Selective cleavage of aryl ether bonds in dimeric lignin model compounds

Lignin is an abundant renewable feedstock with a complicated and ill-defined structure. As beta-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 beta-O-4 aryl ether bond for efficiently utilizing this biomass. Selective cleavage of chemical bonds in beta-O-4 lignin model compounds was investigated by using Fe-2(SO4)(3), HZSM-5 and Pd/C as catalysts under microwave irradiation. When Fe-2(SO4)(3) or HZSM-5 was used as a catalyst, the C-alpha-C-beta 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 beta-O-4 aryl bond of the non-phenolic model compound was selectively cleaved and hydrogenation of C = C on the side chain occurred at the same time. However, the hydrogenation reaction of C = C on the side chain was faster than that of cleavage of the ether bond. Increasing Pd content favored the selective cleavage of the beta-O-4 bond, and microwave irradiation accelerated the cleavage of the beta-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 beta-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 beta-O-4 bonds of the non-phenolic and phenolic model compounds were 120 and 100 degrees C, respectively.