Theoretical Study on Hydrogenolytic Cleavage of Intermonomer Linkages in Lignin

Hydrogenolysis is an important approach for depolymerization of lignin, which provides attractive new sustainable platforms of fuels, chemicals, and materials. The theory of lignin hydrogenolysis is, however, still unsound, which limits the development of this approach and causes inconsistencies among experimental studies. In this paper, density functional theory is employed to investigate the initial hydrogenolytic cleavages of recognized five different types of interaromatic unit linkages of lignin, assuming the presence of hydrogen free radicals. The relative free energies of reactant complexes, reaction free energy changes, and rate constants for candidate reactions are calculated comprehensively at 298-538 K. On the basis of the results of calculation and a rapid equilibrium hypothesis, the major reaction channel is decided for each linkage, and its kinetics is assessed. It is concluded that the hydrogenolysis occurs at β-O-4 ether, diphenyl ether 4-O-5', and β-1' diphenylmethane linkages instantaneously if these are accessible to hydrogen free radicals, while β-5 phenylcoumaran and β-β' pinoresinol linkages are virtually inert to hydrogenolysis.