Abstract
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.
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