Rapid-Mixing In Situ X-ray Diffraction Reveals Precursor-Reorganization Signatures in Additive-Free Zeolitic Imidazolate Framework-8 Nanoparticle Formation

Controlling particle size and phase purity in solution-processed metal–organic frameworks requires understanding how disordered precursors evolve before they crystallize into the final framework. However, capturing this precursor-to-crystal conversion remains a key challenge. Here, we couple a central-collision-type microreactor with time-resolved in situ X-ray diffraction to track the additive-free formation of zeolitic imidazolate framework-8 (ZIF-8) in water–methanol cosolvents, achieving 5 s temporal resolution after submillisecond micromixing. The setup directly captures crystal appearance, apparent crystallization kinetics, and subsequent lattice relaxation. Crystallization is fastest at 40–50 vol % methanol, where Bragg reflections appear earliest and grow most rapidly. Water-rich media approach a competing phase boundary between ZIF-8 and layered zeolitic imidazolate framework-L (ZIF-L) and yield a predominantly amorphous product at the water-rich extreme, whereas methanol-rich media form pure ZIF-8 more slowly. After crystallinity emerges, the apparent lattice constant contracts, indicating that the earliest Bragg-detectable domains undergo lattice relaxation as growth proceeds. Strikingly, raising the total reactant concentration at a fixed metal-to-linker ratio delays crystal appearance and yields larger particles, contrary to the behavior typically expected in a nucleation-dominated system. We propose that concentration-dependent precursor accumulation, coarsening, and interconnection, coupled to metal–ligand network reorganization, link these contrasting trends.