Heteropoly acid catalysts for limonene oxide isomerization: the correlation of catalyst acid strength to activity and selectivity

Abstract

Limonene extracted from plants and obtained from sugar fermentation can be converted into a broad range of chemicals for fuels, fragrances, pharmaceuticals, and novel polymers. A limonene oxide isomer, dihydrocarvone, has been widely used in the food, cosmetics, agrochemical, and pharmaceutical industries. Herein, acid strength effects on limonene oxide isomerization were investigated with a wide range of Brønsted acid catalysts using Keggin heteropoly acids (HPAs) and organosulfonic acids. Temperature-programmed desorption of 2,6-di-tert-butylpyridine (DTBP) was used to measure surface proton density and acid strength of the catalysts. A good correlation of surface acid strength represented by the DTBP desorption temperature with the turnover rate calculated using the amount of DTBP chemisorbed was obtained. Further, good correlations of acid strength, represented by desorption temperatures of ammonia and DTBP, to both the turnover rate and dihydrocarvone selectivity were established. This correlation held for the bulk and supported HPA catalysts and organosulfonic acid catalysts, thereby indicating that the turnover rate and dihydrocarvone selectivity are primarily determined by catalyst acid strength, regardless of the catalyst porosity and support surfaces. The findings not only provide an efficient strategy for upgrading a biomass derivative, limonene oxide, but also present an advancement in the selective characterization for Brønsted acid sites of HPA-type catalysts.