Preparation of ZSM‐5 Coated on Monolithic Interconnected Macroporous Al2O3 Using Cheap n‐Butylamine as Template

Using cheap n‐butylamine as template, ZSM‐5 zeolites have been successfully synthesized and coated on monolithic interconnected macroporous Al₂O₃ by the secondary growth method. The use of cheap n‐butylamine could significantly reduce the synthesis cost. Hierarchical monolithic ZSM‐5 zeolites were prepared from synthetic mixtures with different H₂O/Na₂O or SiO₂/Al₂O₃ ratio. The synthesized samples were characterized by scanning electron microscopy (SEM), X‐ray diffraction (XRD), Fourier transform infrared spectroscopy (FT‐IR) and N₂ adsorption‐desorption. The results show that the hierarchical monolithic zeolites were obtained with cheap n‐butylamine template as template. During the hydrothermal reaction process, the morphology of the micrometer‐sized support was well maintained. The irregular crystals were formed in a wide range of the H₂O/Na₂O or SiO₂/Al₂O₃ ratio of synthetic mixtures and coated on monolithic Al₂O₃. The relative crystallinity of the zeolites was highest at H₂O/Na₂O=250 or SiO₂/Al₂O₃=160. This type of composites exhibited hierarchical porous structures and relatively high specific surface areas.     

Theoretical study of the adsorption of ethylene on alkali‐exchanged zeolites

The structures of alkali‐exchanged faujasite (X–FAU, X = Li⁺ or Na⁺ ion) and ZSM‐5 (Li–ZSM‐5) zeolites and their interactions with ethylene have been investigated by means of quantum cluster and embedded cluster approaches at the B3LYP/6‐31G(d, p) level of theory. Inclusion of the Madelung potential from the zeolite framework has a significant effect on the structure and interaction energies of the adsorption complexes and leads to differentiation of different types of zeolites (ZSM‐5 and FAU) that cannot be drawn from a typical quantum cluster model, H₃SiO(X)Al(OH)₂OSiH₃. The Li–ZSM‐5 zeolite is predicted to have a higher Lewis acidity and thus higher ethylene adsorption energy than the Li–FAU zeolites (16.4 vs. 14.4 kcal/mol), in good agreement with the known acidity trend of these two zeolites. On the other hand, the cluster models give virtually the same adsorption energies for both zeolite complexes (8.9 vs. 9.1 kcal/mol). For the larger cation‐exchanged Na–FAU complex, the adsorption energy (11.6 kcal/mol) is predicted to be lower than that of Li–FAU zeolites, which compares well with the experimental estimate of about 9.6 kcal/mol for ethylene adsorption on a less acidic Na–X zeolite. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem 94: 333–340, 2003     

π–π Interactions Between Aromatic Groups in Amphiphilic Molecules: Directing Hierarchical Growth of Porous Zeolites

The development of hierarchical macro‐ or mesoporous zeolites is essential in zeolite synthesis because the size of the micropores limits mass transport and their use as industrial catalysts for bulky molecules. Although major breakthroughs have been achieved, fabricating crystallographically ordered mesoporous zeolites using a templating strategy is still an unsolved challenge. This minireview highlights our recent efforts on the self‐assembly of amphiphilic molecules to obtain ordered hierarchical MFI zeolites by introducing aromatic groups into the hydrophobic tail of the amphiphilic molecules. Owing to the geometric matching between the self‐assembled aromatic tails and the MFI framework, a) single‐crystalline mesostructured zeolite nanosheets (SCZNs), b) SCZNs with a 90° rotational intergrowth structure, c) a hierarchical MFI zeolite with a two‐dimensional square P4mm mesostructure, and d) a single‐crystalline mesoporous ZSM‐5 with three‐dimensional pores and sheetlike mesopores layered along the a‐axis were successfully synthesized.     

Synthesis,characterization and separation properties of a composite mordenite/ZSM-5/chabazite hydrophilic membrane

Composite mordenite/ZSM5/chabazite membranes were prepared on α-alumina tubular supports by in situ liquid phase hydrothermal synthesis. The membranes obtained were approximately 10 μm thick and were characterized by XRD, SEM and EPMA, as well as permeation of single gases (N₂ and n-butane). The membranes were then used to separate ternary mixtures, containing water, alcohol (methanol, ethanol or propanol), and a permanent gas (O₂). Water permeated faster because of the hydrophilic character of the composite membrane, with water/propanol selectivities as high as 149. The influence of operating conditions (temperature, pressure and feed composition) on the separation performance was analyzed. Also, the behavior of this composite zeolite membrane was compared with that of pure silicalite and ZSM5 membranes, and the differences observed are discussed in terms of relative organophilicity/hydrophilicity of the zeolites involved.     

A Simple Route to Synthesize Mesoporous ZSM‐5 Templated by Ammonium‐Modified Chitosan

Uniform mesoporous zeolite ZSM‐5 crystals have been successfully fabricated through a simple hydrothermal synthetic method by utilizing ammonium‐modified chitosan and tetrapropylammonium hydroxide (TPAOH) as the meso‐ and microscale template, respectively. It was revealed that mesopores with diameters of 5–20 nm coexisted with microporous network within mesoporous ZSM‐5 crystals. Ammonium‐modified chitosan was demonstrated to serve as a mesoporogen, self‐assembling with the zeolite precursor through strong static interactions. As expected, the prepared mesoporous ZSM‐5 exhibited greatly enhanced catalytic activities compared with conventional ZSM‐5 and Al‐MCM‐41 in reactions involving bulky molecules, such as the Claisen–Schmidt condensation of 2‐hydroxyacetophenone with benzaldehyde and the esterification reaction of dodecanoic acid and 2‐ethylhexanol.     

Hierarchical ZSM‐5 Zeolites in Shape‐Selective Xylene Isomerization: Role of Mesoporosity and Acid Site Speciation

The isomerization of o‐xylene, a prototypical example of shape‐selective catalysis by zeolites, was investigated on hierarchical porous ZSM‐5. Extensive intracrystalline mesoporosity in ZSM‐5 was introduced by controlled silicon leaching with NaOH. In addition to the development of secondary porosity, the treatment also induced substantial aluminum redistribution, increasing the density of Lewis acid sites located at the external surface of the crystals. However, the strength of the remaining Brønsted sites was not changed. The mesoporous zeolite displayed a higher o‐xylene conversion than its parent, owing to the reduced diffusion limitations. However, the selectivity to p‐xylene decreased, and fast deactivation due to coking occurred. This is mainly due to the deleterious effect of acidity at the substantially increased external surface and near the pore mouths. A consecutive mild HCl washing of the hierarchical zeolite proved effective to increase the p‐xylene selectivity and reduce the deactivation rate. The HCl‐washed hierarchical ZSM‐5 displayed an approximately twofold increase in p‐xylene yield compared to the purely microporous zeolite. The reaction was followed by operando infrared spectroscopy to simultaneously monitor the catalytic performance and the buildup of carbonaceous deposits on the surface. Our results show that the interplay between activity, selectivity, and stability in modified zeolites can be optimized by relatively simple post‐synthesis treatments, such as base leaching (introduction of mesoporosity) and acid washing (surface acidity modification).     

A Novel Method for Synthesis of ZSM‐5 Coatings on Monolithic Cordierite Substrate

ZSM‐5 coatings, have been synthesized onto a monolithic cordierite substrate by an environmental friendly and high coating selectivity method—Vapor Phase Transport (VPT). With this method, an aluminosilicate gel coated onto the monolithic cordierite substrate has been transformed into a ZSM‐5 layer under vapors of n‐butylamine and water, n‐Butylamine played a key role in the forming of ZSM‐5 layer on the cordierite substrate. The ZSM‐5/cordierite monolith composites prepared by this method were ion‐exchanged with Cu²⁺ and tested for the selective catalytic reduction of NO by propane. The deNO_x activities of Cu/ZSM‐5 monolith catalysts were not only dependent on the ion‐exchange methods, but also on the ZSM‐5 loading of the monolith catalysts. The best result was obtained over the Cu (B3)/ZSM‐5 monolith catalyst, which had a ZSM‐5 loading of about 13% and was prepared by a pressure exchange procedure. At a temperature of 723 K and a space velocity of 10,000 h^?1 (based on the monolith volume), 85% of NO conversion and 93% of C₃H₃ conversion were achieved over the Cu(B3)/ZSM‐5 monolith catalyst.     

甲烷在含镓沸石（MFI）上无氧活化性能的研究

采用水热法合成了不同Ｓｉ／Ａｌ、Ｓｉ／Ｇａ比的含镍分子筛，催化反应活性表征结果表明，地丙烷芳构化反应具有良好催化活性的含镍分子。没有显示出任何催化甲烷芳构化反应的特性；ＭｏＯ３作为添加组分显著地改变了催化剂的反应性能，３％ＭｏＯ３负载的含Ｇａ分子筛，在９７３Ｋ温度给出了约６％甲烷转化率和大于８０％的芳烃选择性，与ＺＳＭ－５相比，城相同的ＭｏＯ３担载条件下，含Ｇａ分子筛催化剂甲烷芳构化活性较低，但是     

Gamma‐Ray Irradiation to Accelerate Crystallization of Mesoporous Zeolites

Gamma‐ray (γ‐ray) irradiation was introduced into zeolite synthesis. The crystallization process of zeolite NaA, NaY, Silicalite‐1, and ZSM‐5 were greatly accelerated. The crystallization time of NaA zeolite was significantly decreased to 18 h under γ‐ray irradiation at 20 °C, while more than 102 h was needed for the conventional process. Unexpectedly, more mesopores were created during this process, and thus the adsorption capacity of CO₂ increased by 6‐fold compared to the NaA prepared without γ‐ray irradiation. Solid experimental evidence and density function theory (DFT) calculations demonstrated that hydroxyl free radicals (OH*) generated by γ‐rays accelerated the crystallization of zeolite NaA. Besides NaA, mesoporous ZSM‐5 with MFI topology was also successfully synthesized under γ‐ray irradiation, which possessed excellent catalytic performance for methanol conversion, suggesting the universality of this new synthetic strategy for various zeolites.     

气固相同晶取代法制备Ti-ZSM-5及其催化性能的研究

以Ｂ－ＺＳＭ－５沸石为母体,经过盐酸洗涤脱硼后,采用气固相同晶取代法制备了Ｔｉ－ＺＳＭ－５沸石．考察了制备条件如反应温度、反应时间和载气流速等对产品沸石中钛物种的配位状态和含量的影响．发现进入Ｔｉ－ＺＳＭ－５骨架中的钛含量存在一个极限值．ＦＴ－ＩＲ、ＵＶ－Ｖｉｓ表征技术和丙烯环氧化反应结果表明：不同气固相反应条件下制备的Ｔｉ－ＺＳＭ－５沸石具有不同的物化性质．骨架钛含量高而非骨架钛含量低的钛沸石具有较高的丙烯环氧化催化活性．     

Characterization of acidity in [B], [Al], and [Ga] isomorphously substituted ZSM‐5: Embedded DFT/UFF approach

The structure and electronic properties of the Brønsted acid site in B, Al or Ga isomorphously substituted ZSM‐5 zeolites were studied by using quantum cluster and embedded ONIOM approaches. In the former approach, zeolites are modeled by 5T and 12T quantum clusters, where T represents a Si or Al atom. In the latter model, called “Embedded ONIOM”, the long‐range interactions of the zeolite lattice beyond the 12T quantum cluster is included via optimized point charges added to the ONIOM(B3LYP/6‐31G(d,p):UFF). Inclusion of the extended zeolitic framework covering the nanocavity has an effect on the structure and adsorption properties. We found that the OH distances and v OH of the acidic proton in zeolite obtained from both models can predict the trend of acid strength as: B‐ZSM‐5 < Ga‐ZSM‐5 < Al‐ZSM‐5, which is in very good agreement with the experimental sequence. Furthermore, the PA data calculated from E‐ONIOM is also consistent with the experimental trend: B‐ZSM‐5 < Ga‐ZSM‐5 < Al‐ZSM‐5. It has, therefore, been demonstrated that our embedded ONIOM model provides accurate performance and can be one of the useful and affordable methods for future mechanistic studies involving petrochemical reactions. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

The porosity, acidity, and reactivity of dealuminated zeolite ZSM-5 at the single particle level: the influence of the zeolite architecture

A combination of atomic force microscopy (AFM), high‐resolution scanning electron microscopy (HR‐SEM), focused‐ion‐beam scanning electron microscopy (FIB‐SEM), X‐ray photoelectron spectroscopy (XPS), confocal fluorescence microscopy (CFM), and UV/Vis and synchrotron‐based IR microspectroscopy was used to investigate the dealumination processes of zeolite ZSM‐5 at the individual crystal level. It was shown that steaming has a significant impact on the porosity, acidity, and reactivity of the zeolite materials. The catalytic performance, tested by the styrene oligomerization and methanol‐to‐olefin reactions, led to the conclusion that mild steaming conditions resulted in greatly enhanced acidity and reactivity of dealuminated zeolite ZSM‐5. Interestingly, only residual surface mesoporosity was generated in the mildly steamed ZSM‐5 zeolite, leading to rapid crystal coloration and coking upon catalytic testing and indicating an enhanced deactivation of the zeolites. In contrast, harsh steaming conditions generated 5–50 nm mesopores, extensively improving the accessibility of the zeolites. However, severe dealumination decreased the strength of the Brønsted acid sites, causing a depletion of the overall acidity, which resulted in a major drop in catalytic activity.     

Direct Imaging of Atomically Dispersed Molybdenum that Enables Location of Aluminum in the Framework of Zeolite ZSM‐5

Integrated differential phase‐contrast scanning transmission electron microscopy (iDPC‐STEM) is capable of directly probing guest molecules in zeolites, owing to its sufficient and interpretable image contrast for both heavy and light elements under low‐dose conditions. This unique ability is demonstrated by imaging volatile organic compounds adsorbed in zeolite Silicalite‐1; iDPC‐STEM was then used to investigate molybdenum supported on various zeolites including Silicalite‐1, ZSM‐5, and mordenite. Isolated single‐Mo clusters were observed in the micropores of ZSM‐5, demonstrating the crucial role of framework Al in driving Mo atomically dispersed into the micropores. Importantly, the specific one‐to‐one Mo‐Al interaction makes it possible to locate Al atoms, that is, catalytic active sites, in the ZSM‐5 framework from the images, according to the positions of Mo atoms in the micropores.     

Effect of weak base modification on ZSM‐5 catalyst for methanol to aromatics

The effect of weak base modification on the catalytic performance of ZSM‐5 catalyst for conversion of methanol to aromatics was investigated. The catalysts were characterized using X‐ray diffraction, X‐ray fluorescence, N₂ adsorption–desorption, NH₃ temperature‐programmed desorption, Fourier transform infrared spectroscopy, scanning electron microscopy and thermogravimetry. The results showed that catalysts treated with weak base (NaHCO₃, Na₂CO₃ and NH₃⋅H₂O) exhibited a pore structure with interconnected micropores and mesopores. The existence of mesopores was beneficial for improving the diffusion of reactants and products, and the coke deposition resistance capacity of treated catalysts was enhanced greatly. Meanwhile, compared to traditional ZSM‐5 zeolite, the ratio of Brønsted to Lewis (B/L) acid sites of ZSM‐5/NH₃⋅H₂O (B/L = 7.35) zeolite slightly increased but the amount of acid sites reduced, while those of ZSM‐5/NaHCO₃ (B/L = 0.127) and ZSM‐5/Na₂CO₃ (B/L = 0.107) significantly reduced. Further, the catalyst treated with NH₃⋅H₂O solution was evaluated in the methanol to aromatics reaction and led to an enhanced aromatization reaction rate. The liquid hydrocarbons product distribution exhibited higher aromatic hydrocarbons yield (56.12%) and selectivity (40.28%) of benzene, toluene and xylene (BTX) with isoparaffin content reducing to 26.17%, which could be explained by appropriate B/L acid sites ratio, higher pore volumes and higher surface area.     

分散法制备的CuCl/ZSM-5分子筛催化剂上C3H6选择性催化还原NO反应的研究

研究了由分散法制备的CuCl/ZSM-5分子筛催化剂上丙烯在过量氧存在下选择催化还原NO反应,发现该法能使活性组份高度分散于载体上,且所制备的高负载量CuCl/H-ZSM-5与离子交换法制备的Cu-ZSM-5相比在较低反应温度下具有更高的反应活性.     

Catalytic Para‐Xylene Maximization. V. Toluene Methylation with Methanol and Disproportionation of Toluene Using Pt/ZSM‐5 and Pt/Mordenite Catalysts

Toluene was methylated with methanol and disproportionated using catalysts containing different Pt contents (0.2, 0.4 and 0.6%) supported on H‐ZSM‐5 or H‐mordenite (H‐M) zeolites in a fixed‐bed flow‐reactor operated atmospherically at temperatures of 300–500 °C in a flow of hydrogen. Platinum dispersion in the zeolite supports and acid sites strength distribution were evaluated using hydrogen chemisorption (1:1 stoichiometry) and ammonia temperature programmed desorption (TPD) in a differential scanning calorimeter (DSC). Toluene methylation was much faster on all catalysts than toluene disproportionation (DISP). Both reactions were more accelerated using H‐ZSM‐5 containing catalysts than H‐M containing catalysts. The yield of xylenes, and in particular para‐xylene, was significantly influenced by the yield of trimethylbenzenes (TMBs) in product. The selectivities for para‐, ortho‐ and meta‐xylenes production were found largely dependent on the Pt content in the catalysts, particularly when supported on H‐ZSM5‐zeolite. However, using Pt/H‐M catalysts, these selectivities were not strictly controlled by Pt content in the catalysts.     

Alcohol Dehydrations over ZSM‐5 Type Zeolites,Montmorillonite Clays and Pillared Montmorillonites

The reactions of aliphatic alcohols (ethanol, 1‐propanol and 2‐propanol) were studied at 1 atm and 150–300°C by using ZSM‐5 type zeolites, montmorillonites, and pillared montmorillonites. With H‐ZSM‐5 (X) Y zeolites, the total number of acid sites increases with a decrease of Si0₂/Al₂O₃ molar ratio (X) and calcining temperature (Y). In addition, apparent increase in the ratio of strong to weak acid sites occurs with increasing X or decreasing Y. The acidities of M‐ZSM‐5 (51) 600 zeolites follow the sequence: Li = Na < K < Cs. Pillared clays exhibit both larger surface areas and more acid sites than the clays. The alcohol conversions decrease in the order of 2‐propanol < 1‐propanol < ethanol, in accordance with the relative stabilities of the corresponding carbenium ions. The catalytic activities are parallel to the total number of catalyst acid sites and the reaction temperature. Favorable formations of ethers are observed at low reaction temperature and small contact time on a catalyst with weak acid strength. Simple consecutive reactions and combined pathway of parallel and consecutive reactions are proposed, respectively, for the dehydration of ethanol and for those of 1‐propanol and 2‐propanol.     

Decarboxylation of Lactones over Zn/ZSM‐5: Elucidation of the Structure of the Active Site and Molecular Interactions

Herein, we report the catalytic decarboxylation of γ‐valerolactone (GVL) over Zn/ZSM‐5 to butene, followed by aromatization at high yield with co‐feeding of water. An evaluation of the catalytic performance after prolonged periods of time showed that a water molecule is essential to maintain the decarboxylation and aromatization activities and avoid rapid catalyst deactivation. Synchrotron X‐ray powder diffraction and Rietveld refinement were then used to elucidate the structures of adsorbed GVL and immobilized Zn species in combination with EXAFS and NMR spectroscopy. A new route for the cooperative hydrolysis of GVL by framework Zn−OH and Brønsted acidic sites to butene and then to aromatic compounds has thus been demonstrated. The structures and fundamental pathways for the nucleophilic attack of terminal Zn−OH sites are comparable to those of Zn‐containing enzymes in biological systems.     

异丁烯在不同硅铝比的NanZSM-5型分子筛中的吸附：分子模拟研究

Frameworks of NanZSM-5 type zeolites with various Si/A1 ratios have been constructed and optimized with molecular dynamic quench simulation. The results show that the structure parameters of NanZSM-5 type zeolite, including the bond length and atomic charges, are consistent with those predicted by ab initio cluster calculations. It was also observed that atomic charges of Si atoms were shifted to higher field in NanZSM-5 type zeolite with lower Si/Al ratio. Then, the adsorption of isobutene on NanZSM-5 with various Si/Al ratios has been investigated using grand canonical ensemble Monte Carlo simulation and Cvff-300-1.01 forcefield. The simulated adsorption amount was in good agreement with the experimental data. Based on these facts, the effects of Si/Al ratio on the adsorption amount and adsorption isotherms of isobutene on NanZSM-5 were predicted. The results indicated that Si/Al ratio was important for the adsorption of isobutene and the adsorption amount was decreased as the Si/Al ratio was increased, which can be explained that the atomic charge of Na^＋ cation would influence greatly the π electrons of the isobutene double bond due to the Coulomb force. In addition, the adsorption sites of isobutene and interaction energy of isobutene with NanZSM-5 were also discussed.     

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.