Catalyzed reactions of organic molecules at clay surfaces: Ester breakdown, dimerizations, and lactonizations

Abstract

The use of montmorillonites as Brønsted acid catalysts has been investigated for a range of reactions: (i) decomposition of cyclohexyl esters, (ii) dimerization of propenylbenzenes, and (iii) the lactonization of cyclooctene-5-carboxylic acid. The ester cracking can be accomplished with a wide range of ion-exchanged clays, with conversions of up to 98% after 7 hr at 140 °C (Al 3+-montmorillonite). Although the cracking rate is dependent upon the interlayer cations in the clay, at least some of the activity is due to sites on the external surfaces of the clay particles. The dimerization of anethole (1-(4-methoxybenzene)-prop-1-ene) is efficient when using either di- or trivalent cation-exchanged montmorillonites, e.g., 44% recoverable yield of metanethole is attainable in 1 hr at 126 °C compared with literature procedures giving 24% yields in 7 hr. For a second propenylbenzene, isohomogenol (1-(3,4-dimethoxybenzene)-prop-1-ene), dimerization could only be effected using the very acidic trivalent ion-exchanged montmorillonites. Yields were typically 60% after 5 hr at 69 °C. It is not apparent why there are large differences in reactivity between anethole and isohomogenol, which are closely related structurally. Cation-exchanged montmorillonites can also catalyze the conversion of cyclooctene-5-carboxylic acid initially to 4-cyclooctane carbolactone and then to a variety of other lactone products. The rate of consumption of the acid is similar with a variety of ion-exchanged clays, but the subsequent rate of isomerization is cation dependent. For the lactonizations the clay acid catalysts proved no better than liquid phase catalysts in producing a high percentage of one particular lactone.