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 N2 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 H2/CH4, CO2/N2 and CO2/CH4. Mixed‐metal MOFs: Dual‐metal zeolitic imidazolate framework (ZIF) crystals with SOD and RHO topologies were prepared from metastable ZIF‐108 (Zn(2‐nitroimidazolate)2) by metal substitution (see figure). The transformation of ZIF‐108 is a dissolution/heterogeneous nucleation process requiring much lower activation energy than one‐step homogenous nucleation (Etwo‐step≪ Eone‐step). Ni‐Zn(nim)2 exhibits high adsorption selectivity for CO2 over N2, and the H2 and CO2 permeability of a polysulfone membrane was improved by using Co‐Zn(nim)2 nanoparticles as filler.