Furfural hydrodeoxygenation (HDO) over silica-supported metal phosphides – the influence of metal - phosphorus stoichiometry on catalytic properties

The gas-phase hydrodeoxygenation (HDO) of furfural, a model compound for bio-based conversion, was investigated over transition metal phosphide catalysts. The HDO activity decreases in the order Ni ₂P ≈ MoP > Co ₂P ≈ WP ≫ Cu ₃P > Fe ₂P. Nickel phosphide phases (e.g., Ni ₂P, Ni ₁₂P ₅, Ni ₃P) are the most promising catalysts in the furfural HDO. Their selectivity to the gasoline additives 2-methylfuran and tetrahydro-2-methylfuran can be adjusted by varying the P/Ni ratio. The effect of P on catalyst properties as well as on the reaction mechanism of furfural HDO were investigated in depth for the first time. An increase of the P stoichiometry weakens the furan-ring/catalyst interaction, which contributes to a lower ring-opening and ring-hydrogenation activity. On the other hand, an increasing P content does lead to a stronger carbonyl/catalyst interaction, i.e., to a stronger η ²(C, O) adsorption configuration, which weakens the C1[sbnd]O1 bond (Scheme 1) in the carbonyl group and enhances the carbonyl conversion. Phosphorus species can also act as Brønsted acid sites promoting C1[sbnd]O1 (Scheme 1) hydrogenolysis of furfuryl alcohol, hence contributing to higher production of 2-methylfuran.