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 Co 2+ substitution mainly occurs at the outer surface of ZIF‐108 and results in a core–shell structure. On the contrary, the Cu 2+ ‐substituted ZIF has a RHO topology with a homogeneous distribution of Cu 2+ ions in the structure. Substitution with Ni 2+ resulted in a remarkable enhancement in adsorption selectivity toward CO 2 over N 2 by a factor of up to 227. With Co 2+ ‐substituted nanoparticles as inorganic filler, a mixed matrix membrane based on polysulfone displayed greatly improved performance in the separation of H 2 /CH 4 , CO 2 /N 2 and CO 2 /CH 4 .