Pressure-dependent branching in initial decomposition of gamma-valerolactone: a quantum chemical/RRKM study

Recently, the potential of gamma-valerolactone (a cyclic ester, C₅H₈O₂) as a bio-fuel and fuel additive has been highlighted. To unravel its chemical behavior in combustion, the reaction kinetics of initial decomposition of gamma-valerolactone (GVL) has been investigated theoretically by utilizing ab initio calculations and transition-state-theory based simulations. The effect of pressure on decomposition rates and, more importantly, on the branching fractions of major products has been explored. The potential energy surface was constructed at the CCSD(T)-F12/cc-pVDZ-f12 level based on B2PLYPD3/cc-pVTZ optimized geometries. Rate coefficients were obtained from the solution of RRKM/master-equations at a number of pressures (within the range of 7.6-76 000 torr). The isomerization of GVL to 4-pentenoic acid (4PA) followed by C-C bond fission to form allyl plus CH₂COOH is confirmed to be the most important route in the initial decomposition of GVL. Aside from isomerization to 4PA, several other routes also occur with significant contributions, during which pressure was found to take a great role in their branching. At pressures above 760 torr, the concerted reactions to form CO + ethene + acetaldehyde and propene + 2-oxiranone account for over 50% of the overall decomposition at the higher temperature end. On the other hand, the "formally direct formation" of allyl + CH₂COOH, which occurs via directly skipping the 4PA well, has a non-ignorable branching above 1400 K at low pressures. As opposed to GVL, the effect of pressure on the branching of 4PA consumption routes is very minor. It is hoped that the present study will establish a firm foundation for more comprehensive kinetic studies on GVL combustion in the future.