Catalysis Today 223 (2014) 115–121 Contents lists available at ScienceDirect Catalysis Today jou rn al hom epage: www.elsevier.com/locate/cattod Transesterification of short chain esters using sulfonic acid-functionalized hybrid silicas: Effect of silica morphology Maria Luisa Testa ∗ , Valeria La Parola, Anna Maria Venezia Istituto per lo Studio dei Materiali Nanostrutturati, UOS-PA CNR, via U. La Malfa 153, I-90146 Palermo, Italy a r t i c l e i n f o Article history: Received 10 April 2013 Received in revised form 24 July 2013 Accepted 11 September 2013 Available online 15 October 2013 Keywords: Transesterification Acid catalyst Sulfonic-acid silica Mesostructured materials a b s t r a c t Sulfonic acid-functionalized hybrid silicas with different structure (amorphous, HMS, SBA-15) were syn- thesized by different methodologies, with a variable amount of organic moieties (propyl SO 3 H). The obtained catalysts, characterized by X-Ray photoelectron spectroscopy, low angle X-Ray diffraction, N 2 adsorption and acid capacity measurements, were tested in the transesterification reaction of short chain esters (from hexanoic to lauric ethyl ester). The optimized reaction was carried out under mild condition in the presence of 15% mol of the corresponding organic acid in order to reproduce a typical low-grade oil. A correlation between the catalytic activity of the materials and their acid capacities was found. The propyl-sulfonic SBA-15 catalyst presented the best performance in terms of activity and structural stability with no leaching of the sulfonic groups. © 2013 Elsevier B.V. All rights reserved. 1. Introduction Transesterification and esterification are industrially important reactions used in several fields, among them, biodiesel production. Biodiesel is a renewable, biodegradable and non-toxic fuel. The first generation biofuels using the edible parts of food plants as feedstock are considered not fully sustainable and with a negative impact on the food supply. On the other hand, second generation biofuels using a low-grade oil feedstock, typically waste cooking oil, represent a greener alternative. However, differently from the refined oil containing less than 0.5% of free fatty acids (FFAs), the low-grade oil has an FFA contents between 0.5–15% which leads to the undesired soap formation [1]. In this case, a pretreatment, including esterification, is necessary to reduce the amount of acids. Most of the industrial processes employ homogeneous acid or basic catalysts. However, their use has several drawbacks. As concern the homogeneous basic catalysts, they must work with refined oil poor in FFA (<0.5%) to avoid soap formation and deactivation of the cata- lyst. As concern the homogeneous acid catalysts, they promote the esterification of FFA and transesterification simultaneously, thus avoiding soap formation, but they are very corrosive and toxic for the environment and are difficult to separate and recover. Employ- ing new and sustainable heterogeneous solid acid catalysts for ∗ Corresponding author at: Istituto per lo Studio dei Materiali Nanostrutturati ISMN-CNR, via U. La Malfa 153, I-90146 Palermo, Italy. Tel.: +39 0916809253; fax: +39 0916809399. E-mail address: marialuisa.testa@ismn.cnr.it (M.L. Testa). biodiesel production facilitates the separation, regeneration and recycling of the catalyst and reduces corrosion problems [2]. A large number of heterogeneous solid acid catalysts, includ- ing modified inorganic oxide such as zirconium [3], titanium [4] and tin [5] oxides, sulfonic ion-exchange resins [6] and heteropoly- acids, [7] have been reported in the literature for transesterification reactions. Special attention has been devoted to those catalysts containing sulfonic groups in particular organosulfonic-silica solids that have been proved to be active at moderate temperatures and pressure [1,8]. According to recent reports [9,10], an effi- cient solid acid catalyst requires large pores to minimize diffusion problems, high concentration of acid sites, high catalytic stability against leaching and poisoning and the possibility to modify the hydrophobicity of the surface to promote the preferential adsorp- tion of substrates and the repulsion of highly polar compounds which could cause deactivation. Due to their high surface area and controlled porosity it is becoming very attractive to use ordered mesoporous silicas (e.g., MCM-41, HMS, SBA-15) as supports [11] for various catalytic applications. The pore sizes, larger than those in zeolites, can be adjusted in the nanometer range by the appro- priate choice of the surfactant templates, making these systems suitable to host large molecules. Many reviews have covered sev- eral aspects of the mesoporous materials related to their synthesis, surface modification and practical applications [12–14]. The main problems associated with such materials consists in their poor hydrothermal stability and lower reactivity as compared to zeo- lites [15]. Among ordered mesoporous silicas HMS and SBA-15 have quite different structure: whereas the former has a wormlike porous structure the latter has an ordered close packed hexago- nal arrangement of the pores forming straight channel. Moreover, 0920-5861/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cattod.2013.09.029