Thermal desorption–degradation of cyclohexylamine over Ni2+- and Al3+-exchanged bentonite studied using evolved gas analysis (TG-EGA) and diffuse reflectance spectroscopy (DRIFTS)

Abstract

Ni²⁺- and Al³⁺-exchanged montmorillonite are known to demonstrate Lewis and Bronsted acidity, respectively, when probed using the diagnostic base pyridine (C. Breen, Clay Miner., 1991, 26, 487). Thermogravimetric analysis (TGA) of pyridine saturated Al³⁺- and Ni²⁺-montmorillonite differ whereas here it was found that cyclohexylamine (CHA) desorption profiles were very similar, albeit CHA was held to a higher temperature on Ni²⁺-SWy-2 than on Al³⁺-SWy-2. Evolved gas analysis (EGA) of the desorbed products revealed significant differences in the amount of cyclohexylamine, cyclohexene, methylcyclopentene, benzene and aniline produced, which demonstrated different decomposition pathways for CHA sorbed to Al³⁺- and Ni²⁺-exchanged SWy-2. For example, cyclohexene is produced over both exchange forms whereas aniline and methylcyclopentene were the major products over Ni²⁺- and Al³⁺-SWy-2, respectively. Variable-temperature DRIFTS was unable to distinguish between CHA-treated Al³⁺- and Ni²⁺-SWy-2 at temperatures <250°C, but at 300°C the Ni²⁺ form exhibited a strong, intense band at 1648 cm⁻¹ which identifies cyclohexylimine as a surface bound intermediate which transforms before desorption.