Location and orientation of adsorbed molecules in zeolites from solid-state REAPDOR NMR

Abstract

The use of rotational echo adiabatic passage double resonance (REAPDOR) solid-state nuclear magnetic resonance (NMR) to determine the site location of an adsorbed polar molecule in a zeolite cage is presented. Nitrogen-15 labeled ammonia is used as a probe molecule to characterize the initial adsorption site in 3A zeolite molecular sieves. The relative position of the ammonia adsorption site in the cage is determined by measuring the internuclear distance between the N on ammonia and both a Na cation site and an Al framework environment using ¹⁵N/²³Na and ¹⁵N/²⁷Al REAPDOR NMR experiments, respectively. The measured internuclear distances are similar to a specific ammonia adsorption site for the zeolite 4A ammonia sorption complex located using X-ray diffraction. Additional details regarding the ammonia hydrogen-bonding environment can be extracted from ¹H/²³Na and ¹H/²⁷Al REAPDOR NMR measurements.