Conversion of methanol to hydrocarbons on zeolite HZSM-5 investigated by in situ MAS NMR spectroscopy under flow conditions and on-line gas chromatography

In situ MAS NMR spectroscopy under flow conditions and on-line gas chromato graphy have been applied to study the onset of the conversion of methanol o n zeolite HZSM-5 at temperatures between 373 and 573 K. In the steady state s of methanol conversion at T greater than or equal to 523 K, by on-line ga s chromatography mainly the formation of ethene and propene was observed. S imultaneously recorded in situ C-13 MAS NMR spectra show signals at 12-25 p pm and at ca. 125-131 ppm indicating the presence of adsorbed C-4-C-6 olefi ns. The observation of these adsorbates on a working catalyst supports the "hydrocarbon pool" mechanism previously proposed for the methanol-to-hydroc arbon conversion on acidic zeolites. Methanol conversion at 473 and 573 K a nd subsequent purging of the catalyst with dry nitrogen at 293 K led to a C -13 MAS NMR signal at 59 ppm due to methoxy groups. No hints to the presenc e of ethoxy, propoxy or butoxy groups and the formation of alkyl oxonium io ns were found by in situ C-13 MAS NMR spectroscopy under flow conditions.