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。     

短b轴ZSM-5分子筛制备方法及应用研究进展

ZSM-5分子筛作为应用最为广泛的催化剂之一,一直是研究关注的重点。由于其沿着b轴直通孔道相比于沿着a轴和c轴的Z形孔道具有更好的扩散性能,其长度的控制对改善分子筛的催化特性具有重要作用。综述了控制ZSM-5分子筛各向生长的主要方法,结构导向法和生长修饰法。对短b轴ZSM-5分子筛在MTP、MTH以及MTG等催化领域的应用进行了具体分析。并指出,特定季铵盐作为结构导向剂制备的片层分子筛,具有均匀良好的自柱撑结构和晶间介孔,由于结构导向剂制备难度较高,采用生长修饰剂部分取代季铵盐更加经济环保。特定生长修饰剂合成难度低且成本低廉,制备的分子筛具有分散或团聚片层结构,新工艺的开发以改善晶间介孔将使其具有更好的扩散性。对片层分子筛厚度和酸性的精准调控、合成机理以及各向长度对催化性能影响的深入研究,以保证高效制备兼顾转化率、选择性以及稳定性的分子筛对短b轴分子筛的发展具有重要意义。     

MFI型沸石分子筛膜的研究进展及应用

综述了MFI型沸石分子筛膜制备方法的研究进展,突出介绍了较为成熟的水热合成法、微波合成法以及气相转移法,讨论了MFI型沸石分子筛膜在应用方面具有优势的有机物提纯、气体分离以及催化反应的研究进展,并提出了MFI型沸石分子筛膜制备和应用方面一些亟待解决的问题。     

MFI型分子筛膜的修复

采用二次生长法制备完备的MFI型分子筛膜,在高温脱除模板剂后测试其CO2/N2分离性能,分离因子低,说明膜存在较大的缺陷。采用纳米SiO2对分子筛膜进行修复,SiO2修复后的分子筛膜对CO2/N2分离因子由1.6提高到5.0,且修复后的分子筛膜的性能很稳定。此外,该修复方法具有良好的重复性,用于多个具有低分离因子的膜都得到了比较好的效果。     

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。     

具有b取向壳层MFI/MFI核壳分子筛的合成与表征

为了研究MFI/MFI核壳分子筛壳层b取向的形成机理,以圆柱形ZSM-5为核相,用碱性溶液对其表面预处理后,水热晶化合成具有高硅b取向壳层的MFI/MFI核壳分子筛,采用SEM、XRD、TEM、XPS和NMR等表征核相表面与晶化产物的结构。结果表明,核壳分子筛的（020）晶面衍射峰强度明显增强,壳层覆盖度为84％。核相碱处理是壳层形成的关键,未处理核相的表面上壳层不能生长,碱处理后在核相上形成的表面非骨架铝对高硅壳层颗粒的形貌和取向影响较大。在低铝含量条件下成核方式可能从非均相成核转变为均相成核,溶液中形成片状晶体,后经表面Si-OH强作用自组装得到二维紧密堆积的b取向壳层。     

MFI型分子筛膜中晶体间隙的在线修补及其二甲苯分离性能研究

为了提高MFI分子筛膜的二甲苯分离性能,采用水热合成法制备了管式MFI型分子筛膜,并通过1,3,5三异丙基苯(TIPB)碳化沉积方法对分子筛膜中的晶体间隙在分离操作过程中进行了在线修补。通过修补,分子筛膜在375~400℃下的对、邻二甲苯分离系数从5左右增加到30,表明膜层中的无选择性的晶体间隙被大幅度减少。     

分子筛膜的制备和研究进展

本文综述了分子筛膜的各种制备方法,如原位水热合成法、晶种法、微波加热法、化学气相沉积法、脉冲激光蒸镀法等,介绍了分子筛膜在物质分离和膜催化反应等方面的应用,同时讨论了在分子筛膜制备过程中膜缺陷的形成及其消除方法,最后提出了分子筛膜的研究展望。     

多孔玻璃表面原位水热合成b轴取向MFI型分子筛膜

采用原位水热合成法,直接用川丙基氢氧化馆(TPAOH)/川乙氧基硅炕(TEOS)/H20合成液在具有较大孔径(约O.I/-lm)的多孔玻璃载体表面合成b轴取向MFI型分了饰膜。考察了载体的放置方式对成膜的影响,结果表明,将多孔玻璃完伞浸入合成液中时,载休表囱片能得到零星分市的MFI细分子筛品休;而将多孔玻璃部分浸入合成液中时,在液面以上的载体表面获得了b轴取向MFI型分子饰膜,靠近液面的载体表由形成的MFI分子筛膜比较致密。优化了合成液的配比和合成时间,发现采用合成i夜配比0.64TPAOH:lTEOS:165H20时165oc合成3h,可以在多孔玻璃表面形成大面积连续的b轴取向MFI型分了筛膜。当合成时间延长至6h时,MFI剖分子筛膜层连续数密,但呈现随机取向。     

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.     

Enhanced compactness of oriented MFI zeolite films by heat treatment of seed layer

The achievement of compact and defect-free film structure is crucial for the application of b-oriented MFI zeolite film. In this work, a novel heat treatment technique was used to treat zeolite seeded substrates prior to secondary growth. The influence of the heat treatment parameters such as treat temperature and time on the final film morphology were systematically investigated. The relationship between film compactness and the parameters was established. Under the optimized treat temperature of 120?°C and treat time of 1?h, compact and uniform b-oriented MFI zeolite film was achieved. The applicability of the optimum heat treatment condition was validated by employing various film substrates.     

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     

Effects of water vapor on gas permeation and separation properties of MFI zeolite membranes at high temperatures

Understanding the effects of water vapor on gas permeation and separation properties of MFI zeolite membranes, especially at high temperatures, is important to the applications of these zeolite membranes for chemical reactions and separation involving water vapor. The effects of water vapor on H₂ and CO₂ permeation and separation properties of ZSM‐5 (Si/Al ～ 80) zeolite and aluminum‐free silicalite membranes were studied by comparing permeation properties of H₂ and CO₂ with the feed of equimolar H₂/CO₂ binary and H₂/CO₂/H₂O ternary mixtures in 300–550°C. For both membranes, the presence of water vapor lowers H₂ and CO₂ permeance to the same extent, resulting in negligible effect on the H₂/CO₂ separation factor. The suppression effect of water vapor on H₂ and CO₂ permeation is larger for the less hydrophobic ZSM‐5 zeolite membrane than for the hydrophobic silicalite membrane, and, for both membranes, is stronger at lower temperatures and higher water vapor partial pressures. © 2011 American Institute of Chemical Engineers AIChE J, 2012     

On the effect of morphological features on the properties of MFI zeolite membranes

MFI type zeolite membranes were prepared on alumina and stainless steel tubular supports by different synthesis procedures, giving rise to different zeolite layer structures and distributions of zeolite material with respect to the support. This was used as a base to establish a classification of zeolite membranes. SEM and EPMA analysis showed clear differences among different types of MFI zeolite membranes, concerning the morphology and location of the zeolite deposits. Three types of membranes were identified, namely: type-A membranes, in which the zeolitic material was located mainly inside the support pores; type-B membranes, with a thin layer of randomly oriented crystals on top of the support; and type-C corresponding to MFI c-oriented membranes. These morphological differences translated into diverse qualitative and quantitative behavior of the corresponding membranes, as shown by the evolution of single-gas permeances and separation selectivity with temperature, and by adsorption and temperature-programmed desorption experiments.     

2,2-Dimethylbutane adsorption and diffusion in MFI zeolite

Adsorption rates of 2,2-dimethylbutane (DMB) from the liquid phase into MFI zeolite crystals were measured by a volumetric/gravimetric method. Desorption rates in the presence of liquid n-hexane were measured by a non-adsorbing solvent (isooctane) method. Dimethylbutane desorbed, and was replaced by n-hexane, approximately three orders of magnitude faster than it adsorbed. Adsorption was slow and reached only 40% of saturation loading in 6-μm crystals after 45 days at 295 K, but this DMB loading desorbed into n-hexane in less than 1 h. Desorption may be faster due to slight expansion of the MFI unit cell by n-hexane adsorption. Although DMB diffused slower than n-hexane in MFI crystals, its transient time through a silicalite-1 membrane was an order of magnitude shorter than the n-hexane time at 313 K because DMB diffused through defects, whereas n-hexane diffused mainly through zeolite pores. The n-hexane transient time was longer because n-hexane: (1) expanded the MFI crystal size slightly and shrank non-zeolitic pores, and (2) adsorbed in the zeolite as it moved through the remaining non-zeolitic pores. A silicalite-1 membrane saturated with DMB at 423 K was sealed so effectively, and DMB desorbed slowly enough, that the helium permeation flux at 313 K was below the detection limit.     

On-stream modification of MFI zeolite membranes for enhancing hydrogen separation at high temperature

-Alumina-supported MFI zeolite membranes were modified by on-stream catalytic thermal cracking of methyldiethoxysilane (MDES) molecules inside the zeolitic channels during the separation of H₂/CO₂ gas mixture at 450 °C and atmospheric pressure. The MDES vapor was carried by the H₂/CO₂ feed gas and the effect of modification was monitored continuously through online analysis of the permeate stream. The modified membrane exhibited a significant increase in H₂ selectivity over CO₂ with a moderate decrease in H₂ permeance. At 450 °C, the modified MFI membrane obtained a H₂/CO₂ permselectivity of 17.5 with H₂ single gas permeance of 1.86 × 10⁻⁷ mol m⁻² s⁻¹ Pa⁻¹ as compared to a permselectivity of 2.78 and permeance of 2.75 × 10⁻⁷ mol m⁻² s⁻¹ Pa⁻¹ for the membrane before modification. The modified membrane also showed good performance and stability in separation of H₂/CO₂ gas mixture containing up to 28.4% water vapor at 450 °C and atmospheric pressure.     

Pure-silica MFI zeolite membrane by cooperative templating approach for ethanol-water separation

Pure-silica MFI zeolite membrane composed of intergrown four prism crystals was fabricated by using tetrabutylammonium (TBA⁺) and tetraethylammonium (TEA⁺) as cooperating templates and the induced function of MFI seeds under synthesis solution with molar composition of 1TEOS:0.2TBAOH:0.2TEABr:180H₂O. The ethanol/water separation factor was 64 with a flux of 1.40 kg m⁻² hr⁻¹ for 5 wt% ethanol/water mixture at 60°C. In contrast, single TBAOH and TEAOH template led to formation of uncontinuous MEL and MFI membrane respectively, without ethanol-selectivity. The behavior of quaternary ammonium (TBA⁺ and TEA⁺) varied in different system: (a) In “TBA⁺+ TEA⁺” system, TBA⁺ and TEA⁺ were incorporated into MFI zeolite channel simultaneously; (b) In “TPA⁺+ M⁺” system (M = TEA or TBA), either TEA⁺ or TBA⁺ could not be occluded into channel, however, appropriate amount of TBA⁺ could accelerate membrane crystallization, meanwhile increase in TEA⁺ slowed down crystallization rate of MFI membrane. This work extends the alternative preparation approaches of ethanol perm-selective MFI membrane.     

Fabrication of titanosilicate pillared MFI zeolites with tailored catalytic activity

Titanosilicate pillared MFI zeolite nanosheets were successfully synthesized by infiltrating the mixed tetraethyl orthosilicate (TEOS)/tetrabutyl orthotitanate (TBOT) solvent into the gallery space between adjacent MFI zeolite layers. The obtained zeolite catalysts were characterized using powder X-ray diffraction, N₂ adsorption/desorption isotherms, scanning electron microscopy, transmission electron microscopy, ultraviolet–visible spectroscopy, X-ray photoelectron spectroscopy, and Fourier-transform infrared spectroscopy techniques. The H₂O₂ oxidation of dibenzothiophene (DBT) was used to evaluate the catalytic performance of the obtained titanosilicate pillared MFI zeolites. The conversion of DBT and selectivity of dibenzothiophene sulfone (DBTS) were most affected by the textural properties of the zeolites. This was attributed to the DBT and DBTS molecules being larger than micropores of the MFI zeolites. The conversion of DBT and yield of DBTS could be systematically tailored by tuning the molar ratio of the TEOS/TBOT solvent. These results implied that a balance between the meso- and microporosity of zeolites and tetrahedrally coordinated Ti(IV) active sites of titanosilicate pillars can be achieved for the preparation of desired catalysts during the oxidation of bulk S compounds.