Revealing Formaldehyde-Mediated Methanol-to-Aromatics Reactions over Zn-Modified Zeolites by Observing the Oxygenated and Polyunsaturated Intermediates

The imperative role of formaldehyde (HCHO) intermediates in the formation of aromatics during the methanol-to-hydrocarbons process has drawn extensive interest. Herein, using synchrotron radiation photoionization mass spectrometry (SR-PIMS) combined with a near-ambient pressure reactor, abundant critical intermediates including HCHO along with oxygenated species (C1–C4 carbonyl compounds, α,β-unsaturated aldehydes, and cyclopentenones) and polyunsaturated hydrocarbons (dienes, polyenes, and fulvene) were identified during the methanol-to-aromatics reaction over Zn-modified HZSM-5. HCHO derived from the direct dehydrogenation of methanol was proved to noticeably promote aromatic selectivity. With the assistance of 13C-labeled HCHO cofeeding, a detailed HCHO-mediated monocyclic and polycyclic aromatic formation network involving Prins, hydroacylation, and aldol condensation reactions of HCHO was proposed. This proposed mechanism provides profound insight into multiple roles of HCHO intermediates via oxygenate-based routes and reveals the fate of the oxygen atom in methanol conversion over zeolite catalysts.