Highly stable polysulfone anion exchange membranes incorporated with bulky alkyl substituted guanidinium cations

Abstract

A series of guanidinium-based cations with various N-substituted groups (methyl, ethyl, isopropyl, cyclobutyl, benzyl and phenyl) were synthesized and their alkaline stabilities were evaluated by ¹H NMR analysis. The results indicated that the contribution of the substituent group toward the stability followed the sequences: isopropyl > ethyl > benzyl > phenyl and ethyl > methyl > cyclobutyl. The relative stability sequences suggested that free and bulky alkyl substituent groups with large steric hindrance can dramatically improve the alkaline stability. Tetraethyl-benzyl-isopropyl guanidinium showed the highest alkaline stability which remained unchanged after treatment with 1 mol L⁻¹ KOH/CD₃OD at 60 °C for 312 h. Novel polysulfone anion exchange membranes (AEMs) with such stable guanidinium cations were prepared via iridium-catalyzed C–H activation and the Suzuki–Miyaura reaction. The obtained guanidinium functionalized AEM exhibited excellent resistance to alkaline solutions (after immersing in 1 mol L⁻¹ NaOH at 60 °C for 30 days, no degradation was observed), indicating that AEMs functionalized by guanidiniums with large steric hindrance are very promising materials for AEM applications.