Insoluble heteropoly compounds as highly active catalysts for liquid-phase reactions

Abstract

Liquid-phase alkylation of m-xylene or trimethylbenzene with cyclohexene was studied using heteropoly compounds and other typical solid acids as the catalysts; the results are discussed in relation to the surface properties of the catalysts. Cs 2.5H 0.5PW 12O 40 (abbreviated as Cs 2.5-salt) was remarkably more active than H 3PW 12O 40, zeolites, Nafion-H, SO 4 2−/ZrO 2, H 2SO 4, and AlCl 3 HCl. Solid-state 31P NMR revealed that Cs 2.5-salt mainly consists of polyanions with one proton (Cs 2HPW 12O 40) and those without a proton (Cs 3PW 12O 40). The quantity of acid sites of Cs 2.5-salt was determined to be 161 μmol g −1 by temperature-programmed desorption (TPD) of NH 3, which was much greater than the estimated quantity of surface acid sites of H 3PW 12O 40, and less than that of zeolites and SO 4 2−/ZrO 2. The acid strength of Cs 2.5-salt measured by Hammett indicators and TPD of NH 3 was smaller than that of SO 4 2−/ZrO 2. It is presumed that the high activity of Cs 2.5-salt is brought about by its large number of surface acid sites and its acid—base bifunctional nature, in which a caesium ion possibly increases the basicity of the heteropoly anion.