The Role of Cyclopentadienium Ions in Methanol-to-Hydrocarbons Chemistry

The “hydrocarbon pool” mechanism for the conversion of methanol to hydrocarbons on H-form zeolites was first introduced more than two decades ago, but the details continue to be a topic of debate. In this contribution, the hydrocarbon pool on zeolites H-ZSM-5 and H-beta was investigated by applying in situ ultraviolet–visible (UV–vis) spectroscopy. The intensity of absorption bands that grew during an induction period were compared with formation rates of gas-phase products. Spectra of both catalysts revealed the presence of alkylbenzenium and alkyl-substituted cyclopentadienium ions; on H-beta, larger polycyclic aromatic compounds were also formed. An absorption band at 295 nm of species on H-ZSM-5, assigned to an alkyl-substituted cyclopentadienium ion with four or five alkyl groups, paralleled the increase in ethene formation rates during an induction period, thus implicating the participation of such cations in the rate-limiting step. The alkylbenzenium ions on H-ZSM-5 were characterized by an average of three to four alkyl groups, whereas the alkylbenzene compounds formed on H-beta were fully alkylated already after 7 min on stream. In both cases, the concentration of alkylbenzenium ions increased almost linearly with time and was not correlated with rates of formation of gas-phase products. Adsorption experiments with hexamethylbenzene and 1,2,3,4,5-pentamethylcyclopentadiene were used to provide a reference for detailed interpretation of UV–vis spectra. Spectra of the latter are the first reported of an alkyl-substituted cyclopentadienium cation on a zeolite.