Spectroscopic Signatures Reveal Cyclopentenyl Cation Contributions in Methanol-to-Olefins Catalysis

The conversion of methanol to olefins on zeolites and zeotypes is an industrially important process, yet the mechanistic details remain unresolved. Various cyclic alkenyl and alkadienyl carbenium ions have been proposed as active intermediates for light olefin production, but direct experimental evidence demonstrating the transformation of these hydrocarbon pool species with product formation is lacking. In this contribution, the interpretations of UV–vis and IR spectra are advanced to provide detail on the species present during catalysis; selected individual species are generated inside the catalyst pores, and their reactivity is tracked. The basis for the interpretation is a multitude of reference spectra of acyclic and cyclic dienes, trienes, and corresponding protonated species in the liquid phase (organic medium or sulfuric acid) or sorbed on dealuminated or acidic zeolites, augmented by trends extracted from these spectra. Accordingly, spectroscopic signatures indicate the presence of both polymethylbenzenium ions and alkyl-substituted cyclopentenyl cations during the conversion of methanol on H-ZSM-5 at 300 °C. To elucidate their role, alkyl-substituted cyclopentenyl cations are generated from acyclic polyenes adsorbed on H-ZSM-5 at room temperature. Cyclization and cleavage are monitored via changes in the electronic and vibrational absorption spectra with increasing temperature. The formation of ethene, propene, and butenes from alkyl-substituted cyclopentenyl cations, in the absence of methylbenzenium ions, is unambiguously demonstrated by temperature-programmed reaction (TPReact) spectroscopy coupled with online gas-phase product analysis. In the UV–vis range, the wavelength of maximum absorption, λ_max, of the alkyl-substituted cyclopentenyl cation is linearly dependent on the total number of carbons in the cation. A blue shift in the position of λ_max of these cyclopentenyl cations during the TPReact indicates a loss of three carbons, which matches the average size of the concomitantly produced olefins.