Mediating Order and Modulating Porosity by Controlled Hydrolysis in a Phosphonate Monoester Metal–Organic Framework

Abstract

A crystalline and permanently porous copper phosphonate monoester framework has been synthesized from a tetraaryl trigonal phosphonate monoester linker. This material has a surface area over 1000 m² g^-1 , as measured by N₂ sorption, the highest reported for a phosphonate-based metal-organic framework (MOF). The monoesters result in hydrophobic pore surfaces that give a low heat of adsorption for CO₂ and low calculated selectivity for CO₂ over N₂ and CH₄ in binary mixtures. By careful manipulation of synthetic conditions, it is possible to selectively remove some of the monoesters lining the pore to form a hydrogen phosphonate while giving an isomorphous structure. This increases the affinity of the framework for CO₂ giving higher ambient uptake, higher heat of adsorption, and much higher calculated selectivity for CO₂ over both N₂ and CH₄ . Formation of the acid groups is noteworthy as complexation with the parent acid gives a different structure.