Differences Between Individual ZSM‐5 Crystals in Forming Hollow Single Crystals and Mesopores During Base Leaching

After base treatment of ZSM‐5 crystals below 100 nm in size, TEM shows hollow single crystals with a 10 nm shell. SEM images confirm that the shell is well‐ preserved even after prolonged treatment. Determination of the Si/Al ratios with AAS and XPS in combination with argon sputtering reveals aluminum zoning of the parent zeolite, and the total pore volume increases in the first two hours of base treatment. In corresponding TEM images, the amount of hollow crystals are observed to increase during the first two hours of base treatment, and intact crystals are visible even after 10 h of leaching; these observations indicate different dissolution rates between individual crystals. TEM of large, commercially available ZSM‐5 crystals shows inhomogeneous distribution of mesopores among different crystals, which points to the existence of structural differences between individual crystals. Only tetrahedrally coordinated aluminum is detected with ²⁷Al MAS NMR after the base leaching of nano‐sized ZSM‐5.     

Creation of hollow zeolite architectures by controlled desilication of Al-zoned ZSM-5 crystals

Hollow zeolite architectures on different length scales have been obtained upon controlled desilication of Al-zoned ZSM-5 zeolites in alkaline medium. The hollow ZSM-5 crystals possess a functional Al-rich exterior and a tunable internal porosity and accessibility.     

An in situ Al K-edge XAS investigation of the local environment of H+- and Cu+-exchanged USY and ZSM-5 zeolites

Aluminum coordination in the framework of USY and ZSM-5 zeolites containing charge-compensating cations (NH4+, H+, or Cu+) was investigated by Al K-edge EXAFS and XANES. This work was performed using a newly developed in-situ cell designed especially for acquiring soft X-ray absorption data. Both tetrahedrally and octahedrally coordinated Al were observed for hydrated H-USY and H-ZSM-5, in good agreement with 27Al NMR analyses. Upon dehydration, water desorbed from the zeolite, and octahedrally coordinated Al was converted progressively to tetrahedrally coordinated Al. These observations confirmed the hypothesis that the interaction of water with Br?nsted acid protons can lead to octahedral coordination of Al without loss of Al from the zeolite lattice. When H+ is replaced with NH4+ or Cu+, charge compensating species that absorb less water, less octahedrally coordinated Al was observed. Analysis of Al K-edge EXAFS data indicates that the Al-O bond distance for tetrahedrally coordinated Al in dehydrated USY and ZSM-5 is 1.67 angstroms. Simulation of k3chi(k) for Cu+ exchanged ZSM-5 leads to an estimated distance between Cu+ and framework Al atoms of 2.79 angstroms.     

Mechanism of hierarchical porosity development in MFI zeolites by desilication: the role of aluminium as a pore-directing agent

The role of the concentration and the nature of aluminium in the creation of hierarchical porosity in both commercial and synthesized MFI zeolites have been investigated through controlled mesoporosity development by desilication in alkaline medium. Framework aluminium controls the process of framework silicon extraction and makes desilication selective towards intracrystalline mesopore formation. An optimal molar Si/Al ratio in the range 25-50 has been identified; this leads to an optimal mesoporosity centred around 10 nm and mesopore surface areas of up to 235 m(2) g(-1) while preserving the intrinsic crystalline and acidic properties. At lower framework Si/Al ratios the relatively high Al content inhibits Si extraction and hardly any mesopores are created, while in highly siliceous ZSM-5 unselective extraction of framework Si induces formation of large pores. The existence of framework Al sites in different T positions that are more or less susceptible to the alkaline treatment, and the occurrence of re-alumination, are tentative explanations for the remarkable behaviour of Al in the desilication process. The presence of substantial extra framework Al, obtained by steam treatment, inhibits Si extraction and related mesopore formation; this is attributed to re-alumination of the extraframework Al species during the alkaline treatment. Removal of extraframework Al species by mild oxalic acid treatment restores susceptibility to desilication, which is accompanied by formation of larger mesopores due to the enhanced Si/Al ratio in the acid-treated zeolite.     

一步处理法制备高水热稳定多级孔ZSM-5分子筛

ZSM-5具有较高的催化活性和独特的择形选择性,因而被广泛用于精细化工和石油炼制等工业过程.但其较小的孔道尺寸导致其在反应中尤其在催化过程中的传质受到影响,从而严重影响催化剂寿命.为了解决反应过程中分子筛中底物及产物的扩散限制问题,近年来关于介微孔复合多级孔道分子筛的研究在分子筛合成领域引起了广泛兴趣,并取得一定进展.但直接合成法存在成本及复杂性问题,因此在量产的分子筛上进行后改性引入介孔表现出明显优势.在这一大类处理过程中,碱处理造介孔因成本低以及可操作性较高而备受青睐.但由于脱硅所形成的介孔往往无序,稳定性较差,因此提高其水热稳定性具有重要意义.文献已有较多报道通过磷元素修饰抑制分子筛在水热环境中脱铝,从而提高分子筛骨架稳定性.但传统的磷元素修饰一般采用后续浸渍法,过程繁琐.本课题组开发了一步法后处理制备高水热稳定多级孔ZSM-5分子筛,将脱硅过程与磷引入过程相结合,以四乙基氢氧化磷(TEPOH)为磷源,直接处理微孔分子筛得到含磷的多级孔分子筛.相比于传统的分子筛磷修饰过程,该磷物种在处理中优先交换分子筛骨架上铝原子附近用来平衡电荷的钠离子,从而增加了磷物种与铝物种相互作用的可能性,提高了稳化骨架的效果.基于此,本文利用氮吸附-脱附、元素分析、X射线粉末衍射(XRD)、扫描电镜(SEM)、透射电镜(TEM)、氨程序升温脱附(NH3-TPD)以及27Al和31P固体核磁(NMR)等一系列表征技术,证明该一步后处理法对分子筛催化性能的作用.SEM和XRD结果表明,分子筛在进行各种处理前后,相对结晶度和分子筛形貌变化不大,说明大部分微孔得到保留.TEM测试表明,经过该一步后处理法得到的分子筛实现了介孔的引入,介孔孔径在10–20 nm.元素分析结果表明,几乎所有的磷源物质在该过程中被引入到分子筛上,实现了介孔和磷物种的同时引入.NH3-TPD测试表明,含磷分子筛在老化前后较无磷分子筛具有更高的酸性位密度保留度,说明磷元素对分子筛上的酸性位起到了稳化作用.孔结构特性数据不仅说明了脱硅过程中成功引入介孔,而且含磷分子筛老化前后的介孔特性数据保留度明显高于无磷样品,实现了磷元素对介孔结构的稳化作用.27Al和31P NMR结果从理论上证明了该样品上磷元素对抑制骨架脱铝的稳化效应,证实了分子筛在处理后水热稳定性的提高.基于前期的研究工作,本文完善了磷元素对分子筛稳化作用的机理过程.TEPBr在水热活化过程中转变成磷酸盐,并修饰分子筛的骨架铝以及部分难以避免的非骨架铝.该过程中形成的磷铝物种在后续的水热老化过程中进一步修饰分子筛骨架铝,使骨架铝得到稳化.而在脱硅过程中存在的"反插铝"过程往往使大部分骨架铝位于介孔孔道中,磷元素与铝元素的相互作用同时也对介孔进行了稳化.通过正辛烯和1,3,5-三异丙苯的裂化测试发现,处理后的分子筛由于其优化后的孔道性能和酸性性质,大大提高了底物分子的转化率以及其自身的容碳能力,从而延长了催化寿命.     

以纯硅silicalite-1为晶种制备空心结构ZSM-5沸石分子筛

微孔分子筛在实际应用过程中常常受到扩散限制的影响,分子筛内部的利用率不高;制备空心结构分子筛对于改善分子筛扩散传质、提高分子筛利用率具有重要意义。以纯硅silicalite-1为晶种,以四丙基氢氧化铵(TPAOH)为结构导向剂制备ZSM-5分子筛;无定型硅铝物种首先在晶种表面晶化生长,同时碱性合成体系的碱度则可以溶解不稳定的silicalite-1晶种,通过控制调变生长与溶解的相对速率制备得到具有空心结构的ZSM-5分子筛;该多级孔道ZSM-5分子筛的制备方法简单、易操作,具有广泛的应用潜力。     

MFI/MFI核壳分子筛合成的影响因素及结晶动力学

以低硅铝比ZSM-5 为核, 采用二次生长法水热合成了MFI/MFI核壳分子筛. 发现对核相分子筛进行预处理是合成的关键步骤. 通过控制壳相合成过程(如合成温度、合成时间和核相分子筛加入量)可有效控制核壳分子筛的壳层生长. 以异丙苯(IPB)及1,3,5-三异丙苯(1,3,5-TIPB)裂解为探针反应, 发现与核相分子筛相比, 核壳分子筛的IPB裂解反应活性相当. 而1,3,5-TIPB裂解活性下降68%, 与外表面Al含量下降程度相近, 表明MFI/MFI核壳分子筛较好地保留了分子筛的核相反应活性. 结晶动力学计算结果表明, MFI/MFI核壳分子筛的成核活化能为51.5 kJ·mol-1, 生长活化能为26.5 kJ·mol-1.     

气相转移法合成ZSM-5/SAPO-5复合分子筛

以磷酸、拟薄水铝石和硅溶胶为原料, 三乙胺为模板剂, 采用气相转移法合成了一系列ZSM-5/SAPO-5复合分子筛. 产物经X射线衍射、扫描电镜、X射线能量散射谱、红外光谱及N2 静态吸附法等手段对其进行了表征, 证明合成材料是以ZSM-5为核、SAPO-5为壳的双结构分子筛. 实验结果表明, 干胶制备条件及液相组成都影响复合分子筛的结晶. 晶化温度的提高和晶化时间的延长有利于分子筛结晶度的提高. VPT法可以减小SAPO-5和复合分子筛颗粒的直径, 改善SAPO-5在ZSM-5分子筛表面的分布. 重油裂化结果表明, 核壳结构复合分子筛对生成低碳烯烃的性能优于机械混合的样品.     

One-Pot Synthesis and Formation Mechanism of Hollow ZSM-5

Coffin-shaped hollow ZSM-5 zeolite (HZZ) particles with shell thickness of about 200 nm and hollow diameter of approximately 1.5 μm were synthesized in one pot by using tetrapropylammonium bromide (TPABr), aluminum triisopropoxide Al[OCH(CH₃)₂]₃ and tetraethoxysilane (TEOS) as the structure-directing agent (SDA), aluminum and silica source, respectively. The appropriate molar ratios of TPABr/SiO₂ and Si/Al as well as suitable crystallization temperature are the key factors for the formation of HZZ. The formation of the HZZ can be attributed to the existence of intrinsic density variation inside the initial amorphous aggregates and the Al zoning in the outer surface of the ZSM-5 particles. Amorphous silica with low crystallinity formed at early stages and low Al concentration, which has been subsequently dissolved and recrystallized on the ZSM-5 particle surface through Ostwald ripening, leading to the formation of HZZ. This approach, which uses a high concentration of SDA, will provide new possibilities and insight into the prospective fabrication of hollow zeolites.     

Hollow STW-Type Zeolite Single Crystals with Aluminum Gradient for Highly Selective Production of p-Xylene from Methanol-Toluene Alkylation

Zeolites with uniform micropores are important shape-selective catalysts. However, the external acid sites of zeolites have a negative impact on shape-selective catalysis, and the microporosity may lead to serious diffusion limitation. Herein, we report on the direct synthesis of hierarchical hollow STW-type zeolite single crystals with a siliceous exterior. In an alkalinous fluoride medium, the nucleation of highly siliceous STW zeolites takes place first, and the nanocrystals are preferentially aligned on the outer surface of the gel agglomerates to grow into single crystalline shells upon crystallization. The lagged crystallization of the internal Al-rich amorphous gels onto the inner surface of nanocrystalline zeolite shells leads to the formation of hollow cavities in the core of the zeolite crystals. The hollow zeolite single crystals possess a low-to-high aluminum gradient from the surface to the core, resulting in an intrinsic inert external surface, and exhibit superior catalytic performance in toluene methylation reactions.     

Formation of small pore SAPO-44 type molecular sieve

The hydrothermal transformation of silico–aluminophosphate gel with cyclohexylamine to SAPO-44 has been examined. The hydrothermal crystallisation products of the SAPO have been investigated by X-ray diffraction, FTIR, nitrogen and water adsorption, thermogravimetric analysis, surface analysis and ²⁷Al, ³¹P, and ²⁹Si MAS NMR. Structural changes were observed in the silico–aluminophosphate gel with and without organic template and during the hydrothermal crystallisation. The silico–aluminophosphate gel converted to pure SAPO-CHA phase in 168 h at 473 K. The surface of SAPO-44 was silicon rich as compared with that of SAPO-34 and SAPO-18. The ²⁷Al MAS NMR signal of tetrahedrally coordinated Al observed in the silico–aluminophosphate gel without the organic template was changed to octa-, penta- and tetrahedrally coordinated aluminium upon the addition of the cyclohexylamine template to the SAPO gel. After 3 h of hydrothermal treatment at 473 K however, the ²⁷Al MAS NMR signals of the octahedral and pentacoordinated aluminium were removed. This was also confirmed by ³¹P and ²⁹Si MAS NMR. The tetrahedrally coordinated P and Si were detected within 3 h at 473 K. The sorption capacity and adsorption–desorption trends of the SAPO gels and the crystallisation products were found to be different. ²⁹Si MAS NMR results indicated that the percentage of Si (4Al) and its distribution were significantly affected by the crystallization period. SAPO-44 was thermally stable up to 973 K with phase change observed over the calcination temperature of 1193 K. The SAPO gels and the crystallisation products have also been investigated for their catalytic behaviour in n-hexane and ethanol conversion reactions.     

Magic-Angle-Spinning NMR (MAS-NMR) Spectroscopy and the Structure of Zeolites

After outlining the chemical features and properties which make zeolites such an important group of catalysts and sorbents, the article explains how high-resolution solid-state NMR with magic-angle spinning reveals numerous new insights into their structure. ²⁹Si-MAS-NMR readily and quantitatively identifies five distinct Si(OAl)_n(OSi)_4-n structural groups in zeolitic frameworks (n = 0, 1,….4), corresponding to the first tetrahedral coordination shell of a silicon atom. Many catalytic and other chemical properties of zeolites are governed by the short-range Si, Al order, the nature of which is greatly clarified by ²⁹Si-MAS-NMR. It is shown that, as expected from Pauling's electroneutrality principle and Loewenstein's rule, both in zeolite X and in zeolite A (with Si/Al = 1.00) there are no ? Al? O? Al? linkages. In zeolite A and zeolite X with Si/Al = 1.00 there is strict alternation of Si and Al on the tetrahedral sites. Ordering models for Si/Al ratios up to 5.00 (in zeolite Y) may also be evaluated by a combination of MAS-NMR experiments and computational procedures. ²⁹Si-MAS-NMR spectra reveal the presence of numerous crystallographically distinct Si(OSi)₄ sites in silicalite/ZSM-5, suggesting that the correct space group for these related porosilicates is not Pnma. ²⁷Al-MAS-NMR clearly distinguishes tetrahedrally and octahedrally coordinated aluminum, proving that, contrary to earlier claims, Al in silicalite is tetrahedrally substituted within the framework. In combination, ²⁹Si- and ²⁷Al-MAS-NMR is a powerful tool for monitoring the course of solid-state processes (such as ultrastabilization of synthetic faujasites) and of gas-solid reactions (dealumination of zeolites with silicon tetrachloride vapor at elevated temperatures). They also permit the quantitative determination of framework Si/Al ratios in the region 1.00 < Si/Al < 10 000. Since most elements in the periodic table may be accommodated within zeolite structures, either as part of the exchangeable cations or as building units of the anionic framework, there is immense scope for investigation by MAS-NMR and its variants (cross-polarization, multiple pulse and variable-angle spinning) of bulk, surface and chemical properties. Some of the directions in which future research in zeolite science may proceed are adumbrated.     

同晶外延生长法合成MFI型核壳复合分子筛

采用二次热液结晶法,以四丙基氢氧化铵水溶液预处理过的低硅ZSM-5分子筛为晶核,通过调控p H值、水量和晶化时间等二次结晶条件,在晶核上外延生长了高硅ZSM-5壳,制备了MFI/MFI核壳型复合分子筛。通过X射线衍射、扫描电镜、能量色散谱仪、透射电子显微镜、N2吸附-脱附和NH3-程序升温脱附等手段表征了所合成的核壳分子筛的晶体结构、表面形态及核/壳界面,并对它们的结构参数以及酸性进行了初步评估。结果表明,核壳复合分子筛的壳层由多层200 nm的MFI沸石晶粒组成;高硅ZSM-5分子筛壳层的生成,引入了介孔结构,显著增大了外比表面积;同时,核壳结构的形成降低了复合分子筛酸性和外表面的酸密度,但增加了弱酸量。当二次晶化母液p H值为8.5,H2O/SO2物质的量比为30,晶化时间为24 h时,高硅分子筛壳层更易可控生长。     

Mesopore Engineering for Well-Defined Mesoporosity in Al-Rich Aluminosilicate Zeolites

Desilication has been proven an effective approach for the construction of well-defined hierarchical porosities inside zeolites with an optimal framework Al content (Si/Al=25–50). However, for the Al-rich aluminosilicate zeolites, desilication is constrained by the excess and extensive shielding effects from high Al-contents. The developments in the desilication of siliceous zeolites convey a simplified principle of controlled dissolution of the microporous matrix for the construction of hierarchical porosities, which benefits the innovation of synthetic approaches for Al-rich zeolites. The perturbations to the environments of framework Al species may alleviate the excess shielding effects. This review highlights two corresponding protocols of sequential “fluorination–desilication” and “steaming–desilication” for the construction of hierarchical porosities inside Al-rich ZSM-5 zeolites. The success of these two protocols revitalizes the prevailing understanding of the interplay between dealumination and desilication, and implies the necessity of investigating the overlooked roles of extra-framework Al species. Despite the long history and significant achievements in the last decade, fundamental understandings at the molecule level are still limited for the desilication-based top-down approaches. In particular, the investigations on Al-rich zeolites just find their growing. The bridging of dealumination and desilication is essential for other industrially relevant Al-rich zeolites (e.g., faujasite zeolites). The complexities in the inherent characters (topology, spatial distribution, proximity, etc.) and apparent parameters (morphology, crystal/particle size, etc.) demand constructive synthetic toolboxes and further fundamental understanding.     

Syntheses and Characterization of ZSM-5 Zeolites Containing Diethylamine and Ethylenediamine

Syntheses of ZSM-5 zeolites from R-SiO2-Al2O3-H2O-HF(R = diethylamine and ethylenediamine, respectively) were investigated by using the hydrothermal crystallization. The large single crystals of ZSM-5 containing diethylamine and ethylenediamine were synthesized. The ZSM-5 precursors were characterized by means of X-ray powder diffraction, scanning electron microscopy, thermal analysis and electron microprobe analysis. The analysis result of aluminium distribution shows that the interior aluminium content of large crystals of ZSM-5 zeolites is rather low.     

129Xe NMR study of ZSM-5 type zeolites Effect of cationic sites

A comprehensive ¹²⁹Xe NMR spectroscopy study on H-ZSM-5 zeolites having different aluminum contents and on cation-exchanged ZSM-5 zeolites is reported. The parent H-ZSM-5 zeolites were ion-exchanged with Group I–III metal ions ( K, Ca, Sr, Ba, Al, La) to varying degrees. The chemical shift of adsorbed ¹²⁹Xe is seen to be a function of the pentasil structure of ZSM-5, of the number of free Brønsted acid sites and of the number of metal cations in the framework. Differences in the chemical shift of ¹²⁹Xe are seen between cations due to their different polarizing forces against xenon. The amount of cations has also an effect on the δ_xe-xe term in Fraissard's equation that may be caused by changes in the diffusional characteristics of Xe atoms in the ZSM-5 framework.     

A theoretical study of hydrothermal stability of P-modified ZSM-5 zeolites

Density functional theory was employed to study the hydrothermal stability of P-modified ZSM-5 zeolites using cluster models. The calculations of hydrolysis energies indicated that the introduction of phos-phorus increases the hydrothermal stability of ZSM-5 zeolites. The initial paths of dealumination were studied with explicit water molecules. It was found that the framework Al—O coordination bond can be replaced by coodination bonds between water molecules and the aluminium. One to three water molecules ...     

Oxidative dehydrogenation of isobutane to isobutylene over supported transition metal oxide catalysts

Para-selectivity of ZSM-5 zeolites with similar bulk Si/Al ratio, but different particle size and surface Al concentration has been investigated in toluene disproportionation. Results showed that enhancedpara-selectivity is a consequence not only of the particle size but also of the external surface aluminium concentration in the particles.     

Synthesis of mesoporous zeolite single crystals with cheap porogens

Mesoporous zeolite (silicalite-1, ZSM-5, TS-1) single crystals have been successfully synthesized by adding soluble starch or sodium carboxymethyl cellulose (CMC) to a conventional zeolite synthesis system. The obtained samples were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), transmission electron microscopy (TEM), nitrogen sorption analysis, ²⁷Al magic angle spinning nuclear magnetic resonance (²⁷Al MAS NMR), temperature-programmed desorption of ammonia (NH₃-TPD) and ultraviolet-visible spectroscopy (UV-vis). The SEM images clearly show that all zeolite crystals possess the similar morphology with particle size of about 300 nm, the TEM images reveal that irregular intracrystal pores are randomly distributed in the whole crystal. ²⁷Al MAS NMR spectra indicate that nearly all of the Al atoms are in tetrahedral co-ordination in ZSM-5, UV-vis spectra confirm that nearly all of titanium atoms are incorporated into the framework of TS-1. The catalytic activity of meso-ZSM-5 in acetalization of cyclohexanone and meso-TS-1 in hydroxylation of phenol was also studied. The synthesis method reported in this paper is cost-effective and environmental friendly, can be easily expended to prepare other hierarchical structured zeolites.     

ZSM－5（核）／AlPO4－5（壳）双结构分子筛的合成与催化裂化性能

 采用X射线衍射、扫描电镜、X射线能量散射谱、红外光谱和核磁共振等多种物化方法对合成的ZSM－5（核）／AlPO4－5（壳）双结构分子筛进行了表征，证明这种材料具有以ZSM－5为核层、以AlPO4－5为壳层的双结构特征．考察了合成条件对ZSM－5（核）／AlPO4－5（壳）分子筛形貌的影响，发现ZSM－5分子筛的加入方式对产物的形貌有较大影响．重油裂化反应结果表明，ZSM－5（核）／AlPO4－5（壳）双结构分子筛的催化性能比ZSM－5和ZSM－5／AlPO4－5机械混合分子筛样品好，表现为原油转化率和低碳烯烃、汽油及柴油收率提高．