¹⁵N solid-state NMR characterization of ammonia adsorption environments in 3A zeolite molecular sieves

The quantitative analysis of ammonia binding sites in 3A zeolite molecular sieves using solid-state ¹⁵N MAS NMR spectroscopy is reported. By utilizing ¹⁵N-enriched ammonia ( ¹⁵NH ₃) gas, the different adsorption/binding sites within the zeolite were characterized as a function of NH ₃ loading. Using ¹⁵N MAS NMR, multiple sites were resolved that have distinct cross-polarization dynamics, relaxation, and chemical shift behavior. A combination of ¹⁵N/ ²³Na and ¹⁵N/ ²⁷Al TRAPDOR NMR methods was used to demonstrate significant dipolar coupling between adsorbed ammonia molecules and both Na cations and Al framework species in the zeolite cage. An estimate of the ²³Na- ¹⁵N and ²⁷A1- ¹⁵N distance was obtained from simulation of the TRAPDOR results. Two-dimensional ¹H → ¹⁵N CP-MAS NMR exchange spectroscopy was implemented to probe dynamics between the two primary adsorbed ammonia environments. In the 3A zeolite, the observed exchange process between the ammonia sites does not display a significant temperature dependence, indicating a spin-diffusion mechanism.