MFI zeolite membranes from a- and randomly oriented monolayers

c-Oriented columnar MFI films made by secondary growth of randomly oriented seed monolayers, deposited using sonication-assisted covalent attachment, exhibit n-hexane/2,2-dimethylbutane separation factor of up to 10⁴, n-/i-butane separation factor of up to 50, and p-/o-xylene separation factor of up to 2. A MFI film from a-oriented seed layer shows lower separation factors for the linear vs. branched isomers but higher separation factor for p-/o-xylene.     

Ab initio quantum chemical study on the zeolite catalyzed transformations of para-xylene

Unimolecular 1,2-methyl shift reaction and disproportionation of para-xylene is investigated by means of ab initio quantum chemical calculations. Isolated gas-phase models are used to estimate the optimum volume of the reaction complex. Space filling of the computed models is examined in zeolite Y, Beta and ZSM-5. It is suggested that the shape selectivity effect of zeolite molecular sieves on the disporportionation reaction can be associated with the steric conditions of the intrinsic rearrangement of the reaction complex in the course of the chemical transformation.     

High-resolution physical adsorption on supported borosilicate MFI zeolite membranes: comparison with powdered samples

We have performed physical adsorption measurements to analyze the structure of intact MFI membranes with a nominal Si:B ratio of 12.5:1. Argon adsorption isotherms of membranes at 77 K are nearly identical to isotherms of corresponding zeolite powders throughout most of the domain of relative pressures, the exception being the micropore-filling region (between 10⁻⁵ and 10⁻⁴ relative pressure). The isotherm in the micropore-filling region is steeper by a factor of 4–5 for the borosilicate membranes than it is for a powder made under the same conditions, while the saturation loadings are found to be similar. This implies a difference in how the zeolite pores fill with argon – either thermodynamic or kinetic in origin – and that fluxes for argon permeation through such powder and membrane samples may differ. Our membrane adsorption measurements reveal no hysteresis and hence no mesoporsity in the zeolite membranes. This finding, from adsorption measurements, stands in contrast to recent alkane permeation measurements on these membranes, which suggest the presence of mesopores. We reaffirm the conclusion, found in previous work, that zeolite membranes can be approximated as a zeolite powder plus a support for the purposes of adsorption characterization.     

MFI型分子筛膜的两段变温合成及对二甲苯异构体的分离性能

分别采用恒温和变温两种方法在氧化铝支撑体上原位制备了MFI型分子筛膜. 恒温法合成的MFI型分子筛膜晶体颗粒较大, 在高温脱除模板剂时会形成较大的缺陷, 没有对二甲苯/邻二甲苯(PX/OX)分离性能. 变温法合成的MFI型分子筛膜晶体在a, b方向尺寸较小, 在高温脱除模板时不会形成较大缺陷, 对PX/OX有良好的分离性能, 在300 ℃下, PX/OX分离因子高达42, PX的渗透性为9.57×10^-9 mol·m^-2·s^-1·Pa^-1. 采用低温臭氧脱除模板剂能够有效减小分子筛晶体热收缩产生的应力, 提高MFI型分子筛膜的分离性能. 两种方法合成的分子筛膜在低温臭氧的条件下脱除模板剂后, 都具有PX/OX分离性能, 其中变温法合成的分子筛膜PX/OX分离因子高达76, PX的渗透性为1.02×10^-8 mol·m^-2·s^-1·Pa^-1.     

Separation of alcohols and alcohols/O2 mixtures using zeolite MFI membranes

Two different types of supported silicalite membranes were employed for the separation of alcohols and alcohols/O₂ mixtures: in one of them the zeolite material was deposited on the top of the γ-alumina supports, while in the other the zeolitic material was mainly present in the porous structure of the α-alumina supports. While both kinds of membranes were able to separate the above mixtures, the second type of membranes having the zeolite material inside the support performed more efficiently. The maximum selectivity reported in this work is 7415 for the ethanol/O₂ separation in an ethanol/methanol/O₂ mixture. For a better understanding of the separation mechanism, the performance of both zeolite membranes was compared to that of a mesoporous silica membrane. Also, the adsorption of methanol and propanol on silicalite crystals was measured using a microbalance.     

薄片状MFI晶体无凝胶蒸汽辅助晶化制备b轴取向沸石膜

采用新型薄片状MFI沸石晶体作为晶种,通过无凝胶蒸汽辅助晶化法(gel-less steam-assisted crystallization,GLSAC)在烧结氧化硅多孔载体上制备了致密平整、b轴取向的MFI沸石膜。考察了GLSAC中模板剂浓度、釜底水量、生长温度及时间对MFI沸石膜的影响。扫描电子显微镜和X射线衍射结果显示,适量的模板剂和釡底水量能抑制晶种的面外生长。成功制备了厚度750 nm、致密平整的MFI沸石膜。制备的对丁烷异构体气体分离测试表明,25℃时,等物质的量的丁烷异构体的分离因子(SF_A/B)可达36,对正丁烷的渗透速率为1.5×10^-7mol·m^-2·s^-1·Pa^-1。     

CVD法制备Sn掺杂MFI分子筛膜及其对乙醇/水体系的分离性能

采用化学气相沉积(CVD)法对MFI分子筛膜进行Sn掺杂,制备了一种Sn-MFI分子筛膜,并研究了其渗透汽化分离乙醇/水体系的性能. XRD,29Si NMR, UV-Vis及分离实验结果表明,采用CVD法在将Sn引入MFI膜时,膜层结构基本得到保持, Sn可以进入分子筛骨架,有效地减少了膜表面的硅羟基缺陷,提高了膜分离乙醇/水体系时的稳定性.在SnCl4用量为3 mL、修饰时间为1 h时,所得到的Sn-MFI分子筛膜的渗透汽化分离性能最佳,并可在60℃下分离5%(质量分数)乙醇/水混合物时保持良好的稳定性.在经过连续50 h渗透气化分离后,其渗透通量仅从1.52 kg·m-2·h-1下降至1.38 kg·m-2·h-1,分离因子从18下降至16.     

MFI型分子膜的制备与H2S/CH4分离性能

采用二次生长法在多孔α-Al₂O₃载体上制备MFI型（ZSM-5和silicate-1）分子筛膜;通过XRD和SEM检测,证明所合成的分子筛膜为致密、交联和无取向的MFI型分子筛膜,厚度为5 μm;单组分气体渗透实验检测中,所制备样品膜的N₂渗透量均小于10^-11 mol/（m²·s·Pa）,可认为其无缺陷;同时,考察了样品分子筛膜对H₂S/CH₄混合气的分离效果,在渗透压分别为0.3和0.5 MPa时,silicate-1分子筛膜的H₂S/CH₄的分离因子分别为1.99和4.44,而ZSM-5分子筛膜的CH₄/H₂S的分离因子分别为6.71和12.85。     

MFI纳米片沉积层气相转化制备超薄b轴取向沸石膜

利用乙二胺-水蒸汽进行气相转化(VPT)制备超薄、取向MFI沸石膜,通过将MFI纳米片沉积层转化为致密的沸石膜,实现了膜厚度的有效控制。扫描电子显微镜和X射线衍射表明,制备的沸石膜膜厚度约为280 nm,具有高度b轴取向的致密结构。丁烷异构体双组分分离测试结果表明,在333 K下,等物质的量的正丁烷/异丁烷混合物的正丁烷渗透速率和分离因子分别为1.5×10^-7 mol·m^-2·s^-1·Pa^-1和14.8。Na₂SiO₃作为低聚硅源在MFI沸石纳米片二次生长过程中能够提供硅源和碱度,通过在胺类蒸汽中实现MFI沸石纳米片间的融合生长,进一步提高了膜的取向度和致密性。     

MFI纳米片沉积层气相转化制备超薄b轴取向沸石膜

利用乙二胺-水蒸汽进行气相转化（VPT）制备超薄、取向MFI沸石膜,通过将MFI纳米片沉积层转化为致密的沸石膜,实现了膜厚度的有效控制。扫描电子显微镜和X射线衍射表明,制备的沸石膜膜厚度约为280 nm,具有高度b轴取向的致密结构。丁烷异构体双组分分离测试结果表明,在333 K下,等物质的量的正丁烷/异丁烷混合物的正丁烷渗透速率和分离因子分别为1.5×10^-7 mol·m^-2·s^-1·Pa^-1和14.8。Na₂SiO₃作为低聚硅源在MFI沸石纳米片二次生长过程中能够提供硅源和碱度,通过在胺类蒸汽中实现MFI沸石纳米片间的融合生长,进一步提高了膜的取向度和致密性。     

Microwave synthesis of zeolite membranes: A review

Significant progress has been achieved in the last years on microwave synthesis of zeolite membranes. In many cases, microwave synthesis has proven to remarkably reduce the synthesis time. In addition, microwave synthesis could also result in different membrane morphology, orientation, composition, and thus the different permeation characteristics as compared with those synthesized by conventional heating. This review attempts to summarize the obtained progress in microwave synthesis of zeolite membranes. Some topics are discussed, including: (1) case study of microwave synthesis of zeolite membranes, e.g. LTA, MFI, AFI, and other types of zeolite membranes; (2) differences between conventional and microwave synthesis; (3) formation mechanism and the so called “specific microwave effect” in the case of microwave synthesis of zeolite membranes; (4) scaling-up of zeolite membrane production by employing microwave heating. The latter three topics are mainly focused on LTA type zeolite membranes. Concluding remarks and future perspective are also suggested in the end.     

Removal of pollutants from indoor air using zeolite membranes

MFI-type zeolite membranes prepared by liquid phase hydrothermal synthesis on tubular commercial supports were used to remove model pollutants n-hexane, formaldehyde and benzene present at very low concentration levels (2–230 ppmv) in indoor air. The influence of several operating parameters was studied both in batch and continuous separation experiments. Depending on the operation conditions, permeation fluxes of the organic compound up to 3300, 130 and 30 mg/(m² h) and organic/air separation factors of 250, 6.3 and 38 were achieved for n-hexane, formaldehyde and benzene, respectively.     

Layered Zeolite for Assembly of Two-Dimensional Separation Membranes for Hydrogen Purification

Membrane separation is an energy-efficient and environmentally friendly process. Two-dimensional (2D) molecular sieving membranes featuring unique nanopores and low transport resistance have the potential to achieve highly permeable and selective mixture separation with low energy consumption. High-aspect-ratio zeolite nanosheets with intrinsic molecular-sieving pores perpendicular to the layers are desirable building blocks for fabricating high-performance 2D zeolite membrane. However, a wider application of 2D zeolitic membranes is restricted by the limited number of recognized zeolite nanosheets. Herein, we report a swollen layered zeolite, ECNU-28, with SZR topology and eight-member ring (8-MR, 3.0 Å×4.8 Å) pores normal to the nanosheets. It can be easily exfoliated to construct 2D membrane, which shows a high hydrogen selectivity up to 130 from natural gas and is promising for hydrogen purification and greenhouse gas capture.     

高稳定性金属有机配位聚合物膜的制备及二甲苯分离性能

以常见的多孔二氧化硅为载体, 采用晶种-二次生长法在溶剂热条件下制备了高稳定性的金属有机配位聚合物膜材料. 通过X射线衍射、 扫描电子显微镜、 单组分气体透过率和二甲苯同分异构体分离等测试手段对材料的结构、 形貌和性质等进行了表征. 二甲苯同分异构体的渗透汽化分离结果表明, 这种多孔膜材料对二甲苯混合物具有选择性吸附分离的性质. 进一步通过热处理、 超声处理和性能的重复表征, 证明了该膜材料具有优良的热稳定性和机械稳定性, 适于实际应用.     

多孔石墨烯分离CH4/CO2的分子动力学模拟

采用分子动力学方法模拟CH₄/CO₂混合气体在多孔石墨烯分离膜中的分离过程, 分析了3 种纳米孔功能化修饰(N/H 修饰、全H修饰和N/―CH₃修饰)对分离过程的影响规律. 模拟结果表明气体分子会在石墨烯表面形成吸附层, CO₂分子的吸附强度高于CH₄分子. 纳米孔的功能化修饰不仅减小了纳米孔的可渗透面积, 还通过影响纳米孔边缘原子的电荷分布提高了气体分子的吸附强度, 进而影响了混合气体分子在多孔石墨烯分离膜中的渗透性和选择性. CO₂分子在多孔石墨烯中的渗透率能达到10⁶ GPU (1 GPU=3.35×10^-10 mol·s^-1·m^-2·Pa^-1), 远远高于传统的聚合物分离膜. 研究表明多孔石墨烯分离膜在天然气处理、CO₂捕获等工业气体分离过程中具有广泛的应用前景.     

Synthesis of Fe‐loaded MFI zeolite using silatrane as precursor and its CO activity

Fe‐MFI zeolite was successfully synthesized using silatrane as precursor and tetrapropyl ammonium bromide as template via the sol–gel process and microwave heating technique. The effects of ageing time, heating temperature, heating time and iron concentration were investigated, and it was found that Fe‐MFI synthesis favors higher heating temperatures, but is limited by the degradation of the incorporation of a template molecule. Moreover, longer ageing and heating times promote the incorporation of higher amounts of iron atom in the MFI structure. However, too long an ageing time decreases the incorporation of iron. The lower the percentage Fe loading, the greater the percentage of Fe³⁺ ions incorporated into the MFI framework. The catalytic activity of Fe‐MFI catalyst for the oxidation of CO was studied and it was found that these synthesized catalysts catalyzed the oxidation of CO very well. Copyright © 2005 John Wiley & Sons, Ltd.     

Preparation and application of zeolite/ceramic microfiltration membranes for treatment of oil contaminated water

NaA zeolite microfiltration (MF) membranes were prepared on α-Al₂O₃ tube by in situ hydrothermal synthesis method and investigated for water separation and recovery from oily water. NaA/α-Al₂O₃ MF membranes with average inter-particle pore sizes of 1.2 μm, 0.4 μm and 0.2 μm were prepared. The membranes were characterized by scanning electron microscope (SEM) and the inter-particle pore size distribution (PSD) was determined by gas bubble pressure method. Membranes with pore sizes of 1.2 μm (NaA1) and 0.4 μm (NaA2) were used to treat an oil-in-water emulsion containing 100 mg/L oil. Better than 99% oil rejection was obtained and water containing less than 1 mg/L oil was produced at 85 L m⁻² h⁻¹ by NaA1 at a membrane pressure of 50 kPa. Consistent membrane performance was maintained by a regeneration regime consisting of frequent backwash with hot water and alkali solution.     

Crack formation in α-alumina supported MFI zeolite membranes studied by in situ high temperature synchrotron powder diffraction

Cracks are frequently formed in α-alumina supported MFI membranes during calcination. To better understand crack formation, in situ powder diffraction data were collected during calcination of a type of MFI membrane (ca. 1800 nm thick) which is known to crack reproducibly. In addition, data for MFI powder and a blank support were also collected. Both a synchrotron radiation facility and an in-house instrument were used. The unit cell parameters were determined with the Rietveld method, and the strain in the direction perpendicular to the film surface was calculated for the film as well as for the support. The microstrain in the support was also estimated. Based on the results obtained here, a model for crack formation in this type of MFI membrane was proposed. The lack of cracks in other types of MFI membranes (ca. 500 nm) prepared in our laboratory is also explained by the model. In thicker MFI films, the crystals are well intergrown. During heating, the MFI crystals contract and the α-alumina support expands. Consequently, a thermal stress develops in the composite which eventually leads to formation of cracks in the film and structural defects in the support. In thinner films, the crystals are less well intergrown and the thermal expansion mismatch leads to opening of grain boundaries rather than cracks.     

Separation of organic mixtures by pervaporation using crosslinked rubber membranes

Natural rubber (NR) and poly(styrene-co-butadiene) rubber (SBR) has been crosslinked with sulfur and accelerator with three different doses of varied accelerator to sulfur ratios to obtain three crosslinked membranes from each of these two rubbers (NR-1, NR-2 and NR-3 and SBR-1, SBR-2 and SBR-3). These six rubber membranes were used for pervaporative separation of toluene–methanol mixture up to 10 wt% of toluene in feed. It has been found that with increase in accelerator to sulfur ratio from membrane-3 to membrane-1, the vulcanization system shifts from conventional to efficient system resulting in higher degree of crosslink density and permeation selectivity. All of these membranes showed reasonably good range of flux (45.26 gm/m² h for NR-3 to 12.0 gm/m² h for SBR-1) and separation factor (162 for SBR-1 to 35.12 for NR-3) for 0.55 wt% of toluene in feed. Among these membranes NR-1 and SBR-1 with highest crosslink density showed maximum separation factor for toluene along with good flux. It has also been found that for comparable crosslink density SBR membranes showed better separation factor than NR membranes.     

Separation of CO_2–N_2 using zeolite NaKA with high selectivity

The adsorption method based on solid adsorbents is one of feasible ways to capture and store CO_2. Using the ion exchange method, different zeolites Na KA varying in K+content were produced. The adsorption isotherms and kinetic uptakes were measured. The experimental results show that the optimal NaKA could adsorb significant quantities of CO_2 and little N_2. On the zeolite Na KA with 14.7 at.% K+, the adsorption capacity for pure CO_2 is over 3.10 mmol g~(-1) and the CO_2–N_2 selectivity is about 149 at ambient pressure and temperature. The kinetic CO_2–N_2 selectivity could also achieved 200 within 3 min according to the uptake data. To demonstrate the separation effectiveness, breakthrough curves of pure components and binary mixtures were investigated experimentally and theoretically in a fixed bed. It is found that the breakthrough points of CO_2 and N_2 are almost at the same time under the atmospheric pressure at 348 K with the raw gas composition CO_2/N_2(20:80, v/v). If the pressure has been increased higher than 0.1 MPa, CO_2 would break through the bed much slower than N_2. Therefore, the pressure may become the limiting factor for the separation performance of zeolites NaKA.