Abstract
Abstract The maximum value in the number of acid sites per unit of surface area of the gypsum formed in the reaction between Ca(NO3)2 and H2SO4 was observed at the heat-treatment temperature of 200 °C, corresponding to the dehydration temperature for the change from its hemihydrate form to soluble anhydrite. The IR-absorption spectra of the gypsum showed no bands assignable to isolated OH groups or Ca–OH species. The IR spectra of the pyridine adsorbed on the gypsum showed only the bands due to Lewis-acid sites. The gypsum heat-treated at temperatures lower than 500 °C showed catalytic activities for the double-bond isomerization of 1-butene. The cis-/trans- ratio of 2-butene was 2–2.5, showing that the catalytic activity was due to Lewis acid, not to Brönsted acid. The isomerization of cyclopropane to propene was scarcely promoted by the gypsum at 250 °C. This fact is another indication of the absence of Brönsted acid sites on the heat-treated gypsum. Most of the weak Lewis acid sites (H0≤+4.8) were ascribable to exposed Ca2+ ions surrounded by monodentate sulfate ions on the surface of metastable anhydrite III-CaSO4.
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