Construction of a Stable Crystalline Polyimide Porous Organic Framework for C2H2/C2H4 and CO2/N2 Separation

Utilization of porous materials for gas capture and separation is a hot research topic. Removal of acetylene (C₂H₂) from ethylene (C₂H₄) is important in the oil refining and petrochemical industries, since C₂H₂ impurities deactivate the catalysts and terminate the polymerization of C₂H₄. Carbon dioxide (CO₂) emission from power plants contributes to global climate change and threatens the survival of life on this planet. Herein, 2D crystalline polyimide porous organic framework PAF-120, which was constructed by imidization of linear naphthalene-1,4,5,8-tetracarboxylic dianhydride and triangular 1,3,5-tris(4-aminophenyl)benzene, showed significant thermal and chemical stability. Low-pressure gas adsorption isotherms revealed that PAF-120 exhibits good selective adsorption of C₂H₂ over C₂H₄ and CO₂ over N₂. At 298 K and 1 bar, its C₂H₂ and CO₂ selectivities were predicted to be 4.1 and 68.7, respectively. More importantly, PAF-120 exhibits the highest selectivity for C₂H₂/C₂H₄ separation among porous organic frameworks. Thus PAF-120 could be a suitable candidate for selective separation of C₂H₂ over C₂H₄ and CO₂ over N₂.     

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.     

Ethylene/ethane separation by CuCl/AC adsorbent prepared using CuCl<Subscript>2</Subscript> as a precursor

Activated carbon (AC) supported CuCl (CuCl/AC) for ethylene/ethane separation has been prepared with CuCl₂ as precursor by a solid-state dispersion method. The samples are characterized by inductively coupled plasma optical emission spectrometry, X-ray diffraction, N₂ adsorption/desorption and X-ray photoelectron spectroscopy, and investigated for ethylene (C₂H₄) and ethane (C₂H₆) adsorptions. The characterization results reveal that CuCl₂ supported on AC can be highly dispersed on the surfaces of AC support and completely converted to CuCl after activation at 543 K in N₂. The resultant adsorbent displays high ethylene adsorption capacity, high C₂H₄/C₂H₆ adsorption selectivity and excellent reversibility. The adsorption isotherms of ethylene and ethane on CuCl/AC at temperatures up to 333 K can be well fitted by the Sips models, and the corresponding isosteric heats of adsorption are calculated from the Clausius–Clapeyron equation. The value of isosteric heat of adsorption suggests that the interaction of ethylene with CuCl/AC is between physisorption and chemisorption.     

A Microporous Metal-Organic Framework with Unique Aromatic Pore Surfaces for High Performance C2H6/C2H4 Separation

Developing adsorptive separation processes based on C₂H₆-selective sorbents to replace energy-intensive cryogenic distillation is a promising alternative for C₂H₄ purification from C₂H₄/C₂H₆ mixtures, which however remains challenging. During our studies on two isostructural metal–organic frameworks ( Ni-MOF 1 and Ni-MOF 2 ), we found that Ni-MOF 2 exhibited significantly higher performance for C₂H₆/C₂H₄ separation than Ni-MOF-1 , as clearly established by gas sorption isotherms and breakthrough experiments. Density-Functional Theory (DFT) studies showed that the unblocked unique aromatic pore surfaces within Ni-MOF 2 induce more and stronger C−H⋅⋅⋅π with C₂H₆ over C₂H₄ while the suitable pore spaces enforce its high C₂H₆ uptake capacity, featuring Ni-MOF 2 as one of the best porous materials for this very important gas separation. It generates 12 L kg⁻¹ of polymer-grade C₂H₄ product from equimolar C₂H₆/C₂H₄ mixtures at ambient conditions.     

A Robust Metal-Organic Framework with Scalable Synthesis and Optimal Adsorption and Desorption for Energy-Efficient Ethylene Purification

Adsorptive separation is an energy-efficient alternative, but its advancement has been hindered by the challenge of industrially potential adsorbents development. Herein, a novel ultra-microporous metal-organic framework ZU-901 is designed that satisfies the basic criteria raised by ethylene/ethane (C₂H₄/C₂H₆) pressure swing adsorption (PSA). ZU-901 exhibits an “S” shaped C₂H₄ curve with high sorbent selection parameter (65) and could be mildly regenerated. Through green aqueous-phase synthesis, ZU-901 is easily scalable with 99 % yield, and it is stable in water, acid, basic solutions and cycling breakthrough experiments. Polymer-grade C₂H₄ (99.51 %) could be obtained via a simulating two-bed PSA process, and the corresponding energy consumption is only 1/10 of that of simulating cryogenic distillation. Our work has demonstrated the great potential of pore engineering in designing porous materials with desired adsorption and desorption behavior to implement an efficient PSA process.     

Dynamic Entangled Porous Framework for Hydrocarbon (C2–C3) Storage,CO2 Capture,and Separation

Storage and separation of small (C1–C3) hydrocarbons are of great significance as these are alternative energy resources and also can be used as raw materials for many industrially important materials. Selective capture of greenhouse gas, CO₂ from CH₄ is important to improve the quality of natural gas. Among the available porous materials, MOFs with permanent porosity are the most suitable to serve these purposes. Herein, a two‐fold entangled dynamic framework {[Zn₂(bdc)₂(bpNDI)]?4DMF}_n with pore surface carved with polar functional groups and aromatic π clouds is exploited for selective capture of CO₂, C2, and C3 hydrocarbons at ambient condition. The framework shows stepwise CO₂ and C₂H₂ uptake at 195 K but type I profiles are observed at 298 K. The IAST selectivity of CO₂ over CH₄ is the highest (598 at 298 K) among the MOFs without open metal sites reported till date. It also shows high selectivity for C₂H₂, C₂H₄, C₂H₆, and C₃H₈ over CH₄ at 298 K. DFT calculations reveal that aromatic π surface and the polar imide (RNC=O) functional groups are the primary adsorption sites for adsorption. Furthermore, breakthrough column experiments showed CO₂/CH₄ C₂H₆/CH₄ and CO₂/N₂ separation capability at ambient condition.     

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

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

Low-Concentration C2H6 Capture Enabled by Size Matching in the Ultramicropore

Low-concentration ethane capture is crucial for environmental protection and natural gas purification. The ideal physisorbent with strong C₂H₆ interaction and large C₂H₆ uptake at low-concentration level has rarely been reported, due to the large pK_a value and small quadrupole moment of C₂H₆. Herein, we demonstrate the perfectly size matching between the ultramicropore (pore size of 4.6 Å) and ethane (kinetic diameter of 4.4 Å) in a nickel pyridine-4-carboxylate metal–organic framework (IISERP-MOF 2 ), which enables the record-breaking performance for low concentration C₂H₆ capture. IISERP-MOF 2 exhibits the large C₂H₆ adsorption enthalpy of 56.7 kJ/mol, and record-high C₂H₆ uptake at low pressure of 0.01–0.1 bar and 298 K (1.8 mmol/g at 0.01 bar). Molecule simulations and C₂H₆-loading crystal structure analysis revealed that the maximized interaction sites in IISERP-MOF 2 with ethane molecule originates the strong C₂H₆ adsorption. The dynamic breakthrough experiments for gas mixtures of C₂H₆/N₂(1/999, v/v) and C₂H₆/CH₄ (5/95, v/v) proved the excellent low-concentration C₂H₆ capture performance.     

Behavior of ethylene and ethane within single-walled carbon nanotubes. 1-Adsorption and equilibrium properties

Endohedral adsorption properties of ethylene and ethane onto single-walled carbon nanotubes were investigated using a united atom (2CLJQ) and a fully atomistic (AA-OPLS) force fields, by Grand Canonical Monte Carlo and Molecular Dynamics techniques. Pure fluids were studied at room temperature, T=300 K, and in the pressure ranges 4×10⁻⁴<p<47.1 bar (C₂H₄) and 4×10⁻⁴<p<37.9 bar (C₂H₆). In the low pressure region, isotherms differ quantitatively depending on the intermolecular potential used, but show the same qualitative features. Both potentials predict that ethane is preferentially adsorbed at low pressures, and the opposite behavior was observed at high loadings. Isosteric heats of adsorption and estimates of low pressure Henry’s constants, confirmed that ethane adsorption is the thermodynamically favored process at low pressures. Binary mixtures of C₂H₄/C₂H₆ were studied under several (p,T) conditions and the corresponding selectivities towards ethane, S, were evaluated. Small values of S<4 were found in all cases studied. Nanotube geometry plays a minor role on the adsorption properties, which seem to be driven at lower pressures primarily by the larger affinity of the alkane towards the carbon surface and at higher pressures by molecular volume and packing effects. The fact that the selectivity towards ethane is similar to that found earlier on carbon slit pores and larger diameter nanotubes points to the fact that the peculiar 1-D geometry of the nanotubes provides no particular incentive for the adsorption of either species.     

Thermal and gas (CH4 and C2H4) adsorption characteristics of nitric acid-treated clinoptilolite

In this study, in order to consider the effect of the time of the nitric acid treatment on thermal, structural and gas adsorption properties, clinoptilolite was modified with 1.0 M acid solutions at 80 °C for 2, 4, 6, 12 and 24 h. Structural and thermal properties of natural and acid-treated clinoptilolites were investigated by powder X-ray diffraction, X-ray fluorescence, thermogravimetric analysis, differential thermal analysis and nitrogen adsorption methods. Methane (CH₄) and ethylene (C₂H₄) are hazardous gases for human and plant health, respectively. Therefore, some measures should be taken to reduce emissions of methane and ethylene. CH₄ and C₂H₄ adsorption capacities of clinoptilolite (CLN) from Turkey and that of acid-treated forms at 273 K up to 100 kPa were found between 0.556–0.683 and 0.470–1.295 mmol g^?1, respectively.

     

Trapping of dichlorosilane (H2SiCl2) gas by transition metals doped fullerene nanostructured materials

Dichlorosilane is a flammable and poisonous gas which is very toxic when inhaled. When handling this gas, extreme precautions must be taken to prevent exposure and so therefore there is need to develop a more sensitive and affordable sensor to detect and measure the concentration of gas in the environment in case of unintentional release of the gas into the air. Overtime, structured materials have been used in the adsorption of target gas. Herein, the detection of dichlorosilane (H₂SiCl₂) gas by transition metals (X = Cr, Fe, Ni, Ti, and Zn) anchored fullerene is studied using the density functional theory (DFT) computation at theωB97X-D/gen/6-311++G(d,p)/LanL2DZ level of theory. From the electronic properties, large energy gap signifies lower electrical conductivity and sensitivity. The result showed an increase in the energy gap on adsorption of the gas on nanocages except for N1 and T1 where the energy gap was lesser than that of the nanocages. Calculations shows among the five studied surfaces, C₂₃–Ti surface emerged with the highest adsorption energy value of ?2.231 eV and corresponding energy gap value of 5.200 eV. Also, the decreasing trend of adsorption energies: H₂Cl₂SiC₂₃Ti (T1), (?2.231) > H₂SiCl₂C₂₃Ni (N1), (?2.095) > H₂SiCl₂C₂₃Zn (Z1) (?2.068) > H₂SiCl₂C₂₃Cr (C1) (?1.796) > H₂SiCl₂C₂₃Fe (F1) (?1.742) was observed. High negative value of adsorption energy, the lesser the recovery time. H₂Cl₂SiC₂₃Ti (T1) complex with high negative value of adsorption energy has a lesser recovery time exhibits better sensing attributes. The C₂₃–Ti surface is relatively a better candidate in the adsorption and, hence, confirmed as suitable nanosensor material for the detection and adsorption of toxic dichlorosilane (H₂SiCl₂) gas molecule.     

Sieving Effect for the Separation of C2H2/C2H4 in an Ultrastable Ultramicroporous Zinc-Organic Framework

The separation of C₂H₂ from C₂H₄ is one of the most challenging tasks due to the similarity of their physical properties. In addition, green synthetic protocol and adsorbent's stability are also the major concerns during the separation. Herein, under hydrothermal green synthesis conditions, an ultrastable ultramicroporous Zn-MOF was designed and synthesized with a high yield. The pore diameter of the Zn-MOF is 3.6 Å, which lies in between the diameters of C₂H₂ (3.3 Å) and C₂H₄ (4.2 Å) molecules, leading to an efficient separation of the C₂H₂/C₂H₄ mixtures by the sieving effect. The practical separation performance of C₂H₂/C₂H₄ was confirmed by the dynamic breakthrough experiments. Moreover, the high stability enables the adsorption capacity of the Zn-MOF to C₂H₂, which can be maintained under a wide range of pH (1–13). Molecular simulations were also performed to identify the different C₂H₂- and C₂H₄-binding sites in Zn-MOF.     

New α,ω-dicarboxylate coordination polymers with 4,4′-bipyridine: Cu(bpy)(C5H6O4), Zn(bpy)(C5H6O4), Zn(bpy)(C6H8O4) and Mn(bpy)(C8H12O4) · H2O

Four 4,4′-bipyridine α,ω-dicarboxylate coordination polymers Cu(bpy)(C₅H₆O₄) (1), Zn(bpy)(C₅H₆O₄) (2), Zn(bpy)(C₆H₈O₄) (3) and Mn(bpy)(C₈H₁₂O₄) · H₂O (4) have been synthesized and structurally characterized by single crystal X-ray diffraction methods (bpy = 4,4-bipyridine, (C₅H₆O₄)²⁻ = glutarate anion, (C₆H₈O₄)²⁻ = adipate anion, (C₈H₁₂O₄)²⁻ = suberate anion). Their crystal structures are featured by dimeric metal units, which are co-bridged by 4,4′-bipyridine ligands and dicarboxylate anions such as glutarate, adipate and suberate anions to generate 2D layers with a (4,4) topology in 1, 2 and 4 as well as to form 3D frameworks in 3. Two 3D frameworks in 3 interpenetrate with each other to form a topology identical to the well-known Nb₆F₁₅ cluster compound. Over 5–300 K, the paramagnetic behavior of 4 follows the Curie–Weiss law χ_m(T − Θ) = 4.265(5) cm³ mol⁻¹ with the Weiss constant Θ = −6.3(2) K. Furthermore, the thermal behavior of 3 and 4 is also discussed.     

A new perchlorate-based hybrid ultramicroporous material with rich bare oxygen atoms for high C2H2/CO2 separation

Adsorptive separation of acetylene (C₂H₂) from carbon dioxide (CO₂) is of great significance in petrochemical industry, but still remains as a daunting challenge by reason of their very similar molecular sizes/shapes and physical properties. Herein, we reported a new perchlorate-based hybrid ultramicroporous material ZJU-194 that features the unique flexible-robust network decorated with rich bare oxygen atoms. By integrating the refined pore space as well as specific binding sites, the activated ZJU-194 (ZJU-194a) enables a selective two-step gate-opening adsorption toward C₂H₂, but blocks off the further uptake of CO₂. It thus exhibits a very high C₂H₂/CO₂ selectivity (22.4) at ambient conditions, which is superior to most reported MOF materials. Its complete separation for 50/50 C₂H₂/CO₂ mixtures is further evidenced by the dynamic breakthrough experiments.     

Ultramicroporous Covalent Organic Framework Nanosheets with Functionality Pair for Membrane C2H2/C2H4 Separation

Gas separation efficiency of covalent organic framework (COF) membrane can be greatly elevated through precise functionalization. A pair-functionalized COF membrane of 1,3,5-triformylphloroglucinol (TP) and isoquinoline-5,8-diamine (IQD) monomers in two and three nodes is designed and synthesized. TP-IQD is crystallized in a two-dimensional structure with a pore size of 6.5 Å and a surface area of 289 m² g⁻¹. This COF possesses N−O paired groups which cooperatively interact with C₂H₂ instead of C₂H₄. TP-IQD nanosheets of ≈10 μm in width and ≈4 nm in thickness are prepared by mechanical exfoliation; they are further processed with 6FDA-ODA polymer into a hybrid membrane. High porosity and functionality pair of TP-IQD offer the membrane with significantly increased C₂H₂ permeability and C₂H₂/C₂H₄ selectivity which are 160 % and 430 % higher of pure 6FDA-ODA. The boosted performance demonstrates high efficiency of the pair-functionality strategy for the synthesis of separation-led COFs.     

New Microporous Materials for Acetylene Storage and C2H2/CO2 Separation: Insights from Molecular Simulations

Force‐field based grand‐canonical Monte Carlo simulations are used to investigate the acetylene and carbon dioxide uptake capacity, as well as the C₂H₂/CO₂ adsorption selectivity of three novel microporous materials: Magnesium formate, Cu₃(btc)₂, and cucurbit[6]uril. Because no comparable computational studies of acetylene adsorption have been reported so far, the study focuses on systems for which experimental data are available to permit a thorough validation of the simulation results. The results for magnesium formate are in excellent agreement with experiment. The simulation predicts a high selectivity for acetylene over CO₂, which can be understood from a detailed analysis of the structural features that determine the affinity of Mg‐formate towards C₂H₂. For Cu₃(btc)₂, preliminary calculations reveal the necessity to include the interaction of the sorbate molecules with the unsaturated metal sites, which is done by means of a parameter adjustment based on ab‐initio calculations. In spite of the high C₂H₂ storage capacity, the C₂H₂/CO₂ selectivity of this material is very modest. The simulation results for the porous organic crystal cucurbit[6]uril show that the adsorption characteristics that have been observed experimentally, particularly the very high isosteric heat of adsorption, cannot be understood when an ideal structure is assumed. It is postulated that structural imperfections play a key role in determining the C₂H₂ adsorption behavior of this material, and this proposition is supported by additional calculations.     

Two –COOH Decorated Anionic Metal‐organic Frameworks with Open Cu2+ Sites Afforded Highly C2H2/CO2 and C2H2/CH4 Separation and Removal of Organic Dyes

An anionic multifunctional porous metal organic framework (MOF), [Cu₂THBA(H₂O)₂] · (C₃H₇NO)₁₂ · (H₂O)₁₀ ( 1 ) (H₄THBA = p‐terphenyl‐3,2′′,3′′,5,5′′,5′′′‐ hexcarboxylic acid) with NbO‐type topology was synthesized and characterized. Due to multiple functional sites and suitable pore size, the desolvated compound 1a exhibits high separation selectivity for C₂H₂/CO₂ of 30 and C₂H₂/CH₄ of 131 at 1 kPa at room temperature. Compound 1 can also efficiently and completely separate methylene blue (MB⁺) molecules of low concentrations from aqueous solution in 12 h.     

Light Hydrocarbon Separations Using Porous Organic Framework Materials

Light hydrocarbons (C₁–C₃) are used as basic energy feedstocks and as commodity organic compounds for the production of many industrially necessary chemicals. Due to the nature of the raw materials and production processes, light hydrocarbons are generated as mixtures, but the high-purity single-component products are of vital importance to the petrochemical industry. Consequently, the separation of these C₁–C₃ products is a crucial industrial procedure that comprises a significant share of the total global energy consumption per year. As a complement to traditional separation methods (distillation, partial hydrogenation, etc.), adsorptive separations using porous solids have received widespread attention due to their lower energy costs and higher efficiency. Extensive research has been devoted to the use of porous materials such as zeolites and metal-organic frameworks (MOFs) as solid adsorbents for these key separations, owing to the high porosity, tunable pore structures, and unsaturated metal sites present in these materials. Recently, porous organic framework (POF) materials composed of organic building blocks linked by covalent bonds have also shown excellent properties in light hydrocarbon adsorption and separation, sparking interest in the use of these materials as adsorbents in separation processes. This Minireview summarizes the recent advances in the use of POFs for light hydrocarbon separations, including the separation of mixtures of methane/ethane, methane/propane, ethylene/ethane, acetylene/ethylene, and propylene/propane, while highlighting the relationships between the structural features of these materials and their separation performances. Finally, the difficulties, challenges, and opportunities associated with leveraging POFs for light hydrocarbon separations are discussed to conclude the review.     

Soot formation from C2H2 and C2H4 pyrolysis at different temperatures

A study of the pyrolysis of two hydrocarbons, C₂H₂ and C₂H₄, at different temperatures has been carried out in order to compare their behaviour in terms of soot and gas yields and gas composition. Pyrolysis experiments have been performed in the same conditions for both hydrocarbons: an inlet hydrocarbon concentration of 15,000 ppmv and a temperature range of 1000–1200 °C. For C₂H₂ and C₂H₄ pyrolysis tests, the results present the same trend when increasing the temperature: an increase in soot yield, a decrease in gas yield and a similar evolution of the outlet gases. Comparatively, it can be observed that acetylene is a more sooting hydrocarbon than ethylene for a given temperature. Additionally, the study of soot reactivity with O₂ and NO shows that the soot samples obtained from ethylene show a slightly higher reactivity towards O₂ and NO than the soot samples formed from acetylene.     

Higher hydrogen uptake capacity of C2H4Ti+ than C2H4Ti: a quantum chemical study

Using density functionals theory, we show that gravimetric hydrogen uptake of C₂H₄Ti complex and its cation, C₂H₄Ti⁺, differ by about 2 wt%. Six and five hydrogen molecules are found to be adsorbed on C₂H₄Ti⁺ and C₂H₄Ti complexes thereby showing a hydrogen-uptake capacity of 13.74 and 11.72 wt%, respectively. All hydrogen molecules are adsorbed in molecular form on C₂H₄Ti⁺ ion with an increase in metal bond strength, whereas in some cases, the hydrogen molecules are found to be dissociated on C₂H₄Ti neutral complex. The uptake capacity of neutral C₂H₄Ti complex shown in this work is in excellent agreement with that reported experimentally, Phillips and Shivaram (Phys Rev Lett 100:105505, 2008). The H₂ adsorption energy and its dependence on exchange and correlation functions in density functionals method were illustrated. Even after the adsorption of maximum number of hydrogen molecules on C₂H₄Ti and C₂H₄Ti⁺ complexes, Ti and Ti⁺ remain strongly bound to C₂H₄ substrate.