Comparative study of adsorbents performance in ethylene/ethane separation

Some potential adsorbents for ethylene/ethane separation are ethylene selective while the others are ethane selective. Among different adsorbents, i.e., zeolites and metal organic frameworks (MOFs), a comparative study is critical to find the more suitable adsorbent for the separation. In this paper, binary ethylene/ethane adsorption performances of zeolites and MOFs, i.e., equilibrium selectivities and adsorption capacities are investigated utilizing ideal adsorbed solution theory (IAST). IAST model is applied at different gas compositions (0.1–0.9 ethylene mole fractions) and pressures up to 100 kPa. The results revealed that the most selective adsorbent toward ethylene is 5A zeolite while MOFs have higher equilibrium adsorption capacities. Among zeolites and MOFs, 5A and Fe₂(dobdc) have the highest selectivity (27.4 and 13.6) and capacity (≈2.8 and 5.8 mmol ethylene/g) at 100 kPa and 298 K for a 50/50 mixture. Among ethane selective adsorbents, Silicalite-1 zeolite and UTSA-33a (MOF) have the highest selectivity and capacity (≈2.9 and ≈1.5 mmol ethane/g) at 100 kPa and 298 K for a 50/50 mixture, respectively. Investigation showed that adsorption capacity of ethylene selective adsorbents is higher than that of ethane selective ones.     

MIL‐53(Al) as a Versatile Platform for Ionic‐Liquid/MOF Composites to Enhance CO2 Selectivity over CH4 and N2

Five different imidazolium‐based ionic liquids (ILs) were incorporated into a metal–organic framework (MOF), MIL‐53(Al), to investigate the effect of IL incorporation on the CO₂ separation performance of MIL‐53(Al). CO₂, CH₄, and N₂ adsorption isotherms of the IL/MIL‐53(Al) composites and pristine MIL‐53(Al) were measured to evaluate the effect of the ILs on the CO₂/CH₄ and CO₂/N₂ selectivities of the MOF. Of the composite materials that were tested, [BMIM][PF₆]/MIL‐53(Al) exhibited the largest increase in CO₂/CH₄ selectivity, 2.8‐times higher than that of pristine MIL‐53(Al), whilst [BMIM][MeSO₄]/MIL‐53(Al) exhibited the largest increase in CO₂/N₂ selectivity, 3.3‐times higher than that of pristine MIL‐53(Al). A comparison of the CO₂ separation potentials of the IL/MOF composites showed that the [BMIM][BF₄]‐ and [BMIM][PF₆]‐incorporated MIL‐53(Al) composites both showed enhanced CO₂/N₂ and CO₂/CH₄ selectivities at pressures of 1–5 bar compared to composites of CuBTC and ZIF‐8 with the same ILs. These results demonstrate that MIL‐53(Al) is a versatile platform for IL/MOF composites and could help to guide the rational design of new composites for target gas‐separation applications.     

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 ¹H-¹H spin diffusion MAS NMR spectroscopy and further verified by computational density distributions. The ¹H 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.     

Interfacial Enhancement by γ‐Al2O3 of Electrochemical Oxidative Dehydrogenation of Ethane to Ethylene in Solid Oxide Electrolysis Cells

Oxidative dehydrogenation of ethane (ODE) is limited by the facile deep oxidation and potential safety hazards. Now, electrochemical ODE reaction is incorporated into the anode of a solid oxide electrolysis cell, utilizing the oxygen species generated at anode to catalytically convert ethane. By infiltrating γ‐Al₂O₃ onto the surface of La_0.6Sr_0.4Co_0.2Fe_0.8O_3‐δ‐Sm_0.2Ce_0.8O_2‐δ (LSCF‐SDC) anode, the ethylene selectivity reaches as high as 92.5 %, while the highest ethane conversion is up to 29.1 % at 600 °C with optimized current and ethane flow rate. Density functional theory calculations and in situ X‐ray photoelectron spectroscopy characterizations reveal that the Al₂O₃/LSCF interfaces effectively reduce the amount of adsorbed oxygen species, leading to improved ethylene selectivity and stability, and that the formation of Al‐O‐Fe alters the electronic structure of interfacial Fe center with increased density of state around Fermi level and downshift of the empty band, which enhances ethane adsorption and conversion.     

A Microporous Hydrogen Bonded Organic Framework for Highly Selective Separation of Carbon Dioxide over Acetylene

The separation of acetylene (C₂H₂) from carbon dioxide (CO₂) is a very important but challenging task due to their similar molecular dimensions and physical properties. In terms of porous adsorbents for this separation, the CO₂-selective porous materials are superior to the C₂H₂-selective ones because of the cost- and energy-efficiency but have been rarely achieved. Herein we report our unexpected discovery of the first hydrogen bonded organic framework (HOF) constructed from a simple organic linker 2,4,6-tri(1H-pyrazol-4-yl)pyridine (PYTPZ) (termed as HOF-FJU-88) as the highly CO₂-selective porous material. HOF-FJU-88 is a two-dimensional HOFs with a pore pocket of about 7.6 Å. The activated HOF-FJU-88 takes up a high amount of CO₂ (59.6 cm³ g⁻¹) at ambient conditions with the record IAST selectivity of 1894. Its high performance for the CO₂/C₂H₂ separation has been further confirmed through breakthrough experiments, in situ diffuse reflectance infrared spectroscopy and molecular simulations.     

Combination of Optimization and Metalated‐Ligand Exchange: An Effective Approach to Functionalize UiO‐66(Zr) MOFs for CO2 Separation

The strategy to functionalize water‐stable metal–organic frameworks (MOFs) in order to improve their CO₂ uptake capacities for efficient CO₂ separation remains limited and challenging. We herein present an effective approach to functionalize a prominent water‐stable MOF, UiO‐66(Zr), by a combination of optimization and metalated‐ligand exchange. In particular, by systematic optimization, we have successfully obtained UiO‐66(Zr) of the highest BET surface area reported so far (1730 m² g^?1). Moreover, it shows a hybrid Type I/IV N₂ isotherm at 77 K and a mesopore size of 3.9 nm for the first time. The UiO‐66 MOF underwent a metalated‐ligand‐exchange (MLE) process to yield a series of new UiO‐66‐type MOFs, among which UiO‐66‐(COONa)₂‐EX and UiO‐66‐(COOLi)₄‐EX MOFs have both enhanced CO₂ working capacity and IAST CO₂/N₂ selectivity. Our approach has thus suggested an alternative design to achieve water‐stable MOFs with high crystallinity and gas uptake for efficient CO₂ separation.     

A Series of Mesoporous Metal-Organic Frameworks with Tunable Windows Sizes and Exceptionally High Ethane over Ethylene Adsorption Selectivity

The NIIC-20 (NIIC stands for Nikolaev Institute of Inorganic Chemistry) is a family of five isostructural metal-organic frameworks (MOFs) based on dodecanuclear wheel-shaped carboxylate building blocks {Zn₁₂(RCOO)₁₂(glycol)₆} (glycol is deprotonated diatomic alcohol: ethylene glycol, 1,2-propanediol, 1,2-butanediol, 1,2-pentanediol or glycerol), quantitatively crystallized from readily available starting chemicals. The crystal structures contain large mesoporous cages of 25 Å connected through {Zn₁₂} rings, of which inner diameter and chemical nature depend solely on the chosen glycol. The NIIC-20 compounds feature high surface area and rarely observed inversed adsorption affinity for saturated hydrocarbon (ethane) over the unsaturated ones (ethylene, acetylene). The corresponding IAST (Ideal Adsorbed Solution Theory) adsorption selectivity factors reach as much as 15.4 for C₂H₆/C₂H₄ and 10.9 for C₂H₆/C₂H₂ gas mixtures at ambient conditions, exceeding those for any other porous MOF reported so far. The remarkable combination of high adsorption uptakes and high adsorption selectivities makes the NIIC-20 series a new benchmark of porous materials designed for ethylene separation applications.     

Effect of methane co-feeding on the selectivity of ethylene produced from oxidative dehydrogenation of ethane with CO2 over a Ni-La/SiO2 catalyst

A Ni-La/SiO₂ catalyst was prepared through the incipient wetness impregnation method and tested in the oxidative dehydrogenation of ethane (ODHE) with CO₂. The fresh and used catalysts were characterized by XRD and SEM techniques. The Ni-La/SiO₂ catalyst exhibited catalytic activity for the oxidative dehydrogenation of ethane, but with low ethylene selectivity in the absence of methane. The selectivity to ethylene increased with increasing molar ratio of methane in the feed. The carbon deposited on the catalyst surface in the sole ODHE with CO₂ was mainly inert carbon, while much more filamentous carbon was formed in the presence of methane. The filamentous carbon was easy to be removed by CO₂, which might play a role in improving the conversion of ethane to ethylene. The introduction of methane might affect the equilibrium of the CO₂ reforming of ethane and the ODHE with CO₂. As a consequence, the synthesis gas produced from CO₂ reforming of methane partly inhibited the reaction of ethane and promoted the ODHE with CO₂, thus increasing the selectivity of ethylene.     

Tailoring a robust Al-MOF for trapping C2H6 and C2H2 towards efficient C2H4 purification from quaternary mixtures

Light hydrocarbon separation is considered one of the most industrially challenging and desired chemical separation processes and is highly essential in polymer and chemical industries. Among them, separating ethylene (C₂H₄) from C2 hydrocarbon mixtures such as ethane (C₂H₆), acetylene (C₂H₂), and other natural gas elements (CO₂, CH₄) is of paramount importance and poses significant difficulty. We demonstrate such separations using an Al-MOF synthesised earlier as a non-porous material, but herein endowed with hierarchical porosity created under microwave conditions in an equimolar water/ethanol solution. The material possessing a large surface area (793 m² g⁻¹) exhibits an excellent uptake capacity for major industrial hydrocarbons in the order of C₂H₂ > C₂H₆ > CO₂ > C₂H₄ > CH₄ under ambient conditions. It shows an outstanding dynamic breakthrough separation of ethylene (C₂H₄) not only for a binary mixture (C₂H₆/C₂H₄) but also for a quaternary combination (C₂H₄/C₂H₆/C₂H₂/CO₂ and C₂H₄/C₂H₆/C₂H₂/CH₄) of varying concentrations. The detailed separation/purification mechanism was unveiled by gas adsorption isotherms, mixed-gas adsorption calculations, selectivity estimations, advanced computer simulations such as density functional theory (DFT), grand canonical Monte Carlo (GCMC) and ab initio molecular dynamics (AIMD), and stepwise multicomponent dynamic breakthrough experiments.

Industrially important C₂H₄ purification from multi-component hydrocarbon mixtures.     

Preparation and Gas Separation Properties of Metal‐Organic Framework Membranes

Continuous and intergrown metal‐organic framework (MOF) membranes, MIL‐100(In) (MIL represents Materials Institute Lavoisier), were prepared directly on porous anodic alumina oxide (AAO) membranes using an in situ crystallization method. The pore surface of MIL‐100(In) is conferred with polarity due to the presence of the 1, 3,5‐benzenetricarboxylic acid. The thickness of MIL‐100(In) membranes was tuned by varying the reactant concentration of indium chloride and 1, 3,5‐benzenetricarboxylic acid. Single gas permeation measurements on this MOF membrane indicate the large permeances of 0.90 × 10^–6 and 0.81 × 10^–6 mol · m^–2·s^–1·Pa^–1 for CO₂ and CH₄, and relatively high ideal selective factors of 3.75 and 3.38 for CO₂/N₂ and CH₄/N₂, respectively.     

Identifying Different Types of Catalysts for CO2 Reduction by Ethane through Dry Reforming and Oxidative Dehydrogenation

The recent shale gas boom combined with the requirement to reduce atmospheric CO₂ have created an opportunity for using both raw materials (shale gas and CO₂) in a single process. Shale gas is primarily made up of methane, but ethane comprises about 10 % and reserves are underutilized. Two routes have been investigated by combining ethane decomposition with CO₂ reduction to produce products of higher value. The first reaction is ethane dry reforming which produces synthesis gas (CO+H₂). The second route is oxidative dehydrogenation which produces ethylene using CO₂ as a soft oxidant. The results of this study indicate that the Pt/CeO₂ catalyst shows promise for the production of synthesis gas, while Mo₂C‐based materials preserve the C? C bond of ethane to produce ethylene. These findings are supported by density functional theory (DFT) calculations and X‐ray absorption near‐edge spectroscopy (XANES) characterization of the catalysts under in situ reaction conditions.     

Catalytic performance of silica-supported chromium oxide catalysts in ethane dehydrogenation with carbon dioxide

The oxidative dehydrogenation of ethane into ethylene by CO₂ over a series of silica-supported chromium oxide catalysts was investigated. The results showed that the catalysts were effective for the reaction and CO₂ in the feed promoted the catalytic activity. The 5%Cr/SiO₂ catalyst exhibited the excellent performance with 30.7% ethane conversion and 96.5% ethylene selectivity at 700^oC. ESR and UV-DRS were used to probe the active sites and the species with high valent states (Cr⁵⁺ and/or Cr⁶⁺) were found to be important for the reaction.     

High CO2‐Capture Ability of a Porous Organic Polymer Bifunctionalized with Carboxy and Triazole Groups

A new porous organic polymer, SNU‐C1 , incorporating two different CO₂‐attracting groups, namely, carboxy and triazole groups, has been synthesized. By activating SNU‐C1 with two different methods, vacuum drying and supercritical‐CO₂ treatment, the guest‐free phases, SNU‐C1‐va and SNU‐C1‐sca , respectively, were obtained. Brunauer–Emmett–Teller (BET) surface areas of SNU‐C1‐va and SNU‐C1‐sca are 595 and 830 m²g^?1, respectively, as estimated by the N₂‐adsorption isotherms at 77 K. At 298 K and 1 atm, SNU‐C1‐va and SNU‐C1‐sca show high CO₂ uptakes, 2.31 mmol g^?1 and 3.14 mmol g^?1, respectively, the high level being due to the presence of abundant polar groups (carboxy and triazole) exposed on the pore surfaces. Five separation parameters for flue gas and landfill gas in vacuum‐swing adsorption were calculated from single‐component gas‐sorption isotherms by using the ideal adsorbed solution theory (IAST). The data reveal excellent CO₂‐separation abilities of SNU‐C1‐va and SNU‐C1‐sca , namely high CO₂‐uptake capacity, high selectivity, and high regenerability. The gas‐cycling experiments for the materials and the water‐treated samples, experiments that involved treating the samples with a CO₂‐N₂ gas mixture (15:85, v/v) followed by a pure N₂ purge, further verified the high regenerability and water stability. The results suggest that these materials have great potential applications in CO₂ separation.     

An Electrochemical Sensor for the Detection of Cu2+ Based on Gold Nanoflowers‐modifed Electrode and DNAzyme Functionalized Au@MIL‐101 (Fe)

In this study, we developed an electrochemical sensor for sensitive detection of Cu²⁺ based on gold nanoflowers (AuNFs)‐modified electrode and DNAzyme functionalized Au@MIL‐101(Fe) (MIL: Materials of Institute Lavoisier). The AuNFs‐modified indium tin oxide modified conductive glass electrode(AuNFs/ITO) prepared via electrodeposition showed improved electronic transport properties and provided more active sites to adsorb large amounts of oligonucleotide substrate (DNA1) via thiol‐gold bonds. The stable Au@MIL‐101(Fe) could guarantee the sensitivity because of its intrinsic peroxidase mimic property, while the Cu²⁺‐dependent DNA‐cleaving DNAzyme linked to Au@MIL‐101(Fe) achieved the selectivity toward Cu²⁺. After the DNAzyme substrate strand (DNA2) was cleaved into two parts due to the presence of Cu²⁺, the oligonucleotide fragment linked to MIL‐101(Fe) was able to hybridize with DNA1 adsorbed onto the surface of AuNFs/ITO. Due to the peroxidase‐like catalytic activity of MIL‐101(Fe) and the affinity recognition property of DNAzyme toward Cu²⁺, the electrochemical biosensor showed a sensitive detection range from 0.001 to 100 μM, a detection limit of 0.457 nM and a high selectivity, demonstrating its potential for Cu²⁺ detection in real environmental samples.     

Olefin Recovery by *BEA-Type Zeolite Membrane: Affinity-Based Separation with Olefin−Ag+ Interaction

Ag⁺ was introduced into *BEA-type zeolite membrane by an ion-exchange method to enhance olefin selectivity. Ag−*BEA membrane exhibited superior olefin separation performance for both ethylene/ethane and propylene/propane mixtures. Particularly, the separation factor for ethylene at 373 K reached 57 with the ethylene permeance of 1.6×10⁻⁷ mol m⁻² s⁻¹ Pa⁻¹. Adsorption properties of olefin and paraffin were evaluated to discuss contribution of Ag⁺ to separation performance enhancement. A strong interaction between olefin and Ag⁺ in the membrane caused preferential adsorption of olefin against paraffin, leading to selective permeation of olefin. Ag−*BEA membrane also exhibited high olefin selectivities from olefin/N₂ mixtures. The affinity-based separation through Ag−*BEA membrane showed a high potential for olefin recovery and purification from various gas mixtures.     

Robust Ultramicroporous Metal–Organic Frameworks with Benchmark Affinity for Acetylene

Highly selective separation and/or purification of acetylene from various gas mixtures is a relevant and difficult challenge that currently requires costly and energy‐intensive chemisorption processes. Two ultramicroporous metal–organic framework physisorbents, NKMOF‐1‐M (M=Cu or Ni), offer high hydrolytic stability and benchmark selectivity towards acetylene versus several gases at ambient temperature. The performance of NKMOF‐1‐M is attributed to their exceptional acetylene binding affinity as revealed by modelling and several experimental studies: in situ single‐crystal X‐ray diffraction, FTIR, and gas mixture breakthrough tests. NKMOF‐1‐M exhibit better low‐pressure uptake than existing physisorbents and possesses the highest selectivities yet reported for C₂H₂/CO₂ and C₂H₂/CH₄. The performance of NKMOF‐1‐M is not driven by the same mechanism as current benchmark physisorbents that rely on pore walls lined by inorganic anions.     

Elimination of 4‐chlorophenol in aqueous solution by the Novel Pd/MIL‐101(Cr)‐Hydrogen‐Accelerated catalytic fenton system

Excessive consumption of Fe (II) and massive generation of sludge containing Fe (III) from classic Fenton process remains a major obstacle for its poor recycling of Fe (III) to Fe (II). Therefore, the MHACF‐MIL‐101(Cr) system, by introducing H₂, Pd⁰ and MIL‐101(Cr) into Fenton reaction system, was developed at normal temperature and pressure. In this system, the reduction of Fe^III back to Fe^II by solid catalyst Pd/MIL‐101(Cr) for the storage and activation of H₂, was accelerated significantly by above 10‐fold and 5‐fold controlled with the H₂‐MIL‐101(Cr) system and H₂‐Pd⁰ system, respectively. However, the concentration of Fe (II) generated by the reduction of Fe (III) could not be detected with the only input of H₂ and without the addition of MOFs material. In addition, the apparent consumption of Fe (II) in MHACF‐MIL‐101(Cr) system was half of that in classical Fenton system, while more Fe (II) might be reused infinitely in fact. Accordingly, only trace amount of Fe (II) vs H₂O₂ concentration was needed and hydroxyl radicals through the detection of para‐hydroxybenzoic acid (p‐HBA) as the oxidative product of benzoic acid (BA) by·OH could be continuously generated for the effective degradation of 4‐chlorophenol(4‐CP). The effects of initial pH, concentration of 4‐CP, dosage of Fe²⁺, H₂O₂ and Pd/MIL‐101(Cr) catalyst, Pd content and H₂ flow were investigated, combined with systematic controlled experiments. Moreover, the robustness and morphology change of Pd/MIL‐101(Cr) were thoroughly analyzed. This study enables better understanding of the H₂‐mediated Fenton reaction enhanced by Pd/MIL‐101(Cr) and thus, will shed new light on how to accelerate Fe (III)/Fe (II) redox cycle and develop more efficient Fenton system.     

Defect‐Free Mixed‐Matrix Membranes with Hydrophilic Metal‐Organic Polyhedra for Efficient Carbon Dioxide Separation

Defect‐free mixed‐matrix membranes (MMMs) were prepared by incorporating hydrophilic metal‐organic polyhedra (MOPs) into cross‐linked polyethylene oxide (XLPEO) for efficient CO₂ separation. Hydrophilic MOPs with triethylene glycol pendant groups, which were assembled by 5‐tri(ethylene glycol) monomethyl ether isophthalic acid and Cu^II ions, were uniformly dispersed in XLPEO without particle agglomeration. Compared to conventional neat XLPEO, the homogenous dispersion of EG₃‐MOPs in XLPEO enhanced CO₂ permeability of MMMs. Upon increasing the amount of EG₃‐MOPs, the membrane performance such as CO₂/N₂ selectivity was steadily improved because of unsaturated Cu^II sites at paddle‐wheel units, which was confirmed by Cu K‐edge XANES and TPD analysis. Therefore, such defect‐free MMMs with unsaturated metal sites would contribute to enhance CO₂ separation performance.     

Influence of Solvent‐Like Sidechains on the Adsorption of Light Hydrocarbons in Metal–Organic Frameworks

A variety of strategies have been developed to adsorb and separate light hydrocarbons in metal–organic frameworks. Here, we present a new approach in which the pores of a framework are lined with four different C3 sidechains that feature various degrees of branching and saturation. These pendant groups, which essentially mimic a low‐density solvent with restricted degrees of freedom, offer tunable control of dispersive host–guest interactions. The performance of a series of frameworks of the type Zn₂(fu‐bdc)₂(dabco) (fu‐bdc^2?=functionalized 1,4‐benzenedicarboxylate; dabco=1,4‐diazabicyclo[2.2.2]octane), which feature a pillared layer structure, were investigated for the adsorption and separation of methane, ethane, ethylene, and acetylene. The four frameworks exhibit low methane uptake, whereas C2 hydrocarbon uptake is substantially higher as a result of the enhanced interaction of these molecules with the ligand sidechains. Most significantly, the adsorption quantities and selectivity were found to depend strongly upon the type of sidechains attached to the framework scaffold.     

Covalent and electrostatic incorporation of amines into hypercrosslinked polymers for increased CO2 selectivity

Two methods of incorporating functional groups rich in nitrogen into low cost microporous hypercrosslinked polymers (HCPs) have been evaluated and the effects on the carbon dioxide CO₂/N₂ IAST selectivity were measured. Electrostatic incorporation of an ammonium salt into a sulfonic acid-containing HCP polymer afforded a static CO₂ uptake of 2.5 mmol g⁻¹ with a CO₂/N₂ IAST selectivity of 42:1 at 1 bar and 298 K. Using column breakthrough measurements with a 15:85 CO₂/N₂ mixture at 298 K and 1 bar, a selectivity of 17:1 was obtained. However, varying the counterion resulted in polymers with lower CO₂/N₂ selectivity values. Decoration of the parent polymer with CO₂-philic imidazole followed by electrostatic ammonium salt incorporation blocked some of the micropores reducing the selectivity which re-emphasizes the role and importance of pore width for CO₂/N₂ selectivity. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018 , 56, 2513–2521