Acidity and catalytic activity of mildly acid-treated Mg-rich montmorillonite and hectorite

Abstract

XRD, FTIR spectroscopy, cyclohexylamine desorption, potentiometric titrations and catalytic activity in an ether-forming test reaction have been used to investigate mildly acid-treated montmorillonite (SAz-1) and hectorite. The number of acid sites obtained from the cyclohexylamine desorption agreed with the accepted cation exchange capacity values for all samples. No evidence of any free protons was found in the potentiometric titration curves, which proved that complete autotransformation of the acid-treated samples had occurred over a much shorter period than previously recorded. Acid sites associated with Al³⁺ (Fe³⁺) released from the SAz-1 structure during acid treatment and subsequent autotransformation were of sufficient strength to catalyse the test reaction and produce tetrahydropyranyl ether in 80% yield. The complete absence of catalytic activity in the acid-treated hectorite samples confirmed that acid sites associated with interlayer Mg²⁺ and Li⁺ were unable to catalyse the test reaction.