Parameters influencing the selectivity to propene in the MTO conversion on 10-ring zeolites: directly synthesized zeolites ZSM-5, ZSM-11, and ZSM-22

• 10-ring zeolites ZSM-5, ZSM-11, and ZSM-22 were studied in the MTO-conversion.
• Adjustment of T, WHSV and acid site density increase the propene selectivity.
• Propene selectivities over 50% and enhanced lifetimes were obtained.
• The concentration of reactive intermediates determines the dominant catalytic cycle.
• A proper acid site density is as important as the product shape selectivity.

Zeolites ZSM-5, ZSM-11, and ZSM-22 (nSi/nAl = 20–1000) were applied as methanol-to-olefin conversion (MTO) catalysts and optimized for high propene selectivities at high methanol conversions, high weight hourly space velocities, and for long catalyst lifetimes. On zeolites ZSM-5 and ZSM-11 with optimized Brønsted acid site densities of 0.13 and 0.15 mmol/g, propene selectivities of 51 and 52%, respectively, at a reaction time of 25 h were reached. In contrast, zeolite ZSM-22 with an optimized acid site density of 0.30 mmol/g showed a maximum propene selectivity of 38% only and a significantly shorter lifetime. Under these conditions, no aromatics could be detected by in situ UV–vis spectroscopy and on-line GC. Thus, an optimized acid site density can suppress the aromatic-based reaction mechanisms by hindering intermolecular hydrogen transfer reactions. Therefore, the acid site optimization could be a promising way for tuning the product selectivity of MTO catalysts on significantly different pore systems.

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