Crystalline Structure and Thermal Stability of an Unknown ZIF-L300 Phase.

ABSTRACT

In recent years, the synthesis, crystalline structure, and applications of zeolite imidazole frameworks (ZIFs) have attracted extensive attention. Since the ZIF-L phase was synthesized, a new phase was observed during the heating process, but its crystal structure is unknown. The unknown new phase, which was named ZIF-L300 in this study, was confirmed again. In this study, the X-ray powder diffraction technique and Rietveld refinement were used to solve the crystalline structure of the unknown ZIF-L300 phase. The results demonstrate that ZIF-L300 has the same chemical formula (ZnC8N4H10) as in ZIF-8 and belongs to a hexagonal structure with a space group of P61. The lattice parameters have been determined as follows a = b = 8.708(7) Å, c = 24.195(19) Å, α = ß = 90°, and γ = 120°. The X-ray absorption fine structure (XAFS) technique was also used to extract the local atomic structures. The in situ X-ray diffraction (XRD) technique was used to monitor the structural evolution of the as-prepared ZIF-L in a temperature range from room temperature to 600 °C. The results show that the sample experiences a change process from the initial ZIF-L orthorhombic phase (<210 °C), to the ZIF-L300 hexagonal phase (∼300 °C), then to an amorphous phase (∼390 °C), and finally to a zincite ZnO phase (>420 °C). These sorts of structural information are helpful to the application of ZIF materials and enrich the knowledge of the thermal stability of ZIF materials.