Journal of Catalysis 215 (2003) 57–65 www.elsevier.com/locate/jcat Structure–reactivity correlations in sulphated-zirconia catalysts for the isomerisation of α -pinene Muriel A. Ecormier, a Karen Wilson, b and Adam F. Lee a,∗ a Department of Chemistry, University of Hull, Hull HU6 7RX, UK b Department of Chemistry, University of York, Heslington, York YO10 5DD, UK Received 5 August 2002; revised 7 October 2002; accepted 4 November 2002 Abstract A range of mesoporous sulphated zirconias with tuneable structural and catalytic properties have been prepared by direct impregnation. The surface sulphate coverage can be readily varied, achieving a maximum value of ∼ 0.2 monolayers. High-temperature calcination induces the crystallisation of tetragonal zirconia while suppressing the monoclinic phase and enhances surface acidity. Superacid sites only appear above a critical threshold SO 4 coverage of 0.08 mL (corresponding to 0.44 wt% total S). Sulphated zirconias show good activity towards α-pinene isomerisation of under mild conditions. Conversion correlates with the number Brønsted acid sites, while the selectivity towards mono- versus polycyclic products depends on the corresponding acid site strength; superacidity promotes limonene formation over camphene.  2003 Elsevier Science (USA). All rights reserved. Keywords: Solid acid catalysts; Clean technology; Green chemistry; Zirconia; α-pinene 1. Introduction The syntheses of many fine and speciality chemicals often rely on homogeneous mineral acids, bases, or metal salts, which are frequently used in stoichiometric amounts. Tightening legislation on the treatment and disposal of excessive toxic waste, produced during the separation and neutralisation of products from these reaction media, is driving industry to consider cleaner technologies, including the use of heterogeneous catalysis. Of particular concern are the wide range of liquid-phase industrial reactions which rely on the use of inorganic or mineral acids. While many of these processes are catalytic, some require stoichiometric amounts of acid (e.g., acylation using AlCl 3 ). While zeolites are widely employed as solid acid catalysts in gas-phase chemistry, their application in liquid-phase organic synthesis is limited by their small pore sizes (< 8 Å), which make them unsuitable for reactions involving bulky substrates. However, recent developments in materials chemistry have led to the discovery of the M41S family of mesoporous molecular sieves [1,2] offering pore sizes in the range * Corresponding author. E-mail address: a.f.lee@chem.hull.ac.uk (A.F. Lee). 20–100 Å and thus new avenues for liquid-phase solid acid catalysis. In contrast to liquid acids, which possess well-defined acid properties, solid acids may contain a variety of acid sites. Generally they are categorised by their Brønsted and/or Lewis acidity, the strength and number of these sites, and the textural properties of the support (surface area and porosity) [3]. The synthesis of pure Brønsted and pure Lewis acid catalysts has attracted great academic interest, although the latter has proven more difficult because Brønsted acidity often arises from Lewis acid–base complexation [4]. In order to achieve high selectivity towards the desired products in a synthesis all these properties must be considered and if possible controlled. For example, acetal formation and hydrolysis reactions generally require medium-acid- strength sites, while electrophilic additions of alcohols or water to olefins, skeletal rearrangements, and esterification and alkylation reactions require strong acid sites. The importance of the type of acid site has likewise been shown for Friedel Crafts alkylation reactions, where Lewis acid sites are required for toluene alkylation by benzyl chloride, while Brønsted sites are preferred for reactions using benzyl alcohol [5]. There has been much interest in the use of sulphated metal oxides as strong solid acids, in particular sulphated 0021-9517/03/$ – see front matter  2003 Elsevier Science (USA). All rights reserved. doi:10.1016/S0021-9517(02)00150-1