X-ray diffraction and Al-27 multiquantum MAS NMR combined with global eleme
ntal analysis have been used to identify and quantify extra-framework alumi
num (EFAL) in Y zeolites stabilized by steaming followed by EFAL extraction
using nitric acid, ammonium hexafluorosilicate (AHFS) or ethylenediaminete
traacetic acid (EDTA). Both the ease of extraction and the type of EFAL ext
racted are found to vary with the strength and nature of the leaching agent
. Polymeric amorphous alumina, removed easily at low concentrations of nitr
ic acid, is extracted progressively with increasing concentrations of other
agents. This type of EFAL accounts for less than half of the extra-framewo
rk aluminum. Aluminum atoms linked to the zeolite framework in a distorted
tetrahedral environment are also removed by acid leaching but are resistant
to high concentrations of AHFS and EDTA, whereas isolated cationic EFAL sh
ows the reverse behavior. (C) 2000 Elsevier Science B.V. All rights reserve
d.