Mapping Al Distributions in SSZ-13 Zeolites from 23Na Solid-State NMR Spectroscopy and DFT Calculations

Al distributions and locations of corresponding nonframework cations in SSZ-13 zeolites have significant impact on their catalytic and adsorption properties. Herein, we demonstrate the feasibility of elucidating Na+ locations and correlated Al distributions in a series of Na-SSZ-13 zeolites with Si/Al ratios ranging from 4 to 48 by combining 23Na MQ MAS NMR spectroscopy and DFT calculations. As only one crystallographically inequivalent T site exists in CHA cages, the isolated Al and the Al pair (Al–O–Si–O–Si–O–Al) and (Al–O–Si–O–Al) in 6-membered ring (MR) distributions and the corresponding Na+ locations are clearly discriminated. In the high-silica Na-SSZ-13 zeolite, Na+ ions are mainly located at the 6-MR SIIa0 site corresponding to one Al substitution in the CHA cage. However, a small amount of the Al pair in 6-MR is still observable even with an Si/Al ratio of 48, corresponding to two Na+ ions located at 6-MR SIIa1 and 8-MR SIII’a1 sites, respectively. With more Al substitutions in the CHA cage, a portion of Na+ ions with high mobility among different SII sites may result in the SIII’b site. In Al-rich SSZ-13 zeolite (Si/Al = 4), the presence of Si(2Al) and Si(3Al) may lead to Na+ being located at 6-MR SIIa2, with 23Na NMR signals exhibiting relatively large quadrupolar interactions, and at 8-MR SIII’a2, with NMR signals overlapped with that of the SIII’a1 site. This approach may expand its application in discrimination of Al distributions in other zeolites to gain deeper understanding of the relationship between the structure and the property for their applications in catalysis and adsorption.