Pervaporative Dehydration of Organic Solvents Using High-Silica CHA-Type Zeolite Membrane

Bis(triethoxysilyl)ethane (BTESE)-Organosilica Membranes for H2O/DMF Separation in Reverse Osmosis (RO): Evaluation and Correlation of Subnanopores via Nanopermporometry (NPP), Modified Gas Translation (mGT) and RO Performance

A 40 cm length Bis(triethoxysilyl)ethane (BTESE) membrane having different pore sizes was successfully prepared by changing the number of coating times for gas permeation (GP) and organic solvent reverse osmosis (OSRO) separation study. It was found that BTESE-6 membranes prepared through six-time coating consisted of small-sized pores in the range 0.56 to 0.64 nm estimated using modified Gas Translation (mGT) method and 0.59 to 0.67 nm estimated by nanopermporometry (NPP) method, respectively. These membranes demonstrated a high DMF rejection, RDMF > 95% with total flux, Jv total > 5 kg m-2 h-1 at operating condition feed pressure, Pf: 8 MPa; feed temperature, Tf : 50 °C; and feed flowrate, Qf : 30 mL/min; and they exhibited a high degree selectivity of He/SF6 in the range of ~ 260-3400 at a permeation temperature 200 °C. On the other hand, the larger pore sizes of the BTESE-4 membranes (pore size estimates > 0.76 nm to 1.02 nm) exhibited low DMF rejection and a low degree selectivity of He/SF6 around ~30% and 25, respectively, at the same operating condition as BTESE-6. Both GT and NPP methods can be considered as an indicator of the measurement membrane pore size. From this study, it was found that He and SF6 gases can be some of the potential predictors for water and DMF permeance. Furthermore, by comparing our OSRO membrane with other PV membranes for DMF/H2O separation, our BTESE-6 membranes still exhibited high flux in the range of 3-6 kg m-2 h-1 with a separation factor H2O/DMF in the range of 80-120.

Hydrogen-bonded structures and low temperature transitions of the confined water in subnano channels

The interfacial and confined water have long been attractive objects due to their crucial roles in biological, geological processes, etc. In this paper, we investigate the hydrogen-bonded structures of water and their low temperature transitions in the subnano channels of AlPO4-11 for the first time on the basis of infrared spectroscopy. The number of the adsorbed water molecules is estimated to be 8.45 per channel in one unit cell by thermogravimetric analysis. It is found that the confined water molecules are involved in saturated and unsaturated coordination with different hydrogen bond strengths at ambient temperature. The former refers to ice-like four-coordinated water and the latter includes liquid-like structures, Al-coordinated and relatively free water molecules. Unique coordination between water molecules and framework Al sites is responsible for the ice-like structures in the channels above the ice melting point. The appearance of liquid-like structures is closely related to the strong channel confinement, which does not allow the formation of extensive tetrahedral hydrogen-bonded configuration. As temperature decreases, a structural transformation of confined water happens in the channels of AlPO4-11. Isolated small water oligomers and two new components with stronger hydrogen bonds, such as low-density amorphous ice-like structures and a kind of low-density liquid-like structures are preferred. Our results provide important insights into the structural organizations and thermal-dynamic behaviors of confined water in extreme narrow channels.

Organic Non-Nucleophilic Electrolyte Resists Carbonation during Selective CO2 Electroreduction

The spontaneous reaction of CO₂ with water and hydroxide to form (bi)carbonates in alkaline aqueous electrolytes compromises the energy and carbon efficiency of CO₂ electrolyzers. We hypothesized that electrolyte carbonation could be mitigated by operating the reaction in an aprotic solvent with low water content, while also employing an exogenous non-nucleophilic acid as the proton donor to prevent parasitic capture of CO₂ by its conjugate base. However, it is unclear whether such an electrolyte design could simultaneously engender high CO₂ reduction selectivity and low electrolyte carbonation. We herein report selective CO₂ electroreduction with low carbonate formation on a polycrystalline Au catalyst using dimethyl sulfoxide as the solvent and acetic acid/acetate as the proton-donating medium. CO₂ is reduced to CO with over 90% faradaic efficiency at potentials relative to the reversible hydrogen electrode that are comparable to those in neutral aqueous electrolytes. ¹H and ¹³C NMR studies demonstrate that only millimolar concentrations of bicarbonates are reversibly formed, that the proton activity of the medium is largely unaffected by exposure to CO₂, and that low carbonation is maintained upon addition of 1 M water. This work demonstrates that electrolyte carbonation can be attenuated and decoupled from efficient CO₂ reduction in an aprotic solvent, offering new electrolyte design principles for low-temperature CO₂ electroreduction systems.

Estimation of CO2 Separation Performances through CHA-Type Zeolite Membranes Using Molecular Simulation

Chabazite (CHA)-type zeolite membranes are a potential material for CO₂ separations because of their small pore aperture, large pore volume, and low aluminum content. In this study, the permeation and separation properties were evaluated using a molecular simulation technique with a focus on improving the CO₂ separation performance. The adsorption isotherms of CO₂ and CH₄ on CHA-type zeolite with Si/Al = 18.2 were predicted by grand canonical Monte Carlo, and the diffusivities in zeolite micropores were simulated by molecular dynamics. The CO₂ separation performance of the CHA-type zeolite membrane was estimated by a Maxwell-Stefan equation, accounting for mass transfer through the support tube. The results indicated that the permeances of CO₂ and CH₄ were influenced mainly by the porosity of the support, with the CO₂ permeance reduced due to preferential adsorption with increasing pressure drop. In contrast, it was important for estimation of the CH₄ permeance to predict the amounts of adsorbed CH₄. Using molecular simulation and the Maxwell-Stefan equation is shown to be a useful technique for estimating the permeation properties of zeolite membranes, although some problems such as predicting accurate adsorption terms remain.

Ethanol dehydration performance of three types of commercial-grade zeolite permselective membranes

BACKGROUND

Many organic solvents form difficult-to-separate mixtures with water and have an affinity for water, making drying a potential reuse prerequisite. Pervaporation (PV) and vapor permeation (VP) membrane technologies hold promise for energy-efficient solvent drying. Several water-selective membrane materials are commercially available, but performance data is limited, particularly for two recently commercialized membrane materials: chabazite (CHA) and T-type zeolites. In this work, commercial-grade samples of CHA and T-type membranes, along with a NaA zeolite membrane, were evaluated for the removal of water from ethanol.

RESULTS

The CHA sample had the highest initial PV water permeance (6820 GPU) and water permselectivity (3430) with 5 wt% water in ethanol at 50 °C. Initial NaA membrane performance was slightly lower (6060 GPU and 3260), while the T-type membrane had the lowest initial permeance and selectivity (4260 GPU and 1090). Performance declined over time, most notably for the NaA membrane, for which water permeance fell over 50% through 39 days of testing. The T-type membrane exhibited the steadiest PV water permeance, but the most variable ethanol permeance.

CONCLUSION

The PV performance of the three membranes largely overlapped the predicted range for T-type membranes. That performance generally exceeds the anticipated ethanol drying performance of non-zeolitic PV membranes but is less than that predicted for NaA and CHA membranes. The present CHA membrane results, along with other recent reports, refine earlier predictions of the ethanol dehydration performance of that type of zeolite. The changing performance with time should be understood to properly design a solvent dehydration system.

Pervaporation as a Successful Tool in the Treatment of Industrial Liquid Mixtures

Pervaporation is one of the most active topics in membrane research, and it has time and again proven to be an essential component for chemical separation. It has been employed in the removal of impurities from raw materials, separation of products and by-products after reaction, and separation of pollutants from water. Given the global problem of water pollution, this approach is efficient in removing hazardous substances from water bodies. Conventional processes are based on thermodynamic equilibria involving a phase transition such as distillation and liquid-liquid extraction. These techniques have a relatively low efficacy and nowadays they are not recommended because it is not sustainable in terms of energy consumption and/or waste generation. Pervaporation emerged in the 1980s and is now becoming a popular membrane separation technology because of its intrinsic features such as low energy requirements, cheap separation costs, and good quality product output. The focus of this review is on current developments in pervaporation, mass transport in membranes, material selection, fabrication and characterization techniques, and applications of various membranes in the separation of chemicals from water.

Zeolite Membrane: From Microstructure to Separation Performance

Zeolite membrane have been investigated all over the world as an attractive tool in the development of separation processes for both liquid and gaseous components [...]

Biodegradable Polymeric Membranes for Organic Solvent/Water Pervaporation Applications

Biodegradable polymers are a green alternative to apply as the base membrane materials in versatile processes. In this study, two dense membranes were made from biodegradable PGS (poly(glycerol sebacate)) and APS (poly(1,3-diamino-2-hydroxypropane-co-polyol sebacate)), respectively. The prepared membranes were characterized by FE-SEM, AFM, ATR-FTIR, TGA, DSC, water contact angle, and degree of swelling, in comparison with the PDMS (polydimethylpolysiloxane) membrane. In the pervaporation process for five organic solvent/water systems at 37 °C, both biodegradable membranes exhibited higher separation factors for ethanol/water and acetic acid/water separations, while the PDMS membrane attained better effectiveness in the other three systems. In particular, a positive relationship between the separation factor and the swelling ratio of organic solvent to water (DS_o/DS_w) was noticed. In spite of their biodegradability, the stability of both PGS and APS membranes was not deteriorated on ethanol/water pervaporation for one month. Furthermore, these two biodegradable membranes were applied in the pervaporation of simulated ABE (acetone-butanol-ethanol) fermentation solution, and the results were comparable with those reported in the literature.

Development of Methanol Permselective FAU-Type Zeolite Membranes and Their Permeation and Separation Performances

The separation of non-aqueous mixtures is important for chemical production, and zeolite membranes have great potential for energy-efficient separation. In this study, the influence of the framework structure and composition of zeolites on the permeation and separation performance of methanol through zeolite membranes were investigated to develop a methanol permselective zeolite membrane. As a result, the FAU-type zeolite membrane prepared using a solution with a composition of 10 SiO₂:1 Al₂O₃:17 Na₂O:1000 H₂O showed the highest permeation flux of 86,600 μmol m^-2 s^-1 and a separation factor of 6020 for a 10 wt% methanol/methyl hexanoate mixture at 353 K. The membrane showed a molecular sieving effect, reducing the single permeation flux of alcohol with molecular size for single-component alcohols. Moreover, the permeation flux of methanol and the separation factor increased with an increase in the carbon number of the alcohols and methyl esters containing 10 wt% methanol. In this study, the permeation behavior of FAU-type zeolite membranes was also discussed based on permeation data. These results suggest that the FAU-type zeolite membrane has the potential to separate organic solvent mixtures, such as solvent recycling and membrane reactors.

Alicyclic Polyimide/SiO2 Mixed Matrix Membranes for Water/n-Butanol Pervaporation

Alicyclic polyimides (PIs) have excellent properties in solubility, mechanical strength, thermal property, etc. This study developed two types of alicyclic PI-based mixed matrix membranes (MMMs) for water/n-butanol pervaporation application, which have never been investigated previously. The fillers were hydrophilic SiO₂ nanoparticles. The synthesized PI was mixed with SiO₂ nanoparticles in DMAc to make the casting solution, and a liquid film was formed over PET substrate using doctor blade. A dense MMM was fabricated at 80 °C and further treated via multi-stage curing (100–170 °C). The prepared membranes were characterized by FTIR, TGA, FE-SEM, water contact angle, and solvent swelling. The trends of pure solvent swelling effects agree well with the water contact angle results. Moreover, the pervaporation efficiencies of alicyclic PI/SiO₂ MMMs for 85 wt% n-butanol aqueous solution at 40 °C were investigated. The results showed that BCDA-3,4′-ODA/SiO₂ MMMs had a larger permeation flux and higher separation factor than BCDA-1,3,3-APB/SiO₂ MMMs. For both types of MMMs, the separation factor increased first and then decreased, with increasing SiO₂ loading. Based on the PSI performance, the optimal SiO₂ content was 0.5 wt% for BCDA-3,4′-ODA/SiO₂ MMMs and 5 wt% for BCDA-1,3,3-APB/SiO₂ MMMs. The overall separation efficiency of BCDA-3,4′-ODA-based membranes was 10–30-fold higher.

Recent Advances of Pervaporation Separation in DMF/H2O Solutions: A Review

N,N-dimethylformamide (DMF) is a commonly-used solvent in industry and pharmaceutics for extracting acetylene and fabricating polyacrylonitrile fibers. It is also a starting material for a variety of intermediates such as esters, pyrimidines or chlordimeforms. However, after being used, DMF can be form 5–25% spent liquors (mass fraction) that are difficult to recycle with distillation. From the point of view of energy-efficiency and environment-friendliness, an emergent separation technology, pervaporation, is broadly applied in separation of azeotropic mixtures and organic–organic mixtures, dehydration of aqueous–organic mixtures and removal of trace volatile organic compounds from aqueous solutions. Since the advances in membrane technologies to separate N,N-dimethylformamide solutions have been rarely reviewed before, hence this review mainly discusses the research progress about various membranes in separating N,N-dimethylformamide aqueous solutions. The current state of available membranes in industry and academia, and their potential advantages, limitations and applications are also reviewed.

Influence of Organic Solvent Species on Dehydration Behaviors of NaA-Type Zeolite Membrane

In this study, an NaA-type zeolite membrane was prepared, and the dehydration performances of the membrane were determined by the pervaporation for several organic solvents to understand the lower dehydration performances of zeolite membranes for NMP solutions than those for alcohols. For a 90 wt% ethanol solution at 348 K, the permeation flux and separation factor of the membrane were 3.82 kg m⁻² h⁻¹ and 73,800, respectively. The high dehydration performances were also obtained for alcohols and low boiling solvents (acetonitrile, acetone, methyl ethyl ketone (MEK) and tetrahydrofuran (THF)). However, the permeation flux and separation factors decreased significantly for high boiling solvents, such as N,N-dimethylacetamide (DMA), N,N-dimethyl formamide (DMF), dimethyl sulfoxide (DMSO) and N-methyl-2-pyrrolidone (NMP). The influences of the water content and temperature on the dehydration performances for the NMP solutions were determined to understand the lower dehydration performances for those solvents. Those results suggest that the lower dehydration performances for the high boiling solvents were attributed to the lower vapor pressures of water and the higher permeances of those solvents. Furthermore, this study proposes that the permeation behaviors through zeolite membranes could be understood by the determination of the effect of temperature on the permeance of individual components.

Gas Permeation Properties of High-Silica CHA-Type Zeolite Membrane

The polycrystalline CHA-type zeolite layer with Si/Al = 18 was formed on the porous α-Al₂O₃ tube in this study, and the gas permeation properties were determined using single-component H₂, CO₂, N₂, CH₄, n-C₄H₁₀, and SF₆ at 303-473 K. The membrane showed permeation behavior, wherein the permeance reduced with the molecular size, attributed to the effect of molecular sieving. The separation performances were also determined using the equimolar mixtures of N₂-SF₆, CO₂-N₂, and CO₂-CH₄. As a result, the N₂/SF₆ and CO₂/CH₄ selectivities were as high as 710 and 240, respectively. However, the CO₂/N₂ selectivity was only 25. These results propose that the high-silica CHA-type zeolite membrane is suitable for the separation of CO₂ from CH₄ by the effect of molecular sieving.