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.     

Scale-up of NaA zeolite membranes onα-Al2O3 hollow fibers by a secondary growth method with vacuum seeding

NaA zeolite membranes were prepared by secondary growth method on the outer surface ofα-Al2O3 hollow fiber supports. Vacuum seeding method was used for planting zeolite seeds on the support surfaces. Hydrother-mal crystallization was then carried out in a synthesis solution with molar ratio of Al2O3:SiO2:Na2O:H2O=1:2:2:120 at 100 °C for 4 h. Effects of seeding conditions on preparation of hollow fiber NaA zeolite membranes were extensively investigated. Moreover, hollow fiber membrane modules with packing membrane areas of ca. 0.1 and 0.2 m2 were fabricated to separate ethanol/water mixture. It is found that the thickness of seed layer is obviously affected by seed suspension concentration, coating time and vacuum degree. Close-packing seed layer is required to obtain high-quality membranes. The optimized seeding conditions (seed suspension mass concentration of 0.5%–0.7%, coating time of 5 s and vacuum degree of 10 kPa) lead to dense NaA zeolite layer with a thickness of 6–8μm. Typically, an as-synthesized hollow fiber NaA zeolite membrane exhibits good pervaporation performance with a permeation flux of 7.02 kg·m?2·h?1 and separation factor N 10000 for sepa-ration of 90%(by mass) ethanol/water mixture at 75 °C. High reproducibility has been achieved for batch-scale production of hollow fiber NaA zeolite membranes by the hydrothermal synthesis approach.     

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).     

Preparation of NaA Zeolite Membranes Using Poly(Ethyleneimine) as Buffer Layer,and Study of Their Permeation Behavior

Zeolite NaA membranes were prepared hydrothermally by secondary crystallization process at different temperatures (55°C–75°C) on porous α‐alumina‐based support tubes (inner side) precoated with poly(ethyleneimine) (PEI) buffer layer and NaA seed particles. The NaA seed crystals synthesized at 65°C/2 h in the size range 100–200 nm having BET surface area of 71.57 m²g^?1 were used for secondary crystallization of the membranes. The secondary crystallization at 65°C for (4 + 4) h (double‐stage) showed highly dense NaA grains in the microstructure of the membrane with a thickness of 5 μm. It rendered the permeance values of 50.6 × 10^?8, 2.47 × 10^?8, and 0.55 × 10^?8 molm^?2s^?1Pa^?1 for H₂, N₂, and CO₂, respectively, with their permselectivity of 20.48 (H₂/N₂), 92 (H₂/CO₂), and 4.49 (N₂/CO₂). A tentative mechanism was illustrated for the interaction of PEI with the support substrate and NaA seed crystals.     

Synthesis of NaA zeolite from kaolin source

In this work, zeolite NaA was successfully synthesized by a hydrothermal method using kaolin as a combined source for silica and alumina. Zeolite NaA with high static water adsorption was synthesized from the low-cost raw material, kaolin, and the reaction parameters were optimized. Metakaolin was obtained by calcining kaolin at temperatures ranged from 953 K to 1173 K. The synthesis mixture was pre-crystallized at 343 K and crystallized at 373 K successively. Zeolite NaA was obtained, which was confirmed by SEM, XRD and the water adsorption analysis. The optimized metakaolinization temperature was found at 973 K. The influence of Na₂O/SiO₂ molar ratio, pre-crystallization time and seed on the crystallization of NaA zeolite was investigated. A thorough mixing of metakaolin and NaOH solution was favourable for the nucleation/crystallization rate. The obtained NaA zeolite under the optimized conditions shows excellent crystallinity and static water adsorption of 28.0 wt-%, which was higher than 25.9 wt-% of the commercial NaA zeolite. Kaolin was suggested to be a feasible and economical raw material for the practical industrial applications for NaA zeolite.     

Single-step process without organic template for the formation of zeolite A from RHA

Dice-shaped zeolite A (NaA) was prepared via direct dissolution of rice husk ash (96.5% SiO₂) in the presence of NaOH and sodium aluminate solution by autoclave process at 90°C/6 h without using any templating agent. The prepared particles were characterized by DTA/TG, XRD, FTIR, N₂ adsorption-desorption physisorption analysis, FESEM, and TEM. XRD results confirmed crystallization of pure NaA zeolite phase. FTIR study shows the characteristic bands at 554 cm⁻¹ for double 4 membered ring (D4R) of NaA zeolite. The total BET surface area of the product was found to be 31.6 m² g⁻¹. FESEM and TEM images show dice shaped NaA particles of size around 1 μm which is formed via oriented crystalline aggregation of primary particles (30-40 nm). A tentative mechanism was proposed for the formation of NaA crystals through direct dissolution of rice husk ash. The synthesized NaA zeolite could be used for purification of alcohol and separation of toxic and radioactive ions from waste water via a cost-effective process.     

Structure,stability and permeation properties of NaA zeolite membranes for H2O/H2 and CH3OH/H2 separations

NaA zeolite membranes were synthesised in the secondary growth hydrothermal method based on the seeding of the inner surface of a ceramic α-alumina tube. The impacts of crystallisation time and zeolite precursor concentration (in H₂O) were investigated. The structure and stability of the prepared NaA zeolite membranes were also investigated with operating temperatures, times and pressures. The results indicate that the optimal synthesis gel molar composition was 3Na₂O: 2SiO₂: Al₂O₃: 200H₂O. This led to cubic-shaped NaA zeolite which showed good stability. The optimal NaA zeolite membrane had H₂O and CH₃OH fluxes of 2.77 and 0.19 kg/m²h, with H₂O/H₂ and CH₃OH/H₂ separation factors of ∞ and 0.09 at a temperature of 30 °C. The NaA zeolite membrane had high thermal stability, but poor separation performance at high temperature (240 °C). The results suggested that the H₂ permeation flux is significantly influenced by preferential adsorption of vapour in the NaA zeolite membrane.     

Synthesis of NaA and NaX Zeolite Membranes by Fumed Silica Via Clear Solution Gel

NaA zeolite membranes have been used for dehydration of organic solvents in laboratory and commercial scales. There are many synthesis methods and conditions for manufacturing the membranes. The use of lactescent and clear gel solutions are two of the most important choices for high quality hydrothermal synthesis of NaA zeolite membrane in gas separation or dehydration of organic solvents. In this article, effect of hydrothermal synthesis time was investigated using the clear gel solution (Al₂O₃:5SiO₂:50Na₂O:1000H₂O). Phase transformation from NaA to NaX was recognized as the most possible event when the clear gel solution was used. XRD spectra, SEM images, and GC analyses were used for evaluation of zeolite crystal phase, the membrane layer quality, and the contents of organic substances (ethanol or 2-propanol) in feed and permeate streams in pervaporation tests, respectively. From synthesis of zeolite membrane at different times, it was revealed that for achieving the pure NaA zeolite phase the synthesis time should not exceed 3 h at 100°C.     

High‐performance zeolite T membrane for dehydration of organics by a new varying temperature hot‐dip coating method

A new seeding method, namely, varying‐temperature hot‐dip coating (VTHDC), is proposed for synthesis of zeolite T membranes by secondary hydrothermal growth. The VTHDC method is composed of hot‐dip coating at higher temperature, rubbing off the superfluous crystals, and hot‐dip coating at lower temperature. It was found that the method was flexible and effective for combined control over the seed suspension concentration, seed size, and coating temperature, leading to combined control of properties of the seed layer over the seed size, thickness, coverage, and defect. A thin continuous, smooth defect‐free asymmetric seed layer was achieved consisting of large and small zeolite T seed crystals. The resulting zeolite T membrane M5 exhibited high pervaporation performance with the flux reaching 2.12 and 2.52 kg/m² h for the dehydration of 90 wt % EtOH/H₂O and IPA/H₂O mixture, respectively, at 348 K. The corresponding separation factor was up to 1301 and 10,000, respectively. © 2012 American Institute of Chemical Engineers AIChE J, 59: 936–947, 2013     

Preparation of zeolite NaA membranes on the inner side of tubular supports by means of a controlled seeding technique

Zeolite NaA membranes have been reproducibly prepared by seeded hydrothermal synthesis on the internal surface of porous -alumina tubular supports. A cross-flow filtration technique has been developed to allow a controlled seeding of zeolite NaA crystals from a suspension according to transmembrane pressure, pH, seed suspension flow rate, crystal size and concentration. The optimal seeding weight gain was found to be around 0.40 mg cm⁻² of membrane area. With this procedure, zeolite membranes with selectivities up to 600 at fluxes of 0.50 kg m⁻² h⁻¹ were obtained in the pervaporation of 92:8 wt.% ethanol/water mixtures at 323 K.     

支撑体材料对NaA型沸石分子筛膜形成的影响

采用水热合成法制备NaA型沸石分子筛膜,实验比较了α-AI2O3、ZrO2及TiO2三种支撑体对NaA型沸石分子筛膜形成的影响。XRD测定所合成的沸石分子筛膜是NaA型。SEM和渗透实验结果表明,沸石分子筛膜的性能与支撑体有关,TiO2优于ZrO2和α-AI2O3。TiO2支撑体上合成沸石分子筛膜的H2、N2渗透系数大小基本与膜两侧平均压力无关,理想分离系数约为8,高于Kundsen扩散分离因子3．74,表现有一定的分子筛分效应。     

PEA-NaA /α-Al2OPEA-NaA /α-Al2O3复合支撑分子筛膜的微波水热合成

采用微波水热合成法在聚醚酰亚胺(PEI)-NaA分子筛/α-Al2O3复合载体表面合成了具有高选择性的致密NaA型分子筛膜,重点考察了微波辐射时间对成膜的影响。采用X射线衍射图谱(XRD)和扫描电子显微镜(SEM)对NaA型分子筛膜进行了表征。XRD结果表明,复合载体表面生成的膜中只有NaA分子筛的晶相;SEM结果表明,复合载体基膜表面覆盖了一层致密连续的NaA型分子筛膜。合成的NaA型分子筛膜在不同质量分数乙醇中的渗透汽化性能结果表明,渗透通量随乙醇质量分数增大而减小,分离因子则反之,当乙醇质量分数为95%时,渗透通量仅0.05 kg/(m2.h),而分离因子高达13 000。     

Parametric studies of DD3R particles seeding on the modified surface of α-alumina support and synthesis of DD3R zeolite membrane

The effects of suspension concentration, number of coating stages and dipping time on seeding quality of DD3R particles on the ceramic supports were investigated. The supports were immersed in aqueous suspensions (0.1, 0.2 and 0.3?wt. %) of the DD3R seeds for three different dipping times 30, 90 and 240?s with different stages of 1, 2 and 3. The SEM analysis was used to study the quality of the seeded layers. The optimized seeding conditions of 0.2?wt. % suspension concentration, 2 number of coating stages and 30?s for dipping time leaded to obtain a uniform seeded layer with monolayer structure. The DD3R zeolite membrane was synthesized via hydrothermal method under the optimized seeding conditions. The XRD and SEM analyses confirmed the synthesis of DD3R membrane with proper quality. The single gas permeation results showed a good performance in the separation of CO₂ from CH₄.     

分子筛膜晶种涂敷机的研制

研制了基于800~1000 mm长工业规格不锈钢或陶瓷管状支撑体的晶种涂敷设备。采用先喷涂后摩擦工艺,结合自动化控制技术,实现了分子筛膜大规模制备中晶种的快速涂敷。详细介绍了涂敷机工作原理和主要结构,分析研究了影响设备涂敷性能的主要因素。对不锈钢支撑体进行晶种涂敷制备NaA透水膜实验,在温度为75℃、原料液中乙醇质量分数为90%的条件下,膜渗透通量为4.2~5.8 kg·m^-2·h^-1;渗透液中水质量分数为99.2%~99.8%;生产率为30根/小时,成品率大于98%。     

T型分子筛膜的制备及其对含酸乙腈-水的分离性能

石化医药行业会产生大量乙腈废液,通过无机分子筛膜分离技术进行回收可以创造可观的经济价值。然而,乙腈废液水含量较高,并且具有一定的酸性,目前应用比较广泛的NaA型分子筛膜不能在该体系下长期使用。使用自制的平均粒径为600 nm的T型分子筛作为晶种,采用真空涂敷法将晶种负载在α-Al₂O₃载体管表面,在150 ℃下水热晶化16 h制备出T型分子筛膜,并对其做膜性能评价实验。结果表明,自制的T型分子筛膜在进料流量为3.5 L/h、温度为100 ℃条件下对含有1%（质量分数）乙酸和10%水（质量分数）的乙腈混合溶液进行膜分离脱水实验,渗透通量为2.51 kg/（m ²·h）;在120 h长时间运行过程中,分离系数保持在2 250以上,T型分子筛膜性能基本保持不变。     

Nanocrystalline Zeolite Tetraethylammonium‐Beta Membrane for Preferential Sorption and Transport of CO2 over N2

CO₂/N₂ gas separation was performed over a nanocrystalline zeolite tetraethylammonium (TEA)‐beta membrane prepared on a stainless‐steel porous disc by repeated hydrothermal crystallization. Two to three consecutive hydrothermal syntheses were required to form a membrane comprised of a continuous and compact layer of zeolite beta nanocrystals on the support. The membrane TEA‐BEA3 obtained by three consecutive syntheses, in which the membrane from two consecutive syntheses was used as support, exhibited the highest structural order. When the separation experiment was performed over this membrane without applying any external applied pressure, 100 % selectivity of CO₂ over N₂ was observed. The separation was driven by differences in chemical potentials of the molecules generated only by the adsorption‐desorption behavior of the gases into the membrane. The novel zeolite TEA‐beta membrane provided promising results for the separation of small gas molecules due to the combined influence of diffusion and sorption selectivity.     

Effects of bubbles on the structure and performance of zeolite membranes

Defects in the zeolite layer restricts the applications of the zeolite membrane. In this study, we found that the bubbles in the synthesis solution and the interface between the support and the synthesis solution impeded the movement of the Si/Al active ingredients to the surface of the support during the membrane synthesis process, leading to the formation of defects. Accordingly, we pre-degassed the synthesis solution containing 170-mm-long ceramic tube supports under a vacuum of 0.1 MPa at room temperature before the membrane synthesis. The characterization results showed that the degassing pretreatment drove out the gases in the synthesis solution and the interface between the synthesis solution and the support, and thus eliminated pinhole defects in the zeolite layer. The permselectivity of H₂ over C₃H₈ increased from 8.8 to 20.2 when the synthesis solution was degassing pretreated. In conclusion, the degassing pretreatment clearly eliminated the defects in the membrane.     

Continuous flow ZIF-8/NaA composite membrane microreactor for efficient Knoevenagel condensation

A ZIF-8/NaA composite membrane microreactor was fabricated on multi-channel stainless steel plate. NaA zeolite membrane was first introduced into the microchannel as a modification layer via a secondary growth. Subsequently, a layer of ZIF-8 membrane was grown over the NaA zeolite layer as a catalytic layer by using ZnO-induced synthesis method. Such ZIF-8/NaA composite membrane microreactor was applied in continuous flow Knoevenagel condensation of benzaldehyde and ethyl cyanoacetate. Nearly 100% product yield was achieved in a short residence time under mild conditions. Importantly, no obvious deactivation was observed even after 50 h, indicating its excellent stability.     

Factors affecting the formation of zeolite seed layers and the effects of seed layers on the growth of zeolite silicalite-1 membranes

The present study investigates the formation of silicalite-1 seed layers on a porous carbon support of 0.5 μm pore size and α-A1₂O₃ supports with different pore sizes (0.1 μm and 4 μm) via the slip-casting technique. The effects of support property, seed size and solvent on the formation of seed layers were investigated in detail. The growth of silicalite-1 membranes on different seeded supports by hydrothermal synthesis was also evaluated. The scanning electron microscopy (SEM) and X-ray diffraction (XRD) characterizations indicate that a continuous seed layer can be obtained on the smooth support of 0.1 μm pore size by using any seed of 100 nm, 600 nm or 2.2 μm in size, whereas, on the coarse supports with either 0.5 μm or 4 μm pore size, a continuous seed layer cannot be formed using the above seed sizes and the same seeding time. At a longer contact time, a seed layer can also be formed using 100 nm seed on the supports with larger pore size. However, the layer is not uniform and smooth. For a hydrophobic porous carbon support, seeding ethanol suspension, which has weak polarity, favors the formation of a continuous seed layer. The seed layers and membranes grown from the smaller seed are more uniform and continuous and possess smoother surfaces than those from the larger seed. The seed layer and respective grown membrane formed from nanosized seed (100 nm) are the most uniform and compact. With this method of seeded secondary synthesis of zeolite membranes, the quality of a membrane mainly depends on the quality of the seed layer. Translated from Journal of Chemical Engineering of Chinese Universities, 2006, 20(4): 520–526 [译自: 高校化学工程学报]     

Synthesis and preliminary gas permeation studies of a tubular NaA zeolite membrane (NZM)

Different from traditional seeded method, NaA zeolite membranes (NZMs) were prepared by in situ synthesis onto the inner side of porous α-alumina tubular supports in a hydrothermal synthesis reactor. The influences of pretreatment of porous tubular support, temperature, time, and synthetic cycle for the synthesis of the zeolite membranes were investigated. The operating conditions were optimized. Characterization of the membranes by scanning electron microscopy and X-ray diffraction showed that the crystalline materials on the inner surface of the porous α-alumina tubes were NaA-type zeolite. Single- and binary-gas permeation tests were conducted. Single-component permeabilities of hydrogen and nitrogen through the NZM changed slightly when the transmembrane pressure difference varied from 80 to 420?kPa. Its selectivity for H₂ relative to N₂ was about 5.3, which was greater than that of the Knudsen diffusion. The separation factors of binary gases H₂/N₂ and H₂/CO₂ at 473?K were 3.9 and 5.7, respectively, again exceeding the Knudsen diffusion level. The separation of binary gases suggests that the NaA-type zeolite membranes on α-alumina substrate were defect free and able to provide molecular sieving. The results demonstrate that the unseeded synthetic method presented in this work is successful and reliable.