Metal oxides as heterogeneous catalysts for esterification of fatty acids obtained from soybean oil Vinicius M. Mello, Gabriella P.A.G. Pousa, Mírian S.C. Pereira, Ingrid M. Dias, Paulo A.Z. Suarez ⁎ Laboratório de Materiais e Combustíveis, Instituto de Química, Universidade de Brasília, CP 4478, CEP 70919-970, Brasília-DF, Brazil abstract article info Article history: Received 2 July 2010 Received in revised form 24 August 2010 Accepted 29 August 2010 Keywords: Biodiesel Esterification Tin Aluminum Zinc Heterogeneous catalysis The growing demand for renewable energy sources stimulates the development of new technologies for biofuel production. Biodiesel synthesis by esterification of fatty acids is a favorable route, because, differently from transesterification, it does not produce glycerin and uses cheaper raw materials. In this work the study of metal oxides and their performance as Lewis acid catalysts in the esterification of fatty acids obtained from soybean oil presented promising results in heterogeneous catalysis, with reaction yields as high as 89%. The influence of variables such as temperature, reaction time and the amount of catalyst in the reaction yield was also evaluated. The possibility of recycling tin oxide was also studied, showing that it was possible to reuse the catalyst up to ten times without significant losses in its catalytic activity. © 2010 Elsevier B.V. All rights reserved. 1. Introduction The growing concern around the world regarding fuel supply security as well as the emission of polluting and green house gases stimulates the search for new routes to produce liquid fuels [1]. In this context, biofuel production from renewable sources has been pointed out as an elegant alternative to partially replace petroleum-derived fuels, being biodiesel, a mixture of different methyl or ethyl fatty acid esters, the most accepted choice for diesel engines [2]. Indeed, some examples of the several advantages of the use of this type of biofuel may be cited, particularly the reduction on the emission of sulfur (SO x ) and carbon (CO and CO 2 ) compounds while burning biodiesel in diesel engines and the possibility to use it neat or blended with diesel without any mechanical change [3]. As a result, as in several other countries around the world, the Brazilian government's incentives to biodiesel production gave a new boost in the search for different synthesis routes and the development of new catalysts, especially after the creation of a National Program of Biodiesel Production (PNPB) [4]. Biodiesel can be produced by transesterification (or alcoholysis) of oils and fats (from vegetable or animal sources) or by esterification of fatty acids. The alcoholysis of oils and fats by homogeneous basic catalysis using mono-alcohols (usually methanol and ethanol) producing methyl or ethyl esters and glycerin, shown in Fig. 1, is the most commonly used process for biodiesel production [5]. However this biodiesel production using alkaline basic catalysts has some disadvantages [6,7], such as: (i) the presence of free fatty acids or water in vegetable oils can be an obstacle to biodiesel production due to soap formation; hence, high-quality raw materials are required; (ii) large amounts of glycerin as well as mono- and di- acylglycerides may be formed as by-products, hampering the production of biodiesel with desirable purity. The esterification process seems to be a good solution to some of these problems. In this reaction, shown in Fig. 2, there is no formation of glycerin or other glyceride products. It can also be performed using low-cost stocks as raw material, such as acid grounds produced during edible oil neutralization or fats and oils with high amount of free fatty acids, such as those produced by algae, palm-trees or animals. Many studies report on catalytic systems based on Brönsted or Lewis acids, in both homogeneous and two-phase heterogeneous conditions, as suitable candidates for esterification of fatty acids [6–16]. Homoge- neous systems with strong mineral acids are often related to equipment corrosion problems and difficulty in separating from the products. On the other hand, heterogeneous systems have lower yields because of difficulties during reagents phase transition. Therefore, the challenge is to develop a heterogeneous acid catalyst for the esterification process that combines high activity, easy separation from the products and no corrosiveness. The aim of this work is the study of esterification reactions of fatty acids obtained from soybean oil with methanol, assisted by Lewis acid solid metal oxides such as alumina, tin oxide and doped alumina, which are known to be Lewis acid catalysts for transesterification reactions [17,18]. The influence of different reaction parameters (temperature, time, and amount of catalyst) in the reaction yield was Fuel Processing Technology 92 (2011) 53–57 ⁎ Corresponding author. Tel.: +55 61 31073852. E-mail address: psuarez@unb.br (P.A.Z. Suarez). 0378-3820/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.fuproc.2010.08.019 Contents lists available at ScienceDirect Fuel Processing Technology journal homepage: www.elsevier.com/locate/fuproc