Lignocellulosic biomass is an abundant renewable resource that can be upgraded to chemical and fuel products through a range of thermal conversion processes. In particular, fast pyrolysis uses high temperatures (300-1000 °C) in an inert environment to transform lignocellulose into bio-oils in high yields. 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. For the atomic/molecular level investigation of the pyrolysis mechanism, quantum chemistry modelling is a powerful strategy. Despite having a brief history of only a few decades, its application to the study of biomass pyrolysis mechanisms has advanced rapidly with the development of supercomputers and computational codes in the twenty-first century.<br/><br/>In this work, the pyrolytic reactivity of β-D-xylopyranose (xylose) and its dimer and trimer is thoroughly investigated using computational strategies rooted in quantum chemistry, to check the role of xylan polymerization. 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). Moreover, reference data are obtained employing "cheap" schemes (ChS). ChS is a recently developed family of schemes entirely based on the frozen core coupled cluster ansatz including single, double, and (perturbative) triple excitation calculations in conjunction with a triple-zeta quality basis set, including the contributions due to the extrapolation to the complete basis set limit and core-valence effects using second-order Møller-Plesset perturbation theory. The ChS results are used as a reference to benchmark the accuracy of the other density functionals and well-known black-box composite schemes, i.e. G4 and CBS-QB3. Given the high flexibility of the molecules under investigation, conformational analysis for minima and transition states is performed with state-of-art semiempirical quantum chemical methods coupled with metadynamics simulations to provide reasonable guesses for the potential energy surfaces calculation.

Quantum chemical modelling of hemicellulose pyrolytic reactivity: β-D-xylopyranose as a model compound

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