Production of Biofuel Additives from Esterification and Acetalization of ...

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Production of Biofuel Additives from Esterification and Acetalization of Bioglycerol over SnO2‑Based Solid Acids Baithy Mallesham, Putla Sudarsanam, and Benjaram M. Reddy* Inorganic and Physical Chemistry Division, CSIRIndian Institute of Chemical Technology, Uppal Road, Hyderabad 500 007, India S Supporting Information *

ABSTRACT: Owing to significant environmental and economical concerns of fossil fuels, the search for alternative renewable fuels has received a explicit research interest in recent times. In this work, we prepared efficient solid acid catalysts, namely, SnO2, WO3/SnO2, MoO3/SnO2, and SO42−/SnO2 for the production of bioadditive fuels from bioglycerol under ecofriendly conditions. The synthesized samples were meticulously analyzed by means of X-ray diffraction, X-ray photoelectron spectroscopy, Brunauer−Emmett−Teller (BET) surface area, Barrett−Joyner−Halenda pore size distribution, Fourier transform infrared (FT-IR) spectroscopy, FT-IR analysis of adsorbed pyridine, NH3-temperature programmed desorption, and other techniques. The catalytic efficiency of these solid acids was investigated for esterification and acetalization of glycerol with acetic acid and benzaldehyde, respectively. Characterization studies revealed that the textural properties (crystallite size and BET surface area) of pristine SnO2 were outstandingly improved after the addition of promoters to it. Remarkably, large amounts of acidic sites were found for promoted SnO2 samples as compared to pristine SnO2, attributed to a synergetic effect between SnO2 and promoters. The SO42−/SnO2 sample shows superior concentration of acidic sites accompanied by adequate superacidic sites. Concurrently, the promoters showed a favorable effect on the catalytic performance of pristine SnO2 toward glycerol valorization. The observed activity order for both esterfication and acetalization of glycerol was SO42−/SnO2 > MoO3/SnO2 > WO3/SnO2 > SnO2. The high activity of the SO42−/SnO2 is due to the presence of enhanced acidic sites, along with ample superacidic sites. Interestingly, the activity of SO42−/SnO2 catalyst was decreased with repeated use due to the reduced concentration of acidic sites along with leaching of sulfur content, decreased BET surface area, and formation of larger crystallites.

1. INTRODUCTION The production of biobased fuels to replace the fossil fuels is a major research interest nowadays due to ever-increasing environmental and economical concerns of fossil fules.1,2 Particularly, the fossil fuel resources are diminishing and their extensive use has led to serious environmental problems related to the generation of greenhouse gases.3,4 Hence, the European Union has proposed to increase the biofuel utilization with a target of 10% in commercial fuels by 2015.5 Biodiesel, produced from the transesterification of oils and fats with methanol, represents one of the currently used biofuels worldwide.6 Compared to petroleum-derived diesel fuel, biodiesel shows numerous attractive advantages, like renewability, biodegradability, and nontoxicity, and it also emits a negligible amount of exhaust pollutants, and thereby, its production has tremendously increased in recent times.6−9 The transesterification reaction also affords glycerol as the main byproduct with ∼10 wt % of the total biodiesel produced. The economical utilization of surplus glycerol is one of the key challenging issues for sustainability of the biodiesel industry. Presently, glycerol is found to be useful in more than two thousand applications in several industries, such as cosmetics, pharmaceuticals, tobacco, food and drinks, paper, inks and printing, resins, polyesters, etc.10 Nonetheless, all these industries are still unable to utilize all the glycerol produced from the biodiesel industry. Consequently, great research efforts have been undertaken to develop alternative and efficient glycerol utilization applications. The transformation of glycerol into oxygenated © 2014 American Chemical Society

compounds by esterification and acetalization are considered to be the potential routes for glycerol valorization. The esterification of glycerol with acetic acid principally yields three products, namely, monoacetin, diacetin, and triacetin as shown in Scheme 1. These acetins are highly useful products as biofuel additives directly and as precursors in the synthesis of polyesters.11 Especially, triacetin is used as an antiknock additive for gasoline, and the addition of small amounts of triacetin improves certain critical properties of the biodiesel.12 On the other hand, the acetalization of glycerol with benzaldehyde has been reported to produce branched oxygenated compounds, namely, (2-phenyl-1,3-dioxolan-4-yl)methanol (five-membered acetal) and 2-phenyl-1,3-dioxan-5ol (six-membered acetal) as shown in Scheme 2. These oxygenated products are versatile additives for diesel fuel. Further, the addition of glycerol acetals remarkably reduces the harmful emissions release from diesel fuel. Above all, owing to the biomass origin the glycerol-derived oxygenated compounds can be considered themselves as biofuels.10 Traditionally, both esterification and acetalization of glycerol are performed by means of homogeneous mineral acid catalysts.13,14 However, the inherent disadvantages of homogeneous catalysis, such as difficulty in separation and purification Special Issue: Ganapati D. Yadav Festschrift Received: Revised: Accepted: Published: 18775

March 17, 2014 May 24, 2014 May 27, 2014 May 27, 2014 dx.doi.org/10.1021/ie501133c | Ind. Eng. Chem. Res. 2014, 53, 18775−18785

Industrial & Engineering Chemistry Research

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Scheme 1. Esterification of Glycerol with Acetic Acid to Yield Mono-, Di-, and Tri-acetins

typical procedure, the desired amounts of precursors (NaNO3:SnCl2·2H2O = 1:5) were dissolved in double distilled water and vigorously stirred for 1 h at room temperature. The subsequent evaporation of excess water from the solution at 383 K led to a porous and foam-like solid product. The reaction temperature was further increased to ∼523 K and continued at the same temperature until the formation of white powder tin oxide. The obtained products were cooled to room temperature, thoroughly washed with deionized water and oven-dried at 373 K for 12 h. Some portions of the sample were calcined at 923 K for 5 h in air with a heating ramp of 5 K/min. The promoted SnO2 solid acids with 10 wt % of MoO3 and WO3 were prepared by a wet-impregnation method. In brief, the required quantities of (NH 4 ) 6 Mo 7 O 24 ·4H 2 O or (NH4)6H2W12O40·xH2O (Aldrich, AR grade) were dissolved in double distilled water. Then, the calculated amount of finely powdered tin oxide was added to the precursor solution. The excess amount of water was evaporated on a hot plate under vigorous stirring. The obtained samples were oven-dried at 393 K for 12 h and finally calcined at 923 K for 5 h in air with a heating ramp of 5 K/min. The sulfate ion promoted-SnO2 (SO42−/SnO2) sample was also prepared by a wet-impregnation method. In this procedure, the oven-dried tin oxide was taken in a calculated amount of H2SO4 solution (10 wt % SO42− with respect to SnO2). Then, the excess water was evaporated on a hot plate with constant stirring. The resulting sample was oven-dried at 393 K for 12 h followed by calcination at 923 K for 5 h in air with a heating ramp of 5 K/min. 2.2. Characterization Studies. The crystalline structure of the prepared solid acids was analyzed by powder XRD technique using a Rigaku Multiflex instrument equipped with a nickel filtered Cu Kα (0.15418 nm) radiation source and a scintillation counter detector. The XRD patterns were scanned in the 2θ range of 2−80° with a 0.028 step size and using a counting time of 1 s/point. The BET surface areas of samples were determined by N2-adsorption−desorption measurements on a Micromeritics 110 ASAP 2020 instrument. Prior to analysis, the samples were oven-dried at 393 K for 12 h and flushed with argon gas for 2 h. The pore size distribution of solid acids was estimated by means of the Barrett−Joyner− Halenda (BJH) method applied to the desorption leg of the isotherms. The XPS studies were conducted on a Shimadzu (ESCA 3400) spectrometer using Mg Kα (1253.6 eV) radiation. The analysis was done at room temperature and samples were maintained in high vacuum in the order of less than 10−8 Pa to avoid noise in the spectra. The recorded binding energies were corrected with respect to the adventitious carbon (C 1s) peak at 284.6 eV. A Micromeritics AutoChem 2910 instrument was used for the NH3-TPD experiments. Prior to TPD analysis, a quantity of 30 mg of the catalyst was filled in a quartz tube and degassed up to 573 K under the helium flow. Then, the NH3 gas was passed through the catalyst surface for 30 min and subsequently

Scheme 2. Acetalization of Glycerol with Benzaldehyde to Cyclic Acetals, Namely, Five-Membered and Six-Membered Acetals

of the products and generation of large amounts of waste have strongly confined their applicability in the glycerol valorization.12 To overcome these problems, a large number of heterogeneous solid acids have been developed, including promoted metal oxides, hydroxylated magnesium fluorides, SO3H-functionalized ionic liquids, Amberlyst-15, mesoporous silica with sulfonic acid groups, zeolites, and heteropolyacids.8,15−19 Among them, promoted metal oxides are of significant research interest due to various attractive features, such as facile synthesis procedure, remarkable thermal stability, ample acidic sites, and improved catalytic performance. Especially, SnO2 is an imperative metal oxide due to its favorable structural properties, such as mixed oxidation state (Sn4+/Sn2+) and the presence of adequate oxygen vacancies. Thus, SnO2 has found promising applications in catalysis, lithium-ion batteries, solar cells, gas-sensors, etc.20 It has been reported that the catalytic performance of the metal oxides (e.g., ceria, titania, zirconia, titania-zirconia, etc.) can be outstandingly enhanced by the addition of various promoters (SO42−, MoO3, and WO3) through optimiztically modifying their physicochemical characteristics.7,21−23 Therefore, it is believed that the addition of these promoters to the tin oxide can improve its catalytic efficiency for glycerol valorization. Accordingly, in the present investigation, various promoted SnO2 solid acid catalysts (SO42−, MoO3, and WO3) were prepared by a wet-impregnation method. The resulting samples were systematically analyzed with the help of various characterization techniques, namely, X-ray diffraction (XRD), Brunauer−Emmett−Teller (BET) surface area, Barrett−Joyner−Halenda (BJH) pore size distribution, inductively coupled plasma optical emission spectroscopy (ICP-OES), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared (FT-IR) spectroscopy, FT-IR analysis of adsorbed pyridine, NH3-temperature-programmed desorption (NH3-TPD), and other methods. The catalytic performance of the synthesized catalysts was studied for the esterification and acetalization of glycerol with acetic acid and benzaldehyde, respectively. The catalytic results were well correlated with the physicochemical properties of the solid acid catalysts.

2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation. A fusion method was employed to synthesize pristine SnO2 using SnCl2·2H2O (Aldrich, AR grade) and NaNO3 (Aldrich, AR grade) as the precursors. In a 18776

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flushed with helium gas to remove the physisorbed NH3 gas. A thermal conductivity detector was used for continuous monitoring of the desorbed gas, and the areas under the peaks were integrated. The chemisorbed amount of NH3 was determined in flowing helium gas with a flow rate of 20 mL min−1 from 323 to 1073 K at a heating rate of 10 K min−1. The FT-IR analysis was performed on a Nicolet 740 FT-IR spectrometer at ambient conditions using KBr discs with a nominal resolution of 4 cm−1 and averaging 100 spectra. For pyridine adsorbed-FTIR analysis, the catalyst was oven-dried at 373 K for 1 h. The oven-dried catalyst (∼50 mg), in a sample cup, was contacted with pyridine (∼0.1 cm3) directly. The sample cup was kept in a vacuum oven at 393 K for 1 h to remove the physisorbed pyridine on the catalyst surface. After cooling to room temperature, the spectrum was recorded with a nominal resolution of 4 cm−1 in the spectral range of 1400− 1900 cm−1. 2.3. Activity Studies. Different reaction conditions were used for valorization of bioglycerol over the SnO2-based solid acids. 2.3.1. Esterification of Glycerol with Acetic Acid. The catalytic performance of the solid acid catalysts was examined for the esterification of glycerol with acetic acid under atmospheric pressure conditions. The catalysts were preactivated at 423 K for 2 h to remove surface adsorbed residual moisture before catalytic runs. The catalytic experiments were performed in a 10 mL round-bottom flask with 1:6 molar ratio of glycerol to acetic acid, 373 K of reaction temperature, 5 wt % of catalyst loading (with respect to glycerol), and vigorous stirring for an appropriate reaction time. After completion of the reaction, the reaction mixture was cooled to room temperature, and subsequently the solid catalyst was separated from the reaction mixture by centrifugation. The assignment of reaction products was done through a gas chromatograph− mass spectrometer (GC−MS) with a DB-5 capillary column and a flame ionization detector (FID). The reaction mixture was taken periodically and analyzed by GC equipped with BP20 (Wax) capillary column and FID. 2.3.2. Acetalization of Glycerol with Benzaldehyde. The acetalization of glycerol with benzaldehyde was investigated over the SnO2-based solid acids under inert atmospheric conditions. The catalytic experiments were carried out using 5 wt % of catalyst loading (with respect to glycerol), an equivalent molar ratio of glycerol to benzaldehyde, and 30 min of reaction time under room temperature and solvent-free conditions. The catalysts were preactivated at 423 K for 2 h to remove surface adsorbed residual moisture before the catalytic runs. After completion of the reaction, centrifugation was employed to separate the reaction products from the catalyst. The reaction products were identified by GC−MS equipped with DB-5 capillary column, and also with 1H NMR spectroscopy. Finally, the glycerol conversion and products selectivity were quantified by the GC equipped with BP-20 (Wax) capillary column and a flame ionization detector. The conversion of glycerol and selectivity of products were calculated on the basis of the following equations:

selectivity (%) =

amount of glycerol converted to a product (mole) 100 total amount of glycerol converted (mole)

3. RESULTS AND DISCUSSION 3.1. Characterization Studies. The XRD patterns of the SnO2-based solid acids are displayed in Figure 1. Pristine SnO2

Figure 1. Powder X-ray diffraction patterns of pure SnO2, WO3/SnO2, MoO3/SnO2, and SO42−/SnO2 samples.

exhibited standard diffraction peaks of the tetragonal structure with two theta values corresponding to the (110), (101), (200), (211), (220), (002), (310), (112), (301), (202), and (321) planes (PDF No. 880287). It is interesting to note that the WO3, MoO3, and SO42−-promoted SnO2 catalysts also show diffraction patterns of tetragonal structured SnO2. To our surprise, no XRD peaks corresponding to promoters are observed in the present study. However, the XRD profiles of the promoted SnO2 samples are broad in comparison to bare SnO2, indicating the decrease of SnO2 crystallite size. The mean crystallite size of SnO2 (D) was estimated by the following Scherrer equation, and the obtained values are presented in Table 1. D=

0.9λ β cos θ

Table 1. BET Surface Area (S), Crystallite Size (D), Acidic Sites, Pore Volume (V), and Pore Size (P) of SnO2, WO3/ SnO2, MoO3/SnO2, and SO42−/SnO2 Samples

conversion (%) amount of glycerol converted (mole) = 100 total amount of glycerol in the feed (mole)

sample

S (m2/g)

Da (nm)

acidic sitesb (μmol g−1)

Vc (cm3/g)

Pc (nm)

SnO2 WO3/SnO2 MoO3/SnO2 SO42−/SnO2 SO42−/SnO2d SO42−/SnO2e

11 32 56 41 35 n.d.

13.47 8.76 6.06 7.91 9.73 11.54

46.47 61.81 81.45 186.98 135.81 125.85

0.074 0.064 0.068 0.162 0.129 n.d.

9.035 7.759 4.845 15.797 7.276 n.d.

a

From XRD spectra. bFrom NH3-TPD studies. cFrom BJH analysis. For spent catalyst (glycerol + benzaldehyde). eFor spent catalyst (glycerol + acetic acid); n.d. = not determined. d

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Figure 2. Pore size distribution profiles of SnO2, WO3/SnO2, MoO3/SnO2, and SO42−/SnO2 catalysts.

where λ is the wavelength of the Cu Kα radiation, β is the full width at half-maximum (fwhm) of the diffraction peak (110) and θ is the Bragg’s diffraction angle of the peak (110). The obtained crystallite size of pristine SnO2 is ∼13.47 nm. A remarkable decrease in the crystallite size of SnO2 was noticed after the addition of promoters to it. The average crystallite sizes were found to be ∼8.76, 6.06, and 7.91 nm for WO3/ SnO2, MoO3/SnO2, and SO42−/SnO2 samples, respectively. This interesting observation is indeed due to the beneficial role of the promoters toward inhibition of crystal growth against higher thermal treatments. Similar results were also reported in the literature for various promoted metal oxides, such as TiO2, ZrO2, Fe2O3, CeO2, etc.16,19,24−26 The textural properties of promoted SnO2 samples were determined using N2 physical adsorption/desorption method and the obtained results are listed in Table 1. It can be seen that the N2 adsorption/desorption isotherms for promoted SnO2 samples are very similar to that of the pristine SnO 2 (Supporting Information, Figure S1). All samples exhibited type IV with an H1 hysteresis loop, indicating typical mesoporous nature of the materials.27,28 The BJH pore size distribution profiles of SnO2, WO3/SnO2, MoO3/SnO2, and SO42−/SnO2 solid acids are presented in Figure 2. A unimodal distribution was noticed for the SnO2, MoO3/SnO2, and SO42−/SnO2 catalysts, whereas the WO3/SnO2 sample showed multimodal distribution. The pore size of SnO2, WO3/SnO2, MoO3/SnO2, and SO42−/SnO2 solid acids were found to be ∼9.035, 7.759, 4.845, and 15.797 nm, respectively (Table 1). The promoted SnO2 catalysts showed larger pore volumes when compared to that of pristine SnO2 (Table 1). The average pore volumes were found to be ∼0.074, 0.064, 0.068, and 0.162 cm3/g for SnO2, WO3/SnO2, MoO3/SnO2, and SO42−/SnO2 samples, respectively. The estimated BET surface areas of all SnO2-based samples are shown in Table 1. It was found that the addition of SO42−, MoO3 and WO3 promoters to SnO2 largely increases its BET surface area in agreement with the crystallite size decrease (Table 1). Generally, higher temperature

treatments cause a gradual sintering and consequently, unusual crystallite growth, which eventually leads to low surface area. The enhancement in the BET surface area of the promoted samples is due to the existence of synergetic effect between the promoters and the SnO2. The measured BET surface area of SnO2, WO3/SnO2, MoO3/SnO2 and SO42−/SnO2 samples are ∼11, 32, 56, and 41 m2/g, respectively. The XPS analysis was performed to know the chemical states of the elements present in the SnO2-based catalysts. The binding energies of W 4f7/2, Mo 3d5/2, and S 2p3/2 XP bands are found to be ∼35.6, 232.7, and 169.3 eV, respectively, which are well correlated with that of the reported values (Supporting Information, Figure S2).7,29−31 These binding energies confirm the presence of W 6+ , Mo 6+ , and S 6+ species in the corresponding promoted SnO2 catalysts. Goswami and Ganguli reported that the presence of bands at 163−164 and 167.5 eV indicates the elemental sulfur and SO32− species, respectively.32 However, no XPS peaks related to elemental sulfur and SO32− species were found in the present study. The S 2p3/2 fitted by a Gaussian function centered at ∼169.3 eV is in line with the binding energy of S 2p3/2 in the sulfate ion group.29,33 The presence of sulfate ion groups in the SO42−/SnO2 catalyst is also identified by FT-IR study (Supporting Information, Figure S3). On the other hand, no peaks related to other species are found in the present investigation (i.e., Mo4+ at ∼231.4 eV and Mo5+ at ∼237.5 eV).9 The Sn 3d XP spectra of the SnO2-based solid acids are shown in Figure 3. The Sn 3d spectra of all samples contain two symmetrical bands at around 487.3 and 495.8 eV, which can be assigned to Sn 3d5/2 and Sn 3d3/2, respectively.34 Interestingly, after the incorporation of promoters, there is a small shift toward the higher binding energy side of the Sn 3d spectrum, which might be due to the electron deficiency of Sn in the promoted SnO2 catalysts. A plausible explanation for this unusual observation could be the shifting of electron density of the Sn toward the promoter species, which indicates electronic modification of the Sn−O species after the addition of the 18778



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Figure 3. Sn 3d XP spectra of SnO2, WO3/SnO2, MoO3/SnO2, and SO42−/SnO2 samples.

Figure 4. NH3-TPD profile of SnO2, WO3/SnO2, MoO3/SnO2, and SO42−/SnO2 catalysts.

promoters to the SnO2.35 It was reported that the presence of dopants/promoters results in a decrease in the electronic density of the support, hence the enhanced acidic nature of the promoted SnO2 catalysts.36,37 Therefore, it can be expected that the promoted SnO2 catalysts prepared in the present work show improved acidic nature compared to the pristine SnO2, which could be one of the key reasons for the observed high catalytic performance of the promoted SnO2 catalysts as discussed in the activity part. The obtained O 1s spectra were fitted with two Gaussian functions, which indicate the presence of different oxygen species in the SnO2 samples (Supporting Information, Figure S4). A major band is identified at around 530.2 eV which can be assigned to the oxygen species in the SnO2 lattice.38−40 Another peak was noticed at about 532.2 eV, which reveals the oxygen species present in −OH, CO, CO32− adsorbed groups on the catalyst surface, and surface oxygen ions caused by the change of a static charge distribution within the catalysts.29,41 The acidic properties of the prepared SnO2 catalysts were evaluated by NH3-TPD analysis (Figure 4). The total number of acidic sites were calculated with the help of NH3 desorption peak areas and the obtained values are summarized in Table 1. As can be seen from Figure 4, various peaks were identified in the temperature region of 323−1073 K, which might be due to the variation in the activation energy of NH3 desorbed from the different acidic sites.42 The observed TPD peaks at 900−673 K, 673−473 K, and below 473 K indicate the strong-, medium-, and weak-acidic sites, respectively.7,43 Interestingly, the SO42−/ SnO2 catalyst exhibited a NH 3 desorption peak at a temperature greater than 900 K due to the contribution of superacidic sites, which are very useful for improving its catalytic performance toward glycerol valorization as discussed in the activity part. In contrast, the MoO3 and WO3 promoted SnO2 samples show only strong-, medium-, and weak-acidic sites. It was clear from Figure 4 and Table 1 that the addition of promoters to the SnO2 greatly improve its acidic properties. Among these promoters, more number of acidic sites as well as superacidic sites (>900 K) were observed for the SO42− ion promoted SnO2 catalyst. The sulfate groups present on the surface of the SO42−/SnO2 sample (evidenced by O 1s XPS and FT-IR analyses) (Supporting Information, Figures S3 and S4)

are responsible for its improved amounts of acidic sites.43 The estimated concentration of acidic sites was found to be ∼46.47, 61.81, 81.45, and 186.98 μmol g−1 for SnO2, WO3/SnO2, MoO3/SnO2, and SO42−/SnO2 catalysts, respectively. It is a well-known fact that the NH3-TPD measurements provide information about the acidic strength of the catalysts. Concurrently, the identification of Brønsted- as well as Lewisacidic sites is rather difficult from NH3-TPD studies. Hence, we investigated the pyridine adsorbed FT-IR analysis over the WO3, MoO3, and SO42− promoted SnO2 solid acids (Figure 5).44 All samples show a variety of bands in the region of 1700−1300 cm−1. Particularly, three absorption bands observed at ∼1639, ∼1516, and ∼1550 cm−1 indicate the presence of Brønsted acidic sites on the catalyst surface, whereas the absorption band observed at around ∼1455 cm−1 can be assigned to coordinatively bound pyridine with Lewis acid sites

Figure 5. Pyridine adsorption FT-IR spectra of the SnO2, WO3/SnO2, MoO3/SnO2, and SO42−/SnO2 catalysts. B, Brønsted; and L, Lewis acidic sites. 18779



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on the synthesized catalysts.7,45,46 The observed IR band at ∼1490 cm−1 could be attributed to pyridine associated with both Brønsted- and Lewis-acidic sites on the catalyst surface.9,46 These results evidently reveal that all promoted SnO2 samples contain both Brønsted- and Lewis-acidic sites, of which more number of Brønsted acidic sites could be found compared to that of Lewis acidic sites over all samples. 3.2. Catalytic Activity Studies. 3.2.1. Esterification of Glycerol with Acetic Acid. To find the best solid acid catalyst among the prepared samples, we compared the catalytic activity of the WO3-, MoO3-, and SO42−-promoted SnO2 solid acids for the esterification of glycerol with acetic acid and the results are shown in Figure 6. The experiments were carried out at an

optimized temperature of 373 K, 1:6 molar ratio of glycerol to acetic acid, 5 wt % of catalyst (with respect to glycerol), and 2 h of reaction time with constant stirring (∼800 rpm). In glycerol esterification, the formation of monoacetin is highly feasible when compared to that of further esterification of monoacetin into di- and triacetins (Scheme 1). Therefore, the catalytic efficiency of these solid acids was measured based on the conversion rate toward di- and triacetin products. Activity results revealed that the addition of promoters to the SnO2 significantly improve its catalytic performance. Among the solid acids studied, the SO42−/SnO2 exhibited superior glycerol conversion and high product selectivity toward the di- and triacetins. The obtained glycerol conversions over the SnO2, WO3/SnO2, MoO3/SnO2, and SO42−/SnO2 solid acids were ∼41.6, 66.3, 71.5, and 89%, respectively. As well, the selectivity of the di- and triacetins was found to be 21.2 and 5.8% for the SO42−/SnO2 catalyst. As can be noted from Figure 4 and Table 1, the SO42−/SnO2 solid acid contains a large amount of acidic sites with adequate superacidic sites, and thereby, exhibits enhanced catalytic performance toward glycerol esterification. We compared the catalytic activity of the sulfated SnO2 with other solid acid catalysts published in the literature47−52 (Supporting Information, Table S1). Interestingly, it was observed that the SO42−/SnO2 catalyst shows relatively higher activity toward glycerol esterification with acetic acid than the reported catalysts. The better catalytic performance of the SO42−/SnO2 catalyst could be due to the presence of abundant superacidic sites. Figure 7 shows the influence of molar ratio of glycerol to acetic acid on the catalytic performance of SO42−/SnO2 as a function of reaction time. The reactions were carried out by changing the molar ratio of glycerol to acetic acid from 1:1 to 1:9 at a reaction temperature of 373 K and catalyst loading of 5

Figure 6. Esterification of glycerol with acetic acid over SnO2, WO3/ SnO2, MoO3/SnO2, and SO42−/SnO2 catalysts. Reaction conditions: molar ratio of glycerol to acetone = 1:6; reaction time = 2 h; reaction temperature = 373 K; catalyst amount = 5 wt % (with respect to glycerol).

Figure 7. Effect of molar ratio of glycerol to acetic acid as a function of reaction time on the esterification of glycerol with acetic acid over SO42−/ SnO2 catalyst. (A) 1:1; (B) 1:3; (C) 1:6; and (D) 1:9. Reaction conditions: reaction temperature = 373 K; catalyst amount = 5 wt % (with respect to glycerol). 18780



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obtained results are displayed in Figure 9. The catalytic experiments were performed at 30 min of reaction time, an

wt % (with respect to glycerol) at a constant stirring (∼800 rpm). The obtained results obviously reveal the rapid glycerol conversion and high monoacetin selectivity in the initial reaction times because, as stated earlier, the formation of monoacetin is much easier when compared to that of di- and triacetins. However, the selectivity toward di- and triacetins was remarkably increased at the expense of monoacetin selectivity with increasing molar ratio of glycerol to acetic acid as a function of reaction time. As well, better glycerol conversions were achieved at higher molar ratios of glycerol to acetic acid. At the initial 0.5 h reaction time, the observed glycerol conversion was ∼41, 53, 64, and 72% with 1:1, 1:3, 1:6, and 1:9 molar ratio of glycerol to acetic acid, respectively. Afterward, the glycerol conversion and selectivity toward the triacetin increases enormously with the increase of reaction time. A maximum selectivity of triacetin was achieved (55.2%) at 1:9 molar ratio of glycerol to acetic acid after 10 h of reaction time attributed to consecutive esterification of mono- and diacetins to triacetin. Therefore, it can be concluded that the glycerol conversion and selectivity of triacetin strongly depends on the molar ratio of glycerol to acetic acid. Figure 8 shows the influence of time-on-stream on the glycerol conversion and products selectivity over the SO42−/

Figure 9. Acetalization of glycerol with benzaldehyde over SnO2, WO3/SnO2, MoO3/SnO2, and SO42−/SnO2 catalysts. Reaction conditions: molar ratio of glycerol to benzaldehyde = 1:1; reaction time = 30 min; catalyst amount = 5 wt % (with respect to glycerol).

equivalent molar ratio of glycerol to benzaldehyde and 5 wt % of catalyst loading under solvent-free and room temperature conditions. As can be noticed from Figure 9, all solid acids exhibited considerable catalytic activity and product selectivity toward five- and six-membered acetals. Remarkably, promoted SnO2 solid acids showed superior catalytic performance than that of pristine SnO2. The glycerol conversion over the SnO2, WO3/SnO2, MoO3/SnO2, and SO42−/SnO2 catalysts was found to be ∼48, 61, 69 and 77%, respectively. Interestingly, the selectivity of the six-membered acetal is comparatively higher than that of the five-membered acetal, indicating that the fivemembered acetal isomerizes to give the six-membered acetal. The highest glycerol conversion was observed over the SO42−/ SnO2 catalyst attributed to the presence of a greater number of acidic sites, accompanied by ample superacidic sites. The selectivity to five- and six-membered acetals was found to be ∼41 and 59% over the SO42−/SnO2 solid acid, respectively. Figure 10 shows the effect of the molar ratio of glycerol to benzaldehyde on the glycerol conversion and product selectivity as a function of reaction time. A slight enhancement in the glycerol conversion was found for the 1:2 molar ratio of glycerol to benzaldehyde when compared to the 1:1 molar ratio of glycerol to benzaldehyde. The conversion of glycerol was found to be ∼75 and 82% for 1:1 and 1:2 of glycerol to benzaldehyde molar ratios, respectively, within 30 min of reaction time. Despite different molar ratios of glycerol to benzaldehyde, the glycerol conversion was found to increase with the increase of reaction time. The obtained glycerol conversions were ∼99 and 100% for 1:1 and 1:2 molar ratios of glycerol to benzaldehyde for 150 min of reaction time, respectively. Interestingly, no significant variation in the selectivity of products was observed with the variation of molar ratio as well as increasing the reaction time. 3.2.3. Catalyst Reusability Study. We have studied the reusability of SO42−/SnO2 catalyst to understand its stability for the esterification of glycerol with acetic acid (Supporting Information, Figure S5). The catalytic experimental conditions remained the same as described in Figure 6. After each cycle, the catalyst was separated from the reaction mixture by centrifugation and washed with 100 mL of methanol to remove the adsorbed glycerol and the products from the catalyst

Figure 8. Effect of reaction time on the esterification of glycerol with acetic acid over the SO42−/SnO2 catalyst. Reaction conditions: molar ratio of glycerol to acetic acid = 1:6; reaction temperature = 373 K; catalyst loading = 5 wt % (with respect to glycerol).

SnO2 solid acid. The reactions were performed at different time intervals up to 35 h with molar ratio of glycerol to acetic acid of 1:6, reaction temperature of 373 K, and catalyst loading of 5 wt % (with respect to glycerol). As can be seen from Figure 8, high selectivity of monoacetin was found at initial times. Interestingly, the selectivity toward di- and triacetins was outstandingly increased with the increase of reaction time. After 10 h of reaction time, the selectivity of triacetin was further increased with time and selectivity of mono- and diacetins were decreased due to the further consecutive reactions of monoand diacetins to yield the stable triacetin product. The SO42−/ SnO2 solid acid exhibited a maximum selectivity of ∼75% triacetin at 30 h of reaction time and afterward, there was no significant change in the selectivity of triacetin. 3.2.2. Acetalization of Glycerol with Benzaldehyde. The catalytic efficiency of the SnO2-based solid acids was also studied for the glycerol acetalization with benzadehyde and the 18781



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Figure 10. Effect of the molar ratio of glycerol to benzaldehyde as a function of reaction time on the acetalization of glycerol with benzaldehyde over SO42−/SnO2 (SSn) catalyst: (a) 1:1; (b) 1:2; reaction conditions: catalyst amount = 5 wt % (with respect to glycerol).

surface, and preactivated at 423 K for 3 h before the next catalytic run. The achieved glycerol conversions with acetic acid were ∼89, 78, 66, and 51% for the first, second, third, and fourth cycle, respectively. On the other hand, the selectivity toward the di- and triacetins was decreased with repeated use of the catalyst. The reusability analysis of the SO42−/SnO2 solid acid for the acetalization of glycerol with benzaldehyde was also studied (Supporting Information, Figure S6). The reaction conditions remained the same as in Figure 9. After each reaction, the catalyst was separated from the reaction mixture by centrifugation and washed with 100 mL of methanol to remove any adsorbed reaction ingredients from the catalyst surface. The catalyst was then preactivated at 423 K for 3 h. It was found that the conversion of glycerol decreases continuously with repeated use of the catalyst. The obtained glycerol conversions at the first, second, third, and fourth cycles were ∼77, 71, 63, and 52%, respectively. However, it is interesting to note that the catalyst can be recyclable up to a fourth run without considerable variation in the product selectivity. The selectivity of the five- and six-membered acetals was found to be 42 and 58%, respectively, for all cycles. Interestingly, the decrease in glycerol conversion in the case of glycerol esterification with the acetic acid is more when compared to the decrease in the glycerol conversion for the acetalization of glycerol with the benzaldehyde. This observation indicates the fast deactivation of catalyst in the case of glycerol esterification with acetic acid. Some efforts to know the deactivation of the SO42−/SnO2 catalyst after both reactions have been undertaken by analyzing the textural and structural properties of the SO42−/SnO2 after the first cycle. The obtained results were compared with that of the fresh catalyst. It can be noticed from Figure 11 that the intensity of the diffraction peaks of the SO42−/SnO2 catalyst was notably increased after the repeated use for both the reactions. It is well-documented that an increase in the crystallite size of metal oxides results in an enhancement in the intensity of their XRD patterns. The estimated average crystallite size of the fresh and spent catalyst evidence the above explanation (Table 1). The N2 adsorption−desorption isotherms and pore size distribution curves of the fresh and spent catalysts are shown in Supporting Information, Figure S7 panels A and B, respectively. It was found that the spent catalyst exhibits decreased BET surface area (∼35 m2/g) compared to

Figure 11. Powder X-ray diffraction patterns of the fresh and spent SO42−/SnO2 catalysts: (A) fresh catalyst; (B) spent catalyst (glycerol + benzaldehyde); (C) spent catalyst (glycerol + acetic acid).

fresh catalyst (∼41 m2/g). Concurrently, both catalysts showed type IV isotherms with a H1 hysteresis loop, which suggests the mesoporous nature of the samples.27 However, the pore volume and pore size were found to decrease from 0.162 cm3/g and 15.797 nm to 0.129 cm3/g and 7.276 nm (Table 1), respectively, after the first cycle. ICP-OES analysis was performed to estimate the sulfur content present in the fresh and spent SO42−/SnO2 catalysts. The leaching of sulfur content was found to be ∼13 and 9% after the first cycle for esterification and acetalization of glycerol, respectively. To understand this, we also carried out both reactions, namely, glycerol esterification and acetalization with acetic acid and benzaldehyde, respectively, with homogeneous S species dissolved in the reaction mixture after hot filtration of the catalyst. Interestingly, a considerable glycerol conversions of 94 and 81% were found for the esterification and acetalization, respectively, which strongly indicates the leaching of the S species after the reaction. Moreover, we also investigated the NH3-TPD of the spent catalyst (SO42−/SnO2) after the reactions of esterification and acetalization of glycerol (Figure 12). The calculated total amount of acidic sites present on the surface of SO42−/SnO2 catalyst after the esterification and 18782



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surface area and increased crystallite size are found to be the key factors for the observed catalytic deactivation of the SO42−/ SnO2 solid acid.

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ASSOCIATED CONTENT

* Supporting Information S

BET, XPS, FT-IR, and reusability test of glycerol acetalization and esterification profiles pertaining to SnO2, WO3/SnO2, MoO3/SnO2, and SO42−/SnO2 samples. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; [email protected]. Tel.: +91 40 2719 3510. Fax: +91 40 2716 0921. Notes

The authors declare no competing financial interest.

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Figure 12. NH3-TPD profile of fresh and spent SO42−/SnO2 catalysts: (A) fresh catalyst, (B) spent catalyst (glycerol + benzaldehyde), and (C) spent catalyst (glycerol + acetic acid).

ACKNOWLEDGMENTS B.M. and P.S. thank the Council of Scientific and Industrial Research (CSIR), New Delhi, for the award of research fellowships. Financial support was received from Department of Science and Technology, New Delhi, under SERB Scheme (SB/S1/PC-106/2012).

acetalization were found to be ∼125.85 and 135.81 μmol g−1, respectively (Table 1). It was clear from these results that less number of acidic sites were found for the glycerol esterificationcatalyst than that of the glycerol acetalization-catalyst, which is one of the key reasons for a large decrease in the glycerol esterification reaction (Supporting Information, Figure S5). Therefore, it can be concluded that the presence of a large amount of acidic sites is essential for enhancing the esterification and acetalization of glycerol. Besides, the decreased BET surface area and increased crystallite size are also responsible for the decreased catalytic performance of the SO42−/SnO2 solid acid after the repeated use.

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REFERENCES

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4. CONCLUSIONS We prepared various promoted SnO2-based solid acid catalysts for production of bioadditive fuels from esterification and acetalization of glycerol with acetic acid and benzaldehyde, respectively. A wet-impregnation method was employed for the synthesis of WO3/SnO2, MoO3/SnO2, and SO42−/SnO 2 catalysts. The physicochemical properties of the SnO2-based solid acids were investigated by means of various characterization techniques. It was found that all samples show tetragonal structured SnO2, whereas no XRD peaks related to promoters were observed in this study. The addition of promoters to pristine SnO2 remarkably improved its textural properties. XPS analysis revealed the presence of S6+, Mo6+, and W6+ oxidation states in the corresponding promoted SnO2 catalysts. Large amounts of acidic sites were found for promoted SnO2 samples in comparison to pristine SnO2. Among them, the SO42−/SnO2 sample showed ample superacidic sites. Activity results revealed that the promoted SnO2 samples exhibit outstanding catalytic performance when compared to pristine SnO2, attributed to the presence of superior concentration of acidic sites. Among the studied solid acid catalysts, the SO42−/SnO2 sample showed excellent catalytic activity for both esterification and acetalization of glycerol mainly due to the presence of adequate superacidic sites. Interestingly, the catalytic performance of the SO42−/ SnO2 sample was considerably decreased after the repeated use for both esterification and acetalization reactions. The decreased amount of acidic sites, along with decreased BET 18783



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