An FT-IR study on Diels±Alder reactions catalysed by heteropoly acid containing sol±gel silica A Â . Kukovecz * , Z. Ko Ânya, I. Kiricsi Department of Applied and Environmental Chemistry, University of Szeged, Rerrich B ter 1, H-6720 Szeged, Hungary Received 31 August 2000; revised 27 November 2000; accepted 27 November 2000 Abstract Following the general trend of using heterogeneous catalysts whenever possible, the replacement of traditional homogeneous Lewis acid catalysts for the Diels±Alder reaction is currently considered in the literature. In this contribution, we report the successful application of sol±gel derived tungstophosphoric acid±silica composites as catalysts in the reaction of 1,3-cyclo- hexadiene and 2-propenal. The reaction was monitored by in situ FT-IR spectroscopy following the changes in the ®ne structure of the n (CyO) band. The catalytic activity of the sol±gel immobilised heteropoly acids seems to be comparable with that of more conventional sol±gel silica±aluminas and their selectivity is even better because Broensted acid sites are destroyed upon calcination of the dry gel. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Diels±Alder reaction; FT-IR spectroscopy; Tungstophosphoric acid; Sol±gel method; Heterogeneous catalysis 1. Introduction The Diels±Alder type (4,2) cycloaddition reaction is a very useful tool of synthetic organic chemistry [1] because it makes it possible to prepare complex, opti- cally active molecules in one simple reaction step, traditionally performed using homogeneous Lewis acid catalysts [2]. However, following the general trend of replacing homogeneous acidic catalysts by their heterogeneous counterparts, quite a number of studies are currently focused on Diels±Alder reac- tions catalysed by solid organic networks [3], clays [4], zeolites [5] and immobilised heteropoly acids [6]. We have recently performed a detailed investiga- tion [7] on the acidity of silica±tungstophosphoric acid (H 3 PW 12 O 40 , abbrev.: HPWA) composites prepared by the sol±gel method and are currently developing a membrane reactor for heterogeneous Diels±Alder reactions. In this paper, we present FT- IR spectroscopic evidence for the applicability of HPWA±SiO 2 catalysts in the reaction of 1,3-cyclo- hexadiene and 2-propenal with results comparable to those achieved on conventional silica±alumina systems. The key differences between our studies and a similar work by Meuzelaar et al. [6] are that: (i) our catalysts are prepared in one-step sol±gel incorporation instead of impregnation; (ii) we attempt to utilise FT-IR spectroscopy for reaction monitoring instead of capillary GC and (iii) we have followed the reaction in the adsorbed phase, not in the gas phase. 2. Experimental Silica±HPWA composites containing 0, 5 and 20 wt% tungstophosphoric acid were prepared by the hydrolysis of tetraethyl orthosilicate (TEOS) in Journal of Molecular Structure 565±566 (2001) 121±124 0022-2860/01/$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S0022-2860(01)00459-8 www.elsevier.nl/locate/molstruc * Corresponding author. Tel./fax: 136-62-544-619. E-mail address: kakos@chem.u-szeged.hu (A. Kukovecz).