An elementary-reaction mechanism is developed for the pyrolytic generation of furfural from xylose, a web of pericyclic reactions with transition states and intermediates identified by computational quantum chemistry. Furfural is a heteroatomic aromatic species derived primarily through treatment of xylose. There are varied industrial applications for furfural as a solvent, specialty chemical feedstock, and polymer precursor. Furfural is naturally formed in various biological systems alongside metabolism of biopolymers. Better understanding of the mechanism by which furfural is generated has the potential to yield improvements in the efficient production of furfural as well as understanding the conditions under which it is likely to form in systems where it is an undesirable contaminant. Stoichiometrically, furfural results from dehydration of three moles of water from every mole of xylose. The mechanism of the dehydration and related isomerizations for the conversion to furfural is rarely discussed with regard to the specific elementary chemical transitions at play.
Stable intermediate species and transition state structures included in the proposed mechanism were simulated using computational quantum chemistry software (Gaussian 09). Initial structure exploration and optimization was carried out using DFT methods at a B3LYP/6-311++G(d,p) level of theory. Further refinement of the structures and calculation of their thermochemical properties was carried out using the composite method CBS-QB3. Transition states with accessible activation energies were used to calculate reaction rates using the Mesmer master equation code.
Investigation of this mechanism has shown it to be a complex network, constrained by the chiral specificity of saccharide structures and involving a large number of interconnected, parallel reaction pathways. Over 70 intermediate species and over 150 distinct transition states were identified. Reaction flux analysis was carried out on this network to identify which pathways contributed most significantly to the product formation. Additional analysis was also carried out to discover transition states where the presence of water or alcohol functional groups in nearby species could participate in the reaction to catalyze certain transitions, favoring them over a purely unimolecular reaction.
