Selective cleavage of aryl ether bonds in dimeric lignin model compounds

Abstract

Lignin is an abundant renewable feedstock with a complicated and ill-defined structure. As β-O-4 aryl ether bonds are dominant among all the linkages in lignin, it is important to explore lignin depolymerization targeting the cleavage of the β-O-4 aryl ether bond for efficiently utilizing this biomass. Selective cleavage of chemical bonds in β-O-4 lignin model compounds was investigated by using Fe₂(SO₄)₃, HZSM-5 and Pd/C as catalysts under microwave irradiation. When Fe₂(SO₄)₃ or HZSM-5 was used as a catalyst, the C_α–C_β bond of the C3 side chain in the model compound was broken to form aldehyde, secondary alcohol or ketone compounds. When Pd/C and formate were used as the catalyst, the β-O-4 aryl bond of the non-phenolic model compound was selectively cleaved and hydrogenation of CC on the side chain occurred at the same time. However, the hydrogenation reaction of CC on the side chain was faster than that of cleavage of the ether bond. Increasing Pd content favored the selective cleavage of the β-O-4 bond, and microwave irradiation accelerated the cleavage of the β-O-4 bond dramatically. At a high dosage of formate or high temperature, the condensation reaction among phenolic products was promoted due to the presence of an active phenolic hydroxyl group. The β-O-4 bond of the phenolic model compound was also selectively cleaved with Pd/C as the catalyst, and the reaction temperatures of cleaving about one half β-O-4 bonds of the non-phenolic and phenolic model compounds were 120 and 100 °C, respectively.