Publication
The methanol-to-olefins (MTO) process on zeolite-based catalysts enables the production of olefins from green methanol. Selectivity control requires an understanding of the nature and evolution of hydrocarbon species that act as catalytically active centers within a hydrocarbon pool. To this end, we combined transient operando diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) with gas chromatography (GC), in which by dosing methanol intermittently, the appearance and evolution of adsorbed intermediates and carbenium ions could be followed. Carbenium ions of different natures were strongly chemisorbed and favorably stabilized by interaction with the zeolite framework of ZSM-5 (MFI), SSZ-13 (CHA), and ferrierite (FER). Quantitative correlations were obtained between intensities of DRIFTS bands of carbenium ions and the concentration of olefin products simultaneously detected by GC. In ferrierite, allylic carbocations correlated with the production of C2-C6 hydrocarbons. Polymethylbenzenium ions and alkyl-substituted cyclopentenyl cations were pinpointed in ZSM-5, where the former species quantitatively correlated with the concentration of the short olefins, while the latter one may be implicated only at the very initial stage of the reaction. Similarly, in SSZ-13, polymethylbenzenium ions correlated with ethene and propene as well as propane. Our findings provide unique relationships between the zeolite topology, the nature of the active carbenium ions, and the selectivity in the MTO process.