Synthesis of silicalite-1 by dry-gel conversion method: factors affecting its crystal size and morphology

Silicalite-1 crystals were synthesized from clear and diluted solutions by the dry-gel conversion method and the hydrothermal method was used as reference to do a comparative study. The effect of the structure-directing agent, silica source, aging time and crystallization time was investigated. Zeolite crystals, about 200–500 nm and 1 micron, were obtained by the liquid phase hydrothermal (LPH) and steam-assisted crystallization (SAC), called as dry-gel conversion method, respectively. The crystals were identified by XRD and SEM. The results show that the aging time and the water content are crucial in the synthesis of zeolites. Indeed, well formed zeolite crystals are obtained by the SAC technique using a lower amount of water than in the LPH method. However, the presence of the structure directing agent (SDA) into the dry gel determines the crystallization of silicalite-1.     

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.     

干胶转化法合成整体式SSZ-13分子筛

采用干胶转化法合成了整体式SSZ-13分子筛,先成型后晶化,系统地考察了物料配比及晶化条件对合成结果的影响,通过X射线衍射（XRD）、X射线荧光（XRF）、热重-差热分析仪（TG-DTA）、扫描电子显微镜（SEM）、氨气的程序升温吸附-脱附（NH₃-TPD）、傅里叶红外光谱仪（FI-IR）、液体核磁共振谱（NMR）对产物进行表征分析,并讨论了混合模板剂N,N,N-三甲基-1-金刚烷氢氧化铵和氯化胆碱在晶化过程中起的作用。结果表明,合成的整体式SSZ-13分子筛具有典型的介孔堆积孔、较大的比表面积,在空气中热稳定性良好。氯化胆碱加速了分子筛晶化,缩短了晶化时间,提高产物的结晶度,明显降低了N,N,N-三甲基-1-金刚烷氢氧化铵的使用量,起到了辅助结构导向的作用,且混合模板剂均填充产物孔道。     

Kinetic analysis of crystallization of zeolite beta synthesized by direct heating

To quantitatively investigate the initial crystallization of zeolite beta synthesized by direct heating, the extent of the reaction was precisely evaluated by X-ray diffraction measurements and Rietveld structural refinement, and a kinetic analysis of crystallization was performed using the Avrami-Erofe'ev equation. The activation energy for crystallization was lower than that for hydrothermal synthesis. Reaction and synthesis time curves revealed that the initial zeolite beta crystallization consisted of three stages. The first was an induction period with nucleation by the generation of building units and the formation of an initial coordinated structure. The second stage was crystal growth by a diffusion-controlled reaction, and the third stage involved slowing down of crystallization by the limitation of dehydrocondensation. These stages could be analyzed by calculation of the rate constant and Avrami exponent for each stage.     

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     

The role of alkali cations in zeolite synthesis from silicate solutions containing N,N,N-trimethyl-1-adamantammonium cations

Zeolite synthesis from aqueous N,N,N-trimethyl-1-adamantammonium (TMAA⁺)-alkali (Na⁺, K⁺, Rb⁺, and Cs⁺) silicate mixtures is studied using X-ray diffraction, elemental analysis, scanning electron microscopy, and²⁹Si magic angle spinning (MAS),¹H-¹³C cross-polarization (CP) MAS and¹H-²⁹Si CP MAS NMR spectroscopies. SSZ-24 forms in the presence of potassium cations, and SSZ-31 crystallizes in the presence of sodium cations. This is the first report of SSZ-31 synthesis from Na-TMAA silicate mixtures. Unknown silicates form in the presence of rubidium and cesium cations, whereas no crystalline material is observed in synthesis mixtures devoid of alkali cations. The alkali cations do not appear to serve as templates or void fillers during zeolite crystallization, nor do they stabilize soluble silicate anions which serve as building blocks during zeolite crystallization. Rather, the alkali cations appear to regulate the transformation of the amorphous synthesis gel into either crystalline zeolite or other silicate phases.     

ZSM-35沸石原位固相转化法合成过程控制

以乙二胺(EDA)为模板剂,采用原位固相转化法合成了ZSM-35沸石,并通过XRD对沸石晶化过程中的影响因素和晶化机理作了详细考察。结果表明,制备ZSM-35沸石适宜的硅铝摩尔比为21n(SiO2)/n(Al2O3)37;Na+与模板剂EDA协同作用,共同对ZSM-35沸石起着结构导向作用;延长晶化时间,ZSM-35沸石容易向ZSM-5沸石转晶,合成ZSM-35沸石合适的晶化时间为14～21 h。对晶化机理的考察表明ZSM-35沸石的合成过程具有固相机理的特征,并且由于ZSM-35沸石与ZSM-5沸石具有相同的次级结构单元,在晶化过程中容易发生转晶。     

SSZ-13分子筛合成及改性研究进展

SSZ-13分子筛由于其特殊的孔道结构和物化性质，被广泛应用于许多领域。本文综述了近年来SSZ-13分子筛的一些常规合成方法，包括水热法、固相研磨法、干胶转换法、转晶法以及超声、微波或加入晶种辅助合成，并对这些方法的优点和不足进行了评价；介绍了模板剂、原料组成及配比和晶化条件对SSZ-13分子筛合成的影响，指出寻找廉价的模板剂、明确这些因素的影响方式具有重要的研究意义；概述了金属改性和引入介孔改性对SSZ-13分子筛催化性能的影响，并提出寻找更为有效的改性方法、深化机理研究是SSZ-13分子筛今后研究的重点。     

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。     

锡硅分子筛：新型杂原子分子筛催化材料

锡硅分子筛能高效催化含氧烃转化，因此其具有重要潜在工业应用价值。本文简述了锡硅分子筛的骨架锡判定、合成方法、多级孔合成研究及应用领域。XRD、NMR和EXAFS等表征方法可以判别锡硅分子筛内锡原子的配位形式及落位。合成锡硅分子筛通常采用水热晶化法、后插入法和干胶转化法。碱性水热合成过程中匹配锡源和硅源的水解速率是使锡原子高效插入分子筛骨架的关键。中性水热合成方法制备的锡硅分子筛结晶度较高，但存在使用剧毒含氟试剂和晶粒较大等问题。干胶转化法可在短时间内制备高结晶度含锡分子筛，但该法存在传质传热差等弊端。采用后插入法可将锡原子插入到不同拓扑结构的分子筛骨架，但锡原子易发生聚集。同时，采用碱处理、硬模板剂法和软模板剂法合成多级孔锡硅分子筛，可提升骨架锡活性中心的可接近性。其中，采用软模板剂法展示出独特的优势，其可制备介孔分布均匀且贯通性良好的多级孔锡硅分子筛。另外，锡硅分子筛的骨架锡原子对含氧官能团具有优异的活化作用，因此其在Meerwein-Ponndorf-Verley-Oppenauer反应、葡萄糖异构化反应、乳酸及乳酸酯制备及Baeyer-Villiger氧化反应中都展示了良好的催化效果。     

Synthesis of MFI-type zeolites under atmospheric pressure

Silicalite and highly silicious ZSM-5 were synthesized using two reaction mixtures with different crystal growth environments, a dispersed low density mixture and a separated high density mixture, at 93 ±3 ‡C under the atmospheric pressure. Nucleation behavior and the transformation process of two mixtures were investigated utilizing various analytical techniques such as XRD, FT-IR, TGA, SEM, and pH measurement. During the induction period, the same intermediate phase was observed in both mixtures. The presence of this phase was found to be dependent on the degree of dilution of the reaction mixture. After the induction period, a sharp increase in both the degree of crystallization and the pH of the reaction mixture was detected. This indicates that the pH change in the reaction mixture is closely related to the crystallization process. From these observations, a crystallization mechanism is proposed on the basis of the appearance of stable silicate species and the role of OH^- ions during the induction period. According to this mechanism, MFI-type zeolite grows by condensation reaction among the stable silicate species formed during the induction period.     

高岭土原位合成Y型沸石过程中的吸附研究

采用N2静态吸附法和IR光谱法研究了高岭土原位晶化合成Y型沸石过程中的孔结构和沸石的形成过程。结果显示，在原位晶化过程中，从天然高岭土到晶化产物，孔结构发生了很大的变化，从大孔到中孔发达的过程，焙烧微球在碱溶液的作用下，形成了发达的孔道结构，进而在孔道的可接近表面上均匀地生长出了Y型沸石；原位晶化过程是焙烧微球在液相组分中先转变为硅铝酸钠凝胶，随后硅铝酸钠凝胶逐步地转变为Y型沸石。     

气相法制备沸石分子筛的研究进展

综述了气相法,包括气相转移法和干胶法在合成沸石分子筛、磷铝分子筛和其它杂原子分子筛及分子筛膜方面的研究进展。介绍了合成过程中一些影响因素,如时间、温度、干胶组分或有机模板剂对合成的影响。并且对气相法制备分子筛成型体作了简单介绍。     

In Situ Ultrasonic Measurements as a Tool to Characterize the Stages of the Zeolite A Crystallization Process

Zeolite A (LTA) was synthesized at 80 °C under hydrothermal conditions from a gel with the molar composition of 2.5 Na₂O : 2 SiO₂ : Al₂O₃ : 80 H₂O. In the present study, we report the use of home-made ultrasound devices as a real-time, in situ diagnostic tool for monitoring the progress of zeolite formation. In addition, the specific aim of this study was to show that the measured ultrasound parameters – ultrasonic (US) velocity and attenuation – simultaneously provide independent information about the changes in the liquid and solid phase, respectively, during the crystallization process. Both views are necessary for extracting kinetic data about the zeolite crystallization processes and allow elucidating the different stages of zeolite LTA formation process. The obtained in situ US data were compared with conventional ex situ powder X-ray diffraction measurements following the formation of zeolite LTA from dense gels. The results demonstrate that a good correlation exists between the crystallization progress of zeolite LTA in terms of crystallinity changes and the ultrasonic attenuation and velocity.     

红辉沸石两步水热法合成A型分子筛机理研究

以红辉沸石为原料,采用两步水热法将其转化为A型分子筛,系统地探讨A型分子筛的转化机理。对不同晶化时间的样品进行系统表征,深入解析天然红辉沸石转化A型分子筛的成核机制、生长控制、晶型稳定转化等结构基础问题。研究表明,A型分子筛的生长可分为3个阶段：在成核诱导期（晶化时间≤0.5 h）,凝胶中的β笼通过双四元环构筑成α笼,生成较多晶核及少量1μm小晶体;此后为快速生长期（晶化时间1～4 h）,大量晶核通过聚集生长形成2μm大晶体;在生长稳定期（晶化时间5～8 h）,通过二次成核凝胶将会转化为更多的分子筛晶体。A型分子筛的形成遵循：铝硅酸盐凝胶→四元环、六元环→双四元环、β笼→α笼的晶化过程,因而其转化过程符合液相转化机理。     

Zeolite beta monoliths with hierarchical porosity by the transformation of bimodal pore silica gel

Zeolite beta monoliths with hierarchical porosity were prepared by the steam-assisted conversion of pre-seeded bimodal pore silica gel using the Layer-by-Layer process. The bimodal pore silica gel acts as both macrotemplates and silica source. The zeolite beta monoliths were characterized by X-ray diffraction, scanning electron microscopy, IR absorption spectra and nitrogen adsorption measurements. This zeolite beta monolith had hierarchical porosity: the unique micropores within the zeolite, the small macropores generated by aggregation of crystallites and three dimensionally interconnected macropores formed by template of silica gel. It is believed that the hierarchical structured zeolite monoliths will show good properties and potential applications the fields of catalyst, ion exchange, and adsorption.     

Synthesis of hierarchical ZSM-5 microspheres with superior performance for catalytic methanol-to-olefin conversion

Methanol-to-olefin (MTO) conversion on zeolites has encountered severe coke deposition and rapid deactivation. Creating different levels of porosity is essential to mitigate such issues. Herein, we demonstrate a facile and green strategy to synthesize uniform and hierarchically macro/mesoporous ZSM-5 microspheres by combining spray-freeze drying and steaming-assisted crystallization (SAC). The structure, crystallinity, and porosity of the zeolite microspheres are controlled by adjusting the water/gel mass ratio and time in the SAC process. The structure evolution during the SAC process is revealed. In the catalytic MTO reaction, the representative hierarchically porous ZSM-5 catalyst exhibits superior catalytic performance. At a very high weight hourly space velocity of 18 h⁻¹, it shows a dramatically prolonged lifetime (47 h at >99% conversion) and much-improved selectivity to ethylene and propylene compared with the conventional microporous ZSM-5 and nano-sized ZSM-5. The enhanced performance is originated from the hierarchical structure and suitable acidity of the ZSM-5 microspheres.     

晶种导向剂法制备纳米ZSM-5沸石

在水热合成体系中添加自制晶种导向剂成功制备了纳米ZSM-5沸石,考察了晶种导向剂、晶化温度和合成体系硅铝物质的量比对合成纳米ZSM-5沸石的影响。制备的晶种导向剂是全硅的Silicalite-1沸石纳米颗粒、沸石初级或次级结构单元的SiO₂以及模板剂TPAOH的混合胶体溶液,结果表明,在ZSM-5沸石制备体系中添加晶种导向剂可有效降低有机模板剂的使用量,缩短晶化时间,并能得到纳米尺寸的ZSM-5沸石,降低晶化温度和合成体系硅铝比有利于减小纳米ZSM-5沸石晶体尺寸。     

Synthesis of Ti-MWW by a dry-gel conversion method

MWW type titanosilicate, Ti-MWW, has been synthesized by the dry-gel conversion (DGC) method, and its physicochemical properties and catalytic performance in the liquid-phase epoxidation of alkene have been compared with that of hydrothermally synthesized (HTS) Ti-MWW. The roles in the crystallization of silica source, alkali cation, cyclic amine as a structure-directing agent (SDA), and boric acid structure-supporting agent have been investigated. The crystallization of Ti-MWW did not occur for the dry gels free of boric acid, but was feasible at a Si/B molar ratio as high as 12 in marked contrast to the ratio of 0.75 required in the hydrothermal synthesis. The sodium as a mineralization agent was not necessary and on the contrary inhibited the crystallization particularly at a high content. The seeding technique using deboronated MWW effectively accelerated the crystallization speed and reduced the amount of boric acid required. As-synthesized Ti-MWW-DGC lamellar precursors contained both tetrahedral and octahedral species but the latter was selectively removed by acid treatment. Ti-MWW-DGC catalysts showed lower intrinsic activity than Ti-MWW-HTS in the epoxidation of hex-1-ene with hydrogen peroxide probably because the crystal size of the former was 10–20 times as large as that of the latter and then imposed significant diffusion problems for both the substrates and the products.     

Dry gel conversion synthesis of shape controlled MFI type zeolite materials

Acid treated silica fibers, coming either from asbestos cord waste or calcium aluminosilicate glass, were engaged as reactants to product MFI type zeolite fibers applying a dry gel conversion (DGC) type synthesis. The impact of amorphous silica fiber shape and synthesis conditions on morphology of the final product was investigated. Enhanced zeolite nucleation and restricted crystal growth are necessary to maintain the initial fiber morphology. Whereas a DGC type crystallization in hydroxide media results in the formation of zeolite fibers, the use of fluoride ions leads to the formation of large individual zeolite particles. Synthesis conditions have to be adapted to form zeolite particles with an average size smaller than the silica fiber diameter to allow a good conservation of the fiber morphology. Final zeolite fibers have textural properties comparable to a standard zeolite product and are promising for adsorption applications.