Jared B. DeCoste

Destruction of chemical warfare agents using metal–organic frameworks

Chemical warfare agents containing phosphonate ester bonds are among the most toxic chemicals known to mankind. Recent global military events, such as the conflict and disarmament in Syria, have brought into focus the need to find effective strategies for the rapid destruction of these banned chemicals. Solutions are needed for immediate personal protection (for example, the filtration and catalytic destruction of airborne versions of agents), bulk destruction of chemical weapon stockpiles, protection (via coating) of clothing, equipment and buildings, and containment of agent spills. Solid heterogeneous materials such as modified activated carbon or metal oxides exhibit many desirable characteristics for the destruction of chemical warfare agents. However, low sorptive capacities, low effective active site loadings, deactivation of the active site, slow degradation kinetics, and/or a lack of tailorability offer significant room for improvement in these materials. Here, we report a carefully chosen metal-organic framework (MOF) material featuring high porosity and exceptional chemical stability that is extraordinarily effective for the degradation of nerve agents and their simulants. Experimental and computational evidence points to Lewis-acidic Zr(IV) ions as the active sites and to their superb accessibility as a defining element of their efficacy.

Stability and degradation mechanisms of metal–organic frameworks containing the Zr6O4(OH)4 secondary building unit

Metal–organic frameworks (MOFs) with the Zr6O4(OH)4 secondary building unit (SBU) have been of particular interest for potential commercial and industrial uses because they can be easily tailored and are reported to be chemically and thermally stable. However, we show that there are significant changes in chemical and thermal stability of Zr6O4(OH)4 MOFs with the incorporation of different organic linkers. As the number of aromatic rings is increased from one to two in 1,4-benzene dicarboxylate (UiO-66, ZrMOF–BDC) and 4,4′-biphenyl dicarboxylate (UiO-67, ZrMOF–BPDC), the Zr6O4(OH)4 SBU becomes more susceptible to chemical degradation by water and hydrochloric acid. Furthermore, as the linker is replaced with 2,2′-bipyridine-5,5′-dicarboxylate (ZrMOF–BIPY) the chemical stability decreases further as the MOF is susceptible to chemical breakdown by protic chemicals such as methanol and isopropanol. The results reported here bring into question the superior structural stability of the UiO-67 analogs as reported by others. Furthermore, the degradation mechanisms proposed here may be applied to other classes of MOFs containing aromatic dicarboxylate organic linkers, in order to predict their structural stability upon exposure to solvents.

The effect of water adsorption on the structure of the carboxylate containing metal–organic frameworks Cu-BTC, Mg-MOF-74, and UiO-66

Metal–organic frameworks (MOFs) with metal–carboxylate bonds, including Cu-BTC (HKUST-1), Mg-MOF-74 (Mg/DOBDC), and UiO-66, have been shown to have varying degrees of water stability. The three MOFs in this study are three of the most highly studied MOFs in the literature. We investigate here how each MOF degrades at several temperature and humidity conditions over the course of 28 days. At conditions of 90% relative humidity (RH) and 25 °C, water uptake for Cu-BTC is shown to be higher than at 90% RH and 40 °C, causing the degradation of the inner structure of Cu-BTC to occur more readily at the lower temperature. However the external surfaces of Cu-BTC degrade more readily, as shown through SEM images, at conditions of 90% RH and 40 °C. Mg-MOF-74 has a nearly complete loss of surface area after just one day of exposure to each of the conditions studied, however the PXRD patterns show only a change in the [100] peak. We offer here a novel mechanism for the degradation of Mg-MOF-74, involving a 6-coordinate Mg intermediate, which leaves the 1-dimensional channels of Mg-MOF-74 intact. Furthermore, we conclude that UiO-66 is stable to each of the aging conditions for the full 28 days of this study.

Enhanced Stability of Cu-BTC MOF via Perfluorohexane Plasma-Enhanced Chemical Vapor Deposition

Metal organic frameworks (MOFs) are a leading class of porous materials for a wide variety of applications, but many of them have been shown to be unstable toward water. Cu-BTC (1,3,5 benzenetricarboxylic acid, BTC) was treated with a plasma-enhanced chemical vapor deposition (PECVD) of perfluorohexane creating a hydrophobic form of Cu-BTC. It was found that the treated Cu-BTC could withstand high humidity and even submersion in water much better than unperturbed Cu-BTC. Through Monte Carlo simulations it was found that perfluorohexane sites itself in such a way within Cu-BTC as to prevent the formation of water clusters, hence preventing the decomposition of Cu-BTC by water. This PECVD of perfluorohexane could be exploited to widen the scope of practical applications of Cu-BTC and other MOFs.

Polymer-Metal-Organic Frameworks (polyMOFs) as Water Tolerant Materials for Selective Carbon Dioxide Separations.

Recently, polymer-metal-organic frameworks (polyMOFs) were reported as a new class of hybrid porous materials that combine advantages of both organic polymers and crystalline MOFs. Herein, we report a bridging coligand strategy to prepare new types of polyMOFs, demonstrating that polyMOFs are compatible with additional MOF architectures besides that of the earlier reported IRMOF-1 type polyMOF. Gas sorption studies revealed that these polyMOF materials exhibited relatively high CO2 sorption but very low N2 sorption, making them promising materials for CO2/N2 separations. Moreover, these polyMOFs demonstrated exceptional water stability attributed to the hydrophobicity of polymer ligands as well as the cross-linking of the polymer chains within the MOF.

Effective, Facile, and Selective Hydrolysis of the Chemical Warfare Agent VX Using Zr6-Based Metal–Organic Frameworks

The nerve agent VX is among the most toxic chemicals known to mankind, and robust solutions are needed to rapidly and selectively deactivate it. Herein, we demonstrate that three Zr6-based metal-organic frameworks (MOFs), namely, UiO-67, UiO-67-NH2, and UiO-67-N(Me)2, are selective and highly active catalysts for the hydrolysis of VX. Utilizing UiO-67, UiO-67-NH2, and UiO-67-N(Me)2 in a pH 10 buffered solution of N-ethylmorpholine, selective hydrolysis of the P-S bond in VX was observed. In addition, UiO-67-N(Me)2 was found to catalyze VX hydrolysis with an initial half-life of 1.8 min. This half-life is nearly 3 orders of magnitude shorter than that of the only other MOF tested to date for hydrolysis of VX and rivals the activity of the best nonenzymatic materials. Hydrolysis utilizing Zr-based MOFs is also selective and facile in the absence of pH 10 buffer (just water) and for the destruction of the toxic byproduct EA-2192.

A UiO-66 analogue with uncoordinated carboxylic acids for the broad-spectrum removal of toxic chemicals

Zirconium-based metal–organic frameworks (MOFs) are of great importance as sorbents due to their increased chemical and thermal stability when compared to other MOF families. Here we report a novel analogue of UiO-66 modified with oxalic acid via solvent-assisted ligand incorporation. This analogue has the ability to remove ammonia, cyanogen chloride, sulphur dioxide, nitrogen dioxide and octane at levels greater than or equal to the base UiO-66. We report here the highest known capacities exhibited by a MOF for SO2 and NO2, at pressures less than 0.10 bar and at room temperature, by UiO-66-ox. Furthermore, we show here the importance of the secondary building unit of the MOF in the removal of ammonia and cyanogen chloride.

Tailoring the Pore Size and Functionality of UiO-Type Metal–Organic Frameworks for Optimal Nerve Agent Destruction

Evaluation of UiO-66 and UiO-67 metal-organic framework derivatives as catalysts for the degradation of soman, a chemical warfare agent, showed the importance of both the linker size and functionality. The best catalysts yielded half-lives of less than 1 min. Further testing with a nerve agent simulant established that different rate-assessment techniques yield similar values for degradation half-lives.

Metal–Organic Frameworks for Oxygen Storage

We present a systematic study of metal-organic frameworks (MOFs) for the storage of oxygen. The study starts with grand canonical Monte Carlo simulations on a suite of 10,000 MOFs for the adsorption of oxygen. From these data, the MOFs were down selected to the prime candidates of HKUST-1 (Cu-BTC) and NU-125, both with coordinatively unsaturated Cu sites. Oxygen isotherms up to 30 bar were measured at multiple temperatures to determine the isosteric heat of adsorption for oxygen on each MOF by fitting to a Toth isotherm model. High pressure (up to 140 bar) oxygen isotherms were measured for HKUST-1 and NU-125 to determine the working capacity of each MOF. Compared to the zeolite NaX and Norit activated carbon, NU-125 has an increased excess capacity for oxygen of 237% and 98%, respectively. These materials could ultimately prove useful for oxygen storage in medical, military, and aerospace applications.

Extraordinary NO2 Removal by the Metal–Organic Framework UiO-66-NH2

Here we discuss the removal of nitrogen dioxide, an important toxic industrial chemical and pollutant, from air using the MOF UiO-66-NH2 . The amine group is found to substantially aid in the removal, resulting in unprecedented removal capacities upwards of 1.4 g of NO2  /g of MOF. Furthermore, whereas NO2 typically generates substantial quantities of NO on sorbents, the amount generated by UiO-66-NH2 is significantly reduced. Of particular significance is the formation of a diazonium ion on the aromatic ring of the MOF, and the potential reduction of NO2 to molecular nitrogen.