A silica-supported palladium oxide catalyst (PdO@MCM-41) selectively cleaves ether linkages in lignin model compounds...

Lignin is a potential renewable feedstock for the production of aromatic chemicals but due to the recalcitrant nature of its aryl ether bonds (C-O), and recondensation of depolymerized products, it is challenging to produce aromatic compounds with selectivity in high yield. Here we present that a heterogeneous catalyst containing highly dispersed palladium oxide (PdO) particles supported on mesoporous silica (MCM-41) catalyzes oxidant-free oxidation (dehydrogenation) of hydroxyl group at α-carbon of β-O-4 linkage in lignin model compounds and alkali lignin. The catalyst was synthesized via a molecular approach utilizing molecular designed dispersion of palladium diketonate complex followed by calcination. The oxidized lignin models provide high individual yields of monomeric products such as phenol (97%) at moderate temperature (120 °C) through intramolecular hydrogen transfer in green solvents (ethanol and water). The process, therefore, doesn't require any external oxidant, or reductant for cleaving the most abundant β-O-4 linkage of lignin model compounds and tolerates electron donating or withdrawing substitutes at the benzene ring. The approach was successfully extended to alkali lignin where 89% of lignin oil was produced from alkali lignin containing high yield (26 wt%) of monomeric products such as vanillin (2 wt%), benzaldehyde (12 wt%) and benzoic acid (12 wt%).