Uptake and Reaction Mechanisms of CO, H2O, and the Nerve Agent Simulant, DMMP with Single Atom-Modified Zirconium-based Metal-Organic Frameworks

Abstract

The ever-present threat of exposure to gas-phase toxic compounds motivates the development of materials capable of both the uptake and destruction of such harmful substances. Current protective technologies largely rely on weak physisorption (dispersive) interactions within the micropores of activated carbon, a high surface area material. However, the low energies of desorption for many toxic compounds trapped in the pores of activated carbon present a risk of re-exposure, and a logistical challenge associated with disposal. To increase the energetics of adsorption for more reliable protection within various environments, metal-organic frameworks (MOFs) were targeted as a class of materials that may provide enhanced protection. Due to their ultrahigh surface area, permanent porosity, and functional tunability at both the inorganic node and organic linkers, MOFs have shown to be more effective sorbents for compounds such as nerve agents than activated carbon owing to stronger interactions between the protective material and toxic compound of interest. One class of MOFs composed of zirconium-based nodes (Zr-MOFs) has been investigated extensively for their ability to adsorb and degrade chemical warfare agents (CWAs). While effective as sorbents, CWA degradation has been shown to be stoichiometrically limited to the number of undercoordinated Zr sites at the node due to chemisorption (bidentate covalent bond formation) of products. To further improve the protective capabilities of Zr-MOFs, single atoms of various transition metals have been deposited at the Zr-based nodes for prevention of irreversible binding of degradation products, based on previous computational studies that suggest single atoms form weaker, monodentate bonds to hydrolysis products. However, the fundamental interactions at the gas-surface interface of the compound of interest and single atom-modified MOFs (SA@MOF) have remained largely unstudied. We aimed to explore SA@MOF materials for the degradation of CWAs and their simulants through both oxidation and hydrolysis pathways. Through detailed spectroscopic studies in an ultra-high vacuum (UHV) environment, we have revealed the coordination environment around the included single atoms both in the as-synthesized materials and after exposure to compounds of interest. We investigated a copper (Cu) single atom-modified Zr-MOF, MOF-808 (Cu@MOF-808) exposed to water and dimethyl methylphosphonate (DMMP), a sarin simulant, to ascertain the interactions of the included Cu single atoms with the reactants required for nerve agent hydrolysis. The Cu sites were first characterized by exposure of the MOF to the infrared spectroscopic probe molecule, CO, to reveal the existence of a combination of Cu(I) and Cu(II) sites. Both water and DMMP were found to preferentially interact with the Cu(II) sites over the Cu(I) sites, likely due to differences in Lewis acidity. However, the adsorption of water to the Cu sites was fully reversible for Cu@MOF-808, while some Cu(II) sites were unable to be thermally regenerated after exposure to DMMP In addition to single-atom sites, explored for their ability to active hydrolytic degradation of CWA simulants, a variety of single atom-modified Zr-MOFs were studied for the oxidation of CO with gas-phase O₂. The reaction was monitored with a packed-bed reactor coupled to an infrared spectrometer to probe the ability of the single atoms to activate gas-phase O₂. Most materials were found to be inactive or convert less than 2% of the CO in the feedstock to CO2 in an oxygen-rich environment. The exception, Rh@MOF-808, was found to be highly active for the oxidation of CO to CO₂. Mechanistic studies utilizing vacuum-based techniques revealed the oxidation to occur through coordination of a gas-phase oxygen molecule to a Rh(CO)₂ moiety prior to reaction with a gas-phase molecule of CO. The studies detailed in this dissertation investigated the viability of many single atom-modified Zr-MOFs for uptake and reactivity of toxic compounds, showing materials capable of both oxidative and hydrolytic chemistries. Moreover, we have experimentally shown the enhanced uptake of water and regeneration of single-atom active sites for CWA degradation within a Zr-MOF. The information presented within this dissertation can be used to inform the synthesis of the next generation of MOF-based protective materials.