Metal-organic framework-based materials for the abatement of air pollution and decontamination of wastewater

Developing new and efficient technologies for environmental remediation is becoming significant due to the increase in global concerns such as climate change, severe epidemics, and energy crises. Air pollution, primarily due to increased levels of H₂S, SO_x, NH₃, NO_x, CO, volatile organic compounds (VOC), and particulate matter (PM) in the atmosphere, has a significant impact on public health, and exhaust gases harm the natural sulfur, nitrogen, and carbon cycles. Similarly, wastewater discharged to the environment with metal ions, herbicides, pharmaceuticals, personal care products, dyes, and aromatics/organic compounds is a risk for health since it may lead to an outbreak of waterborne pathogens and increase the exposure to endocrine-disrupting agents. Therefore, developing new and efficient air and water quality management systems is critical. Metal-organic frameworks (MOFs) are novel materials for which the main application areas include gas storage and separation, water harvesting from the atmosphere, chemical sensing, power storage, drug delivery, and food preservation. Due to their versatile structural motifs that can be modified during synthesis, MOFs also have a great promise for green applications including air and water pollution remediation. The motivation to use MOFs for environmental applications prompted the modification of their structures via the addition of metal and functional groups, as well as the creation of heterostructures by mixing MOFs with other nanomaterials, to effectively remove hazardous contaminants from wastewater and the atmosphere. In this review, we focus on the state-of-the-art environmental applications of MOFs, particularly for water treatment and air pollution, by highlighting the groundbreaking studies in which MOFs have been used as adsorbents, membranes, and photocatalysts for the abatement of air and water pollution. We finally address the opportunities and challenges for the environmental applications of MOFs.

Removal of hydrogen sulfide from a binary mixture with methane gas, using IRMOF-1: a theoretical investigation

The natural gas is mainly composed by methane, ethane, propane, and contaminants. Among these contaminants, the H₂S gas has some specific characteristics such as its toxicity and corrosion, besides reducing the combustion power efficiency of natural gas. In this context, metal-organic frameworks appear as promising materials for purification of natural gas by adsorption, due to their large surface area and pore volume. In this work, Grand Canonical Monte Carlo method was used to study the adsorption and separation of CH₄:H₂S mixture by IRMOF-1. The adsorption isotherms were computed for the pure components, and at different compositions of binary mixture (90:10, 75:25, 50:50, 25:75, and 10:90). Interaction energy obtained with the semiempirical method confirmed that the inorganic unit is the preferred site for CH₄ and H₂S adsorption. Moreover, in a gas mixture with 50:50 proportion of CH₄:H₂S mixture, methane adsorbs preferentially in the inorganic unit only at pressures close to 20 bar. Non-covalent interaction (NCI) analyses indicated that the interactions involving H₂S are more effective than that for CH₄, due to an electrostatic character in the H₂S interaction. The simulations also showed that the separation of gases occurs in all compositions and pressures studied, suggesting that IRMOF-1 has a promising potential for this application.

Hydrogen Sulfide (H2S) Removal via MOFs

The removal of the environmentally toxic and corrosive hydrogen sulfide (H₂S) from gas streams with varying overall pressure and H₂S concentration is a long-standing challenge faced by the oil and gas industries. The present work focuses on H₂S capture using a relatively new type of material, namely metal-organic frameworks (MOFs), in an effort to shed light on their potential as adsorbents in the field of gas storage and separation. MOFs hold great promise as they make possible the design of structures from organic and inorganic units, but also as they have provided an answer to a long-term challenging objective, i.e., how to design extended structures of materials. Moreover, in designing MOFs, one may functionalize the organic units and thus, in essence, create pores with different functionalities, and also to expand the pores in order to increase pore openings. The work presented herein provides a detailed discussion, by thoroughly combining the existing literature on new developments in MOFs for H₂S removal, and tries to provide insight into new areas for further research.

Porous Material Screening and Evaluation for Deep Desulfurization of Dry Air

We employ molecular simulations to screen the best microporous materials for deep desulfurization of dry air. Pressure-swing adsorption and temperature-swing adsorption in desulfurization processes are investigated. The selectivity, working ability, selection parameters, and diffusivity of mixed gases are examined to evaluate those materials. The results show that UiO-66, ZIF-71, ZIF-69, and ZIF-97 exhibit good performance for the separation of H₂S from air. The selectivity and adsorption capacity of H₂S are larger than 300 and 0.01 mmol/g at room temperature and atmospheric pressure, respectively. UiO-66, ZIF-71, ZIF-69, MIL-100, Zn-DOBDC, ZnBDC, IRMOF-11, and MIL-140B are ideal materials to remove SO₂ in air. The selectivity of SO₂ is higher than 500 and the adsorption capacity is higher than 0.06 mmol/g. The diffusivity of sulfides is determined by the competition between the sterically hindered effect and the intermolecular synergistic effect. Comprehensive analysis found that zeolitic imidazolate frameworks (ZIFs) are good materials for the removal of sulfides.

Investigation of structural, electronic and magnetic properties of breathing metal-organic framework MIL-47(Mn): a first principles approach

The structural, electronic and magnetic properties of the MIL-47(Mn) metal-organic framework are investigated using first principles calculations. We find that the large-pore structure is the ground state of this material. We show that upon transition from the large-pore to the narrow-pore structure, the magnetic ground-state configuration changes from antiferromagnetic to ferromagnetic, consistent with the computed values of the intra-chain coupling constant. Furthermore, the antiferromagnetic and ferromagnetic configuration phases have intrinsically different electronic behavior: the former is semiconducting, the latter is a metal or half-metal. The change of electronic properties during breathing posits MIL-47(Mn) as a good candidate for sensing and other applications. Our calculated electronic band structure for MIL-47(Mn) presents a combination of flat dispersionless and strongly dispersive regions in the valence and conduction bands, indicative of quasi-1D electronic behavior. The spin coupling constants are obtained by mapping the total energies onto a spin Hamiltonian. The inter-chain coupling is found to be at least one order of magnitude smaller than the intra-chain coupling for both large and narrow pores. Interestingly, the intra-chain coupling changes sign and becomes five times stronger going from the large pore to the narrow pore structure. As such MIL-47(Mn) could provide unique opportunities for tunable low-dimensional magnetism in transition metal oxide systems.

Probing Metal-Organic Framework Design for Adsorptive Natural Gas Purification

Parent and amine-functionalized analogues of metal-organic frameworks (MOFs), UiO-66(Zr), MIL-125(Ti), and MIL-101(Cr), were evaluated for their hydrogen sulfide (H₂S) adsorption efficacy and post-exposure acid gas stability. Adsorption experiments were conducted through fixed-bed breakthrough studies utilizing multicomponent 1% H₂S/99% CH₄ and 1% H₂S/10% CO₂/89% CH₄ natural gas simulant mixtures. Instability of MIL-101(Cr) materials after H₂S exposure was discovered through powder X-ray diffraction and porosity measurements following adsorbent pelletization, whereas other materials retained their characteristic properties. Linker-based amine functionalities increased H₂S breakthrough times and saturation capacities from their parent MOF analogues. Competitive CO₂ adsorption effects were mitigated in mesoporous MIL-101(Cr) and MIL-101-NH₂(Cr), in comparison to microporous UiO-66(Zr) and MIL-125(Ti) frameworks. This result suggests that the installation of H₂S binding sites in large-pore MOFs could potentially enhance H₂S selectivity. In situ Fourier transform infrared measurements in 10% CO₂ and 5000 ppm H₂S environments suggest that framework hydroxyl and amine moieties serve as H₂S physisorption sites. Results from this study elucidate design strategies and stability considerations for engineering MOFs in sour gas purification applications.

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.

Effective removal of hydrogen sulfide using 4A molecular sieve zeolite synthesized from attapulgite

In this work, 4A molecular sieve zeolite was synthesized from attapulgite (ATP) in different conditions and was applied initially for H₂S removal. The sorbent was characterized by scanning electron microscopy, X-ray diffraction, Fourier transform infrared spectra and N₂ adsorption/desorption. The effects of the synthesis condition and adsorption temperature were studied by dynamic adsorption experiment. The optimal adsorption temperature is 50°C. The H₂S adsorption results have showed that the optimal synthesis conditions are as follows: the ratio of silicon to aluminum and ratio of sodium to silicon are both 1.5, the ratio of water to sodium is 30, crystallization temperature and crystallization time is 90°C, 4h, respectively. The breakthrough and saturation sulfur sorption capacities of zeolite synthesized under optimum conditions are up to nearly 10 and 15mg/g-sorbent, respectively, and the H₂S removal rate is nearly 100%. The adsorption kinetics nonlinear fitting results show that the adsorption system follows Bingham model. These results indicate that 4A molecular sieve zeolite synthesized from attapulgite can be used for H₂S removal promisingly.

Metal–organic frameworks for the removal of toxic industrial chemicals and chemical warfare agents

Toxic gases can be captured or degraded by metal–organic frameworks.

Facile synthesis of a Fe3O4/MIL-101(Fe) composite with enhanced catalytic performance

The as-prepared Fe₃O₄/MIL-101(Fe) shows enhanced catalytic performance for the dimerization reaction of o-phenylenediamine via synergistic peroxidase-like activity.

Metal-organic framework based highly selective fluorescence turn-on probe for hydrogen sulphide

Hydrogen sulphide (H₂S) is known to play a vital role in human physiology and pathology which stimulated interest in understanding complex behaviour of H₂S. Discerning the pathways of H₂S production and its mode of action is still a challenge owing to its volatile and reactive nature. Herein we report azide functionalized metal-organic framework (MOF) as a selective turn-on fluorescent probe for H₂S detection. The MOF shows highly selective and fast response towards H₂S even in presence of other relevant biomolecules. Low cytotoxicity and H₂S detection in live cells, demonstrate the potential of MOF towards monitoring H₂S chemistry in biological system. To the best of our knowledge this is the first example of MOF that exhibit fast and highly selective fluorescence turn-on response towards H₂S under physiological conditions.

A new one-dimensional nickel metal-organic framework: catena-poly[[[diaquabis(4-{[(1-phenyl-1H-tetrazol-5-yl)sulfanyl]methyl}benzoato-κO)nickel(II)]-μ-4,4'-bipyridine-κ(2)N:N'] monohydrate]

The title complex, {[Ni(C15H11N4O2S)2(C10H8N2)(H2O)2]·H2O}n, was synthesized by the reaction of nickel chloride, 4-{[(1-phenyl-1H-tetrazol-5-yl)sulfanyl]methyl}benzoic acid (HL) and 4,4'-bipyridine (bpy) under hydrothermal conditions. The asymmetric unit contains two half Ni(II) ions, each located on an inversion centre, two L(-) ligands, one bpy ligand, two coordinated water molecules and one unligated water molecule. Each Ni(II) centre is six-coordinated by two monodentate carboxylate O atoms from two different L(-) ligands, two pyridine N atoms from two different bpy ligands and two terminal water molecules, displaying a nearly ideal octahedral geometry. The Ni(II) ions are bridged by 4,4'-bipyridine ligands to afford a linear array, with an Ni...Ni separation of 11.361 (1) Å, which is further decorated by two monodentate L(-) ligands trans to each other, resulting in a one-dimensional fishbone-like chain structure. These one-dimensional fishbone-like chains are further linked by O-H...O, O-H...N and C-H...O hydrogen bonds and π-π stacking interactions to form a three-dimensional supramolecular architecture. The thermal stability of the title complex was investigated via thermogravimetric analysis.