CH4 Separation Research Articles

Study on the advantageous effect of nano-clay and polyurethane on structure and CO2 separation performance of polyethersulfone based ternary mixed matrix membranes

In recent years, many efforts have been made to develop new membrane materials for natural gas sweetening (CO2/CH4) and post-combustion CO2 capture from flue gas (CO2/N2), especially in coal-fired and natural gas-fired power plants. This study focused on the effect of polyurethane and nano-clay on structure and CO2 separating properties of polyethersulfone (PES) based three-component mixed matrix membranes (MMMs). The different weight fractions polyurethane (PU) (5−30 wt.%) together with clay nano-sheets (Cloisite® 15A) (0.5−10 wt.%) were blended to fabricate a novel PES-based ternary MMMs. The principal purpose was to investigate the effect of loading PU polymer and layered clay particles with 2D channels on the gas separation performance of the PES membranes. Pure gas permeability for N2, CH4, and CO2, and ideal gas separation for CO2/N2 and CO2/CH4 mixtures were examined through neat PES and modified PES membranes. By incorporating 20 wt.% of PU into PES, the permeability of CO2 and selectivities of CO2/N2 and CO2/CH4 were improved by 4, 1.4, and 1.7 times higher than the neat PES membrane, while by loading 20 wt.% of PU and 2 wt.% of nano-clay, the CO2 permeability, and CO2/N2, and CO2/CH4 selectivities were improved by 7.8, 1.8, and 2.2 times higher than the neat membrane, respectively. In general, PU and clay nano-sheets together as a filler into the PES matrix exhibited a meaningful improvement in the gas permeation and separation properties of PES and can be admitted as a worthy membrane for CO2 separation from the flue gas and natural gas.

A Microporous Metal-Organic Framework with Channels Constructed from Nonpolar Aromatic Rings for the Selective Separation of Ethane/Ethylene Mixtures.

The separation of ethane and ethylene is an important segment in the purification of chemical raw materials in industrial production. However, due to their similar physical and chemical properties, the separation of C2 H6 /C2 H4 is challenging. Herein, we report the selective adsorption of ethane over ethylene by a microporous metal-organic framework with nonpolar aromatic rings constructed channels, [Co1.5 (TATB)(H2 O)0.5 ] ⋅ 5DMA ⋅ 3H2 O (Co-TATB, H3 TATB=4,4',4''-(s-triazine-2,4,6-triyl) tribenzoic acid). This compound showed a higher ethane capacity than that of ethylene, and a low adsorption enthalpy of ethane only of 19.4 kJ mol-1 . Further, the dynamic breakthrough experimental confirmed that Co-TATB can selectively adsorb ethane from ethane/ethylene separation.

A Hybrid Zeolite Membrane-Based Breakthrough for Simultaneous CO2 Capture and CH4 Upgrading from Biogas.

Biogas is an environmentally friendly and sustainable energy resource that can substitute or complement conventional fossil fuels. For practical uses, biogas upgrading, mainly through the effective separation of CO2 (0.33 nm) and CH4 (0.38 nm), is required to meet the approximately 90-95% purity of CH4, while CO2 should be concomitantly purified. In this study, a high CO2 perm-selective zeolite membrane was synthesized by heteroepitaxially growing a chabazite (CHA) zeolite seed layer with a synthetic precursor that allowed the formation of all-silica deca-dodecasil 3 rhombohedral (DDR) zeolite (with a pore size of 0.36 × 0.44 nm2). The resulting hydrophobic DDR@CHA hybrid membrane on an asymmetric α-Al2O3 tube was thin (ca. 2 μm) and continuous, thus providing both high flux and permselectivity for CO2 irrespective of the presence or absence of water vapor (the third largest component in the biogas streams). To the best of our knowledge, the CO2 permeance of (2.9 ± 0.3) × 10-7 mol m-2 s-1 Pa-1 and CO2/CH4 separation factor of ca. 274 ± 73 at a saturated water vapor partial pressure of ca. 12 kPa at 50 °C have the highest CO2/CH4 separation performance yet achieved. Furthermore, we explored the membrane module properties of the hybrid membrane in terms of the recovery and purity of both CO2 and CH4 under dry and wet conditions. Despite the high intrinsic membrane properties of the current hybrid membrane, reflected by the high permeance and SF, the corresponding module properties indicated that high-performance separation of CO2 and CH4 for the desired biogas upgrading was achieved at a limited processing capacity. This supports the importance of understanding the correlation between the membrane and module properties, as this will provide guidance for the optimal operating conditions.

Methane separation and capture from nitrogen rich gases by selective adsorption in microporous Materials: A review

The rapid increase of natural gas consumption has induced imbalance between supply and demand of natural gas, and the continuous growth of methane emissions has severely exacerbated greenhouse effects and destroyed the atmospheric methane cycle. High-efficiency CH4 capture and separation from N2 rich gases has become a significant challenge for the recovery and utilization of low-grade natural gas and also industrial tail gas, which was important for sustainable energy and environment and attracted much attention from both researchers and engineers in recent years. This review provides an overview of selective separation techniques for N2 removal from natural gas including the benefits and drawbacks of these typical methods. Most notably, the review primarily outlines the progress of metal–organic frameworks, zeolites and activated carbons as superior adsorbents for separating CH4-N2 mixtures in worldwide laboratories, respectively, with an emphasis on the relationship of the pore size, surface chemistry and composition of adsorbents with the CH4/N2 selectivity and methane capacity. We also highlight the potential for the development of the new adsorbents with the best possible matching between aperture control and atomic-scale tuning of the composition for CH4 capture and separation from N2 rich gases, and indicate that the MOFs possess both the advantages of activated carbons and zeolites and will become one of the most promising candidates for high-efficiency CH4 separation from N2 rich gases in the future.

Amphipathic Solvent-Assisted Synthetic Strategy for Delamination of the Hierarchical Clinoptilolites with Flower-Like Morphology and Thinner Nanosheet for Adsorption and Separation of Co2 and Ch4

Amphipathic Solvent-Assisted Synthetic Strategy for Delamination of the Hierarchical Clinoptilolites with Flower-Like Morphology and Thinner Nanosheet for Adsorption and Separation of Co2 and Ch4

Highly steam-stable CHA-type zeolite imidazole framework ZIF-302 membrane for hydrogen separation

Metal-organic frameworks (MOFs) as molecular sieve membrane materials have received extensive attention on the gas separation application. However, the membrane separation performance is extremely limited by its intrinsic stability of frameworks. The successful preparation of high-quality gas separation membranes with excellent stability remains a grand challenge. ZIF-302 is based on hydrophobic ligand in-situ design with a high-stabile chabazite (CHA) topology. Herein, we synthesized uniform ZIF-302 nanocrystals with exceptional stability by introducing a modulator. Correspondingly, the continuous dense ZIF-302 polycrystalline membranes were also prepared for the separation of H2, N2, CO2, CH4 and other gases. The permeance of H2 and separation factor for H2/CH4 mixtures at 35 °C and 1 bar were 2.8 × 10-8 mol m−2 s−1 Pa−1 and 51.5, respectively, exceeding the Robeson upper bound in 2008. Furthermore, the separation performance of the super-hydrophobic ZIF-302 membranes for humidified mixed gases can maintain more than 168 h at 35 °C, indicating reliable long-term operational stability. Therefore, the CHA-type ZIF-302 polycrystalline membrane is promising for actual industrial gas separation.

C2H2 capture and separation in a MOF based on Ni6 trigonal-prismatic units.

A honeycomb MOF, based on rare Ni6 trigonal-prismatic supermolecular building blocks, was fabricated by utilizing an unexploited [1,1'-biphenyl]-3,3',5,5'-tetracarboxylic acid linker with -NH2 substituent groups. The MOF contains novel building blocks and an enchanting structure, and also exhibits water-stable characteristics. Uniquely, the accessible adsorption sites, arising due to the high-density Lewis-basic amino-coordinated groups and uncoordinated carboxylate O atoms in the pores, endow the MOF with excellent capture and separation capabilities for C2H2.

Construction of two novel non-penetrating Co-MOFs derived from designed 2,4,6-tri(2,4-dicarboxyphenyl)pyridine: synthesis, structure and gas adsorption properties

Two Co(ii)-based metal–organic frameworks have been successfully constructed from a polycarboxyl group ligand. Both MOFs have non-penetrating network structures, and show moderately high selective separation of C2H2, C2H4 and CO2 from CH4.

Design of ultra-stable Yttrium-organic framework adsorbents for efficient methane purification and storage

As a cleaner, cheaper and globally distributed fuel, methane (CH4) has considerable environmental, economic, and political advantages over petroleum as a source of energy for the transportation sector. However, it is still challenging to achieve efficient CH4 purification from CO2 or C2-hydrocarbons and its following storage for practical application, where the design of high-performance adsorbent materials is critical. Here we report two isostructural ultra-stable double-interpenetrated yttrium-based metal–organic frameworks (MOFs), SNNU-324 and SNNU-325, which are rationally designed by a topology-guided strategy with 1,3,5-tris(4-carboxyphenyl)benzene (BTB) and 2,4,6-tris(4-carboxyphenyl)- 1,3,5-triazine (TATB) as tritopic linkers and [Y3(OH)2(H2O)4(COO)8] and [Y4O2(H2O)4(COO)8] clusters as secondary building units (SBUs), respectively. Notably, thanks to the high-connectivity and double-interpenetrated architectures, SNNU-324–325 MOFs exhibit extra-high thermal, water and pH stability. Low-pressure gas adsorption, IAST selectivity and breakthrough curves reveal a remarkable CO2/C2-hydrocarbons over CH4 separation performance of SNNU-324 and SNNU-325. In particular, the breakthrough interval times can reach up to 20, 45, 38, and 61 min g−1 for CO2/CH4, C2H2/CH4, C2H4/CH4 and C2H6/CH4 with 2 mL min−1 for SNNU-325 at 298 K, which exceed most well-known MOF adsorbents. High-pressure gas uptake experiments further indicate SNNU-324 and SNNU-325 are promising CH4 storage adsorbents. Specially, at room temperature, SNNU-325 exhibits a total CH4 uptake of 120 cm3 cm−3 under 35 bar and deliver capacity of 65/78 cm3 cm−3 under 35/65 bar, which are comparable with most top-level methane storage MOFs.

Efficient Ch4 Separation and Vapor Uptakes in a Porous Mof Featuring 2d Interlaced Channels: Experiment and Simulation Exploration

Efficient Ch4 Separation and Vapor Uptakes in a Porous Mof Featuring 2d Interlaced Channels: Experiment and Simulation Exploration

K-Chabazite Zeolite Nanocrystal Aggregates for Highly Efficient Methane Separation.

Methane (CH4 ) enrichment and purification is of great significance for increasing the heating value of unconventional natural gas sources and curtailing its effect on global warming. For a long time, commercial adsorbents for CH4 separation have been hampered by low adsorption capacities, sub-optimal adsorption selectivities, or slow diffusion rates, which have significantly restricted separation productivity. Herein, we report a facile and green seed-passaging method to fabricate donut-like macro-meso-micro hierarchical zeolite K-Chabazite nanocrystal aggregates. This consecutive seed-inducing method requires no organic template. By utilizing this unique nanocrystallization technique, the CH4 adsorption capacity, gas diffusion rate, and separation productivity of the resultant material are dramatically increased compared with those of commercially available adsorbents, thus setting a new benchmark in CH4 /N2 separation. More importantly, production of this adsorbent can be easily scaled-up to the order of 100 kg using readily available raw materials in this environmentally friendly synthetic route, enabling potential industrial implementation.

K‐Chabazite Zeolite Nanocrystal Aggregates for Highly Efficient Methane Separation

AbstractMethane (CH4) enrichment and purification is of great significance for increasing the heating value of unconventional natural gas sources and curtailing its effect on global warming. For a long time, commercial adsorbents for CH4 separation have been hampered by low adsorption capacities, sub‐optimal adsorption selectivities, or slow diffusion rates, which have significantly restricted separation productivity. Herein, we report a facile and green seed‐passaging method to fabricate donut‐like macro–meso–micro hierarchical zeolite K‐Chabazite nanocrystal aggregates. This consecutive seed‐inducing method requires no organic template. By utilizing this unique nanocrystallization technique, the CH4 adsorption capacity, gas diffusion rate, and separation productivity of the resultant material are dramatically increased compared with those of commercially available adsorbents, thus setting a new benchmark in CH4/N2 separation. More importantly, production of this adsorbent can be easily scaled‐up to the order of 100 kg using readily available raw materials in this environmentally friendly synthetic route, enabling potential industrial implementation.

Three-in-One C2 H2 -Selectivity-Guided Adsorptive Separation across an Isoreticular Family of Cationic Square-Lattice MOFs.

Energy-efficient selective physisorption driven C2 H2 separation from industrial C2-C1 impurities such as C2 H4 , CO2 and CH4 is of great importance in the purification of downstream commodity chemicals. We address this challenge employing a series of isoreticular cationic metal-organic frameworks, namely iMOF-nC (n=5, 6, 7). All three square lattice topology MOFs registered higher C2 H2 uptakes versus the competing C2-C1 gases (C2 H4 , CO2 and CH4 ). Dynamic column breakthrough experiments on the best-performing iMOF-6C revealed the first three-in-one C2 H2 adsorption selectivity guided separation of C2 H2 from 1:1 C2 H2 /CO2 , C2 H2 /C2 H4 and C2 H2 /CH4 mixtures. Density functional theory calculations critically examined the C2 H2 selective interactions in iMOF-6C. Thanks to the abundance of square lattice topology MOFs, this study introduces a crystal engineering blueprint for designing C2 H2 -selective layered metal-organic physisorbents, previously unreported in cationic frameworks.

Three‐in‐One C2H2‐Selectivity‐Guided Adsorptive Separation across an Isoreticular Family of Cationic Square‐Lattice MOFs

AbstractEnergy‐efficient selective physisorption driven C2H2 separation from industrial C2‐C1 impurities such as C2H4, CO2 and CH4 is of great importance in the purification of downstream commodity chemicals. We address this challenge employing a series of isoreticular cationic metal‐organic frameworks, namely iMOF‐nC (n=5, 6, 7). All three square lattice topology MOFs registered higher C2H2 uptakes versus the competing C2‐C1 gases (C2H4, CO2 and CH4). Dynamic column breakthrough experiments on the best‐performing iMOF‐6C revealed the first three‐in‐one C2H2 adsorption selectivity guided separation of C2H2 from 1:1 C2H2/CO2, C2H2/C2H4 and C2H2/CH4 mixtures. Density functional theory calculations critically examined the C2H2 selective interactions in iMOF‐6C. Thanks to the abundance of square lattice topology MOFs, this study introduces a crystal engineering blueprint for designing C2H2‐selective layered metal‐organic physisorbents, previously unreported in cationic frameworks.

Acetylene Separation by a Ca-MOF Containing Accessible Sites of Open Metal Centers and Organic Groups.

Efficient separation of acetylene from a ternary acetylene-containing mixture is an important and vital task in petrochemical industry, which is difficult to achieve using a single material. Herein, a new Ca2+-based metal-organic framework (MOF) [Ca(dtztp)0.5(DMA)]·2H2O (1) was constructed using the N,O-donor ligand 2,5-di(2H-tetrazol-5-yl)terephthalic acid and the less-studied alkaline earth Ca2+ ions. The MOF shows a 3D honeycomb framework based on unique metal-carboxylate-azolate rod secondary building units. Owing to the presence of high-density organic hydrogen-bonding acceptors and open metal sites (OMSs), the activated MOF shows high adsorption capacity for C2H2 and selectivity for C2H2 over CO2, C2H4, C2H6, and CH4. Dynamic breakthrough experiments indicated the actual C2H2 separation potential of the MOF from binary (C2H2-C2H4 and C2H2-CO2) and ternary (C2H2-C2H4-CO2 and C2H2-C2H4-C2H6) mixtures. Simulations revealed that the synergistic interactions between the OMSs and N atoms in MOF and C2H2 molecules play an important role in the separation of C2H2.

Amide-Functionalized Metal-Organic Frameworks Coupled with Open Fe/Sc Sites for Efficient Acetylene Purification.

Acetylene (C2H2) purification is of great importance for many chemical synthesis and processes. Metal-organic frameworks (MOFs) are widely used for gas adsorption and separation due to their variable structure and porosity. However, the exploitation of ideal MOF adsorbents for C2H2 keeps a challenging task. Herein, a combination of open metal sites (OMSs) and Lewis basic sites (LBSs) in robust MOFs is demonstrated to effectively promote the C2H2 purification performance. Accordingly, SNNU-37(Fe/Sc), two isostructural MOFs constituted by [Fe3O(COO)6] or [Sc3O(COO)6] trinuclear clusters and amide-functionalized tricarboxylate linkers, were designed with extra-stable 3,6-connected new architectures. Derived from the coexistence of high-density OMSs and LBSs, the C2H2 adsorption amounts of SNNU-37(Fe/Sc) are much higher than those values for C2H4 and CO2. Theoretical IAST selectivity values of SNNU-37(Fe) are 2.4 for C2H2/C2H4 (50/50, v/v) and 9.9 for C2H2/CO2 (50/50, v/v) at 298 K and 1 bar, indicating an excellent C2H2 separation ability. Experimental breakthrough curves also revealed that SNNU-37(Fe) could effectively separate C2H2/C2H4 and C2H2/CO2 under ambient conditions. GCMC simulations further indicate that open Fe or Sc sites and amide groups mainly contribute to stronger adsorption sites for C2H2 molecules.

Molecular insights into hybrid CH4 physisorption-hydrate growth in hydrophobic metal–organic framework ZIF-8: Implications for CH4 storage

Adsorption-hydration hybrid method has been widely concerned due to its great application potential in CH4 storage/transportation. Monte Carlo and molecular dynamics simulations were performed to investigate the hybrid CH4 physisorption-hydrate growth in ZIF-8. The two-way migration phenomenon between water molecules inside the cavities and CH4 molecules outside the cavities first occurs in the physisorption process, which increases the CH4 adsorption capacity to a level close to that of dry ZIF-8. Due to the competitive adsorption for CH4 between ZIF-8 and hydrate surface, it is not conducive to the stability of intergranular CH4 hydrate before the CH4 adsorption capacity of ZIF-8 attains the maximum. After the maximum, intergranular hydrate could exist stably and grow continuously with the dissolution of CH4. Meanwhile, partially adsorbed CH4 molecules in the cavities migrate outwards and promote hydrate growth. These constitute the link between physisorption and hydrate growth. Because of the phase separation of CH4 and water molecules, hydrate cannot form in the ZIF-8 cavities. As a result, under the synergistic effect of physisorption and intergranular hydrates, CH4 storage capacity is significantly increased in H2O-oversaturated ZIF-8 compared with that in dry ZIF-8. Further, hydrophobic ZIF-8 surface facilitates hydrogen bonds formation in interfacial water by inducing ordered structures, and interfacial water as a medium promotes the contact between hydrate cage and ZIF-8 surface. An analysis of the diffusion properties of CH4/water molecules reveals that ZIF-8 cavities, hydrate cages, and intergranular liquid water exert inhibition on their self-diffusion coefficients in different degree. This study provides important microscopic insight into CH4 storage by adsorption-hydration hybrid in ZIF-8.

Molecular dynamics simulation of diffusion and separation of CO2/CH4/N2 on MER zeolites

The adsorption behaviors of CO2, CH4 and N2 on MER zeolites were investigated by Grand Canonical Monte Carlo (GCMC) simulation method. The calculated pure gas uptake agreed well with the experimental data, which proved that the current simulation model and COMPASS force field are reliable. On this basis, molecular dynamics (MD) simulations were carried out for diffusion and separation of CO2, CH4 and N2 on K-MER zeolites, with the silicon MER zeolite as the reference. The results show that mean squared displacement (MSD) versus simulation time is sublinear. In MER zeolites, the configuration diffusion regime is dominant resulted from the tight fit of the gas molecules and the zeolite pore size. The diffusion of CO2, CH4 and N2 in MER zeolites with three-dimensional cage structures is anisotropic. The gas molecules diffuse preferentially along with the direction of x axis in K-MER zeolites. Extra-framework cations in zeolite plays important influence on the gas diffusion. In K-MER zeolites, the self-diffusion coefficients of CO2 and N2 are negative correlated with loading, whereas for the self-diffusion coefficient of CH4, it firstly increases and then decreases with the increase of loading. All of the self-diffusion coefficients of CO2, CH4 and N2 increase with the elevation of temperature. The order of diffusion activation energy is N2 (16.51 kJ/mol)>CH4 (8.39 kJ/mol)>CO2 (4.38 kJ/mol). K-MER zeolite membrane has good separation selectivity for gas mixture system of CO2/CH4, CO2/N2 and N2/CH4.The permeance of CO2 and N2 through K-MER zeolite membrane is as high as 104 GPU (1 GPU=3.35×10−10 mol/(s·m2·Pa)).

Modelling adsorption based on an isoreticular MOF‐series of IFPs—Part II: Dynamic adsorption in fixed beds

AbstractBased on experimental pure component data for the characterization of the isostructural imidazolate framework Potsdam (IFP) series reported in Part I, a model for the simulation of non‐isothermal dynamic adsorption of CO2/CH4‐mixtures in fixed‐bed columns is presented in this Part II. The robustness of the model is examined and validated, by comparison to experimental breakthrough data at different process conditions, such as varying concentration, temperature, and pressure. Thereby, different predictive methods for the estimation of adsorption equilibria of mixtures are compared (RAST, IAST, ML). The results show that ideal behaviour can be assumed with good accuracy for the system under consideration, except for IFP‐2, which shows significant deviations at increased pressures and temperatures. A detailed kinetic analysis reveals that mass transfer is significantly influenced by micropore diffusion. Thus, only for IFP‐1 the dynamic separation of CO2 and CH4 is equilibrium‐driven, while for the remaining IFPs the kinetic regime dominates the process, which in some cases increases the separation efficiency (IFP‐2 to ‐7) but can also inhibit it (IFP‐8). The determined intracrystalline diffusion coefficients show very good agreement with values for metal organic framework (MOF) compounds of similar structure reported in the literature.

Interpenetration Symmetry Control Within Ultramicroporous Robust Boron Cluster Hybrid MOFs for Benchmark Purification of Acetylene from Carbon Dioxide

AbstractThe separation of C2H2/CO2 is an important process in industry but challenged by the trade‐off of capacity and selectivity owning to their similar physical properties and identical kinetic molecular size. We report the first example of symmetrically interpenetrated dodecaborate pillared MOF, ZNU‐1, for benchmark selective separation of C2H2 from CO2 with a high C2H2 capacity of 76.3 cm3 g−1 and record C2H2/CO2 selectivity of 56.6 (298 K, 1 bar) among all the robust porous materials without open metal sites. Single crystal structure analysis and modeling indicated that the interpenetration shifting from asymmetric to symmetric mode provided optimal pore chemistry with ideal synergistic “2+2” dihydrogen bonding sites for tight C2H2 trapping. The exceptional separation performance was further evidenced by simulated and experimental breakthroughs with excellent recyclability and high productivity (2.4 mol kg−1) of 99.5 % purity C2H2 during stepped desorption process.