Understanding hydroxide reactions with guanidinium-based anion exchange polymers under conditions relevant to bipolar membrane electrodialysis

Abstract

Anion exchange polymers are susceptible to loss of cationic functionality as a result of nucleophilic attack by hydroxide ions. This research investigated the stability of two guanidinium-based cations as anion exchange functional groups under conditions relevant to bipolar membrane electrodialysis. Density functional theory simulations were performed to investigate reaction energies and activation barriers for reactions of hydroxide ions with pentamethylguanidinium (PMG) and hexamethylguanidinium (HMG) cations bound to a diaryl ketone polymer backbone. The effect of physically adsorbed chloride and hydroxide ions, and chemically adsorbed hydroxide ions, on reaction energetics were determined. Fukui functions for nucleophilic attack were used to identify locations most likely to undergo reactions with hydroxide ions. For the PMG species, the most likely bond cleavage reactions were highly exergonic, with ΔGrxno values as large as −46 kcal/mol and activation barriers less than 10 kcal/mol. Chemisorption of hydroxide ions on both PMG and HMG cations was energetically favorable, with activation barriers of 5.3 and 2.7 kcal/mol, and resulted in loss of cationic functionality. Anion adsorption changed the reactivity of both PMG and HMG structures towards nucleophilic attack. For nucleophilic attack at the phenyl carbon atom, adsorption of OH− on the guanidinium carbon atom made PMG less reactive, while adsorption of Cl− made HMG less reactive. Bond cleavage and loss of cationic functionality was a two or three step process involving addition of OH− to the phenyl or guanidinium carbon atoms, followed by bond stretching or deprotonation of the added hydroxide species. For PMG species, deprotonation of the hydroxide resulted in bond cleavage or produced metastable species that decomposed with activation barriers less than 2 kcal/mol. HMG species were more stable with respect to this degradation mechanism, having activation barriers for bond cleavage ranging from 17 to 29 kcal/mol.