An infrared spectroscopy study of simple alcohols adsorbed on H-ZSM-5

We have examined the adsorption of methanol, ethanol, 1-propanol, 1-butanol, 2-propanol, and 2-methyl-2-propanol on H-ZSM-5 using transmission infrared spectroscopy. In agreement with previous results from temperature-programmed desorption (TPD) and thermogravimetric analysis [M. T. Aronson, R. J. Gorte, and W. E. Farneth, J. Catal.98, 434 (1986)], each of the alcohols adsorbs in the vicinity of the Al atoms, as evidenced by the disappearance of the hydroxyl band associated with the hydrogen cation. For 2-methyl-2-propanol adsorbed at 295 K and 2-propanol at 360 K, changes in the IR spectral features and isotopic scrambling following exposure to D2O indicate that these molecules dehydrate to form carbenium ion-like intermediates prior to desorbing as olefin products. Methanol, ethanol, and 1-propanol remain unreacted up to their desorption temperatures. While 1-butanol is shown to be very reactive in TPD, no isotopic scrambling was observed during exposure to D2O and reaction appears to occur through a bimolecular mechanism. The results of this study are explained in terms of a model which assumes that the primary interaction between the alcohols and the zeolite is due to proton transfer at the Al sites. The model pictures the adsorbed species as ion pairs consisting of oxonium ions or carbenium ions coupled with the anionic zeolite framework.