Acidic properties of synthetic saponites studied by pyridine IR and TPD[ndash ]TG of n-propylamine

Abstract

The acidity of synthetic Zn and Mg saponites exchanged with H⁺, Al³⁺, Na⁺ and K⁺ has been studied using DRIFTS of adsorbed pyridine and TPD–TG measurements of adsorbed n-propylamine. The characterisation with IR of adsorbed pyridine revealed the presence of a large amount of Lewis acid sites owing to coordinatively unsaturated metal atoms at the edges of the clay platelets. Hardly any Brönsted sites were observed. However, with TPD–TG measurements of n-propylamine it was possible to quantify the number of Brönsted acid sites. In the H⁺ exchanged clays with high Si/Al ratios the amine/Al molar ratio was found to be near unity. At lower Si/Al ratios the number of acid sites increased non-linearly with the Al content. This study demonstrates that pyridine is a useful probe for determining Lewis acidity. However, to study the Brönsted acidity of H⁺ exchanged synthetic saponites, reliable quantitative information can only be obtained using n-propylamine.