5–5 Lignin Linkage Cleavage over Ru: A Density Functional Theory Study
2021
Li, Qiang | Vlachos, D. G. (Dionisios G.)
Lignin is the most abundant natural, aromatic-containing biopolymer. Among all the C–C and C–O bonds being cleaved in catalytic fractionation, the 5–5 linkage is the strongest, and its scission requires harsh conditions. Theoretical investigations of the mechanism and kinetics could provide insights into developing better catalysts but are essentially lacking. We perform extensive density functional theory calculations on 2-methoxy-1,1′-biphenyl, a model compound, with various substitutions at all ring locations on Ru(0001). We analyze the competition between the 5–5 bond cleavage and the defunctionalization of the side functional groups at multiple degrees of depolymerization. The role of ring functional groups in the adsorption of lignin oligomers and the 5–5 bond scission and, conversely, the effect of the aromatic group on the −OCH₃ decomposition are also discussed. We show that increasing the number of methoxy groups decreases the C–C barrier, and thus, we expect the following depolymerization ranking: grass > softwood > hardwood. While Ru exposes modest 5–5 bond scission reaction barriers from some intermediates, rapid side group chemistry prevents the formation of these intermediates; instead, scission happens most probably from defunctionalized compounds whose C–C scission barriers are high. Our results also expose the existence of multiple Brønsted–Evans–Polanyi relations in the catalytic transformation of biphenyl-based molecules that open up the possibility of modeling depolymerization of large lignin chains.
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