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Several catalysts containing metals such as Al, Zr, Ti and Nb were incorporated in SBA-15 with Si/M ratio = 20 using the hydrothermal process. These catalysts were evaluated for their reactivity during acid-catalyzed condensation of glycerol with acetone to yield a mixture of branched compounds, namely (2,2-dimethyl- [1,3] dioxane-4-yl)-methanol (solketal) and 2,2-dimethyl- [1,3] dioxane-5-ol, which are being used as fuel additives. The synthesised catalyst samples were characterized by ICP-AES analysis, N adsorption–desorption measurements, X-ray diffraction, FT-IR, SEM–EDX, UV–visible diffuse reflectance spectroscopy, TPD of ammonia and ex situ pyridine-adsorbed FT-IR spectroscopy. The various characteriza - tion results confirm that metal oxides were incorporated in the pore wall of the SBA-15 matrices. The results of NH -TPD and ex situ adsorbed pyridine FT-IR analyses showed that the acidity of the samples increased after incorporation of metal into the pure SBA-15 samples. Among various metals incorporated into the SBA-15, the Nb–SBA-15 (Si/Nb = 20) showed higher catalytic activity toward the acetalization of glycerol in liquid phase compared to that of other samples investigated. Under the optimal reaction conditions, the Nb–SBA-15 (Si/Nb = 20) exhibited 95% glycerol conversion with 100% solketal selectivity. The catalyst reusability studies indicated that the Nb–SBA-15 sample is regenerable and highly stable in the acetalization of glycerol. Keywords Glycerol · Acetalization · Solketal · SBA-15 · Metal-incorporated SBA-15 Introduction limited resource of petroleum and natural gas coupled with price volatility, there have been substantial research efforts The world energy crisis has become an exceptionally cru- on finding alternative renewable sources. A variety of alter - cial topic of research in recent years with diminishing petro- native energy sources have been developed, such as hydro- leum reserve. Thus, biomass has become the only promis- electric energy, wind power, geothermal energy and solar ing renewable resource of energy, which is environmental energy. However, it should be realized that the application of friendly, industrially feasible, biodegradable and offers these energy sources might take a longer time than expected. less emission of greenhouse gases during combustion, to The development of efficient processes to convert biomass the sustainable development of society. With the increase resource into liquid fuels would be an important research in energy utilization in transport and domestic sectors, the area in the next few decades. Of particular interest is the issue of energy security and climate change has gained much utilization of triglycerides of vegetable or animal oil for the attention for exploration of new catalysts. Considering the production of biodiesel as a liquid fuel. It is a process of transesterification of triglycerides with methanol in the pres - ence of acidic or basic catalysts. Biodiesel has been consid- Electronic supplementary material The online version of this ered to be one of the most promising alternatives to fossil article (https ://doi.org/10.1007/s1320 3-018-0197-6) contains supplementary material, which is available to authorized users. fuel resources [1–4]. In addition, the process produces sub- stantial amounts of glycerol as the main by-product, which * Komandur V. R. Chary is equivalent to 10 wt% of the total biodiesel production . firstname.lastname@example.org Glycerol is currently surplus in the biodiesel industry and Catalysis and Fine Chemicals Division, CSIR-Indian is available at very low prices. Presently, it has very limited Institute of Chemical Technology, Uppal Road, Tarnaka, applications in the area of cosmetics, soaps and medicines Hyderabad, India Vol.:(0123456789) 1 3 108 Applied Petrochemical Research (2018) 8:107–118 . Therefore, the conversion of glycerol into value- added allows metals to disperse uniformly into the pores of SBA- products is an attractive proposition in the current scenario. 15 [20, 21]. Hence, our interest is to synthesize these metal Glycerol is considered as one of the most important platform ions-incorporated SBA-15 materials into the framework for chemicals to produce various chemicals such as acrolein, the acetalization of glycerol into solketal. acrylic acid, propane diols, glyceraldehyde, glyceric acid, In the present investigation, we report a systematic study dihydroxy acetone and acetylated glycerol, using various on the characterization and application of various metals chemical transformations [7, 8]. Among the value addition [Zr, Al, Ti, Nb] incorporated into the SBA-15 catalyst for the processes of glycerol, the acetalization of glycerol with an acetalization of glycerol with acetone to produce solketal. aldehyde or a ketone is an important transformation to pro- The synthesized catalysts were characterized by various duce oxygenated compounds such as acetals and solketal techniques such as N sorption analysis, X-ray diffraction, . In general, the acetalization of glycerol with acetone FT-IR, SEM, UV-DRS, NH -TPD and ex situ adsorbed pyri- gives branched oxygenated compounds, namely (2,2-dime- dine FT-IR methods. The purpose of this work is to examine thyl- [1,3] dioxane-4-yl)-methanol (solketal) and 2,2-dime- the catalytic properties of the metals-incorporated SBA-15 thyl- [1,3] dioxane-5-ol. Solketal is an excellent component catalysts for the acetalization of glycerol with acetone and for the formulation of gasoline, as ignition accelerator and also to find the relation between the surface structural prop- antiknock additives in combustion engines, biodiesel fuels erties and catalytic functionalities during acetalization of and also has great industrial applications in cosmetics, fra- glycerol. grances and pharmaceuticals . This reaction generally involves the use of solid acid cata- lysts such as amberlyst, zeolites and supported heteropoly Experimental section acids . However, these catalysts find limited usage due to poor thermal and textural properties and suffer from low Preparation of pure and metal‑incorporated SBA‑15 catalytic activity toward acetalization reactions . Our earlier research efforts on acetalization of glycerol with The pure and M–SBA-15 materials (where M = Al, Zr, Ti acetone to solketal over molybdenum phosphate supported on SBA-15 catalyst yielded solketal with a high yield of and Nb; Si/M ratio = 20) were synthesized according to a previously reported procedure [22–24]. The zirconium 98% . However, this catalytic system faces severe prob- lem in regaining its original activity during reaction and (IV) n-propoxide solution (70 wt% in 1-propanol, Aldrich), tetraisopropyl o-titanate (TiPOT, > 98%, Merc), niobium (V) regeneration, due to leaching of the active species into the reaction mixture. Therefore, lot of efforts has been made to chloride (99%, Sigma-Aldrich) and aluminum isopropoxide (Sigma-Aldrich) were used as precursors for preparing the design the catalyst with high catalytic activity along with stability during the reaction conditions. In this context, metal M–SBA-15 materials, where M = Zr, Ti, Nb and Al respec- tively. Pluronic P123 (Aldrich, MW = 5800) and tetraethyl oxide catalysts received a great deal of attention due to their excellent catalytic behavior in terms of activity, stability and o-silicate (TEOS, 98%, Aldrich) were used as template and silica source, respectively. regenerability in the reactions. Recently, the discovery of mesoporous materials has In a typical synthesis, 18 g of P123 was dissolved in the mixture of 140 g of deionized water; 10.5 g of hydrochloric stimulated extensive interest because of their wider appli- cations in catalysis, separation and adsorption due to their acid (HCl, 37%, Hartim) and the calculated amount of metal precursors were added at 40 °C. The HCl was not used in high specific surface area, uniform pore size distribution and large pore size. Among the mesoporous materials, SBA-15 the case of niobium-incorporated SBA-15 samples. Then, the TEOS was added very slowly (dropwise addition) to the materials have received particular attention due to various properties such as hydrothermal and thermal stability, larger solution. Thereafter, this mixture was stirred at 40 °C for 24 h and then transformed into Teflon-lined autoclave and pore size and thicker pore wall, which makes it a promising catalytic material [14–16]. However, pure SBA-15 materials kept at 100 °C for another 24 h in static conditions. Then, the product mixture was filtered and washed with a large amount are not as active in catalyzing chemical reactions as would be desired due to lack of various properties such as redox of deionized water followed by drying at 100 °C in an oven for 24 h. The as-synthesized metal-incorporated SBA-15 and acidity/basicity, and this could be achieved by incorpo- rating various transition metals into the mesoporous SBA-15 samples were calcined at 500 °C for 5 h in air using a muffle furnace. The synthesized various metals-incorporated SBA- matrix . However, various metal ions of Al, Ti, Zr and Nb are well known for their application in the acid-catalyzed 15 samples are represented as M–SBA-15, where M denotes the metal such as Nb, Zr, Ti and Al. The pure SBA-15 mate- reaction such as dehydration and esterification [18, 19]. In the present work, for the metal incorporation into SBA-15, rial was obtained by the same procedure as described previ- ously except for the usage of the metal precursor. the direct synthesis method was chosen since this method 1 3 Applied Petrochemical Research (2018) 8:107–118 109 samples were cooled to ambient temperature and saturated Catalyst characterization with pyridine under N flow at 200 °C. The FT-IR spectra of the catalysts were recorded on IR (Model: GC-FT-IR Nico- Powder X-ray diffraction patterns of SBA-15 and metal- incorporated SBA-15 catalysts were recorded with Bruker let 670) spectrometer using the KBr disk method at room temperature. D phaser X-ray diffractometer using Cu Kα radiation (1.5406 Å) at 40 kV and 30 mA with high-resolution Lynx- Catalytic reaction eye detector. The measurements were recorded in steps of 0.045° with a step size of 0.5 s in the range of 2°–65°. The liquid phase acetalization of glycerol with acetone was The FT-IR spectra of the catalysts were recorded on an IR (Model: GC-FT-IR Nicolet 670) spectrometer by the KBr accomplished under solvent-free condition at atmospheric pressure and room temperature in a 25 ml round bottom disc method at room temperature. Scanning electron micro- scope (SEM) images were recorded on Zeiss microscope to flask with a magnetic stirrer using 100 mg of catalyst and the required amount of acetone (3 mmol) and glycerol (1 mmol). investigate the surface morphology. Pore size distribution measurements were performed Prior to the reaction, the catalyst was activated at 200 °C for 1 h in a muffle furnace. 1 mmol of glycerol and 3 mmol on Autosorb-1 instrument (Quantachrome, USA) using the nitrogen physisorption method. The UV–Vis diffused acetone was added to the 25 ml round bottom flask contain- ing 100 mg catalyst. This reaction mixture was stirred for reflectance spectra were recorded on a GBC UV–visible Cintra 10 spectrometer with an integrating sphere reflec - 1 h and the catalyst was separated by filtration. The filtrate was collected systematically for analysis using a gas chro- tance accessory. The spectra were recorded in air at room temperature and the data were transformed according to the matograph GC-2014 (Shimadzu) equipped with a DB-wax 123-7033 (Agilent) capillary column (0.32 mm i.d., 30 m Kubelka–Munk equation f(R) = (1 − Rα) /2Rα. The NH -TPD experiments were conducted on Auto- long) and a flame ionization detector (FID). chem 2910 instrument. The acidity of the parent SBA-15 and metal-incorporated SBA-15 catalysts was determined Results and discussion by temperature-programmed desorption ammonia equipped with thermal conductivity detector TCD using He as a Chemical composition carrier gas. In all the experiments, 100 mg of oven-dried sample was out-gassed at 250 °C for 1 h in flowing He gas −1 ICP-AES analysis was used to determine the quantitative (50 ml min ) and then cooled down to room temperature and then the sample was saturated with 10% NH -He for amount of metal present in the samples and the results are illustrated in Table 1. The analysis results showed that 45 min. After saturation with ammonia, the sample was flushed in He flow for 1 h at 100 °C to remove physisorbed various metal-incorporated SBA-15 samples had the Si/M ratio in the range of 17–22 and it was quite consistent with ammonia. The temperature of the furnace was then brought down to 50 °C before starting the analysis. The desorption the theoretically calculated Si/M ratio (Si/M ratio = 20) employed during the catalyst preparation. Hence, this result of ammonia was recorded with a temperature program from ambient temperature to 700 °C with a heating rate of 10 °C confirmed the existence of the metals in the various samples. −1 In addition, SEM-EDAX analysis results are (Table 1) also min . The amount of NH desorbed was calculated using the GRAMS/32 software. in good agreement with the ICP-AES results. The metals (Nb, Zr, Ti, Al) and Si composition of the catalysts were analyzed by elemental analysis (ICP-AES M/s Nitrogen adsorption–desorption analysis Thermo Scientific iCAP6500 DU) by dissolving the sample in aqua regia along with a few drops of hydrofluoric acid in Figure 1 shows the nitrogen adsorption–desorption analysis of the pure SBA-15 and metal-incorporated SBA-15 sam- a microwave oven for 2 h and further diluted with de-ionized water to analyze the metal (Nb, Zr, Ti, Al) content. This ples. The isotherm of pure SBA-15 and metal-incorporated SBA-15 sample showed typical type IV nitrogen adsorp- enables finding the actual metals (Nb, Zr, Ti, Al) to Si mole ratio in the catalysts. tion–desorption isotherms with H1 hysteresis loop according to the IUPAC classification. This result confirmed the exist- The Brønsted and Lewis acidic sites of SBA-15 and metal-incorporated SBA-15 catalysts were investigated by ence of the hexagonal arrangement of mesopores in all the samples and this was not affected even after incorporation ex situ pyridine adsorption study using the FT-IR spectra recorded in absorbance mode in the wavelength range from of the metals into the parent SBA-15. However, in the case −1 of the metal-incorporated SBA-15 sample, the position and 1400 to 1600 cm . Before adsorption of pyridine, the sam- ple was preheated at 300 °C for 1 h in the furnace to remove shape of the hysteresis loop of all samples changed depend- ing on the metal incorporated in the SBA-15. The hysteresis the absorbed water in the sample. Thereafter, the activated 1 3 110 Applied Petrochemical Research (2018) 8:107–118 Table 1 Textural properties and TPD profiles of the various M–SBA-15 samples a b c ◦ Name of the sample Surface area Pore volume Average pore Si/M ratioSi/M ratio T ( C) NH max 3 (m /g) (cc/g) diameter (Å) desorbed (µmol/g) SBA-15 830 1.34 63.4 – – 140 – Zr–SBA-15 715 0.80 40.3 22 18 450 252 Nb–SBA-15 773 0.83 54.5 19 20 580 316 Ti–SBA-15 672 0.97 39.1 21 19 563 180 Al–SBA-15 620 1.03 28.5 18 19 350 140 Spent Zr–SBA-15 550 0.62 35.5 19 17 560 219 Spent Nb–SBA-15 590 0.69 47.7 17 17 438 260 Calculated from the ICP-AES analysis Calculated from the SEM–EDX analysis Ammonia desorbed temperature positions depend mainly on the nature of metal incorporated in SBA-15. All of the synthesized SBA-15-based materials showed higher specific surface areas (620–830 m /g) and porosity (0.8–0.6 cm /g) as compared to the bare SBA-15. After incorporation of the metals into the parent SBA-15, the surface area of the sample decreased compared to SBA-15 due to blocking of the pores of SBA-15. Among the sam- ples investigated, Nb–SBA-15 showed the highest surface area (772 m /g) with 0.82 cc/g total pore volume, whereas Al–SBA-15 showed the lowest surface area (620 m /g) with 1.03 cc/g total pore volume. Decreasing surface area is asso- ciated with narrower hysteresis loops in comparison to bare SBA-15. The pore size distribution of spent catalysts (In Fig. S1, Supplementary Information) reveals that the surface area 0.20.4 0.60.8 1.0 and pore size distribution of the parent catalysts decreased Relative pressure(P/P ) considerably after the reaction. Fig. 1 Nitrogen adsorption–desorption isotherms of various M/SBA- Low‑angle XRD analysis 15 (Si/M = 20) catalysts. a Pure SBA-15, b Zr–SBA-15, c Nb–SBA- 15, d Ti–SBA-15, e Al–SBA-15, f spent Nb–SBA-15 and g spent Zr– SBA-15 The low-angle XRD patterns of pure SBA-15 and various M–SBA-15 samples are shown in Fig. 2. The pure SBA-15 loop for all the samples was shifted to low relative pressure sample showed diffraction lines at 2θ = 0.92, 1.64 and 1.85 which can be indexed to the characteristic (100), (110) and approximately in the range of 0.4 and 0.7, suggesting the decrease in the pore size of the parent SBA-15. This phe- (200) planes of SBA-15 respectively, confirming the for - mation of hexagonally ordered mesoporous structure with nomenon can be explained by the occupation of metals in the mesopores of pure SBA-15, which decreased the pore p6mm symmetry. The characteristic peak of the M–SBA-15 related to the (100) plane was shifted compared with the size of the parent SBA-15. The BJH pore size distribution analysis of the pure pure SBA-15 sample in the low-angle XRD pattern, depend- ing on the metal incorporated. However, after introduction SBA-15 and metal-incorporated SBA-15 are shown in Fig. S1. From (Fig. S1, Supplementary Information), it is of metals into the pure SBA-15, the intensities of the peaks decreased due to occupation of mesopores of the SBA-15 observed that all samples showed a narrow pore size dis- tribution around 50–80 Å. The textural properties of all the by metals. Although the metal loadings of the SBA-15 are same (Si/M = 20) (M = Zr, Al, Ti and Nb) in all the samples, samples determined by nitrogen sorption experiments are summarized in Table 1. It is interesting to see that all the a clear change in the peak intensity and shift can by noticed. This might be due to restructuring of silica and metal precur- samples have shown unimodal pore size distribution with clear indication of shift in the peak positions. These peak sors, during the step of absorption in the solution. However, 1 3 3 -1 Amount adsorbed(cm /g ) Applied Petrochemical Research (2018) 8:107–118 111 are isopropoxides) and the same Si/M ratio except the nature (100) of the metal [28, 29]. (110) (200) FT‑IR spectra The FT-IR spectra of pure SBA-15 and the metal-incorpo- rated SBA-15 samples are shown in Fig. 4. All the samples −1 exhibited IR bands in the region 3400–2400 cm due to sur- −1 face –OH groups and the IR band at 1630 cm is assigned to the bending mode of the water molecule . In general, the typical siliceous materials exhibit IR bands in the range −1 of 400–2000 cm . The IR spectra of the pure SBA-15 sam- −1 ple exhibited the main IR band at 1080 cm and weaker −1 band at 800 and 960 cm due to asymmetric and symmetric stretching modes of the Si–O–Si bond, respectively. In addi- −1 1234 tion, a strong IR band at 458 cm can be attributed to the rocking of the Si–O–Si bond. The IR spectra of all metal- 2Theta incorporated SBA-15 samples showed the IR bands similar to that of the pure SBA-15 sample. However, with close Fig. 2 Low-angle XRD profiles of various M/SBA-15(Si/M = 20) cat- alysts. a Pure SBA-15, b Zr-SBA- 15, c Nb–SBA-15, d Ti–SBA-15 observation of the IR spectra of metal ions-incorporated −1 and e Al–SBA-15 SBA-15 samples, the intensity of the IR band at 960 cm corresponds to the Si–O–Si bond decrease. It appears to be the relative intensity of the (110) and (200) peaks decreased possible evidence of the isomorphism substitution of Si by − + or disappeared after the introduction of metal into SBA-15, metal ions (Si–O M ) (M: Zr, Al, Ti and Nb) . Hence, which might be due to the disruption of the textural uniform- the IR spectra gave evidence related to the formation of the ity, i.e., hexagonal structure of SBA-15 . Si–O–M bond in the metal-incorporated sample. UV‑DRS analysis Wide‑angle XRD analysis Generally, UV–Vis DRS analysis has been widely used to The wide-angle X-ray diffraction patterns of the calcined find the nature and coordination of metal ions in substi- pure SBA-15 and metal-incorporated SBA-15 samples are tuted molecular sieves. The UV–Vis DRS spectra of pure shown in Fig. S2 (Supplementary Information). The XRD SBA-15 and metal-incorporated SBA-15 in the region of pattern of all the samples exhibits a broad reflection peak 200–800 nm are shown in Fig. S3 (Supplementary informa- between 2θ = 15°–35° due to amorphous silica . Interest- tion). The pure SBA-15 showed no characteristic absorp- ingly, the characteristic XRD peaks of incorporated metals tion band in the spectra, whereas all the metal-incorporated are not appeared in the XRD pattern, suggesting that the SBA-15 samples showed absorption bands in the region incorporated metals were well dispersed on the SBA-15 sup- of 200–400 nm with different intensities depending on the port and no metal oxide crystallites were formed or the metal metal incorporated. content was below the XRD detection limit. The diffuse reflectance UV–visible spectra of the Ti–SBA-15 sample exhibited a very intense absorption band Scanning electron microscopy around 210–230 nm wavelength, characteristic of titanium 4+ ion (Ti ) present in the tetrahedral environment [32, 33]. SEM analysis was used to investigate the surface physical The intensity of the absorption peak decreased from 240 nm morphology of pure SBA-15 and various metals-incorpo- to 380 nm indicating the existence of less number of octa- 6+ rated SBA-15 samples and the respective images are shown hedral environment titanium (T i ). In addition, the absence in Fig. 3. The pure SBA-15 sample showed earmarks of of the absorption band beyond 390 nm reveals the chance of rice grains-like structure and the metal-incorporated sam- existence of T iO in the form of anatase in the synthesised ples exhibited irregular spherical-like morphology . The sample. The spectra of Zr–SBA-15 sample exhibited a band morphology of the sample varies with the type of metals at 210 nm which could be responsible for transitions of eight 2− 4+ incorporated, since these materials were prepared by using coordinate tetravalent Zr (O –Zr ) . The absorption a similar procedure and with the same metal precursors (all band at 210–240 nm for Nb–SBA-15 sample is attributed 2− +4 to the charge transfer band for the O –Nb . This result 1 3 Intensity(a.u) 112 Applied Petrochemical Research (2018) 8:107–118 Fig. 3 SEM images of various metal-doped SBA-15 samples. a Pure SBA-15, b Zr–SBA-15, c Nb–SBA-15, d Ti–SBA-15 and e Al–SBA-15 confirms the presence of the Nb species in the form of tet- as weak (150–300 °C), moderate (300–450 °C) and strong rahedral coordinate in the sample. The Al–SBA-15 sample (450–650 °C) . Pure SBA-15 showed low-intensity showed an absorption band at 210 nm, indicating that alu- broad peak in the medium and strong acidic region in the mina is present in tetrahedral coordination in the SBA-15 NH -TPD measurements. In the case of metal-incorporated framework, along with a weak band at 260 nm due to the SBA-15 sample, the intensity of this peak, i.e., medium to +3 2− charge transfer transition of Al –O . strong acidic region increased in the NH -TPD measure- ments for all samples irrespective of the type of metals. Temperature‑programmed desorption of ammonia This mainly depends on the exposure of the incorporated (NH TPD ‑ ) metal over the surface of the SBA-15 and also the interac- tion between the metal and silica of SBA-15. In the present investigation, acidity measurements were The ammonia uptake volumes are given in Table 1. As carried out by the ammonia-TPD method. The ammonia- can be seen from the results in Table 1, the total acidity TPD profiles of the calcined samples of pure SBA-15 of the catalysts decreases in the following order: Nb–SBA- and metal-incorporated SBA-15 samples are presented in 15 > Zr–SBA-15 > Al–SBA-15 > Ti–SBA-15. This result Fig. 5. Generally, the acidic strength of solid acid cata- clearly indicates that the acidity of the catalysts was closely lyst in the NH -TPD profiles can be classified into three related to the nature of the metal incorporated in SBA-15, types depending on their strength in the temperature and niobium-incorporated SBA-15 showed higher number of region of the TPD profile. These acidic sites are denoted medium to strong acidic sites compared to all other catalysts 1 3 Applied Petrochemical Research (2018) 8:107–118 113 B+L 1400 1450 1500 1550 1600 1650 -1 Wavenumber (cm ) Fig. 6 Ex situ adsorbed profiles of various M/SBA-15 (Si/M = 20) catalysts. a Pure SBA-15, b Nb–SBA-15, c Zr–SBA-15, d Ti–SBA-15 and e Al–SBA-15 Fig. 4 FT-IR spectra of pure SBA-15 and various M/SBA-15 (Si/M = 20) catalysts. a Pure SBA-15, b Zr–SBA-15, c Nb–SBA-15, d saturated with pyridine vapor at 200 °C for 2 h. The ex Ti–SBA-15 and e Al–SBA-15 situ pyridine-adsorbed IR profile of pure SBA-15 exhib- −1 ited two peaks at 1444 and 1595 cm . Except these peaks, pure SBA-15 does not show any peaks, indicating the poor acidic nature of pure SBA-15. The weak acidic sites appear- ing in the pure SBA-15 profile is due to the silanol groups (Si–OH) of SBA-15 [36, 37]. Similar to the pure SBA-15 sample, the various metal-incorporated SBA-15 samples −1 exhibited peaks at 1440–1450 cm . The intensive peaks are due to hydrogen-bonded pyridine with the sample and c also one cannot exclude the existence of Lewis acidic sites of these samples, as both peaks appear in the same region. −1 In addition, these samples exhibited peaks 1490–1500 cm , corresponding to a combination of both Brønsted and Lewis −1 (B + L) acid sites, and at 1540–1548 cm , characteristic bands of Brønsted (B) acidic sites. The appearance of these peaks is probably due to the existence of the Si–O–M bond 200300 400500 600 in the metal-incorporated SBA-15 sample. However, the intensity of these peaks is varied in different proportions Temperature( C) depending on the nature of metal present in the catalyst. The intensity of the IR bands is proportional to the concentration Fig. 5 TPD profiles of various M/SBA-15 (Si/M = 20) catalysts. a Pure SBA-15 b Zr–SBA-15 c Nb–SBA-15 d Ti–SBA-15 and e Al– of acidic sites. From the pyridine-adsorbed FT-IR spectra, −1 SBA-15 the intensity of IR absorption bands at 1498 cm (B + L) has increased in the following order: Nb–SBA-15 > Zr–SBA- 15 > Ti–SBA-15 > Al–SBA-15. The intensity of the IR band −1 (Fig. 5). The acidity of the used catalysts after the acetaliza- at 1490–1500 cm corresponds to the (B + L) acidity and shows the same trend of increase of the total acidity of the tion reaction varied only to a marginal extent compared to samples before the reaction. catalysts in TPD of ammonia. Interestingly, Zr–SBA-15 and Nb–SBA-15 catalysts showed significant amount of Brøn- Ex situ pyridine‑adsorbed FT‑IR analysis sted (B) acidic peaks in the FT-IR profiles than the other samples. The ex situ pyridine-adsorbed FT-IR spectra of different metals incorporated into SBA-15 samples are illustrated in Fig. 6. Prior to FT-IR analysis, the catalyst samples were 1 3 TCD signal(a.u) Intensity (%) 114 Applied Petrochemical Research (2018) 8:107–118 (M = Zr, Nb, Ti and Al) were screened and the results are Acetalization of glycerol presented in Fig. 7. All the reactions were performed at room temperature for 1 h reaction time with 100 mg catalyst The acetalization of glycerol with acetone over pure SBA-15 amount and 3:1 molar ratio of acetone to glycerol. As can be and various metals (M) incorporated on SBA-15 catalysts seen from Fig. 7, pure SBA-15 sample showed 10% glycerol conversion with 55% solketal selectivity. The Nb–SBA-15 and Zr–SBA-15 samples showed better glycerol conversion con 100 than Ti- and Al-incorporated SBA-15 samples, because sel Nb–SBA-15 and Zr–SBA-15 catalysts contain high surface area and acidity because the availability of the reacting mol- ecules on the surface area is more compared to Al–SBA-15 and Ti–SBA-15. Among all the samples, the Nb–SBA-15 sample showed the highest activity, with a maximum con- version of 95% with 100% solketal selectivity. As expected, pure SBA-15 sample showed less activity for the acetaliza- tion reaction, probably due to its less acidic nature com- pared to other supported metal-incorporated catalysts. The blank reaction was also carried out without a catalyst, which resulted in very low glycerol conversion (less than 0.1%). These results clearly suggest the role of the catalyst in the acetalization of the glycerol. The schematic representation of acetalization of glycerol P-SBA-15 Nb-SBA-15Zr-SBA-15 Ti-SBA-15Al-SBA-15 with acetone over metal-incorporated SBA-15 catalysts to Sample name (2, 2-dimethyl-1,3-dioxolan-4-yl) methanol (solketal) and 2, 2-dimethyl-1,3-dioxan-5-ol are shown in Scheme 1. In Fig. 7 Acetalization of glycerol with acetone over various M/SBA-15 general, the acetalization reaction proceeds through the (Si/M = 20) catalysts Scheme 1 Schematic repre- sentation of the acetalization of glycerol with acetone 1 3 Conversion&Selectivity(%) Applied Petrochemical Research (2018) 8:107–118 115 following two reversible reaction steps: a first step in which enhancement of the conversion of glycerol. Moreover, the the alcohol group reacts with the acetone molecule, lead- low amount of acetone could not disperse the glycerol ing to the formation of the corresponding hemiacetal, i.e., homogeneously in the reaction medium, as this reaction was unstable intermediate is formed. In the second step, the carried out in a solventless medium. With the increase of elimination of water molecule from one of the two hydroxyl acetone to glycerol molar ratio further to 4:1 ratio, there was groups of hemiacetal, leading to the formation of solketal no significant change in the glycerol conversion observed, and acetals . Scheme 1 represents the detailed mecha- indicating that too much acetone does not influence glycerol nism of acetalization of glycerol with acetone. Interestingly, conversion. By increasing the molar ratio, the active sites on the product formation (solketal and acetals) depends on the the catalyst decrease due to their blockage with acetone on o o types of alcoholic (1 -OH or 2 -OH) groups of the hemi- the surface of the catalyst. Hence, acetone to glycerol molar catal involved in the reaction, i.e., if 1 alcoholic groups ratio of 3:1 was chosen for further reaction studies. are involved in the reaction, it produces a six-membered unstable transition state and leads to acetal as the product, Eec ff t of catalyst amount and when 2 alcoholic groups are involved in the reaction, it produces 5-membered stable transition state, leading to To study the effect of catalyst amount on glycerol acetali- solketal as the product. zation for various metal-incorporated SBA-15 samples, the catalytic activity study was carried out at acetone to glycerol Eec ff t of molar ratio molar ratio 3:1 and the catalyst amount was varied from 25 to 100 mg. As expected, the conversion of glycerol increased Table 2 shows the effect of acetone to glycerol molar ratio on with the increase of catalyst loading from 25 to 100 mg. glycerol acetalization over various metal-incorporated sam- This is due to the increase in the total number of acidic sites ples. The catalyst amount was kept constant. From Table 3, of the catalysts available for the acetalization of glycerol it can be seen that the glycerol conversion increased with reaction. It is interesting to note that the selectivity of the the increase in acetone to glycerol ratio from 2:1 to 3:1. It is product does not change with the amount of catalyst. Hence, well known that the acetalization of glycerol to solketal is the 100 mg catalyst weight was chosen for further reaction the equilibrium reaction and the excess amount of acetone is studies. necessary to push the equilibrium toward the simultaneous Table 2 Effect of molar ratios on M/SBA-15 (Si/M = 20) catalysts Catalysts Acetone to glycerol molar ratio with catalyst 2:1 3:1 4:1 Conversion (%) Selectivity (%) Conversion (%) Selectivity (%) Conversion (%) Selectivity (%) Blank test < 0.1 < 0.1 < 0.1 Nb–SBA-15 65 98 95 100 97 90 Zr–SBA-15 75 98 92 98 94 88 Ti–SBA-15 32 98 65 98 70 88 Al–SBA-15 28 98 60 98 64 88 Reaction conditions: Cat wt, 100 mg; time: 1 h; room temperature Table 3 Effect of catalyst Catalysts Catalyst amount amount on various M/SBA-15 (Si/M = 20) catalysts 25 mg 50 mg 100 mg Conver- Selectivity (%) Conver- Selectivity (%) Conver- Selectivity (%) sion (%) sion (%) sion (%) Nb–SBA-15 62 98 72 98 95 100 Zr–SBA-15 64 98 78 98 92 98 Ti–SBA-15 36 98 40 98 65 98 Al–SBA-15 30 98 35 98 60 98 Reaction conditions: acetone and glycerol ratio: 3:1; time: 1 h; room temperature 1 3 116 Applied Petrochemical Research (2018) 8:107–118 Table 4 The catalytic activities of the fresh and reused catalysts %Conversion %Selectivity Number of runs Catalyst Nb–SBA-15 Zr–SBA-15 Selectivity (%) Con- Selectivity (%) Con- version version (%) (%) Run1 95 100 92 98 Run2 90 100 85 98 Run3 82 97 78 95 Run4 80 94 75 95 Reaction conditions: acetone and glycerol ratio: 3:1; cat wt: 100 mg,;time: 1 h; room temperature procedure was repeated to test the reusability of the catalyst R.T 45 CR.T 45 by recycling it through four consecutive batch runs and the Temperature( C) results are presented in Table 4. The conversion of glycerol and selectivity of solketal for the Nb–SBA-15 sample during Fig. 8 Effect of reaction temperature over the Nb–SBA-15 and Zr– the first run was found to be 95 and 100%, respectively, and SBA-15 samples. a Nb–SBA-15 and b Zr–SBA-15 it was decreased to 80 and 94% in the fourth run. However, Zr–SBA-15 sample exhibited glycerol conversion around 92 with 98% selectivity toward solketal in the first run. This has Eec ff t of temperature been considerably decreased to 75% conversion and 95% solketal selectivity in the fourth run. In the case of Ti–SBA- An attempt also been made to study the impact of reaction 15, the conversion of glycerol and selectivity of solketal temperature on the conversion and selectivity of the prod- decreased from 65 to 98% for the first run to 30 and 70% ucts using Nb–SBA-15 and Zr–SBA-15 samples to find out respectively, for the second run. Further experiments with the optimum conditions for maximum yield and the results Al–SBA-15 sample show that the conversion of glycerol are shown in Fig. 8. As can be seen from Fig. 8, there is and selectivity of solketal decreased from 60 to 98% for the an increase in the conversion of glycerol with increase first run to 26 and 70% in the second run. The decrease of of reaction temperature from room temperature (R.T) to solketal selectivity increases the formation of the by-product 45 °C. However, the formation of a five-member ring acetal (acetal). Though there was slight reduction in the catalytic (solketal) was decreased as the temperature increased from activities of these samples (Nb–SBA-15 and Zr–SBA-15), RT to 45 °C. These findings can explain the favorable for - it showed an excellent stability in its catalyst activity in the mation of the five-membered ring acetal (solketal) at room fourth run, while in the second run only these two samples temperature and its formation was decreased with the rise of (Ti–SBA-15 and Al–SBA-15) showed considerable decrease temperature (45 °C). This is most likely due to the differene of glycerol conversion and selectivity of solketal. These in formation energy of the compound which is strongly results clearly suggest that Nb–SBA-15 and Zr–SBA-15 are dependent on the reaction temperature. The observed results the most industrially viable heterogeneous catalysts for the in this study are in good agreement with the results reported synthesis of oxygenated fuel additives. in literature . From this result, one can easily conclude that the room temperature is the best reaction temperature Spent sample characterization for the formation of solketal. The surface acidity of the spent catalyst (Nb–SBA-15, Catalyst stability tests Zr–SBA-15) (Fig. S4) and the total acidities are mentioned in Table 1. From the TPD profiles, the acidic strength was To know about the stability of the catalysts, the reusability decreased for the spent samples than the fresh ones. Ex study was carried out for the Nb–SBA-15 and Zr–SBA-15 situ pyridine-adsorbed FT-IR analysis (Fig. S5) was also samples. After each run, the sample was separated by cen- performed to examine the acidic properties of the spent trifugation and washed with methanol followed by drying it (Nb–SBA-15, Zr–SBA-15) catalysts. The band intensities in an oven at 100 °C for 10 h. Thereafter, the reaction was corresponding to Brønsted (B) and B + L acidic sites of spent carried out with the spent sample by using the same reac- catalysts were decreased compared to the fresh catalyst. tion conditions mentioned in “Experimental section”. 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