Adsorption of methylamines on dehydrated NaX, NaY and KY zeolites

Abstract

The isosteric heats of adsorption (q_st) of methylamines over dehydrated NaX, NaY and KY zeolites have been determined by measuring the adsorption isotherms. The q_st curves for NaX differ remarkably in shape from those for NaY, although the gross features of the q_st curves for NaY resemble those for KY. For NaX, site III′ cations act as the most selective and energetically heterogeneous sites for the adsorption of methylamines, while for NaY and KY methylamines are preferentially adsorbed on site II cations. The basicity of methylamines increases in the order NH₃ < MeNH₂ < Me₂NH < Me₃N for NaY and NH₃ < MeNH₂≈ Me₃N < Me₂NH for KY. The potential energy of amine molecules on site II cations of NaY and KY was calculated on the basis of an electrostatic-interaction model in which the repulsion term results from a configuration in which the dipole axis of the molecule coincides with the line joining the site II cation and the centre of the supercage. Despite the smaller size of the Na⁺ ion, the magnitudes of the electrostatic term for both zeolites are nearly the same, whereas the dispersion and repulsion interactions of an adsorbed molecule with the oxygen ions surrounding the site II cation are larger for NaY. It is concluded that the different orders of the methylamines' basicity over NaY and KY are due to the different environments of the site II cations. The systematic dependences of q_st on the degree of adsorption have also been observed and analysed by comparison with the calculated energies of the lateral interaction between adsorbed molecules.