| Popis: |
The lattice structure of a synthetic ammonium dioctahedral clay (designated SMM) is shown to lie similar to muscovite mica. The two differ in that SMM has irregularly interstratified expandable and nonexpandable layers before activation. Thermal activation of SMM causes deamination and a loss of structural hydroxyl, accompanied by a collapse of the interlayer spacing (d001) to 9.4 A. This dehydroxylation, which probably occurs by pairwise loss of adjacent hydroxyls, is partially reversible leading to high temperature water Sorption (PH2O = 24 Torr, T = 500 °C, for example) by activated SMM. During synthesis, FOH substitutions are possible; the presence of fluoride in hydroxyl sites enhances the water sorption capacity. The appearance of an hydroxyl band at 3470 cm−1 in the activated material is attributed to the presence of protons in the tetrahedral vacancies of the octahedral layer. Ammonia and pyridine adsorption experiments show the presence of both Lewis and Bronsted acidity, the relative amounts of each being dependent on the severity of previous outgassing. Total acidity (2.4 × 1019 sites/g) estimated by pyridine adsorption, is small in comparison to that of the zeolitic catalysts (32 × 1019 sites/g). The high activity of SMM is thus believed to be due to decreased diffusion limitations. |
