Synthesis and adsorption properties of ZIF-76 isomorphs

ZIF-76, Zn(Im)x(5-ClbIm)y (Im = imidazolate, 5-ClbIm = 5-chlorobenzimidazolate), is, among the known ZIF structures, one of the few materials that combine a relatively large pore aperture with a large pore volume. Moreover, it is thermally stable up to 400 °C and can be stored under ambient atmosphere without undergoing a measurable degradation. ZIF-76, therefore, appears to be an interesting material for applications in adsorption and separation. The published synthesis protocol leads to very low yields and is, thus, not applicable on a larger scale [1]. In this work, we have succeeded in preparing ZIF-76 in suitable yields by changing the solvent and adding NaOH to the synthesis mixture. The addition of the base increases the yield and leads to smaller crystal sizes, by favoring nucleation over crystal growth. By using the same synthesis strategy, four new isomorphs of ZIF-76 were obtained, with different substituents on the benzimidazole ligand (Me, Br and NO2) and different metal sources (Co and Zn). All the samples were tested in the adsorption and separation of CO2/CH4 in order to highlight the influence of the linker and of the metal center on the adsorption/separation properties.

Graphical abstractFigure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Synthesis of ZIF-76 is optimized by changing the solvent and adding NaOH. ► Applying the same strategy leads to obtain different isomorphs of ZIF-76. ► All these solids were tested in the separation CO2/CH4. ► The results highlight the contribution of the linker and the nature of the metal center.