Control of Metal–Organic Framework Crystallization by Metastable Intermediate Pre-equilibrium Species

The phenomenon of crystallization is poorly understood and yet is critical to the design of new materials, such as metal-organic frameworks (MOFs), for which predictive formation mechanisms remain elusive. We report a tandem in situ X-ray diffraction and pH investigation into the prototypical MOF, ZIF-8, in which it is found that the crystallization mechanism changes from phase boundary-limited to diffusion-limited across the course of each reaction, and crystallization rates decrease with increasing concentration. These counterintuitive observations are explained using a non-linear pre-equilibrium model, in which network formation depends on the rates of Zn-imidazole coordination and imidazole deprotonation. Whilst assembly of under-coordinated species is associative and fast, over-coordinated species act to inhibit network formation. The work rationalises apparent contradictions between previous studies and opens up new opportunities for the creation of MOFs with control over particle size, composition and local structure.