First Application of ²⁷Al Multiple Quantum Magic-Angle Spinning Nuclear Magnetic Resonance at 16.4 T to Inorganic Matter in Natural Coals

Abstract

27Al multiple quantum magic-angle spinning (MQMAS) spectroscopy at high magnetic field (16.4 T) was applied for the first time to the structural analysis of inorganic matter in natural coals. Resolution of 3QMAS spectra at 7.0 T was insufficient for identification of the inorganic matter in natural coals, because of incomplete cancellation of the second-order quadrupolar effects, although 3QMAS spectra gave better resolution than conventional magic-angle spinning (MAS) spectra. On the other hand, an increased magnetic field (16.4 T) greatly improved 3QMAS spectral resolution, compared to that obtained at 7.0 T; hence, it facilitated the assignment of some minerals. Moreover, 3QMAS spectra recorded at 16.4 T led to signal enhancement by a factor of ∼4, compared to spectra taken at 7.0 T. Higher field strengths became useful for the analysis of inorganic matter in natural coals that have a low concentration of aluminum (∼2.0 mass %). 3QMAS and 5QMAS spectra at 16.4 T were also compared. As far as inorganic matter in natural coals is concerned, the spectral resolution was almost unchanged between 3QMAS and 5QMAS, although the excitation of 5 quantum coherences is less efficient than that for 3 quantum coherences. It is concluded that the combination of MQMAS techniques with high magnetic field is a very effective for characterization of inorganic matter in natural coals and is especially well-suited to the analysis of clay minerals that have low crystallinity.

Practical comparison of sensitivity and resolution between STMAS and MQMAS for 27Al

“…These values are identical whichever STMAS and MQMAS spectra were selected, and found to be d iso = 8.9 ± 0.3 ppm and P Q = 3.5 ± 0.3 MHz for [6] Al site of kaolin, and d iso = 71.5 ± 0.5 ppm and P Q = 6.8 ± 0.6 MHz for [4] Al site of glass. These values are reasonably consistent with those of the previous studies [16,17,19]. However, it should be noted that the P Q estimated for glass is just an average value of widely-distributed quadrupolar interaction.…”

High magnetic field solid‐state NMR analyses by combining MAS, MQ‐MAS, homo‐nuclear and hetero‐nuclear correlation experiments

“…where S is the spin quantum number and ν 0 is the Larmor frequency for 27 Al. The P Q value generally reveals the degree of the structural symmetry (smaller P Q values indicate higher symmetry) . Note that the estimated values are an average value of widely distributed quadrupolar interactions .…”

“…Virtually, the single‐pulse MAS NMR techniques cannot completely average out the second‐order quadrupolar interactions . Identifying the actual number of atomic sites based on the MAS NMR results is difficult.…”

“…We first determined the isotropic chemical shifts ( δ cs ) and quadrupolar products ( P Q ) of these three main tetrahedral Al sites in anorthite, Al i , Al ii , and Al iii . These parameters can be determined from the center of gravity positions ( δ F 1 in ppm) along the F 1 dimension and the MQ isotropic shift positions ( δ F 2 in ppm) along the F 2 dimension in the 3Q‐MAS spectrum, Al i (62.0 and 66.0 ppm), Al ii (56.0 and 61.0 ppm), and Al iii (57.5 and 71.5 ppm); according to the following equations …”

Analyses of Aluminum Structures in a Chinese Coal Ash and Its Slag by STMAS NMR

“…The P Q value generally reveals the degree of the structural symmetry (smaller P Q value indicates higher symmetry) (Kanehashi and Saito, 2004). Nevertheless, the STMAS spectrum is able to distinguish the effects of interference from impurity adequately.…”