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Alkaline‐earth metal cations impact the rate of crystallization and the adsorption of N2 and CO2 on nanosized chabazite zeolites. In this study, either calcium or barium is added to precursor mixtures containing alkali‐metal cations (Na+, K+, Cs+) to prepare chabazite (labeled Ca‐CHA and Ba‐CHA), and are compared to a reference sample (Ref‐CHA) synthesized using exclusively Na+, K+, and Cs+. Partial substitution of the Na+ and the pore‐blocking Cs+ extra‐framework cations is observed for Ca‐CHA depending on the molar ratio of K2O used in the synthesis, while a change to the amount of Na+ cations only is observed for Ba‐CHA. The type of alkaline‐earth metal cation affects the crystallization rate; slower in the presence of Ca2+ (10 h to full crystallinity) and similar rates in the presence of Ba2+ (4 h to full crystallinity); the crystallite size and morphology remained similar. The presence of Ca2+ or Ba2+ extra‐framework cations leads to N2 uptake values of 290 and 169 mmol g−1 (−196 °C, 100 kPa), respectively, while at low CO2 pressure (<1 kPa, 25 °C), the physisorbed CO2 capacity for Ref‐CHA, Ca‐CHA, and Ba‐CHA zeolites is 0.63, 0.66, and 0.59 mmol g−1, respectively. Interestingly, an opposite effect is observed for the amount of chemisorbed CO2 species. [ABSTRACT FROM AUTHOR]

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]

Silanols are key players in the application performance of zeolites, yet, their localization and hydrogen bonding strength need more studies. The effects of post-synthetic ion exchange on nanosized chabazite (CHA), focusing on the formation of silanols, were studied. The significant alteration of the silanols of the chabazite nanozeolite upon ion exchange and their effect on the CO2 adsorption capacity was revealed by solid-state nuclear magnetic resonance (NMR), Fourier-transform infrared (FTIR) spectroscopy, and periodic density functional theory (DFT) calculations. Both theoretical and experimental results revealed changing the ratio of extra-framework cations in CHA zeolites changes the population of silanols; decreasing the Cs+/K+ ratio creates more silanols. Upon adsorption of CO2, the distribution and strength of the silanols also changed with increased hydrogen bonding, thus revealing an interaction of silanols with CO2 molecules. To the best of our knowledge, this is the first evidence of the interplay between alkali-metal cations and silanols in nanosized CHA. CHA zeolites display high selectivity toward CO2 adsorption, but the influence of the nature and concentration of their extra-framework cations on the formation of silanol sites, and the respective impact on CO2 sorption, is poorly understood. Here, the authors use high-resolution solid-state magic-angle spinning 1H NMR and Fourier-transform infrared spectroscopy to study the effects of post-synthetic exchange of extra-framework alkali-metal cations on silanol sites in nanosized CHA zeolites. [ABSTRACT FROM AUTHOR]

In this work, a new post‐synthetic hydrothermal approach leading to simultaneous dealumination and silylation of zeolite nanocrystals is presented. Starting from a nanosized chabazite (CHA) type zeolite with a Si/Al ratio equal to 2, a set of samples with Si/Al ratios up to 4 are obtained by altering the acidity and concentration of the added silica modifier during the post‐synthesis treatment leading to concurrent dealumination and silylation. The effect of the acidity during the post‐synthetic silylation treatment at different pH of 1.0–8.5 at a constant silicon concentration is investigated. The nanosized CHA samples are characterized in details by Fourier transform infrared spectroscopy and NMR spectroscopy (1H, 29Si, 27Al), X‐ray diffraction, and thermogravimetric analyses. The post‐synthesis approach reported here can be applied to other zeolite types for fine‐tuning of their structural features including type and location of T‐atoms, silanol sites, and active sites. [ABSTRACT FROM AUTHOR]

Intensive research on improving the catalytic properties of zeolites is focused on modulating their acidity and the distribution of associated Al sites. Herein, by studying a series of ZSM‐5 zeolites over a broad range of Al content, we demonstrate how the nature of the mineralizing agent (F− or OH−) used in hydrothermal syntheses directly impacts Al sites distribution. The proportions of Al sites, probed by 27Al NMR, depend on the Si/Al ratio for F−, but remain identical for OH− (from Si/Al=30 to 760). This leads to contrasting variations in weak and strong acidities. Such opposite effect of mineralizers is explained by the spatial location of negative charges and the resulting balance between short‐ and long‐range electrostatic interactions. This understanding paves the way for additional and simple opportunities to control zeolites' acidity. [ABSTRACT FROM AUTHOR]

Intensive research on improving the catalytic properties of zeolites is focused on modulating their acidity and the distribution of associated Al sites. Herein, by studying a series of ZSM‐5 zeolites over a broad range of Al content, we demonstrate how the nature of the mineralizing agent (F− or OH−) used in hydrothermal syntheses directly impacts Al sites distribution. The proportions of Al sites, probed by 27Al NMR, depend on the Si/Al ratio for F−, but remain identical for OH− (from Si/Al=30 to 760). This leads to contrasting variations in weak and strong acidities. Such opposite effect of mineralizers is explained by the spatial location of negative charges and the resulting balance between short‐ and long‐range electrostatic interactions. This understanding paves the way for additional and simple opportunities to control zeolites' acidity. [ABSTRACT FROM AUTHOR]

This paper deals with the extension of the synthesis field of microporous zeolite-type materials and types of organic structure-directing agents (OSDA) that can be used to promote their crystallization. The highly hydrophilic hexamethylenetetramine (urotropine), with its C/N ratio = 1.5, which is unusual to act as a structure-directing agent in the crystallization of open-framework silica polymorphs, is used to exemplify the novelty of the employed approach. Namely, the protonation of urotropine in an acidic fluorine-containing medium transforms it into a very efficient OSDA that yields dodecasil 3C (MTN-type). This novel synthesis also allows gaining insights into OSDA-framework interactions in the MTN-type structure. The comprehensive 29Si and 19F MAS NMR indicate a small number of point defects of the framework T sites and the multiple bonding of F− ions to Si in a disordered manner. Based on this finding, a unit cell model has been generated using Monte Carlo simulation and validated with Rietveld refinement using experimental powder X-ray diffraction data. In the model, protonated urotropine cations are located in the center of the big hexakaidecahedral [51264] cages at full occupancy with random orientations. The charge balance is provided by the disordered F− ions. [ABSTRACT FROM AUTHOR]

In this work we show the use of high-resolution 1H MAS NMR to distinguish between two kinds of aggregation states of (1R,2S)-ephedrine, a chiral organic structure directing agent, occluded within AFI-type microporous aluminophosphates. We investigate in particular the supramolecular assembly of the molecules through π⋯π type interactions of their aromatic rings when confined within the one-dimensional AFI channels. A series of high-resolution two-dimensional spin diffusion spectra combined with molecular simulations and DFT calculations allowed us to distinguish different aggregation states of ephedrine molecules and precisely estimate the distances between the aromatic rings and their closest protons inside the zeolite channels as a consequence of distinct proton spin diffusion profiles. [ABSTRACT FROM AUTHOR]

The amount and location of acid sites (strong Brønsted and weak silanols) in zeolites are crucial for their applications. In this work we revealed the enigma of the complex H-bonded silanol networks in pure silica MFI-type zeolites using 1H solid-state NMR and IR spectroscopy combined with DFT calculations. The silanol sites have a determining role in phase selectivity during the zeolite synthesis and also play an imperative role in setting their acidity, stability, lifetime and hydrophobicity. The spectral signatures of silanol defects in pure silica zeolites are disclosed. Based on the experimental and theoretical observations we identified four types of silanols in pure silica zeolites which varied depending on the crystallite size: (i) isolated (free) silanols, not participating in hydrogen bonds either as proton-donors nor as proton-acceptors, (ii) proton-acceptor silanols, participating in hydrogen bonds only as proton-acceptors, (iii) proton-donor silanols, participating in weak hydrogen bonds and (iv) medium and strong hydrogen-bonding silanols, participating as proton-donors or simultaneously as proton-donors and proton-acceptors. The main factor determining the strength of the hydrogen bond of a specific silanol group is the possibility of its proton to approach closer to the oxygen center of the proton accepting silanol. This suggested that the flexibility of the zeolite fragment at which the proton-donating and proton-accepting silanols are bound is the main factor determining the formation and strength of hydrogen bonds and their corresponding spectral features. [ABSTRACT FROM AUTHOR]

Zeolites have been game-changing materials in oil refining and petrochemistry over the last 60 years and have the potential to play the same role in the emerging processes of the energy and environmental transition. Although zeolites are crystalline inorganic solids, their structures are not perfect and the presence of defect sites – mainly Brønsted acid sites and silanols – influences their thermal and chemical resistance as well as their performances in key areas such as catalysis, gas and liquid separations and ion-exchange. In this paper, we review the type of defects in zeolites and the characterization techniques used for their identification and quantification with the focus on diffraction, spectroscopic and modeling approaches. More specifically, throughout the review, we will focus on silanol (Si–OH) defects located within the micropore structure and/or on the external surface of zeolites. The main approaches applied to engineer and heal defects and their consequences on the properties and applications of zeolites in catalysis and separation processes are highlighted. Finally, the challenges and opportunities of silanol defect engineering in tuning the properties of zeolites to meet the requirements for specific applications are presented. [ABSTRACT FROM AUTHOR]

Still, the challenging questions are how to control hydroxyls in zeolites using different synthesis methodologies and how to identify them and relate them to the reactivity of zeolites. Optimizing the combination of alkali metal cations and organic cationic SDAs for obtaining new zeolites will be crucial for controlling the Al site distribution and performance of new zeolite catalysts by controlling the bridged hydroxyls. The ideal crystalline framework of zeolites is perturbed by hydroxyl groups. [Extracted from the article]

The structure of the silica AST zeolites (octadecasil) synthesized in fluoride medium using tetramethylammonium (TMA) as the organic structure-directing agent has been reinvestigated using 14N NMR quadrupolar parameters and DFT calculations. The value of the experimental 14N quadrupolar coupling constant ( CQ = 27 kHz) is larger than expected for a TMA cation possessing a high degree of motion. The analysis of a DFT-optimized octadecasil cluster along with the comparison between measured and calculated 14N NMR parameters demonstrate the presence of weak C-H...O hydrogen bonds between the TMA in the [46612] cages and the silica skeleton. These intermolecular interactions can be related to the presence of Si...F tetrel bonds within the [46] cages. These new results provide additional information with regard to the formation mechanisms and structure of the octadecasil zeolites. [ABSTRACT FROM AUTHOR]

The local structure of as-synthesised silicalite-1 zeolites is modified using asymmetric R(Pr)3N+ structure directing agents. Using multi-nuclear NMR (1H, 13C, 14N, 19F, 29Si), we show for the first time the ability of these cations to adopt preferential orientations at the zeolite channels’ crossing. [ABSTRACT FROM AUTHOR]

Local order in as-synthesised zeolites templated by tetraalkylammonium cations is proven from solid-state 14N NMR and related quadrupolar parameters, opening new perspectives in the study of porous materials. [ABSTRACT FROM AUTHOR]