Microporous 18 Å Al-pilared vermiculites:: preparation and characterization

A new method for obtaining 18 Angstrom Al-pillared vermiculites is reported. In a first step, the vermiculite is submitted to an acid treatment followed by calcination. These treatments lead to a permanent reduction of the global negative charge of the mineral layers. The extra-framework species are removed by a subsequent leaching with a complexing acid, and the solid is converted to the sodium form by ion exchange. The charge-reduced Na (or Ca)-vermiculite is then brought in contact with the aluminum pillaring solution. The calcined Al-pillared vermiculites exhibit the characteristic features of this class of materials, namely, 18 Angstrom spacing, high surface area, microporosity, and both Bronsted and Lewis acidities. These textural properties are preserved at higher temperatures (up to 800 degrees C) compared with Al-pillared smectites, owing to the higher thermal stability of the vermiculite structure (higher dehydroxylation temperatures). The Bronsted acid content of the Al-pillared vermiculites established by IR spectroscopy of adsorbed pyridine is about twice as high as that of an Al-pillared saponite. (C) 2000 Elsevier Science B.V. All rights reserved.