Effect of Carbon Dioxide on the Degradation of Chemical Warfare Agent Simulant in the Presence of Zr Metal Organic Framework MOF-808

Developing novel and more efficient filters for chemical warfare agent (CWA) decomposition remains an important challenge for modern technology due to the continuous threat those weapons present in the event of use. Recently, metal-organic frameworks (MOFs) have attracted attention as potential catalysts for nerve agent decomposition. However, in order to improve their performance under battlefield conditions, it is crucial to understand the influence of ambient contaminants such as carbon dioxide on CWA adsorption and decomposition. Here, we present a comprehensive experimental and computational study on the influence of CO₂ on the adsorption and decomposition of the CWA simulant dimethyl methyl phosphonate (DMMP) by the Zr-based MOF-808. The study combined in situ synchrotron powder X-ray diffraction (PXRD) with variable-temperature infrared spectroscopy (VTIR) and computations. PXRD and experiments with pure CO₂ revealed that the MOF framework contracts by 0.2% after CO₂ saturation and CO₂ adsorbs within the MOF-808 framework via dispersion forces with an enthalpy of adsorption of -32.2 ± 1.8 kJ/mol. From five DRIFTS experiments conducted in various gas flow regimes we established that CO₂ interferes with the decomposition of DMMP due to the formation of carbonate species, while if it is used as a regeneration agent, it accelerates the desorption of DMMP and DMMP decomposition products. In situ PXRD experiments showed that CO₂ also limited expansion of the framework due to the formation of carbonate species by 40% in comparison to values in a control experiment with N₂. The main conclusions of the computational study are (a) CO₂ reversibly adsorbs onto MOF-808 via both μ₃-OH groups located within the tetrahedral pores of the MOF and/or to the MOF surface with stabilizing dispersion interactions, (b) the presence of DMMP provides stability to the CO₂-(μ₃-OH) moiety, which hinders the μ₃-OH transfer to the DMMP molecule required for the nerve-agent decomposition until the carbonyl to carbonate transformation is completed, and (c) CO₂ molecules are not sufficient for the regeneration of the MOF when DMMP decomposition products are present but rather there is desorption of bound DMMP.