Pre-programed hydroxy double salt templates for room-temperature controlled synthesis of mixed-metal zeolitic imidazolate frameworks

Abstract

Mixed-metal metal–organic frameworks (mmMOFs) are promising for many envisioned applications such as gas adsorption, separations, catalysis and electrochemical energy storage. However, it is still very challenging to achieve controlled stoichiometric composition and uniform distribution of the metal-containing secondary building units in mmMOFs at room temperature. Here, we report a room-temperature conversion strategy using pre-programed hydroxy double salts (HDSs) as templates to synthesize mixed-metal zeolitic imidazolate frameworks (mmZIFs) including ZIF-7, ZIF-8 and ZIF-71. We found that the HDS prepared via coprecipitation enables precise control over the Co : Zn ratio in the mmZIF-7 with a discrepancy of only 3.0% between the metal ratio in the final product and the precursor form. A two-step mechanism was found for the HDS conversion process involving intercalation of organic linker anions in the HDS and subsequent reaction between the linker and the cationic nanosheets. We further investigated our mmZIF-7 and found that the Co : Zn ratio of the mmZIF-7 affects the BET surface area and CO₂ uptake at 298 K. With facilitated CO₂ transport, these mmZIF-7 fillers enable excellent performance for CO₂/N₂ separation in Pebax-2533 mixed matrix membranes.