The Influence of Solvent on the Acidity and Activity of Supported Sulfonic Acid Catalysts
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Catalysis Letters Vol. 98, No. 4, December 2004 (Ó 2004)
The influence of solvent on the acidity and activity of supported sulfonic acid catalysts S. Koujout and D.R. Brown* Department of Chemical and Biological Sciences, University of Huddersfield, Huddersfield HD1 3DH, UK
Received 19 August 2004; accepted 26 August 2004
The acid strengths and catalytic activities of sulfonic acids supported on polystyrene resins and ordered mesoporous HMS and SBA-15 silicas are compared. Acid strengths are measured by acid–base titration calorimetry in terms of the molar enthalpies of neutralisation with either NaOH or n-butylamine in water, acetonitrile and cyclohexane. Catalytic activities (turnover numbers) are reported in model reactions in water, 1,2-dichlorobenzene and anisole, and compared with acid strengths. In water, sulfonated resins are both stronger acids and more active catalysts than sulfonated silicas. Catalytic activities in water correlate well with these measured acid strengths. In acetonitrile the order of acid strengths is reversed and the sulfonated silicas are the stronger acids. Catalytic measurements in 1,2-dichlorobenzene, a similar dipolar aprotic solvent, show the same reversed order of activities. In the non-polar solvent cyclohexane (where only macroporous sulfonated resins show measurable acidity) the sulfonated silicas are again the stronger acids but by a larger margin. Catalytic activities in anisole, which is also only very weakly solvating towards sulfonic acid groups, show a similar trend. The results illustrate the role of the solvent in controlling the acid strength of solid acid catalysts, and the importance of taking this into account when designing acid catalysts for liquid phase processes. KEY WORDS: acid catalysis; solid acid; ion-exchange resin; sulfonated silica; mesoporous molecular sieve; sulfonated polystyrene; titration calorimetry; enthalpy of neutralisation.
1. Introduction Sulfonic acids supported on polymeric resins are well known acid catalysts, widely used in MTBE, TAME, bis-phenol A and other syntheses [1–3]. Analogous materials in which sulfonic acids are supported on inorganic rigid solids such as porous silicas have only been reported recently [4–6] and, as far as we know, have not yet been adopted for industrial application. A comparison between sulfonic acids on these two very different supports provides the opportunity to observe directly the effect of the support on the acidity and catalytic activity of the supported acid group. In previous work we have shown how sulfonated polystyrene resin catalysts exhibit significantly higher acid strengths and specific catalytic activities (per acid group) than sulfonic acids supported on silica in the presence of water [7,8]. The implication of this result is that the way in which the solvent interacts with both the catalyst and the support might play an important role in controlling the catalytic properties of the acid groups. It therefore seemed important to examine the effects of other solvents on the relative acidities and a
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