Thermogravimetric and infrared study of the desorption of butylamine, cyclohexylamine and pyridine from Ni- and Co-exchanged montmorillonite

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Abstract
The acidity of Ni2+- and Co2+-exchanged montmorillonite has been probed using the diagnostic bases n-butylamine, cyclohexylamine and pyridine. Derivative thermograms for the desorption of pyridine from Ni2+- and Co2+-exchanged montmorillonite exhibited strong maxima at 40°, 90° and 360°C together with a weak maximum near 170°C The desorption maximum at 360°C is usually attributed to desorption of base from Bronsted acid sites. However, IR spectra of pyridine adsorbed on Ni2+- and Co2+-exchanged montmorillonite, at 250°C were dominated by intense bands near 1450 and 1607 cm−1 which are diagnostic of Lewis-bound pyridine. Consequently, the desorption maximum at 360°C must, in this instance, be attributed to desorption of pyridine from Lewis acid centres. Other bases, including cyclohexylamine and butylamine, also desorb at temperatures which have previously been attributed to the desorption of protonated base. Ni2+- and Co2+-exchanged clay contained predominatly Lewis acid or electron accepting sites, which is in marked contrast to the behaviour of trivalent cation exchanged clays.