Absolute molecular sieve separation of ethylene/ethane mixtures with silver zeolite A

Molecular recognition with resolution below 0.2 angstroms through thermoregulatory oscillations in covalent organic frameworks

Crystalline materials with uniform molecular-sized pores are desirable for a broad range of applications, such as sensors, catalysis, and separations. However, it is challenging to tune the pore size of a single material continuously and to reversibly distinguish small molecules (below 4 angstroms). We synthesized a series of ionic covalent organic frameworks using a tetraphenoxyborate linkage that maintains meticulous synergy between structural rigidity and local flexibility to achieve continuous and reversible (100 thermal cycles) tunability of "dynamic pores" between 2.9 and 4.0 angstroms, with resolution below 0.2 angstroms. This results from temperature-regulated, gradual amplitude change of high-frequency linker oscillations. These thermoelastic apertures selectively block larger molecules over marginally smaller ones, demonstrating size-based molecular recognition and the potential for separating challenging gas mixtures such as oxygen/nitrogen and nitrogen/methane.

Balancing the Kinetic and Thermodynamic Synergetic Effect of Doped Carbon Molecular Sieves for Selective Separation of C2H4/C2H6

Selective separation of ethylene and ethane (C2H4/C2H6) is a formidable challenge due to their close molecular size and boiling point. Compared to industry-used cryogenic distillation, adsorption separation would offer a more energy-efficient solution when an efficient adsorbent is available. Herein, a class of C2H4/C2H6 separation adsorbents, doped carbon molecular sieves (d-CMSs) is reported which are prepared from the polymerization and subsequent carbonization of resorcinol, m-phenylenediamine, and formaldehyde in ethanol solution. The study demonstrated that the polymer precursor themselves can be a versatile platform for modifying the pore structure and surface functional groups of their derived d-CMSs. The high proportion of pores centered at 3.5 Å in d-CMSs contributes significantly to achieving a superior kinetic selectivity of 205 for C2H4/C2H6 separation. The generated pyrrolic-N and pyridinic-N functional sites in d-CMSs contribute to a remarkable elevation of Henry selectivity to 135 due to the enhancement of the surface polarity in d-CMSs. By balancing the synergistic effects of kinetics and thermodynamics, d-CMSs achieve efficient separation of C2H4/C2H6. Polymer-grade C2H4 of 99.71% purity can be achieved with 75% recovery using the devised d-CMSs as reflected in a two-bed vacuum swing adsorption simulation.

Enhanced Adsorption of Trace Ethylene on Ag/NZ5 Modified with Ammonia: Hierarchical Structure and Metal Dispersion Effects

Trace ethylene poses a significant challenge during the storage and transportation of agricultural products, causing over-ripening, reducing shelf life, and leading to food waste. Zeolite-supported silver adsorbents show promise for efficiently removing trace ethylene. Herein, hierarchical Ag/NZ5(X) adsorbents were prepared via different ammonia modifications, which featured enhanced ethylene adsorption ability. Ag/NZ5(2.5) exhibited the largest capacity and achieved near-complete removal at room temperature with prolonged efficacy. Characterization results indicated that the ammonia modification led to the formation of a hierarchical structure in the zeolite framework, reducing diffusion resistance and increasing the accessibility of the active sites. Additionally, desilication effects increased the defectiveness, generating a stronger metal-support interaction and resulting in a higher metal dispersion rate. These findings provide valuable insights into the development of efficient adsorbents for removing trace ethylene, thereby reducing food waste and extending the shelf life of agricultural products.

Adsorptive interaction between typical VOCs and various topological zeolites: Mixture effect and mechanism

Adsorption is one of the most feasible and effective methods to alleviate the volatile organic compounds (VOCs) pollution. However, the mixture effect and mechanism for competitive adsorption of VOCs on zeolites are barely addressed. In this study, toluene, acetone, and ethyl acetate as prevalent VOCs species were removed by four potential zeolites (13X, USY, Beta, ZSM-5) in both single- and multi-component systems. The structure-property relationship between adsorbate-adsorbent pairs was revealed by X-ray diffraction, scanning electron microscopy, energy dispersive spectrometer, X-ray fluorescence, N2 adsorption and density function theory calculation. The molecular polarity and volatility of VOCs species played key roles in adsorption and the dynamic uptakes were generally listed as follows: ethyl acetate > toluene > acetone. As for the above VOCs mixtures, 13X zeolite selectively adsorbed oxygenated VOCs rather than toluene. In contrast, USY exhibited a preference to trap toluene. Ethyl acetate could be readily enriched by ZSM-5 and Beta selectively. The possible explanations and implications are discussed based on the subtle change in electron density. The results obtained are vital for understanding the mixture effect of VOCs adsorption and may guide the selection of proper adsorbent for real applications.

Inverse Adsorption Separation of N2 O/CO2 in AgZK-5 Zeolite

Nitrous oxide (N2 O), as the third largest greenhouse gas in the world, also has great applications in daily life and industrial production, like anesthetic, foaming agent, combustion supporting agent, N or O atomic donor. The capture of N2 O in adipic acid tail gas is of great significance but remains challenging due to the similarity with CO2 in molecular size and physical properties. Herein, the influence of cation types on CO2 -N2 O separation in zeolite was studied comprehensively. In particular, the inverse adsorption of CO2 -N2 O was achieved by AgZK-5, which preferentially adsorbs N2 O over CO2 , making it capable of trapping N2 O from an N2 O/CO2 mixture. AgZK-5 shows a recorded N2 O/CO2 selectivity of 2.2, and the breakthrough experiment indicates excellent performance for N2 O/CO2 separation. The density functional theory (DFT) calculation shows that Ag+ has stronger adsorption energy with N2 O, and the kinetics of N2 O is slightly faster than that of CO2 on AgZK-5.

A Compact Monitor for Ethylene and Other Plant-Produced Volatile Organic Compounds for NASA Space Missions

In this work, we discuss the development of a compact analytical instrument for monitoring ethylene in compact greenhouses utilized by NASA to grow fresh vegetables in space. Traditionally, ethylene measurements are conducted by GC-MS systems. However, in space, they are not applicable due to their bulky size, heavy weight, special carrier gas requirement and high maintenance. Our group developed a compact and robust battery-powered ethylene monitor based on the principles of analytical gas chromatography. The device utilizes purified ambient air as a carrier gas and a metal oxide sensor as a GC detector. Implementation of a CarboWax 20 M packed column from Restek together with a Tenax TA pre-concentrator allowed us to achieve a 20 ppb limit of detection for ethylene. Full automation of measurements and reporting of concentrations was accomplished via the implementation of a Raspberry Pi 4 computer and a 7″ 720P LED capacitive touchscreen utilized for data output. Based on a feasibility study, a fully automated, industrial-grade ethylene monitoring and removal system for greenhouses was developed.

Revolutionizing Porous Liquids: Stabilization and Structural Engineering Achieved by a Surface Deposition Strategy

Facile approaches capable of constructing stable and structurally diverse porous liquids (PLs) that can deliver high-performance applications are a long-standing, captivating, and challenging research area that requires significant attention. Herein, a facile surface deposition strategy is demonstrated to afford diverse type III-PLs possessing ultra-stable dispersion, external structure modification, and enhanced performance in gas storage and transformation by leveraging the expeditious and uniform precipitation of selected metal salts. The Ag(I) species-modified zeolite nanosheets are deployed as the porous host to construct type III-PLs with ionic liquids (ILs) containing bromide anion , leading to stable dispersion driven by the formation of AgBr nanoparticles. The as-afforded type-III PLs display promising performance in CO2 capture/conversion and ethylene/ethane separation. Property and performance of the as-produced PLs can be tuned by the cation structure of the ILs, which can be harnessed to achieve polarity reversal of the porous host via ionic exchange. The surface deposition procedure can be further extended to produce PLs from Ba(II)-functionalized zeolite and ILs containing [SO4 ]2- anion driven by the formation of BaSO4 salts. The as-produced PLs are featured by well-maintained crystallinity of the porous host, good fluidity and stability, enhanced gas uptake capacity, and attractive performance in small gas molecule utilization.

Probing sub-5 Ångstrom micropores in carbon for precise light olefin/paraffin separation

Olefin/paraffin separation is an important but challenging and energy-intensive process in petrochemical industry. The realization of carbons with size-exclusion capability is highly desirable but rarely reported. Herein, we report polydopamine-derived carbons (PDA-Cx, where x refers to the pyrolysis temperature) with tailorable sub-5 Å micropore orifices together with larger microvoids by one-step pyrolysis. The sub-5 Å micropore orifices centered at 4.1-4.3 Å in PDA-C800 and 3.7-4.0 Å in PDA-C900 allow the entry of olefins while entirely excluding their paraffin counterparts, performing a precise cut-off to discriminate olefin/paraffin with sub-angstrom discrepancy. The larger voids enable high C₂H₄ and C₃H₆ capacities of 2.25 and 1.98 mmol g^-1 under ambient conditions, respectively. Breakthrough experiments confirm that a one-step adsorption-desorption process can obtain high-purity olefins. Inelastic neutron scattering further reveals the host-guest interaction of adsorbed C₂H₄ and C₃H₆ molecules in PDA-Cx. This study opens an avenue to exploit the sub-5 Å micropores in carbon and their desirable size-exclusion effect.

Zeolites in Adsorption Processes: State of the Art and Future Prospects

Zeolites have been widely used as catalysts, ion exchangers, and adsorbents since their industrial breakthrough in the 1950s and continue to be state-of the-art adsorbents in many separation processes. Furthermore, their properties make them materials of choice for developing and emerging separation applications. The aim of this review is to put into context the relevance of zeolites and their use and prospects in adsorption technology. It has been divided into three different sections, i.e., zeolites, adsorption on nanoporous materials, and chemical separations by zeolites. In the first section, zeolites are explained in terms of their structure, composition, preparation, and properties, and a brief review of their applications is given. In the second section, the fundamentals of adsorption science are presented, with special attention to its industrial application and our case of interest, which is adsorption on zeolites. Finally, the state-of-the-art relevant separations related to chemical and energy production, in which zeolites have a practical or potential applicability, are presented. The replacement of some of the current separation methods by optimized adsorption processes using zeolites could mean an improvement in terms of sustainability and energy savings. Different separation mechanisms and the underlying adsorption properties that make zeolites interesting for these applications are discussed.

On the Role of Flexibility for Adsorptive Separation

Chemical separations, mostly based on heat-driven techniques such as distillation, account for a large portion of the world's energy consumption. In principle, differential adsorption is a more energy-efficient separation method, but conventional adsorbent materials are still not effective for many industry-relevant mixtures. Porous coordination polymers (PCPs), or metal-organic frameworks (MOFs), are attractive for their well-defined, designable, modifiable, and flexible structures connecting to various potential applications. While the importance of the structural flexibility of MOFs in adsorption-based functions has been demonstrated, the understanding of this special feature is still in its infancy and mostly stays at the periodic structural transformation at the equilibrium state and the special shapes of single-component adsorption isotherms. There are many confusions about the categorization and roles of various types of flexibility. This Account discusses the role of flexibility of MOFs for adsorptive separation, mainly from the thermodynamic and kinetic points of view.As the classic type of framework flexibility, guest-driven structural transformations and the corresponding adsorption isotherms can be thermodynamically described by the energies of the host-guest system. The highly guest-specific pore-opening action showing contrasting single-component adsorption isotherms is regarded as a strategy for achieving molecular sieving without the need for aperture size control, but its effect and role for mixture separation are still controversial. Quantitative mixture adsorption/separation experiments showed that the common periodic (cooperative) pore-opening action leads to coadsorption of molecules smaller than the opened aperture, while the aperiodic (noncooperative) one can achieve inversed molecular sieving under a thermodynamic mechanism.The energy barrier and structure in the nonequilibrium state are also important for flexibility and adsorption/separation. With suitable energy barriers between metastable structures, new types of framework flexibility such as aperture gating can be realized. While kinetically controlled gating flexibility is usually ignored because of the difficulty of characterization or considered as disadvantageous for separation because of the variable aperture size, it plays a critical role in most kinetic separation systems, including adsorbents conventionally regarded as rigid. With the concept of gating flexibility, the meanings of aperture and guest sizes for judging molecular sieving need to be reconsidered. Gating flexibility depends on not only the host itself but also the guest, the host-guest interaction, and the external environment such as temperature, which can be rationally tuned to achieve special adsorption/separation behaviors such as inversed temperature dependence, molecular sieving, and even inversed thermodynamic selectivity. The comprehensive understanding of the thermodynamic and kinetic bases of flexibility will give a new horizon for next-generation separation materials beyond MOFs and adsorbents.

In Situ Synthesis of Multivariate Zeolitic Imidazolate Frameworks for C2 H4 /C2 H6 Kinetic Separation

Herein, a new approach for the in situ synthesis of zeolitic imidazolate framework (ZIF) nanoparticles with triple ligands, referred to as Sogang ZIF-8 (SZIF-8), is reported for enhanced C₂ H₄ /C₂ H₆ kinetic separation. SZIF-8 consists of tetrahedral zinc metals coordinated with tri-butyl amine (TBA), 2,4-dimethylimidazole (DIm), and 2-methylimidazole (MIm). SZIF-8(x) with different DIm contents in x (up to 23.2 mol%) are synthesized in situ because TBA preferably deprotonates DIm ligands due to the much lower pK_a of DIm over MIm, allowing for the Zn-DIm coordination. The Zn-DIm coordination reduces the window size of ZIF-8 with suppressed linker flipping motion due to bulky DIm ligands and simultaneously enhances the interfacial interaction between 6FDA-DAM polyimide (6FDA) and SZIF-8 via electron donor-acceptor interactions. Consequently, 6FDA/SZIF-8(13) mixed matrix membrane exhibits an excellent C₂ H₄ permeability of 60.3 Barrer and C₂ H₄ /C₂ H₆ selectivity of 4.5. The temperature-dependent transport characterization reveals that such excellent C₂ H₄ /C₂ H₆ kinetic separation is attained by the enhancement in size discrimination-based energetic selectivity. Our hybrid multi-ligand approach can offer a useful tool for the fine-tuning of molecular structures and textural properties of other metal organic frameworks.

Temperature-Dependent Superhydrophobic Functionalized Coordination Polymers (SFCPs) for Selective Adsorption of C2H4 over C2H6

We prepared two new superhydrophobic functionalized coordination polymers (SFCPs) [Zn₄(OH)₂(BTMB)₂(4,4'-Bipy)₂]_∞ ⊃ solvent, 1, and [Cd₄(OH)₂(BTMB)₂(4,4'-Bipy)₃]_∞ ⊃ solvent, 2, by solvothermal methods. For 1, the single-crystal XRD structure revealed that it contains two crystallographically distinct Zn²⁺ ions with two different types of coordination geometries of 4 and 6, exhibiting a unique superhydrophobic behavior with microporosity. Compound 1 exhibits superhydrophobicity with a contact angle of 155.5° (at 30 °C), which is stable even at high temperatures, whereas for the SFCP 2, all of the Cd²⁺ ions have only 6-coordination and exhibit a superhydrophobic character at room temperature with a contact angle of 156.7°(at 30 °C). However, surprisingly, this superhydrophobic character is stable only up to 60 °C, above which it is converted to hydrophilic nature, in contrast to the SFCP 1. Moreover, in this study, we also report a selective gas adsorption study of two C2 gases with similar kinetic diameters (∼3.9 Å) of ethylene over ethane.

Recent Advances in Ethylene Gas Detection

The real-time detecting and monitoring of ethylene gas molecules could benefit the agricultural, horticultural and healthcare industries. In this regard, we comprehensively review the current state-of-the-art ethylene gas sensors and detecting technologies, covering from preconcentrator-equipped gas chromatographic systems, Fourier transform infrared technology, photonic crystal fiber-enhanced Raman spectroscopy, surface acoustic wave and photoacoustic sensors, printable optically colorimetric sensor arrays to a wide range of nanostructured chemiresistive gas sensors (including the potentiometric and amperometric-type FET-, CNT- and metal oxide-based sensors). The nanofabrication approaches, working conditions and sensing performance of these sensors/technologies are carefully discussed, and a possible roadmap for the development of ethylene detection in the near future is proposed.

Direct Visualization of Supramolecular Binding and Separation of Light Hydrocarbons in MFM-300(In)

The purification of light olefins is one of the most important chemical separations globally and consumes large amounts of energy. Porous materials have the capability to improve the efficiency of this process by acting as solid, regenerable adsorbents. However, to develop translational systems, the underlying mechanisms of adsorption in porous materials must be fully understood. Herein, we report the adsorption and dynamic separation of C₂ and C₃ hydrocarbons in the metal-organic framework MFM-300(In), which exhibits excellent performance in the separation of mixtures of ethane/ethylene and propyne/propylene. Unusually selective adsorption of ethane over ethylene at low pressure is observed, resulting in selective retention of ethane from a mixture of ethylene/ethane, thus demonstrating its potential for a one-step purification of ethylene (purity > 99.9%). In situ neutron powder diffraction and inelastic neutron scattering reveal the preferred adsorption domains and host-guest binding dynamics of adsorption of C₂ and C₃ hydrocarbons in MFM-300(In).

Evaluation of Iron-Based Metal-Organic Framework Activation Temperatures in Acetylene Adsorption

One of the major issues regarding long-term human space exploration is the need for a breathable atmosphere. A major component toward achieving this goal is both the removal of exhaled carbon dioxide (CO₂) and the generation or recovery of oxygen (O₂). NASA's current technology only operates at about 50% efficiency due to the need to vent the methane that is produced during the CO₂ reduction process. One method of improving the efficiency of this process is through plasma pyrolysis, wherein the methane is pyrolyzed to produce hydrogen and various dehydrogenated carbon byproducts. In this process, acetylene is one of the main components of this byproduct stream. Unfortunately, while the concentration of this effluent is generally high in hydrogen (>90% typically), the presence of the acetylene waste product can act as a poison for the ruthenium-alumina catalyst used in the CO₂-reducing Sabatier process, requiring a removal step. Metal-organic frameworks (MOFs) represent a valuable method for removing these unsaturated hydrocarbons due to their high tunability, particularly through the incorporation of open metal sites. In this study, two common iron-based MOFs, MIL-100 and PCN-250, were studied for their ability to adsorb acetylene. A combination of gas adsorption analysis and density functional theory calculation results shows the ability of these materials to undergo a thermal-induced reduction event, which results in an improvement in gas adsorption performance. This improvement in gas performance appears to be at least partially due to the increased presence of π-backbonding toward the acetylene molecules.

Preferential adsorption sites for propane/propylene separation on ZIF-8 as revealed by solid-state NMR spectroscopy

Solid-state NMR spectroscopy in conjunction with theoretical calculation was employed to investigate the adsorbent-adsorbate host-guest interactions during propane/propylene separation on ZIF-8. ¹H NMR chemical shifts of free gaseous and adsorbed propane/propylene are unambiguously assigned with the assistance of two-dimensional (2D) ¹H-¹H correlation spectroscopy (COSY) MAS NMR spectra. Meanwhile, the adsorption selectivity for propane/propylene mixtures on ZIF-8 at a pressure in range of 1.9-9.6 bar is quantitatively determined using ¹H MAS NMR experiments, which agreed well with the ideal adsorbed solution theory (IAST) predictions. The preferential adsorption of propane compared with propylene on ZIF-8 is directly visualized from the 2D ¹H-¹H spin diffusion homo-nuclear correlation (HOMCOR) MAS NMR spectroscopy. Moreover, the preferential adsorption sites for propane and propylene are deduced from the ¹H-¹H spin diffusion buildup curves, which is further confirmed by DFT theoretical calculations. This work provides insights to understand the structure-property relationship during the propane/propylene separation on ZIF-8 as adsorbent.

Porous frameworks from Ecuadorian clays

This research provides a literature review on several topics as a foundation to comprehend porous materials, their structure, and behavior to explore how they can be derived from clays and nanoclays. In this case, considering the several minerals present in some Ecuadorian clays, which are a potential starting material for the synthesis of porous frameworks, they constitute a solid source of metal atoms such as Silicon or Aluminum. This research presents the evaluation and characterization via XRD and AAS of clay samples collected in the southeast of Ecuador in the provinces of Azuay, Morona Santiago and Zamora Chinchipe, which present diversified soil mineralogy with many chemical and crystallographic features for suitable precursors in nanomaterials design.

Recent Advances in Mixed-Matrix Membranes for Light Hydrocarbon (C1–C3) Separation

Light hydrocarbons, obtained through the petroleum refining process, are used in numerous applications. The separation of the various light hydrocarbons is challenging and expensive due to their similar melting and boiling points. Alternative methods have been investigated to supplement cryogenic distillation, which is energy intensive. Membrane technology, on the other hand, can be an attractive alternative in light hydrocarbon separation as a phase change that is known to be energy-intensive is not required during the separation. In this regard, this study focuses on recent advances in mixed-matrix membranes (MMMs) for light hydrocarbon (C1–C3) separation based on gas permeability and selectivity. Moreover, the future research and development direction of MMMs in light hydrocarbon separation is discussed, considering the low intrinsic gas permeability of polymeric membranes.

Incorporating Flexibility Effects into Metal-Organic Framework Adsorption Simulations Using Different Models

High-throughput calculations based on molecular simulations to predict the adsorption of molecules inside metal-organic frameworks (MOFs) have become a useful complement to experimental efforts to identify promising adsorbents for chemical separations and storage. For computational convenience, all existing efforts of this kind have relied on simulations in which the MOF is approximated as rigid. In this paper, we use extensive adsorption-relaxation simulations that fully include MOF flexibility effects to explore the validity of the rigid framework approximation. We also examine the accuracy of several approximate methods to incorporate framework flexibility that are more computationally efficient than adsorption-relaxation calculations. We first benchmark various models of MOF flexibility for four MOFs with well-established CO₂ experimental consensus isotherms. We then consider a range of adsorption properties, including Henry's constants, nondilute loadings, and adsorption selectivity, for seven adsorbates in 15 MOFs randomly selected from the CoRE MOF database. Our results indicate that in many MOFs adsorption-relaxation simulations are necessary to make quantitative predictions of adsorption, particularly for adsorption at dilute concentrations, although more standard calculations based on rigid structures can provide useful information. Finally, we investigate whether a correlation exists between the elastic properties of empty MOFs and the importance of including framework flexibility in making accurate predictions of molecular adsorption. Our results did not identify a simple correlation of this type.

Post-Synthesis Functionalization Enables Fine-Tuning the Molecular-Sieving Properties of Zeolites for Light Olefin/Paraffin Separations

Zeolite molecular sieves are widely used in gas separation and shape-selective catalysis, but these applications often require discriminating differences as little as 0.1 Å. Molecular sieving with such size selectivity demands zeolites with highly tunable pore diameters and adsorption properties, which are technically challenging to prepare. Nevertheless, it is shown that a wide range of organic functional groups can be covalently functionalized onto the interior pore walls of the zeolites, MOR, LTL, FAU, and MFI, to systematically "tune" their effective pore diameters with respect to the size of organic groups. For organic functionalization, small and aggressive organic electrophiles are used (e.g., organo-halide and -diazonium) as grafting agents, which are accessible to the intracrystalline void space, forming a C-O_zeolite bond in a reaction with a bridging oxygen as proved by multiple analysis data. It is demonstrated that the post-functionalization can be used to tailor the molecular sieving action of a parent zeolite to give size-selective adsorbents for light olefin/paraffin separations. 4-Methoxybenzene-functionalized MOR separates ethylene from ethane with an ideal-adsorbed-solution-theory selectivity of ≈5873, whereas toluene-grafted MOR completely separates propylene/propane mixtures. Therefore, tailoring the molecular-sieving properties of zeolites by organic functionalization broadens their applications to challenging separations.

A Molecular Compound for Highly Selective Purification of Ethylene

Purification of C₂ H₄ from an C₂ H₄ /C₂ H₆ mixture is one of the most challenging separation processes, which is achieved mainly through energy-intensive, cryogenic distillation in industry. Sustainable, non-distillation methods are highly desired as alternatives. We discovered that the fluorinated bis(pyrazolyl)borate ligand supported copper(I) complex {[(CF₃ )₂ Bp]Cu}₃ has features very desirable in an olefin-paraffin separation material. It binds ethylene exclusively over ethane generating [(CF₃ )₂ Bp]Cu(C₂ H₄ ). This molecular compound exhibits extremely high and record ideal adsorbed solution theory (IAST) C₂ H₄ /C₂ H₆ gas separation selectivity, affording high purity (>99.5 %) ethylene that can be readily desorbed from separation columns. In-situ PXRD provides a "live" picture of the reversible conversion between [(CF₃ )₂ Bp]Cu(C₂ H₄ ) and the ethylene-free sorbent in the solid-state, driven by the presence or removal of C₂ H₄ . Molecular structures of trinuclear {[(CF₃ )₂ Bp]Cu}₃ and mononuclear [(CF₃ )₂ Bp]Cu(C₂ H₄ ) are also presented.

Ethylene/ethane separation in a stable hydrogen-bonded organic framework through a gating mechanism

Porous materials are very promising for the development of cost- and energy-efficient separation processes, such as for the purification of ethylene from ethylene/ethane mixture-an important but currently challenging industrial process. Here we report a microporous hydrogen-bonded organic framework that takes up ethylene with very good selectivity over ethane through a gating mechanism. The material consists of tetracyano-bicarbazole building blocks held together through intermolecular CN···H-C hydrogen bonding interactions, and forms as a threefold-interpenetrated framework with pores of suitable size for the selective capture of ethylene. The hydrogen-bonded organic framework exhibits a gating mechanism in which the threshold pressure required for guest uptake varies with the temperature. Ethylene/ethane separation is validated by breakthrough experiments with high purity of ethylene (99.1%) at 333 K. Hydrogen-bonded organic frameworks are usually not robust, yet this material was stable under harsh conditions, including exposure to strong acidity, basicity and a variety of highly polar solvents.

Elimination or Removal of Ethylene for Fruit and Vegetable Storage via Low-Temperature Catalytic Oxidation

Ethylene acts as an important hormone to trigger the ripening and senescence of fruits and vegetables (F&V). Thus, it is essential to eliminate trace ethylene and prevent F&V losses effectively. There are several technologies currently applying to control the ethylene concentration in the storage and transportation environment, including adsorption, gene modification, oxidation, etc. These protocols will be compared, and special attention will be paid to the low-temperature catalytic oxidation that has already been applied to practical production in this review. The active sites, supports, and reaction and deactivation mechanism of the catalysts for the low-temperature ethylene oxidation will be discussed and evaluated systematically to provide new insights for the development of effective catalysts, along with the suggestion of some perspectives for future research on this important catalytic system for F&V preservation.

Separation and Purification of Hydrocarbons with Porous Materials

This Minireview focuses on the developments of the adsorptive separation of methane/nitrogen, ethene/ethane, propene/propane mixtures as well as on the separation of C8 aromatics (i.e. xylene isomers) with a wide variety of materials, including carbonaceous materials, zeolites, metal-organic frameworks, and porous organic frameworks. Some recent important developments for these adsorptive separations are also highlighted. The advantages and disadvantages of each material category are discussed and guidelines for the design of improved materials are proposed. Furthermore, challenges and future developments of each material type and separation processes are discussed.

Graphyne-3: a highly efficient candidate for separation of small gas molecules from gaseous mixtures

Two-dimensional nanosheets, such as the general family of graphenes have attracted considerable attention over the past decade, due to their excellent thermal, mechanical, and electrical properties. We report on the result of a study of separation of gaseous mixtures by a model graphyne-3 membrane, using extensive molecular dynamics simulations and density functional theory. Four binary and one ternary mixtures of H[Formula: see text], CO[Formula: see text], CH[Formula: see text] and C[Formula: see text]H[Formula: see text] were studied. The results indicate the excellence of graphyne-3 for separation of small gas molecules from the mixtures. In particular, the H[Formula: see text] permeance through the membrane is on the order of [Formula: see text] gas permeation unit, by far much larger than those in other membranes, and in particular in graphene. To gain deeper insights into the phenomenon, we also computed the density profiles and the residence times of the gases near the graphyne-3 surface, as well as their interaction energies with the membrane. The results indicate clearly the tendency of H[Formula: see text] to pass through the membrane at high rates, leaving behind C[Formula: see text]H[Formula: see text] and larger molecules on the surface. In addition, the possibility of chemisorption is clearly ruled out. These results, together with the very good mechanical properties of graphyne-3, confirm that it is an excellent candidate for separating small gas molecules from gaseous mixtures, hence opening the way for its industrial use.

Zeolitic Octahedral Metal Oxides with Ultra-Small Micropores for C2 Hydrocarbon Separation

Separation of C₂ hydrocarbons, C₂ H₆ , C₂ H₄ , and C₂ H₂ , remains significant challenges in chemical industry. However, there are only few adsorbents that can effectively isolate C₂ hydrocarbons from their mixtures particularly at a high temperature. Herein, we design a zeolitic octahedral metal oxide based on ϵ-Keggin polyoxometalates with metal ion linkers. Single gas adsorption of the material shows the different adsorption performances for the C₂ hydrocarbons and the strong interaction of the material with the C₂ hydrocarbons. Dynamic competitive adsorption experiments show that the material efficiently separates each of the binary C₂ hydrocarbon mixtures and even the ternary C₂ hydrocarbon mixtures with high selectivity. The material keeps high separation performance even the temperature was increased to 85 °C. The material is stable and is able to be reused without loss of the separation performance.

Host-Guest Interaction in Ethylene and Ethane Separation on Zeolitic Imidazolate Frameworks as Revealed by Solid-State NMR Spectroscopy

The separation of ethane/ethylene mixture by using metal-organic frameworks (MOFs) as adsorbents is strongly associated with the pore size-sieving effect and the adsorbent-adsorbate interaction. Herein, solid-state NMR spectroscopy is utilized to explore the host-guest interaction and ethane/ethylene separation mechanism on zeolitic imidazolate frameworks (ZIFs). Preferential access to the ZIF-8 and ZIF-8-90 frameworks by ethane compared to ethylene is directly visualized from two-dimensional ¹ H-¹ H spin diffusion MAS NMR spectroscopy and further verified by computational density distributions. The ¹ H MAS NMR spectroscopy provides an alternative for straightforwardly extracting the adsorption selectivity of ethane/ethylene mixture at 1.1∼9.6 bar in ZIFs, which is consistent with the IAST predictions.

Octanuclear Cobalt(II) Cluster-Based Metal-Organic Framework with Caged Structure Exhibiting the Selective Adsorption of Ethane over Ethylene

A novel metal-organic framework (MOF) of [Co₈(OH)₄(TCA)₄(H₂O)₄]_n (abbreviation: JXNU-9) based on the unique octanuclear Co₈(μ₃-OH)₄ clusters linked by 4,4',4″-nitrilotribenzoate (TCA^3-) ligands featuring small caged structures and one-dimensional channels was prepared and characterized. JXNU-9 shows a high C₂H₆ uptake capacity of 3.60 mmol g^-1 (4.46 mmol cm^-3) at 298 K and 1 atm with a small isosteric heat of adsorption (23.6 kJ mol^-1) and a moderate C₂H₆/C₂H₄ adsorption selectivity of 1.7, resulting in excellent C₂H₆/C₂H₄ separation performance. The pore walls decorated by plenty of aromatic rings provide π-electron-cloud-surrounding environments to accommodate the large polarizable C₂H₆ molecules. The calculations demonstrate that the rich π-systems in JXNU-9 facilitate an adsorption affinity for large C₂H₆ molecules through multiple C-H···π interactions. Additionally, the open metal sites located in the concave pores with a close Co···Co separation (4.21 Å) in octanuclear Co₈(μ₃-OH)₄ clusters make the open metal sites inaccessible for the C₂H₄ molecule with a kinetic diameter of 4.163 Å. Thus, the annihilation of open metal sites in this structure is achieved, which further facilitates the C₂H₆-selective C₂H₆/C₂H₄ separation.

Benchmark C2 H2 /CO2 Separation in an Ultra-Microporous Metal-Organic Framework via Copper(I)-Alkynyl Chemistry

Separation of acetylene from carbon dioxide remains a daunting challenge because of their very similar molecular sizes and physical properties. We herein report the first example of using copper(I)-alkynyl chemistry within an ultra-microporous MOF (Cu^I @UiO-66-(COOH)₂ ) to achieve ultrahigh C₂ H₂ /CO₂ separation selectivity. The anchored Cu^I ions on the pore surfaces can specifically and strongly interact with C₂ H₂ molecule through copper(I)-alkynyl π-complexation and thus rapidly adsorb large amount of C₂ H₂ at low-pressure region, while effectively reduce CO₂ uptake due to the small pore sizes. This material thus exhibits the record high C₂ H₂ /CO₂ selectivity of 185 at ambient conditions, significantly higher than the previous benchmark ZJU-74a (36.5) and ATC-Cu (53.6). Theoretical calculations reveal that the unique π-complexation between Cu^I and C₂ H₂ mainly contributes to the ultra-strong C₂ H₂ binding affinity and record selectivity. The exceptional separation performance was evidenced by breakthrough experiments for C₂ H₂ /CO₂ gas mixtures. This work suggests a new perspective to functionalizing MOFs with copper(I)-alkynyl chemistry for highly selective separation of C₂ H₂ over CO₂ .

Different behavior of two iso-structural zeolitic imidazolate frameworks in ethane-ethylene separation: Study of H2 -CO2 and CH4 -CO2 separation capacity

The separation of ethane-ethylene mixtures was evaluated using two zeolitic imidazolate frameworks, which are only different from the assemble metal, as stationary phase from inverse gas chromatography data. The chromatographic profiles exhibited peaks adequately resolved. Separation is attributed to the pore sizes of the adsorbents that discern between ethane and ethylene molecule shapes and closed kinetic diameters. The adsorption heats were estimated for each probe molecule from the slopes of the straight lines at four temperatures. The evaluated materials are iso-structural compounds with the unique difference of assembling metal (cobalt and zinc). Cobalt material showed atypical adsorption of ethane over ethylene, which was observed taking into account the retarded elution of the paraffin. Therefore, the anomalous behavior could be ascribed to the presence of cobalt(II). Structural characterization of both materials was performed by X-ray powder diffraction, X-ray photoelectronic spectroscopy, and thermogravimetry, while morphological characterization was performed by scanning electron microscopy. H₂ -CO₂ and CH₄ -CO₂ mixtures separation were also evaluated by inverse gas chromatography. Both materials were able to separate these two mixtures. CO₂ was the highest retained probe molecule due to the presence of quadrupole moment.

Understanding the Effect of Water on CO2 Adsorption

Carbon capture from large sources and ambient air is one of the most promising strategies to curb the deleterious effect of greenhouse gases. Among different technologies, CO₂ adsorption has drawn widespread attention mostly because of its low energy requirements. Considering that water vapor is a ubiquitous component in air and almost all CO₂-rich industrial gas streams, understanding its impact on CO₂ adsorption is of critical importance. Owing to the large diversity of adsorbents, water plays many different roles from a severe inhibitor of CO₂ adsorption to an excellent promoter. Water may also increase the rate of CO₂ capture or have the opposite effect. In the presence of amine-containing adsorbents, water is even necessary for their long-term stability. The current contribution is a comprehensive review of the effects of water whether in the gas feed or as adsorbent moisture on CO₂ adsorption. For convenience, we discuss the effect of water vapor on CO₂ adsorption over four broadly defined groups of materials separately, namely (i) physical adsorbents, including carbons, zeolites and MOFs, (ii) amine-functionalized adsorbents, and (iii) reactive adsorbents, including metal carbonates and oxides. For each category, the effects of humidity level on CO₂ uptake, selectivity, and adsorption kinetics under different operational conditions are discussed. Whenever possible, findings from different sources are compared, paying particular attention to both similarities and inconsistencies. For completeness, the effect of water on membrane CO₂ separation is also discussed, albeit briefly.

Chemically Stable Hafnium-Based Metal-Organic Framework for Highly Efficient C2H6/C2H4 Separation under Humid Conditions

Realization of ethane-selective porous materials for efficient ethane/ethylene (C₂H₆/C₂H₄) separation is an important task in the petrochemical industry. Although a number of C₂H₆-selective adsorbents have been realized, their adsorption capacity and selectivity might be mostly dampened under humid conditions due to structure decomposition or co-adsorption of water vapor. A desired material should have simultaneously high C₂H₆ uptake and selectivity, excellent water stability, and ultralow water adsorption uptake for industrial applications, but such a material is elusive. Herein, we report a chemically stable hafnium-based material (Hf)DUT-52a, featuring the suitable pore apertures and less hydrophilicity for highly efficient C₂H₆/C₂H₄ separation under humid conditions. Gas sorption results reveal that (Hf)DUT-52a exhibits both high ethane adsorption capacity (4.02 mmol g^-1) and C₂H₆/C₂H₄ selectivity (1.9) at 296 K and 1 bar, which are comparable to the majority of the top-performing materials. Most importantly, the less pore hydrophilicity enables (Hf)DUT-52a to exhibit a negligible water uptake of 0.036 g g^-1 before 40% relative humidity (RH), effectively minimizing the impact of humidity on separation capacity. This material thus shows excellent separation capacity even under 40% RH with a high polymer-grade ethylene production capacity up to 8.43 L kg^-1 at ambient conditions, as evidenced by the breakthrough experiments.

Modifying a partial corn-sql layer-based (3,3,3,3,4,4)-c topological MOF by substitution of OH- with Cl- and its highly selective adsorption of C2 hydrocarbons over CH4

Modifying VNU-18 (a MOF with a partial cone-sql layer pillared by the chains of pillars decorated with OH^-) by substitution of OH^- with Cl^-, a new isoreticular structure, [Cu₆(L)₅·(Cl^-)₂·H₂O·4DMF]·4DMF·6(H₂O) (SNNU-Bai67, SNNU-Bai = Shaanxi Normal University Bai's group), has been successfully synthesized. Furthermore, we deeply investigated the selective C2 hydrocarbon separation properties of SNNU-Bai67 and VNU-18 by single-component gas adsorption experiments, breakthrough experiments and simulation studies. They exhibit highly selective adsorption for C2 hydrocarbons over CH₄ compared to many reported MOFs for the separation of C2 hydrocarbons from CH₄, due to the suitable pore sizes of the partial corn sql-layer built from the isophthalic acid analogy and Cu-paddlewheel units. Interestingly, with the counterion of OH^- in VNU-18 tuned by Cl^- in SNNU-Bai67, the adsorption uptake values of C2 hydrocarbons were apparently improved by 15.0% for C₂H₂, 20.4% for C₂H₄ and 25.3% for C₂H₆, respectively, while the IAST selectivities of C2 hydrocarbons/CH₄ were still nearly the same, which may be because the synergistic effect of interactions of H_C2/1O_COO, H_C2/1N/C_Py, H_PyC_C2, ππ or H_C2C_C2 between the gas molecules and the framework is enhanced by the weaker polarity of Cl^- decorating the framework.

Adsorptive Molecular Sieving of Styrene over Ethylbenzene by Trianglimine Crystals

The separation of styrene (ST) and ethylbenzene (EB) mixtures is of great importance in the petrochemical and plastics industries. Current technology employs multiple cycles of energy-intensive distillation due to the very close boiling points of ST and EB. Here, we show that the molecular sieving properties of easily scalable and stable trianglimine crystals offer ultrahigh selectivity (99%) for styrene separation. The unique molecular sieving properties of trianglimine crystals are corroborated by DFT calculations, suggesting that the incorporation of the nonplanar EB requires a significant deformation of the macrocyclic cavity whereas the planar ST can be easily accommodated in the cavity.

Facile synthesis of anionic porous organic polymer for ethylene purification

The removal of acetylene from ethylene is of vital significance in the petroleum and chemical industry, the presence of trace acetylene impurities in ethylene polymerization process could lead to the interruption of ethylene polymerization. Herein, we construct a new anionic porous organic polymer using potassium tetraphenylborate via Friedel-Crafts alkylation reaction under mild conditions. The resulting material, APOP, possesses good thermal stability and a decent BET surface area, as exemplified by thermogravimetric analysis measurement and nitrogen gas sorption experiment. Acetylene and ethylene adsorption isotherms reveal that APOP has a higher adsorption capacity of acetylene than that of ethylene under same conditions. Ideal adsorbed solution theory calculations and breakthrough experiments both demonstrate that APOP is capable of selective adsorption of acetylene over ethylene. To the best of our knowledge, APOP represents the first anionic porous organic polymer material capable of selective adsorption of acetylene over ethylene, and the exploration of APOP may provide a new way for these key gas separations using ionic porous organic polymer materials.