Exceptional Adsorption and Binding of Sulfur Dioxide in a Robust Zirconium-Based Metal-Organic Framework

A stable Zr(IV)-MOF for efficient removal of trace SO2 from flue gas in dry and humid conditions

Obtaining suitable adsorbents for selective separation of SO2 from flue gas still remains an important issue. A stable Zr(IV)-MOF (Zr-PTBA) can be conveniently synthesized through the self-assembly of a tetracarboxylic acid ligand (H4L = 4,4',4'',4'''-(1,4-phenylenebis(azanetriyl))tetrabenzoic acid) and ZrCl4 in the presence of trace water. It exhibits a three-dimensional porous structure. The BET surface area is 1112.72 m2/g and the average pore size distribution focus on 5.9, 8.0 and 9.3 Å. Interestingly, Zr-PTBA shows selective adsorption of SO2. The maximum uptake reaches 223.21 cm3/g at ambient condition. While it exhibits lower adsorption uptake of CO2 (30.50 cm3/g) and hardly adsorbs O2 (2.57 cm3/g) and N2 (1.31 cm3/g). Higher IAST selectivities of SO2/CO2 (21.9), SO2/N2 (912.7), SO2/O2 (2269.9) and SO2/CH4 (85.0) have been obtained, which reveal its' excellent gas separation performance. Breakthrough experiment further confirms its application for flue gas deep desulfurization both in dry and humid conditions. Furthermore, the gas adsorption results and mechanisms have also been studied by theoretical calculations.

Unravelling the structure of CO2 in silica adsorbents: an NMR and computational perspective

This comprehensive review describes recent advancements in the use of solid-state NMR-assisted methods and computational modeling strategies to unravel gas adsorption mechanisms and CO2 speciation in porous CO2-adsorbent silica materials at the atomic scale. This work provides new perspectives for the innovative modifications of these materials rendering them more amenable to the use of advanced NMR methods.

Carboxyl position-directed structure diversity in zirconium-tricarboxylate frameworks

Herein, three tritopic carboxylic acids were used to construct three Zr-MOFs, HIAM-4033, HIAM-4034, and HIAM-4035, to investigate the effect of carboxyl position on the MOF structures. The results showed that HIAM-4033 and HIAM-4034 possess (3,9)-c models with different underlying nets, whereas HIAM-4035 exhibits the same underlying net as UiO-68. Nanosized HIAM-4033 exhibits excellent sensitivity and selectivity for detecting aromatic acids, such as benzoic acid and 2-fluorobenzoic acid, compared with aliphatic acids and inorganic acids. This study offers new insights into achieving an organic linker directed structure evolution of Zr-MOFs, which might facilitate the discovery of unprecedented underlying nets.

Modulator-Dependent Dynamics Synergistically Enabled Record SO2 Uptake in Zr(IV) Metal-Organic Frameworks Based on Pyrene-Cored Molecular Quadripod Ligand

Developing innovative porous solid sorbents for the capture and storage of toxic SO2 is crucial for energy-efficient transportation and subsequent processing. Nonetheless, the quest for high-performance SO2 sorbents, characterized by exceptional uptake capacity, minimal regeneration energy requirements, and outstanding recyclability under ambient conditions, remains a significant challenge. In this study, we present the design of a unique tertiary amine-embedded, pyrene-based quadripod-shaped ligand. This ligand is then assembled into a highly porous Zr-metal-organic framework (MOF) denoted as Zr-TPA, which exhibits a newly discovered 3,4,8-c woy net structure. Remarkably, our Zr-TPA MOF achieved an unprecedented SO2 sorption capacity of 22.7 mmol g-1 at 298 K and 1 bar, surpassing those of all previously reported solid sorbents. We elucidated the distinct SO2 sorption behaviors observed in isostructural Zr-TPA variants synthesized with different capping modulators (formate, acetate, benzoate, and trifluoroacetate, abbreviated as FA, HAc, BA, and TFA, respectively) through computational analyses. These analyses revealed unexpected SO2-induced modulator-node dynamics, resulting in transient chemisorption that enhanced synergistic SO2 sorption. Additionally, we conducted a proof-of-concept experiment demonstrating that the captured SO2 in Zr-TPA-FA can be converted in situ into a valuable pharmaceutical intermediate known as aryl N-aminosulfonamide, with a high yield and excellent recyclability. This highlights the potential of robust Zr-MOFs for storing SO2 in catalytic applications. In summary, this work contributes significantly to the development of efficient SO2 solid sorbents and advances our understanding of the molecular mechanisms underlying SO2 sorption in Zr-MOF materials.

Synergistic Adsorption and In Situ Catalytic Conversion of SO2 by Transformed Bimetal-Phenolic Functionalized Biomass

SO2 removal is critical to flue gas purification. However, based on performance and cost, materials under development are hardly adequate substitutes for active carbon-based materials. Here, we engineered biomass-derived nanostructured carbon nanofibers integrated with highly dispersed bimetallic Ti/CoOx nanoparticles through the thermal transition of metal-phenolic functionalized industrial leather wastes for synergistic SO2 adsorption and in situ catalytic conversion. The generation of surface-SO32- and peroxide species (O22-) by Ti/CoOx achieved catalytic conversion of adsorbed SO2 into value-added liquid H2SO4, which can be discharged from porous nanofibers. This approach can also avoid the accumulation of the adsorbed SO2, thereby achieving high desulfurization activity and a long operating life over 6000 min, preceding current state-of-the-art active carbon-based desulfurization materials. Combined with the techno-economic and carbon footprint analysis from 36 areas in China, we demonstrated an economically viable and scalable solution for real-world SO2 removal on the industrial scale.

Feasible bottom-up development of conjugated microporous polymers (CMPs) for boosting the deep removal of sulfur dioxide

A pain-point for material development is that computer-screened structures are usually difficult to realize in experiments. Herein, considering that linkages are crucial for building functional nanoporous polymers with diverse functionalities, we develop an efficient approach for constructing target-specific conjugated microporous polymers (CMPs) based on screening feasible polymerization pathways. Taking the deep removal of SO2 from a SO2/CO2 mixture as the specific target, we precisely screen the linkages and fabricate different CMPs by manipulating the porosity and hydrophobicity. Based on the optimized Buchwald-Hartwig amination, the obtained CMPs can achieve SO2/CO2 selectivity as high as 113 and a moderate Qst of 30 kJ mol-1 for feasible regeneration. Furthermore, the potential of CMPs for practical SO2/CO2 separation is demonstrated through continued breakthrough tests. The SO2 binding sites are consistent with the screening results and proved by in situ Fourier transform infrared spectroscopy and grand canonical Monte Carlo simulation, providing solid feasibility for synthesis realizability for future boosts of task-specific CMPs.

Detection of SO2 using a chemically stable Ni(II)-MOF

The MOF-type Ni2(dobpdc) shows a high chemical stability towards SO2, high capacity for SO2 capture at low pressure (4.3 mmol g-1 at 298 K and up to 0.05 bar), and exceptional cycling performance. Fluorescence experiments demonstrated the SO2 detection properties of Ni2(dobpdc) with a remarkable SO2 detection selectivity. Finally, time-resolved photoluminescence experiments provided a plausible mechanism of SO2 detection by this Ni(II)-based MOF material.

A robust ultra-microporous cationic aluminum-based metal-organic framework with a flexible tetra-carboxylate linker

Al-based cationic metal-organic frameworks (MOFs) are uncommon. Here, we report a cationic Al-MOF, MIP-213(Al) ([Al₁₈(μ₂-OH)₂₄(OH₂)₁₂(mdip)₆]6Cl·6H₂O) constructed from flexible tetra-carboxylate ligand (5,5'-Methylenediisophthalic acid; H₄mdip). Its crystal structure was determined by the combination of three-dimensional electron diffraction (3DED) and high-resolution powder X-ray diffraction. The structure is built from infinite corner-sharing chains of AlO₄(OH)₂ and AlO₂(OH)₃(H₂O) octahedra forming an 18-membered rings honeycomb lattice, similar to that of MIL-96(Al), a scarce Al-polycarboxylate defective MOF. Despite sharing these structural similarities, MIP-213(Al), unlike MIL-96(Al), lacks the isolated μ₃-oxo-bridged Al-clusters. This leads to an ordered defective cationic framework whose charge is balanced by Cl^- sandwiched between two Al-trimers at the corner of the honeycomb, showing strong interaction with terminal H₂O coordinated to the Al-trimers. The overall structure is endowed by a narrow quasi-1D channel of dimension ~4.7 Å. The Cl^- in the framework restrains the accessibility of the channels, while the MOF selectively adsorbs CO₂ over N₂ and possesses high hydrolytic stability.

Metal-Organic Frameworks for Greenhouse Gas Applications

Using petrol to supply energy for a car or burning coal to heat a building generates plenty of greenhouse gas (GHG) emissions, including carbon dioxide (CO₂ ), water vapor (H₂ O), methane (CH₄ ), nitrous oxide (N₂ O), ozone (O₃ ), fluorinated gases. These up-and-coming metal-organic frameworks (MOFs) are structurally endowed with rigid inorganic nodes and versatile organic linkers, which have been extensively used in the GHG-related applications to improve the lives and protect the environment. Porous MOF materials and their derivatives have been demonstrated to be competitive and promising candidates for GHG separation, storage and conversions as they shows facile preparation, large porosity, adjustable nanostructure, abundant topology, and tunable physicochemical property. Enormous progress has been made in GHG storage and separation intrinsically stemmed from the different interaction between guest molecule and host framework from MOF itself in the recent five years. Meanwhile, the use of porous MOF materials to transform GHG and the influence of external conditions on the adsorption performance of MOFs for GHG are also enclosed. In this review, it is also highlighted that the existing challenges and future directions are discussed and envisioned in the rational design, facile synthesis and comprehensive utilization of MOFs and their derivatives for practical applications.

Exploring the Potential of a Highly Scalable Metal-Organic Framework CALF-20 for Selective Gas Adsorption at Low Pressure

In this study, the ability of the highly scalable metal-organic framework (MOF) CALF-20 to adsorb polar and non-polar gases at low pressure was investigated using grand canonical Monte Carlo (GCMC) and molecular dynamics (MD) simulations. The results from the simulated adsorption isotherms revealed that the highest loading was achieved for SO₂ and Cl₂, while the lowest loading was found for F₂ molecules. The analysis of interaction energies indicated that SO₂ molecules were able to form the strongest adsorbent-adsorbate interactions and had a tight molecular packing due to their polarity and angular structure. Additionally, Cl₂ gas was found to be highly adsorbed due to its large van der Waals surface and strong chemical affinity in CALF-20 pores. MD simulations showed that SO₂ and Cl₂ had the lowest mobility inside CALF-20 pores. The values of the Henry coefficient and isosteric heat of adsorption confirmed that CALF-20 could selectively adsorb SO₂ and Cl₂. Based on the results, it was concluded that CALF-20 is a suitable adsorbent for SO₂ and Cl₂ but not for F₂. This research emphasizes the importance of molecular size, geometry, and polarity in determining the suitability of a porous material as an adsorbent for specific adsorbates.

Creating High-Number Defect Sites through a Bimetal Approach in Metal-Organic Frameworks for Boosting Trace SO2 Removal

Herein, we represent a bimetallic approach to enhance the defect number, leading to eight defect sites per node in a metal-organic framework, showing both a higher SO₂ adsorption capacity and higher SO₂/CO₂ selectivity. The results can be further strongly supported by density functional theory calculations.

Surface, Interface and Structure Optimization of Metal-Organic Frameworks: Towards Efficient Resourceful Conversion of Industrial Waste Gases

Industrial waste gas emissions from fossil fuel over-exploitation have aroused great attention in modern society. Recently, metal-organic frameworks (MOFs) have been developed in the capture and catalytic conversion of industrial exhaust gases such as SO₂ , H₂ S, NO_x , CO₂ , CO, etc. Based on these resourceful conversion applications, in this review, we summarize the crucial role of the surface, interface, and structure optimization of MOFs for performance enhancement. The main points include (1) adsorption enhancement of target molecules by surface functional modification, (2) promotion of catalytic reaction kinetics through enhanced coupling in interfaces, and (3) adaptive matching of guest molecules by structural and pore size modulation. We expect that this review will provide valuable references and illumination for the design and development of MOF and related materials with excellent exhaust gas treatment performance.

Structural and Dynamic Analysis of Sulphur Dioxide Adsorption in a Series of Zirconium‐Based Metal–Organic Frameworks

We report reversible high capacity adsorption of SO₂ in robust Zr‐based metal–organic framework (MOF) materials. Zr‐bptc (H₄bptc=biphenyl‐3,3′,5,5′‐tetracarboxylic acid) shows a high SO₂ uptake of 6.2 mmol g⁻¹ at 0.1 bar and 298 K, reflecting excellent capture capability and removal of SO₂ at low concentration (2500 ppm). Dynamic breakthrough experiments confirm that the introduction of amine, atomically‐dispersed Cu^II or heteroatomic sulphur sites into the pores enhance the capture of SO₂ at low concentrations. The captured SO₂ can be converted quantitatively to a pharmaceutical intermediate, aryl N‐aminosulfonamide, thus converting waste to chemical values. In situ X‐ray diffraction, infrared micro‐spectroscopy and inelastic neutron scattering enable the visualisation of the binding domains of adsorbed SO₂ molecules and host–guest binding dynamics in these materials at the atomic level. Refinement of the pore environment plays a critical role in designing efficient sorbent materials.

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.

Stable Porous Organic Polymers Used for Reversible Adsorption and Efficient Separation of Trace SO2

The development of porous solid adsorbents for selective adsorption and separation of SO₂ has attracted much attention recently. Herein, we design porous organic polymers (POPs) decorated with pyridine ligands as building units (POP-Py) through a radical polymerization of the 2,5-divinylpyridine (v-Py) monomer. Due to its high BET surface area, nanoporosity, and excellent stability, the prepared POP-Py can be used for reversible adsorption and efficient separation of SO₂. The POP-Py possesses a SO₂ capacity of 10.8 mmol g^-1 at 298 K and 1.0 bar, which can be well retained after 6 recycles, showing an excellent reversible adsorption capacity. The POP-Py also shows superior separation performance for SO₂ from a ternary SO₂/CO₂/N₂ mixture (0.17/15/84.83v%), giving a breakthrough time and a saturated SO₂ capacity at 178 min g^-1 and 0.4 mmol g^-1. The retention time was well maintained even under high moisture conditions, confirming its superior water resistance. Furthermore, when other vinyl-functionalized organic ligand monomers (bipyridine, pyrimidine, and pyrazine) were employed for radical polymerization, all of the resultant porous organic ligand polymers (POP-BPy, POP-PyI, and POP-PyA) exhibited superior performance for reversible adsorption and efficient separation of SO₂. The combined features of reversible adsorption, efficient separation, and water resistance are important for the industrial applications of these materials as SO₂ adsorbents.

Adsorption of Sulfur Dioxide in Cu(II)-Carboxylate Framework Materials: The Role of Ligand Functionalization and Open Metal Sites

The development of efficient sorbent materials for sulfur dioxide (SO₂) is of key industrial interest. However, due to the corrosive nature of SO₂, conventional porous materials often exhibit poor reversibility and limited uptake toward SO₂ sorption. Here, we report high adsorption of SO₂ in a series of Cu(II)-carboxylate-based metal–organic framework materials. We describe the impact of ligand functionalization and open metal sites on the uptake and reversibility of SO₂ adsorption. Specifically, MFM-101 and MFM-190(F) show fully reversible SO₂ adsorption with remarkable capacities of 18.7 and 18.3 mmol g^–1, respectively, at 298 K and 1 bar; the former represents the highest reversible uptake of SO₂ under ambient conditions among all porous solids reported to date. In situ neutron powder diffraction and synchrotron infrared microspectroscopy enable the direct visualization of binding domains of adsorbed SO₂ molecules as well as host–guest binding dynamics. We have found that the combination of open Cu(II) sites and ligand functionalization, together with the size and geometry of metal–ligand cages, plays an integral role in the enhancement of SO₂ binding.

Heteroatom-Doped Porous Carbons as Effective Adsorbers for Toxic Industrial Gasses

Ammonia (NH3), often stored in large quantities before being used in the production of fertilizer, and sulfur dioxide (SO2), a byproduct of fossil fuel consumption, particularly the burning of coal, are highly toxic and corrosive gases that pose a significant danger to humans if accidentally released. Therefore, developing advanced materials to enable their effective capture and safe storage is highly desired. Herein, advanced benzimidazole-derived carbons (BIDCs) with an exceptional capacity for NH3 and SO2 have been designed and tested. These heteroatom-doped porous carbon adsorbents were synthesized by thermolysis of imidazolate-potassium salts affording high surface area and controlled heteroatom content to optimize for rapid NH3 and SO2 gas uptake and release under practical conditions. According to gas uptake measurements, these nitrogen-doped carbons exhibit exceptional gas adsorption capacity, with BIDC-3-800 adsorbing 21.42 mmol/g SO2 at 298 K and 1 bar, exceeding most reported porous materials and BIDC-2-700 adsorbing 14.26 mmol/g NH3 under the same conditions. The NH3 uptake of BIDC-2-700 surpassed reported activated carbons and is among the best adsorbents including metal organic frameworks (MOFs). Our synthetic method allows for control over both textural and chemical properties of the carbon and enables heteroatom functionality to be incorporated directly into the carbon framework without the need for postsynthetic modification. These materials were also tested for recyclability; all adsorbents showed almost complete retention of their initial gas uptake capacity during recyclability studies and maintained their structural integrity and their previous adsorption capacity of both NH3 and SO2, highlighting their potential for practical application.

Multi-Level Computational Screening of in Silico Designed MOFs for Efficient SO2 Capture

SO₂ presence in the atmosphere can cause significant harm to the human and environment through acid rain and/or smog formation. Combining the operational advantages of adsorption-based separation and diverse nature of metal-organic frameworks (MOFs), cost-effective separation processes for SO₂ emissions can be developed. Herein, a large database of hypothetical MOFs composed of >300,000 materials is screened for SO₂/CH₄, SO₂/CO₂, and SO₂/N₂ separations using a multi-level computational approach. Based on a combination of separation performance metrics (adsorption selectivity, working capacity, and regenerability), the best materials and the most common functional groups in those most promising materials are identified for each separation. The top bare MOFs and their functionalized variants are determined to attain SO₂/CH₄ selectivities of 62.4-16899.7, SO₂ working capacities of 0.3-20.1 mol/kg, and SO₂ regenerabilities of 5.8-98.5%. Regarding SO₂/CO₂ separation, they possess SO₂/CO₂ selectivities of 13.3-367.2, SO₂ working capacities of 0.1-17.7 mol/kg, and SO₂ regenerabilities of 1.9-98.2%. For the SO₂/N₂ separation, their SO₂/N₂ selectivities, SO₂ working capacities, and SO₂ regenerabilities span the ranges of 137.9-67,338.9, 0.4-20.6 mol/kg, and 7.0-98.6%, respectively. Besides, using breakdowns of gas separation performances of MOFs into functional groups, separation performance limits of MOFs based on functional groups are identified where bare MOFs (MOFs with multiple functional groups) tend to show the smallest (largest) spreads.

Customized Synthesis: Solvent- and Acid-Assisted Topology Evolution in Zirconium-Tetracarboxylate Frameworks

Metal-organic frameworks (MOFs) demonstrate strong potential for various important applications due to their well tunable structures and compositions through metal and organic linker engineering. As an effective approach, topology evolution by controlling linker conformation has received considerable attention, where solvents and acids have crucial effects on structural formation. However, a systematic study of such effects remains under investigated. Herein, we carried out a methodical study on the topology evolution in Zr-MOFs directed by solvothermal conditions with various combinations of three common solvents and six different acids. As a result, three Zr-MOFs with different topologies, scu (HIAM-4007), scp (HIAM-4008), and csq (HIAM-4009), were obtained using the same Zr₆-cluster and tetratopic carboxylate linker, in which structure diversity shows significant influence on their corresponding photoluminescence quantum yields. Further experiments revealed that the acidity of acids and the basicity of solvents strongly influenced the linker conformation in the resultant MOFs, leading to the topology evolution. Such a solvent- and acid-assisted topology evolution represents a general approach that can be used with other tetratopic carboxylate linkers to realize structural diversity. The present work demonstrates an effective structure designing strategy by controlling synthetic conditions, which may prove to be powerful for customized synthesis of MOFs with specific structure and functionality.

Reversible ammonia uptake at room temperature in a robust and tunable metal-organic framework

Ammonia is useful for the production of fertilizers and chemicals for modern technology, but its high toxicity and corrosiveness are harmful to the environment and human health. Here, we report the recyclable and tunable ammonia adsorption using a robust imidazolium-based MOF (JCM-1) that uptakes 5.7 mmol g⁻¹ of NH₃ at 298 K reversibly without structural deformation. Furthermore, a simple substitution of NO₃⁻ with Cl⁻ in a post-synthetic manner leads to an increase in the NH₃ uptake capacity of JCM-1(Cl⁻) up to 7.2 mmol g⁻¹.

Recyclable and tunable ammonia adsorption with JCM-1 and JCM-1(Cl⁻) at room temperature occurs reversibly without structural decomposition.

Creating Optimal Pockets in a Clathrochelate-Based Metal-Organic Framework for Gas Adsorption and Separation: Experimental and Computational Studies

The rational design and synthesis of robust metal-organic frameworks (MOFs) based on novel organic building blocks are fundamental aspects of reticular chemistry. Beyond simply fabricating new organic linkers, however, it is important to elucidate structure-property relationships at the molecular level to develop high-performing materials. In this work, we successfully targeted a highly porous and robust cage-type MOF (NU-200) with an nbo-derived fof topology through the deliberate assembly of a cyclohexane-functionalized iron(II)-clathrochelate-based meta-benzenedicarboxylate linker with a Cu₂(CO₂)₄ secondary building unit (SBU). NU-200 exhibited an outstanding adsorption capacity of xenon and a high ideal adsorbed solution theory (IAST) predicted selectivity for a 20/80 v/v mixture of xenon (Xe)/krypton (Kr) at 298 K and 1.0 bar. Our extensive computational simulations with grand canonical Monte Carlo (GCMC) and density functional theory (DFT) on NU-200 indicated that the MOF's hierarchical bowl-shaped nanopockets surrounded by custom-designed cyclohexyl groups─instead of the conventionally believed open metal sites (OMSs)─played a crucial role in reinforcing Xe-binding affinity. The optimally sized pockets firmly trapped Xe through numerous supramolecular interactions including Xe···H, Xe···O, and Xe···π. Additionally, we validated the unique pocket confinement effect by experimentally and computationally employing the similarly sized probe, sulfur dioxide (SO₂), which provided significant insights into the molecular underpinnings of the high uptake of SO₂ (11.7 mmol g^-1), especially at a low pressure of 0.1 bar (8.5 mmol g^-1). This work therefore can facilitate the judicious design of organic building blocks, producing MOFs featuring tailor-made pockets to boost gas adsorption and separation performances.

Schematic Design of Metal-Free NHC-Mediated Sequestering and Complete Conversion of SO2 to Thiocarbonyl S-Oxide Derivatives at Room Temperature

The sequestering and complete conversion of SO₂ to valuable chemicals in a metal-free pathway is highly demanded. The recent success of SO₂ fixation by N-heterocyclic carbenes instigated further studies in this regard. Previous reports were confined within the carbene-SO₂ reaction mechanism and the stability of oxathiirane S-oxide derivatives. The complete conversion of captured SO₂ to precious chemicals was not studied. The present inquisition has accomplished the scarcity of the earlier studies. It is observed that in the presence of an excess amount of carbene, the registered SO₂ is converted to the ketone derivative and thiocarbonyl S-oxide derivative. An electronic level investigation of these reactions is carried out. From the change of the molecular orbitals along the reaction path, it is concluded that the reaction between the oxathiirane S-oxide derivative and carbene follows a frog's hunting mechanism.

Investigating the Influence of Hexanuclear Clusters in Isostructural Metal-Organic Frameworks on Toxic Gas Adsorption

The efficient capture of toxic gases, such as ammonia (NH₃) and sulfur dioxide (SO₂), can protect the general population and mitigate widespread air pollution. Metal-organic frameworks (MOFs) comprise a tunable class of adsorbents with high surface areas that can meet this challenge by selectively capturing these gases at low concentrations. In this work, we explored how modifying the metal ions in the node of an isostructural MOF series from a transition metal to a lanthanide or actinide influences the electronic environment of the node-based active site. Next, we investigated the adsorption properties of each MOF toward the relatively basic NH₃ and relatively acidic SO₂ gases. Within the NU-907 family of MOFs, we found that Zr₆-NU-907 exhibits the best uptake toward NH₃ at low pressures, while Th₆-NU-907 demonstrates the best low-pressure performance for SO₂ adsorption. Tracking the infrared (IR) stretching frequency of the node-based μ₃-OH groups provides insights into the electronegativity of the metal ion and suggests that the most electronegative metal ion (Zr) affords the node with the best NH₃ uptake at low pressures. In contrast, the Th₆ node contains additional coordinated water groups relative to the other M₆ nodes, which appears to yield the MOF with the greatest affinity for SO₂ uptake that occurs predominately through reversible physisorption interactions. Finally, in situ NH₃ IR spectroscopic studies indicate that both NH₄⁺ and Lewis-bound NH₃ species form during adsorption. Combined, these results suggest that tuning the electronic properties and structure of the node-based active site in an MOF presents a viable strategy to change the affinity of an MOF toward toxic gases.

Acid/base responsive assembly/dis-assembly of a family of zirconium(IV) clusters with a cyclic imide-dioxime ligand

The hydrolytically stable dioxime ligand (2Z-6Z)-piperidine-2,6-dione (H₃pidiox) acts as a strong chelator mainly with hard metals in high oxidation states, a pre-requisite for potential applications in metal sequestering processes from aqueous solutions. Reaction of ZrCl₄ with H₃pidiox in methanol gives the mononuclear compound [Zr^IV(η¹,η¹,η²-H₂pidiox-O,N,O')₂(OH₂)₂]Cl₂·H₂O·CH₃OH (1), while the same reaction mixture in the presence of KOH gave the pentanuclear ZrOC [ZrIV5(μ₂-OH)₄(OH₂)₄(μ₂-η¹,η¹,η²-Hpidiox-O,N,O')₄(η¹,η¹,η¹-HpidioxO,N,O')₄]·5KCl·3CH₃OH·8H₂O (2). Compound 1 is formed at very acidic pH = 0, and the pentanuclear ZrOC 2 at higher pH values (pH = 2). Compounds 1 and 2 were characterized by single crystal X-ray structure analysis, multi-nuclear NMR spectroscopy and ESI-MS spectrometry. The single crystal X-ray structure analysis of 1 revealed a mononuclear zirconium(IV) compound containing an eight-coordinate zirconium atom bound to two singly deprotonated H₂pidiox^- ligands and two water molecules in a severely distorted bicapped octahedral geometry. The pentanuclear ZrOC 2 constitutes the second example of a Zr₅ cluster to be reported and the first one in which the four zirconium atoms are arranged in a tetrahedral arrangement with the fifth occupying the center of the tetrahedron. 1D and 2D NMR spectroscopies of the acidic CD₃OD solutions of complex 1 reveal a fast equilibrium between 1 and 2. Addition of KOH into a CH₃OH solution of 2 results in the controlled fast transformation of 2 to an asymmetric hexanuclear ZrOC 3 as evidenced by the NMR and real-time ESI-MS solution studies. Further addition of KOH to the solution of 3 leads to the ZrOC 4, and on the basis of NMR and ESI-MS data and in comparison with the known hexanuclear titanium(IV)/H₃pidiox cluster, it is concluded that the cluster 4 should have a hexanuclear structure. Electrospray ionization mass spectrometry (ESI-MS) demonstrated not only the structural stability 1 and 2 in solution, but also revealed the reversible pH driven dis-assembly/re-assembly process between the monomeric 1 and the pentanuclear ZrOC 2.

A {Ni12}-Wheel-Based Metal-Organic Framework for Coordinative Binding of Sulphur Dioxide and Nitrogen Dioxide

Air pollutions by SO 2 and NO 2 have caused significant risks on the environment and human health. Understanding the mechanism of active sites within capture materials is of fundamental importance to the development of new clean-up technologies. Here we report the crystallographic observation of reversible coordinative binding of SO 2 and NO 2 on open Ni(II) sites in a metal-organic framework (NKU-100) incorporating an unprecedented {Ni 12 }-wheel, which exhibits six open Ni(II) sites on desolvation. Immobilised gas molecules are further stabilised by cooperative host-guest interactions comprised of hydrogen bonds, π ··· π interactions and dipole interactions. At 298 K and 1.0 bar, NKU-100 shows adsorption uptakes of 6.21 and 5.80 mmol g -1 for SO 2 and NO 2 , respectively. Dynamic breakthrough experiments have confirmed the selective retention of SO 2 and NO 2 at low concentrations under dry conditions. This work will inspire the future design of efficient sorbents for the capture of SO 2 and NO 2 .

Recent advances and future challenges in the synthesis of 2,4,6-triarylpyridines

2,4,6-Triarylpyridines are key building blocks to access functional molecules that are used in the design of advanced materials, metal-organic frameworks, supramolecules, reactive chemical intermediates and drugs. A number of synthetic protocols to construct this heterocyclic scaffold have been developed to date, the most recent of which (2015-present) are included and discussed in the present review. An emphasis has been placed on the utility of each synthetic approach in view of the scope of aryl/hetaryl substituents, limitations and an outlook of each method to be used in applied sciences.

Cucurbit[6]uril@MIL-101-Cl: loading polar porous cages in mesoporous stable host for enhanced SO2 adsorption at low pressures

The robust cucurbituril-MOF composite CB6@MIL-101-Cl was synthesized by a wet impregnation method and a concomitant OH-to-Cl ligand exchange {CB6 = cucurbit[6]uril, 31 wt% content in the composite, MIL-101-Cl = [Cr₃(O)Cl(H₂O)₂(BDC)₃], BDC = benzene-1,4-dicarboxylate}. MIL-101-Cl was formed postsynthetically from standard fluorine-free MIL-101 where Cr-OH ligands were substituted by Cl during treatment with HCl. CB6@MIL-101-Cl combines the strong SO₂ affinity of the rigid CB6 macrocycles and the high SO₂ uptake capacity of MIL-101, and shows a high SO₂ uptake of 438 cm³ g^-1 (19.5 mmol g^-1) at 1 bar and 293 K (380 cm³ g^-1, 17.0 mmol g^-1 at 1 bar and 298 K). The captured SO₂ amount is 2.2 mmol g^-1 for CB6@MIL-101-Cl at 0.01 bar and 293 K (2.0 mmol g^-1 at 298 K), which is three times higher than that of the parent MIL-101 (0.7 mmol g^-1) under the same conditions. The near zero-coverage SO₂ adsorption enthalpies of MIL-101 and CB6@MIL-101-Cl are -35 kJ mol^-1 and -50 kJ mol^-1, respectively, reflecting the impact of the incorporated CB6 macrocycles, having higher affinity towards SO₂. FT-IR spectroscopy confirms the interactions of the SO₂ with the cucurbit[6]uril moieties of the CB6@MIL-101-Cl composite and SO₂ retention for a few minutes under ambient air. Comparative experiments demonstrated loss of crystallinity and porosity after dry SO₂ adsorption for MIL-101, while CB6@MIL-101-Cl exhibits nearly complete retention of crystallinity and porosity under the exposure to both dry and wet SO₂. Thus, CB6@MIL-101-Cl can be an attractive adsorbent for SO₂ capture because of its excellent recycling stability, high capacity and strong affinity toward SO₂ at low pressure.

SO2 Capture by Two Aluminum-Based MOFs: Rigid-like MIL-53(Al)-TDC versus Breathing MIL-53(Al)-BDC

Metal-organic frameworks MIL-53(Al)-TDC and MIL-53(Al)-BDC were explored in the SO₂ adsorption process. MIL-53(Al)-TDC was shown to behave as a rigid-like material upon SO₂ adsorption. On the other hand, MIL-53(Al)-BDC exhibits guest-induced flexibility of the framework with the presence of multiple steps in the SO₂ adsorption isotherm that was related through molecular simulations to the existence of three different pore opening phases narrow pore, intermediate pore, and large pore. Both materials proved to be exceptional candidates for SO₂ capture, even under wet conditions, with excellent SO₂ adsorption, good cycling, chemical stability, and easy regeneration. Further, we propose MIL-53(Al)-TDC and MIL-53(A)-BDC of potential interest for SO₂ sensing and SO₂ storage/transportation, respectively.

Capture and Separation of SO2 Traces in Metal-Organic Frameworks via Pre-Synthetic Pore Environment Tailoring by Methyl Groups

Herein, we report a pre-synthetic pore environment design strategy to achieve stable methyl-functionalized metal-organic frameworks (MOFs) for preferential SO2 binding and thus enhanced low (partial) pressure SO2 adsorption and SO2 /CO2 separation. The enhanced sorption performance is for the first time attributed to an optimal pore size by increasing methyl group densities at the benzenedicarboxylate linker in [Ni2 (BDC-X)2 DABCO] (BDC-X=mono-, di-, and tetramethyl-1,4-benzenedicarboxylate/terephthalate; DABCO=1,4-diazabicyclo[2,2,2]octane). Monte Carlo simulations and first-principles density functional theory (DFT) calculations demonstrate the key role of methyl groups within the pore surface on the preferential SO2 affinity over the parent MOF. The SO2 separation potential by methyl-functionalized MOFs has been validated by gas sorption isotherms, ideal adsorbed solution theory calculations, simulated and experimental breakthrough curves, and DFT calculations.

SO2 Capture Using Porous Organic Cages

We report the first experimental investigation of porous organic cages (POCs) for the demanding challenge of SO2 capture. Three structurally related N-containing cage molecular materials were studied. An imine-functionalized POC (CC3) showed modest and reversible SO2 capture, while a secondary-amine POC (RCC3) exhibited high but irreversible SO2 capture. A tertiary amine POC (6FT-RCC3) demonstrated very high SO2 capture (13.78 mmol g-1 ; 16.4 SO2 molecules per cage) combined with excellent reversibility for at least 50 adsorption-desorption cycles. The adsorption behavior was investigated by FTIR spectroscopy, 13 C CP-MAS NMR experiments, and computational calculations.

Deep Desulfurization with Record SO2 Adsorption on the Metal-Organic Frameworks

Selective elimination of sulfur dioxide is significant in flue gas desulfurization and natural gas purification, yet developing adsorbents with high capture capacity especially at low partial pressure as well as excellent cycling stability remains a challenge. Herein, a family of isostructural gallate-based MOFs with abundant hydrogen bond donors decorating the pore channel was reported for selective recognition and dense packing of sulfur dioxide via multiple hydrogen bonding interactions. Multiple O···H-O hydrogen bonds and O···H-C hydrogen bonds guarantee SO₂ molecules are firmly grasped within the framework, and appropriate pore apertures afford dense packing of SO₂ with high uptake and density up to 1.86 g cm^-3, which is evidenced by dispersion-corrected density functional theory calculations and X-ray diffraction resolution of a SO₂-loaded single crystal. Ultrahigh adsorption uptake of SO₂ at extremely low pressure (0.002 bar) was achieved on Co-gallate (6.13 mmol cm^-3), outperforming all reported state-of-the-art MOFs. Record-high IAST selectivity of SO₂/CO₂ (325 for Mg-gallate) and ultrahigh selectivity of SO₂/N₂ (>1.0 × 10⁴) and SO₂/CH₄ (>1.0 × 10⁴) were also realized. Breakthrough experiments further demonstrate the excellent removal performance of trace amounts of SO₂ in a deep desulfurization process. More importantly, M-gallate shows almost unchanged breakthrough performance after five cycles, indicating the robust cycling stability of these MOFs.

Zirconium and Aluminum MOFs for Low-Pressure SO2 Adsorption and Potential Separation: Elucidating the Effect of Small Pores and NH2 Groups

Finding new adsorbents for the desulfurization of flue gases is a challenging task but is of current interest, as even low SO₂ emissions impair the environment and health. Four Zr- and eight Al-MOFs (Zr-Fum, DUT-67(Zr), NU-1000, MOF-808, Al-Fum, MIL-53(Al), NH₂-MIL-53(Al), MIL-53(tdc)(Al), CAU-10-H, MIL-96(Al), MIL-100(Al), NH₂-MIL-101(Al)) were examined toward their SO₂ sorption capability. Pore sizes in the range of about 4-8 Å are optimal for SO₂ uptake in the low-pressure range (up to 0.1 bar). Pore widths that are only slightly larger than the kinetic diameter of 4.1 Å of the SO₂ molecules allow for multi-side-dispersive interactions, which translate into high affinity at low pressure. Frameworks NH₂-MIL-53(Al) and NH₂-MIL-101(Al) with an NH₂-group at the linker tend to show enhanced SO₂ affinity. Moreover, from single-gas adsorption isotherms, ideal adsorbed solution theory (IAST) selectivities toward binary SO₂/CO₂ gas mixtures were determined with selectivity values between 35 and 53 at a molar fraction of 0.01 SO₂ (10.000 ppm) and 1 bar for the frameworks Zr-Fum, MOF-808, NH₂-MIL-53(Al), and Al-Fum. Stability tests with exposure to dry SO₂ during ≤10 h and humid SO₂ during 5 h showed full retention of crystallinity and porosity for Zr-Fum and DUT-67(Zr). However, NU-1000, MOF-808, Al-Fum, MIL-53(tdc), CAU-10-H, and MIL-100(Al) exhibited ≥50-90% retained Brunauer-Emmett-Teller (BET)-surface area and pore volume; while NH₂-MIL-100(Al) and MIL-96(Al) demonstrated a major loss of porosity under dry SO₂ and MIL-53(Al) and NH₂-MIL-53(Al) under humid SO₂. SO₂ binding sites were revealed by density functional theory (DFT) simulation calculations with adsorption energies of -40 to -50 kJ·mol^-1 for Zr-Fum and Al-Fum and even above -50 kJ·mol^-1 for NH₂-MIL-53(Al), in agreement with the isosteric heat of adsorption near zero coverage (ΔH_ads⁰). The predominant, highest binding energy noncovalent binding modes in both Zr-Fum and Al-Fum feature μ-OH^δ+···^δ-OSO hydrogen bonding interactions. The small pores of Al-Fum allow the interaction of two μ-OH bridges from opposite pore walls with the same SO₂ molecule via OH^δ+···^δ-OSO^δ-···^δ+HO hydrogen bonds. For NH₂-MIL-53(Al), the DFT high-energy binding sites involve NH^δ+···^δ-OS together with the also present Al-μ-OH^δ+···^δ-OS hydrogen bonding interactions and C₆-π^δ-···^δ+SO₂, N^δ-···^δ+SO₂ interactions.