Effect of countercation on the water stability of an anionic metal–organic framework

Abstract

Methods for adjusting and potentially improving the water stability of metal–organic frameworks (MOFs) are important for many potential applications. Herein, we investigate how the identity of uncoordinated countercations housed within the channels of an anionic MOF affect its structure and porosity after exposure to liquid water and water vapour. bMOF-1, Zn₈(ad)₄(BPDC)₆O·2 cations (ad = adeninate; BPDC = biphenyldicarboxylate), was used for this study and different alkylammonium countercations were incorporated into the framework via cation exchange. In general, it was determined that cations having larger alkyl chains resulted in a more water stable bMOF-1 material; conversely, more hydrophilic cations detrimentally affected bMOF-1 stability to water. Finally, bMOF-1 was loaded with mixtures of both hydrophilic and hydrophobic cations. It was found that these bMOF-1 samples, despite the presence of the hydrophilic cations, still maintained their structure and porosity after exposure to water vapour.