Exploring the Na+ Locations and Al Distributions in SSZ-39 Zeolite by Solid-State NMR Spectroscopy and DFT Calculations

Abstract

The catalytic performance of zeolite SSZ-39 with AEI structure is significantly affected by the locations of its extra-framework cation and aluminum distributions. The AEI cage contains three crystallographically inequivalent T sites that tend to be substituted by aluminum. In this work, the Na+ locations and Al distributions in Na-SSZ-39 with different Si/Al ratios were studied by 27Al/23Na MQ MAS NMR spectroscopy together with density functional theory (DFT) calculations. For isolated Al substitution, the T3 site was found to be preferentially occupied by Al, and Na+ was mainly located in the 6-MR (SIIa0) or 8-MR (SIII'a0) sites of the AEI cage, although the priority of SIII'a0 site was slightly higher, and minor Na+ was located inside the hexagonal prism (SIa0). For paired Al substitution, stable AlSiSiAl structure was found to be located in 6-MR, and the corresponding Na+ cations were located at SIIa1 and SIII'a1 sites, respectively. In post-modified zeolites with partial destruction of the framework structure, some free Na+ cations were found to form distinct SIII'b sites. This study deepened the understanding on the structure-reactivity correlation of SSZ-39 zeolite and provided insights into how to fine-tune its catalytic performance.

Keywords

• solid-state nuclear magnetic resonance
• ssz-39 zeolite
• al distribution
• cation location
• density functional theory (dft) calculations