Spatially Confined π-Complexation within Pore-Space-Partitioned Metal-Organic Frameworks for Enhanced Light Hydrocarbon Separation and Purification

Ultramicroporous metal-organic frameworks (MOFs) are demonstrated to be advantageous for the separation and purification of light hydrocarbons such as C2H2, C2H4, and CH4. The introduction of transition metal sites with strong π-complexation affinity into MOFs is more effective than other adsorption sites for the selective adsorption of π-electron-rich unsaturated hydrocarbon gases from their mixtures. However, lower coordination numbers make it challenging to produce robust MOFs directly utilizing metal ions with π-coordination activity, such as Cu+, Ag+, and Pd2+. Herein, a series of novel π-complexing MOFs (SNNU-33s) with a pore size of 4.6 Å are precisely constructed by cleverly introducing symmetrically matched C3-type [Cu(pyz)3] (pyz = pyrazine) coordinated fragments into 1D hexagonal channels of MIL-88 prototype frameworks. Benifit from the spatial confinement combined with π-complex-active Cu+ of [Cu(pyz)3], pore-space-partitioned SNNU-33 MOFs all present excellent C2H2/CH4, C2H4/CH4, and CO2/CH4 separation ability. Notably, the optimized SNNU-33b adsorbent demonstrates top-level IAST selectivity values for C2H2/CH4 (597.4) and C2H4/CH4 (69.8), as well as excellent breakthrough performance. Theoretical calculations further reveal that such benchmark light hydrocarbon separation and purification ability is mainly ascribed to the extra-strong binding affinity between Cu+ and π-electron donor molecules via a spatially confined π-complexation process.

Microporous Fluorinated MOF with Multiple Adsorption Sites for Efficient Recovery of C2H6 and C3H8 from Natural Gas

Purifying C2H6/C3H8 from a ternary natural gas mixture through adsorption separation is an important but challenging process in the petrochemical industry. To address this challenge, the industry is exploring effective strategies for designing high-performance adsorbents. In this study, we present two metal-organic frameworks (MOFs), DMOF-TF and DMOF-(CF3)2, which have fluorinated pores obtained by substituting linker ligands in the host material. This pore engineering strategy not only provides suitable pore confinement but also enhances the adsorption capacities for C2H6/C3H8 by providing additional binding sites. Theoretical calculations and transient breakthrough experiments show that the introduction of F atoms not only improves the efficiency of natural gas separation but also provides multiple adsorption sites for C2H6/C3H8-framework interactions.

Halogen-modified metal-organic frameworks for efficient separation of alkane from natural gas

As a rich green energy source, natural gas is widely used in many fields such as the chemical industry, automobile energy, and daily life. However, it is very challenging to separate and recover C2H6 and C3H8 from natural gas. Metal-organic frameworks (MOFs) as an emerging type of multi-pore porous materials show huge potential in gas adsorption separation. Herein, we report pillar-layered MOFs, Ni (BDC)(DABCO)0.5 (DMOF-X), modified by halogen atoms (F, Cl, Br), and investigate their CH4/C2H6/C3H8 separation performance. The experimental results show that DMOF-Cl exhibited a extremely high adsorption capacity for C3H8 and C2H6. Under the conditions of 298 K and 100 kPa, the adsorption capacities for C3H8 and C2H6 on DMOF-Cl are as high as 6.23 and 4.94 mmol g-1, which are superior to the values for most of the porous materials that have been reported. In addition, DMOF-Cl also shows high C3H8/CH4 (5: 85, V/V) and C2H6/CH4 (10: 85, V/V) separation selectivities, with values of 130.9 and 12.5, respectively. Finally, DMOF-Cl also demonstrated great potential as an adsorbent for separating C3H8/C2H6/CH4.

An ethynyl-modified interpenetrated metal-organic framework for highly efficient selective gas adsorption

An ethynyl-modified interpenetrated MOF material with lvt topology, [Cu2(BTEB)(NMF)2]·NMF·8H2O (compound 1, H4BTEB = 4,4',4'',4'''-(benzene-1,2,4,5-tetrayltetrakis(ethyne-2,1-diyl))tetrabenzoic acid, NMF = N-Methylformamide), was successfully synthesized by using an alkynyl-functionalized H4BTEB organic ligand under solvothermal conditions. Structural analysis shows that compound 1, consisting of a tetradentate carboxylic acid ligand and classical [Cu2(CO2)4] paddle-wheel structure building units, has a rare 4-connected lvt topology with dual interpenetrating structure, which can improve the framework stability, as well as the gas adsorption capacity and selectivity due to the restricted pore channel. According to the study of gas adsorption performance, compound 1 with a larger surface area, boasts a superior adsorption capacity for small gas molecules. Also, ideal adsorption solution theory (IAST) computational simulation shows that compound 1 has good gas adsorption selectivity for C3H8/CH4, indicating its potential application in gas separation.

Supramolecular isomerism and structural flexibility in coordination networks sustained by cadmium rod building blocks

Bifunctional N-donor carboxylate linkers generally afford dia and sql topology coordination networks of general formula ML₂ that are based upon the MN₂(CO₂)₂ molecular building block (MBB). Herein, we report on a new N-donor carboxylate linker, β-(3,4-pyridinedicarboximido)propionate (PyImPr), which afforded Cd(PyImPr)₂via reaction of PyImPrH with Cd(acetate)₂·2H₂O. We observed that, depending upon whether Cd(PyImPr)₂ was prepared by layering or solvothermal methods, 2D or 3D supramolecular isomers, respectively, of Cd(PyImPr)₂ were isolated. Single crystal X-ray diffraction studies revealed that both supramolecular isomers are comprised of the same carboxylate bridged rod building block, RBB. We were interested to determine if the ethylene moiety of PyImPr could enable structural flexibility. Indeed, open-to-closed structural transformations occurred upon solvent removal for both phases, but they were found to be irreversible. A survey of the Cambridge Structural Database (CSD) was conducted to analyse the relative frequency of RBB topologies in related ML₂ coordination networks in order to provide insight from a crystal engineering perspective.

Latest insights on separation and storage of carbon compounds in buildings towards sustainable environment: Recent innovations, challenges, future perspectives and application of machine learning

Throughout the past few decades, scientific agencies have paid a lot of attention to environmental issues such as acid rain, water poisoning, and global warming. In order to solve these environmental problems, metal-organic frameworks (MOFs), which are made up of metal ions and/or clusters attached to organic ligands, have shown some promise. With a focus on the usage of MOFs, this paper examines the most recent developments, difficulties, and potential future directions in the separation and storage of carbon compounds in buildings for a sustainable environment. The importance of using MOFs in decarbonizing water systems and lowering environmental concerns in buildings is highlighted in the research. It addresses the most recent developments in the use of MOFs for renewable energy, such as the elimination of dangerous gases like CO2 and CH4 from water systems. The article also looks at how MOFs might be used to decarbonize water systems in structures, with a focus on how carbon-containing compounds are stored chemically and physically using artificial neural network models. MOFs are a potential solution for renewable energy and environmental remediation in buildings because they have special physical and chemical characteristics like adjustable pores, high porosity, and tiny pore size. The report offers insights into existing treatments and invites academics to investigate MOFs' potential for resolving environmental problems in order to create a sustainable environment in buildings.

Control over Phase Transformations in a Family of Flexible Double Diamondoid Coordination Networks through Linker Ligand Substitution

In this work, we present the first metal-organic framework (MOF) platform with a self-penetrated double diamondoid (ddi) topology that exhibits switching between closed (nonporous) and open (porous) phases induced by exposure to gases. A crystal engineering strategy, linker ligand substitution, was used to control gas sorption properties for CO₂ and C3 gases. Specifically, bimbz (1,4-bis(imidazol-1-yl)benzene) in the coordination network X-ddi-1-Ni ([Ni₂(bimbz)₂(bdc)₂(H₂O)]_n, H₂bdc = 1,4-benzenedicarboxylic acid) was replaced by bimpz (3,6-bis(imidazol-1-yl)pyridazine) in X-ddi-2-Ni ([Ni₂(bimpz)₂(bdc)₂(H₂O)]_n). In addition, the 1:1 mixed crystal X-ddi-1,2-Ni ([Ni₂(bimbz)(bimpz)(bdc)₂(H₂O)]_n) was prepared and studied. All three variants form isostructural closed (β) phases upon activation which each exhibited different reversible properties upon exposure to CO₂ at 195 K and C3 gases at 273 K. For CO₂, X-ddi-1-Ni revealed incomplete gate-opening, X-ddi-2-Ni exhibited a stepped isotherm with saturation uptake of 3.92 mol·mol^-1, and X-ddi-1,2-Ni achieved up to 62% more gas uptake and a distinct isotherm shape vs the parent materials. Single-crystal X-ray diffraction (SCXRD) and in situ powder X-ray diffraction (PXRD) experiments provided insight into the mechanisms of phase transformation and revealed that the β phases are nonporous with unit cell volumes 39.9, 40.8, and 41.0% lower than the corresponding as-synthesized α phases, X-ddi-1-Ni-α, X-ddi-2-Ni-α, and X-ddi-1,2-Ni-α, respectively. The results presented herein represent the first report of reversible switching between closed and open phases in ddi topology coordination networks and further highlight how ligand substitution can profoundly impact the gas sorption properties of switching sorbents.

Pore Environmental Modification by Alkoxy Groups in Pore-Space-Partitioned Metal-Organic Frameworks to Achieve Gas Uptake-Selectivity Balance

Due to the trade-off barrier between high storage capacity and high selectivity, the controllable and systematic design of metal-organic frameworks (MOFs) aiming at performance optimization is still challenging. Herein, considering the effectiveness of alkoxy group functionalization and a pore-space partition strategy, a series of rigid Mg-pacs-MOFs (SNNU-10-n, n = 1-6) with flexible side chains are built for the first time, realizing systematic pore environmental modification. The steric hindrance effects, electron-donating ability, and the flexibility of alkoxy groups are considered as key factors, which lead to a regular change of gas adsorption capacity and selectivity. Notably, methoxy-modified SNNU-10-1 with moderately high storage capacities of C₂H₂ (139.4 cm³ g^-1), C₂H₄ (100.4 cm³ g^-1), CO₂ (105.0 cm³ g^-1), and high selectivity values for equimolar C₂H₂/CH₄ (431.8), C₂H₄/CH₄ (164.2), and CO₂/CH₄ (16.1) mixture separation at 273 K and 100 kPa achieves an ideal gas uptake-selectivity balance. Breakthrough experiments verified that it could effectively separate the above-mentioned mixtures under ambient conditions, and GCMC simulation provides a deep understanding of methoxy group functionalization. Undoubtedly, this work not only realizes controllable regulation of gas adsorption behavior but also proves the validity of improving selectivity by alkoxy groups in those platforms with high gas-uptake potential to overcome the trade-off barrier.

MIL-101-Cr/Fe/Fe-NH2 for Efficient Separation of CH4 and C3H8 from Simulated Natural Gas

Adsorptive separation based on porous solid adsorbents has emerged as an excellent effective alternative to energy-intensive conventional separation methods in a low energy cost and high working capacity manner. However, there are few stable mesoporous metal-organic frameworks (MOFs) for efficient purification of methane from other light hydrocarbons in natural gas. Herein, we report a series of stable mesoporous MOFs, MIL-101-Cr/Fe/Fe-NH2, for efficient separation of CH4 and C3H8 from a ternary mixture CH4/C2H6/C3H8. Experimental results show that all three MOFs possess excellent thermal, acid/basic, and hydrothermal stability. Single-component adsorption suggested that they have high C3H8 adsorption capacity and commendable selectivity for C3H8 and C2H6 over CH4. Transient breakthrough experiments further certified the ability of direct separation of CH4 from simulated natural gas and indirect recovery of C3H8 from the packing column. Theoretical calculations illustrated that the van der Waals force proportional to the molecular weight is the key factor and that the structural integrity and defect can impact separation performances.

Structural Regulation and Light Hydrocarbon Adsorption/Separation of Three Zirconium-Organic Frameworks Based on Different V-Shaped Ligands

On the basis of different V-shaped ligands, three zirconium-organic frameworks (JLU-Liu45, Zr-SDBA, and Zr-OBBA) have been successfully constructed. By regulating spatial configuration and functional groups of organic ligands, these as-synthesized Zr-MOFs (MOF = metal-organic framework) display distinct structures and different light hydrocarbon adsorption/separation capabilities. JLU-Liu45, with a double-walled interpenetrated 3D primitive cubic (pcu) framework, exhibits good gas-adsorption capacity but not prominent selective separation ability. Through regulating sizes and torsion angles of the organic ligands, Zr-SDBA possesses a 2D square lattice (sql) network, while Zr-OBBA displays a non-interpenetrated 3D pcu framework. Furthermore, by regulating functional groups on the ligands, Zr-SDBA shows prominent C₂H₂ uptake (101.2 cm³·g^-1) and the best C₂H₂/CH₄ selectivity (230.5, 1:1) among the three Zr-MOFs, and Zr-OBBA shows a significant C₃H₈/CH₄ selectivity (105.6, 1:1). This work demonstrates the feasibility of structural regulation for MOF materials in the light hydrocarbon adsorption/separation field.

Flexible Microporous Copper(II) Metal-Organic Framework toward the Storage and Separation of C1-C3 Hydrocarbons in Natural Gas

A flexible and robust microporous copper(II) metal-organic framework (MOF) based on a methyl-functionalized ligand, namely, [Cu₃(μ₃-OH)₂(L)₂(DMF)] (LIFM-ZZ-1; L = 2,2'-dimethyl-4,4'-biphenyldicarboxylic acid and DMF = N,N-dimethylformamide), was constructed. Its sorption performance for the separation of CH₄, C₂H₆, and C₃H₈ was investigated. LIFM-ZZ-1 showed a breathing behavior that led to a transition between large- and narrow-pore states. The sample also showed outstanding water stability. Gas adsorption experiments revealed that desolvated LIFM-ZZ-1 exhibited higher adsorption capacities for C₂H₆ and C₃H₈ (2.80 and 4.06 mmol·g^-1) than for CH₄ (0.39 mmol·g^-1) at 298 K and 1 bar. Breakthrough experiments showed that a CH₄/C₂H₆/C₃H₈ mixture was completely separated at 298 K, demonstrating the promising potential applications of this material for separating low contents of C2/C3 hydrocarbons from natural gas.

A flexible microporous framework with temperature-dependent gate-opening behaviours for C2 gases

Herein we report a two-fold interpenetrating pillar-layer microporous material, whose framework severely shrinks after losing guest molecules and transforms into a stable nonporous one. More importantly, the guest-free framework has rarely seen temperature-dependent gate-opening behaviours for C2 gases around room temperature.

Highly C2/C1-Selective Covalent Organic Frameworks Substituted with Azo Groups

A series of covalent organic frameworks substituted with azo groups (AzoCOFs) have been synthesized via imine condensation. The obtained frameworks show crystallinity and high stability. More importantly, the AzoCOFs exhibit exceptionally high ideal adsorption solution theory (IAST) selectivity in adsorption of C₂H₂ (35-2891) over CH₄ at 273 K and 1 bar, owing to the favorable interactions between azo groups and acetylene molecules. The dependence of the gas adsorption property on pore size and polarity of the frameworks was also studied. The triethylene glycol substituted Tg-AzoCOF shows the highest C₂H₂/CH₄ selectivity (IAST selectivity of 2891), which represents the highest reported for all porous materials. The AzoCOFs also exhibit high IAST adsorption selectivity of C₂H₄/CH₄ (11-20), C₂H₆/CH₄ (15-22), and CO₂/CH₄ (12-37), which is comparable with most porous materials, thus showing their great potential in gas separation applications.

Ti4(embonate)6 Cage-Ligand Strategy on the Construction of Metal-Organic Frameworks with High Stability and Gas Sorption Properties

Herein we report an efficient strategy to construct cage-based metal-organic frameworks (MOFs) via the assembly of Ti₄L₆ (L = embonate) cages and N-contained ligands with Mn²⁺ and Zn²⁺ ions, respectively, and two Ti₄L₆-cage-based MOFs (PTC-219 and PTC-220) with high stability and gas sorption properties have been generated through two-step reactions, respectively.

Tuning the packing, interpenetration, and porosity of two-dimensional networks by metal ions and ligand side groups

A methyl-modified bent pyridyl-carboxylate ligand reacts with three metal ions to yield three sql coordination networks, showing different packing and interpenetration modes and porosities.

A flexible doubly interpenetrated metal–organic framework with gate opening effect for highly selective C2H2/C2H4 separation at room temperature

A flexible doubly interpenetrated MOF is realized at room temperature, whose pore spaces could accommodate a significant amount of C₂H₂ at room temperature via the gate-opening effect but exclude C₂H₄ and C₂H₆ outside.