Conversion of methanol to hydrocarbons: Spectroscopic characterization of Carbonaceous species formed over H-ZSM-5

Very recently, catalytic measurements based on isotopic labeling disclosed details of the reaction mechanism in the methanol to hydrocarbons reaction over H-ZSM-5 and demonstrated that there are major mechanistic differences from one catalyst topology to another. In addition to having consequences for the catalyst selectivity, the catalyst topology may also lead to differences in the mechanism for the deactivation process. However, the understanding of the catalyst deactivation and the recognition of all of the species formed (coke) are rather complex. Most recent studies have reported the analysis (gas chromatography-mass spectrometry) of the fraction of extracted carbonaceous material soluble in an organic solvent only. This contribution describes the nature and evolution of all of the involved species formed both inside the channels and on the exterrial surface of the catalyst during the reaction by spectroscopic techniques, (diffuse reflectance) DR-UV-visible and IR. Both DR-UV-visible and IR reveal the presence of methylated aromatic carbocationic species. These compounds slowly evolve into larger species, i.e., precursors of graphitic coke. In particular, the bands at 410 nm and 15 10 cm(-1) in the DR-UV-visible and IR spectra, respectively, point to the formation of cationic species precursors of coke. These spectroscopic features appear at a rather short time on stream and grow during the course of the reaction, but at a certain point, they start to decrease, while bigger species are formed in a parallel manner.