Vapour phase esterification of butyric acid with 1-pentanol over Al-MCM-41 mesoporous molecular sieves A. Palani, M. Palanichamy, and A. Pandurangan* Department of Chemistry, Anna University, Chennai 600 025, India Received 4 February 2007; accepted 14 February 2007 Al-MCM-41 molecular sieves with Si/Al ratios 25, 50, 75 & 100 were synthesized hydrothermally and characterized systematically by various analytical and spectroscopy techniques. Their catalytic activity was evaluated for the vapour phase reaction of butyric acid with 1-pentanol. Pentyl butyrate was obtained as the only product. Reaction parameters such as temperature, molar ratio and feed rate were optimized for higher butyric acid conversion. The time-on-stream study was carried out at optimum conditions resulting in gradual decrease in the activity of the catalyst. KEY WORDS: esterification; Al-MCM-41 molecular sieves; 1-pentanol; butyric acid; pentyl butyrate. 1. Introduction Esterification of carboxylic acid with alcohols is industrially carried out using homogeneous acid catalyst viz., mineral acids, metal hydroxides and metal chlorides [1]. Organic esters find extensive application in per- fumery, solvents, flavours, pharmaceuticals, plasticizer and chiral auxiliaries [2]. The traditional industrial process of synthesizing esters using homogeneous acid catalyst is conveniently replaced by solid acid catalyst, ion exchange resins, clay etc [3–9]. Esterification reac- tions have been already reported over solid acid catalyst such as Nafion-H [10], zeolite Y [11, 12] and oxide cat- alyst [13–15]. In heterogeneous esterification, a per fluorinated resin sulponic acid gave high yields of vari- ous esters in the gas–solid system [10]. But these resins are expensive. It is well known that the esterification is a reversible reaction catalyzed by an acid environment and usually performed in the liquid phase. In order to drive the reaction towards the desired product, the excess amount of alcohol is used or the by-product water is removed continuously. Chen et al. [16] carried out the reaction between 3-methyl-1-butanol and buty- ric acid using sodium hydrogen sulfate as catalyst and toluene as solvent. Mao et al. [17] reported synthesis of pentyl butyrate using ZnCl 2 as the catalyst. It is a well- known fact that the reaction is thermodynamically favoured when performed in the vapour phase due to the higher values of equilibrium constants in com- parison with those of the liquid phase [18]. Further advantages of the vapour phase esterification are to prevent corrosivity of equipment, to save energy and to reduce the cost of materials as well as to simplify tech- nological process. When the reaction is carried out in vapour phase, there is no need to use a solvent. The MCM-41 materials have high thermal stability and large surface area, which lead to good adsorption capacity for organic molecules. Acid sites can be generated by the incorporation of aluminum in the framework. For these reasons, in our present study, we carried out esterifica- tion of butyric acid with 1-pentanol over Al-MCM-41 molecular sieves. 2. Experimental 2.1. Materials All the reagents viz., sodium meta silicate, aluminium sulphate, cetyltrimethylammonium bromide (CTAB), sulfuric acid and butyric acid were purchased from Merck and used as such. 1-Pentanol (Merck) was dis- tilled and used. 2.2. Synthesis of Al-MCM-41 Al-MCM-41 molecular sieves (Si/Al = 25, 50, 75 & 100) were synthesized hydrothermally as follows. Sodium meta silicate and Aluminium sulphate were used as the source for Si and Al respectively. These meso- porous materials were crystalised by autoclaving the gel composition SiO 2 ; x Al 2 O 3 ; 0.2 CTAB; 0.89 H 2 SO 4 ; 120 H 2 O(x various with Si/Al ratios). The as synthes- ised sample was calcined at 550 ° C in air for 6 h. *To whom correspondence should be addressed. E-mail: pandurangan_a@yahoo.com Catalysis Letters, Vol. 115, Nos. 1–2, May 2007 (Ó 2007) 40 DOI: 10.1007/s10562-007-9073-1 1011-372X/07/0500–0040/0 Ó 2007 Springer Science+Business Media, LLC