Molecular Routes of Coke Species on HZSM-5 Zeolite with Atomic-Resolution Structural Identification

Catalyst deactivation has drawn continuing concern from both the academia and industry. Preventing deactivation and optimizing the regeneration process necessitate molecular deciphering of coke deposits. Key to achieving this goal is to identify previously less unexplored, more condensed coke species. Herein, taking commercially relevant HZSM-5 zeolite-catalyzed methanol-to-hydrocarbon as a prototypical reaction, we decode the "structural code" of the previously elusive coke molecules. This is made possible by integrating multiple techniques, especially scanning tunneling microscopy (STM) and noncontact atomic force microscopy (nc-AFM) techniques, which enables the single-molecule direct imaging of coke species with atomic resolution in real space. Combined with complementary techniques, such as gas chromatograph-mass spectrometry and matrix-assisted laser desorption/ionization Fourier-transform ion cyclotron resonance mass spectrometry, coke species with an explicit molecular structure covering soluble and insoluble ranges are identified. With this, their molecular routes are consequently unveiled. Molecular imaging with atomic precision resolves the previous ambiguity about "average structures" of "ensemble coke" given by the traditional means with group-based structural identification. This work showcases the potential of STM and nc-AFM as powerful mechanistic tools for resolving mechanistic hypotheses in catalysis.