Colin A. Fyfe

27Al and29Si magic angle spinning nuclear magnetic resonance spectroscopy of Al-substituted tobermorites

Solid-state27Al and29Si NMR spectroscopy with magic angle spinning (MAS) of samples was used to study several 1.13 nm tobermorites, most of which were deliberately substituted with aluminium.27Al MASNMR clearly showed that aluminium is tetrahedrally co-ordinated in tobermorite structures. In addition two different aluminium environments resonating at ∼ 57 and 64 ppm from [Al(H2O)6]3+ were detected.29Si MASNMR of pure, anomalous tobermorites showed resonances at −85.7 and −95.7 ppm from tetramethylsilane representing chain middle groups (Q2) and branching sites (Q3), respectively. Anomalous Al-substituted tobermorites, on the other hand, showed two to four resonances representing different silicon environments. One Al-substituted tobermorite showed two resonances at −84.6 and −91.5 ppm which were assigned to Q2(0 Al) and Q3 (1 Al), respectively. In the above tobermorite aluminium appeared to have substituted into branching sites only. Two other Al-substituted tobermorites, however, showed four distinct resonances at ∼ −82.0, −85.2, −92.0 and −96.0 and these were assigned to Q2 (1 Al), Q2 (0 Al), Q3 (1 Al) and Q3 (0 Al), respectively. Thus these two tobermorites showed substitution of aluminium in the chain middle groups as well as branching sites. Another Al-substituted tobermorite which showed a normal thermal behaviour exhibited, as expected, only Q2(0 Al) and Q2 (1 Al) sites resonating at −84.7 and ∼ −80.2 ppm, respectively. No Q3 sites were detected because few or no branching sites are present in this normal tobermorite. The results reported here clearly demonstrate the usefulness of solid-state27Al and29Si MASNMR spectroscopy for the investigation of short-range order in alumino-silicate materials.

Evaluation of the quantitative reliability of the 13C CP/MAS technique for the analysis of coals and related materials

Abstract The percentages of aromatic carbon in three representative solid coal samples and one pitch sample have been determined by three different n.m.r. techniques. The first two involve the use of cross-polarization/magic-angle spinning (CP/MAS) and in one case, the magnetization curves were fitted to a theoretical expression and the fit parameters used to calculate the percentage of aromatic carbon. In the second case, the percentage of aromatic carbons was obtained from a choice of contact time in the cross-polarization pulse sequence which gave maximum signal intensity. The third technique used MAS together with a simple 90° carbon pulse sequence with a 5 × T1, (for the carbon-13 nuclei) recycle time and was assumed to give the absolute quantitative values. It was found that the results obtained using cross-polarization techniques could be substantially different from those obtained using 90° pulses and that caution must be exercised in the application of these techniques to the quantitative analysis of solid carbonaceous fuels.

Structural States of Mg-Cordierite I: Order Parameters from Synchrotron X-Ray and NMR Data

The hexagonal to orthorhombic phase transition in synthetic Mg-cordierite has been studied by (i) measuring the spontaneous strain associated with the transition using Synchrotron X-ray powder diffraction and (ii) measuring the degree of Al, Si order in terms of the number of Al-O-Al bonds per formula unit using solid state NMR spectroscopy. This defines the two order parametersQ andQodrespectively, and their relationship as a function of annealing temperature and time is used to define the structural states of cordierite during the ordering sequence. The formation of modulated hexagonal cordierite within which a high degree of Al, Si order can be attained, results in a strongly non-linear relationship betweenQ andQod.The transition from modulated to orthorhombic cordierite is strongly first-order under all temperature conditions studied and involves a large step inQ, whileQodchanges continuously throughout the ordering sequence with no marked discontinuity at the phase transition. The lattice distortion, traditionally defined in cordierite by the Δ index provides no full information on the degree of Al, Si order in anhydrous Mg-cordierite, and both order parameters must be used to define its structural state. Transmission electron microscopy has been used to study the mechanism of the transformation from hexagonal to modulated to orthorhombic cordierite.

Correlations between lattice structures of zeolites and their 29Si MAS n.m.r. spectra: zeolites KZ-2, ZSM-12, and Beta

Si MAS n.m.r. spectra are presented for highly siliceous samples of zeolites KZ-2, ZSM-12, and Beta. KZ-2 shows four resonances of relative intensities 2:1:1:2; ZSM-12, seven resonances of equal intensity; and Beta, a minimum of nine resonances of different intensities. The data are correlated with the recently proposed structures for KZ-2 and ZSM-12.

Correlation of 13C chemical shifts with torsional angles from high-resolution, 13C-C.P.-M.A.S. N.M.R. studies of crystalline cyclomalto-oligosaccharide complexes, and their relation to the structures of the starch polymorphs

The chemical shifts and multiplicities of the resonances in high-resolution, 13C-c.p.-m.a.s. n.m.r. spectra of cyclomalto-oligosaccharide inclusion complexes are characteristic of the crystalline structure of the different complexes. In particular, the 13C chemical shifts of C-1 and C-4 correlate with the torsion angles φ2and φ1 respectively (related to Ψ and φ, respectively, in an alternative terminology), which describe the orientation of the D-glucosyl residues about the α-D-(1→ 4) glycosidic linkage. The 13C chemical shift of C-6 correlates with the torsion angle x, which describes the orientation of O-6 about the exocyclic, C-5-C-6 bond. The cyclomalto-oligosaccharide inclusion complexes are good models for the interpretation of the characteristic chemical shifts and multiplicities previously observed in the 13C-c.p.-m.a.s. n.m.r. spectra of the natural starch polymorphs. From these chemical-shift correlations, values for the torsion angles φ2, φ1 and X are predicted for starches that crystallize as “A” and “B” structures. These predicted values are in agreement with the limited data currently available from X-ray fiber diffraction studies.

Detailed interpretation of the 29Si and 27Al high-field MAS n.m.r. spectra of zeolites offretite and omega

Abstract The 29Si MAS n.m.r. spectra of highly dealuminated zeolites are shown to exhibit separate Si(OAl) resonances due to crystallographically inequivalent lattice sites. The chemical shift dispersion due to crystallographic inequivalence is of the same order of magnitude as that from the presence of aluminium atoms in the first coordination sphere and can result in the overlapping of resonances. Thus, the interpretation of the 29Si MAS n.m.r. spectra of zeolites of low Si Al ratios can be ambiguous. The correct peak assignments are presented for the zeolites offretite and omega and the problems involved in obtaining quantitatively reliable Si Al ratios from the spectra discussed. In the case of offretite it is not possible to completely describe the distribution of silicon and aluminium over its two non-equivalent crystallographic sites, although the Si Al ratios for a number of samples are shown to agree with a random distribution. For zeolite omega it is possible to completely describe the non random Si distribution and to predict the aluminium distribution, which is in excellent agreement with that obtained independently from the high field (14.1T) 27Al MAS n.m.r. spectrum. All data indicate that Si and Al distributions in omega are not random in nature.

STRUCTURE AND CHEMISTRY OF NAFION-H: A FLUORINATED SULFONIC ACID POLYMER

Abstract This paper deals with the structure and chemistry of Nafion-H, the acidic form of Nafion, which is the trade name for a perfluorinated polymer manufactured by E. I. du Pont de Nemours & Co., Inc. Nafion-H and its salts are available from Dupont in both membrane and powder forms. Nafion composites with Teflon mesh (polytetrafluoroethylene) added to give increased mechanical strength are also available. In addition, sulfonated fluorocarbons modified with other chemically active polymers have been made by Dupont and by competing companies such as Ashai Chemical Industry Co., Ashai Glass Co., Tokuyama Soda Co., and Toya Soda Co.

An Investigation of the Dynamic Structures of Ferrocene, Ruthenocene and Dibenzenechromium in the Solid State and in Solution

Abstract Pulse nuclear magnetic resonance measurements have been used to determine the activation enthalpies for the reorientation of the organic rings in ferrocene, ruthenocene and dibenzenechromium. The contribution from non-bonded interactions to the total potential for reorientation of the rings has been calculated and is compared to the experimental activation enthalpies. The results indicate that there is no contribution to the potential from the non-bonded interactions between the two rings on a single molecule and that the observed potential barrier is due to a combination of crystal packing forces and bonding forces within the molecule. The crystal structure of ferrocene is thought to be disordered above 135[ddot]K, and a model is proposed for the order-disorder transition in the solid state. On this basis, dynamic models are proposed for the structures of the three metallocenes in solution.

Naturally occurring 1.4 nm tobermorite and synthetic jennite: Characterization by 27Al and 29Si MASNMR spectroscopy and cation exchange properties

Abstract A naturally occurring 1.4 nm tobermorite and a synthetic jennite were characterized by 27 Al and 29 Si magic angle spinning nuclear magnetic resonance (MASNMR) spectroscopy and their cation exchange properties were measured. 27 Al MASNMR spectroscopy revealed that both the 1.4 nm tobermorite and synthetic jennite contained trace quantities of Al in tetrahedral coordination. The 1.4 nm tobermorite also contained octahedrally coordinated Al probably due to the presence of an aluminum compound as a trace impurity. The 29 Si MASNMR spectrum of 1.4 nm tobermorite exhibited a strong resonance at −85.2 ppm and a small shoulder at −80 ppm which are attributed to chain middle groups (Q 2 ) and end groups (disilicates) respectively. 29 Si MASNMR spectroscopy of the synthetic jennite showed a strong resonance at −85.7 ppm and a moderately strong resonance at −81.4 ppm which correspond to single chains and end groups respectively. Jennite showed a considerably larger quantity of end groups than the 1.4 nm tobermorite. The unsubstituted 1.4 nm tobermorite and synthetic jennite exhibited only small cation exchange capacities.

Al,Si ordering on cordierite using ?magic angle spinning? NMR: I. Si29 spectra of synthetic cordierites

Silicon-29 “magic angle spinning” nuclear magnetic resonance (NMR) spectroscopy has been used to study the changes in local Si environment during Al, Si ordering in synthetic cordierite, Mg2Al4Si5O18. In the most disordered form, crystallized from a glass, eight distinct tetrahedral sites for silicon can be identified and assigned, while there are only two distinguishable Si sites in the well-annealed ordered form. This allows the changes in the Si site environments to be determined as a function of annealing time for the transformation from the disordered to the ordered form. The first crystallized state has a considerable degree of partitioning between T1 and T2 sites with the following site occupancies: T1 − Al:Si=0.80:0.20, T2−Al:Si=0.27:0.73 The changes in Si environment are approximately linear with log time. The measured values of 29Si isotropic chemical shift do not fit well to previously determined correlations of shift with various structural parameters.