Journal of Molecular Catalysis A: Chemical 316 (2010) 30–44 Contents lists available at ScienceDirect Journal of Molecular Catalysis A: Chemical journal homepage: www.elsevier.com/locate/molcata Transesterification reaction of triglycerides in the presence of Ag-doped H 3 PW 12 O 40 A. Zieba, L. Matachowski, J. Gurgul, E. Biela ´ nska, A. Drelinkiewicz ∗ Institute of Catalysis and Surface Chemistry Polish Academy of Sciences, 30-239 Kraków, Niezapominajek 8, Poland article info Article history: Received 17 June 2009 Received in revised form 25 September 2009 Accepted 28 September 2009 Available online 4 October 2009 Keywords: Biodiesel Transesterification Triacetin Castor oil Heteropolyacid abstract Present work is concerned with examination of catalytic properties of 12-tungstophosphoric acid and its silver salts Ag x H 3-x PW 12 O 40 with Ag-content ranging from x = 0.5 up to x = 3 for transesterification of triglycerides with methanol to form methyl ester (biodiesel) under mild conditions (atmospheric pres- sure, temperature of 50–60 ◦ C). Reaction was studied for triacetin, a model triglyceride and for vegetable oil, castor oil. Various techniques (BET, FTIR, XRD, XPS, laser diffraction, electron microscopy, SEM, and EDS) were used to characterize as-received Ag-salts and the samples separated after the catalytic tests. Because of the presence of methanol, which is the reactant, silver salts formed colloidal dispersion during the catalytic reaction. Under such conditions, all silver salts were active for transesterification of triglyc- erides. The conversion of triglycerides gradually decreases as the protons in heteropolyacid are replaced by Ag + cations. However, due to the leaching of parent H 3 PW 12 O 40 upon dissolving in methanol, the con- tribution of homogeneous catalysis was observed, especially in the presence of Ag-salts with low silver content such as Ag 0.5 H 2.5 PW 12 O 40 . Catalytic performance of Ag-salts in methanolysis of a short-chain triglyceride, triacetin, differs remarkably from that in castor oil comprising natural, long-chain triglyc- erides. After transesterification of castor oil, initially crystalline particles of Ag-salts partially rearrange to “gel-type” material making their further processing difficult. Although, no “gel-type” particles are formed after reaction of triacetin, the formation of “nanowires” due to partial reduction of some of Ag + ions is observed. © 2009 Elsevier B.V. All rights reserved. 1. Introduction Biodiesel, a nonpetroleum-based fuel consists of fatty acid methyl-esters (FAME) formed through a transesterification reac- tion. In this process, the triglycerides of fatty acids (C 14 –C 20 ) from renewable sources like vegetable oils, animal fats and recycled greases from food industry are converted to methyl-esters and glyc- erol as a by-product in transesterification reaction with methanol (methanolysis). The reaction has traditionally been catalysed by homogeneous catalysts, such as K- or Na-alkoxides or -hydroxides, and mineral acids [1–3]. However, the application of heterogeneous catalysts is more desirable from economic, technological and envi- ronmental points of view. In particular, solid acid catalysts seem to be very useful since they are able to catalyse both the transesteri- fication of triglycerides and esterification of free fatty acids, which becomes important when using recycled greases. Therefore, great research efforts have recently been undertaken to find efficient solid acid catalysts and the results of these studies were reviewed in general papers [1–3]. ∗ Corresponding author. E-mail address: ncdrelin@cyf-kr.edu.pl (A. Drelinkiewicz). The most commonly tested vegetable oils include palm oil, soy- bean oil, sunflower oil, coconut oil and rapeseed oil. Moreover, castor oil derived from Riccinus communis plants is mentioned fre- quently in the literature as a potential raw material for biodiesel. The main constituent of castor oil is triglyceride of 12-hydroxy-9- octadecenoic acid (ricinoleic acid, Scheme 1). Due to the presence of OH group at C-12 carbon, castor oil exhibits unique chemical and physical properties. Castor oil is well soluble in both methanol and methyl-esters formed and the homogeneity of reaction mixture is attained during transesterification reaction [4–6]. It is well known that Keggin type heteropolyacids and their acidic salts are useful catalysts for reactions requiring strong acid- ity. Although they are useful solid catalysts for gas-phase reactions, they are highly soluble in polar media like water and methanol. Therefore, when studied in methanolysis of rapeseed oil, het- eropolyacids (H 3 PW 12 O 40 ,H 4 SiW 12 O 40 , and H 3 PMo 12 O 40 ) acted as homogeneous catalysts and their activity was higher than those of mineral acids (H 2 SO 4 and H 3 PO 4 ) [7]. However, quite recently published data demonstrated that heteropolyacids (H 3 PW 12 O 40 and H 3 PMo 12 O 40 ) supported on acid-treated K-10 clay were effec- tive and stable catalysts in the transesterification of different oils with alcohols [8]. On the other hand, by combining vari- ous amounts of heteropolyacid H 3 PW 12 O 40 and large monovalent 1381-1169/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.molcata.2009.09.019