Microwave-assisted catalytic transfer hydrogenolysis of lignin-derived aromatic ethers over Ru/C

Abstract

Cleavage of aromatic ether bonds through hydrogenolysis is one of the most promising routes for depolymerisation and transformation of lignin into value-added chemicals. Herein, we investigate the catalytic transfer hydrogenolysis (CTH) of benzyl phenyl ether (BPE), as a model molecule of α-O-4 lignin linkages, over commercial Ru/C under microwave irradiation using 2-propanol as H-donor. By tuning the reaction parameters, such as time, temperature and catalyst amount, BPE was fully converted, at 150 °C in 5 min of irradiation, with a related aromatics selectivity of 92%. Phenylethylphenyl ether (PPE), and diphenyl ether (DPE), as a models of β-O-4 and 4-O-5 linkages, respectively, were also considered to this scope and under optimized reaction conditions, >99% and 72% conversion, respectively, were achieved with >89% aromatics selectivity. Remarkably, catalytic tests with moderate conversions clearly showed that the cleavage of etheric C-O bond was the primary step under CTH conditions, while the hydrogenations of the aromatic rings take place in a subsequent step. Overall, the screening of the reaction parameters was crucial to avoid the hydrogenation of aromatic rings. No less significant was the Ru/C stability, which has been recycled at least six times without any noticeable activity loss.