Selective Gas Adsorption Research Articles

Phenolic Resin-type Microporous Organic Polymers for High-Performance Carbon Dioxide Adsorption.

Three new types of Si-centered porous organic polymer (Si-POPs) were successfully prepared using phenolic resin-type chemistry to form C-C bonds. This new family of microporous Si-POPs manifests as uniform, microporous, spherical particles with a high specific surface area. Notably, Si-POPs were engineered to possess varying numbers of hydroxyl (-OH) groups by altering the monomer in the synthetic process. Among these materials, the variant with the highest number of hydroxyl groups exhibited ultra-high CO2 adsorption capacity, reaching up to 4.3 mmol g-1 at 273 K and 1.0 bar, which surpasses the performance of most porous polymers. Furthermore, Si-POPs also demonstrated remarkable selectivity adsorption for carbon dioxide over nitrogen (17-50, IAST at 273 K and 1.0 bar). This study not only highlighted the superior CO2 adsorption properties of Si-POPs but also explored their potential application in selective gas adsorption.

A Fluorine-Functionalized Tb(III)-Organic Framework for Ba2+ Detection.

The development of lanthanide-organic frameworks (Ln-MOFs) using for luminescence sensing and selective gas adsorption applications is of great significance from an energy and environmental perspective. This study reports the solvothermal synthesis of a fluorine-functionalized 3D microporous Tb-MOF with a face-centered cubic (fcu) topology constructed from hexanuclear clusters (Tb6O30) bridged by fdpdc ligands, formulated as {[Tb6(fdpdc)6(μ3-OH)8(H2O)6]·4DMF}n (1), (fdpdc = 3-fluorobiphenyl-4,4'-dicarboxylate). Complex 1 displays a 3D framework with the channel of 7.2 × 7.2 Å2 (measured between opposite atoms) perpendicular to the a-axis. With respect to Ba2+ cation, the framework of activated 1 (1a) exhibits high selectivity and reversibility in luminescence sensing function, with an LOD of 4.34665 mM. According to the results of simulations, compared to other small gas molecules (CO2, N2, H2, CO, and CH4), activated 1 (1a) shows a high adsorption selectivity for C2H2 at 298 K.

Selective Gas Adsorption in Permanently Microporous Coordination Cages with Exposed Metal Sites.

Porous coordination cages (PCCs), molecular analogs of metal-organic frameworks, offer modular platforms for studying the adsorption properties of small molecules, with coordinatively unsaturated metal centers playing a pivotal role in tuning these behaviors. In this work, we present the synthesis, activation, and detailed gas adsorption studies of second-row transition metal-based M24L24 cuboctahedral cages, specifically Mo24(bdc)24, Rh24(bdc)24, and [Ru24(bdc)24]Cl12. These materials represent rare examples of Mo-, Rh-, and Ru-based hybrid porous solids. The synthesis and activation of these cages were optimized to maximize porosity, yielding BET surface areas of up to 832 m2/g. Gas adsorption studies with CO2 and CO reveal distinctive uptake behaviors linked to the metal cations, with Mo24(bdc)24 demonstrating the highest gravimetric CO2 uptake (2.12 mmol/g at 298 K) and [Ru24(bdc)24]Cl12 exhibiting the strongest CO binding (-75 kJ/mol). Additionally, we explore the selective adsorption of unsaturated hydrocarbons, such as ethylene and propylene, revealing strong binding interactions at low pressures as a result of strong metal-hydrocarbon interactions based on pi-backbonding interactions.

Unveiling the role of metal-doped BNC2 monolayer for selective gas adsorption: A DFT investigation

Developing effective gas sensors is crucial for environmental monitoring, safety, and industrial applications. Two-dimensional (2D) materials are increasingly investigated as gas-sensing platforms due to their high surface-to-volume ratios and tunable electronic properties. In this study, we utilize density functional theory (DFT) to investigate the gas-sensing properties of the BNC2 monolayer (ML), focusing on the adsorption of CO, CO2, NO, NO2, and HCN on pristine and metal-doped (Li, Mg, and Al) BNC2. Our results reveal that all doped metal atoms are strongly bound to the BNC2 surface, though slightly protruding from the plane. Among the dopants, Al-doped BNC2 exhibits the strongest interaction with gas molecules, particularly NO and NO2, while Mg-doped BNC2 shows a balanced interaction, making it highly suitable for selective gas sensing. Li-doped BNC2, in contrast, demonstrates weaker interactions, leading to faster desorption times. The charge transfer analysis indicates that most gas molecules act as charge acceptors, with NO and NO2 showing significant electron gain from Mg- and Al-doped surfaces. Additionally, recovery time calculations suggest that metal doping significantly enhances gas-sensing performance compared to pristine BNC2, particularly for NO and NO2 detection. This work underscores the versatility of metal-doped BNC2 monolayers, both in gas sensing and gas capture, contributing to the development of advanced sensing technologies and environmental sustainability solutions.

Mechanistic Insights into Gas Adsorption on 2D Materials.

Owing to their exceptional characteristics, such as one-atom thickness, high specific surface area, and tunability of surfaces, 2D materials are excellent templates to study the surface-dependent gas adsorption phenomenon. Moreover, the properties of 2D materials like morphology, bandgap, structure, and carrier mobility can be modulated easily by modification methods such as functionalization, defect and doping engineering. These modifications create and activate unconventional inert and active sites, leading to the selective adsorption of gases via mechanisms such as charge transfer kinetics, Schottky-barrier modification, and surface interactions. These methods enhance the adsorption sites by adding covalent and non-covalent moieties to the 2D surface and play a critical role in developing ultrafast gas sensing with high sensitivity, selectivity, fast response/recovery rates, and low detection limits. Here, this perspective is presented on the mechanism of the adsorption process of gases on modified 2D surfaces based on recent studies related to adsorption-dependent applications of 2D materials.

Tailored microporous graphitic carbon adsorbents from the urea-assisted method for efficient selective gas adsorption and separation of CH4/N2 and CH4/H2

Currently, the adsorption and separation of methane (CH4) from CH4/N2 and CH4/H2 have attracted global attention as an essential technique for the utilization of unconventional natural gas and to meet energy demand. This technique completely relies on the adsorbent; the development of the adsorbent can achieve selective CH4 adsorption and separation. In this context, microporous graphitic carbon adsorbents were tailored using hydrothermal and calcination methods. The textural features of adsorbents were precisely tailored using a hydrothermal approach by optimizing the glucose-urea ratio, and they were then activated with the non-corrosive reagent K2CO3, which gives a specific pore structure that aids in performance. Adsorbents’ textural characteristics, such as surface area and porosity, were significantly influenced by the glucose-urea ratio, which plays an essential role in gas adsorption and separation. A series of tailored microporous graphitic carbon adsorbents named 0.5-HTAC, 1-HTAC, 1.5-HTAC, 2-HTAC, 1.5-HTAC-a, 1.5-HTAC-b, and 1.5-HTAC-c. Among them, 1.5-HTAC adsorbent displayed superior CH4 gas adsorption (36.86 cm3/g) and excellent selectivity of CH4/N2 (6.29) at room temperature, owing to its microporosity, good surface area, and morphological effect. Meanwhile, H2 gas adsorption was not detected at room temperature, indicating that the 1.5-HTAC adsorbent is extremely selective for CH4 gas. However, an additional study of H2 uptake performed at 77 K revealed excellent H2 adsorption of about 314.3 cm3/g. Furthermore, the 1.5-HTAC adsorbent exhibited a low heat of CH4 adsorption as well as impressive reversibility, indicating that it can be reused more effectively.

Carbothermally synthesized, lignin biochar-based, embedded and surface deposited nano zero-valent iron composites: Comparative material characterization, selective gas adsorption and nitroaromatics remediation

Biochar (BC) with nanoscale zero-valent iron (nZVI) incorporation offers advantageous materials for water purification. While the most common approach for nZVI incorporation is the deposition on -a carrier surface, embedding in -a support matrix has also been reported. However, the behavior of the embedded material in contaminant removal has not been adequately studied -nor the characteristics and the remediation capabilities of the two materials have been compared. Present study focuses on preparing and extensively characterizing two materials: nZVI embedded in (Lig-e-nZVI) and surface deposited on (Lig-s-nZVI) lignin BC followed by a comparative study of remedial action for two nitroaromatics, p-nitroaniline (pNA) and p-nitrophenol (pNP). The synthesis of Lig-e-nZVI and Lig-s-nZVI involved simultaneous and subsequent pyrolysis of lignin and carbothermal reduction of the iron salt, respectively. Lig-e-nZVI showed enhanced porosity. XRD confirmed the formation of Fe0. HR-TEM images proved the core-shell structure of nZVI, and an interlayer spacing of 0.36 nm of the shell verified that the Fe0 particles were encapsulated with graphene while an iron carbide inner layer was also observed, thinner in Lig-e-nZVI and thicker in Lig-s-nZVI. A band gap energy of 2.54 eV suggested photocatalytic activity for both materials. Best fitted Sips isotherms showed 23.1 and 13.1 mg g−1 capacities for Lig-s-nZVI in pNP and pNA adsorption respectively. Highest stability was portrayed by Lig-e-nZVI over 4 regeneration cycles. The physicochemical features of the developed materials further enabled selective gas adsorption. Findings provide new insights into physicochemical characteristics and remedial actions of differently synthesized nZVI-BC composites.

High Performance of the Metal Organic Framework CPO‐27 for Toxic Gas Capture (NO2)

AbstractMetal‐organic frameworks of the CPO‐27 (MOF‐74) series are known for their high adsorption capacities for nitrogen oxides. On the other hand, significantly varying and partly contradictory results were reported regarding their stability to moisture. This aspect has hampered the use of these MOFs in air filtration applications. Here, we show that the stability of CPO‐27 materials towards moisture and CO2 crucially depends on the synthesis parameters. By precisely adjusting the synthetic parameter‐property relationship, it is possible to prepare a highly stable CPO‐27(Ni) MOF by a simple precipitation in water. To demonstrate the capacity for NO2, breakthrough experiments were performed under dry and wet conditions (55% relative humidity). Extremely high values of nearly 0.74 g/g in dry conditions and 1.23 g/g under wet conditions were achieved. These results pave the way for the toxicologically safe and cost‐effective preparation of a moisture‐stable CPO‐27(Ni), bridging the gap to a potential industrial application of the material for the selective adsorption of toxic gases, such as NO2.

Solvent-Regulated Formation of Metal/Metal-Oxo Nodes in Two Indium Metal-Organic Frameworks: Syntheses, Structures, Selective Gas Adsorption and Fluorescence Sensor Properties.

Two water-stable indium metal-organic frameworks, (NH2Me2)3[In3(BTB)4] ⋅ 12DMA ⋅ 4.5H2O (In-MOF-1) and (NH2Me2)9[In9O6(BTB)8(H2O)4(DMSO)4] ⋅ 27DMSO ⋅ 21H2O (In-MOF-2) (BTB=4,4',4''-benzene-1,3,5-tribenzoate) with 3D interpenetrated structure has been constructed by regulating solvents. Structure analysis revealed that In-MOF-1 has a three-dimensional (3D) structure with a single metal core, while In-MOF-2 features an octahedron cage constructed by three kinds of metal clusters to further form a 3D structure. The fluorescence investigations showed that In-MOF-1 and In-MOF-2 are potential MOF-based fluorescent sensors to detect acetone and Fe3+ ions in EtOH or water with high sensitivity, excellent selectivity, recyclability and a low limit of detection. Moreover, the fluorescence mechanisms of In-MOF-1 and In-MOF-2 toward acetone and Fe3+ ions were further explained. In addition, In-MOF-2 has higher thermal and framework stability than In-MOF-1. The activated In-MOF-2 presents a high BET surface area of 998.82 m2g-1 and a pore size distribution of 8 to 16 Å. At the same time, In-MOF-2 exhibits high selective CO2 adsorption for CO2/CH4 and CO2/N2, respectively. Furthermore, the adsorption sites and adsorption isotherms were predicted using grand canonical Monte Carlo (GCMC) simulations, and the adsorption energy of the lowest-energy adsorption configuration was calculated using molecular dynamics (MD) simulations.

Electrostatic-assisted fabrication of hollow carbon sphere-based porous liquids for gas selective adsorption

Porous liquids represent a category of materials that combine the porosity of solid structures with the fluidity of liquids, drawing substantial research interest. In this work, we synthesized a novel class of porous carbon liquids, termed H-PLs, which exhibit stable flow at room temperature. This stability is achieved through cation-π interactions between an imidazolium-based polymeric ionic liquid ([EMIM][TF2N]) and a porous carbon network. Notably, the CO2 adsorption capacity of H-PLs reached 3.73 wt% at 298 K and 8 bar, significantly outperforming parent [EMIM][TF2N] ionic liquid and demonstrating a CO2 solubility approximately 37 times greater than that of N2 under identical conditions. These findings reveal the potential of H-PLs for selective gas adsorption, and suggest that electrostatic-assisted synthesis method offers a rapid, efficient and scalable pathway for future production of porous liquids.

Chromium(II)-isophthalate 2D MOF with Redox-Tailorable Gas Adsorption Selectivity.

Redox-active metal-organic frameworks (MOFs) are very promising materials due to their potential capabilities for postsynthetic modification aimed at tailoring their application properties. However, the research field related to redox-active MOFs is still relatively underdeveloped, which limits their practical application. We investigated the self-assembly process of Cr(II) ions and isophthalate (m-bdc) linkers, which have been previously demonstrated to yield 0D metal-organic polyhedra. However, using the diffusion-controlled synthetic approach, we demonstrate the selective preparation of a 2D-layered Cr(II)-based MOF material [Cr(m-bdc)]·H2O (1·H2O). Remarkably, the controlled oxidation of the developed 2D MOF using nitric oxide or dry oxygen resulted in modified porous materials with excellent H2/N2 adsorption selectivities.

A Flexible Dihydrogen-Bonded Organic Framework with Selective Gas Adsorption Properties

A Flexible Dihydrogen-Bonded Organic Framework with Selective Gas Adsorption Properties

Role of catalyst oxidation states in the selective detection of reducing gases: A comparative study of nanocomposites based on Au and chemically/thermally reduced graphene oxide

Catalyst-loaded reduced graphene oxide (rGO) is a promising material for developing high-performance gas sensors. In this work, surface functionalization is achieved by loading different quantities of nanogold (Au) on chemically and thermally reduced graphene oxide (CRGO and TRGO). These materials showed selective gas response toward reducing gases such as hydrogen (H2), carbon monoxide (CO), and methane (CH4) in the temperature range RT to 150 °C. The quantity of Au catalysts and their oxidation states in the films influenced the selectivity attributes of the studied sensors. The surface oxidation states of the Au catalyst are different for CRGO-Au and TRGO-Au films. It is apparent from the results that the difference in the Au oxidation states is due to the variation in surface oxygen functionalities of the base matrix CRGO and TRGO. The experimental results are tallied with the proposed gas selectivity mechanism of CRGO-Au and TRGO-Au films.Novelty of the Work:In reduced graphene oxide based gas sensors, the combined influence of the graphene oxide reduction method, gold (Au) catalyst quantity, and valence oxidation states (in the Au catalyst) on the gas selectivity is rarely reported. The available reports only focus on examining the metallic gold (Au0) phase of the catalyst. However, it is necessary to note that Au can also exist in various higher oxidation states (Au+1, Au+2, Au+3, etc.), which provide selective gas adsorption sites. Therefore, this work addresses the above issue by employing chemically reduced graphene oxide (CRGO) and thermally reduced graphene oxide (TRGO) decorated with Au nanoparticles (Au wt% = 1 %, 50 % & 90 %) for the selective sensing of reducing gases.

Pore engineering in highly stable hydrogen-bonded organic frameworks for efficient CH4 purification

The unsatisfactory stability of hydrogen-bonded organic frameworks (HOFs) arising from the weakness and flexibility of hydrogen-bonding (H-bonding) has generally hindered their practical applications. By synergistically employing three key strategies: (Ⅰ) introducing charge-assisted H-bonding to increase the polarity of the framework, (Ⅱ) optimizing the length of the building unit by extending the core structure of the linker molecule, and (III) incorporating functional sites containing methoxy and trifluoromethyl, to increase the stability of HOFs and the adsorption affinity for gas molecules, we successfully synthesized three charge-assisted hydrogen-bonded HOFs (NKM-HOF-3, NKM-HOF-4, and NKM-HOF-5; NKM = Nankai Materials). These HOFs exhibit distinct space configurations of nonporous packing, discrete cavities or continuous one-dimensional channels. Notably, NKM-HOF-5 exhibits commendable thermal and chemical stability and can be readily regenerated by re-solvothermal reaction. The activated NKM-HOF-5a exhibits a low affinity for CH4 but captures C2H6 and C3H8 with a relatively high affinity. According to theoretical calculations, the suitable pore size, together with functionalized and polarized surface environments in NKM-HOF-5a contribute to the excellent selective gas adsorption performance of NKM-HOF-5a. Furthermore, the dynamic breakthrough experiments demonstrate the highly efficient separation performance of NKM-HOF-5a for C2H6/CH4, C3H8/CH4, and C3H8/C2H6/CH4.

An isostructural series of metal-organic frameworks: selective gas adsorption and sensing studies

The solvothermal reaction of Ln(NO3)3•6H2O (Ln = Nd, Sm, Eu, Tb, and Dy) with 9,10-bis(4-carboxylatopyridinium-1-methylene)anthracene dibromide (H2L)Br2 resulted in the formation of an isostructural series of lanthanide-based metal organic frameworks (Ln-MOFs), {[Nd(L)3/2(H2O)2]·(NO3)3·(S)}n (1), {[Sm(L)3/2(H2O)2]·(NO3)3·(S)}n (2), {[Eu(L)3/2(H2O)2]·(NO3)3·(S)}n (3), {[Tb(L)3/2(H2O)2]·(NO3)3·(S)}n (4) and {[Dy(L)3/2(H2O)2]·(NO3)3·(S)}n (5), where S denotes squeezed disordered solvent molecules and out of three nitrate ions in the voids, one nitrate is squeezed due to the high disorder. These MOFs were characterized with various techniques and single crystal X-ray analysis revealed a three dimensional (3D) (4,4,6)-connected metal organic framework with mononuclear metallic nodes in bicapped trigonal prismatic geometry. From the present series, we selected {[Sm(L)3/2(H2O)2]·(NO3)3·(S)}n (2) as a representative example for structural description and gas adsorption study whereas {[Dy(L)3/2(H2O)2]·(NO3)3·(S)}n (5) was used for the sensing of nitro compounds. All MOFs showed stability up to 200 °C. 2 demonstrates a higher CO2 uptake, despite exhibiting a low specific surface area (SABET) in N2 sorption studies. The sensing result shows that 5 act as selective sensors towards TNP with quenching efficiency greater than 99%.

Metal-organic framework-based porous liquids via surface functionalization engineering for selective gas adsorption

Porous liquids (PLs) combine the distinct porosity of solids with the fluidity of liquids, offering new solutions for carbon capture, membrane separation, and more. This study introduces the development of unique UIO-66-PLs through a meticulous surface functionalization strategy, using surface-modified UIO-66-(OH)2 with the organic oligomer SIT8378.3 (SIT) and polyether amines (PEAs) of various molecular weights (M1000, M2070, and M3085) as the hindering solvents. The resulting UIO-66-PLs exhibit exceptional stability and dispersibility in both aqueous and organic environments. Importantly, benefiting from preserving UIO-66-(OH)2′s porosity in PLs, the affinity of PEAs’ ethylene oxide segments for CO2 via Lewis acid-base interactions, and additional free space within organic chains, UIO-66-PLs revealed enhanced CO2 adsorption capabilities, particularly for CO2 over N2. Encouragingly, CO2 adsorption of UIO-66-PLs was enhanced by 117.54 % (UIO-66-M1000), 106.01 % (UIO-66-M2070), and 22.45 % (UIO-66-M3085) in capacity relative to the respective pure organic outer layer. It is also observed that while surface engineering improves gas adsorption performance, PEAs with a high molecular weight may partially block UIO-66-(OH)2 pores, decreasing gas adsorption capacities. Our research showcases the potential of UIO-66-PLs in gas storage and separation applications, offering a new approach to developing materials functionalized with customized organic molecular weights for advanced CO2 capture technologies.

Effect of Boron-Doping on Gate-Opening CO2 Adsorption in Zinc-Benzimidazolate Coordination Networks.

Flexible metal-organic frameworks (MOFs) have attracted much attention as selective gas adsorption and storage. This report describes boron doping in zeolitic imidazolate framework-7 (B-ZIF-7), which exhibits reversible phase transition during CO2 adsorption/desorption. We have successfully prepared B-ZIF-7 coordination networks using boron-bridged benzimidazolate (B(bim)4-) as organic ligands. Powder X-ray diffraction (PXRD) measurements and infrared spectroscopy revealed that B-ZIF-7 has a crystal structure similar to that of ZIF-7 while containing boron bridging in the coordination network. Since B-ZIF-7 forms a cationic coordination network, the guest anions are encapsulated within the pore. CO2 adsorption/desorption measurements at 300 K showed that B-ZIF-7(NO3), which contains nitrate ions (NO3-) as guest anions in its pores, exhibits a S-shaped CO2 adsorption/desorption isotherm, which is characteristic of gate-opening type MOFs. Compared with ZIF-7, B-ZIF-7(NO3) has superior CO2 adsorption capacity in the low-pressure and superior CO2 storage capacity. The CO2 adsorption and desorption behavior of B-ZIF-7(NO3) was analyzed by in situ temperature-controlled PXRD measurements and thermogravimetric analysis under a CO2 atmosphere, and a reversible phase transition was observed. We have also successfully prepared B-ZIF-7(Cl) and B-ZIF-7(OTf) (OTf = CF3SO3-) with different guest anions. The CO2 adsorption/desorption behaviors of B-ZIF-7(Cl) and B-ZIF-7(OTf) were significantly different from those of B-ZIF-7(NO3) and ZIF-7. B-ZIF-7(Cl) showed gate opening at a higher pressure than ZIF-7, and B-ZIF-7(OTf) did not show S-shaped CO2 adsorption isotherm and showed adsorption behavior in micropores. These results indicate that the CO2 adsorption behavior of B-ZIF-7 depends on the interaction between the guest anions and CO2 molecules or the cationic framework and the bulkiness of the guest anions. Boron doping in a coordination network with boron-bridged imidazolate ligands is a promising strategy to increase the gas adsorption capability of porous materials.

Various dicarboxylates induced six Zn(II)/Cd(II) coordination polymers containing a new benzoimidazolyl terpyridine ligand: syntheses, structural diversity, luminescence and gas adsorption properties

Six Zn(II)/Cd(II) coordination polymers (CPs), namely, {[Zn2(bimphtpy)(p-bdc)1.5(OH)]·(H2O)0.5}n (1), [Zn2(bimphtpy)2(tdc)2]n (2), {[Zn(bimphtpy)(cpoa)]·4H2O}n (3), [Cd2(bimphtpy)(p-bdc)2(H2O)]n (4), {[Cd2(bimphtpy)(m-bdc)2(H2O)2]·(H2O)0.75}n (5), and {[Cd(bimphtpy)(qda)]·H2O}n (6), constructed from N-heterocyclic ligand 4'-(4-(benzimidazol-1-yl)phenyl)-4,2':6',4''-terpyridine (bimphtpy) and five dicarboxylic acids, (p-H2bdc = 1,4-benzenedicarboxylic acid, H2tdc = 2,5-thiophenedicarboxylic acid, H2cpoa =4-carboxyphenoxyacetatic acid, m-H2bdc = 1,3-benzenedicarboxylic acid, H2qda = hydroquinone-o,o'-diacetic acid), have been solvothermally synthesized and structurally characterized mainly by single-crystal X-ray diffraction along with elemental analysis, IR spectroscopy, PXRD and TG analysis. Complex 1 exhibits a 2D + 2D → 3D polycatenated framework with bilayers showing the uninodal 5-c (4,4)Ia topology if Zn2(CO2)(OH) SBU (secondary building unit) are considered. Complex 2 reveals a 2D → 2D polycatenane of two-fold interpenetrated network with rods threading loops showing 63-hcb topology. Complex 3 presents a highly undulated 2D 44-sql sheet with thickness of about 18.5 Å. Complex 4 displays a single 3D network constructed by binuclear Cd2(CO2)4O2 SBU showing the uninodal 6-c pcu topology. Complex 5 features a 2D + 2D → 3D polycatenated framework showing an unidentified topology with point symbol of {42·6·72·9}{42·6}{6·72}{64·8·10}. Complex 6 shows a single 3D network constructed by binuclear Cd2O4C2 SBU with 8-c cds topology. Moreover, the fluorescent properties of complexes 1–6 in the solid state at room temperature as well as the adsorption properties of complex 3 are also investigated, suggesting their potential applications as luminescence materials and gas adsorption selectivity materials.

Tunable Gas Admission via a "Molecular Trapdoor" Mechanism in a Flexible Cationic Metal-Organic Framework Featuring 1D Channels.

Achieving high gas selectivity is challenging when dealing with gas pairs of similar size and physiochemical properties. The "molecular trapdoor" mechanism discovered in zeolites holds promise for highly selective gas adsorption separation but faces limitations like constrained pore volume and slow adsorption kinetics. To address these challenges, for the first time, a flexible metal-organic framework (MOF) featuring 1D channels and functioning as a "molecular trapdoor" material is intoduced. Extra-framework anions act as "gate-keeping" groups at the narrowest points of channels, permitting gas admissions via gate opening induced by thermal/pressure stimuli and guest interactions. Different guest molecules induce varied energy barriers for anion movement, enabling gas separation based on distinct threshold temperatures for gas admission. The flexible framework of Pytpy MOFs, featuring swelling structure with rotatable pyridine rings, facilitates faster gas adsorption than zeolite. Analyzing anion properties of Pytpy MOFs reveals a guiding principle for selecting anions to tailor threshold gas admission. This study not only overcomes the kinetic limitations related to gas admission in the "molecular trapdoor" zeolites but also underscores the potential of developing MOFs as molecular trapdoor adsorbents, providing valuable insights for designing ionic MOFs tailored to diverse gas separation applications.

In Situ Single-crystal X-ray Diffraction Studies of Physisorption and Chemisorption of SO2 within a Metal-Organic Framework and Its Competitive Adsorption with Water.

Living on an increasingly polluted planet, the removal of toxic pollutants such as sulfur dioxide (SO2) from the troposphere and power station flue gas is becoming more and more important. The CPO-27/MOF-74 family of metal-organic frameworks (MOFs) with their high densities of open metal sites is well suited for the selective adsorption of gases that, like SO2, bind well to metals and have been extensively researched both practically and through computer simulations. However, until now, focus has centered upon the binding of SO2 to the open metal sites in this MOF (called chemisorption, where the adsorbent-adsorbate interaction is through a chemical bond). The possibility of physisorption (where the adsorbent-adsorbate interaction is only through weak intermolecular forces) has not been identified experimentally. This work presents an in situ single-crystal X-ray diffraction (scXRD) study that identifies discrete adsorption sites within Ni-MOF-74/Ni-CPO-27, where SO2 is both chemisorbed and physisorbed while also probing competitive adsorption of SO2 of these sites when water is present. Further features of this site have been confirmed by variable SO2 pressure scXRD studies, DFT calculations, and IR studies.