Lignocellulosic biomass is an abundant renewable resource that can be upgraded to chemical and fuel products through a range of thermal conversion processes. Fast pyrolysis is a promising technology which uses high temperatures and fast heating rates to convert lignocellulose into bio-oils in high yields in the absence of oxygen. Hemicellulose is one of three major components of lignocellulosic biomass and has a heterogeneous polymeric composition of pentoses, hexoses, hexuronic acids, and acetyl groups. Xylan is one of the most abundant polysaccharides that comprise hemicelluloses. Thus, comprehension of hemicellulose reactivity relies on our knowledge of the reaction mechanism of xylan.<br/><br/>In this study, we propose β-D-xylopyranose (xylose) as a model structural motif for the essential chemical structure of xylan. The gas-phase pyrolytic reactivity of xylose, xylobiose and xylotriose is thoroughly investigated using computational strategies rooted in quantum chemistry. In particular, the potential energy surfaces of thermal degradation of the aforementioned compounds are computed employing Density Functional Theory (M06-2X, PW6B95, and revDSD-PBEP86), post Hartree-Fock methods, and composite schemes (G4, CBS-QB3, and ChS schemes). Conformational analysis for minima and transition states is performed with state-of-art semiempirical quantum chemical methods coupled with metadynamics simulations. Lastly, rate coefficients of important steps of the reaction pathways are computed with Transition State Theory.<br/><br/>A reaction kinetic model which focuses on understanding the reaction kinetics of the discrete xylose, xylobiose, and xylotriose molecular structures, is created by using the thermodynamical and kinetics information obtained from the aforementioned calculations. The model is then extended by the concept of molecular group additivity to model the more complex real-world biopolymers of xylan and actual softwood and hardwood hemicelluloses. The model is tested against thermal gravimetric analysis using xylan, xylotriose, xylobiose, and xylose to study the effect of the degree of polymerisation. Our results show that the pyrolytic reactivity of xylose and xylobiose are similar, however, that of xylotriose is markedly different. Analysis of the model indicates that the differences in reactivity exhibited by xylotriose are due to a propensity for concerted elimination reactions that are available to the xylotriose molecular structure but not to xylose or xylobiose.

Quantum biomass: Thermogravimetric analysis, quantum chemistry and reaction kinetic model for hemicellulose fast pyrolysis

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