Isolating Clusters of Light Elements in Molecular Sieves with Atom Probe Tomography

Understanding the 3-D distribution and nature of active sites in heterogeneous catalysts is critical to developing structure-function relationships. However, this is difficult to achieve in microporous materials as there is little relative z-contrast between active and inactive framework elements (e.g., Al, O, P, and Si), making them difficult to differentiate with electron microscopies. We have applied atom probe tomography (APT), currently the only nanometer-scale 3-D microscopy to offer routine light element contrast, to the methanol-to-hydrocarbons (MTH) catalyst SAPO-34, with Si as the active site, which may be present in the framework as either isolated Si species or clusters (islands) of Si atoms. Si-29 solid-state NMR data on isotopically enriched and natural abundance materials are consistent with the presence of Si islands, and the APT results have been complemented with simulations to show the smallest detectable cluster size as a function of instrument spatial resolution and detector efficiency. We have identified significant Si-Si affinity in the materials, as well as clustering of coke deposited by the MTH reaction ((CH3OH)-C-13 used) and an affinity between Bronsted acid sites and coke. A comparison with simulations shows that the ultimate spatial resolution that can be attained by APT applied to molecular sieves is 0.5-1 nm. Finally, the observed C-13 clusters are consistent with hydrocarbon pool mechanism intermediates that are preferentially located in regions of increased Bronsted acidity.