Atmospheric Oxidation of Piperazine Initiated by OH: A Theoretical Kinetics Investigation

Piperazine (Pz) mixed with other amines is a proposed carbon capture solvent, but there is concern about the impact of Pz on air quality, including the potential to produce toxic products. Here, the •OH initiated oxidation of Pz has been studied by ab initio modelling and RRKM / master equation kinetic simulations. The Pz + •OH reaction is found to proceed at around the capture rate, consistent with experiment, with abstraction predominantly from C—H sites. The subsequent reaction kinetics of carbon centred Pz radicals with O2 are also studied, so as to determine the first-generation oxidation products. We find that the Pz radical predominantly reacts with O2 to produce a cyclic imine product + HO2• under tropospheric conditions, with the stabilized peroxyl radical formed as a minor product. Subsequent reaction of the peroxyl radical with NO produces an alkoxyl radical that can react with O2 to yield a cyclic amide or undergo unimolecular ring opening followed by a second O2 addition / HO2• elimination step to produce CH2=NCH2CH2NHCHO.