The thermal decomposition mechanism of amino acid ionic liquids (AAILs) is an important feature, which affects their industrial application, especially in the gas absorption process. In this study, thermal decomposition mechanisms including SN2, alkyl rearrangement, and Hoffman elimination mechanisms were investigated by density functional theory. AAILs consist of 1-ethyl-3-methylimidazolium ([C2mim]) as the cation and lycine (Lys-), arginine (Arg-), histidine (His-), methionine (Met-), and proline (Pro-) as the anions. The polarizable continuum model was used to examine the effect of environment polarity on the thermal decomposition mechanism of the AAILs. The SN2 and alkyl rearrangement mechanisms are the most probable routs from kinetic and thermodynamic viewpoints. In the cases of [Emim][Arg], the energy barrier of the reaction is lower than that of other AAILs that shows the rate of reaction increases by increasing the number of amine groups in amino acid anions. Moreover, the environment polarity is a key factor that has negative effects on the thermodynamic and kinetic of different mechanisms. Also, the effect of environment polarity in the alkyl rearrangement route is more than other mechanisms.

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