The methanol-to-hydrocarbons (MTH) reaction over ZSM-5 is a well-established chemical process operation, with the zeolite providing the internal structure, acid strength and distribution of acid sites to facilitate the conversion of methanol to gasoline grade hydrocarbons. The reaction has contemporary significance as a variety of carbon sources, including biomass, can be used to produce methanol which, via the MTH reaction, provides access to transportation fuels. The mode of reaction is understood in terms of the renown ‘Hydrocarbon Pool’ mechanism. Just recently, synchrotron infrared micro-spectroscopic studies have demonstrated a role for propene as an intermediate species in MTH chemistry over large crystals of ZSM-5. This presentation follows up on that premise but features the technique of inelastic neutron scattering (INS) and ancillary analytical techniques, primarily IR spectroscopy, to discern how propene could be incorporated within the MTH hydrocarbon pool catalytic cycle. Reaction profiles for the reaction of propene over a commercial grade ZSM-5 catalyst as a function of temperature are correlated with vibrational spectra (INS and IR) and confirm the connectivity of the cyclopentadienyl cation acting as an intermediary between the alkene and arene cycles of the hydrocarbon pool. Minor deviations from classical MTH chemistry, as manifested in a restricted range of substituted products, arises because of the absence of a methylation reaction pathway. Referring to the Symposium call details, this presentation connects to Topic 4, i.e., detailed mechanistic and structure-property relationship studies of well-defined materials.

Propene as an intermediate species in the methanol-to-hydrocarbons reaction over ZSM-5

ACS Spring 2022

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