Methanol to hydrocarbons: spectroscopic studies and the significance of extra-framework aluminium

FTIR and C-13 NMR studies are reported of methanol and dimethylether adsorption in four different HZSM-5 zeolites containing different concentrations of framework and extra-framework aluminium. From comparisons of FTIR spectra in the different zeolites, it is possible to identify four different adsorbed species obtained from methanol, methanol hydrogen-bonded to Bronsted acid sites, chemisorbed methanol as methoxy groups formed on Bronsted acid sites, silanol groups and extra-framework AlOH sites. Dimethylether is hydrogen-bonded to silanol groups and to Bronsted acid sites, and at higher temperatures forms the same methoxy species generated from methanol. C-13 NMR spectra show hydrogen-bonded methanol, hydrogen-bonded dimethylether, and a signal at 60 ppm attributed to methoxy groups. The different types of methoxy group are not resolved by NMR. The spectra, coupled with analysis of initial hydrocarbon products, indicate that the Bronsted acid sites and the methoxy groups formed on them ate the key to hydrocarbon formation. Extra-framework AlOH sites play no direct role in initial carbon-carbon bond formation. (C) 1999 Elsevier Science B.V. All rights reserved.