Improved hydrodeoxygenation of bio-oil model compounds with polymethylhydrosiloxane by Brønsted acidic zeolites

Bio-oil, one of significant renewable energy, was blocked from its direct application by thermodynamic instability due to the high oxygen content, so its deoxygenation needs urgent solution. Avoiding the disadvantages of the traditional catalytic method, such as harsh reaction conditions, potential explosion risk and environmental pollution, we utilize solid-acidic zeolites as low-load Pd-based catalyst supports instead of corrosive acid additives, with polymethylhydrosiloxane (PMHS) as the hydrogen-supplying agent, to achieve efficient hydrodeoxygenation of bio-oil models (carbonyl compounds) under mild conditions. The reaction conditions such as Si/Al ratio of HZSM-5 zeolite, temperature, solvent, and the type of Pd salts precursor are optimized. In particular, we have found that polar protic solvents improve catalytic efficiency by promoting proton transfer in the reaction. In an open-to-air, 97.9% ethylbenzene yield can be obtained for acetophenone conversion under mild conditions (0.5 wt% Pd/HZSM-5(18), 65 °C, 3 h, n-butanol as solvent), which is more efficient and environmental friendly than currently reports. Meanwhile, hydrogenation–dehydration mechanism was proposed, and the Brønsted acid in HZSM-5 promotes the dehydration of the alcohol (rate-limiting step) by efficiently accelerating the removal of hydroxyl groups and the proton transfer of the reaction. Furthermore, the catalytic scheme exhibits the excellent stability (reusable seven times) and versatility. The potential of a green catalytic technology using with PMHS opens attractive opportunities for bio-oil upgrading.