Characterization of surface acidity of an acid montmorillonite activated with hydrothermal, ultrasonic and microwave techniques Beena Tyagi * , Chintan D. Chudasama, Raksh V. Jasra Silicates and Catalysis Discipline, Central Salt and Marine Chemicals Research Institute (CSMCRI), G.B. Marg, 364 002, Bhavnagar, Gujarat, India Received 1 July 2004; received in revised form 31 May 2005; accepted 4 July 2005 Available online 29 August 2005 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 8C. Strong Lewis acid sites were also developed at higher temperature (N 300 8C), which were able to retain pyridine at 450 8C. The results show that surface acidity of similar strength can be generated with conventional hydrothermal treatment at 80 8C 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 H 2 SO 4 . Therefore, microwave- and ultrasonic-assisted acid digestions are potential and rapid alternative methods to conventional hydrothermal acid digestion for preparing acid-activated clays. D 2005 Elsevier B.V. All rights reserved. Keywords: Surface acidity; Montmorillonite; DRIFT spectroscopy 1. Introduction Acid-activated montmorillonites find diverse appli- cations in chemical, foodstuff, beverage and paper industry. They are also increasingly used as catalysts, catalytic support and adsorbents (Christidis et al., 1997; Vaccari, 1999). Acid-treated montmorillonite is one of the widely studied solid acid catalysts for many organic transformations such as isomerisation (Yadav et al., 2004), alkylation (Hart and Brown, 2004; Rhodes et al., 1991) and acylation (Jasra, 2003) reactions. The treatment of purified montmor- illonite with hot mineral acids imparts surface acidity, which improves its adsorption and catalytic properties. 0169-1317/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.clay.2005.07.001 * Corresponding author. Tel.: +91 278 2471793; fax: +91 278 2567562. E-mail addresses: btyagi@csmcri.org (B. Tyagi), rvjasra@csmcri.org (R.V. Jasra). Applied Clay Science 31 (2006) 16 – 28 www.elsevier.com/locate/clay