Cation‐Exchange Porosity Tuning in Anionic Metal–Organic Frameworks for the Selective Separation of Gases and Vapors and for Catalysis

Abstract

The outperforming adsorptive properties of the so-called open metal–organic frameworks (MOFs) or porous coordination polymers (PCPs) rely on their fully accessible porous structure and the easy tuning of the shape, size, and chemical nature of their pores. The ability of some of these systems to mimic the structure and properties of zeolites has also been realized. There are, however, unsolved problems related to the general lower thermal and chemical stability (hydrolysissensitive nature) of MOFs compared to their zeolite counterparts. Consequently, the search for highly robust MOFs capable of withstanding the working conditions typically found in industrial processes is a highly desirable challenge. In this regard, the robustness of the metal–nitrogen(heterocycle) coordinative bonds leads to the formation of MOF materials with enhanced chemical and thermal stabilities. It should also be noted that, in contrast to the wellknown cation-exchange features of zeolites, the zeomimetic coordination polymers generally possess neutral or cationic frameworks and consequently do not usually give rise to cation-exchange processes. Herein, we report the synthesis, structural characterization, thermal/chemical stability, and adsorptive, separation, and catalytic properties of the anionic MOF NH4[Cu3(m3-OH)(m3-4-carboxypyrazolato)3] (NH4@1). In addition, we have examined the plausible modulation of its porous network by means of ion-exchange processes of the extraframework cations. The results show that the ion-exchange processes on these systems lead to profound changes in the textural properties of their porous surface and in the adsorption selectivity of different separation processes of gases and vapors. The crystal structure of NH4@1 [10] is based on an anionic 3D porous framework built up of trinuclear Cu3(m3-OH) clusters connected to another six through m3-4-carboxypyrazolato bridges (Figure 1). In this way, tetrahedral cages with