Surface acidity study of Mn+-montmorillonite clay catalysts by FT-IR spectroscopy:: Correlation with esterification activity

A simple method for evaluating the surface acidity of different cation-exchanged montmorillonite (mont) clay catalysts, Mn+-mont (Mn+ =Al3+, Fe3+, Cr3+, Zn2+, Ni2+, Cu2+, and H+), involving treatment with pyridine is described. After treating with pyridine, the samples were heated at 120 degrees C and the FT-IR spectra were directly recorded in the region 1650 and 1350 cm(-1). The data obtained show the presence of both Lewis and Bronsted acid sites. The activities of the catalysts to bring about Bronsted acid catalysed esterification of succinic acid with iso-butanol to yield di-(iso-butyl) succinate have been studied. The Bronsted acidity data obtained for M2+-mont correlated well with activity in the esterification reaction. The activities of the catalysts were found to decrease in the order of exchange ions Al3+ > Fe3+ > Cr3+ > Zn3+ > Ni2+ > Cu2+ > Na+-mont. They also correlated well with the charge to radius ratio of the cations. The catalysts exchanged with trivalent cations showed stronger absorption bands attributed to Bronsted acidity (1540 cm(-1)) whereas those exchanged with divalent cations showed an increased Lewis acidity (1450 cm(-1)) and reduced Bronsted acidity along with charge to radius ratio. Zn2+-, Cu2+- and Ni2+-exchanged clays showed an additional peak around 1605 cm(-1) which is attributed to the pyridine adsorption on surface sites through its pi electrons. The method suggested here to evaluate the acidity is suitable for active sites which are thermally unstable such as water molecules in the hydration shell of a cation in exchanged clay. (c) 2006 Elsevier B.V. All rights reserved.