Facile synthesis of Cd-substituted zeolitic-imidazolate framework Cd-ZIF-8 and mixed-metal CdZn-ZIF-8

Zeolitic-imidazole framework ZIF-8 has attracted tremendous interests for the high-resolution kinetic separation of propylene/propane mixture due to its effective aperture size in between the sizes of propylene and propane molecules. It is of great interest to fine-tune the effective aperture size of ZIF-8 either to improve its propylene/propane separation performances or to extend its use to the separation of other gas mixtures. It has been shown that substituting Zn with other metal nodes (e.g. Co) is a potential means to fine-tune the effective aperture size of ZIF-8. Here, we attempt to introduce another metal center, Cd, into ZIF-8 in a facile and scalable manner. Phase-pure Cd-ZIF-8 was successfully synthesized in methanol using a conventional solvothermal method, although it showed a narrow synthesis window. The presence of an organic base (triethylamine, TEA) was found critical not only for the facile synthesis of phase-pure Cd-ZIF-8 but also for the suppression of its phase transformation. A battery of characterizations including single-crystal X-ray structure solutions confirmed that the effective aperture size of Cd-ZIF-8 is the largest among its iso-structures (Zn-ZIF-8 and Co-ZIF-8). Finally, for the first time, mixed-metal CdZn-ZIF-8 crystals with various Cd/Zn ratios were solvothermally synthesized, demonstrating a further opportunity for varying the effective aperture sizes of ZIF-8 and its iso-structures.