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.     

High flux mesoporous MCM-48 membranes: Effects of support and synthesis conditions on membrane permeance and quality

This communication reports experimental efforts to synthesize defect-free mesoporous MCM-48 membranes with improved gas flux. We demonstrate a facile and inexpensive method of synthesizing defect-free supported MCM-48 membranes with improved N₂ and CO₂ permeance (>2 × 10⁻⁷ mol/m² s Pa) employing asymmetric supports for the membrane synthesis which contain layers of macropores possessing different pore sizes. The membranes prepared on asymmetric -alumina supports displayed higher gas permeance than those fabricated on symmetric supports (N₂ permeance <10⁻⁷ mol/m² s Pa) as confirmed by unsteady-state gas permeation experiments. Further improvement in gas permeance was achieved by covering one face and the sides of the support with a ceramic tape during membrane synthesis.     

Dehydration of water/1-1-dimethylhydrazine mixtures by zeolite membranes

In this research, dehydration of water/1-1-dimethylhydrazine (UDMH) mixtures by zeolite NaA and hydroxy sodalite membranes has been investigated. Support of these membranes has been tubular mullites that have been made by extruding a mixture of about 67–75% kaolin clay and 33–25% distilled water using an extruder. Zeolite NaA and hydroxy sodalite membranes have been coated on the external surface of the porous supports by the hydrothermal synthesis.

UDMH/water mixtures have been separated at ambient temperature and pressure by pervaporation (PV) using these zeolite membranes. These membranes showed very high selectivity of water for all UDMH mixtures. For the UDMH/water mixtures, separation factor as high as 10 000 has been obtained for UDMH feed concentration of 2%. Total mass fluxes of 1.05–0.2 kg/(m² h) have been also obtained.     

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.     

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。     

含甲基纤维素的NaA沸石膜的制备和表征

NaA沸石膜具有规则的孔道结构,利于分子传输,在有机物脱水领域有一定的应用。为使沸石膜生长更连续均匀,提高渗透汽化性能,本文以甲基纤维素作为空间限制剂加入合成液,探究碱度、晶化温度以及晶化时间对膜的影响,按最优条件制备合成液,并依据质量比m(MC)∶m(H₂O)=1∶100添加甲基纤维素,制备NaA沸石膜。表征方法采用XRD、SEM和渗透汽化3种方式,结果表明添加甲基纤维素的沸石膜表面结构完整,生长致密且性能优良,在75℃下对0.6mol/L的NaCl水溶液做渗透汽化测试时,通量达8.33kg/(m²·h),盐离子截留率为99.95%。在0.6mol/L的NaCl的水溶液中测试72h,结果表明添加甲基纤维素的NaA沸石膜时间依存性更好,通量保持在8.30kg/(m²·h)左右,离子截留率稳定在99.90%。渗透汽化分离ω(C₂H₆O)=90%乙醇的水溶液,随着温度从60℃升高到75℃,沸石膜的通量由1.55kg/(m²·h)升高到2.56kg/(m²·h),渗透侧水含量保持在99.90%左右。     

Partial oxidation of methane in Ba0.5Sr0.5Co0.8Fe0.2O3−δ membrane reactor at high pressures

Oxygen permeation fluxes through dense disk-shaped Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCFO) membranes were investigated as a function of temperature (973–1123 K), pressure (2–10 atm), and membrane thickness (1–2 mm) under an air/helium gradient. A high oxygen permeation flux of 2.01 ml/cm² min was achieved at 1123 K and 10 atm under an air/He oxygen partial pressure gradient. Based on the dependence of the oxygen permeation flux on the oxygen partial pressure difference across the membrane and the membrane thickness, it is assumed that bulk diffusion of oxygen ions was the rate-controlling step in the oxygen transport across the BSCFO membrane disk under an air/He gradient. The partial oxidation of methane (POM) to syngas using LiLaNiO_x/γ-Al₂O₃ as catalyst in a BSCFO membrane reactor was successfully performed at high pressure (5 atm). Ninety-two percent methane conversion, 90% CO selectivity, and 15.5 ml/cm² min oxygen permeation flux were achieved in steady state at a temperature of 1123 K and a pressure of 5 atm. A syngas production rate of 79 ml/cm² min was obtained. Characterization of the membrane surface by SEM and XRD after reaction showed that the surface exposed to the air side preserved the Perovskite structure while the surface exposed to the reaction side was eroded.     

含氟体系中MFI型分子筛膜的制备及其乙醇/水分离性能

以氟化铵为矿化剂、四丙基溴化铵为模板剂,在负载晶种的钇稳定氧化锆(YSZ)中空纤维支撑体表面合成了MFI型分子筛膜,并用于乙醇/水的分离;系统考察了氟硅比(n_NH4F/n_SiO2)、合成时间等条件对膜分离性能的影响,在n_NH4F/n_SiO2为0.8、合成时间为8 h下合成出高性能膜,其通量达8.2 kg·m^-2·h^-1、乙醇/水分离因子为47;同时研究了MFI型分子筛膜在乙醇/水体系中的分离稳定性,揭示出该方法所合成膜表面无Si-OH,从而避免了Si-OH与乙醇反应而带来膜分离性能的下降.     

体型化NaA分子筛的制备研究

以粉煤灰一步酸溶法提取氧化铝剩余的酸溶渣为原料制备具有一定强度的地质聚合物,经过水热反应原位转化NaA分子筛。通过研究合成过程中的硅铝比、碱量、液固比等因素对制备体型化分子筛的影响,得到了制备酸溶渣基地质聚合物原位转化NaA型分子筛的最佳条件：在二氧化硅与三氧化二铝物质的量比为1.8、氧化钠与三氧化二铝物质的量比为1.0、水与酸溶渣质量比为1.2条件下制备出地质聚合物,之后在0.5 mol/L的75 mL氢氧化钠溶液中晶化,在晶化温度为100 ℃、晶化时间为8 h条件下可以制备出晶相单一、结晶度高的NaA体型化分子筛。制备的NaA体型化分子筛具有明显的立方体形貌,颗粒分布均匀,孔径为0.685 nm,比表面积为86.75 m²/g,抗压强度为4.5 MPa。     

Oxygen permeation study of perovskite hollow fiber membranes

The first oxygen permeation data of a dense hollow fiber perovskite membrane based on BaCo_xFe_yZr_zO_3 − δ are reported. The hollow fiber was prepared by a phase inversion process. Dense fibers were obtained with the following typical geometries: outer diameter, 800–900 μm; inner diameter, 500–600 μm; length, 30 cm. The O₂-permeation through the hollow fiber perovskite membrane was studied in a high-temperature gas permeation cell under different operation conditions. The increase of the helium gas flow rate reduces the oxygen partial pressure (p_O2) on the core side and a higher oxygen permeation flux is observed. High oxygen flux of 0.73 m³ (O₂)/(m² (membrane) h) was achieved at 850 °C under the operation parameters F_air (shell side) = 150 ml/min and F_He (core side) = 30 ml/min. The oxygen partial pressure dependence of the O₂ permeation flux indicated an interplay of both surface reaction and bulk diffusion as rate limiting steps. During 5 days of permeation a high and stable oxygen flux was observed. X-ray diffraction patterns of fresh and spent membranes after the permeation measurements revealed that no degradation after oxygen permeation appears.     

Influence of exchanged cations (Na, Cs, Sr and Ba) on xylene permeation through ZSM-5/SS tubular membranes

Na-ZSM-5 membranes were synthesized by secondary growth on the outer surface of stainless steel porous tubes. The membranes were ion-exchanged with Cs⁺, Ba²⁺ and Sr²⁺ to investigate their effect upon the separation of p-xylene from m-xylene and o-xylene. The permeation through the membranes was measured between 150 and 400 °C using each xylene isomer separately and a ternary mixture. All the membranes were selective to p-xylene in the temperature range studied. N₂ and xylene permeation measurements together with SEM observations were used to determine whether or not cracks and/or pinholes developed after exposure to the xylene isomers at high temperature (400 °C). Neither pore blockage nor extra-zeolitic pores developed after the ion exchange procedure and subsequent calcination. Furthermore, duplicate synthesized membranes of each cation form had similar separation factors and permeances. The duplicate values differ much less than the measurement error. The p-xylene permeation flux decreased in the order: Na-ZSM-5 > Ba-ZSM-5 > Sr-ZSM-5  Cs-ZSM-5 while the permeation flux of the m- and o-xylene decreased in the order Na-ZSM-5 > Sr-ZSM-5 > Ba-ZSM-5 > Cs-ZSM-5. The membrane that exhibited the best performance was Ba-ZSM-5, with a maximum p/o separation factor of 8.4 and a p-xylene permeance of 0.54 × 10⁻⁷ mol s⁻¹ m⁻² Pa⁻¹ at 400 °C.     

管式支撑体内表面NaA分子筛膜的合成与表征

采用转动合成方式通过二次生长法在管式α-Al₂O₃支撑体内表面合成了NaA分子筛膜,采用X射线衍射仪（XRD）、场发射扫描式电子显微镜（FE-SEM）和渗透汽化分离技术对所合成的膜进行了系统表征。考察了合成釜转速对分子筛膜合成的影响,结果表明合成釜转速的提高有利于分子筛膜合成。在转速为4 r·min^-1时所合成NaA分子筛膜分离因子高达2370,渗透通量1.86 kg·h^-1·m^-2左右（乙醇/相似文献   

Preparation of ITQ-29 (Al-free zeolite A) membranes

Membranes consisting of Al-free ITQ-29 (a zeolite with the LTA-type structure) have been prepared on alumina tubular supports by seeded liquid phase hydrothermal synthesis. A supramolecular organic structure-directing agent (SDA) was used for the synthesis of ITQ-29 that must be removed from the zeolite pores to activate the membrane for permeation. The high temperature needed to decompose SDA from small-pore zeolites could create thermal stresses that affect the final membrane quality. Because of this, the effect of the atmosphere (N₂, O₂, air and O₃/air) and temperature of the membrane calcination process has been studied. An ITQ-29 membrane activated by means of the combination of pyrolytic (at 723 K) and ozonization (at 473–573 K) treatments gave the best results, with N₂/CH₄ and N₂/propane selectivities well above the corresponding Knudsen values.     

Preparation of zeolite A membranes by microwave heating

Molecular sieve membranes consisting of NaA zeolite crystals have been successfully synthesized on symbol -Al₂O₃ substrate by means of microwave heating. In this way, the reaction time is greatly reduced to only 15–20 min and the membranes obtained are very stable and dense. Moreover, the thickness of the membranes can be easily controlled by varying the amount of the reaction mixture. To study the formation process of the membranes, the samples in various reaction stages have been characterized by field emission scanning electron microscopy and X-ray diffraction.     

Preparation, performances and reaction mechanism for the synthesis of H2O2 from H2 and O2 based on palladium membranes

A series of new tubular catalytic membranes (TCM's) have been prepared and tested in the direct synthesis of H₂O₂. Such TCM's are asymmetric -alumina mesoporous membranes supported on macroporous -alumina, either with a subsequent carbon coating (CAM) or without (AAM). Pd was introduced by two different impregnation techniques. Deposition–precipitation (DP) was applied to CAM's to obtain an even Pd particles distribution inside the membrane pore network, whereas electroless plating deposition (EPD) was successfully applied to AAM's to give a 1–10 μm thick nearly-dense Pd layer. Both type of membranes were active in the direct synthesis of H₂O₂. Catalytic tests were carried out in a semi-batch re-circulating reactor under very mild conditions. Concentrations as high as 250–300 ppm H₂O₂ were commonly achieved with both CAM's and AAM's after 6–7 h time on stream, whereas the decomposition rate was particularly high in the presence of H₂. Important features are the temperature control and pre-activation. In order to slow down the decomposition and favor the synthesis of H₂O₂ a smooth metal surface is needed.     

1,2-Diolates of titanium as suitable precursors for the preparation of photoactive high surface titania

1,2-Diolates of titanium of stoichiometric formula [Ti(OCHRCH₂O)₂] (R = −H, −CH₃, −CH₂CH₃) were prepared by reacting a titanium source (titanium(IV) isopropoxide (TIP) or titanium(IV) chloride) with an excess of the respective 1,2-diol. The diol was employed in a great excess; no other solvent was present in the synthesis procedure. The titanium diolates obtained have a white powder appearance and show a high degree of crystallinity as deduced from powder X-ray diffraction. It has been observed that their stability towards moisture is determined by the R group (R = −H, −CH₃, −CH₂CH₃).

Abstract

Hydrolysis of these titanium 1,2-diolates in water was a simple way to produce high surface porous titania with BET surface area up to 300 m² g⁻¹. These as-synthesised titania contain anatase crystallites and show photocatalytic activity versus formaldehyde degradation prior to calcination. Nevertheless, the decomposition rate of formaldehyde increased notably when titania samples were annealed at 773 K. This thermal treatment increases their crystallinity at the expense of a BET surface area decrease down to 75 m² g⁻¹ or less. The photocatalytic performance of the annealed samples is superior than that of commercial TiO₂ (Degussa P-25, Hombikat UV-100).     

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。     

Microwave-enhanced catalytic degradation of phenol over nickel oxide

A mix-valenced nickel oxide, NiO_x, was prepared from nickel nitrate aqueous solution through a precipitation with sodium hydroxide and an oxidation by sodium hypochlorite. Further, pure nickel oxide was obtained from the NiO_x by calcination at 300, 400 and 500 °C (labeled as C300, C400 and C500, respectively). They were characterized by thermogravimetry (TG), X-ray diffraction (XRD), nitrogen adsorption at −196 °C and temperature-programmed reduction (TPR). Their catalytic activities towards the degradation of phenol were further studied under continuous bubbling of air through the liquid phase. Also, the effects of pH, temperature and kinds of nickel oxide on the efficiency of the microwave-enhance catalytic degradation (MECD) of phenol have been investigated. The results indicated that the relative activity affected significantly with the oxidation state of nickel, surface area and surface acidity of nickel oxide, i.e., NiO_x (>+2 and S_BET = 201 m² g⁻¹)  C300 (+2 and S_BET = 104 m² g⁻¹) > C400 (+2 and S_BET = 52 m² g⁻¹) > C500 (+2 and S_BET = 27 m² g⁻¹). The introduction of microwave irradiation could greatly shorten the time of phenol degradation.     

SAPO-34 membranes for CO2/CH4 separations: Effect of Si/Al ratio

Silicoaluminophosphate (SAPO) membranes with Si/Al gel ratios from 0.05 to 0.3 were synthesized by in situ crystallization onto porous, tubular stainless steel support. Pure SAPO-34 membranes were obtained when the Si/Al ratio was 0.15 or higher. The adsorbate polarizability correlated with the adsorption capacity on SAPO-34, and the amounts of gases adsorbed were in the order: CO₂ > CH₄ > N₂ > H₂. The Si/Al ratio did not affect the pore volume significantly, but it changed the CO₂ and CH₄ adsorption equilibrium constants. The SAPO-34 membranes effectively separated CO₂ from CH₄ for feed pressures up to 7 MPa. At 295 K, for a pressure drop of 138 kPa and a 50/50 feed, the CO₂/CH₄ selectivity was 170 for a membrane with a Si/Al gel ratio of 0.15. At 7 MPa, the CO₂/CH₄ selectivity was 100 and the CO₂ permeance was 4 × 10⁻⁸ mol/(m² · s · Pa) at 295 K. This membrane was also separated CO₂/N₂ (selectivity = 21) and H₂/CH₄ (selectivity = 32) mixtures at 295 K and a pressure drop of 138 kPa. Competitive adsorption and difference in diffusivities are responsible for CO₂/CH₄ and CO₂/N₂ separations, whereas the H₂/CH₄ separation was due to diffusivity differences. For a membrane with Si/Al gel ratio of 0.1, a mixture of SAPO-34 and SAPO-5 formed, and the CO₂/CH₄ selectivity was lower.     

Synthesis and oxygen transport characteristics of dense and porous cerium/gadolinium oxide materials: Interest in membrane reactors

Cerium/gadolinium oxide (CGO)-based ceramic ion conductive membranes (CICMs) have potential uses in catalytic membrane reactors (CMRs) and solid oxide fuel cells (SOFCs). A supercritical CO₂ aided sol–gel process allowed the synthesis of CGO materials with the composition Ce_0.9Gd_0.1O_1.95. The produced nanophase powders were non-agglomerated, with a controlled morphology, a high purity and a high specific surface area (>100 m²/g). The CGO cubic crystalline phase has been obtained at temperatures <300 °C, lower than those of conventional solid state chemistry routes. With respect to ionic oxygen transport, a high conductivity at intermediate temperature (2 × 10⁻² S cm⁻¹ at 600 °C), almost equivalent in dense and porous samples, has been obtained on sintered materials prepared from these powders. In relation to their porosity characteristics, a modelling approach successfully explained the high ionic oxygen transport of some specific porous samples. Future directions for preparing porous conductive ceramics well adapted to CMR or SOFC applications can be anticipated from this model.