Hemicellulose pyrolysis: mechanism and kinetics of functionalized xylopyranose

This work analyzes the thermochemical kinetic influence of the most prominent functionalizations of the β-D-xylopyranose motif, specifically 4-methoxy, 5-carboxyl, and 2-O-acetyl, regarding the pyrolytic depolymerization mechanism. The gas-phase potential energy surface of the initial unimolecular decomposition reactions is computed with M06-2X/6-311++G(d,p), following which energies are refined using the G4 and CBS-QB3 composite methods. Rate constants are computed using the transition state theory. The energies are integrated within the atomization method to assess for the first time the standard enthalpy of formation of β-D-xylopyranose, 4-methoxy-5-carboxy-β-D-xylopyranose, and 2-O-acetyl-β-D-xylopyranose: -218.2, -263.1, and -300.0 kcal mol^-1, respectively. For all isomers, the activation enthalpies of ring-opening are considerably lower, 43.8-47.5 kcal mol^-1, than the ring-contraction and elimination processes, which show higher values ranging from 61.0-81.1 kcal mol^-1. The functional groups exert a notable influence, lowering the barrier of discrete elementary reactions by 1.9-8.3 kcal mol^-1, increasing thus the reaction rate constant by 0-4 orders of magnitude relative to unsubstituted species. Regardless of the functionalization, the ring-opening process appears to be the most kinetically favored, characterized by a rate constant on the order 10¹ s^-1, exceeding significantly the values associated with ring-contraction and elimination, which fall in the range 10^-4-10^-10 s^-1. This analysis shows the decomposition kinetics are contingent on the functionalization specificities and the relative orientation of reacting centers. A relatively simple chemical reactivity and bonding analysis partially support the elaborated thermokinetic approach. These insights hold significance as they imply that many alternative decomposition routes can be quickly, yet accurately, informed in forthcoming explorations of potential energy surfaces of diverse hemicellulose motifs under pyrolysis conditions.