Limonene oxide isomerization: a broad study of acid catalyst effects

In this work, a broad screening of possible catalysts for limonene oxide isomerization was performed. The catalysts included both Brønsted and Lewis acids as homogeneous or heterogeneous catalysts. The aim was to show the efficiency of different catalysts in the studied reaction and use one homogeneous (methanesulfonic acid) and two heterogeneous (zeolite beta-25, montmorillonite K10) catalysts to evaluate the influence of the acid site type on the selectivity. When the selected catalysts were used, almost total conversion up to 1 h was observed (2?50% of the catalyst was based on type, 80 °C, and 16.6 V% of the solution was limonene oxide in toluene). The strength of the acid/acid catalyst influenced the composition of the products. When a strong acid (methanesulfonic acid) was used, the products formed by dehydration (p-cymene) and isomerization of the double bond (carvenone) were detected in large amounts. Using the majority of catalysts, the main products were cis- and trans-dihydrocarvone, and the minor products were carveols and carvone. The water content of the acid catalyst strongly affected the selectivity. Limonene diol was formed as a hydration product, which decreased the selectivity for the desired products. We have described a wide spectrum of possible catalysts for limonene isomerization. Moreover, our study enables the simple choice of catalyst on the basis of the desired composition of the products.