Ideal Selectivity Research Articles

Nematic liquid crystalline polymer films for gas separation

ABSTRACT The gas separation performances of free-standing planar aligned nematic LC polymer films were investigated for gas separations of He, CO2, CH4 and Xe. The films consist of derivatives of 1,4-phenylene bis(4-((6-(acryloyloxy)hexyl)oxy)benzoate)s with respective cyano, chloro, methyl and phenyl substituents on the central aromatic cores. Two new LC derivatives of 1,4-phenylene bis(4-((6-(acryloyloxy)hexyl)oxy)benzoate)s were successfully synthesised and fully characterised. Single gas permeation and sorption data show increasing gas permeabilities with increasing steric size of the substituents while the ideal gas selectivity of He over CH4 and He over CO2 decreases. The sorption coefficient of all films is independent of the LC substituents, while the subsequently extracted diffusion coefficient for the films with a phenyl substituent is three times higher compared to the films with a cyano substituent, demonstrating that the steric size of the LC substituents mainly affects the diffusion of gasses rather than the solubility of the gases. Irrespective of a methyl or a phenyl substituent, a larger kinetic diameter of Xe gives a 20 times lower diffusion coefficient compared to the smaller species (CO2).

Ethylene Recovery via Pebax-Based Composite Membrane: Numerical Optimization

Membrane technology, particularly polymeric membranes, is utilized in major industrial ethylene recovery owing to the very convenient and robust process. Thus, in this paper, a composite membrane (CM) comprising SAPO-34 and Pebax-1657 was employed to conduct a separation performance under two operating conditions, including temperatures and pressures, ranging from 25.0–60.0 °C and 3.5–10.0 bar, respectively. CO2 permeability and CO2/C2H4 ideal selectivity values that ranged from 105.68 to 262.86 Barrer and 1.81 to 3.52, respectively, were obtained via the experimental works. The separation of carbon dioxide (CO2) from ethylene (C2H4) has then been optimized using response surface methodology (RSM) by adopting a central composite design (CCD) method. As a result, the ideal operational conditions were discovered at a temperature of 60.0 °C and pressure of 10.0 bar with the maximum CO2 permeability of 233.62 Barrer and CO2/C2H4 ideal selectivity of 3.22. The typical discrepancies between experimental and anticipated data for CO2 permeability and CO2/C2H4 ideal selectivity were 1.67% and 3.10%, respectively, demonstrating the models’ validity. Overall, a new combination of Pebax-1657 and SAPO-34 composite membrane could inspire the latest understanding of the ethylene recovery process.

Design and Screening of Metal-Organic Frameworks for Ethane/Ethylene Separation.

Separation of ethane and ethylene is considered to be industrially important for various chemical processes, but their similarities make the process expensive. In this study, we integrated computational screening with machine learning to find optimal metal-organic frameworks (MOFs) with high ethane/ethylene selectivity. Using our algorithm, a hypothetical MOF structure with an ideal adsorption solution theory selectivity of 3.6 at 298 K and 1 bar was discovered. Furthermore, structural analysis was implemented, and the full adsorption isotherm of some of the top structures was obtained.

Dibenzomethanopentacene-Based Polymers of Intrinsic Microporosity for Use in Gas-Separation Membranes.

Dibenzomethanopentacene (DBMP) is shown to be a useful structural component for making Polymers of Intrinsic Microporosity (PIMs) with promise for making efficient membranes for gas separations. DBMP-based monomers for PIMs are readily prepared using a Diels-Alder reaction between 2,3-dimethoxyanthracene and norbornadiene as the key synthetic step. Compared to date for the archetypal PIM-1, the incorporation of DBMP simultaneously enhances both gas permeability and the ideal selectivity for one gas over another. Hence, both ideal and mixed gas permeability data for DBMP-rich co-polymers and an amidoxime modified PIM are close to the current Robeson upper bounds, which define the state-of-the-art for the trade-off between permeability and selectivity, for several important gas pairs. Furthermore, long-term studies (over ≈3 years) reveal that the reduction in gas permeabilities on ageing is less for DBMP-containing PIMs relative to that for other high performing PIMs, which is an attractive property for the fabrication of membranes for efficient gas separations.

Dibenzomethanopentacene‐Based Polymers of Intrinsic Microporosity for Use in Gas‐Separation Membranes

AbstractDibenzomethanopentacene (DBMP) is shown to be a useful structural component for making Polymers of Intrinsic Microporosity (PIMs) with promise for making efficient membranes for gas separations. DBMP‐based monomers for PIMs are readily prepared using a Diels–Alder reaction between 2,3‐dimethoxyanthracene and norbornadiene as the key synthetic step. Compared to date for the archetypal PIM‐1, the incorporation of DBMP simultaneously enhances both gas permeability and the ideal selectivity for one gas over another. Hence, both ideal and mixed gas permeability data for DBMP‐rich co‐polymers and an amidoxime modified PIM are close to the current Robeson upper bounds, which define the state‐of‐the‐art for the trade‐off between permeability and selectivity, for several important gas pairs. Furthermore, long‐term studies (over ≈3 years) reveal that the reduction in gas permeabilities on ageing is less for DBMP‐containing PIMs relative to that for other high performing PIMs, which is an attractive property for the fabrication of membranes for efficient gas separations.

Sulfonated Phenol–Formaldehyde Resins for Highly Efficient, Selective, and Reversible Adsorption of NH3

Currently available liquid absorbents and porous adsorbents for capturing NH3 are often unfavorable in terms of possessing high stability and reusability. Herein, three novel sulfonated phenol–formaldehyde (SPF) resins with almost non-porosity were prepared through a facile polymerization of various phenol sulfonates with formaldehyde and applied to the adsorption of NH3. It is demonstrated that the SPF–Na resin displayed excellent reversibility and good stability for efficient NH3 adsorption (12.87 mmol·g–1) and superior ideal adsorption solution theory selectivities for 3% NH3/97% N2 (5076) and 3% NH3/97% H2 (2619) at 298.2 K and 1.0 bar even in 20 adsorption–desorption cycles, whereas common PF resin only achieved a very low capacity of 1.9 mmol·g–1. Column breakthrough results further confirmed that the SPF–Na resin showed good performance in selective adsorption of 3% NH3 in the mixture of NH3/N2/H2. Nonporous SPF resins with high stability, desired capacity, and outstanding reversibility are perceived as promising solid adsorbents for effectively capturing NH3 in the ammonia industry.

A simple ZnO-embedded method to prepare flexible ZIF-8 composite PESf hollow fiber membrane

Highly flexible PESf-HF(hollow fiber)-supported ZIF-8 membrane was synthesized through a ZnO-embedded homologous seed induction method involving preparation of ZnO-containing-PESf-HF and growth of ZIF-8 on the outer-surface of PESf-HF with I.D. and O.D. of ∼1.9 mm and ∼2.5 mm. ZIF-8 membrane exhibited favorable molecular-sieve property of H2 over other gases. ZnO particles inside PESf-HF were used as nucleation and anchoring sites for ZIF-8 membrane growth to embed the membrane into support, which enhanced the binding strength between membrane and PESf-HF, thus obtaining excellent membrane stability and reproducibility. Because of this, the membrane displayed constant H2 permeance (∼1.06 × 10−7 mol·m−2·s−1·Pa−1) and H2/CO2 ideal selectivity (∼5.46) during the 1800 min-run.

Study on the Performance of Cellulose Triacetate Hollow Fiber Mixed Matrix Membrane Incorporated with Amine-Functionalized NH2-MIL-125(Ti) for CO2 and CH4 Separation

The increase in the global population has caused an increment in energy demand, and therefore, energy production has to be maximized through various means including the burning of natural gas. However, the purification of natural gas has caused CO2 levels to increase. Hollow fiber membranes offer advantages over other carbon capture technologies mainly due to their large surface-to-volume ratio, smaller footprint, and higher energy efficiency. In this work, hollow fiber mixed matrix membranes (HFMMMs) were fabricated by utilizing cellulose triacetate (CTA) as the polymer and amine-functionalized metal-organic framework (NH2-MIL-125(Ti)) as the filler for CO2 and CH4 gas permeation. CTA and NH2-MIL-125(Ti) are known for exhibiting a high affinity towards CO2. In addition, the utilization of these components as membrane materials for CO2 and CH4 gas permeation is hardly found in the literature. In this work, NH2-MIL-125(Ti)/CTA HFMMMs were spun by varying the air gap ranging from 1 cm to 7 cm. The filler dispersion, crystallinity, and functional groups of the fabricated HFMMMs were examined using EDX mapping, SEM, XRD, and FTIR. From the gas permeation testing, it was found that the NH2-MIL-125(Ti)/CTA HFMMM spun at an air gap of 1 cm demonstrated a CO2/CH4 ideal gas selectivity of 6.87 and a CO2 permeability of 26.46 GPU.

Experimental study on CO2 separation using hydrophobic deep eutectic solvent based supported liquid membranes

Deep eutectic solvents (DESs) have been proposed as a viable and more sustainable solvent for CO2 capture, owing to their low volatility and tunable properties. In this work, polymer membranes were combined with hydrophobic DESs to prepare supported liquid membranes (DES-SLMs), and the potential of using SLMs for CO2/N2 separation was investigated. COSMO-RS model simulations, bond energy analysis as well as stability tests were used to select the polymeric porous support. Hydrophobic polyvinylidene fluoride (PVDF) with a nominal pore size of 0.1 μm was chosen to support three selected hydrophobic DESs. The gas permeabilities through three selected DES-SLMs were experimentally determined, where the thymol-coumarin (1:1) based SLM showed the best performance with a CO2 permeability of 175.0 Barrer and selectivity of 30.2 at room temperature. The separation capability was further improved by mixing the DES with traditional physical absorbents. In addition, the effects of water presence in the gas phase on the DES-SLM were investigated. The membrane was physically stable, but the ideal selectivity of CO2/N2 through the membrane deteriorated with time on steam. By predicting the permeabilities of CO2 and N2 through traditional organic liquid based SLMs, the prospects of developing DESs for SLM applications were demonstrated. Finally, an advanced membrane capture process for a natural gas-fired combined cycle (NGCC) power plant was studied with process simulation and economic analysis, where the SLMs are used to purify the CO2 product.

A lysine-based 2:1-[α/aza]-pseudopeptide series used as additives in polymeric membranes for CO2 capture: synthesis, structural studies, and application.

The current study presents for the first time the synthesis of a new 2:1-[α/aza]-pseudopeptide series possessing charged amino acids (i.e., lysine) and aims at studying the influences of chirality, backbone length, and the nature of the lysine side chains on the conformation of the 2:1-[α/aza]-oligomers in solution using NMR, FTIR spectroscopy and molecular dynamic calculations. The spectroscopic results emphasized the conservation of the β-turn conformation adopted by the trimers regardless of the chirality which demonstrated a noticeable effect on the conformation of homochiral hexamer (8c) compared with the hetero-analogue (8d). The molecular dynamic calculations predicted that the chirality and the side chain of the lysine residues caused a little distortion from the classical β-turn conformation in the case of short trimer sequences (7c and 7d), while the chirality and the backbone length exerted more distortion on the β-turn adopted by the longer hexamer sequences (8c and 8d). The large disturbance in hexamers from classical β-turn was attributed to increasing the flexibility and the possibility of molecules to adopt a more energetically favorable conformation stabilized by non-classical β-turn intramolecular hydrogen bonds. Thus, alternating d- and l-lysine amino acids in the 2:1-[α/aza]-hexamer (8d) decreases the high steric hindrance between the lysine side chains, as in the homo analogue (8c), and the distortion is less recognized. Finally, short sequences of aza-pseudopeptides containing lysine residues improve CO2 separation when used as additives in Pebax® 1074 membranes. The best membrane performances were obtained with a pseudopeptidic dimer as an additive (6b'; deprotected lysine side chain), with an increase in both ideal selectivity α CO2/N2 (from 42.8 to 47.6) and CO2 permeability (from 132 to 148 Barrer) compared to the virgin Pebax® 1074 membrane.

High-entropy oxides as photocatalysts for organic conversion.

A strategy involving organic photocatalytic conversion using hydrothermal synthesis of high-entropy oxide (HEO) (CoCuZnMnNa)Ox nanoparticles was developed. Under mild conditions, HEO nanoparticles were driven by visible light to achieve ideal yields and selectivity in sulfide oxidative coupling reactions and benzimidazole cyclization reactions, with a wide substrate range. This study is expected to contribute to the use of high-entropy oxides in organic photocatalysis.

Method of Liquid-Phase Fluorination of Polymers to Increase the Selectivity of the Air Separation

The one-sided modification of homogeneous polymer films of polyvinyltrimethylsilane (PVTMS), poly(2,6-dimethylphenyl-1,4 oxide) (PPO), and polybenzodioxane (PIM-1) was carried out by liquid-phase fluorination method using a fluorine-nitrogen mixture in perfluorodecalin. The fluorination time ranged from 10 to 60 min. For PPO films the amorphous and β-nanocrystalline phase (48%) was found by X-ray diffraction analysis of the original films. It was shown that fluorination does not significantly affect the degree of PPO crystallinity. The effect of fluorination time on the effective coefficients of permeability, diffusion, and solubility of oxygen and nitrogen has been studied. It was found that the fluorination leads to a decrease in both effective diffusion coefficients and effective gas solubility coefficients, while the resulting increase in permeability selectivity ranged from 30% for PVTMS to a 2 times increase in the case of PPO and PIM-1. It has been noted that this improvement in permeability selectivity is achieved mainly by the solubility selectivity increasing. The permeability of O2/N2 mixture for original and modified films was studied. It was found that the achieved values of the separation factors are close to the ideal selectivity. The results obtained demonstrate the possibility of effective application of the liquid-phase modification method not only for the homogeneous polymer films of the studied polymers treatment, but also for membranes with a selective non-porous layer based on them.

A cation-π interaction confined graphene oxide membrane for separation of light paraffins and olefins.

A supported graphene oxide membrane is endowed with selective function for olefins by a cation intercalation method. The metal-cation fixed GO membrane exhibits a high propane to propylene ideal selectivity of 1817 for single gas and a separation factor of 7.1 for binary mixtures with fast gas permeance in the order of 10-7 mol m-2 s-1 Pa-1 and reliable permeation stability.

Alumina-PEBA/ PSf Multilayer composite membranes for CO2 separation: experimental and molecular simulation studies

In this research, Polyether block amide (PEBA) containing different loadings of α-Al2O3 particles was deposited on top of the polysulfone (PSf ) supports to form PEBA 1657‒α-Al2O3/PSf multilayer composite mixed matrix membranes (MCMMMs). Multilayer Composite structure was used to overcome the sedimentation of fillers in the polymer matrix. Moreover, alpha phase of the Al2O3 particles was applied to improve the distribution of these particles at higher loadings. SEM, XRD, and FTIR tests were applied to study morphology, crystalline structure, and chemical structure of the membranes, respectively. Gas permeation properties of the membranes were also measured using three different pure gases (CO2, CH4, and N2) at the pressure of 7 bar and temperature of 25 ºC. CO2 permeance and ideal selectivity of CO2/CH4, and CO2/N2 for the optimum MCMMM with 20 wt% loading of α-Al2O3 particles were 25% (117.5 Barrer), 81.5 % (32), and 86.5% (57) higher than that of multilayer composite neat membrane (MCNM), respectively. The molecular simulation results confirmed the results of the experimental studies and approved that the α-Al2O3 particles are right candidates for improving the PEBA performance for CO2 separation.

Poly(ethylene oxide)-Based Copolymer-IL Composite Membranes for CO2 Separation.

Poly(ethylene oxide) (PEO)-based copolymers are at the forefront of advanced membrane materials for selective CO2 separation. In this work, free-standing composite membranes were prepared by blending imidazolium-based ionic liquids (ILs) having different structural characteristics with a PEO-based copolymer previously developed by our group, targeting CO2 permeability improvement and effective CO2/gas separation. The effect of IL loading (30 and 40 wt%), alkyl chain length of the imidazolium cation (ethyl- and hexyl- chain) and the nature of the anion (TFSI-, C(CN)3-) on physicochemical and gas transport properties were studied. Among all composite membranes, PEO-based copolymer with 40 wt% IL3-[HMIM][TFSI] containing the longer alkyl chain of the cation and TFSI- as the anion exhibited the highest CO2 permeability of 46.1 Barrer and ideal CO2/H2 and CO2/CH4 selectivities of 5.6 and 39.0, respectively, at 30 °C. In addition, almost all composite membranes surpassed the upper bound limit for CO2/H2 separation. The above membrane showed the highest water vapor permeability value of 50,000 Barrer under both wet and dry conditions and a corresponding H2O/CO2 ideal selectivity value of 1080; values that are comparable with those reported for other highly water-selective PEO-based polymers. These results suggest the potential application of this membrane in hydrogen purification and dehydration of CO2 gas streams.

Biocompatible ammonium-based ionic liquids/ZIF-8 composites for CO2/CH4 and CO2/N2 separations

The development of sorbent materials with high carbon dioxide (CO2) selectivity is of vital importance to minimize the impact of global warming by separating and capturing this major greenhouse gas. In this work, for the first time five ammonium-based ionic liquids (ILs) were impregnated into the metal-organic framework (MOF) ZIF-8 for gas sorption applications. Characterization results showed that the produced IL@ZIF-8 composites are thermally stable and of microporous nature. IL impregnation was successfully accomplished and did not affect the crystalline structure of ZIF-8. Sorption-desorption equilibrium isotherms of different gases, including CO2, methane (CH4) and nitrogen (N2) were measured at 303 K for the pristine MOF and the composites. Given the careful choice of the ILs, the anion and cation effects were studied in terms of their impact on the sorption capacity and selectivity performance of each composite. Acetate-based composites showed high CO2 selectivity when compared to ZIF-8; specifically, the composite [N2 1 1 4][Ac]@ZIF-8 showed 51% increase, at 1 bar and for flue gas conditions. Because of the good CO2/N2 selectivity of the acetate-based composites, sorption-desorption equilibrium isotherms of the above-mentioned gases were also measured at 323 K, along with ZIF-8. Ideal selectivities were thus calculated to study the temperature impact on the selectivity performance of the materials. It can be observed that the higher the temperature, the less selective the materials are. Nevertheless, at 323 K and when compared to ZIF-8, the composite [N2 1 1 4][Ac]@ZIF-8 showed 56% increase, at 1 bar and for flue gas conditions. Finally, the isosteric heats of adsorption of ZIF-8 and the acetate-based ILs were calculated revealing that all gases are physisorbed by the three materials.

Layered Ti3C2Tx MXene/CuO spindles composites for NH3 detection at room-temperature

Ammonia (NH3) detection is important for human health and environmental protection in our daily life. In this direction, the multilayers Ti3C2Tx MXene/CuO spindles (M-Ti3C2Tx MXene/CuO) composite was synthesized by a simple solvothermal approach for room-temperature (RT) NH3 detection. The CuO spindles as active center components were homogeneously anchored on the surface and the interlayers of the lamelleted Ti3C2Tx MXene. The structure and sensing characteristics of the as-fabricated composites were measured applying an array of analytical technique. The M-Ti3C2Tx MXene/CuO-based sensor indicated excellent sensitivity (46.7% towards 5 ppm NH3) and ideal selectivity. Moreover, the sensor also exhibited the shorter response time (12 s) and recovery time (25 s), brilliant repeatability and great long-term stability. A possible sensing mechanism based on the formation of Ti3C2Tx MXene/CuO heterojunctions was explained in detail in conjunction with density functional theory (DFT) simulations. The designed M-Ti3C2Tx MXene/CuO hybrid material can be considered as a promising candidate for high-performance NH3 detection at room temperature.

Spinning of Polysulfone Hollow Fiber Membranes Using Constant Dope Solution Composition: Viscosity Control via Temperature.

The dope solution viscosity is an important parameter that largely determines the properties of hollow fiber membranes. In the literature available today, the change in viscosity is carried out only by changing the quantitative and/or qualitative dope solution compositions. However, such an important spinning parameter as temperature should significantly affect the dope solution viscosity. For the first time, the influence of the dope solution viscosity of a constant composition on polysulfone hollow fiber membrane properties was studied. The hollow fiber membranes were obtained by the phase separation method induced by a non-solvent (NIPS). The change in the dope solution temperature was carried out in the temperature range of 17-27 °C, providing a dope solution viscosity range of 34.3-21.6 Pa∙s. This work shows that even in such a narrow temperature range, the properties of polysulfone hollow fiber membranes change significantly. With a decrease in the viscosity in this temperature range, the wall thickness of the hollow fiber membrane decreases by 2.8 times; the permeance for the individual gases He and CO2 increases by 1.6-1.8 times, respectively; the ideal selectivity decreases by 1.12 times; the mean flow pore size increases by 1.63 times; and the surface porosity increases about 3 times.

Effect of bulk and surface composition of Ni + Ga intermetallic compound catalysts in propane steam/wet reforming: Origins of nearly ideal experimental product selectivity

In our prior studies, Ni + Ga IMCs were isolated as promising wet reforming catalysts that achieved nearly ideal product distributions of H2 and either CO or CO2 and exhibited excellent stability (reduced coke formation). A systematic understanding of IMC surface chemistry as a function of constituent element selection directed our studies to the Ni + Ga IMC compositional space. However, Ni + Ga IMC surface chemistry towards oxygen and in oxidation reaction steps remained to be understood. This study focuses upon exploring the origin of the performance of Ni + Ga IMCs in the wet reforming of propane with a specific focus on controlling the fate of atomic H and understanding the control of oxidation steps to dictate CO vs CO2 production. Additionally, a simple kinetic coupling parameter was developed to predict coke formation and catalyst stability with excellent agreement between predictions and experimentally observed stability.

Enhancement of H2 Separation Performance in Ring-Opened Tröger’s Base Incorporating Modified MOFs

Energy-efficient hydrogen (H2) recovery technology is of great significance to developing H2-based economy and accomplishing global carbon-neutrality with minimized worldwide energy consumption. A ring-opened Tröger’s base (TBOR) polymer membrane as a highly H2-selective membrane holds great promise for recovering H2 while partial microporosity loss induced by ring opening reaction deteriorates the H2 permeability across the pure membrane. Herein we introduce polydopamine-modified ZIF-8 (PDA-ZIF-8) nanoparticles into TBOR to develop a permeable membrane for selective H2 separation. The resultant mixed matrix membranes demonstrate the significantly improved H2 permeability, owing to the porous structure of ZIF-8, and efficiently exclude large molecules of CH4 and N2, giving rise to promising H2/CH4 and H2/N2 ideal selectivities. The PDA-ZIF-8 interacts with the secondary amines of TBOR via hydrogen bonds, creating good interfacial adhesion, thus preventing a sharp selectivity loss that occurred in most mixed matrix membranes upon increasing filler loadings. At a maximum ZIF-8 loading of 50 wt %, the pure H2 permeability is 457 Barrer, increasing to 4.2 times as compared with the undoped membrane, and the ideal H2/CH4 selectivity of 57 and ideal H2/N2 selectivity of 59 are achieved. The H2/CH4 separation properties remain unchanged upon increasing feed pressures, indicating the membrane’s good stability when operated at realistic high pressures. The ring-opening functionalization of Tröger’s base has proven to be very useful for fabricating selective mixed matrix membranes and can be extended for designing H2-selective hybrid membranes with other porous fillers.