Chemical Engineering Journal 134 (2007) 123–130 Synthesis of biodiesel with heterogeneous NaOH/alumina catalysts: Comparison with homogeneous NaOH G. Arzamendi a , I. Campo a , E. Argui˜ narena a , M. S´ anchez b , M. Montes b , L.M. Gand´ ıa a,∗ a Departamento de Qu´ ımica Aplicada, Edificio de los Acebos, Universidad P ´ ublica de Navarra, Campus de Arrosad´ ıa s/n, E-31006 Pamplona, Spain b Departamento de Qu´ ımica Aplicada, Facultad de Ciencias Qu´ ımicas de San Sebasti ´ an, Universidad del Pa´ ıs Vasco, P o Manuel de Lardiz´ abal 3, E-20018 San Sebasti ´ an, Spain Abstract The activity and selectivity of a series of NaOH catalysts supported on alumina for the transesterification with methanol of refined sunflower oil have been investigated. The performance of the supported catalysts has been compared with that of homogeneous NaOH. The influence of the molar methanol/oil and catalyst/methanol ratios and of the NaOH loading of the supported catalysts has been considered. The results have shown that the transesterification rate largely depends on the catalyst/methanol ratio, whereas the reaction selectivity seems to be governed by the methanol/oil molar ratio. Calcination of the NaOH/-Al 2 O 3 catalysts had a negative effect on their activity. On the other hand, the performance of the NaOH/alumina catalysts just dried at 393 K was not far of that of homogeneous NaOH. Measurements of sodium leached during the progress of the transesterification reactions have evidenced some lack of chemical stability of the supported NaOH catalysts under reaction conditions. Therefore, progress should be made to prepare more stable catalysts based on supported NaOH for this application. © 2007 Elsevier B.V. All rights reserved. Keywords: Biodiesel synthesis; Transesterification selectivity; Heterogeneous catalysts; Alumina 1. Introduction Biodiesel is a fuel obtained from renewable biomass feed- stocks that can be used in diesel engines as neat fuel or blended at various proportions with conventional fossil diesel fuel. It consists of mono-alkyl esters usually produced by transesterifi- cation with an alcohol (alcoholysis), most commonly methanol (methanolysis), of the triglycerides found in vegetable oils, ani- mal fats or waste cooking oils [1–4]. However, production costs are high, which is due to the high vegetable oil prices and some technological issues. As a result, biodiesel produced in the Euro- pean Union breaks even at petroleum prices around D 60 per barrel [5]. Most of the commercial biodiesel is produced from plant oils (mainly colza, soybean and sunflower) using very effec- tive homogeneous base catalysts such as NaOH or KOH, which are soluble in methanol. Nevertheless, these alkali compounds have some drawbacks: they must be neutralized giving rise to wastewaters and cannot be reutilized, they favour the forma- ∗ Corresponding author. Tel.: +34 948 169605; fax: +34 948 169606. E-mail address: lgandia@unavarra.es (L.M. Gand´ ıa). tion of stable emulsions making separation of the methyl esters (biodiesel) difficult, glycerol is obtained as an aqueous solution of relatively low purity and the reaction becomes very sensitive to the presence of water and free fatty acids. These problems can be alleviated with the use of heterogeneous transesterifica- tion catalysts. Therefore, there is an increasing interest in the possibility of replacing the homogeneous alkaline hydroxides, carbonates or metal alkoxides by heterogeneous solid catalysts insoluble in methanol that could potentially lead to easier refin- ing of the produced biodiesel and glycerol, recycling of the catalysts and hence, lower production costs [6]. As a matter of fact, Axens (Institut Franc ¸ais du P´ etrole Group Technologies), commercializes the Esterfip-H TM process, which is operated in the continuous mode and includes two successive fixed-bed reactors [6,7]. In recent years a considerable research effort is being devoted to the heterogeneously catalyzed methanolysis of vegetable oils. As a result, a great variety of catalysts such as alkaline-earth metals oxides and hydroxides [8–10], alkali metals (Na and K) hydroxides or salts supported on -alumina [11–13], zeolites [14], hydrotalcites [15] as well as some acid solids [16] have been checked to date at very different reaction conditions and with a variable degree of success. Nevertheless, it is generally 1385-8947/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.cej.2007.03.049