Deciphering Metal–Organic Framework Synthesis from Hydroxy Double Salts: In-Situ Insights via Synchrotron X‑ray Diffraction and Absorption Spectroscopy

Abstract

Developing rapid room-temperature synthesis is key to sustainable manufacturing of metal–organic frameworks (MOFs). Using layered compounds such as hydroxy double salts (HDSs) as precursors significantly promotes the reaction kinetics and lowers the required synthesis temperature. However, limited understanding of the reaction mechanism impedes the further exploration of new routes for MOF synthesis. Here, we report for the first time the use of combined in-situ synchrotron X-ray diffraction and X-ray absorption spectroscopy to monitor the dynamic processes to form MOFs in solution. The conversion from a (Zn,Co) HDS to a mixed-metal zeolitic imidazolate framework-8 (mmZIF-8) was chosen as our model reaction. Time-resolved diffraction patterns exclude the presence of intercalated HDS structures with altered d-spacing and any other crystalline intermediate phase during the synthesis. The activation energies of nucleation and growth were found as 25.5 ± 2.5 and 64.0 ± 7.9 kJ·mol–1, respectively. In addition, we captured the evolution of local structures from mixed coordination states in the HDS to tetrahedral coordination in the mmZIF-8. Furthermore, two possible reaction pathways were proposed to account for the fast conversion from HDS to mmZIF-8. The fundamental understanding towards the HDS-based synthesis obtained in this work is expected to guide future development of new fabrication methods for MOF materials.