PNNL

Abstract

Zeolites are microporous crystalline aluminosilicates, which are widely used in a variety of commercial applications, including but not limited to catalysis, adsorption, and ion exchange. As a catalyst, their properties are uniquely determined by a combination of the pore architecture (shape selectivity) and the chemical composition. The latter is of particular importance for catalytic processes, which often involves Brønsted acidity. These Brønsted acid sites are formed by the substitution of AlO4 for SiO4 tetrahedra. In order to understand the specific catalytic activity associated with a particular acid site it is of utmost importance to have detailed information on the number, precise location, and information of the neighboring sites of aluminum in the zeolite framework. Unfortunately, such detailed information has not yet been obtained, mainly due to the lack of suitable characterization methods, which most often average out information on the location of the aluminum sites. Here, we present a unique characterization approach, which involves combining well-defined and characterized zeolite ZSM-5 crystals with a powerful nanoscale probing method, namely atom probe tomography (APT). It is shown that Al atoms are homogeneously distributed within the parent zeolite ZSM-5 crystals with an average Al-Al neighbor distance of 17±7 Å and an average Al density of 0.025 nm3. In the steamed ZSM-5 crystals the Al is preferentially located in clusters of approximately 10 atoms with an average Al density of 0.5 nm3, either accumulated in patches or at the grain boundaries,