Thermal desorption and infrared studies of methylamines adsorbed on dehydrated alkaline-earth-metal X zeolites

Abstract

The effect of the methyl substituent on the interaction between the exchangeable cations in alkaline-earth-metal X zeolites and methyl-substituted amines has been examined by measuring the thermal desorption spectra and the infrared absorption spectra of adsorbed amines. The methylamines adsorbed on NaX, SrX, CaX and MgX gave two types of desorption peaks (α and β). The temperature range of the α peak, which appears on the lower-temperature side, was constant independent of a combination of exchangeable cations and amines, while the β peak, on the higher-temperature side, was shifted towards higher temperatures in the order NaX < SrX < CaX < MgX for a given methylamine. Furthermore, with SrX and CaX the temperature range of the β peak increased in the order NH₃ < MeNH₂ < Me₂NH < Me₃N, while for NaX and MgX it was determined solely by the cation, regardless of the amine. These facts lead to the conclusion that the β peak on the dehydrated alkaline-earth-metal X zeolites originates from the desorption of molecules interacting with site II cations in the supercages. Me₂NH adsorbed on site II cations of the alkaline-earth-metal X zeolites gave rise to a broad band centred at ca. 3500 cm^–1 due to the NH stretching vibration, although their free NH stretching frequency is 3384 cm^–1. NH₃ adsorbed on site II cations of SrX and CaX gave a very broad band centred at ca. 1580 cm^–1 due to the NH₃ antisymmetric bending vibration, contrary to the relatively sharp bands at 1650 and 1600 cm^–1 observed for NH₃ on MgX. Such a difference is explained on the basis of the size effect of the bivalent cations.