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A novel silver ion-exchange treatment of zeolite was introduced in this work to change the physical and chemical adsorption properties of penetrants in the zeolite. EDX data confirm the complete replacement of sodium ion in zeolite NaA by silver ion, whereas XRD patterns and BET results show no changes in some physical properties of zeolite after the silver ion-exchange treatment. Polyethersulfone (PES)-zeolite NaA mixed matrix membranes (MMMs) and PES-zeolite AgA MMMs were fabricated at high processing temperatures with different zeolite loadings. Cross-sectional SEM images of these two types of MMMs indicate the interface between polymer and zeolite phases is comparable before and after the silver ion-exchange treatment. The effects of silver ion-exchange treatment of zeolite and zeolite loadings on the gas separation performance of these MMMs were investigated. CO2 permeability of PES-zeolite AgA MMMs is higher than that of PES-zeolite NaA MMMs, whereas their CH4 permeability is lower than that of PES-zeolite NaA MMMs. This trend is the result of the reversible reaction between silver ion and CO2 molecule. CO2 and CH4 permeability of PES-zeolite AgA MMMs decreases with increasing zeolite content arising from the effects of partial pore blockage of zeolite and polymer chain rigidification, whereas their CO2/CH4 selectivity increases with an increase in zeolite loadings and the highest value reaches 59.6 at 50 wt % zeolite loading because of a combined effect of the facilitated transport mechanism of silver ion and the molecular sieving mechanism of zeolite. Both CO2-induced plasticization test and CO2/CH4 mixed gas measurement were performed to examine the applicability of these developed PES-zeolite AgA MMMs in industry. Results prove that this type of composite membrane material is a superior candidate for the practical separation of natural gas. [ABSTRACT FROM AUTHOR]

It has been demonstrated that a novel p-xylenediamine/methanol soaking method could efficiently remove the polymer--zeolite interface defects of the mixed-matrix structure. In this work, the mixed-matrix structure is in the form of an ultrathin (1.5-3 µm) polysulfone/ zeolite beta mixed-matrix layer that is supported by a neat Matrimid® layer in dual-layer composite hollow fibers. The particle's loading in this thin layer has reached 30 wt %. The ideal selectivities of the mixed-matrix hollow fibers (30 wt % of zeolite)for O2/N2 and CO2/CH4 separation were roughly 30 and 50% superior to that of the neat PSF/Matrimid® hollow fibers, respectively. Investigation of the morphology of the mixed-matrix selective layer and its relation with gas separation performance indicated that without p-xylenediamine/methanol solution treatment, the outer layer showed various polymer--zeolite interface structures in different fibers with the same heat treatment procedures; this situation might lead to the different selectivities after coating. However, by applying p-xylenediamine/methanol processing on the fibers before thermal treatment, the fibers obtained a more uniform structure and improved attachment between polymer matrix and zeolite surface. Hydrogen bonding was proposed as the possible mechanism for the tighter attachment between the two phases. The improvement of separation efficiency was presumably related to the polymer chain rigidification, partial pore blockage, and/or favorable interaction between the gas penetrants and zeolite framework. [ABSTRACT FROM AUTHOR]

Abstract: Hollow fibers with a thin zeolite beta-polysulfone mixed matrix selective layer and enhanced selectivity for He/N2 and O2/N2 separation have been developed in this study. It is believed that the heat treatment and the coating processes combine to bring out the separation properties of zeolite beta imbedded in the polymer matrix. The fibers were initially formed by using the dual-layer co-extrusion and dry-jet wet/wet spinning technology. Owing to the loose nature of the mixed matrix skin, the as-spun fibers show Knudsen diffusion even after silicone rubber coating. The effectiveness of heat treatment and surface coating in healing the hollow fiber has been investigated. SEM pictures show that heat treatment at above T g can significantly densify the loose mixed matrix layer and produce hollow fibers with a thin mixed matrix selective layer of around 1.5–12μm. However, heat treatment at above T g does not necessarily produce defect-free hollow fiber membranes. Three types of silicone rubber coatings were applied and compared. It is found that, after a two-step coating, the ideal selectivity of mixed matrix fibers increases to surpass that of hollow fiber with a neat polysulfone outer layer. The sealing of the mixed matrix outer layer instead of the caulking of the inner layer is confirmed to be the cause for the selectivity increment. The reaction between the two chemicals in the two-step coating is contributive to the efficiency of this coating method. In addition, it is also observed that the thickness of the mixed matrix composite outer layer influences permeance and selectivity. The activation energies (E p) for O2 and N2 calculated from the permeance or permeability at different temperatures indicate that the addition of zeolite beta in the polysulfone has reduced the energy barrier for O2 permeation. [Copyright &y& Elsevier]

Abstract: Membrane separation process using NaA zeolite membrane is a good alternative process for ethanol production in terms of both energy consumption and environmental friendliness. However, the highest cost for the membrane separation process is from the membrane production step. The reduction of the membrane production cost by optimization of the synthesis time is, thus, necessary to the cost-effective production of ethanol production. The main focus of this work is to obtain NaA zeolite membranes synthesized on a tubular alumina support using a conventional heating or autoclave technique. Effect of the synthesis time on the performance of the membrane for water–ethanol separation via pervaporation at 343K is studied. It is found that the optimum synthesis time to produce a high purity of ethanol (>99.5% by volume) is only 13h with a total water flux and separation factor of 2.82kg/m2/h and >10,000, respectively. The thickness of the membrane obtained is around 7–9μm. The overall results and the reproducibility of the membranes are discussed. [Copyright &y& Elsevier]

Abstract: The performance of sodium A (NaA) zeolite membranes with a surface area of 4.91cm2, synthesized on an alumina support via microwaving (MW) and autoclaving (AC, or conventional heating) using the recycle-continuous pervaporation of a water–ethanol mixture (10:90vol.%), was studied. It was found that the process could produce ethanol having a concentration close to 100% (higher than 99.5%) after passing the mixture through the membrane for 120–140h. The time-dependence separation factor and the total water flux were 3350–6050 and 0.4–1.0kg/m2/h, respectively. The time-dependence separation factor of this process was increased to higher than 10,000 when the percentage of ethanol at the retentate side was higher than 98%. The overall results showed good stability of the membranes for the water–ethanol separation. [Copyright &y& Elsevier]

Abstract: Dual-layer PES/P84 hollow fiber membranes with a PES–zeolite beta mixed matrix dense-selective layer of 0.55μm have been successfully fabricated in this study by adjusting the ratio of outer layer flow rate to inner layer flow rate during the spinning with the aid of heat-treatment at 235°C and two-step coating. To our best knowledge, this is the thinnest thickness that has ever been reported for dual-layer hollow fiber membranes with the mixed matrix outer layer. The newly developed dual-layer hollow fibers exhibit enhanced O2/N2 and CO2/CH4 selectivity of around 10–20% compared with that of neat PES dense films. Both the polymer chain rigidification and partial pore blockage of zeolite beta may be the main causes for the selectivity increment. The outer surface of wet-spun hollow fibers shows fewer defects than that of hollow fibers spun at 1.5cm air gap probably because of the overlarge elongation stress with an increase in air gap. Scanning electron microscope (SEM) images demonstrate that heat-treatment can effectively reduce or eliminate the voids induced by the unfavorable interaction between polymer and zeolite phases. Differential scanning calorimetry (DSC) results prove that heat-treated dual-layer hollow fiber membranes with a mixed matrix outer layer possess a higher glass transition temperature (T g) over neat PES dense film. The performance of these newly developed dual-layer hollow fiber membranes has also been confirmed in mixed gas tests. [Copyright &y& Elsevier]

Abstract: In this study, a series of three-layer zeolite-embedded poly(vinyl alcohol) (PVA) composite membranes have been successfully fabricated with a casting machine. Zeolites, examined with a loading of 20wt%, include 3A, 4A, 5A, NaX, NaY, silicalite and beta. These hydrophilic composite membranes have been evaluated in the dehydration of ethanol aqueous solution by means of pervaporation. The unfilled PVA membrane is observed to exhibit much higher separation factor than two commercial PERVAP 2210 and PERVAP 2510 membranes. After adding zeolites into the PVA matrix, higher separation factor and higher fluxes or higher selectivity and higher penetrant permeances are both achieved by these resultant zeolite-incorporated membranes, indicating that ethanol/water separation has been enhanced with the aid of incorporated zeolites. Through evaluating the pervaporation performance in terms of water permeance, ethanol permeance and selectivity, we have revealed that the separation performances of zeolite-filled membranes are strongly related to the zeolite pore dimension, its hydrophilic/hydrophobic nature as well as its crystal framework. The temperature dependence of the pervaporation behaviors like the penetrant fluxes and permeances has been discussed in detail in terms of Arrhenius activation energy. The evaluated results have revealed that the permeance and selectivity (i.e., the membrane intrinsic properties) are less dependent on the operating temperature than the flux and separation factor. Zeolite addition has led to decreased activation energies for water and ethanol, and more considerable drop of the water activation energy has subsequently resulted in the increased selectivity in ethanol dehydration. [Copyright &y& Elsevier]

Abstract: A novel silane coupling agent, (3-aminopropyl)-diethoxymethyl silane (APDEMS) was used in this work to modify zeolite surface for mixed matrix membranes (MMMs). Both elementary analysis and XPS spectra confirm the chemical modification, while BET measurements show no changes in zeolite surface area and total pore volume after the modification. Polyethersulfone (PES)–zeolite 3A, 4A and 5A MMMs were fabricated at high processing temperatures using unmodified and chemical modified zeolite. SEM images of these MMMs indicate the interface between polymer and zeolite phases becomes better if modified zeolite is used. The effects of chemical modification of zeolite surface and zeolite loadings on the gas separation performance of these MMMs were investigated. Both permeability and selectivity of MMMs made from APDEMS modified zeolite are higher than those of MMMs made from unmodified zeolite at 20wt% zeolite loading because of a decrease in the degree of partial pore blockage of zeolites. The permeability of all studied gases decreases with increasing zeolite content for PES–zeolite 4A-NH2 MMMs, while for PES–zeolite 5A-NH2 MMMs, the gas permeability decreases and then increases with an increase in zeolite loadings. This unique phenomenon implies that using large pore-size zeolite for MMMs would potentially offset the negative effects of partial pore blockage and polymer chain rigidification on permeability. A modified Maxwell model with adjusted parameters was applied to study the PES–zeolite 4A-NH2 MMM system. The predicted permeability and selectivity show very good agreement with experimental data, indicating the modified Maxwell model is fully capable of predicting the gas separation performance of MMMs made from both unmodified and modified zeolite. [Copyright &y& Elsevier]

Abstract: We have revealed, for the first time, the evolution of nano-particle distribution and the mechanisms of nano-particle movement during the formation of single-layer and dual- membranes. Both single-layer and dual-layer mixed matrix hollow fibers have been fabricated using nano-size zeolite beta. It is found that three factors play important roles in determining the nano-particle distribution in the resultant fibers. They are (1) shear within the spinneret, (2) die swell when exiting from the annulus spinneret and (3) elongation drawing in the air gap region. The shear stress induced within the spinneret determines the initial particle distribution. As a result, the particle distributions in both wet-spun single-layer and dual-layer hollow fibers follow the parabolic shape of dope''s axial velocity profile in the spinneret. Die swell flattens and levels off the particle distribution, while elongation stretch redistributes the particles to the outer surface with the aid of (1) moving outer surface inward and (2) the outflow of solvents during the phase inversion process. The particle redistribution phenomenon is significantly magnified and enhanced during the co-extrusion of dual-layer hollow fiber membranes with an outer layer made of mixed matrix materials. The proposed causes and mechanisms for particle distribution have been verified by the EDX line scanning spectra of silicon and SEM pictures. Preliminary gas separation data indicate that the outer mixed matrix layer of the as-spun hollow fiber membrane has a defective skin for Knudsen diffusion. Further post-treatment is needed to eliminate defects for gas separation. [Copyright &y& Elsevier]