High field MAS NMR, with simulations of the effects of disorder on lineshape, applied to thermal transformations of alumina hydrates

Abstract

Samples of gibbsite, boehmite, pseudoboehmite and bayerite, calcined at temperatures up to 1000"C, were studied using 27AI MAS NMR with spinning speeds up to 15 kHz in magnetic fields of 14.1, 11.8 and 7 T (156, 130 and 78 MHz 27A1 frequencies). The high magnetic fields reduce the effects of the second order quadrupolar interactions, giving much better resolution of the peaks than previous studies at 7 T and aUowing quantitation of the different species. The samples generally show only octahedral and tetrahedral species, with the latter increasing with calcination temperature. The exceptions are p-alumina and ;~-alumina which show a peak at 33 ppm (in the 130 MHz spectra) with about 20% and 7% respectively of the total intensity. The peaks show large upfield tails anda dependence on magnetic field which differs from that expected fora single electric field gradient. Spectra were simulated assuming a random distribution of electric field gradients, and the dependence of the speetra on the field strength, quadrupolar parameters and the range of electric field gradients is illustrated and empirical equations derived. The method provides a good fit to the observed spectra at all 3 field strengths. These materials show considerable disorder which can be categorized by the parameter used to describe the range of electric field gradients.

27Al NMR Studies of Aluminosilicate Glasses

“…In contrast to DOR, however, DAS is a 2D NMR technique, utilizing that each A 20 (Ω) and A 40 (Ω) factor [Eq. (26)] depends on the precise rotor inclination angle β r ; the protocol may be expressed schematically as t 1 (β r ¼ 37.38 ∘ )-(rotor reorientation)-t 2 (β r ¼ 79.19 ∘ ), with z-magnetization stored while the rotor-axis position is altered. With this combination of orientations (one out of several options [90,196,207]), the phase accumulated by the À1QC evolution at the time-point t 2 becomes free of both second/fourth-rank tensor contributions from the second-order quadrupolar interaction, leading to the formation of an "echo" with solely the isotropic frequency component encoded [196].…”

“…As for the case of the CT, the ST ssb peakwidth is ideally broadened solely by the MAS-surviving fourth-rank component [A 40 (Ω)] of the second-order quadrupolar interaction [Eq. (26)]. However, since the CT is invariant to H ð1Þ Q , this property holds regardless of the precise rotor-axis inclination angle β r (relative to B 0 ), whereas each ST ssb broadens by % ν Q (3cos 2 Δβ f gÀ 1) for a deviation Δβ from the exact magic angle β m ¼ arctan ð ffiffi ffi 2 p Þ % 54:7356 ∘ [36].…”

“…This broadening mechanism applies to any spatial NMR interaction portion that transforms as a second-rank tensor, such as the CSA and the A 20 (Ω) part of H ð2Þ Q [Eq. (26)]. Fortunately, the relatively small size (kHz range) of these anisotropies sets much lower demands on the precise β r -value.…”

“…One option involves generating normal distributions of two EFG tensor components of {V xx ,V yy ,V zz }, whereas the third one is deduced by the restriction V xx +V yy +V zz ¼ 0 of the traceless property of the tensor, from which either each distribution of C Q and η is generated [105], or η is (arbitrarily) assumed to be fixed [26,106]. Simpler approaches (albeit not physically sound, as discussed by de Lacaillerie [107]), assume a Gaussian distribution of C Q , while the asymmetry parameter is either kept constant [108][109][110][111] or distributed independently [112].…”

“…We highlight the distinctions between 27 Al NMR applications to well-ordered structures and glasses, whose amorphous nature yields distributions of the NMR observables (Section 5). Shortly after the mid 1980s when solid-state 27 Al NMR investigations of inorganic materials begun to expand significantly, some excellent reviews that addressed practical concerns of 27 Al NMR experimentation appeared in the early 1990s-a few targeted specifically 27 Al MAS NMR applications to disordered structures and were published in a special volume of Applied Magnetic Resonance [25][26][27]. More recent reviews merely focus on new techniques and/or theoretical aspects of NMR on quadrupolar nuclei.…”