Correlation Experiments Involving Half-integer Quadrupolar Nuclei

Deschamps, Michaël; Massiot, Dominique

doi:10.1002/9780470034590.emrstm1207

Cited by 10 publications

(9 citation statements)

References 95 publications

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“…Robust 17 Al- or 1 H-excited D-HMQC sequences were used, as it requires lower power and shorter mixing time for recoupling and importantly, distance measurements can be obtained from the Al–H buildup curves. ,, For the D-HMQC experiment, recoupling was applied with the SR 4 1 2 sequence at various mixing times as stated in the text accordingly. For 27 Al detected D-HMQC experiments, an adiabatic WURST (wideband, uniform rate, smooth truncation) type of pulse was applied before each transient for a 2-fold signal sensitivity enhancement; 80 and 50 rotor-synchronized t 1 increments were required in 27 Al- and 1 H- detected experiments, respectively, to avoid truncations in the indirect dimension.…”

Section: Methodsmentioning

confidence: 99%

“…In order to develop meaningful structure–activity relationships for H-ZSM-5 catalysts, as well as address the current literature debate about water’s impact on both reactivity and stability of the various Al moieties and their associated hydroxyl groups, , there exists a need to systematically assess and discuss the accurate detection of all Al species in industrially relevant H-ZSM-5 catalysts in both hydrated and dehydrated states. Excellent descriptions of the unique requirements for doing quantitative NMR on quadrupolar nuclei already exist in the literature. − Here, the primary goal is to identify how signals from the different Al species in H-ZSM-5 catalysts manifest themselves as a function of catalyst history, exposure to moisture, and the specific NMR experiments used to detect them. Particular focus will be given to the types of Al species that exist when the catalyst is dehydrated, as such conditions should more accurately reflect the high-temperature reactions relevant to hydrocarbon catalysis.…”

Section: Introductionmentioning

confidence: 99%

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Distribution of Aluminum Species in Zeolite Catalysts: ²⁷Al NMR of Framework, Partially-Coordinated Framework, and Non-Framework Moieties

et al. 2021

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The structure of aluminum-containing moieties in and within zeolite H-ZSM-5 catalysts is a complex function of the elemental composition of the catalyst, synthesis conditions, exposure to moisture, and thermal history. 27Al NMR data collected at field strengths ranging from 7.05 to 35.2 T, i.e., 1H Larmor frequencies from 300 to 1500 MHz, reveal that Al primarily exists as framework or partially coordinated framework species in commercially available dehydrated H-ZSM-5 catalysts with Si/Al ranging from 11.5 to 40. Quantitative direct-excitation and sensitivity-enhanced 27Al NMR techniques applied over the wide range of magnetic field strengths used in this study show that prior to significant hydrothermal exposure, detectable amounts of nonframework Al species do not exist. Two-dimensional 27Al multiple-quantum magic-angle spinning (MQMAS) along with 1H–27Al and 29Si–27Al dipolar correlation (D-HMQC) NMR experiments confirm this conclusion and show that generation of nonframework species following varying severities of hydrothermal exposure are clearly resolved from partially coordinated framework sites. The impact of hydration on the appearance and interpretation of conventional direct-excitation 27Al spectra, commonly used to assess framework and nonframework Al, is discussed. Aluminum sites in dehydrated catalysts, which are representative of typical operating conditions, are characterized by large quadrupole interactions and are best assigned by obtaining data at multiple field strengths. On the basis of the results here, an accurate initial assessment of Al sites in high-Al content MFI catalysts prior to any hydrothermal treatment can be used to guide reaction conditions, anticipate potential water impacts, and identify contributions from hydroxyl groups other than those associated with the framework bridging acid site.

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Section: Methodsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Distribution of Aluminum Species in Zeolite Catalysts: ²⁷Al NMR of Framework, Partially-Coordinated Framework, and Non-Framework Moieties

et al. 2021

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“…To achieve this, methods relying on 1 H-magnetization transfer are particularly well suited, thanks to the presence of hydroxyl groups at the surface of alumina. The most classical method derived from spin 1/2 is cross-polarization-magic-angle spinning (CP-MAS), which is used to characterize alumina-type materials. − As described below in the case of γ-Al 2 O 3 , it is well-known that CP-MAS is subject to severe limitations when applied to a quadrupolar nucleus such as 27 Al and should be avoided in such a context. , An alternative method used to study alumina and derived materials is dipolar heteronuclear multiple quantum coherence (D-HMQC), which does not rely on spin locking . It can be implemented with a dipolar recoupling scheme in order to highlight those sites which are in the vicinity of protons .…”

Section: Introductionmentioning

confidence: 99%

“…11−14 As described below in the case of γ-Al 2 O 3 , it is well-known that CP-MAS is subject to severe limitations when applied to a quadrupolar nucleus such as 27 Al and should be avoided in such a context. 15,16 An alternative method used to study alumina and derived materials is dipolar heteronuclear multiple quantum coherence (D-HMQC), which does not rely on spin locking. 17 It can be implemented with a dipolar recoupling scheme in order to highlight those sites which are in the vicinity of protons.…”

Section: Introductionmentioning

confidence: 99%

Caveat on the Actual Robustness of Heteronuclear NMR Methods for Probing the Surface of γ-Alumina and Related Catalysts

et al. 2019

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Transitional aluminas are a very important class of materials in catalysis. The detailed understanding of their surface chemistry has attracted much attention with input from spectroscopic and theoretical studies. In this context, NMR is a key technique which, thanks to recent methodological developments, provides insights into local structure and relative arrangements. In the present paper, we first assess two 1 H-27 Al heteronuclear correlation NMR methods for studying the surface of γ-Al2O3 dehydroxylated at 500 °C. The commonly-used cross-polarization (CP) transfer method is compared to the D-HMQC (Dipolar Heteronuclear Multiple Quantum Coherence) experiment, revealing that the latter is significantly more robust and efficient with less signal distortion that hampers the recording of meaningful signals. Furthermore, a new MQ-D-HMQC pulse sequence is proposed to confirm the presence of sites, with a large quadrupolar coupling constant at the surface of alumina. The robustness of D-HMQC is further probed by two additional alumina-based catalysts, namely aluminum hydrides supported on γ-Al2O3, which are active in olefin hydrogenation and polymerization, and ZrCp2Me2 grafted onto γ-Al2O3, a single-site olefin polymerization catalyst. In both cases, this provides information on the configuration of surface aluminum species in the vicinity of catalytically active sites, and on the residual hydroxyl networks. The present study combines advanced NMR methods and complex catalytic materials for which a critical, yet fruitful, spectroscopic approach is required for deeper understanding.

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“…Second, it will also be possible to utilize the high spectral resolution in the 13 C dimension to separate overlapping 17 O NMR signals, which often suffer from second-order quadrupole broadenings even under magic-angle spinning (MAS) conditions. While two-dimensional (2D) heteronuclear correlation solid-state NMR spectroscopy between quadrupolar and spin-1/2 nuclei has been widely reported in the literature, − the only attempt for 13 C– 17 O correlation was the recent work by Keeler et al, where they successfully demonstrated the Z-Filtered Transferred-Echo Double Resonance (ZF-TEDOR) experiment on a sample of N-acetyl-[U- 13 C, 15 N; 70% 17 O]- l -Val- l -Leu. However, because the reported sensitivity of the ZF-TEDOR experiment was exceedingly low, it is necessary to develop new strategies that can improve both sensitivity and spectral resolution.…”

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confidence: 99%

Two- and Three-Dimensional ¹³C–¹⁷O Heteronuclear Correlation NMR Spectroscopy for Studying Organic and Biological Solids

Hung

Gan

2021

J. Phys. Chem. Lett.

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We report two-and three-dimensional (2D and 3D) 13 C− 17 O heteronuclear correlation solid-state NMR experiments under magic-angle spinning (MAS) conditions. These experiments utilize the D-RINEPT (Dipolar-mediated Refocused Insensitive Nuclei Enhanced by Polarization Transfer) scheme with symmetry-based SR4 1 2 recoupling blocks for coherence transfer between 13 C and 17 O nuclei. First, a 2D 17 O → 13 C correlation experiment was performed for the [1-13 C, 17 O]-Gly/Gly•HCl cocrystal and [U-13 C, 1-17 O]-α/β-D-glucose samples. Second, a 2D 17 O → 13 C MQ-D-RINEPT correlation experiment where the indirect dimension incorporates the multiple-quantum MAS (MQMAS) scheme was tested for obtaining isotropic 17 O resolution with [U-13 C, 1-17 O]-α/β-D-glucose. Third, a new 3D 17 O → 13 C → 13 C correlation experiment was demonstrated where 17 O → 13 C and 13 C → 13 C correlations are achieved by D-RINEPT and DARR (Dipolar Assisted Rotational Resonance) sequences, respectively (thus termed as a 3D D-RINEPT/DARR OCC experiment). This new 3D 17 O NMR experiment is implemented with the aim for site-resolved solid-state 17 O NMR studies.T he oxygen element is an essential constituent of organic and biological molecules and its importance in the structure and function of these molecules can be readily appreciated. Unlike other key constituents of organic/biological molecules such as H, C, N, and P, the only NMR-active oxygen isotope, 17 O (I = 5/2, natural abundance 0.037%), has not been widely used in NMR spectroscopic studies of biological molecules. 1−6 Two major obstacles have contributed to the paucity of 17 O NMR studies of biological macromolecules such as proteins and nucleic acids. One is the difficulty of introducing 17 O-isotopes into biological molecules (i.e., 17 O-labeling) and the other is the intrinsically low spectral resolution often associated with a half-integer quadrupolar nucleus such as 17 O. In a recent study, Lin et al. 7 demonstrated that it is possible to incorporate [1-13 C, 17 O]-doubly labeled amino acids into recombinant proteins so that selective protein backbone carbonyl groups are isotope labeled in the form of 13 C 17 O. Lin et al. 7 further proposed that this [1-13 C, 17 O]double labeling scheme can serve two purposes. First, it will be possible to use 13 C− 17 O heteronuclear correlation spectroscopy to aid 17 O NMR signal assignment, because 13 C signal assignment can be readily achieved with the conventional solid-state 13 C and 15 N NMR approaches for uniformly 13 C/ 15 N labeled proteins. Second, it will also be possible to utilize the high spectral resolution in the 13 C dimension to separate overlapping 17 O NMR signals, which often suffer from second-order quadrupole broadenings even under magic-angle spinning (MAS) conditions. While two-dimensional (2D) heteronuclear correlation solid-state NMR spectroscopy

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Correlation Experiments Involving Half-integer Quadrupolar Nuclei

Cited by 10 publications

References 95 publications

Distribution of Aluminum Species in Zeolite Catalysts: ²⁷Al NMR of Framework, Partially-Coordinated Framework, and Non-Framework Moieties

Distribution of Aluminum Species in Zeolite Catalysts: ²⁷Al NMR of Framework, Partially-Coordinated Framework, and Non-Framework Moieties

Caveat on the Actual Robustness of Heteronuclear NMR Methods for Probing the Surface of γ-Alumina and Related Catalysts

Two- and Three-Dimensional ¹³C–¹⁷O Heteronuclear Correlation NMR Spectroscopy for Studying Organic and Biological Solids

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Correlation Experiments Involving Half-integer Quadrupolar Nuclei

Cited by 10 publications

References 95 publications

Distribution of Aluminum Species in Zeolite Catalysts: 27Al NMR of Framework, Partially-Coordinated Framework, and Non-Framework Moieties

Distribution of Aluminum Species in Zeolite Catalysts: 27Al NMR of Framework, Partially-Coordinated Framework, and Non-Framework Moieties

Caveat on the Actual Robustness of Heteronuclear NMR Methods for Probing the Surface of γ-Alumina and Related Catalysts

Two- and Three-Dimensional 13C–17O Heteronuclear Correlation NMR Spectroscopy for Studying Organic and Biological Solids

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Distribution of Aluminum Species in Zeolite Catalysts: ²⁷Al NMR of Framework, Partially-Coordinated Framework, and Non-Framework Moieties

Distribution of Aluminum Species in Zeolite Catalysts: ²⁷Al NMR of Framework, Partially-Coordinated Framework, and Non-Framework Moieties

Two- and Three-Dimensional ¹³C–¹⁷O Heteronuclear Correlation NMR Spectroscopy for Studying Organic and Biological Solids