ACIDIC PROPERTIES OF SYNTHETIC SAPONITES STUDIED BY PYRIDINE IR AND TPD-TG OF N-PROPYLAMINE

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 I R of adsorbed pyridine revealed the presence of a large amount of Lewi s acid sites owing to coordinatively unsaturated metal atoms at the ed ges of the clay platelets. Hardly any Bronsted sites were observed. Ho wever, with TPD-TG measurements of n-propylamine it was possible to qu antify the number of Bronsted acid sites. In the H+ exchanged clays wi th high Si/Al ratios the amine/Al molar ratio was found to be near uni ty. At lower Si/Al ratios the number of acid sites increased non-linea rly with the Al content. This study demonstrates that pyridine is a us eful probe for determining Lewis acidity. However, to study the Bronst ed acidity of H+ exchanged synthetic saponites, reliable quantitative information can only be obtained using n-propylamine.