(7f) Distributions of Al Atoms in Chabazite Zeolite Frameworks and Their Effects on Adsorption and Catalytic Reaction Properties

The reaction properties of zeolites depend on the local compositions and structures of the nanoporous channels and cations that charge-balance anionic sites on zeolite framework. For example, small pore (0.4 nm) chabazite-type zeolites 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 1-2 O-Si-O linkages. The local Al distributions in zeolites can be varied by zeolite synthesis mixture or post-synthetic treatments, such as hydrothermal aging. However, 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. Through a combination of two-dimensional ²⁷Al{²⁹Si} and ²⁹Si{²⁹Si} through-bond NMR correlation experiments, direct evidence of “paired” framework Al atoms in aluminosilicate chabazite zeolite catalysts is obtained. In particular, second-nearest-neighbor aluminum configurations are identified by correlations between ²⁷Al NMR signals from framework Al and ²⁹Si Q⁴(2Al) NMR signals which otherwise overlap with ²⁹Si NMR signals from silanols for many zeolite frameworks, and 3NN configurations are identified by pairs of Q⁴(1Al) ²⁹Si species that are covalently linked. In situ ¹³C NMR measurements, furthermore, enable the distributions of adsorbed reactant species on chabazite catalysts to be resolved, quantified, and correlated with macroscopic catalytic reaction properties for methanol dehydration. The results yield new atomic-level insights on how different distributions of framework heteroatoms, such as aluminum, affect the activity and selectivity of zeolite catalysts.