Fine-Tuning the Aperture Size of Co-ZIF-67 via Dehydration, Demethylation, and Co-Zn Hybridization

Abstract Co-ZIF-67 (Zeolitic Imidazolate Framework) is a porous material where cobalt (Co) metal ions bond with organic ligands through tetrahedral coordination. It exhibits high chemical and thermal stability similar to that of zeolite structures and is utilized for gas separation, making it an ideal target for research. A large single crystal of Co-ZIF-67 was successfully synthesized using a simple hydrothermal method. Dehydration and demethylation of the 2-methylimidazole in Co-ZIF-67, as well as altering the central metal, change the aperture size and affect the types of gases that can be separated. Therefore, in this work, dehydrated, demethylated, and hybridized Co-ZIF-67s with cobalt (Co) and zinc (Zn) were prepared in this work. These crystal structures were analyzed using single-crystal synchrotron X-ray diffraction techniques. In the untreated Co-ZIF-67 single crystal, water molecules were located at two sites opposite the 6-ring, with eight molecules per unit cell. Heat treatment at 673 K completely removed the water molecules without loss of crystallinity, and demethylation began at 683 K. As the temperature increased, the number of carbon atom at C3 per unit cell decreased, indicating the gradual removal of methyl groups. Measurement of the 6-ring aperture size revealed that heat-treated crystals had larger aperture sizes compared to untreated crystal 1. However, at 753 K, single-crystal X-ray diffraction data could not be obtained due to the loss of Co-ZIF-67 crystallinity. In the case of Co/Zn-ZIF-67 single crystals synthesized with different molar ratios, the structures were similar to those of Co-ZIF-67 single crystals, with Co exhibiting higher occupancy rates compare to Zn. Additionally, the Co/Zn-ZIF-67 demonstrated smaller unit cell parameters compared to Co-ZIF-67 (crystal 1). Consequently, the aperture size of the Co/Zn-ZIF-67 with tuning of the amount of Co and Zn was smaller than that of Co-ZIF-67 (crystal 1).