Research Progress in Microporous Materials for Selective Adsorption and Separation of Methane from Low-Grade Gas

Given that methane (CH4) and nitrogen (N2) have similar properties, achieving high-purity enrichment of CH4 from nitrogen-rich low-grade gas is extremely challenging and is of great significance for sustainable development in energy and the environment. This paper reviews the research progress on carbon-based materials, zeolites, and MOFs as adsorbent materials for CH4/N2 separation. It focuses on the relationship between the composition, pore size, surface chemistry of the adsorbents, CH4/N2 selectivity, and CH4 adsorption capacity. The paper also highlights that controlling pore size and atomic-scale composition and optimizing these features for the best match are key directions for the development of new adsorbents. Additionally, it points out that MOFs, which combine the advantages of carbon-based adsorbents and zeolites, are likely to become the most promising adsorbent materials for efficient CH4/N2 separation.

Using ligand regulation, metal replacement, and ligand doping strategies on Zr-FUM to improve methane separation from coalbed gas

Developing adsorbents suitable for industrial applications that can effectively enhance the separation of methane (CH4) from nitrogen (N2) in coalbed gas is crucial to improve energy recovery and mitigate greenhouse gas emissions. In this study, three modification strategies were implemented on Zr-FUM, including ligand regulation, metal replacement, and ligand doping, to synthesize Zr-FDCA, Al-FUM, and Zr-FUM-FA, with the aim of improving the performance of CH4/N2 separation under humid conditions. The results demonstrated that the promotion of robust orbital overlap and strengthened electrovalent bonding on adsorbents can selectively enhance CH4 adsorption. As a result, Zr-FUM-FA achieved a saturated CH4 adsorption capacity of 1.37 mmol/g, a CH4 working window of 307 s, and a CH4/N2 sorbent selection parameter (Ssp) of 47.31, exceeding the performance of most reported adsorbents. Analyses of the pore structure, surface morphology, and functional groups revealed that the presence of an ultramicropore proximity to CH4, reduced static resistance, and enhanced electrovalent bond were key factors for CH4 separation. Grand Canonical Monte Carlo and Density Functional Theory studies indicated that the introduction of -C-H- in FA played a crucial role in enhancing CH4 adsorption. Optimization of adsorption parameters using the Aspen adsorption package showed that in a dual-adsorbent bed system, the recovery and purity of CH4 in Zr-FUM-FA reach 99.5% and 97.3%, respectively, providing important theoretical support for the improvement of CH4 recovery in the pressure swing adsorption process from coalbed gas.

Adsorption of Natural Gas in Metal-Organic Frameworks: Selectivity, Cyclability, and Comparison to Methane Adsorption

Evaluation of metal-organic frameworks (MOFs) for adsorbed natural gas (ANG) technology employs pure methane as a surrogate for natural gas (NG). This approximation is problematic, as it ignores the impact of other heavier hydrocarbons present in NG, such as ethane and propane, which generally have more favorable adsorption interactions with MOFs compared to methane. Herein, using quantitative Raman spectroscopic analysis and Monte Carlo calculations, we demonstrate the adsorption selectivity of high-performing MOFs, such as MOF-5, MOF-177, and SNU-70, for a methane and ethane mixture (95:5) that mimics the composition of NG. The impact of selectivity on the storage and deliverable capacities of these adsorbents during successive cycles of adsorption and desorption, simulating the filling and emptying of an ANG tank, is also demonstrated. The study reveals a gradual reduction in the storage performance of MOFs, particularly with smaller pore volumes, due to ethane accumulation over long-term cycling, until a steady state is reached with substantially degraded storage performance.

Nitrogen Rejection from Methane via a "Trapdoor" K-ZSM-25 Zeolite

Nitrogen (N₂) rejection from methane (CH₄) is the most challenging step in natural gas processing because of the close similarity of their physical-chemical properties. For decades, efforts to find a functioning material that can selectively discriminate N₂ had little outcome. Here, we report a molecular trapdoor zeolite K-ZSM-25 that has the largest unit cell among all zeolites, with the ability to capture N₂ in favor of CH₄ with a selectivity as high as 34. This zeolite was found to show a temperature-regulated gas adsorption wherein gas molecules' accessibility to the internal pores of the crystal is determined by the effect of the gas-cation interaction on the thermal oscillation of the "door-keeping" cation. N₂ and CH₄ molecules were differentiated by different admission-trigger temperatures. A mild working temperature range of 240-300 K was determined wherein N₂ gas molecules were able to access the internal pores of K-ZSM-25 while CH₄ was rejected. As confirmed by experimental, molecular dynamic, and ab initio density functional theory studies, the outstanding N₂/CH₄ selectivity is achieved within a specific temperature range where the thermal oscillation of door-blocking K⁺ provides enough space only for the relatively smaller molecule (N₂) to diffuse into and through the zeolite supercages. Such temperature-regulated adsorption of the K-ZSM-25 trapdoor zeolite opens up a new approach for rejecting N₂ from CH₄ in the gas industry without deploying energy-intensive cryogenic distillation around 100 K.

Alkyl decorated metal-organic frameworks for selective trapping of ethane from ethylene above ambient pressures

The trapping of paraffins is beneficial compared to selective olefin adsorption for adsorptive olefin purification from a process engineering point of view. Here we demonstrate the use of a series of Zn2(X-bdc)2(dabco) (where X-bdc2- is bdc2- = 1,4-benzenedicarboxylate with substituting groups X, DM-bdc2- = 2,5-dimethyl-1,4-benzenedicarboxylate or TM-bdc2- = 2,3,5,6-tetramethyl-1,4-benzenedicarboxylate and dabco = diazabicyclo[2.2.2.]octane) metal-organic frameworks (MOFs) for the adsorptive removal of ethane from ethylene streams. The best performing material from this series is Zn2(TM-bdc)2(dabco) (DMOF-TM), which shows a high ethane uptake of 5.31 mmol g-1 at 110 kPa, with a good IAST selectivity of 1.88 towards ethane over ethylene. Through breakthrough measurements a high productivity of 13.1 L kg-1 per breakthrough is revealed with good reproducibility over five consecutive cycles. Molecular simulations show that the methyl groups of DMOF-TM are forming a van der Waals trap with the methylene groups from dabco, snuggly fitting the ethane. Further, rarely used high pressure coadsorption measurements, in pressure regimes that most scientific studies on hydrocarbon separation on MOFs ignore, reveal an increase in ethane capacity and selectivity for binary mixtures with increased pressures. The coadsorption measurements reveal good selectivity of 1.96 at 1000 kPa, which is verified also through IAST calculations up to 3000 kPa. This study overall showcases the opportunities that pore engineering by alkyl group incorporation and pressure increase offer to improve hydrocarbon separation in reticular materials.

Preparation of Activated Carbon Doped with Graphene Oxide Porous Materials and Their High Gas Adsorption Performance

It is still a great challenge to develop a new porous carbon adsorbent with excellent separation performance and to recover low-concentration CH₄ in coal mine gas. This work provides a new idea for the study of CH₄ adsorption on activated carbon (AC) composites. Composite materials with microporous structures were prepared from coconut-shell activated carbon (CAC) doped with graphene oxide (GO) by a chemical activation process in this paper. The expansion and dissociation of GO at high temperatures indirectly improve the specific surface area (SSA) of the composite. The interlayer aggregation is reduced, the activation effect is improved, and a new low-cost adsorption material is prepared. The SSA of CAC-50 is more than 3000 m²·g^-1. A high SSA and a narrow pore size distribution lead to a higher total adsorption capacity of CH₄. The breakthrough test also confirmed that AC/GOs had a better adsorption capacity for CH₄. The separation performance of the CH₄/N₂ mixture is not good at room temperature, which is due to the influence of a high SSA and average pore size. As a low-cost and rich material, CAC has a wide range of application prospects. The composite is a potential material for recovering low-concentration CH₄ from the coal mine, which is worthy of attention. In the future, the selectivity of AC/GOs to CH₄ can be increased by loading functional groups or modification.

An automated multi-component gas adsorption system (MC GAS)

The knowledge gap on adsorption of complex mixtures in the literature relative to single component data represents a persistent obstacle to developing accurate process models for adsorption separations. The collection of mixed gas adsorption data is an imminent need for improved understanding of the behavior of adsorbent systems in these diverse adsorption applications. Current approaches to understanding mixture adsorption using predictive theories based on pure component adsorption experiments often fail to capture the behavior of more complex, non-ideal systems. In this work, we present an automated volumetric instrument for the measurement of mixed gas adsorption isotherms. This instrument was validated by comparison to other in-house instruments and data available in the literature, and the binary adsorption measurements were found to be thermodynamically consistent. The automation of this instrument allows for rapid collection of high-quality mixture adsorption data.

Solvent-Free and Phase-Selective Synthesis of Aluminum Trimesate Metal-Organic Frameworks

Aluminum-based metal-organic frameworks (Al-MOFs) have shown promise as commercially valuable materials due to the variety of applications, excellent thermal, hydrothermal, and chemical stabilities, and the abundance of aluminum. In this work, for the first time, we report the solvent-free synthesis of the aluminum trimesate (Al-BTC) MOFs (MIL-100(Al), MIL-96(Al), and MIL-110(Al)) with phase selectivity and high yield. These MOFs were traditionally prepared with HF, HNO₃, and bulk solvents, but these methods struggled to produce pure-phase isolations. The solvent-free strategy provides valuable insight into the future industrial scale-up production of the Al-MOFs and promotes the potential commercialization of such materials.

Vapor-phase linker exchange of metal-organic frameworks

Metal-organic frameworks (MOFs) have been attracting intensive attention because of their commendable potential in many applications. Postsynthetic modification for redesigning chemical characteristics and pore structures can greatly improve performance and expand functionality of MOF materials. Here, we develop a versatile vapor-phase linker exchange (VPLE) methodology for MOF modification. Through solvent-free and environment-friendly VPLE processing, various linker analogs with functional groups but not for straightforward MOF crystallization are inserted into frameworks as daughter building blocks. Besides single exchange for preparing MOFs with dual linkers, VPLE can further be performed by multistage operations to obtain MOF materials with multiple linkers and functional groups. The halogen-incorporated ZIFs exhibit good porosity, tunable molecular affinity, and impressive CO₂/N₂ and CH₄/N₂ adsorption selectivities up to 31.1 and 10.8, respectively, which are two to six times higher than those of conventional adsorbents. Moreover, VPLE can substantially enhance the compatibility of MOFs and polymers.

Tuning the Molecular and Cationic Affinity in a Series of Multifunctional Metal-Organic Frameworks Based on Dodecanuclear Zn(II) Carboxylate Wheels

A series of new zinc(II)-thiophene-2,5-dicarboxylate (tdc) MOFs based on novel dodecanuclear wheel-shaped building blocks has been synthesized in almost quantative yields. Single-crystal X-ray diffraction analyses reveal 3D porous frameworks with a complex composition [Zn₁₂(tdc)₆(glycolate)₆(dabco)₃] where glycolate is a deprotonated polyatomic alcohol (ethylene glycol, EgO₂, 1; 1,2-propanediol, PrO₂, 2; 1,2-butanediol, BuO₂, 3; 1,2-pentanediol, PeO₂, 4; glycerol, GlO₂, 5) and dabco is 1,4-diazo[2.2.2.]bicyclooctane. All compounds 1-5 are isostructural except for pendant groups of the diols decorating the surface of channels. The adsorption of small gases (N₂, CO₂, CH₄, C₂H₂, C₂H₄, C₂H₆) and larger hydrocarbons (benzene, cyclohexane) both in liquid and vapor phases was thoroughly investigated for all compounds. The zero-coverage adsorption enthalpies, Henry constants, and selectivity factors by various models are calculated and discussed. The versatile adsorption functionality of the title series results from the variable nature of the diol and could be tailored for a specific adsorbate system. For example, 1 shows excellent selectivity of benzene over cyclohexane (20:1 for vapors, 92:1 for liquid phase), while 4 demonstrates unprecedented adsorption preference of cyclohexane over benzene (selectivity up to 5:1). The compound 5 demonstrates great adsorption selectivity for CO₂/N₂ (up to 75.1), CO₂/CH₄ (up to 7.7), C₂H₂/CH₄ (up to 14.2), and C₂H₄/CH₄ (up to 9.4). Also, due to polar nature of the pores, 5 features size-selective sorption of alkaline metal cations in order Li⁺ > Na⁺ > K⁺ > Cs⁺ as well as a notable luminescent response for cesium(I) ions and urea.

Improving CO2 Separation Performance of MIL-53(Al) by Incorporating 1-n-Butyl-3-Methylimidazolium Methyl Sulfate

1-n-Butyl-3-methylimidazolium methyl sulfate is incorporated into MIL-53(Al). Detailed characterization is done by X-ray fluorescence, Brunauer-Emmett-Teller surface area, scanning electron microscopy, X-ray diffraction, Fourier-transform infrared spectroscopy, and thermogravimetric analysis. Results show that ionic liquid (IL) interacts directly with the framework, significantly modifying the electronic environment of MIL-53(Al). Based on the volumetric gas adsorption measurements, CO2, CH4, and N2 adsorption capacities decreased from 112.0, 46.4, and 19.6 cc (STP) gMIL-53(Al) -1 to 42.2, 13.0, and 4.3 cc (STP) gMIL-53(Al) -1 at 5 bar, respectively, upon IL incorporation. Data show that this postsynthesis modification leads to more than two and threefold increase in the ideal selectivity for CO2 over CH4 and N2 separations, respectively, as compared with pristine MIL-53(Al). The isosteric heat of adsorption (Qst) values show that IL incorporation increases CO2 affinity and decreases CH4 and N2 affinities. Cycling adsorption-desorption measurements show that the composite could be regenerated with almost no decrease in the CO2 adsorption capacity for six cycles and confirm the lack of any significant IL leaching. The results offer MIL-53(Al) as an excellent platform for the development of a new class of IL/MOF composites with exceptional performance for CO2 separation.

Phenolics isolation from bio-oil using the metal–organic framework MIL-53(Al) as a highly selective adsorbent

The selective uptake of phenolic compounds, a key step of bio-refining, was achieved on MIL-53(Al) and Basolite A100 both from a simulated bio-oil and a real pyrolysis bio-oil.

Acidity and Cd2+ fluorescent sensing and selective CO2 adsorption by a water-stable Eu-MOF

A new luminescent Eu-MOF from an amino-group modified tetracarboxylic acid ligand was designed, which could perform as an efficient pH acidity and Cd²⁺ PL sensor and CO₂ selector.

Rigorous prognostication and modeling of gas adsorption on activated carbon and Zeolite-5A

Gas adsorption on various adsorbents is of highly important issue for the separation of gas mixtures in many industrial processes. In this work, estimation of pure gases (CH₄, N₂, CO₂, H₂, C₂H₄) adsorption on activated carbon (AC) and CO₂, CH₄, N₂ on Zeolite-5A adsorbent were studied by developing four different computing techniques, namely MLP-ANN, ANFIS, LSSVM, and PSO-ANFIS for a broad range of experimental data found in the literature. Temperature, pressure, pore size (only for AC) and kinetic diameter of adsorbed gases are considered as the inputs and the gas adsorption as the output parameters of the developed models. We also used several statistical and graphical tools to assess the accuracy and applicability of the proposed models. The results of the study suggest the reliability and validity of all the models developed for estimating the equilibrium adsorption of gases on the adsorbents. Also, it is found that of all the models developed, the ANN model estimates experimental data of the gas adsorption on AC more accurately due to its values of R² and AARD%, 0.9865 and 0.8948, respectively. Besides, PSO-ANFIS is the best model to prognosticate gas adsorption on zeolite 5A with R² and AARD%, 0.9897 and 0.9551, respectively.

SO2 gas adsorption on carbon nanomaterials: a comparative study

Owing to their high stability against corrosive gases, carbon-based adsorbents are preferentially used for the adsorptive removal of SO₂. In the present study, SO₂ adsorption on different carbon nanomaterials namely carbon nanohorns (CNHs), multiwalled carbon nanotubes (MWNTs), single-walled carbon nanotubes (SWNTs) and vertically aligned carbon nanotubes (VACNTs) are investigated and compared against the adsorption characteristics of activated carbon and graphene oxide (GO). A comprehensive overview of the adsorption behavior of this family of carbon adsorbents is given for the first time. The relative influence of surface area and functional groups on the SO₂ adsorption characteristics is discussed. The isosteric heat of adsorption values are calculated to quantify the nature of the interaction between the SO₂ molecule and the adsorbent. Most importantly, while chemisorption is found to dominate the adsorption behavior in activated carbon, SO₂ adsorption on carbon nanomaterials occurs by a physisorption mechanism.

Exploring the electronic structure of aluminum metal–organic framework Basolite A100: solid-state synchronous fluorescence spectroscopy reveals new charge excitation/relaxation pathways

Solid-state synchronous fluorescence spectroscopy is superior to “conventional” fluorescence; it identifies absorption/emission by the linkers, bandgap and excimers in Al-MOF Basolite A100.

Controlled synthesis of conjugated polycarbazole polymers via structure tuning for gas storage and separation applications

A series of conjugated microporous polymers (CMPs) based on 1,3,6,8-tetrabromocarbazole (N4CMP-1-5) is synthesized via Suzuki cross-coupling or Sonogashira polycondensation. The porosity properties and surface area of these polymer networks can be finely tuned by using a linker with different geometries or strut length. These polymers show the Brunauer-Emmett-Tellerthe (BET) surface areas ranging from 592 to 1426 m2 g-1. The dominant pore sizes of the polymers on the basis of the different linker are located between 0.36 and 0.61 nm. Gas uptake increases with BET surface area and micropore volume, N4CMP-3 polymer can capture CO2 with a capacity of 3.62 mmol g-1 (1.05 bar and 273 K) among the obtained polymers. All of the polymers show high isosteric heats of CO2 adsorption (25.5-35.1 kJ mol-1), and from single component adsorption isotherms, IAST-derived ideal CO2/N2 (28.7-53.8), CO2/CH4 (4.6-5.2) and CH4/N2 (5.7-10.5) selectivity. Furthermore, N4CMPs exhibit the high CO2 adsorption capacity of 542-800 mg g-1 at 318 K and 50 bar pressure. These data indicate that these materials are a promising potential for clean energy application and environmental field.

Hierarchically structured nanoporous carbon tubes for high pressure carbon dioxide adsorption

Mesoscopic, nanoporous carbon tubes were synthesized by a combination of the Stoeber process and the use of electrospun macrosized polystyrene fibres as structure directing templates. The obtained carbon tubes have a macroporous nature characterized by a thick wall structure and a high specific surface area of approximately 500 m²/g resulting from their micro- and mesopores. The micropore regime of the carbon tubes is composed of turbostratic graphitic areas observed in the microstructure. The employed templating process was also used for the synthesis of silicon carbide tubes. The characterization of all porous materials was performed by nitrogen adsorption at 77 K, Raman spectroscopy, infrared spectroscopy, thermal gravimetric analysis (TGA), scanning electron microscopy (SEM) as well as transmission electron microscopy (TEM). The adsorption of carbon dioxide on the carbon tubes at 25 °C at pressures of up to 30 bar was studied using a volumetric method. At 26 bar, an adsorption capacity of 4.9 mmol/g was observed. This is comparable to the adsorption capacity of molecular sieves and vertically aligned carbon nanotubes. The high pressure adsorption process of CO2 was found to irreversibly change the microporous structure of the carbon tubes.

CO2 Capture and Separations Using MOFs: Computational and Experimental Studies

This Review focuses on research oriented toward elucidation of the various aspects that determine adsorption of CO₂ in metal-organic frameworks and its separation from gas mixtures found in industrial processes. It includes theoretical, experimental, and combined approaches able to characterize the materials, investigate the adsorption/desorption/reaction properties of the adsorbates inside such environments, screen and design new materials, and analyze additional factors such as material regenerability, stability, effects of impurities, and cost among several factors that influence the effectiveness of the separations. CO₂ adsorption, separations, and membranes are reviewed followed by an analysis of the effects of stability, impurities, and process operation conditions on practical applications.

Role of porosity and polarity of nanoporous carbon spheres in adsorption applications

Structure-properties (i.e., porosity and polarity) of carbons are judiciously considered for the specific adsorption applications.

Selective separation of CO2–CH4 mixed gases via magnesium aminoethylphosphonate nanoparticles

Magnesium 2-aminoethylphosphonate (Mg₂O(2AEP) × 4H₂O) nanoparticles (particle diameter: 20–30 nm; specific surface area: 360 m² g⁻¹) are presented for selective separation of CO₂ and CH₄.

Adsorptive Separation of Olefin/Paraffin Mixtures with ZIF-4

The microporous zeolitic imidazolate framework ZIF-4 has been synthesized, and its ethylene/ethane and propylene/propane separation potentials have been evaluated by single-component adsorption isotherms and breakthrough experiments of the respective binary mixtures. In all experiments, a higher selectivity for the paraffin is observed that is manifested by a steeper equilibrium isotherm as well as a later breakthrough in the fixed-bed adsorber experiments. Microporous adsorbents with paraffin selectivity are rare but highly interesting for cyclic adsorption processes such as pressure-swing adsorption (PSA).