The Formation and Deconstruction of Lignin Carbohydrate Complexes (LCCs) in Lignocellulosic Biomass
Creators: Beck, Seth
Publisher: name: University of Alberta Library
Year: 2024

<p>Current biomass fractionation technologies are not atom efficient, which is primarily a result of its recalcitrance. Although covalent linkages between lignin and cellulose/hemicellulose, commonly referred to as lignin carbohydrate complexes (LCCs), have been identified to directly correlate with biomass recalcitrance, their formation mechanism and role in deconstruction remains largely speculative. Hence, in this thesis, the kinetics and thermodynamics of the reaction pathways resulting in the formation of the various types of LCC linkages (benzyl ether, benzyl ester and phenyl glycoside (PG)) are quantified. For the stable LCC linkages that are anticipated to contribute to the biomass’ recalcitrance the most, reaction mechanisms and energetics of the various deconstruction pathways are also computed. All-electron density functional theory (DFT) calculations were performed to quantify the reaction energetics as well as elucidate reaction pathways. </p>

<p>Independent of the source of biomass, the predominant lignin linkage is the β-O-4 linkage, which requires the re-aromatization of a quinone methide (QM) intermediate via electrophilic addition followed by nucleophilic addition. The electrophile is commonly assumed to be a proton from the acidic conditions of the plant cell wall and the nucleophile is assumed to be water. In such a case, both reaction sites would be terminated, and lignin would only interact physically with cellulose/hemicellulose. The present work explores the electrophilic and nucleophilic addition of hemicellulose, both of which would lead to LCC linkages as well as the nucleophilic addition of other monoglinols, leading to novel lignin linkages. The electrophilic addition of hemicellulose demonstrates a novel mechanism for PG formation and is kinetically facile and thermodynamically favoured (exergonic) over competing mechanisms at the reducing end of hemicellulose, thereby suggesting it is the likely mechanism resulting in the formation of PG linkages experimentally reported. The nucleophilic addition of hemicellulose is kinetically facile and thermodynamically more favorable (exergonic) than the nucleophilic addition of water, indicating the formation of benzyl ether and ester LCC linkages are a preferred synthesis route during lignin polymerization. However, formation of the benzyl ester linkage is kinetically limited compared to the formation of benzyl ether linkages, suggesting the benzyl ester linkages are not abundant in vivo. Moreover, the nucleophilic addition of other monolignols is kinetically facile and exergonic, suggesting that the predominant β-O-4 linkage comprising lignin can act as a branching point via an α-O-γ lignin linkage.</p>

<p>Given the thermodynamic preference for benzyl ether linkages to occur along the hemicellulose chain and PG linkages to occur at the reducing end of hemicellulose, these linkages represent the most probable LCC linkages contributing to biomass recalcitrance. Therefore, the reaction mechanisms, kinetics and thermodynamics associated with the deconstruction of the benzyl ether and PG LCC linkages in biomass under acidic conditions are elucidated. Competing reactions are also essential to identify to ensure the desired deconstruction products are obtained during biomass deconstruction. As such, the possible reaction pathways identified include degrading the lignin structure, degrading the hemicellulose structure, or cleaving the LCC linkage. The deconstruction energetics demonstrate that breaking PG linkages is kinetically and thermodynamically favored in acid catalyzed deconstruction, indicating that PG linkages are unlikely to be the LCC linkage significantly contributing to biomass recalcitrance. Cleaving the benzyl ether linkages demonstrated the lowest activation barriers, however, possessed positive reaction energies (endergonic). Comparatively, the degradation of hemicellulose possessed higher activation barriers and the greatest thermodynamic feasibility, with negative reaction energies (exergonic). Computing the reaction energetics as a function of temperature suggested that increasing the temperature during the deconstruction of lignocellulosic biomass would result in the activation barriers associated with hemicellulose degradation being surmounted before favorable reaction free energies would be established for cleaving benzyl ether linkages. As a result, benzyl ether linkages are likely the LCC linkage significantly contributing to biomass recalcitrance and cleaving these linkages to obtain lignin and carbohydrates in their chemically intact form is a thermodynamically controlled process. The present work utilizes computational tools to pinpoint the origin of biomass recalcitrance, and compute the preferred deconstruction pathways, providing insight into the convoluted structure of cellulose, hemicellulose and lignin within lignocellulosic biomass. Ultimately, this work will lay the foundation for designing novel solvent-based deconstruction technologies and play a key role in making bio-derived specialty chemicals and materials a sustainable reality.</p>