Zeolites often act as host materials to encapsulate guest molecules to enrich their functions and applications, particularly in heterogeneous catalysis. Unfortunately, directly imaging guest molecules residing in the intact microporous structure of a zeolite has long been a challenge. Here, we report that integrated differential phase-contrast scanning transmission electron microscopy (iDPC-STEM) is capable of directly probing guest molecules in zeolites, due to its sufficient and interpretable image contrast for both heavy and light elements under low-dose conditions. We first demonstrate this unique ability by imaging volatile organic compounds adsorbed in zeolite Silicalite-1; we then use iDPC-STEM to investigate molybdenum supported on various zeolites including Silicalite-1, ZSM-5, and mordenite. We observe isolated single-Mo clusters in the micropores of ZSM-5, and demonstrate the crucial role of framework Al in driving Mo atomically dispersed into the micropores. Importantly, the specific one-to-one Mo-Al interaction makes it possible to locate Al atoms, i.e. catalytic active sites, in the ZSM-5 framework from the images, according to the positions of Mo atoms in the micropores. Our results indicate the feasibility of directly imaging guest components in fragile crystals at atomic resolution, paving the way to investigate host-guest interactions in nanoporous materials in unprecedented detail.
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