A theoretical study on the mechanism of the formation of furfural and furan from the pyrolysis of α-D-galactose

Biomass is a carbon neutral renewable resource. In order to solve a series of environmental pollution and energy shortage caused by the massive use of fossil fuels, biomass is gradually being developed to replace traditional fossil fuels. Pyrolysis is an important method for efficient utilization of biomass, and a large number of biomass resources in nature can be transformed into high value-added chemical products through pyrolysis. In the current research on biomass pyrolysis, the hemicellulose monosaccharide model compounds represented by galactose are not fully studied, and the formation mechanism of typical pyrolysis products has not been clarified. In this study, B3LYP/CBSB7 method of density functional theory was used to calculate the quantum chemistry and rate constant of the initial pyrolysis reaction of the hemicelluloses model compound α-D-galactose, and the competitiveness of different initial reaction types was discussed. The results showed that the energy barrier of the ring-opening reaction of α-D-galactose was the lowest during the pyrolysis process. It is 190.07 kJ/mol, which has obvious advantages compared with other initial reactions. With the increase of temperature, the competition between reaction paths becomes more intense, and the yield of other products is improved under high temperature conditions. In addition, for furfural and furan, two high value-added pyrolysis products, 21 possible detailed reaction paths were discussed based on the collaborative reaction mechanism, and the structure optimization and frequency calculation of the reactants, transition states, intermediates and products on the reaction path were carried out. The high-precision energy was calculated by CBS-QB3 composite method. By comparing the whole barrier of different reaction paths, the dominant reaction paths of the two products were summarized, and the kinetic parameters of the reaction at different temperatures were calculated. The results showed that α-d-galactose formed furfural through ring-opening, isomerization, hemiacetal, two-step dehydration, combined dehydration and dealdehyde in sequence. The overall energy barrier of this reaction path was 291.53 kJ/mol, and the small molecular volatiles formed were water and formaldehyde. For furan, the energy barrier of the rate-determining step is 287.21 kJ/mol and 288.51 kJ/mol, respectively. The former forms glycolic acid and water, and the latter formic acid, formaldehyde and water. In this study, the pyrolysis process of α-D-galactose, half of the cellulose monosaccharide model compound, was calculated by DFT theory and kinetic parameters, which is helpful to understand the pyrolysis mechanism of galactose in depth and promote the comprehensive utilization of hemicellulose. However, the temperature variation in the actual pyrolysis process and the complexity of the reaction mechanism pose more challenges to current theoretical research. In future studies, the pyrolysis experiments of α-D-galactose at different temperatures will be carried out in a jet stirring reactor to quantitatively calculate the active intermediates and the generation process of important pyrolysis products during the pyrolysis process. At the same time, based on the density functional theory, the α-D-galactose pyrolysis reaction network was further expanded from the aspects of free radical reaction mechanism and interaction mechanism, the thermodynamics and kinetics database of α-D-galactose pyrolysis reaction was established, and the detailed pyrolysis model of α-D-galactose was constructed. Chemkin-pro software was used to simulate the generation process of active intermediates and important pyrolysis products during the pyrolysis process, and the sensitivity of each reaction path during the generation process was analyzed to gain an in-depth understanding of the pyrolysis behavior of α-D-galactose, and then guide the design, optimization and control of biomass conversion process.