Published: April 22, 2011 r2011 American Chemical Society 6620 dx.doi.org/10.1021/ie102066q | Ind. Eng. Chem. Res. 2011, 50, 6620–6628 ARTICLE pubs.acs.org/IECR Biodiesel Production by Esterification of Free Fatty Acids over 12-Tungstophosphoric Acid Anchored to MCM-41 Varsha Brahmkhatri † and Anjali Patel* ,† † Department of Chemistry, Faculty of Science, M. S. University of Baroda, Vadodara 390002, India ABSTRACT: Heterogeneous acid catalyst comprising 12-tungstophosphoric acid (30%) and MCM-41 was synthesized and characterized by X-ray diffraction (XRD), surface area measurement (BET method), and solid state 29 Si NMR. The use of synthesized catalyst was explored for biodiesel production by esterification of free fatty acid, palmitic acid with methanol. The effect of various reaction parameters such as catalyst concentration, acid/alcohol molar ratio, and temperature were studied to optimize the conditions for maximum conversion. The catalyst shows high activity in terms of 100% conversion toward palmitic acid and a high turnover number, 1992. The kinetic studies as well as the Koros Nowak test were also carried out, and it was found that esterification of palmitic acid follows first order kinetics and the rates are not mass transfer limited. The excellent catalytic performance is attributed to the large surface area and pore diameter of the mesoporous support, MCM-41 as well as the Bronsted acid strength of TPA, as active sites. The catalyst shows the potential of being used as a recyclable catalytic material after simple regeneration without significant loss in conversion. As an application, preliminary studies were carried out for biodiesel production from waste cooking oil, as feedstock without any pretreatment, and with methanol over the present catalyst. Studies also reveal that the catalyst can be used for biodiesel production from waste cooking oil. 1. INTRODUCTION Biodiesel is gaining much attention in recent years as a renewable fuel. It is a nonpetroleum based fuel that consists of alkyl esters derived from either the transesterification of trigly- cerides (TGs) or the esterification of free fatty acids (FFAs) with low molecular weight alcohols. 1À4 The conventional biodiesel production technology involves the use of alkaline homogeneous catalysts such as NaOH and KOH, but sometimes NaOCH 3 or KOCH 3 are also employed mainly in large-scale production plants. These are not compatible for feedstocks with large amounts of free fatty acids (FFAs) and moisture due to the formation of soaps that strongly affect the feasibility of glycerol separation which is an important coproduct of transesterification reaction. The traditional liquid acids such as HCl and H 2 SO 4 were found to be more efficient, but they need a very long reaction time and a very high molar ratio of methanol to oil. Also corrosion of reaction vessels and problem of recycling are the key issues with traditional liquid acids. Therefore commercializa- tion of biodiesel production is difficult due to the technological drawbacks such as separation and purification steps that increase the cost factor to maximum. An interesting alternative for low cost biodiesel production is the utilization of low quality raw materials as feedstocks such as waste cooking oil obtained from canteens restaurants and from houses which are rich in free fatty acids. A literature survey shows that biodiesel production becomes a two step process where high free fatty acid-containing feedstocks are used as raw materials. 5 Simultaneous FFA esterification and TGs transesterification using acid catalysts provides an alternative single step process for poor quality oils containing high FFA levels. 6À10 This is only possible with heterogeneous acid catalysts. Considering heterogeneous solid acid catalysts, a literature survey shows that there are fewer reports than those on solid bases. Compared with solid base catalysts, solid acid catalysts have lower activity but higher stability, thus they can be applied for feedstock with large amounts of free fatty acids without catalyst deactivation. Catalysis by supported HPAs has been greatly expanded during the past few years from the viewpoint of their variety of structures and compositions. They provide the opportunities for tuning their chemical properties, such as acidities, and reactivities by choice of appropriate support. Considering acidic properties, they have found enormous applications in various industrially important classes of reactions such as alkylations, acylations, and esterifications. 11À16 Recently, they have gained tremendous interest, in the synthesis of biodiesel. In the present contribution esterification of free fatty acid, palmitic acid was taken as a model reaction for biodiesel production, as it is present in most of the fatty acids derived Table 1. Elemental Analysis (EDS) elemental analysis (weight %) W P catalyst O Si by EDS theoretical by EDS theoretical TPA 3 /MCM-41 53.9 27.8 18.0 19 0.30 0.32 R-TPA 3 /MCM-41 53.9 27.9 17.8 19 0.30 0.32 Received: October 11, 2010 Accepted: April 12, 2011 Revised: January 5, 2011