The gas-phase pyrolysis of cyclopropylamine: A computational study on the kinetics and reaction mechanism

Abstract

Small amines represent an accessible model to understand the reactivity of the nitrogenous components of biomass during combustion and pyrolysis processes. In this work, the kinetics and mechanism of the gas-phase pyrolysis of cyclopropylamine, c-C3H5NH2, have been studied by ab initio and density functional calculations. From the derived information high-pressure limit rate coefficients were estimated using the transition state theory over the 400 – 1400 K temperature range. The results support a stepwise process, in which the initial ring-opening stage determines the rate through competitive direct, biradical and carbene mechanisms to give 1-aminopropene, CH3CH=CHNH2, and 1-propanimine, CH3CH2CH=NH, as reactive intermediates. The Arrhenius equation obtained at the G3(MP2)//B3LYP/6–311++G(3df,3pd) level of theory, log10 (koverall,∞/s-1) = (15.47 ± 0.11) – (57.68 ± 0.32) kcal mol-1 (2.303 RT)-1, agrees very well with the experimental expression determined by Parry and Robinson. According to these findings, the carbenic pathway is most important up to ∼500 K, above which the biradical mechanism predominates. Subsequent reaction steps include the addition of the CH3CH=CHNH2 and CH3CH2CH=NH species to another c-C3H5NH2 molecule to produce c-C3H5NHCH(NH2)CH2CH3, which afterwards decomposes into c-C3H5N=CHCH2CH3 and NH3. The results also reveal relatively high electronic barriers for the interconversion between the geometric isomers of CH3CH=CHNH2 and CH3CH2CH=NH, suggesting that the contribution of these processes is negligible. Further calculations of the title reaction in the presence of selected aliphatic amines RNH2 (R = Me, Et, n-Pr, i-Pr, allyl) confirm that the unimolecular fission of the ring is the rate determining step.