Heterogeneously Catalyzed Reductive Depolymerization of Lignin to Value-Added Chemicals.

Lignin is an abundant renewable source of aromatics, but its complex heterogeneous structure poses challenges for its depolymerization and valorization. Heterogeneously catalyzed reductive depolymerization (HCRD) has emerged as a promising approach, utilizing heterogeneous catalysts to facilitate selective bond cleavage in lignin and hydrogen transfer to stabilize the products under mild conditions. This review provides a comprehensive understanding of the hydrogen transfer mechanisms in HCRD, involving different hydrogen sources, including molecular hydrogen, alcohols, formic acid, etc., and the native hydrogen donor groups in lignin. The interaction between hydrogen sources and catalyst design is explored, emphasizing how catalyst characteristics must align with specific hydrogen transfer pathways to optimize efficiency and selectivity. Precious metal-based and non-precious metal-based catalysts are examined, highlighting advances that enhance hydrogen activation and transfer. Comparative analyses of hydrogen sources reveal distinct advantages and limitations. The significance of HCRD in lignin valorization and the development of integrated biorefineries is underscored, emphasizing its potential to contribute to a sustainable bioeconomy through improved process integration and economic viability.