Location and transport properties of ammonia molecules in a series of faujasite zeolite structures as studied by FT-IR and 2H-NMR spectroscopies

Abstract

The interaction, the location and the transport properties of ammonia molecules in a series of cationic faujasite zeolites have been investigated by means of infrared and 2 H-NMR spectroscopies. The results have been correlated with the nature of counter-ions, the Si/Al ratio, the negative charge of oxygen atoms of the zeolite framework and the acido-basicity of the studied zeolites. The present work evidenced experimentally for the first time two kinds of interaction between ammonia and cationic zeolites. The ammonia can interact not only with alkali cations via the lone pair on nitrogen atoms but also with the negatively charged oxygen atoms of the framework via its hydrogen atoms. However, the Lewis acidity of counter-ions is the dominating factor for the ammonia interaction. The correlation times and the activation energies of ammonia in Na + -exchanged faujasite zeolites have been determined from 2 H-NMR spin-lattice relaxation experiments. At low temperature, a rotation around 3-fold axis of the ammonia molecule is detected. At high temperature, an isotropic motion of ammonia is evidence. Moreover, an enhancement of the ammonia mobility from Na-X to N-Y d has been observed and explained.