Closer Look at Adsorption of Sarin and Simulants on Metal-Organic Frameworks

The development of effective protection against exposure to chemical warfare agents (CWAs), such as sarin, relies on studies of its adsorption on the capturing materials and seeking candidates capable of adsorbing large amounts of sarin gas. Many metal-organic frameworks (MOFs) are promising materials for the effective capture and degradation of sarin and simulant substances. Among the simulants capable of mimicking thermodynamic properties of the agent, not all of them have been investigated on the ability to act similarly in the adsorption process, in particular, whether the agent and a simulant have similar mechanisms of binding to the MOF surface. Molecular simulation studies not only provide a safe way to investigate the aforementioned processes but can also help reveal the mechanisms of interactions between the adsorbents and the adsorbing compounds at the molecular level. We performed Monte Carlo simulations of the adsorption of sarin and three simulants, dimethyl methylphosphonate (DMMP), diisopropyl methylphosphonate (DIMP), and diisopropyl fluorophosphate (DIFP), on selected MOFs that have previously shown strong capabilities to adsorb sarin. On the basis of the calculated adsorption isotherms, enthalpy of adsorption, and radial distribution functions, we revealed common mechanisms among the particularly efficient adsorbents as well as the ability of simulants to mimic them. The findings can help in selecting a suitable simulant compound to study CWA adsorption on MOFs and guide further synthesis of efficient MOFs for the capture of organophosphorus compounds.

Insights and Heuristics for Predicting Diffusion Rates of Chemical Warfare Agents in Zirconium Metal-Organic Frameworks

Designing nanoporous catalysts to destroy chemical warfare agents (CWAs) and environmental contaminants requires consideration of both intrinsic catalytic activity and the mass transfer of molecules in and out of the pores. Polar adsorbates such as CWAs experience a heterogeneous environment in many metal-organic frameworks (MOFs) due to the arrangement of the metal nodes and organic linkers of the MOF. However, quantitative relationships between the pore architecture and the resulting diffusion properties of polar molecules have not been established. We used molecular dynamics simulations to calculate the diffusion coefficients of the CWA simulant dimethyl methyl phosphonate (DMMP) in a diverse set of 776 MOFs with Zr₆ nodes. We developed a 4-parameter machine learning model to predict DMMP diffusivities in Zr₆ MOFs and found the model to be transferable to the CWA sarin. We then developed a simplified heuristic based on the machine learning model that the node-node distance and accessible surface area should be maximized to find MOFs with rapid CWA diffusion.

Computational Screening of Metal-Organic Frameworks for Ammonia Capture from H2/N2/NH3 Mixtures

The separation of ammonia from H₂/N₂/NH₃ mixtures is an important step in ammonia decomposition for hydrogen production and ammonia synthesis from H₂ and N₂ based nonaqueous technologies. Metal-organic frameworks (MOFs) are considered as potential materials for capturing ammonia. In the present work, high-throughput screening of 2932 Computation-Ready Experimental MOFs (CoRE MOFs) was carried out for ammonia capture from H₂/N₂/NH₃ mixtures by Grand Canonical Monte Carlo (GCMC) simulations. It was found that the high-performing MOFs are characterized by tube-like channels, moderate LCD (largest cavity diameter) (4-7.5 Å), and high Q _st ⁰(NH₃) (the isosteric heat of NH₃ adsorption) (>45 kJ/mol). MOFs with high NH₃ adsorption capacity often feature moderate surface area, while the surface area of MOFs with high NH₃ selectivity is relatively lower, which limits the NH₃ adsorption capacity. Q _st ⁰ and the Henry's constant (K _H ) are two energy descriptors describing the interactions between adsorbents and adsorbates. The former has a stronger correlation with the adsorption selectivity, while the latter has a stronger correlation with the adsorption capacity. By analyzing the molecular density distribution of adsorbates in high-performing MOFs, it was found that unsaturated coordinated metal sites provide the main functional binding sites for NH₃. Most MOFs with high NH₃ selectivity have multiple different metal nodes or other atoms except C, O, and H, such as N and P. Multiple metal nodes and nonmetallic atoms provide more functional binding sites. Finally, the adsorption behavior with various concentrations of gas mixtures was examined to verify the universality of the screening calculations, and the effect of framework flexibility on adsorption performance was explored.

Nano-silver functionalized spherical activated carbon with enhanced dipropyl sulfide adsorption capacity and antibacterial properties

Owing to the large dynamic adsorption performance and excellent mechanical strength, spherical activated carbon (SAC) has been widely applied in the field of biochemical protection. However, the adsorbed chemical warfare agent molecules might easily escape from the pores of SAC due to the impact of ambient temperature and humidity, resulting in secondary pollution. Herein, to improve the adsorption performance of SAC, an excessive impregnation method was used to fabricate nano-silver functionalized spherical activated carbon (Ag-SAC). The surface physicochemical structure of the obtained Ag-SAC was extensively studied, and dipropyl sulfide (DPS), a simulant of sulfur mustard (HD), was employed as the adsorbate to evaluate its adsorption capability. The effects of AgNO₃ impregnation concentration, reaction time, initial concentration and temperature on the adsorption performance, were investigated. The equilibrium adsorption capacity of Ag-SAC towards DPS increased by 13.41% compared with that of pristine SAC. Kinetic models, adsorption isotherm models, and adsorption thermodynamics were used to study the adsorption mechanism. The results revealed that the adsorption of DPS by Ag-SAC is a mixed synergistic process, which includes chemical adsorption and physical adsorption. Moreover, the Ag-SAC exhibited good antibacterial characteristics, with an antibacterial rate over 99.28% against Escherichia coli. We anticipate that the Ag-SAC could be a promising material for the development of high performance breathable biochemical protection clothing.

A kind of functional composite was prepared by loading silver nanoparticles firmly on the surface of spherical activated carbon, which showed chemical adsorption properties for chemical warfare agents and satisfactory antibacterial activity.

Methylcellulose/tannic acid complex particles coated on alginate hydrogel scaffold via Pickering for removal of methylene blue from aqueous and quinoline from non-aqueous media

Adsorbents reported for liquid phase decontamination under both aqueous and non-aqueous media are all dispersed phase sorbents that further require a tedious separation step post adsorption. Herein, a monolith, highly porous, and mechanically robust scaffold was synthesized for the adsorption of pollutants from both aqueous and non-aqueous media with facile separation and regeneration. Methylcellulose-tannic acid complex particles were prepared and systematically decorated on the surface of interpenetrating polymer network (IPN) scaffold via Pickering emulsion. Due to the surface coating of the particles, plausible amphiphilic adsorption of quinoline (QUI) and methylene blue (MB) was achieved from fuel and water, respectively. The hydroxyl (OH-) and carboxyl (COOH-) groups of tannic acid, alginate, and polyacrylic acid created hydrogen bonding, electrostatic interaction, acid-base interaction, and π-π stacking. Maximum adsorption capacity of 791.17 mg/g MB and 460.92 mg/g QUI was recorded with facile separation, excellent adsorbent regeneration, and reusability. Although both followed the pseudo-second-order adsorption kinetic model, a different mechanism was identified to govern the adsorption under aqueous and non-aqueous environment i.e. only the surface particles were active for QUI adsorption while the scaffold was also involved for MB adsorption.

Incorporating Flexibility Effects into Metal-Organic Framework Adsorption Simulations Using Different Models

High-throughput calculations based on molecular simulations to predict the adsorption of molecules inside metal-organic frameworks (MOFs) have become a useful complement to experimental efforts to identify promising adsorbents for chemical separations and storage. For computational convenience, all existing efforts of this kind have relied on simulations in which the MOF is approximated as rigid. In this paper, we use extensive adsorption-relaxation simulations that fully include MOF flexibility effects to explore the validity of the rigid framework approximation. We also examine the accuracy of several approximate methods to incorporate framework flexibility that are more computationally efficient than adsorption-relaxation calculations. We first benchmark various models of MOF flexibility for four MOFs with well-established CO₂ experimental consensus isotherms. We then consider a range of adsorption properties, including Henry's constants, nondilute loadings, and adsorption selectivity, for seven adsorbates in 15 MOFs randomly selected from the CoRE MOF database. Our results indicate that in many MOFs adsorption-relaxation simulations are necessary to make quantitative predictions of adsorption, particularly for adsorption at dilute concentrations, although more standard calculations based on rigid structures can provide useful information. Finally, we investigate whether a correlation exists between the elastic properties of empty MOFs and the importance of including framework flexibility in making accurate predictions of molecular adsorption. Our results did not identify a simple correlation of this type.

Emergence of Coupled Rotor Dynamics in Metal-Organic Frameworks via Tuned Steric Interactions

The organic components in metal–organic frameworks (MOFs) are unique: they are embedded in a crystalline lattice, yet, as they are separated from each other by tunable free space, a large variety of dynamic behavior can emerge. These rotational dynamics of the organic linkers are especially important due to their influence over properties such as gas adsorption and kinetics of guest release. To fully exploit linker rotation, such as in the form of molecular machines, it is necessary to engineer correlated linker dynamics to achieve their cooperative functional motion. Here, we show that for MIL-53, a topology with closely spaced rotors, the phenylene functionalization allows researchers to tune the rotors’ steric environment, shifting linker rotation from completely static to rapid motions at frequencies above 100 MHz. For steric interactions that start to inhibit independent rotor motion, we identify for the first time the emergence of coupled rotation modes in linker dynamics. These findings pave the way for function-specific engineering of gear-like cooperative motion in MOFs.

Force Fields for Molecular Modeling of Sarin and its Simulants: DMMP and DIMP

Even three decades after signing the Chemical Weapons Convention, organophosphorus chemical warfare agents (CWAs), such as sarin, remain a threat. The development of novel methods for the detection of CWAs, protection from CWAs, and CWA decontamination motivates research on their physicochemical properties. Due to the extreme toxicity of sarin, most of the experimental studies are carried out using less toxic simulant compounds. In addition to experimental studies of sarin simulants, both sarin and simulants can be studied using in silico experiments-molecular simulations. The results of classical molecular modeling of the compounds and their agreement with experimental data rely on the force field used to describe the system. In recent years, there have been several force fields proposed for sarin and its most common simulant dimethyl methylphosphonate (DMMP). However, other simulants frequently used in experiments received less attention from the molecular simulation perspective, for example, to date, there is no force field and no simulation data for diisopropyl methylphosphonate (DIMP). Here, we compare the literature force fields for sarin and DMMP, focusing specifically on the vapor-liquid equilibrium for the pure species. We carried out Monte Carlo and molecular dynamics simulations using the existing literature force fields from which we predicted the liquid densities and vapor pressures developing the entire binodal curves. We compared the predictions to the experimental data and showed that the TraPPE-UA force field outperformed the other force fields. Thus, we extended TraPPE-UA for DIMP, utilized this force field in molecular simulations, and predicted the liquid densities and vapor pressures for a range of temperatures (binodal curve), which agreed well with the published experimental data. From the binodal, we calculated the critical properties of DIMP and demonstrated that these parameters can be used in the Peng-Robinson equation of state for this compound.