15N Solid-State NMR Characterization of Ammonia Adsorption Environments in 3A Zeolite Molecular Sieves

Abstract

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 (15NH3) gas, the different adsorption/binding sites within the zeolite were characterized as a function of NH3 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 27Al−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...