MQMAS NMR Spectroscopy Research Articles

Mapping Al Distributions in SSZ-13 Zeolites from 23Na Solid-State NMR Spectroscopy and DFT Calculations

Al distributions and locations of corresponding nonframework cations in SSZ-13 zeolites have significant impact on their catalytic and adsorption properties. Herein, we demonstrate the feasibility of elucidating Na+ locations and correlated Al distributions in a series of Na-SSZ-13 zeolites with Si/Al ratios ranging from 4 to 48 by combining 23Na MQ MAS NMR spectroscopy and DFT calculations. As only one crystallographically inequivalent T site exists in CHA cages, the isolated Al and the Al pair (Al–O–Si–O–Si–O–Al) and (Al–O–Si–O–Al) in 6-membered ring (MR) distributions and the corresponding Na+ locations are clearly discriminated. In the high-silica Na-SSZ-13 zeolite, Na+ ions are mainly located at the 6-MR SIIa0 site corresponding to one Al substitution in the CHA cage. However, a small amount of the Al pair in 6-MR is still observable even with an Si/Al ratio of 48, corresponding to two Na+ ions located at 6-MR SIIa1 and 8-MR SIII’a1 sites, respectively. With more Al substitutions in the CHA cage, a p...

Solid-state dynamics in the closo-carboranes: a (11)B MAS NMR and molecular dynamics study.

This work explores the dynamic behavior of the three closo-carborane isomers (formula C2B10H12) using modern solid-state magic angle spinning (MAS) NMR techniques and relates the experimental measurements to theoretical results obtained using molecular dynamics simulations. At high temperatures and at B0 = 9.4 T, the (11)B MAS line widths are narrow (40-90 Hz) for the three isomers. The rotational correlation times (τc) calculated by molecular dynamics are on the picosecond time scale, showing a quasi-isotropic rotation at these temperatures, typical for liquid systems. For all three isomers, the values of the (11)B spin-lattice relaxation times (T1) show discontinuities as the temperature is decreased, confirming the phase changes reported in the literature. At low temperatures, the (11)B MAS spectra of all three isomers exhibit much broader lines. The simulations showed that the molecular reorientation was anisotropic around different symmetry axes for each isomer, and this was supported by the values of the reduced quadrupolar parameter PQ(eff) derived from "dynamic shift" measurements using (11)B MQMAS NMR spectroscopy. The behavior of PQ(eff) as a function of temperature for p-carborane suggests that molecular reorientation is about the C5 symmetry axis of the molecule at low temperatures, and this was supported by the molecular dynamics simulations.

Local silico-aluminophosphate interfaces within phosphated H-ZSM-5 zeolites

In order to elucidate phosphorus-zeolite H-ZSM-5 interactions, a variety of phosphorus-modified zeolite H-ZSM-5 materials were studied in a multi-spectroscopic manner. By deploying single pulse (27)Al, (31)P MAS NMR, 2D heteronuclear (27)Al-(31)P correlation (HETCOR), (27)Al MQ MAS NMR spectroscopy, TPD of pyridine monitored by FT-IR spectroscopy, and Scanning Transmission X-ray Microscopy (STXM) the interplay and influence of acidity, thermal treatment and phosphorus on the structure and acidity of H-ZSM-5 were established. It was found that while acid treatment did not affect the zeolite structure, thermal treatment leads to the breaking of Si-OH-Al bonds, a decrease in the strong acid site number and the formation of terminal Al-OH groups. No extra-framework aluminium species was observed. Phosphorus precursors interact with the zeolitic framework through hydrogen bonds and physical coordination, as phosphorus species can be simply washed out with hot water. After phosphatation and thermal treatment two effects occur simultaneously, namely (i) phosphorus species transform into water insoluble condensed poly-phosphoric acid and (ii) phosphoric acid binds irreversibly to the terminal Al-OH groups of partially dislodged four-coordinated framework aluminium, forming local silico-aluminophosphate interfaces. These interfaces are potentially the promoters of hydrothermal stability in phosphated zeolite H-ZSM-5.

Effects of Adsorbate Molecules on the Quadrupolar Interaction of Framework Aluminum Atoms in Dehydrated Zeolite H,Na-Y

The effect of adsorbate molecules on the quadrupolar interaction of framework aluminum atoms with the electric field gradient in dehydrated zeolite H,Na-Y has been studied by (27)Al MAS NMR and (27)Al MQMAS NMR spectroscopy at magnetic fields of 9.4 and 17.6 T. Upon adsorption of molecules interacting with bridging OH groups by hydrogen bonds (acetonitrile and acetone), the quadrupole coupling constant of framework aluminum atoms was found to decrease from 16.0 MHz (unloaded zeolite) to 9.4 MHz. Adsorption of molecules, which cause a proton transfer from the zeolite framework to the adsorbates (ammonia and pyridine), reduces the quadrupole coupling constant to 3.8 MHz for coverages of 0.5-2 molecules per bridging OH group. The experiments indicate that the quadrupole coupling constant of framework aluminum atoms in dehydrated zeolite H,Na-Y reflects the chemical state of adsorbate complexes formed at bridging OH groups. In agreement with earlier investigations it was found that a proton affinity of the adsorbate molecules of PA = 812-854 kJ/mol is necessary to induce a proton transfer from the zeolite framework to the adsorbed compounds. This proton transfer is accompanied by a strong improvement of the tetrahedral symmetry of zeolitic framework AlO(4) tetrahedra and a decrease of the electric field gradient.

Reaction of layered sodium disilicate SKS-6 with calcium chloride solution as revealed by solid state NMR spectroscopy: Exploring the calcium ion extracting mechanism of SKS-6

Solid-state 1H, 29Si NMR and 23Na MQMAS NMR spectroscopy combined with X-ray powder diffraction (XRD) technique was used to study the reaction between layered sodium disilicate SKS-6 and calcium chloride solution with different concentrations. Based on the experimental results the calcium ion extracting mechanism of SKS-6 was demonstrated to be via ion replacement, during which calcium ions replace sodium ions and combine with the non-bridging oxygen in SKS-6 for charge compensation, and simultaneously sodium ions leave the interlayer. This ion-replacement process was greatly influenced by the substitution of sodium ions by protons as well as the calcium ions concentration. Increasing calcium ions concentration favors the replacement of sodium ions by calcium ions, but prevents sodium ions from being substituted by protons.

Toward the aluminum coordination in dealuminated mordenite and amorphous silica–alumina: A high resolution 27Al MAS and MQ MAS NMR study

27Al MAS and MQ MAS NMR spectroscopy were used to study the aluminum coordination in dealuminated mordenite. A 27Al MAS NMR signal ascribed to distorted tetrahedrally coordinated Al (DTetrAl) was detected. By comparison with amorphous silica–alumina and alumina materials, in which no such signal was seen, the presence of DTetrAl species could be a typical feature in crystalline zeolites after activation and dealumination. An ammonium–water solution treatment under pressure in an autoclave was introduced; besides realumination of the activated zeolite, this treatment can transform six- and five- coordinate Al species to a tetrahedral coordination state, both in dealuminated zeolites and in amorphous silica–alumina, but not in alumina. From these experimental results we suggest that: (i) amorphous silica–alumina is a better model for the extra-framework material in dealuminated zeolites than alumina; (ii) DTetrAl species, which are absent in thermally treated amorphous silica–alumina, might be regarded as transitional from the crystalline framework lattice state to the amorphous silica–alumina state during the process of dealumination.

Characterization of framework and extra-framework aluminum species in non-hydrated zeolites Y by 27Al spin-echo, high-speed MAS, and MQMAS NMR spectroscopy at B0= 9.4 to 17.6 T

27Al spin-echo, high-speed MAS (nu(rot) = 30 kHz), and MQMAS NMR spectroscopy in magnetic fields of B0 = 9.4, 14.1, and 17.6 T were applied for the study of aluminum species at framework and extra-framework positions in non-hydrated zeolites Y. Non-hydrated gamma-Al2O3 and non-hydrated aluminum-exchanged zeolite Y (Al,Na-Y) and zeolite H,Na-Y were utilized as reference materials. The solid-state 27Al NMR spectra of steamed zeolite deH,Na-Y/81.5 were found to consist of four signals. The broad low-field signal is caused by a superposition of the signals of framework aluminum atoms in the vicinity of bridging hydroxyl protons and framework aluminum atoms compensated in their negative charge by aluminum cations (delta(iso) = 70 +/- 10 ppm, C(QCC) = 15.0 +/- 1.0 MHz). The second signal is due to a superposition of the signals of framework aluminum atoms compensated by sodium cations and tetrahedrally coordinated aluminum atoms in neutral extra-framework aluminum oxide clusters (delta(iso) = 65 +/- 5 ppm, C(QCC) = 8.0 +/- 0.5 MHz). The residual two signals were attributed to aluminum cations (delta(iso) = 35 +/- 5 ppm, C(QCC) = 7.5 +/- 0.5 MHz) and octahedrally coordinated aluminum atoms in neutral extra-framework aluminum oxide clusters (delta(iso) = 10 +/- 5 ppm, C(QCC) = 5.0 +/- 0.5 MHz). By chemical analysis and evaluating the relative solid-state 27Al NMR intensities of the different signals of aluminum species occurring in zeolite deH,Na-Y/81.5 in the non-hydrated state, the aluminum distribution in this material was determined.

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.

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.

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

Paramagnetic Effect of Oxygen in the 23Na MAS NMR and 23Na MQMAS NMR Spectroscopy of Zeolite LiNaX

The paramagnetic effects of oxygen molecules on the sodium cations in zeolite LiNaX (≈70% Li; 30% Na) were investigated using variable-temperature 23Na MAS NMR and 23Na MQMAS NMR. The presence of the oxygen caused a downfield paramagnetic shift for the site III resonance, indicating the accessibility of these cation sites. Although a shift was realized, it is much smaller than what is observed for 7Li by others. To understand these results, 23Na MQMAS NMR was used to calculate the isotropic chemical shifts, quadrupolar shifts, and the quadrupolar coupling constants. The quadrupolar coupling constant and quadrupolar shift of the site III sodium remain the same irrespective of the presence or absence of oxygen; however, these values were affected by temperature changes and were found to increase at lower temperatures.

Two-Dimensional Triple-Quantum 27Al MAS NMR Spectroscopic Study of the High-Temperature Phase Transformation of Microporous VPI-5

Two-dimensional (2D) triple-quantum 27Al magic-angle spinning (3Q MAS) NMR spectra of VPI-5 have been recorded in situ at room temperature and 85 °C. The results confirm that at 70−80 °C VPI-5 undergoes a reversible phase transformation from space group P63 to P63cm. This is the first study of a phase transformation by 2D MQ MAS NMR spectroscopy.