α-Pinene isomerisation over heteropoly acid catalysts in the gas-phase

Abstract

The isomerisation of α-pinene was studied in the gas phase over solid heteropoly acid (HPA) catalysts in a fixed-bed continuous flow reactor at 200 °C and ambient pressure. The catalysts included bulk and SiO 2-supported H 3PW 12O 40 and bulk Cs 2.5H 0.5PW 12O 40 possessing strong Brønsted acid sites, as well as composites prepared by supporting 15 wt% H 3PW 12O 40 on Nb 2O 5, ZrO 2 and TiO 2 possessing both Brønsted and Lewis acid sites of moderate strength. The reaction yielded camphene as the main product in a mixture with monoterpene by-products such as limonene, terpinolenes, terpinenes, β-pinene, p-cymene and others. The HPA catalysts with strong Brønsted acid sites exhibited high initial activities, but suffered from catalyst deactivation, resulting in low camphene yields. Conversely, the HPA catalysts supported on Nb 2O 5, ZrO 2 and TiO 2, although weaker acids, showed more stable performance in α-pinene isomerisation. The HPA catalyst supported on TiO 2 gave a camphene yield of 51% and a total camphene and limonene yield of 58%.