Pyrolytic Conversion of Glucose into Hydroxymethylfurfural and Furfural: A Survey of Mechanisms and Benchmark Quantum-Chemical Calculations

Quantum chemical methods have been intensively applied to study the pyrolytic conversion of glucose into hydroxymethylfurfural (HMF) and furfural (FF). Herein, we collect the most relevant mechanistic proposals from the recent literature and organize them into a single reaction network. The transition structures (TSs) and intermediates are characterized using high level ab initio methods that predict relative energies within chemical accuracy. The reaction pathways are assessed in terms of the Gibbs free energy differences of the TSs and intermediates with respect to β-glucopyranose, selecting a 2D ideal-gas standard state at 773K to represent the usual pyrolysis conditions. After having scored all the possible pathways throughout the network assuming reversible reaction steps, several pathways, which present various changes with respect to the former proposals, can lead to the formation of both HMF and FF passing through rate-determining TSs that have ∆G‡ values of ~ 49-50 kcal/mol. Interestingly, the catalysis by auxiliary water molecules and the non-specific environmental effects as modelled by solvent continuum methods, have only a minor impact on the Gibbs free energy profiles of the most favoured routes. Since the HMF fragmentation (HMF→FF+CH2O) is predicted to have a small ∆rxnG value and an accessible ∆G‡ barrier, the HMF/FF molecular ratio may be partly determined by equilibrium conditions. In addition, the benchmark energies and structures are employed to study the performance of density functional methodologies. Finally, we show that the computational results are in consonance with the kinetic parameters derived from lumped models, the results of isotopic labelling experiments and the reported HMF/FF molecular ratios. Eventually, they could be useful in future computational studies focused on the kinetic modelling of the pyrolysis mechanisms including non-equilibrium kinetic effects, which could render much more detailed information about product yields and the importance of the various pathways.