In situ hydroprocessing of lignocellulosic biomass-derived molecules into fuels and chemicals using heterogeneous catalysts

Valorization of lignocellulosic biomass, an abundantly available renewable hydrocarbon source, to produce value-added chemicals, drop-in chemicals, and biofuels is indispensable, considering the limited sources of fossil fuels and their adverse environmental effects. The application of efficient heterogeneous catalysts is a promising method of sustainable biomass processing with low energy consumption and minimal waste generation. Hydroprocessing of biomass using a hydrogen source is considered to be a pivotal strategy for the synthesis of industrially important chemicals and fuels. As the use of molecular hydrogen gas results in several problems and requires harsh reaction conditions, the utilization of safe and clean liquid hydrogen carriers is preferred to address the economic and sustainable aspects of biorefinery. In this review, we address the significance of in situ hydrogen generation for biomass hydroprocessing. The primary focus of the review is the catalytic transfer hydrogenation (CTH) of lignocellulosic biomass-derived molecules using various liquid hydrogen carriers. Various heterogeneous catalysts, including bulk, nanosized, and single-atom catalysts, and the role of their BET surface area, pore size, particle size/morphology, surface chemistry, acid-base, and redox properties in biomass hydrogenation to obtain desirable products are meticulously discussed with the support of kinetic, mechanistic, and theoretical studies. The challenges associated with the in situ generation of hydrogen and its selective adsorption/activation on the catalyst surface for the CTH processing of biomass-derived molecules as well as the prospects for a rational design of novel heterogeneous catalysts and the utilization of new hydrogen carriers for biomass valorization are elucidated.