Difference between the mechanisms of propylene production from methanol and ethanol over ZSM-5 catalysts

• The mechanism of methanol conversion over ZSM-5 depended on the Si/Al2 ratio.
• At a high Si/Al2 ratio, propylene was produced from methanol via dimethyl ether.
• At a low Si/Al2 ratio, dimethyl ether was dehydrated into ethylene.
• Propylene was directly produced from ethanol via ethylene.
• Ethylene was transformed to a carbene (CH2) as a transient intermediate.

We investigated the difference between the mechanisms of methanol conversion and ethanol conversion to propylene over ZSM-5 catalysts. The mechanism of methanol conversion depended on the Si/Al2 ratio, whereas the mechanism of ethanol conversion did not. We believe that at a high Si/Al2 ratio, propylene was produced from methanol via dimethyl ether by means of the hydrocarbon pool mechanism. In contrast, at a low Si/Al2 ratio, dimethyl ether was dehydrated to ethylene, which was then directly converted to propylene. In the case of ethanol conversion, propylene was produced from ethylene, which was formed by intramolecular dehydration. We propose that ethylene was transformed to a carbene species, which was a transient intermediate for direct production of propylene.

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