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

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 SiO2-supported H3PW12O40 and bulk Cs2.5H0.5PW12O40 possessing strong Brønsted acid sites, as well as composites prepared by supporting 15 wt% H3PW12O40 on Nb2O5, ZrO2 and TiO2 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 Nb2O5, ZrO2 and TiO2, although weaker acids, showed more stable performance in α-pinene isomerisation. The HPA catalyst supported on TiO2 gave a camphene yield of 51% and a total camphene and limonene yield of 58%.