Homogeneous and heterogeneous reactions of the n-C5H11O, n-C5H10OH and OOC5H10OH radicals in oxygen. Analytical steady state solution by use of the Laplace transform

Abstract

The different reaction routes of n-pentoxy radicals in O₂/N₂ mixtures were investigated in the temperature range 423–523 K under atmospheric pressure. Flow experiments were performed in several reactors, with wall efficiencies increasing from passivated quartz to Pyrex, and with a great variety of mol fractions of O₂ (% of O₂), which was varied from ∽0 to 100% O₂; the products, a pentanal-hydroperoxide C₅H₁₀O₃ of M_r 118, pentanal, a methylfuranone (γ-valerolactone), pentanol, and three methylfurans were analyzed and identified by GC-MS. Pentanal-hydroperoxide, also called 4-hydroperoxypentanal, OCH(CH₂)₂CH(OOH)CH₃, is the major product in passivated quartz vessels. It is formed by two consecutive isomerizations: (i) a fast one, RO→R_-HOH, via a six-membered ring transition state and (ii) a much slower one, involving an OOR_-HOH radical, OOR_-HOH→HOOR_-2HOH, via a seven-membered ring intermediate: CCCCCO ——min CCCCCOH ——minCC(OO)CCCOH ——min CC(OOH)CCCOH ——min CC(OOH)CCCO+HO₂. A chemical mechanism, taking into account all of the experimental results is proposed for reactions of the n-pentoxy radical in oxygen. An analytical steady state solution, based on the Laplace transform, has been helpful in rejecting or validating candidate models. Rate constants appearing in the proposed mechanism have been evaluated by the use of an optimization method. This analysis shows that (i) isomerization is the predomi-nant reaction route, accounting for the diversity of end products; (ii) homogeneous pentanal is formed by reaction with O₂ of anisomerized pentoxy radical: CCCCCO ——min CCCCCOH ——min CCCCCOH ——min CCCCCO+HO₂; (iii) methylfurans are also homogeneously produced through an analogous reaction; (iv) γ-valerolactone is formed by a heterogeneous reaction; pentanal and methylfurans are produced in significant quantities through heterogeneous reactions, especially at low concentrations of oxygen (<5% O₂). The implications of heterogeneous reactions of RO radicals in atmospheric chemistry and in combustion are discussed for those reactions can correspond to a sink for radicals.