(71c) Effect of Solvents on Lignin-Surface Interactions Via Molecular Dynamics Simulations

Lignocellulosic biomass is an abundant and renewable resource which can be an alternative to petroleum feedstocks for the fuel and chemical industries. It primarily consists of cellulose, hemicellulose, and lignin; cellulose and hemicellulose have been extensively studied for applications in the biofuels and paper industries, while lignin is the most abundant natural source of aromatic monomers for the polymer industry and chemical production^{1, 2}. However, challenges in lignin valorization process arise from its complex heteropolymer structure, broad molecular weight distributions and high variability in molecular structure, leading to numerous studies of catalytic, thermal and biological approaches to convert lignin into monomers and oligomers. One approach called reductive catalytic fractionation (RCF) is reported to produce high yields of phenolic monomers and oligomers under different catalytic conditions³. RCF in an alcohol solvent over a heterogeneous Pd/C catalyst has been found to favor the generation of product mixtures that can be microbially converted to valuable products at high yield⁴. An important choice in optimizing RCF is the selection of solvent; however, there is a lack of detailed information regarding how solvent selection impacts lignin behaviors and interactions. This gap motivates our study of the solvation of lignin, the interaction of lignin with the catalyst surface in solvent, and the structural changes of lignin in the liquid phase.

In this work, we perform all-atom molecular dynamics (MD) simulations of an oligomeric lignin model compound in various organic solvents including methanol, ethanol, a binary mixture of ethanol+water (85:15, v:v), and water at both the RCF reaction temperature (473 K) and room temperature. Analysis of structural features such as the radius of gyration and solvent accessible surface area of lignin suggests that these three organic solvents can better solvate lignin, resulting in a more extended configuration suitable for catalytic conversion to valuable chemicals. We further introduce the presence of either a model Pd or C surface to understand how the choice of solvent impacts adsorption onto the catalytic surface or support and to quantify the competition among the reactant and solvent for the surface. Unbiased simulations suggest that there is strong adsorption of lignin on both Pd and C surfaces at 473 K. To further quantify adsorption energetics, we computed adsorption energies by employing an earlier reported⁵ approach which involves separately calculating contributions due to lignin-solvent, lignin-solvent-surface, solvent-surface, and lignin-solvent-surface interactions. Analysis of adsorption energies indicates strong adsorption of lignin on both surfaces with notable solvent-mediated differences in adsorption energy. Our results further indicate that lignin adsorption is also driven by the entropy gain associated with liberation of solvent molecules from the surface. Overall, our study provides a molecular perspective of adsorption of lignin onto Pd and carbon surfaces, which is a first step towards understanding and optimizing the catalytic conversion of lignin into valuable chemicals.

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