Mechanistic outlook on thermal degradation of 1,3-dialkyl imidazolium ionic liquids and organoclays

摘要

The thermal degradation mechanisms of ionic liquids (ILs) 1-butyl-3-methylimidazolium chloride (BMImCl) and 1-butyl-3-methylimidazolium tetrafluoroborate (BMImBF(4)) have been established using pyrolysis-GCMS (Py-GC-MS) and B3LYP/6-311+G(d,p) level of density functional theory (DFT). BMImCl decompose through a bimolecular nucleophilic substitution (S(N)2) while BMImBF(4) exhibit S(N)2 along with a competitive E2 elimination pathway. Activation energy parameters obtained using Kissinger-Akahira-Sunose method and Ozawa-Flynn-Wall method is compared with the computed activation barriers. The montmorillonite based organoclay prepared using these ionic liquids exchange only the cation part ([BMIm](+)) into the clay gallery leading to an expansion of d-spacing from 12.08 to 13.64 angstrom. The organoclay showed the maximum decomposition at 462 degrees C in the TGA experiment and the decomposition products were identified as methyl imidazole and 1-butene using Py-GC-MS. DFT studies employing a model compound Si(OH)(3)O- suggested a mechanism involving an imidazole-2-ylidine (carbene) intermediate for the decomposition of [BMIm](+) in the clay. Theoretical results were further supported by C-13 NMR analysis of IL in presence of colloidal silica which showed a characteristic carbene NMR signal at 187.6 ppm.