(602d) A Microkinetic Model for Understanding the Synthesis of Thin Film Metal Organic Framework Using Solution Shearing Based Evaporative Crystallization

Thin-film metal-organic frameworks (MOFs) are attracting an increasing interest due to their wide range of applicability in separations, storage, sensing, and catalysis. They have been fabricated using many techniques, and synthesis using solution shearing-based evaporative crystallization is one. Although the effect of different variables was studied experimentally, there is still a need for a theoretical understanding of the effect of variables on crystallinity, coverage, and film thickness. Recently, a microkinetic model was developed to understand the mechanism of crystallization in a batch reactor. However, there have been no previous reports on the theoretical understanding of the synthesis of thin-film MOFs. In this work, a microkinetic model consisting of eight parameters, optimized using the time-resolved in-situ X-ray scattering experimental data, was developed to get insights into the nucleation and growth mechanisms of the thin film MOF UiO-66. The formation of secondary building units, which follows the autocatalytic initiation reaction, depends on the number of available sites for the attachment of metal-ligand and organic linker in the bulk solution as well as the adsorbed growth units on the substrate. Reaction dynamics of the adsorbed growth units is driven by the effective rate constant, which is dependent on the concentration of growth units formed, rate constant from the batch reaction, and diffusion limited rate constant. We observed that the reaction dynamics of the first layer formed on this substrate are different from other layers. These first layer dynamics determine the thickness of the film formed, crystal size distribution, and fractional coverage obtained from a single solution shearing. Overall, a micro-kinetic model was devised, that explains the dynamics of growth of the thin film MOF, which is challenging to obtain even with the much developed state-of-the-art experimental and computational techniques.