STRUCTURAL LOCALIZATION OF AL3+ IONS IN ALUMINOSILICATES: APPLICATION OF HETERONUCLEAR CHEMICAL SHIFT CORRELATION TO 2:1 PHYLLOSILICATES

Abstract

The main objective of this paper is to apply an NMR-based methodology to clay minerals that provides resolution enhancement in 27Al NMR spectra, permitting the differentiation of aluminum ions with one same coordination number, but with different chemical environments. For this purpose, we have performed two-dimensional 1H→27Al cross-polarization experiments, which facilitate the identification of the different aluminum crystallographic sites by means of the nature of the proton polarization source. For the development of this methodology, we have carried out a systematic study of a set of well-characterized 2:1 phyllosilicates with aluminum in different crystallographic sites. The structural locations of aluminum in the selected layered silicates have been established unequivocally. Once the correlation between all different aluminum and proton sites in smectite was well established, an unknown material was examined, the utility of this methodology being illustrated. This methodology can be applied to a great variety of materials, such as zeolites, ALPOs and mesoporous aluminosilicates, in which a precise determination of the aluminum location is demanded.