MQMAS NMR Research Articles

Generation and Characterization of Well-Defined Zn2+ Lewis Acid Sites in Ion Exchanged Zeolite BEA

Bifunctional, Lewis, and Brønsted acidic molecular sieves were prepared by stepwise zinc ion exchange of zeolite BEA. The relation between the location of the Zn2+ cations in the zeolite structure and the Lewis acidity of the metal cations was explored. Several techniques were used for the detailed characterization of the material, including IR spectroscopy, with pyridine and acetonitrile as probe molecules, temperature programmed desorption, with ammonia and 2-propylamine as probe molecules, X-ray absorption spectroscopy, and 27Al MAS and MQMAS NMR spectroscopy. At low zinc concentrations (<0.15 Zn/Al) the cations are preferentially incorporated in the vicinity of two framework aluminum atoms. With increasing zinc loading (0.15 ≤ Zn/Al ≤ 0.26), additional cation sites are created at nearby framework aluminum pairs with two zinc cations being bridged by an oxygen atom. At higher zinc loading (0.26 < Zn/Al < 0.77) zinc oxide is formed in addition to the other two Zn2+ species.

Observation of Two 4-Coordinated Al Sites in Montmorillonite using High Magnetic Field Strength 27Al MQMAS NMR

AbstractAnalyses of the layer structure of Na-montmorillonite have been performed using 27Al MAS and 27Al MQMAS NMR techniques. Results of 27Al MAS NMR measurements at higher magnetic field strength (16.4 T) suggest that the 4-coordinated Al site in Na-montmorillonite has two different structures. This was confirmed by the fact that two peaks corresponding to 4-coordinated Al are observed in the 27Al MQMAS NMR at high magnetic field strength. The ratio of two 4-coordinated Al sites was found to be affected by water in the interlayer space because the area ratio of cross peaks corresponding to two 4-coordinated Al sites changes with the water content.

Observation of Two 4-coordinated Al Sites in Montmorillonite Using High Magnetic Field Strength 27al Mqmas Nmr

Observation of Two 4-coordinated Al Sites in Montmorillonite Using High Magnetic Field Strength 27al Mqmas Nmr

Flexible Aluminum Coordination in Alumino−Silicates. Structure of Zeolite H−USY and Amorphous Silica−Alumina

The aluminum coordination in amorphous silica−alumina with different aluminum concentrations as well as in pure γ-Al2O3 has been studied by means of 27Al (MQ) MAS NMR spectroscopy. The effect of ammonia adsorption at 120 °C has been studied and compared with that on zeolite H−USY, as well as with zeolites with different topologies. As for protonic zeolites, part of the octahedral aluminum reversibly changed to tetrahedrally coordinated upon ammonia adsorption on amorphous silica−alumina. The distinction between flexible aluminum coordination in zeolites and silica−alumina was possibly based on the quadrupolar parameters. Ammonia adsorption on pure γ-Al2O3 did not influence the aluminum coordination. A parallel between the structure of zeolite USY and that of amorphous silica−alumina was made on the basis of the results of 27Al MQ MAS NMR spectroscopy. A partial conversion from octahedral to tetrahedral coordination upon ammonia adsorption was observed for zeolite H−USY. Most of the flexible aluminum speci...

NUCLEAR MAGNETIC RESONANCE STUDY OF Al:Si AND F:OH ORDER IN ZUNYITE

Zunyite [Al13Si5O20(OH,F)18Cl] possesses an unusual combination of an Al 13O16(OH)24 Keggin structure and a Si 5O16 pentamer unit. We used 19 F, 27 Al, 29 Si MAS and 27 Al MQMAS NMR spectroscopy of a series of samples with varying Al:Si and F:OH ratios to investigate the degree of atomic order in zunyite. The quadrupole coupling constant (C Q) and asymmetry parameter () obtained from simulations of 27 Al MAS and MQMAS spectra were compared with those derived from the electric field gradients (EFG), which were calculated from an ab initio cluster quantum model using the program Gaussian 98W. This comparison led to unequivocal assignments for 29 Si, 27 Al and 19 F MAS NMR peaks and showed that the number of Al substituting for Si(1) in the pentamer increases with Al content. The number of Al octahedra with one F atom increases with F content, whereas the number with two F atoms remains negligible, indicating that F preferentially occupies AlO 4OHF octahedra.

Insight into the dry-gel synthesis of gallium-rich zeolite [Ga]Beta

The conversion of a dry gel with an n Si/ n Ga ratio of 8.5 to zeolite [Ga]Beta (structure-type BEA) with a framework n Si/ n Ga ratio of 11.6 was studied by scanning electron microscopy, X-ray diffraction (XRD), and multi-nuclear solid-state NMR spectroscopy. XRD indicated a fast formation of the long-range order while, as revealed by NMR spectroscopy, the rearrangement of the local structure occurs during a period of up to 65 h. This rearrangement of the local structure during the dry-gel conversion (DGC) process consists of different concerted mechanisms: The dominating step at the beginning of the DGC is a breakage of chemical bonds leading to a strong increase in the concentration of defect SiOH groups as indicated by 1H MAS NMR spectroscopy. After a conversion time of 16 h, the intensity of the 1H MAS NMR signals of defect SiOH groups and of the 29Si MAS NMR signals of Q 1, Q 2, and Q 3 silicon species decreases significantly, indicating a condensation of terminal bonds. High-field ( B 0=17.6 T) 71Ga MAS NMR investigations evidenced that the silanol condensation is accompanied by an incorporation of gallium into the zeolite framework. This finding is corroborated by the increase of the 29Si MAS NMR signals of Si(1Ga) and Si(2Ga) species. Two-dimensional 71Ga MQMAS NMR spectroscopy shows the presence of two types of tetrahedrally coordinated gallium atoms at crystallographically non-equivalent framework positions, but also distorted tetrahedrally coordinated extra-framework gallium species. The existence of the latter species explains the difference between the bulk (determined by AES/ICP) and the framework (determined by 29Si MAS NMR spectroscopy) n Si/ n Ga ratios. 1H and 13C MAS NMR spectroscopy indicate that the template molecules (tetraethylammonium hydroxide) are not affected by the DGC process, excluding a loss of mobility in zeolite [Ga]Beta obtained after a DGC time of 65 h.

Influence of F content on the composition of Al-rich synthetic phlogopite: Part II. Probing the structural arrangement of aluminum in tetrahedral and octahedral layers by27Al MQMAS and1H/19F-27Al HETCOR and REDOR experiments

The influence of F substitution on the local structure of Al in the tetrahedral and octahedral sheets of synthetic Al-rich phlogopite samples with nominal gel compositions of K(Mg 3-x Al x )[Al 1+x Si 3-x O 10 ] (OH) y (F) 2-y between 0.0 ≤ x ≤ 0.8 and 0.5 ≤ y ≤ 1.8, was studied by 27 Al MAS, MQMAS, { 1 H/ 19 F} → 27 Al 2 D CPMAS (HETCOR) and { 1 H/ 19 F} 27 Al REDOR solid-state NMR and by IR spectroscopy. Changes in intensity of the absorption bands in the OH-stretching region of the IR spectra clearly indicate the incorporation of octahedral Al. Signals from the different phases can be separated in the 27 Al MQMAS NMR spectra by generation of an isotropic dimension in F1. The 27 Al quadrupolar parameters of the four phases were estimated from 27 Al MAS NMR spectra obtained at 104.26 and 208.42 MHz. The quadrupolar coupling constant and isotropic chemical shift increases with increasing Al content for the IV Al site in phlogopite. The VI Al site shows a clear increase of the asymmetry parameter and C Q with increasing F content. The estimated 27 Al signal areas show the lowest amount of impurity phases at high OH contents and a stabilization of VI Al sites by hydroxyl groups. The {1H} → 27 Al 2D CPMAS (HETCOR) NMR experiment at short contact times provides information about site neighborhoods of tetrahedral Al sites and Mg 3 OH as well as Mg 2 AlOH sites, whereas magnetization is only transferred to the octahedral Al sites from hydroxyl groups in Mg 2 AlOH sites. The { 19 F} → 27 Al 2D CPMAS (HETCOR) NMR spectrum is dominated by IV Al sites coupled to the Mg 3 F complex in phlogopite. Resonances from Mg 2 AlF complexes are not observed. Finally, the { 1 H/ 19 F} 27 Al REDOR experiments support the results of the 2D CPMAS (HETCOR) experiments.

Optimization of zeolite Beta by steaming and acid leaching for the acylation of anisole with octanoic acid: a structure–activity relation

This paper presents a catalytic and spectroscopic study of the dealumination of a commercial zeolite, Beta (Zeolyst). After dealumination by hydrochloric or oxalic acid and steaming (up to 973 K), increased activity and selectivity were found in the acylation of anisole with octanoic acid. The treatments led to the extraction of aluminum from the crystallographic T-sites in the framework structure, giving amorphous extraframework aluminum(–silicon)–oxide species in the case of steaming, while treatment with mineral acids or complexing agents removed framework and extraframework aluminum from the zeolite. 27Al MQ MAS NMR revealed a changing of the aluminum coordinations and a shift from framework to extraframework species after steaming. Steaming removes aluminum from other crystallographic T-sites than acid leaching. Using nitrogen physisorption, no differences in the texture properties could be observed and XRD showed that the crystallinity was preserved, indicating high stability of the commercial zeolite Beta. The nature of the enhanced activity is suggested to result from higher accessibility of the active sites that are proposed to be associated with framework-connected aluminum atoms.

Water solubility mechanism in hydrous aluminosilicate glasses: information from 27Al MAS and MQMAS NMR

New 27Al NMR data are presented in order to clarify the discrepancies in the interpretation of the previous 27Al Magic Angle Spinning (MAS) spectra from hydrous aluminosilicate glasses. The 27Al MAS data have been collected at much higher magnetic field (14.1 and 17.6 T) than hitherto, and in addition, multiple quantum (MQ) MAS NMR data are presented for dry and hydrous nepheline glasses and NaAlSi 7.7O 17.4 glass that, according to the model of Zeng et al. (Zeng Q., Nekvasil H., and Grey C. P. 2000. In support of a depolymerisation model for water in sodium aluminosilicate glasses: Information from NMR spectroscopy. Geochim. Cosmochim. Acta 64, 883–896), should produce a high fraction (up to 30%) of Al in Al Q 3-OH on hydration. Although small differences in the MAS spectra of anhydrous and hydrous nepheline glasses are observed, there is no evidence for the existence of significant (>∼2%) amounts of Q 3 Al-OH in these glasses in either the MAS or MQMAS data.

23Na 2D 3QMAS NMR and 29Si, 27Al MAS NMR investigation of Laponite and synthetic saponites of variable interlayer charge

Abstract29Si, 27Al MAS NMR is used to characterize Laponite RD and synthetic saponites of variable interlayer charge. The Si/Al ratios are in good agreement with the calculated charge from chemical analysis except for the lowest-charged saponite. In contrast to the 29Si MAS NMR spectra in which resolved signals are detected, the 27Al MAS NMR spectra show one signal whose linewidth increases with the clay charge. The water content of the clay samples was obtained from 1H MAS NMR.The 2D MQMAS NMR technique is required to obtain a high-resolution spectrum of nuclei with strong quadrupolar interaction. This method was applied to the 23Na nucleus of clay counterions and to the 27Al structural nucleus. One well-defined 23Na NMR signal is observed for all the clays studied except the highest-charged saponite. Possible explanations for this different behaviour are discussed. The calculated isotropic chemical shift evolves progressively with the clay charge whereas the deduced quadrupolar interaction does not change significantly. The 27Al 2D 3QMAS technique was not able to resolve more than one signal.

Small 51V chemical shift anisotropy for LaVO 4 from MQMAS and MAS NMR spectroscopy

51V MQMAS NMR of the triple-quantum transitions is shown to be particularly useful in the determination of the sign and magnitude of the chemical shift anisotropy (CSA) parameter δ σ (= δ iso– δ zz ) along with the asymmetry parameter ( η σ ) for a vanadium environment with a small CSA and a rather strong quadrupole coupling. This is demonstrated for the orthovanadate LaVO 4 for which 51V magic-angle spinning (MAS) NMR of the central and satellite transitions at 14.1 T gives precise values for the quadrupole coupling parameters, however, an ambiguous sign for δ σ . The CSA parameters are reliably obtained from analysis of the spinning sidebands observed in a 51V triple-quantum MAS experiment. Combining these data with least-squares analysis of the manifold of spinning sidebands in the single-pulse MAS NMR spectrum results in a precise determination of the magnitudes and relative orientation of the 51V quadrupole coupling and CSA tensors for LaVO 4.

&lt;sup&gt;27&lt;/sup&gt;Al Multiple Quantum MAS NMR Study on the Distribution of Aluminium in the Zeolite MCM-22

The distribution of aluminium in the zeolite MCM-22 lattice has been investigated by Al-27 MQ MAS NMR and quantum chemical calculation. Three different aluminium species in tetrahedral coordination were clearly observed in the 2D 5Q MAS NMR spectrum of zeolite MCM-22. From the spectrum, we can calculate the isotropic chemical shifts(delta 50. 5, delta 57. 3, delta 62. 4) and the quadrupolar coupling constant (1. 74, 1. 68, 1. 92 MHz) of the three species. The structure of MCM-22 has the 8 crystallographically distinct T sites. From the simulated Al-27 5Q MAS NMR spectrum, it is evident that 8 T sites can be divided into 3 groups. Sites T2 and T6 correspond to the signals at delta 61, delta 49, respectively; while sites T1, T3, T4, T5, T7, T8 give rise to signal at delta 56. Al is randomly distributed in the framework of MCM-22 zeolite when the Si/Al atom ratio is varied from 10 to 15.

Structural Study of Silyl−Carborane Hybrid Diethynylbenzene−Silylene Polymers by High-Resolution Solid-State 11B, 13C, and 29Si NMR Spectroscopy

The structure of silyl-carborane (1,7-C2B10H10) hybrid diethynylbenzene-silylene polymers with thermal treatment over a wide range of temperatures has been characterized by using 11B MQ-MAS NMR, and 13C and 29Si CP-MAS NMR methods. From the experimental results, the following was found: (1) naphthalene rings form by diene reaction between the Ph−C≡C group and C≡C group at temperatures from 250 to 500 °C; (2) three-dimensional network forms by addition reaction between the vinyl group and C≡C group at curing temperatures from 250 to 350 °C and addition reaction between m-carborane group and the vinyl group and/or the C≡C group at curing temperatures from 250 to 350 °C; (3) the hybrid polymers are decomposed by oxidation at curing temperature of 500 °C; and (4) structural change of the m-carborane hybrid polymer leads to desirable modification of the oxygen index, heat deflection temperature, and flexural strength.

On the acid-dealumination of USY zeolite: a solid state NMR investigation

The dealumination of USY (ultrastable Y) zeolites by nitric acid and oxalic acid treatment was systematically investigated by multinuclear solid-state NMR and MQ MAS NMR experiments. The results show that both acids are very effective in removing non-framework Al as well as framework Al but that aluminum is extracted from the lattice at a higher rate by oxalic acid even at low concentrations. The presence of different species (e.g. silanol nest, Al–OH, four-coordinated framework Al, six-coordinated framework Al, six-coordinated non-framework Al and five-coordinated non-framework Al) was detected, and their changes were followed during the dealumination. The investigation gives evidence that the breakdown of the parent USY zeolite mainly depends on the degree of dealumination and that non-framework Al exerts a great effect on the acidity of the USY zeolite. Leaching-induced increase in the Brønsted acidity of the USY zeolite was also observed by 1H MAS NMR spectroscopy. The different distribution of Al species in these samples accounted for the different catalytic performance of n-dodecane cracking.

Towards guest-zeolite interactions: an NMR spectroscopic approach.

Guest(metal)-zeolite interactions in a two component heterogeneous catalyst have been investigated by high-field and high-speed (27)Al MAS NMR, and two-dimensional (27)Al MQ MAS NMR experiments as well as ab initio DFT methods. It was established that strong interactions between guest and zeolite occur in a metal/zeolite system, with the metal anchored to the tetrahedral aluminum framework site through two oxygen bridges. It disturbs the tetrahedral environment of associated aluminum framework, changing AlO(4) geometry from near T(d) to C(2v); this enables us to resolve this species from the undisturbed aluminum framework species in high-field (27)Al MAS NMR and two-dimesional (27)Al MQ MAS NMR experiments.

Hydration in Alkali-silicate Glasses Studied by Two Dimensional Multi-Quantum Magic Angle Spinning

The hydration in Na2O-3SiO2 glass was studied by 23Na Magic Angle Spinning (MAS) and 2D MQMAS NMR spectroscopy. It was found that one-dimensional MAS spectra for the hydrated glasses with more than 8.4 wt% water consist of two signals at around -10 and -40 ppm. On the other hand, anhydrous glass and glass with low water content (2.8 wt%) give a single resonance line at around -10 ppm. From 2D MQMAS spectra, the isotropic chemical shift ( δiso) and the quadrupole coupling frequency (VQ) for two sites (Na(1) and Na(2)) were estimated: δiso = 4.2 ppm and VQ = 0.6 MHz for Na(1) and δiso = 10.2 ppm and VQ = 1.3 MHz for Na(2). These results are discussed together with our previous results of 29Si and 1H NMR, and infrared spectra. It is speculated that Na(1) may exist in a similar environment as that in anhydrous sodium-silicate glasses, while Na(2) may be attached directly to water molecules

27Al MQ-MAS NMR as a Tool for Structure Determination in Nanocomposite Materials: The Nature of Al Pillars in “Al13−heidi” Pillared Clays

27Al 3QMAS 2D NMR was used to study the molecular-level organization of a clay pillared with the “Al13(heidi)63+” polycation, whose unusual structure is stabilized by hydroxyethyliminodiacetate (heidi) ligands. The quadrupolar line shape and the isotropic chemical shifts were first unambiguously determined on a solid salt of the polycation. This compound was then used to identify the fingerprint of the Al13(heidi)x3+ polycation in a sample of saponite clay that had been intercalated with a solution containing Al(III) and heidi ligands, showing that the basic structure of the Al polymer had been preserved upon intercalation. A more detailed analysis of 3QMAS 2D NMR line shapes indicated, however, that a selective partial hydrolysis reaction had occurred, most probably leading to Al13(heidi)33+ polycations (loss of three heidi ligands). Thus, 3QMAS NMR allowed to choose one structural model for these nanocomposites from the several compatible with previously published characterizations.

Unified representation of MQMAS and STMAS NMR of half-integer quadrupolar nuclei

High resolution solid state NMR has been achieved for nuclei that exhibit quadrupolar interaction to the second order by MQMAS and more recently by STMAS. Both methods extend to several variants, 3QMAS, 5QMAS, … for MQMAS as well as ST1MAS, ST2MAS,… for STMAS. These experiments contain the same information content i.e. anisotropic second-order powder pattern and isotropic terms. A unified representation allows us to display the 2D spectra independently of the method used. Direct comparison of any such spectra is therefore straightforward and does not need any further renormalization.

Direct Evidence for the Nonrandom Nature of Al Substitution in Zeolite ZSM-5: An Investigation by27Al MAS and MQ MAS NMR

Der Einbau von Al-Atomen auf bestimmten Positionen in der Struktur von ZSM-5 wird durch das Si/Al-Verhältnis bei der Synthese gesteuert, und ihre Verteilung auf die zwölf unterschiedlichen Tetraederzentren (T1–T12) bei der Kristallisation erfolgt nicht per Zufall – so die Ergebnisse von 27Al-MAS-NMR-Messungen an ZSM-5-Zeolith-Derivaten mit Si/Al-Verhältnissen zwischen 14 und 250. Gestützt werden diese Folgerungen durch simulierte Spektren (Beispiel siehe Bild) auf der Grundlage von Röntgenbeugungsdaten.

Procedures for Labeling the High-Resolution Axis of Two-Dimensional MQ-MAS NMR Spectra of Half-Integer Quadrupole Spins

The increasing development and application of the multiple-quantum MAS NMR for half-integer quadrupole spins has led to various RF pulse sequences for improving the excitation of multiple-quantum coherences and their conversion to single-quantum coherences. As a result, several conventions for labeling the F1 dimension of a 2D MQ-MAS spectrum appear in the literature. The corresponding relations for extracting the isotropic chemical shift, the quadrupole coupling constant, and the asymmetry parameter from experimental data are not always provided. We analyze these various conventions systematically and propose a new one, similar to that introduced by J.-P. Amoureux and C. Fernandez (2000, Solid State NMR10, 339–343). These various conventions are illustrated with 27Al (I=5/2) nuclei in aluminum acetylacetonate Al(CH3COCHCOCH3)3. Another experimental problem often met, the aliasing of peaks in the 2D spectrum, is analyzed and illustrated with 27Al (I=5/2) in NH4Y zeolite and 23Na (I=3/2) in sodium pyrophosphate Na4P2O7.