Acidity scale of carbenium ions in ZSM-5

During the methanol-to-olefins process, hydrocarbon residues build inside the porosity of the zeolite catalyst and are considered precursors of the olefin products. The underlying chemistry responsible for this transformation is complex and is based on the dual site mechanism where a catalytic cycle considered responsible for the production of ethylene relies on the aromatics immobilized in the zeolite, while higher olefins are produced by the olefin cycle. In this work, we have attempted at titrating the residual acidity in zeolite ZSM-5 after constructing carbenium ions from interaction with methanol at 543 K. Exposure to excess acetonitrile produced the (reversible) disappearance of only the IR signal of benzenium ions as a result of selective interaction of the probe molecule with Brönsted acid sites. The shift of the cyclopentenyl cations (to form the corresponding neutral hydrocarbon species) occurred only when using a stronger base (pyridine), which points to the different scale of acidity and proton affinity of hydrocarbon species confined in zeolites. Density functional theory simulations confirm this behaviour.

The similar properties of methanol and acetonitrile suggest that the methanol-to-olefin process relies on the acid properties of Brönsted acid sites and benzenium ions.

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