Gas-Phase Pyrolysis of Diisopropyl Methylphosphonate

Abstract

Gas-phase pyrolysis studies of diisopropyl methylphosphonate (DIMP) in nitrogen have been conducted to gain insight into the decomposition behavior of organophosphorus chemical warfare nerve agents. Experiments were conducted in a quartz-lined atmospheric flow reactor between 700 and 800 K, at residence times ranging from 15 to 90 ms. Propylene, isopropanol, isopropyl methylphosphonate (IMP), and methylphosphonic acid (MPA) were identified as decomposition products of DIMP. FTIR spectrometry was used to quantify parent, propylene, and isopropanol mole fractions in the reactor. The proposed pyrolysis mechanism for DIMP comprises two stages. The first corresponds to the unimolecular decomposition of the parent into IMP and propylene. The second involves two competing pathways for the unimolecular decomposition of IMP, one leading to isopropanol and the very reactive methyl dioxophosphorane; the other to propylene once again and MPA. In the range of temperatures studied, an isopropanol to propylene mole fraction ratio close to 0.25 suggests a branching ratio of 1.5 between these two pathways in favor of propylene production. The Arrhenius expression for the unimolecular decomposition of DIMP was found to be: k[s −1]=10 (12.0±1.5)[s −1] exp(−36.7±4.9[ kcal.mole −1]/(RT)). Pyrolysis experiments with isopropyl and t-butyl acetates, which have well-known decomposition rates, were performed to illustrate the apparatus’ ability to produce valid chemical kinetic data. An investigation of the effects of surface to volume ratio on the DIMP decomposition process shows that wall reactions are significant in a 4 mm i.d. quartz tube, but less important in an 8 mm i.d. tube. Their effects are expected to be small in the 45 mm i.d. reactor.