Isolating\nClusters of Light Elements in Molecular\nSieves with Atom Probe Tomography

Understanding the\n3-D distribution and nature of active sites in\nheterogeneous catalysts is critical to developing structure–function\nrelationships. However, this is difficult to achieve in microporous\nmaterials as there is little relative z-contrast between active and\ninactive framework elements (e.g., Al, O, P, and Si), making them\ndifficult to differentiate with electron microscopies. We have applied\natom probe tomography (APT), currently the only nanometer-scale 3-D\nmicroscopy to offer routine light element contrast, to the methanol-to-hydrocarbons\n(MTH) catalyst SAPO-34, with Si as the active site, which may be present\nin the framework as either isolated Si species or clusters (islands)\nof Si atoms. 29Si solid-state NMR data on isotopically\nenriched and natural abundance materials are consistent with the presence\nof Si islands, and the APT results have been complemented with simulations\nto show the smallest detectable cluster size as a function of instrument\nspatial resolution and detector efficiency. We have identified significant\nSi–Si affinity in the materials, as well as clustering of coke\ndeposited by the MTH reaction (13CH3OH used)\nand an affinity between Brønsted acid sites and coke. A comparison\nwith simulations shows that the ultimate spatial resolution that can\nbe attained by APT applied to molecular sieves is 0.5–1 nm.\nFinally, the observed 13C clusters are consistent with\nhydrocarbon pool mechanism intermediates that are preferentially located\nin regions of increased Brønsted acidity.