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

FTIR and C-13 NMR studies are reported of methanol and dimethylether adsorp tion in four different HZSM-5 zeolites containing different concentrations of framework and extra-framework aluminium. From comparisons of FTIR spectr a in the different zeolites, it is possible to identify four different adso rbed species obtained from methanol, methanol hydrogen-bonded to Bronsted a cid sites, chemisorbed methanol as methoxy groups formed on Bronsted acid s ites, silanol groups and extra-framework AlOH sites. Dimethylether is hydro gen-bonded to silanol groups and to Bronsted acid sites, and at higher temp eratures forms the same methoxy species generated from methanol. C-13 NMR s pectra show hydrogen-bonded methanol, hydrogen-bonded dimethylether, and a signal at 60 ppm attributed to methoxy groups. The different types of metho xy group are not resolved by NMR. The spectra, coupled with analysis of ini tial hydrocarbon products, indicate that the Bronsted acid sites and the me thoxy groups formed on them ate the key to hydrocarbon formation. Extra-fra mework AlOH sites play no direct role in initial carbon-carbon bond formati on. (C) 1999 Elsevier Science B.V. All rights reserved.