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Molybdenum-modified HZSM-5 catalysts are widely used for methane dehydroaromatization (MDA) but suffer from rapid deactivation due to coke formation. The limited diffusion in zeolitic catalysts promotes coke accumulation in micropores, creating concentration gradients of reactants and products within the crystals. In this study, we propose a synthesis approach to evaluate the impact of removing the crystal core in the Mo/HZSM-5 catalyst on the MDA performance. To achieve this, a parent zeolite (ZP) comprising a silicalite-1 core encased by an HZSM-5 crystal is subjected to two distinct treatments: fluorine (ZF) and alkaline (ZOH). These treatments dissolve the silicon-rich core, creating a central macropore in the crystals leading to the formation of donut-shaped catalysts. The alkaline treatment further generates additional mesopores and silanols by leaching atoms from the zeolite framework. The MDA reaction on molybdenum-impregnated zeolite catalysts (ZP, ZF, and ZOH) at 973 K was carried out. Interestingly, the "donut"-shaped catalysts modify the nature of the coke deposition, relocating the coke from the micropores to the macropores. Consequently, the removal of the crystal core effectively mitigates catalyst deactivation caused by coke formation. Moreover, the alkaline-treated catalyst exhibits additional beneficial properties, such as increased silanols and a higher surface area, which help to limit the sintering of molybdenum carbides during the MDA reaction. The new strategy improves the performance and stability of Mo/HZSM-5 catalysts in methane dehydroaromatization. [ABSTRACT FROM AUTHOR]

The mesopores formation in zeolite crystals has long been considered to occur through the stochastic hydrolysis and removal of framework atoms. Here, we investigate the NH4F etching of representative small, medium, and large pore zeolites and show that the zeolite dissolution behavior, therefore the mesopore formation probability, is dominated by zeolite architecture at both nano‐ and sub‐nano scales. At the nano‐scale, the hidden mosaics of zeolite structure predetermine the spatio‐temporal dissolution of the framework, hence the size, shape, location, and orientation of the mesopores. At the sub‐nano scale, the intrinsic micropore size and connectivity jointly determine the diffusivity of reactant and dissolved products. As a result, the dissolution propensity varies from removing small framework fragments to consuming nanodomains and up to full digestion of the outmost part of zeolite crystals. The new knowledge will lead to new understanding of zeolite dissolution behavior and new adapted strategies for tailoring hierarchical zeolites. [ABSTRACT FROM AUTHOR]

A commercial zeolite L is subjected to chemical etching with aqueous solutions of NH4F and NH4HF2 to generate secondary porosity. The derived samples are characterized by X‐ray diffraction, N2 adsorption, scanning, and transmission electron microscopy, 27Al MAS NMR, thermogravimetric analysis, and their acid sites are quantified by FTIR spectroscopy of adsorbed basic probe molecules. The catalytic properties of the pristine and modified zeolites are evaluated in the dealkylation of 1,3,5‐triisopropylbenzene. Tuning the etching conditions allows to generate hierarchical zeolites without losing micropore volume. Furthermore, both fluoride etchants do not change the acid properties of the zeolite substantially, which is confirmed by chemical composition analysis (ICP‐OES) and by FTIR spectroscopy of pyridine. The dissolution rate of zeolite L is much faster with NH4HF2 than NH4F. The set of experimental data shows that the 1 wt% NH4HF2 solution is an efficient etchant able to generate mesopores without affecting the parent zeolite properties as lower amounts of fluoride source are required than with concentrated (20–40 wt%) NH4F solutions. [ABSTRACT FROM AUTHOR]

Green and efficient synthesis of titanium‐containing molecular sieves is limited by the quantity of environmentally unfriendly additives and complicated synthesis procedures required. Oligomerization of Ti monomers into anatase TiO2 is the typical outcome of such procedures because of a mismatch between hydrolysis rates of Si and Ti precursors. We report a simple and generic additive‐free route for the synthesis of Ti‐containing molecular sieves (MFI, MEL, and BEA). This approach successfully reverses the formation of Ti oligomers to match hydrolysis rates of Ti and Si species with the assistance of hydroxyl free radicals generated in situ from ultraviolet irradiation. Moreover, fantastic catalytic performance for propene epoxidation with H2 and O2 was observed. Compared with the conventional hydrothermal method, this approach opens up new opportunities for high‐efficiency, environmentally benign, and facile production of pure titanium‐containing molecular sieves. [ABSTRACT FROM AUTHOR]

This study reports the seeded synthesis of MFI‐type (ZSM‐5) zeolite in fluoride medium at pH = 8.5. Crystal growth kinetics of the resulting zeolite (ZSM‐5‐F) as a function of seed content and crystallization temperature is studied. The crystallization time is reduced to 1.5 h and crystals with sizes below 200 nm and a Si/Al ratio of 23.6 are obtained. A zeolite with similar characteristics but synthesized in a hydroxyl medium (ZSM‐5‐OH) is used to evaluate ZSM‐5s synthesized in different crystallization media. Their physicochemical properties are compared and particular attention is paid to the nature, number, and distribution of silanol sites. The two zeolites exhibit similar number of Brφnsted acid sites; however the material synthesized in a hydroxyl medium contains a substantially larger number of surface and internal silanols that impact significantly its catalytic performance in methanol to hydrocarbon transformation. While the two materials exhibit similar selectivity in methanol transformations, the catalyst synthesized in fluoride medium shows superior activity and resistance to deactivation. The results suggest that seeded synthesis in a fluoride medium can be used for the preparation of superior zeolite catalysts. [ABSTRACT FROM AUTHOR]

Desilication in alkaline medium has been widely used in construction of hierarchical zeolites for industrially relevant catalytic processes. The built of hierarchy in zeolites, especially with low aluminum stability or high Si/Al ratio, often suffers from uncontrolled destruction of zeolitic framework, accompanied by a significant loss of microporous domains and intrinsic acidity after desilication. Here, we report a novel and simple methodology for preparation of hierarchical zeolites with highly complete framework and minimum sacrifice of microporosity and acidity. The pre-impregnated amines in zeolite micropores act as inner pore-directing agents (iPDAs), largely protecting the zeolitic framework and moderating the silicon extraction during the alkaline treatment. The resulting hierarchical zeolites exhibit high crystallinity, tunable hierarchy, stable framework, and well-preserved acidity, endowing them with significantly improved mass transport properties and enhanced activities in catalytic conversion of methanol or furfural., (© 2024 Wiley‐VCH GmbH.)