Understanding the Nucleation and Growth of ZIF-8 Polymorphs

Polymorphism is an important concept in crystallization and has been widely studied for many systems, including calcium carbonate and zeolite materials. Metal-organic frameworks (MOFs) are crystalline materials that exhibit polymorphism. MOF polymorphism has been widely studied from the perspective of which synthetic factors, such as the ligand to metal ratio, can be used to control the final polymorph. However, limited studies on the nucleation mechanism of multipolymorph MOFs have been performed. Here we study the formation of a model zeolitic imidazole framework-8 and the mechanism that drives the formation of the two most commonly observed polymorphs, sodalite and diamondoid. To understand the mechanism and factors that affect polymorph formation, we performed time-resolved in situ wide-field X-ray scattering, electrospray ionization mass spectrometry, and time-resolved cryogenic transmission electron microscopy. The collective data reveal a clear correlation between the size of the prenucleation clusters and the final polymorph. By gaining a deeper understanding of the mechanisms governing polymorph control in MOF systems, we can improve the design of synthetic conditions, allowing the tailoring of crystal properties.