Acidic Mesoporous Silica for the Catalytic Conversion of Fatty Acids in Beef Tallow Isa K. Mbaraka, Kyle J. McGuire, and Brent H. Shanks* Department of Chemical and Biological Engineering, Iowa State UniVersity, Ames, Iowa 50011 Propylsulfonic acid-functionalized mesoporous silica materials were synthesized using a co-condensation technique. The catalytic performance of the resulting acidic mesoporous materials was evaluated in the methanol esterification of free fatty acids in beef tallow as a pretreatment step for alkyl ester production. The multicycle stability of the acid-functionalized mesoporous silica catalysts was studied. Issues concerning impurities in the feedstocks as well as a means of improving performance of the acidic solid catalyst through alternative strategies are discussed. Introduction of a hydrophobic group into the organosulfonic acid-functionalized mesoporous silica catalyst significantly enhanced the catalytic performance of the catalyst. The catalytic activity of the synthesized catalysts was compared to commercially available homogeneous and heterogeneous acidic catalysts. Introduction Alkyl esters from the transesterification of vegetable oils with short-chain alcohols in the presence of an alkaline catalyst to form alkyl esters have application as intermediates for value- added chemicals as well as alternative transportation fuel (biodiesel). 1-6 However, alkyl ester production has been limited due to the inability to compete economically because of high raw material and production costs. To improve the economic outlook of alkyl esters, the selection of raw materials, reaction conditions, and the use of efficient catalysts become critical. The availability of oil and fat feeds such as beef tallow and yellow grease provide alternative less expensive feedstocks than vegetable oils. 7-10 Unfortunately, these oil feeds have high free fatty acid (FFA) content, which is not compatible with the homogeneous catalyst used in the transesterification reaction since the FFAs are saponified by the alkali catalyst causing catalyst depletion through formation of soap. The soap formation also creates subsequent difficulties in product separation. For these feedstocks to be processed in standard transesteri- fication process technology, the FFA content needs to be lowered to no more than 0.5 wt %. 11 This reduction can be achieved by first esterifying the FFA in the presence of an acidic catalyst. Such a pretreatment process has been successfully demonstrated using sulfuric acid; however, the use of a homogeneous catalyst adds a neutralization step as well as separation steps to the overall alkyl ester process. 12 Conse- quently, it would be desirable to use a heterogeneous acidic catalyst to esterify the FFAs, thereby simplifying the pretreat- ment process. Inorganic mesoporous silica materials are of interest in catalysis applications due to their combination of large surface area, flexible pore sizes, and the ability of controlling the catalytic functionalities at the molecular level. 13-16 The physical and chemical properties of these mesostructured materials can be modified by incorporating organic groups, either by grafting on the preformed mesopore surface or by one-step co-condensa- tion during synthesis. 17-20 Several studies have shown that incorporation of alkylsulfonic acid groups onto the mesoporous silicas framework generate acidic solid catalysts that can catalyze esterification reactions. 20-24 We recently demonstrated the application of organosulfonic acid-functionalized mesoporous silicas in the esterification of a model feed containing palmitic acid in refined soybean oil. 25 The acidic mesoporous catalysts exhibited good catalytic performance in the methanol esterifi- cation of palmitic acid. While the amount of palmitic acid used in the model feed was similar in quantity to the FFA level found in beef tallow or yellow grease, the fatty acid distribution in animal fats and other impurities could affect the catalytic performance of the solid acid catalyst. Animal fats typically have a high concentra- tion of saturated fatty acids (e.g. palmitic and stearic acids) and low concentration of polyunsaturated fatty acids (e.g. linoleic and linolenic acids) as compared to vegetable oils, which is expected to have minimal effect on the performance of the acid solid catalyst. However, impurities in these high-FFA feedstocks might adversely affect the performance of the catalyst system. For example, beef tallow contains low levels of nitrogenous materials (e.g. proteinaceous matter and phosphatides). There- fore, the use of acid-functionalized mesoporous silica for the catalytic esterification of FFAs ultimately needs to be validated using real beef tallow. Herein, we report the catalytic performance of propylsulfonic acid-functionalized mesoporous silica catalysts in the esterifi- cation of FFAs in beef tallow. The results for the catalytic performance of the acid-functionalized mesoporous silicas are also compared to those from commercial acidic catalysts. Experimental Section Materials. Flotation-grade beef tallow (Tyson Foods Inc., Lincoln, NE) with approximately a 7 wt % FFA concentration, as measured according to the AOCS method Ca 3a-63, was used in this study. Anhydrous methanol (MeOH) was obtained from Fisher Scientific Inc. Activated charcoal (Acros) and TriSyl 300 (Grace Davison Co., USA), an activated silica, were used in some experiments as a pretreatment of the flotation beef tallow. Propylsulfonic acid-functionalized mesoporous silica (SBA-15- SO 3 H) was synthesized according to the procedure described previously. 25 Tetraethoxysilane (TEOS; 98%, Aldrich) and (3- mercaptopropyl)trimethoxysilane (MPTMS; 85%, Acros) were used as purchased for synthesizing the organic-inorganic hybrid materials. The MPTMS was oxidized in situ with H 2 O 2 during * To whom correspondence should be addressed Tel.: 515-294-1895. Fax: 515-294-2689. E-mail: bshanks@iastate.edu. 3022 Ind. Eng. Chem. Res. 2006, 45, 3022-3028 10.1021/ie0601089 CCC: $33.50 © 2006 American Chemical Society Published on Web 03/22/2006