Catalytic Transfer Hydrogenation of Biomass-Derived Substrates to Value-Added Chemicals on Dual Functional Catalysts: Opportunities and Challenges.

Aqueous phase hydrodeoxygenation (APH) of bio-derived feedstocks into useful chemical building blocks is one the most important processes for biomass conversion. But several technological challenges, such as elevated reaction temperature (220-280 oC), high H2 pressure (4-10 MPa), uncontrollable side reactions and intensive capital investment have bottlenecked further development of existing APH processes. Catalytic transfer hydrogenation (CTH) under much milder conditions using non-fossil based H2 have attracted extensive interests due to several advantageous features, including (i) high atom efficiency (~100%), (ii) low energy intensity, and (iii) green H2 obtained from renewable sources. Typically, CTH can be categorized as (a) internal H2 transfer (sacrificing small amounts of feedstocks for H2 generation) and (b) external H2 transfer from H2 donors (e.g., alcohols, formic acid). Although recent decade has witnessed a few successful applications of conventional APH technologies, CTH is still relatively new for biomass conversion. Very limited attempts have been made in both academia and industry. Understanding the fundamentals on precise control of catalyst structures is key for tunable dual functionality, combining simultaneous H2 generation and hydrogenation. Therefore, this review is focused on rational design of dually functionalized catalysts synchronizing H2 generation and hydrogenation of bio-feedstocks into value-added chemicals via CTH technologies. Most recent studies, published from 2015-2018, on transformation of selected model compounds including glycerol, xylitol, sorbitol, levulinic acid, hydroxylmethylfurfural, furfural, cresol, phenol and guaiacol will be critically reviewed in this article. The relationship between nanostructures of heterogeneous catalysts and catalytic activity and selectivity for C-O, C-H, C-C and O-H bond cleavage, will be particularly discussed to provide insights into future design for atom economical conversion of biomass to fuels and chemicals.