Correlation of catalytic activity with infra-red, 29Si MAS NMR and acidity data for HCl-treated fine fractions of montmorillonites

The <2 μm fractions of SAz-1 (Cheto, Arizona, USA) and JP (Jelšový Potok, Slovakia) montmorillonites were treated with 6 M HCl for 30, 300 and 900 minutes at 95°C. The materials obtained were investigated by X-ray fluorescence (XRF), X-ray diffraction (XRD), thermogravimetry (TG), infrared (IR) spectroscopy, and 29Si MAS NMR spectroscopy. The number of acid sites was determined from the thermal desorption of cyclohexylamine and the catalytic activity was evaluated by reacting 2,3-dihydropyran with methanol to yield the tetrahydropyranyl ether. All the investigative methods utilised, which each provided discrete evidence for the depopulation of the octahedral sheet, confirmed that treatment of SAz-1 for 300 and 900 minutes caused complete destruction of the original structure. In contrast JP was more resistant to acid attack and the treatments resulted in materials with different levels of structural decomposition. Samples of JP and SAz-1 treated for 30 minutes exhibited Brønsted acidities commensurate with the exchange capacities of the parent materials and this was reflected in their catalytic activity. A reduction in both acidity and catalytic activity was observed after longer treatment times and this was attributed to the presence of fewer exchange sites due to the depopulation of the octahedral sheet. The Ca back-exchanged samples did not catalyse the ether forming reaction.