Hydrodeoxygenation of lignin-derived phenolics - a review on the active sites of supported metal catalysts

Hydrodeoxygenation (HDO) of lignin-derived phenolic compounds is one of the most promising strategies for the practical conversion of biomass materials to chemicals and fuels. Several consecutive and parallel reactions such as hydrogenation, hydrogenolysis, dehydration and others occur in HDO and give various products over a wide distribution, depending on reaction conditions and catalysts used. It is necessary, therefore, to optimize reaction conditions and choose suitable catalysts for the selective synthesis of target products. Frequently used catalysts are multifunctional supported metal ones, in which different functions come from inherent and cooperative properties of their metal and support components. This article mainly reviews the state-of-the-art achievements of supported metal catalysts and the characteristics and actions of their active sites, working in the reactions involved in HDO of lignin-derived phenolic compounds. We first survey the present research situation of HDO reactions with reaction conditions, most studied model substrates and heterogeneous catalysts and reaction mechanisms proposed so far. Then we focus on the functions of different active sites in supported metal catalysts, such as metal sites, acid sites (liquid or solid) and defective sites on oxide supports, which show their intrinsic (independent) and cooperative (synergistic) characteristics in HDO reactions.