Investigating acid-catalyzed biomass reactions and solvent effects in humins formation, using multiscale molecular modeling
Creators: Velasco Calderon, Jose Carlos
Publisher: name: University of Alberta Library
Year: 2023

<p>The conversion of lignocellulosic biomass to produce chemicals has gained significant interest recently. A broad spectrum of specialty chemicals can be obtained from lignocellulose-derived molecules. One of the most promising reactions in this category is the dehydration of cellulose-derived sugars such as glucose or fructose to 5-hydroxyl methyl furfural (5-HMF). 5-HMF is a valuable chemical platform from which polymers and biofuels can be produced. However, acid-catalyzed dehydration of fructose or glucose to 5-HMF is accompanied by side reactions, forming soluble polymers and insoluble humins. Humins are carbonaceous, polymeric by-products formed during acid-catalyzed condensed phase transformation of biomass-derived moieties. They are responsible for significant carbon loss and catalyst deactivation, and little is known about the chemistry and composition of humins. Additionally, condensed phase catalytic transformation of biomass is highly susceptible to solvent composition, where addition of aprotic solvents has shown to alter conversion and selectivity and can also avoid the formation of unwanted dehydration and rehydration by-products like humins and smaller molecular weight acids. Thus, revealing the mechanism of humins formation and elucidating the effect of solvent medium on humins formation chemistry are crucial knowledge gaps to fill to make quantum leaps in the development of biomass to chemicals technology.<br>
In this thesis, for the first time, the reaction pathways and the activation and reaction-free energies of all the elementary reaction steps of the 5-HMF initiated acid-catalyzed humins formation are computed using density functional theory (DFT)-based calculations. The reaction pathway elucidated in this work explains the subsequent chemistry of further polymerization leading to humins. The computed mechanism and suggested step growth polymerization scheme are in excellent agreement with the experimental spectroscopic data in the literature. This work provides mechanistic details of humins chemistry for the subsequent higher-scale kinetic modelling of humins formation and optimizes the reaction conditions to minimize them.<br>
The solvent effects are studied at two levels: physical interactions of the solvent with reacting species and preferential solvation as well as their chemical effects in altering reaction kinetics and thermodynamics. A computational approach of molecular mechanics-based molecular dynamics and well-tempered metadynamics methods is implemented to investigate physical effects. The effect of aprotic solvent on the<br>
interaction of reacting species with the hydronium ions (acid catalyst) is studied for the fructose dehydration to 5-HMF as a simulation system model. Analogous to heterogeneous catalysis, the interaction between the reactant and the catalyst and the competition between reactant and solvent to interact with the catalyst surface is believed to govern the reactivity. We demonstrate a remarkable enhancement in the interaction between the hydronium ions and the fructose hydroxyl group as the DMSO concentration increased. Also, adding DMSO improves the stabilization of the hydronium ions in the first solvation shell of fructose compared to that in the bulk solvent. On the other hand, hydronium ions became less stable near 5-HMF as the concentration of DMSO increased. These observations help to better understand the improved conversion and selectivity during the acid dehydration of fructose to 5-HMF experimentally observed when adding aprotic polar solvent to the reacting system.<br>
Three solvation modelling approaches are systematically compared to study the chemical effects of the solvent environment, i.e., implicit solvent, explicit, non-polarizable, equilibrated solvent molecules without dynamics and explicit solvent molecules with dynamics with polarizability. The model reaction is the acid-catalyzed protonation of 5-HMF, the first charge transfer step, leading to humins formation. The effect of the solvent environment on activation-free energy barriers was only captured when solvent and reaction dynamics were considered and when the solvent molecules were polarizable. The charge redistribution in the reacting species was observed to lead to the dynamic solvent reorganization; as the solvent composition changes, the Marcus reorganization energy changes, altering the reaction-free energy barriers. Additionally, the changes in the timescale of solvent reorganization led to non-equilibrium solvation.<br>
This thesis investigates the molecular-level formation of humins from biomass and their derivatives, identifying key intermediates and modeling humin formation in condensed-phase reactions during acid-catalyzed biomass conversion. These findings guide the selection of catalysts, solvents, and operating conditions to enhance desired product selectivity and reduce humin formation in biomass-to-chemical transformations.</p>