NaA Membranes Research Articles

Effect of sol–gel-based silica buffer layer on the formation of NaA zeolite membrane

The support substrates were modified with sol–gel-based silica coating as intermediate buffer layer followed by NaA zeolite seed (prepared hydrothermally at 85 °C for 3 h) coating with 2 wt% aqueous dispersion. The secondary crystallization of NaA zeolite in the silica-seed-coated supports was carried out hydrothermally through crystal growth process at 65 °C for 4 and 8 h (single-stage each), (6 + 4) and (6 + 6) h (double-stage each), and (6 + 4 + 2) and (6 + 4 + 4) h (triple-stage each). The crystallization behaviours of the prepared NaA membranes were studied by X-ray diffraction, and the microstructures of the same films were observed by field emission scanning electron microscope. The efficiency of the membranes was tested with gas permeation study. The double-stage secondary crystallization at 65 °C for (6 + 4) h showed highly interlocked NaA grains in the membrane and it rendered the permeance values of 3.7 × 10−8 mol m−2 s−1 Pa−1 and 15.9 × 10−8 for single gas, nitrogen (N2) and hydrogen (H2) respectively with the ideal selectivity of 4.3.

Influences of the zeolite loading and particle size in composite hollow fiber supports on properties of zeolite NaA membranes

NaA/polymer composite hollow fibers (CHF) used as zeolite NaA membrane supports are fabricated from the dope slurry of NaA zeolite, N-methyl-2-pyrrolidone (NMP) and polyethersulfone (PES) using a dry–wet spinning process. The CHF supports can avoid seeding process and have great potential for developing low-cost zeolite membranes with high reproducibility. The influences of the spinning condition, the zeolite loading and particle size on the properties of the CHF supports and zeolite NaA membranes are investigated in detail. The supports and membranes were characterized by SEM, bending strength tests and water/ethanol pervaporation separation tests. Dense membranes with the separation factor >10,000 were obtained on the Z1 (4.0μm)-85 (85wt.% NaA) and Z2 (1.5μm)-75 supports after the synthesis of 5h, while the zeolite membranes obtained on the Z1-70 and Z1-75 supports had pinholes. A dense membrane with the separation factor >10,000 was obtained on the Z2-85 support after the synthesis of 4h. The results indicate that the CHF supports with smaller zeolite particles were better as the supports for zeolite NaA membranes. The flux of the zeolite membrane on the CHF support increased with the zeolite loading because the increase of the zeolite loading resulted in the increase of the support porosity. However, the increase of the zeolite loading can reduce the mechanical strength of the supports.

The preparation of zeolite NaA membranes on the inner surface of hollow fiber supports

Abstract Zeolite NaA membranes with excellent water/ethanol separation factors are successfully prepared on the inner surface of hollow fibers in a rotating autoclave for the first time. NaA/PES composite hollow fibers (CHF) are first used to investigate the effects of different synthesis conditions such as synthesis composition, synthesis time and synthesis method (static and dynamic) because there are uniform seed crystals on the inner surface of CHF supports. The seeded growth of zeolite NaA membranes on the inner surface of alumina hollow fibers (AHF) supports (1.0 mm i.d.) are carried out at 373 K. The optimal synthesis conditions obtained are applied to AHF supports to investigate the effects of seed suspensions by the dip-coating seeding method. The effects of the synthesis conditions for AHF supports on the membrane morphology and properties are also investigated. The synthesized membranes are characterized by scanning electron microscopy. The separation properties of the membranes are evaluated by pervaporation of 90 wt.% ethanol/water mixture at 348 K. Zeolite membranes with high separation factor up to 10,000 can be obtained under the conditions: synthesis composition of 7.5Na 2 O:2SiO 2 :Al 2 O 3 :600H 2 O, synthesis time of 5 h, using a rotating autoclave (30 rpm).

Zeolite membrane-assisted preparation of high fatty esters

The esterification of high fatty acid with alcohol was performed by coupling the vapor permeation of zeolite membrane and p-toluenesulfonic acid-catalyzed reaction. The use of a NaA zeolite membrane to in situ remove the water generated by the reaction could shift the equilibrium toward completion within a short reaction time. The VP-aided esterification process was well simulated using a simple model. Zeolite NaA membrane worked steadily in the vapor phase for a long time. The influence of operating parameters such as reaction temperature, ratio of membrane area to initial reaction volume, initial molar ratio of alcohol to acid and amount of catalyst on the performance of the combined process was investigated systematically. Based on the study, a series of high fatty esters were synthesized.

P1.032] Dpermeation by Zeolite NaA Membranes

P1.032] Dpermeation by Zeolite NaA Membranes

Desalination of aqueous solutions by LTA and MFI zeolite membranes using pervaporation method

LTA and MFI zeolite membranes were hydrothermally grown on the surface of an α-alumina porous support. The synthesized membranes were used for removal of cationic and anionic species from aqueous solutions by the pervaporation method. The perfection of the membranes was improved by employing the multi-stage synthesis method. The membranes were characterized by XRD, SEM and IR methods. The membranes were initially evaluated by the pervaporation separation of water from aqueous 2-propanol mixtures. The separation factors obtained were 7081 and 105 for NaA and ZSM-5 membranes, respectively. The ability of membranes for desalination of some aqueous solutions containing I-, Cs+ and Sr2+ ions was examined in various conditions. These ionic species were chosen because of their importance in the nuclear sciences. Both membranes effectively removed (more than 99 wt%) I-, Cs+ and Sr2+ from their singlesalt solutions (0.001mol dm-3) over a temperature range of 298-338 K. The effects of parameters such as time and temperature on the separation factors and fluxes were investigated. This work shows that, due to their excellent chemical, thermal and mechanical stability, the zeolitic membranes are useful for desalination of aqueous solutions and treating saline wastewaters by pervaporation. Therefore, this method has the ability to desalinate harsh environment solutions involving strong solvent and radioactive components.

Influences of Seeds on the Properties of Zeolite NaA Membranes on Alumina Hollow Fibers

The influences of size (90–1500 nm) and amount (1–4 wt %) of as-synthesized seed crystals on the properties of zeolite NaA membranes on alumina hollow fibers are investigated. Seeds and membranes a...

Emulsion-based synthesis of NaA zeolite nanocrystals and its integration towards NaA membranes

NaA zeolite nanoparticles (seed crystals) of size 50–65 nm were synthesized using water-in-oil (w/o) type emulsions at a considerably low temperature of 65 ± 1°C in a short duration of 2 h. The emulsions were stabilized using non-ionic surfactants e.g. sorbitan monooleate (Span 80), sorbitan monolaurate (Span 20), polyoxyethylene(5)nonylphenylether with ethoxy numbers of 5 (Igepal CO-520) and polyoxyethylene sorbitan monooleate (Tween 80) of hydrophilic-lipophilic balance (HLB) values of 4.3, 8.6, 10 and 15 respectively. Among the surfactants, the intermediate HLB values of 8.6 (Span 20) and 10 (Igepal CO-520) were effective in synthesizing highly dispersible NaA nanoparticles of size 50–65 nm. The membrane prepared hydrothermally in multi-steps at 65 ± 1°C, using the Span 20-derived seed crystals deposited on porous support, showed the formation of high quality interlocked NaA coating. Single gas nitrogen (N2) permeation of the membrane exhibited a permeance value of 1.01 × 10−8 mol m−2 s−1 Pa−1 at ambient temperature (30°C).

Influence of zeolite crystal expansion/contraction on NaA zeolite membrane separations

In situ powder XRD measurements showed that adsorption causes the NaA zeolite unit cell to contract or expand, and these changes depend on adsorbate loading. Changes in zeolite crystal size correlated with permeation changes through NaA zeolite membranes. These membranes had high water/alcohol pervaporation selectivities, even though gas permeation was mainly through defects, as indicated by Knudsen selectivities for single gas permeation. At 300 K and a thermodynamic activity of 0.03, water contracted the NaA crystals by 0.22 vol%, and this contraction increased the helium flux through two NaA membranes by approximately 80%. Crystal contraction also increased the fluxes of i-butane during vapor permeation and i-propanol (IPA) during pervaporation (∼0.03 wt% water). At activities above 0.07, water expanded NaA crystals and correspondingly decreased the fluxes of helium, i-butane, and IPA through NaA zeolite membranes. Methanol also contracted NaA crystals, but by only 0.05 vol% at an activity of 0.02, and this contraction slightly increased the helium and i-butane fluxes through a NaA membrane. Above an activity of 0.06, methanol expanded the NaA crystals, and the fluxes of helium and i-butane through a NaA membrane decreased. These adsorbate-induced changes explain some pervaporation behavior reported by others, and they indicate that crystal expansion and contraction may increase or decrease zeolite NaA membrane selectivity by changing the defect sizes.

Permeation mechanism through zeolite NaA and T-type membranes for practical dehydration of organic solvents

Commercially available zeolite NaA and T-type membranes, which were grown on the surface of porous cylindrical mullite tubes by hydrothermal treatment, showed excellent water permselective performance in pervaporation (PV) and vapor permeation (VP) for water/organic mixtures. Both of zeolite membranes were composed of inter-grown zeolite crystal layer on the outer surface of the tube, each of which is ca. 10 μm for NaA and ca. 20 μm for T-type membrane in thickness. Their zeolite crystal layer was composed of two layers, namely the top surface layer and the intermediate layer under the top surface. Average Kelvin diameters were 1.4 nm for the NaA membrane and 0.8 nm for the T-type membrane, respectively. Zeolite NaA membranes showed extremely high PV and VP performance even for water/methanol mixture. T-type membranes also showed extremely high PV and VP performance for water/alcohol mixtures except for water/methanol mixture. The experimental data were discussed from the new theoretical consideration. The consideration consists of the following: a very fine and narrow non-zeolitic pore penetrates from the zeolite crystal layer to the mullite support. The majority of zeolitic pores in top surface layer are assembling in the non-zeolitic pore. The water molecules in the feed are selectively adsorbed in zeolitic pores in the top surface layer, and then are transported to the non-zeolitic pore through the zeolitic pores. At narrower space in the non-zeolitic pore, the capillary condensation occurs, and then the condensate evaporates and diffuses into the permeation side.

Influence of PVP buffer layer on the formation of NaA zeolite membrane

The support substrates were modified with the aqueous solutions of 1 and 3 wt% of polyvinyl pyrrolidine (PVP) as intermediate buffer layer followed by NaA zeolite seed (prepared hydrothermally at 85 °C for 2 h) coating with 2.5 wt% aqueous dispersion in each case. A better surface coverage with the oriented layer of NaA seed crystals was found with 1 wt% PVP buffer layer. The secondary crystallization of NaA membranes in the PVP-seed-coated supports was carried out hydrothermally at 65 °C for 2, 4, 6 h (single-stage each) and (2 + 2), (4 + 2)h (double-stage each) crystal growth processes. The crystallization behaviours of NaA membranes were studied by X-ray diffraction (XRD) while the microstructures of the same films were observed by scanning electron microscope (SEM). The single-stage secondary crystallization at 65 °C for 4 h showed highly interlocked and oriented NaA grains in the membranes and it rendered the permeance value of 2.2 × 10−8 mol m−2s−1Pa−1 for single gas, nitrogen (N2) at ambient temperature (30 °C).

The effect of intermediate layer on synthesis and gas permeation properties of NaA zeolite membrane

NaA zeolite membrane coating was successfully synthesized on an alumina porous disc by hydrothermal treatment. The effects of synthesis parameters, such as seeding condition (in situ, ex situ), synthesis time, synthesis stages, application of intermediate layer, etc., on membrane characteristics were investigated. Surface seeding accelerates the zeolite crystallization process on the support surface, and also enhances the formation of homogeneous NaA zeolite layer. But the main problem associated with membrane coating synthesis is crack formation. Formation of crack was reduced by applying intermediate layer, between the support surface and seed layer. A thin Boehmite layer was applied to the support surface before applying seed crystals to enhance the adherence between zeolite seed layer and boehmite layer by hydrogen bonding and also to increase the mechanical strength of the membrane layer. The quality of the membrane layer can be improved by employing the multi-stage coating methods. The permeance of O2, N2 decreased as kinetic diameter of gases increased, which shows the molecular sieving effect of the NaA membrane. The permselectivity of O2/N2 was 1.9–2.0. This value of permselectivity ratio is higher than Knudsen diffusion ratio 0.94; it was also confirmed the molecular sieving properties of synthesized NaA zeolite membrane.

Technical and economical feasibility of zeolite NaA membrane-based reactors in liquid-phase etherification reactions

This paper describes the pervaporation performance of film-like zeolite NaA membranes synthesized in our laboratory by template-free seeded hydrothermal towards the dehydration of n-pentanol/water and n-pentanol/water/DNPE mixtures (DNPE = di- n-pentyl ether, C 10H 22O). DNPE is a linear symmetric ether that can be used as blending additive in reformulated diesel fuels and that can be produced from C 4 feedstocks via n-pentanol, obtained in its turn by selective hydroformylation of linear butenes. The membranes showed selectivities up to 3000 in the dehydration of n-pentanol/water binary mixtures, and water/DNPE and n-pentanol/DNPE selectivities approaching infinity (i.e. DNPE does not pervaporate) in the dehydration of n-pentanol/water/DNPE ternary mixtures. On the basis of the separation results presented in this work, and using experimental reaction kinetics data, we discuss the feasibility of zeolite NaA membrane reactors to carry out the liquid-phase etherification reaction of n-pentanol to di- n-pentyl ether (DNPE) catalyzed by ion-exchange sulfonated resins. These catalysts suffer from strong deactivation in the progress of the reaction due to the generation of water.

Continuous synthesis of NaA zeolite membranes

Zeolite NaA membranes were obtained by using a continuous recirculating flow, 0.25–5 ml min −1 of synthesis solution along the inner side of asymmetric porous alumina tubular supports. The membranes of 5 μm in thickness showed a N 2 permeance of 5 × 10 −7–1 × 10 −6 m −2 s −1 Pa −1. The procedure developed in this work provides control of the synthesis and crystallization conditions at any position on the tubular support, thus implying scale-up.

Modeling Pervaporation of Ethanol/Water Mixtures within ‘Real' Zeolite NaA Membranes

A modified version of the adsorption-diffusion model derived form the Maxwell−Stefan theory developed in a previous study (Pera-Titus, et al. Catal. Today 2006, 118, 73) is presented in this paper to describe the dehydration behavior of zeolite NaA membranes for pervaporation of ethanol/water mixtures. Compared to the former version, two additional contributions are included in the model: (1) the adsorbed solution theory of Myers and Prausnitz is used instead of the extended Langmuir isotherm to account for binary adsorption equilibria of water and ethanol on zeolite A, and (2) the explicit role of pressure-driven mechanisms in large intercrystalline defects (macrodefects) to permeation is considered. These refinements in the Maxwell−Stefan equations provide a superior description of solvent dehydration using zeolite NaA membranes. The fitted surface diffusivities at 323 K and at zero loading of water and ethanol for weak confinement show values in the order of 10-12 and 10-13 m2·s-1, respectively. The former values are 3−4 orders of magnitude higher than those that have been measured from water adsorption kinetics experiments. This difference might be ascribed to a certain role of nanosized grain boundaries between adjacent zeolite A crystals. Grain boundaries might behave as fast diffusion paths or nanoscopic shortcuts due to anisotropy of zeolite layers, resulting in higher apparent water surface diffusivities and lower apparent activation energies for surface diffusion.

Knoevenagel condensation reaction between benzaldehyde and ethyl acetoacetate in microreactor and membrane microreactor

The Knoevenagel condensation reaction between benzaldehyde and ethyl acetoacetate was performed in microreactor using Cs-exchanged NaX catalyst and NaA membrane. The laminar flow and slow diffusion of the bulky product molecules in the microchannel resulted in the further reactions of the main Knoevenagel condensation product, 2-acetyl-3-phenylacrylic acid ethyl ester that led to byproduct formation and poorer reaction selectivity. Replacing the powder catalyst in the microchannels with a thin CsNaX film improved the selectivity (78% vs. 55%), while increasing the microchannel height-to-width aspect ratio from 2 to 5 doubled the conversion from 25% to 60%. The selective removal of water byproduct in the membrane microreactors benefited the Knoevenagel condensation reactions. Higher conversions were observed for the reactions between benzaldehyde and (1) ethyl cyanoacetate (ECA), (2) ethyl acetoacetate (EAA) and (3) diethyl malonate (DEM), but the improvement diminished with increasing difficulty of the reaction. Higher selectivity and product yield for 2-acetyl-3-phenylacrylic acid ethyl ester were obtained for the membrane microreactor with a hybrid NaA membrane-CsNaX catalyst film deposited in the microchannels.

On a rapid method to characterize intercrystalline defects in zeolite membranes using pervaporation data

This work presents a mass-balance-based model devoted to the characterization of intercrystalline defects active to permeation in film-like zeolite membranes using pervaporation data. Relevant structural information concerning such domains can be obtained from the slope and intercept of the line that describes the increase of the flux pervaporated through a membrane with the absolute pressure in the liquid feed. Capillary forces appear to govern mass transfer within intercrystalline pores. The suitability of the model is assessed by using a collection of zeolite NaA membranes prepared on our premises for the separation of ethanol/water mixtures. The results show that, although a membrane displays high selectivity, a reduced number of defects in the zeolite layers cannot be ruled out. The model offers the advantage of enabling the characterization of large defects in pervaporation membranes at experimental conditions similar to those in which they are likely to be used.

Selective Oxidation of Carbon Monoxide and Ethene Mixture on Zeolite NaA Membranes Coated Catalysts

Defect-free zeolite NaA membranes were coated onto the surface of spherical Pt/Al2O3 particles using a two-step hydrothermal method. The structure and morphology of the synthesized composite catalysts were characterized using XRD and SEM techniques, respectively. The results indicated a layer of compact and uniform NaA molecular sieve membrane with a thickness of about 20 m was coated on the spherical Pt/Al2O3 particles after the two-step hydrothermal synthesis. The prepared NaA membrane coated catalysts were used in the oxidation of a mixture of CO and C2H4 to study the reactant selectivity over the coated zeolite NaA membranes. Under the optimized conditions, the oxidation selectivity for CO over C2H4 on the composite catalyst was as high as 96%. The feasible application of this composite membrane coated catalyst to the selective removal of CO in the presence of C2H4 was anticipated.

Lattice Constants of Tubular Zeolite NaA Membranes by Parallel Beam X-Ray Diffraction

A parallel beam X-ray diffraction analysis (XRD) machine equipped with a parabolic multilayer, Soller slit and long horizontal slit has been applied to tubular zeolite NaA (LTA) membranes supported by a porous alumina tube, prepared by hydrothermal synthesis for the dehydration of ethanol. The present XRD is a suitable method used to analyze tubular LTA membranes and give a sharp XRD pattern, because the sample form does not affect the diffraction angles. For lattice-constant measurements, six diffraction lines in the 2θ region of 40∼60° were used, and their diffraction angles were corrected using 220 and 311 lines of silicon powder (NIST SRM640c) as an external standard. The XRD measurements were carried out for LTA membranes after pervaporation experiments of an ethanol/water (90/10) mixture, and the observed lattice constants were 24.573∼24.599 A, corresponding to the hydrothermal synthesis condition. Thus, the lattice constants of the tubular LTA membranes were first reported.

Preparation of higher flux NaA zeolite membrane on asymmetric porous support and permeation behavior at higher temperatures up to 145 °C in vapor permeation

NaA zeolite membranes were synthesized on an asymmetric porous alumina support with a lower mass-flow resistance for development of more economically feasible membranes with higher permeation performance. The support influence on permeation fluxes through the membrane using asymmetric support was investigated by vapor permeation at 100–145 °C in a mixture of water (10 wt.%)/ethanol (90 wt.%) in which the higher permeation fluxes up to 37 kg m −2 h −1 or water permeances up to 3.2 × 10 −6 mol m −2 s −1 Pa −1at 145 °C were observed. The performance was higher than those in the previously reported NaA membrane on a monolayer porous alumina support of 31 kg m −2 h −1 or water permeances of 2.5 × 10 −6 mol m −2 s −1 Pa −1at 145 °C. These results are experimental evidence to show the effect of asymmetric support utilization in membrane preparation on the higher membrane performance. The estimate of the pressure drop over the both types of support indicates that the improvement of higher permeation fluxes in the asymmetric type membrane could be attributed to the suppression of pressure drop in the support layer due to lower mass-flow resistance there. The water permeances are decreased at elevated temperatures, resulting in that the permeation flux cannot be increased linearly for the applied higher driving force of vapor pressure. The elevated temperatures are significant inhibition to the increasing of permeation fluxes through NaA zeolite membranes.