Novel Ni/ZnO–HZSM-5 adsorbents were synthesized by incipient wetness impregnation. The Ni/ZnO–HZSM-5 adsorbent can achieve deep desulfurization and olefin aromatization at the same time. Thiophene sulfur was removed from 495 to less than 10 ppm via reactive adsorption desulfurization (RADS). Olefins were also converted into aromatics. HZSM-5 did not only support adsorbents but also cooperated with active Ni sites to catalyze olefins into aromatic hydrocarbons. Aromatization of 1-pentene, 2-pentene, 2-methyl-2-butene, and 1-hexene on adsorbents was investigated. The adsorbents were character- ized by the Brunauer–Emmett–Teller, X-ray diffraction, temperature-programmed reduction, and temperature-programmed desorption of ammonia and thermogravimetric analysis. The experimental results showed that strong acids on the adsorbent disappeared after HZSM-5 loaded active metal sites, and almost no coke was generated on adsorbents in RADS. Keywords Ni/ZnO-HZSM-5 · Desulfurization · Olefin aromatization Introduction of FCC gasoline will improve, and the fuel refining industry will also save high amounts of energy. Deep desulfurization of fossil fuels and upgrading the The traditional adsorbent of RADS consists of Ni/ quality of gasoline will be an inevitable tendency for strin- ZnO–Al O –SiO ; adsorbents have also performed supe- 2 3 2 gent environmental legislations in various countries. Fluid riorly in deep desulfurization of FCC gasoline [2–4]. The catalytic cracking (FCC) gasoline contains large amounts active component Ni plays a key role in adsorbing sulfur of olefins and thiophene sulfides [ 1]; besides, olefins are atoms . Organic sulfide compounds are first adsorbed by unstable. Therefore, if olefins can be efficiently converted Ni atoms, and Ni continuously removes the sulfur atoms to other high-octane and stable components such as aro- from organic sulfides, generating NiS [ 6]. NiS would be matics or isoparaffins, the quality of gasoline will signifi - reduced by H and release H S. Finally, H S is captured 2 2 2 cantly improve, and the octane number of gasoline will also rapidly by ZnO and produces ZnS based on chemical be maintained at the same time. Currently, the technology Eqs. (1), (2), and (3) . The mechanism of adsorption of deep desulfurization is very mature, especially reactive has been put forward by Song  and Velu et al. , who adsorption desulfurization (RADS). If RADS can couple considered that organic sulfide compounds were adsorbed olefin aromatization or isomerization together, the quality on active metal Ni sites via direct sulfur–adsorbent (S–M) interaction, forming organometallic complexes, rather than by π-complexation. Wang et al.  also confirmed that thiophene was adsorbed via direct S–M interaction through * Yonghong Li the study of adsorption heat, whereas olefins were adsorbed firstname.lastname@example.org through π-complexation. However, although S–M inter- Key Lab for Green Chemical Technology of Ministry actions are much stronger than π-complexation, they still of Education, School of Chemical Engineering exhibit fierce competitive adsorption due to the larger quan - and Technology, Tianjin University, Tianjin 300350, China tity of olefins than sulfides in FCC gasoline. As a result, National Engineering Research Center for Distillation this competitive adsorption would decrease the selectivity Technology, Tianjin 300350, China of desulfurization on the adsorbents, and olefins would satu - Collaborative Innovation Center of Chemical Science rate in the H atmosphere. Therefore, studies attempt to find and Engineering, Tianjin 300350, China Vol.:(0123456789) 1 3 144 J. Du et al. methods to enhance the selectivity of adsorption and avoid trans 95.0%), 2-methyl-2-butene (Aladdin, 99%), n-heptane olefin saturation. Khare [11, 12] put forward a formulation (Aladdin, 98.5%), thiophene (TCI, 98.0%), Ni(NO ) ·6H O 3 2 2 in which active sites consist of Ni and Co and expected that (Aladdin, 99.0%) and Zn(NO ) ·6H O (Aladdin, 99.0%), 3 2 2 Ni and Co could play a synergistic role to enhance the abil- HZSM-5 (Nankai University Catalyst Co., Ltd, China), and ity of RADS. Wang et al.  demonstrated that Ni and Co γ-Al O (Aladdin, 99.0%). The model hydrocarbon com- 2 3 are the best active metal sites by density functional theory. prised 35.0 wt% olefin and 65.0 wt% n-heptane. Sulfur Skrzypski et al.  used Mo as auxiliaries to prepare adsor- concentration reached 495 mg/kg and was represented by bents which could increase the specific surface area and pore 1300 mg/kg thiophene in the model feedstock. volume. Ju et al.  used Ca to promote the dispersion of active Ni sites and enhance desulfurization and regeneration Adsorbent Preparation ability of adsorbents. Ni/ZnO–HZSM-5 adsorbents with different ratios of Ni/ Ni + RSH = NiS + RH, (1) ZnO were synthesized by incipient wetness impregna- tion. HZSM-5 was calcinated at 500 °C for 3.0 h before NiS + H = Ni + H S (2) 2 2 using. A total of 4.95 g of Ni(NO ) ·6H O was dissolved 3 2 2 ZnO + H S = ZnS + H O in 6.0 mL distilled water and followed by the addition of (3) 2 2 6.1 g of Zn(NO ) ·6H O under stirring at 90 °C in water However, although the adsorbent Ni/ZnO–Al O –SiO 3 2 2 2 3 2 bath. The mixture solution was dropwise added into 4.0 g could realize deep desulfurization and control the concen- HZSM-5 and ultrasound-treated for 15 min, followed by stir- tration of sulfur under 10 mg/kg, a part of olefin would react ring at 90 °C in water bath for 4 h, evaporating the redundant through hydrogenation saturation [16–18], resulting in the water. The mixture was aged at 25 °C for 8 h. After aging, loss of octane value. ZnO, Ni, and Al O would easily com- 2 3 the mixture was dried at 120 °C overnight and followed by bine together and generate irreversible spinel of ZnAl O 2 4 calcination at 550 °C in air for 6 h in a muffle furnace. The and inactive NiAl O in Ni/ZnO–Al O –SiO adsorbent 2 4 2 3 2 temperature of the muffle furnace was heated up to 550 °C . Therefore, novel adsorbents must be developed for with a ramp rate of 4 °C per min. Finally, the powder was desulfurization and maintaining octane number. Porous tableted and ground to 20–40 mesh adsorbents. For com- materials, such as Al O or SiO and HZSM-5, could be 2 3 2 parison purposes, Ni/ZnO–Al O adsorbent was synthesized used as matrix owing to their large specific surface area 2 3 using the same method, and Ni/ZnO was synthesized by . HZSM-5 possesses large specific surface area, unique co-precipitation. The adsorbents with different ratios of Ni/ micropore channels, and stable structural characteristics. ZnO were numbered, as shown in Table 1. Particularly, HZSM-5 can resist coking and olefin aromati- zation [20–22]. Accordingly, HZSM-5 is the ideal material Adsorbent Performance Evaluation and Analytic for olefin aromatization and isomerization [23– 25]. HZSM-5 Procedure loading Ni or Zn would increase the amount of Lewis acid sites and decrease Bronsted acid sites . Lewis acid sites RADS performance of adsorbents was evaluated in a fixed favor the olefin aromatization and isomerization . bed microreactor using thiophene as a model sulfur com- We aim to convert olefins into aromatics and isoparaffin pound. A steel microreactor with an internal diameter of during RADS, achieving the deep desulfurization and main- 6 mm and 600 mm in length was used for experiment. A taining the octane number at the same time. In this paper, total of 1.0 g (20–40 mesh) adsorbent was loaded into the HZSM-5 was used as carrier to load Ni–ZnO and to syn- constant-temperature zone of the microreactor column and thesize difunctional Ni/ZnO–HZSM-5 adsorbents for deep embedded between glass wool plugs. Before RADS experi- desulfurization and reduction of olefins. The adsorbents ment, Ni/ZnO–HZSM-5 was first reduced for conversion of were evaluated with RADS using four different olefins as NiO into Ni in the presence of H with a flow rate of 30 mL/ feedstock, and the samples were characterized with a series 2 min under 2.0 MPa and at 470 °C for 2 h. After reduction, of techniques. Table 1 Adsorbent numbers No. Adsorbent Experiment 1 Ni/ZnO-HZSM-5-1:1 2 Ni/ZnO-HZSM-5-1:1.5 Chemicals and Feedstock 3 Ni/ZnO-HZSM-5-1:2 4 Ni/ZnO–Al O 2 3 The chemicals and feedstock included 1-hexene (Macklin, 5 Ni/ZnO 99%), 1-pentene (TCI, 99.0%), 2-pentene (TCI, 99.0%, 1 3 Difunctional Adsorbents Ni/ZnO–HZSM-5 on Adsorption Desulfurization and Aromatization of… 145 the temperature of adsorbent bed was set to 400 °C, and the derived from the adsorption branch of isotherms based on pressure was unchanged at 2.0 MPa. Then, the model fuel the Horvath–Kawazoe method. The profiles of temperature- was preheated at 120 °C and then fed into the microreactor programmed desorption of ammonia (NH -TPD) were car- −1 by liquid pump at a weight hourly space velocity of 4.1 h ried out on a Xianquan TP-5076 TPD analyzer with a ther- and H /oil volume ratio of 100. The deactivated adsorbents mal conductivity detector (TCD). Finally, thermogravimetric were regenerated at 480 °C with an air flow rate of 30 mL/ analysis (TG) was performed in air flow (25 mL/min) on min under 0.5 MPa. Product composition was analyzed with Shimadzu-TGA-50 apparatus. Temperature ranged from 35 gas chromatography–mass spectrometry (GC–MS) (Agilent to 750 °C with a heating rate of 10 °C per min. 7890) and a Fuli 9790 gas chromatograph using a flame ioni- zation detector. The total sulfur content in the liquid product was analyzed with a flame photometric detector (Fuli 9790). Results and Discussion Adsorption capacity of thiophene sulfur was calculated by Eq. (4): RADS Performance of Adsorbents C = C V − C V ∕m (4) 0 1 where C refers to the initial sulfur concentration; V is fuel Figure 1 presents the RADS performance and sulfur adsorp- flow; C is the sulfur concentration in RADS product; and tion capacities (mg/g) of Ni/ZnO–HZSM-5-X adsorbents; m is weight of adsorbents. the model fuel comprises 34.3% 1-hexene and 65.5% n-hep- tane. The Ni/ZnO–HZSM-5-1:1 adsorbent exhibited an Adsorbent Characterization excellent RADS performance with high thiophene conver- sion of 98.5% before the first 12 mL model fuel and gradu- X-ray diffraction (XRD) measurements were taken on a ally decreased to 95.6% with the model fuel of 20 mL. The Bruker D 8-Focus and advanced X-ray diffractometer (Cu corresponding cumulative sulfur adsorption capacity meas- Kα λ = 0.15418 nm, 40 kV, and 40 mA) in the step scan- ured 7.34 mg/g. In the case of Ni/ZnO–HZSM-5-1:1.5 and ning mode with 2θ between 20° and 80° at a scanning step Ni/ZnO–HZSM-5-1:2, sulfur adsorption capacity reached of 5°/min. The reducibility of adsorbents was investigated 7.21 and 6.87 mg/g, respectively. Thiophene conversion by hydrogen temperature-programmed reduction (H -TPR) of Ni/ZnO–HZSM-5-1:1.5 remained above 98.0% before technique using the apparatus of Xianquan TP-5079, and 12 mL. After 20 mL of model fuel, thiophene conversion reduction temperature ranged from 60 to 770 °C with a reduced drastically. The order of desulfurization ability ramp rate of 10 °C per min. N sorption experiments of Ni/ is as follows: Ni/ZnO–HZSM-5-1:1 > Ni/ZnO–HZSM-5- ZnO–HZSM-5 were performed at 77 K by a Micromeritics 1:1.5 > Ni/ZnO–HZSM-5-1:2. The difference in RADS ASAP 2020 analyzer. Prior to the analysis, adsorbents were activity is mainly ascribed to the number of active Ni degassed at 300 °C for 6 h under N . The surface area was atoms distributed on the adsorption surface. The used Ni/ calculated from the adsorption branch in the range of relative ZnO–HZSM-5-1:1.5 adsorbents were regenerated, and pressure from 0.050 to 0.295 by Brunauer–Emmett–Teller Ni/ZnO–HZSM-5-1:1.5 (R) was applied again for RADS (BET) method. The micropore volume was calculated by test. As shown in Fig. 1b, the RADS performance of Ni/ t-Polt method, and the pore size distribution (PSD) was ZnO–HZSM-5-1:1.5 (R) was as good as the fresh materials. Fig. 1 a Sulfur adsorption capacity of adsorbents at 24 mL model fuel and b RADS profile of adsorbents 1 3 146 J. Du et al. Olefin aromatization was also investigated. Aromatics the Ni/ZnO–HZSM-5-1:1.5 adsorbent to study the perfor- yield of Ni/ZnO–HZSM-5-1:1, Ni/ZnO–HZSM-5-1:1.5, mance of desulfurization and aromatization. Unless special and Ni/ZnO–HZSM-5-1:2 reached 58.5, 67.1, and 63.7%, instructions were provided, the adsorbent considered was respectively. The conversion of olefins to aromatics showed Ni/ZnO–HZSM-5-1:1.5. a nonlinear correlation with the amount of Ni atoms. Studies have reported that ZnO could also enhance the catalysis of Comparison of Thiophene Desulfurization olefin aromatization . Thus, ZnO would act synergisti-and Aromatization in C Olefins cally with Ni in aromatization. Otherwise, aromatics yield of Ni/ZnO–HZSM-5-1:1 would not reach lower than that of Ni/ To investigate the influences between aromatization and ZnO–HZSM-5-1:2. The aromatics yield of Ni/ZnO–HZSM- desulfurization, three kinds of C isomeride olefins were 5-1:1.5 was higher than those of Ni/ZnO–HZSM-5-1:2 tested on Ni/ZnO–HZSM-5-1:1.5. Table 2 presents the com- orand Ni/ZnO–HZSM-5-1:1. Desulfurization performance position distributions of RADS product with different model of Ni/ZnO–HZSM-5-1:1.5 adsorbent was almost the same fuels. as that of Ni/ZnO–HZSM-5-1:1 before the 15 mL model In comparison with the three olefins, 2-methyl-2-butene fuel. Thus, in the following study, we focused on using (35.2%) exhibited the best performance in desulfurization but also the poorest performance in aromatization. Model fuel volume was about 15 mL when the concentration of Table 2 Product composition with different model fuels sulfur in 2-methyl-2-butene product reached 10 ppm, whereas model fuel volumes of 1-pentene (35.0%) and Product Product composition (wt%) 2-pentene (35.4%) were 9 mL and 11 mL, respectively. The 1-pentene 2-pentene 2-methyl- three converted olefins decreased in the order of 2-pen- (35.0%) (35.4%) 2-butene tene > 2-methyl-2-butene > 1-pentene, as shown in Fig. 2a. (35.2%) However, aromatization reactivity of the three isomers is 2-Methylpropene 0.5 1.6 1.7 opposite to the olefin conversion reactivity, whose order is 2-Methylbutane 4.6 4.8 5.3 1-pentene > 2-pentene > 2-methyl-2-butene, as shown in Butane – – 0.6 Fig. 2b. The results suggest that the positions of carbon–car- Pentane 0.6 1.2 0.8 bon double bonds and branched chains in the olefins would 1-Pentene 7.5 – – significantly affect their reactivity, but olefin aromatization 2-Pentene – 5.7 – reactivity showed no positive correlation with conversion 2-Methyl-2-butene – – 6.1 reactivity. Although 2-pentene and 2-methyl-2-butene pro- Cyclopentane 2.1 4.5 5.6 duced relatively less aromatics than 1-pentene, they pro- Benzene 1.3 1.0 0.9 duced relatively larger amounts of isoparaffins and cyclo- n-Heptane 59.8 63.3 64.0 pentane, respectively, which are also superior high-octane Toluene 12.6 8.3 7.4 supporter in gasoline. Xylene 10.6 8.8 6.8 Olefins are adsorbed on adsorbents via the 1-Ethyl-2-methylbenzene 0.4 0.6 0.5 π-complexation that connects the carbon atom with Ni . Fig. 2 a Olefin aromatics yield/olefin conversion (I, II, III: conversion; IV, V, VI: yield. I, IV: 1-pentene; II, V: 2-pentene; III, VI: 2-methyl- 2-butene) and b RADS profiles of adsorbents using different model fuels as feedstock 1 3 Difunctional Adsorbents Ni/ZnO–HZSM-5 on Adsorption Desulfurization and Aromatization of… 147 π-Complexation would be enhanced if the carbon atoms Table 4 Product composition of different adsorbent used model fuel: 1-hexene (34.3%) and n-heptane (65.5%) with double bonds connect the electron-donating group, similar to methyl. Therefore, the adsorption ability of the Product Product composition (wt%) three isomers decreased in the following order: 2-methyl- Ni/ZnO–HZSM- Ni/ZnO–Al O 2 3 2-butene > 2-pentene > 1-pentene. The carbon atoms with 5-1:1.5 double bonds connecting the electron-donating group would 2-Butene – 0.7 be more conducive to be adsorbed on adsorbents and thus Isobutane 1.8 – would feature more opportunities to participate in reactions. Butane 3.5 – On the other hand, in olefin aromatization, the first step of 2-Methyl-1-propene – 0.6 the reaction results in carbocation , in which olefins with 2-Methylbutane 5.4 – branched chains show advantage for their thick electron den- 2-Pentene – 1.1 sity. However, the intermediate carbocation with branches is Pentane 1.3 – not conducive to oligomerization and cyclizing as positive 2-Methylpentane 2.3 – charges are weakened by the electronic effect, which would 2-Methyl-1-pentene – 1.3 decrease the yield of aromatics. 3-Methyl-2-pentene – 1.7 The heptane proportion in the product of 1-pentene 3-Methylpentane 0.3 – decreased by 5.2%, while heptane in the product of 2-pen- n-Hexane 0.4 1.4 tene or 2-methyl-2-butene remained almost unchanged. 4-Methyl-2-pentene – 2.1 Notably, this result indicates that heptane in the 1-pentene 2,3-Dimethyl-2-butene – 1.5 model fuel participates in reactions. This finding is also 1-Hexene 3.8 20.4 ascribed to 1-pentene (the weakest among the three isomers) Benzene 1.0 – adsorption on the adsorbent. Thus, heptane possesses more n-Heptane 57.6 68.8 opportunities to be adsorbed on the adsorbent and undergo 2-Methyl-2-hexene – 0.3 chemical reaction. The product of pure heptane showed Toluene 12.2 – almost no contribution to aromatic hydrocarbon, as shown Xylene 8.7 – in Table 3. 1-Ethyl-2-methylbenzene 1.3 – 2-Methylnaphthalene 0.3 – Comparison of Thiophene Desulfurization and Aromatization Between 1‑Pentene and 1‑Hexene Comparing 1-hexene (34.3%) with 1-pentene (35.0%), the hydrocarbons included toluene and xylene rather than the low-molecular-weight benzene or other higher aromatics. conversion rate of 1-hexene (88.9%) was higher than that of 1-pentene (79.4%) at the reaction time of 1.0 h on the Ni/ A low benzene yield enables the quality of product to meet the strict regulation on benzene content in clean gasoline. ZnO–HZSM-5-1:1.5, as shown in Table 4. The aromatics yield of 1-hexene was lower than that of 1-pentene, whereas Based on the above analysis and olefin product analysis in Tables 2 and 4, C olefins easily generated naphthene, 1-hexene exhibited better desulfurization performance than 1-pentene, as shown in Fig. 3. The results agree with the whereas 1-hexene experienced difficulty in generating the same compound, resulting in almost no naphthene in the discussion in Section “Comparison of thiophene desulfuri- zation and aromatization in C olefins.” In a word, under product. These findings imply that the reaction mechanism of olefin aromatization between C and C may be different. good olefin aromatization, thiophene desulfurization would 5 6 be weakened. One of the reasons for the weaker aromati- Reaction Mechanism zation of 1-hexene than 1-pentene is that the pore size of HZSM-5 was fixed and measured less than 0.45 nm (as The product distribution of Ni/ZnO–Al O was analyzed shown in Fig. 7), and HZSM-5 featured very high selec- 2 3 tivity. The fixed pore resulted in higher molecular volume, by GC–MS, as shown in Table 4. The main products were 1-hexene isomerides with branched chains, accounting causing the more disadvantageous generation of aromatics . This result also explains why the predominant aromatic for 8.0%. The results imply that rearrangement reaction Table 3 Product composition of C4 Pentane 3-Methylhexane hexane n-Heptane Methyl-cyclohexane Toluene Xylene 100% n-heptane (wt%) 1.7 1.2 0.6 92.9 0.8 1.3 1.5 C4: butane and 2-methyl-1-propene 1 3 148 J. Du et al. Fig. 3 a Aromatics yield and b RADS profiles of adsorbents using 1-pentene and 1-hexene model fuel as feedstock occurred on the active Ni sites. Based on the above dis- cussion and experimental phenomena, we infer that the competition between sulfides and olefins did not only exist in physical adsorption on the adsorbent surface but also in subsequent chemical reaction on active Ni sites. In this process, hydrogen molecules reacted with Ni to produce H-positive and H-negative ions. Then, the H-pos- itive ions attacked the 1-hexene adsorbed on Ni by π-bond formation to generate carbocation . Next, the carboca- tion molecules underwent structural rearrangements and generated branched isomers on the Ni surface , which follows the minimum energy principle. However, while HZSM-5 was used as carrier, almost no 1-hexene isomers were present in the product. Instead, a large amount of Fig. 4 XRD patterns of Ni/ZnO–HZSM-5-X, X = 1:1 (I), 1:1.5 (II), aromatics toluene and xylene were generated, as shown 1:2 (III) in Table 4. Therefore, we speculate that in olefin aroma- tization, Ni played a major role in activating olefins into carbocation; then, carbocation followed oligomerization, cyclizing dehydrogenation in the pore of HZSM-5 [29, 32, 33]. On the other hand, nickel also played a key role in desulfurization via removing S atoms from the ring of thiophene. Thus, the high aromatics yield would weaken desulfurization performance because desulfurization and olefin aromatization both require the participation of active Ni sites at the same time. XRD Results As shown in Fig. 4, the diffraction peaks at 2θ = 37.0°, 43.1°, 62.8°, 74.7°, 78.8° are attributed to NiO (JCPDS-PDF No. 78-0423). The diffraction peaks at 2θ = 31.7°, 34.4°, 36.3°, 47.5°, 58.6°, 63.0° belong to ZnO (No. 79-2205). The dif- Fig. 5 XRD patterns of fresh and regenerated Ni/ZnO–HZSM-5-1:1.5 fraction peaks at 2θ = 23.1°, 23.7°, 24.4° agree with those of HZSM-5 (No. 49-0657). No NiAl O or ZnAl O was 2 4 2 4 detected in the adsorbents. The spinel would not gener- adsorbent was compared with the fresh one, and the peak 2+ ate due to excess Ni or ZnO, and all Ni existed as Ni in widths and heights were unchanged, suggesting that the Ni/ZnO–HZSM-5. As shown in Fig. 5, the regenerated 1 3 Difunctional Adsorbents Ni/ZnO–HZSM-5 on Adsorption Desulfurization and Aromatization of… 149 internal structure of regenerated adsorbent has recovered as ZnO. Comparing curves I and II in Fig. 6, the temperature fresh adsorbents. peak of signals increased with decreasing Ni to ZnO ratio, suggesting that the intensity of interaction between NiO and H TPR A ‑ nalysis ZnO would be reinforced as the content of nickel decreased. Ni actively breaks C–S bonds and releases H S from sul- BET Features and NH TPD A ‑ nalysis 2 3 fur-containing compounds. Cleavage of C–S bonds was considered to be the rate-limiting step in RADS , which Table 5 provides the textural properties and acid amounts of strongly depends upon the reducibility of NiO . There- Ni/ZnO–HZSM-5-X adsorbents. N adsorption/desorption fore, H -TPR technique was used to measure the reduction isotherms are typical microporous adsorption belonging to temperature of NiO and to better understand NiO reduc- type I isotherms according to IUPAC (Fig. 7a). The PSD of tion. H -TPR profile of adsorbent Ni/ZnO–HZSM-5 exhib- synthetic adsorbents indicated that pore size mainly con- ited one broad H consumption peak, as shown in Fig. 6. centrated at the range of 0.35–0.40 nm, as shown in Fig. 7b. The intensity of TCD signal presented a positive correla- This result suggests that active metal sites caused no dam- tion with Ni content. With higher nickel content, more H age in the structure of HZSM-5 pore after loading Ni and was consumed by nickel oxide reduction, whereas zinc ZnO compared with the parent HZSM-5. This phenomenon oxide cannot be reduced without consuming hydrogen in is also consistent with the result of XRD characterization. the range of 360–570 °C . The temperature peak of Ni/ A more pronounced ascent of the isotherms at high relative ZnO–HZSM-5 was the same as that of Ni/ZnO adsorbent, pressure (P/P > 0.9) was discerned, and this condition was and temperature range of H reduction peak also distributed associated with nitrogen adsorption in macropores [35, 36]. in 360–570 °C, indicating that the NiO sites weakly inter- Metal oxides changed the acid amount on adsorbent acted with HZSM-5 matrix. The temperature peak of the surface, especially the drastic decrease in strong acidity, signal appeared at 480 °C, as shown in Fig. 6; the reduction as shown in Fig. 8. Strong acids play a key role in steps temperature of Ni/ZnO–HZSM-5 was higher than that of of dehydrocyclization formation of aromatics , and pure NiO as the temperature peak of pure NiO is 390 °C strong acid sites are indispensable in olefin aromatization , suggesting the strong interaction between NiO and . Song et al.  further reported that weak acid sites could catalyze olefin into diolefin or cycloolefin, which are intermediates of olefin aromatization, and strong acids could IV directly transform mono-olefins into aromatics through hydrogen transfer. Although the strong acid amount of Ni/ ZnO–HZSM-5-X remarkably reduced, the capacity of cata- lyzing olefins into aromatics showed no decline. This result implies that Ni or ZnO assists catalysis during the conver- sion of olefins into aromatics, and condition of decreased II strong acid could also efficiently avoid coking and deep III cracking resulting from excessive strong acidity . Coke Analysis 200300 400 500 600 700 The adsorbents were analyzed by TG analysis after react- ( C) Temperature ing for 3 h. Figure 9 shows the TG measurements on used adsorbents and used HZSM-5 under air flow. In addition, Fig. 6 H -TPR of Ni/ZnO–HZSM-5-X (X = 1:1 (I), 1:1.5 (II), 1:2 hydrogen-reduced fresh Ni/ZnO–HZSM-5-1:2(F) adsorbent (III)) and Ni/ZnO-1:1 (IV) Table 5 Textural properties and 2 Adsorbent S (m /g) Micropore Strong acid Weak acid Total acid BET acid amounts of adsorbents volume amount amount amount (cm /g) (mmol/g) (mmol/g) (mmol/g) HZSM-5 287.1 0.1464 0.235 0.575 0.81 Ni/ZnO-HZSM-5-1:1 131.1 0.06454 0.058 0.479 0.537 Ni/ZnO-HZSM-5-1:1.5 136.8 0.06574 0.021 0.527 0.548 Ni/ZnO-HZSM-5-1:2 146.2 0.07172 0.033 0.433 0.466 1 3 TCD Signal 150 J. Du et al. Fig. 7 a N adsorption/desorption isotherms and b PSD of Ni/ZnO–HZSM-5-X and HZSM-5 and 650 °C. This result was ascribed to the gradual oxidation of Ni in the internal adsorbents into NiO in air flow at high temperature. As for the fresh reduction Ni/ZnO–HZSM-5 Ni/ZnO-HZSM-5-1:2 (F) adsorbent, the TG curve also rises up and exceeds the initial weight. We can conclude that almost no coke was Ni/ZnO-HZSM-5-1:1.5 generated on Ni/ZnO–HZSM-5 adsorbents, and deactiva- Ni/ZnO-HZSM-5-1:1 tion of adsorbents mainly resulted from sulfur rather than coking. Coking of pure HZSM-5 was more significant than HZSM-5 that of Ni/ZnO–HZSM-5. The possible reason is that acid distribution resulted in such situation as strong acids disap- peared after HZSM-5 loaded the active metal Ni and ZnO 100 200 300 400 500 600 700 sites, whereas much strong acids distributed on HZSM-5. Temperature ( C) Fig. 8 NH -TPD profiles of Ni/ZnO–HZSM-5-X and HZSM-5 Conclusion The difunctional Ni/ZnO–HZSM-5-X adsorbents can achieve deep desulfurization and convert olefins into aro- matics at the same time. In olefin aromatization, the active Ni sites cooperated with HZSM-5 to catalyze olefins into aromatics. The Ni/ZnO–HZSM-5-1:1.5 adsorbent could remove 98.0% thiophene with a processing amount 12 mL of model fuel flow, corresponding to the concentration of sulfur Ni/ZnO-HZSM-5-1:2 under 10 ppm. A competitive relationship exists between Ni/ZnO-HZSM-5-1:1.5 Ni/ZnO-HZSM-5-1:1 thiophene desulfurization and olefin aromatization as these HZSM-5 processes require the active Ni sites to participate in reac- Ni/ZnO-HZSM-5-1:2(F) 0 100 200 300 400 500 600 700 800 tion at same time. The performance of the different chemical Temperature ( C) structures in olefin aromatization decreased in the follow - ing order: 1-pentene > 2-pentene > 2-methyl-2-butene, and Fig. 9 TG curves of used Ni/ZnO–HZSM-5, used HZSM-5, and 1-pentene > 1-hexene. hydrogen-reduced fresh Ni/ZnO–HZSM-5-1:2(F) under air flow Open Access This article is distributed under the terms of the Crea- tive Commons Attribution 4.0 International License (http://creat iveco was used as a contrast. 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Transactions of Tianjin University – Springer Journals
Published: May 28, 2018
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