Nuclear energy is among the most viable alternatives to our current fossil fuel-based energy economy. The mass deployment of nuclear energy as a low-emissions source requires the reprocessing of used nuclear fuel to recover fissile materials and mitigate radioactive waste. A major concern with reprocessing used nuclear fuel is the release of volatile radionuclides such as xenon and krypton that evolve into reprocessing facility off-gas in parts per million concentrations. The existing technology to remove these radioactive noble gases is a costly cryogenic distillation; alternatively, porous materials such as metal–organic frameworks have demonstrated the ability to selectively adsorb xenon and krypton at ambient conditions. Here we carry out a high-throughput computational screening of large databases of metal–organic frameworks and identify SBMOF-1 as the most selective for xenon. We affirm this prediction and report that SBMOF-1 exhibits by far the highest reported xenon adsorption capacity and a remarkable Xe/Kr selectivity under conditions pertinent to nuclear fuel reprocessing.

The mechanism and kinetics of interactions between dimethyl methylphosphonate (DMMP), a key chemical warfare agent (CWA) simulant, and Zr6-based metal organic frameworks (MOFs) have been investigated with in situ infrared spectroscopy (IR), X-ray photoelectron spectroscopy (XPS), powder X-ray diffraction (PXRD), and DFT calculations. DMMP was found to adsorb molecularly to UiO-66 through the formation of hydrogen bonds between the phosphoryl oxygen and the free hydroxyl groups associated with Zr6 nodes on the surface of crystallites and not within the bulk MOF structure. Unlike UiO-66, the infrared spectra for UiO-67 and MOF-808, recorded during DMMP exposure, suggest that uptake occurs through both physisorption and chemisorption. The XPS spectra of MOF-808 zirconium 3d electrons reveal a charge redistribution following exposure to DMMP. In addition, analysis of the phosphorus 2p electrons following exposure and thermal annealing to 600 K indicates that two types of stable phosphorus-containing species exist within the MOF. DFT calculations, used to guide the IR band assignments and to help interpret the XPS features, suggest that uptake is driven by nucleophilic addition of an OH group to DMMP with subsequent elimination of a methoxy substituent to form strongly bound methyl methylphosphonic acid (MMPA). The rates of product formation indicate that there are likely two distinct uptake processes, requiring rate constants that differ by approximately an order of magnitude. However, the rates of molecular uptake were found to be nearly identical to the rates of reaction, which strongly suggests that the reaction rates are diffusion-limited. The final products were found to inhibit further reactions within the MOFs, and these products could not be thermally driven from the MOFs prior to decomposition of the MOFs themselves.

New materials for the rapid decomposition of chemical warfare agents (CWAs) are in high demand for protecting military and civilian populations from these weapons of mass destruction. The need for novel sorbents and decontamination catalysts has gained great urgency as terrorists groups demonstrate the ability to synthesize and deploy agents in chemical attacks. Although many new materials, such as metal-organic frameworks (MOFs), have been proposed to use as CWA filtration media, their eventual transition requires a detailed understanding of the atomic-scale reaction mechanisms. Zr-based MOFs were recently shown to be among the fastest catalysts of nerve-agent hydrolysis reaction in solution. We show the results of a detailed study of the adsorption and decomposition of a nerve-agent simulant, dimethyl methylphosphonate (DMMP), on UiO-66, UiO-67, MOF-808 and NU-1000 MOFs (that have different pore sizes and connectivities) using synchrotron-based X-ray powder diffraction, X-ray absorption and infrared spectroscopies, which reveals key aspects of the reaction mechanism.[1] This study describes the implementation of a newly developed experimental setup for delivering vaporized DMMP to a reaction cell containing a MOF sample. The diffraction measurements indicate that all four MOFs adsorb DMMP (introduced at atmospheric pressures through a flow of helium or in air) within the pore space. In addition, the combination of X-ray absorption and infrared spectra suggests direct coordination of DMMP to the Zr 6 cores of all MOFs and its subsequent decomposition to phosphonate products. Further, we show that DMMP is actively adsorbed from air with good selectivity, even in the presence of humidity or other ambient gases, demonstrating that Zr 6-based MOFs may serve as effective sorbents for CWAs under ambient conditions. These experimental probes into the mechanism of adsorption and decomposition of chemical warfare agent simulants on Zr-based MOFs open new opportunities for rational design of new and superior decontamination materials.

The cryogenic separation of noble gases is energy-intensive and expensive, especially when low concentrations are involved. Metal-organic frameworks (MOFs) containing polarizing groups within their pore spaces are predicted to be efficient Xe/Kr solid-state adsorbents, but no experimental insights into the nature of the Xe-network interaction are available to date. Here we report a new microporous MOF (designated SBMOF-2) that is selective toward Xe over Kr under ambient conditions, with a Xe/Kr selectivity of about 10 and a Xe capacity of 27.07 wt % at 298 K. Single-crystal diffraction results show that the Xe selectivity may be attributed to the specific geometry of the pores, forming cages built with phenyl rings and enriched with polar -OH groups, both of which serve as strong adsorption sites for polarizable Xe gas. The Xe/Kr separation in SBMOF-2 was investigated with experimental and computational breakthrough methods. These experiments showed that Kr broke through the column first, followed by Xe, which confirmed that SBMOF-2 has a real practical potential for separating Xe from Kr. Calculations showed that the capacity and adsorption selectivity of SBMOF-2 are comparable to those of the best-performing unmodified MOFs such as NiMOF-74 or Co formate.

A thermally stable, microporous calcium coordination network shows a reversible 5.75 wt % CO2 uptake at 273 K and 1 atm pressure, with an enthalpy of interaction of ∼31 kJ/mol and a CO2/N2 selectivity over 45 under ideal flue gas conditions. The absence of open metal sites in the activated material suggests a different mechanism for selectivity and high interaction energy compared to those for frameworks with open metal sites.