Metal-organic frameworks (MOFs) have shown significant potential in various applications, particularly in gas capture technologies. Their use extends to energy storage, food preservation, and notably, the remediation of chemical warfare agents (CWAs). Existing CWA remediation technologies face challenges including re-emission, low capacity, and disposal issues. To improve these technologies, a detailed understanding of the CWA uptake and decomposition mechanisms at the atomic level is crucial. Zr-MOFs are emerging as promising candidates due to their Lewis-acidic Zr metal centers, which catalyze organophosphonate hydrolysis, and their high stability under harsh conditions. Previous studies, combining experiments and theoretical calculations, have identified hydrolysis products and their coordination environments, advancing our understanding of CWA decontamination. Density functional theory (DFT) calculations suggest that organophosphorus degradation involves nucleophilic hydroxyl addition, forming pentacoordinated phosphorus intermediates that rapidly decompose. Experimental evidence from in situ X-ray powder diffraction (XRPD), extended X-ray absorption fine structure (EXAFS), and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) supports this, showing DMMP sorption, binding, and decomposition into phosphonate products, particularly irreversibly bidentate-bound methyl methylphosphonate (MMPA). While promising, optimizing these materials requires space-resolved studies across broad length scales, combining experimental probes with in situ capabilities and atomic-level sensitivity. Various techniques, including single crystal or powder diffraction and spectroscopic methods, have been used to study adsorbate-adsorbent interactions. However, molecular-level resolution often relies on ordered structures, limiting applicability to less-ordered systems. Pair distribution function (PDF) analysis offers a solution by using all coherent scattering information, providing a real-space fingerprint of atom-pair distances irrespective of long-range order. PDF analysis has been used to characterize MOF structures, defects, and gas adsorption, but its application to track reactive changes in guest substructures is less explored. This study aims to advance the understanding of DMMP uptake and decomposition in UiO-67 using in situ total scattering data and PDF analysis, tracking the changes in both the framework and DMMP with high real-space resolution, from activation to gas adsorption, desorption, and regeneration.

The extensive research on MOFs over two decades has led to industrial applications primarily in gas capture. This includes applications in energy storage, food preservation, and filtration, including CWA remediation. Zr-MOFs have gained importance in CWA destruction, leveraging the Lewis acidity of Zr centers for organophosphonate hydrolysis and the framework's exceptional stability. Previous in situ studies, coupled with DFT calculations, provided insights into hydrolysis reactions and the coordination environments of phosphonic acid products. DFT suggests a mechanism involving nucleophilic hydroxyl addition to organophosphorus molecules, creating pentacoordinated phosphorus intermediates that decompose. Experimental techniques like XRPD, EXAFS, and DRIFTS have further supported these findings in Zr-MOF systems, illustrating DMMP sorption, binding, and decomposition into phosphonate products, including MMPA. However, the optimization of these materials requires a more comprehensive understanding of sorption and decomposition mechanisms at the atomic level. Existing techniques like single-crystal/powder diffraction and various spectroscopic methods have their limitations in providing the necessary atomic-scale resolution, especially in less-ordered systems. This study overcomes these limitations by employing PDF analysis, which provides real-space information regardless of the level of long-range order in the material.

The study utilized UiO-67, a Zr-based MOF, synthesized using a modified literature procedure. In situ X-ray total scattering measurements were conducted on beamline 28-ID-2 at NSLS-II using a high-energy X-ray Powder Diffraction beamline. Rapid acquisition mode was employed with a large-area 2D detector, at an incident energy of 66.41 keV. Powdered UiO-67 was loaded into a 1 mm ID polyimide capillary gas flow cell, with gas flowing at 10 mL/min. The experiment involved several stages: (I) measurement of unprocessed UiO-67 at room temperature (RT); (II) activation by heating to 80 °C under He; (III) reheating under He from RT to 80 °C; (IV) DMMP vapor/He mixture introduction at RT; (V) He flow at RT after DMMP removal; (VI) temperature ramp under He from RT to 80 °C; and (VII) re-activation at 80 °C under He. Data reduction involved calibration, polarization correction, azimuthal integration, and background subtraction using pyFAI and PDFgetX3 within xPDFsuite to obtain the total scattering structure function F(Q), followed by Fourier transformation to obtain the pair distribution function (PDF), G(r). Rietveld refinement was performed using TOPAS v6, employing a structure model with F432 symmetry to describe the UiO-67 framework. Pseudo-atoms (Dy) were introduced to model DMMP, allowing their positions and occupancies to refine via simulated annealing. Real-space PDF refinements used PDFgui. DFT calculations employed VASP 5.4.1 with the PBE functional, projected-augmented-wave pseudopotentials, and a plane-wave basis. The UiO-67 unit cell with a missing-linker defect was modeled, with under-coordinated Zr atoms saturated by hydroxo and aqua ligands. DMMP binding and reaction to MMPA were simulated. Difference PDFs (dPDFs) were generated by comparing PDFs from different experimental steps and were used to analyze changes in short-range order. Structural models were constructed and refined to fit the PDFs, incorporating both framework and guest components. This involved refining scale factors for simulated dPDFs of DMMP and MMPA in different bound states.

The study revealed that UiO-67 rapidly adsorbs and decomposes DMMP, leading to a ~1.14% increase in cell volume. DMMP preferentially occupies the smaller tetrahedral pores. Upon He gas flow at RT, the framework re-contracts due to the exchange of some bulk DMMP for He, but the majority of DMMP remains. Heating above ~40 °C further removes DMMP, potentially aided by the framework's negative thermal expansion (NTE). Bound MMPA remains after re-activation, indicating stronger binding interactions. In situ total scattering and PDF analysis, coupled with DFT simulations, enabled simultaneous tracking of lattice expansion/contraction, gas uptake and decomposition, and structural changes in the Zr6 clusters. The analysis of difference PDFs (dPDFs) showed the appearance of peaks corresponding to free DMMP and the formation of P-Zr bonds due to MMPA binding. Heating led to the removal of free DMMP, while bound MMPA persisted. Refinement using simulated PDFs of free and bound states revealed a preference for monodentate and bidentate MMPA binding over bound DMMP. Analysis of Zr-Zr distances showed only slight changes upon DMMP sorption and decomposition, suggesting that binding of DMMP/MMPA does not cause significant distortions in the Zr6 clusters. The study demonstrated the potential of the in situ total scattering and PDF analysis method for studying reactive changes in MOFs, providing high real-space resolution insights.

This research successfully demonstrates a powerful methodology for obtaining atomic-scale insights into the dynamic interactions within MOFs. The observation of DMMP adsorption and decomposition within UiO-67, accompanied by measurable cell volume changes, provides valuable data for understanding CWA capture mechanisms. The preference for DMMP occupancy in smaller pores suggests stronger adsorption forces in these sites, while the persistent MMPA binding highlights the importance of linker vacancies in creating reactive sites. The combined in situ total scattering and PDF analysis approach, complemented by DFT simulations, offers a comprehensive way to study complex material responses to gas exposure. The ability to simultaneously track multiple structural features and changes at high real-space resolution provides a superior approach compared to conventional methods. The identified limitations of previous techniques are addressed by this approach, paving the way for more detailed investigations of MOF interactions with guest molecules. The findings strongly suggest the potential of this method for evaluating a broad range of materials, enabling improved design of protective materials.

This study presents a novel method for achieving atomic-scale visualization of MOF structural changes during gas sorption and reaction. The application to UiO-67 and DMMP revealed detailed information on DMMP adsorption, decomposition, and the resulting changes in the MOF framework. The in situ total scattering and PDF analysis, aided by DFT, provides a powerful tool for studying such complex systems, offering insights into reaction mechanisms and informing the design of improved CWA capture materials. Future research should explore different MOF structures with varying defect concentrations to optimize CWA reactivity and understand the influence of defects on the overall material performance.

The study's findings are primarily based on one specific MOF, UiO-67, and a single CWA simulant, DMMP. The generalizability of the findings to other MOFs and CWAs requires further investigation. The modelling approach, although sophisticated, may not fully capture all the complexity of the interactions, and more comprehensive models are possible. The limited number of linker vacancies in the sample used might have influenced the observed binding of MMPA; studying MOFs with a higher density of defects could provide further insights.