Metal-Substituted Zeolitic Imidazolate Framework ZIF-108: Gas-Sorption and Membrane-Separation Properties

A series of dual-metal zeolitic imidazolate framework (ZIF) crystals with SOD and RHO topologies was synthesised by metal substitution from ZIF-108 (Zn(2-nitroimidazolate)(2), SOD topology) as the parent material. This was based on the concept that metal substitution of ZIF-108 requires a much lower activation energy than homogenous nucleation owing to the metastability of ZIF-108. In-depth investigations of the formation processes of the daughter ZIFs indicated that the transformation of ZIF-108 is a dissolution/heterogeneous nucleation process. Typical isostructural Co2+ substitution mainly occurs at the outer surface of ZIF-108 and results in a core-shell structure. On the contrary, the Cu2+-substituted ZIF has a RHO topology with a homogeneous distribution of Cu2+ ions in the structure. Substitution with Ni2+ resulted in a remarkable enhancement in adsorption selectivity toward CO2 over N-2 by a factor of up to 227. With Co2+-substituted nanoparticles as inorganic filler, a mixed matrix membrane based on polysulfone displayed greatly improved performance in the separation of H-2/CH4, CO2/N-2 and CO2/CH4.