11-41: Quantum Chemical Study of the Remarkably Diverse Linkages in Lignin

Lignin in plant cell walls is a potential renewable source of biofuels, chemicals, and value-added products. It consists of various aryl ethers, irregularly connected by a variety of linkages creating a complex structural network; hence, it is difficult to identify selective bond breaking events. In this study, we predict bond dissociation tendencies using the density functional theoretical (DFT) approach of a diverse set of lignin linkages encompassing 65 lignin model compounds with different substituents on the arene rings and aliphatic carbons connecting the rings. We have considered 33 ether-linked lignin model compounds from β-O-4, α-O-4, and 4-O-5 subtypes and 32 model compounds containing the subtypes of β-1, α-1, β-5, and 5-5 C-C linkages. Results from our study identify the weakest and strongest linkages connecting arene rings in different classes of lignin model compounds. The ether bond in the α-O-4 linkage type is the weakest linkage among the different ether lignin linkages studied in this investigation. The C-C bond of β-1 linkages has the lowest bonding strength among the lignin model compounds with C-C bond linkages. The ether linkage is substantially weakened when substituted by electron donating methoxy groups on the adjacent arene ring. The strength of C−C linkage is influenced by hydrocarbon, methyl, and hydroxyl group substitutions at the aliphatic carbon positions in the linkage. The reported results provide strategic avenues for delignification.