Lignin is a primary component of lignocellulosic biomass and has a large potential for valorization due to its aromatic nature. Even so, the industrial utilization of lignin trails both cellulose and hemicellulose. One of the steps toward valorizing lignin is through its depolymerization to smaller aromatic building blocks, which can be achieved through fast pyrolysis. Understanding the reaction mechanism and kinetics that govern the thermal degradation of lignin is necessary to optimize conversion processes and make lignin valorization a reality. The complex and variable nature of lignin makes it difficult to investigate degradation at the polymer scale mechanistically. Instead, density functional theory (DFT) has been employed to investigate the mechanism of smaller, model lignin monomers and dimers. Previous studies have primarily focused on major linkages in lignin, such as the β-O-4 linkage. Several less prominent linkages have not received much investigation. Our goal is to bridge the gap by investigating the less understood β-β and 4-O-5 linkages.<br/>In this work, we investigated two model tetramers containing β-β, 4-O-5, and β-O-4 linkages. The β-β and 4-O-5 linkages make up approximately 3-12% and 2% of lignin linkages, respectively. This work provides detailed insight into the degradation mechanism, through homolytic cleavage, of these two linkages via bond dissociation enthalpies (BDE). A Monte Carlo conformational analysis identified the lowest energy conformers of each relevant stereoisomer. Geometry optimization and frequency analysis of both the tetramers and the homolytic cleavage products were calculated at the M06-2X/6-311++G(d,p) level of theory. The BDEs of each relevant bond scission along the interunit linkages provide a comparison point to determine the cleavage hierarchy between several linkage types in a more representative lignin substructure.

Investigation of the thermal deconstruction of β-β and 4-O-5 linkages in lignin model oligomers by density functional theory

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