Clinoptilolites for nitrogen/methane separation

One Scalable and Stable Metal-Organic Framework for Efficient Separation of CH4/N2 Mixture

Separating CH4 from coal bed methane is of great importance but challenging. Adsorption-based separation often suffers from low selectivity, poor stability, and difficulty to scale up. Herein, a stable and scalable metal-organic framework [MOF, CoNi(pyz-NH2)] with multiple CH4 binding sites was reported to efficiently separate the CH4/N2 mixture. Due to its suitable pore size and multiple CH4 binding sites, it exhibits excellent CH4/N2 selectivity (16.5) and CH4 uptake (35.9 cm3/g) at 273 K and 1 bar, which is comparable to that of the state-of-the-art MOFs. Theoretical calculations reveal that the high density of open metal sites and polar functional groups in the pores provide strong affinity to CH4 than to N2. Moreover, CoNi(pyz-NH2) displays excellent structural stability and can be scale-up synthesized (22.7 g). This work not only provides an excellent adsorbent but also provides important inspiration for the future design and preparation of porous adsorbents for separations.

Screening of Alkali Metal-Exchanged Zeolites for Nitrogen/Methane Separation

Methane (CH4) is the primary component of natural gas and must be purified to a certain level before it can be used as pipeline gas or liquified natural gas (LNG). In particular, nitrogen (N2), a common contaminant in natural gas needs to be rejected to increase the heating value of the gas and meet the LNG product specifications. The development of energy-efficient N2 removal technologies is hampered by N2's inertness and its resemblance to CH4 in terms of kinetic size and polarizability. N2-selective materials are so rare. Here, for the first time, we screened 1425 alkali metal cation exchange zeolites to identify the candidates with the best potential for the separation of N2 from CH4. We discovered a few extraordinary zeolite frameworks capable of achieving equilibrium selectivity toward N2. Particularly, Li+-RRO-3 zeolite with a specific two-dimensional structure demonstrated a selective N2 adsorption capacity of 2.94 mmol/g at 283 K and 1 bar, outperforming the capacity of all known zeolites. Through an ab initio density functional theory study, we found that the five-membered ring of the RRO framework is the most stable cationic site for Li+, and this Li+ can interact with multiple N2 molecules but only one CH4, revealing the mechanism for the high capacity and selectivity of N2. This work suggests promising adsorbents to enable N2 rejection from CH4 in the gas industry without going for energy-intensive cryogenic distillations.

Natural Zeolite Clinoptilolite Application in Wastewater Treatment: Methylene Blue, Zinc and Cadmium Abatement Tests and Kinetic Studies

In recent decades, several abatement techniques have been proposed for organic dyes and metal cations. In this scenario, adsorption is the most known and studied. Clinoptilolite was considered, since it is a zeolite with a relatively low cost (200-600 $ tons^-1) compared to the most well-known adsorbent used in wastewater treatment. In this work, Clinoptilolite was used for the adsorption of Methylene Blue (MB) at three different concentrations, namely, 100, 200, and 250 ppm. Furthermore, the adsorption capacity of the natural zeolite was compared with that of Activated Charcoal (250 ppm of MB). The two adsorbents were characterized by complementary techniques, such as N₂ physisorption at -196 °C, X-ray diffraction, and field emission scanning electron microscopy. During the adsorption tests, Clinoptilolite exhibited the best adsorption capacities at 100 ppm: the abatement reached 98% (t = 15 min). Both Clinoptilolite and Activated Charcoal, at 250 ppm, exhibited the same adsorption capacities, namely, 96%. Finally, at 250 ppm MB, the adsorption capacity of Clinoptilolite was analyzed with the copresence of Zn²⁺ and Cd²⁺ (10 ppm), and the adsorption capacities were compared with those of Activated Charcoal. The results showed that both adsorbents achieved 100% MB abatement (t = 40 min). However, cation adsorption reached a plateau after 120 min (Zn²⁺ = 86% and 57%; Cd²⁺ = 53% and 50%, for Activated Charcoal and Clinoptilolite, respectively) due to the preferential adsorption of MB molecules. Furthermore, kinetic studies were performed to fully investigate the adsorption mechanism. It was evidenced that the pseudo-second-order kinetic model is effective in describing the adsorption mechanism of both adsorbents, highlighting the chemical interaction between the adsorbent and adsorbate.

Explorations on Thermodynamic and Kinetic Performances of Various Cationic Exchange Durations for Synthetic Clinoptilolite

Various cation–exchanged clinoptilolites (M–CPs, M = Li⁺, Cs⁺, Ca²⁺, Sr²⁺) were prepared, and their exchanged thermodynamic (and kinetic) properties and adsorption performances for CH₄, N₂, and CO₂ were investigated. The results demonstrated that the relative crystallinity of M–CP_S decreased with the increase of exchange times. Their chemisorbed water weight loss gradually increased with the increasing exchange times, except that of Cs–x–CP. The Δ_rGmθ values of exchange process of Li⁺, Cs⁺, Ca²⁺, or Sr² presented the increased trend with the enhanced exchange times, but they decreased as the temperature increased. The negative Δ_rGmθ values and the positive Δ_rHmθ and Δ_rSmθ values suggested that the exchanged procedure belonged to spontaneous, endothermic, and entropy-increasing behaviors; their kinetic performances followed a pseudo–second–order model. However, the calculated E_a values of exchange process showed the increased tendencies with the enhanced exchange times, indicating that the exchange process became more difficult. Finally, the preliminary adsorption results indicated that the maximum adsorption amount at 273 K and 1 bar was 0.51 mmol/g of CH₄ and 0.38 mmol/g of N₂ by (Na, K)–CP, and 2.32 mmol/g of CO₂ by Li–6–CP.

Tuning the Pore Environment of MOFs toward Efficient CH4/N2 Separation under Humid Conditions

Adsorption separation technology using adsorbents is promising as an alternative to the energy-demanding cryogenic distillation of natural gas (CH₄/N₂) separation. Although a few adsorbents, such as metal-organic frameworks (MOFs), with high performance for CH₄/N₂ separation, have been reported, it is still challenging to target the desired adsorbents for the actual CH₄/N₂ separation under humid conditions because the adsorption capacity and selectivity of the adsorbents might be mainly dampened by water vapor. Except for the high CH₄ uptake and CH₄/N₂ selectivity, the adsorption material should simultaneously have excellent stability against moisture and relatively low-water absorption affinity. Here, we tuned the ligands and metal sites of reticular MOFs, Zn-benzene-1,4-dicarboxylic acid-1,4-diazabicyclo[2.2.2]octane (Zn-BDC-DABCO) (DMOF), affording a series of isostructural MOFs (DMOF-N, DMOF-A₁, DMOF-A₂, and DMOF-A₃). Because of the finely engineered pore size and introduced aromatic rings in the functional DMOF, gas sorption results reveal that the materials show improved performance with a benchmark CH₄ uptake of 37 cm³/g and a high CH₄/N₂ adsorption selectivity of 7.2 for DMOF-A₂ at 298 K and 1.0 bar. Moisture stability experiments show that DMOF-A₂ is a robust MOF with low water vapor capacity even at ∼40% relative humidity (RH) because of the presence of more hydrophobic aromatic rings. Breakthrough experiments verify the excellent CH₄/N₂ separation performances of DMOF-A₂ under high humidity.

Low-Energy Adsorptive Separation by Zeolites

Separation of mixture is always necessarily required in modern industry, especially in fine chemical, petrochemical, coal chemical, and pharmaceutical industries. The challenge of separation process is usually associated with small molecules with very similar physical and chemical properties. Among the separation techniques, the commonly used high-pressure cryogenic distillation process with combination of high-pressure and very low temperature is heavily energy-consumed and accounts for the major production costs as well as 10–15% of the world's energy consumption. To this end, the adsorptive separation process based on zeolite sorbents is a promising lower-energy alternative and the performance is directly determined by the zeolite sorbents. In this review, we surveyed the separation mechanisms based on the steric, equilibrium, kinetic, and ‘trapdoor’ effect, and summarized the recent advances in adsorptive separation via zeolites including CO2, light olefins, C8 aromatics, and hydrogen isotopes. Furthermore, we provided the perspectives on the rational design of zeolite sorbents for the absolute separation of mixtures.

Effect of Gallium as an Additive Over Corresponding Ni–Mo/γ-Al2O3 Catalysts on the Hydrodesulfurization Performance of 4,6-DMDBT

Experiments were carried out to research the different contents of Ga₂O₃ modification effects on the hydrodesulfurization (HDS) performance of 4,6-dimethyldibenzothiophene (4,6-DMDBT) catalyzed by the stepwise impregnation method. Characterization techniques such as XRD, BET, HRTEM, NH₃-TPD, and Py-FTIR were performed to determine the effects of each modification of the catalyst by Ga on the properties of the prepared supports and catalysts. The catalytic effect of gallium is reflected in the fact that the empty d-orbitals of Ga elements participate in the formation of molecular orbitals in the active center and change their orbital properties, thus generating a direct desulfurization active phase suitable for complex sulfides for endpoint adsorption. The characterization results indicated that the introduction of Ga₂O₃ with appropriate content (2 wt.%) promoted Ni and Mo species to disperse uniformly and doping of more Ni atoms into the MoS₂ crystals, which also increased the average stacking number and the length of MoS₂. As a result, more NiMoS active phases were favored to form in the system. The specific surface area and the amounts of acid sites were increased, facilitating the adsorption of reactant molecules and the HDS reactions. The HDS results also suggested the effects of Ga modification play a very important role in the catalytic performance of the corresponding catalysts. The catalyst Ga–Ni–Mo/Al₂O₃ exhibited the highest conversion rate towards 4,6-DMDBT HDS when the amount of Ga₂O₃ loading was 2 wt.% with an LHSV of 2.5 h⁻¹ at 290°C and Ga modification also can effectively improve the direct desulfurization (DDS) route selectivity in varying degrees.

Highly efficient and reversible adsorption of ammonia by incorporation of deep eutectic solvents into silica gel and Al2O3

The uptake of NH3 in the industrial tail gas will improve environmental quality for human health and ecology. Adsorption separation has the advantages of simple operation and low energy consumption....

Construction of a Porous Metal-Organic Framework with a High Density of Open Cr Sites for Record N2 /O2 Separation

The removal of low concentration N₂ is of great significance and challenging in the industrial production of high-purity O₂ . Herein, a chromium-based metal-organic framework, namely, TYUT-96Cr, is reported, which has an unprecedented N₂ capture capacity of 37.46 cm³ cm^-3 and N₂ /O₂ (5:95, v/v) selectivity up to 26.95 (298 K and 1 bar), thus setting new benchmarks for all reported metal-organic frameworks and commercially used ones (Li-LSX and 13X). Breakthrough experiments reveal that N₂ can be directly extracted from various N₂ /O₂ (79:21, 50:50, 5:95, and 1:99, v/v) mixtures by this material, affording a record-high O₂ -production scale with 99.99% purity. Density functional theory calculations and in situ infrared spectroscopy studies demonstrate that the high-density open Cr (III) sites in TYUT-96Cr can behave as effective Lewis acidic sites, thus resulting in a strong affinity toward N₂ . The high N₂ adsorption selectivity, exceptional separation performance, and ultrahigh structural stability render this porous material with great potential for this important industrial application.

Separation and Purification of Hydrocarbons with Porous Materials

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

Accurate and Compatible Force Fields for Molecular Oxygen, Nitrogen, and Hydrogen to Simulate Gases, Electrolytes, and Heterogeneous Interfaces

Gas molecules and interfaces with liquids and solids play a critical role in living organisms, sorption, catalysis, and the environment. Monitoring adsorption and heterogeneous interfaces remains difficult in experiments, and earlier models for molecular simulations lead to errors over 100% in fundamental molecular properties. We introduce conceptually new force field parameters for molecular oxygen, nitrogen, and hydrogen that reduce deviations to <5%. We employ a combination of a harmonic bond stretching potential and Lennard-Jones parameters with 12-6 and 9-6 options, leading to computed bond lengths, Raman peaks, liquid densities, vaporization enthalpies, and free energies of hydration in impressive agreement with experiments. Reliable free energies of hydration were obtained upon validation of density and vaporization energy without significant further parameter adjustments. We illustrate applications to O₂ adsorption on Pt electrocatalysts and N₂ adsorption in zeolites, showing <5% deviation in adsorption energies measured in experiments without additional fitting parameters. We discuss the chemical interpretation of all parameters and explain the reasons for discrepancies in earlier models. Compatibility with the Interface Force Field (IFF), CHARMM, AMBER, OPLS-AA, GROMOS, DREIDING, CVFF, PCFF, COMPASS, and QM/MM methods enables reliable simulations of gases and liquid/solid interfaces with biopolymers, minerals, and metals. The parametrization protocol can be applied to similar molecules.

How Reliable Is the Ideal Adsorbed Solution Theory for the Estimation of Mixture Separation Selectivities in Microporous Crystalline Adsorbents?

Microporous crystalline adsorbents such as zeolites and metal-organic frameworks (MOFs) have potential use in a wide variety of separation applications. The adsorption selectivity S _ads is a key metric that quantifies the efficacy of any microporous adsorbent in mixture separations. The Ideal Adsorbed Solution Theory (IAST) is commonly used for estimating the value of S _ads, with unary isotherms of the constituent guests as data inputs. There are two basic tenets underlying the development of the IAST. The first tenet mandates a homogeneous distribution of adsorbates within the pore landscape. The second tenet requires the surface area occupied by a guest molecule in the mixture to be the same as that for the corresponding pure component. Configurational-bias Monte Carlo (CBMC) simulations are employed in this article to highlight several scenarios in which the IAST fails to provide a quantitatively correct description of mixture adsorption equilibrium due to a failure to conform to either of the two tenets underpinning the IAST. For CO₂ capture with cation-exchanged zeolites and MOFs with open metal sites, there is congregation of CO₂ around the cations and unsaturated metal atoms, resulting in failure of the IAST due to an inhomogeneous distribution of adsorbates in the pore space. Thermodynamic non-idealities also arise due to the preferential location of CO₂ molecules at the window regions of 8-ring zeolites such as DDR and CHA or within pockets of MOR and AFX zeolites. Thermodynamic non-idealities are evidenced for water/alcohol mixtures due to molecular clustering engendered by hydrogen bonding. It is also demonstrated that thermodynamic non-idealities can be strong enough to cause selectivity reversals, which are not anticipated by the IAST.

A nanoprecursor method for successfully synthesizing clinoptilolite with high-crystallinity and resultant effects on CO2/CH4 selective adsorption

Nanoprecursors used as a structural promoter (SP) were prepared by a hydrothermal method and named sol-SP. After centrifugation, the supernatant and precipitate were denoted as solution-SP and solid-SP, respectively. The effect of the additive amount on the structures and properties of the synthesized clinoptilolite was investigated using various characterization techniques. The activation energies of crystallization kinetics during induction and growth periods were calculated. The results showed that the induction period is the control step during the synthesis of clinoptilolite, while additive sol-SP or solid-SP was beneficial to shorten the induction period and therefore enhance the formation of the crystal nucleus. When their pre-crystallization time was too long or the additive amount was too much, the impure phase (phillipsite) in the synthesized clinoptilolite was easily generated. Although the addition of solution-SP had no obvious effect on the induction period, it promoted the growth of crystals after nucleation. Finally, the adsorption performances for CO₂ and CH₄ were preliminarily assessed using synthetic clinoptilolite as the adsorbent, showing the promising application for the separation of CO₂/CH₄.

Nanoprecursors used as a structural promoter (SP) were prepared by a hydrothermal method and named sol-SP.

The rational design of Li-doped nitrogen adsorbents for natural gas purification

Separation of nitrogen (N2) and methane (CH4) is one of the most challenging and energy-intensive processes in the natural gas industry, due to their close physico-chemical properties. The quest for an effective N2-selective adsorbent has long been the focus of research; however, the results have been sparse. In this work, a first-principle study has been used to construct and investigate Li-doped polycyclic aromatic hydrocarbons (PAHs) for N2 rejection in natural gas purification. We doped lithium on a series of linear/nonlinear PAHs consisting of two to six benzene rings. The adsorption affinity of the Li-doped organic molecular systems toward N2 and CH4 was evaluated by calculating the interaction energy using density functional theory. From the gas adsorption selectivities for different Li-doped PAHs, Li-doped phenanthrene and chrysene showed the highest N2 over CH4 equilibrium selectivities, with values of 119.7 and 80.8, respectively. It was found that the Li atom enabled the π bond of the aromatic substrate to interfere with the N2 lowest unoccupied molecular orbital, resulting in strong physisorption of N2. These results indicate the high potential of Li-doped phenanthrene and chrysene for N2 removal from natural gas.

CO2 capture on natural zeolite clinoptilolite: Effect of temperature and role of the adsorption sites

In this study, the adsorption capacity of the low-cost zeolite clinoptilolite was investigated for capturing carbon dioxide (CO₂) emitted from industrial processes at moderate temperature. The CO₂ adsorption capacity of clinoptilolite (a commercial natural zeolite) and ion-exchanged (with Na⁺ and Ca²⁺) clinoptilolite were tested under both dynamic (using a fixed-bed reactor operating with 10% vol. CO₂ in N₂) and equilibrium conditions (measuring single component adsorption isotherms). The dynamic CO₂ adsorption capacity of bare clinoptilolite and ion-exchanged clinoptilolite were evaluated in the temperature range from 293 K to 338 K and the obtained breakthrough curves were compared with those of the commercial zeolite 13X (Z13X). Although the adsorption capacity of Z13X exceeded those of bare clinoptilolite and ion-exchanged clinoptilolite at 293 K, the clinoptilolite exhibited the highest CO₂ uptake at a moderate temperature of 338 K (i.e. 25 % higher than Z13X). This feature appears in agreement with the lower isosteric heat of CO₂ adsorption on clinoptilolite compared to the other samples. The surface species affecting the q_iso and adsorption capacity were investigated through the FTIR spectroscopy using CO₂ as probe molecule. As a whole, it has been observed that CO₂ forms linear adducts onto K⁺ and Mg²⁺ cations of the bare clinoptilolite, and carbonate-like species onto its basic sites. With the Na-exchanged clinoptilolite, Na⁺ ions led to a decrease in surface basicity and to the formation of both single (Na⁺···OCO) and dual (Na⁺···OCO⋯Na⁺) cationic sites available for the formation of linear adducts. As a result of the remarkable adsorption capacity of clinoptilolite at 338 K, this material appears to be a promising adsorbent for the direct CO₂ removal from different flue gases sources operating at such temperatures.

Effects of natural zeolites on field-scale geologic noble gas transport

Improving predictive models for noble gas transport through natural materials at the field-scale is an essential component of improving US nuclear monitoring capabilities. Several field-scale experiments with a gas transport component have been conducted at the Nevada National Security Site (Non-Proliferation Experiment, Underground Nuclear Explosion Signatures Experiment). However, the models associated with these experiments have not treated zeolite minerals as gas adsorbing phases. This is significant as zeolites are a common alteration mineral with a high abundance at these field sites and are shown here to significantly fractionate noble gases during field-scale transport. This fractionation and associated retardation can complicate gas transport predictions by reducing the signal-to-noise ratio to the detector (e.g. mass spectrometers or radiation detectors) enough to mask the signal or make the data difficult to interpret. Omitting adsorption-related retardation data of noble gases in predictive gas transport models therefore results in systematic errors in model predictions where zeolites are present.Herein is presented noble gas adsorption data collected on zeolitized and non-zeolitized tuff. Experimental results were obtained using a unique piezometric adsorption system designed and built for this study. Data collected were then related to pure-phase mineral analyses conducted on clinoptilolite, mordenite, and quartz. These results quantify the adsorption capacity of materials present in field-scale systems, enabling the modeling of low-permeability rocks as significant sorption reservoirs vital to bulk transport predictions.

Metrics for Evaluation and Screening of Metal-Organic Frameworks for Applications in Mixture Separations

For mixture separations, metal-organic frameworks (MOFs) are of practical interest. Such separations are carried out in fixed bed adsorption devices that are commonly operated in a transient mode, utilizing the pressure swing adsorption (PSA) technology, consisting of adsorption and desorption cycles. The primary objective of this article is to provide an assessment of the variety of metrics that are appropriate for screening and ranking MOFs for use in fixed bed adsorbers. By detailed analysis of several mixture separations of industrial significance, it is demonstrated that besides the adsorption selectivity, the performance of a specific MOF in PSA separation technologies is also dictated by a number of factors that include uptake capacities, intracrystalline diffusion influences, and regenerability. Low uptake capacities often reduce the efficacy of separations of MOFs with high selectivities. A combined selectivity-capacity metric, Δq, termed as the separation potential and calculable from ideal adsorbed solution theory, quantifies the maximum productivity of a component that can be recovered in either the adsorption or desorption cycle of transient fixed bed operations. As a result of intracrystalline diffusion limitations, the transient breakthroughs have distended characteristics, leading to diminished productivities in a number of cases. This article also highlights the possibility of harnessing intracrystalline diffusion limitations to reverse the adsorption selectivity; this strategy is useful for selective capture of nitrogen from natural gas.

One-step synthesis of hydrophobic clinoptilolite modified by silanization for the degradation of crystal violet dye in aqueous solution

Hydrophobic clinoptilolite (CP) was successfully synthesized via a silanization method using methyltriethoxysilane (MTS) or diethoxydimethylsilane (DMTS) as silane coupling agents. The structural and textural properties of the resultant hydrophobic CP were characterized using various methods. The effect of the amount of MTS or DMTS additive on the induction (nucleation) and growth of CP were also investigated, and the apparent activation energy values for induction and growth periods were calculated, suggesting that the induction period is kinetically controlled, while the rapid growth process is thermodynamically controlled. Meanwhile, DMTS modification enhanced the hydrophobicity of CP compared with its MTS-modified counterpart and pure CP. Finally, various ZnO-supported CPs were used as photocatalysts for the removal of crystal violet from aqueous solution, demonstrating that ZnO/hydrophobic CP has the largest adsorption capacity and best removal performance. These results suggest that hydrophobic CP, as an adsorbent or support, has the most potential for applications in separation and catalysis.

Large-Scale Screening and Design of Metal-Organic Frameworks for CH4 /N2 Separation

CH₄ /N₂ separation is one of the great challenges in gas separation, which is of scientific and practical importance, such as in the upgrading of unconventional natural gas. Unfortunately, the separation performance is still quite low so far mainly due to their very close physical properties. In this work, a high-throughput computational screening method was performed to develop metal-organic frameworks (MOFs) for efficient CH₄ /N₂ separation. General designing rules as well as the correlation between selectivity and our proposed adsorbility (AD) parameter were obtained by carrying out systematic GCMC simulations of the existing 5109 CoRE MOFs. With the aid of this information, five virtual MOFs were screened out from the large database with 303 991 generated MOFs constructed in our previous work, exhibiting much higher selectivities than all the reported values. Among them, the selectivity of Zn-PYZ-BPY-1 can reach over 29.0, about 2.4 times of the highest value reported in the literature. These results may not only suggest promising candidates for CH₄ /N₂ separation but also provide useful information for large screening of MOFs for other specific separation mixtures.

The characterization and methane adsorption of Ag-, Cu-, Fe-, and H-exchanged chabazite-rich tuff from Turkey

In this study, a chabazite-rich tuff (CHA) from the Bala deposit of Ankara region (Turkey) and its modified forms (CuCHA, AgCHA, FeCHA, and HCHA samples) were investigated at 273 and 298 K using volumetric apparatus up to 100 kPa. The chabazite samples were characterized by using thermal analysis (TG-DTG-DTA), X-ray diffraction (XRD), X-ray fluorescence (XRF), scanning electron microscopy with detector X-ray energy dispersive (SEM-EDX), Fourier transform infrared (FT-IR), and N₂ adsorption methods. It was found that natural chabazite is composed of predominantly chabazite with small amounts of clinoptilolite and erionite. XRD showed that there are major structural changes to Fe- and H-exchanged chabazite samples. Capacity of chabazites for CH₄ ranged from 0.168 and 1.341 mmol/g. Among all the modified forms, it was observed that Ag form of chabazite zeolite had the greatest methane adsorption capacity at both temperatures.

Highlighting the Influence of Thermodynamic Coupling on Kinetic Separations with Microporous Crystalline Materials

The main focus of this article is on mixture separations that are driven by differences in intracrystalline diffusivities of guest molecules in microporous crystalline adsorbent materials. Such "kinetic" separations serve to over-ride, and reverse, the selectivities dictated by mixture adsorption equilibrium. The Maxwell-Stefan formulation for the description of intracrystalline fluxes shows that the flux of each species is coupled with that of the partner species. For n-component mixtures, the coupling is quantified by a n × n dimensional matrix of thermodynamic correction factors with elements Γ _ij ; these elements can be determined from the model used to describe the mixture adsorption equilibrium. If the thermodynamic coupling effects are essentially ignored, i.e., the Γ _ij is assumed to be equal to δ _ij , the Kronecker delta, the Maxwell-Stefan formulation degenerates to yield uncoupled flux relations. The significance of thermodynamic coupling is highlighted by detailed analysis of separations of five different mixtures: N₂/CH₄, CO₂/C₂H₆, O₂/N₂, C₃H₆/C₃H₈, and hexane isomers. In all cases, the productivity of the purified raffinate, containing the tardier species, is found to be significantly larger than that anticipated if the simplification Γ _ij = δ _ij is assumed. The reason for the strong influence of Γ _ij on transient breakthroughs is traceable to the phenomenon of uphill intracrystalline diffusion of more mobile species. The major conclusion to emerge from this study is that modeling of kinetic separations needs to properly account for the thermodynamic coupling effects.

Separation of CH4/N2 of Low Concentrations From Coal Bed Gas by Sodium-Modified Clinoptilolite

Clinoptilolite is a widely distributed tectosilicate, mainly composed of Al₂O₃, SiO₂ with exchangeable cations such as Ca, K, Mg, and Na. In this research, raw clinoptilolite was ground, gravimetrically concentrated and ion-exchanged using different concentrations of NaCl solution. Then the modified clinoptilolite powder was formulated into particles as adsorbents. The adsorbents were applied to CH₄ separation in coal bed gas. The raw and modified clinoptilolites were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM), atomic emission spectrometer (ICP-AES), Fourier transform infrared spectrometer (FTIR), and Brunauer Emmett Teller (BET) specific surface area. The CH₄ absorptivity by raw and modified clinoptilolites was evaluated using pressure swing adsorption (PSA) to assess the CH₄ separation ability. The results indicated that the ion-exchanged clinoptilolite using 0.2 mol/L NaCl solution was found to be promising for the kinetic PSA separation of CH₄/N₂, giving a better absorptivity for CH₄ separation under different influence factors. Based on the simulated static experiments, it was indicated that both CH₄ and N₂ were capable of diffusing into clinoptilolite while N₂ adsorption by clinoptilolite was excellent. The experiment results also indicated that ion-exchanged clinoptilolite using a 0.2 mol/L NaCl solution was the optimal adsorbent for separating CH₄/N₂ at the low pressure condition. From the simulated dynamic experiments, the ion-exchanged clinoptilolite using a 0.2 mol/L NaCl solution as a potential sorbent in kinetic PSA processes for N₂/CH₄ separation, exhibited the best performance at 648 K under 0.2 MPa within 28 min, in comparison to the raw clinoptilolite and clinoptilolite under other modification conditions. In the next phase of research, the modified clinoptilolite will be tested for CH₄ separation in real coal bed gas.

CH4/N2 Adsorptive Separation on Zeolite X/AC Composites

A series of zeolite X/activated carbon (AC) composites were prepared from the same starting materials at various activation time. The corresponding modified samples were obtained by being treated with diluted NH4Cl solution. The relationship between porosity development, surface properties, and CH4/N2 adsorption performance was investigated. The increase of micropore volume is beneficial to the improvement of CH4 and N2 adsorption capacity, but more sensitive for CH4. In addition, the polar functional groups of zeolite X/AC composites may enhance CH4 adsorption capacity. More importantly, both developing micropore structure and surface modification contributed to enhance the adsorption selectivity αCH4/N2. As the optimum sample of these studies, HZAC(24) showed CH4 adsorption capacity of 17.3 cm3/g and the highest adsorption selectivity αCH4/N2 of 3.4. The CH4 and N2 adsorption isotherms of all samples can be well fitted by the Langmuir–Freundlich model. HZAC(24) showed an excellent cyclability of adsorption/desorption of CH4 with a neglectable capacity loss after subsequent cycles. Moreover, HZAC(24) displayed relatively rapid adsorption kinetics. These properties of zeolite X/AC composites are essential for the adsorptive separation of CH4 from N2 in the pressure swing adsorption (PSA) process.

Adsorption Properties of Modified Clinoptilolite for Methane and Nitrogen

Coalbed methane (CBM) is a kind of unconventional gas. CBM often contains a great deal of air when it comes out of the well. So, it must be condensed and purified before it can be applied. In this paper, raw clinoptilolite (Cp) was treated with grinding, gravimetric concentration, and ion-exchange using different aqueous solutions of salts. Then, the modified Cp powder was prepared into particles as adsorbents. Then, the adsorbents were used for nitrogen/methane separation in pressure swing adsorption (PSA) at the same condition of 0.2 MPa and 298 K. Research results indicated that there were micropores and lots of mesopores in the Cp, and the pores were mainly slit holes formed by sheet stacking. The adsorbents of NH₄-Cp, Cs-Cp, and Cu-Cp showed good equilibrium selectivity for CH₄, and the equilibrium separation factors of CH₄ and N₂ were 2.56, 2.31, and 1.95, respectively. The adsorbents of Na-Cp and Ag-Cp showed good equilibrium selectivity for N₂, and the equilibrium separation factors of N₂ and CH₄ were 7.25 and 6.53, respectively. Consequently, the adsorbent of Na-Cp was suitable for nitrogen/methane mixture separation, which could make the concentration of methane concentrated from 19.7% to 30.72%.