Immobilized glycerol-based liquid membranes in hollow fibers for selective CO2 separation from CO2–N2 mixtures

Glycerol-based liquid membranes immobilized in the pores of hydrophilic microporous hollow fibers have been studied for selective separation of CO₂ from a mixed gas (CO₂, N₂) feed having low concentrations of CO₂ characteristic of gases encountered in space walk and space cabin atmosphere. The immobilized liquid membranes (ILMs) investigated consist of sodium carbonate–glycerol or glycine-Na–glycerol solution. Based on the performances of such liquid membranes in flat hydrophilic porous substrates [Chen et al., Ind. Eng. Chem. Res. 38 (1999) 3489; Chen et al., Ind. Eng. Chem. Res. 39 (2000) 2447], hollow fiber-based ILMs were studied at selected CO₂ partial pressure differentials (Δp_CO2 range 0.36–0.50 cmHg), relative humidities (RH range 45–100%), as well as carrier concentrations. The sodium carbonate concentration was primarily 1.0 mol/dm³; the glycine-Na concentration was 3.0 mol/dm³. The sweep gas was always dry helium and it flowed on the shell side. Very high CO₂/N₂ selectivities were observed with porous polysulfone microfiltration membranes as substrate. As in the case of flat film-based ILMs (see references above), feed side RH is an important factor determining the ILM performances. Generally, lower permeances and greater CO₂/N₂ selectivity values were observed at lower feed stream RHs. When the feed side average RH=60%, p_CO2,f=0.005 atm and glycine-Na concentration was 3.0 M, the CO₂/N₂ separation factor observed was over 5000. Prolonged runs lasting for 300 h showed that the hollow fiber-based ILM permeation performances were stable.     

Halogen–C2H2 Binding in Ultramicroporous Metal–Organic Frameworks (MOFs) for Benchmark C2H2/CO2 Separation Selectivity

Acetylene (C₂H₂) capture is a step in a number of industrial processes, but it comes with a high-energy footprint. Although physisorbents have the potential to reduce this energy footprint, they are handicapped by generally poor selectivity versus other relevant gases, such as CO₂ and C₂H₄. In the case of CO₂, the respective physicochemical properties are so similar that traditional physisorbents, such as zeolites, silica, and activated carbons cannot differentiate well between CO₂ and C₂H₂. Herein, we report that a family of three isostructural, ultramicroporous (<7 Å) diamondoid metal–organic frameworks, [Cu(TMBP)X] (TMBP=3,3′,5,5′-tetramethyl-4,4′-bipyrazole), TCuX (X=Cl, Br, I), offer new benchmark C₂H₂/CO₂ separation selectivity at ambient temperature and pressure. We attribute this performance to a new type of strong binding site for C₂H₂. Specifically, halogen ⋅⋅⋅ HC interactions coupled with other noncovalent in a tight binding site is C₂H₂ specific versus CO₂. The binding site is distinct from those found in previous benchmark sorbents, which are based on open metal sites or electrostatic interactions enabled by inorganic fluoro or oxo anions.     

Pebax/TSIL blend thin film composite membranes for CO_2 separation

In this study a thin film composite (TFC) membrane with a Pebax/Task-specific ionic liquid (TSIL) blend selective layer was prepared. Defect-free Pebax/TSIL layers were coated successfully on a polysulfone ultrafiltration porous support with a polydimethylsiloxane (PDMS) gutter layer. Different parameters in the membrane preparation (e.g. concentration, coating time) were investigated and optimized. The morphology of the membranes was studied by scanning electron microscopy (SEM), while the thermal properties and chemical structures of the membrane materials were investigated by thermo-gravimetric analyzer (TGA), differential scanning calorimetry (DSC) and Fourier transform infrared spectroscopy (FTIR). The CO₂ separation performance of the membrane was evaluated using a mixed gas permeation test. Experimental results show that the incorporation of TSIL into the Pebax matrix can significantly increase both CO₂ permeance and CO₂/N₂ selectivity. With the presence of water vapor, the membrane exhibits the best CO₂/N₂ selectivity at a relative humidity of around 75%, where a CO₂ permeance of around 500 GPU and a CO₂/N₂ selectivity of 46 were documented. A further increase in the relative humidity resulted in higher CO₂ permeance but decreased CO₂/N₂ selectivity. Experiments also show that CO₂ permeance decreases with a CO₂ partial pressure increase, which is considered a characteristic in facilitated transport membranes.     

Solubility of Native Gold in H—O—S Fluids at 100–400°C and High H2S Content

Reaction of MgS, water, and air in sealed gold capsules at 100 to 400°C and0.15 GPa is used to generate an aqueous fluid with very high (20.6 m) H₂Scontent and to remobilize significant quantities of native gold as gold sulfides.A combination of X-ray photoelectron and Auger electron spectroscopy (XPS,AES), analytical scanning-electron microscopy (SEM—EDX),electron-micro-probe analysis (EPMA), and calculated solution properties shows that the goldsulfides precipitated during quenching and later perforation of the capsulesrepresent native gold dissolved as Au(I)-bisulfide under the experimental conditions.The equilibrium constant (logK) for the reaction:Au(s) H₂S(aq) + HS^– = Au(HS)₂ ^– + 1/2H₂(g)ranges from –3.96 ± 0.40 at 115°C to –1.06 ± 0.32 at 400°C; it is in goodagreement with literature values for 25°C and 300–350°C, and varies inverselywith absolute temperature T[–H ⁰ ₁/(2.303R)= –2644 ± 33K; r = 1.00]. Themaximum solubility of native gold in this study (29.4 g/kg at 200°C) issignificantly greater than that from published studies on Au(I)-bisulfides and maystimulate interest in developing bisulfides as gold-complexing agents in goldextraction technology.     

Mixed Metal–Organic Framework with Multiple Binding Sites for Efficient C2H2/CO2 Separation

The separation of C₂H₂/CO₂ is particularly challenging owing to their similarities in physical properties and molecular sizes. Reported here is a mixed metal–organic framework (M′MOF), [Fe(pyz)Ni(CN)₄] ( FeNi-M′MOF , pyz=pyrazine), with multiple functional sites and compact one-dimensional channels of about 4.0 Å for C₂H₂/CO₂ separation. This MOF shows not only a remarkable volumetric C₂H₂ uptake of 133 cm³ cm⁻³, but also an excellent C₂H₂/CO₂ selectivity of 24 under ambient conditions, resulting in the second highest C₂H₂-capture amount of 4.54 mol L⁻¹, thus outperforming most previous benchmark materials. The separation performance of this material is driven by π–π stacking and multiple intermolecular interactions between C₂H₂ molecules and the binding sites of FeNi-M′MOF . This material can be facilely synthesized at room temperature and is water stable, highlighting FeNi-M′MOF as a promising material for C₂H₂/CO₂ separation.     

Critical temperatures,pressures, and densities for the mixtures CO2–C3H8, CO2–nC4H10, C2H6–C3H8, and C3H8–nC4H10

A simple method is developed to estimate mixture critical temperatures (T_c), pressures (P_c), and densities (ρ_c) as a function of overall composition (X) from near critical region experimental coexistence data. This three-step method is applied to four mixtures, CO₂–C₃H₈, CO₂–nC₄H₁₀, C₂H₆–C₃H₈, and C₃H₈–nC₄H₁₀. Isothermal liquid–vapor coexistence data, which includes temperature, vapor pressure, coexisting densities (ρ^ℓ and ρ^v), and coexisting compositions for the more volatile component (x₁^v and x₁^ℓ) are used. In the first step, the difference of the saturated liquid and vapor densities (ρ^ℓ−ρ^v) is fitted to an empirical function in ((P_c−P)/P_c) to obtain P_c. Then P/P_c and ((ρ^ℓ+ρ^v)/2ρ_c) are simultaneously fitted to functions of a polynomial in (X₁−(x₁^v+x₁^ℓ)/2) yielding estimates of ρ_c and X₁. Finally, the discrete estimated critical data points are fitted with an equation to provide a continuous representation of the critical lines. The method is successfully tested for the mixtures, CO₂–C₃H₈ and CO₂–nC₄H₁₀, for which there is a reasonable amount of isothermal data. The procedure is then applied to the mixtures, C₂H₆–C₃H₈ and C₃H₈–nC₄H₁₀, for which there are sparse data. For all four mixtures, the critical temperature line, T_c vs. X₁, matches literature values within ±0.5%. The critical pressure line, P_c vs. X₁, and critical density line, ρ_c vs. X₁, match literature values, in general, within ±2%.     

Stabilized superhydrophobic composite membranes prepared by electrospinning for oil–water separation

In dealing with the oil–water separation process, improving the oil–water selectivity of the membrane surface and increasing the porosity within the membrane are effective means to achieve durable and efficient oil–water separation. Therefore, polystyrene/polyacrylonitrile-polyvinylidene fluoride/Polydimethylsiloxane Fe₃O₄ nanoparticles (PS/PAN-PVDF/PDMS@Fe₃O₄) composite membranes with superhydrophobicity and lipophilicity were prepared by electrospinning technique. By controlling the filling concentration of Fe₃O₄ nanoparticles, a stable superhydrophobic and lipophilic rough structure was constructed, and micro–nano multilayer rough voids existed inside the membrane. The results showed that the composite fiber membrane exhibited exceptional superhydrophobicity (156.2°), thermal stability (338°C), mechanical properties (tensile strength of 3.0 MPa, elongation of 33.9%), and oil adsorption capacity (30–100 g/g). Moreover, even under corrosive environments, this composite fiber membrane maintained its superhydrophobic properties (above 152°) and achieved high oil–water separation efficiency (above 97%). Remarkably, after 40 cycles, the composite membrane could sustain a separation flux of 5000 L m⁻² h⁻¹. Consequently, the composite fiber membrane manufactured using this strategy exhibits promising potential for applications in oil–water separation.     

Metal–organic framework-based mixed-matrix membranes for gas separation: An overview

Mixed-matrix membranes (MMMs) have been studied widely in the field of gas separation due to their potential to overcome performance barriers found in traditional polymeric membranes. Most polymeric membranes exhibit a trade-off between permeation and selectivity, which has limited their development in many challenging separation applications. One solution to this issue utilizes the introduction of fillers into the polymer matrix to produce MMMs. Out of the many different fillers, metal–organic frameworks stand out as a promising candidate due to their highly tunable structure, molecular sieving effect, and superior compatibility with the polymer matrix. This review will provide an in-depth look into the basic mechanisms of MMMs for gas separation and different approaches to model the permeation of gases through the membrane. In addition, challenges facing the field and recent research trends for MMMs will be discussed as well as their many applications for different gas separations. Finally, some insight on the future direction for MMMs will be covered, focusing on many intriguing opportunities and challenges that must be further explored to advance this technology.     

De-Linker-Enabled Exceptional Volumetric Acetylene Storage Capacity and Benchmark C2H2/C2H4 and C2H2/CO2 Separations in Metal–Organic Frameworks

An ideal adsorbent for separation requires optimizing both storage capacity and selectivity, but maximizing both or achieving a desired balance remain challenging. Herein, a de-linker strategy is proposed to address this issue for metal–organic frameworks (MOFs). Broadly speaking, the de-linker idea targets a class of materials that may be viewed as being intermediate between zeolites and MOFs. Its feasibility is shown here by a series of ultra-microporous MOFs (SNNU-98-M, M=Mn, Co, Ni, Zn). SNNU-98 exhibit high volumetric C₂H₂ uptake capacity under low and ambient pressures (175.3 cm³ cm⁻³ @ 0.1 bar, 222.9 cm³ cm⁻³ @ 1 bar, 298 K), as well as extraordinary selectivity (2405.7 for C₂H₂/C₂H₄, 22.7 for C₂H₂/CO₂). Remarkably, SNNU-98-Mn can efficiently separate C₂H₂ from C₂H₂/CO₂ and C₂H₂/C₂H₄ mixtures with a benchmark C₂H₂/C₂H₄ (1/99) breakthrough time of 2325 min g⁻¹, and produce 99.9999 % C₂H₄ with a productivity up to 64.6 mmol g⁻¹, surpassing values of reported MOF adsorbents.     

Chitosan/N-methylol nylon 6 blend membranes for the pervaporation separation of ethanol–water mixtures

Blend membranes of chitosan and N-methylol nylon 6 were prepared by solution blending. Their pervaporation performances for the separation of ethanol–water mixtures were investigated in terms of acid (H₂SO₄) post-treatment, feed concentration, blend ratio and temperature. The pervaporation performance of the blend membranes was significantly improved by ionizing with H₂SO₄. The blend ratio of chitosan and N-methylol nylon 6 plays a different role at feed solutions of low and high water content. At a feed solution having low water content, an increase in chitosan content caused a decrease in permeability and an increase in separation factor. At a feed solution having high water content, the permeability increases with an increase in chitosan content, while the separation factor shows a maximum value around 60 wt% chitosan. It is proposed that extra permeation channels generated from the phase separation boundary between ionized chitosan and N-methylol nylon 6 account for the abnormal temperature dependence of pervaporation performance of the blend membranes.     

Solubility and Crystallization in the System MgCl2–MgSO4–H2O at 50 and 75°C

The system MgCl₂–MgSO₄–H₂O has been investigated experimentally and modeled thermodynamically according to the Pitzer method at 50 and 75°C. It was found that, even when seemingly all requirements for reaching the stable thermodynamic equilibrium are fulfilled, the crystallization of higher hydrates as metastable phases is possible, and cannot be avoided in each crystallization field of a stable lower hydrate of magnesium sulfate. Crystallization of MgSO₄ · x H₂O (x = 1, 4, 6) and MgCl₂ · 6 H₂O at 50°C and of MgSO₄ · H₂O and MgCl₂ · 6 H₂O at 75°C as stable phases has been observed. Three metastable crystallization fields of MgSO₄ · x H₂O (x = 4, 6, 7) have been detected at 50°C and two of MgSO₄ · x H₂O (x = 4, 6) at 75°C. The results obtained and the contradictions existing in the literature with respect to the solubility and the crystallizing solid phases are discussed in terms of the crystal structures.     

Structural and mechanical characterizations of microporous silica–boron membranes for gas separation

This paper presents structural and mechanical characterizations of microporous silica membranes for gas separation. The membrane separative layer is made of microporous silica–B₂O₃ produced via a sol–gel process. This layer of about 200 nm of thickness is deposited on the internal surface of a tubular asymmetric γ-alumina/α-alumina support. FTIR and Raman analyses indicate the presence of the boron in the silica net and the above methods in conjunction with ¹¹B MAS NMR analyses of the samples indicate that boron is located mainly in the tetrahedral framework position. Such membranes present interesting gas separation properties at temperatures up to 500 °C and transmembrane pressures lower than 8 bar. He permeance values close to 10⁻¹⁰ kmol m⁻² s⁻¹ Pa⁻¹ are obtained, associated with ideal selectivity α(He/CO₂) which can reach 55. Mechanical properties of separative silica-modified layers are measured by nanoindentation and the coefficient of thermal expansion is obtained from pure material.     

Synthesis and morphology of polycaprolactone–block-polyimide–block-polycaprolactone triblock copolymers for film separation membranes

Triblock copolymers with the central polyimide block and peripheral polycaprolactone blocks were synthesized by controlled ring-opening anionic coordination polymerization of ε-caprolactone on polyimide macroinitiators with terminal hydroxy groups. The chemical structure of the products was determined by spectroscopic methods, and the molecular-mass characteristics of the triblock copolymers and their separate blocks were determined by size-exclusion chromatography. Examination of the phase morphology of strong self-supporting films formed from the copolymer solutions revealed microphase segregation on the nanometer level in the films.     

Polymeric proton conducting membranes for medium temperature fuel cells (110–160°C)

The availability of stable polymeric membranes with good proton conductivity at medium temperatures is very important for the development of methanol PEM fuel cells. In view of this application, a systematic investigation of the conductivity of Nafion 117 and sulfonated polyether ether ketone (S-PEEK) membranes was performed as a function of relative humidity (r.h.) in a wide range of temperature (80–160°C). The occurrence of swelling/softening phenomena at high r.h. values prevented conductivity determinations above certain temperatures. Nevertheless, when r.h. was maintained at values lower than 80%, measurements were possible up to 160°C. The results showed that Nafion is a better proton conductor than S-PEEK at low r.h. values, especially at temperatures lower than 120°C. The differences in conductivity were, however, leveled out with the increasing r.h. and temperature. While at 100°C and 35% r.h. the conductivity of S-PEEK 2.48 was about 30 times lower than the conductivity of Nafion, both membranes reached a comparable conductivity (4×10⁻² S cm⁻¹) at 160°C and 75% r.h. The effect of superacidity and crystallization of the polymers on the conductivity, as well as the possibility of using Nafion and S-PEEK membranes in medium temperature fuel cells, are discussed.     

n-Pentane/i-pentane separation by using zeolite–PDMS mixed matrix membranes

The performances of various zeolite filled polymeric membranes in the separation of n-pentane from i-pentane were investigated as a function of zeolite loading and various experimental conditions. Polydimethylsiloxane (PDMS) was chosen as the polymer phase and HZSM-5, NaZSM-5, 4A and 5A were used as zeolite fillers. Different Si/Al ratios and different activation temperatures were tested for ZSM-5 and A type zeolites, respectively. No improvement with respect to the n-pentane/i-pentane ideal selectivity of the original polymeric membrane could be obtained for the mixed matrix membranes investigated in this study. Interactions occurring in the zeolite–polymer interface seem to play a significant role in the results obtained. n-Pentane consistently showed lower permeability than for the pure polymer. On the other hand, i-pentane permeability was not reduced and was even increased with increased zeolite loading in some cases. It is hypothesized that these trends reflect differences in interaction between the two chemically similar penetrants with the “interphase” region connecting the bulk polymer and dispersed zeolite phases. The Si/Al ratio, the cation type and the activation temperature of the zeolite employed also seem to affect the performance of the zeolite–PDMS mixed matrix membranes.     

Aluminum electrode for electrochemical studies in cryolite-alumina melts at 700–960°C

The paper presents the results of tests of different aluminum electrode designs for electrochemical studies in cryolite-alumina melts in the temperature range of 700?C960°C. Their operation as regards stability and reproducibility of the potential and the highest activity of metallic aluminum is analyzed. The new design of the aluminum electrode is suggested that is characterized by a more stable and reproducible potential, as compared to the designs earlier suggested. Herewith, the activity of metallic aluminum is higher. It is shown that the suggested electrode is suitable for operation in cryolite-alumina melts in the temperature range from 700°C. The electrode can be used for a prolonged time. Herewith, its potential remains stable and reproducible.     

Carbon electrode for electrochemical studies in cryolite-alumina melts at 700–960°C

The paper presents the results of tests of different carbon electrode designs for electrochemical studies in cryolite-alumina melts in the temperature range of 700?C960°C. Operation of the electrode and its applicability as the reference electrode is analyzed. The theoretical value of the maximum possible instability of its potential related to instability of the gas composition is calculated. A new design of the carbon electrode is suggested in which the gas composition is much more stable. It is shown that the suggested electrode is suitable for application as the reference electrode in cryolite-alumina melts in the temperature range of 750°C and above. The electrode can be used for a prolonged time. Herewith, its potential remains stable and reproducible.     

The ion product of H2O,dissociation constants of H2CO3 and pitzer parameters in the system Na+/H+/OH−/HCO 3 − /CO 3 2- /ClO 4 − /H2O at 25°C

The ion product of water and the dissociation constants of carbonic acid have been determined in 0.1, 1.0, 3.0, and 5.0M NaClO₄ at 25°C. The ion product of water K _w ^' has been evaluated by emf measurements with a combined glass electrode in NaClO₄ solutions containing 0.001–0.1M HCLO₄ or NaOH. The product K _H ^' K _l ^' K ₂ ^' of the Henry constant for CO₂ and the dissociation constants for H₂CO₃ have been determined by titration of carbonate solutions equilibrated with pCO₂ =10^–3.52 atm, and K ₂ ^' has been evaluated by potentiometric titration and by measuring the H⁺ concentration at fixed HCO ₃ ^– and CO ₃ ^2- concentrations. The ion interaction (Pitzer) equations are applied to describe the constants K _w ^' , K ₂ ^' and K _H ^' H ₁ ^' K ₂ ^' as a function of the NaClO₄ concentration. The experimental data are used to evaluate the mixing parameters _{i/ClO 4} and _{i/ClO 4} ^-/Na⁺ fori = OH ^-,HCO ₃ ^- andCO ₃ ^2-     

Micro–nano structural engineering of filter paper surface for high selective oil–water separation

Cellulose-based lotus-leaf-like filter paper for selective separation of oil/water was prepared. Experimentally, cellulose nanofibril aerogel microspheres prepared by ultrasonic atomization method were coated on commercial filter papers to form unique “micro–nano” structured surface. By controlling both the morphology and chemistry of the surface, the papers could be either under-water superoleophobic or under-oil superhydrophobic. It was found that the filter papers could be engineered to effectively filter only oil or only water from their mixtures.