Shape controlled zeolite EU-1 (EUO) catalysts: Dry gel conversion type synthesis, characterization and formation mechanisms

Shape controlled zeolite EU-1 (EUO) catalysts were synthesized by a dry gel conversion (DGC) type synthesis. Silica extrudates as starting material were impregnated to the incipient wetness point and subjected to a hydrothermal treatment under water vapor. All-zeolite extrudates with the same size and shape as initial silica extrudates were obtained. Formation mechanisms were investigated by varying the initial extrudate water content. Hydrothermal treatment was carried out in the presence or in the absence of an external water source. Extrudate H₂O/SiO₂ molar ratios of at least 1.5 were required to obtain zeolite extrudates with high crystallinity. In the presence of an external water source, a non-uniform progression of zeolite crystallization from the outer to the inner extrudate surface was observed. On the contrary in the absence of an external water source, first crystals appeared throughout the extrudates. For both methods, mesopores of silica extrudates disappeared during impregnation processes and micro- and macropores were formed during crystallization. In a m-xylene isomerization reaction shape controlled zeolite EU-1 extrudates display an enhanced p-xylene selectivity.     

Synthesis of mesoporous MFI zeolite by dry gel conversion with ZnO particles and the catalytic activity on TMB cracking

A facile synthesis method for mesoporous MFI zeolite (MMZ) has been developed. MFI zeolite was synthesized by a dry gel conversion in the presence of ZnO nanoparticles with a size of 20 nm. The as-synthesized MFI zeolite included crystalline layered zinc silicate and already possessed 5–15 nm mesopores. After calcination, MMZ/zinc silicate composite was treated with hydrochloric acid to remove unreacted ZnO particles. The micropore (1–2 nm) volume was increased after the HCl treatment, suggesting that ZnO nanoparticles (1–2 nm) remained during crystallization as well as zinc silicate. The catalytic activity of MMZ on 1,3,5-trimethylbenzene (TMB) cracking was compared with that of conventional MFI nanocrystals with a size of 80–100 nm. The conversion of TMB on MMZ was much higher than that on the MFI nanocrystals even though crystal size of MMZ is larger than the conventional MFI zeolite. These results suggest that acid sites on the internal surface of mesopores of MMZ contribute to the high conversion of TMB.     

Hydrothermal synthesis of giant single crystals of MFI type zeolite: Modified bulk material dissolution method

In the present study, we aimed at the development of an easy and reproducible synthesis method of giant MFI zeolite crystals. We used a new synthesis method, modified bulk material dissolution (M-BMD) method, in which a piece of quartz glass tube was inserted as a silica source in a small PTFE sleeve together with ZSM-5 powder as seed crystals. We adopted a higher HF concentration (25 mmol/25 ml) of the synthesis solution than that usually used. By using this method, only a few crystal nuclei were formed separately from each other on the surface of the quartz glass tube in the PTFE sleeve and giant zeolite crystals were reproducibly synthesized on it. Giant single crystals of MFI as large as 5.0 mm were successfully synthesized at 453 K for 90 days by using the M-BMD method.     

Seed-induced synthesis of functional MFI zeolite materials: Method development,crystallization mechanisms,and catalytic properties

As an important zeolite material, MFI zeolites, as well as their controllable synthesis, are of great interest in both basic and applied science. Among the developed synthetic approaches, the seed-induced method has gradually evolved into a facile, low-cost, and even green alternative to give zeolites the desirable physicochemical properties. In this review, we briefly summarize the development of seed-induced syntheses of diverse functional MFI zeolites, where the “living” seed crystals not only direct the formation of zeolitic framework but also function as special “templates” or “units” to fine-tune the zeolite materials with diverse sizes, shapes, compositions, morphologies and pore structures. Moreover, on the basis of their structural features and crystallization behaviors in seed-induced synthesis, we reveal the roles of seeds and discuss the related crystallization mechanisms including both classical and non-classical pathways. We also want to guide readers to investigate the structure-performance relationships between these functional MFI zeolite catalysts and suitable catalytic reactions.     

无模板剂二次生长法合成MFI分子筛膜的生长机理

Based on two template-free secondary growth methods, the formation mechanism of single-layer and multi-layer zeolite membranes was investigated. The composition of the synthesis solution did not affect the membrane structure, whereas the position-setting of the seeded support in the synthesis solution and the ageing process of the synthesis solution were two critical factors in the formation of the membrane. The synthesis solution with lower viscosity could easily penetrate into the gap between seeds boundaries, resulting in the formation of multi-layer membranes.     

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。     

Effect of mesoporosity against the deactivation of MFI zeolite catalyst during the methanol-to-hydrocarbon conversion process

The effects of mesoporosity on catalyst longevity of methanol-to-hydrocarbon (MTH) reactions have been investigated using a number of MFI zeolites with different degrees of mesoporosity, which were obtained via the post-synthetic desilication, dry-gel conversion in nanocarbon templates, and hydrothermal synthesis with the addition of organosilane surfactants. The MTH catalytic lifetime could be increased by more than three times due to the generation of mesopores. The cause for catalyst longevity was investigated by probing the location of coke that formed during the reaction using argon adsorption measurements. The result showed that the coke formed mainly on mesopore walls in the case of mesoporous zeolite. On the other hand, the coke was more heavily deposited inside micropores in the case of solely microporous zeolites. The short diffusion path lengths and hence facile diffusion of coke precursors were most likely the cause of the improvement in the catalytic lifetime.     

微波技术在沸石分子筛材料合成中的应用研究进展

综述微波技术在多种常规沸石分子筛、纳米沸石分子筛以及分子筛膜合成过程中的研究进展。微波较常规水热合成能够大大缩短反应时间,有利于生成晶粒尺寸更小的晶体。对国内外开展微波合成沸石分子筛材料的反应设备进行简述,结合作者前期开展的微波合成β分子筛试验,分析并讨论微波和常规水热合成之间的主要区别以及反应机理,对微波加热技术未来应用于实际工业生产进行展望。     

Synthesis of SUZ-4 zeolite by a dry gel conversion method

SUZ-4 zeolite was synthesized by the dry gel conversion (DGC) process with water vapor as gas phase, and characterized by XRD, SEM and N₂ adsorption. The dry gel was prepared with the assistance of a small amount of crystalline seed and organic template tetraethylammonium hydroxide (TEAOH). Molar ratios of SiO₂/Al₂O₃, KOH/SiO₂, TEAOH/SiO₂ and H₂O/SiO₂, amounts of seed and dry gel, types of silica sources, and crystallization temperature and time, were changed to optimize synthesis conditions. The results show that the DGC method leads to formation of SUZ-4 zeolite in a broad range of crystallization temperature 120–180 °C. Under the optimal conditions, i.e., SiO₂/Al₂O₃ = 22.5, KOH/SiO₂ = 0.44, TEAOH/SiO₂ = 0.044, H₂O/SiO₂ = 22.2, seed amount = 0.1 wt%, fumed silica as the silica source, 160 °C and 5 days, SUZ-4 zeolite is obtained with a high crystallinity. Compared to hydrothermal synthesis, the present DGC approach employs far less amount of organic template and allows using inert fumed silica as silica source, producing smaller rod-like SUZ-4 zeolite crystals.     

预成型高岭土干胶转化合成NaY型分子筛

通过干胶转化法制备了整体式NaY分子筛,采用XRD、SEM及XRF分析表征原料及产物,系统考察了合成体系的n（SiO₂）/n（Al₂O₃）、n（Na₂O）/n（SiO₂）、水量、晶化温度和晶化时间对整体式NaY分子筛制备的影响。结果表明,合成体系n（SiO₂）/n（Al₂O₃）=7.5时,骨架硅铝比（二氧化硅与氧化铝物质的量比）最大为6.12;n（Na₂O）/n（SiO₂）逐渐增大,整体式NaY分子筛结晶度逐渐升高,当n（Na₂O）/n（SiO₂）增至0.35时,会导致P型分子筛的生成;晶化温度从90 ℃逐渐增至120 ℃时,整体式NaY分子筛结晶度也随之升高;在110 ℃下晶化20 h,产物的结晶度达到98%并趋于稳定。干胶转化制备整体式NaY分子筛必须有水的参与,n（H₂O）/n（SiO₂）为4.2左右对反应物的成型及整体式NaY分子筛的晶化较为适宜。     

One‐pot synthesis of binderless zeolite A spheres via in situ hydrothermal conversion of silica gel precursors

One‐pot synthesis route was explored for preparing binderless zeolite A via an in situ hydrothermal transformation process involving the preformation of silica gel precursors. Synthesis process was optimized and resulting samples were characterized using XRD, Raman spectroscopy, EDS, ²⁹Si and ²⁷Al NMR spectroscopy, SEM, HRTEM, and pore structure analyses. Furthermore, the kinetics for the in situ synthesis was investigated and the underlying crystallization mechanism was interpreted. The transformation of silica precursors was accomplished via controlling the synthesis parameters which govern the crystallization through determining the dissolution rate of silica nanoparticles, diffusion of Al species into silica precursors and crystal nuclei growth. Subsequently, Al species could contact and react with released Si species from silica nanoparticles to build the structural units that finally construct the LTA framework through self‐organized arrangement. On pure zeolite A phase basis, the synthesized binderless sample exhibits higher specific surface area and n‐paraffins adsorption capacity than binder‐containing zeolite. © 2018 American Institute of Chemical Engineers AIChE J, 64: 4027–4038, 2018     

Methanol conversion to lower olefins over RHO type zeolite

Eight-membered ring small-pore zeolite of RHO-type topology has been synthesized, characterized and tested for methanol-to-olefin (MTO) reaction. The zeolite was hydrothermally crystallized from the gel with Si/Al ratio of 5.0. It showed a high BET specific surface area (812 m² g^–1), micropore volume (0.429 cm³ g^–1), and acid amount (2.53 mmol g^–1). Scanning electron microscopy observations showed small crystallites of about 1 μm. The zeolite was active for MTO reaction with 100% methanol conversions at 623–723 K, whereas selectivity to lower olefins changed with time.     

多级孔MFI分子筛合成及催化性能研究

采用水热合成法,以四丙基氢氧化铵为微孔模板剂、十六烷基三甲基溴化铵为介孔结构导向剂合成了多级孔ZSM-5分子筛,又进一步对合成出的ZSM-5分子筛进行后处理改性。结果表明,经过后处理的多级孔ZSM-5分子筛使芳烃收率进一步提高,其中负载Zn的多级孔ZSM-5分子筛的催化性能最好。     

多级孔MFI分子筛合成及催化性能研究

采用水热合成法,以四丙基氢氧化铵为微孔模板剂、十六烷基三甲基溴化铵为介孔结构导向剂合成了多级孔ZSM-5分子筛,又进一步对合成出的ZSM-5分子筛进行后处理改性。结果表明,经过后处理的多级孔ZSM-5分子筛使芳烃收率进一步提高,其中负载Zn的多级孔ZSM-5分子筛的催化性能最好。     

Synthesis of hierarchical MFI zeolite microspheres with stacking nanocrystals

MFI nanozeolite microspheres with intercrystal mesopores were synthesized by a steam-assisted crystallization–aggregation method, starting with close packed amorphous SiO₂ or Al-containing SiO₂ colloidal nanoparticles. The obtained samples with different Si/Al ratios were characterized by X-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy (SEM)/ transmission electron microscopy (TEM) observations, nitrogen adsorption analyses and ²⁷Al magic angle spinning nuclear magnetic resonance (NMR). The SEM images clearly showed that the mean size of nanozeolite microspheres was ca. 0.5–3 μm, agglomerated from zeolite nanocrystals and varied with different Si/Al ratios. N₂ sorption analyses as well as TEM micrographs indicated the dual-porosity of samples, one is from intracrystal micropores, and the other is from intercrystal mesopores. ²⁷Al MAS-NMR suggested nearly all of the Al atoms were located at tetrahedral-coordinated sites in the framework. Furthermore, nanozeolite microspheres with various Si/Al ratios, even Si/Al equal to five, could be prepared by this facile route and less structure-directing agent was needed compared to conventional approach. This simple method could be easily extended to prepare other hierarchically structured zeolites.     

Aromatization activity of gallium containing MFI and TON zeolite catalysts in n-butane conversion: Effects of gallium and reaction conditions

A kinetic study of n-butane conversion over acidic and gallium (Ga) containing MFI and TON zeolites has revealed that Ga active sites create a new pathway for aromatics formation via dehydrogenation reaction steps. This pathway does not involve bulky bimolecular hydrogen transfer steps of the aromatization process over acidic zeolites, and, as a consequence, leads to considerably higher enhancement of the aromatization activity of the one-dimensional TON catalyst when compared to the three-dimensional MFI catalyst. This finding highlights fundamentally different spatial requirements for alkane aromatization over acidic and Ga containing zeolites and indicates that the zeolites with severe spatial constraints could become very selective catalysts for alkane aromatization after their modification with Ga. It is anticipated that these results will initiate the search for new, highly selective aromatization catalysts based on zeolites with different structures. The second important finding of this work is the evolution of the aromatization activity of GaH-TON and GaH-MFI catalysts during n-butane reaction that is likely associated with formation of catalytically active Ga⁺ ions. In our study, this process was completed in about 15 min, i.e. much faster then similar processes that were reported earlier in the literature for the GaH-MFI catalysts. To the best of our knowledge, no data on the evolution of the aromatization activity was reported up to day for the one-dimensional GaH-TON catalysts.     

Hydrothermal synthesis of an ITH-type germanosilicate zeolite in a non-concentrated gel system

A germanosilicate zeolite with the ITH framework type (designated JLG-18) has been hydrothermally synthesized. Compared with other ITH-type materials synthesized in concentrated systems, JLG-18 was the first one synthesized in a non-concentrated system. In addition, the organic structure-directing agent (OSDA) for the synthesis of JLG-18 was N,N,N′,N′-tetramethyl-1,6-hexanediamine (TMHDA), which was less expensive than those used for the synthesis of other ITH-type materials. The optimal molar ratio of the initial gel was 0.5 SiO₂: 0.5 GeO₂: 7 TMHDA: 1.4 HF: 42 H₂O. The effects of various synthetic parameters on the formation of final products, such as source materials, GeO₂:SiO₂ ratio, H₂O:(SiO₂ + GeO₂) ratio, TMHDA:(SiO₂ + GeO₂) ratio, crystallization temperature, and reaction time, were investigated. The as-synthesized samples were characterized by powder X-ray diffraction (XRD), scanning electron microscopy (SEM), inductively coupled plasma (ICP), elemental analysis, thermogravimetric analysis (TG), and N₂ adsorption. JLG-18 with a Si:Ge ratio of 2:1 exhibited excellent thermal stability, even after calcination at 700 °C in air. The microporous surface area of JLG-18 was 317.07 m²/g.     

Hydrothermal synthesis of gallosilicate (MFI) zeolite: Factors influencing crystallization kinetics and crystal size/growth

The kinetics of the hydrothermal synthesis of gallosilicate (MFI) zeolite (under autogeneous pressure at 180°C) from a gel containing a source of silica (Na-trisilicate or fumed silica), gallium nitrate, templating agent (TPA-Br or TPA-OH), deionized water and sulfuric acid have been thoroughly investigated in order to understand the influence of various synthesis parameters (viz. gel composition, source of silica, template, method of gel preparation and pH of gel) on the crystallization kinetics and crystal size/growth of the zeolite. The zeolite samples were characterized for their crystallinity by XRD and crystal size and morphology by SEM. The crystallization rate and crystal size of the zeolite were found to be strongly influenced by the above synthesis parameters. © 1998 SCI     

Hydrothermal conversion of FAU zeolite into aluminous MTN zeolite

The hydrothermal conversion of FAU zeolite into aluminous MTN zeolite is described here. In the presence of both benzyltrimethylammonium hydroxide (BTMAOH) and sodium chloride (NaCl) the highly crystalline and pure MTN zeolites with Si/Al ratios of 21-23 could be obtained from the hydrothermal conversion of FAU zeolite. Based on powder XRD refinement and ¹³C CP/MAS NMR spectra, BTMA⁺ ions were not present in cages of the obtained zeolites, but TMA⁺ ions existed instead. It means that BTMAOH underwent degradation during the conversion. Moreover, the effects of Si/Al ratio of starting FAU zeolite, synthesis parameters (BTMAOH/SiO₂ and H₂O/SiO₂ ratios) and the addition of alkali metal chlorides on the hydrothermal conversion of FAU zeolite into MTN zeolite are discussed. As compared to amorphous SiO₂/γ-Al₂O₃, which produced impurity, the hydrothermal conversion of FAU zeolite showed a fast crystallization rate and a high selectivity to MTN zeolite formation. These phenomena indicate that the assembly of locally ordered aluminosilicate species coming from the decomposition or dissolution of FAU zeolite should be taking part in the conversion process.