(639h) Pericyclic Reactions in Xylose Pyrolysis and Implications for Xylan Pyrolysis

Isomerization and decomposition kinetics of xylose is studied with computational quantum chemistry in this work as a window into elementary reactions of hemicellulose pyrolysis. Xylose and acetylated units of xylose are major monomer components in many varieties of the biopolymer hemicellulose. Conversion of plant biomass into biofuels is often carried out through the thermal process of pyrolysis. Chemical mechanisms for the decomposition of biomass and its constituent monomers have been widely studied but are generally presented as lumped models where the individual elementary reaction steps are not known. It is the goal of this work to develop a mechanism of elementary steps for xylose degradation. As one of the major constituents of hemicellulose in biomass, the development of thermal degradation mechanisms for xylose is important to the development of mechanisms for hemicellulose and biomass as a whole.

Reactions were considered proceeding from the five major isomer structures of xylose. Both four- and six-centered pericyclic transition states were investigated. Quantum-chemistry modeling was carried out using the Gaussian 09 software package. Initial structure exploration and optimization were carried out using DFT methods at a B3LYP/6-311++G(d,p) level of theory. Additional refinement of stable species and transition states was then carried out using the compound method CBS-QB3. Reaction rate constants were calculated for significant reactions using Mesmer master equation code. Over 200 transition states were discovered for reactions initiating directly from xylose, including a class of ring contraction reactions believed to be novel. Additional investigation was performed on xylose structures acetylated at various positions on the ring to account for reactions of acetylated monomer units in hemicellulose varieties such as xylans. Reaction types identified in xylose were then applied to model compounds, such as xylobiose, to evaluate their effect in the chemistry of xylan chains by analogy. The ability of water or neighboring alcohol functional groups to act catalytically in these reactions was also simulated due to their potential interactions with these reactions in condensed phases.