Understanding the role of local condensed phase environments in pyrolytic and catalytic biomass conversion

Abstract

Biomass conversion generally involves two major sets of chemical transformations – (1) thermal breakdown of macromolecules in the feedstock, such as cellulose, to smaller sugars and oxygenates via fast pyrolysis followed by (2) catalytic upgrading to the desired fuels or precursor chemicals. These reactions of biomass conversion usually occur in the condensed phase – either in the melt phase for pyrolytic reactions or in the solvent phase for catalytic upgrading reactions. The work in this thesis sheds light on the molecular complexity of such condensed phase environments. Explicit molecular modeling of these condensed phase environments coupled with first-principles simulation techniques such as density functional theory (DFT) and ab initio molecular dynamics (AIMD) are used to elucidate the influence of such environments on the kinetics of biomass conversion reactions. Examples from cellulose pyrolysis and hydrogenation chemistry are studied to demonstrate the critical importance of considering the role of condensed phase environments in biomass conversion.Using DFT calculations, constrained AIMD and experimental kinetics from the Pulsed Heated Analysis of Solid Reactions (PHASR) set-up, it is shown that vicinal hydroxyl groups which are present in the cellulose matrix in abundance can directly participate in the activation of cellulose by promoting facile proton transfer as well as stabilizing transition states through hydrogen bonding. The kinetic influence of calcium ions, naturally present in such feedstocks, is also examined in this thesis. It is shown that calcium interacts with cellulosic melt environment such that the native hydrogen bonding is disrupted. Such disruption of the hydrogen bonding network coupled with Lewis acid stabilization of the transition states leads to dual catalytic cycles for cellulose activation and second order rate dependence on calcium. Explicit modeling of the cellulosic environment is critical towards capturing such kinetic behavior. Furthermore, the influence of hydroxyl groups, calcium ions and more generally the cellulosic condensed phase environment, is examined more broadly and extended to other ring opening and fragmentation pathways that lead to glycolaldehyde, a side product of pyrolysis. The work from this part of the thesis helps establish the ubiquitous involvement of the local condensed phase environment in mediating biomass pyrolysis reactions. Finally, aqueous phase hydrogenation of C=C bonds in phenol over Pt particles inside zeolites is studied as a model reaction to demonstrate the importance of solvent environment in catalytic upgrading. Through explicit modeling of local water clusters around the reaction centers, it is shown that increasing the acidity of the zeolite supports can alter the local acidity of the water clusters. This in turn is shown to not just open up proton coupled electron transfer (PCET) pathways but also improve the efficacy of such mechanisms for hydrogenation. Thus, this study helps demonstrate that one can alter the solvent environment to enhance reactions of biomass conversion, especially those that involve proton transfer. More generally, the collective body of work in this thesis could act as a framework for future studies that seek to understand the role of condensed phase environments in biomass conversion as well as to develop strategies that use such environments for improved reactivity and selective chemical transformations.