(53a) Influence of Al Distributions in Cu-CHA Zeolite Frameworks on Catalytic Activity for Selective Catalytic Reduction of NOx

Copper-exchanged aluminosilicate chabazite (CHA) zeolites are crystalline nanoporous materials used as industrial catalysts for environmental emission control of NO_x in diesel engines. The locations of framework Al heteroatoms, which impart local anionic charges to the framework, strongly influence the siting and dynamics of the catalytically active copper cation sites. The catalytic and hydrothermal stability properties of Cu-CHA materials depend strongly on the distribution of framework Al atoms, in particular paired Al configurations that compensate Cu²⁺ cations and influence the distribution of Cu active species. However, assessing the distributions of Al-Al paired moieties is challenging by conventional scattering, spectroscopy, and titration techniques. By comparison, advanced solid-state 2D ²⁷Al-²⁹Si and ²⁹Si-²⁹Si NMR techniques can establish unambiguously the local distributions of aluminum heteroatoms in the zeolite frameworks.¹ Using such methods, we report detailed insights on distinct differences in the types and relative populations of local paired Al configurations in CHA zeolites that are synthesized under different conditions. These findings are correlated with complementary X-ray diffraction, transmission electron microscopy, and macroscopic reaction kinetics data for the selective catalytic reduction of NO_x to N₂ and H₂O, along with the atomic-scale structural moieties that account for the different kinetic behavior of Cu-CHA catalysts. The findings reveal structure-activity relationships that provide design criteria for the synthesis of Cu-CHA catalysts with superior properties for deNO_x applications.

References:

1. Schmithorst, M. B., et al., J. Am. Chem. Soc. 145, 18215–18220 (2023).