Zr-MOFs for Hydrolysis Reactions: Computational Evaluation of Interactions, Kinetics, and Molecular Accessibility

Abstract

Metal-organic frameworks (MOFs) are a class of porous crystalline solids that present as promising instruments for the removal of chemical warfare agents (CWAs). Owing to their remarkable porosities and large surface areas, MOFs possess superior adsorption, reactivity, and catalytic abilities, providing an ideal environment for target species capture and decomposition. The tunable networks of MOFs also allow for customization of their chemical functionalities, making them practicable in personal protective equipment and adjustable to dynamic environments. While progress has been made in the tailoring of MOF-based materials for toxic chemical degradation applications, issues of small pore apertures and poor water stability have hindered their practicality. Recent breakthroughs have shown that zirconium-based MOFs have the highest potential among different MOFs for hydrolytic and oxidative degradation of CWAs; however, it remains unclear what combination of features enables efficient breakdown in the solid phase and under realistic environmental conditions of humidity. Furthermore, many puzzles still exist regarding how those features may change with respect to the specific toxic chemical and the mechanism of detoxification. Characterizing structure-property relationships of different Zr-MOFs with various CWAs in the presence of atmospheric moisture is essential to establishing design rules that will lead to effective degradation under relevant field conditions. This dissertation focuses on the use of computational modeling to gain insight into the role of structure and topology in the molecular accessibility, mass transport, kinetics, and adsorption of water, nerve agents, and their simulants in Zr-based MOFs. With combined molecular dynamics (MD) and density functional theory (DFT) approaches, the effects of pore size, connectivity, and hydrophobicity/hydrophilicity on adsorbed species distribution, binding, residence time, and diffusion in Zr-MOFs are explored. In the first section, we provide a comprehensive overview of intrinsic catalytic reaction mechanisms in MOFs, the design of efficient degradation strategies in the aqueous and solid phases, and the tuning and functionalization of MOFs to enhance CWA removal under realistic battlefield conditions. In the second section, we utilize a combination of equilibrium and non-equilibrium MD simulations to investigate the transport diffusion properties of water in two main classes of Zr-MOFs. In the third section, we propose a combination of MOF design rules that lead to promising performance characteristics for hydrolysis operations in conditions of varying humidity, supported by MD simulations. We also we develop a code for calculating the radial distribution function of adsorbed molecules in nonuniform systems. In the fourth section, we use MD and DFT approaches to evaluate whether those design rules remain relevant for degradation of different types of nerve agents, both by themselves and in the presence of water. We also investigate whether reaction sites and mechanisms are likely to remain the same for all nerve agents in all Zr-MOFs. The results of this dissertation contribute to the advancement of MOF-based strategies for the destruction of CWAs and highlight the potential of these materials to address the challenges associated with chemical warfare.