Insight into Fast Nucleation and Growth of Zeolitic Imidazolate Framework-71 by In Situ Time-Resolved Light and X-ray Scattering Experiments

The current rather poor understanding of the mechanisms of crystallization of zeolitic imidazolate frameworks (ZIFs), an important subclass of porous metal organic framework (MOF) materials, greatly hampers rational synthesis of predicted ZIF phases/polymorphs and control of crystal size/shape. In this contribution, we present the results of in situ studies on a recently developed fast ZIF-71 nanocrystal synthesis from solution. By taking advantage of the combined use of time-resolved static and dynamic light scattering and stopped-flow synchrotron small-angle and wide-angle X-ray scattering, we were able to reveal the whole nanocrystal formation process: a population of small amorphous particles (termed clusters) first forms via coagulation, followed by the formation of a second population of bigger amorphous particles that grow via addition of significantly smaller monomers, the nature of which remains as yet unknown (clusters and/or smaller units). The latter particles transform into the periodic ZIF-71 structure with RHO topology, probably via intraparticle nucleation and subsequent particle reorganization. The reaction kinetics speeds up with increasing linker-to metal concentration ratio, yielding nanocrystals with decreasing size, while the life times of amorphous intermediates become very short and challenging to be observed experimentally. We discuss how our results complement and extend recent findings on the crystallization of ZIF-8 with SOD topology under similar synthetic conditions.