Etherification of Glycerol with Isobutylene to Produce Oxygenate

Nov 18, 2010 - With the increasing demand for biodiesel, the production of crude .... was supplied by China National Petroleum Corporation, Lanzhou, C...
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Ind. Eng. Chem. Res. 2010, 49, 12399–12404

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Etherification of Glycerol with Isobutylene to Produce Oxygenate Additive Using Sulfonated Peanut Shell Catalyst Weiqin Zhao,† Bolun Yang,*,‡ Chunhai Yi,‡ Zhao Lei,‡ and Jie Xu‡ School of Life Science and Technology, Department of Chemical Engineering, The State Key Laboratory of Multiphase Flow in Power Engineering, Xi’an Jiaotong UniVersity, Xi’an Shaanxi 710049, P.R. China

A carbon-based solid acid catalyst was prepared by sulfonation of partially carbonized peanut shell, and characterized by SEM, EDS, BET analysis, FTIR spectroscopy, NH3 TPD, and TGA. The analytic results indicate that sulfonated peanut shell catalyst has an amorphous porous structure with a high acid capacity and good thermal stability and exhibits better catalytic activity for the glycerol etherification reaction than cation-exchange resin. With a molar ratio of isobutylene to glycerol of 4:1, a catalyst-to-glycerol mass ratio of 6 wt %, a reaction temperature of 343 K, and a reaction time of 2 h, glycerol was completely transformed into a mixture of glycerol ethers including mono-tert-butylglycerols (MTBGs), di-tert-butylglycerols (DTBGs), and tri-tert-butylglycerol (TTBG), and the selectivity toward the sum of the desired DTBGs and TTBG of 92.1% was obtained. Moreover, excellent reusability of the catalyst was also confirmed by repeated experiments. 1. Introduction Biodiesel (fatty acid methyl ester, FAME) has received increasing attention as a substitute for fossil fuels. It is generally produced through the methanolysis of triglycerides in the presence of catalyst.1 Glycerol is the main byproduct in this process and is normally generated at the rate of 1 mol of glycerol for every 3 mol of methyl esters, approximately equivalent to 10 wt % of the total product. With the increasing demand for biodiesel, the production of crude glycerol is excessive,2 which leads to the reduction of the current glycerol market price. To utilize the low-cost glycerol, many researchers have paid much attention to the catalytic conversion of glycerol into valueadded chemicals, for example, through selective oxidation,3 hydrogenolysis to propylene glycol,4 dehydration to aldehydes,5 reforming to syngas,6 fermentation to 1,3-propanediol,7 transesterification to glycerol carbonate,8 and synthesis of epichlorohydrin.9 Etherification of glycerol with an olefin such as isobutylene toward fuel oxygenates recently has been considered a promising and economically viable use for glycerol.10 Because of low solubility and poor thermal stability, glycerol cannot be directly added into fuel because it will lead to significant engine problems at high temperatures. Thus, glycerol should be transformed into a derivative that is compatible with diesel and biodiesel. In the presence of acid catalyst, glycerol reacts with isobutylene and yields a glycerol ether mixture that includes mono-tert-butylglycerols (MTBGs), di-tert-butylglycerols (DTBGs), and tri-tert-butylglycerols (TTBGs). MTBGs have low solubility in diesel fuel. However, DTBGs and TTBGs are miscible with diesel fuel and contain high oxygen contents of 23.5% (DTBGs) and 18.5% (TTBGs), together with high octane numbers of 112-128 (blending research octane number, BRON) and 91-99 (blending motor octane number, BMON).11 As a result, DTBGs and TTBGs have been reported to be ignition accelerators and octane boosters, taking the place of methyl tert-butyl ether (MTBE), which can enhance the combustion efficiency in internal combustion engines and * To whom correspondence should be addressed. Tel.: +86-2982663189; Fax: +86-29-82668789. E-mail address: blunyang@ mail.xjtu.edu.cn. † School of Life Science and Technology. ‡ Department of Chemical Engineering.

decrease the polluting emissions. Because of poor viscosity, cloud points, and pour points, biodiesel cannot be widely used. When 20% glycerol ethers are blended with biodiesel, the cloud point can be reduced by 5 K, and the viscosity can be reduced by 8%.12 Moreover, the mixed fuel shows burning characteristics similar to those of petroleum-based diesel. The etherification reaction not only makes use of byproduct glycerol but also increases the total yield of biodiesel by approximately 20 vol. %. The etherification reaction can be catalyzed by both homogeneous and heterogeneous acid catalysts. In the homogeneousacid-catalyzed system, H2SO4 and para-toluene sulfonic acid are the most extensively investigated catalysts.13 However, these liquid acids cause equipment corrosion and require neutralization and separation from the products after reaction, resulting in considerable energy consumption and material wastage. Solid acid catalysts can be easily separated from products by decantation or filtration and can be repeatedly used without neutralization. Thus, it is attractive to replace liquid acid catalysts by solid acid catalysts. An ideal solid acid catalyst for glycerol etherification reaction should have high stability and high density of strong protonic acid sites. Cation-exchange resins and zeolites have been applied for the glycerol etherification reaction.14,15 However, the major disadvantage of cation-exchange resins is poor thermal stability. For zeolites, the low density of strong protonic acid sites abates the reaction rate, and the small pore diameter leads to steric hindrance for the formation of TTBGs. Moreover, the high costs of these catalysts correspondingly increases the costs of the final products. Because of their high efficiency and stability, sulfonated carbon-based solid acid catalysts have been considered as promising catalysts. They can be inexpensively produced by partial carbonization of sulfopolycyclic aromatic hydrocarbons or sulfonation of partially carbonized organic compounds, such as sugar.16-18 Carbon-based solids are rigid materials. Sulfonation will introduce sulfonic acid groups onto the surface of this material. These solid catalysts exhibit an amorphous carbonbased framework composed of highly dispersed small polycyclic aromatic carbon sheets containing sulfonic acid and have a high density of active sites with good thermal stability. Nakajima et al. reported that carbon-based solid acid showed higher catalytic

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Figure 1. SEM images of (a) partially carbonized peanut shell and (b) partially carbonized and sulfonated peanut shell catalyst.

activity for biodiesel synthesis than strongly acidic cationexchange resin (Nafion).19 Peanut shell is an inedible byproduct with main components of large cellulose and hemicellulose. As to utilizing waste biomass, peanut shell can be considered as a raw material for the preparation of a green catalyst of carbon-based solid acid. In this work, a sulfonated peanut shell catalyst was prepared by a partial carbonization and sulfonation method and applied in the etherification of glycerol with isobutylene. To obtain the highest glycerol conversion and selectivity toward DTBGs and TTBGs, the reaction conditions, such as reaction temperature, reaction time, catalyst-to-glycerol mass ratio, and isobutyleneto-glycerol molar ratio, were optimized in terms of catalytic activity. 2. Experimental Section 2.1. Catalyst Preparation. Peanut shell was heated at 723 K for 15 h under a N2 flow to obtain a partially carbonized carbon carrier, and then the solid was ground into powder. To sulfonate the carbon carrier, 50 g of the partially carbonized peanut shell was heated with 1 L of concentrated H2SO4 at 483 K in an oil bath under reflux and as N2 flow for 10 h. After treatment, the mixture was diluted with deionized water, filtered to collect the solid, and then dried at 393 K in a vacuum for 4 h to obtain the sulfonated peanut shell catalyst. 2.2. Materials. Isobutylene (99.9%) was supplied by China National Petroleum Corporation, Lanzhou, China. Cationexchange resin (Amberlyst-15) was purchased from Rohm and Haas, Paris, France. Glycerol (analytical-grade purity) was obtained from Xi’an Chemical Reagent Factory, Xi’an, China. 2.3. Catalyst Characterization. The morphologies and particle sizes of samples were characterized by scanning electron microscopy (SEM) using a JSM-6390A SEM analyzer (JEOL, Tokyo, Japan). The atomic ratios of samples were measured by energy-dispersive spectroscopy (EDS) using a JSM-6390A EDS analyzer (JEOL, Tokyo, Japan). The BET (Brunauer-Emmett-Teller) surface area and pore size of the catalysts were measured by the multipoint N2 adsorption-desorption method at liquid nitrogen temperature (77 K) using a JW-K instrument (Beijing Gaobo High-Tech Development Center, Beijing, China). Prior to each measurement, the sample was pretreated at 393 K and 10-3 mbar for 3 h. The sulfonic acid groups in the sample were investigated using a Fourier transform infrared (FTIR) spectrometer (Nicolet Avatar 360 FTIR, Madison, WI) for skeletal spectra (infrared fundamental vibrations from 400 to 4000 cm-1). The surface acidity of the catalyst was determined by the ammonia temperature-programmed adsorption-desorption (NH3 TPD) technique on a PX200 instrument (Tianjin Pengxiang

Technology Co., Ltd., Tianjing, China). Before each test, 200 mg of the catalyst was preheated at 393 K to remove adsorbed water. The adsorption was conducted at 308 K with pulsed NH3 inputs, and the NH3 consumption per pulse was detected. The thermal stability of the samples was evaluated by thermogravimetric analysis (TGA), which was performed on an HCT-2 analyzer (Beijing Scientific Instrument Factory, Beijing, China). A flow of air at 50 mL/min was used, and the temperature was ramped from 313 to 1173 K at 10 K/min. 2.4. Catalytic Reaction Procedure. The reactions were carried out in a stainless steel stirred autoclave (1 L). Prior to reaction, the catalysts were heated at 373 K in a vacuum for 1 h to remove adsorbed water. The reaction mixture consisted of 92 g of glycerol (1 mol), 180-450 mL of liquid isobutylene (2-5 mol), and 0.92-7.36 g of catalysts (1-8 wt %, referred to the glycerol mass). For a typical run, the glycerol and catalyst were first added into the reactor. When the pressure in the reactor increased to 1.5 MPa with nitrogen, liquid-phase pressurized isobutylene was injected into the reactor by a pump. The temperature was maintained in the range of 323-363 K. The stirring rate for all experiments was adjusted to 1000 rpm to overcome external diffusional limitations. After the reaction, the reactor was opened, reducing the pressure to atmospheric pressure, and unreacted isobutylenes were vaporized. To monitor the reaction progress, samples of the reaction mixture were withdrawn from the reactor at different times and analyzed by gas chromatography. 2.5. Product Analysis. The samples of reaction products were analyzed in an Agilent4890D gas chromatograph, with an HP-Innowax column (30 m × 0.32 mm × 0.25 µm) and a flame ionization detector. Analyses were carried out with the temperature program increasing from 313 to 493 K (with a slope of 10 K/min) and holding at 493 K for 10 min isothermally. The glycerol conversion is defined as XG )

M2 M1

(1)

and the selectivity to glycerol ethers is defined as SY )

M3 M2

(2)

where XG is the conversion of glycerol (mol %), SY (Y represents MTBGs, DTBGs, and TTBGs) is the selectivity toward different glycerol ethers (mol %), M1 is the amount of starting glycerol (mol), M2 is the total amount of glycerol ethers (mol), and M3 is the amount of corresponding glycerol ether (mol).

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Figure 2. EDS spectrum of sulfonated peanut shell catalyst.

Figure 4. TGA of sulfonated peanut shell catalyst in air.

Figure 3. FTIR spectra of partially carbonized peanut shell and sulfonated peanut shell catalyst.

3. Results and Discussion 3.1. Characterization of the Catalyst. The microscopic characterizations by SEM are presented in Figure 1. After partial carbonization, the peanut shell exhibited an amorphous porous structure that was composed of polycyclic aromatic carbon sheets. The carbon material did not show an obvious difference between when it was untreated and when it was treated by concentrated H2SO4 at 483 K. This implies that the prepared carbon material catalyst had good stability in the sulfonation. The BET surface area and pore diameter were investigated to obtain detailed information on the pores. The surface area was 10.45 m2 /g, and the average pore size was 41.95 nm. In Figure 2, the EDS analysis shows that the contents of sulfur and oxygen in the sulfonated peanut shell catalyst were 6.18 and 36.25 wt %, respectively. It can thus be calculated that the acidity contributed by sulfur was 1.93 mmol/g. FTIR spectroscopy was employed to estimate the functional groups on the sulfonated peanut shell catalyst. As shown in Figure 3, two bands appeared at 1040 and 1181 cm-1 in the sulfonated peanut shell catalyst compared with the unsulfonated one that could be assigned to the SO2 asymmetric and symmetric stretching modes.20 These result indicated that the covalently linked sulfonic acid group formed on the surface of the sulfonated peanut shell catalyst. The band at 1720 cm-1 can be attributed to the CdO stretching mode of the sCOOH groups. The broad band centered at 3424 cm-1 was assigned to the sOH stretching mode of the sCOOH group. Therefore, sCOOH and sSO3H were found to be the functional groups of the sulfonated peanut shell catalyst and could be used as proton conductors. The total acid capacity of the sulfonated peanut shell catalyst was 2.07 mmol/g, which was estimated by the NH3 TPD technique. The TGA profile of sulfonated peanut shell catalyst in air is shown in Figure 4. Within the temperature range of 323-393 K, the TGA profile of the peanut shell catalyst presented only

Figure 5. Reaction scheme for glycerol etherification with isobutylene.

4.7% weight loss, which was due to the desorption of water. No further weight loss was observed from 393 to 573 K. Then, a rapid weight loss was detected in the range from 573 to 803 K, implying that nongraphitic and graphitic carbons were oxidized. The sample weight reached a plateau with further increase in temperature. Similar TGA analysis results for sulfonated glucose catalyst have also been obtained by other researchers.21 These results indicate that the sulfonated peanut shell catalyst is stable at high reaction temperature, even to 573 K. 3.2. Etherification of Glycerol with Isobutylene. When the glycerol ether synthesis was carried out, the hydroxyl groups of glycerol reacted with isobutylene, and five glycerol ether isomers were potentially produced, including two MTBGs (3tert-butoxy-1,2-propanediol and 2-tert-butoxy-1,3-propanediol), two DTBGs (2,3-di-tert-butoxy-1-propanol and 1,3-di-tertbutoxy-2-propanol), and one TTBG (1,2,3-tri-tert-butoxy propane), as shown in Figure 5. The DTBGs and TTBG were the desired products in this reaction. In addition, isobutylene might be dimerized forming di-isobutylenes (DIBs) as a side reaction. 3.2.1. Effect of Reaction Time. To study the influence of reaction time on the glycerol etherification reaction, experiments using the same loading of sulfonated peanut shell catalyst were carried out at 20, 40, 60, 90, 120, 180, and 240 min. The results are shown in Figure 6.

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Figure 6. Influence of the reaction time on the glycerol etherification reaction. Conditions: Reaction temperature, 343 K; isobutylene/glycerol molar ratio, 4/1; catalyst loading, 6 wt %.

Figure 7. Influence of the reaction temperature on the glycerol etherification reaction. Conditions: Isobutylene/glycerol molar ratio, 4/1; catalyst loading, 6 wt %; reaction time, 2 h.

As shown in Figure 6, the conversion of glycerol rapidly increased from 20 to 120 min and reached 100% after 120 min. The selectivity of glycerol ethers was also affected by the reaction time. In the mixture of glycerol ethers, the selectivity toward MTBGs obviously decreased from 20 to 120 min and then slightly reduced with a further increase in the reaction time from 120 to 240 min. The combined selectivity toward DTBGs and TTBG presented the opposite tendency with respect to the selectivity toward MTBGs and reached 92.1% after 120 min. With increasing reaction time, the content of DIBs in the products also increased. These results can be explained as follows: The glycerol etherification reaction consisted of three consecutive reversible steps: glycerol and isobutylene produced MTBGs; then, MTBGs reacting with isobutylene were transformed into DTBGs; and finally, reaction of DTBGs with isobutylene led to TTBG. The first step was important for the conversion of glycerol, and a great amount of MTBGs formed initially. The transformation of DTBGs and TTBG was enhanced with increasing reaction time. Considering that the purpose of optimization was to obtain a high glycerol conversion and a low selectivity toward MTBGs, 120 min was thus considered as a suitable reaction time in this experimental system. 3.2.2. Effect of Reaction Temperature. To study the influence of the reaction temperature on the glycerol etherification reaction, experiments using sulfonated peanut shell catalyst were conducted at 323, 333, 343, 353, and 363 K. As shown in Figure 7, the conversion of glycerol increased with the increase in temperature from 323 to 343 K, and the complete conversion of glycerol was obtained at 343 K after 2 h. However, the results showed that the conversion exhibited a

Figure 8. Influence of the isobutylene-to-glycerol molar ratio on the glycerol etherification reaction. Conditions: Reaction temperature, 343 K; catalyst loading, 6 wt %; reaction time, 2 h.

slightly decrease from 343 to 363 K. The selectivity toward the sum of DTBGs and TTBG presented a similar tendency, which increased from 323 to 343 K and slightly reduced from 353 to 363 K. In general, a comparatively higher temperature would be required to obtain a higher reaction rate in a short reaction time. Because glycerol etherification is a reversible reaction, higher temperature could enhance the back-reaction, which would lead to an increase in the dealkylation rate of higher ethers (DTBGs, TTBGs), forming glycerol or lower ethers and isobutylene.22 On the other hand, the dimerization of isobutylene is an undesired side reaction that depends on reaction time and temperature. Higher reaction temperature would promote the dimerization reaction, which would consume a large amount of isobutylene and might influence the glycerol conversion and selectivity toward DTBGs and TTBG. Based on these comprehensive considerations, 343 K was chosen as the optimum temperature. 3.2.3. Effect of Molar Ratio of Isobutylene to Glycerol. DTBGs and TTBG are all desired products of the glycerol etherification reaction. By stoichiometric equations, at least 2 mol of isobutylene is required for each mole of glycerol to synthesize each mole of DTBGs. Because etherification is a reversible reaction, in the reaction mixture, the amount of isobutylene must be in excess to drive the reaction toward the formation of DTBGs and TTBG. The effect of the isobutyleneto-glycerol molar ratio on the etherification reaction was studied using four different ratios within the range from 2:1 to 5:1. As shown in Figure 8, the glycerol conversion increased and the selectivity toward MTBGs in the glycerol ether mixture decreased with an increase in the molar ratio from 2 to 4. With a further increase in the molar ratio from 4 to 5, the selectivity of MTBGs decreased slightly. Although the selectivity of TTBG went up with decreasing selectivity of DTBGs, the selectivity toward the sum of DTBGs and TTBG did not obviously increase when the molar ratio was increased from 4 to 5. Considering that a higher amount of TTBG would consume more isobutylene, a molar ratio of 4 was considered to be optimal. 3.2.4. Effect of Catalyst Loading. The catalyst loading is one of the important factors influencing the reaction rate. Therefore, when sulfonated peanut shell was used as the catalyst, the effect of catalyst loading on the glycerol etherification reaction was investigated with various loadings of catalyst in the range from 1 to 8 wt % on the basis of glycerol. As shown in Figure 9, the conversion of glycerol markedly climbed with increasing catalyst loading from 1 to 6 wt %. The highest conversion was obtained when the catalyst loading was 6 wt %. The glycerol conversion was insignificantly influenced

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Figure 9. Influence of the catalyst loading on the glycerol etherification reaction. Conditions: Reaction temperature, 343 K; isobutylene/glycerol molar ratio, 4/1; reaction time, 2 h.

Figure 10. Reusability of the catalyst. Conditions: Reaction temperature, 343 K; isobutylene/glycerol molar ratio, 4/1; catalyst loading, 6 wt %; reaction time, 2 h.

by a further increase in catalyst loading from 6 to 8 wt % because of the equilibrium limit. Therefore, the optimum catalyst loading for this experimental system was found to be 6 wt %. 3.2.5. Reusability of the Catalyst. Sulfonated peanut shell catalyst was repeatedly used for the glycerol etherification reaction to investigate its reusability. The catalyst was collected by filtering the reactant mixture after reaction and directly used in a new reaction cycle without any treatment. Glycerol and isobutylene were added in similar amounts as for the initial reaction. As shown in Figure 10, a high conversion of glycerol and a good selectivity toward the sum of DTBGs and TTBG were obtained every time. Moreover, the activity of sulfonated peanut shell catalysts remained almost unchanged even after five cycles. Similarly, it has been reported that sulfonated ordered mesoporous carbons catalyst could retain a sustained activity for biodiesel production.23 The cost of commercial solid acid catalysts is high because of the expensive raw materials and complex preparation process used, whereas the peanut shell catalyst is easily produced from cheaper waste biomass. From an economic point of view, the prepared peanut shell catalyst with stability and sustained activity could significantly lower the cost of glycerol ether production. 3.2.6. Comparison of the Peanut Shell Catalyst and Cation-Exchange Resin in the Glycerol Etherification Reaction. To rationally evaluate the catalytic performance of

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Figure 11. Comparison of the peanut shell catalyst and cation-exchange resin in the glycerol etherification reaction. Conditions: Reaction temperature, 343 K; isobutylene/glycerol molar ratio, 4/1; catalyst loading, 6 wt %; reaction time, 2 h.

sulfonated peanut shell catalyst in the etherification reaction, experiments were carried out using the peanut shell catalyst and cation-exchange resin (Amberlyst-15) under the optimized reaction conditions for 1.5 h. As shown in Figure 11, comparative experiments indicated that sulfonated peanut shell catalyst could provide a higher glycerol conversion and better selectivity toward the sum of DTBGs and TTBG. There might be several reasons for this result. As shown in Table 1, the average pore size of the sulfonated peanut shell catalyst (41.95 nm) was larger than that of the cation-exchange resin (30 nm). In addition, the average particle size of the sulfonated peanut shell catalyst was about 50 µm, as shown in SEM images, which was smaller than that of Amberlyst-15 (700 µm). Owing to this structure, sulfonated peanut shell catalyst might be more favorable to molecular internal diffusion than cation-exchange resin. On the other hand, basic carbonized peanut shell is a hydrophobic adsorbent, which could exhibit good adsorption for hydrophobic isobutylene. After sulfonation, the carbonized peanut shell was decorated with -COOH and -SO3H groups and could provide good access for hydrophilic glycerol to reach the active sites. For this reason, the catalyst developed in this work might more easily adsorb all of the reactants, including hydrophobic isobutylene and hydrophilic glycerol, compared to cation-exchange resin.24 This might be another reason why peanut shell catalyst had a high catalytic activity. It has been reported that D-glucose-derived solid acid catalyst was successfully used for biodiesel preparation and resulted in a higher initial reaction rate and yield than Amberlyst-15.25 Moreover, sulfonated peanut shell catalyst showed better heat stability than cation-exchange resin, which is another appealing aspect of the sulfonated carbon materials in addition to its low cost. 4. Conclusions Utilizing agricultural byproduct peanut shell as the starting material, a novel solid acid catalyst was prepared by the sulfonation of partially carbonized peanut shell. The resultant catalyst consisted of an amorphous porous carbon-based framework decorated with highly dispersed polycyclic aromatic

Table 1. Physicochemical Properties of Sulfonated Peanut Shell Catalyst and Amberlyst-15 catalyst

surface area (m2/g)

average pore diameter (mmol/g)

acid capacity (mmol/g)

Tmax (K)

average particle size (µm)

sulfonated peanut shell catalyst Amberlyst-15

10.45 53

41.95 30

2.07 4.7

573 393

50 700

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hydrocarbons. The sulfonic acid groups were covalently attached to the surface of the carbon-based material, forming Bro¨nsted acid centers. Moreover, the catalyst was thermally stable to temperatures up to 573 K. The experimental results indicated that sulfonated peanut shell catalyst showed a good catalytic activity and could be used as a green catalyst for glycerol etherification with isobutylene. The complete conversion of glycerol and a selectivity toward the sum of DTBGs and TTBG of 92.1% were obtained using a 4:1 molar ratio of isobutylene to glycerol, a 6 wt % mass ratio of catalyst to glycerol, and a temperature of 343 K in 2 h. Because of low cost and easily availability, this catalyst has good application prospects. Acknowledgment Financial support for this work from the National Basic Research Program of China (973 Program, 2009CB219906), Key Project of National High Technology Research and Development Program (863 Program) of China (2009AA050703), National Natural Science Foundation of China (20776117, 20976144), and Specialized Research Fund for the Doctoral Program of Higher Education of China (20070698037) is gratefully acknowledged. Literature Cited (1) Yuan, H.; Yang, B. L.; Zhu, G. L. Synthesis of Biodiesel Using Microwave Absorption Catalysts. Energy Fuels 2008, 23, 548. (2) Behr, A.; Eilting, J.; Irawadi, K. Improved utilisation of renewable resources: New important derivatives of glycerol. Green Chem. 2008, 10, 13. (3) Dimitratos, N.; Lopez-Sanchez, J.; Lennon, D.; Porta, F.; Prati, L.; Villa, A. Effect of Particle Size on Monometallic and Bimetallic (Au, Pd)/C on the Liquid Phase Oxidation of Glycerol. Catal. Lett. 2006, 108, 147. (4) Meher, L. C.; Gopinath, R.; Naik, S. N.; Dalai, A. K. Catalytic Hydrogenolysis of Glycerol to Propylene Glycol over Mixed Oxides Derived from a Hydrotalcite-Type Precursor. Ind. Eng. Chem. Res. 2009, 48, 1840. (5) Lehr, V.; Sarlea, M.; Ott, L.; Vogel, H. Catalytic dehydration of biomass-derived polyols in sub- and supercritical water. Catal. Today 2007, 121, 121. (6) Luo, N.; Fu, X.; Cao, F.; Xiao, T.; Edwards, P. P. Glycerol aqueous phase reforming for hydrogen generation over Pt catalystsEffect of catalyst composition and reaction conditions. Fuel. 2008, 87, 3483. (7) Lin, R.; Liu, H.; Hao, J.; Cheng, K.; Liu, D. Enhancement of 1,3propanediol production by Klebsiella pneumoniae with fumarate addition. Biotechnol. Lett. 2005, 27, 1755. (8) Aresta, M.; Dibenedetto, A.; Nocito, F.; Pastore, C. A study on the carboxylation of glycerol to glycerol carbonate with carbon dioxide: The role of the catalyst, solvent and reaction conditions. J. Mol. Catal. A: Chem. 2006, 257, 149.

(9) Santacesaria, E.; Tesser, R.; Di Serio, M.; Casale, L.; Verde, D. New Process for Producing Epichlorohydrin via Glycerol Chlorination. Ind. Eng. Chem. Res. 2009, 49, 964. (10) Pagliaro, M.; Ciriminna, R.; Kimura, H.; Rossi, M.; Della Pina, C. From Glycerol to Value-Added Products. Angew. Chem., Int. Ed. 2007, 46, 4434. (11) Wessendorf, R. Glycerol Derivatives as Fuel Components. Erdo¨l Kohle Erdgas Petrochem. 1995, 48, 138. (12) Noureddini, H.; Daily, W. R.; Hunt, B. A. Production of ethers of glycerol from crude glycerolsthe by-product of biodiesel production. Chem. Biomol. Eng. Res. 1998, 13, 121. (13) Behr, A.; Obendorf, L. Development of a Process for the AcidCatalyzed Etherification of Glycerine and Isobutene Forming Glycerine Tertiary Butyl Ethers. Eng. Life Sci. 2002, 2, 185. (14) Klepa´cˇova´, K.; Mravec, D.; Bajus, M. Etherification of glycerol with tert-butyl alcohol catalysed by ion-exchange resins. Chem. Pap. 2006, 60, 224. (15) Klepa´cˇova´, K.; Mravec, D.; Kaszonyi, A.; Bajus, M. Etherification of glycerol and ethylene glycol by isobutylene. Appl. Catal. A: Gen. 2007, 328, 1. (16) Okamura, M.; Takagaki, A.; Toda, M.; Kondo, J. N.; Domen, K.; Tatsumi, T.; Hara, M.; Hayashi, S. Acid-Catalyzed Reactions on Flexible Polycyclic Aromatic Carbon in Amorphous Carbon. Chem. Mater. 2006, 18, 3039. (17) Toda, M.; Takagaki, A.; Okamura, M.; Kondo, J. N.; Hayashi, S.; Domen, K.; Hara, M. Green chemistry: Biodiesel made with sugar catalyst. Nature 2005, 438, 178. (18) Shu, Q.; Zhang, Q.; Xu, G.; Nawaz, Z.; Wang, D.; Wang, J. Synthesis of biodiesel from cottonseed oil and methanol using a carbonbased solid acid catalyst. Fuel Process. Technol. 2009, 90, 1002. (19) Nakajima, K.; Hara, M.; Hayashi, S. Environmentally Benign Production of Chemicals and Energy Using a Carbon-Based Strong Solid Acid. J. Am. Ceram. Soc. 2007, 90, 3725. (20) Shu, Q.; Zhang, Q.; Xu, G.; Wang, J. Preparation of biodiesel using s-MWCNT catalysts and the coupling of reaction and separation. Food Bioprod. Process. 2009, 87, 164. (21) Wang, Y.; Iqbal, Z.; Mitra, S. Rapidly Functionalized, WaterDispersed Carbon Nanotubes at High Concentration. J. Am. Chem. Soc. 2005, 128, 95. (22) Melero, J. A.; Vicente, G.; Morales, G.; Paniagua, M.; Moreno, J. M.; Rolda´, R.; Ezquerro, A.; Pe´rez, C. Acid-catalyzed etherification of bio-glycerol and isobutylene over sulfonic mesostructured silicas. Appl. Catal. A: Gen. 2008, 346, 44. (23) Liu, R.; Wang, X.; Zhao, X.; Feng, P. Sulfonated ordered mesoporous carbon for catalytic preparation of biodiesel. Carbon 2008, 46, 1664. (24) Takagaki, A.; Toda, M.; Okamura, M.; Kondo, J. N.; Hayashi, S.; Domen, K.; Hara, M. Esterification of higher fatty acids by a novel strong solid acid. Catal. Today 2006, 116, 157. (25) Zong, M. H.; Duan, Z. Q.; Lou, W. Y.; Smith, T. J.; Wu, H. Preparation of a sugar catalyst and its use for highly efficient production of biodiesel. Green Chem. 2007, 9, 434.

ReceiVed for reView July 8, 2010 ReVised manuscript receiVed October 27, 2010 Accepted November 8, 2010 IE101461G