Bronsted and Lewis acidity of modified montmorillonite clay catalysts determined by FT-IR spectroscopy

The surface acidity of different modified montmorillonite clay catalysts, Al3+-exchanged montmorillonite (Al3+-Mont) and aluminium pillared montmorillonite (AI-PILC) was determined by DRIFTS (diffuse reflectance Fourier transform infrared spectra) using pyridine as probe molecule. The method involved treating the clay sample directly with pyridine, drying the sample at 120 degrees C and recording FT-IR spectra in the region 1650 and 1350 cm(-1). The spectra obtained showed well resolved absorption bands for Lewis and Bronsted acid sites in the clay catalysts. In order to understand the role of acid sites present on clay catalysts, esterification of propionic acid with p-cresol has been studied. The Bronsted acidity data obtained by FT-IR study for modified montmorillonite clay catalysts correlated well with the catalytic activity in the esterification reaction. Among the modified clay catalysts, Al3+-Mont and H+-Mont clay catalysts showed good activity and AI-PILC showed negligible activity in esterification. The inactivity of AI-PILC catalysts is attributed to the absence of Bronsted acidity. (c) 2008 Elsevier B.V. All rights reserved.