Recent advances in the preparation of high-value-added chemicals by catalytic hydrogenolysis of lignin

The development and utilization of renewable biomass resources is an effective way to achieve CO₂ reduction. Biomass has a complex structure with low overall reactivity and utilization. Lignin is the only renewable aromatic polymer with high energy density in nature, and its conversion and utilization have attracted much attention worldwide. However, the complexity of the lignin structure, the uncertainty of the linkages, the stability of the side-chain connections, and the inevitable recondensation of the reactive fragments make the depolymerization of lignin into biofuels or aromatic chemicals a formidable challenge. Catalytic hydrogenolysis technology converts lignin into highly selective, high-yield phenolic monomers with high heating value, low oxygen content, and high carbon utilization of the product. However, the mechanism of the conversion between products and structures remains unclear with respect to the directed bond shearing during the catalytic depolymerization of lignin. In this paper, in view of the latest research progress on the catalytic hydrogenolysis of lignin for the production of high-value chemicals. We focus on the catalytic hydrogenolysis of lignin to summarize the coupling correlation between the catalysts and their products of high-value chemicals and focus on the influence of different catalyst systems on the process mechanism of lignin depolymerization products. For metal-based catalysts in particular, a detailed review of recent advances in the effects of noble metal-based catalysts, transition metal-based catalysts, hydrotalcite catalysts, and metal-organic framework catalysts on product distribution is presented. And further summarized the problems and conversion mechanisms of different catalysts. Meanwhile, the solvent in the lignin catalytic hydrogenolysis cracking process is the key to promote lignin dissolution, accelerating the heat and mass transfer, and promoting the homogeneous dispersion of reactants and catalysts in the reactor. In this paper, the main solvents for lignin liquefaction, such as water, alcohol, and new solvent systems, are reviewed for their depolymerization impact on lignin. And further, outline the effect of the solvent system on the properties of lignin conversion products. Nevertheless, there are still many difficulties in the catalytic hydrogenolysis of lignin for the preparation of high-value chemicals. The complexity of the macromolecular structure of lignin, the directed depolymerization of the C−O and C−C structures is still difficult, and the preparation of efficient catalysts as well as the mechanism of directional regulation of the products are still to be further investigated. Due to the insolubility of lignin, no solvent system that can completely dissolve lignin has been found yet; secondly, the research on the solvent effect is still only in the preliminary exploration stage. Novel technology for favorable conversion of lignin is still only at the stage of laboratory research. And the efficient conversion of renewable lignin into valuable chemicals and fuels is of great significance in solving the energy crisis and slowing down global warming, and at the same time, it will help our country to realize the energy-dependence transition from oil to renewable biomass. So finally, the opportunities and challenges facing the field are summarized and outlooked, providing a theoretical reference for efficient targeted conversion and high-value utilization of lignin.