Computational modeling of the stability and reactivity of zeolitic imidazolate frameworks

Author / Creator

Weng, Tingting, 1992- author

Available as

Online

Physical

Summary

Targeted synthesis of metal-organic frameworks (MOFs) is challenging, which requires developing a comprehensive and predictive model of MOF crystallization under synthetic conditions to provide mec...

Targeted synthesis of metal-organic frameworks (MOFs) is challenging, which requires developing a comprehensive and predictive model of MOF crystallization under synthetic conditions to provide mechanistic insights. With this ultimate goal, we initiate our effort in this thesis with computational modeling of the stability and reactivity of zeolitic imidazolate frameworks (ZIFs), a subclass of MOFs. First, we develop a transferable ab-initio intramolecular force field "ZIF-FF", that is capable of properly describing the relative stability of ZIF polymorphs, a crucial element in ZIF nucleation and crystal growth. Beginning with a general Amber force field (GAFF), Zn-related force field parameters are optimized against dispersion-corrected DFT-calculated properties using a genetic algorithm. We validate the resulting force field by examining bond and angle distributions, phonon density of states, mechanical properties, diffusion properties and via modeling a ZIF amorphization process. Furthermore, we find that ZIF-FF is transferable in ZIF surfaces and functionalized ZIFs. Second, we examine the diversity of surface terminations of ZIFs under a variety of conditions including gas-phase, "post-synthetic" and solution-phase conditions. We construct surface phase diagrams to predict the most stable ZIF surface terminations as a function of external parameters (including temperature, adsorbate pressures, and pH), and find that the resulting phase diagrams can be used to explain the results of prior experimental studies of ZIF terminations across a variety of conditions and provide important insights into the factors that govern the structures of ZIF interfaces. Third, we explore a special crystal growth process of ZIFs, linker exchange of ZIF-8. Using DFT and nudged elastic band method for transition state search, we investigate the possible reaction pathways in dry gas phase and wet vapor phase. To further elucidate the explicit solvation effect in solution, we apply DFTB approach combining with enhanced sampling well-tempered metadynamics simulations. The proposed reaction mechanisms well explain prior experimental results on linker exchange. We anticipate that our models can be further extended to reveal ZIF or other MOF crystal growth mechanisms at interfaces.

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