Glycerol Valorization as Biofuel Additives by Employing a Carbon

Sep 30, 2014 - A two-step process was developed for the preparation of triacetylglycerol (TAG), a biofuel additive employing a solid acid catalyst der...
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Glycerol Valorization as Biofuel Additives by Employing a CarbonBased Solid Acid Catalyst Derived from Glycerol Ummadisetti Chandrakala, Rachapudi B. N. Prasad, and Bethala L. A. Prabhavathi Devi* Centre for Lipid Research, CSIR - Indian Institute of Chemical Technology, Tarnaka, Hyderabad-500007, India S Supporting Information *

ABSTRACT: A two-step process was developed for the preparation of triacetylglycerol (TAG), a biofuel additive employing a solid acid catalyst derived from glycerol. The first step involves esterification of glycerol with acetic acid (1:4 mol) at 115 °C for 1 h in the presence of catalyst (5 wt %) to obtain 100% conversion of glycerol with 22, 67, and 11% MAG, DAG, and TAG, respectively. In the second step, the esterified product mixture after removal of excess acetic acid and moisture was acetylated with acetic anhydride (1 mol with respect to glycerol) at 115 °C for 30 min to obtain 100% TAG. The recovered catalyst was reused for five cycles without any significant loss of its initial activity. The protocol is cost-effective due to the use of highly stable and recyclable SO3H-carbon catalyst for the production of TAG with minimum moles of acetic acid and acetic anhydride in shorter reaction time. gasoline.15 The acetylated glycerols can be produced by esterification/acetylation of glycerol with acetic acid/acetic anhydride with or without homogeneous or heterogeneous catalysts. Homogeneous acid catalysts such as HCl, H3PO4, and p-TSA have been employed for the acetylation of glycerol. However, homogeneous catalysts generate several problems at the end of the reactions, including difficulty in separation of the catalysts, production of toxic pollutants, and corrosion of the reactor. To overcome these drawbacks, research attention has been mainly impelled to discover ecofriendly and highly stable heterogeneous catalysts, such as solid acid or base catalysts. In the last few years, several studies were reported by using different solid acid catalysts such as Amberlyst-15, zirconia, niobic acid, HPAs, and zeolites for the preparation of glycerol acetates from glycerol and acetic acid.16−18 Acidic mesoporous silica catalyst was reported for 90% glycerol conversion with over 85% of combined selectivity toward DAG and TAG.15 When sulfuric acid treated activated carbon was used as catalyst, 91% glycerol conversion with selectivity of 38, 28, and 34% for mono-, di-, and triacetyl glycerols, respectively, was achieved at 120 °C and 3 h of reaction time.5 When Amberlyst-15 acid resin was used as a catalyst, a selectivity of 54 and 13% toward DAG and TAG, respectively, was achieved.4 Dodecatungstophosphoric acid immobilized onto a silica matrix as a heterogeneous catalyst achieved 65% combined selectivity (62 and 3% for DAG and TAG, respectively) to DAG and TAG.19 Whereas in the presence of niobic acid-supported tungstophosphoric acid (TPA) as catalyst, a relatively high combined selectivity of 77% (57 and 20% of DAG and TAG, respectively) to DAG and TAG was recorded.7 When MoOx/TiO2−ZrO2 catalyst was used, 100% conversion with selectivity of 53, 40, and 7% of MAG, DAG, and TAG, respectively, was reported.8

1. INTRODUCTION In recent years, the commercial use of biodiesel has been rapidly expanding throughout the world as a potential substitute for the petroleum based fuels because it is one of the renewable, biodegradable, and biocompatible new energy sources. The rapid growth of the biodiesel industry resulted in oversupply of glycerol which has become a burden to the biodiesel industry until new markets are created for glycerol by way of new product development.1,2 Glycerol is a highly fascinating molecule for the preparation of a variety of chemical intermediates, and also it is a platform chemical for the production of biofuel additives. Several groups are working for the conversion of glycerol into value added products through different strategies and/or approaches such as hydrogenolysis, esterification/acetylation, reforming, etherification, etc.3−13 Most of these reactions are reported to be favored in the presence of Brønsted acid catalysts, especially for the production of oxygenated compounds like fuel additives from biodiesel byproduct glycerol. Valorization of glycerol as fuel additives can not only improve the engine performance by reducing the diesel fuel particulate emissions, pour point, and viscosity of biodiesel on blending but also reduce the cost and increase the quality of biofuel.14 Acetylated glycerol derivatives, namely, monoacetylglycerol (MAG), diacetylglycerol (DAG), and triacetylglycerol (TAG), have gained potential interest to find applications for the better utilization of excess glycerol produced from the biodiesel process. The industrial production of acetylglycerols is known since decades ago, and all of them have several applications in the chemical industry. MAG is used in the manufacture of explosives, as tanning agent, and as solvent for dyes. DAG is used as a plasticizer and softening agent and solvent. TAG is used as a solvent for dissolving or diluting drugs and organic compounds. In addition, DAG and TAG are valuable additives that cause improved properties with respect to viscosity, flash point, and oxidation stability when used as a supplement to diesel fuel and also as antiknocking agent when added to © XXXX American Chemical Society

Received: August 1, 2014 Revised: September 30, 2014 Accepted: September 30, 2014

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Sulfated zirconia catalysts were considerably less active.20 Most recently, sulfated zircon-based mixed oxides (SO42−/CeO2− ZrO2) solid acid catalyst was reported with 100% conversion with 25, 59, and 16% selectivity for MAG, DAG, and TAG, respectively.10 However, some of these catalysts cannot be applied industrially because HPAs are soluble in polar media, some silicate-based catalysts are difficult to functionalize, zeolites have low thermal stability, and some materials are too expensive.5 Moreover, water also affects the catalytic behavior of the catalyst because of the hygroscopic nature of some of these catalysts. Even though there are many studies for the preparation of acetylated glycerol derivatives with acetic acid in a single step using different solid acid catalysts, none of them showed 100% selectivity to TAG even with high amounts of acetic acid: glycerol ratio (>6:1 mmol) and after longer reaction periods (>4 h) at higher temperatures (>120 °C). As the rate of acetylation reaction is higher than esterification, acetic anhydride as an acylating reagent can achieve 100% selectivity for TAG either through esterification followed by acetylation with the acid resin Amberlyst-35 as a catalyst in the presence of a high excess of acetic acid or by direct acetylation with acetic anhydride with K-10 Montmorillonite as catalyst.21,17 According to Liao et al.,21 first step of esterification was carried out with 9 equiv of acetic acid (with respect to glycerol) for 4 h at 105 °C, and in the second step acetic anhydride was added to complete the reaction with 100% selectivity to TAG in 15 min. But the main disadvantage of this method is use of high moles of acetic acid for the esterification step and longer reaction time with less selectivity to TAG. Recently, carbon-based solid acid catalysts have gained significant attraction over homogeneous catalysts as they are highly efficient, sustainable, and eco-friendly and can be reused several times without appreciable loss of activity.22 In this context, we have demonstrated that sulfonic acid functionalized heterogeneous carbon catalysts derived from glycerol (biodiesel byproduct)/glycerol pitch (waste from fat splitting industry) are promising catalysts for the esterification of fatty acids, tetrahydropyranyl protection and deprotection of alcohols and phenols, simultaneous esterification and transesterification of nonedible oils, acetylation of alcohols, phenols, and amines, and also the regioselective azidolysis of epoxides.23−28 In continuation of our efforts toward exploring the applications of the carbon acid catalyst having 1.6 mmol/g acid density with surface area of 0.21 m2/g, we herein report a simple and highly efficient two-step process for the production of TAG, a biofuel additive by esterification of glycerol with acetic acid followed by acetylation of glycerol acetates with acetic anhydride at 115 °C within 1.5 h with high selectivity and reusability.

XPS, IR, 13C MAS, and Raman spectroscopy. The methods are discussed in Supporting Information. 2.2. Two-Step Methodology for the Preparation of Triacetylglycerol (TAG). In the first step, glycerol was reacted with acetic acid followed by acetic anhydride in the second step to achieve 100% TAG in quantitative yields. 2.2.1. First Step: Esterification of Glycerol with Acetic Acid. A mixture of glycerol (9.2 g, 0.1 mol), acetic acid (23 mL, 0.4 mol), and catalyst (0.46 g, 5 wt % of glycerol) were heated at 115 °C for 1 to 4 h under stirring. The reaction was monitored by TLC and GC. After complete conversion of glycerol the catalyst was separated from the reaction mixture by filtration, washed with methanol, and dried at 110 °C for reuse. The product mixture was concentrated and dried by distilling out the unreacted acetic acid and moisture formed during the reaction in a rotary evaporator under reduced pressure. Optimization of reaction conditions for the complete conversion of glycerol to glycerol acetates was conducted by varying the catalyst loading from 5 to 20 wt % (with respect to glycerol) and mole ratio of glycerol:acetic acid (1:4 to 1:10) for different reaction periods (1 to 4 h) at 115 °C. The composition of the reaction mixture for the formation of MAG, DAG, and TAG was determined at regular intervals by GC. There is a possibility of the formation of the isomers of MAG (1-MAG and 2-MAG) and DAG (1,3-DAG and 1,2DAG) during esterification reaction. However, separation of the isomers of MAG and DAG was not observed in the GC analysis and hence it was reported as MAG and DAG. 2.2.2. Second Step: Acetylation of Glycerol Ester Mixture with Acetic Anhydride. The dried esterified products mixture obtained after 1 h and the recovered carbon acid catalyst from the first step were treated with acetic anhydride (9.56 mL, 0.1 mol, 1:1 with respect to glycerol used in the first step) at 115 °C for 30 min to get 100% TAG. The reaction was monitored by TLC and GC. After completion of the reaction, catalyst was separated by filtration, and the unreacted acetic anhydride and acetic acid formed during the reaction were distilled out under vacuum in a rotary evaporator to obtain 100% TAG in quantitative yield. In another protocol the reaction mixture from the first step (without separation of catalyst and drying) after 1 h was treated with acetic anhydride (0.1 mol, 1:1 with respect to glycerol) at 115 °C for 4 h. The effect of drying of the esterified product and catalyst in the acetylation reaction for the preparation of TAG was also studied by conducting the reactions with and without catalyst and also by varying the acetic anhydride concentration and reaction period. 2.3. Analysis of Reaction Products. The reaction samples were collected at 1 h intervals and were qualitatively analyzed using gas chromatography (GC) for conversion percentages. The reaction sample (100 μL) was diluted in CHCl3:MeOH (1:1, 2 mL), and 1 μL of the liquid was injected into a capillary nonpolar GC column HP-1 (30 m × 0.32 mm × 0.10 μm) mounted in a gas chromatograph GC 6850 system (Agilent Technologies, USA). The analysis was performed in a temperature-programmed mode of 80 °C (0 min)−10 °C/ min−300 °C (5 min). The typical retention times (tR) of the components were 4.04, 5.03, and 5.93 min for MAG, DAG, and TAG, respectively.

2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation.28 A mixture of glycerol (10 g) and concentrated sulfuric acid (30 g) were taken in a 500 mL glass beaker and gently heated on hot plate from ambient temperature to 220 °C to facilitate in situ partial carbonization and sulfonation. The contents of the reaction were left at that temperature for about 20 min until the foaming ceased. The resultant black crystalline product was cooled to ambient temperature and washed with hot water under agitation until the wash water became neutral to pH. The product was filtered using filter cloth (90 μ pore size) under vacuum and dried in the oven for 2 h at 120 °C in order to ensure that it was free of moisture to obtain glycerol based carbon acid catalyst (5.60 g). The catalyst was fully characterized by CHN analysis, XRD,

3. RESULTS AND DISCUSSION Generally, the synthesis of biofuel additives by acetylation of glycerol using acetic anhydride proceeds with higher reaction rates and is more difficult to handle than that of esterification. B

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Scheme 1. Carbon Acid Catalyzed Two-Step Process for the Preparation of Triacetylglycerol

However, if only acetic acid was used for esterification reaction with glycerol, MAG and DAG were found to be major components restricting TAG to not more than 20%. So there is a need to use acetic anhydride for completing the reaction to prepare TAG in high yields. But the price of the acetic anhydride is as high as four times that of acetic acid, and hence it is not commercially feasible for industry to use acetic anhydride alone to prepare TAG. In order to reduce the cost of the overall process for higher production of TAG, we employed a two-step protocol to achieve TAG quantitatively in short time by employing highly stable and recyclable carbon-based solid acid catalyst derived from glycerol. In the first step, glycerol was esterified with cheap acetic acid to the mixture of MAG, DAG, and TAG in the presence of carbon acid catalyst. In the second step, both MAG and DAG were acetylated with acetic anhydride to TAG (Scheme 1). 3.1. First Step: Esterification of Glycerol with Acetic Acid. Glycerol esterification reactions were conducted with acetic acid in the presence of carbon acid catalyst at 115 °C. To obtain maximum conversion and selectivity in the first step of the process, different reaction parameters such as glycerol to acetic acid molar ratio, reaction time, and catalyst loading were studied. Reactions were also performed without catalyst as a control, to evaluate the diversity of the reactions performed in the presence of catalyst. 3.1.1. Effect of Reaction Time and Acetic Acid Mole Ratio. Initially, effect of reaction period on the esterification of glycerol with 1:4 mol ratio of acetic acid in the presence of 5 wt % of the catalyst at 115 °C for 1 to 4 h was studied. The composition of the acetylated product mixture (glycerol conversion to MAG, DAG, and TAG) obtained was analyzed by GC, and the results in wt % are given in Table 1. From this study, complete esterification of glycerol with acetic acid resulting in acetylated glycerols with high content of DAG (67%) along with MAG (22%) and TAG (11%) was observed within 1 h. With further increase of reaction period to 4 h, TAG content increased from 11 to 25%, whereas MAG and DAG decreased slightly. In the case of the control reaction (without catalyst), only 60% glycerol conversion was observed, and the esterified product mixture after 1 h of reaction was found to contain only MAG (45%) and DAG (15%) along with 40% glycerol without any TAG (Table 1). This study revealed that the carbon acid catalyst has the capability to complete the

Table 1. Effect of Acetic Acid Mole Ratio and Time on the Esterification of Glycerola glycerol:AcOH (mole ratio) 1:4 time (h)

product

1

glycerol MAG DAG TAG glycerol MAG DAG TAG MAG DAG TAG MAG DAG TAG

2

3

4

1:6

1:8

1:10

control 1:4

1:10

[composition (wt %) of the product by GC] 22.0 67.0 11.0 20.1 63.4 16.5 17.1 61.5 21.4 12.7 61.9 25.4

15.8 72.2 12.0 20.4 62.2 17.4 11.0 65.3 23.7 8.0 64.0 28.0

12.3 73.2 14.5 9.8 70.3 19.9 65.2 34.8 61.1 38.9

8.4 71.8 19.8 7.5 70.2 22.3 63.0 37.0 60.2 39.8

40.4 44.6 15.0 34.6 21.8 41.2 2.4 41.5 54.5 4.0 31.1 62.0 6.9

40.6 34.2 24.8 0.4 33.2 30.9 33.8 2.1 48.9 46.6 4.5 22.8 65.7 11.5

a

Reaction conditions: glycerol = 9.2 g, 0.1 mol; catalyst = 0.46 g, 5 wt % of glycerol; T = 115 °C.

esterification of glycerol (1 mol) with less moles of acetic acid (4 mol) to its acetylated glycerols (MAG, DAG, and TAG) within 1 h. Effect of acetic acid mole ratio on the esterification of glycerol to TAG was investigated by varying the acetic acid mole ratio from 1:4 to 1:10 in the presence of catalyst (5 wt %) at 115 °C for 1 to 4 h, and this was compared with the control (1:10 and without catalyst) reaction (Table 1). The results clearly indicated that a marginal increase in the formation of DAG (67 to 73%) and TAG (11 to 20%) was observed in 1 h even by increasing the amount of acetic acid from 1:4 to 1:10. Whereas, in the control reaction even with high amount of acetic acid (1:10 mol), only 60% glycerol conversion with selectivity of MAG (34%), DAG (25%), and traces of TAG was observed under similar conditions. In order to see the effect of reaction period on the complete conversion of glycerol to DAG and TAG, the study was extended for 4 h with varying acetic acid ratio from 1:4 to 1:10, and the results are given in Table 1. By increasing the acetic acid ratio from 1:4 to 1:8, selective esterification of glycerol to C

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Efforts were also made to push the reaction for selective formation of TAG by using Dean−Stark apparatus for the azeotropic removal of water formed in the reaction. However, this study also does not have any encouraging results to obtain 100% TAG conversion even after 4 h and also with increase of acetic acid ratio from 1:4 to 1:10. From the above discussion, the optimal reaction conditions in the first step for obtaining 100% conversion of glycerol to glycerol acetate derivatives is the reaction of 1:4 mol of glycerol to acetic acid with 5 wt % of catalyst for a period of 1 h at 115 °C temperature. After completion of reaction, the catalyst was filtered, washed with methanol, and dried at 110 °C for 1 h to make it moisture free for reuse. 3.2. Second Step: Acetylation of Glycerol Ester Mixture with Acetic Anhydride. In the second step, according to Scheme 1, acetic anhydride (0.1 mol, 1:1 with respect to glycerol used in the first step) was added to the reaction mixture of the first step after 1 h, and the reaction continued for 4 h at 115 °C. The product obtained was found to contain only 50% TAG along with 50% DAG. In order to accelerate the reaction toward 100% TAG, the esterified reaction product obtained after 1 h in the first step was subjected to a pretreatment step involving separation of the catalyst by filtration and removal of excess acetic acid and water formed during esterification reaction by distillation under reduced pressure. The pretreated product was acetylated with acetic anhydride (9.56 mL, 0.1 mol, 1:1 with respect to glycerol in the first step) in the presence of 5 wt % of catalyst at 115 °C for 30 min and resulted in 100% selectivity for TAG in quantitative yield. In the case of the control reaction, only 23% TAG formation along with 77% DAG was observed even after 4 h indicating the efficient catalytic activity of the catalyst in acetylation of MAG and DAG to 100% TAG. Attempts were also made for the acetylation by heating a mixture of glycerol with acetic acid and acetic anhydride (1:4:2 mol ratio) at 115 °C for 4 h in the presence of 5 wt % of the catalyst and resulted in 45 and 55% of DAG and TAG, respectively. Increase in the reaction time (4 h) and acetic anhydride content (1:2 mol with respect to glycerol in the first step) also did not result in 100% TAG. On the basis of the above results, the optimized reaction conditions for 100% selectivity to TAG are esterification of glycerol with acetic acid (1:4 mol) using 5 wt % of catalyst at 115 °C for 1 h followed by acetylation of pretreated esterified product with acetic anhydride (1 mol with respect to glycerol) using 5 wt % of catalyst at 115 °C for 30 min. The advantage of this protocol is use of less moles of acetic acid (1:4), shorter reaction time (1 h), low catalyst loading (5 wt %), and easy recovery and reusability of the catalyst compared to the reported two-step synthesis of TAG.21 3.3. Reusability of Catalyst. To check the reusability of the carbon acid catalyst, esterification reactions of glycerol with acetic acid (1:4) were conducted at 115 °C for 1 h for five cycles (Figure 1). After each cycle the catalyst was recovered by filtration, washed with methanol, and reused after drying in an oven at 110 °C for 1 h. The solid acid catalyst has excellent reusability without any significant reduction in the conversion of glycerol to acetylated glycerols due to its high stability without any deactivation and leaching under the reaction conditions.

DAG and TAG was observed with considerable TAG content after 3 h of reaction (Table 1). However, maximum selectivity of TAG obtained is 39% with 1:8 and 40% with 1:10 glycerol to acetic acid ratio even after 4 h of reaction. Whereas, for control reaction with 1:10 ratio of acetic acid complete glycerol conversion was observed only after 3 h and the product composition obtained after 4 h was found to be almost similar to that of solid acid catalyzed reaction with 1:4 mol ratio of acetic acid after 1 h. On the basis of this study, it was found that 1:4 mol ratio of glycerol:acetic acid and reaction time of 1 h is optimum for the complete conversion of glycerol to biofuel additives. 3.1.2. Effect of Catalyst Loading. The effect of catalyst loading on the complete conversion of glycerol (1 mol) to acetylated glycerol derivatives with acetic acid (4 mol) was investigated by varying from 5 to 20 wt % for a period of 1 to 4 h, and the results are correlated with control reaction (without catalyst). Results in Table 2 clearly indicate that 100% Table 2. Effect of Catalyst Loading on the Esterification of Glycerol with Acetic Acida catalyst (wt % of glycerol) 5 time (h)

product

1

glycerol MAG DAG TAG glycerol MAG DAG TAG MAG DAG TAG MAG DAG TAG

2

3

4

10

15

20

control

[composition (wt %) of the product by GC] 22.0 67.0 11.0 20.1 63.4 16.5 17.1 61.5 21.4 12.7 62.9 24.4

22.4 66.4 11.2 15.7 64.8 19.5 9.1 67.0 23.9 7.3 66.0 26.7

14.4 71.5 14.1 11.1 67.2 21.7 8.7 66.3 25.0 4.3 68.4 27.3

12.3 73.1 14.5 9.0 67.4 23.6 8.0 66.7 25.2 3.8 68.4 27.8

40.4 44.6 15.0 34.6 21.8 41.2 2.4 41.5 54.5 4.0 31.1 62.0 6.9

a

Reaction conditions: glycerol (9.2 g, 0.1 mol); AcOH (23 mL, 0.4 mol); T = 115 °C.

conversion of glycerol to biofuel additives with selectivity of MAG (22%), DAG (67%), and TAG (11%) was observed with minimum amount of catalyst loading (5 wt %) in 1 h. Whereas, for control reaction, 100% of glycerol conversion with selectivity of 42% MAG, 54% DAG, and 4% TAG was obtained only after 3 h of reaction, thus indicating that without catalyst the esterification reaction is slow. However, selectivity of TAG after 4 h in the presence of catalyst increases from 25 to 28% indicates that increase in selectivity of TAG is very little with respect to catalyst percentage and hence there is no need to go for high catalyst loading. The highest catalytic activity signified by glycerol conversion (100%) and the highest selectivity toward MAG, DAG, and TAG attributes the acidity of the catalyst with enhanced surface interactions between acetic acid and glycerol. The carbon acid catalyzed esterification protocol succeeded in resulting 100% conversion of glycerol to acetylated glycerols within 1 h and was demonstrated to be superior compared to the reported solid acid catalysts with respect to acetic acid molar ratio and reaction time. D

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(2) Behr, A.; Eilting, J.; Irawadi, K.; Leschinski, J.; Lindner, F. Improved utilization of renewable resources: new important derivatives of glycerol. Green Chem. 2008, 10, 13−30. (3) Rodrigues, R.; Isoda, N.; Gonçalves, M.; Figueiredo, F. C. A.; Mandelli, D.; Carvalho, W. A. Effect of niobia and alumina as support for Pt catalysts in the hydrogenolysis of glycerol. Chem. Eng. J. 2012, 198−199, 457−467. (4) Gonçalves, V. L. C.; Pinto, B. P.; Silva, J. C.; Mota, C. J. A. Acetylation of glycerol catalyzed by different solid acids. Catal. Today 2008, 133−135, 673−677. (5) Khayoon, M. S.; Hameed, B. H. Acetylation of glycerol to biofuel additives over sulphated activated carbon catalyst. Bioresour. Technol. 2011, 102 (19), 9229−9235. (6) Khayoon, M. S.; Hameed, B. H. Synthesis of hybrid SBA-15 functionalized with molybdophosphoric acid as efficient catalyst for glycerol esterification to fuel additives. Appl. Catal., A 2012, 433−434, 152−161. (7) Balaraju, M.; Nikhitha, P.; Jagadeeswaraiah, K.; Srilatha, K.; Sai Prasad, P. S.; Lingaiah, N. Acetylation of glycerol to synthesize bioadditives over niobic acid supported tungstophosphoric acid catalysts. Fuel Process. Technol. 2010, 91, 249−253. (8) Reddy, P. S.; Sudarsanam, P.; Raju, G.; Reddy, B. M. Synthesis of bio-additives: Acetylation of glycerol over zirconia-based solid acid catalysts. Catal. Commun. 2010, 11, 1224−1228. (9) Reddy, P. S.; Sudarsanam, P.; Raju, G.; Reddy, B. M. Selective acetylation of glycerol over CeO2-M and SO42‑/ CeO2-M (M= ZrO2 and Al2O3) catalysts for synthesis of bioadditives. J. Ind. Eng. Chem. 2012, 18 (2), 648−654. (10) Sudarsanam, P.; Mallesham, B.; Reddy, P. S.; Reddy, B. M. Highly promising sulfate ion promoted M-ZrO2 (M= Al2O3 and CeO2) heterogeneous solid acids for biodiesel derived glycerol esterification. J. Chem. Sci. Technol. 2013, 2 (3), 161−168. (11) Wang, C.; Dou, B.; Chen, H.; Song, Y.; Xu, Y.; Du, X.; Zhang, L.; Luo, T.; Tan, C. Renewable hydrogen production from steam reforming of glycerol by Ni−Cu−Al, Ni−Cu−Mg, Ni−Mg catalysts. Int. J. Hydrogen Energy 2013, 38, 3562−3571. (12) Janaun, J.; Ellis, N. Glycerol etherification by t-butanol catalysed by sulfonated carbon catalyst. J. Appl. Sci. 2010, 10 (21), 2633−2637. (13) Ayoub, M.; Khayoon, M. S.; Abdullah, A. Z. Synthesis of oxygenated fuel additives via the solvent less etherification of glycerol. Bioresour. Technol. 2012, 112, 308−312. (14) Rahmat, N.; Abdullah, A. Z.; Mohamed, A. R. Recent progress on innovative and potential technologies for glycerol transformation into fuel additives: A critical review. Renewable Sustainable Energy Rev. 2010, 14, 987−1000. (15) Melero, J. A.; Grieken, R. V.; Morales, G.; Paniagua, M. Acedic mesoporous silica for the acetylation of glycerol: Synthesis of bioadditives to petrofuel. Energy Fuels 2007, 21, 1782−1791. (16) Jagadeeswaraiah, K.; Balaraju, M.; Prasad, P. S. S.; Lingaiah, N. Selective esterification of glycerol to bioadditives over heteropoly tungstate supported on Cs-containing zirconia catalysts. Appl. Catal., A 2010, 386, 166−170. (17) Silva, L. N.; Goncalves, V. L. C.; Mota, C. J. A. Catalytic acetylation of glycerol with acetic anhydride. Catal. Commun. 2010, 11, 1036−1039. (18) Ferreira, P.; Fonseca, I. M.; Ramos, A. M.; Vital, J.; Castanheiro, J. E. Acetylation of glycerol over heteropolyacids supported on activated carbon. Catal. Commun. 2011, 12, 573−576. (19) Ferreira, P.; Fonseca, I. M.; Ramos, A. M.; Vital, J.; Castanheiro, J. E. Glycerol acetylation over dodecatungstophosphoric acid immobilized into a silica matrix as catalyst. Appl. Catal., B 2009, 91, 416−422. (20) Dosuna-Rodríguez, I.; Adriany, C.; Gaigneaux, E. M. Glycerol acetylation on sulphated zirconia in mild conditions. Catal. Today 2011, 167 (1), 56−63. (21) Liao, X.; Zhu, Y.; Wang, S. G.; Li, Y. Producing triacetylglycerol with glycerol by two steps: Esterification and acetylation. Fuel Process. Technol. 2009, 90, 988−993.

Figure 1. Reusability of the catalyst on the esterification reaction with acetic acid. (Reaction conditions: glycerol = 9.2 g, 0.1 mol; acetic acid = 23 mL, 0.4 mol; catalyst = 0.46 g, 5 wt % with respect to glycerol; T = 115 °C, time = 1 h).

4. CONCLUSIONS In conclusion, the present study demonstrated excellent catalytic activity of the glycerol-based solid acid catalyst for the preparation of TAG in a two-step method with lower molar ratios of acetic acid and acetic anhydride in shorter reaction times. The optimized conditions for the complete esterification of glycerol with acetic acid was found to be 1:4 glycerol to acetic acid at 115 °C for 1 h followed by acetylation with one mole of acetic anhydride for 30 min employing 5 wt % catalyst. This protocol would be commercially feasible for industrial applications with high selectivity and conversion rate of glycerol to biofuel additives with outstanding recyclability of the catalyst.

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ASSOCIATED CONTENT

S Supporting Information *

Glycerol-based carbon acid catalyst characterization [XRD (Figure 1), XPS (Figure 2), FTIR (Figure 3), 13C MAS NMR spectra (Figure 4)], effect of reaction time (Figure 5) and effect of acetic acid mole ratio (Figure 6) on the esterification of glycerol, and GC chromatograms of glycerol esters obtained by esterification of glycerol with AcOH for 1 h (Figure 7a) followed by acetylation with Ac2O for 30 min (Figure 7b). 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]. Tel.: +91 40 27191845. Fax: +91 40 27193370. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The author U.C. gratefully acknowledges the Council for Scientific and Industrial Research (CSIR), New Delhi, India, for the financial support as Project Research Fellow under 11th FYP FAC project.

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REFERENCES

(1) Zhou, C. H.; Beltramini, J. N.; Fan, Y. X.; Lu, G. Q. Chemoselective catalytic conversion of glycerol as a biorenewable source to valuable commodity chemicals. Chem. Soc. Rev. 2008, 37, 527−549. E

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dx.doi.org/10.1021/ie503079m | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX