The surface acidity of different modified montmorillonite clay catalysts, Al3+-exchanged montmorillonite
(Al3+-Mont) and aluminium pillared montmorillonite (Al-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 8C and recording FT-IR spectra in
the region 1650 and 1350 cm1. The spectra obtained showed well resolved absorption bands for Lewis
and Brønsted 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 Brønsted 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 Al-PILC showed negligible activity in esterification. The inactivity of
Al-PILC catalysts is attributed to the absence of Brønsted acidity.