An initial degradation reaction before ring-opening in imidazolium-based anion exchange membranes: A DFT study

Abstract

Based on density functional theory, we report an initial degradation reaction before a ring-opening reaction which is a traditional degradation reaction in imidazolium-based anion exchange membranes (AEMs) and has been reported by several experimental groups. In the present work, a 1-ethyl-3-(4-vinylbenzyl) imidazolium (EVIm) monomer is focused as a part of imidazolium-based AEMs. Firstly, a hydroxide anion is put to the EVIm monomer and the geometry is optimized. Secondly, the minimum reaction path along the ring-opening reaction is calculated and it is concluded that the EVIm monomer is resistant to the ring-opening reaction because of the high energy barrier. Further, when the hydroxide anion is put near the EVIm monomer, the C-H bond at the C2 position of imidazole ring is elongated. As the result, the feasibility of a dehydrogenation reaction where the hydrogen bonding to the C2 position desorbs is suggested. Finally, in order to compare the energy barrier to that of the ring-opening reaction, the minimum reaction path along the dehydrogenation reaction is calculated. From these analyses, we propose that the dehydrogenation reaction precedes the ring-opening reaction and it is also much more dominant rather than the ring-opening reaction in the EVIm monomer. © 2013 The Surface Science Society of Japan.