CO2-CH4 Mixture Research Articles

A Multiple-physics coupled model for nanoconfined CO2-CH4 mixture transport behavior: Implications for CO2 geological storage in ultra-tight formation

Considerable efforts have been made to examine the feasibility of CO2-enhanced gas recovery (CO2-EGR), a promising approach for improving gas recovery by using CO2 to displace in-situ CH4 in ultra-tight gas reservoirs rich in nanopores. However, most research has focused on either single-component gas flow physics or competitive adsorption of multiple gas components in nanopores. The flow behavior of nanoconfined CO2-CH4 mixtures remains an area of knowledge gap. This work establishes a theoretical framework for CO2-CH4 mixture flow capacity by coupling mixture flow in the bulk region with surface diffusion in the adsorption area. Results indicate that: a) Total mixture flow conductance increases rapidly at pressures below ∼10 MPa, with CO2 contributing over 99 % to surface diffusion conductance; b) Increasing CO2 molar ratio from 0.2 to 0.8 can lead to a 167 % enhancement in total mixture conductance; c) Variations in CO2-CH4 competitive adsorption characteristics have little impact on total conductance.

Microwave plasma-assisted hydrogen production via conversion of CO2–CH4 mixture

A method concerning re-use of the main greenhouse gases: CO2 and CH4 at the same time producing value-added chemicals (H2 + CO) is presented in this paper. The method involves the methane dry reforming using an atmospheric pressure operated 915 MHz microwave plasma source with a swirl gas flow. The effect of the CO2 to CH4 flow rates ratio, the total gas flow rate and the absorbed microwave power on the volume concentration of the gaseous plasma component in the outlet gas (%), CO2 and CH4 conversion rate (%), hydrogen production rate (g h−1) and energy yield of hydrogen production (g kWh−1) was investigated experimentally. The recorded results of the CO2 and CH4 conversion rate were about 82% and 88%, respectively. The best results in terms of the hydrogen production rate and energy yield of hydrogen production were equal to 156 g h−1 and 24 g kWh−1, respectively.

Competitive adsorption mechanisms of multicomponent gases in kaolinite under electric fields: A molecular perspective

The investigation of the competitive adsorption mechanisms of gas mixtures within the inorganic nanopores of shale influenced by an electric field holds paramount importance for improving shale gas recovery and enhancing CO2 storage efficiency. This study employs molecular dynamics (MD) simulations to investigate the competitive adsorption mechanisms of gas mixtures on kaolinite surfaces. Subsequently, the engineering parameters governing electric field-enhanced CO2 sequestration and enhanced gas recovery (CS-EGR) are explored and optimized. In the absence of an electric field, the analysis reveals that water molecules exhibit the highest adsorption capacity in a ternary H2O–CO2–CH4 mixture, resulting in the formation of a water film of a specific thickness. CO2 displays a superior adsorption capability over CH4. As the CO2 volume injected into the reservoir increases, CH4 undergoes desorption due to the dominant adsorption effect of CO2. However, the efficiency of CO2 displacing CH4 diminishes significantly. The introduction of an electric field alters the equilibrium state of the gas mixture, leading to a further increase in CO2 adsorption and enhancing its selectivity relative to that of CH4 by 33%. These findings lay the foundation for the practical implementation of electric field-enhanced CS-EGR in shale reservoirs.

Water content in CO2–CH4 mixtures: New saturation setup and measurements using a quartz-crystal microbalance.

This study presents experimental data on water content in CO2, methane, and their mixtures. A new saturation process was developed using a PVT cell connected to a variable-volume cylinder filled with deionized water. Dry gas with a known composition was injected into the cylinder, and the temperature and pressure conditions were adjusted accordingly. The water content of the saturated gas was then analyzed using a quartz-crystal microbalance (QCM). Various mixtures with different CO2 and methane mole ratios were investigated at different pressures for a given temperature. The PC-SAFT and CPA equations of state were applied to correlate the moisture data. The association scheme employed included 2B, 3B, and 4C types, considering the cross-association between water and CO2. The results for pure methane showed that the water content increased as the pressure decreased across all pressure ranges. However, for pure CO2 above 70 bar, the water content increased due to the condensation behavior of CO2. The same trend was observed for mixtures, where the water content increased with increasing pressure for CO2 mole fractions above 0.74. For all the systems studied, the PC-SAFT and CPA equations of state successfully correlated the water content data using a single constant binary interaction parameter and considering the cross-association between water and CO2.

Selective adsorption and transport of CO2–CH4 mixture under nano-confinement

Selective adsorption and transport of CO2–CH4 mixture under nano-confinement

Confinement effect on transport diffusivity of adsorbed CO2–CH4 mixture in coal nanopores for CO2 sequestration and enhanced CH4 recovery

The transport of CO2–CH4 mixture in coal nanopores is crucial to CO2-enhanced coalbed (CO2-ECBM) recovery, but is highly challenging on adsorption-diffusion dominated nanoscale due to the gas-surface and intermolecular interactions. Using Monte Carlo and molecular dynamics simulation, the confinement effect of nanopores with two slit widths (2 and 6 nm) on the CO2–CH4 binary diffusion in the adsorption layer are investigated. A modified computational method for multi-component transport diffusivity in the adsorption layer is proposed by incorporating the survival probability function. The results show that the transport diffusivity in the semi-confined adsorption layer shows a general decreasing tendency with the increase of pressure, while it slightly decreases when the pressure <6.4 MPa in the confined adsorption layer, following an inverse parabola trend with the maximum value at around 12 MPa. Three mechanisms are classified regarding this pressure-dependent evolution. Firstly, the collision between molecules of weaker attraction and surface dominates the transport capacity, and this collisional frequency reduces at the low-pressure region. Accompanying the pressure exceeds the critical threshold, surface diffusion becomes the primary contribution. Lastly, significant intermolecular interactions appear after the adsorption saturation. During the CO2-ECBM process, pressure drawdown is the primary method to release the adsorbed CH4, and the largest recovery ratio can reach 25.9% in 2 nm micropore and 30.1% in 6 nm mesopore. After the early CO2 injection, the transport diffusion of CH4 exceeds that of CO2, and the selectivity of CH4 over CO2 increases in the subsequent CO2 huff-n-puff cycles. The CO2–CH4 mixture under greater confinement effect manifests higher diffusion selectivity, which benefits the separation of the CO2–CH4 mixture and further contributes to the CH4 recovery.

Investigation of CO2 and CH4 capture from syngas in adsorption process and breakthrough modeling by modified Lattice-Boltzmann mesoscale method

A modified Lattice Boltzmann method (LBM) was developed to model and analyze the transient flow and multi-component adsorption in a dispersive packed bed column of activated carbon as a new kind of meso-scale modeling. The model using D2Q9 (two dimensions and 9 speed) lattice solves the two-dimensional space, convection-dispersion adsorption model for the CO2–CH4 mixture in rich H2 at transient condition. The sink/source term model was considered as the adsorption/desorption kinetic rate based on Extended Langmuir theory for multicomponent mixture. The mole balances in the solid phase were considered as the lumped kinetic model of adsorption-desorption reaction. The results of the developed model was presented as the flow velocity and molar fractions of the components in axial and radial directions of the bed as well as breakthrough curves of CO2 and CH4 from their mixture in H2 gas stream under pressures of 3 and 5 bar and inlet linear velocities of 0.01, 0.04, 0.08 and 0.1m/min. The breakthrough curves were validated with the experimental data and the average absolute relative deviations (AARD) were calculated for both components. In addition, the results of LBM were compared with finite difference method in which AARDs were dtermined as 3% and 8% for CO2,4% and 7% for CH4 , by LBM and FDM, respectively.

Influence of impurities on reactive transport of CO2 during geo-sequestration in saline aquifers

Carbon capture and storage (CCS) is a promising technology to reduce greenhouse gas emissions by storing CO2 in geological formations. Storing less pure CO2, which has low percentages of impurities, can reduce the cost of the process. However, impurities can affect the dynamics of CO2 propagation and its trapping mechanisms in porous media, potentially impacting the overall effectiveness of the storage process. Therefore, this study investigates the effect of impurities on the reactive transport of CO2 during geo-sequestration in a saline aquifer using a reliable reactive transport model. The effect of co-injected H2S and CH4 in the CO2 stream on CO2 plume migration, CO2 trapping mechanisms, minerals dissolution and precipitation, and cap rock integrity, was studied here. To do so, reactive transport modeling was performed using a compositional simulator based on actual data from Sleipner storage site. The model was run for pure CO2 and mixtures (CO2 with 5 wt% H2S and CO2 with 5 wt% CH4). The results showed that the impurities co-injected with CO2 affect the CO2 plume migration in the aquifer. For instance, injecting the CO2-CH4 mixture increased the CO2 horizontal migration below the cap rock compared to the pure CO2 injection case. However, the buoyancy effect of the injected gas was slightly reduced by adding H2S to the CO2 stream. The impurities influenced the pH of the system, and hence minerals dissolution and precipitation, and cap rock porosity changes. The impurities (CH4 and H2S) injected with CO2 were found to suppress an anticipated increase in the cap rock porosity and accelerated the mineral trapping of CO2 compared to pure CO2 storage. While, among the studied cases, the CO2-CH4 mixture case had the highest structural trapping and the lowest solubility trapping of CO2. Thus, this study suggests that storage of impure CO2 can enhance mineral trapping, maintain cap rock sealing, and change CO2 distribution in porous media while reducing separation costs.

Title Supersonic Condensation and Separation Characteristics of CO2-Rich Natural Gas under Different Pressures

Supersonic separation technology is a new natural gas sweetening method for the treatment of natural gas with high CO2 (carbon dioxide) content. The structures of the Laval nozzle and the supersonic separator were designed, and the mathematical models of supersonic condensation and swirling separation for CO2-CH4 mixture gas were established. The supersonic condensation characteristics of CO2 in natural gas and the separation characteristics of condensed droplets under different inlet pressures were studied. The results show that higher inlet pressure results in a larger droplet radius and higher liquid phase mass fraction; additionally, the influence of centrifugal force is more pronounced, and the separation efficiency and removal efficiency of CO2 are higher. When the inlet pressure is 6 and 9 MPa, the liquefaction efficiency at the Laval nozzle outlet increases from 56.90% to 79.97%, and the outlet droplet radius increases from 0.39 to 0.72 μm, and the removal efficiency is 31.25% and 54.52%, respectively. The effects of inlet pressures on the removal efficiency of the supersonic separator are complicated and are controlled by the combined effects of liquefaction capacity of the nozzle and centrifugal separation capacity of the swirl vane.

Efficient separation of CO2 from methane at high pressures by modified microporous titanosilicates; design and breakthrough studies

In this work, type-4 titanosilicate (TS-4) having a micro-pore size of 3–4 Å, was synthesized from inexpensive source materials, such as commercial silicon dioxide, with the template-free hydrothermal method. The XRD analysis confirmed formation of the TS-4 crystalline phase. Then, strontium cation modification was performed using SrCl2 solution to adjust the pore structure and increase the basicity of the surface to improve CO2 selectivity versus methane. In this study, the adsorption isotherms of methane and CO2 were measured in the range of 0 to 50 bars, at two different temperatures of 298 K and 308 K, then the ideal selectivities of CO2/CH4 were calculated at different pressures. The ideal CO2/CH4 selectivity was determined at 19.04 and 38.49 for TS-4 and Sr-TS-4, respectively, at the pressure of 50 bar and the temperature of 298 K. The isosteric heat of adsorption of CO2 was calculated by Van’t Hoff equation as −29.63 kJ/mol and −26.22 kJ/mol, respectively. Low CO2 heat of adsorption of Sr-TS-4 and high selectivity of CO2/CH4 at high pressures, against zeolites, offer Sr-TS-4 as an economic and efficient adsorbent for cyclic adsorption processes. To study the dynamic selectivity of CO2/CH4, dynamic adsorption experiments were conducted at the temperature of 298 K, pressures of 7 and 15 bars, flow rates of 10 and 20 Nml/min in a bed of 2.7 gr adsorbent, with the inlet molar fraction of 5–95 % CO2-CH4 mixture. Comparison of the breakthrough curves indicated that Sr-TS-4 had a longer breakpoint and a larger used ratio of the length at any feed flow rate. The maximum relative dynamic selectivity of CO2/CH4 at 15 bar, by the Sr-TS-4, was obtained 95.29 at CO2 breakpoint of 40 min, whereas maximum relative dynamic selectivity, by the TS-4, was obtained 63.54 at CO2 breakpoint of 30 min. It is concluded that Sr-TS-4 would be an efficient adsorbent for separation of CO2 from natural gas in commercial scales at high pressures with high methane recovery.

Quantitative Raman Spectroscopic Determination of the Composition, Pressure, and Density of CO2-CH4 Gas Mixtures

The Raman spectra for pure CO2 and CH4 gases and their ten gas mixtures were collected at pressures and temperatures ranging from 2 MPa to 40 MPa and room temperature (∼24°C) to 300°C, respectively. A systematic analysis was carried out to establish a methodology for the quantitative determination of the composition, pressure, and density of CO2-CH4 mixtures. The shift in the peak position of the υ1 band for CH4 was sufficiently large to enable the accurate determination of the pressure of pure CH4 and CH4-dominated fluids (&gt;50 mol% CH4). An equation representing the observed relationship of the peak position of the υ1 band of CH4, density, and composition was developed to calculate the density of CO2-CH4 mixtures. The Raman quantification factor F (CH4)/F (CO2) was demonstrated to be near a constant value of 5.048 ± 0.4 and was used to determine the CH4 to CO2 molar ratio in an unknown CO2-CH4‐bearing fluid with high internal pressure (&gt;10 MPa) based on the Raman peak area ratio. The effect of temperature on the variation in Raman spectral parameters was also investigated at temperatures up to 300°C. The results showed that the effect of temperature must be considered when Raman spectral parameters are used to calculate the pressure, density, and composition of CO2-CH4 gas mixtures. Raman spectroscopic analysis results obtained for six samples prepared in fused silica capillary capsules were validated by comparison with the results obtained from microthermometry measurements.

Ultrathin DDR Films with Exceptionally High CO2 Flux and Uniformly Adjustable Orientations

AbstractThin and oriented zeolite films are important for advanced separations, catalysis, and sensing. Strategies for tailoring zeolites for applications include controlling their crystal size, shape, and orientation. Here, three siliceous DDR zeolite ultrathin films with different orientations achieved by homoepitaxial growth from 60 nm‐sized seed particles are reported. The 0.5 µm thick membrane shows a separation selectivity of 400 for CO2–CH4 mixtures and CO2 permeance of 25 × 10‐7 mol m‐2 s‐1 Pa‐1 at 20 °C and 1 bar, leading to a record‐high performance among all reported DDR membranes. Furthermore, the seed nanoparticles are grown into mono‐dispersed DDR sub‐micron crystals with trigonal and tabular habits. These crystals are assembled in monolayers for the growth of ultrathin and uniformly (h0h)‐oriented and c‐oriented films with maximum surface pore diameter of 0.365 and 0.263 nm, respectively, by using the 1‐adamantanamine template in fluoride medium. The novel strategy not only provides high‐performance membrane candidates for industrial CO2 separation, but also inspires interfacial engineering, pore size, and orientation controlling for other microporous crystals, and their membranes.

Genetic model for Jurassic shale-hosted Zn-Pb deposits of the Arak Mining District, Malayer-Esfahan metallogenic belt: Insight from sedimentological, textural, and stable isotope characteristics

The genetic model, including conditions under which mineralization formed, and relative timing of mineralization are critical questions for SEDEX (or shale-hosted massive sulfide, SHMS) deposits. There is increasing awareness that sub-seafloor replacement is an important process in the formation of some SEDEX deposits. We have studied the Hossein-Abad and Western Haft-Savaran Zn-Pb SEDEX deposits located in the Arak basin of the Malayer-Esfahan Metallogenic Belt, Iran, to address these questions of genesis. This metallotect formed in a back-arc paleotectonic setting as a result of the subduction of the Neo-Tethys oceanic plate beneath the Sanandaj-Sirjan Zone. The rocks that host the mineralization are Jurassic organic matter-bearing, fine-grained sandstones, siltstones, and shales. Asymmetric lenticular bedding, unidirectional flow (based on oblique silt lamination direction relative to horizontal bedding), graded bedding, and clay-rich interbeds indicate sediments were deposited from turbidity currents in a low-energy basin environment.There are three ore facies in the Hossein-Abad and Western Haft-Savaran Zn-Pb deposits: 1) bedded ore; 2) massive ore; 3) feeder zone. Bedded ore contains pyrite framboids and polyframboidal clusters. The size range of the pyrite framboids (3 to 6 µm in diameter) indicates they formed in the water column and not in the subsurface. Characteristic structures in bedded ore are: 1) sulfide-bearing silt injections into clay-filled burrows, 2) injection of sulfide-bearing silt into flame structures of claystone laminae, and 3) organic matter in claystone oriented obliquely relative to bedding. These structures are the result of seismic deformation induced by synsedimentary earthquakes, whereby sulfides that formed in permeable unconsolidated sediment were injected into the organic matter-bearing claystone unit.The δ18O and δ13C values of siderite, calcite and dolomite in veins from the feeder zone and massive ore range from 12.2 to 23.8‰ and −16.7 to 1.7‰, respectively. These values indicate that formational water, seawater and organic matter oxidation-decomposition all played a role in hydrothermal carbonate formation. Melting and homogenization temperatures for CO2 for CO2-bearing fluid inclusions range from −57.5 to −60 °C and 6.6 to 29.5 °C, respectively, and indicate the presence of < 15 mol percent CH4. The CO2 homogenization temperature range suggests the CO2-rich phase is a CO2-CH4 mixture. The CH4 and CO2 in the H2S-bearing fluid were likely generated by biodegradation and oxidation of organic matter via BSR and methanogenesis.The sulfur isotope compositions of pyrite, galena, sphalerite and chalcopyrite from the feeder zone and the massive ores range from δ34S −4.3 to + 7.2‰ and display equilibrium fractionations, indicating that the sulfur originated from two processes: (a) bacterial reduction of seawater sulfate (BSR) (distal from mineralization site), and (b) thermochemical reduction of seawater sulfate (TSR) (in both massive ore and feeder zone). Sulfur isotope geothermometric calculations (Δgalena-sphalerite) for two samples give temperatures in the range 209–224 °C for the massive ore facies. Such high temperatures negate a contribution of sulfur from BSR. However, BSR likely occurred in sediments distal to the feeder zone, where seawater sulfate was bacterially reduced to H2S, and this H2S migrated to the mineralization site during diagenesis. Collectively, the stable isotope data indicate mineralization formed in response to mixing occurred between ascending hot metalliferous hydrothermal fluid that rose up the fault and fracture network, with H2S-bearing fluid and sulfate-bearing percolating seawater, triggering sulfide deposition.

Experimental Studies on Gas Hydrate‐Based CO2 Storage: State‐of‐the‐Art and Future Research Directions

Hydrate‐based CO2 storage is considered a potentially effective way of reducing greenhouse gas emissions and slowing down global warming. Herein, the locations in the ocean and permafrost that meet the requirements for hydrate‐based CO2 storage are summarized. Furthermore, research progress and shortcomings of hydrate‐based CO2 storage are analyzed. The two main methods for hydrate‐based CO2 storage are direct CO2 hydrate storage and CO2–CH4 hydrate replacement. Direct CO2 hydrate storage on the seabed and in the subsea (or permafrost) is proposed, and CO2 formation experiments on different scales are conducted. Various porous media are used to simulate the natural sediment and investigate the hydrate formation equilibrium and kinetics. CO2–CH4 replacement is preferred because it combines the benefits of CO2 storage and CH4 production. The hydrate equilibrium of different CO2–CH4 mixture hydrates is measured using different methods. Nuclear magnetic resonance and Raman spectroscopy are commonly used to study the hydrate structure characteristics and mass transfer properties. Macrokinetics are usually conducted in a high‐pressure vessel to analyze the effects of the experimental conditions on the replacement results. Nevertheless, the replacement rate and hydrate properties should be investigated further.

Fast synthesis of thin SSZ-13 membranes by a hot-dipping method

Among zeolite membranes with CHA structure, SSZ-13 membrane has certain advantages over all-silica CHA and SAPO-34 membranes, like easy synthesis, good stability and insensitivity to moisture, however, its separation performance is not satisfactory. In this study, SSZ-13 membranes were prepared by a hot-dipping method, where the hot contact of seeded support and mother liquor at synthesis temperature resulted into fast crystallization kinetics and thin membranes. The obtained sub-micron SSZ-13 membranes exhibited outstanding CO2–CH4 mixture separation performance, CO2 permeance ~28 × 10−7 mol/(m2 s Pa) and CO2–CH4 selectivity ~180 at 0.14 MPa and room temperature. The hot-dipping method achieved the complete separation of seeding and crystallization (or membrane growth) by preventing the contact of mother liquor and seeded support before reaching synthesis temperature. This treatment avoids any undesirable change of the seed layer before membrane growth, which may help us to correlate the membrane performance with the properties of seed layer, like seed density, thickness and seed size. On the contrary, the CHA seed layer might be dissolved partially by mother liquor during the heating process of conventional membrane synthesis, which led to thick membranes due to unsatisfactory seeding. This method could be used for the synthesis of other zeolite membranes.

Study on Competitive Adsorption and Displacing Properties of CO2 Enhanced Shale Gas Recovery: Advances and Challenges

CO2 enhanced shale gas recovery (CO2-ESGR) draws worldwide attentions in recent years with having significant environmental benefit of CO2 geological storage and economic benefit of shale gas production. This paper is aimed at reviewing the state of experiment and model studies on gas adsorption, competitive adsorption of CO2/CH4, and displacement of CO2-CH4 in shale in the process of CO2-ESGR and pointing out the related challenges and opportunities. Gas adsorption mechanism in shale, influencing factors (organic matter content, kerogen type, thermal maturity, inorganic compositions, moisture, and micro/nano-scale pore), and adsorption models are described in this work. The competitive adsorption mechanisms are qualitatively ascertained by analysis of unique molecular and supercritical properties of CO2 and the interaction of CO2 with shale matrix. Shale matrix shows a stronger affinity with CO2, and thus, adsorption capacity of CO2 is larger than that of CH4 even with the coexistence of CO2-CH4 mixture. Displacement experiments of CO2-CH4 in shale proved that shale gas recovery is enhanced by the competitive adsorption of CO2 to CH4. Although the competitive adsorption mechanism is preliminary revealed, some challenges still exist. Competitive adsorption behavior is not fully understood in the coexistence of CO2 and CH4 components, and more experiment and model studies on adsorption of CO2-CH4 mixtures need to be conducted under field conditions. Coupling of competitive adsorption with displacing flow is key factor for CO2-ESGR but not comprehensively studied. More displacement experiments of CO2-CH4 in shale are required for revealing the mechanism of flow and transport of gas in CO2-ESGR.

Raman spectral behavior of N2, CO2, and CH4 in N2–CO2–CH4 gas mixtures from 22°C to 200°C and 10 to 500 bars, with application to other gas mixtures

AbstractThe Raman spectral behavior of N2, CO2, and CH4 in ternary N2–CO2–CH4 mixtures was studied from 22°C to 200°C and 10 to 500 bars. The peak position of N2 in all mixtures is located at lower wavenumbers compared with pure N2 at the same pressure (P)–temperature (T) (PT) conditions. The Fermi diad splitting in CO2 is greater in the pure system than in the mixtures, and the Fermi diad splitting increases in the mixtures as CO2 concentration increases at constant P and T. The peak position of CH4 in the mixtures is shifted to higher wavenumbers compared with pure CH4 at the same PT conditions. However, the relationship between peak position and CH4 mole fraction is more complicated compared with the trends observed with N2 and CO2. The relative order of the peak position isotherms of CH4 and N2 in the mixtures in pressure–peak position space mimics trends in the molar volume of the mixtures in pressure–molar volume space. Relationships between the direction of peak shift of individual components in the mixtures, the relative molar volumes of the mixtures, and the attraction and repulsion forces between molecules are developed. Additionally, the relationship between the peak position of N2 in ternary N2–CO2–CH4 mixtures with pressure is extended to other N2‐bearing systems to assess similarities in the Raman spectral behavior of N2 in various systems.

Synthesis and characterization of 6FDA/3,5-diamino-2,4,6-trimethylbenzenesulfonic acid-derived polyimide for gas separation applications

A sulfonic acid-functionalized trimethyl-substituted polyimide was synthesized by reacting 4,4′-(hexafluoroisopropylidene)diphthalic anhydride (6FDA) and 3,5-diamino-2,4,6-trimethylbenzenesulfonic acid (TrMSA). The properties of 6FDA-TrMSA were compared to the related 6FDA-derived polyimide analogues made from 2,4,6-trimethylbenzene-1,3-diamine (6FDA-TrMPD) and 3,5-diamino-2,4,6-trimethylbenzene benzoic acid (6FDA-TrMCA). Compared to 6FDA-TrMPD and 6FDA-TrMCA, sulfonic acid functionalization resulted in significantly lower surface area, reduced fractional free volume, and tighter chain d-spacing. Consequently, 6FDA-TrMSA displayed lower gas permeabilities with a commensurate increase in permeability-based gas-pair selectivities. The enhanced CO2/CH4 selectivity of 6FDA-TrMSA was caused exclusively by higher diffusion selectivity, which was promoted by strong hydrogen bonding induced by the SO3H functionalization. Permeation experiments of 6FDA-TrMSA with a 1:1 CO2-CH4 mixture revealed the occurrence of competitive sorption effects (depressing CO2 gas permeability) and CO2-induced polymer matrix plasticization, which reduced the polymer selectivity by enhancing CH4 permeability. At ~20 atm total pressure, 6FDA-TrMSA showed a CO2 permeability of ~15 Barrer and an equimolar CO2/CH4 mixed-gas selectivity of 55, which are ~2-fold higher performance values than those of the state-of-the-art polymer used for industrial scale natural gas sweetening, i.e., cellulose triacetate.

Efficient C2Hn Hydrocarbons and VOC Adsorption and Separation in an MOF with Lewis Basic and Acidic Decorated Active Sites.

To help address efficient separation of C2Hn light hydrocarbons and C2H2/CO2 in the chemical industry, the self-assembly via an azolate-carboxylate ligand and Co(II) ion gave rise to a new porous MOF material, [Co(btzip)(H2btzip)]·2DMF·2H2O (1) (H2btzip = 4,6-bis(triazol-1-yl)isophthalic acid). In the MOF, the pores are modified by rich uncoordinated triazolyl Lewis basic N atoms and acidic -COOH groups, which strengthen interactions with C2Hn hydrocarbons and CO2 molecules, leading to high adsorption amounts for C2H2, C2H4, C2H6, and CO2 and remarkable separation efficiency for C2Hn-CH4, CO2-CH4, and C2H2-CO2 mixtures, as confirmed by breakthrough experiments on the realistic gas mixtures. The MOF also reveals outstanding selective adsorption ability for benzene/toluene, methanol/1-propanol, methanol/2-propanol, and 2-propanol/1-propanol isomers. Molecular simulations disclose the different adsorption sites in the MOF for various adsorbates.

Synthesis of thin SAPO-34 zeolite membranes in concentrated gel

Synthesis of thin zeolite membranes in the sub-micron range is desirable but a great challenge. Despite the successful synthesis of ultrathin Silicalite-1 and CHA membranes, the pursuit of SAPO-34 membranes thinner than 1 μm remains difficult. Herein, thin SAPO-34 membranes were prepared in concentrated gel by conventional hydrothermal synthesis. Concentrated membrane gel (TEAOH/Al2O3 = 4 and H2O/Al2O3 = 10) enhanced nucleation and facilitated the formation of small SAPO-34 crystals and therefore thin membranes. High TEAOH/Al2O3 ratio and special mixing sequence resulted into a homogeneous membrane gel with low viscosity which is beneficial for membrane synthesis with high reproducibility. Al(OH)3 as Al source led to formation of cubic SAPO-34 crystals which facilitated crystal intergrowth and reduced defects. These membranes (∼1.2 μm thick) exhibited high CO2 permeance of 3.9 × 10−6 mol/(m2 s Pa) and CO2–CH4 selectivity of 152 for equimolar CO2–CH4 mixture at room temperature and 0.14 MPa pressure drop. Further optimization of aging treatment or limiting the excessive membrane growth by a two-step hydrothermal synthesis + dry gel conversion method, the membrane thickness was successfully reduced to ∼0.9 μm, which increased the CO2 permeance to ∼5.0 × 10−6 mol/(m2 s Pa) without sacrificing CO2–CH4 selectivity.