Selective lignin depolymerization via transfer hydrogenolysis using Pd/hydrotalcite catalysts: model compounds to whole biomass

Cleavage of lignin ether bonds via transfer hydrogenolysis is a promising route to valorize lignin, thus processes that use mild reaction conditions and exploit renewable hydrogen donor solvents (rather than molecular hydrogen) are economically advantageous. Herein we demonstrate the efficient catalytic transfer hydrogenolysis and tandem decarbonylation of lignin model compounds possessing aromatic ether bonds (alpha-O-4, beta-O-4 and 4-O-5 linkages), over transition metal-modified Pd hydrotalcite catalysts with ethanol as the hydrogen donor and solvent. Quantitative conversions and yields were attained for all model compounds, except for 4-O-5 models, which possess inherently strong sp2 C-O bonds. The latter demonstrates the utility of Pd hydrotalcite catalysts for transfer hydrogenolysis of model compounds. This process was employed to achieve whole pine biomass delignification with 97% yield and a 22% phenolic monomer yield, with 64% selectivity for 4-(3-hydroxypropyl)-2-methoxyphenol.