Interconnected Hierarchical ZSM-5 with Tunable Acidity Prepared by a Dealumination-Realumination Process: A Superior MTP Catalyst

Unravelling the Crucial of Spatial Al Distribution to Realize Precise Alkali-Treatment for Target Acid-Catalyzed Reactions

Constructing mesoporous structure within zeolites by alkali-treatment is an effective protocol to improve their diffusion properties. However, undesirable changes in Brönsted acid site (BAS) densities always offset this advantage in acid-catalyzed reactions. In this context, the crucial roles of spatial aluminum (Al) distribution were unraveled during alkali-treatment of MFI zeolite and the desirable BAS density was achieved in obtained hierarchical samples for the target reactions. Various characterization methods, particularly the multiple one- and two-dimensional magic-angle spinning (MAS) NMR techniques, were performed to track the alkali-treatment processes. For the sample with a more uniform spatial Al distribution, more tetrahedral Al sites would fall off and migrate around the Si-OH in zeolite as Al(OH)4 -. Those re-deposited Al(OH)4 - sites were easily transformed into NMR-invisible Al sites during the calcination process, which contributed negligibly to both Brönsted and Lewis acidities, thus being referred to "acid-free" Al species. While most tetrahedral Al sites were preserved after the alkali-treatment of sample with non-uniform Al distribution and the BAS density gradually increased with treatment time. According to the requirements of typical acid-catalyzed reactions, such as catalytic cracking of 1,3,5-triisopropylbenzene and methanol-to-olefins, the desired hierarchical zeolite catalysts were developed by matching the amounts of extracted Si and generated "acid-free" Al during the precise alkali-treatment.

Construction of diverse hollow MFI zeolites through regulating the micropore filling agents

Constructing hollow structure into microporous zeolites can improve the accessibility of acid sites located at the inner part and the diffusion property. Hence, the development of an efficient synthesis strategy to acquire zeolites with tunable hollow structures and acidity has attracted much attention. In this work, an innovative tandem synthesis route was proposed to prepare MFI zeolites with diverse hollow structure while maintaining solid yields exceeding 90 %. The substitution of ethanol molecules, which previously occupied the micropores, with tetrapropylammonium cations was proved to be the key factor to construct hollow structure. And a crystallization-driven particle dissolution mechanism was proposed. The dimension of the hollow cavity, particle size, and Si/Al ratio can be flexibly regulated. Interestingly, hollow MFI samples featuring the common cavity structure, "eye-like" cavity structure, or double-cavity structure can be directly synthesized by controlling the dissolution of core parts. In the 1-butene catalytic cracking reactions, a much higher conversion of 67.2 % was acquired over hollow ZSM-5 compared with that over conventional ZSM-5 (35.8 %) after 64 h of reaction. This improvement can be attributed to the eightfold increase of diffusivity in hollow ZSM-5. This facile and efficient synthesis method endows accurate regulation of the hollow structure, which is meaningful for both fundamental research and industrial applications of hollow zeolites.

Catalytic performance and mechanism study of the isomerization of 2,5-dichlorotoluene to 2,4-dichlorotoluene

This work investigates the influence of catalyst HZSM-5 on the isomerization of 2,5-dichlorotoluene (2,5-DCT) to produce 2,4-dichlorotoluene (2,4-DCT). We observe that hydrothermal treatment leads to a decrease in total acidity and Brønsted/Lewis ratio of HZSM-5 while generating new secondary pores. These characteristics result in excellent selectivity for post-hydrothermal modified HZSM-5 in the isomerization reaction from 2,5-DCT to 2,4-DCT. Under atmospheric pressure at 350 °C, unmodified HZSM-5 achieves a selectivity of 66.4% for producing 2,4-DCT, however after hydrothermal modification the selectivity increases to 78.7%. Density Functional Theory (DFT) calculations explore the thermodynamic aspects of adsorption between the HZSM-5 surface and 2,4-DCT. The kinetic perspective investigates the mechanism involving proton attack on the methyl group of 2,5-DCT followed by rearrangement leading to formation of 2,4-DCT during isomerization. The consistency between simulation and experimental results provides evidence for the feasibility of isomerizing 2,5-DCT to 2,4-DCT. This work fills the gap in the low value-added product 2,5-DCT isomer conversion, indicating its significant practical application potential and provides a valuable reference and guidelines for industrial research in this field.

Realizing of ZSM-5 microspheres with enhanced catalytic properties prepared from iron ore tailings via solid-phase conversion method

The comprehensive utilization of iron ore tailings (IOTs) not only resolved environmental problems but also brought huge economic benefits. In this study, the synthetic route presented herein provides a novel method for the synthesis of ZSM-5 microspheres from IOTs. The effects of Si/Al molar ratios and the pH of the precursor solution on the formation of zeolite was evaluated by various analytical methods. The catalytic performance of the catalyst prepared by the solid-phase conversion method (denoted as MP-ZSM-5) was evaluated by methanol-to-propylene (MTP) reaction. Compared with the zeolite catalyst that synthesized via the conventional hydrothermal method (denoted as HM-ZSM-5), MP-ZSM-5 not only prolongs catalytic lifetime from 18.7 to 36.0 h but also has higher selectivity for propylene by MP-ZSM-5 (43.7%) than that for HM-ZSM-5 (38.6%). In addition, Kissinger-Akahira-Sunose (KAS) model is applied to the TG result to study the template removal process kinetics. The average activation energy values required for the removal of CTAB and TPABr are 201.11 ± 13.42 and 326.88 ± 16.91 kJ∙mol^-1, respectively. Furthermore, this result is well coupled with the model-free kinetic algorithms to determine the conversion and isoconversion of the TPABr and CTAB decomposition in ZSM-5, which serves as important guidelines for the industrial production process.

Construction of a One-Dimensional Al-Rich ZSM-48 Zeolite with a Hollow Structure

Diffusion limitation and acid deficiency are two main challenges that the ZSM-48 zeolite faces in practical application. To date, there have been few effective strategies to solve both problems, simultaneously. Also, it is also a challenge to construct a hollow structure in a one-dimensional (1D) zeolite. Herein, an Al-rich ZSM-48 zeolite with a hollow structure is constructed through an alumination-recrystallization strategy, thereby solving the problems related to diffusion and acidity simultaneously. The hollowness and enrichment of aluminum can be controlled by judiciously matching the desilication and recrystallization. The silica to alumina ratio (SAR) of the ZSM-48 zeolite can be tuned from 130 to 45, which breaks the SAR limitation of conventional synthesis. On the basis of the different characterization results, the whole crystallization can be divided into two stages: rapid desilication-alumination and time-consuming recrystallization. In the selective desilication-recrystallization process, the preferential special distribution of the organic template leads to the formation of a hollow structure and the healing of mesopores at the outer shell, as evidenced by structured illumination microscopy images. Due to the enhancement in diffusion ability and acid density, the obtained hollow Al-rich ZSM-48 zeolite exhibits excellent catalytic stability and high p-xylene yield (∼26%) in the m-xylene isomerization reaction (WHSV = 18 h^-1), indicating its strong industrial application potential.

Hollow ZSM-5 zeolite encapsulating Pt nanoparticles: Cage-confinement effects for the enhanced catalytic oxidation of benzene

A zeolitic cage was introduced and rationally fabricated by encapsulating Pt nanoparticles (NPs) in hollow ZSM-5, a nanomaterial with a cavity and porous shell, for efficient catalytic oxidation of benzene. The structure and formation of the zeolitic cage were systematically investigated and characterized using transmission electron microscopy, nitrogen sorption investigations, X-ray photoelectron spectroscopy, nuclear magnetic resonance spectroscopy, and X-ray diffraction. The obtained hollow 0.2 Pt@ZSM-5 exhibited a comparable low-temperature catalytic activity with 0.5Pt/ZSM-5 with T₉₀ value of 178 °C. Various characterization techniques combined with adsorption experiments uncover the tremendous role of the zeolitic cage in the catalytic activity toward benzene oxidation. The porous shell prevented benzene dilution and the acidity originating from the hollow interior of ZSM-5 promoted the storage of benzene, thereby forming a high local concentration of benzene around Pt NPs, resulting in excellent catalytic performance. These findings provide valuable insights into the rational design of efficient catalysts for the catalytic oxidation of volatile organic compounds.

Effect of acid distribution and pore structure of ZSM-5 on catalytic performance

Zeolite with crystal intracrystalline pore structure with less acid on the outer surface.

Constructing single-crystalline hierarchical plate-like ZSM-5 zeolites with short b-axis length in the synthesis for catalyzing MTO reaction

ZSM-5 zeolite with hierarchical and lamellar structure is highly desired in industrial application. This paper reports an efficient additive, tetramethylguanidine (TMG), modifying crystal growth of the zeolite to this morphology....

Direct Preparation of *MRE Zeolites with Ultralarge Mesoporosity: Strategy and Working Mechanism

Introduction of mesopore is critical for applications where mass-transport limitations within microporous networks, especially for zeolite with one-dimensional microporous network, hinder their performance. Generally, the creation of mesopore in zeolite through a direct synthesis route is strongly dependent on complex and expensive organic molecules, which limits their commercial application. Here, we successfully developed a facile synthesis route for preparing ZSM-48 zeolite (*MRE topology) with ultralarge mesoporosity in which typical 1,6-hexylenediamine worked as an organic structure-directing agent, innovatively assisted by a simple crystal growth modifier (tetraethylammonium bromide, TEABr). The working mechanism of TEABr during crystallization was revealed and proposed on the basis of TEM, thermal gravimetric mass spectrum, and ¹³C cross-polarization magic angle spinning NMR characterization results. In the process, TEA⁺ ions preferentially interacted with the solid during the induction period, which effectively suppressed the aggregation of ZSM-48 primary nanorods. As a result, ultralarge mesoporosity of 0.97 cm³·g^-1 was constructed through the stacking of the nanorods. Interestingly, TEA⁺ ions only took part in the crystallization process and did not occlude in the pores of the final zeolites indicating its potential in recyclability. Moreover, similar synthesis strategy could be applied for the preparation of hierarchical ferrierite zeolites, implying the universality of this strategy. Compared with a conventional sample, ZSM-48 zeolite with ultralarge mesoporosity showed superior catalytic stability in the m-xylene isomerization reaction due to its significantly enhanced diffusion and mass transfer capability, which will greatly promote the practical application of ZSM-48 zeolite.

Manganese supported on controlled dealumination Y-zeolite for ozone catalytic oxidation of low concentration toluene at low temperature

Low-temperature catalytic degradation of VOCs with ozone has received widespread attention recently. In this work, a combination method of steam and nitric acid was used to control the dealuminization of Y zeolite, and then manganese oxide was loaded on the Y zeolite by impregnation method. It was found that MnO_x was highly dispersed in the dealumination zeolite, and the adsorbed oxygens were more easily activated in the active oxygen vacancies. The MnO_x supported on dealumination Y zeolite showed better catalytic effect than that supported on the parent Y. At low humidity (0.8%) in 30 °C, the degradation efficiency of toluene reached above 94% by using the catalyst with mild dealumination. When more water vapor was introduced, the degradation of toluene was inhibited. However, the catalytic performance of the catalyst with deep dealumination was not affected. With the help of in-situ DRIFTS, it was observed that the intermediates and reaction by-products had changed under different humidity conditions.

The effect of hierarchical single-crystal ZSM-5 zeolites with different Si/Al ratios on its pore structure and catalytic performance

Hierarchical single-crystal ZSM-5 zeolites with different Si/Al ratios (Hier-ZSM-5-x, where x = 50, 100, 150 and 200) were synthesized using an ordered mesoporous carbon-silica composite as hard template. Hier-ZSM-5-x exhibits improved mass transport properties, excellent mechanical and hydrothermal stability, and higher catalytic activity than commercial bulk zeolites in the benzyl alcohol self-etherification reaction. Results show that a decrease in the Si/Al ratio in hierarchical single-crystal ZSM-5 zeolites leads to a significant increase in the acidity and the density of micropores, which increases the final catalytic conversion. The effect of porous hierarchy on the diffusion of active sites and the final catalytic activity was also studied by comparing the catalytic conversion after selectively designed poisoned acid sites. These poisoned Hier-ZSM-5-x shows much higher catalytic conversion than the poisoned commercial ZSM-5 zeolite, which indicates that the numerous intracrystalline mesopores significantly reduce the diffusion path of the reactant, leading to the faster diffusion inside the zeolite to contact with the acid sites in the micropores predominating in ZSM-5 zeolites. This study can be extended to develop a series of hierarchical single-crystal zeolites with expected catalytic performance.

Direct Ink Writing of Metal Oxide/H-ZSM-5 Catalysts for n-Hexane Cracking: A New Method of Additive Manufacturing with High Metal Oxide Loading

Previously, 3D printing of porous materials and metal oxides was limited to low loading metal loadings, as increasing the nitrate salt concentrations, which are used to generate the oxide component, gave rise to poor rheological properties beyond 10 wt %. In this study, we addressed this problem by directly printing insoluble oxides alongside H-ZSM-5 zeolite, which allowed for increased oxide loadings. Various metal oxides such as V₂O₅, ZrO₂, Cr₂O₃, and Ga₂O₃ were doped onto H-ZSM-5 through the additive manufacturing method. Characterization and correlation between the X-ray diffraction, NH₃-temperature-programmed desorption, O₂-temperature programmed oxidation, temperature-programmed reduction, scanning electron microscopy-energy dispersive spectroscopy, and in situ CO₂ DRIFTS experiments revealed that directly 3D printing metal oxides/H-ZSM-5 inks leads to significant modification in the surface properties and oxide bulk dispersion, thereby enhancing the composites' reducibility and giving rise to widely differing product distributions in n-hexane cracking reaction. The obtained metal oxide/zeolite structured materials were used as bifunctional structured catalysts for the selective formation of light olefins from hexane at 550-600 °C and GHSV = 9000 mL/g_catalst·h in a packed-bed reactor. Among the various compositions of metal oxides/H-ZSM-5 examined (i.e., 15 wt % Ga₂O₃, 15 wt % ZrO₂, 15 wt % V₂O₅, 15 wt % Cr₂O₃, or 5 wt % Cr/10 wt % ZrO₂/10 wt % V₂O₅/10 wt % Ga₂O₃ balanced with H-ZSM-5), the 15 wt % Cr/ZSM-5 monolith displayed the best n-hexane cracking performance, as it achieved 80-85% conversion of hexane with a 40% selectivity toward propylene, 30% selectivity toward ethylene, and 10% selectivity toward butene at 550 °C. The sample also showed zero benzene/toluene/xylene selectivity and no deactivation after 6 h. This study represents a proof-of-concept for tailoring customizable heterogeneous structured catalysts by directly 3D printing high loading of metal oxides/porous zeolite and is a breakthrough in material science.

State of the Art and Perspectives of Hierarchical Zeolites: Practical Overview of Synthesis Methods and Use in Catalysis

Microporous zeolites have proven to be of great importance in many chemical processes. Yet, they often suffer from diffusion limitations causing inefficient use of the available catalytically active sites. To address this problem, hierarchical zeolites have been developed, which extensively improve the catalytic performance. There is a multitude of recent literature describing synthesis of and catalysis with these hierarchical zeolites. This review attempts to organize and overview this literature (of the last 5 years), with emphasis on the most important advances with regard to synthesis and application of such zeolites. Special attention is paid to the most common and important 10- and 12-membered ring zeolites (MTT, TON, FER, MFI, MOR, FAU, and *BEA). In contrast to previous reviews, the research per zeolite topology is brought together and discussed here. This allows the reader to instantly find the best synthesis method in accordance to the desired zeolite properties. A summarizing graph is made available to enable the reader to select suitable synthesis procedures based on zeolite acidity and mesoporosity, the two most important tunable properties.

Surface-Protection-Induced Controllable Restructuring of Pores and Acid Sites of the Nano-ZSM-5 Catalyst and Its Influence on the Catalytic Conversion of Methanol to Hydrocarbons

Creating mesopores for the nano-ZSM-5 catalyst could further promote the diffusion of molecules in its micropores and improve the catalytic activity and stability. Inorganic alkali treatment of ZSM-5 usually removes internal silica for the existence of an aluminum distribution gradient and leads to a hollow structure. Herein, surface TPA⁺ adsorption-induced protective desilication and recrystallization successively occurred during hydrothermal treatment, and controllable mesopore fabrication was achieved. The evolution of mesopores and acid sites was characterized by N₂ physisorption, XRD, XRF, TEM, NH₃-TPD, Py-IR, ²⁷Al MAS NMR, ²⁹Si MAS NMR, and TG techniques. It was found that the TPAOH concentration influenced the formation of internal cavity and mesopores in the shell. Introducing TPABr into TPAOH solution increased the surface protection because of the increased TPA⁺ adsorption, and coated hollow ZSM-5 was obtained. The acidity was restructured during the above mesopore fabrication. High-concentration TPAOH solution promoted the insertion of destructive Al into the skeleton structure to form strong acid sites, and the catalytic lifetime was recovered and even obviously prolonged. This reflected the key role of strong acid sites on the catalytic performance. Applying hollow nano-ZSM-5 with a mesoporous shell and strong acidity increased the lifetime by 50% and the conversion capacity for liquid hydrocarbon by 20% compared to the parent sample.

Highly Selective Hierarchical ZnO/ZSM-5 Catalysts for Propane Aromatization

Hierarchical ZnO/ZSM-5 catalysts were prepared by desilication and impregnation with 2 wt % metallic ZnO. X-ray diffraction and Fourier transform infrared (FTIR) results showed that the structures of the hierarchical zeolites were relatively preserved despite desilication but were accompanied with sequential loss in crystallinity, likewise Bro̷nsted acidity causing decline in conversion or activity of the catalyst. However, pyridine FTIR shows enhancement of the Bro̷nsted acidic sites. Throughout the activity test, the hierarchical ZnO/ZSM-5 catalysts showed an outstanding performance within 5 h on stream with the average aromatic (benzene, toluene, and xylenes) selectivity trend, represented by their NaOH concentrations 0.3 M > 0.4 M > 0.2 M > 0.1 M corresponding to 61.0, 53.5, 40.3, and 36.8%, respectively. Their average propane conversions within the same period followed a consecutive trend 0.1 M > 0.2 M > 0.3 M > 0.4 M conforming to 34.1, 24.8, 17.3, and 10.2%, respectively. These were compared with that of the reference (ZnO/ZSM-5), which exhibited an average aromatic selectivity of 25.2% and propane conversion of 39.7%. Furthermore, the hierarchical catalyst generally displayed a low amount of C9+ heavier aromatics with the ZnO/ZSM-5(0.3 M) catalyst having the lowest C9+ selectivity of 23.7% compared to the reference catalyst with 72.7% at the same time on stream.

Direct Synthesis of Aluminosilicate IWR Zeolite from a Strong Interaction between Zeolite Framework and Organic Template

A large amount of zeolite structures are still not synthetically available or not available in the form of aluminosilicate currently. Despite significant progress in the development of predictive concepts for zeolite synthesis, accessing some of these new materials is still challenging. One example is the IWR structure as well. Despite successful synthesis of Ge-based IWR zeolites, direct synthesis of aluminosilicate IWR zeolite is still not successful. In this report we show how a suitable organic structure directing agent (OSDA), through modeling of an OSDA/zeolite cage interaction, could access directly the aluminum-containing IWR structure (denoted as COE-6), which might allow access to new classes of materials and thus open opportunities in valuable chemical applications. The experimental results reveal that the COE-6 zeolites with a SiO₂/Al₂O₃ ratio as low as 30 could be obtained. Very interestingly, the COE-6 zeolite has much higher hydrothermal and thermal stabilities than those of the conventional Ge-Al-IWR zeolite. In methanol-to-propylene (MTP) reaction, the COE-6 zeolite exhibits excellent selectivity for propylene, offering a potential catalyst for MTP reaction in the future.

Facile creation of hierarchical nano-sized ZSM-5 with a large external surface area via desilication–recrystallization of silicalite-1 for conversion of methanol to hydrocarbons

Mesoporous ZSM-5 with large external surface and strong acidity were created through desilication–recrystallization of nano-sized silicalite-1, and the catalytic lifetime of its regenerated catalyst is 2.5 times longer than fresh catalyst.

Combined solid-state NMR, FT-IR and computational studies on layered and porous materials

Understanding the structure-property relationship of solids is of utmost relevance for efficient chemical processes and technological applications in industries. This contribution reviews the concept of coupling three well-known characterization techniques (solid-state NMR, FT-IR and computational methods) for the study of solid state materials which possess 2D and 3D architectures and discusses the way it will benefit the scientific communities. It highlights the most fundamental and applied aspects of the proactive combined approach strategies to gather information at a molecular level. The integrated approach involving multiple spectroscopic and computational methods allows achieving an in-depth understanding of the surface, interfacial and confined space processes that are beneficial for the establishment of structure-property relationships. The role of ssNMR/FT-IR spectroscopic properties of probe molecules in monitoring the strength and distribution of catalytic active sites and their accessibility at the porous/layered surface is discussed. Both experimental and theoretical aspects will be considered by reporting relevant examples. This review also identifies and discusses the progress, challenges and future prospects in the field of synthesis and applications of layered and porous solids.

Protective desilication of highly siliceous H-ZSM-5 by sole tetraethylammonium hydroxide for the methanol to propylene (MTP) reaction

Protective desilication of highly siliceous H-ZSM-5 was effectively realized by dissolution and recrystallization in tetraethylammonium hydroxide (TEAOH) solution. With better balance between dissolution of OH⁻ and recrystallization of TEA⁺, intracrystalline mesopores could be generated by selective dissolution of Si by the drilling effects of TEAOH on the micropores, and then Si species in the mother liquor near the external surface could be recrystallized into ZSM-5 shell. With a significantly reduced diffusion length provided by the intracrystalline mesopores, TEAOH-treated samples exhibited longer lifetime and higher propylene selectivity than the parent H-ZSM-5 zeolite. The mediumly-treated T-16 h sample possessed the longest MTP lifetime of 140 h, 5.6 times that of the parent H-ZSM-5 zeolite. Furthermore, the coke content and adsorbed methyl benzene species on the T-16 h sample were heavier than those on the parent H-ZSM-5 sample, which were related to the intracrystalline mesopore structure.

Protective desilication of highly siliceous H-ZSM-5 was effectively realized by dissolution and recrystallization in tetraethylammonium hydroxide (TEAOH) solution.