Thanks to the coupling between chemical precipitation reactions and hydrodynamics, new dynamic phenomena may be obtained and new types of materials can be synthesized.

The evolution of different confined precipitation patterns is determined by the ratio of the chemical and hydrodynamic time scales.

Exploring the kinetics of metal−organic framework crystallization is crucial in order to design novel synthesis methods benefiting from far-from-equilibrium conditions, such as flow, microfluidic, or gel-phase reactors. Herein we focus on zeolitic imidazolate frameworks (ZIFs) obtained in the reaction of 2-methylimidazolate and zinc ions as a model system. The room-temperature synthesis carried out in pure water solvent without the addition of any alkaline chemical yields a highly crystalline product with good conversion within minutes. The short-term kinetic characteristics were determined by a high-speed camera, the result of which was complemented by a UV−vis photometer based long-term investigation. At low concentrations and moderate linker excess, which facilitate sluggish coordination and precipitation, a fast colloid formation is followed by a delayed and remarkable recrystallization, which leads to the formation of a dense dia(Zn) polymorph of ZIF. Although this is the thermodynamically favored route, it should be avoided due to the nonporous structure of the product. The same stoichiometric excess of the linker ions together with higher concentrations gives rise to the production of the thermodynamically less stable polymorph of ZIF: i.e., ZIF-8 with a sodalite-like structure is obtained via kinetic control. Similar conclusions are drawn in the case of cobalt(II)-containing imidazolate frameworks, which highlights that kinetic control based polymorph selection might be achieved for other MOFs as well. Our kinetic results were supported by scanning electron microscopy, powder X-ray diffraction, nitrogen gas adsorption, and Fourier-transformed infrared and Raman spectroscopy.

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Flow-driven precipitation experiments are performed in model porous media shaped within the confinement of a Hele-Shaw cell. Precipitation pattern formation and the yield of the reaction are investigated when borosilicate...