Efficient and strategical installations of quaternary ammonium groups in metal-organic frameworks for hydroxide conductivity

Positively charged aromatic quaternary ammonium and aliphatic quaternary ammonium groups were successfully installed in metal-organic frameworks (MOFs) for hydroxide conductivity studies. Two representative solid-state MOF functionalization strategies, post-synthetic covalent modification (PSM) and post-synthetic ligand exchange (PSE), were used and their charge-loading efficiencies were compared. PSE processes were successfully performed for both aromatic and aliphatic quaternary ammonium groups, and the PSM technique was restricted to the aromatic ammonium functionalization of MOFs. Although all ammonium installations induced significantly enhanced hydroxide conductivities compared to the bare MOF, the aliphatic ammonium group via the triazole linker from the CuAAC click chemistry showed the best performance. This study discusses the practicality of PSE for MOF functionalization and the additional effects of functional group modification.