Pore Environment Research Articles

A separation MOF with O/N active sites in nonpolar pore for One-step C2H4 purification from C2H6 or C3H6 mixtures

Here, a 2-fold interpenetrated metal–organic framework (MOF) was synthesized by imidazole derivative and carboxyl co-ligands, which features nonpolar pore environment with accessible N/O sorption sites, resulting in the suitable pore sizes (8.78 Å × 11.45 Å) to optimally interact with C2H6 and C3H6 compared to C2H4. The C3H6- and C2H6-selective phenomena were rigorously studied by combining pure-component sorption isotherms, ideal adsorbed solution theory (IAST) calculations, molecular simulations and breakthrough curves. The MOF exhibits high C3H6 uptake (53.6 cm3 g−1 at 10 kPa and 298 K) and benchmark C3H6/C2H4 selectivity (17.1), surpassing all of the reported porous materials for C3H6/C2H4 separation. Molecular simulations revealed both nonpolar pore environment and accessible O/N sites synergistically “match” better with C3H6 and C2H6 to provide stronger multiple interactions than C2H4. Experimental breakthrough curves and transient breakthrough simulations revealed that the MOF not only can realize one-step C2H4 purification (purity ≥ 99.9%) from various ratios of C2H6/C2H4 and C3H6/C2H4 mixtures with great recyclability, but also greatly increase the purity of C3H6 (such as from 28.5% to 87.7% for 2/5 C3H6/C2H4 mixture).

Free-Standing Metal–Organic Framework Membranes Made by Solvent-Free Space-Confined Conversion for Efficient H2/CO2 Separation

Metal-organic frameworks (MOFs) are promising candidates for the advanced membrane materials based on their diverse structures, modifiable pore environment, precise pore sizes, etc. Nevertheless, the use of supports and large amounts of solvents in traditional solvothermal synthesis of MOF membranes is considered inefficient, costly, and environmentally problematic, coupled with challenges in their scalable manufacturing. In this work, we report a solvent-free space-confined conversion (SFSC) approach for the fabrication of a series of free-standing MOF (ZIF-8, Zn(EtIm)2, and Zn2(BIm)4) membranes. This approach excludes the employment of solvents and supports that require tedious pretreatment and, thus, makes the process more environment-friendly and highly efficient. The free-standing membranes feature a robust and unique architecture, which comprise dense surface layers and highly porous interlayer with large amounts of irregular-shaped micron-scale pore cavities, inducing satisfactory H2/CO2 selectivities and exceptional H2 permeances. The ZIF-8 membrane affords a considerable H2 permeance of 2653.7 GPU with a competitive H2/CO2 selectivity of 17.1, and the Zn(EtIm)2 membrane exhibits a high H2/CO2 selectivity of 22.1 with an excellent H2 permeance (6268.7 GPU). The SFSC approach potentially provides a new pathway for preparing free-standing MOF membranes under solvent-free conditions, rendering it feasible for scale-up production of membrane materials for gas separation.

Metal‐Organic Frameworks with Rod Yttrium Secondary Building Units

AbstractWe report a new hydroxamate‐based yttrium MOF, named MOF‐419 [Y(HCOO)(BDH)], with rod‐SBUs. The compound was synthesized employing chelating construction of benzene‐1,4‐hydroxamate (BDH2−) linkers, and the use of formic acid as the modulator was found to be crucial for the formation of rod‐shaped SBUs. MOF‐419 shows permanent porosity and has a BET surface area of 1130 m2/g. Its hydrophobic pore environment and gas sorption properties were demonstrated through the combination of single crystal X‐ray diffraction and nitrogen, carbon dioxide, and water sorption experiments. We envision that these results will aid the study and understanding of rod MOFs with chelating linkages.

Engineering Pore Environments of Sulfate-Pillared Metal-Organic Framework for Efficient C2 H2 /CO2 Separation with Record Selectivity.

Engineering pore environments exhibit great potential in improving gas adsorption and separation performances but require specific means for acetylene/carbon dioxide (C2 H2 /CO2 ) separation due to their identical dynamic diameters and similar properties. Herein, a novel sulfate-pillared MOF adsorbent (SOFOUR-TEPE-Zn) using 1,1,2,2-tetra(pyridin-4-yl) ethene (TEPE) ligand with dense electronegative pore surfaces is reported. Compared to the prototype SOFOUR-1-Zn, SOFOUR-TEPE-Zn exhibits a higher C2 H2 uptake (89.1 cm3 g-1 ), meanwhile the CO2 uptake reduces to 14.1 cm3 g-1 , only 17.4% of that on SOFOUR-1-Zn (81.0 cm3 g-1 ). The high affinity toward C2 H2 than CO2 is demonstrated by the benchmark C2 H2 /CO2 selectivity (16 833). Furthermore, dynamic breakthrough experiments confirm its application feasibility and good cyclability at various flow rates. During the desorption cycle, 60.1 cm3 g-1 C2 H2 of 99.5% purity or 33.2 cm3 g-1 C2 H2 of 99.99% purity can be recovered by stepped purging and mild heating. The simulated pressure swing adsorption processes reveal that 75.5 cm3 g-1 C2 H2 of 99.5+% purity with a high gas recovery of 99.82% can be produced in a counter-current blowdown process. Modeling studies disclose four favorable adsorption sites and dense packing for C2 H2 .

Isomerization of DASA Molecules in the Nanopores of Metal-Organic Frameworks: What Determines Its Reversibility?

In recent years, light-responsive molecules have been incorporated in metal-organic frameworks (MOFs) to fabricate light-responsive intelligent devices, where reversible isomerization of the guest molecules in the nanopores is crucial. However, how to design a porous environment of MOFs to achieve a reversible isomerization remains unknown until now. In this work, donor-acceptor Stenhouse adducts (DASAs), a new kind of visible light responsive compound, were confined in the nanopores of different MOFs to study their isomerization upon visible-light irradiation/mild heating. We found that the polarity of the pore environment is the key to control the reversibility of isomerization of such guest molecules. Under the guidance of this principle, MIL-53(Al) was screened to investigate the proton conductivity and switching performance of the DASA-confined MOF. The proton conductance was up to 0.013 S cm-1 at 80 °C and 98 % RH, and at least 30 switching cycles were achieved thanks to the Grotthuss-type mechanism and the low polarity of MIL-53(Al) pore environment.

Auxiliary ligand-directed assembly of a non-interpenetrated cage-within-cage metal-organic framework for highly efficient C2H2/CO2 separation

The development of metal-organic frameworks (MOFs) with highly efficient adsorption and separation of acetylene is very important and challenging in chemical industry due to the explosive nature of acetylene. Porous MOFs can be constructed by inserting a second auxiliary ligand, which allows the use of large ligands to construct non-interpenetrated structures and increase pore utilization. Herein, SNNU-205 is successfully synthesized, which connects two sets of interpenetrated structures to form a double walled cage-within-cage structure by using the introduction of a second auxiliary ligand. The modified pore environment enables SNNU-205 to efficiently selectively adsorb C2H2 over CO2. At 298 K and 1 atm, SNNU-205 can uptake much more C2H2 (76.3 cm3 g−1) than CO2 (47.3 cm3 g−1), resulting in a high substance ratio of C2H2-to-CO2 (1.6). More importantly, the ideal adsorbed solution theory selectivity calculations and column breakthrough tests further indicate SNNU-205 to be promising adsorbents for C2H2 adsorption and purification.

Hierarchical Microtubular Covalent Organic Frameworks Achieved by COF-to-COF Transformation.

Pore environment and aggregated structure play a vital role in determining the properties of porous materials, especially regarding the mass transfer. Reticular chemistry imparts covalent organic frameworks (COFs) with well-aligned micro/mesopores, yet constructing hierarchical architectures remains a great challenge. Herein, we reported a COF-to-COF transformation methodology to prepare microtubular COFs. In this process, the C3 -symmetric guanidine units decomposed into C2 -symmetric hydrazine units, leading to the crystal transformation of COFs. Moreover, the aggregated structure and conversion degree varied with the reaction time, where the hollow tubular aggregates composed of mixed COF crystals could be obtained. Such hierarchical architecture leads to enhanced mass transfer properties, as proved by the adsorption measurement and chemical catalytic reactions. This self-template strategy was successfully applied to another four COFs with different building units.

Control of the pore chemistry in metal-organic frameworks for efficient adsorption of benzene and separation of benzene/cyclohexane

Control of the pore chemistry in metal-organic frameworks for efficient adsorption of benzene and separation of benzene/cyclohexane

Fine-Tuned Ultra-Microporous Metal-Organic Framework in Mixed-Matrix Membrane: Pore-Tailoring Optimization for C2 H2 /C2 H4 Separation.

Effective separation of ethyne from ethyne/ethylene (C2 H2 /C2 H4 ) mixtures is a challenging and crucial industrial process. Herein, an ultra-microporous metal-organic framework (MOF) platform, Cd(dicarboxylate)2 (ditriazole), with triangular channels is proposed for high-efficiency separation of C2 H2 from C2 H4 . The targeted structures are constructed via a mixed-ligand strategy by selecting different-sized ligands, allowing for tunable pore sizes and volumes. The pore properties can be further optimized by additional modification via pore environment tailoring. This concept leads to the successful synthesis of three ultra-microporous Cd-MOFs (JLU-MOF87-89). As intended, C2 H2 uptake and C2 H2 /C2 H4 selectivity gradually increase with progressively optimizing the pore structure by adjusting ligand length and substituents. JLU-MOF89, functionalized with methyl groups, features the most optimal pore chemistry and shows selective recognition of C2 H2 over C2 H4 , owing to the framework-C2 H2 host-guest interactions. Furthermore, JLU-MOFs are fabricated into mixed-matrix membranes for C2 H2 /C2 H4 separation. C2 H2 permeability and C2 H2 /C2 H4 permselectivity are substantially enhanced by ≥400% and ≥200%, respectively, after hybridization of JLU-MOF88 and JLU-MOF89 with a polyimidepolymer(6FDA-ODA). These membranes can work efficiently and are stable under different conditions, demonstrating their potential in actual ethyne separation.

Pore-Environment-Dependent Photoresponsive Oxidase-Like Activity in Hydrogen-Bonded Organic Frameworks.

Mimicking the bioactivity of native enzymes through synthetic chemistry is an efficient means to advance the biocatalysts in a cell-free environment, however, remains long-standing challenges. Herein, we utilize structurally explicit hydrogen-bonded organic frameworks (HOFs) to mimic photo-responsive oxidase, and uncover the important role of pore environments on mediating oxidase-like activity by means of constructing isostructural HOFs. We discover that the HOF pore with suitable geometry can stabilize and spatially organize the catalytic substrate into a favorable catalytic route, as with the function of the native enzyme pocket. Based on the desirable photo-responsive oxidase-like activity, a visual and sensitive HOFs biosensor is established for the detection of phosphatase, an important biomarker of skeletal and hepatobiliary diseases. This work demonstrates that the pore environments significantly influence the nanozymes' activity in addition to the active center.

Pore‐Environment‐Dependent Photoresponsive Oxidase‐Like Activity in Hydrogen‐Bonded Organic Frameworks

AbstractMimicking the bioactivity of native enzymes through synthetic chemistry is an efficient means to advance the biocatalysts in a cell‐free environment, however, remains long‐standing challenges. Herein, we utilize structurally explicit hydrogen‐bonded organic frameworks (HOFs) to mimic photo‐responsive oxidase, and uncover the important role of pore environments on mediating oxidase‐like activity by means of constructing isostructural HOFs. We discover that the HOF pore with suitable geometry can stabilize and spatially organize the catalytic substrate into a favorable catalytic route, as with the function of the native enzyme pocket. Based on the desirable photo‐responsive oxidase‐like activity, a visual and sensitive HOFs biosensor is established for the detection of phosphatase, an important biomarker of skeletal and hepatobiliary diseases. This work demonstrates that the pore environments significantly influence the nanozymes’ activity in addition to the active center.

Boosting Xe/Kr separation by a Mixed-linker strategy in Radiation-Resistant Aluminum-Based Metal − Organic frameworks

Capturing and separating radioactive isotopes of Xenon (Xe) and Krypton (Kr) from off-gas streams in the reprocessing of used nuclear fuel (UNF) gaseous radioactive nuclides emerges as a major concern to meet the rapid growth and sustainable development of nuclear energy. Owing to the inert atomic gases (Xe and Kr) nature without dipole or quadruple moments and their existing extremely low concentrations in radioactive off-gas mixture, it is a grand challenge to design radiation-resistant porous adsorbents for efficient Xe capture and Xe/Kr separation at dilute conditions. Herein, we report a mixed-linker strategy to realize a confined aliphatic pore environment in high radioactive stable CAU-10 type Aluminum-based MOFs using isophthalate and 5-methylisophthalate as mixed linkers for Xe/Kr separation at low concentration. By systematically tune the ratio of the two linkers, the precise and appropriate amount of aliphatic methyl groups could be immobilized on the framework to achieve delicate regulation of pore structure (pore shape and pore size), which significantly increase Xe binding affinity. Mixed-linker CAU-10-H-CH3-0.21–0.79 shows significantly enhanced Xe capture and separation performance at dilute conditions compared to pristine CAU-10-H and fully methyl functionalized CAU-10-CH3, demonstrated by adsorption isotherms and breakthrough curves. The high radioactive stability of CAU-10 type Al-MOFs was also identified by gamma-ray irradiation experiments, suggesting a great potential for practical application.

Control of pore environment in nickel-based metal-organic frameworks for SF6/N2 separation

Control of pore environment in nickel-based metal-organic frameworks for SF6/N2 separation

Self‐Standing Covalent Organic Framework Membranes for H2/CO2 Separation

AbstractCovalent organic frameworks (COFs) are proposed as promising candidates for engineering advanced molecular sieving membranes due to their precise pore sizes, modifiable pore environment, and superior stability. However, COFs are insoluble in common solvents and do not melt at high temperatures, which presents a great challenge for the fabrication of COF‐based membranes (COFMs). Herein, for the first time, a new synthetic strategy is reported to prepare continuous and intact self‐standing COFMs, including 2D N‐COF membrane and 3D COF‐300 membrane. Both COFMs show excellent selectivity of H2/CO2 mixed gas (13.8 for N‐COF membrane and 11 for COF‐300 membrane), and especially ultrahigh H2 permeance (4319 GPU for N‐COF membrane and 5160 GPU for COF‐300 membrane), which is superior to those of COFMs reported so far. It should be noted that the overall separation performance of self‐standing COFMs exceeds the Robeson upper bound. Furthermore, a theoretical study based on Grand Canonical Monte Carlo (GCMC) simulation is performed to explain the excellent separation of H2/CO2 through COFMs. Thus, this facile preparation method will provide a broad prospect for the development of self‐standing COFMs with highly efficient H2 purification.

Highly Robust Microporous Metal‐Organic Frameworks for Efficient Ethylene Purification under Dry and Humid Conditions

AbstractTwo C2H6‐selective metal‐organic framework (MOF) adsorbents with ultrahigh stability, high surface areas, and suitable pore size have been designed and synthesized for one‐step separation of ethane/ethylene (C2H6/C2H4) under humid conditions to produce polymer‐grade pure C2H4. Experimental results reveal that these two MOFs not only adsorb a high amount of C2H6 but also display good C2H6/C2H4 selectivity verified by fixed bed column breakthrough experiments. Most importantly, the good water stability and hydrophobic pore environments make these two MOFs capable of efficiently separating C2H6/C2H4 under humid conditions, exhibiting the benchmark performance among all reported adsorbents for separation of C2H6/C2H4 under humid conditions. Moreover, the affinity sites and their static adsorption energies were successfully revealed by single crystal data and computation studies. Adsorbents described in this work can be used to address major chemical industrial challenges.

Highly Robust Microporous Metal-Organic Frameworks for Efficient Ethylene Purification under Dry and Humid Conditions.

Two C2 H6 -selective metal-organic framework (MOF) adsorbents with ultrahigh stability, high surface areas, and suitable pore size have been designed and synthesized for one-step separation of ethane/ethylene (C2 H6 /C2 H4 ) under humid conditions to produce polymer-grade pure C2 H4 . Experimental results reveal that these two MOFs not only adsorb a high amount of C2 H6 but also display good C2 H6 /C2 H4 selectivity verified by fixed bed column breakthrough experiments. Most importantly, the good water stability and hydrophobic pore environments make these two MOFs capable of efficiently separating C2 H6 /C2 H4 under humid conditions, exhibiting the benchmark performance among all reported adsorbents for separation of C2 H6 /C2 H4 under humid conditions. Moreover, the affinity sites and their static adsorption energies were successfully revealed by single crystal data and computation studies. Adsorbents described in this work can be used to address major chemical industrial challenges.

One-step ethylene separation from ternary C2 hydrocarbon mixture with a robust zirconium metal–organic framework

One-step separation of high-purity ethylene (C2H4) from C2 hydrocarbon mixture is critical but challenging because of the very similar molecular sizes and physical properties of C2H4, ethane (C2H6), and acetylene (C2H2). Herein, we report a robust zirconium metal–organic framework (MOF) Zr–TCA (H3TCA = 4,4′,4″-tricarboxytriphenylamine) with suitable pore size (0.6 nm × 0.7 nm) and pore environment for direct C2H4 purification from C2H4/C2H2/C2H6 gas-mixture. Computational studies indicate that the abundant oxygen atoms and non-polar phenyl rings created favorable pore environments for the preferential binding of C2H2 and C2H6 over C2H4. As a result, Zr–TCA exhibits not only high C2H6 (2.28 mmol·g−1) and C2H2 (2.78 mmol·g−1) adsorption capacity but also excellent C2H6/C2H4 (2.72) and C2H2/C2H4 (5.64) selectivity, surpassing most of one-step C2H4 purification MOF materials. Dynamic breakthrough experiments confirm that Zr–TCA can produce high-purity C2H4 (>99.9%) from a ternary gas mixture (1/9/90 C2H2/C2H6/C2H4) in a single step with a high C2H4 productivity of 5.61 L·kg−1.

Pore Environment Optimization of Microporous Metal–Organic Frameworks with Huddled Pyrazine Pillars for C2H2/CO2 Separation

Metal-organic frameworks (MOFs) have been proven promising in addressing many critical issues related to gas separation and purification. However, it remains a great challenge to optimize the pore environment of MOFs for purification of specific gas mixtures. Herein, we report the rational construction of three isostructural microporous MOFs with the 4,4',4"-tricarboxyltriphenylamine (H3TCA) ligand, unusual hexaprismane Ni6O6 cluster, and functionalized pyrazine pillars [PYZ-x, x = -H (DZU-10), -NH2 (DZU-11), and -OH (DZU-12)], where the building blocks of Ni6O6 clusters and huddled pyrazine pillars are reported in porous MOFs for the first time. These building blocks have enabled the resulting materials to exhibit good chemical stability and variable pore chemistry, which thus contribute to distinct performances toward C2H2/CO2 separation. Both single-component isotherms and dynamic column breakthrough experiments demonstrate that DZU-11 with the PYZ-NH2 pillar outperforms its hydrogen and hydroxy analogues. Density functional theory calculations reveal that the higher C2H2 affinity of DZU-11 over CO2 is attributed to multiple electrostatic interactions between C2H2 and the framework, including strong C≡C···H-N (2.80 Å) interactions. This work highlights the potential of pore environment optimization to construct smart MOF adsorbents for some challenging gas separations.

Combination of Low-Polar and Polar Binding Sites in Aliphatic MOFs for the Efficient C2H6/C2H4 Separation

The selective capture of C2H6 from C2H6/C2H4 mixtures is of critical importance to realize the efficient one-step purification of C2H4 but remains challenging due to their similar properties and smaller quadrupole moment of C2H6 that usually result in C2H4-preferring adsorption. Herein, we reported two isostructural pillared-layer metal-organic frameworks, ZUL-C3 and ZUL-C4, which were constructed by mixed polycycloalkane-type ligands. Their low-polar pore environment along with more accessible low-polar C-H binding sites on the pore surface are conducive to generate more van der Waals interactions with C2H6 while the carboxylic groups distributed at four corners of pores form stronger and more dipolar interactions with C2H6, cooperatively resulting in a good C2H6/C2H4 uptake ratio of 1.50 for ZUL-C3 and 1.72 for ZUL-C4 in static adsorption experiments and a high C2H4 (>99.99% purity) productivity of 10.1 L/kg for ZUL-C3 and 14.6 L/kg for ZUL-C4 from an equimolar C2H6/C2H4 mixture in breakthrough experiments.

Efficient C2H2-selective separation in a microporous Zn(II)-based metal-organic framework via the dual-ligand strategy.

Implementing the dual-ligand strategy, a microporous Zn-based MOF 1 with nitro and amino groups was effectively produced. The activated interconnected pores of 1 exhibited high C2H2 uptake capacity and preferential adsorption behaviour for C2H2 over CO2, as identified by the experiments and simulations. This work provides a new approach for designing and synthesizing the MOFs with desired structures and properties by optimizing their pore environment via the dual-ligand strategy.