Amorphous-to-Crystalline Transformation: How Cluster Aggregation Drives the Multistep Nucleation of ZIF-8.

Nucleation, the pivotal first step of crystallization, governs the essential characteristics of crystallization products, including size distribution, morphology, and polymorphism. While understanding this process is paramount to the design of chemical, pharmaceutical, and industrial production processes, major knowledge gaps remain, especially with respect to the crystallization of porous solids. Also for nanocrystalline ZIF-8, one of the most widely studied metal-organic frameworks, questions regarding the species involved in the nucleation pathway and their structural and chemical transformations remain unanswered. By combining harmonic light scattering, inherently sensitive to structural changes, with NMR spectroscopy, which reveals molecular exchanges between particles and solution, we were able to capture the crystallization mechanism of ZIF-8 in unprecedented detail. This dual approach provides concurrent structural and chemical insights, revealing key processes not previously observed in ZIF crystallization. Upon mixing, small charged prenucleation clusters (PNCs) are formed, exhibiting an excess of ligands and net positive charge. We show that nucleation is initiated by aggregation of PNCs, through the release of ligands and associated protons to the liquid. This leads to the formation of charge neutral amorphous precursor particles (APPs), which incorporate neutral monomers from the solution and crystallized ZIF-8. Our work highlights chemical dynamics as a vital, yet often overlooked, dimension in the multistage structural evolution of MOFs. By establishing the critical role of PNCs in the nucleation of ZIF-8, new pathways open up for controlling crystallization of metal-organic frameworks through targeted chemical interactions with these species.