Tetramethylbenzenium radical cations as major active intermediates of methanol-to-olefin conversions over phosphorous-modified HZSM-5 zeolites

The reaction intermediates of methanol-to-olefin (MTO) conversion over phosphorous-modified HZSM-5 (P-MFI) catalysts were investigated by electron spin resonance (ESR) and gas chromatography–mass spectroscopy (GC–MS). Phosphorous modification partially neutralized the strong acid sites in HZSM-5. The phosphorous compounds suppress the migration of polymethylbenzenium radical cations, lowering spin–spin interactions and thus preserved their hyperfine splitting. The ESR spectra of the organic species formed in P-MFI zeolites during MTO conversion were similar to those of the cation species generated from 1,2,4,5-tetramethylbenzene in mordenite. GC–MS of organic extracts revealed the predominant formation of polymethylbenzenes with 4–6 methyl groups. The correlation between conversion and the number of spins of tetramethylbenzenium radical cations exhibited that these radical cations might be major active intermediates of MTO conversion over P-MFI catalysts.