Journal of Molecular Catalysis A: Chemical 223 (2004) 143–147 Esterification by solid acid catalysts—a comparison S. Shanmugam, B. Viswanathan ∗ , T.K. Varadarajan Department of Chemistry, Indian Institute of Technology, Chennai 600036, India Received 15 July 2003; received in revised form 14 February 2004; accepted 25 February 2004 Available online 12 September 2004 Abstract The esterification of triethylene glycol (TEG) with methacrylic acid (MAA) was carried out over various solid acid catalysts and the results were compared with conventional acid catalysts. Among the catalysts studied, heteropolyacids showed higher activities than the conventional acids. These were attributed to the acidity of the catalysts. A systematic investigation was carried out for the esterification of triethylene glycol with methacrylic acid over phosphotungstic acid without any free radical inhibitor. The effects of catalyst concentration, alcohol/acid molar ratio and temperature were also studied. Parameters such as activation energy, reaction enthalpy and reaction entropy were derived for the above reaction over phosphotungstic acid in the temperature range of 353–383 K. The obtained results were also compared with silicotungstic acid. © 2004 Elsevier B.V. All rights reserved. Keywords: Esterification; Solid acids; Heteropolyacids; Activation energy; Multifunctional methacrylate 1. Introduction The drive to develop green processes has led to the de- velopment of solid acid catalysts to a significant increase in research activities both in academic and industrial sec- tions. These materials can replace the corrosive liquid acids currently used in many industries. A variety of materials have been used as solid acid catalysts such as clays, zeo- lites, sulfated metal oxides, heteropolyacids, etc. [1,2]. Each of these materials offers unique properties that can influence the catalytic activity. Among these solid acids, heteropoly compounds are unique in the sense, they can be tuned at atomic/molecular level to exhibit a wide variation on proper- ties like acidity and redox behavior [3–5]. They behave like mineral acids having the protons in protected environments and exhibit Brønsted acidity, which of several orders of mag- nitude higher in strength than that of conventional mineral acids [6]. In the recent years, heteropoly compounds with Keggin structures have received much attention as solid acid cata- ∗ Corresponding author. Tel.: +91 44 2257 8250; fax: +91 44 2257 8250. E-mail address: bviswanathan@hotmail.com (B. Viswanathan). lysts for both homogeneous and heterogeneous reactions due to their high acid strength and high thermal stability [7–17]. Augustine et al. utilized heteropoly compounds as anchoring agents to heterogenize the homogeneous catalysts [18–20]. The protons in the secondary structure of the heteropoly acids can be easily exchanged, completely or partially, with differ- ent cations without affecting the primary Keggin structure. These materials exhibit the so called pseudoliquid phase be- havior, where polar molecules like water, alcohols, amines enter into the bulk, expanding or contracting the distance be- tween the Keggin anions in the crystal lattice, while nonpolar molecules such as hydrocarbons just adsorb on the surface with out entering the bulk [21]. This is because the protons present in the heteropolyacid form hydrogen bonds with the water present in the crystal structure and exist as guarded pro- tons, H 3 O + or H 2 O 5 + [22]. The water molecules can be re- placed by incoming polar molecules and thus get protonated stiochiometrically. The isolated protons in heteropolyacids are delocalized in individual polyanions as well as neighbor- ing polyanions at 10 3 to 10 6 times faster than the transforma- tion of the organic molecules [23]. This delocalization is an unique property of heteropolyacids. In the case of zeolites, the acidic protons are bound in the proximity of Al O Si 1381-1169/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.molcata.2004.02.030