Solvothermal Crystallization Kinetics and Control of Crystal Size Distribution of MOF-808 in a Continuous Flow Reactor

Abstract

A fundamental understanding of the crystallization pathways for metal–organic frameworks (MOFs) allows for exploring the untapped combinatorial space of the organic and inorganic building units, creating possibilities to synthesize highly crystalline frameworks with desired physicochemical properties. In this work, we employ a continuous flow reactor to elucidate the kinetics of crystallization for the Zr-based MOF-808 using time-resolved powder X-ray diffraction measurements. Specifically, we fit the crystallization curves obtained experimentally using the Gualtieri model to determine the rate constants for nucleation (kN) and growth (kG) for different linker concentrations and temperatures. Higher linker concentrations reduce the competitive coordination of the formate ligand (growth modulator) with the secondary building unit, resulting in higher nucleation and growth rates. The activation energies obtained from Arrhenius plots for nucleation (Ea(N)) and growth (Ea(G)) are 64.7 ± 4 and 59.2 ± 5 kJ mol–1, respectively. At constant residence time, temperature, and composition, higher flow velocities increase the advective transport of precursor species to nucleation sites in the slugs resulting in increased crystal growth rates and thus higher average crystal sizes. Variation in the total flow rate from 0.334 to 1.067 mL/min increased the average crystal sizes from ∼105 ± 22 to ∼180 ± 19 nm, with other parameters held constant. We demonstrate that performing crystallization in the flow reactor provides a unique opportunity to tailor MOF crystal sizes. By strictly controlling the temperature, residence time, and mixing parameters, our results showcase the advantages of flow systems for performing rigorous crystallization and structural evolution studies that can be applied for the synthesis of other MOFs with tailored physicochemical properties.

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