29Si MAS NMR Spectra Research Articles

Effect of the cation mobility on the X-ray diffraction pattern and the Si MAS NMR spectra of Na,CoY zeolites

Abstract The paramagnetic Co 2+ cations in partially exchanged CoNaY occupy sites I′ and II′ where their mobility is strongly restricted. The NMR resonance line of Si immediately adjacent to these sites disappears by broadening. The mobile Co 2+ cation influences the residual Si resonance lines differently, in that it decreases the intensity of the Si resonance in the 4 Q(2Al) and 4 Q(3Al) environments. The modification of the scattering factor at the sites where cobalt is not mobile provokes strong intensity modifications in the X-ray reflections. The relative mobile and non-mobile cobalt amount in the Na–Co exchange is quantitatively related.

29Si MAS NMR systematics of calcic and sodic-calcic amphiboles

We report the compositional systematics of the 29 Si MAS NMR spectra of richterite, (super [A]) (Na,K,Rb) 1 (Na,Ca,Sr) 2 Mg 5 Si 8 O 22 (OH,F) 2 ; pargasite, NaCa 2 Mg 4 AlSi 6 Al 2 O 22 (OH) 2 ; and fluor-edenite, NaCa 2 Mg 5 Si 7 AlO 22 F 2 . (super [A]) Na causes (1) splitting of T1 into two sites, while leaving T2 unsplit, and (2) the Q 3 chemical shift to be 2.5 to 3 ppm less negative than Q 3 when the A site is empty as in tremolite and magnesiohornblende. The preferential splitting of T1 is explained in terms of ordering of the A cation at Am. The effects of M4 and A-site chemistry upon the richterite spectra aid the assignment of peaks for pargasite and fluor-edenite. The long-range ordering of Al and Si over T1 and T2 sites in fluor-edenite synthesized at 2 kbar, and 1000 degrees C and pargasite synthesized at 1 kbar, and 930 degrees C has been calculated from their 29 Si MAS NMR spectra assuming that Al avoidance operates. The extent of long-range order is calculated from the intensities of the Q 2 (2Al), Q 2 (1Al), and Q 2 (0Al) peaks. An equation is derived that allows the extent of long-range Al-Si order to be calculated from 29 Si MAS NMR spectra of amphiboles. The spectrum of fluor-edenite is consistent with all (super [4]) Al being at T1 with maximal short-range disorder within the constraints of Al avoidance. The pargasite spectrum is more complex, because there is a probable peak coincidence of Q 2 (1Al) and Q 3 (2Al) at -82 ppm that must be considered. The presence of a Q 3 (0Al) peak in the pargasite spectrum indicates unambiguously that some long-range disorder exists, and this implies that Q 3 (2Al) groupings also occur. The calculated extent of long-range disorder in pargasite is 55 = 10%. This value is consistent with the single-crystal X-ray data of Oberti et al. (1995a) for natural amphiboles extrapolated to (super [4]) Al = 2 apfu at 900 degrees C. The different long-range Al-Si ordering behavior of flour-edenite and pargasite is explained in terms of the bond-valence requirements of O4. At high temperatures, configurational entropy becomes an important stabilizing factor, and structural distortion around O4 in pargasite and hornblende allows Al into T2, provided that O4 is coupled to Al at an adjacent M2 site, as in pargasite. The results for fluoredenite, which has no (super [6]) Al, show that the low O4 bond-strength sum of the T2 Al M2 Mg M4 Ca configuration cannot be accommodated by sufficient structural relaxation, even at 1000 degrees C. Coupling between Al at M2 and T2 is an important control on Al-Si long-range order-disorder.

Changes in the crystal structure of tsaregorodtsevite [N(CH3)4][Si2(S (sub 0.5) Al (sub 0.5) )O6]2on heating

Tsaregorodtsevite [N(CH 3 ) 4 ][Si 2 (Si (sub 0.5) Al (sub 0.5) )O 6 ] 2 is a unique feldspathoid with a tetrame-thylammonium (TMA-) organic cation in an ordered, sodalite-like framework of orthorhombic symmetry (I222) with a = 8.984(3), b = 8.937(2), and c = 8.927(2) Aa. Here, 13 C cross polarization MAS and 1 H MAS NMR spectra give direct evidence that the organic material in the cavities in tsaregorodtsevite is TMA (super +) . The 27 Al and 29 Si MAS NMR spectra, and XRD data show that the framework is well ordered with Si in the T1 and T2 sites and both Si and Al in the T3 site. Upon heating, the colorless tsaregorodtsevite crystals change color becoming yellow, then brown, and finally black. In this study 1 H, 13 C, 27 Al, and 29 Si MAS NMR, powder XRD, and electron microprobe analyses are used to investigate the structural changes in tsaregorodtsevite on heating. The cell dimensions and T-O bond lengths decrease, and there are two phase changes. At 690 degrees C, the TMA- molecule breaks down to form aromatic rings and acidic groups, possibly including benzene and the pyridinium ion, with ammonia and other gases produced. The Al-Si framework changes slightly to give a less well-ordered tetragonal structure (I422), with a = 8.925(1), c = 8.908(1) Aa, and a small amount of Al enters a separate phase. After heating at 970 degrees C, the organic material has further broken down with little C and H visible by NMR spectroscopy. The Al-Si framework forms at least two new phases: one with a cubic crystalline structure (I432) with a = 8.817(3) Aa, and an amorphous aluminosilicate phase. There is a small residue of the initial structure even after heating for one hour at over 900 degrees C.

Al, Si, and Mg occupancies in tetrahedrally and octahedrally coordinated sites in synthetic aluminous tremolite

A series of aluminous tremolites was synthesized at 800‐850 8C and 6‐13 kbar along the join Ca1.8Mg5.2Si8O22(OH)2-Ca1.8(Mg4.2Al)(AlSi7)O22(OH)2 by means of the Mg-Tschermaks substitution. Although the amphibole yields were high (97‐99 wt%), traces of orthopyroxene, clinopyroxene, amorphous silica, and, for the most aluminous mixture, corundum, were identified in some synthesis products. The samples were characterized by electron microprobe (EMP), X-ray diffraction Rietveld structure refinement (XRD), Fourier-transform infrared (FTIR) spectroscopy, and magic-angle-spinning nuclear magnetic resonance (MAS NMR) spectroscopy for the purpose of identifying the extent of Al, Mg, and Si order-disorder in the octahedral and tetrahedral sites. Both EMP and 27 Al NMR verify that the Mg-Tschermaks substitution, with even distribution of Al between tetrahedral and octahedral sites, is obeyed in these amphiboles up to a limit of 1.9 total Al atoms per formula unit (apfu). 29 Si MAS NMR spectra indicate that Al substitutes essentially equally into both the T1 and T2 sites. For octahedral sites, the FTIR spectra indicate the presence of Al on the M1, M3, or both sites (presently indistinguishable) by the presence of an (MgMgAl)-OH band at 3652 cm 21 , which increases steadily with increasing Al content. The FTIR and 27 Al MAS NMR spectra show that Al is always present in the M1-M3 sites as well as the M2 site but partitioned among these sites in ways that are not clearly resolved at present. It is clear in this study that Al is more widely distributed over the octahedral and tetrahedral sites in synthetic aluminous tremolite with only a moderate Al content (,2.0 Al apfu) than has been traditionally thought.

Metal Oxide (TiO2, MoO3, WO3) Substituted Silicate Xerogels as Catalysts for the Oxidation of Hydrocarbons with Hydrogen Peroxide

TiO2, MoO3, and WO3have been dispersed in amorphous silica using the low temperature sol–gel procedure for xerogel preparation. These simply prepared amorphous compounds are proposed as possible alternatives to metal-substituted crystalline molecular sieves in H2O2oxidations. The metallosilicate compounds are catalytically active in the 30% aqueous H2O2oxidation of alkenes and alcohols provided the metal oxide precursor in the xerogel synthesis is a metal–dichlorodialkoxy compound yieldingMOx(Cl)–SiO2, and not the tetraalkoxy derivative yieldingMOx–SiO2. Catalyst efficiency is increased by using low loading of metal oxide in the silica framework. Excess H2O2reduces yield due to the detrimental effect of water, so more hydrophobic silicates with phenyl-silicon units increases catalyst efficiency. IR studies show that in the xerogels, the absorption at ∼950 cm−1is mainly due to the Si–OH vibrations in (SiO)3Si–OH units and not (SiO)3Si–OM as has often been reported in studies of titanium-substituted zeolites.29Si MAS NMR spectra, sensitive to second neighbor atoms, of catalytically activeMOx(Cl)–SiO2versusinactiveMOx–SiO2reveals that the former have largerQ3peaks and therefore more (SiO)3Si–OM units, indicating higher molecular dispersion of the metal oxide in the xerogels. Diffuse reflectance UV-vis measurements indicate, however, that this molecular dispersion is not complete as absorptions attributable to polymeric forms of metal oxide are observable. ESR spectra of the metal oxide substituted silicates in the presence of hydrogen peroxide or in the reduced form are not useful in differentiating between active and inactive xerogel compounds. Atomic force microscopy imaging of the xerogels at ∼10 nm resolution shows that the xerogel has a basically smooth surface. Large cylindrical pits of 500–700 nm diameter and depth of 15–40 nm are also observable as imperfections in the xerogel. There is also formation of small silicate droplets on the surface with dimensions similar to that of the pits. The catalytic xerogels are microporous with an average pore diameter of 15 Å and a surface area of 750 m2/g.

Nucleation and growth behavior of lithium disilicate at pressures to 0.5 GPa investigated by NMR spectroscopy

Glasses of the homogeneously nucleating lithium disilicate, Li 2O–2SiO 2, exhibit a distinct nucleation behavior with pressure, in that the nucleation maximum moves from 451°C at ambient pressure to 475°C at 0.1 GPa and to 490°C at 0.31 GPa. At 0.5 GPa no nucleation is observed. Glasses quenched at or close to these conditions are investigated by 29 Si , 7 Li and 6 Li magic angle spinning nuclear magnetic resonance (MAS NMR) and nutation NMR to track related structural changes. NMR spectra of 6 Li and 7 Li do not differ from those of the virgin glass, while 29 Si MAS NMR spectra show changes of the peak width, and the peak maximum becomes more negative with pressure. The 29 Si MAS NMR spectrum of crystals produced from the glass at 0.31 GPa is narrower in width than spectra of crystals grown at ambient conditions.

Synthesis and characterization of mesoporous niobium-doped silica molecular sieves

Hexagonally-packed mesoporous Nb-doped silica molecular sieves were hydrothermally synthesized with various siliceous sources and dopant concentration levels. The microstructure of mesoporous materials was characterized by a variety of physico-chem-ical methods. The pore packing order of the Nb-doped material depended strongly on the synthesis conditions such as aging temperature, pH of reaction mixture, surfactant-to-Si ratio, and dopant concentration. Different silica precursors gave rise to different reactivity with the Nb dopants and interactions with the surfactants. The pore size and surface area of the materials can be systematically varied. The chemical environment of the Nb dopants in the silica framework was studied by XPS, EPR PA-FTIR, UV-Vis, and 29 Si MAS NMR. The results indicated that the Nb 5+ ions were well dispersed in the framework of the mesoporous silica. The diffuse-reflectance UV-Vis spectrum of the mesoporous Nb-doped silica is red-shifted with respect to pure mesoporous silica. The 29 Si MAS NMR spectrum contained a broad component at about - 105 ppm, which may have been attributed to the presence of Nb-O-Si bonding in the mesostructure.

A reassessment of the 29Si MAS-NMR spectra of sepiolite and aluminated sepiolite

AbstractA 29Si MAS-NMR one-pulse and ﹛1H﹜ CP comparative study of sepiolite, folded sepiolite, aluminated sepiolite, and palygorskite was conducted. The three main resonances of sepiolite at -92.0 ± 0.1, —94.3 + 0.1, and — 97.8 ± 0.1 p.p.m. were assigned with respect to the position of the Si atoms within the three pairs of symmetrical sites in the basal plane. The line at —97.8 p.p.m. was attributed to the central sites by comparison with the resonances and chemical environments of Si in talc and palygorskite. The line at —94.3 p.p.m. was attributed to the edge sites, because it is strongly reinforced by cross-polarization. Alumination of sepiolite led to a broadening of the peaks and new downfield resonances in line with what can be observed in other phyllosilicates and zeolites.

NMR spectra of moganite and chalcedony

29 Si MAS NMR, 29 Si { 1 H} CP MAS NMR and 1 H MAS NMR spectra of the water-containing microcrystalline silica varieties chalcedony, moganite and three intermediate samples are reported. The 29 Si MAS NMR spectra give evidence that, unlike quartz, the silicon atoms occupy two symmetrically different sites in moganite. The 1 H MAS NMR spectra show signals of molecular water and four different species of silanol groups

Feldspars with trivalent non-tetrahedral cations: Experimental studies in the system NaAlSi3O8 - CaAl2Si2O8 - LaAl3SiO8

Feldspars with up to about 45% trivalent cations on M positions were synthesized by heating gels of composition Ca 8−2x Na x La x Al 16 Si 16 O 64 at 1300°C and 1 bar. In the range O 3 this substitution is accompanied by the substitution Ca 2+ + Al 3+ → Na + + Si 4+ . As x increases, the triclinic anorthite structure gradually becomes monoclinic at x(gel) = 3.05(3). Electron diffraction indicates transformation from P1 through I1 to I2/c with increasing x. 29 Si MAS NMR spectra indicate considerable Al/Si disorder, as well as low long- and medium-range order of the Q 4 (4Al) units. The latter is due to distortion of the [SiO 4 ] tetrahedra and considerable variation of the Si−O−T angles caused by rotation of the [TO 4 ] tetrahedra as they adjust to the different M cations

Changes in the environment of silicon and aluminum in SAPO-37 zeolite during acidity measurements

The interaction of bases (pyridine, 2,6-dimethylpyridine) is studied on SAPO-37, pretreated at nonusual high temperatures from 873 to 1173 K. XRD, infrared, and 31 P, 27 Al, and 29 Si MAS NMR spectroscopies are used, the spectra being recorded in the presence of the bases. Several types of interactions are shown by infrared and NMR, depending on the pretreatment temperatures. The 27 Al and 29 Si MAS NMR spectra are modified in the presence of bases. The protonic acidity, studied with the two probes, decreases when the temperature rises, as usual in zeolites for the bridging Si-OH-Al hydroxyls, but very strong acid sites are created after the 1173 K heating. At least three kinds of Lewis sites are seen by infrared

Phase discrimination with silicon-29 MAS NMR in EMT/FAU zeolite intergrowths

In the 29 Si MAS NMR spectra of Na-exchanged FAU/EMT zeolite intergrowths, the signals from the FAU and EMT phases coincide. However, after Li ion exchange of such zeolites, an upfield shift of the FAU signals by 2-3 ppm occurs and, consequently, partial resolution of the resonance signals is possible. Thus the combination of 29 Si MAS NMR spectra of the Na- and Li-exchanged forms of an EMT/FAU zeolite allows one to determine the Si,Al distribution in the FAU and EMT phases separately as well as the relative amounts of each phase in the sample. The method is applicable to EMT/FAU samples containing Si(4Al) environments and is illustrated for ZSM-2 and ZSM-3

High-resolution aluminum-27 and silicon-29 MAS NMR investigation of silica-alumina glasses

{sup 27}Al and {sup 29}Si MAS NMR spectra have been obtained for SiO{sub 2}-Al{sub 2}O{sub 3} glasses quenched from the melt at two rates: 10{sup 5}-10{sup 6} {degree}C s{sup {minus}1} (SQ) and 10{sup 2}-10{sup 3} {degree}C s{sup {minus}1} (NQ). TEM experiments detected phase separation in the S{sub 76}A{sub 24}SQ and S{sub 52}A{sub 48} SQ samples but no phase separation for the S{sub 41}A{sub 59} SQ composition. The NQ glasses were phase separated and, in the diffraction TEM experiments, showed diffuse spots indicating microcrystallinity. {sup 27}Al spectra for the SQ glasses show three peaks due to aluminum in 4-, 5-, and 6-coordination, whereas NQ glasses show only peaks associated with aluminum in 4- and 6-coordination. Quantitative results for the {sup 27}Al spectra are discussed. The effect of spin speed on the {sup 27}Al MAS NMR spectra for SQ and NQ samples is demonstrated. Magnitudes of mean quadrupolar coupling constants and mean isotropic chemical shifts have been estimated. {sup 29}Si spectra of SQ glasses reveal a distribution of 4-coordinate Si sites, whereas NQ glasses show two inequivalent 4-coordinate sites.

Silicon-29 NMR characterization of .alpha.- and .beta.-silicon nitride

Fine structure in the {sup 29}Si MAS NMR spectra of Si{sub 3}N{sub 4} has been observed, prompting a reevaluation of the current interpretation of Si{sub 3}N{sub 4} spectra. On the basis of variable field studies, this fine structure indicates the presence of quadrupole-perturbed residual dipolar coupling to {sup 14}N. This is apparently the first such observation in {sup 29}Si NMR. The peaks in the spectra of {alpha}- and {beta}-Si{sub 3}N{sub 4} are shown to arise from differences in the non-nearest-neighbor environments. Arguments based on a sphere of influence approach appear to be an attractive interpretive tool for these and similar ceramic materials. Some speculation is also presented concerning the relationship between the {sup 29}Si-{sup 14}N splitting and the structure of Si{sub 3}N{sub 4}.