Vibrational Spectroscopy of NH₄⁺ Ions in Zeolitic Materials:  An IR Study

It is now well accepted that the interaction of NH 3 with the Brönsted acid sites of zeolites leads to the formation of NH 4 + ions that are strongly H-bonded to the basic oxygens of the lattice; nevertheless, unambiguous evidence of their local siting is still lacking. In this contribution we report a detailed assignment of the IR vibrational features of NH 4 + ions which reveals that mainly bidentate and tridentate species with various local symmetry are stabilized inside the cages and channels of ZSM-5, MORD, β, SAPO-34, and Y zeolites. Tetradentate species, mainly in MORD, are also found. In addition, it is shown that Fermi-type resonance interactions lead to the appearance of several bands in the ν(NH) stretching region of the vibrational spectrum that do not correspond to different NH oscillators. Finally, the solvation of NH 4 + in the presence of excess ammonia is also investigated, and the IR manifestations, observed when reversibly adsorbed NH 3 is present in the channels and cavities, are explained in terms of N 2 H 7 + and N 2 H 7 + · n NH 3 structures.