Thermal decomposition and isomerization of furfural and 2-pyrone: a theoretical kinetic study

Abstract

We have studied the decomposition and isomerization of furfural in the gas phase using quantum chemical and statistical reaction rate theory techniques. This work uncovers a variety of new reaction channels in furfural pyrolysis that lead to formation of the experimentally observed products, including CO₂, which was previously unexplained. In addition to the known mechanism for furan + CO production, furfural is shown to isomerize directly to 2-pyrone, with a barrier height of 69 kcal mol⁻¹, from where it can decompose to vinylketene + CO (highest barrier of 65 kcal mol⁻¹) or to CO₂ + 1,3-cyclobutadiene (highest barrier of 66 kcal mol⁻¹). Alternative pathways to vinylketene + CO and 4-pyrone are also described. An RRKM theory/master equation model is developed to describe reactions on the C₅O₂H₄ surface and used to simulate the decomposition kinetics of furfural and 2-pyrone. For both molecules, decomposition at 1400–2100 K is dominated by the formation of furan + CO, which represents around 75% of the total products, compared to around 19% and 6% for vinylketene + CO and total CO₂, respectively. The model also predicts significant formation of stabilized 2-pyrone under these conditions. Rate coefficient expressions are reported as a function of both temperature and pressure for the main decomposition and isomerization channels identified in the pyrolysis of furfural and 2-pyrone, to facilitate detailed chemical kinetic modelling of these important oxygenated hydrocarbons.