SURFACE-CHEMISTRY OF RH-MO GAMMA-AL2O3 - AN ANALYSIS OF SURFACE-ACIDITY/

This study highlights the nature of the interactions between the trans ition metals in the Rh-Mo/gamma-Al2O3 system and the underlying alumin a surface. The investigation was carried out in the context of explori ng the retrograde dehydration to dimethyl ether of primary alcohols pr oduced at active metal sites during CO hydrogenation. Both native gamm a-Al2O3 and Rh-Mo/gamma-Al2O3 samples were doped with KNO3 in an attem pt to create an alkaline-metal oxide layer on the support surface whic h would screen alcohols from underlying Lewis acidity. The samples wer e characterized by microreactor-based methanol dehydration studies, an alysis of infrared spectra of adsorbed pyridine, and ammonia saturatio n and temperature-programmed desorption. As expected, the dehydration activity of the samples was strongly correlated to their potassium loa ding and, hence, their Lewis acidity. More intriguing, however, are th e insights gained in this analysis concerning the nature of the intera ctions between Mo, Rh, and the Lewis acid sites on the alumina surface . Our results are consistent with the principle, suggested in the Mo/g amma-Al2O3 literature, that our Mo(CO)(6) precursor interacts primaril y with nonacidic hydroxyl groups on the dehydrated alumina surface. Fu rthermore, we provide strong evidence for the diminution of acidity br ought on by the adsorption of Rh on Lewis acid sites of Mo/gamma-Al2O3 . (C) 1995 Academic Press, Inc.