(580g) Distributions of Al Atoms in Chabazite Zeolite Frameworks and Their Effects on Catalytic Reaction Properties

Zeolite catalysts are used for a wide range of industrial and environmental applications. The reaction properties of zeolites depend on the local compositions and structures of the nanoporous channels and the cations that charge-balance the anionic zeolite framework. For example, small pore (0.4 nm) chabazite-type materials are used for methanol-to-hydrocarbon conversion in the H⁺-form and selective catalytic reduction of NO_x in the Cu²⁺-form. Reaction properties of these catalysts, including hydrothermal stability and product selectivity, depend on the distributions of Al heteroatoms in the zeolite framework, particularly “paired” Al heteroatoms separated by 1-3 O-Si-O linkages.¹ Recent work has suggested that the fraction of “paired” Al heteroatoms in aluminosilicate chabazite can be influenced by judicious selection of structure-directing agents during synthesis.² However, direct atomic-level evidence for these paired framework Al sites has been challenging to obtain, along with their effects on cation distributions and catalytic reaction properties.

Solid-state NMR is a powerful means of probing the nanoscale proximities of zeolite framework moieties and their interactions with exchangeable cations or adsorbed species in the zeolite nanopores.³ Through a combination of 2D ²⁷Al{²⁹Si} and ²⁹Si{²⁹Si} through-bond NMR correlation experiments, we have obtained direct evidence of “paired” framework Al atoms in aluminosilicate chabazite zeolite catalysts. Furthermore, ²⁷Al{¹H} and ²³Na{¹H} NMR experiments provide insight into the interactions between the organic structure-directing agents and the framework that promote the formation of paired Al configurations. These atomic-level insights are correlated with methanol-to-hydrocarbon reaction results over aluminosilicate chabazite to demonstrate that different distributions of framework Al affect the catalytic properties of the material.

References:

1. Deimund, et al., ACS Catalysis 6, 542–550 (2016).
2. di Iorio, & Gounder, Chemistry of Materials 28, 2236–2247 (2016).
3. Li, S. et al., Advanced Materials 32, 2002879 (2020).