In situ MAS NMR spectroscopy of surface compounds formed from methanol andfrom a toluene/methanol mixture on basic zeolite X

The conversion of methanol and a toluene/methanol mixture on a basic form o f zeolite X has been investigated by in situ MAS NMR spectroscopy, both und er batch and flow conditions at temperatures between 573 K and 723 K. The z eolite CsNaX/24CsOH used as basic catalyst was prepared by cesium ion excha nge and subsequent impregnation with cesium hydroxide followed by calcinati on. By C-13 MAS NMR spectroscopy, the formation of surface compounds that c aused signals at 166 and 171 ppm was observed. The compound that gave rise to the signal at 166 ppm was formed by conversion of methanol on zeolite Cs NaX/24CsOH at 573 K. Upon purging the catalyst with dry nitrogen at tempera tures from 623 K to 723 K, the compound, at 166 ppm, was transformed into t he compound at 171 ppm. This process could be reversed by addition of metha nol at 573 K. Based on a comparison between the MAS and CP/MAS spectra of t he compounds at 166 and 171 ppm, and on literature data, the compound at 16 6 ppm was assigned to formate species. These formate species are formed by dehydrogenation of methanol to formaldehyde and subsequent reaction with th e zeolite framework. The main content of the compound at 171 ppm, occurring as MAS spinning sideband pattern with a chemical shift anisotropy of Delta sigma = -40 +/- 2 ppm and an asymmetry parameter of eta = 0.8 +/- 0.1, is due to carbonate species. In situ C-13 MAS NMR studies of the conversion of toluene/methanol mixtures on zeolite CsNaX/24CsOH indicated that the above -mentioned formate species could play an active role as alkylating agent in the side-chain alkylation of toluene. (C) 2000 Elsevier Science B.V. All r ights reserved.