Systemic regulation of binding sites in porous coordination polymers for ethylene purification from ternary C2 hydrocarbons.

The global demand for poly-grade ethylene (C2H4) is increasing annually. However, the energy-saving purification of this gas remains a major challenge due to the similarity in molecular properties among the ternary C2 hydrocarbons. To address this challenge, we report an approach of systematic tuning of the pore environment with organic sites (from -COOH to -CF3, then to -CH3) in porous coordination polymers (PCPs), of which NTU-73-CH3 shows remarkable capability for the direct production of poly-grade C2H4 from ternary C2 hydrocarbons under ambient conditions. In comparison, the precursor structure of NTU-73-COOH is unable to purify C2H4, while NTU-73-CF3 shows minimal ability to harvest C2H4. This is because the changed binding sites in the NTU-73-series not only eliminate the channel obstruction caused by the formation of gas clusters, but also enhance the interaction with acetylene (C2H2) and ethane (C2H6), as validated by in situ crystallographic and Raman analysis. Our findings, in particular the systematic tuning of the pore environment and the efficient C2H4 purification by NTU-73-CH3, provide a blueprint for the creation of advanced porous families that can handle desired tasks.

Fine-regulation of gradient gate-opening in nanoporous crystals for sieving separation of ternary C3 hydrocarbons.

The adsorptive separation of ternary propyne (C3H4)/propylene (C3H6)/propane (C3H8) mixtures is of significant importance due to its energy efficiency. However, achieving this process using an adsorbent has not yet been accomplished. To tackle such a challenge, herein, we present a novel approach of fine-regulation of the gradient of gate-opening in soft nanoporous crystals. Through node substitution, an exclusive gate-opening to C3H4 (17.1 kPa) in NTU-65-FeZr has been tailored into a sequential response of C3H4 (1.6 kPa), C3H6 (19.4 kPa), and finally C3H8 (57.2 kPa) in NTU-65-CoTi, of which the gradient framework changes have been validated by in situ powder X-ray diffractions and modeling calculations. Such a significant breakthrough enables NTU-65-CoTi to sieve the ternary mixtures of C3H4/C3H6/C3H8 under ambient conditions, particularly, highly pure C3H8 (99.9%) and C3H6 (99.5%) can be obtained from the vacuum PSA scheme. In addition, the fully reversible structural change ensures no loss in performance during the cycling dynamic separations. Moving forward, regulating gradient gate-opening can be conveniently extended to other families of soft nanoporous crystals, making it a powerful tool to optimize these materials for more complex applications.

Solvents regulate the packing porosity of a bilayer metal-organic cage.

Metal-organic cages (MOCs) are an emerging class of porous materials with promising applications. However, controlling the configuration of the cage packing, which can influence the overall porosity of the materials, remains a difficulty, as many factors can influence the cage assembly and stacking. Herein, we report a solvent strategy to fine-tune the packing configuration of a bilayer MOC, a small triangular prism cage (six Cu ions act as vertices, three nitrate ions act as pillars, and six nitrate ions act as caps) incorporated into a large triangular prism cage (another six Cu ions act as vertices, a couple of oxygen atoms act as pillars and six ligands (L1: 3,5-bis(pyridine-3-yl)-4H-1,2,4-triazole) act as a jointed cap) by the coordination between the triazole nitrogen from L1 and the inner vertex Cu ions. The involved solvents water, acetonitrile (MeCN) and N,N'-dimethylformamide (DMF) form hydrogen bonds with this bilayer MOC, resulting in three different types of packing associated with systemically tuned porosity (NTU-93: 12.2%, NTU-94: 19.3%, and NTU-95: 42.1%). Gas adsorption and breakthrough tests demonstrate that NTU-95 has potential ability for C2H2/C2H4 separation. This work not only shows a case of finely tuned packing of coordination cages, but also provides a powerful tool that may be extended to other cage families.

Immobilization of Amino-site into a Pore-Partitioned Metal-Organic Framework for Highly Efficient Separation of Propyne/Propylene.

Adsorptive separation of propyne/propylene (C3H4/C3H6) is a crucial yet complex process, however, it remains a great difficulty in developing porous materials that can meet the requirements for practical applications, particularly with an exceptional ability to bind and store trace amounts of C3H4. Functionalization of pore-partitioned metal-organic frameworks (ppMOFs) is methodically suited for this challenge owing to the possibility of dramatically increasing binding sites on highly porous and confined domains. We here immobilized Lewis-basic (-NH2) and Lewis-acidic (-NO2) sites on this platform. Along with an integrated nature of high uptake of C3H4 at 1 kPa, high uptake difference of C3H4-C3H6, moderated binding strength, promoted kinetic selectivity, trapping effect and high stability, the NH2-decorated ppMOF (NTU-100-NH2) can efficiently produce polymer-grade C3H6 (99.95 %, 8.3 mmol ⋅ g-1) at room temperature, which is six times more than the NO2-decorated crystal (NTU-100-NO2). The in situ infrared spectroscopy, crystallographic analysis, and sequential blowing tests showed that the densely packed amino group in this highly porous system has a unique ability to recognize and stabilize C3H4 molecules. Moving forward, the strategy of organic functionalization can be extended to other porous systems, making it a powerful tool to customize advanced materials for challenging tasks.

Tuning Multiple Counter-Anions in Porous Coordination Polymers with lcy Topology for Acetylene/Ethylene Separation.

The efficient separation of acetylene (C2H2) and ethylene (C2H4) is an important and complex process in the industry. Herein, we report a new family of lcy-topologic coordination frameworks (termed NTU-90 to NTU-92) with Cu3MF6 (M = Si, Ti, and Zr) nodes. These charged frameworks are compensated by different counterbalanced ions (MF62-, BF4-, and Cl-), yielding changes in the size of the window apertures. Among these frameworks, NTU-92-a (activated NTU-92) shows good adsorption selectivity of C2H2/C2H4 and also significant ability in recovering both highly pure C2H4 (99.95%) and C2H2 (99.98%). Our work not only presents a potential alternative for energy-saving purification of C2 hydrocarbons but also provides a new approach for tuning the function of charged porous materials.

Molecular Sieving of Propyne/Propylene by a Scalable Nanoporous Crystal with Confined Rotational Shutters.

Soft porous coordination polymers (PCPs) have the remarkable ability to recognize similar molecules as a result of their structural dynamics. However, their guest-induced gate-opening behaviors often lead to issues with selectivity and separation efficiency, as co-adsorption is nearly unavoidable. Herein, we report a strategy of a confined-rotational shutter, in which the rotation of pyridyl rings within the confined nanospace of a halogen-bonded coordination framework (NTU-88) creates a maximum aperture of 4.4 Å, which is very close to the molecular size of propyne (C3 H4 : 4.4 Å), but smaller than that of propylene (C3 H6 : 5.4 Å). This has been evidenced by crystallographic analyses and modelling calculations. The NTU-88o (open phase of activated NTU-88) demonstrates dedicated C3 H4 adsorption, and thereby leads to a sieving separation of C3 H4 /C3 H6 under ambient conditions. The integrated nature of high uptake ratio, considerable capacity, scalable synthesis, and good stability make NTU-88 a promising candidate for the feasible removal of C3 H4 from C3 H4 /C3 H6 mixtures. In principle, this strategy holds high potential for extension to soft families, making it a powerful tool for optimizing materials that can tackle challenging separations with no co-adsorption, while retaining the crucial aspect of high capacity.

Delicate Softness in a Temperature-Responsive Porous Crystal for Accelerated Sieving of Propylene/Propane.

Soft nanoporous crystals with structural dynamics are among the most exciting recently discovered materials. However, designing or controlling a porous system with delicate softness that can recognize similar gas pairs, particularly for the promoted ability at increased temperature, remains a challenge. Here, we report a soft crystal (NTU-68) with a one-dimensional (1D) channel that expands and contracts delicately around 4 Å at elevated temperature. The completely different adsorption processes of propane (C3H8: kinetic dominance) and propylene (C3H6: thermodynamic preference) allow the crystal to show a sieving separation of this mixtures (9.9 min·g-1) at 273 K, and the performance increases more than 2-fold (20.4 min·g-1) at 298 K. This phenomenon is contrary to the general observation for adsorption separation: the higher the temperature, the lower the efficiency. Gas-loaded in situ powder X-ray analysis and modeling calculations reveal that slight pore expansion caused by the increased temperature provides plausible nanochannel for adsorption of the relatively smaller C3H6 while maintaining constriction on the larger C3H8. In addition, the separation process remains unaffected by the general impurities, demonstrating its true potential as an alternative sorbent for practical applications. Moving forward, the delicate crystal dynamics and promoted capability for molecular recognition provide a new route for the design of next-generation sieve materials.

Synergistic Catalysis of Cobalt Tetroxide and Bamboo-Shaped Carbon Nanotubes Doped with Nitrogen for Oxygen Reduction in Zn-Air Batteries.

Zinc-air batteries (ZABs) have been considered as one of the most emerging systems for energy conversion and storage. However, the preparation of highly efficient oxygen reduction reaction (ORR) catalysts on an air cathode is still faced with significant challenges. Herein, we report a secondary nitrogen source strategy for fine-tuning the active center, which provides a carbon-based hierarchical porous catalyst (termed Co3O4@N/CNT-1000) for highly efficient ORR activity (E1/2 = 0.87 V, JL = 5.32 mA cm-2, and Eonset = 1.021 V) and excellent stability. Controlled experiments demonstrate that such high activity derives from the synergistic effect of cobalt tetroxide and bamboo-shaped carbon nanotubes doped with nitrogen, prepared by the pyrolysis of a two-dimensional metal-organic framework nanosheet (termed NTU-70) and melamine. Furthermore, the ZAB assembled with Co3O4@N/CNT-1000 displays high specific capacity (854 mA h g-1Zn) and power density (179 mW cm-2), excellent long-term cycling (330 h), and durable charging/discharging ability.

Highly Efficient CO2 Capture from Wet-Hot Flue Gas by a Robust Trap-and-Flow Crystal.

Highly selective CO2 capture from flue gas based on adsorption technology is among the largest challenge on the horizon, due to its high temperature (>333 K), lower partial pressure (0.1-0.2 bar), and competition from water. Due to the designable and tunable pore system, porous coordination polymers (PCPs) have been considered as the most exciting discoveries in porous materials. However, the rational design and function-led preparation of the pore system that permits highly selective CO2 capture from flue gas (CO2/N2/O2/CO/H2O) remains a great challenge. Herein, we report a highly selective CO2 capture from wet-hot (363 K, RH = 40%) flue gas by a robust trap-and-flow crystal (NTU-67). Crystallographic analysis showed that the flow channel provides plausible CO2 traffic, while the confined trap works as an accommodation for captured gas molecules. Further, the hydrophobic pore surface endows the function of the channels that are not influenced by hot moisture, a major obstacle to overcome direct CO2 capture by PCPs. The integral nature of NTU-67, including good stability in SO2, meets the key prerequisites that are usually considered for practical applications. The molecular insight and highly efficient CO2 capture make us believe that different nanospace with their own duties may be extended into ingenious design of more advanced adsorbents for cost-effective and promising for CO2 capture from flue gas.

Formation and fine-tuning of metal-organic frameworks with carboxylic pincers for the recognition of a C2H2 tetramer and highly selective separation of C2H2/C2H4.

Highly efficient ethylene (C2H4) and acetylene (C2H2) separation is a great challenge and an important process in current industries. Herein, we finely tune a new family of 6-c metal-organic frameworks (MOFs) with crab-like carboxylic pincers for the recognition of a C2H2 tetramer and afford NTU-72 with high adsorption C2H2/C2H4 selectivity (56-441, 298 K) as well as unprecedented recovery of both highly pure C2H4 (99.95%) and C2H2 (99.36%). Furthermore, the effective binding of a C2H2 tetramer by NTU-72's carboxylic pincers has been revealed by gas-loaded crystallography and Raman spectral studies. Our work provides a novel approach for the selective binding of a small molecular cluster for designing high-performance MOFs.

Polycrystalline Metal-Organic Framework Membranes for Separation of Light Hydrocarbons.

Due to facile designability and versatile nanospace, metal-organic frameworks (MOFs) have been considered as promising membrane materials. Compared to the mixed matrix membranes that incorporated with MOF particles, the polycrystalline MOF membranes demonstrates significant advantages in maximum utilizing the crystalline nanospace, and thus yielding a fruitful of achievements in the last twenty years. Although some reviews have summarized the development of MOF-based membranes, the theoretical framework for oriented design and preparation of polycrystalline MOF membranes for highly efficient separation of light hydrocarbons remains in infancy. Herein, in this review, the fabrication strategies of polycrystalline MOF membranes and the corresponding performance in the separation of light hydrocarbons were classified and summarized. Particularly, the MOF membranes with global and local dynamics have been proposed as an interesting topic promoted performance.

Metal-Organic Framework Nanoparticles with Universal Dispersibility through Crown Ether Surface Coordination for Phase-Transfer Catalysis and Separation Membranes.

Dispersing metal-organic framework (MOF) solids in stable colloids is crucial for their availability and processibility. Herein, we report a crown ether surface coordination approach for functionalizing the surface-exposed metal sites of MOF particles with amphiphilic carboxylated crown ether (CEC ). The surface-bound crown ethers significantly improve MOF solvation without compromising the accessible voids. We demonstrate that CEC -coated MOFs exhibit exceptional colloidal dispersibility and stability in 11 distinct solvents and six polymer matrices with a wide range of polarities. The MOF-CEC can be instantaneously suspended in immiscible two-phase solvents as an effective phase-transfer catalyst and can form various uniform membranes with enhanced adsorption and separation performance, which highlights the effectiveness of crown ether coating.

Confining Water Nanotubes in a Cu10O13-Based Metal-Organic Framework for Propylene/Propane Separation with Record-High Selectivity.

Energy-efficient separation of propylene (C3H6)/propane (C3H8) is in high demand for the chemical industry. However, this process is challenging due to the imperceptible difference in molecular sizes of these gases. Here, we report a continuous water nanotube dedicatedly confined in a Cu10O13-based metal-organic framework (MOF) that can exclusively adsorb C3H6 over C3H8 with a record-high selectivity of 1570 (at 1 bar and 298 K) among all the porous materials. Such a high selectivity originates from a new mechanism of initial expansion and subsequent contraction of confined water nanotubes (∼4.5 Å) caused by C3H6 adsorption rather than C3H8. Such unique response was further confirmed by breakthrough measurements, in which one adsorption/desorption cycle yields each component of the binary mixture high purity (C3H6: 98.8%; C3H8: >99.5%) and good C3H6 productivity (1.6 mL mL-1). Additionally, benefiting from the high robustness of the framework, the water nanotubes can be facilely recovered by soaking the MOF in water, ensuring long-term use. The molecular insight here demonstrates that the confining strategy opens a new route for expanding the function of MOFs, particularly for the sole recognition from challenging mixtures.

Rotation configuration control of the sp2 bond in the diimidazole-dicarboxylate linker for the isomerism of porous coordination polymers.

Porous isomers constructed from the same building blocks but different topology break the general preferred coordination rule of organic linkers and metal nodes, representing an invaluable opportunity for enriching their pore chemistry. Herein, a new group of porous isomers (termed as NTU-69 and NTU-70) was prepared from a C2v symmetric diimidazole-dicarboxylate ligand and mononuclear Cu ion. The structural differences arise from the different rotation configuration of the sp2 bond in the ligand, leading them to exhibit completely different topologies of unc (NTU-69) and sod (NTU-70) as well as framework rigidness. This rotation configuration of the sp2 bond can be controlled by the different acidity of the synthetic solution and the metal/ligand ratio. Gas adsorption and IAST selectivity show that NTU-70 features high potential for CH4 purification from C2H4, C2H6, C3H6 and CO2 mixtures at room temperature. The insight from this work establishes a new bridge between the ligand design and controlled construction of porous isomers.

Anion Regulates scu Topological Porous Coordination Polymers into the Acetylene Trap.

The development of efficient porous absorbents with high uptake and selectivity remains a great challenge, especially for the recovery of acetylene (C2H2) from its carbon dioxide (CO2)-containing mixtures. Here, we propose and report an anion-planting strategy for regulating the scu topological porous coordination polymers (PCPs) into the C2H2 trap. The three electronegative anions SiF62-, TiF62-, and ZrF62-, in addition to the ligand of 3,5-di(1H-imidazol-1-yl)benzoic acid (HL) and Cu2+ ion, were employed to construct highly porous PCPs (NTU-60, NTU-61, and NTU-62) with varied window aperture. Especially, due to a matching distance (dF-F) of 5.7 Å along the c-axis, the limited space that can be assigned as a single C2H2 trap enables NTU-61 to show optimal ability for C2H2 (van der Waals (vdW) parameters of the two H atoms: ∼5.72 Å) recognition, validated by Grand Canonical Monte Carlo (GCMC) calculations and Raman spectra. These characteristics allow the NTU-series to show higher C2H2 uptake, as well as excellent C2H2/CO2 separation performance under dynamic conditions. The molecular insight and strategy here not only permit balanced adsorption and separation in a single domain but also exhibit an opportunity to develop advanced adsorbents in nearly all frameworks with lattice or coordinated ions, which may act as the platforms for various selective guest trappings with on-demand time and/or spatial resolution.

Double-Walled Zn36@Zn104 Multicomponent Senary Metal-Organic Polyhedral Framework and Its Isoreticular Evolution.

Metal-organic polyhedral frameworks are attractive in gas storage and separation due to large voids with windows that can serve as traps for guest molecules. Introducing multivariant/multicomponent functionalities in them are ways of improving performances for certain targets. The high compatibility of organic linkers can generate multivariant MOFs, but by far, the diversity of secondary building units (SBUs) in a single metal-organic framework is still limited (no more than two in most cases). Here we report a new double-walled Zn36@Zn104 metal-organic polyhedral framework (HHU-8) with five types of topologically distinct SBUs and its isoreticular evolution to the Zn36@Zn136 counterpart (HHU-8s). Both MOFs are the first to be constructed with such high numbers of topologically distinct SBUs as well as topologically distinct nodes, and their formation and evolution provide new insight into SBU's controllability.

Pore engineering of metal-organic frameworks for ethylene purification.

Ethylene production is an important and direct indicator related to the development of the petrochemical industry in a country. However, the separation and purification of ethylene is an extremely energy-consuming process. In this review, the latest progress in the purification of ethylene using metal organic frameworks (MOFs), a new type of physical adsorbent, is summarized according to four classifications of pore engineering, including pore surface functionalization, molecular sieving, controlled framework softness and dynamic pore-dominated molecular diffusion. Finally, the current challenges and future prospects in this field are also discussed.

Tuning Gate-Opening of a Flexible Metal-Organic Framework for Ternary Gas Sieving Separation.

In comparison with the fast development of binary mixture separations, ternary mixture separations are significantly more difficult and have rarely been realized by a single material. Herein, a new strategy of tuning the gate-opening pressure of flexible MOFs is developed to tackle such a challenge. As demonstrated by a flexible framework NTU-65, the gate-opening pressure of ethylene (C2 H4 ), acetylene (C2 H2 ), and carbon dioxide (CO2 ) can be regulated by temperature. Therefore, efficient sieving separation of this ternary mixture was realized. Under optimized temperature, NTU-65 adsorbed a large amount of C2 H2 and CO2 through gate-opening and only negligible amount of C2 H4 . Breakthrough experiments demonstrated that this material can simultaneously capture C2 H2 and CO2 , yielding polymer-grade (>99.99 %) C2 H4 from single breakthrough separation.

Finely Tuned Framework Isomers for Highly Efficient C2H2 and CO2 Separation.

Obtaining the optimal physiadsorbents based on the same starting materials is one of the crucial technologies that can address the increasing problem of energy-consuming separation. Herein, a group of porous coordination isomers (NTU-51 to NTU-54) with topologies of sql, dia, nbo, and kgm has been newly designed and prepared from a 4-c square node (paddlewheel cluster) and a 2-c linker (isophthalic acid derivative). Pure gas measurements revealed that they have a varied ability for selective C2H2 capture from C2H2/CO2 mixtures, originating from the fine arrangement of functional sites within these isomers as well as size-exclusive effects. Further dynamic breakthrough experiments exhibited good C2H2/CO2 (1/1, v/v) separation performance of the two isomers (NTU-53 and NTU-54) in both dry and humid gas phases (R.H. = 45%). More interestingly, stability tests and long-term measurements demonstrated a high potential of them to be used under realistic conditions.

Accelerated C2H2/CO2 Separation by a Se-Functionalized Porous Coordination Polymer with Low Binding Energy.

High-quality pure acetylene (C2H2) is a kind of crucial starting material for various value-added products. However, selective capture of C2H2 from the main impurity of CO2 via porous absorbents is a great challenge, as they possess extremely similar kinetic diameters and boiling points, as well as the explosive and reactive properties of C2H2. Herein, we report a porous coordination polymer (PCP), (NTU-55), which assembled from the coordination between a Cu dimer and a newly designed ligand with a nonmetal selenium (Se) site. Static single-component adsorption and dynamic breakthrough experiments reveal that desolvated NTU-55 can completely adsorb C2H2 from the C2H2/CO2 mixture (1/4, v/v) at 298 K, along with higher C2H2 capacity and much lower binding energy. The origin of this separation, as comprehensively revealed by density functional theory (DFT) calculations, is derived from the interaction discriminatory of C2H2 and CO2 toward accessible Se and Cu adsorption sites. To the best of our knowledge, this is the first time to find the positive effect of nonmetal Se sites for selective C2H2 capture.