Oxygenated intermediates drive stepwise deep cracking of polycyclic aromatics over steam-regenerated zeolite...

Coking and decoking chemistry remains a long-standing yet critical issue in zeolite catalysis. A steam-induced decoking strategy offers a promising regeneration route that converts polycyclic aromatic hydrocarbons (PAHs) into valuable intermediates or products, but the dynamic evolution of coke and the underlying mechanism remain ambiguous. Here, we found that high-temperature steam treatment of coked SAPO-34 catalysts not only fully restores methanol conversion activity but also remarkably enhances ethene selectivity. To understand the origin of this regeneration and selectivity improvement, we traced the involvement trajectory of H₂ ¹⁸O and captured previously unrecognized oxygenated intermediates by integrating ¹⁸O isotopic labeling with GC-MS. The hydroxylated species are primary products from the steam scission of cross-linked macromolecular coke, whereas the phenalenone species is proposed to participate in the transformation of polycyclic pyrene to naphthalene-derived species, which work as hydrocarbon pool species and favor the enhancement of ethene selectivity. DFT calculations further clarified a reasonable evolution pathway encompassing these oxygenated intermediates and hierarchical aromatic species, which outlines a stepwise deep cracking trajectory of PAHs. This work thus elucidates the transformation of polycyclic aromatics to active species during steam regeneration, and deepens fundamental insights into zeolite decoking chemistry as well as rational regulation of catalytic performance.