Hydroxide Degradation Pathways for Substituted Trimethylammonium Cations: A DFT Study

Abstract

Substituted trimethylammonium cations serve as small molecule analogues for tetherable cations in anion exchange membranes. In turn, these membranes serve as the basis for alkaline membrane fuel cells by allowing facile conduction of hydroxide. As these cations are susceptible to hydroxide attack, they degrade over time and greatly limit the lifetime of the fuel cell. In this research, we performed density functional theory calculations to investigate the degradation pathways of substituted trimethylammonium cations to probe the relative durability of cation tethering strategies in alkyl and aromatic tethers. Our results show that significant changes in calculated energy barriers occur when substitution groups change. Specifically, we have found that, when available, the Hofmann elimination pathway is the most vulnerable pathway for degradation; however, this barrier is also found to depend on the carbon chain length and number of hydrogens susceptible to Hofmann elimination. SN2 barriers were also investigated for both methyl groups and substitution groups. The reported findings give important insight into potential tethering strategies for trimethylammonium cations in anion exchange membranes.

Hydroxide Degradation Pathways for Substituted Benzyltrimethyl Ammonium: A DFT Study

“…This is consistent with the conclusion that the benzyl S N 2 pathway is the dominant degradation pathway for unsubstituted BTMA + . 18,20,26,27 For the double-meta −OCH 3 substitution, the benzyl S N 1 G = is 136.4 kJ/mol, close to the value of the unsubstituted BTMA + . For the double-meta −N(CH 3 ) 2 substitution, the benzyl S N 1 G = is 115.9 kJ/mol, still larger than its benzyl S N 2 G = (104.2 kJ/mol), although the gap between S N 1 G = and S N 2 G = is much smaller.…”

“…20 In short, Gaussian 09 (G09) 21 was used to optimize the reactants and TS structures by B3LYP 22 method, 6-311++G(2d,p) basis set, and PCM solvation model. During the geometry optimization calculations, no molecular symmetry was applied so that asymmetric structures could be explored.…”

“…G = was then obtained by comparing the total free energies of the ground states of reactants (cation + OH − ) with the free energy of the TS state. 20 …”

Reactivity of the Ionic Liquid Pyr14TFSI with Superoxide Radicals Generated from KO2or by Contact of O2with Li7Ti5O12

“…Moreover, the methyl group can also be removed in a S N 2 reaction, which is, however, less favorable. 55 The final product of this S N 2 reaction would be methanol, which could not be detected by NMR in this study due to the overlap with the H g of pyrrolidinium. Yet, it was possible to distinguish signals of decomposition products with a chemical shift similar to the pyrrolidinium cation, which are highlighted by the orange boxes in Figure 7b, even though they could not be reasonably integrated due to their overlap with the Pyr + 14 signals and satellites.…”

“…55 Even though such rules provide some hints how a Hofmann elimination reaction might be prevented, the safest rule is the complete avoidance of any β-H. Moreover, the methyl group can also be removed in a S N 2 reaction, which is, however, less favorable.…”