A review on reactivity and stability of heterogeneous metal catalysts for deoxygenation of bio-oil model compounds

Catalytic deoxygenation is a fundamental process for bio-oil upgrading due to its high oxygen content which will result in lower heating value, corrosion and instability issues. The discovery of an excellent heterogeneous deoxygenation metal catalyst with high deoxygenation activity is a necessary breakthrough for an optimized bio-oil catalytic deoxygenation. For an effective deoxygenation supported metal catalyst, properties such as high H₂ sticking coefficient, optimal metal-oxygen bond strength and suitable acid strength from support are needed to ensure facile scission of C–O bonds and activation of H₂ and O-containing compounds. Metals such as Fe, Ru, Sn, W, Zr and supports such as C, TiO₂, ZrO₂ which are oxophilic were also observed to enhance direct removal of oxygen from O-containing compounds due to their high C–O and C[dbnd]O bond affinities. The choice of support is important to ensure it has optimal physicochemical properties for facile deoxygenation and the optimal acid strength to enhance C–O hydrogenolysis activity while minimizing coke formation. The choice of metal is dependent on the type of model compound since different metals catalyze different reaction pathways of the deoxygenation of model compounds. This review presents on the use of heterogeneous metal catalysts in the deoxygenation of bio-oil model compounds through several perspectives which are catalytic properties, reaction conditions, deactivation and regeneration of metal catalysts. In addition, several outlooks on the feasible range of reaction condition for catalytic deoxygenation and criteria of excellent deoxygenation supported metal catalysts were also expressed in this article based on the studies on the literatures.