Flexibility in zeolites: origin, limits, and evaluation

Numerous pieces of evidence in the literature suggest that zeolitic materials exhibit significant intrinsic flexibility as a consequence of the spring-like behavior of Si-O and Al-O bonds and the distortion ability of Si-O-Si and Al-O-Si angles. Understanding the origin of flexibility and how it may be tuned to afford high adsorption selectivity in zeolites is a big challenge. Zeolite flexibility may be triggered by changes in temperature, pressure, or chemical composition of the framework and extra-framework compounds, as well as by the presence of guest molecules. Therefore, zeolite flexibility can be classified into three categories: (i) temperature and pressure-induced flexibility; (ii) guest-induced flexibility; and (iii) compositionally-induced flexibility. An outlook on zeolite flexibility and the challenges met during the precise experimental evaluations of zeolites will be discussed. Overcoming these challenges will provide an important tool for designing novel selective adsorbents.

Interplay between alkali-metal cations and silanol sites in nanosized CHA zeolite and implications for CO2 adsorption

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.

Impact of Mineralizing Agents on Aluminum Distribution and Acidity of ZSM-5 Zeolites

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 ²⁷ Al 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.

Acidic medium synthesis of zeolites - an avenue to control the structure-directing power of organic templates

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.

Unraveling the Effect of Silanol Defects on the Insertion of Single-Site Mo in the MFI Zeolite Framework

The preparation of defect-free MFI crystals containing single-site framework Mo through a hydrothermal postsynthesis treatment is reported. The insertion of single Mo sites in the MFI zeolite samples with different crystal sizes of 100, 200, and 2000 nm presenting a diverse concentration of silanol groups is revealed. The nature of the silanols and their role in the incorporation of Mo into the zeolite structure are elucidated through an extensive spectroscopic characterization (²⁹Si NMR, ¹H NMR, ³¹P NMR, and IR) combined with X-ray diffraction and HRTEM. In addition, a DFT-based theoretical modeling of a large Si₁₅₄O₃₅₄H₉₂ nanoparticle containing 600 atoms is carried out to understand the expansion of the unit cell volume measured by X-ray diffraction. An accurate quantification of the silanols in the MFI crystals with different particle sizes and the insertion of Mo in the zeolitic framework is reported for the first time. The results confirmed that the non-H-bonded silanols seem to be the gateway for the insertion of single Mo atoms in the zeolite structure. Such materials with single metal sites present high crystallinity and perfect structure, thus providing great stability in catalytic applications.

Hydroxyl environments in zeolites probed by deuterium solid-state MAS NMR combined with IR spectroscopy

Deuterium (2D) solid-state MAS NMR spectroscopy is used for the first time for identifying both silanols and Brønsted acid sites in zeolites. The environments of hydroxyl groups in faujasite-type zeolites...

The challenge of silanol species characterization in zeolites

The chemistry of silica-based materials, including zeolites, is strongly influenced by the nature and amount of their silanols. In zeolites, they are either isolated (non H-bonded) silanol species, or silanol...

A combination of Proton Spin Diffusion NMR and molecular simulations to probe supramolecular assemblies of organic molecules in nanoporous materials

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...

Silanol defect engineering and healing in zeolites: opportunities to fine-tune their properties and performances

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.

The role of mixed alkali metal cations on the formation of nanosized CHA zeolite from colloidal precursor suspension

A clear understanding of the crystal formation pathways of zeolites remains one of the most challenging issues to date. Here we investigate the synthesis of nanosized chabazite (CHA) zeolites using organic template-free colloidal suspensions by varying the time of aging at room temperature and the time of hydrothermal treatment at 90 °C. The role of mixed alkali metal cations (Na⁺, K⁺, Cs⁺) on the formation of CHA in the colloidal suspensions was studied. Increasing the aging time of the precursor colloidal suspension from 4 to 17 days resulted in faster crystallization of CHA nanocrystals (3 h instead of 7 h at 90 °C) to afford significantly smaller particles (60 nm vs 600 nm). During the crystallization a considerable change in the content of inorganic cations in the recovered solid material was observed to coincide with the formation of the CHA nanocrystals. The Na⁺ cations were found to direct the formation of condensed and pre-shaped aluminosilicate particles in the colloidal precursor suspensions, while K⁺ cations facilitated the formation of secondary building units (SBUs) of the CHA type framework structure such as d6r and cha cages, and the Cs⁺ cations promoted the long-range crystalline order facilitating the crystallization of stable zeolite nanocrystals.

Zeolite Structure Direction: Identification, Strength and Involvement of Weak CH⋅⋅⋅O Hydrogen Bonds

We demonstrate that weak CH⋅⋅⋅O hydrogen bonds (HBs) are important host-guest interactions in zeolite assemblies involving structure directing organocations. This type of HB is identified between alkyl groups of the organic structure directing agent (OSDA) and the silica framework in as-synthesized silicalite-1 of complex topology (MFI) using a combination of experimental and theoretical data obtained at low and room temperatures. The 28 weak CH⋅⋅⋅O HBs, evidenced along dynamics simulation at room temperature, represent 30 % of the energy of the Coulomb electrostatic interaction between OSDA and the zeolite framework. The strongest and most stable HB found here connects the OSDA to the [4¹ 5² 6² ] cage containing F atoms and should contribute to preserve zeolite topology during crystal growth. An inspection of other as-synthesized zeolites of very different framework topology indicates that the directional CH⋅⋅⋅O HBs have to be considered when discussing zeolite structure directing phenomena.

ZSM-5 Zeolite: Complete Al Bond Connectivity and Implications on Structure Formation from Solid-State NMR and Quantum Chemistry Calculations

Al site distribution in the structurally complex and industrially important ZSM-5 zeolite is determined by studying the spectroscopic response of Al(OSi)₄ units and using a self-consistent combination of up-to-date solid-state NMR correlations (²⁹Si-²⁷Al and ¹H-²⁷Al D-HMQC) and quantum chemistry methods (DFT-D). To unravel the driving forces behind specific Al sitting positions, our approach focuses on ZSM-5 containing its more efficient OSDA, tetrapropylammonium.

One-pot synthesis of silanol-free nanosized MFI zeolite

The synthesis of nanostructured zeolites enables modification of catalytically relevant properties such as effective surface area and diffusion path length. Nanostructured zeolites may be synthesized either in alkaline media, and so contain significant numbers of hydrophilic silanol groups, or in expensive and harmful fluoride-containing media. Here, we report and characterize, using a combination of experimental and theoretical techniques, the one-pot synthesis of silanol-free nanosized MFI-type zeolites by introducing atomically dispersed tungsten; this prevents silanol group occurrence by forming flexible W-O-Si bridges. These W-O-Si bonds are more stable than Si-O-Si in the all-silica MFI zeolite. Tungsten incorporation in nanosized MFI crystals also modifies other properties such as structural features, hydrophobicity and Lewis acidity. The effect of these is illustrated on the catalytic epoxidation of styrene and separation of CO₂ and NO₂. Silanol-free nanosized W-MFI zeolites open new perspectives for catalytic and separation applications.

Intermolecular interactions in AST zeolites through 14N NMR and DFT calculations

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.

Preferential orientations of structure directing agents in zeolites

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

Assessment of safety, cardiovascular and subjective effects after intravenous cocaine and lofexidine

The primary objective of this study was to determine the safety of lofexidine, an α2 receptor agonist, alone and concurrent with cocaine in non-treatment seeking cocaine-dependent or cocaine-abusing participants. After screening, eligible participants received double-blind, randomized infusions of saline and 20mg of cocaine on Day 1, and saline and 40mg of cocaine on Day 2. Subjects were randomized and started receiving daily administration of placebo (N=4) or lofexidine on Day 3 and continued on this schedule until Day 7. Two dosing regimens for lofexedine were investigated: 0.8 QID (N=3) and 0.2mg QID (N=11). On Days 6 and 7, subjects received double-blind infusions of saline and 20mg of cocaine on Day 6, and saline and 40mg of cocaine on Day 7. The data reveal a notable incidence of hemodynamic-related AEs over the course of the study. Two of the three participants at the 0.8mg dose level discontinued, and five of 11 participants at the 0.2mg dose level were withdrawn (or voluntarily discontinued) after hemodynamic AEs. Subjective effects and cardiovascular data were derived from all participants who were eligible to receive infusions (i.e., did not meet stopping criteria) on Days 6 and 7 (6 received lofexidine 0.2mg, QID and 4 received placebo, QID). As expected, cocaine significantly increased heart rate and blood pressure, as well as several positive subjective effects. There was a trend for lofexidine to decrease cocaine-induced cardiovascular changes and cocaine-induced ratings for "any drug effect", "good effects", and "desire cocaine", but sample size issues limit the conclusions that can be drawn. Despite the trends to reduce cocaine-induced subjective effects, cardiovascular AEs may limit future utility of lofexidine as a treatment for this population.