The change of the unit cell dimension of different zeolite types by heating and its influence on supported membrane layers

In comparison with the well-known swelling of organic polymer membranes, zeolite membranes have been considered for a long time as relatively stiff. The problem of the different thermal expansion coefficients of the α-Al₂O₃ support and the zeolite layer was usually reduced on a possible crack formation during the template removal of MFI membranes. However, the crack formation could be avoided by an extremely slow heating rate and by membrane layers formed of small and non-oriented zeolite crystallites. Several papers have shown that the isomorphous incorporation of Al into the MFI structure (silicalite → ZSM-5) results in an increase of the non-selective intercrystalline transport and – as a consequence – the separation selectivity drops. In accordance with this observation, no shape-selective gas separation is reported for the Al-rich zeolite membranes MOR, FAU and LTA. On the other hand, these membranes allow the highly selective separation of water from organic mixtures due to hydrophilic interactions. In the present paper the change of the unit cell dimension (UCD) for zeolites LTA, FAU, MOR and MFI was studied as a function of temperature and water content using Rietveld refinement. Parallel to the determination of the linear and volume expansion coefficients by in situ-heating XRD, the de-watering and de-templating was studied by thermogravimetry (TGA). A strong UCD-change was found for all Al-rich zeolite types as a result of the de-watering. In contrast, the smallest changes of the UCD in the temperature range 50–450 °C were found for MFI crystals. Nevertheless, also for MFI membrane layers with rising temperature an increasing tension between the expanding α-Al₂O₃ support and the slightly shrinking membrane layer takes place. Zeolite layers of small crystals with either a random or a preferential crystal orientation relative to the support and an only partial de-watering in the case of Al-rich zeolite layers can minimize the tension in the support-membrane system. Thus, the irreversible formation of macroscopic cracks can be avoided and the formation of small intercrystalline pores in the mesopore region becomes reversible.     

动态水热法制备Silicalite-1分子筛膜包覆多孔缺陷Al2O3微球

采用晶种涂覆-预晶化-晶化成膜的动态水热法成功在具有多孔缺陷的氧化铝微球上合成Silicalite-1分子筛膜。以乙醇作为润湿试剂在氧化铝表面涂覆一层晶种,将涂覆过晶种的载体加入到分子筛合成液中预晶化,一层分子筛完全覆盖载体并与载体结合牢固。预晶化后的载体在动态水热条件下处理3天,得到致密分子筛膜包覆的Al₂O₃微球。运用X射线衍射（XRD）和扫描电镜（SEM）对所得材料进行表征。结果表明,包覆的分子筛膜具有典型的MFI结构,晶粒交互生长,厚度约为3μm。考察了TPAOH用量和水量对分子筛膜微观结构的影响,结果发现TPAOH用量主要影响Silicalite-1分子筛膜的形貌,当TPAOH用量为0.17时,合成的Silicalite-1分子筛膜连续致密,而水量对分子筛膜微观结构的影响较小。这种晶种涂覆-预晶化-晶化成膜的方法有助于在多孔缺陷的Al₂O₃微球上制备高质量的分子筛膜。     

ZrO2 和Al2O3支撑的MFI 型沸石分子筛膜:支撑体材料对膜性能的影响

采用水热合成法，以纯SiO2为源物质，在介孔Y2O3掺杂的ZrO2(YZ)及大孔α-Al2O3支撑体上制备出高质量的MFI型沸石分子筛膜，通过H2／n-C4H10气体混合物的渗透分离和p-xylene的蒸发研究了不同支撑体上MFI型沸石分子筛膜分离性能，在较低温度范围，YZ支撑的MFI型沸石分子筛膜中n-C4H10的渗透率比Al2O3支撑的膜高很多，最大n-C4H10与H2的分离率达到500，Al2O3支撑的膜中py-xylene的蒸发流量随时间下降很快，而YZ支撑的膜中的蒸发流量则变化缓慢，用XRD对膜的晶体结构进行分析，通过多种温度下热处理不同支撑体上的膜样品研究了其热稳定性与支持体材料的关系，YZ支撑的MFI型沸石分子筛膜的MFI结构在1000摄氏度后仍能保持，而Al2O3支撑的膜950摄氏度时已完全转变为石英相，研究结果表明，YZ支撑的MFI型沸石分子筛膜比Al2O3支撑的膜表现出更好的厌不性，热稳定性以及抗阻塞性。     

Effect of substrate curvature on microstructure and gas permeability of hollow fiber MFI zeolite membranes

Literature data show that gas permeability of MFI zeolite membrane varies depending on the geometry of supports. The present work investigates the effects of the surface curvature of substrates on the microstructure and the gas permeation property of supported zeolite membranes. MFI zeolite membranes were grown on porous alumina hollow fibers with different diameters (surface curvature) by the secondary growth method. Single gas permeation and H₂/CO₂ binary gas separation from 25 to 300 were conducted to study the membrane quality. The zeolite membranes on supports of larger surface curvature have higher permeability and lower selectivity due to the presence of more inter‐crystalline gaps in the zeolite layer formed during the template removal step. The effects of the support surface curvature (and geometry) on zeolite membrane microstructure and gas permeation characteristics are semi‐quantitatively analyzed by a transport model considering both structural change and gas diffusion in micropores. © 2018 American Institute of Chemical Engineers AIChE J, 64: 3419–3428, 2018     

On the combination of different transport mechanisms for the simulation of steady-state mass transfer through composite systems using permeation through stainless steel supported silicalite-1 membranes as a model system

A method to calculate the steady-state multicomponent mass transfer in heterogeneous structures is presented. For this, different transport mechanisms represent different regions inside the composite system. The solving scheme allows the calculation of the molar flow rates along the different transport pathways and the estimation of unknown transport parameters based on experimental data.

The steady-state mass transfer of hydrogen and sulfur hexafluoride through a sinter metal supported MFI zeolite membrane is examined as a model system. The support is simulated using the dusty gas model, the molecules passing the zeolite layer may follow transport mechanisms like Knudsen diffusion, surface diffusion or activated gas diffusion. The configuration used, with data from single gas experiments, can predict the binary system only at high temperatures. The problems arising at low temperatures are due to the oversimplified assumption that adsorbed and desorbed molecules do not interact on their way through the MFI zeolite micropores.

However, because of the modular structure of the approach, it can be easily extended to more advanced transport models to account for interactions between the different molecules. And the method can be applied equally well to other systems, such as palladium composite membranes or membranes with catalytic activity.     

Zeolite membranes: microstructure characterization and permeation mechanisms

Since their first synthesis in the 1940s, zeolites have found wide applications in catalysis, ion-exchange, and adsorption. Although the uniform, molecular-size pores of zeolites and their excellent thermal and chemical stability suggest that zeolites could be an ideal membrane material, continuous polycrystalline zeolite layers for separations were first prepared in the 1990s. Initial attempts to grow continuous zeolite layers on porous supports by in situ hydrothermal synthesis have resulted in membranes with the potential to separate molecules based on differences in molecular size and adsorption strength. Since then, further synthesis efforts have led to the preparation of many types of zeolite membranes and better quality membranes. However, the microstructure features of these membranes, such as defect size, number, and distribution as well as structure flexibility were poorly understood, and the fundamental mechanisms of permeation (adsorption and diffusion), especially for mixtures, were not clear. These gaps in understanding have hindered the design and control of separation processes using zeolite membranes. In this Account, we describe our efforts to characterize microstructures of zeolite membranes and to understand the fundamental adsorption and diffusion behavior of permeating solutes. This Account will focus on the MFI membranes which have been the most widely used but will also present results on other types of zeolite membranes. Using permeation, x-ray diffraction, and optical measurements, we found that the zeolite membrane structures are flexible. The size of defects changed due to adsorption and with variations in temperature. These changes in defect sizes can significantly affect the permeation properties of the membranes. We designed methods to measure mixture adsorption in zeolite crystals from the liquid phase, pure component adsorption in zeolite membranes, and diffusion through zeolite membranes. We hope that better understanding can lead to improved zeolite membranes and eventually facilitate the large-scale application of zeolite membranes to industrial separations.     

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.     

Effect of Mesoporous Silica Buffer Layer on the Orientation of MFI Zeolite Membranes

Growth of MFI zeolite membranes on porous α-alumina substrates with and without precoated mesoporous silica buffer layer has been performed by in situ crystallization. A preferentially b -oriented MFI membrane can be prepared on a mesoporous silica-coated substrate. A membrane with a randomly oriented and eventually [ h 0 h ]-oriented top layer is obtained on an uncoated α-alumina substrate. The chemical properties of the substrate are suggested to have significant effects on the membrane orientation.     

Synthesis and characterization of silicalite-1 membrane prepared on a novel support by the pore plugging method

The effect of the support pore size on the membrane morphology was investigated for zeolite silicalite-1 membranes synthesized by pore plugging method on supports with zirconium oxide and/or titanium oxide active layer. Parameters including surface coverage, zeolite layer thickness, crystal size and shape, zeolite penetration depth were used to quantify the membrane morphology. Five supports with different pore sizes for their active layer in the range of 0.14–1.4 μm were investigated. The X-ray diffraction (XRD) analysis showed a typical silicalite-1 zeolite structure with a high internal crystalline order grown inside the pores as well as on top of all supports. The XRD results also showed that the silicalite-1 crystals in the synthesized membranes are not randomly oriented. The crystallographic preferred orientation (CPO) analysis revealed that the degree of orientation toward either the a-axis or b-axis perpendicular to the support surface, increased by decreasing the pore size of the support. The 0.45 μm support had the most preferably oriented zeolite layer for access of molecules entering into the membrane structure with the highest number of crystals oriented with the b-axis (the one with straight channels) perpendicular to the support surface. The scanning electron micrographs (SEM) analysis of the membranes revealed a dense and continuous surface morphology with the highest crystal size of silicalite-1 around 1.5 μm on the surface of the support with the 0.45 μm pore size. SEM micrographs also showed a continuous layer grown over four supports out of five supports with different pore sizes that were investigated, with no layer observed on the 1.4 μm pore size support. The average thickness of the zeolite layer was in the range of 0.7–1.4 μm, depending on the pore size of the support. The supports with 0.2 and 0.45 μm pore sizes had the most uniform zeolite layer thickness while the support with 0.8 μm pore size active layer had the least uniform zeolite layer thickness. The electron diffraction spectrometer (EDS) analysis confirmed the formation of pure silicalite-1 layer at the surface as well as inside the pores of all supports. The highest silicalite-1 crystal penetration was for the supports with 0.45 and 1.4 μm pore sizes. Single gas permeation experiments with He and N₂ gases at 293 K illustrated that regardless of the pore size of the support, the He and N₂ permeances were constant despite the change of the pressure across the membranes. The highest permeances were observed for the membrane prepared using the 0.45 μm pore size support, while the lowest permeances were for the membrane prepared using the 1.4 μm pore size support. These results confirmed the selective properties of the prepared membranes. No matter what is the pore size of the support or the feed pressure, N₂ permeances were around three times higher than those for He.     

b轴取向MFI分子筛膜的制备及应用研究进展

分子筛膜是一种新型的无机膜,具有孔道结构规整、化学和热稳定性好、机械强度高、抗污染性能好、易于改性等优点。b轴取向MFI分子筛膜因可以缩短传质路径、降低传质阻力、提高扩散效率,在膜分离和膜反应器领域有着广泛的应用前景,受到国内外学者的普遍关注。综述了b轴取向分子筛膜的制备方法及应用研究进展。详细介绍了原位水热合成法、二次生长法、微波辅助合成法、无凝胶法、固相转化法及纳米片法等。二次生长法可以控制分子筛膜的微观结构,且受载体表面性质的影响较小;微波辅助法可以缩短结晶时间,降低能耗,对工业化生产具有重要意义;无凝胶法具有制备工艺简单、环境友好等优点。在上述方法的基础上,将纳米片作为晶种可以降低膜厚。最后,展望了b轴取向MFI分子筛膜的发展前景,在制备b轴取向连续无缺陷MFI分子筛膜方面仍面临许多挑战,包括提高膜的机械强度和长期运行稳定性、实现粗糙或弯曲以及大尺寸载体表面取向膜层的制备等。     

TMCS修饰MFI分子筛膜的制备及乙醇/水分离稳定性的研究

采用三甲基氯硅烷（TMCS）作为修饰源对MFI分子筛膜进行表面改性,系统考察了TMCS浓度以及修饰时间对于MFI分子筛膜在分离乙醇/水混合物时的性能影响。SEM、XRD、²⁹Si NMR、FT-IR、接触角实验及分离实验结果表明,TMCS可以与硅羟基反应,嫁接分子筛膜表面,在消除膜表面硅缺陷的同时提高膜的疏水性及膜分离性能的稳定性。随着TMCS浓度以及反应时间的增加,修饰后MFI分子筛膜的通量及分离因子略有下降,但稳定性增强。在TMCS的浓度为0.4%（质量）,修饰时间为2 h时,所得到的膜具有最佳渗透汽化分离性能,并可在60℃下分离5%（质量）乙醇/水混合物时保持良好的稳定性。在连续90 h渗透汽化分离过程中,其渗透通量稳定在1.61 kg·m^-2·h^-1 左右,分离因子保持在20以上。     

Modification of carbon membranes and preparation of carbon-zeolite composite membranes with zeolite growth

A novel method for the modification of carbon membranes was developed by zeolite growth on the surface of porous carbon tubes using a hydrothermal synthesis method and carbon-zeolite composite membranes were successfully obtained. Zeolite seeds for the zeolite growth were introduced to the inner surface of the tubes by using a slip-casting technique in 1 wt.% seed ethanol solution and continuous and highly intergrown zeolite layers on the seeded tubes were formed by subsequent hydrothermal synthesis. Different types of zeolite layers can be grown on porous carbon tubes by using different types of zeolite seeds. Without zeolite seeds, a zeolite layer could not be formed on the carbon surface. SEM, XRD and pure gas permeation characterization indicates that the carbon-zeolite membranes are continuous and defect-free.     

Mathematical modeling and parameter estimation of MFI membranes for para/ortho-xylene separation

A mathematical model of a membrane with a thin, oriented, and selective MFI layer, which also includes contributions from defects, pore blockages, support layer, and external mass transfer, was formulated based on the Maxwell-Stefan equations. It was validated using reported (Kim et al., Angew Chemie Int Ed, 2018, 57:480–485; Jeon et al., Nature, 2017, 543:690–694) para/ortho-xylene separation data from five MFI membranes. The diffusivities of the xylenes were considered to be the same for all membranes, while the thickness and the density of defects and pore blockage were treated as unique properties of each membrane, and their contributions were estimated by fitting the model to the corresponding separation data. The effects of these properties and the role of permeate pressure on the separation performance were subsequently analyzed. The proposed modeling, parameter estimation, and analysis framework allow one to quantitatively interpret the variation of separation performance, to understand separation bottlenecks, and to provide guidance for designing membranes with desired performance.     

MFI型分子筛膜的制备及表征

采用无模板剂的二次生长合成方法,在-αA l2O3基膜上合成了MFI型分子筛膜,用XRD,SEM和气体渗透实验等方法进行表征,表明合成在-αA l2O3基膜的物质为MFI型分子筛。二次生长分子筛膜的正/异丁烷理想分离系数在298 K和473 K时分别为77和70,气体分离数据表明,2种分子筛膜对气体分离是由分子筛分占主导,同时分子筛膜完整无裂缺。不同温度,通过MFI分子筛膜渗透汽化分离质量分数分别为5%、50%和95%的乙醇/水的渗透通量和分离因子,结果表明渗透通量随温度的升高而升高,而分离因子随温度的升高却降低;渗透通量随乙醇质量分数的升高而降低,分离因子却随质量分数的升高而升高。     

Thin-layer template-free polycrystalline mordenite membranes on cylindrical mullite supports

Continuous nanoporous mordenite membranes were reproducibly fabricated on tubular microporous mullite supports. The ceramic supports were prepared during calcination of kaolin clay, followed by a leaching post-treatment. A template-free seeded hydrothermal synthesis in a brass autoclave was used to prepare the zeolite membranes. The membranes and the powder crystals collected from the bottom of the autoclave were characterized with XRD, SEM and EDAX. PV properties of the membranes were assessed using dehydration of different ethanol–water mixtures. XRD patterns showed that mordenite was the only zeolitic material present in the membrane. SEM examination of the membrane revealed that thickness of the mordenite crystals filling the voids as a dense thin-layer among the mullite support was 5–10 μm. The continuous top-layer of the membrane was formed by many large and oriented crystals. EDAX analysis showed a Si to Al ratio of 6 for the mordenite powder. The polycrystalline membranes were found to be selective for dehydration of different ethanol/water mixtures. It was concluded that selective adsorption of water on the zeolite pores, small intercrystalline defects and a pinhole-free structure of the membrane layer leaded to high water selectivity of the mordenite membranes studied.     

Solution‐processable exfoliated zeolite nanosheets purified by density gradient centrifugation

Highly crystalline exfoliated MFI‐nanosheets can pave the way for large‐scale deployment of sub‐500‐nm zeolite membranes due to their processing and packing advantages. Exfoliated MFI‐nanosheets prepared by melt compounding contain a large amount of polymer and unexfoliated particles which are detrimental to the fabrication of ultrathin zeolite membranes. Complete removal of polystyrene from the nanosheet suspension in toluene is demonstrated by centrifugation of the suspension across chlorobenzene as confirmed by thermogravimetric analysis (TGA) data and transmission electron microscopy (TEM) images. Rate‐zonal centrifugation in a nonlinear density gradient fractionated exfoliated MFI‐nanosheets from unexfoliated particles. The purified nanosheets were highly crystalline as indicated by high‐resolution TEM (HRTEM) and electron diffraction (ED). Coating of purified MFI‐nanosheets on a smooth α‐alumina support, fabricated by filtration of α‐alumina suspension, led to a compact, b‐oriented, 80‐nm‐thick film. A mild hydrothermal treatment of the film led to a 200‐nm‐thick membrane, which demonstrated molecular sieving properties. © 2013 American Institute of Chemical Engineers AIChE J, 59: 3458–3467, 2013     

α-Al2O3载体上b-轴取向MFI分子筛膜的制备

在α-Al2O3支撑的PVA表面手工组装紧密排列的b-轴取向silicalite-1分子筛晶体层,煅烧除去高聚物层后可得到很好的晶种层。采用两步法二次合成工艺,在多孔氧化铝载体表面可直接合成完备的b-轴取向MFI型分子筛膜。n（TEOS）：n（TPAOH）：n（H20）为1．0：0．2：200．0的合成液经150℃预加热处理可有效减少二次生长过程中α-轴孪晶的生成,保证制备的MFI膜为很好的b-轴取向。SEM及XRD检测表明所制备膜层的完备性及b-轴取向。另外采用多层取向晶种层经过一次晶化3h即可制备完备的b-轴取向多层MFI型分子筛膜。实现了在大孔载体上直接合成b-轴取向的MFI膜,对提高膜的应用性能有重要意义。     

Tuning the framework polarity in MFI membranes by deboronation: Effect on mass transport

The polarity of a zeolite is an important parameter determining the transport and separation properties in zeolite packed beds and membranes. This study focuses on the systematic variation of the zeolite polarity for membrane applications by varying the amount of silanol groups within the framework of the MFI zeolite by deboronation and consecutive heat treatment from 9 to 4 silanol groups per unit cell. The decrease in the number of silanol groups after heat treatment is confirmed by diffuse reflection infrared Fourier transform (DRIFT) and ²⁹Si Magic Angle Spinning Nuclear Magnetic Resonance (MAS NMR). The permeation of pure water through the modified silicalite membranes is insensitive to the number of silanol groups, attributed to a decreased adsorption compensated by an increased diffusivity. The ethanol selectivity in water/ethanol mixtures shows an increase with decreasing amount of silanol groups. Residual boron in the zeolite may react with ethanol and decrease its adsorption capacity. A reduction in flux through the debronated MFI membranes was not observed after ethanol/water permeation.     

Effect of nanowhisker-modified zeolites on mechanical and thermal properties of poly(vinyl acetate) composites with pure-silica MFI

The effect of nanoscale surface morphology of pure-silica MFI zeolite on the interfacial, mechanical, and thermal properties of pure-silica MFI zeolite/poly(vinyl acetate) (PVAc) composites was investigated under different annealing conditions. Mg(OH)₂ inorganic whisker- or asperity-like nanostructures were achieved on MFI nano- and micro-particle surfaces via Grignard or solvothermal treatment. The creation of nano-roughness on the MFI surface promoted compatibility between the zeolite and the polymer matrix, resulting in void-free interfaces. PVAc composites containing surface-modified particles showed increased tensile strength and elongation at break as compared with composites containing unmodified zeolite. Surface modification of the microparticles exhibited interfacial and mechanical enhancement over a wider range of annealing temperatures than nanoparticles. Differential scanning calorimetry revealed that surface treatment of MFI resulted in broader glass transitions compared to composites containing unmodified MFI. This is explained by improved interfacial adhesion and associated slower chain relaxation dynamics. Furthermore, X-ray diffraction demonstrated that enhanced adhesive interactions between the PVAc and the MFI surface are associated with surface-induced orientation of the MFI particles within the polymer matrix. The optimal surface morphology, associated with the most enhanced mechanical and thermal properties of composites, was produced with the solvothermal method.