Tunable Photocatalytic Singlet Oxygen Generation by Metal-Organic Frameworks via Functionalization of Pyrene-Containing Linkers.

Metal-organic frameworks (MOFs) are highly versatile materials that have shown great promise in chemical warfare agent (CWA) adsorption and decontamination. Sulfur mustard has been one of the most prominently used CWAs over the last century; therefore, the development of effective detoxification strategies is of utmost importance. However, typical routes of detoxification are slow and/or result in the production of harmful byproducts. NU-1000 has previously shown promise as a "soft" oxidizer that can readily detoxify sulfur mustard and its simulant 2-chloroethyl ethyl sulfide (2-CEES) through the generation of singlet oxygen in the presence of either UV (396 nm) or blue (465 nm) light. Several variants of NU-1000 were synthesized (MOF-R, R = -Cl, -NO2, -CH3) with functional groups positioned either ortho or meta to the carboxylic acid on the linker. NU-1000-o-(Cl)4 and NU-1000-m-(Cl)4 showed significant enhancement of photooxidation of 2-CEES due to spin-orbit coupling, enhancing the intersystem crossing into the MOF triplet (T1) state. Furthermore, substitution of MOF linkers led to pyrene-phenyl rotation. Linkers with substituents in the ortho-position were shown to have smaller pyrene-phenyl torsion angles, leading to enhanced conjugation between the rings and a subsequent red shift in the absorption spectra. This red shift leads to enhanced reactivity of NU-1000-o-(Cl)4 under blue light conditions and gives perspective on making materials with enhanced reactivity utilizing visible light.

BEAMS: a workforce development program to bridge the gap between biologists and material scientists.

To maximize innovation in materials science and synthetic biology, it is critical to master interdisciplinary understanding and communication within an organization. Programming aimed at this juncture has the potential to bring members of the workforce together to frame new networks and spark collaboration. In this article, we recognize the potential synergy between materials and synthetic biology research and describe our approach to this challenge as a case study. A workforce development program was devised consisting of a lecture series, laboratory demonstrations and a hands-on laboratory competition to produce a bacterial cellulose material with the highest tensile strength. This program, combined with support for infrastructure and research, resulted in a significant return on investment with new externally funded synthetic biology for materials programs for our organization. The learning elements described here may be adapted by other institutions for a variety of settings and goals.

High-Throughput Screening of MOFs for Breakdown of V-Series Nerve Agents.

Metal-organic frameworks (MOFs) have shown promise for the catalytic decomposition of chemical weapons. Finding the best materials for the degradation of nerve agents requires the ability to screen a high number of samples and elucidate the key parameters of effective catalysis. In this work, a high-throughput screening (HTS) method has been developed to evaluate MOFs as catalysts, specifically against the V-class of nerve agents. Over 100 MOFs have been tested using the V-class simulant, O,O-diethyl S-phenyl phosphorothioate (DEPPT), revealing good activity for some UiO-66 derivatives. A medium-throughput hydrolysis assay for the nerve agent O-ethyl S-[2-(diisopropylamino)ethyl]methylphosphonothioate (VX) was also performed using six MOFs selected from HTS and was validated by 31P NMR. The results demonstrated that the DEPPT-based assay is a good indicator of V-series agent reactivity and should be considered in addition to the common (4-nitrophenyl)phosphate (DMNP) assay that is used for G-series agents.

Insights into the solvent-assisted degradation of organophosphorus compounds by a Zr-based metal-organic framework.

The degradation of a chemical warfare agent simulant using a catalytically active Zr-based metal-organic framework (MOF) as a function of different solvent systems was investigated. Complementary molecular modelling studies indicate that the differences in the degradation rates are related to the increasing size in the nucleophile, which hinders the rotation of the product molecule during degradation. Methanol was identified as an appropriate solvent for non-aqueous degradation applications and demonstrated to support the MOF-based destruction of both sarin and soman.

Spectroscopically Resolved Binding Sites for the Adsorption of Sarin Gas in a Metal-Organic Framework: Insights beyond Lewis Acidity.

Here we report molecular level details regarding the adsorption of sarin (GB) gas in a prototypical zirconium-based metal-organic framework (MOF, UiO-66). By combining predictive modeling and experimental spectroscopic techniques, we unambiguously identify several unique bindings sites within the MOF, using the P═O stretch frequency of GB as a probe. Remarkable agreement between predicted and experimental IR spectrum is demonstrated. As previously hypothesized, the undercoordinated Lewis acid metal site is the most favorable binding site. Yet multiple sites participate in the adsorption process; specifically, the Zr-chelated hydroxyl groups form hydrogen bonds with the GB molecule, and GB weakly interacts with fully coordinated metals. Importantly, this work highlights that subtle orientational effects of bound GB are observable via shifts in characteristic vibrational modes; this finding has large implications for degradation rates and opens a new route for future materials design.

Metal Hydroxide/Polymer Textiles for Decontamination of Toxic Organophosphates: An Extensive Study of Wettability, Catalytic Activity, and the Effects of Aggregation.

Electrospun nanofibers (NFs) incorporated with catalytically active components have gained significant interest in chemical protective clothing. This is because of the desirable properties of the NFs combined with decontamination capability of the active component. Here, a series of metal hydroxide catalysts Ti(OH)x, Zr(OH)4, and Ce(OH)4 were incorporated into three different polymer NF systems. These new polymer/metal hydroxide composite NFs were then evaluated for their catalytic activity against a nerve agent simulant. Two methods were utilized to incorporate the metal hydroxides into the NFs. Method one used direct incorporation of Ti(OH)x, Zr(OH)4, and Ce(OH)4 catalysts, whereas method two employed incorporation of Ti(OH)x via a precursor molecule. Composite NFs prepared via method one resulted in greatly improved reaction rates over the respective pure metal hydroxides due to reduced aggregation of catalysts, with polymer/Ce(OH)4 composite NFs having the fastest reaction rates out of method one materials. Interestingly, composite samples prepared by method two yielded the fastest reaction rates overall. This is because of the homogeneous distribution of the metal hydroxide catalyst throughout the NF. This homogeneous distribution created a hydroxyl-decorated NF surface with a greater number of exposed active sites for catalysis. The hydroxyl-decorated NF surface also resulted in an unexpected highly wettable composite NF, which also was found to contribute to the observed reaction rates. These results are not only promising for applications in chemical protective clothing but also show great potential for application in areas which need highly wettable membrane materials. This includes areas such as separators, antifouling membranes, and certain medical applications.

Insight into organophosphate chemical warfare agent simulant hydrolysis in metal-organic frameworks.

Metal-organic frameworks (MOFs) are porous 3-dimensional crystalline structures that have shown promise for a variety of applications including adsorption, catalysis, and sensing. Modern warfare has placed chemical warfare agent (CWA) destruction at the forefront of chemical applications for MOFs. However, experiments involving CWAs can only be performed by a small number of highly trained individuals as they are extremely dangerous and available only to certain laboratories. As such, it is imperative that suitable chemical simulants and reaction conditions are determined for CWAs of interest. In this work, we determine the reaction rate for heterogeneous catalytic hydrolysis of eight commonly used G-agent simulants with zirconium-based MOFs. Of the simulants tested, only dimethyl chlorophosphate (DMCP), diisopropylfluorophosphate (DFP), and dimethyl p-nitrophenylphosphate (DMNP) exhibit the ability to be catalytically hydrolyzed in a manner similar to the G-agents by the MOFs studied. Two different base-catalyzed reaction mechanisms are proposed for the hydrolysis reaction on the different MOF secondary building units, and the effect of pH and buffer properties is determined using an N-ethylmorpholine (NEM) buffer at pH 8-10 and a 3-(cyclohexylamino)-1-propanesulofinic acid (CAPS) buffer at pH 10-11.

Solid-Phase Detoxification of Chemical Warfare Agents using Zirconium-Based Metal Organic Frameworks and the Moisture Effects: Analyze via Digestion.

Zirconium-based metal organic frameworks (Zr-MOFs) are highly chemically and thermally stable and have been of particular interest as reactive sorbents for chemical warfare agent (CWA) removal due to their fast and selective reactivity toward CWAs reported in buffer solutions. However, we find that decontamination of neat CWAs directly on Zr-MOFs, UiO-66, UiO-66-NH2, and NU-1000 is rather slow, and the reactivity trend and products generated are very different from those in solution. Furthermore, we show that their decontamination rates are affected by the amount of moisture present in the MOFs. Although the effects are minor for UiO-66-NH2 and NU-1000, the hydrolytic activity of UiO-66 toward CWAs dramatically improves as the amount of water present increases. Specifically, the initial hydrolysis rate of methyl paraoxon by UiO-66 increases from 6 µmol/d with 0 wt % water loading to 140 µmol/d with 400 wt % water loading. The results reported here suggest that decontamination of CWAs by Zr-MOFs in solid phase behaves very differently than solution decontamination. Additionally, we present for the first time a digestion method for analyzing and quantifying solid-phase decontamination, which is a daunting challenge itself due to the lack of a convenient analytical method.

Single-component frameworks for heterogeneous catalytic hydrolysis of organophosphorous compounds in pure water.

Amine modified Zr6-based metal-organic frameworks (MOFs) were synthesized through solvent-assisted linker incorporation (SALI) and utilized as single-component heterogeneous catalysts for the hydrolysis of organophosphorous compounds under solely aqueous conditions at room temperature. These materials display unprecidentedly fast catalytic hydrolysis for dimethyl p-nitrophenyl phosphate (DMNP) and nerve agent VX without the use of a buffered solution.

High-throughput screening of solid-state catalysts for nerve agent degradation.

A high-throughput screening (HTS) method was devised to increase the rate of discovery and evaluation of nerve agent degradation catalysts. Using this HTS method, >90 solid state materials, predominantly metal-organic frameworks (MOFs), were analyzed for their ability to hydrolyze the nerve agent simulant methyl paraoxon at two pH values (8.0 and 10.0).

Metal-Organic Framework Modified Glass Substrate for Analysis of Highly Volatile Chemical Warfare Agents by Paper Spray Mass Spectrometry.

Paper spray mass spectrometry has been shown to successfully analyze chemical warfare agent (CWA) simulants. However, due to the volatility differences between the simulants and real G-series (i.e., sarin, soman) CWAs, analysis from an untreated paper substrate proved difficult. To extend the analytical lifetime of these G-agents, metal-organic frameworks (MOFs) were successfully integrated onto the paper spray substrates to increase adsorption and desorption. In this study, several MOFs and nanoparticles were tested to extend the analytical lifetimes of sarin, soman, and cyclosarin on paper spray substrates. It was found that the addition of either UiO-66 or HKUST-1 to the paper substrate increased the analytical lifetime of the G-agents from less than 5 min detectability to at least 50 min.

Enhancing Van der Waals Interactions of Functionalized UiO-66 with Non-polar Adsorbates: The Unique Effect of para Hydroxyl Groups.

UiO-66 is a highly stable metal-organic framework (MOF) that has garnered interest for many adsorption applications. For small, nonpolar adsorbates, physisorption is dominated by weak Van der Waals interactions limiting the adsorption capacity. A common strategy to enhance the adsorption properties of isoreticular MOFs, such as UiO-66, is to add functional groups to the organic linker. Low and high pressure O2 isotherms were measured on UiO-66 MOFs functionalized with electron donating and withdrawing groups. It was found that the electron donating effects of -NH2 , -OH, and -OCF3 groups enhance the uptake of O2 . Interestingly, a significant enhancement in both the binding energy and adsorption capacity of O2 was observed for UiO-66-(OH)2 -p, which has two -OH groups para from one another. Density functional theory (DFT) simulations were used to calculate the binding energy of oxygen to each MOF, which trended with the adsorption capacity and agreed well with the heats of adsorption calculated from the Toth model fit to multi-temperature isotherms. DFT simulations also determined the highest energy binding site to be on top of the electron π-cloud of the aromatic ring of the ligand, with a direct trend of the binding energy with low pressure adsorption capacity. Uniquely, DFT found that oxygen molecules adsorbed to UiO-66-(OH)2 -p prefer to align parallel to the -OH groups on the aromatic ring. Similar effects for the electron donation of the functional groups were observed for the low pressure adsorption of N2 , CH4 , and CO2 .

Tailoring the Adsorption and Reaction Chemistry of the Metal-Organic Frameworks UiO-66, UiO-66-NH2, and HKUST-1 via the Incorporation of Molecular Guests.

Metal-organic frameworks (MOFs) are versatile materials highly regarded for their porous nature. Depending on the synthetic method, various guest molecules may remain in the pores or can be systematically loaded for various reasons. Herein, we present a study that explores the effect of guest molecules on the adsorption and reactivity of the MOF in both the gas phase and solution. The differences between guest molecule interactions and the subsequent effects on their activity are described for each system. Interestingly, different effects are observed and described in detail for each class of guest molecules studied. We determine that there is a strong effect of alcohols with the secondary building unit of UiO MOFs, while Lewis bases have an effect on the reactivity of the -NH2 group in UiO-66-NH2 and adsorption by the coordinatively unsaturated copper sites in HKUST-1. These effects must be considered when determining synthesis and activation methods of MOFs toward various applications.

Chemical Warfare Agents Detoxification Properties of Zirconium Metal-Organic Frameworks by Synergistic Incorporation of Nucleophilic and Basic Sites.

The development of protective self-detoxifying materials is an important societal challenge to counteract risk of attacks employing highly toxic chemical warfare agents (CWAs). In this work, we have developed bifunctional zirconium metal-organic frameworks (MOFs) incorporating variable amounts of nucleophilic amino residues by means of formation of the mixed ligand [Zr6O4(OH)4(bdc)6(1-x)(bdc-NH2)6x] (UiO-66-xNH2) and [Zr6O4(OH)4(bpdc)6(1-x)(bpdc-(NH2)2)6x] (UiO-67-x(NH2)2) systems where bdc = benzene-1,4-dicarboxylate; bdc-NH2= benzene-2-amino-1,4-dicarboxylate; bpdc = 4,4'-biphenyldicarboxylate; bpdc-(NH2)2 = 2,2'-diamino-4,4'-biphenyldicarboxylate and x = 0, 0.25, 0.5, 0.75, 1. In a second step, the UiO-66-xNH2 and UiO-67-x(NH2)2 systems have been postsynthetically modified by introduction of highly basic lithium tert-butoxide (LiOtBu) on the oxohydroxometallic clusters of the mixed ligand MOFs to yield UiO-66-xNH2@LiOtBu and UiO-67-x(NH2)2@LiOtBu materials. The results show that the combination of pre and postsynthetic modifications on these MOF series gives rise to fine-tuning of the catalytic activity toward the hydrolytic degradation of both simulants and real CWAs in unbuffered aqueous solutions. Indeed, UiO-66-0.25NH2@LiOtBu is able to hydrolyze both CWAs simulants (diisopropylfluorophosphate (DIFP), 2-chloroethylethylsulfide (CEES), and real CWAs (soman (GD), sulfur mustard (HD)) quickly in aqueous solution. These results are related to a suitable combination of robustness, nucleophilicity, basicity, and accessibility to the porous framework.

MOFabric: Electrospun Nanofiber Mats from PVDF/UiO-66-NH2 for Chemical Protection and Decontamination.

Textiles capable of capture and detoxification of toxic chemicals, such as chemical-warfare agents (CWAs), are of high interest. Some metal-organic frameworks (MOFs) exhibit superior reactivity toward CWAs. However, it remains a challenge to integrate powder MOFs into engineered materials like textiles, while retaining functionalities like crystallinity, adsorptivity, and reactivity. Here, we present a simple method of electrospinning UiO-66-NH2, a zirconium MOF, with polyvinylidene fluoride (PVDF). The electrospun composite, which we refer to as "MOFabric", exhibits comparable crystal patterns, surface area, chlorine uptake, and simulant hydrolysis to powder UiO-66-NH2. The MOFabric is also capable of breaking down GD (O-pinacolyl methylphosphonofluoridae) faster than powder UiO-66-NH2. Half-life of GD monitored by solid-state NMR for MOFabric is 131 min versus 315 min on powder UiO-66-NH2.

Filtration of chlorine and hydrogen chloride gas by engineered UiO-66-NH2 metal-organic framework.

Chlorine (Cl2) and hydrogen chloride (HCl) are heavily utilized industrial chemicals that present significant respiratory health risks. The metal-organic framework UiO-66-NH2 has shown an unprecedented ability in powder form to remove chlorine gas. Here, we engineered UiO-66-NH2 into 20×40 mesh granules and evaluated their ability to remove chlorine and hydrogen chloride gas challenges. The exposed materials were characterized with nitrogen isotherms, powder X-ray diffraction, and attenuated total reflectance - Fourier transform infrared spectroscopy. Breakthrough results revealed that UiO-66-NH2 sorption of chlorine and hydrogen chloride met or exceeded sorption of state-of-the-art metal-impregnated activated carbon materials on a mass and volume basis in engineered form.

Postsynthetic Incorporation of a Singlet Oxygen Photosensitizer in a Metal-Organic Framework for Fast and Selective Oxidative Detoxification of Sulfur Mustard.

A fullerene-based photosensitizer is incorporated postsynthetically into a Zr6 -based MOF, NU-1000, for enhanced singlet oxygen production. The structural organic linkers in the MOF platform also act as photosensitizers which contribute to the overall generation of singlet oxygen from the material under UV irradiation. The singlet oxygen generated by the MOF/fullerene material is shown to oxidize sulfur mustard selectively to the less toxic bis(2-chloroethyl)sulfoxide with a half-life of only 11 min.

Detoxification of Chemical Warfare Agents Using a Zr6 -Based Metal-Organic Framework/Polymer Mixture.

Owing to their high surface area, periodic distribution of metal sites, and water stability, zirconium-based metal-organic frameworks (Zr6 -MOFs) have shown promising activity for the hydrolysis of nerve agents GD and VX, as well as the simulant, dimethyl 4-nitrophenylphosphate (DMNP), in buffered solutions. A hurdle to using MOFs for this application is the current need for a buffer solution. Here the destruction of the simulant DMNP, as well as the chemical warfare agents (GD and VX) through hydrolysis using a MOF catalyst mixed with a non-volatile, water-insoluble, heterogeneous buffer is reported. The hydrolysis of the simulant and nerve agents in the presence of the heterogeneous buffer was fast and effective.

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.

High volumetric uptake of ammonia using Cu-MOF-74/Cu-CPO-27.

Cu-MOF-74 (also known as Cu-CPO-27) was identified as a sorbent having one of the highest densities of Cu(ii) sites per unit volume. Given that Cu(ii) in the framework can be thermally activated to yield a five-coordinate Cu(ii) species, we identified this MOF as a potential candidate for maximal volumetric uptake of ammonia. To that end, the kinetic breakthrough of ammonia in Cu-MOF-74/Cu-CPO-27 was examined under both dry and humid conditions. Under dry conditions the MOF exhibited a respectable performance (2.6 vs. 2.9 NH3 per nm(3) for the current record holder HKUST-1), and under 80% relative humidity, the MOF outperformed HKUST-1 (5.9 vs. 3.9 NH3 per nm(3), respectively).