Unimolecular decomposition of 2,5-dimethylfuran: a theoretical chemical kinetic study

The unimolecular decomposition of 2,5-dimethylfuran (DMF), a promising next-generation biofuel, was studied at the CBS-QB3 level of theory. As most of its decomposition routes remain unknown, a large number of pathways were explored: initial C-H bond fission, biradical ring opening, H-atom and CH3-group transfers involving carbene intermediates. Based on the computed potential energy surfaces, thermochemical data and high-pressure limit rate constants were determined and included in a small detailed kinetic mechanism for DMF unimolecular decomposition. Simulations performed under the conditions of recent experimental data focusing on the initial steps in DMF thermal decomposition showed that the unimolecular decomposition of DMF is dominated by two product channels. For typical combustion conditions, the major product channel (similar to 70% at 1500 K and 1 bar) is reached by a 3,2-hydrogen transfer in DMF leading to a carbene intermediate that rearranges to hexa-3,4-dien-2-one which in turn decomposes into CH3CO and C4H5 by initial C-C bond fission. In contrast with previous studies, the initial C-H bond fission in DMF has not been found to be the major decomposition channel (similar to 30% at 1500 K and 1 bar). Pressure effects were probed using a semi-quantitative approach and were found to be negligible above 1 bar. Below atmospheric pressure, initial C-H bond cleavage yields increase while CH3CO + C4H5 branching ratios decrease. This study brings a new understanding of the nature of the initial radicals and molecules created upon thermal activation of DMF.