Triptycene-supported bimetallic salen porous organic polymers for high efficiency CO2 fixation to cyclic carbonates

Triptycene units in bimetallic salen POPs are envisaged to support the alignment of bimetallic salen macrocycles in side walls of channels for exposing more metal active sites resulting in the high efficiency coupling reaction of epoxides with CO₂.

Fine-Tuning Pore Dimension in Hybrid Ultramicroporous Materials Boosting Simultaneous Trapping of Trace Alkynes from Alkenes

Removing trace amounts of alkynes from alkenes is one of the most critical and challenging steps to produce high-purity alkenes, the fundamental raw materials in petrochemical industry. Selective hydrogenation using noble metal catalysts under harsh conditions can convert trace alkynes to alkenes, but suffers from limited selectivity, over-hydrogenation, and energy-intensive consumption. Herein, the simultaneously adsorptive removal of trace propyne (C₃ H₄ ) and acetylene (C₂ H₂ ) from quaternary C₂ H₂ /C₂ H₄ /C₃ H₄ /C₃ H₆ mixture is reported for the first time using an anion-pillared hybrid ultramicroporous material ZU-16-Co (or TIFSIX-3-Co) by finely tuning the pore dimensions and introducing different binding sites to match the shape of alkynes. ZU-16-Co with contracted aperture size and judiciously extended cell dimension simultaneously exhibits superior trapping capacity for propyne under low concentration (2.45 mmol g^-1 at 5000 ppm) and surprisingly high C₂ H₂ uptake (4.18 and 1.4 mmol g^-1 at 1.0 and 0.01 bar, respectively) through synergistic host-guest and guest-guest interactions. Importantly, the ability of ZU-16-Co to capture trace alkynes (C₂ H₂ and C₃ H₄ ) in one step is confirmed by breakthrough experiments for quaternary C₃ H₄ /C₂ H₂ /C₃ H₆ /C₂ H₄ mixtures, presenting ZU-16-Co as a promising material for alkyne trapping.

Fine-Tuning and Selective-Binding within an Anion-Functionalized Ultramicroporous Metal-Organic Framework for Efficient Olefin/Paraffin Separation

Olefin/paraffin separation is one of the intrinsically challenging tasks, mainly realized through the energy-intensive cryogenic distillation techniques. Here, we report the efficient separation of both propylene/propane (C₃H₆/C₃H₈) and ethylene/ethane (C₂H₄/C₂H₆) mixtures for the first time by a cheap and customized anion-functionalized metal-organic framework, ZU-36 (also termed as GeFSIX-3-M, M = Ni²⁺, Co²⁺), through molecular recognition and cross-section matching mechanism. Specifically, the pore window size of the fine-tuned ZU-36-Ni (4.42 × 4.42 Ų) decorated with high density of electronegative anions matches well with the cross section of C₃H₆ (4.16 × 4.65 Ų) and C₂H₄ (3.28 × 4.18 Ų), leading to efficient separation of C₃H₆/C₃H₈ and C₂H₄/C₂H₆. The high uptake capacity and separation selectivity of ZU-36-Ni were confirmed by adsorption isotherms and breakthrough tests. Binary dynamic breakthrough results showed that ZU-36-Ni can trap 1.35 mmol g^-1 C₃H₆ and 1.08 mmol g^-1 C₂H₄ from C₃H₆/C₃H₈ and C₂H₄/C₂H₆ mixtures, respectively. The separation selectivity for C₃H₆/C₃H₈ on ZU-36-Ni calculated from the breakthrough tests is 19, which sets a new benchmark for C₃H₆/C₃H₈ separation. The proposed cross-section matching mechanism together with proper selective binding affinity may serve as a guide for the design of effective porous materials for other important gas separation.

Metrics for Evaluation and Screening of Metal-Organic Frameworks for Applications in Mixture Separations

For mixture separations, metal-organic frameworks (MOFs) are of practical interest. Such separations are carried out in fixed bed adsorption devices that are commonly operated in a transient mode, utilizing the pressure swing adsorption (PSA) technology, consisting of adsorption and desorption cycles. The primary objective of this article is to provide an assessment of the variety of metrics that are appropriate for screening and ranking MOFs for use in fixed bed adsorbers. By detailed analysis of several mixture separations of industrial significance, it is demonstrated that besides the adsorption selectivity, the performance of a specific MOF in PSA separation technologies is also dictated by a number of factors that include uptake capacities, intracrystalline diffusion influences, and regenerability. Low uptake capacities often reduce the efficacy of separations of MOFs with high selectivities. A combined selectivity-capacity metric, Δq, termed as the separation potential and calculable from ideal adsorbed solution theory, quantifies the maximum productivity of a component that can be recovered in either the adsorption or desorption cycle of transient fixed bed operations. As a result of intracrystalline diffusion limitations, the transient breakthroughs have distended characteristics, leading to diminished productivities in a number of cases. This article also highlights the possibility of harnessing intracrystalline diffusion limitations to reverse the adsorption selectivity; this strategy is useful for selective capture of nitrogen from natural gas.

Ultramicropore Engineering by Dehydration to Enable Molecular Sieving of H2 by Calcium Trimesate

The high energy footprint of commodity gas purification and increasing demand for gases require new approaches to gas separation. Kinetic separation of gas mixtures through molecular sieving can enable separation by molecular size or shape exclusion. Physisorbents must exhibit the right pore diameter to enable separation, but the 0.3-0.4 nm range relevant to small gas molecules is hard to control. Herein, dehydration of the ultramicroporous metal-organic framework Ca-trimesate, Ca(HBTC)⋅H₂ O (H₃ BTC=trimesic acid), bnn-1-Ca-H₂ O, affords a narrow pore variant, Ca(HBTC), bnn-1-Ca. Whereas bnn-1-Ca-H₂ O (pore diameter 0.34 nm) exhibits ultra-high CO₂ /N₂ , CO₂ /CH₄ , and C₂ H₂ /C₂ H₄ binary selectivity, bnn-1-Ca (pore diameter 0.31 nm) offers ideal selectivity for H₂ /CO₂ and H₂ /N₂ under cryogenic conditions. Ca-trimesate, the first physisorbent to exhibit H₂ sieving under cryogenic conditions, could be a prototype for a general approach to exert precise control over pore diameter in physisorbents.

Calcium-Based Metal-Organic Framework for Simultaneous Capture of Trace Propyne and Propadiene from Propylene

The simultaneous capture of trace propyne and propadiene from propylene is one of the important but energy demanding industrial processes because of their similar physicochemical properties as well as the ultralow concentration in the mixtures. Herein, a highly stable Ca-based MOF, constructed from an inexpensive precursor (CaCO₃) and rigid squaric acid, is capable of preferentially capturing trace propyne and propadiene with record-high uptake capacities of 2.44 and 2.64 mmol/g at pressures as low as 5 mbar, respectively. Direct multicomponent breakthrough experiments confirm that Ca-based MOF exhibits an excellent performance for simultaneous removal of trace propyne and propadiene from propylene. DFT simulation and in situ single-crystal X-ray diffraction of propadiene- and propyne-adsorbed Ca-based MOFs reveal that the strong affinity of the framework toward two species is ascribed to the multiple types of cooperative binding including π-π stacking and C-H···O interactions. The calcium squarate framework sets a new benchmark for adsorptive purification of propylene, showing great potential in the practical application.

Reversed ethane/ethylene adsorption in a metal-organic framework via introduction of oxygen

Separation of ethane from ethylene is a very important but challenging process in the petrochemical industry. Finding an alternative method would reduce the energy needed to make 170 million tons of ethylene manufactured worldwide each year. Adsorptive separation using C2H6-selective porous materials to directly produce high-purity C2H4 is more energy-efficient. We herein report the "reversed C2H6/C2H4 adsorption" in a metal-organic framework Cr-BTC via the introduction of oxygen on its open metal sites. The oxidized Cr-BTC(O2) can bind C2H6 over C2H4 through the active Cr-superoxo sites, which was elucidated by the gas sorption isotherms and density functional theory calculations. This material thus exhibits a good performance for the separation of 50/50 C2H6/C2H4 mixtures to produce 99.99% pure C2H4 in a single separation operation.

Separation of Xe from Kr with Record Selectivity and Productivity in Anion-Pillared Ultramicroporous Materials by Inverse Size-Sieving

The separation of xenon/krypton (Xe/Kr) mixture is of great importance to industry, but the available porous materials allow the adsorption of both, Xe and Kr only with limited selectivity. Herein we report an anion-pillared ultramicroporous material NbOFFIVE-2-Cu-i (ZU-62) with finely tuned pore aperture size and structure flexibility, which for the first time enables an inverse size-sieving effect in separation along with record Xe/Kr selectivity and ultrahigh Xe capacity. Evidenced by single-crystal X-ray diffraction, the rotation of anions and pyridine rings upon contact of larger-size Xe atoms adapts cavities to the shape/size of Xe and allows strong host-Xe interaction, while the smaller-size Kr is excluded. Breakthrough experiments confirmed that ZU-62 has a real practical potential for producing high-purity Kr and Xe from air-separation byproducts, showing record Kr productivity (206 mL g^-1 ) and Xe productivity (42 mL g^-1 , in desorption) as well as good recyclability.

Two Co-based MOFs assembled from an amine-functionalized pyridinecarboxylate ligand: inorganic acid-directed structural variety and gas adsorption properties

A pair of Co-based MOFs exhibited inorganic acid-driven structural diversities, and one of them displayed the C₂H₂ separation and purification potential.

Energy-efficient separation alternatives: metal–organic frameworks and membranes for hydrocarbon separation

The diversity of metal–organic frameworks enables the design of highly efficient adsorbents and membranes towards hydrocarbon separations for energy consumption mitigation.

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.

Metal-Organic Framework Materials for the Separation and Purification of Light Hydrocarbons

The separation and purification of light hydrocarbons (LHs) mixtures is one of the most significantly important but energy demanding processes in the petrochemical industry. As an alternative technology to energy intensive traditional separation methods, such as distillation, absorption, extraction, etc., adsorptive separation using selective solid adsorbents could potentially not only lower energy cost but also offer higher efficiency. The need to develop solid materials for the efficiently selective adsorption of LHs molecules, under mild conditions, is therefore of paramount importance and urgency. Metal-organic frameworks (MOFs), emerging as a relatively new class of porous organic-inorganic hybrid materials, have shown promise for addressing this challenging task due to their unparalleled features. Herein, recent advances of using MOFs as separating agents for the separation and purification of LHs, including the purification of CH₄ , and the separations of alkynes/alkenes, alkanes/alkenes, C₅ -C₆ -C₇ normal/isoalkanes, and C₈ alkylaromatics, are summarized. The relationships among the structural and compositional features of the newly synthesized MOF materials and their separation properties and mechanisms are highlighted. Finally, the existing challenges and possible research directions related to the further exploration of porous MOFs in this very active field are also discussed.

The stepwise substitution in the hierarchical building of {Co11Cd6} cluster-based MOFs from {Co14} precursor

A heterometallic aggregation of {Co₁₁Cd₆} can be hierarchical built from a {Co₁₄} precursor directly. HRESI-MS technique was used to track the stepwise Cd²⁺-capture: {Co₁₄} → {Co₈} → {Co₈Cd₂} → {Co₈Cd₃} → {Co₈Cd₄} → {Co₈Cd₅} → {Co₈Cd₆} → {Co₁₁Cd₆}.

Engineering the Pore Size of Pillared-Layer Coordination Polymers Enables Highly Efficient Adsorption Separation of Acetylene from Ethylene

The pore size of adsorbents plays a vital role in determining the overall separation performance of gas separation and purification by adsorption. In this work, the pore apertures of the coordination pillared layer (CPL) was systematically controlled by adjusting the length of pillared ligands. We used pyrazine, 4,4'-bipyridine, and 1,2-di(4-pyridyl)-ethylene with increased length to synthesize CPL-1 (L = pyrazine), CPL-2 (L = 4,4'-bipyridine), and CPL-5 [L = 1,2-di(4-pyridyl)-ethylene], respectively. The aperture size of these CPLs varies from 4 to 11 Å: CPL-1 (4 × 6 Ų), CPL-2 (9 × 6 Ų), and CPL-5 (11 × 6 Ų). Among the three frameworks, CPL-2 exhibits the highest C₂H₂ uptake at ambient conditions as it has moderate pore size and porosity. However, CPL-1 has the best separation performance in the breakthrough experiments with binary gas mixture of C₂H₂/C₂H₄, thanks to the optimal pore size nearly excluding C₂H₄, which is only observed in the state-of-the-art UTSA-300a so far. The DFT calculations were carried out to elucidate the specific adsorption sites for both acetylene and ethylene among these frameworks. The modeling results suggest that binding strength is highly related to aperture size and that CPL-1 shows the highest adsorption selectivity owing to the optimal pore size. This work demonstrates that engineering pore size enables us to fabricate the highly efficient metal-organic framework (MOF)-based adsorbents for specific gas separation on the basis of the isoreticular chemistry.

Fine-Tuning the Pore Environment of the Microporous Cu-MOF for High Propylene Storage and Efficient Separation of Light Hydrocarbons

Ethylene (C2H4) and propylene (C3H6) are important energy sources and raw materials in the chemical industry. Storage and separation of C2H4 and C3H6 are vital to their practical application. Metal-organic frameworks (MOFs) having adjustable structures and pore environments are promising candidates for C3H6/C2H4 separation. Herein, we obtained a Cu-based MOF synthesized by H3TTCA and pyrazine ligands. By adding different functional groups on the ligands within the MOFs, their pore environments are adjusted, and thus, the C3H6 storage capacity and C3H6/C2H4 separation efficiency are improved. Eventually, the fluoro- and methyl-functionalized iso-MOF-4 exhibits a better gas storage and C3H6/C2H4 separation performance compared with iso-MOF-1 (nonfunctionalized), iso-MOF-2 (fluoro-functionalized), and iso-MOF-3 (methyl-functionalized). A record-high C3H6 uptake of 293.6 ± 2.3 cm3 g-1 (273 K, 1 atm) is achieved using iso-MOF-4. Moreover, iso-MOF-4 shows excellent repeatability, and only 3.5% of C3H6 storage capacities decrease after nine cycles. Employing Grand Canonical Monte Carlo (GCMC) simulations, it is indicated that iso-MOF-4 preferentially adsorbs C3H6 rather than C2H4 at low pressure. Single-crystal X-ray diffraction on C3H6-adsorbed iso-MOF-4 crystals precisely demonstrates the adsorption positions and arrangement of C3H6 molecules in the framework, which is consistent with the theoretical simulations. Remarkably, gas sorption isotherms, molecular simulations, and breakthrough experiments comprehensively demonstrate that this unique MOF material exhibits highly efficient C3H6/C2H4 separation. Additionally, iso-MOF-4 also possesses efficient separation of C3H8/CH4 and C2H6/CH4, indicating its promising potential in storage/separation of light hydrocarbons in industry.

Porous metal-organic frameworks for gas storage and separation: Status and challenges

Gases are widely used as energy resources for industry and our daily life. Developing energy cost efficient porous materials for gas storage and separation is of fundamentally and industrially important, and is one of the most important aspects of energy chemistry and materials. Metal-organic frameworks (MOFs), representing a novel class of porous materials, feature unique pore structure, such as exceptional porosity, tunable pore structures, ready functionalization, which not only enables high density energy storage of clean fuel gas in MOF adsorbents, but also facilitates distinct host-guest interactions and/or sieving effects to differentiate different molecules for energy-efficient separation economy. In this review, we summarize and highlight the recent advances in the arena of gas storage and separation using MOFs as adsorbents, including progresses in MOF-based membranes for gas separation, which could afford broader concepts to the current status and challenges in this field.

Soft Porous Crystal Based upon Organic Cages That Exhibit Guest-Induced Breathing and Selective Gas Separation

Soft porous crystals (SPCs) that exhibit stimuli-responsive dynamic sorption behavior are attracting interest for gas storage/separation applications. However, the design and synthesis of SPCs is challenging. Herein, we report a new type of SPC based on a [2 + 3] imide-based organic cage (NKPOC-1) and find that it exhibits guest-induced breathing behavior. Various gases were found to induce activated NKPOC-1 crystals to reversibly switch from a "closed" nonporous phase (α) to two porous "open" phases (β and γ). The net effect is gate-opening behavior induced by CO₂ and C3 hydrocarbons. Interestingly, NKPOC-1-α selectively adsorbs propyne over propylene and propane under ambient conditions. Thus, NKPOC-1-α has the potential to separate binary and ternary C3 hydrocarbon mixtures, and the performance was subsequently verified by fixed bed column breakthrough experiments. In addition, molecular dynamics calculations and in situ X-ray diffraction experiments indicate that the gate-opening effect is accompanied by reversible structural transformations. The adsorption energies from molecular dynamics simulations aid are consistent with the experimentally observed selective adsorption phenomena. The understanding gained from this study of NKPOC-1 supports the further development of SPCs for applications in gas separation/storage because SPCs do not inherently suffer from the recyclability problems often encountered with rigid materials.

A Metal-Organic Framework Based Methane Nano-trap for the Capture of Coal-Mine Methane

As a major greenhouse gas, methane, which is directly vented from the coal-mine to the atmosphere, has not yet drawn sufficient attention. To address this problem, we report a methane nano-trap that features oppositely adjacent open metal sites and dense alkyl groups in a metal-organic framework (MOF). The alkyl MOF-based methane nano-trap exhibits a record-high methane uptake and CH₄ /N₂ selectivity at 298 K and 1 bar. The methane molecules trapped within the alkyl MOF were crystalographically identified by single-crystal X-ray diffraction experiments, which in combination with molecular simulation studies unveiled the methane adsorption mechanism within the MOF-based nano-trap. The IAST calculations and the breakthrough experiments revealed that the alkyl MOF-based methane nano-trap is a new benchmark for CH₄ /N₂ separation, thereby providing a new perspective for capturing methane from coal-mine methane to recover fuel and reduce greenhouse gas emissions.

Our journey of developing multifunctional metal-organic frameworks

Metal-organic frameworks (MOFs) are organic-inorganic hybrid solids constructed from the coordination interaction of metal ions/clusters with organic linkers, which currently represent one of the most rapidly expanding platforms for new functional materials. Based on well-established approaches, involving tuning the pore sizes, incorporation of functional sites and post-synthetic modification, the pore structures of MOFs can be readily controlled for multifunctional applications. In this brief review, we summarize and highlight our research progresses during our journey on developing functional MOFs for various applications including gas storage, gas separations, luminescent sensing, proton conduction, and molecular recognitions.

Reaction duration-dependent formation of two Cu(ii)-MOFs with selective adsorption properties of C3H4 over C3H6

Reaction duration was found to play an important role in the formation of two different MOFs, BUT-301 and BUT-302, which all show three-dimensional framework structures, permanent porosity and selective adsorption properties of C₃H₄ over C₃H₆.