetics of the oxidation of phenanthrene are hand, switch condensers allow for recovering close to 90% best described with empirical power law equations [38]. It is of the maleic anhydride produced in the reaction. Refining worth mentioning that the rate of oxidation of phenanthrene of this product is completed later on by batch distillation. is less than that of anthracene over the same V O –K SO / 2 5 2 4 The remaining 10% of maleic anhydride is recovered in a SiO catalyst. two-stage scrubber unit using water and dilute alkali. Fuma- ric acid is also obtained in this process. The unconverted Reactor engineering in aromatic oxidation benzene is separated from the exhaust air by adsorption with activated carbon, and it is returned as part of the reactor Vapor-phase catalytic aromatic oxidations are strongly exo- feed. thermic reactions. Furthermore, the heat released is always Control of the reaction temperature during catalytic aro- greater than the calculated heat of reaction of the main reac- matic oxidation can also be achieved using fluidized bed tion, due to side reactions leading to the formation of C O . reactors [23]. Some commercial plants employ this tech- For instance, while the heat of oxidation of naphthalene to nology to oxidize naphthalene to phthalic anhydride. Thus, phthalic anhydride corresponds to 1880 kJ/mol, in prac- during typical operation, liquid naphthalene is fed at the tice, due to over-oxidation the total heat liberated can reach bottom of the catalyst bed; after instant evaporation, it dis- 2900 kJ/mol [39]. tributes over the entire fluidized bed. Oxygen fed through a Aromatic oxidation requires large amounts of heat to be distributor plate is mixed with the vaporized naphthalene. removed from the reaction zone to maintain the temperature The reaction temperature is kept in the range 345–385 °C and keep the overall process under control. An uncontrolled through heat exchanger tubes in the fluidized bed. Moreo- reaction would cause a severe reduction of yields (complete ver, a uniform temperature profile is obtained because of the combustion would be favored) as well as the loss of catalyst intense agitation and mixing of the catalyst in the fluidized life [23]. Furthermore, it is a self-amplifying problem, i.e., bed. High-pressure steam (> 4 MPa) is produced from the an increase in temperature causes more combustion reactions heat removed [40]. that would further increase the temperature. Proper reactor The increase in size for aromatic hydrocarbons, which is design and heat management are therefore key aspects in represented by the number of aromatic rings in a molecule, 1 3 64 Applied Petrochemical Research (2018) 8:55–78 Fig. 7 Ruhrol–Lurgi process for production of maleic anhydride by benzene oxidation Table 3 Heat evolved during Aromatic hydrocarbon Overall oxidation reaction ∆H (kJ/mol) References aromatic oxidation Benzene − 1917 [16] C H +4 O → C H O +2CO +2H O 6 6 2 4 2 3 2 2 Naphthalene − 1881 C H +4 O → C H O +2CO +2 H O 10 8 2 8 4 3 2 2 Anthracene − 559 C H + O → C H O +2H O 14 10 2 14 8 2 2 Theoretical heat of reaction (heat released per mol of aromatic hydrocarbon reacted) has an effect on the calculated (theoretical) heat of reaction. Oxidation of heterocyclic aromatics According to Table 3, the heat liberated during oxidation to the main oxidation product decreases in the order benzene > naph- The selective oxidation of heterocyclic aromatics involves thalene > anthracene. As the molecules become heavier, fewer the same types of reaction as noted for the aromatic hydro- oxygen atoms react per mole of aromatic substrate to produce carbons [9], but added differentiation is required to distin - the main product, and so considerably less heat is released. guish between carbon oxidation and heteroatom oxidation. Irrespective of the aromatic feed, successful selective oxida- The general types of reactions are (Fig. 8): tion requires proper heat management. 1 3 Applied Petrochemical Research (2018) 8:55–78 65 aromatic rings, e.g., o-nitrobenzo-propenoic acid from quinoline. (e) Formation of oxygenates by oxidation of the side chains attached to the aromatic rings. (f) e.g., oxidation of 4-methyl-pyridine to pyridine-4-car- boxylic acid. Of these reaction types, only (a)–(d) will be considered. It is also possible for oxidation of the multinuclear aromatics to take place on a carbocyclic ring that does not contain a heteroatom, as was discussed in the previous section. One variation on reaction types (a) and (b) is reactions leading to the formation of addition products. Oxidative addition does not necessarily lead to oxygen incorporation into the product. Even though selective oxidation is a direct and economic route to convert aromatic hydrocarbons into valuable chemi- cals, the scenario for heterocyclic aromatics is a little dif- ferent. Oxidation with air of these compounds as a synthe- sis route to obtain added value products is not reported in standard texts [42, 43]. Their oxidation chemistry is rather discussed as part of the crude oil processing and fuel stabil- ity. For instance, sulfur- and nitrogen-containing compounds are typically undesired materials in crude oil. The oxidation of these compounds (i.e., oxidative desulfurization and oxi- dative denitrogenation) to give chemically modified products with properties that favor their separation or removal has been explored [44, 45]. Furthermore, it is known that autoxi- dation of nitrogen heterocyclic aromatics plays a role in the formation of heavy addition products, e.g., nitrogen-con- taining compounds seem to be related with the formation of Fig. 8 Examples of heteroaromatic oxidation involving a carbon sludge in fuels [46], thereby affecting fuel storage stability. oxidation without the cleavage of the aromatic rings, b heteroatom The oxidation of the distinct aromatic heterocyclic oxidation without the cleavage of the aromatic rings, c cleavage of carbon–carbon bonds of the aromatic rings, d cleavage of carbon–het- compound classes is considered in the following sections. eroatom bonds of the heterocyclic aromatic rings, e oxidation of the Emphasis is placed on the chemistry of five-membered side chains attached to the aromatic rings heterocyclic compounds that contain sulfur, nitrogen, and oxygen, as well as on their benzologs. Some comments (a) Formation of oxygenates by carbon oxidation without regarding the chemistry of pyridine, a nitrogen-containing the cleavage of the aromatic rings, e.g., formation of six-membered heterocyclic compound, are also included as coumaranone from benzofuran and the polymerization part of the discussion. of pyrrole. (b) Formation of oxygenates by heteroatom oxidation with- Oxidation of aromatic S‑heterocyclic compounds out the cleavage of the aromatic rings, e.g., formation of benzothiophene-1,1-dioxide from benzothiophene. The first class of aromatic heterocyclic compounds con- (c) Formation of oxygenates accompanied by the cleav- sidered in this work corresponds to the sulfur-containing age of carbon–carbon bonds of the aromatic rings, e.g., compounds, which are represented by thiophene and its ben- formation of N,2-(cyclopenta-1,5-dione)-aniline from zologs, e.g., benzothiophene and dibenzothiophene (Fig. 9). 2,3-cyclopenteno-indole [41]. However, it should be These molecules are typically found in the heavier fractions noted that oxidative carbon–carbon scission in hetero- of crude oil [47]. Refining of crude oil to final products, cyclic aromatic rings appears to be rare. such as fuels and some petrochemicals, requires desulfuri- (d) Formation of oxygenates accompanied by the cleav- zation of the oil. However, the sulfur contained in aromatic age of carbon-heteroatom bonds of the heterocyclic rings is more difficult to remove than aliphatic sulfur using 1 3 66 Applied Petrochemical Research (2018) 8:55–78 two separate steps. First, the reaction between an oxidant and sulfur alters the nature of the sulfur-containing com- pounds. Second, the properties of the oxidized products are exploited to facilitate their removal through a separa- tion process [44]. Oxidation with air has been extensively evaluated. Autoxidation of S-heterocyclic aromatics involves the formation of hydroperoxides by oxidation of aliphatic compounds that are already present or are added to the reaction mixture. Hydroperoxides are key intermediates formed in  situ by the oxygen. The thiophenic sulfur is subsequently oxidized by the hydroperoxides, rather than by oxygen directly (free radical chemistry). The chemistry taking place during the first step of this conversion illus- trates the oxidative behavior of S-heterocyclic compounds in the presence of air. The thiophenic sulfur is oxidized to Fig. 9 Examples of heterocyclic aromatic compounds sulfoxides and sulfones (Fig. 10). Moreover, thiophenes that could be considered refractive traditional technologies, e.g., hydrodesulfurization (HDS) to HDS, are readily oxidized because of their increased elec- and thermal conversion [47]. tron density on the sulfur atom [48]. Temperatures below Oxidation of thiophene and its benzologs as an alterna- 200 °C and pressures near atmospheric pressure are usual tive approach to HDS for sulfur removal has received a for oxidation of heterocyclic sulfur. Typical conditions for lot of attention. Oxidative desulfurization (ODS) involves oxidative desulfurization processes are given in Table 4. Fig. 10 Oxidation of sulfur compounds by oxygen in air, as illustrated by the oxidation of thiophene Table 4 Desulfurization of model mixtures and industrial feedstocks by liquid-phase oxidation with air Feed Oxidant Catalyst Oxidation conditions Comments References Straight-run kerosene O Non-catalytic oxidation 120–200 °C Stirring rate and resident [49] 5–20 min time affected the degree of High pressure oxidation of sulfur-containing Presence of water compounds Model oil and hydrodesulfur- Air Non-catalytic oxidation 140 °C Lactones (e.g., γ-butyrolactone) [50] ized diesel fuel Atm. Pressure used as solvents play a role in oxygen transfer Model jet fuel and real jet fuel O Fe(III) salts (nitrate and bro- 25 °C Oxidation reactivity sequence [51] (JP-8) mide) 2–5 h between ASBP, BP and DBP was evaluated Model oil and diesel fuel O Co (II) salts (acetate and 40 °C Aldehydes were used as sac- [52] chloride) Atm. pressure rificial materials to produce, with the oxygen fed, in situ peracids to oxidize the sulfur- containing compounds Atmospheric residue Air/O Pt, Pd, Ni, V (their salts or 130–180 °C Metals catalyze decomposition [53] oxides) 2–20 h of organic hydroperoxides ASBP alkyl-substituted benzothiophene, BP benzothiophene, DBP dibenzothiophene 1 3 Applied Petrochemical Research (2018) 8:55–78 67 Autoxidation of S-heterocyclic aromatics can also be observed for aromatic compounds, due to the high selectiv- performed using a catalyst or oxygen carrier, instead of ity of autoxidation to oxidize the heterocyclic sulfur. In this employing free radical autoxidation. Catalysts facilitate the case, there are no complex reaction networks. The forma- decomposition of hydroperoxides, and in doing so, acceler- tion of oxygenates by oxidation of the heteroatom is actually ate the propagation step during autoxidation. Oxygen carri- the main oxidative transformation. Even for addition reac- ers, on the other hand, are more active oxidation agents than tions to take place, oxidation of the sulfur plays a role and oxygen. Depending on their nature, they can be viewed as a autoxidation of the aromatic carbons is not observed except type of catalyst which can be regenerated using molecular at extreme (combustion) conditions. oxygen, i.e., oxidation by the Mars–Van Krevelen mecha- nism; this characteristic makes their behavior similar to the Oxidation of aromatic N‑heterocyclic compounds one observed for metal oxides in the selective oxidation of aromatic compounds, as discussed in the previous section. The second class of aromatic heterocyclic compounds exam- Additional details on the topic can be found in [54]. ined in this work corresponds to the nitrogen-containing The C–S bond strength is decreased when the sulfur is compounds. Examples of N-heterocyclic compounds are oxidized, which in principle allow the cleavage of the C–S pyrrole, indole and pyridine (Fig. 9). Aromatic nitrogen- bonds by catalytic [55], or by thermal decomposition. These containing compounds present in crude oil are normally assertions are based on the published homolytic bond disso- classified in two categories: basic (derivatives of pyridine) ciation energies for sulfones. In the case of a diaryl sulfone, and non-basic (derivatives of pyrrole). Irrespective of the which is representative of the sulfone group in dibenzothio- category, these compounds are associated with a number of phene-1,1-dioxide, the bond dissociation energy is report- different problems during oil processing and their removal edly 288 kJ/mol [56]. However, this value was found to be is desirable [45]. inconsistent with the thermal stability of diaryl sulfones that Oxidation of nitrogen-containing compounds has been suggested a much stronger bond [57]. explored as an alternative approach to traditional practices Autoxidation of sulfur-containing compounds can also such as hydrodenitrogenation (HDN) by hydrotreating. Even lead to the formation of addition products. For instance, though the fundamental idea behind this process, i.e., oxi- model oil consisting of n-heptane and dibenzothiophene dative denitrogenation, is very similar to those of oxidative reacted with air at 145–170 °C and atmospheric pressure desulfurization (the aim is to produce chemically modified resulted in the formation of some heavier products [58]. products that can be more efficiently extracted from oils In this case, besides the typical products involving the for- using polar solvents), very little has been reported on the mation of both sulfoxides (dibenzothiophene-1-oxide) and topic [45]. sulfones (dibenzothiophene-1,1-dioxide), the formation of Few studies dealing with the oxidation of industrial feed- sediments was observed. This precipitation process involves stocks have been reported, and in those cases the oxidation a type of chemistry analogous to that explaining the gum for- of the S- and N-containing compounds has been performed mation, a common problem that undermines storage stability simultaneously [60, 61]. The purpose of oxidative denitro- of transportation fuels. In the proposed reaction pathway, genation is mainly to remove basic nitrogen-containing com- a sulfoxide reacts with a hydroperoxide to give an inter- pounds. Typical reaction conditions involve temperatures mediate product that may be decomposed by reaction with around 70 °C and atmospheric pressure. Peracetic acid or another sulfoxide to yield an addition product [58]. mixtures of hydrogen peroxide and acetic acid have been The thiophene ring system, unless carrying electron- used as oxidizing agents to convert pyridines into N oxides. donating substituents, is relatively stable to atmospheric While sulfones and sulfoxides are easily obtained during oxidation [59], and resist the action of moderate oxidizing oxidative desulfurization, products of polymeric nature are agents [42, 43]. Oxygen and peracids are known to pref- obtained after the oxidation of N-heterocyclic aromatics erentially attack the sulfur atom [43], while stronger oxi- [60]. It will be shown that this is due to the oxidation of dants are required to actually attack the aromatic carbons. pyrroles and not due to the oxidation of pyridines. Ozone, for example, attacks the double bond between the Because the oxidation chemistry of pyrroles and pyri- aromatic carbons which results in the cleavage of the ring; dines is quite different, some comments for each class of thus, oxidation of benzothiophene with ozone yields o-mer- compound will be made in the following sub-sections in a captobenzaldehydae [59]. Similarly, strong oxidants, e.g., separate manner. nitric acid, are able to break down the aromatic ring to give maleic and oxalic acids, while the ring sulfur is oxidized to Oxidation of pyrroles sulfuric acid [59]. It is evident that oxidation chemistry of sulfur-contain- Simple pyrroles are easily oxidized. Compounds contain- ing compounds, using O as oxidant, is simpler that the one ing a pyrrole ring, such as indoles and carbazoles, have a 1 3 68 Applied Petrochemical Research (2018) 8:55–78 negative effect on the oxidative stability of jet fuels [62]. which ultimately leads to oxidative coupling and formation The effect of pyrroles on color and sediment formation in of polymeric or addition products [64]. fuels has been documented and explained through a num- Depending on the conditions of the reaction, indole addi- ber of oxidation and polymerization reactions [63]. These tion may or may not involve the incorporation of oxygen. In are all industrially important fouling reactions. the presence of air and light, indole is autoxidized to give However, the nature of the products depends on the indoxyl which further reacts to produce indigo [43]. Indigo strength of the oxidizing agent. Strong oxidants, such as is a molecule containing two oxygen atoms and it is the reac- chromium trioxide in aqueous sulfuric acid, often lead to tion product from two indole molecules. The nature of the complex breakdown products. When the ring survives, reaction products found after the catalytic oxidation of indole maleimide derivatives are obtained [42]. Milder oxidants using Mn(II)porphyrins as catalyst, suggested that the mecha- such as hydrogen peroxide convert pyrroles to pyrrolines nism of indole addition involves both oxygen incorporation [59]. In the case of oxygen, which is the oxidant of inter- and hydrogen disproportionation [66]. On the other hand, est in this work, pyrrolic compounds autoxidize to form selectivity close to 90% towards oxidative addition product dark colored pigments known as “pyrrole black” [64, 65]. formation was found during the low temperature air oxidation In oil fractions exposed to air, pyrroles autoxidize to form of indole [67]. In this case, no evidence for oxygen incorpora- the so-called “red tars” [43, 64]. Irrespective of the name, tion in the products was found suggesting that the addition pyrrole black or red tar, these are all mixtures of pyrrole mechanism did not require oxygen incorporation even though addition products. Oxidative addition of pyrroles is due to oxygen was necessary for the reaction to proceed. aromatic carbon oxidation [64], a variation on the reaction type (a) shown in Fig. 8. Oxidation of pyridines The autoxidation of 2,5-dimethylpyrrole is presented in Fig. 11 to illustrate how a typical reaction of a pyrrolic The lone pair of electrons on the nitrogen atom that is avail- compound with air takes place. The first step corresponds able for bonding, without disturbing the aromaticity of the to the formation of a molecular association complex (I) ring, makes pyridine chemistry quite distinct from that of between the oxygen and the pyrrolic compound. Then, the pyrrole [67, 68]. Pyridines are therefore bases. Because electron transfer from the pyrrole nucleus of the complex the pyridine ring is π-electron deficient and oxidants act as to the oxygen produces a charge-transfer complex (II), electron acceptors, the oxidation of the ring is difficult [43]. which is suggested to be in equilibrium with an endoper- In fact, strong oxidants, e.g., neutral aqueous potassium oxide (III) [65]. Alkylated pyrroles and pyrroles having permanganate, and vigorous conditions are required for its electron-donating substituents are more susceptible to oxi- breakdown. Side chains may be oxidized to the correspond- dation than those having electron-withdrawing substituents ing carboxylic acid group without breaking the ring [42]. [64]. Pyrroles that have a ketone or ester substituent are The nitrogen atom, on the other hand, is a center of high also more resistant to ring degradation and yield side- electron density and as such can be easily oxidized to give chain oxidation products [42]. pyridine N-oxide [43]. Alkaline hydrogen peroxide and vari- The oxidation chemistry of pyrrole and the correspond- ous peracids have been used with this purpose [69]. ing benzopyrrole (indole) is quite similar. The presence of It has been reported that quinoline is not autoxidized the benzene ring changes the preferred position of attack under mild reaction conditions (oxidation with air at 130 °C for oxidants from carbon in the 2- and 5-positions in the for 6 h) [67]. Oxygen, hydrogen peroxide and hydroperox- former to carbon in the 3-position in the latter [59]; how- ides in the absence of carboxylic acids are incapable of ever, it does not suppress the susceptibility of the pyrrole oxidizing the pyridinic nitrogen. The acid–base interaction ring to give addition products [64]. The carbon adjacent between the peracid and the pyridine is consequently impor- to the nitrogen atom in a pyrrole ring is readily oxidized, tant for the reaction to proceed. Fig. 11 Oxidation of pyrrolic compounds by oxygen in air, as illustrated by the oxidation of 2,5-dimethylpyrrole 1 3 Applied Petrochemical Research (2018) 8:55–78 69 on in standard texts [42, 43, 62], or in specialized texts [74]. Oxidation of aromatic O‑heterocyclic compounds In fact, it seems that oxidation of this oxygen-containing compound requires the use of a catalyst to enable oxidative The last class of aromatic heterocyclic compounds consid- ered in this work, corresponds to the oxygen-containing addition (coupling) reactions [75–77]. On the other hand, low temperature autoxidation of benzofuran resulted in the compounds, for example, furan and benzofuran (Fig. 9). Of the mononuclear five-membered heterocyclic com- formation of addition products [67]. In this case, the exper- imental evidence indicated that coumaranone, the ketone pounds, i.e., thiophene, pyrrole and furan, furan has the least aromatic character [68]. As a result, its chemistry is, to some obtained by oxidation of benzofuran, was a key intermediate in the oxidative dimerization process. Carbon–carbon cou- extent, different from that described for thiophene and pyr - role. Furan is particularly sensitive to oxidation. In the pres- pling through the 2- and 3-positions of the furan ring in this ketone compound led to the addition product. ence of oxygen or air, it is unstable [43]. The autoxidation of furan involves the 1,4-addition of oxygen to the diene system to give the transannular peroxide. Further reaction opens the aromatic ring by producing succinaldehyde (Fig. 12) [43]. Discussion On the other hand, the vapor-phase catalytic oxidation of furan and its derivatives, using air as oxidant, produces Reactivity of aromatic hydrocarbons to oxidation maleic acid as the main reaction product [70]. Formation of small amounts of acetic and oxalic acids have also been The aromatic sextet reported [71]. Typical oxidation conditions involve the use of vanadium-based catalysts for reaction in the temperature Even though aromatic compounds have been the center of countless studies, it is valuable to refer to two specific contri- range 290–410 °C [70, 72]. During catalytic oxidation, furan derivatives have to undergo oxidative elimination of the butions in relation to the relative stability of aromatic com- pounds compared to cyclic aliphatic compounds and conju- side chain [72]. Substituents are oxidized to furan carbox- ylic acids, which are then decarboxylated to furan (Fig. 13). gated polyenes. Hückel’s rule, given by the formula 4n + 2, where n is zero or an integer, successfully explained the As of this point, the reaction involves the formation of an endoperoxide, followed by the formation of malealdehyde stability of benzene, compared to other monocyclic conju- gated polyenes [78]. On the other hand, the Clar’s π-sextet and finally maleic acid [71]. Substituents on the aromatic ring influence the reaction rate and decrease the yield to rule has enabled a description of the stability of polycyclic systems [79]. maleic acid [72]. Details on the oxidation of furan using singlet oxygen According to Clar’s rule, the aromatic sextet or π-sextet is denoted by a circle. The appropriate way to draw Clar struc- (photochemical oxidation) as well as other oxidants common to the organic chemists can be found in [42, 71, 73]. tures is explained in [80]. Due to the importance of the Clar description of multinuclear aromatics as way to explain the The reactivity of benzofuran is different from that of furan. The oxidation of benzofuran with air is not reported oxidation of aromatics, a brief description is included here. Fig. 12 Oxidation of furan with air as oxidant Fig. 13 Oxidation of furan and furan derivatives using V O as catalyst and air as oxidant 2 5 1 3 70 Applied Petrochemical Research (2018) 8:55–78 Three simple rules are followed to build aromatic structures formation of true double bonds. For instance, the outer [80, 81]: rings in phenanthrene exhibit a local aromaticity and have more aromatic character than the central ring [79]; (1) Circles denoting π-sextets cannot be drawn in adjacent as a result, two external π-sextets and a central double benzenoid rings. This representation would erroneously bond are formed (Fig.  14c). This double bond is as indicate the presence of 12 π-electrons, when in reality reactive as any olefinic double bond [35]. only 10 π-electrons are present. (4) Empty rings: The so-called empty rings refer to those (2) On ignoring the rings with π-sextets, all other benze- rings without π-electrons. They are found in “fully ben- noid rings must have a Kekulé structure. Hence, rings zenoids” structures (Fig. 14d). might have one or two double bonds, or might be empty but there must not be unpaired electrons. For multinuclear aromatics in which different Clar struc- (3) Conditioned to the above constrains, a Clar struc- tures are possible, the one with the greatest number of aro- ture must contain the maximum number possible of matic sextet carbons is preferred [83]. Aromatic compounds π-sextets. When dealing with a linear string of benze- with a higher fraction of aromatic sextet carbons are more noid rings (e.g., anthracene), the π-sextet can occupy stable, compared to those with a higher fraction of isolated any of the rings, which is designated using an arrow. double bonds [83]. Clar’s rule classifies aromatic rings into four categories Influence of structure on oxidation reactivity [35], as illustrated by Fig. 14: The Clar formalism is a valuable tool to rationalize the reac- (1) Rings having localized π-sextets: Aromatic rings having tivity of multinuclear aromatics towards oxidation. Formulas a localized π-sextet, also known as benzenoid rings, with π-sextets not only indicate the stability of a molecule display benzenoid-like stability [82]. Benzenoid rings but also point out to the reactive positions on the ground are considered to be the most aromatic centers in the state [82]. Thus, the structure of an aromatic compound can polyaromatic hydrocarbons; other rings are less aro- be related with the type of oxidation chemistry observed; it matic in comparison, and so are chemically more reac- also explains the differences in reactivity among aromatic tive than the benzenoid rings [79]. hydrocarbons. Figure 15 presents the structures of multinu- (2) Rings sharing a migrating π-sextet: Some multinuclear clear aromatics consisting of three fused rings arranged in aromatics can be represented by more than one Clar different geometric configurations. The oxidation chemistry structure. The aromatic sextet in naphthalene can exist of these compounds, using oxygen as oxidant, will be dis- in any of the two rings. However, resonance makes cussed to illustrate the use of the Clar formalism in explain- the rings in this molecule equal in terms of electron ing oxidation reactivity of multinuclear aromatics. density, and so it is not possible to assign the π-sextet The triangular building principle results in aromatic to any one ring alone. Instead, the π-sextet migrates hydrocarbons that form radicals [82]. The first member of between the rings as denoted by the arrow in Fig. 14b. this series corresponds to perinaphthyl (Fig. 15a). Four reso- In linear acenes, the benzenoid character of the π-sextet nance structures of this compound have been reported [84]. is diluted when increasing the length of the hydrocar- An aromatic sextet and double bonds are possible, but more bon [82]; higher acenes are much more reactive due importantly, a permanent free radical is present. Due to the to migration of the π-sextet and loss of the aromatic diradical nature of oxygen, it can directly perform addition nature. without prior hydrogen abstraction or any other electronic (3) Rings with localized double bonds: In some cases, the rearrangement (Fig.  16a). This radical–radical coupling presence of rings with localized π-sextets leads to the should have the lowest activation energy when comparing Fig. 14 Types of aromatic rings according to Clar’s π-sextet 1 3 Applied Petrochemical Research (2018) 8:55–78 71 Fig. 15 Clar formalism applied to aromatic compounds having three fused rings Fig. 16 Pre-oxidative behavior of a perinaphthyl, b phenanthrene and c anthracene the oxidation chemistry of (a) (b) and (c) in Fig. 15. Once the weaker O–O bond in the hydroxide species can be broken oxygen is added, the resonance in perinaphthyl is restricted. to give a ketone. On the other hand, oxidation of the second Only two of the rings can share a π-sextet, while, a double carbon of the double bond might involve hydrogen abstrac- bond is formed on third one. Oxygen attack is likely to occur tion. It is expected that oxidation of phenanthrene requires in the latter (reactive position). more energy than oxidation of perinaphthyl. The angular configuration of phenanthrene leads to Because anthracene is a linear aromatic hydrocarbon, the formation of two aromatic sextets in the outer rings one migrating π-sextet is shared between the three rings (Fig. 15b). The carbon atoms in these rings are quite sta- (Fig.  15c). Although all carbons should have a similar ble, and so difficult to oxidize. On the contrary, the double reactivity, oxygen preferentially attacks the middle ring, bond in the middle ring is very reactive. Oxygen can per- namely the 9,10-positions. This corresponds to the reso- form direct addition without previous hydrogen abstraction nance structure with two aromatic sextets in the outer rings to give a diradical species: a free radical located on one of and a diradical on the 9,10-positions. During autoxida- the carbons involved in the double bond, and another radical tion, the resonance electronic structure of the molecule is situated on the second oxygen of the O molecule that was disturbed and the two π-electrons at the 9- and 10-posi- added (Fig. 16b). Further oxidation involves some electronic tion become localized, allowing the oxygen to be added rearrangement and bond breaking. The hydrogen bonded to to these two non-adjacent carbons through a para-attack the carbon initially attacked can migrate to stabilize the free (Fig. 16c). Moreover, the transannular addition of oxygen charge on the second carbon. Similarly, additional hydrogen is favored by the formation of two aromatic sextets in the can stabilize the radical on the second oxygen atom (from outer rings of the oxidized product, i.e., 9,10-anthraqui- the O molecule added) forming a hydroperoxide. The now none. The reactivity of the para-position, which is caused 1 3 72 Applied Petrochemical Research (2018) 8:55–78 by the localization of two π-electrons, is important in the Reactivity of  O towards heterocyclic aromatics addition reactions of acenes [82]. The transannular addi- tion of oxygen to the 9,10-positions in anthracene takes It has been shown that oxygen can directly interact with place more readily than oxygen addition to the 9,10-posi- multinuclear aromatics (carbocyclic rings) to oxidize the tion in phenanthracene [6]. aromatic carbons. The way in which O interacts is influ- By comparing the oxidation chemistry of aromatic com- enced by the peculiarities of the structure of the aromatic pounds consisting of three fused rings, it becomes evident substrate, which dictates the most reactive positions (See that the structure influences the way in which oxygen can “Influence of structure on oxidation reactivity”). The reac- interact with a given substrate. The reactivity towards oxi- tivity towards oxidation in heterocyclic hydrocarbons is in dation for linear, angular and condensed aromatic hydro- turn influenced by the presence of a heteroatom in an aro- carbons is different. matic ring. In the case of S-heterocyclic aromatic compounds, autoxi- dation in the liquid phase involves the formation of hydrop- Pre‑oxidative behavior the non‑catalytic oxidation eroxides species by oxidation of molecules other that the of multinuclear aromatics S-heterocyclic aromatic. Oxidation of thiophenes is achieved by the hydroperoxides, rather than by oxygen itself. Further- Based on the work that appeared around the time that Tip- more, the process is highly selective and the oxidation of son [6] was writing his review, advances were made in the heterocyclic sulfur, to give asulfoxides and sulfones, is the understanding of how oxygen interacts with aromat- the main oxidative transformation. Vapor-phase oxidation of ics. The first “pre-oxidation” step, which also explained S-heterocyclic aromatic compounds requires different con- why fluorescence of aromatics was quenched in the pres- ditions and takes place by a catalytic process. In this case, ence of dissolved oxygen, was that the oxygen acted as an the oxidation of the S-containing compounds yields sulfur electron acceptor, with the aromatic molecule being the dioxide and sulfur trioxide and smaller aromatic compounds electron donor [6]. The initiation of oxidation of aromatic compared to the initial molecules. Metal oxides or salts of hydrocarbons appears to be different in its sequence to metals having several oxidation states are used as catalysts that found in aliphatic hydrocarbon oxidation. The lat- [88]. ter involves hydrogen abstraction from the hydrocarbon Autoxidation of N-heterocyclic aromatic compounds fol- being oxidized to form the free radicals required to start lows different chemical routes depending on the basic or the chain process [85, 86]. Hydrogen abstraction is not non-basic character of the nitrogen atom. Oxidation of the necessary for the pre-oxidative step in aromatic oxidation. pyridinic nitrogen with air is difficult. Other oxidants, e.g., In anthracene, the oxygen first adds in a transannu- peracids, have been used to convert pyridines into N oxides. lar fashion to two carbons, which causes the carbons to On the other hand, oxidation of pyrroles is readily achieved 2 3 change from sp to sp hybridized carbons. Only after this by oxygen. In this case, the main oxidative transformation step is the hydrogen on the sp hybridized carbon (non- involves the oxidation of the aromatic carbons to form prod- aromatic hydrogen) abstracted, or transferred. Intermedi- ucts of polymeric nature (addition products), in which oxy- ate products, such as anthrone and oxanthrone were identi- gen might or might not be present. fied and reported in reaction products from the oxidation In the case of O-heterocyclic aromatic compounds, the of anthracene [87]. In phenanthrene, the oxygen first adds reactivity towards autoxidation of furan and its derivatives to one of the carbons in the double bond. This reaction is quite different of that of benzofuran and its benzologs. can be described as radical addition to an olefin, which Oxygen can readily attack the aromatic carbons of furan to causes C–O bond formation with one of the carbons of the ring-open the structure. On the other hand, autoxidation of C=C group and the formation of a carbon centered radi- the aromatic carbons of the 5-membered ring in benzofuran cal on the other carbon. These carbons are no longer aro- leads to addition products. matic, but aliphatic, which facilitates hydrogen migration, transfer and/or abstraction. In perinaphthyl, the oxygen can directly perform addition; radical–radical coupling is Catalysis in aromatic oxidation energetically favorable. It can be seen that regardless of the type of oxidation chemistry taking place, i.e., transan- Transition metal oxides, in particular vanadium oxides, are nular oxygen addition, oxygen addition to a double bond the most important catalysts in industrially successful pro- or free radical chemistry, the first “pre-oxidation” step in cesses for the selective oxidation of aromatics with air (see aromatic oxidation does not involve hydrogen abstraction “Oxidation of aromatic hydrocarbons”). as is the case in aliphatic autoxidation. In fact, it facilitates A detailed discussion on the catalytic chemistry of vana- subsequent hydrogen abstraction. dium oxides dealing with topics such as redox behavior, 1 3 Applied Petrochemical Research (2018) 8:55–78 73 −1 geometry of the catalyst surface, role of the carrier, among M–O bond strength, i.e., values of Q of 210–250 kJ mol −1 others, can be found in [89]. (50–60 kcal mol ). The vapor-phase oxidation of aromatic hydrocarbons Regardless of the complexity of the chemistry of metal occurs mainly by the two-step Mars–Van Krevelen mecha- oxide catalysts in aromatic oxidation, there are a few com- nism [25]. In the first step, the aromatic hydrocarbon is oxi- ments that are worth making. First, the action of oxygen dized by oxygen from the oxide lattice. Desorption of the as oxidant of multinuclear aromatics includes the addition oxygenated product forms an oxygen vacancy and leaves to a double bond, addition to p-centers (transannular addi- an active site in a reduced state at the surface of the catalyst tion) and formation of o-, pseudo-p- or p-diones (Fig. 17) [1]. In the second step, reoxidation of the catalyst active site [6]. These primary pathways do not rely on the presence takes place. This process is achieved either by incorporation of a metal oxide catalyst, e.g., V O . However, the selec- 2 5 of gas phase oxygen or by diffusion of bulk lattice oxygen tive formation of oxygenates accompanied by the cleavage [1, 90]. of the aromatic rings has only been successful at industrial Gaseous oxygen is adsorbed on metal oxides surfaces in level when catalyzed by metal oxides, e.g., V O . Second, 2 5 the form of activated electrophilic species, i.e., O (oxide) or catalytic aromatic oxidation has a significant effect on mini- O (superoxide) species, before being incorporated into the mizing undesired side reactions, i.e., network of free radical 2− lattice in the form of nucleophilic oxygen, i.e., O (oxide reactions and total combustion. Metal oxide catalysts may ion) species [8]. The contribution of non-lattice oxygen to enable transition states that would not require the aromatic oxidation has not been completely ruled out [89]. The role of carbons to assume a free radical nature. Also, catalysts the different oxygen species in catalytic oxidation processes enable conversion processes requiring lower oxidation tem- has been extensively discussed and a rigorous discussion peratures, which inherently reduce losses due to combustion on the topic can be found in [91]. For metal oxides with a reactions. Third, metal oxides accelerate selective oxidation significant mobility of lattice oxygen, regeneration of the of aromatics. By accelerating selective oxidation compared catalyst by diffusion of bulk oxide ions is very efficient; in to other oxidation reactions, catalysts facilitate the produc- these cases, the dead time of the active sites is short and the tion of intermediate oxidation products, namely the desired turn-over frequency is high [1]. products, to be isolated in commercial yields under con- It has been suggested that the activity and selectivity of trolled reaction conditions. metal oxides towards hydrocarbon oxidation depends on the metal–oxygen (M–O) bond strength within the catalyst [92]. Implications for industrial use Oxides with strong M–O bonds are likely to retain their oxy- gen making them inactive for oxidation. On the contrary, Production of oxygenated chemicals by the oxidation of oxides with weak M–O bonds are likely to give their oxygen aromatic hydrocarbons with oxygen can be considered an away, which might lead to rapid but unselective oxidation. industrially proven technology (See “Oxidation of aro- To achieve high selectivity, oxides with M–O bonds of inter- matic hydrocarbons”). These processes make use of puri- mediate strength should be present in the catalyst. fied hydrocarbons as starting materials. The same principle M–O bond strengths have been measured using different applies to oxidation with oxygen for the manufacturing of approaches. Andersen and Kung [93] correlated the differ - smaller volume chemical products, which were not reviewed. ential heat of reoxidation, measured by calorimetry, with As explained in the introduction, the review was moti- the reaction selectivity for the oxidative dehydrogenation of vated by the potential use of autoxidation to add value to butane on V O /Al O . In this case, the selectivity was low materials that contain a high concentration of multinuclear 2 5 2 3 when the heat of reoxidation was small but it increased with aromatics in a mixture, such as heavy oil, bitumen, and coal the increase in the heat of reoxidation. Results suggested that liquids. The application that appears to have the most scope the heat of reoxidation measures the M–O bond strength and for value addition is the use of O instead of H as reagent 2 2 indicates the ease of the removal of lattice oxygen. Simons for ring-opening of multinuclear aromatics. The purpose et  al. [94] studied the interaction between 1-butene and of ring-opening is to ultimately convert the multinuclear butadiene with a series of metal oxides ( MnO, V O , CuO, aromatics into smaller and/or less refractory molecules for 2 2 5 Co O, Fe O , NiO, T iO, SnO , ZnO and Cr O ) over the refining to transport fuels and chemicals. 3 4 2 3 2 2 2 3 temperature range 300–600 °C. They related the M–O bond Oxidative degradation is a recurring theme in the litera- strength with Q , a single parameter that represented the heat ture on coal conversion, and in particular, the wet oxida- necessary to dissociate ½ O from the oxide catalysts. Their tive conversion of coal to lower molecular mass coal acids work indicated that catalytic activity decreased with increas- [95–98]. Although it was found that oxidation using O was ing the M–O bond strength, i.e., high values of Q , and that slower compared to chemical oxidation [99], the outcome a maximum of selectivity could be obtained for intermediate was the similar. 1 3 74 Applied Petrochemical Research (2018) 8:55–78 Fig. 17 Types of diones obtained by oxidation of multinuclear aromatics The oxidative ring-opening of some multinuclear aro- would enable acceptable catalyst cycle times and catalyst matic compounds appears to be quite facile, for example, life time for practical industrial application. oxidation pathway of benzo[a]pyrene with O leads pri- marily to the ring-opened product [100]. Low temperature wet oxidative dissolution of coal with O was reported to achieve dissolution yields of around 70% [97, 98]. Conclusions Analogous claims were made for petroleum derived asphaltenes, but these could not be substantiated [101]. Heavy aromatic feed materials, e.g., coals liquids, bitumen Oxidative hardening appears to dominate the autoxidation and heavy oils could potentially benefit from conversion of heavy petroleum fractions [7]. Conversion leading to by oxidation with air. Oxygen may act as a reagent to ring- oxidative addition reactions is related to the presence of open multinuclear aromatics and give smaller and/or less specific compound classes [67]. refractory compounds for refining to transportation fuels The implication for the potential industrial application and petrochemicals. Based on these premises, this work of autoxidation for ring-opening of multinuclear aromatics reviewed the autoxidation chemistry of the atoms that are is far reaching. It implies that when addition-prone com- part of an aromatic ring structure, i.e., aromatic carbons pound classes are present, a non-radical pathway for oxida- and heteroatoms contained in an aromatic ring. tive ring-opening must be employed. This is possible by The following observations were made from the avail- catalytic oxidation, for example, by making use of metal able literature on selective catalytic oxidation of aromatic oxide catalysts. It remains to be seen whether catalysts hydrocarbons: can be employed with heavy oil and bitumen in a way that 1 3 Applied Petrochemical Research (2018) 8:55–78 75 (a) Production of oxygenates accompanied by cleavage of ture, whereas oxidation of benzofuran leads to addition the aromatic rings has only been successful at industrial products. The most common outcome is the formation scale when catalyzed by transition metal oxides, in par- of heavier products by oxygen-mediated addition reac- ticular vanadium oxides. Catalyzed processes operate tions. in the temperature range 320–450 °C. (i) With exception of furan and its derivatives, the oxida- (b) The catalytic action of metal oxides involves (1) mini- tion with air of heterocyclic aromatics does not lead to mizing the undesired reactions, i.e., free radical chem- ring-opening except at severe non-selective oxidation istry and total combustion, by limiting the formation conditions (total combustion conditions). of transition states that are of free radical nature (2) lowering the oxidation temperatures, which helps con- The Clar formalism is a valuable tool in the understand- trolling the selectivity losses due to total combustion, ing of the fundamentals of aromatic oxidation and its impli- and (3) accelerating selective oxidation compared to cations for industrial use. Some relevant comments are: other oxidation reactions, which facilitates the isola- tion of intermediate oxidation products at commercial (j) The interaction of oxygen with multinuclear aromat- yields. ics is particular to their structure. As a result, funda- (c) The chemistry of metal oxide catalysts in aromatic oxi- mentally different oxidation chemistries are possible, dation is complex. Even though processes have been namely transannular oxygen addition, oxygen addition described by the two-step Mars–Van Krevelen mecha- to a carbon–carbon double bond, or free radical chem- nism, a redox mechanism which states that oxidation istry. takes place by transfer of lattice oxygen, the contribu- (k) The sequence during the initiation of the oxidation of tion of non-lattice oxygen to oxidation has not been aromatic hydrocarbons seems to be different compared completely ruled out. to that found in aliphatic hydrocarbons. Regardless of (d) The absence of a metal oxide catalyst is detrimental the type of chemistry taking place, the first “pre-oxida - for the reaction selectivity. Depending on the tempera- tion” step does not involve hydrogen abstraction. ture of reaction, non-catalyzed oxidation favors total (l) Free radical oxidation can likely not be used for con- combustion over partial oxidation or promotes addition version of petroleum due to the loss in selectivity, e.g., reactions (condensation and polymerization reactions). addition reactions. The only realistic strategy for oxida- tive ring-opening of heavy aromatic feed materials is There are significant differences in the autoxidation catalytic oxidation. chemistry of heterocyclic aromatics compared to that of multinuclear hydrocarbon aromatics (carbocyclic rings that Open Access This article is distributed under the terms of the Crea- do not contain heteroatoms). Some points to consider are: tive Commons Attribution 4.0 International License (http://creat iveco mmons.or g/licenses/b y/4.0/), which permits unrestricted use, distribu- (e) The way in which oxygen interacts with heterocyclic tion, and reproduction in any medium, provided you give appropriate aromatic compounds during oxidation is influenced credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. by the nature of the heteroatom present in an aromatic ring. References (f) Liquid-phase oxidation of S-heterocyclic aromatics is achieved by hydroperoxides species, rather than by 1. Warren BK, Oyama ST (eds) (1996) Heterogeneous hydrocarbon oxygen itself. The oxidation of the aromatic sulfur to oxidation (ACS Symp. Ser. 638). American Chemical Society, form sulfoxides and sulfones species is the main oxida- Washington, DC tive transformation. 2. Sittig M (1962) Combine oxygen and hydrocarbons for profit. 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Gordon and Breach Science Publishers, Amsterdam, pp Publisher’s Note Springer Nature remains neutral with regard to 93–101 jurisdictional claims in published maps and institutional affiliations. 1 3 http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Applied Petrochemical Research Springer Journals

Autoxidation of aromatics

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Abstract

Autoxidation is a conversion pathway that has the potential to add value to multinuclear aromatic-rich coal liquids, heavy oils and bitumens, which are typically considered low-value liquids. In particular, autoxidation of these heavy materials could lead to products that may have petrochemical values, e.g., lubricity improvers and emulsifiers. Proper assessment of an oxidative transformation to ring-open the multinuclear aromatics present in heavy feeds relies on the understanding of the fundamentals of aromatic oxidation. This work reviews the selective oxidation chemistry of atoms that form part of an aromatic ring structure using oxygen (O ) as oxidant, i.e., the oxidation of aromatic carbons as well as heteroatoms contained in an aromatic ring. Examples of industrially relevant oxidations of aromatic and heterocyclic aromatic hydrocarbons are provided. The requirements to produce oxygenates involving the selective cleavage of the ring C–C bonds, as well as com- peting non-selective oxidation reactions are discussed. On the other hand, the Clar formalism, i.e., a rule that describes the stability of polycyclic systems, assists the interpretation of the reactivity of multinuclear aromatics towards oxidation. Two aspects are developed. First, since the interaction of oxygen with aromatic hydrocarbons depends on their structure, oxida- tion chemistries which are fundamentally different are possible, namely, transannular oxygen addition, oxygen addition to a carbon–carbon double bond, or free radical chemistry. Second, hydrogen abstraction is not necessary for the initiation of the oxidation of aromatics compared to that of aliphatics. Keywords Autoxidation · Catalytic oxidation · Multinuclear aromatics · PAHs · Heterocyclic aromatics · Clar formalism Introduction the petrochemical industry. The breadth of the industrial application of oxidation can be seen from books that discuss Processes for the oxidation of organic materials cover the industrially practiced oxidation processes [4, 5]. entire selectivity spectrum ranging from total or near total The scope of this review is much narrower. The review combustion to give carbon oxides, to mild partial oxida- is concerned only with oxidation of atoms that form part tion to functionalize organic molecules to produce chemical of an aromatic ring structure. This is a topic that received intermediates [1]. Selective oxidation is a direct and eco- relatively little attention in the literature since the report by nomic route to convert petroleum products into petrochemi- Tipson [6]. Oxidation of aromatic carbons as well as heter- cals [2]. This type of chemistry is present in the large-scale oatoms contained in aromatic rings will be considered. production of commodities, as well as in the synthesis of Heavy aromatic feed materials can in principle be con- small amounts of pharmaceuticals and fine chemicals [ 3]. verted into higher value petrochemicals that are also mix- Selective oxidation of alkanes (paraffins), alkenes (olefins) tures, for example, lubricity improvers, emulsifiers and and aromatics feedstocks produce oxygenated compounds, greases. Oxidized aromatic compounds may also be low-cost such as alcohols, aldehydes, ketones, carboxylic acids, anhy- intermediates that can be used as feedstocks for the produc- drides and epoxides, which are important commodities in tion of petrochemical products, or that are useful products in their own right. For example, autoxidation (oxidation using air) is commercially employed for asphalt hardening [7], * Natalia Montoya Sánchez which is an important process for producing asphalt for road nmontoya@ualberta.ca construction. Autoxidation is also a conversion pathway by which value can potentially be added to other predominantly Department of Chemical and Materials Engineering, aromatic feed materials, such as coal liquids, petroleum University of Alberta, 9211-116th Street, Edmonton, AB T6G 1H9, Canada Vol.:(0123456789) 1 3 56 Applied Petrochemical Research (2018) 8:55–78 vacuum residues, heavy aromatic oils and bitumens. Such complex mixtures are not normally considered for petro- chemical production. Petrochemicals are usually produced from feed materials that are purified compounds, or well- defined mixtures of compounds. However, due to the low value associated with heavy aromatic feedstocks, there is an economic incentive to increase their quality if possible. Oxidative transformation of heavy aromatic oils is likely challenging and one of the objectives of this review is to assess the potential of oxidation as pathway for the conver- sion of aromatics and in particular multinuclear aromatics. When selecting an oxidant there are some points to con- sider: price, ease of handling, nature of the by-products and percentage of oxygen available. Depending on the scale, pro- duction options change. Bulk industrial oxidation processes are largely limited to air, but fine chemistry can afford the luxury of using more expensive oxidants, as illustrated by Table 1 adapted from [8]. When converting low-value heavy aromatic oils, air and possibly purified oxygen, are likely the only oxidants that make economic sense. Without proving the point, the review was restricted to O as oxidant. It also 2 Fig. 1 Examples of aromatic oxidation involving a non-cleavage of touches on the catalytic selective oxidation of multinuclear the aromatic ring, b cleavage of the aromatic ring, and c oxidation of the side chains attached to the aromatic ring aromatic compounds. Oxidation of aromatic hydrocarbons (b) Formation of oxygenates accompanied by the cleav- age of the aromatic rings, e.g., the formation of maleic The selective oxidation of aromatic hydrocarbons involves anhydride from benzene. three general types of reactions [9], as illustrated by Fig. 1: (c) Formation of oxygenates by oxidation of the side chains attached to the aromatic rings, e.g., the formation of (a) Formation of oxygenates without the cleavage of the benzaldehyde or benzoic acid from toluene. aromatic rings, e.g., the formation of phenol or benzo- quinone from benzene. Table 1 Common oxidants for liquid phase oxidation. Adapted from [8] Oxidant Active oxygen By-product Comments (wt%) Air 23 N /Ar Oxidation forms hydroperoxides; lowest cost for low-pressure open-loop processes a a Purified O 100 None Air separation needed, but purified O becomes more efficient for higher pressure 2 2 and closed-loop processes H O 47 H O Usually available as aqueous solution 2 2 2 c a O 33 O Generated from O by corona discharge 3 2 2 NaClO 22 NaCl ClO can produce chlorocarbon by-products (CH ) CO H 18 (CH ) COH Usually available in solution 3 3 2 3 3 CH COO H 21 CH COOH Usually produced in situ by H O added to acetic acid 3 2 3 2 2 KHSO 11 KHSO Water compatible 5 4 C H IO 7 C H I High cost; metal-catalyzed oxidations often selective 6 5 6 5 Depends on purity, but in practice purified O from an air separation unit is ~ 99.5 wt% Depends on purity, but in practice the H O in water is 35 wt% or less 2 2 Depends on efficiency of corona discharge, typically ~ 2 wt% or less Depends on purity, typically available in solution 1 3 Applied Petrochemical Research (2018) 8:55–78 57 Of these reactions, only (a) and (b) are considered in this In industrial operation benzene is oxidized with 67–72% of review, since (c) does not involve the oxidation of an atom the theoretical yield of maleic anhydride [18]. that is part of the aromatic ring structure. The mechanism of benzene oxidation has been the subject Aromatic C–C bonds are strong and substitution reac- of considerable research work. Yet, the literature indicates tions, reactions of type (a), are more readily achieved. For some disagreements regarding the nature of the intermedi- example, oxidation to produce quinonoids is well-described ates. The generally adopted reaction network for benzene [10]. Reactions of type (a) may also lead to addition products oxidation is presented in Fig. 2. This scheme is the basis of that do not necessarily incorporate oxygen; these oxidative most kinetic studies. Benzene oxidizes in two independent aromatic coupling reactions can be both intra- and intermo- ways to produce either maleic anhydride or total combus- lecular [11]. tion products. Both reactions are the result of a number of Even though aromatic oxidation involving splitting of the short-lived intermediates [17]. Further oxidation of maleic ring C–C bonds represented a milestone in the heterogene- anhydride also leads to the production of carbon oxides and ous catalyzed oxidation technology [12], the literature on water. In the absence of a catalyst, thermal decomposition of the topic is somewhat limited. The literature on ring scission benzene is favored. Transition metal oxides have been suc- is mostly focused on the discussion of a few examples of cessfully used to control the selectivity of the reaction and industrially successful selective catalytic oxidations, e.g., promote the production of maleic anhydride. Industrially, the commercial production of phthalic anhydride from the metal oxides are of interest because the temperature window, vapor-phase oxidation of naphthalene. This type of chem- in which they catalyze the desired oxidation at appreciable istry is of particular interest for the present work, due to its rate, is wider than the one offered by most metal catalysts potential application for the conversion of heavy aromatic [16]. materials. As a result, the oxidation of some key aromatic For benzene oxidation, vanadium, molybdenum and tung- hydrocarbons such as benzene, naphthalene, anthracene and sten oxides have proved to be very selective [16, 18]. How- phenanthrene will be discussed in the following sections. ever, systems based on V O have received special attention. 2 5 There are important differences in the way oxidation of these According to the patent literature, the development of active four compounds proceed. and selective vanadium-based catalysts became an important research topic. A number of different catalyst compositions Benzene oxidation have been proposed and evaluated. Mixtures of vanadium and molybdenum oxides, in combination with one or more The vapor-phase oxidation of benzene to produce maleic oxides of other elements, e.g., P, W, Co, Fe, Ni or their mix- anhydride, using air as oxidant, is one of the first successful tures, or in combination with salts, e.g., sodium borate, have catalytic oxidations of aromatic hydrocarbons. Commer- been tested in the temperature range 340–450 °C [20–22]. cial production of maleic anhydride from benzene started Industrial catalysts for benzene oxidation are based on mix- around 1933. Currently, the industrial process for benzene tures of V O and MoO . The latter can be found in concentra- 2 5 3 to maleic anhydride autoxidation is only of historical and tions up to 30% mol [18]. Typical promoters used are oxides educational interest [12]. The selective vapor-phase oxida- or salts of elements such as W, Bi, Sn, P, Ag, Cu, Na, B, Ti tion of n-butane over vanadium phosphorous oxide (VPO) and Ni [23]. The special catalytic behavior of vanadium-based catalysts has been the dominant chemical route to produce catalysts is related to the valence changes of the metal during maleic anhydride since the mid-1970s [13]. Because of its reaction [16]. Some interesting discussion on the structure, the availability and lower cost, n-butane became an attractive redox behavior and the phases formed in the catalyst during raw material [14]. The industrial routes for phenol production from benzene all employ intermediate products and there are no processes to produce phenol by direct oxidation of benzene [15]. Even though benzene is the simplest aromatic hydro- carbon, its oxidation to maleic anhydride is a complicated chemical reaction, with many side-products. Produc- tion of phenol, quinone and hydroquinone from benzene autoxidation has been reported [16–18]. The presence of small amounts of other compounds such as formaldehyde, biphenyl, acrylic and formic acid has also been mentioned [17–19]. In this case, maleic anhydride is the only product obtained in yields high enough for commercial production. Fig. 2 Benzene oxidation reaction scheme 1 3 58 Applied Petrochemical Research (2018) 8:55–78 reaction as well as the interactions of oxygen and benzene on In steady state, the reaction rates of these two steps the catalysts surface can be found in [23, 24]. (reduction–oxidation cycle) are equal. Also, if for one aro- Mars and Van Krevelen studied the catalytic oxidation matic molecule, β molecules of oxygen are required, it can of some aromatic hydrocarbons, namely benzene, toluene, be written as naphthalene and anthracene [25]. Their work suggested that k p  = k p (1 − ), 1 Ar 2 o (5) the oxidation of these compounds occurred through a redox mechanism (appropriately known as the Mars–Van Krevelen k p mechanism), which involves two independent steps. First, (6) k p + k p . the reaction between the aromatic hydrocarbon and the metal 1 Ar 2 oxide catalyst (MO ) takes place (Eq. 1). Second, the partially reduced catalyst (MO ) is reoxidized with gas phase oxygen Combining Eq. 3 and 6, the overall reaction rate can be n−1 expressed as (Eq. 2), Aromatic hydrocarbon + MO → Oxidation products r = . Ar + MO +ΔQ , (7) (1) n−1 ox + k p k p 1 Ar 2 MO + 1∕2O → MO . (2) n−1 2 n The validity of the kinetic expression (Eq.  7) with Due to the generality of the kinetics following from this m = 1 for the oxidation of benzene was confirmed by mechanism, it is worthwhile providing a brief description. Mars and Van Krevelen [25]. Furthermore, this type of Assuming that the first step is proportional to the partial pres - equation was also valid for the oxidation of naphthalene sure of the aromatic hydrocarbon (p ), i.e., first-order reac- Ar and anthracene; the value of the rate constant k of the tion, and to the fraction of catalyst surface covered with active reoxidation process on a V O –Mo O/Al O catalyst was 2 5 3 2 3 oxygen (θ), the reaction rate can be expressed according to found to be the same for the catalytic oxidation of ben- Eq. 3. On the other hand, if the rate of reoxidation of the cata- zene, naphthalene and anthracene. lyst is assumed to be proportional to a certain power (m) of The reaction kinetics of benzene oxidation over a number oxygen partial pressure and to the catalyst surface area not of vanadium-based catalysts have been reported. A valu- covered with active oxygen (1 − θ), the reaction rate for the able compilation of kinetic studies for benzene oxidation second step can be expressed according to Eq. 4, can be found in [17, 24]. Results from selected studies are r = k p , (3) Ar 1 Ar presented in Table  2. At high concentrations of benzene, reactions (1) and (3) in Fig. 2 appear to be of pseudo-first r = k p (1 − ). o 2 o (4) order on benzene [23]. The effect of the mass transfer rate on the pseudo-first-order rate constants has been discussed Table 2 Catalytic oxidation of benzene: kinetics Reaction conditions Catalyst Kinetic expressionKinetic order Refer- ences T (°C) P (kPa) 375–400 p = 0.3–0.6 V O –MoO on Al O r = kp 1st on B [17] B 2 5 3 2 3 B p = 20 ~ 2nd on M 450–530 p ~ 1.3–2.6 AgO, V O, MoO, Al O on SiC – 1st on B [17] B 2 2 5 3 2 3 p ~ 22 325–450 p = 1–2 V O on Al O – 1st on B [17] B 2 5 2 3 p = 20 377 p = 0.1–4 V O –MoO on Al O 1∕r = 1∕k p + ∕k p p < 6, 1st on B [25] B 2 5 3 2 3 1 B 2 O B p = 10–101 p > 6, 0th on B O B p < 200, 1st on O O 2 p > 300, 0th on O O 2 −3 −3 2 −3 2 380–440 C = (0.1–1.4) × 10 V O –MoO on corundum C < 4 × 10 , r = [O ] / C < 4 × 10 , 2nd on O [18] B 2 5 3 O 2 O 2 2 0.74 0.78 0.71 −3 2 mol/L [M] (k [B] + k [B] ) C < 4 × 10 , 0th on O 1 3 O −3 −3 C = (1–3.2) × 10 C > 4 × 10 , O O 2 2 0.74 mol/L r = 1/[M] 0.78 0.71 (k [B] + k [B] ) 1 3 Kinetic order for the overall disappearance of benzene (B benzene, M maleic anhydride) 1 3 Applied Petrochemical Research (2018) 8:55–78 59 [17]. The dependence of reaction (2) on the concentration of maleic anhydride is less clear [23]. It has been accepted that kinetic models derived from the reaction scheme presented in Fig. 2 are a good approximation when the concentration of benzene is high and the formation of side-products, such as phenol and quinone, can be neglected. More comprehen- sive studies have taken into account the oxidation of benzene as well as the oxidation of maleic anhydride, quinone, and phenol [18]. Naphthalene oxidation The oxidation of naphthalene to phthalic anhydride is com- mercially one of the most important vapor-phase catalytic oxidations of aromatic compounds. The first commercial process started operation in 1916. Industrially, naphthalene oxidation is presently carried out in fixed-bed reactors operating in the temperature range 340–380 °C and using supported vanadium oxide catalysts [26, 27]. Current processes are flexible and allow for using dual feedstocks, i.e., mixtures of naphthalene and o-xylene [26]. In recent years, the use of o-xylene as the main raw material has spread due to its lower cost, good availability and ease of transportation [28]. Fig. 3 Naphthalene oxidation reaction scheme Compared to benzene, the presence of a second aromatic ring in the naphthalene molecule increases the complexity of the reaction network. Several parallel and consecutive be simplified to reactions ( 1)–(4) [27], as discussed later on in this section. reactions take place in this case. Even though naphthoqui- none is a key intermediate in the reaction sequence, phthalic It is believed that the stepwise oxidation of the naph- thalene ring to form phthalic anhydride proceeds by a anhydride is not formed exclusively through this compound. Production of maleic anhydride and carbon oxides as sec- similar mechanism to that of benzene oxidation to form maleic anhydride. The formation of the 1,4-naphthoquinone ondary products has been reported [28]. Figure 3 presents the generally adopted reaction scheme. (α-quinone), which is the equivalent of the quinone in ben- zene oxidation, supports this premise [16]. The formation Naphthalene is oxidized by three independent routes. Pro- duction of phthalic anhydride and 1,4-naphthoquinone rep- of the 1,4-naphthoquinone also points out the difficulty of breaking the aromatic structure. The oxidation of the naph- resents the most important primary reactions [17]. The dis- tribution of these products may vary according to the initial thalene molecule may be analyzed in three steps: addition of oxygen into the structure, breaking of one of the two rings point of attack in the naphthalene molecule [17]. It has been stated that for the sake of completeness, 1,2-naphthoqui- in naphthalene and further destruction of the remaining ben- zene ring [16]. none should be indicated as an intermediate in reaction (1); however, this compound is usually neglected because of its At ambient conditions, naphthalene is inert to air (oxy- gen) and cannot be partially oxidized. Therefore, the use rapid oxidation to phthalic anhydride [27]. Naphthalene is also directly oxidized, to a much lesser extent, to maleic of higher temperatures is required. In the absence of a catalyst, two scenarios are possible [16]. First, oxidation at anhydride and carbon oxides [17, 27]. Secondary reactions are also important in this network. Typically, a large frac- high temperature (i.e., at a temperature high enough for the reaction rate to be significant) favors total combustion over tion of the 1,4-naphthoquinone formed is further oxidized to give phthalic anhydride, while a small portion contributes partial oxidation. Second, oxidation at moderate conditions promotes addition reactions (condensation reactions) and to the production of maleic anhydride and carbon oxides. Similarly, part of the phthalic anhydride formed by reactions polymerization of the intermediates due to the longer reac- tion times needed to achieve significant conversion. The use (1) and (3) is oxidized to give maleic anhydride and carbon oxides. Production of aromatic acids, i.e., phthalic and ben- of an appropriate catalyst is essential to control the reaction selectivity. zoic acids has also been reported [16]. For reactor design purposes, the kinetic scheme of naphthalene oxidation may 1 3 60 Applied Petrochemical Research (2018) 8:55–78 Transition metal oxides of the fifth and sixth groups using vanadium-based catalysts, namely V O and K S O 2 5 2 2 7 of the periodic table have been the candidates of choice. or K SO supported on SiO , are in agreement with this 2 4 2 Vanadium and molybdenum oxides as well as tin and bis- behavior. Mars and Van Krevelen investigated the effect muth vanadates have been tested as catalysts for this reac- of naphthalene and oxygen pressure on the rate of naph- tion [16, 17]. Oxides of magnesium, aluminum, silicon, thalene oxidation [25]. According to their work, the rate titanium, zirconium, copper and cobalt have shown rather is approximately first-order with respect to naphthalene poor performance in naphthalene oxidation [16]. Cata- at p < 0.5 kPa, but it is almost independent of the naph- lysts based on vanadium oxides are quite active and can thalene concentration at p > 1 kPa. A similar trend was cause over-oxidation to maleic anhydride or carbon oxides. observed by Calderbank [30], who studied the reaction Strict control of the reaction residence time or addition of over the range of naphthalene concentrations of 2–10 kPa, less active oxides to the catalyst (e.g., manganese, cop- and by Ioffe and Sherman [31], who explored the pressure per or cobalt oxides) is common practices to avoid this range 0.7–1.5 kPa. On the other hand, the dependence of problem [16]. For vanadium-based catalysts, yields to reaction rate on the concentration of oxygen was found phthalic anhydride of more than 70% have been reported to vary almost linearly with the O concentration in the [17]. Similarly, selectivities towards phthalic anhydride region of oxygen partial pressures below 40 kPa [25]. of nearly 90% have been achieved using mixtures of V O Valuable compilations of kinetic studies for naphthalene 2 5 and K SO supported on SiO , i.e., commercial type of oxidation were prepared by Dixon and Longfield [17] and 2 4 2 catalysts [29]. A great deal of information on the cata- Wainwright and Foster [28]. In general, reactions (1)–(4) in lysts used for naphthalene oxidation is found in the pat- Fig. 3 appear to depend on the concentration of both oxy- ents while only some is found in the journal literature. gen and the organic reactant to an order between 0.5 and There is no recent review dealing with commercially used 1.0 [17]. The observed differences in individual studies are catalysts for naphthalene oxidation. A valuable discussion attributed to the variation in the reaction conditions rather on the parameters affecting the catalytic performance of than a real effect. There is little consistency in the reaction vanadium-based catalysts (e.g., V O concentration, active temperature, catalyst, reactor type and conversion achieved 2 5 sites, role of alkali metal sulfates, among others) can be in the reported studies; therefore, comparisons are not easy found in [28]. In addition, a compilation of the catalysts [28]. and conditions used in naphthalene oxidation can be found in [17]. Anthracene and phenanthrene oxidation The reaction mechanism and kinetics of naphthalene oxidation have been extensively studied. Yet, they are not The catalytic vapor-phase autoxidation of anthracene to completely elucidated. As mentioned in the previous sec- anthraquinone with yields of 85–90% has been described in tion on benzene oxidation, it has been found that naphtha- patents in the 1930s and has found some industrial applica- lene oxidation also proceeds through a redox mechanism tions [2, 17]. The vapor-phase oxidation of anthracene with [25]. In other words, the aromatic hydrocarbon reacts with air is the preferred synthesis method to produce 9,10-anth- a metal oxide catalyst to give the oxidized products and raquinone [32], and > 90% yield is possible. However, the a partially reduced catalyst; then, the catalyst is reoxi- most widespread use of oxidation in relation to anthracene is dized by gas phase oxygen. Hence, the kinetic expression the use of the autoxidation-and-reduction cycle of 9,10-dihy- derived before, Eq. 7, can be used to describe this reac- droxyanthracene and anthraquinone to produce hydrogen tion too. At constant oxygen partial pressure, Eq. 7 can be peroxide [2], as illustrated by Fig. 4. rearranged to Eq. 8. Vapor-phase catalytic oxidations of anthracene and phen- anthrene have been less extensively investigated compared k p 1 N r = , to benzene and naphthalene oxidation. The additional aro- (8) 1 + k Cp 1 N matic ring further increases the complexity of the reaction network. Thus, depending on the position of attack and the extent of the reaction, oxidation products from the inner and where C = . (9) outer rings are possible. As in other aromatic oxidations, k p achieving selectivity is a challenge and using an appropriate catalyst to promote the production of the desired products The latter expression shows that at low partial pressures is necessary. of naphthalene (p ), the oxidation rate should approach Almost complete conversion of anthracene with high first order in naphthalene concentration; whereas at high selectivity to 9,10-anthraquinone is possible and 9,10-anth- partial pressures of naphthalene, it should approach zero raquinone of 99% purity is industrially produced in this way order in this reactant [17]. A number of kinetic studies [32]. Secondary and complete combustion products are also 1 3 Applied Petrochemical Research (2018) 8:55–78 61 addition of oxygen causes the carbons at the 9,10-positions 2 3 to change from sp to sp hybridized carbons. In other words, these carbons are now benzylic aliphatic carbons with a much weaker C–H bond, which facilitates hydrogen abstrac- tion by further reaction with oxygen. It has been suggested that the catalytic conversion takes place in two steps, which are in agreement with the Mars–Van Krevelen mechanism [17, 25]. First, anthracene is oxidized by a metal oxide catalyst producing anthraquinone and water; during this step the oxide catalyst is reduced. Second, the catalyst is reoxidized by gas phase oxygen to return to its original state. Vanadium oxide, iron vanadate or vanadic acid doped with alkali or alkaline earth metal ions are suitable cata- Fig. 4 Catalytic hydrogen peroxide production through the anthraqui- lysts for anthracene oxidation [16, 33, 34]. Industrially, the none process reaction has been carried out at temperatures in the range 320–390 °C and using catalysts consisting of V O, K SO 2 5 2 4 obtained is small amounts. Carbons in the 9- and 10-posi- and Fe O on pumice; yields of 95–97 wt% of anthraqui- 2 3 tions are the most reactive centers of attack in the anthracene none were reported [17]. A reaction scheme derived from molecule [6]. The attack of oxygen on these carbons results the oxidation of anthracene using V O –MoO –P O and 2 5 3 2 5 in the production of 9,10-anthraquinone. On the other hand, V O –Fe O as catalysts can be found [33]. In this case, 2 5 2 3 the unselective oxidation of the carbons in the 1- to 4-posi- oxidation at the inner and outer rings of anthracene leads to tions, i.e., carbons in the outer ring, gives ketone (1,4-anth- 9,10-anthraquinone, 1,4-anthraquinone and 2,3-naphthalic raquinone) and anhydride (2,3-naphthalic anhydride and anhydride. At high conversion, the selectivities towards pyromellitic anhydride) species as by-products [33]. The these compounds decreases and production of phthalic anhy- production of small quantities of phthalic anhydride, maleic dride, pyromellitic anhydride and CO is favored. anhydride, CO and CO has also been reported [16, 33, The liquid phase oxidation of anthracene and 9,10-anth- 34]. The higher selectivity of the oxidation reaction can be raquinone has been investigated by thermal analysis [36]. In explained in terms of the Clar formalism [35], noting that this study, a number of metal oxides (V O, MoO, Fe O 2 5 3 2 3 in anthracene additional stabilization of the outermost rings and NiO) were evaluated as catalysts. Among them, V O 2 5 can be achieved if the π-electrons on the 9,10-positions are showed the highest catalytic contribution during reaction; more localized. it measurably accelerated the autoxidation of both aromatic The overall reaction for vapor-phase anthracene oxida- compounds. Experiments to determine whether this oxide tion is shown in Fig. 5. The oxidation process involves the just activated the O from air, or whether oxidation took interaction of one hydrocarbon molecule with three oxygen place by transfer of lattice oxygen were also performed. atoms. However, the actual reaction mechanism is more Interestingly, reaction under nitrogen atmosphere confirmed complex than that. With the localization of the π-electrons that V O was capable of using the lattice oxygen for oxi- 2 5 on the 9,10-positions, not only does the transannular addi- dation of anthracene and 9,10-anthraquinone in the liquid tion of oxygen take place, but two aromatic sextets are also phase. A Mars–Van Krevelen mechanism was likely. formed. Even more, the removal of hydrogen from an aro- Phenanthrene is used for industrial production of matic carbon is not required for oxidation to take place. The 9,10-phenanthrene quinone as well as diphenic acid by Fig. 5 Vapor-phase oxidation of anthracene 1 3 62 Applied Petrochemical Research (2018) 8:55–78 vapor-phase oxidation over vanadium-based catalysts [33]. routes, which are governed by the position of attack, i.e., Same as anthracene, the 9 and 10 carbons in the phenan- oxidation in the inner or outer rings. threne molecule are much more reactive than the other Compared to the benzene or naphthalene cases, there ones, and so the 9- and 10-positions are the preferred are a lot fewer publications dealing with the kinetics of the centers of attack [6]. As in the case of anthracene, this is vapor-phase anthracene and phenanthrene oxidation. Mars explained by the Clar formalism [35]. Selective oxidation and Van Krevelen found that at anthracene partial pres- on these carbons gives the 9,10-dione; whereas further sures > 0.1 kPa, the reaction rate was essentially independ- reaction produces anhydride (diphenic anhydride), lactone ent with respect to the concentration of the aromatic reactant (2-hydroxy-diphenyl-2′-carboxylic acid lactone) and ketone [25]. In addition, the reaction rate showed some dependence (9-fluorenone) species as side-products [33]. When oxygen on oxygen over the entire concentration range (oxygen par- attacks the carbons on the outer rings (not preferred path- tial pressures up to 100 kPa). Pyatnitskii derived a kinetic way), 1,2-naphthalic anhydride is produced [33]. On the expression for aromatic oxidations [38] based on the work other hand, a number of experimental studies taking place of Mars and Van Krevelen. In his work, the two independent in the temperature range 370–440 °C and using vanadium steps of the redox mechanism (Eqs. 1 and 2) were written as pentoxide, mixtures of vanadium and molybdenum or iron r = k p  , (10) Ar Ar Ar oxides and tin vanadates as oxidation catalysts resulted in the production of phthalic anhydride and phthalic acid [17]. m b r = k p (1 − ) , o o (11) The reaction network for phenanthrene oxidation is pre- 2 2 sented in Fig. 6. The scheme was derived from the study of where a and b are the orders with respect to adsorbed oxygen the reaction over V O supported on SiO with and without 2 5 2 and to oxygen vacancies on the catalyst surface, respectively. K SO [37]. Phenanthrene is oxidized by two independent 2 4 Fig. 6 Phenanthrene oxidation reaction scheme 1 3 Applied Petrochemical Research (2018) 8:55–78 63 These equations reduce to Eqs. 3 and 4 when a = 1 and b = 1. industrial processes. Reaction at near-isothermal conditions In steady state, is preferred to maintain optimal catalyst efficiency and prod- uct selectivity. Multitubular reactors and fluidized beds with r = r ∕v, Ar o (12) heat exchange internals have been widely used for this pur- where the number of oxygen molecules required to oxidize pose; after all, these configurations are suitable to minimize one aromatic molecule (Ar) to the products P , P , …, P , is 1 2 i the temperature gradients across the catalyst bed. represented by the stoichiometric coefficient (v ) as follows: Most of the processes for industrial production of maleic anhydride from benzene use multitubular reactors. A typical v = v S + v S + v S +… + v S , (13) 1 1 2 2 3 3 i i reactor contains a bundle of 10,000–15,000 vertical tubes, where S is the selectivity for P and v corresponds to the i i i ~ 25 mm in diameter, enclosed by a jacket through which number of oxygen molecules necessary to oxidize one aro- cooling medium circulates [23]. During operation, the reac- matic molecule to P . By replacing Eqs. 10 and 11 in Eq. 5, tion gas flows through the tubes and over the catalyst, while it can be seen that the surface concentration of oxygen is a the process pressure is kept between 0.15 and 0.25 MPa [19]. function of the ratio of the partial pressures of oxygen and Circulating heat-transfer fluids or agitated eutectic salt mix- the aromatic hydrocarbon ( P ∕P ): O Ar tures are commonly used to deal with the large duty [23]. Steam, which can be used in other operations, is generated a k p o o 2 2 from the heat removed. Nearly 27 MJ of heat are dissipated = . (14) k v p (1 − ) Ar Ar per each tonne of reacted benzene, which translates to the production of almost 10 tonnes of saturated steam per tonne For the simplest case a = b = 1, and combining Eqs. 11 of reacted benzene [19]. Considering that the reaction is and 14, the total rate of oxidation can be expressed as conducted at temperatures > 300 °C, production of high- pressure steam, typically 4–5 MPa steam, is possible. k k p p Ar o Ar o 2 2 r = . A typical flow diagram of a process using a multitubular (15) Ar (k p + vk p ) o o Ar Ar 2 2 reactor for benzene oxidation [23] is presented in Fig. 7. In this configuration, a mixture of benzene and preheated The kinetic expression (Eq. 15) derived in Pyatnitskii’s air is reacted in the multitubular reactor. The conversion of work describes the oxidation of anthracene on V O –K SO 2 5 2 4 benzene is not complete. The feed-product heat exchange with Fe O and S iO , and on C oMoO /SiO [38]. On the 2 3 2 4 2 decreases the duty of the feed preheating. On the other other hand, the kinetics of the oxidation of phenanthrene are hand, switch condensers allow for recovering close to 90% best described with empirical power law equations [38]. It is of the maleic anhydride produced in the reaction. Refining worth mentioning that the rate of oxidation of phenanthrene of this product is completed later on by batch distillation. is less than that of anthracene over the same V O –K SO / 2 5 2 4 The remaining 10% of maleic anhydride is recovered in a SiO catalyst. two-stage scrubber unit using water and dilute alkali. Fuma- ric acid is also obtained in this process. The unconverted Reactor engineering in aromatic oxidation benzene is separated from the exhaust air by adsorption with activated carbon, and it is returned as part of the reactor Vapor-phase catalytic aromatic oxidations are strongly exo- feed. thermic reactions. Furthermore, the heat released is always Control of the reaction temperature during catalytic aro- greater than the calculated heat of reaction of the main reac- matic oxidation can also be achieved using fluidized bed tion, due to side reactions leading to the formation of C O . reactors [23]. Some commercial plants employ this tech- For instance, while the heat of oxidation of naphthalene to nology to oxidize naphthalene to phthalic anhydride. Thus, phthalic anhydride corresponds to 1880 kJ/mol, in prac- during typical operation, liquid naphthalene is fed at the tice, due to over-oxidation the total heat liberated can reach bottom of the catalyst bed; after instant evaporation, it dis- 2900 kJ/mol [39]. tributes over the entire fluidized bed. Oxygen fed through a Aromatic oxidation requires large amounts of heat to be distributor plate is mixed with the vaporized naphthalene. removed from the reaction zone to maintain the temperature The reaction temperature is kept in the range 345–385 °C and keep the overall process under control. An uncontrolled through heat exchanger tubes in the fluidized bed. Moreo- reaction would cause a severe reduction of yields (complete ver, a uniform temperature profile is obtained because of the combustion would be favored) as well as the loss of catalyst intense agitation and mixing of the catalyst in the fluidized life [23]. Furthermore, it is a self-amplifying problem, i.e., bed. High-pressure steam (> 4 MPa) is produced from the an increase in temperature causes more combustion reactions heat removed [40]. that would further increase the temperature. Proper reactor The increase in size for aromatic hydrocarbons, which is design and heat management are therefore key aspects in represented by the number of aromatic rings in a molecule, 1 3 64 Applied Petrochemical Research (2018) 8:55–78 Fig. 7 Ruhrol–Lurgi process for production of maleic anhydride by benzene oxidation Table 3 Heat evolved during Aromatic hydrocarbon Overall oxidation reaction ∆H (kJ/mol) References aromatic oxidation Benzene − 1917 [16] C H +4 O → C H O +2CO +2H O 6 6 2 4 2 3 2 2 Naphthalene − 1881 C H +4 O → C H O +2CO +2 H O 10 8 2 8 4 3 2 2 Anthracene − 559 C H + O → C H O +2H O 14 10 2 14 8 2 2 Theoretical heat of reaction (heat released per mol of aromatic hydrocarbon reacted) has an effect on the calculated (theoretical) heat of reaction. Oxidation of heterocyclic aromatics According to Table 3, the heat liberated during oxidation to the main oxidation product decreases in the order benzene > naph- The selective oxidation of heterocyclic aromatics involves thalene > anthracene. As the molecules become heavier, fewer the same types of reaction as noted for the aromatic hydro- oxygen atoms react per mole of aromatic substrate to produce carbons [9], but added differentiation is required to distin - the main product, and so considerably less heat is released. guish between carbon oxidation and heteroatom oxidation. Irrespective of the aromatic feed, successful selective oxida- The general types of reactions are (Fig. 8): tion requires proper heat management. 1 3 Applied Petrochemical Research (2018) 8:55–78 65 aromatic rings, e.g., o-nitrobenzo-propenoic acid from quinoline. (e) Formation of oxygenates by oxidation of the side chains attached to the aromatic rings. (f) e.g., oxidation of 4-methyl-pyridine to pyridine-4-car- boxylic acid. Of these reaction types, only (a)–(d) will be considered. It is also possible for oxidation of the multinuclear aromatics to take place on a carbocyclic ring that does not contain a heteroatom, as was discussed in the previous section. One variation on reaction types (a) and (b) is reactions leading to the formation of addition products. Oxidative addition does not necessarily lead to oxygen incorporation into the product. Even though selective oxidation is a direct and economic route to convert aromatic hydrocarbons into valuable chemi- cals, the scenario for heterocyclic aromatics is a little dif- ferent. Oxidation with air of these compounds as a synthe- sis route to obtain added value products is not reported in standard texts [42, 43]. Their oxidation chemistry is rather discussed as part of the crude oil processing and fuel stabil- ity. For instance, sulfur- and nitrogen-containing compounds are typically undesired materials in crude oil. The oxidation of these compounds (i.e., oxidative desulfurization and oxi- dative denitrogenation) to give chemically modified products with properties that favor their separation or removal has been explored [44, 45]. Furthermore, it is known that autoxi- dation of nitrogen heterocyclic aromatics plays a role in the formation of heavy addition products, e.g., nitrogen-con- taining compounds seem to be related with the formation of Fig. 8 Examples of heteroaromatic oxidation involving a carbon sludge in fuels [46], thereby affecting fuel storage stability. oxidation without the cleavage of the aromatic rings, b heteroatom The oxidation of the distinct aromatic heterocyclic oxidation without the cleavage of the aromatic rings, c cleavage of carbon–carbon bonds of the aromatic rings, d cleavage of carbon–het- compound classes is considered in the following sections. eroatom bonds of the heterocyclic aromatic rings, e oxidation of the Emphasis is placed on the chemistry of five-membered side chains attached to the aromatic rings heterocyclic compounds that contain sulfur, nitrogen, and oxygen, as well as on their benzologs. Some comments (a) Formation of oxygenates by carbon oxidation without regarding the chemistry of pyridine, a nitrogen-containing the cleavage of the aromatic rings, e.g., formation of six-membered heterocyclic compound, are also included as coumaranone from benzofuran and the polymerization part of the discussion. of pyrrole. (b) Formation of oxygenates by heteroatom oxidation with- Oxidation of aromatic S‑heterocyclic compounds out the cleavage of the aromatic rings, e.g., formation of benzothiophene-1,1-dioxide from benzothiophene. The first class of aromatic heterocyclic compounds con- (c) Formation of oxygenates accompanied by the cleav- sidered in this work corresponds to the sulfur-containing age of carbon–carbon bonds of the aromatic rings, e.g., compounds, which are represented by thiophene and its ben- formation of N,2-(cyclopenta-1,5-dione)-aniline from zologs, e.g., benzothiophene and dibenzothiophene (Fig. 9). 2,3-cyclopenteno-indole [41]. However, it should be These molecules are typically found in the heavier fractions noted that oxidative carbon–carbon scission in hetero- of crude oil [47]. Refining of crude oil to final products, cyclic aromatic rings appears to be rare. such as fuels and some petrochemicals, requires desulfuri- (d) Formation of oxygenates accompanied by the cleav- zation of the oil. However, the sulfur contained in aromatic age of carbon-heteroatom bonds of the heterocyclic rings is more difficult to remove than aliphatic sulfur using 1 3 66 Applied Petrochemical Research (2018) 8:55–78 two separate steps. First, the reaction between an oxidant and sulfur alters the nature of the sulfur-containing com- pounds. Second, the properties of the oxidized products are exploited to facilitate their removal through a separa- tion process [44]. Oxidation with air has been extensively evaluated. Autoxidation of S-heterocyclic aromatics involves the formation of hydroperoxides by oxidation of aliphatic compounds that are already present or are added to the reaction mixture. Hydroperoxides are key intermediates formed in  situ by the oxygen. The thiophenic sulfur is subsequently oxidized by the hydroperoxides, rather than by oxygen directly (free radical chemistry). The chemistry taking place during the first step of this conversion illus- trates the oxidative behavior of S-heterocyclic compounds in the presence of air. The thiophenic sulfur is oxidized to Fig. 9 Examples of heterocyclic aromatic compounds sulfoxides and sulfones (Fig. 10). Moreover, thiophenes that could be considered refractive traditional technologies, e.g., hydrodesulfurization (HDS) to HDS, are readily oxidized because of their increased elec- and thermal conversion [47]. tron density on the sulfur atom [48]. Temperatures below Oxidation of thiophene and its benzologs as an alterna- 200 °C and pressures near atmospheric pressure are usual tive approach to HDS for sulfur removal has received a for oxidation of heterocyclic sulfur. Typical conditions for lot of attention. Oxidative desulfurization (ODS) involves oxidative desulfurization processes are given in Table 4. Fig. 10 Oxidation of sulfur compounds by oxygen in air, as illustrated by the oxidation of thiophene Table 4 Desulfurization of model mixtures and industrial feedstocks by liquid-phase oxidation with air Feed Oxidant Catalyst Oxidation conditions Comments References Straight-run kerosene O Non-catalytic oxidation 120–200 °C Stirring rate and resident [49] 5–20 min time affected the degree of High pressure oxidation of sulfur-containing Presence of water compounds Model oil and hydrodesulfur- Air Non-catalytic oxidation 140 °C Lactones (e.g., γ-butyrolactone) [50] ized diesel fuel Atm. Pressure used as solvents play a role in oxygen transfer Model jet fuel and real jet fuel O Fe(III) salts (nitrate and bro- 25 °C Oxidation reactivity sequence [51] (JP-8) mide) 2–5 h between ASBP, BP and DBP was evaluated Model oil and diesel fuel O Co (II) salts (acetate and 40 °C Aldehydes were used as sac- [52] chloride) Atm. pressure rificial materials to produce, with the oxygen fed, in situ peracids to oxidize the sulfur- containing compounds Atmospheric residue Air/O Pt, Pd, Ni, V (their salts or 130–180 °C Metals catalyze decomposition [53] oxides) 2–20 h of organic hydroperoxides ASBP alkyl-substituted benzothiophene, BP benzothiophene, DBP dibenzothiophene 1 3 Applied Petrochemical Research (2018) 8:55–78 67 Autoxidation of S-heterocyclic aromatics can also be observed for aromatic compounds, due to the high selectiv- performed using a catalyst or oxygen carrier, instead of ity of autoxidation to oxidize the heterocyclic sulfur. In this employing free radical autoxidation. Catalysts facilitate the case, there are no complex reaction networks. The forma- decomposition of hydroperoxides, and in doing so, acceler- tion of oxygenates by oxidation of the heteroatom is actually ate the propagation step during autoxidation. Oxygen carri- the main oxidative transformation. Even for addition reac- ers, on the other hand, are more active oxidation agents than tions to take place, oxidation of the sulfur plays a role and oxygen. Depending on their nature, they can be viewed as a autoxidation of the aromatic carbons is not observed except type of catalyst which can be regenerated using molecular at extreme (combustion) conditions. oxygen, i.e., oxidation by the Mars–Van Krevelen mecha- nism; this characteristic makes their behavior similar to the Oxidation of aromatic N‑heterocyclic compounds one observed for metal oxides in the selective oxidation of aromatic compounds, as discussed in the previous section. The second class of aromatic heterocyclic compounds exam- Additional details on the topic can be found in [54]. ined in this work corresponds to the nitrogen-containing The C–S bond strength is decreased when the sulfur is compounds. Examples of N-heterocyclic compounds are oxidized, which in principle allow the cleavage of the C–S pyrrole, indole and pyridine (Fig. 9). Aromatic nitrogen- bonds by catalytic [55], or by thermal decomposition. These containing compounds present in crude oil are normally assertions are based on the published homolytic bond disso- classified in two categories: basic (derivatives of pyridine) ciation energies for sulfones. In the case of a diaryl sulfone, and non-basic (derivatives of pyrrole). Irrespective of the which is representative of the sulfone group in dibenzothio- category, these compounds are associated with a number of phene-1,1-dioxide, the bond dissociation energy is report- different problems during oil processing and their removal edly 288 kJ/mol [56]. However, this value was found to be is desirable [45]. inconsistent with the thermal stability of diaryl sulfones that Oxidation of nitrogen-containing compounds has been suggested a much stronger bond [57]. explored as an alternative approach to traditional practices Autoxidation of sulfur-containing compounds can also such as hydrodenitrogenation (HDN) by hydrotreating. Even lead to the formation of addition products. For instance, though the fundamental idea behind this process, i.e., oxi- model oil consisting of n-heptane and dibenzothiophene dative denitrogenation, is very similar to those of oxidative reacted with air at 145–170 °C and atmospheric pressure desulfurization (the aim is to produce chemically modified resulted in the formation of some heavier products [58]. products that can be more efficiently extracted from oils In this case, besides the typical products involving the for- using polar solvents), very little has been reported on the mation of both sulfoxides (dibenzothiophene-1-oxide) and topic [45]. sulfones (dibenzothiophene-1,1-dioxide), the formation of Few studies dealing with the oxidation of industrial feed- sediments was observed. This precipitation process involves stocks have been reported, and in those cases the oxidation a type of chemistry analogous to that explaining the gum for- of the S- and N-containing compounds has been performed mation, a common problem that undermines storage stability simultaneously [60, 61]. The purpose of oxidative denitro- of transportation fuels. In the proposed reaction pathway, genation is mainly to remove basic nitrogen-containing com- a sulfoxide reacts with a hydroperoxide to give an inter- pounds. Typical reaction conditions involve temperatures mediate product that may be decomposed by reaction with around 70 °C and atmospheric pressure. Peracetic acid or another sulfoxide to yield an addition product [58]. mixtures of hydrogen peroxide and acetic acid have been The thiophene ring system, unless carrying electron- used as oxidizing agents to convert pyridines into N oxides. donating substituents, is relatively stable to atmospheric While sulfones and sulfoxides are easily obtained during oxidation [59], and resist the action of moderate oxidizing oxidative desulfurization, products of polymeric nature are agents [42, 43]. Oxygen and peracids are known to pref- obtained after the oxidation of N-heterocyclic aromatics erentially attack the sulfur atom [43], while stronger oxi- [60]. It will be shown that this is due to the oxidation of dants are required to actually attack the aromatic carbons. pyrroles and not due to the oxidation of pyridines. Ozone, for example, attacks the double bond between the Because the oxidation chemistry of pyrroles and pyri- aromatic carbons which results in the cleavage of the ring; dines is quite different, some comments for each class of thus, oxidation of benzothiophene with ozone yields o-mer- compound will be made in the following sub-sections in a captobenzaldehydae [59]. Similarly, strong oxidants, e.g., separate manner. nitric acid, are able to break down the aromatic ring to give maleic and oxalic acids, while the ring sulfur is oxidized to Oxidation of pyrroles sulfuric acid [59]. It is evident that oxidation chemistry of sulfur-contain- Simple pyrroles are easily oxidized. Compounds contain- ing compounds, using O as oxidant, is simpler that the one ing a pyrrole ring, such as indoles and carbazoles, have a 1 3 68 Applied Petrochemical Research (2018) 8:55–78 negative effect on the oxidative stability of jet fuels [62]. which ultimately leads to oxidative coupling and formation The effect of pyrroles on color and sediment formation in of polymeric or addition products [64]. fuels has been documented and explained through a num- Depending on the conditions of the reaction, indole addi- ber of oxidation and polymerization reactions [63]. These tion may or may not involve the incorporation of oxygen. In are all industrially important fouling reactions. the presence of air and light, indole is autoxidized to give However, the nature of the products depends on the indoxyl which further reacts to produce indigo [43]. Indigo strength of the oxidizing agent. Strong oxidants, such as is a molecule containing two oxygen atoms and it is the reac- chromium trioxide in aqueous sulfuric acid, often lead to tion product from two indole molecules. The nature of the complex breakdown products. When the ring survives, reaction products found after the catalytic oxidation of indole maleimide derivatives are obtained [42]. Milder oxidants using Mn(II)porphyrins as catalyst, suggested that the mecha- such as hydrogen peroxide convert pyrroles to pyrrolines nism of indole addition involves both oxygen incorporation [59]. In the case of oxygen, which is the oxidant of inter- and hydrogen disproportionation [66]. On the other hand, est in this work, pyrrolic compounds autoxidize to form selectivity close to 90% towards oxidative addition product dark colored pigments known as “pyrrole black” [64, 65]. formation was found during the low temperature air oxidation In oil fractions exposed to air, pyrroles autoxidize to form of indole [67]. In this case, no evidence for oxygen incorpora- the so-called “red tars” [43, 64]. Irrespective of the name, tion in the products was found suggesting that the addition pyrrole black or red tar, these are all mixtures of pyrrole mechanism did not require oxygen incorporation even though addition products. Oxidative addition of pyrroles is due to oxygen was necessary for the reaction to proceed. aromatic carbon oxidation [64], a variation on the reaction type (a) shown in Fig. 8. Oxidation of pyridines The autoxidation of 2,5-dimethylpyrrole is presented in Fig. 11 to illustrate how a typical reaction of a pyrrolic The lone pair of electrons on the nitrogen atom that is avail- compound with air takes place. The first step corresponds able for bonding, without disturbing the aromaticity of the to the formation of a molecular association complex (I) ring, makes pyridine chemistry quite distinct from that of between the oxygen and the pyrrolic compound. Then, the pyrrole [67, 68]. Pyridines are therefore bases. Because electron transfer from the pyrrole nucleus of the complex the pyridine ring is π-electron deficient and oxidants act as to the oxygen produces a charge-transfer complex (II), electron acceptors, the oxidation of the ring is difficult [43]. which is suggested to be in equilibrium with an endoper- In fact, strong oxidants, e.g., neutral aqueous potassium oxide (III) [65]. Alkylated pyrroles and pyrroles having permanganate, and vigorous conditions are required for its electron-donating substituents are more susceptible to oxi- breakdown. Side chains may be oxidized to the correspond- dation than those having electron-withdrawing substituents ing carboxylic acid group without breaking the ring [42]. [64]. Pyrroles that have a ketone or ester substituent are The nitrogen atom, on the other hand, is a center of high also more resistant to ring degradation and yield side- electron density and as such can be easily oxidized to give chain oxidation products [42]. pyridine N-oxide [43]. Alkaline hydrogen peroxide and vari- The oxidation chemistry of pyrrole and the correspond- ous peracids have been used with this purpose [69]. ing benzopyrrole (indole) is quite similar. The presence of It has been reported that quinoline is not autoxidized the benzene ring changes the preferred position of attack under mild reaction conditions (oxidation with air at 130 °C for oxidants from carbon in the 2- and 5-positions in the for 6 h) [67]. Oxygen, hydrogen peroxide and hydroperox- former to carbon in the 3-position in the latter [59]; how- ides in the absence of carboxylic acids are incapable of ever, it does not suppress the susceptibility of the pyrrole oxidizing the pyridinic nitrogen. The acid–base interaction ring to give addition products [64]. The carbon adjacent between the peracid and the pyridine is consequently impor- to the nitrogen atom in a pyrrole ring is readily oxidized, tant for the reaction to proceed. Fig. 11 Oxidation of pyrrolic compounds by oxygen in air, as illustrated by the oxidation of 2,5-dimethylpyrrole 1 3 Applied Petrochemical Research (2018) 8:55–78 69 on in standard texts [42, 43, 62], or in specialized texts [74]. Oxidation of aromatic O‑heterocyclic compounds In fact, it seems that oxidation of this oxygen-containing compound requires the use of a catalyst to enable oxidative The last class of aromatic heterocyclic compounds consid- ered in this work, corresponds to the oxygen-containing addition (coupling) reactions [75–77]. On the other hand, low temperature autoxidation of benzofuran resulted in the compounds, for example, furan and benzofuran (Fig. 9). Of the mononuclear five-membered heterocyclic com- formation of addition products [67]. In this case, the exper- imental evidence indicated that coumaranone, the ketone pounds, i.e., thiophene, pyrrole and furan, furan has the least aromatic character [68]. As a result, its chemistry is, to some obtained by oxidation of benzofuran, was a key intermediate in the oxidative dimerization process. Carbon–carbon cou- extent, different from that described for thiophene and pyr - role. Furan is particularly sensitive to oxidation. In the pres- pling through the 2- and 3-positions of the furan ring in this ketone compound led to the addition product. ence of oxygen or air, it is unstable [43]. The autoxidation of furan involves the 1,4-addition of oxygen to the diene system to give the transannular peroxide. Further reaction opens the aromatic ring by producing succinaldehyde (Fig. 12) [43]. Discussion On the other hand, the vapor-phase catalytic oxidation of furan and its derivatives, using air as oxidant, produces Reactivity of aromatic hydrocarbons to oxidation maleic acid as the main reaction product [70]. Formation of small amounts of acetic and oxalic acids have also been The aromatic sextet reported [71]. Typical oxidation conditions involve the use of vanadium-based catalysts for reaction in the temperature Even though aromatic compounds have been the center of countless studies, it is valuable to refer to two specific contri- range 290–410 °C [70, 72]. During catalytic oxidation, furan derivatives have to undergo oxidative elimination of the butions in relation to the relative stability of aromatic com- pounds compared to cyclic aliphatic compounds and conju- side chain [72]. Substituents are oxidized to furan carbox- ylic acids, which are then decarboxylated to furan (Fig. 13). gated polyenes. Hückel’s rule, given by the formula 4n + 2, where n is zero or an integer, successfully explained the As of this point, the reaction involves the formation of an endoperoxide, followed by the formation of malealdehyde stability of benzene, compared to other monocyclic conju- gated polyenes [78]. On the other hand, the Clar’s π-sextet and finally maleic acid [71]. Substituents on the aromatic ring influence the reaction rate and decrease the yield to rule has enabled a description of the stability of polycyclic systems [79]. maleic acid [72]. Details on the oxidation of furan using singlet oxygen According to Clar’s rule, the aromatic sextet or π-sextet is denoted by a circle. The appropriate way to draw Clar struc- (photochemical oxidation) as well as other oxidants common to the organic chemists can be found in [42, 71, 73]. tures is explained in [80]. Due to the importance of the Clar description of multinuclear aromatics as way to explain the The reactivity of benzofuran is different from that of furan. The oxidation of benzofuran with air is not reported oxidation of aromatics, a brief description is included here. Fig. 12 Oxidation of furan with air as oxidant Fig. 13 Oxidation of furan and furan derivatives using V O as catalyst and air as oxidant 2 5 1 3 70 Applied Petrochemical Research (2018) 8:55–78 Three simple rules are followed to build aromatic structures formation of true double bonds. For instance, the outer [80, 81]: rings in phenanthrene exhibit a local aromaticity and have more aromatic character than the central ring [79]; (1) Circles denoting π-sextets cannot be drawn in adjacent as a result, two external π-sextets and a central double benzenoid rings. This representation would erroneously bond are formed (Fig.  14c). This double bond is as indicate the presence of 12 π-electrons, when in reality reactive as any olefinic double bond [35]. only 10 π-electrons are present. (4) Empty rings: The so-called empty rings refer to those (2) On ignoring the rings with π-sextets, all other benze- rings without π-electrons. They are found in “fully ben- noid rings must have a Kekulé structure. Hence, rings zenoids” structures (Fig. 14d). might have one or two double bonds, or might be empty but there must not be unpaired electrons. For multinuclear aromatics in which different Clar struc- (3) Conditioned to the above constrains, a Clar struc- tures are possible, the one with the greatest number of aro- ture must contain the maximum number possible of matic sextet carbons is preferred [83]. Aromatic compounds π-sextets. When dealing with a linear string of benze- with a higher fraction of aromatic sextet carbons are more noid rings (e.g., anthracene), the π-sextet can occupy stable, compared to those with a higher fraction of isolated any of the rings, which is designated using an arrow. double bonds [83]. Clar’s rule classifies aromatic rings into four categories Influence of structure on oxidation reactivity [35], as illustrated by Fig. 14: The Clar formalism is a valuable tool to rationalize the reac- (1) Rings having localized π-sextets: Aromatic rings having tivity of multinuclear aromatics towards oxidation. Formulas a localized π-sextet, also known as benzenoid rings, with π-sextets not only indicate the stability of a molecule display benzenoid-like stability [82]. Benzenoid rings but also point out to the reactive positions on the ground are considered to be the most aromatic centers in the state [82]. Thus, the structure of an aromatic compound can polyaromatic hydrocarbons; other rings are less aro- be related with the type of oxidation chemistry observed; it matic in comparison, and so are chemically more reac- also explains the differences in reactivity among aromatic tive than the benzenoid rings [79]. hydrocarbons. Figure 15 presents the structures of multinu- (2) Rings sharing a migrating π-sextet: Some multinuclear clear aromatics consisting of three fused rings arranged in aromatics can be represented by more than one Clar different geometric configurations. The oxidation chemistry structure. The aromatic sextet in naphthalene can exist of these compounds, using oxygen as oxidant, will be dis- in any of the two rings. However, resonance makes cussed to illustrate the use of the Clar formalism in explain- the rings in this molecule equal in terms of electron ing oxidation reactivity of multinuclear aromatics. density, and so it is not possible to assign the π-sextet The triangular building principle results in aromatic to any one ring alone. Instead, the π-sextet migrates hydrocarbons that form radicals [82]. The first member of between the rings as denoted by the arrow in Fig. 14b. this series corresponds to perinaphthyl (Fig. 15a). Four reso- In linear acenes, the benzenoid character of the π-sextet nance structures of this compound have been reported [84]. is diluted when increasing the length of the hydrocar- An aromatic sextet and double bonds are possible, but more bon [82]; higher acenes are much more reactive due importantly, a permanent free radical is present. Due to the to migration of the π-sextet and loss of the aromatic diradical nature of oxygen, it can directly perform addition nature. without prior hydrogen abstraction or any other electronic (3) Rings with localized double bonds: In some cases, the rearrangement (Fig.  16a). This radical–radical coupling presence of rings with localized π-sextets leads to the should have the lowest activation energy when comparing Fig. 14 Types of aromatic rings according to Clar’s π-sextet 1 3 Applied Petrochemical Research (2018) 8:55–78 71 Fig. 15 Clar formalism applied to aromatic compounds having three fused rings Fig. 16 Pre-oxidative behavior of a perinaphthyl, b phenanthrene and c anthracene the oxidation chemistry of (a) (b) and (c) in Fig. 15. Once the weaker O–O bond in the hydroxide species can be broken oxygen is added, the resonance in perinaphthyl is restricted. to give a ketone. On the other hand, oxidation of the second Only two of the rings can share a π-sextet, while, a double carbon of the double bond might involve hydrogen abstrac- bond is formed on third one. Oxygen attack is likely to occur tion. It is expected that oxidation of phenanthrene requires in the latter (reactive position). more energy than oxidation of perinaphthyl. The angular configuration of phenanthrene leads to Because anthracene is a linear aromatic hydrocarbon, the formation of two aromatic sextets in the outer rings one migrating π-sextet is shared between the three rings (Fig. 15b). The carbon atoms in these rings are quite sta- (Fig.  15c). Although all carbons should have a similar ble, and so difficult to oxidize. On the contrary, the double reactivity, oxygen preferentially attacks the middle ring, bond in the middle ring is very reactive. Oxygen can per- namely the 9,10-positions. This corresponds to the reso- form direct addition without previous hydrogen abstraction nance structure with two aromatic sextets in the outer rings to give a diradical species: a free radical located on one of and a diradical on the 9,10-positions. During autoxida- the carbons involved in the double bond, and another radical tion, the resonance electronic structure of the molecule is situated on the second oxygen of the O molecule that was disturbed and the two π-electrons at the 9- and 10-posi- added (Fig. 16b). Further oxidation involves some electronic tion become localized, allowing the oxygen to be added rearrangement and bond breaking. The hydrogen bonded to to these two non-adjacent carbons through a para-attack the carbon initially attacked can migrate to stabilize the free (Fig. 16c). Moreover, the transannular addition of oxygen charge on the second carbon. Similarly, additional hydrogen is favored by the formation of two aromatic sextets in the can stabilize the radical on the second oxygen atom (from outer rings of the oxidized product, i.e., 9,10-anthraqui- the O molecule added) forming a hydroperoxide. The now none. The reactivity of the para-position, which is caused 1 3 72 Applied Petrochemical Research (2018) 8:55–78 by the localization of two π-electrons, is important in the Reactivity of  O towards heterocyclic aromatics addition reactions of acenes [82]. The transannular addi- tion of oxygen to the 9,10-positions in anthracene takes It has been shown that oxygen can directly interact with place more readily than oxygen addition to the 9,10-posi- multinuclear aromatics (carbocyclic rings) to oxidize the tion in phenanthracene [6]. aromatic carbons. The way in which O interacts is influ- By comparing the oxidation chemistry of aromatic com- enced by the peculiarities of the structure of the aromatic pounds consisting of three fused rings, it becomes evident substrate, which dictates the most reactive positions (See that the structure influences the way in which oxygen can “Influence of structure on oxidation reactivity”). The reac- interact with a given substrate. The reactivity towards oxi- tivity towards oxidation in heterocyclic hydrocarbons is in dation for linear, angular and condensed aromatic hydro- turn influenced by the presence of a heteroatom in an aro- carbons is different. matic ring. In the case of S-heterocyclic aromatic compounds, autoxi- dation in the liquid phase involves the formation of hydrop- Pre‑oxidative behavior the non‑catalytic oxidation eroxides species by oxidation of molecules other that the of multinuclear aromatics S-heterocyclic aromatic. Oxidation of thiophenes is achieved by the hydroperoxides, rather than by oxygen itself. Further- Based on the work that appeared around the time that Tip- more, the process is highly selective and the oxidation of son [6] was writing his review, advances were made in the heterocyclic sulfur, to give asulfoxides and sulfones, is the understanding of how oxygen interacts with aromat- the main oxidative transformation. Vapor-phase oxidation of ics. The first “pre-oxidation” step, which also explained S-heterocyclic aromatic compounds requires different con- why fluorescence of aromatics was quenched in the pres- ditions and takes place by a catalytic process. In this case, ence of dissolved oxygen, was that the oxygen acted as an the oxidation of the S-containing compounds yields sulfur electron acceptor, with the aromatic molecule being the dioxide and sulfur trioxide and smaller aromatic compounds electron donor [6]. The initiation of oxidation of aromatic compared to the initial molecules. Metal oxides or salts of hydrocarbons appears to be different in its sequence to metals having several oxidation states are used as catalysts that found in aliphatic hydrocarbon oxidation. The lat- [88]. ter involves hydrogen abstraction from the hydrocarbon Autoxidation of N-heterocyclic aromatic compounds fol- being oxidized to form the free radicals required to start lows different chemical routes depending on the basic or the chain process [85, 86]. Hydrogen abstraction is not non-basic character of the nitrogen atom. Oxidation of the necessary for the pre-oxidative step in aromatic oxidation. pyridinic nitrogen with air is difficult. Other oxidants, e.g., In anthracene, the oxygen first adds in a transannu- peracids, have been used to convert pyridines into N oxides. lar fashion to two carbons, which causes the carbons to On the other hand, oxidation of pyrroles is readily achieved 2 3 change from sp to sp hybridized carbons. Only after this by oxygen. In this case, the main oxidative transformation step is the hydrogen on the sp hybridized carbon (non- involves the oxidation of the aromatic carbons to form prod- aromatic hydrogen) abstracted, or transferred. Intermedi- ucts of polymeric nature (addition products), in which oxy- ate products, such as anthrone and oxanthrone were identi- gen might or might not be present. fied and reported in reaction products from the oxidation In the case of O-heterocyclic aromatic compounds, the of anthracene [87]. In phenanthrene, the oxygen first adds reactivity towards autoxidation of furan and its derivatives to one of the carbons in the double bond. This reaction is quite different of that of benzofuran and its benzologs. can be described as radical addition to an olefin, which Oxygen can readily attack the aromatic carbons of furan to causes C–O bond formation with one of the carbons of the ring-open the structure. On the other hand, autoxidation of C=C group and the formation of a carbon centered radi- the aromatic carbons of the 5-membered ring in benzofuran cal on the other carbon. These carbons are no longer aro- leads to addition products. matic, but aliphatic, which facilitates hydrogen migration, transfer and/or abstraction. In perinaphthyl, the oxygen can directly perform addition; radical–radical coupling is Catalysis in aromatic oxidation energetically favorable. It can be seen that regardless of the type of oxidation chemistry taking place, i.e., transan- Transition metal oxides, in particular vanadium oxides, are nular oxygen addition, oxygen addition to a double bond the most important catalysts in industrially successful pro- or free radical chemistry, the first “pre-oxidation” step in cesses for the selective oxidation of aromatics with air (see aromatic oxidation does not involve hydrogen abstraction “Oxidation of aromatic hydrocarbons”). as is the case in aliphatic autoxidation. In fact, it facilitates A detailed discussion on the catalytic chemistry of vana- subsequent hydrogen abstraction. dium oxides dealing with topics such as redox behavior, 1 3 Applied Petrochemical Research (2018) 8:55–78 73 −1 geometry of the catalyst surface, role of the carrier, among M–O bond strength, i.e., values of Q of 210–250 kJ mol −1 others, can be found in [89]. (50–60 kcal mol ). The vapor-phase oxidation of aromatic hydrocarbons Regardless of the complexity of the chemistry of metal occurs mainly by the two-step Mars–Van Krevelen mecha- oxide catalysts in aromatic oxidation, there are a few com- nism [25]. In the first step, the aromatic hydrocarbon is oxi- ments that are worth making. First, the action of oxygen dized by oxygen from the oxide lattice. Desorption of the as oxidant of multinuclear aromatics includes the addition oxygenated product forms an oxygen vacancy and leaves to a double bond, addition to p-centers (transannular addi- an active site in a reduced state at the surface of the catalyst tion) and formation of o-, pseudo-p- or p-diones (Fig. 17) [1]. In the second step, reoxidation of the catalyst active site [6]. These primary pathways do not rely on the presence takes place. This process is achieved either by incorporation of a metal oxide catalyst, e.g., V O . However, the selec- 2 5 of gas phase oxygen or by diffusion of bulk lattice oxygen tive formation of oxygenates accompanied by the cleavage [1, 90]. of the aromatic rings has only been successful at industrial Gaseous oxygen is adsorbed on metal oxides surfaces in level when catalyzed by metal oxides, e.g., V O . Second, 2 5 the form of activated electrophilic species, i.e., O (oxide) or catalytic aromatic oxidation has a significant effect on mini- O (superoxide) species, before being incorporated into the mizing undesired side reactions, i.e., network of free radical 2− lattice in the form of nucleophilic oxygen, i.e., O (oxide reactions and total combustion. Metal oxide catalysts may ion) species [8]. The contribution of non-lattice oxygen to enable transition states that would not require the aromatic oxidation has not been completely ruled out [89]. The role of carbons to assume a free radical nature. Also, catalysts the different oxygen species in catalytic oxidation processes enable conversion processes requiring lower oxidation tem- has been extensively discussed and a rigorous discussion peratures, which inherently reduce losses due to combustion on the topic can be found in [91]. For metal oxides with a reactions. Third, metal oxides accelerate selective oxidation significant mobility of lattice oxygen, regeneration of the of aromatics. By accelerating selective oxidation compared catalyst by diffusion of bulk oxide ions is very efficient; in to other oxidation reactions, catalysts facilitate the produc- these cases, the dead time of the active sites is short and the tion of intermediate oxidation products, namely the desired turn-over frequency is high [1]. products, to be isolated in commercial yields under con- It has been suggested that the activity and selectivity of trolled reaction conditions. metal oxides towards hydrocarbon oxidation depends on the metal–oxygen (M–O) bond strength within the catalyst [92]. Implications for industrial use Oxides with strong M–O bonds are likely to retain their oxy- gen making them inactive for oxidation. On the contrary, Production of oxygenated chemicals by the oxidation of oxides with weak M–O bonds are likely to give their oxygen aromatic hydrocarbons with oxygen can be considered an away, which might lead to rapid but unselective oxidation. industrially proven technology (See “Oxidation of aro- To achieve high selectivity, oxides with M–O bonds of inter- matic hydrocarbons”). These processes make use of puri- mediate strength should be present in the catalyst. fied hydrocarbons as starting materials. The same principle M–O bond strengths have been measured using different applies to oxidation with oxygen for the manufacturing of approaches. Andersen and Kung [93] correlated the differ - smaller volume chemical products, which were not reviewed. ential heat of reoxidation, measured by calorimetry, with As explained in the introduction, the review was moti- the reaction selectivity for the oxidative dehydrogenation of vated by the potential use of autoxidation to add value to butane on V O /Al O . In this case, the selectivity was low materials that contain a high concentration of multinuclear 2 5 2 3 when the heat of reoxidation was small but it increased with aromatics in a mixture, such as heavy oil, bitumen, and coal the increase in the heat of reoxidation. Results suggested that liquids. The application that appears to have the most scope the heat of reoxidation measures the M–O bond strength and for value addition is the use of O instead of H as reagent 2 2 indicates the ease of the removal of lattice oxygen. Simons for ring-opening of multinuclear aromatics. The purpose et  al. [94] studied the interaction between 1-butene and of ring-opening is to ultimately convert the multinuclear butadiene with a series of metal oxides ( MnO, V O , CuO, aromatics into smaller and/or less refractory molecules for 2 2 5 Co O, Fe O , NiO, T iO, SnO , ZnO and Cr O ) over the refining to transport fuels and chemicals. 3 4 2 3 2 2 2 3 temperature range 300–600 °C. They related the M–O bond Oxidative degradation is a recurring theme in the litera- strength with Q , a single parameter that represented the heat ture on coal conversion, and in particular, the wet oxida- necessary to dissociate ½ O from the oxide catalysts. Their tive conversion of coal to lower molecular mass coal acids work indicated that catalytic activity decreased with increas- [95–98]. Although it was found that oxidation using O was ing the M–O bond strength, i.e., high values of Q , and that slower compared to chemical oxidation [99], the outcome a maximum of selectivity could be obtained for intermediate was the similar. 1 3 74 Applied Petrochemical Research (2018) 8:55–78 Fig. 17 Types of diones obtained by oxidation of multinuclear aromatics The oxidative ring-opening of some multinuclear aro- would enable acceptable catalyst cycle times and catalyst matic compounds appears to be quite facile, for example, life time for practical industrial application. oxidation pathway of benzo[a]pyrene with O leads pri- marily to the ring-opened product [100]. Low temperature wet oxidative dissolution of coal with O was reported to achieve dissolution yields of around 70% [97, 98]. Conclusions Analogous claims were made for petroleum derived asphaltenes, but these could not be substantiated [101]. Heavy aromatic feed materials, e.g., coals liquids, bitumen Oxidative hardening appears to dominate the autoxidation and heavy oils could potentially benefit from conversion of heavy petroleum fractions [7]. Conversion leading to by oxidation with air. Oxygen may act as a reagent to ring- oxidative addition reactions is related to the presence of open multinuclear aromatics and give smaller and/or less specific compound classes [67]. refractory compounds for refining to transportation fuels The implication for the potential industrial application and petrochemicals. Based on these premises, this work of autoxidation for ring-opening of multinuclear aromatics reviewed the autoxidation chemistry of the atoms that are is far reaching. It implies that when addition-prone com- part of an aromatic ring structure, i.e., aromatic carbons pound classes are present, a non-radical pathway for oxida- and heteroatoms contained in an aromatic ring. tive ring-opening must be employed. This is possible by The following observations were made from the avail- catalytic oxidation, for example, by making use of metal able literature on selective catalytic oxidation of aromatic oxide catalysts. It remains to be seen whether catalysts hydrocarbons: can be employed with heavy oil and bitumen in a way that 1 3 Applied Petrochemical Research (2018) 8:55–78 75 (a) Production of oxygenates accompanied by cleavage of ture, whereas oxidation of benzofuran leads to addition the aromatic rings has only been successful at industrial products. The most common outcome is the formation scale when catalyzed by transition metal oxides, in par- of heavier products by oxygen-mediated addition reac- ticular vanadium oxides. Catalyzed processes operate tions. in the temperature range 320–450 °C. (i) With exception of furan and its derivatives, the oxida- (b) The catalytic action of metal oxides involves (1) mini- tion with air of heterocyclic aromatics does not lead to mizing the undesired reactions, i.e., free radical chem- ring-opening except at severe non-selective oxidation istry and total combustion, by limiting the formation conditions (total combustion conditions). of transition states that are of free radical nature (2) lowering the oxidation temperatures, which helps con- The Clar formalism is a valuable tool in the understand- trolling the selectivity losses due to total combustion, ing of the fundamentals of aromatic oxidation and its impli- and (3) accelerating selective oxidation compared to cations for industrial use. Some relevant comments are: other oxidation reactions, which facilitates the isola- tion of intermediate oxidation products at commercial (j) The interaction of oxygen with multinuclear aromat- yields. ics is particular to their structure. As a result, funda- (c) The chemistry of metal oxide catalysts in aromatic oxi- mentally different oxidation chemistries are possible, dation is complex. Even though processes have been namely transannular oxygen addition, oxygen addition described by the two-step Mars–Van Krevelen mecha- to a carbon–carbon double bond, or free radical chem- nism, a redox mechanism which states that oxidation istry. takes place by transfer of lattice oxygen, the contribu- (k) The sequence during the initiation of the oxidation of tion of non-lattice oxygen to oxidation has not been aromatic hydrocarbons seems to be different compared completely ruled out. to that found in aliphatic hydrocarbons. Regardless of (d) The absence of a metal oxide catalyst is detrimental the type of chemistry taking place, the first “pre-oxida - for the reaction selectivity. Depending on the tempera- tion” step does not involve hydrogen abstraction. ture of reaction, non-catalyzed oxidation favors total (l) Free radical oxidation can likely not be used for con- combustion over partial oxidation or promotes addition version of petroleum due to the loss in selectivity, e.g., reactions (condensation and polymerization reactions). addition reactions. The only realistic strategy for oxida- tive ring-opening of heavy aromatic feed materials is There are significant differences in the autoxidation catalytic oxidation. chemistry of heterocyclic aromatics compared to that of multinuclear hydrocarbon aromatics (carbocyclic rings that Open Access This article is distributed under the terms of the Crea- do not contain heteroatoms). Some points to consider are: tive Commons Attribution 4.0 International License (http://creat iveco mmons.or g/licenses/b y/4.0/), which permits unrestricted use, distribu- (e) The way in which oxygen interacts with heterocyclic tion, and reproduction in any medium, provided you give appropriate aromatic compounds during oxidation is influenced credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. by the nature of the heteroatom present in an aromatic ring. References (f) Liquid-phase oxidation of S-heterocyclic aromatics is achieved by hydroperoxides species, rather than by 1. Warren BK, Oyama ST (eds) (1996) Heterogeneous hydrocarbon oxygen itself. The oxidation of the aromatic sulfur to oxidation (ACS Symp. Ser. 638). American Chemical Society, form sulfoxides and sulfones species is the main oxida- Washington, DC tive transformation. 2. Sittig M (1962) Combine oxygen and hydrocarbons for profit. 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Applied Petrochemical ResearchSpringer Journals

Published: Mar 28, 2018

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