Impact of lignin–carbohydrate complex (LCC) linkages on cellulose pyrolysis chemistry

Abstract

Understanding the impact of cross-linked cellulose with lignin in lignin–carbohydrate complexes (LCC) on cellulose decomposition reaction kinetics and chemistry is challenging. This study combines first-principles molecular simulations and thin-film experiments to investigate key cellulose decomposition mechanisms, including transglycosylation, ring contraction, and ring opening, which lead to the formation of major bio-oil components (levoglucosan, 5-hydroxymethylfurfural, and glycolaldehyde). Ab initio molecular dynamics and metadynamics are employed to model LCC molecules with β-O-4 benzyl ether linkages at the C2, C3, and C6 carbon positions of cellobiose. Density functional theory (DFT) calculations are used to evaluate the reaction energetics of cellulose activation via these mechanisms. Activation barriers, reaction energies, and frontier molecular orbital interactions are compared between cellobiose with and without LCC, providing insights into the influence of LCC linkages. Experimental product yields from native herbaceous biomass pyrolysis are measured and compared to those from pure cellulose pyrolysis. The results demonstrate that cross-linked cellobiose in LCC exhibits higher activation barriers (2X) and reaction energies (3–4X) compared to pure cellobiose, indicating altered kinetics and thermodynamics. The differences within LCC conformers are minimal, except the blocking of the C6 position due to the LCC linkage. Analysis of HOMO–LUMO interactions reveals a spatial separation of reaction centers in LCC, indicating the favorability of inter-moiety mechanisms over intra-moiety mechanisms. This study underscores the novel role of covalent LCC bonding between lignin and carbohydrates in the reaction kinetics and chemistry of cellulose decomposition in native biomass and in the formation of major bio-oil products.