A mechanochemically synthesized Schiff-base engineered 2D mixed-linker MOF for CO2 capture and cationic dye removal

Developing synthetic strategies for smart materials for the adsorption and separation of toxic chemicals is of great importance. Metal-organic frameworks (MOFs) have been proven to be outstanding adsorbent materials that possess excellent pollutant removal performances in wastewater treatment, including dye recycling. In this work, a neutral Cd(II) based 2D framework with a dual ligand strategy involving -OH functionalized 5-hydroxyisophthalic acid (5-OH-H2IPA) and the amide decorated Schiff base ligand (E)-N'-(pyridin-4-ylmethylene)isonicotinohydrazide (L) has been synthesized by different synthetic routes and characterized by various analytical methods. Thus, crystals of {[Cd(5-OH-IPA)(L)]·CH3OH}n synthesized via diffusion (ADES-7D) and the phase pure bulk product synthesized by conventional reflux (ADES-7C) and the mechanochemical grinding method (ADES-7M) have been established using PXRD data of the respective product showing identical simulated SXRD data to those of ADES-7D. The mechanochemically synthesized ADES-7M possesses a better surface area and CO2 adsorption capability compared to ADES-7C, which is also supported by electron microscopy and particle size measurements. Furthermore, ADES-7 can be used as an efficient adsorbent material for the reversible, selective adsorption (42-99%) and separation of the cationic dyes malachite green (MG), methyl violet (MV), methylene blue (MB), and rhodamine B (RhB) from the mixture of cationic/anionic dyes (methyl orange (MO) and bromocresol green (BCG)) in the aqueous phase. Specifically, ADES-7M possesses better dye capture capability compared to ADES-7C, even in the case of the bigger dye RhB with adsorption differences of 2.38 to 1.01 mg g-1, respectively. The dye adsorption kinetics follows pseudo-second-order kinetics, and the dye adsorption isotherm fits well with the Langmuir/Freundlich adsorption isotherm models. The probable mechanism of adsorption involving the supramolecular interaction between the host MOF and the guest dye has also been proposed.

Solvent-Induced In(III)-MOFs with Controllable Interpenetration Degree Performing High-Efficiency Separation of CO2/N2 and CO2/CH4

Herein, three In(III)-based metal-organic frameworks (In-MOFs) with different degrees of interpenetration (DOI), namely In-MOF-1, In-MOF-2, and In-MOF-3, constructed by In3+ and Y-shaped ligands 4,4',4″-s-triazine-2,4,6-triyltribenzoate (H3TATB), are successfully synthesized through the ionothermal/solvothermal method. Subsequently, three novel In-MOFs, including noninterpenetration polycatenation, 2-fold interpenetrated, and 4-fold interpenetrated structure, are employed as the platform for systematically investigating the separation efficiency of CO2/N2, CO2/CH4, and CO2/CH4/N2 mixture gas system. Among them, In-MOF-2 shows the highest CO2 uptake capacities at 298 K and simultaneously possesses the low adsorption enthalpy of CO2 (26.4 kJ/mol at low coverage), a feature desirable for low-energy-cost adsorbent regeneration. The CO2/N2 (v: v = 15/85) selectivity of In-MOF-2 reaches 37.6 (at 298 K and 1 bar), also revealing outstanding selective separation ability from flue gases and purifying natural gas, affording a unique robust separation material as it has moderate DOI and pore size. In-MOF-2 shows exceptional stability and feasibility to achieve reproducibility. Aperture adjustment makes In-MOF-2 a versatile platform for selectively capturing CO2 from flue gases or purifying natural gas.

Electrochemical activation of CO2 by MOF-(Fe, Ni, Mn) derivatives of 5-aminoisophthalic acid and the thiazole group influence on its catalytic activity

The electrocatalysis of carbon dioxide using earth abundant metals and a functionalized MOF to find a sustainable process was investigated.

Coordination complexes of zinc and manganese based on pyridine-2,5-dicarboxylic acid N-oxide: DFT studies and antiproliferative activities consideration

We report here the design, synthesis, and antiproliferative activity of three coordination complexes [Mn₂(pydco)₂(bpy)₂(H₂O)₂]·2H₂O (1), [Zn(bpy)(Hpydco)₂] (2), and [Zn(bpy)Cl(Hpydco)]·2H₂O (3) (H₂pydco = pyridine-2,5-dicarboxylic acid N-oxide, bpy = 2,2'-bipyridine). Molecular structures of these complexes have been characterized by elemental analysis, Fourier transform infrared spectroscopy, thermogravimetric analysis, and powder and single-crystal X-ray diffraction. According to the structural analysis, 1-3 are discrete complexes containing N- and O-donor ligands (bpy and pydco^2-) in which pydco^2- can be coordinated to the metal centres via the N-oxide oxygen and one carboxylate oxygen to generate a six-membered chelate ring. Also, these structures benefit from extensive intermolecular interactions such as hydrogen bonds and π-interactions which are the major forces to make them more stable in the solid state. The energetic features of the π-stacking interactions observed in compounds 1-3 have been computed and compared to the H-bonds. The interactions in the solid state have been also studied using the independent gradient model approach (IGM plot). The IGM-δg approach uses a new descriptor (δg) that locally represents the difference between a virtual upper limit of the electron density gradient and the true electron density gradient. This newly developed IGM methodology automatically extracts the signature of interactions between two given fragments. Finally, the antiproliferative properties of these complexes were tested on several cancer cell lines by MTT assay and flow cytometry. Also, to compare the antiproliferative activities of these complexes with common chemotherapy drugs, the antiproliferative property of cisplatin was evaluated as a reference and positive control.

A Microporous MOF Constructed by Cross-Linking Helical Chains for Efficient Purification of Natural Gas and Ethylene

Natural gas (NG) and ethylene (C₂H₄) are two raw materials of significant value for manufacturing versatile fine chemicals and/or polymers, and thus the development of solid adsorbing agents such as metal-organic frameworks (MOFs) applied to their depuration is very crucial but remains highly challenging. In this research, we designed and synthesized a ligand containing mixed N and O coordination donors, which was solvothermally assembled with Cu(II) ions to generate a microporous MOF. X-ray crystallography revealed that the title MOF incorporates one-dimensional (1D) homochiral helical chains that are datively cross-linked to form open channels in the three-periodic coordination framework. Furthermore, the behaviors of C₁-C₂ hydrocarbons and carbon dioxide (CO₂) adsorbed in the title MOF were systematically investigated, revealing its promising potential for the purification of both NG and C₂H₄. At 109 kPa and 298 K, the C₂/methane (CH₄), CO₂/CH₄, and acetylene (C₂H₂)/C₂H₄ adsorption selectivities are impressive, reaching as high as 62.9, 28.6, and 3.5, respectively. This work represents a unique MOF based on cross-linked homochiral helical chains exhibiting dual-function separation potentials for NG and C₂H₄ purifications.

A redetermination of the structure and Hirshfeld surface analysis of poly[di-aquadi-μ-hydroxido-tetra-kis-(μ-nicotinato N-oxide)tricopper(II)]

The product obtained from the reaction of pyridine-2,3-di-carb-oxy-lic acid and hydrated copper(II) chloride in hot aqueous NaOH solution was determined by low temperature X-ray diffraction to be [Cu3(C6H4NO3)4(OH)2(H2O)2] n or [Cu3(μ-OH)2(μ-nicNO)4(H2O)2] n (nicNO is pyridine-3-carboxyl-ate N-oxide), a structure obtained from room temperature data and reported previously. The present determination is improved in quality and treatment of the H atoms. A Hirshfeld surface analysis of the inter-molecular inter-actions is presented.

Unprecedented High Temperature CO2 Selectivity and Effective Chemical Fixation by a Copper-Based Undulated Metal-Organic Framework

Post- and precombustion CO₂ capture and separation are the vital challenges from industrial viewpoint, as the accessible technologies are not cost-effective and cumbersome. Thus, the development of functional metal-organic frameworks (MOFs) that are found to be promising materials for selective CO₂ capture, separation, and conversion is gaining an importance in the scientific world. Based on the strategic design, a new functionalized triazine-based undulated paddle-wheel Cu-MOF (1), {[Cu(MTABA)(H₂O)]·4H₂O·2EtOH·DMF}_n (where, H₂MTABA = 4,4'-((6-methoxy-1,3,5-triazine-2,4-diyl)bis(azanediyl))dibenzoic acid), has been synthesized under solvothermal conditions and fully characterized. MOF 1 contains a one-dimensional channel along the a-axis with pore walls decorated with open metal sites, and multifunctional groups (amine, triazine, and methoxy). Unlike other porous materials, activated 1 (1') possesses exceptional increment in CO₂/N₂ and CO₂/CH₄ selectivity with increased temperature calculated by the ideal adsorbed solution theory. With an increase in temperature from 298 to 313 K, the selectivity of CO₂ rises from 350.3 to 909.5 at zero coverage, which is unprecedented till date. Moreover, 1' behaves as a bifunctional heterogeneous catalyst through Lewis acid (open metal) and Brönsted acid sites to facilitate the chemical fixation of CO₂ to cyclic carbonates under ambient conditions. The high selectivity for CO₂ by 1' even at higher temperature was further corroborated with configurational bias Monte Carlo molecular simulation that ascertains the multiple CO₂-philic sites and epoxide binding sites in 1' to further decipher the mechanistic pathway.

An overview on trace CO2 removal by advanced physisorbent materials

This review paper focuses on various gas processing technologies and materials that efficiently capture trace levels of carbon dioxide (CO₂). Fundamental separation mechanisms such as absorption, adsorption, and distillation technology are presented. Liquid amine-based carbon capture (C-capture) technologies have been in existence for over half a century, however, liquid amine capture relies upon chemical reactions and is energy-intensive. Liquid amines are thus not economically viable for broad deployment and offer little room for innovation. Innovative C-capture technologies must improve both the environmental footprint and cost-effectiveness. As a promising alternative, physisorbents have many advantages including considerably lower regeneration energy. Generally, existing classes of physisorbent materials, such as metal-organic frameworks (MOFs) and zeolites are selective toward C-capture. However, their selectivity is currently not high enough to remove trace levels (e.g., ~1%) of CO₂ from various natural gas process streams. This review summarizes the current advancements in physisorbent materials for CO₂ capture. Here, key performance parameters needed to select the most suitable candidate are highlighted. Furthermore, this review discusses the scope for the development of better performing CO₂ selective physisorbents from both environmental and economic perspectives. In addition, hybrid ultra microporous materials (HUMs), characterized mainly by ultra-micro pores (<0.7 nm), are discussed in reference to C-capture. Various characteristics of HUMs result in high selectivity and applicability in difficult separations such as the gas sweetening and C-capture from complex humid mixed gas streams.

Immobilization of N-oxide functionality into NbO-type MOFs for significantly enhanced C2H2/CH4 and CO2/CH4 separations

Two NbO-type MOFs with N-oxide functionality immobilized in the pore surface display significantly enhanced C₂H₂/CH₄ and CO₂/CH₄ separation performance.

Unusual adsorption behaviours and responsive structural dynamics via selective gate effects of an hourglass porous metal-organic framework

An hourglass porous metal–organic framework, LIFM-12, constructed on a T-shaped flexible ligand with Cu²⁺ paddle-wheel clusters, shows temperature and gas adsorption responsive structural dynamics upon reversible molecular guest binding. Temperature-dependent single crystal and powder X-ray diffraction experiments show that the open gate status of the framework with adaptive behaviours facilitates kinetic diffusion of gas molecules resulting in the sequential filling of pores of different sizes, thus creating a breathing behaviour reminiscent of the observation of several steps in adsorption isotherms. In addition, adsorption studies revealed that LIFM-12 performs exceptional adsorption selectivity of 10–25 for CO₂versus light gases N₂, CH₄, and CO and up to 200 for C₃H₆versus CH₄.

A Cu²⁺ based metal–organic framework exhibits dynamic behaviour upon guest inclusion/release process, and performing stepwise sorption isotherms for various gas. We elucidate detailed mechanisms under its exceptional sorption behavior.

MIL-53(Al) as a Versatile Platform for Ionic-Liquid/MOF Composites to Enhance CO2 Selectivity over CH4 and N2

Five different imidazolium-based ionic liquids (ILs) were incorporated into a metal-organic framework (MOF), MIL-53(Al), to investigate the effect of IL incorporation on the CO2 separation performance of MIL-53(Al). CO2 , CH4 , and N2 adsorption isotherms of the IL/MIL-53(Al) composites and pristine MIL-53(Al) were measured to evaluate the effect of the ILs on the CO2 /CH4 and CO2 /N2 selectivities of the MOF. Of the composite materials that were tested, [BMIM][PF6 ]/MIL-53(Al) exhibited the largest increase in CO2 /CH4 selectivity, 2.8-times higher than that of pristine MIL-53(Al), whilst [BMIM][MeSO4 ]/MIL-53(Al) exhibited the largest increase in CO2 /N2 selectivity, 3.3-times higher than that of pristine MIL-53(Al). A comparison of the CO2 separation potentials of the IL/MOF composites showed that the [BMIM][BF4 ]- and [BMIM][PF6 ]-incorporated MIL-53(Al) composites both showed enhanced CO2 /N2 and CO2 /CH4 selectivities at pressures of 1-5 bar compared to composites of CuBTC and ZIF-8 with the same ILs. These results demonstrate that MIL-53(Al) is a versatile platform for IL/MOF composites and could help to guide the rational design of new composites for target gas-separation applications.

Specific K+ Binding Sites as CO2 Traps in a Porous MOF for Enhanced CO2 Selective Sorption

Metal-organic frameworks (MOFs) can be fine-tuned to boost sorbent-sorbate interactions in order to improve gas sorption and separation performance, but the design of MOFs with ideal structural features for gas separation applications remains a challenge. Herein it is reported that unsaturated alkali metal sites can be immobilized in MOFs through a tetrazole based motif and that gas affinity can thereby be boosted. In the prototypal MOF of this type-NKU-521 (NKU denotes Nankai University), K⁺ cations are effectively embedded in a trinuclear Co²⁺ -tetrazole coordination motif. The embedded K⁺ sites are exposed to the pores of NKU-521 through water removal, and the isosteric heat (Q_st ) for CO₂ is boosted to 41 kJ mol^-1 . The nature of the binding site is validated by molecular simulations and structural characterization. The K⁺ cations in effect serve as gas traps and boost the CO₂ -framework affinity, as measured by the Q_st , by 24%. In addition, the impact of unsaturated alkali metal sites upon the separation of hydrocarbons is evaluated for the first time in MOFs using ideal adsorbed solution theory (IAST) calculations and column breakthrough experiments. The results reveal that the presence of exposed K⁺ sites benefits gas sorption and hydrocarbon separation performances of this MOF.

MOFs containing a linear bis-pyridyl-tris-amide and angular carboxylates: exploration of proton conductivity, water vapor and dye sorptions

Four new MOFs were shown to have appreciable proton conductivities, selective adsorption of water vapor over nitrogen and a tendency to selectively adsorb cationic dyes such as methylene blue and crystal violet.

rtl-M-MOFs (M = Cu, Zn) with a T-shaped bifunctional pyrazole-isophthalate ligand showing flexibility and S-shaped Type F-IV sorption isotherms with high saturation uptakes for M = Cu

The flexible, activated MOF rtl-[Cu(HIsa-az-dmpz)] undergoes a reversible phase change into a closed form with gate opening at cryogenic temperatures for N₂ and CO₂.

Effects of N-oxidation on the molecular and crystal structures and properties of isocinchomeronic acid, its metal complexes and their supramolecular architectures: experimental, CSD survey, solution and theoretical approaches

A series of new coordination complexes bearing H₂pydco, an O-donor ligand, were synthesized and compared to corresponding complexes with H₂pydc. The molecular and supramolecular structures were investigated using crystallography, solution phase and high level DFT studies.

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.

Five transition metal coordination polymers driven by a semirigid trifunctional nicotinic acid ligand: selective adsorption and magnetic properties

Five coordination polymers have been synthesized by a new organic linker containing three distinct types of functional groups together with the mixed 2,2′-bipy or 4,4′-bipy co-ligand, revealing various framework structures and selective gas adsorption and magnetic properties.

Efficient separation of C2H2from C2H2/CO2mixtures in an acid–base resistant metal–organic framework

An acid–base resistant metal–organic framework,ZJU-196, for excellent C₂H₂/CO₂separation has been successfully synthesised.

Construction of a Multi-Cage-Based MOF with a Unique Network for Efficient CO2 Capture

As a kind of MOFs, cage-based MOFs usually carry large voids and small windows, which are advantageous to the storage of small molecules that remain kinetically trapped inside the cages (confinement effect). By adjusting the size of windows via reticular synthesis, the cage-based MOFs can selectively capture and separate the suitable size molecules. Here, considering angle-directed and face-directed strategies, a novel multicage-based MOF NUM-3 with a new (3,4,5)-connected topology was successfully constructed in the mixed-ligands assembly. In the framework of NUM-3, there exist four different kinds of cages, which exhibit diverse polyhedral configurations. The four kinds of cages in the order ABCDDCBA as the minimum repeat unit form a 1D tortuous channel along the c axis. Based on the structure characteristics that the 1D channel exhibits different inner diameter (from 4.0 to13.0 Å), NUM-3a (actived NUM-3) can capture CO₂ over C₂H₄ and C₂H₆ by the size selectivity (the empirical kinetic diameters: CO₂ < C₂H₄ < C₂H₆). In addition, it also exhibits commendable selectivity for CO₂ over N₂ and CH₄.