Energy Fuels 2010, 24, 4095–4096 Published on Web 06/24/2010
: DOI:10.1021/ef100333y
Challenges of Phosphate and Carbonate Salts as Catalysts for Biodiesel Synthesis Stephanie L. Britton,† Jesse Q. Bond, and Thatcher W. Root* Department of Chemical and Biological Engineering, University of Wisconsin;Madison, 1415 Engineering Drive, Madison, Wisconsin 53706 † Present address: Air Products and Chemicals, 7201 Hamilton Blvd., Allentown, PA 18195. Received March 19, 2010. Revised Manuscript Received June 18, 2010 FAME synthesis was undertaken based on a common procedure established in the literature.7,16,17 In a slight departure from the traditional synthesis composition, methyl esters were included in the starting mixture to serve as an emulsifier. Most catalyst supports have a hydrophilic surface, because they are covered with hydroxyl groups or physisorbed water.18 When the solid is mixed with oil, the solid adheres to the hydrophilic surface of the glass reaction vessel. This prevents the catalyst from achieving adequate contact with both reactant phases. While other groups have chosen to use a co-solvent19 or an excess of methanol20 to promote phase mixing, we chose to use an emulsifier to facilitate catalyst and oil contact. The methyl esters in the biodiesel product are good emulsifiers, and they do not complicate product purification. Adding 5 wt % of methyl esters to the reaction mixture proved to be sufficient to promote good mixing of all phases. With this addition, we observed no catalyst clumping and, in all cases, the catalyst was well-dispersed. The basic catalyst deprotonates methanol to form methoxide anions, which act as the active species and attack the triglyceride molecules at the carbonyl.20 A survey of bulk basic salts, shown in Figure 1, compared the pKa of these salts to their effectiveness in converting triglycerides to FAME. This experiment revealed that only bases with a pKa of 10 or above are strong enough to deprotonate methanol (pKa = 15) in sufficient quantities to catalyze the reaction. Dibasic phosphate, tetrabasic diphosphate (also known as pyrophosphate), and monobasic carbonate, among others, were not strong enough bases to catalyze the reaction. Tribasic phosphate and dibasic carbonate salts proved sufficiently basic for the conversion. The tribasic phosphate salts were focused on in this study because of the effectiveness of solid-state 31P nuclear magnetic resonance (NMR) to characterize the phosphate active site. Our tests also revealed that these active salts are slightly soluble in the reaction mixture. The solubility of phosphate and carbonate salts in the transesterification reaction has been confirmed by Arzamendi et al.15 Because a heterogeneous catalyst is desired, the active species was stabilized by fixing it to a support. The silica and alumina supports were used as received and were also subjected to two pretreatments to promote catalyst bonding: heat treatment and acid washing. The details of the support activation and catalyst impregnation procedures are given in the Supporting Information. XRD analysis showed that no bulk salt phases were formed on the support surface.
In the search for renewable fuels that can replace petroleum-based fuels, biodiesel has gained distinction as a promising solution.1-6 Biodiesel is a mixture of fatty acid methyl esters (FAMEs) synthesized via the transesterification of vegetable oils with methanol catalyzed by homogeneous sodium methoxide. Salt produced during neutralization of the catalyst contaminates the FAME product. Removal of the salt from the biodiesel fraction requires three or four water washes, which generate copious amounts of wastewater.1,7,8 A heterogeneous catalyst would not require such purification. Some progress in developing a solid catalyst has been made,9-15 but these materials do not yet match the effectiveness of liquid catalysts. This paper discusses the catalytic activity of tribasic phosphate and dibasic carbonate supported on treated silica or alumina. A simple method was developed for facilitating the dispersion of solid catalyst particles in the reaction mixture. Upon acid-treated silica, trisodium phosphate proved to be among the most active catalysts for this reaction, but its stability as a heterogeneous catalyst was shown to be poor. Silica supports were also shown to be unstable in the presence of the highly basic methoxide generated during the reaction, suggesting that silica is unsuitable as a support for any basic catalyst for FAME production. Catalyst and FAME preparation and characterization techniques are described in detail in the Supporting Information. *To whom correspondence should be addressed. E-mail: thatcher@ engr.wisc.edu. (1) National Renewable Energy Laboratory. Biomass Oil Analysis: Research Needs and Recommendations; Technical Report NREL/ TP-510-34796, 2004. http://www1.eere.energy.gov/biomass/pdfs/34796.pdf. (2) Schwab, A. W.; Bagby, M. O.; Freedman, B. Fuel 1987, 66, 1372–1378. (3) Korus, R. A.; Hoffman, D. S.; Bam, N.; Peterson, C. L.; Drown, D. C. Proceedings of the First Biomass Conference of the Americas: Energy, Environment, Agriculture, and Industry; National Renewable Energy Laboratory (NREL): Golden, CO, 1993; Vol. 2. (4) Schumacher, L. G.; Soylu, S.; Van Gerpen, J.; Wetherell, W. Appl. Eng. Agric. 2005, 21, 149–152. (5) Peterson, C. L.; Korus, R. A.; Mora, P. G.; Madsen, J. P. Trans. ASAE 1987, 30, 28–35. (6) Clark, S. J.; Wagner, L.; Schrock, M. D.; Piennaar, P. G. J. Am. Oil Chem. Soc. 1984, 61, 1632–1636. (7) Freedman, B.; Pryde, E. H.; Mounts, T. L. J. Am. Oil Chem. Soc. 1984, 61, 1638–1643. (8) Ma, F. R.; Hanna, M. A. Bioresour. Technol. 1999, 70, 1–15. (9) Leclercq, E.; Finiels, A.; Moreau, C. J. Am. Oil Chem. Soc. 2001, 78, 1161–1165. (10) Peter, S. K. F.; Ganswindt, R.; Neuner, H.-P.; Weidner, E. Eur. J. Lipid Sci. Technol. 2002, 104, 324–330. (11) Meher, L. C.; Sagar, D. V.; Naik, S. N. Renewable Sustainable Energy Rev. 2006, 10, 248–268. (12) Xie, W.; Huang, X.; Li, H. Bioresour. Technol. 2007, 98, 936–939. (13) Cantrell, D. G.; Gillie, L. J.; Lee, A. F.; Wilson, K. Appl. Catal., A 2005, 287, 183–190. (14) Kim, H.-J.; Kang, B.-S.; Kim, M.-J.; Park, Y. M.; Kim, D.-K.; Lee, J.-S.; Lee, K.-Y. Catal. Today 2004, 93-95, 315–320. (15) Arzamendi, G.; Arguinarena, E.; Campo, I.; Zabala, S.; Gandia, L. M. Catal. Today 2008, 133-135, 305–313. (16) Freedman, B.; Butterfield, R. O.; Pryde, E. H. J. Am. Oil Chem. Soc. 1986, 63, 1375–1380. r 2010 American Chemical Society
(17) Dmytryshyn, S. L.; Dalai, A. K.; Chaudhari, S. T.; Mishra, H. K.; Reaney, M. J. Bioresour. Technol. 2004, 92, 55–64. (18) Gates, B. C. Catalytic Chemistry; John Wiley and Sons, Inc.: New York, 1992. (19) Kim, H.-J.; Kang, B.-S.; Kim, M.-J.; Park, Y. M.; Kim, D.-K.; Lee, J.-S.; Lee, K.-Y. Catal. Today 2004, 93-95, 315–320. (20) Carey, F. A. Organic Chemistry, 3rd ed.; The McGraw-Hill Companies, Inc.: New York, 1996.
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: DOI:10.1021/ef100333y described in the Supporting Information. The ICP-OES and NMR results clearly show that these catalysts are not active when reused because the active phosphorus leaches off the silica or alumina surfaces in the presence of polar liquids, such as methanol or water. Silica supports experience some dissolution because of the presence of strong basic species present in the reaction, such as sodium methoxide. The inactivity of the catalyst after one reaction cycle and the instability of silica supports in the reaction medium indicate that this catalyst preparation is unsuitable for industrial use. The dissolution of the silica support has further implications for any group trying to develop novel heterogeneous catalysts for FAME production, because it shows that silica supports are not stable in any transesterification reaction for the production of FAME. Such reactions will contain sodium methoxide as the active agent for the production of FAME, and this methoxide will dissolve any silica support used in a heterogeneous catalyst preparation. Leaching of the active site from the catalyst surface occurred in both silica- and alumina-supported species, and therefore, stability of the alumina support was not fully characterized. However, alumina is known to be very stable under harsh conditions.21 If a means could be found to stabilize an active basic species on the surface, this support may be worth investigating in future studies. Although this work was unsuccessful in creating a stable heterogeneous base catalyst for FAME synthesis, a number of important conclusions were reached. First, although tribasic phosphate is active for this reaction, these authors were unable to permanently bind this species to a support surface without destroying the catalytic activity of the phosphate. Second and perhaps more importantly, it was shown that silica supports, which are among the most common heterogeneous catalyst supports in use today, are not stable in transesterification reactions containing strongly basic species.
Figure 1. Conversion of canola oil to FAME by different basic salts sorted by pKa. Table 1. Conversion Results (%) for Tribasic Phosphate and Dibasic Carbonate Salts on Supports Pretreated Using Different Activation Methodsa catalyst Na3PO4/SiO2 K3PO4/SiO2 Na2CO3/SiO2 Na3PO4/Al2O3 K3PO4/Al2O3 Na2CO3/Al2O3 a
untreated heat treated acid washed unsupported salt 6 3 25 0 0 3
6 1 44 0 47 57
31 12 53 50 6 74
85 89 57 85 89 57
Unsupported salt numbers are included for comparison.
Table 1 compiles the results of testing different combinations of catalyst salts, support materials, and support pretreatments. Figures plotting the reaction conversions from each type of support treatment versus the catalyst loading are shown in the Supporting Information, along with the catalyst loading for each material, as determined by inductively coupled plasma-optical emission spectroscopy (ICP-OES). It was found that the treatment of the catalyst support before exposure to the basic salt made a difference in the activity of the catalyst. For tribasic phosphate salts, heat treating the support yielded a small increase in the catalyst activity compared to a catalyst on an untreated support. However, washing the support in phosphoric acid yielded a much more active catalyst. 31P magic angle spinning (MAS) NMR tests, described in the Supporting Information, revealed that tetrahedral orthophosphate sites were responsible for catalyst activity. The phosphate catalyst preparations were tested further to determine catalyst stability over multiple reaction cycles, as
Acknowledgment. This material is based in part on work supported under a National Science Foundation Graduate Research Fellowship. The authors acknowledge Claudia Wee for her assistance with collecting the carbonate data. Supporting Information Available: Procedure for the catalyst preparation, procedure for FAME synthesis and characterization, procedure for catalyst characterization, and characterization of the active phosphate. This material is available free of charge via the Internet at http://pubs.acs.org. (21) Satterfield, C. N. Heterogeneous Catalysis in Practice; The McGraw-Hill Companies, Inc.: New York, 1980; pp 13 and 82.
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