First observation of 5-coordinate aluminum by MAS aluminum-27 NMR in well-characterized solids

Structure and Thermal Transformations of Imogolite Studied by 29Si and 27Al High-Resolution Solid-State Nuclear Magnetic Resonance

“…After dehydroxylation, but prior to recrystallization, the amount of A1 visible was small and resulted in poor signal-to-noise, but peaks appeared to be present at about 37 and 59 ppm ( Figure 3J). Similar results were found for both dehydroxylated pyrophyllite and metakaolinite by MacKenzie et al (1985aMacKenzie et al ( , 1985b, who suggested that these two peaks represented A1 in slightly different tetrahedral environments, although the peak at 37 ppm could possibly have been due to a more regular 5-coordinated A1 site (Alemany and Kirker, 1986). The separation of these sites was lost on recrystallization of the sample ( Figure 3K), which was accompanied by a further increase in the proportion of detectable tetrahedral AI ( Figure 4B) and a change in the octahedral chemical shift to -15 ppm ( Figure 3L).…”

The stability of nanosized HZSM-5 zeolite: a high-resolution solid-state NMR study

“…A broad signal at ca. 30 ppm appeared gradually upon dealumination, which means that this peak must be associated with nonframework Al, and may be ascribed to the pentacoordinated nonframework Al [19,20]. These facts demonstrate that thermal treatment leads to dealumination of the framework of both the microsized and nanosized HZSM-5 zeolites.…”

27Al Multiple-Quantum MAS NMR of Mechanically Treated Bayerite (α-Al(OH)3) and Silica Mixtures

“…This resonance, which has been reported in many amorphous or poorly crystalline Al-containing materials [20][21][22][23], has been ascribed to five-coordinate Al, primarily on the basis of the similarity of its shift to the isotropic chemical shift of known five-coordinate Al sites in the crystalline mineral andalusite [24] and other samples [25]. An alternative interpretation is that the resonance arises from very distorted tetrahedrally coordinated Al located in the vicinity of oxygen vacancies, such as those that occur in crystalline mullite [20,25].…”