Acetylation of Glycerol over Sulfated Alumina: Reaction Parameter

Dec 2, 2015 - Acetylation of Glycerol over Sulfated Alumina: Reaction Parameter Study and Optimization Using Response Surface Methodology ..... All of...
0 downloads 10 Views 3MB Size
Download PDF
Article pubs.acs.org/EF

Acetylation of Glycerol over Sulfated Alumina: Reaction Parameter Study and Optimization Using Response Surface Methodology Arun P, Satyanarayana Murty Pudi, and Prakash Biswas* Department of Chemical Engineering, Indian Institute of Technology Roorkee, Roorkee-247667, Uttarakhand, India ABSTRACT: A highly active and stable SO42−/γ-Al2O3 catalyst was developed, and the catalytic activity was evaluated for esterification of glycerol. The reactions were performed in a batch reactor under reflux conditions. The influence of reaction conditions, such as the reaction temperature, catalyst amount, and acetic acid/glycerol mole ratio, on the product distribution was investigated. The results demonstrated that the 2 M SO42−/γ-Al2O3 catalyst was very active and showed 100% glycerol conversion with 77.4% selectivity of higher esters [glycerol diacetate (DAG) + glycerol triacetate (TAG)] at 95 °C, after 5 h of reaction time in the presence of acetic acid/glycerol mole ratio of 12:1. Further, response surface methodology (RSM) was used for the development of the empirical process model, followed by the optimization of reaction parameters. The optimization process depicted that 76.1% combined selectivity to DAG and TAG can be achieved at complete conversion of glycerol at 108.8 °C in the presence of 0.36 g of catalyst at the acetic acid/glycerol mole ratio of 12:1.

1. INTRODUCTION Growing world population, depleting fossil fuel reserves, hiking oil prices, and environmental and health issues are demanding alternative sources of energy. In this context, recently, biodiesel has been established as a promising renewable fuel as a result of its versatile features. It is biodegradable and nontoxic, and most importantly, it is free of sulfur and aromatic compounds, thereby reducing the environmental pollution.1−3 In the conventional transesterification of vegetable oil and animal fat for the production of biodiesel, surplus amounts of glycerol are produced as byproduct. Approximately, every 10 kg of total transesterification product comprises 9 kg of biodiesel and 1 kg of glycerol. Hence, finding a new and sustainable route for glycerol utilization is essential to make the biodiesel production more attractive.2,4 Several glycerol value addition processes, such as esterification, hydrogenolysis, etherification, selective oxidation, fermentation, dehydration, carboxylation, etc., have been proposed. Among all of these possibilities, esterification of glycerol with acetic acid is a promoising route for glycerol upgradation. The esterification of glycerol with acetic acid produces glycerol monoacetate (MAG), glycerol diacetate (DAG), and glycerol triacetate (TAG). All of the glycerol acetylation products have their significant commercial importance. MAG is widely used as a food additive, and in combination with DAG and TAG, MAG finds applications in the manufacture of dyes, softening agents, and plasticizers. MAG is also used in the manufacture of explosives and smokeless powder.3,5 DAG and TAG are excellent fuel additives, which reduce the viscosity of fuel and increase engine efficiency, and they also improve the antiknocking properties of gasoline when blended.3 Acetylation of glycerol is known as an acid-catalyzed reaction, and many homogeneous catalysts, such as mineral acid catalysts, have been used for this reaction.5−10 Despite good catalytic activity, mineral acids has several disadvantages of being toxic, corrosive, and difficult to separate from the reaction mixture.5 Therefore, various solid acid catalysts, such as ionexchange resins, niobic acid, zeolites, functionalized meso© 2015 American Chemical Society

porous silica, sulfated zirconia, and supported heteropolyacids, have been developed and evaluated for esterification reaction.3,5,11,12 Zhou et al.3 performed the acetylation reaction in the presence of Amberlyst-15 and optimized the reaction parameters. They have achieved maximum glycerol conversion of 97%, with 40% selectivity to TAG at 110 °C in the presence of acetic acid/glycerol mole ratio of 9:1 after a reaction time of 5 h. However, they have reported low thermal stability of Amberlyst-15. Many previous studies discussed the peformance of zeolites (HUSY and HZSM-5) as catalysts.3,11 The activity of zeolite catalysts was very poor (250 °C referred to the presence of moderate and strong strength acidic sites on the catalysts.17,19 The obtained total acidity of γ-Al2O3 was 1.203 mmol of NH3 g−1 of catalyst. For the 2 M SO42−/γ-Al2O3 catalyst, the desorption peak obtained in the temperature range of 120−220 °C was corresponding to weak acidic sites and a high intensity broad peak was detected over a temperature range of 250−600 °C, in favor of the existence of medium and strong acidic sites. The acidic strength of the 2 M SO42−/γ-Al2O3 catalyst was increased significantly (2.51 mmol of NH3 g−1 of catalyst) as a result of the sulfation process, which was in agreement with the previous results.6 3.2. Experimental Design in RSM. RSM was used to develop an empirical process model, followed by optimization of reaction parameters. From the preliminary factorial experiments and the detailed literature survey,7,8,20 the reaction temperature, acid/glycerol mole ratio in the feed, and catalyst amount were identified as the primary important reaction parameters, which affected the glycerol conversion and product selectivity. The chosen responses of interest were glycerol conversion and selectivity toward the products MAG, DAG, and TAG, respectively. A second-order polynomial was used as the best approximate functional relationship between the chosen factors and the responses. In RSM, a three-factor, three-level face-centered central composite design (CCD) was used. The factors chosen and their corresponding levels are given in Table 2. For each factor, three levels were represented

k

τi = β0 +

k

∑ βjxj +

∑ βjjxj 2 +

j=1

j=1

k

∑ ∑ βijxixj i< j=2

(1)

Using ANOVA, the significance of the quadratic model and each term in the model were examined. The insignificant terms (p value > 0.05) were eliminated from the quadratic polynomial model obtained, and the modified empirical model for glycerol conversion and selectivity toward different products is provided in Table 4. Equations 2−6 shown in Table 4 were empirical model equations in terms of coded factor levels, and eqs 7−11 presented in Table 4 were empirical equations in terms of actual factor values. The model fitness was tested using the statistical measure R2, the determination coefficient, which indicated closeness of the experimental (actual) data to the predicted data or fitted regression line. However, the R2 value 586

DOI: 10.1021/acs.energyfuels.5b01901 Energy Fuels 2016, 30, 584−593

Article

Energy & Fuels Table 3. Experimental Design for Acetylation of Glycerol with Acetic Acid Using the 2 M SO42−/γ-Al2O3 Catalyst run number

T (°C)

MR

C (g)

IT

IM

IC

XG (%)

SMAG (%)

SDAG (%)

STAG (%)

SDAG + TAG (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

110 110 95 125 95 110 110 110 110 125 95 125 95 95 110 125 125 110 110 110

12 6 9 6 6 9 9 9 9 6 12 9 12 6 9 12 12 9 9 9

0.50 0.50 0.50 0.75 0.75 0.50 0.75 0.50 0.50 0.25 0.25 0.50 0.75 0.25 0.50 0.25 0.75 0.50 0.25 0.50

0 0 −1 +1 −1 0 0 0 0 +1 −1 +1 −1 −1 0 +1 +1 0 0 0

+1 −1 0 −1 −1 0 0 0 0 −1 +1 0 +1 −1 0 +1 +1 0 0 0

0 0 0 +1 +1 0 +1 0 0 −1 −1 0 +1 −1 0 −1 +1 0 −1 0

100 96.8 97.1 97.1 95.6 98.6 98.9 99.2 98.5 94.5 96.5 98.8 99.0 90.4 98.1 96.4 99.7 99.0 96.3 98.7

24.5 42.4 33.3 44.5 44.0 28.9 29.6 32.0 31.2 44.3 24.9 32.3 22.6 47.7 31.4 26.2 26.2 30.4 32.0 30.9

49.0 43.4 46.6 42.5 43.0 48.8 47.8 47.1 47.5 42.5 50.5 47.1 48.2 43.3 48.0 50.6 48.1 47.9 48.4 47.3

26.5 14.2 20.1 13.0 13.0 22.3 22.6 20.9 21.3 13.2 24.6 20.6 29.2 9.00 20.6 23.2 25.7 21.7 19.6 21.8

75.5 57.6 66.7 55.5 56.0 71.1 70.4 68.0 68.8 55.7 75.1 67.7 77.4 52.3 68.6 73.8 73.8 69.6 67.4 69.1

Table 4. Empirical Process Models for Glycerol Conversion and Product Selectivity Obtained from Experimental Design in Terms of Coded Factor Levelsa in terms of coded factors XG = 98.69 + 0.79IT + 1.72IM + 1.62IC − 0.62ITIM − 1.06IT2 − 1.41IC2 SMAG = 30.84 + 0.1IT − 9.85IM − 0.82IC + 0.98ITIM + 0.78ITIC + 1.80IT2 + 2.45IM2 SDAG = 47.76 − 0.08IT + 3.17IM − 0.57IC − 0.56IMIC − 0.56IT2 − 1.21IM2 STAG = 21.40 − 0.020IT + 6.68IM + 1.39IC − 1.14IMIT − 0.79ICIT − 1.24IT2 − 1.24IM2 SDAG + TAG = 69.16 − 0.1IT + 9.85IM + 0.82IC − 0.98ITIM − 0.78ITIC − 1.80IT2 − 2.45IM2 in terms of actual factors XG = 8.320 + 1.21T + 2.10M + 28.98C − 0.0139TM − 0.00469T2 − 22.5C2 SMAG = 212.963 − 2.052T − 10.567M − 26.013C + 0.0217TM + 0.2067TC + 0.008T2 + 0.2722M2 SDAG = −4.167 + 0.538T + 3.844M + 4.47C − 0.75MC − 0.00247T2 − 0.134M2 STAG = −115.929 + 1.5473T + 7.4947M + 28.16C − 0.0253TM − 0.210TC − 0.005527T2 − 0.1382M2 SDAG + TAG = −112.963 + 2.052T + 10.567M + 26.013C − 0.0217TM − 0.2067TC − 0.008T2 − 0.2722M2

(2) (3) (4) (5) (6) (7) (8) (9) (10) (11)

a IT, IM, and IC, coded factors (1, 0, or −1); T, temperature (95, 110, or 125 °C); M, acetic acid/glycerol molar ratio (6:1, 9:1, or 12:1); C, catalyst loading (0.25, 0.50, or 0.75 g); I, coded value; XG, conversion of glycerol; and S, selectivity toward different products (MAG, DAG, and TAG).

responses accurately. The parity plots indicating the model fitness for all responses are shown in Figure 3. 3.4. Effect of Reaction Parameters on Glycerol Conversion and Product Selectivity. 3.4.1. Effect of the Mole Ratio. As shown in Table 3, glycerol conversion was increased with an increasing acetic acid concentration in the feed. The maximum glycerol conversion of 100% was achieved at the highest acetic acid/glycerol mole ratio of 12:1 at 110 °C and in the presence of 0.5 g of catalyst. Esterification of glycerol with acetic acid is known as an equilibrium-limited reaction; therefore, high conversion of glycerol can be attainable when one of the reactants is in excess.20 For all of the experiments conducted, it was observed that, after 5 h of reaction, the glycerol conversion was >90%, which might be due to the high acidic nature of the 2 M SO42−/γ-Al2O3 catalyst. It was also noticed that the higher concentration of acetic acid in the reaction medium showed a positive influence on the formation of higher acetates, i.e., DAG and TAG. The selectivity to TAG was increased from 14.2 to 26.5%, while MAG selectivity was decreased from 42.4 to 24.5% with an increasing acetic acid/ glycerol mole ratio from 6:1 to 12:1 at a constant temperature

sometimes acted as a biased estimate as a result of the fact that increasing the number of terms simply increased the value of R2. A more unbiased estimation of model goodness was adjusted R2. For all models, obtained high values (>0.95) of R2 and adjusted R 2 indicated that the model fitted the experimental data very well (Table 5). In addition, the closeness of values of adjusted R2 and predicted R2 (within ±0.1) indicated that the models not only fitted the experimental values satisfactorily but also predicted the Table 5. ANOVA Details response

R2

adjusted R2

predicted R2

adequate precision

glycerol conversion MAG selectivity DAG selectivity TAG selectivity combined DAG and TAG selectivity

0.957 0.991 0.969 0.989 0.991

0.937 0.986 0.956 0.984 0.986

0.845 0.977 0.935 0.954 0.977

27.181 43.788 24.172 48.306 43.788

587

DOI: 10.1021/acs.energyfuels.5b01901 Energy Fuels 2016, 30, 584−593

Article

Energy & Fuels

Figure 3. Comparison of model data versus experimental data: (A) glycerol conversion, (B) MAG selectivity, and (C) combined selectivity to DAG and TAG.

Figure 4. Three-dimensional response surface plots and contour showing the interaction effect of the acetic acid/glycerol mole ratio (6:1−12:1) and temperature (95−125 °C) on glycerol conversion in the presence of 0.50 g of catalyst.

the reaction temperature from 95 to 125 °C at a high acetic acid/glycerol mole ratio of 9:1 in the presence of 0.5 g of catalyst, as shown in Table 3. However, at a low acetic acid/ glycerol mole ratio of 6:1 and in the presence of 0.25 g of catalyst, conversion of glycerol was increased from 90.4 to 94.5% with increasing temperature from 95 to 125 °C (Table

of 110 °C and in the presence of 0.5 g of catalyst (Table 3). A high acetic acid concentration may enhance the diffusion and, thereby, the formation of large molecules, such as DAG and TAG, by lowering the viscosity of the reaction mixture.3,5 3.4.2. Effect of the Reaction Temperature. Conversion of glycerol was almost unaffected (∼95−100%) with increasing 588

DOI: 10.1021/acs.energyfuels.5b01901 Energy Fuels 2016, 30, 584−593

Article

Energy & Fuels

Figure 5. Three-dimensional response surface plots and contour showing the interaction effect of the acetic acid/glycerol mole ratio (6:1−12:1) and temperature (95−125 °C) on the selectivity to MAG in the presence of 0.50 g of catalyst.

Figure 6. Three-dimensional response surface plots and contour showing the interaction effect of the acetic acid/glycerol mole ratio (6:1−12:1) and temperature (95−125 °C) on the selectivity to DAG in the presence of 0.50 g of catalyst.

favored the selectivity to MAG. Mufrodi et al.22 suggested that, in the presence of sulfuric acid, triacetin synthesis was an exothermic reaction and, hence, higher temperatures (>118 °C) were not beneficial. In addition, they also have found that the selectivity to triacetin decreased at high temperatures (>115 °C) as a result of the evaporation of acetic acid. 3.4.3. Effect of the Catalyst Amount. Upon doubling the catalyst amount from 0.25 to 0.5 g, glycerol conversion was increased by 3%, i.e., from 96.3 to 98.7%, at a constant mole ratio of 9:1 and at a temperature of 110 °C (Table 3). Similar kinds of results were also reported earlier.8,20,23 Product selectivity was almost unaffected with the catalyst amount. The observed positive effect of the catalyst amount on glycerol conversion and selectivity to higher esters was negligibly small. 3.4.4. Effect of the Interaction among the Temperature and Mole Ratio. The interaction effect of the temperature and acetic acid/glycerol mole ratio on glycerol conversion and product selectivity is discussed using of response surface plots, as shown in Figures 4−7. It was observed that, for glycerol

3). Thus, the positive effect of the temperature on glycerol conversion was more pronounced at a lower acetic acid/ glycerol mole ratio of 6:1 in the presence of 0.25 g of catalyst. From Table 3, it was also observed that, with increasing temperature from 95 to 110 °C, the formation of MAG was suppressed, while the selectivity to higher esters was promoted at the constant mole ratio of 9:1 and in the presence of 0.5 g of catalyst. These results suggested that, at a higher temperature, MAG was converted to higher acetates through consecutive reaction.3,5 Moreover, with a further rise in the temperature from 110 to 125 °C, the selectivity to higher esters was decreased slightly (DAG, from 47.8 to 47%; TAG, from 21.4 to 20.6%) and, simultaneously, the selectivity to MAG was slightly increased (from 30.8 to 32.3%). The drop in selectivity to DAG was due to the conversion of more DAG into TAG.3,5 In addition, the result indicated that higher temperatures (>110 °C) could lead to the deactivation of some of the acidic sites of the catalyst, which might have caused the drop in selectivity of the catalyst toward higher esters (both DAG and TAG) and 589

DOI: 10.1021/acs.energyfuels.5b01901 Energy Fuels 2016, 30, 584−593

Article

Energy & Fuels

Figure 7. Three-dimensional response surface plots and contour showing the interaction effect of the acetic acid/glycerol mole ratio (6:1−12:1) and temperature (95−125 °C) on the selectivity to TAG in the presence of 0.50 g of catalyst.

Table 6. Comparison of Experimental Values versus Predicted Values for Conversion of Glycerol and Product Selectivitya reaction condition

experimental value

predicted value

MR

T (°C)

C (g)

XG (%)

SMAG (%)

SDAG (%)

STAG (%)

SDAG + TAG (%)

XG (%)

SMAG (%)

SDAG (%)

STAG (%)

SDAG + TAG (%)

10 11 8

100 105 115

0.30 0.35 0.40

96.8 98.5 97.9

29.0 25.3 34.5

49.2 50.1 46.9

21.8 24.2 18.9

70.9 74.6 65.8

96.2 98.1 97.4

29.4 25.9 34.7

49.1 49.9 46.6

21.7 24.4 18.6

70.6 74.0 65.2

a MR, mole ratio of acetic acid/glycerol; T, reaction temperature; C, amount of catalyst; XG, conversion of glycerol; and S, selectivity toward different products (MAG, DAG, and TAG).

presence of 0.5 g of catalyst. A high acetic acid concentration may enhance the diffusion and, thereby, the formation of large molecules, such as DAG and TAG, by lowering the viscosity of the reaction mixture.3,5 A high temperature increases the rate of reaction, but the associated evaporation of acetic acid retards the successive acetylation reactions.23 3.5. Model Validation. The accuracy of prediction by the empirical models obtained through the design of experiments was verified at different operating conditions inside the design space. For this purpose, prior to the experiments, empirical models in terms of actual factors, eqs 7−11 (Table 4), were used to obtain the predicted values for glycerol conversion and selectivity to different products at various temperatures (100, 105, and 115 °C), acetic acid/glycerol mole ratios (10, 11, and 8), and catalyst loadings (0.30, 0.35, and 0.40 g), respectively. Further, the experiments were carried out at these reaction conditions, and the experimental results were compared to the model prediction. The experimental and predicted values obtained for all of the responses are given in Table 6. Results showed that the predicted responses were in good agreement with the corresponding experimental values. Hence, it was confirmed that the empirical model not only fitted the experimental data very well but also predicted the responses accurately. 3.6. Optimization of Reaction Parameters. The numerical optimization package in Design-Expert 7.0 software was used for the optimization of reaction parameters by the simplex search method.10 The empirical models obtained from the experimental analysis were used to search for the optimum factor. All factors and responses were assigned with appropriate goals and importance level based on the main objective of

conversion, the interaction effect was predominant at higher temperatures (>110 °C) and higher acetic acid/glycerol mole ratios (>9:1). Figure 4 shows that the interaction of the temperature and acetic acid/glycerol mole ratio imparted the curvature effect on the response surface for glycerol conversion, which altered the contour plots from straight lines to curves at the areas of relatively higher temperature or mole ratio, where high glycerol conversion (>97%) was achieved. The results reported in Figure 4 suggested that a higher acetic acid/glycerol mole ratio (12:1) and higher reaction temperatures (>100 °C) were beneficial for higher glycerol conversion. A concave response surface was obtained for MAG selectivity, as shown in Figure 5, which suggested the negative effect of the acetic acid/ glycerol mole ratio and reaction temperature on MAG selectivity. The influence of the interaction term, which brings the curvature effect, was less for MAG selectivity, and probably, as a result of this reason, the three-dimensional (3D) surface plot of MAG selectivity shown in Figure 5 possessed relatively less curvature.16 The contour plot (Figure 6) of DAG selectivity shows that the selectivity to DAG increased with an increasing acetic acid/glycerol mole ratio from 6:1 to 12:1, and with an increasing temperature from 95 to 125 °C, the selectivity of DAG passed through maxima. The result reported in Figure 6 suggested that a high acetic acid/glycerol mole ratio of 12:1 and a temperature of 110 °C were favorable for higher selectivity to DAG in the presence of 0.5 g of catalyst. A similar trend in DAG selectivity was observed by Zhou et al.3 for glycerol acetylation using the Amberlyst-15 catalyst. The 3D surface plot for TAG selectivity (Figure 7) suggested that a higher acetic acid/glycerol mole ratio (12:1) and lower temperatures ( 0.95). Numerical optimization using RSM indicated that optimum reaction conditions for the glycerol acetylation reaction were acetic acid/glycerol mole ratio of 12:1, reaction temperature of 108.8 °C, and catalyst amount of 0.36 g. At this optimum reaction condition, 99.1% conversion of glycerol was achieved, with 76.1% combined selectivity to DAG and TAG. Reusability tests indicated that the glycerol conversion and product selectivity were almost unaffected, even after four consecutive experiments.

to the deactivation of some of the acidic sites of the catalysts, which might have caused the drop in selectivity of the catalysts toward higher esters (TAG). Another possible reason for decreasing the selectivity toward higher esters might be due to the blockage of acidic sites of catalysts by the esters formed during the reaction. To verify this argument, the FTIR spectra of the fresh and used catalysts after every run were compared (Figure 8). Results showed that the peak corresponding to

■

AUTHOR INFORMATION

Corresponding Author

*Telephone: +91-1332-28-5820. Fax: +91-1332-27-6535. Email: [email protected] and/or [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS Arun P and Satyanarayana Murty Pudi are thankful to the Ministry of Human Resource Development (MHRD), Government of India, for the award of fellowship to carry out this work in the Department of Chemical Engineering at the Indian Institute of Technology Roorkee, Roorkee, Uttarakhand, India.

■

REFERENCES

(1) Acevedo, J. C.; Hernández, J. A.; Valdés, C. F.; Khanal, S. K. Bioresour. Technol. 2015, 188, 117−123. (2) Yang, F.; Hanna, M. A.; Sun, R. Biotechnol. Biofuels 2012, 5, 13. (3) Zhou, L.; Al-Zaini, E.; Adesina, A. A. Fuel 2013, 103, 617−625. (4) Nakagawa, Y.; Tomishige, K. Catal. Sci. Technol. 2011, 1 (2), 179−190. (5) Zhou, L.; Nguyen, T. H.; Adesina, A. A. Fuel Process. Technol. 2012, 104, 310−318. (6) Ferreira, P.; Fonseca, I. M.; Ramos, A. M.; Vital, J.; Castanheiro, J. E. Catal. Commun. 2011, 12 (7), 573−576. (7) Sanchez, J. A.; Hernandez, D. L.; Moreno, J. A.; Mondragon, F.; Fernandez, J. J. Appl. Catal., A 2011, 405, 55−60. (8) Ferreira, P.; Fonseca, I. M.; Ramos, A. M.; Vital, J.; Castanheiro, J. E. Appl. Catal., B 2009, 91, 416−422. (9) Trejda, M.; Stawicka, K.; Dubinska, A.; Ziolek, M. Catal. Today 2012, 187, 129−134. (10) Troncea, S. B.; Wuttke, S.; Kemnitz, E.; Coman, S. M.; Parvulescu, V. I. Appl. Catal., B 2011, 107 (3−4), 260−267. (11) Gonçalves, V. L. C.; Pinto, B. P.; Silva, J. C.; Mota, C. J. A. Catal. Today 2008, 133−135, 673−677. (12) Jagadeeswaraiah, K.; Balaraju, M.; Prasad, P. S. S.; Lingaiah, N. Appl. Catal., A 2010, 386 (1−2), 166−170. (13) Mekhemer, G. A. H.; Khalaf, H. A.; Mansour, S. A. A.; Nohman, A. K. H. Monatsh. Chem. 2005, 136, 2007−2016. (14) Yang, T.; Chang, T.; Yeh, C. J. Mol. Catal. A: Chem. 1997, 115, 339−346. (15) Ji, X.; Chen, Y.; Wang, X.; Liu, W. Kinet. Catal. 2011, 52 (4), 555−558. (16) Bezerra, M. A.; Santelli, R. E.; Oliveira, E. P.; Villar, L. S.; Escaleira, L. A. Talanta 2008, 76, 965−977. (17) Ma, L.; Han, Y.; Sun, K.; Lu, J.; Ding, J. Energy Convers. Manage. 2015, 98, 46−53. (18) Melero, J. A.; van Grieken, R.; Morales, G.; Paniagua, M. Energy Fuels 2007, 21, 1782−1791. (19) Dosuna-Rodriguez, I.; Adriany, C.; Gaigneaux, E. M. Catal. Today 2011, 167, 56−63. (20) Liao, X.; Zhu, Y.; Wang, S. G.; Li, Y. Fuel Process. Technol. 2009, 90, 988−993.

Figure 8. FTIR spectra of the fresh and used 2 M SO42−/γ-Al2O3 catalyst.

sulfate species at around 1100 cm−1 is intact in the used catalyst. However, the additional peak detected at 1735 cm−1 corresponds to the esters formed in the reaction, and the additional peak detected at 1360 cm−1 corresponds to alcohols.24 Therefore, the reusability experiment suggested that the 2 M SO42−/γ-Al2O3 catalyst can be considered as a stable catalyst for the acetylation of glycerol with acetic acid. In fact, γ-Al2O3 has been recognized as a widely used catalyst support as a result of its excellent thermal and mechanical stability, with the exposed pore structure bearing Lewis acid sites up to temperatures above 700 °C.14 The structure of acidic species on γ-Al2O3 has been proposed to be a surface complex consisting of a metal cation and sulfate ions with two covalent SO bonds, which stabilize the alumina support.13

4. CONCLUSION The esterification of glycerol with acetic acid was performed in the presence of the 2 M SO42−/γ-Al2O3 catalyst prepared by impregnation of sulfuric acid on γ-Al2O3. Results demonstrated that sulfated alumina exhibited very high catalytic activity and showed the glycerol conversion of >90% at all of the experimental conditions used. Complete glycerol conversion was achieved at 110 °C with an acetic acid/glycerol mole ratio of 12:1 in the presence of 0.50 g of catalyst. Results suggested that the effect of the reaction parameter on glycerol conversion and the yield of higher esters followed the order: acetic acid/ glycerol mole ratio > interaction term as a result of the mole ratio and temperature > reaction temperature > catalyst amount. RSM was used to develop the empirical process 592

DOI: 10.1021/acs.energyfuels.5b01901 Energy Fuels 2016, 30, 584−593

Article

Energy & Fuels (21) Carley, K. M.; Kamneva, N. Y.; Reminga, J. Response Surface Methodology; School of Computer Science, Carnegie Mellon University: Pittsburgh, PA, 2004; CASOS Technical Report CMUISRI-04-136. (22) Mufrodi, Z.; Rochmadi, S.; Budiman, A. Int. J. Chem. 2012, 4, 101−107. (23) Dosuna-Rodriguez, I.; Gaigneaux, E. M. Catal. Today 2012, 195, 14−21. (24) Dean, J. Lange’s Handbook of Chemistry, 15th ed.; McGraw-Hill: New York, 1999.

593

DOI: 10.1021/acs.energyfuels.5b01901 Energy Fuels 2016, 30, 584−593