Characterization of surface acidity of an acid montmorillonite activated with hydrothermal, ultrasonic and microwave techniques

The surface acidity of montmorillonite treated by conventional hydrothermal reaction with acids as well as ultrasonic- and microwave-assisted acid digestion was studied using diffuse reflectance FT-IR (DRIFT) spectroscopy of adsorbed pyridine molecules. An attempt for semi-quantitative measurement of Brønsted acidity generated during the acid digestion was also carried out. The conventional hydrothermal, ultrasonic- and microwave-assisted acid digestion yielded strong Brønsted acid sites of similar nature and strength, which retained pyridine up to 400–450 °C. Strong Lewis acid sites were also developed at higher temperature (> 300 °C), which were able to retain pyridine at 450 °C. The results show that surface acidity of similar strength can be generated with conventional hydrothermal treatment at 80 °C for 4 h or ultrasonic-treatment at room temperature for 1 h or microwave-assisted acid digestion for 10 min using 3 N concentration of H2SO4. Therefore, microwave- and ultrasonic-assisted acid digestions are potential and rapid alternative methods to conventional hydrothermal acid digestion for preparing acid-activated clays.