Membranes and Membrane Technologies最新文献

The process of oxidative dehydrogenation of ethane on titanium-vanadium oxide catalysts deposited on silica gel by the molecular layer deposition method was studied. It was shown that the deposition of titanium and vanadium on the surface of silica occurs uniformly. When at least two layers are deposited titanium oxide crystallizes in the anatase phase while vanadium oxide remains X-ray amorphous in all catalysts. The influence of the titanium-to-vanadium ratio on the activity of the obtained catalysts was studied. It was shown that the conversion of ethane and the selectivity of its conversion into ethylene increase with increasing both the number of deposited titanium and vanadium layers and temperature. The maximum values of ethane/ethylene conversion and ethylene selectivity reach 19 and 85%, respectively. The possibility of separating ethylene and ethane in a mixture of gases leaving a catalytic reactor cooled to 30°C using a grafted ion-exchange membrane with ethylene enrichment up to 90% in three runs is demonstrated. The ethylene permeability during separation achieves 36 Barrer, and the separation factor of the ethane-ethylene mixture is 3.8.

Hydrogenation is one of the most crucial and widespread chemical processes. This review addresses the rapidly developing electrochemical hydrogenation technology using membrane reactors. A membrane acts simultaneously as a cathode for the electrochemical formation of hydrogen and a separator dividing the electrochemical compartment from the hydrogenation compartment with a substrate. Electrochemical membrane hydrogenation allows producing high-purity hydrogen directly from water, avoiding catalyst poisoning. In this case, hydrogen reaches the membrane surface in the hydrogenation compartment in a highly active atomic state. The choice of catalyst and process conditions enables one to tune selectivity of the process, and the separation of chemical processes occurring in the system can make it possible to eliminate the stage of products purification from at least some of the starting compounds and solvents. The various types of membranes that can be used in this technology are considered, as well as the processes of electrochemical hydrogenation of various organic and inorganic compounds in membrane reactors, including the processes of electrolytic hydrogen production and the operation of fuel cells. In conclusion, the prospects for the development of this technology are discussed.

The influence of gas-phase fluorination conditions on the gas-selective properties of laboratory-scale hollow fiber membrane modules based on Matrimid 5218^® for the main components of biogas separation was investigated. The modification was carried out directly in laboratory membrane modules with a F₂/He gas mixture in the F₂ concentration range of 2–10 vol %. It was found that the interaction with fluorine leads to a change in the chemical structure of the selective membrane layer due to the replacement of hydrogen atoms by fluorine in the polymer structure. Increasing the fluorine concentration in the fluorination mixture and the fluorination time results in a decrease in the permeability of the modules to the target gases. As a result, the CO₂ permeability decreases by a factor of 4-5 and the CH₄ permeability by more than an order of magnitude, leading to an increase in selectivity (from 50 to 136) compared to the original hollow fibers. The high convergence of the ideal selectivity and separation factor of the СО₂/СН₄ mixture for hollow fibers modified under various conditions has been experimentally demonstrated. The temperature effect on the gas transfer parameters for modified hollow fiber modules based on Matrimid 5218^® was investigated. In the case of biogas separation under real conditions at T = 50°C (average temperature of vital activity of methanogenic bacteria), the selectivity of СО₂/СН₄ separation remains at a high level. For example, the CO₂/CH₄ selectivity for the module modified by 2% F₂ for 2 min was equal to 61, and the СО₂ permeability turns out to be 1.5 times higher in comparison with the original membranes. Based on the experimental results, operational schemes have been proposed and modelling of the biogas separation process with release of methane with high recovery rates (up to 99%) and ballast CO₂ using highly selective modified Matrimid 5218^® membranes has been carried out, moreover, with fluoridation of the modules in “soft” conditions, the indicators improve over time.

The development of membrane processes requires new materials for the fabrication of highly efficient membranes. In this work, a composite based on copolyimide P84 with a novel modifier, the titanosilicate mineral natisite, used an additive has been created. For this purpose, natisite has been synthesized and identified. The composite P84/natisite (5 wt %) prepared in a DMF solution was used to obtain a flat film membrane. The physicochemical, mechanical, and gas transport properties of the P84/natisite membrane were studied in comparison with the P84 membrane. The transport properties were evaluated by the permeability of the membranes to He, O₂, N₂, and CO₂. The gas permeability through the membranes made of the composite is lower than that of the pure P84 membrane, and the H₂/N₂, CO₂/N₂, and O₂/N₂ selectivities are improved by including the modifier natisite. It is shown that the presence of 5 wt % natisite does not significantly alter the mechanical properties of the P84/natisite (5%) membrane, which meet the performance requirements.

Resource-efficient and environmentally sustainable electrodialysis (ED) is increasingly being used for the separation and purification of organic acids, including the extraction of their anions from wines, juices, and biochemically processed waste products. In this study, tartaric acid transport through the CJMA-3 anion-exchange membrane was investigated using voltammetry, chronopotentiometry, and ED experiments. It was shown that when using a Na_xH_(2–x)T solution at pH 9.0, which contains only divalent tartrate anions T²⁻, the transport patterns are similar to those well-known for strong electrolytes. However, at pH 2.5 or 3.0, the solution contains a mixture of undissociated tartaric acid molecules H₂T and monovalent anions HT⁻. Upon entering the membrane, some of the HT⁻ anions dissociate. Protons are expelled into the depleted solution due to the Donnan effect, while the newly formed divalent anions T²⁻ migrate through the CJMA-3 membrane. The reduction in HT⁻ concentration near the membrane stimulates the irreversible dissociation of H₂T. Under the influence of the electric field, protons are removed from the reaction zone and migrate into the solution, while anions move into the membrane. Thus, tartrate transport through the anion-exchange membrane occurs even when the feed solution primarily contains undissociated acid molecules. These mechanisms lead to empirical limiting currents significantly exceeding theoretical limiting current values. The energy consumption for extracting 20% of tartrates from a 0.022 M Na_xH_(2–x)T solution is 0.22 (pH 9.0), 0.32 (pH 3.0), and 0.57 kWh/kg (pH 2.5). The duration of ED increases in the following order: pH 3.0 ( ll ) pH 9.0 < pH 2.5.

A one-sided modification of homogeneous poly(1-trimethylsilylpropyne) (PTMSP) films was carried out by the method of liquid-phase fluorination with elemental fluorine in perfluorodecalin medium. The structure of the initial and modified samples was studied by X-ray photoelectron spectroscopy (XPS) and cross-sections of fluorinated films were tested by scanning electron microscopy (SEM). The differential gas chromatographic method was used to obtain the transport and separation characteristics of the studied samples for O₂, N₂, CO₂, CH₄, He, H₂. Fluorination of PTMSP films was shown to occur not only in the near-surface layer of the sample, but also to a depth of up to 50 µm. It was found that the gas permeability of the fluorinated films decreases, while the change in the flow through the membrane depends on the size of the penetrant molecule, as a result there is a significant increase in the selectivity of fluorinated PTMSP samples for H₂–CO₂, H₂–CH₄, H₂–N₂ and O₂–N₂ pairs relative to the virgin polymer. It was shown that in the process of liquid-phase fluorination PTMSP films are saturated with perfluorodecalin, which leads to a significant decrease in gas permeability. An approach is proposed to increase the flow of gases through a fluorinated membrane by removing perfluorodecalin by holding samples in hexafluorobenzene, leading to a significant increase in gas permeability while maintaining selectivity. The gas separation data for fluorinated PTMSP films after treatment in hexafluorobenzene are located significantly above the upper bound of 2008 in the permeability-selectivity plots. Thus, the method of liquid-phase fluorination followed by hexafluorobenzene treatment leads to a significant improvement in the gas separation characteristics of PTMSP.

The paper presents the results of using ultrafiltration for purification of solutions simulating liquid radioactive waste, produced during decontamination of parts by electrolytic-plasma treatment, to remove chromium(III) radionuclides (⁵¹Cr(III)). The main performance characteristics and transport properties of ultrafiltration membranes made of hydrophilized polysulfone, polyethersulfone, and regenerated cellulose with different molecular weight cutoff (MWCO) values have been determined. The dependences of membrane flux and ⁵¹Cr(III) rejection coefficient on the pH of solutions and thermostatting time have been established. It has been shown that in 8% (NH₄)₂SO₄ solution at pH 7–8, the ⁵¹Cr(III) radionuclide occurs in the form of polynuclear hydroxo complexes, which are retained by ultrafiltration membranes and precipitate during centrifugation. A regenerated-cellulose membrane with MWCO = 10 is the most effective, rejecting ∼97% of ⁵¹Cr(III) at pH 8. Increasing the time of solution thermostatting before membrane separation leads to an increase in the rejection of ⁵¹Cr(III) due to a deeper hydrolysis process with the formation of polynuclear hydroxo complexes.

One of the main trends in the development of metal-ion batteries concerns the transition to lithium anodes, the safe use of which is impossible without replacing liquid membranes with solid ones, primarily inorganic membranes. Lithium–niobium–chromium phosphates with calculated compositions Li_3–2xNb_xCr_2–x(PO₄)₃ (х = 0.95, 1.00, 1.05) were obtained by solid-state synthesis and characterized by XRD analysis and impedance spectroscopy. The obtained complex lithium-niobium-chromium phosphates with the NASICON structure crystallize in hexagonal modification. The unit cell parameters of crystal lattice of the synthesized materials decrease with increasing chromium content. The highest ionic conductivity and the lowest activation energy are exhibited by the material of composition Li_1.1Nb_0.95Cr_1.05(PO₄)₃ (3 × 10^–5 S/cm at 25°С), which indicates a greater mobility of lithium ions by the interstitial mechanism even in the region of its own disorderliness.

The physicochemical and transport characteristics of cast and extruded MF-4SK perfluorinated membranes modified with zirconium hydrogen phosphate in an amount of 3–10% are studied. The inorganic phase is formed directly in the membrane volume by the pore-filling method. The effect of zirconium hydrogen phosphate content on the ion-exchange capacity, water content, diffusion permeability for electrolyte solution, hydrogen gas permeability, and electrical conductivity at the limited humidity of the MF-4SK membrane, as well as the efficiency of its use in a low-temperature proton-exchange membrane fuel cell, are investigated. A nonmonotonic change in the transport characteristics depending on the dopant content is demonstrated. The lowest diffusion permeability and maximum electrical conductivity at low humidity are exhibited by the membrane containing 6% of zirconium hydrogen phosphate. It is shown that zirconium hydrogen phosphate-modified membranes show promise as a polymer electrolyte in a hydrogen-air fuel cell membrane-electrode assembly due to a 17% higher maximum specific power compared to the original MF‑4SK membrane. This fact can be explained by an almost twofold lower ohmic resistance and reduced contribution of kinetic limitations of the membrane-electrode assembly (MEA) with the modified membranes, compared to the unmodified membrane, revealed by analysis of its impedance spectra.

t—Palladium-containing membranes are used for hydrogen separation and purification. However, for sufficiently thin membranes permeation flux can be limited by the kinetics of surface processes. In the present study, in order to overcome the limitation of transition through the surface, the developed Pd₇₆Ag₁₄Au₁₀ alloy membranes were modified with a nanostructured surface layer. The modification was carried out by the deposition of penta-branched bimetallic Pd–Pt nanoparticles on the membrane surface. An increase in hydrogen flux was observed in a wide temperature range (25–400°C). The highest values of permeation flux density were demonstrated for membranes with a penta-branched modifier, up to 52.43 mmol s^–1 m^–2 at 400°С. It is assumed that the complex morphology of the nanoparticles, as well as the presence of synergistic effect from the combination of Pd and Pt, contribute to a decrease in activation barriers and an increase in catalytic activity. The developed membranes demonstrated high and stable selectivity over time, which opens up wide possibilities for their use in steam reforming reactors for producing high-purity hydrogen.