Comparison of two multifunctional catalysts [M/Nb2O5 (M = Pd, Pt)] for one-pot hydrodeoxygenation of lignin

Abstract

Improved lignin valorisation via its catalytic conversion to value-added feedstocks is of essential importance to the development of future bio-refineries. The identification of active binding domains within a catalyst structure provides important insights into the function of catalysts. Vibrational analysis employing inelastic neutron scattering (INS) enables the full investigation of the dynamics of hydrogen in host-guest systems. Here, we report an in situ INS investigation of the catalytic origins of two multifunctional catalysts [M/Nb₂O₅ (M = Pd, Pt)] for the hydrodeoxygenation (HDO) of phenol, a key model compound for lignin. We also report the excellent activity of these two catalysts for the conversion of raw lignin into hydrocarbons showing (i) complete removal of the oxygen content; (ii) near-quantitative yields of C₇–C₉ products based upon lignin monomers and (iii) different reaction networks. The in situ INS study has revealed the molecular details directly underpinning the distinct reaction pathways. We found that on the Pd/Nb₂O₅ catalyst, the hydrogenation of phenyl rings has higher selectivity than the direct cleavage of C–O bonds owing to the strong binding of the phenyl ring to the catalyst surface and the high hydrogenation activity of Pd, while on the Pt/Nb₂O₅ catalyst, the HDO reaction undergoes a combination of a direct dehydroxylation (DDO) process and a tandem route with fast kinetics, leading to a higher selectivity for arenes than that over Pd/Nb₂O₅ at the beginning of the reaction.