Tailoring Zeolite ZSM‐5 Crystal Morphology/Porosity through Flexible Utilization of Silicalite‐1 Seeds as Templates: Unusual Crystallization Pathways in a Heterogeneous System

Diffusion limitation in micropores of zeolites leads to a demand for optimization of zeolite morphology and/or porosity. However, tailoring crystallization processes to realize targeted morphology/porosity is a major challenge in zeolite synthesis. On the basis of previous work on the salt‐aided, seed‐induced route, the template effect of seeds on the formation of micropores, mesopores and even macropores was further explored to selectively achieve desired hierarchical architectures. By carefully investigating the crystallization processes of two typical samples with distinct crystal morphologies, namely, 1) nanocrystallite‐oriented self‐assembled ZSM‐5 zeolite and 2) enriched intracrystal mesoporous ZSM‐5 zeolite, a detailed mechanism is proposed to clarify the role of silicalite‐1 seeds in the formation of diverse morphologies in a salt‐rich heterogeneous system, combined with the transformation of seed‐embedded aluminosilicate gel. On the basis of these conclusions, the morphologies/porosities of products were precisely tailored by deliberately adjusting the synthesis parameters (KF/Si, tetrapropylammonium bromide/Si and H₂O/Si ratios and type of organic template) to regulate the kinetics of seed dissolution and seed‐induced recrystallization. This work may not only provide a practical route to control zeolite crystallization for tailoring crystal morphology, but also expands the knowledge of crystal growth mechanisms in a heterogeneous system.     

Deposition of oriented zeolite A films: in situ and secondary growth

Zeolite A suspensions with a monomodal, narrow particle size distribution have been prepared. The suspended particles in a TMAOH water solution at pH 9 are negatively charged with a zeta potential of −43 mV. Modification of the external surface of the zeolite particles by a silylation reaction produces particles that, when they are suspended in water, are positively charged and have a zeta potential of +40 mV.The suspensions of the negatively or positively charged particles can be used for the preparation of adsorbed layers of particles on oppositely charged substrates by electrostatic attraction. This deposition process leads to a high coverage of the substrate with well-adhered particles. The cubic morphology of the zeolite particles results in preferential orientation after deposition. The particles are oriented with their {h 0 0} planes (cube faces) parallel and perpendicular to the substrate (out-of-plane orientation). The particles are randomly oriented with respect to the direction perpendicular to the substrate (in-plane orientation). Although, under optimized conditions, the coverage is high and only one adsorption cycle is necessary, the particles are not closely packed.Alternately, the zeolite particle suspensions can be used to deposit close-packed arrays of particles by convective particle transport during dip coating on substrates bearing the same charge as the zeolite particles. Using monodispersed zeolite A suspensions and slow speed dip coating close-packed hexagonal colloidal crystals were prepared. The type of colloidal crystal deposits formed range from continuous sublayers, monolayers, or multilayers to isolated discoidal clusters consisting of few zeolite particles. Factors affecting the deposited layer(s) structure are particle concentration of the suspension and withdrawal speed. In addition to close packing, the layers prepared by dip coating exhibit preferred orientation with the particle faces lying parallel and perpendicular to the substrate surface. Moreover, this second route of precursor film formation by colloidal crystallization leads to domains of well-aligned zeolite particles in three dimensions, i.e. with their faces parallel to each other. The oriented domains span the length of several particles; however, low angle boundaries and other defects during colloidal crystallization prevent the formation of macroscopically three-dimensionally ordered zeolite particles.The precursor layers were subjected to secondary growth in order to prepare continuous intergrown films. Secondary growth proceeds initially by local epitaxy on the deposited particles. Later in the process, deposition proceeds by incorporation of particles from solution along with re-nucleation on the growing film. The intergrown films have predominately [h 0 0] out-of-plane orientation; however, after extended secondary growth treatment a population of [h h h] grains appears on the surface of the regrown films.     

Aggregative growth of silicalite-1

Precursor silica nanoparticles can evolve to silicalite-1 crystals under hydrothermal conditions in the presence of tetrapropylammonium (TPA) cations. It has been proposed that in relatively dilute sols of silica, TPA, water, and ethanol, silicalite-1 growth is preceded by precursor nanoparticle evolution and then occurs by oriented aggregation. Here, we present a study of silicalite-1 crystallization in more concentrated mixtures and propose that growth follows a path similar to that taken in the dilute system. Small-angle X-ray scattering (SAXS), cryogenic transmission electron microscopy (cryo-TEM), and high-resolution transmission electron microscopy (HRTEM) were used to measure nanoparticle size and to monitor zeolite nucleation and early-stage crystal development. The precursor silica nanoparticles, present in the clear sols prior to crystal formation, were characterized using two SAXS instruments, and the influence of interparticle interactions is discussed. In addition, SAXS was used to detect the onset of secondary particle formation, and HRTEM was used to characterize their structure and morphology. Cryo-TEM allowed for in situ visual observation of the nanoparticle population. Combined results are consistent with growth by aggregation of silica nanoparticles and of the larger secondary crystallites. Finally, a unique intergrowth structure that was formed during the more advanced growth stages is reported, lending additional support for the proposal of aggregative growth.     

基底表面性质与前驱液化学环境对原位定向构筑钛硅分子筛膜的影响

b取向MFI型分子筛膜能够显著促进分子的传输效率, 被广泛应用于混合物分离及催化领域. 虽然传统的原位水热晶化法已较为成熟, 然而仍难以调控膜层的b轴取向生长. 本文以304不锈钢片为基底, 采用经典的原位水热晶化法研究了基底表面物化性质、 前驱液配比及晶化条件对钛硅分子筛膜b轴取向生长的影响. 结果表明, TiO₂中间层表面的羟基能够定向诱导分子筛晶粒的吸附, 进而提高钛硅分子筛膜的b轴取向程度. 同时, 前驱液中模板剂和水含量对晶粒的大小及膜层的定向生长具有显著影响, 即仅在合适的碱度下才能形成高b轴取向的钛硅分子筛膜.     

Investigation of the Mechanism of Colloidal Silicalite‐1 Crystallization by Using DLS,SAXS, and 29Si NMR Spectroscopy

Colloidal silicalite‐1 zeolite was crystallized from a concentrated clear sol prepared from tetraethylorthosilicate (TEOS) and aqueous tetrapropylammonium hydroxide (TPAOH) solution at 95 °C. The silicate speciation was monitored by using dynamic light scattering (DLS), synchrotron small‐angle X‐ray scattering (SAXS), and quantitative liquid‐state ²⁹Si NMR spectroscopy. The silicon atoms were present in dissolved oligomers, two discrete nanoparticle populations approximately 2 and 6 nm in size, and crystals. On the basis of new insight into the evolution of the different nanoparticle populations and of the silicate connectivity in the nanoparticles, a refined crystallization mechanism was derived. Upon combining the reagents, different types of nanoparticles (ca. 2 nm) are formed. A fraction of these nanoparticles with the least condensed silicate structure does not participate in the crystallization process. After completion of the crystallization, they represent the residual silicon atoms. Nanoparticles with a more condensed silicate network grow until approximately 6 nm and evolve into building blocks for nucleation and growth of the silicalite‐1 crystals. The silicate network connectivity of nanoparticles suitable for nucleation and growth increasingly resembles that of the final zeolite. This new insight into the two classes of nanoparticles will be useful to tune the syntheses of silicalite‐1 for maximum yield.     

Facile Synthesis of Hierarchical Nanosized Single‐Crystal Aluminophosphate Molecular Sieves from Highly Homogeneous and Concentrated Precursors

The synthesis of hierarchical nanosized zeolite materials without growth modifiers and mesoporogens remains a substantial challenge. Herein, we report a general synthetic approach to produce hierarchical nanosized single‐crystal aluminophosphate molecular sieves by preparing highly homogeneous and concentrated precursors and heating at elevated temperatures. Accordingly, aluminophosphate zeotypes of LTA (8‐rings), AEL (10‐rings), AFI (12‐rings), and ‐CLO (20‐rings) topologies, ranging from small to extra‐large pores, were synthesized. These materials show exceptional properties, including small crystallites (30–150 nm), good monodispersity, abundant mesopores, and excellent thermal stability. A time‐dependent study revealed a non‐classical crystallization pathway by particle attachment. This work opens a new avenue for the development of hierarchical nanosized zeolite materials and understanding their crystallization mechanism.     

熵驱大分子胶体粒子的柱状受限结晶

熵是物理化学的基本状态参量,在统计力学和热力学中处于核心位置.按照玻尔兹曼的微观解释,熵可以由孤立系统微观状态的数目(W)给出,即S=kBlnW,这里kB为玻尔兹曼常数[1,2].根据此公式,微观状态数越多,系统越混乱,熵越大,所以熵常被视作体系无序程度的度量.但熵增仅对应体系微观状态数的增加,与可观测的结构有序程度无关[3~5].在一些典型的软物质体系中,结构越有序熵反而越大,如胶体硬球在随机密堆积点的有序结晶[6]及描述各向异性棒状分子从各向同性相到向列相转变的Onsager原理[7].     

Exceptionally small colloidal zeolites templated by Pd and Pt amines

Monomodal colloidal suspensions containing EDI-type zeolite nanocrystals with sizes below 20 nm were prepared via a palladium and platinum amine templating approach. The role of the metal complexes in zeolite crystallization is elucidated using spectroscopic and microscopic characterization techniques in a series of samples containing pure Pd, Pt, and Cu amine complexes as well as mixtures of two compounds. The crystallization process of colloidal zeolites in precursor suspensions containing both [Pd(NH3)4]2+ and [Pt(NH3)4]2+ proceeds faster than in [Cu(NH3)4]2+ systems. The Pd and Pt complexes lead to a faster and enhanced nucleation rate in the precursor aluminosilicate suspensions in comparison to the copper amine complex. The latter explains both the smaller particle size and the higher monodispersity in the samples templated by Pd and Pt as compared to pure Cu-containing samples. Precursor systems containing mixed metal templates were used to control further the particle size and degree of metal loadings in the colloidal molecular sieves.     

Sol-Gel Routes to Zeolite Membranes and Thin Films

The synthesis of zeolite membranes and thin films using the secondary growth process is briefly described. In this process colloidal zeolite particles (sols) are prepared hydrothermally and then subsequently deposited on substrates to produce uniform layers of controlled thickness, as illustrated with silicalite and zeolite-A. The formation and growth of the zeolite sols has been investigated in situ by small angle neutron scattering (SANS). SANS measurements on silicalite sols at progressively higher concentrations have provided details of the colloid interactions which lead to zeolite gel-layer structures which are uniform and free of defects.     

Phase Transformation Behavior of a Two‐Dimensional Zeolite

Understanding the molecular‐level mechanisms of phase transformation in solids is of fundamental interest for functional materials such as zeolites. Two‐dimensional (2D) zeolites, when used as shape‐selective catalysts, can offer improved access to the catalytically active sites and a shortened diffusion length in comparison with their 3D analogues. However, few materials are known to maintain both their intralayer microporosity and structure during calcination for organic structure‐directing agent (SDA) removal. Herein we report that PST‐9, a new 2D zeolite which has been synthesized via the multiple inorganic cation approach and fulfills the requirements for true layered zeolites, can be transformed into the small‐pore zeolite EU‐12 under its crystallization conditions through the single‐layer folding process, but not through the traditional dissolution/recrystallization route. We also show that zeolite crystal growth pathway can differ according to the type of organic SDAs employed.     

Uniformly Oriented Zeolite ZSM‐5 Membranes with Tunable Wettability on a Porous Ceramic

Facile fabrication of well‐intergrown, oriented zeolite membranes with tunable chemical properties on commercially proven substrates is crucial to broadening their applications for separation and catalysis. Rationally determined electrostatic adsorption can enable the direct attachment of a b‐oriented silicalite‐1 monolayer on a commercial porous ceramic substrate. Homoepitaxially oriented, well‐intergrown zeolite ZSM‐5 membranes with a tunable composition of Si/Al=25–∞ were obtained by secondary growth of the monolayer. Intercrystallite defects can be eliminated by using Na⁺ as the mineralizer to promote lateral crystal growth and suppress surface nucleation in the direction of the straight channels, as evidenced by atomic force microscopy measurements. Water permeation testing shows tunable wettability from hydrophobic to highly hydrophilic, giving the potential for a wide range of applications.     

In situ observation of homogeneous nucleation of nanosized zeolite A

The formation of zeolite A (LTA) in the presence of tetramethylammonium cations is studied using in situ small angle and wide angle X-ray scattering (SAXS/WAXS) techniques. The SAXS measurements show the formation of homogeneous precursors 10 nm in size prior to the crystallization of LTA which were consumed during the crystallization. The crystal size is estimated by fitting the SAXS patterns with an equation for a cubic particle, and it is revealed that the final crystal size of the LTA depends on the synthesis temperature. However, although such temperature dependence is noted for the final crystal size, the initial precursor particles size appears to be closely similar (ca. 10 nm) irrespective of the synthesis temperature.     

Silicalite-1修饰的HY型分子筛及其对纤维素水解的催化性能

采用silicalite-1对HY型分子筛进行修饰,得到具有核壳结构的复合分子筛HY/silicalite-1。通过X射线衍射(XRD)、扫描电镜(SEM)、透射电镜(TEM)、N2的吸附-脱附及吡啶吸附红外(Py-FTIR)等手段对不同晶化时间合成的HY/silicalite-1复合分子筛进行了表征,研究了复合分子筛对纤维素水解的催化性能。结果表明,晶化时间直接影响复合分子筛的晶体生长规律和两组分的相对含量,最佳晶化时间为16-24 h,所得到的复合分子筛外貌呈核壳结构,silicalite-1附晶生长在HY型分子筛的表面;随着晶化时间的延长,复合分子筛的表面由胶浊状变为光滑,最终变为鳞片状;其B酸量先减少后增加,而L酸量则先增加后减少。其中,晶化时间为24 h的HY/silicalite-1复合分子筛B酸量最大,L酸量最小,对纤维素水解反应具有良好的催化性能,葡萄糖收率由HY型分子筛催化获得的28.0%大幅提高至45.8%。     

沸石转晶的研究(Ⅲ)——NaY型沸石的稳定性及NaPc型沸石的结构

研究了原始凝胶3、3Na₂O·Al₂O₃·10SiO₂·200H₂O在100℃NaY型沸石向NaPc型沸石转晶的晶化过程,应用电子显微镜、X-射线衍射、TMS-GC和固体高分辨Si²⁹-NMR方法研究了沸石晶核的形成、晶体粒度和外貌的变化及液、固相中硅酸根离子存在状况及转晶中沸石结构的变化,并提出了转晶的液相传质机理。     

Graphene Oxide Facilitates Solvent‐Free Synthesis of Well‐Dispersed,Faceted Zeolite Crystals

Zeolites with molecular dimension pores are widely used in petrochemical and fine‐chemical industries. While traditional solvothermal syntheses suffer from environmental, safety, and efficiency issues, the newly developed solvent‐free synthesis is limited by zeolite crystal aggregation. Herein, we report well‐dispersed and faceted silicalite ZSM‐5 zeolite crystals obtained using a solvent‐free synthesis facilitated by graphene oxide (GO). The selective interactions between the GO sheets and different facets, which are confirmed by molecular dynamics simulations, result in oriented growth of the ZSM‐5 crystals along the c‐axis. More importantly, the incorporation of GO sheets into the ZSM‐5 crystals leads to the formation of mesopores. Consequently, the faceted ZSM‐5 crystals exhibit hierarchical pore structures. This synthetic method is superior to conventional approaches because of the features of the ZSM‐5 zeolite.     

Graphene Oxide Facilitates Solvent‐Free Synthesis of Well‐Dispersed,Faceted Zeolite Crystals

Zeolites with molecular dimension pores are widely used in petrochemical and fine‐chemical industries. While traditional solvothermal syntheses suffer from environmental, safety, and efficiency issues, the newly developed solvent‐free synthesis is limited by zeolite crystal aggregation. Herein, we report well‐dispersed and faceted silicalite ZSM‐5 zeolite crystals obtained using a solvent‐free synthesis facilitated by graphene oxide (GO). The selective interactions between the GO sheets and different facets, which are confirmed by molecular dynamics simulations, result in oriented growth of the ZSM‐5 crystals along the c‐axis. More importantly, the incorporation of GO sheets into the ZSM‐5 crystals leads to the formation of mesopores. Consequently, the faceted ZSM‐5 crystals exhibit hierarchical pore structures. This synthetic method is superior to conventional approaches because of the features of the ZSM‐5 zeolite.     

Laser Control of Zeolite Nucleation

Precursor solutions for the synthesis of zeolites are irradiated by means of a Nd‐YAG laser. These solutions are subsequently submitted to a hydrothermal treatment and the results analyzed by X‐ray diffraction and electron microscopy. Laser irradiation promotes the formation of silica nanoparticles that nucleate into zeolite (silicalite‐1), following a hydrothermal treatment. The average crystal size (in the 0.6–3.6 μm range) of the zeolite exponentially decreases as a function of laser irradiation time. In addition, a longer irradiation time results in a narrower crystal size distribution.     

Electron Microscopy Reveals the Nucleation Mechanism of Zeolite Y from Precursor Colloids

The growth of nanoscale crystals of zeolite Y from colloidal aluminosilicate gel particles has been investigated with high-resolution electron microscopy. Each amorphous gel particle nucleates only a single zeolite crystal, with nucleation beginning at the gel-solution interface (see scheme). Further growth of these nanocrystals is possible through the solution-mediated transport of framework building blocks.     

以氨水做碱源胶态晶种导向法合成小晶粒TS-1分子筛

TS-1分子筛具有MFI拓扑结构,因其独特的择形选择性和优异的催化氧化能力而广受关注.最早报道的TS-1合成方法采用大量四丙基氢氧化铵(TPAOH)作为有机结构导向剂, TPAOH价格昂贵,制约着TS-1分子筛大规模应用.开发廉价、环境友好的合成工艺是TS-1分子筛合成领域的重要课题.以价格相对较低的四丙基溴化铵(TPABr)代替TPAOH做有机结构导向剂,以氨水为碱源可合成TS-1分子筛,但产物晶粒尺寸远远大于以TPAOH做模板的合成结果,影响TS-1分子筛的传质和催化性能.因此,人们对该法进行了改进,选用有机胺作为碱源, TPABr为结构导向剂合成TS-1分子筛,但始终未能将其晶粒尺寸降至1μm以下.在合成体系中引入预先合成的TS-1分子筛或TS-1胶态前驱体作为晶种可以促进成核,缩短成核诱导期,有利于获得小晶粒尺寸的TS-1分子筛.此类方法往往需要辅助以大量有机胺等结构导向剂;胶态TS-1前驱体的制备需要特别小心以保证晶种中Ti的四配位状态,通常需要经历低温水解钛酸四丁酯(TBOT)和高温加热除醇等繁琐步骤.而胶态纯硅silicalite-1制备则相对简单,且已广泛用于导向合成同样具有MFI结构的ZSM-5沸石,但目前鲜有以silicalite-1做晶种合成TS-1分子筛的报道.基于此,本文以纯硅胶态silicalite-1为晶种,以氨水做碱源,辅助以少量TPABr做导向剂,合成了小晶粒TS-1分子筛,并以正己烯环氧化和环己酮氨肟化做探针反应考察了所得TS-1分子筛的催化氧化性能. X射线衍射结果表明,当晶种中SiO2占合成体系中SiO2的10 wt%(晶种引入TPAOH, TPAOH/SiO2=0.35),加入TPABr (TPABr/SiO2=0.03)做辅助结构导向剂,即合成体系中总(TPAOH+TPABr)/SiO2摩尔比低至0.07时,所得样品依然具有良好的结晶度.扫描电镜照片观察不到无定形物存在;晶种中SiO2占合成体系中SiO2的10 wt%时,所得TS-1晶粒尺寸约为250 nm ×150 nm ×50 nm;其他条件不变,胶态晶种用量增加到15 wt%时,初级晶粒尺寸基本保持不变,晶粒-晶粒之间交叉生长,形成孪生形貌;继续增加胶态晶种用量至20 wt%时,晶粒尺寸下降至仅100 nm左右;而用20 wt%胶态晶种所含相同量的TPAOH来代替胶态晶种,得到样品呈近10μm的大块状.与之对应的是,胶态silicalite-1晶种导向得到的小晶粒TS-1分子筛具有比直接用TPAOH得到的大块状样品更大的外比表面积和堆积孔体积.分析结果显示所得TS-1分子筛的体相TiO2/SiO2比在41–43.红外光谱和紫外可见光谱结果表明,胶态晶种导向法所得TS-1分子筛中的Ti主要以四配位状态存在,而直接用TPAOH合成的大块状样品则呈现显著骨架外Ti吸收峰,说明胶态晶种有助于Ti物种进入分子筛骨架.在催化正己烯环氧化反应时,用胶态silicalite-1晶种导向得到的小晶粒TS-1分子筛表现出与大块状TS-1相似的催化性能;而以环己酮氨肟化做探针反应时,小晶粒TS-1分子筛因具有外比表面积大和扩散路径短等优点而表现出远远高于大块状TS-1分子筛的催化活性.但与文献报道的相同SiO2/TiO2比的TS-1分子筛比较,本文所得小晶粒TS-1分子筛催化正己烯环氧化的活性略差.提高该小晶粒TS-1分子筛正己烯环氧化活性和建立构-效关系是下一步工作的重点.     

Preparation and characterization of nanosized ZSM-5 zeolite using kaolin and investigation of kaolin content, crystallization time and temperature changes on the size and crystallinity of products

Nano sized ZSM-5 zeolite samples were synthesized successively from kaolin clay as alumina source having a large amount of quartz (39%) and silicic acid as silica source by hydrothermal treatment with NaOH in the presence of tetrapropylammonium hydroxide as a template. Then the effect of kaolin content, crystallization temperature and time on the size and crystallinity of the products were investigated. The prepared samples were characterized using XRD, SEM, EDS and FT-IR techniques. The results showed that the synthesized ZSM-5 zeolite samples were almost pure and their crystallization was almost complete. The average particle size, as determined by Debye-Scherrer equation, was in the range of 20-42 nm. Increasing kaolin content on crystal size was more effective than increase in crystallization temperature and time. Additional evidences for the nano sized ZSM-5 zeolite were the asymmetric stretch vibration band at 1225 cm^-1 in the FT-IR spectra and TEM images. The scanning electron micrographs of the synthesized zeolites showed that they are spherical shape crystals.