(10c) In Situ Spectroscopic Identification of Active Components in the “Hydrocarbon Pool” during Methanol-to-Hydrocarbons Conversion

Methanol-to-hydrocarbons conversion is believed to proceed under participation of a “hydrocarbon pool” present inside the pores of a zeolite or zeotype catalyst. Many constituents of the pool have been identified, with alkylbenzenes being the most frequently reported species. However, it is debated which components participate in the product formation and how the products are actually formed. The goal of this investigation was to identify active components in the pool. HZSM-5 and H-beta were used as catalysts at temperatures of 275 to 310 °C and a methanol pressure of 10 kPa. The primary tool for species identification was in situ diffuse reflectance UV-vis-NIR spectroscopy.

Spectra of both catalysts during methanol conversion revealed the presence of charged hydrocarbon species, mostly cations of alkylbenzenes and alkyl-substituted cyclopentadienes; on H-beta, cations of larger polycyclic aromatic compounds were formed in addition. The number of alkyl groups on the rings was estimated from the spectroscopic data and found to be larger in the case of H-beta, consistent with the larger pores of this zeolite in comparison to HZSM-5. Correlations between the pool composition and catalyst performance were sought by imposing changes in conditions, monitoring the response, and seeking parallel trends in the catalytic and spectroscopic data. From this analysis, cyclopentenyl species emerge as most promising key intermediates.