A Moisture-Stable 3D Microporous CoII -Metal-Organic Framework with Potential for Highly Selective CO2 Separation under Ambient Conditions

A Highly Selective MOF-Based Probe for Turn-On Luminescent Detection of Al3+, Cr3+, and Fe3+ in Solution and Test Paper Strips through Absorbance Caused Enhancement Mechanism

Trivalent metal ions (Cr³⁺, Al³⁺, and Fe³⁺) constitute a major section of the environmental pollutants, and their excess accumulation has a detrimental effect on health, so their detection in trace quantity has been a hot topic of research. A highly scalable 3D porous Zn-based luminescent metal-organic framework (MOF) has been synthesized by exploiting the mixed ligand synthesis concept. The strategic selection of an aromatic π-conjugated organic linker and N-rich spacer containing the azine functionality as metal ion binding sites immobilized across the pore spaces, have made this MOF an ideal turn-on sensor for Al³⁺, Cr³⁺, and Fe³⁺ ions with very high sensitivity, selectivity, and recyclability. An in-depth study revealed absorbance caused enhancement mechanism (ACE) responsible for such turn-on phenomena. In order to make the detection process straightforward, convenient, portable, and economically viable, we have fabricated MOF test paper strips (the MOF could be simply immobilized onto the paper strips) for naked eye visual detection under UV light, which, thus, manifests its potential as a real-time smart sensor for these trivalent ions.

pH-Stable Luminescent Metal-Organic Frameworks for the Selective Detection of Aqueous-Phase FeIII and CrVI Ions

The development of chemically stable metal-organic framework (MOF)-based luminescent platforms for toxic ion detection in an aqueous medium is highly challenging because most of the classical MOFs are prone to water degradation, and that is the reason why most of the MOF-based luminescent sensors use a nonaqueous medium for sensing. In this contribution, we report two new water-stable luminescent MOFs (Zn-MOF-1 and Zn-MOF-2), assembled from a mixed-ligand synthesis approach. Because of the presence of a hydrophobic trifluoromethyl group to the backbone and stronger metal-N coordination, these MOFs exhibit excellent stability not only in water but also in acidic/alkaline aqueous solutions (pH = 3-10). Here, we report a green sensing approach by exploiting the significant reduction in photoluminescence of these MOFs in the presence of toxic ions. Fe³⁺ and CrO₄^2-/Cr₂O₇^2- ions could be traced with a detection limit (LOD) in the micromolar range (0.045 and 0.745/0.33 μM for Zn-MOF-1; 125.2 and 114.2/83.5 μM for Zn-MOF-2). The mechanistic study reveals that competitive absorption of the excitation energy coupled with fluorescent resonance energy transfer are responsible for the turn-off quenching. The anti-interference ability and recyclability along with the pH stability gave these MOFs high potential to be used as practical sensors toward Fe^III and Cr^VI ions in water as a greenest medium.

Advances in Post-Combustion CO2 Capture by Physical Adsorption: From Materials Innovation to Separation Practice

The atmospheric CO₂ concentration continues a rapid increase to its current record high value of 416 ppm for the time being. It calls for advanced CO₂ capture technologies. One of the attractive technologies is physical adsorption-based separation, which shows easy regeneration and high cycle stability, and thus reduced energy penalties and cost. The extensive research on this topic is evidenced by the growing body of scientific and technical literature. The progress spans from the innovation of novel porous adsorbents to practical separation practices. Major CO₂ capture materials include the most widely used industrially relevant porous carbons, zeolites, activated alumina, mesoporous silica, and the newly emerging metal-organic frameworks (MOFs) and covalent-organic framework (COFs). The key intrinsic properties such as pore structure, surface chemistry, preferable adsorption sites, and other structural features that would affect CO₂ capture capacity, selectivity, and recyclability are first discussed. The industrial relevant variables such as particle size of adsorbents, the mechanical strength, adsorption heat management, and other technological advances are equally important, even more crucial when scaling up from bench and pilot-scale to demonstration and commercial scale. Therefore, we aim to bring a full picture of the adsorption-based CO₂ separation technologies, from adsorbent design, intrinsic property evaluation to performance assessment not only under ideal equilibrium conditions but also in realistic pressure swing adsorption processes.

Unravelling moisture-induced CO2 chemisorption mechanisms in amine-modified sorbents at the molecular scale

This work entails a comprehensive solid-state NMR and computational study of the influence of water and CO₂ partial pressures on the CO₂-adducts formed in amine-grafted silica sorbents. Our approach provides atomic level insights on hypothesised mechanisms for CO₂ capture under dry and wet conditions in a tightly controlled atmosphere. The method used for sample preparation avoids the use of liquid water slurries, as performed in previous studies, enabling a molecular level understanding, by NMR, of the influence of controlled amounts of water vapor (down to ca. 0.7 kPa) in CO₂ chemisorption processes. Details on the formation mechanism of moisture-induced CO₂ species are provided aiming to study CO₂ : H₂O binary mixtures in amine-grafted silica sorbents. The interconversion between distinct chemisorbed CO₂ species was quantitatively monitored by NMR under wet and dry conditions in silica sorbents grafted with amines possessing distinct bulkiness (primary and tertiary). Particular attention was given to two distinct carbonyl environments resonating at δ _C ∼161 and 155 ppm, as their presence and relative intensities are greatly affected by moisture depending on the experimental conditions. 1D and 2D NMR spectral assignments of both these ¹³C resonances were assisted by density functional theory calculations of ¹H and ¹³C chemical shifts on model structures of alkylamines grafted onto the silica surface that validated various hydrogen-bonded CO₂ species that may occur upon formation of bicarbonate, carbamic acid and alkylammonium carbamate ion pairs. Water is a key component in flue gas streams, playing a major role in CO₂ speciation, and this work extends the current knowledge on chemisorbed CO₂ structures and their stabilities under dry/wet conditions, on amine-modified solid surfaces.

Immobilization of a Polar Sulfone Moiety onto the Pore Surface of a Humid-Stable MOF for Highly Efficient CO2 Separation under Dry and Wet Environments through Direct CO2-Sulfone Interactions

The stability of microporous metal-organic frameworks (MOFs) in moist environments must be taken into consideration for their practical implementations, which has been largely ignored thus far. Herein, we synthesized a new moisture-stable Zn-MOF, {[Zn₂(SDB)₂(L)₂]·2DMA}_n, IITKGP-12, by utilizing a bent organic linker 4,4'-sulfonyldibenzoic acid (H₂SDB) containing a polar sulfone group (-SO₂) and a N, N-donor spacer (L) with a Brunauer-Emmett-Teller surface area of 216 m² g^-1. This material displays greater CO₂ adsorption capacity over N₂ and CH₄ with high IAST selectivity, which is also validated by breakthrough experiments with longer breakthrough times for CO₂. Most importantly, the separation performance is largely unaffected in the presence of moisture of simulated flue gas stream. Temperature-programmed desorption (TPD) analysis shows the ease of the regeneration process, and the performance was verified for multiple cycles. In order to understand the structure-function relationship at the atomistic level, grand canonical Monte Carlo (GCMC) calculation was performed, indicating that the primary binding site for CO₂ is between the sulfone moieties in IITKGP-12. CO₂ is attracted to the bonded structure (V-shape) of the sulfone moieties in a perpendicular fashion, where C_CO2 is aligned with S, and the CO₂ axis bisects the SO₂ axis. Thus, the strategic approach to immobilize the polar sulfone moiety with a high number of inherent stronger M-N coordination and the absence of coordination unsaturation made this MOF potential toward practical CO₂ separation applications.

A "Thermodynamically Stable" 2D Nickel Metal-Organic Framework over a Wide pH Range with Scalable Preparation for Efficient C2 s over C1 Hydrocarbon Separations

The design and construction of "thermodynamically stable" metal-organic frameworks (MOFs) that can survive in liquid water, boiling water, and acidic/basic solutions over a wide pH range is highly desirable for many practical applications, especially adsorption-based gas separations with obvious scalable preparations. Herein, a new thermodynamically stable Ni MOF, {[Ni(L)(1,4-NDC)(H₂ O)₂ ]}_n (IITKGP-20; L=4,4'-azobispyridine; 1,4-NDC=1,4-naphthalene dicarboxylic acid; IITKGP stands for the Indian Institute of Technology Kharagpur), has been designed that displays moderate porosity with a BET surface area of 218 m²  g^-1 and micropores along the [10-1] direction. As an alternative to a cost-intensive, cryogenic, high-pressure distillation process for the separation of hydrocarbons, MOFs have recently shown promise for such separations. Thus, towards an application standpoint, this MOF exhibits a higher uptake of C₂ hydrocarbons over that of C₁ hydrocarbon under ambient conditions, with one of the highest selectivities based on the ideal adsorbed solution theory (IAST) method. A combination of two strategies (the presence of stronger metal-N coordination of the spacer and the hydrophobicity of the aromatic moiety of the organic ligand) possibly makes the framework highly robust, even stable in boiling water and over a wide range of pH 2-10, and represents the first example of a thermodynamically stable MOF displaying a 2D structural network. Moreover, this material is easily scalable by heating the reaction mixture at reflux overnight. Because such separations are performed in the presence of water vapor and acidic gases, there is a great need to explore thermodynamically stable MOFs that retain not only structural integrity, but also the porosity of the frameworks.

A hydrostable cage-based MOF with open metal sites and Lewis basic sites immobilized in the pore surface for efficient separation and purification of natural gas and C2H2

Design and construction of stable adsorbents for efficient separation and purification of natural gas and C2H2 is fundamentally important in the chemical industry, and hierarchical cage-based MOFs are attractive in this regard due to their intrinsic structural advantages. In this work, a cage-based MOF (termed ZJNU-15) assembled from a tetranuclear Cu4O-based SBU and an amine-functionalized N,O-mixed donor ligand was solvothermally constructed. Single-crystal X-ray diffraction studies showed that the resulting MOF incorporates two different types of polyhedral cages in the entire network and bears incompatible open copper sites and uncoordinated amine groups immobilized in the pore surface. In view of its intriguing structural features, its gas adsorption properties with respect to C2 hydrocarbons, CO2, and CH4 were systematically investigated, revealing that it could achieve efficient removal of C2 hydrocarbons and CO2 from CH4 as well as separation of a binary C2H2-CO2 mixed gas, which is associated with natural gas and C2H2 separation and purification. At 298 K and 1 atm, for equimolar binary components, the IAST-predicted adsorption selectivities for C2 hydrocarbons over CH4 are above 17.7, while the CO2/CH4 and C2H2/CO2 adsorption selectivities are 5.0 and 4.4, respectively. Notably, stability studies showed that the framework maintained its structural integrity after being immersed in HCl/NaOH aqueous solutions within a pH range of 4-11 at ambient temperature for 24 h, indicating its good hydrolytic stability under harsh chemical conditions, which might lay a solid foundation for its practical applications.

Interchanged Hysteresis for Carbon Dioxide and Water Vapor Sorption in a Pair of Water-Stable, Breathing, Isoreticular, 2-Periodic, Zn(II)-Based Mixed-Ligand Metal-Organic Frameworks

We report the synthesis of two isoreticular mixed-ligand metal-organic frameworks (MOFs), namely, [Zn(μ₂-ia)(μ₂-bpe)] _n· nDMF (1) and [Zn(μ₂-mia)(μ₂-bpe)] _n· nDMF (2), where ia = isophthalate, mia = 5-methoxyisophthalate, bpe = 1,2-bis(4-pyridyl)ethane, and DMF = N, N'-dimethylformamide. Single-crystal X-ray diffraction studies revealed that the structures of 1 and 2 consist of a 2-periodic, layer sql motif. Structures exhibit entanglement through interpenetration of neighboring frameworks to form a two-dimensional bilayer. Variable-temperature powder X-ray diffraction studies confirmed both structures retain crystallinity upon desolvation up to ∼500 K. Although structurally similar, activated samples of 1 and 2 showed differing gas and vapor sorption capabilities. Despite activated 2 having the higher actual void space, activated 1 showed significantly higher sorption for carbon dioxide at 195 K, as well as significant hysteresis upon desorption. Empirical evidence points toward weaker bilayer···bilayer interactions, which allow the separation of the bilayers, illustrating that small changes in functional groups within an isoreticular pair of MOFs may have a large tuning effect on sorption properties.

Effects of synthesis methodology on microporous organic hyper-cross-linked polymers with respect to structural porosity, gas uptake performance and fluorescence properties

The structural characteristics of hyper-cross-linked polymers (HCPs) make them interesting for a wide variety of applications.

A stable luminescent zinc–organic framework as a dual-sensor toward Cu2+ and Cr2O72−, and excellent platform-encapsulated Ln3+ for systematic color tuning and white-light emission

A stable three-fold interpenetrated framework (Zn-MOF), showing chemical sensing for Cu²⁺ and Cr₂O₇²⁻, tunable luminescence and white-light emission after encapsulating lanthanide cations, was synthesized and characterized.

Design of Mo-doped cobalt sulfide hollow nanocages from zeolitic imidazolate frameworks as advanced electrodes for supercapacitors

A Mo-doped CoS HNC with enhanced electrochemical performance was designed by using ZIF-67 as a self-sacrificial template through a dissolution–regrowth process in the presence of NaMoO₄ with an additional sulfurization process.

Two 2D microporous MOFs based on bent carboxylates and a linear spacer for selective CO2 adsorption

Two new 2D microporous MOFs based on bent carboxylates and an unexplored N,N-donor spacer containing imine and amide functionalities exhibited high IAST selectivity for CO₂/N₂ and CO₂/CH₄ mixtures under ambient conditions.

Host-guest selectivity in a series of isoreticular metal-organic frameworks: observation of acetylene-to-alkyne and carbon dioxide-to-amide interactions

We report a series of six isoreticular metal–organic frameworks (MOFs) for selective gas adsorption, specifically for selective adsorption of CO₂ and C₂H₂.

In order to develop new porous materials for applications in gas separations such as natural gas upgrading, landfill gas processing and acetylene purification it is vital to gain understanding of host–substrate interactions at a molecular level. Herein we report a series of six isoreticular metal–organic frameworks (MOFs) for selective gas adsorption. These materials do not incorporate open metal sites and thus provide an excellent platform to investigate the effect of the incorporation of ligand functionality via amide and alkyne groups on substrate binding. By reducing the length of the linker in our previously reported MFM-136, we report much improved CO₂/CH₄ (50 : 50) and CO₂/N₂ (15 : 85) selectivity values of 20.2 and 65.4, respectively (1 bar and 273 K), in the new amide-decorated MOF, MFM-126. The CO₂ separation performance of MFM-126 has been confirmed by dynamic breakthrough experiments. In situ inelastic neutron scattering and synchrotron FT-IR microspectroscopy were employed to elucidate dynamic interactions of adsorbed CO₂ molecules within MFM-126. Upon changing the functionality to an alkyne group in MFM-127, the CO₂ uptake decreases but the C₂H₂ uptake increases by 68%, leading to excellent C₂H₂/CO₂ and C₂H₂/CH₄ selectivities of 3.7 and 21.2, respectively. Neutron powder diffraction enabled the direct observation of the preferred binding domains in MFM-126 and MFM-127, and, to the best of our knowledge, we report the first example of acetylene binding to an alkyne moiety in a porous material, with over 50% of the acetylene observed within MFM-127 displaying interactions (<4 Å) with the alkyne functionality of the framework.

A metal-organic framework based on a custom-designed diisophthalate ligand exhibiting excellent hydrostability and highly selective adsorption of C2H2 and CO2 over CH4

The ligand truncation strategy provides facile access to a wide variety of linkers for the construction of MOFs bearing diverse structures and intriguing properties. In this work, we employed this strategy to design and prepare a novel bent diisophthalate ligand, and used it to successfully construct a copper-based MOF ZJNU-51 with the formula of [Cu2L(H2O)2]·5DMF (H4L = 5,5'-(triphenylamine-4,4'-diyl) diisophthalic acid), which was thoroughly characterized by various techniques including FTIR, TGA, PXRD and single-crystal X-ray diffraction. ZJNU-51 is a two-fold interpenetrated network in which the single network consists of dicopper paddlewheel units connected by the organic ligands and contains open channels as well as six distinct types of metal-organic cages. Furthermore, gas adsorption properties with respect to C2H2, CO2, and CH4 were systematically investigated, demonstrating that ZJNU-51 is a highly promising material for C2H2/CH4 and CO2/CH4 separations. Specifically, the IAST adsorption selectivity at 298 K and 1 atm reaches 35.6 and 5.4 for the equimolar C2H2/CH4 and CO2/CH4 gas mixtures, respectively. More significantly, as revealed by PXRD and N2 adsorption measurements, ZJNU-51 exhibits excellent chemical stability, which lays a good foundation for its practical application.

Two azo-functionalized luminescent 3D Cd(ii) MOFs for highly selective detection of Fe3+ and Al3+

Two 3D luminescent Cd(ii) MOFs (Cd-MOF-1 and Cd-MOF-2) exposing azo functional sites displayed selective detection of Fe³⁺ and Al³⁺ metal ions with high sensitivities.

Multivariant synthesis, crystal structures and properties of four nickel coordination polymers based on flexible ligands

Four new Ni(ii) metal–organic frameworks with diverse structures have been synthesized under different conditions. The structural transformation from compound 4 to 3 has also been investigated.