分子筛膜包覆型催化剂的制备和应用

均匀、连续、致密分子筛膜的合成和应用受到广泛关注。利用分子筛膜具有的筛分和催化作用,在传统颗粒催化剂或载体表面包覆分子筛膜形成复合型催化剂,可以实现膜基分离和催化过程的耦合,增加反应物选择性,提高目标产物收率。本文综述了近年来在不同类型颗粒催化剂或载体表面合成分子筛膜的制备方法,描述了分子筛膜包覆型复合催化剂用于不同催化反应体系的研究结果。同时,在归纳和总结已有研究成果基础上展望了分子筛膜包覆型催化剂的研究发展趋势。     

溶胶-凝胶和浸渍-水热制备方法对TiO_2/AC光催化剂结构和性能的影响

分别采用溶胶-凝胶法和浸渍-水热法制得负载于活性炭(AC)的TiO2催化剂,并用扫描电镜(SEM)、X射线衍射(XRD)、拉曼光谱和氮气吸附等方法对催化剂进行了表征.结果表明:溶胶-凝胶法制得的TiO2以不规则碎片形式涂附在载体表面,而浸渍-水热法制得的球形TiO2颗粒呈柱形生长均匀覆盖在载体表面;不同温度处理的浸渍-水热法制得的TiO2/AC光催化剂的中孔和微孔比表面积均大于溶胶-凝胶法制得的样品,负载的TiO2粒径则小于溶胶-凝胶法制得的样品.对甲基橙(MO)溶液的光催化降解测试结果表明,600℃煅烧为两种方法的最佳热处理温度,浸渍-水热法制得的催化剂光催化效果明显强于溶胶-凝胶法的.     

FeCrAl丝网负载FAU沸石膜的制备及其催化性能

采用水热法制备FeCrAl丝网负载FAU沸石膜,考察了合成液老化、载体预涂晶种以及晶种液中添加聚乙烯醇(PVA)等因素的影响,评价了FeCrAl丝网负载FAU沸石膜对模型吸热燃料正辛烷裂解的催化活性。结果表明,载体表面预涂Y分子筛晶种后有助于增加载体表面分子筛负载量,采用老化后的合成液,效果更显著,并且更易在金属丝上生长出一层连续致密的FAU沸石膜。在相同水热合成条件下,载体表面用含PVA的晶种液预涂晶种后,可增加分子筛负载量。正辛烷在FeCrAl丝网负载FAU沸石膜上的初始裂解转化率(7.2%～13.2%,500 ℃)高于其在空白载体上的裂解转化率(4.4%),FAU沸石膜的初始催化活性不仅与载体表面分子筛负载量有关,也与分子筛膜的形貌有关。     

动态水热合成b轴取向MFI型分子筛膜

在旋转烘箱中采用动态水热法,在不锈钢片支撑体上合成了连续的b轴取向MFI型分子筛膜,并运用扫描电镜和X射线衍射考察了合成釜转速、晶化温度和晶化时间对所得分子筛膜性能的影响.与静态水热法相比,动态水热合成过程中合成液的温差和浓度差大大降低,且合成液持续冲刷支撑体表面,使得动态法具有合成时间短、取向性好和分子筛颗粒粒径分布均一的优点.     

蒸汽相中c轴取向连续完美单层B-AI-ZSM-5沸石膜的原位制备

MFI型沸石分子筛，由于其具有较宽合成条件、合适均的一孔径、可修饰和改 性的骨架和孔道系统，高的热稳定性、水热稳定性、化学稳定性和机械强度，不仅 广泛应用于传统的催化和分离领域，而且也广泛地应用于膜的制备。由于MFI型沸 石晶体体型和晶内孔道系统 的各向异性，控制晶体在载体表面的定向生长对于膜 分离、膜催化、膜传感等都极其重要的。文献中报道的c轴取向MFI型沸石膜层的制 备方法有：（1）晶体在强电场作用下在载体表面上进行定向排 列，然后在排列好 的晶体间喷镀或电镀一层金属膜；（2）在有刻蚀槽的微结构硅片表面进行组装； （3）水热合成法；（4）晶种层的二次生长法等。     

蒸汽相中c轴取向连续完美单层B-AI-ZSM-5沸石膜的原位制备

MFI型沸石分子筛，由于其具有较宽合成条件、合适均的一孔径、可修饰和改 性的骨架和孔道系统，高的热稳定性、水热稳定性、化学稳定性和机械强度，不仅 广泛应用于传统的催化和分离领域，而且也广泛地应用于膜的制备。由于MFI型沸 石晶体体型和晶内孔道系统 的各向异性，控制晶体在载体表面的定向生长对于膜 分离、膜催化、膜传感等都极其重要的。文献中报道的c轴取向MFI型沸石膜层的制 备方法有：（1）晶体在强电场作用下在载体表面上进行定向排 列，然后在排列好 的晶体间喷镀或电镀一层金属膜；（2）在有刻蚀槽的微结构硅片表面进行组装； （3）水热合成法；（4）晶种层的二次生长法等。     

高度b取向Silicalite-1分子筛膜的制备

以正硅酸乙酯(TEOS)为硅源, 四丙基氢氧化铵(TPAOH)为模板剂, 通过调变合成silicalite-1分子筛溶胶组成及反应时间等合成参数, 用原位水热晶化法在玻璃基底上制备了表面平整、连续性好且高度b取向的silicalite-1分子筛膜. 研究了溶胶组成及基底表面粗糙度与分子筛膜中晶体取向的关系, 讨论了晶化条件对分子筛膜厚度及膜中微晶尺寸及分布的影响, 实现了silicalite-1 沸石分子筛在基底表面的高度取向生长和膜中晶体大小及膜层微结构的调控. 本文描述的制备方法简单且重复性好. 此外, 还利用扫描电子显微镜(SEM)和X射线衍射(XRD)等分析方法对样品进行了表征.     

用亚微米级晶种涂层法合成NaA沸石膜及其结构表征

 在大孔α-Al2O3陶瓷管载体上,采用亚微米级晶种涂层法在澄清溶液体系中二次生长成膜,制备了NaA沸石膜. 采用TEM,SEM和XRD等手段对晶种、陶瓷载体及沸石膜的结构、晶体形貌和成膜情况进行了表征. 结果表明,合成晶种的晶粒呈立方体,颗粒小(约150 nm)而均匀,无杂晶,可作为晶种在载体上进行预涂. 载体的孔径大而不均匀,表面粗糙不平整,直接成膜则表面仅有较少的沸石晶粒沉积,不能连续成膜. 经亚微米级晶种涂层后,载体表面形成了一层均匀、光滑的晶种层(厚度为2～3 μm); 水热晶化成膜后,膜表面晶粒相互交织生长完好,无晶间隙,所得膜致密,连续,规整,清晰,无裂缺. 该法是一种制备沸石膜的好方法.     

应用于热控的壳厚可控核壳结构SiO2@TiO2颗粒制备

利用St觟ber方法合成了平均粒径在800 nm,球形度、单分散性良好的SiO2微球,再将其作为制备核壳结构SiO2@TiO2颗粒的内核。利用钛酸四丁酯水解反应,在SiO2内核上包覆制备了壳厚在30～100 nm的TiO2壳层,TiO2壳层厚度可根据水解反应中钛酸四丁酯的量调控。将制得的SiO2@TiO2核壳结构颗粒在550℃煅烧1 h,氧化钛壳层的晶型转变为锐钛矿相,晶型转变为锐钛矿相的TiO2更适合作为填料应用于近红外反射涂层。本文合成厚度可控SiO2@TiO2微球的方法是一种改进的溶胶凝胶方法,即在溶胶凝胶方法的基础上增加水热合成工艺。另外,本合成方法工艺简单,无表面活性剂或者耦合剂的引入。     

勃姆石纤维增强二氧化硅气凝胶的制备及性能

以六水合氯化铝为铝源, 通过水热法制备勃姆石纤维; 以甲基三甲氧基硅烷和正硅酸乙酯为硅源共前驱体, 采用溶胶-凝胶法进而常压干燥制备了勃姆石纤维掺杂的二氧化硅复合气凝胶; 探究了勃姆石纤维的掺杂量对复合气凝胶性能的影响. 当勃姆石纤维的掺杂量(质量分数)为1%时, 气凝胶的机械性能最好, 能够承受17.1%的压缩应变, 最大压缩强度为1.12 MPa, 压缩模量高达2.57 MPa, 复合气凝胶在150 ℃仍然具有较低的导热系数(0.0670 W·m^?1·K^?1). 勃姆石纤维能够一定程度地抑制二氧化硅颗粒在高温下的烧结和相转变, 对二氧化硅气凝胶的耐高温性能有显著的提升作用, 复合气凝胶在1100 ℃高温热处理后, 仍能保持良好的隔热性能和较高的机械强度.     

Preparation and characterization of silicalite-1 membranes prepared by secondary growth of seeds with different crystal sizes

MFI-type zeolite particles of 0.1–1 μm in diameter were prepared by adjusting tetra-n-propylammonium hydroxide (TPAOH) and water contents in synthesis mixtures. Using those particles as seeds, MFI-type zeolite membranes were prepared on the surface of a porous mullite tube by secondary growth. The membranes were formed as polycrystalline zeolite layers on and inside the porous support, and the membranes were composed of the [h 0 h]-oriented crystallites. The membrane consisting of a-oriented crystallites could be also prepared. However, the a-oriented zeolite layers were not active on the permeation properties of butanes. Rather the size and loaded amount of the seed particles influenced on the permeation properties through the membranes. As a result, the n-C₄H₁₀/i-C₄H₁₀ permselectivity could be increased to 220 by adjusting the size and the loaded amount of particles. These results suggest that the number of loaded particles affects on the permeation properties through the membranes.     

Molecular sieving MFI-type zeolite membranes for pervaporation separation of xylene isomers

Molecular sieving MFI-type zeolite membranes were prepared by a secondary growth method without using an organic template. Silicalite membranes with intercrystalline pores minimized or eliminated were obtained by this synthesis method which avoids the template removal step. The silicalite membrane exhibits molecular sieving characteristics with pervaporation separation factor for p-xylene to o-xylene or m-xylene of as high as about 70, the highest ever reported for a pervaporation membrane.     

The preparation and analysis of zeolite ZSM-5 membranes on porous alumina supports

Zeolite ZSM-5 particles formed in the synthesis solution assume a negative charge due to electrical double layer effects. Therefore, we investigated the use of electrophoretic techniques in addition to the hydrothermal synthesis method to attract the zeolite particles to the substrate surface before they precipitate out of the solution. This electrophoretic driving force produces a thin, continuous zeolite ZSM-5 membrane on the porous substrate. This research was conducted in order to produce zeolite membranes on a tubular support that could effect continuous gas phase separations. The thickness of membranes can be controlled by varying the precursor concentration, applied potential, and synthesis time. The selectivity of n-butane over iso-butane is observed on every membrane. The membranes were characterized by using scanning electron microscopy (SEM), X-ray diffractometry (XRD), and energy dispersive X-ray spectrometry (EDX).     

Coating of Sol Particles for Membrane Applications

For improving chemical and thermal stability of γ-Al₂O₃ membranes boehmite (AlOOH) sol-particles are coated with Zr⁴⁺-species with two techniques. These techniques are heterogeneous precipitation (HP-method) and “surface-reaction-followed-by-polycondensation” (SRPC-method). A continuous coating layer is formed at relative low Zr⁴⁺-concentration, about one monolayer coverage of boehmite particles, and with the HP-method. For large concentrations and for the SRPC-method small particles (order 1 nm) of Zr⁴⁺-species are formed on the surface of the boehmite particles. After drying and calcination up to 1000°C no continuous layer of a zirconia phase could be detected for all samples. However the thermal stability of the porous structure is improved. Phase transitions of alumina occur at temperatures of 1100°C and the porous structure of the membrane material is then destabilized.     

Effects of synthesis parameters on zeolite membrane formation and performance by microwave technique

Recently, zeolite membranes on porous supports have been extensively studied in the ethanol–water separation process for further use for gasohol production. This work focuses on a NaA membrane synthesized on an α‐Al₂O₃ support via microwave hydrothermal treatment. Synthesis temperature and time, type of substrate, seed amount and seeding time for the layer growth of the membrane are considered. The formation of as‐synthesized membranes is discussed according to observations by SEM and XRD. In addition, a preliminary study of the performance of the synthesized NaA zeolite membrane was conducted using the pervaporation technique. It was found that, for the synthesized continuous NaA membranes prepared using a 0.5 µm NaA crystal seed concentration of 3 g/l via vacuum seeding, the optimum conditions were 363 K synthesis temperature for 15–20 min via microwave heating. The flux and the separation factor obtained were 1.6 kg/m² h and 1760.5, respectively, for the substrate without an intermediate layer. Interestingly, the substrate with an intermediate layer showed better flux and separation factor at 1.7 kg/m² h and 6532.7, respectively. Copyright © 2007 John Wiley & Sons, Ltd.     

Two-Dimensional MFI-Type Zeolite Flow Battery Membranes

Vanadium flow battery (VFB) is one of the most reliable stationary electrochemical energy-storage technologies, and a membrane with high vanadium resistance and proton conductivity is essential for manufacturing high-performance VFBs. In this study, a two-dimensional (2D) MFI-type zeolite membrane was fabricated from zeolite nanosheet modules, which displayed excellent vanadium resistance (0.07 mmol L⁻¹ h⁻¹) and proton conductivity (0.16 S cm⁻¹), yielding a coulombic efficiency of 93.9 %, a voltage efficiency of 87.6 %, and an energy efficiency of 82.3 % at 40 mA cm⁻². The self-discharge period of a VFB equipped with 2D MFI-type zeolite membrane increased up to 116.2 h, which was significantly longer than that of the commercial perfluorinated sulfonate membrane (45.9 h). Furthermore, the corresponding battery performance remained stable over 1000 cycles (>1500 h) at 80 mA cm⁻². These findings demonstrate that 2D MFI-type membranes are promising ion-conductive membranes applicable for stationary electrochemical energy-storage devices.     

Expanding the Synthesis Field of High-Silica Zeolites

Aluminosilicate zeolites are synthesized under hydrothermal conditions in a basic/alkaline medium in the pH range between 9 and 14. The synthesis of MFI-type zeolite in an acidic medium is presented. The critical parameter determining the zeolite formation in an acidic medium was found to be the isoelectric point (IEP) of gel particles. MFI-type zeolite was synthesized above the isoelectric point of the employed silica source, where the silica species exhibit a negative charge and the paradigm of zeolite formation based on the electrostatic interaction with the positively charged template is retained. No zeolite formation is observed below the isoelectric point of silica. The impact of aluminum on the zeolite formation is also studied. The results of this study will serve to extend the synthesis field of high silica zeolites to the acidic medium and thus open new opportunities to control the zeolite properties.     

Improving the Gas-Separation Properties of PVAc-Zeolite 4A Mixed-Matrix Membranes through Nano-Sizing and Silanation of the Zeolite

Mixed-matrix membranes containing synthesised nano-sized zeolite 4A and PVAc were fabricated to investigate the effect of zeolite loading on membrane morphology, polymer-filler interaction, thermal stability and gas separation properties. SEM studies revealed that, although the membranes with 40 wt % nano-sized zeolite particles were distributed uniformly through the polymer matrix without voids, the membranes with 15 wt % zeolite loading showed agglomeration. With increasing zeolite content, the thermal stability improved, the permeability decreased and the selectivity increased. The effect of silanation on dispersion of 15 wt % zeolite 4A nanoparticles through PVAc was investigated by post-synthesis modification of the zeolite with 3-Aminopropyl(diethoxy)methylsilane. Modification of the nanoparticles improved their dispersion in PVAc, resulting in higher thermal stability than the corresponding unmodified zeolite membrane. Modification also decreased the rigidity of the membrane. Partial pore blockage of the modified zeolite nanoparticles after silanation caused a further decrease in permeability, compared to the 15 wt % unmodified zeolite membrane.     

二次生长法制备高分离性能的PHI分子筛膜

通过二次生长法在α-Al2O3支撑体表面合成了PHI分子筛膜,考察了晶种合成方式、二次生长合成温度及时间对形成PHI分子筛膜的影响.采用X射线衍射(XRD)、扫描电子显微镜(SEM)对合成膜进行表征.结果表明:载体表面合成出了PHI分子筛;二次生长法合成出的PHI分子筛膜连续、致密,膜厚约为20 μm.利用渗透汽化技术对甲醇、乙醇、异丙醇和叔丁醇等不同分子尺寸的醇/水体系进行分离性能的研究,同时考察原料液中水含量对所制备的PHI分子筛膜的分离性能的影响.结果表明:PHI分子筛膜对几种醇水体系都具有良好的分离效果,随着水含量的增加,水的渗透通量呈增大趋势,乙醇和甲醇的理想分离因子有所降低,异丙醇和叔丁醇的理想分离因子增大.