J. Stephen Hartman

A high-resolution 29Si and 27Al NMR study of alkaline earth aluminosilicate glasses

Abstract The abundance of chain- (Q2), sheet- (Q3), and framework-like (Q4) tetrahedral units, and the distribution of Al among those units in CaOAl2O3SiO2 and MgOAl2O3SiO2 glasses, has been investigated by 29Si and 27Al magic-angle spinning nuclear magnetic resonance (MAS-NMR). In each system two compositional series were studied: (i) average polymerization is held constant at one non-bridging oxygen per tetrahedron (Q3), and (ii) silica content held constant at 50 mol% (average polymerization varying from Q2.67 to Q4). The results indicate that (1) Al is entirely tetrahedrally coordinated and is largely in Q4 sites, except in Al-rich Q3 glasses where it is also in Q3 sites, and (2) at constant polymerization of Q3 the ‘reaction’ Q2 + Q4 = 2Q3 goes to the right with increasing Al content. 29Si chemical shifts calculated from 27Al peak positions assuming that all Al is tetrahedrally coordinated and in Q4 sites, and that only Q2, Q3, and Q4 sites are present in the glass, give reasonable agreement with measured values (±2 ppm), but this model applies only to sufficiently silicon-rich compositions in which there is negligible Al in Q3 sites.

Gel synthesis of magnesium silicates: a 29Si Magic Angle Spinning Nmr study

The formation of the magnesium silicate minerals forsterite, enstatite, and roedderite by heating of amorphous “protosilicate” gels precipitated from aqueous solution has been studied by 29Si MAS nmr. Gentle drying of the hydrogels at 110° C gives materials with broad nmr signals that do not differ appreciably with preparation conditions, but the minerals formed by heating at 750° C or higher are greatly dependent on the precipitation and washing conditions of the original gel. The rare mineral roedderite, best known from studies of unequilibrated enstatite chondrite meteorites, becomes a major species along with forsterite when the hydrogels are washed with sodium hydroxide solution before drying and heating to 750° C.

27Al and 23Na NMR spectroscopy and structural modeling of aluminofluoride minerals

Abstract Simulations of high-resolution 19F-decoupled 27Al and 23Na magic-angle spinning nuclear magnetic resonance (MAS NMR) spectra of the aluminofluoride minerals, cryolite, cryolithionite, thomsenolite, weberite, chiolite, prosopite, and ralstonite combined with theoretical modeling have given accurate values of chemical shift (δiso), and quadrupolar interaction parameters (Cq and η), thereby eliminating ambiguities incurred by the complex nuclear interactions. These NMR data have been correlated with local electronic environments in the minerals, which were calculated using Full Potential Linearized Augmented Plane Wave (FP LAPW) modeling based on the structures from X-ray diffraction (XRD) data. This combination of NMR, XRD, and modeling techniques allowed the analysis and optimization of the crystal structures. The electronegativities and distances of neighboring ions, represented here by an environmental parameter χ, are shown to control δiso of both 23Na and 27Al. The calculations using χ, also show that the ions beyond the nearest neighbor play an important role in determining δiso of 27Al and 23Na in these aluminofluoride minerals, and the substitution of OH for F significantly affects the shielding around 27Al in prosopite and ralstonite. There is a positive correlation between the site distortion at the Na and Al sites and the values of Cq in these aluminofluoride minerals.

Direct detection of chlorine-35 multiple-quantum NMR transitions in a single crystal of sodium chlorate

35Cl (spin 3/2) in sodium chlorate has a strong quadrupole coupling, similar in size to its interaction with a magnetic field of 11.7 T (500 MHz for protons). This means that it obeys neither the standard nuclear quadrupole resonance (NQR) nor the nuclear magnetic resonance (NMR) rules. Recently, we have published a theoretical approach that allows us to calculate the transition frequencies of such a system without approximations. This exact solution is applicable to any spin, with any relative ratio of Zeeman to quadrupole coupling, so we can map out how NQR becomes NMR as the magnetic field is increased. In the general situation, the selection rules restricting direct observation to only single quantum transitions break down and nominal multiple-quantum transitions become directly observable. The transition frequencies depend on the Zeeman interaction, the quadrupole coupling and the orientation of the crystal in the magnetic field. Sodium chlorate is an ideal sample, since we can use the 23Na NMR spectrum to determine the orientation of the crystal. 23Na (spin-3/2) has a weak quadrupole coupling and there are four molecules per unit cell. The positions of the quadrupole satellites from the four molecules are sufficient to determine the orientation, so the sodium nucleus acts as an internal goniometer. Since we already know the magnetic parameters for 35Cl, we can predict where to look for its resonances, which can range from less than 10 MHz to greater than 150 MHz. A single spin-3/2 has 6 possible transitions, so the four molecules in the unit cell should show 24 transitions. We have observed 17 of them.

FRIEDEL-CRAFTS CATALYSIS USING SUPPORTED REAGENTS : SYNTHESIS, CHARACTERIZATION AND CATALYTIC APPLICATIONS OF SOL-GEL-DERIVED ALUMINOSILICATES

Aluminosilicate catalyst supports have been prepared from aluminium tri-sec-butoxide and tetraethyl orthosilicate in butanol using sol–gel methodologies, thereby providing useful supports for zinc chloride, to generate heterogeneous Friedel–Crafts alkylation catalysts. The optimum loading for these sol–gel-derived materials is higher than that of K10, the support for the commercially available catalyst ‘Clayzic’, and gives significantly higher catalytic activity in a standard test reaction. A more convenient, ‘one-pot’ approach, in which zinc chloride is added during the sol–gel synthesis, generates catalysts which also show significantly higher activity than Clayzic. The resulting materials have been characterized by 29 Si cross-polarization magic-angle spinning (CP-MAS) and 27 Al MAS NMR, IR spectroscopy and by surface area and pore volume analysis. The Si:Al ratio, drying temperature and relative amounts of water and (non-aqueous) solvent have pronounced effects on catalytic activity. The most active catalysts show relatively high levels of framework aluminium species compared with non-framework sites, and also have higher surface areas and pore volumes than materials with lower activity.

Transition metal cation site preferences in forsterite (Mg2SiO4) determined from paramagnetically shifted NMR resonances

Abstract In marked contrast to the single, narrow 29Si MAS NMR resonance for pure forsterite (Mg2SiO4), the spectra for synthetic forsterite containing 0.05 to 5% of the Mg2+ replaced with Ni2+, Co2+, or Fe2+ display between 4 and 26 additional, small, paramagnetically shifted peaks that are caused by interactions of the unpaired electron spins on the transition metal cations and the nuclear spins. Analyses of these relative peak areas, their numbers, and comparison of their positions to those in spectra of synthetic monticellites (CaMgSiO4) containing similar levels of transition metals, allows at least partial assignment to the effects of cations in either the M1 octahedral site only or to both M1 and M2 sites. More detailed analyses indicate that in forsterite, Ni2+ occupies only M1, Fe2+ occupies M1 and M2 roughly equally, and Co2+ occupies both M1 and M2 in an approximately 3:1 ratio. These findings for low concentrations agree with expectations from previous studies by other methods (e.g., XRD) of olivines with much higher transition metal cation contents. However, even low concentrations of Mn2+ (e.g., 0.1%), as well as higher Fe2+ contents (e.g., in natural San Carlos olivine) can broaden NMR peaks sufficiently to greatly reduce this kind of information content in spectra.

Friedel–Crafts catalysis using supported reagents. Synthesis, characterization and catalytic application of ZnCl2 supported on fluoride‐modified sol–gel‐derived aluminosilicates

ZnCl2–aluminosilicate catalysts were prepared via a sol–gel route involving fluoride‐catalyzed hydrolysis of aluminum and silicon alkoxides in the presence of NaF, KF, NH4F and ZnF2. The catalysts were characterized by employing 29Si, 27Al and 19F solid‐state MAS NMR. The dependence of the activities of the catalysts on the nature and amount of fluoride present in the catalysts were investigated using Friedel–Crafts alkylation reaction of benzene with benzyl chloride.

29 Si and 13C magic angle spinning n.m.r. spectra of silicon carbide polymorphs

Hexagonal and cubic silicon carbide polymorphs give distinctive 29Si and 13C magic spinning n.m.r. spectra that can be related to the number of nonequivalent lattice sites in the structure and variations in the non-nearest-neighbour environment.

Mixed Boron Trihalide Adducts. A Nuclear Magnetic Resonance Study of Halogen Redistribution in Adducts of Trimethylphosphine, Trimethylphosphine Oxide, and Trimethylphosphine Sulfide

Halogen redistribution in the sparingly soluble boron trihalide adducts of Me3P, Me3PO, and Me3PS gives mixed boron trihalide adducts of these bases. Only small proportions of the fluorine-containing mixed boron trihalide adducts are present at equilibrium in the Me3PS adduct system, in accord with previous studies of adducts of other sulfur donors. The effects of the donor on F,Cl and F,Br halogen redistribution are discussed.

Analysis of fluid inclusions using nuclear magnetic resonance

Nuclear magnetic resonance (NMR) spectra from 23Na and 35C1 in fluid inclusions in samples of quartz and beryl show the potential of NMR as a powerful analytical technique for the study of fluid inclusions.