Reaction pathways and energetics of the deconstruction of lignin carbohydrate complexes (LCCs) in lignocellulosic biomass

Abstract

Current biomass fractionation technologies are not atom efficient due to the recalcitrant nature of native biomass. Although covalent linkages between lignin and cellulose/hemicellulose, commonly referred to as lignin carbohydrate complexes (LCCs), have been identified to significantly contribute to the refractory nature of biomass, little is known about their deconstruction pathways and chemistry. This work uses first-principles based computational methods to quantify the reaction mechanisms, kinetics and thermodynamics associated with the deconstruction of the prominent benzyl ether LCC linkages in biomass under acidic conditions. Competing reaction pathways, including the chemical degradation of lignin and hemicellulose moieties are also investigated. Acid catalyzed cleavage of LCC linkages demonstrated the lowest activation barriers but positive reaction free energies, rendering their cleavage thermodynamically less favorable. In contrast, the degradation of hemicellulose possessed higher activation barriers and the greatest thermodynamic feasibility. Evaluating the effect of temperature on the reaction energetics indicated that increasing the temperature during deconstruction would likely result in hemicellulose degradation becoming kinetically feasible before the cleavage of LCC linkages become thermodynamically facile. Hence, we suggest that cleaving benzyl ether LCC linkages selectively, to obtain lignin and carbohydrates in their chemically intact forms, thereby maximizing carbon recovery, is a thermodynamically controlled process. Thus, preserving the chemical and structural integrity of the biopolymers during the deconstruction of biomass will require a suitable solvent environment that is capable of establishing favorable reaction free energies for cleaving LCC linkages.