Ab-initio studies of thermal unimolecular decomposition of furan: A complementary deterministic and stochastic master equation model

The detailed reaction mechanism for the thermal unimolecular decomposition of furan was comprehensively investigated in a wide range of conditions (T = 800–2000 K and P = 0.001–100 atm). The main reaction pathways were explored using different composite electronic structure methods including W1U, CBS-APNO, CBS-QB3, G3, G3B3, and G4. The temperature-/pressure-dependent dynamic behaviors of the furan pyrolysis were characterized using the combined deterministic and stochastic Master Equation/Rice–Ramsperger–Kassel–Marcus (ME/RRKM) model. These calculations embodied the hindered internal rotation and quantum tunneling corrections. Besides the C–H bond fission channels, the pyrolysis mechanism is found to involve the H-transfer reactions yielding α-carbene and β-carbene as intermediates which eventually decompose and/or isomerize producing three final products, viz. C2H2 + H2CO (P1), CH3CCH + CO (P2), and CH2CCH + HCO (P3). While P1 and P2 appear to be the primary products at T > 1300 K, channels P3, 2-furyl + H (P4) and 3-furyl + H (P5) are found insignificant. Our calculations reveal that the title reaction occurs via β-carbene with a contribution of 85–91% between 1600 K and 2000 K and at 1 atm pressure, which agrees well with the recent measurements (Urness et al., J. Chem. Phys., 2013, 139, 124305). The calculated rate coefficients, k(T, P), and the thermodynamic properties of the species involved are found to be in good agreement with the experimental results. Therefore, the reported data in this work are highly recommended for future modeling and simulation of furan-related combustion applications. The performance of the considered electronic structure methods for kinetic purposes was also discussed.