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

Abstract

The conversion of methanol and a toluene/methanol mixture on a basic form of zeolite X has been investigated by in situ MAS NMR spectroscopy, both under batch and flow conditions at temperatures between 573 K and 723 K. The zeolite CsNaX/24CsOH used as basic catalyst was prepared by cesium ion exchange and subsequent impregnation with cesium hydroxide followed by calcination. By 13C MAS NMR spectroscopy, the formation of surface compounds that caused 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 CsNaX/24CsOH at 573 K. Upon purging the catalyst with dry nitrogen at temperatures from 623 K to 723 K, the compound, at 166 ppm, was transformed into the compound at 171 ppm. This process could be reversed by addition of methanol at 573 K. Based on a comparison between the MAS and CP/MAS spectra of the compounds at 166 and 171 ppm, and on literature data, the compound at 166 ppm was assigned to formate species. These formate species are formed by dehydrogenation of methanol to formaldehyde and subsequent reaction with the zeolite framework. The main content of the compound at 171 ppm, occurring as MAS spinning sideband pattern with a chemical shift anisotropy of Δ σ=−40±2 ppm and an asymmetry parameter of η=0.8±0.1, is due to carbonate species. In situ 13C 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.