Quantification of electric-field gradient in the supercage of Y zeolites with 131Xe (I = 3/2) NMR

The electric-field gradients in the supercage of HY and steam-dealuminated Y zeolites have been quantified using one-dimensional nutation NMR applied to physisorbed ¹³¹Xe. Their values are 1.25 and 0.81·10¹⁹ V·m^–2, respectively. The composite-pulse sequence used to detect ¹³¹Xe signal also cancels the ringing signals from the NMR probe head. The fitting program for extracting the quadrupole coupling constant from the nutation NMR data is available in the web site http://www.pascal-man.com.     

NMR crystallography of zeolites: How far can we go without diffraction data?

Nuclear magnetic resonance (NMR) crystallography—an approach to structure determination that seeks to integrate solid-state NMR spectroscopy, diffraction, and computation methods—has emerged as an effective strategy to determine structures of difficult-to-characterize materials, including zeolites and related network materials. This paper explores how far it is possible to go in determining the structure of a zeolite framework from a minimal amount of input information derived only from solid-state NMR spectroscopy. It is shown that the framework structure of the fluoride-containing and tetramethylammonium-templated octadecasil clathrasil material can be solved from the 1D ²⁹Si NMR spectrum and a single 2D ²⁹Si NMR correlation spectrum alone, without the space group and unit cell parameters normally obtained from diffraction data. The resulting NMR-solved structure is in excellent agreement with the structures determined previously by diffraction methods. It is anticipated that NMR crystallography strategies like this will be useful for structure determination of other materials, which cannot be solved from diffraction methods alone.     

How Water and Ion Mobility Affect the NMR Fingerprints of the Hydrated JBW Zeolite: A Combined Computational-Experimental Investigation

An important aspect within zeolite synthesis is to make fully tunable framework materials with controlled aluminium distribution. A major challenge in characterising these zeolites at operating conditions is the presence of water. In this work, we investigate the effect of hydration on the ²⁷Al NMR parameters of the ultracrystalline K,Na-compensated aluminosilicate JBW zeolite using experimental and computational techniques. The JBW framework, with Si/Al ratio of 1, is an ideal benchmark system as a stepping stone towards more complicated zeolites. The presence and mobility of water and extraframework species directly affect NMR fingerprints. Excellent agreement between theoretical and experimental spectra is obtained provided dynamic methods are employed with hydrated structural models. This work shows how NMR is instrumental in characterising aluminium distributions in zeolites at operating conditions.     

17O NMR spectroscopy of crystalline microporous materials

Microporous materials, containing pores and channels of similar dimensions to small molecules have a range of applications in catalysis, gas storage and separation and in drug delivery. Their complex structure, often containing different types and levels of positional, compositional and temporal disorder, makes structural characterisation challenging, with information on both long-range order and the local environment required to understand the structure–property relationships and improve the future design of functional materials. In principle, ¹⁷O NMR spectroscopy should offer an ideal tool, with oxygen atoms lining the pores of many zeolites and phosphate frameworks, playing a vital role in host–guest chemistry and reactivity, and linking the organic and inorganic components of metal–organic frameworks (MOFs). However, routine study is challenging, primarily as a result of the low natural abundance of this isotope (0.037%), exacerbated by the presence of the quadrupolar interaction that broadens the spectral lines and hinders the extraction of information. In this Perspective, we will highlight the current state-of-the-art for ¹⁷O NMR of microporous materials, focusing in particular on cost-effective and atom-efficient approaches to enrichment, the use of enrichment to explore chemical reactivity, the challenge of spectral interpretation and the approaches used to help this and the information that can be obtained from NMR spectra. Finally, we will turn to the remaining challenges, including further improving sensitivity, the high-throughput generation of multiple structural models for computational study and the possibility of in situ and in operando measurements, and give a personal perspective on how these required improvements can be used to help solve important problems in microporous materials chemistry.

Cost-effective and atom-efficient isotopic enrichment enables ¹⁷O NMR spectroscopy of microporous materials to be used to probe local structure and disorder and to explore chemical reactivity.     

Probing the Brønsted Acidity of the External Surface of Faujasite-Type Zeolites

We outline two methodologies to selectively characterize the Brønsted acidity of the external surface of FAU-type zeolites by IR and NMR spectroscopy of adsorbed basic probe molecules. The challenge and goal are to develop reliable and quantitative IR and NMR methodologies to investigate the accessibility of acidic sites in the large pore FAU-type zeolite Y and its mesoporous derivatives often referred to as ultra-stable Y (USY). The accessibility of their Brønsted acid sites to probe molecules (n-alkylamines, n-alkylpyridines, n-alkylphosphine- and phenylphosphine-oxides) of different molecular sizes is quantitatively monitored either by IR or ³¹P NMR spectroscopy. It is now possible, for the first time to quantitatively discriminate between the Brønsted acidity located in the microporosity and on the external surface of large pore zeolites. For instance, the number of external acid sites on a Y (LZY-64) zeolite represents 2 % of its total acid sites while that of a USY (CBV760) represents 4 % while the latter has a much lower framework Si/Al ratio.     

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 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.     

Application of Multinuclear MAS NMR for the in situ Monitoring of Hydrothermal Synthesis of Zeolites

In situ MAS NMR studies on the monitoring of hydrothermal synthesis of zeolites are reviewed. The first part of the review contains information on the experimental techniques used for the in situ NMR studies in static and MAS conditions. In the second part, the main capabilities of the in situ ¹H, ¹¹B, ¹³C, ¹⁴N, ¹⁹F, ²³Na, ²⁷Al, ²⁹Si and ³¹P MAS NMR for the elucidation of the mechanism of hydrothermal synthesis of zeolites are examined and the data on NMR lines identification are summarized. In the last part the main application areas of the techniques are considered and illustrated with examples taken from the mechanistic studies of zeolites A, X, MFI and BEA synthesis. A cross-reference index between the materials studied, the experimental approaches used, the mechanistic information obtained, and the corresponding literature sources is established.     

13C NMR investigations of carbon monoxide adsorbed in zeolites

¹³C NMR spectra of CO and CO₂ molecules adsorbed in zeolites of A, X, Y type were measured as a function of temperature and pore filling. In contrast to other systems, strong resonance shifts to lower fields appear when CO is adsorbed in decationated zeolites. These shifts can be interpteted by an interaction with adsorption sites of Lewis type.     

<Superscript>129</Superscript>Xe NMR spectroscopy of adsorbed xenon: Possibilites for exploration of microporous carbon materials

Despite the extensive use of ¹²⁹Xe NMR for characterization of high surface-to-volume porous solids, particularly zeolites, this method has not been widely used to explore the properties of microporous carbon materials. In this study, commercial amorphous carbons of different origin (produced from different precursors) and a series of activated carbons obtained by successive cyclic air oxidation/pyrolysis treatments of a single precursor were examined. Models of ¹²⁹Xe chemical shift as a function of local Xe density, mean pore size, and temperature are discussed. The virial coefficient arising from binary xenon collisions, σ_Xe-Xe, varied linearly with the mean pore size given by N₂ adsorption analysis; σ_Xe-Xe appeared to be a better probe of the mean pore size than the chemical shift extrapolated to zero pressure, σ_S.     

Temperature-programmed desorption of n-hexane from hydrated HZSM-5 and NH4ZSM-5 zeolites

Temperature-programmed desorption coupled with mass spectrometer as a detector (TPD), IR and ¹³C NMR measurements are used to study the adsorption of n-hexane on hydrated HZSM-5 and NH₄ZSM-5 zeolites. The ¹³C NMR measurements show that n-hexane can access the pore structure of ZSM-5 zeolites previously saturated with water. TPD spectra of n-hexane are monitored in the temperature region 50–300°C, in the case of fully or partially hydrated samples; two-stage desorption of n-hexane is found. Simultaneous desorption of water and n-hexane in the same temperature region are found, in all investigated samples.     

Tailoring Hierarchical Zeolites with Designed Cationic Surfactants and Their High Catalytic Performance

Three hierarchical porous zeolites (H‐*BEA, H‐MTW, and H‐*MRE) were successfully synthesized with the assistance of designed cationic surfactants under hydrothermal synthesis conditions. The as‐synthesized zeolite samples can be easily regulated by changing the number of long hydrophobic n ‐alkyl chains. Also, we investigated the relationship between the length of the surfactant and the formation of the microporous structure of the zeolite. Furthermore, the alkylation of benzene with propene was performed as a probe reaction to evaluate the catalytic performance of the synthesized hierarchical zeolites. The resulting materials were characterized by using a complementary combination of techniques, that is, X‐ray powder diffraction, N₂ adsorption–desorption isotherms, scanning electron microscopy, transmission electron microscopy, Fourier transform IR spectroscopy, ²⁸Si and ²⁷Al MAS NMR spectroscopies, thermogravimetric analysis, and computer simulation. These analysis results indicated that quaternary ammonium surfactants acted as organic structure‐directing agents (OSDAs) in the formation of these hierarchical zeolite samples, whether the surfactant had long hydrophobic tail groups or not. The simulation results indicated that the organic molecules with no long hydrophobic chain could lead to the synthesis of zeolite through charge control, and the hydrophobic molecules with long hydrophobic chains could form zeolites through orbital control. These hierarchical zeolites showed improved catalytic activity towards the industrially relevant alkylation of benzene with propene compared with conventional zeolites with the same frameworks. More importantly, the success of using quaternary ammonium surfactants with no hydrophobic n ‐alkyl tail group in the synthesis of hierarchically structured mesoporous zeolites provides a new pathway for the synthesis of hierarchical porous materials by a soft‐templating method.     

Connecting Rheological Properties and Molecular Dynamics of Egg-Tempera Paints based on Egg Yolk

Egg-tempera painting is a pictorial technique widely used in the Middle Ages, although poorly studied in its physico-chemical aspects until now. Here we show how NMR relaxometry and rheology can be combined to probe egg-tempera paints and shed new light on their structure and behavior. Based on recipes of the 15th century, model formulations with egg yolk and green earth have been reproduced to characterize the physicochemical properties of this paint at the mesoscopic and macroscopic scales. The rheological measurements highlight a synergetic effect between green earth and egg yolk, induced by the interactions between them and the structural organisation of the system. ¹H NMR relaxometry emphasizes the presence and the structure of a network formed by the yolk and the pigment.     

Eine Methode zur ldentifizierung von Gitter-Leerstellen in Zeolithen

A Method for the Identification of T-Atom Vacancies in the Lattices of Zeolites Silanol groups in the lattices of zeolites can be silylated with trimethylchlorosilane and the products can be characterized by means of ²⁹Si (CP) MAS NMR. T-atom vacancies are indicated by the formation of silylation products with three or four siloxy-linkages under mild reaction conditions as this requires the presence of three or silanol groups in favourable geometrical arrangements.     

甲胺与MFI、FAU 及FER型高硅沸石的相互作用

The adsorption of methylamine on highly siliceous MFI, FAU and FER-type zeolites was investigated withXRD, FT-IR, Raman, ^13C and ^29Si MAS NMR, and compared with the adsorption of methanol. As the adsorption of the amine, the relative intensity of XRD peaks of the zeolites has been changed significantly, the high-resolution ^29Si MAS NMR peaks have been broadened and shifted to low field, and the resonance of Si-OH groups has appeared. The vibration of N-H has been shifted to low frequency and C-N vibration moved to high frequency in the IR spectra, and the ^13C resonance peak broadened and shifted to high field for the adsorbed amine. The facts reveal an associating interaction between the perfect framework of the zeolites and the adsorbed methylamine with hydrogen bonds, leading to the formation of Si-OH groups and the high desorption temperature of the methylamine from the zeolites.     

A new method for characterizing solid surface acidity - an infrared spectroscopic method using probe molecules such as N2 and rare gases

A new infrared-spectroscopic method to characterize acid sites of zeolites using small and weakly basic molecules such as diatomic and monoatomic molecules is reviewed. It has been revealed that N2 is an effective probe molecule to characterize both Brønsted acidity and Lewis acidity of H-form zeolites. The characteristics of the N 2 probe are discussed in detail in comparison with the CO probe. O2 and rare gases have also been applied to monitor the strong acid sites in the H-form zeolites. Further, the studies of the adsorption of water on H-form zeolites are shortly reviewed: a recent IR study of the H₂ ¹⁸O adsorption on H-ZSM-5 has given direct experimental evidence that the main feature of the observed IR bands is due to the hydrogen-bonded adsorption of water on the Brønsted acid sites.     

1H‐19F REDOR‐filtered NMR spin diffusion measurements of domain size in heterogeneous polymers

Solid state NMR spectroscopy is inherently sensitive to chemical structure and composition and thus makes an ideal method to probe the heterogeneity of multicomponent polymers. Specifically, NMR spin diffusion experiments can be used to extract reliable information about spatial domain sizes on multiple length scales, provided that magnetization selection of one domain can be achieved. In this paper, we demonstrate the preferential filtering of protons in fluorinated domains during NMR spin diffusion experiments using ¹H‐¹⁹F heteronuclear dipolar dephasing based on rotational echo double resonance (REDOR) MAS NMR techniques. Three pulse sequence variations are demonstrated based on the different nuclei detected: direct ¹H detection, plus both ¹H?¹³C cross polarization and ¹H?¹⁹F cross polarization detection schemes. This ¹H‐¹⁹F REDOR‐filtered spin diffusion method was used to measure fluorinated domain sizes for a complex polymer blend. The efficacy of the REDOR‐based spin filter does not rely on spin relaxation behavior or chemical shift differences and thus is applicable for performing NMR spin diffusion experiments in samples where traditional magnetization filters may prove unsuccessful. This REDOR‐filtered NMR spin diffusion method can also be extended to other samples where a heteronuclear spin pair exists that is unique to the domain of interest.     

Chemoselective detection and discrimination of carbonyl‐containing compounds in metabolite mixtures by 1H‐detected 15N nuclear magnetic resonance

NMR spectra of mixtures of metabolites extracted from cells or tissues are extremely complex, reflecting the large number of compounds that are present over a wide range of concentrations. Although multidimensional NMR can greatly improve resolution as well as improve reliability of compound assignments, lower abundance metabolites often remain hidden. We have developed a carbonyl‐selective aminooxy probe that specifically reacts with free keto and aldehyde functions, but not carboxylates. By incorporating ¹⁵N in the aminooxy functional group, ¹⁵N‐edited NMR was used to select exclusively those metabolites that contain a free carbonyl function while all other metabolites are rejected. Here, we demonstrate that the chemical shifts of the aminooxy adducts of ketones and aldehydes are very different, which can be used to discriminate between aldoses and ketoses, for example. Utilizing the 2‐bond or 3‐bond ¹⁵N‐¹H couplings, the ¹⁵N‐edited NMR analysis was optimized first with authentic standards and then applied to an extract of the lung adenocarcinoma cell line A549. More than 30 carbonyl‐containing compounds at NMR‐detectable levels, six of which we have assigned by reference to our database. As the aminooxy probe contains a permanently charged quaternary ammonium group, the adducts are also optimized for detection by mass spectrometry. Thus, this sample preparation technique provides a better link between the two structural determination tools, thereby paving the way to faster and more reliable identification of both known and unknown metabolites directly in crude biological extracts. Copyright © 2015 John Wiley & Sons, Ltd.     

Triplet photochemistry within zeolites through heavy atom effect, sensitization and light atom effect

Methods to generate triplets of organic molecules within zeolites have been established by employing the Zimmerman rearrangement of barrelenes, oxa-di-π-methane rearrangement of β,γ-unsaturated ketones and photodimerization of acenaphthylene as probe reactions. The two methods, heavy cation effect and triplet sensitization, are well established solution techniques and these work well within zeolites. The Zimmerman rearrangement of dibenzobarrelene is enhanced even within Li⁺ and Na⁺ exchanged zeolites and these are believed to be the result of slowing of the rearrangement to dibenzocyclooctatetraene from S₁ through cation-π interaction. The methods described here provide an opportunity to explore the control afforded by the zeolite environment on triplet reactions.     

Eine neue Methode zur Temperaturbestimmung in Kernresonanzspektrometern

The temperature in the probe of NMR spectrometers can be determined accurately by the clearing point of liquid crystals. This method is suitable for ¹H- and ¹³C-NMR measurements, has a margin of error of ±0.2°C in ¹H- and ±0.3°C in ¹³C-NMR experiments and an accuracy 5 times greater than those of previously published methods. The following experiments have been carried out: re-examination of the ethylene glycol calibration curve, calibration of temperature controllers in NMR spectrometers, determination of temperature gradients in NMR sample tubes, dependence of the probe temperature on the air stream in NMR experiments at variable temperatures and influence of noise- and off-resonance decoupling conditions on the probe temperature.     

2H SOLCOR: A novel tool for reducing volume variation as a source of error in external standard quantitative NMR

Tube to tube volume difference presents a challenge in obtaining correct external standard quantitative NMR (esqNMR) results. Deuterium (²H) NMR is easily observable, intrinsically quantitative, present in all samples, free of interfering signals, and insensitive to probe tune/match and sample saltiness. These properties make ²H peak integral an ideal parameter in esqNMR for correcting volume differences between the reference standard and analyte. We demonstrate a novel and practical technique abbreviated as “²H SOLCOR” (²H SOLvent CORrected), where the ²H peak integral from the solvent is used as a universal internal standard to correct volume variations in NMR tubes, thereby improving accuracy and precision of esqNMR method. Herein, this simple yet effective technique is described, and practical considerations for successful implementation are presented. ²H SOLCOR can be applied anywhere esqNMR is used, including where precious samples need to be accurately quantified for qualification as an authentic analytical standard.