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This study highlights the nature of the interactions between the transition metals in the Rh-Mo/γ-Al2O3 system and the underlying alumina surface. The investigation was carried out in the context of exploring the retrograde dehydration to dimethyl ether of primary alcohols produced at active metal sites during CO hydrogenation. Both native γ-Al2O3 and Rh-Mo/γ-Al2O3 samples were doped with KNO3 in an attempt to create an alkaline-metal oxide layer on the support surface which would screen alcohols from underlying Lewis acidity. The samples were characterized by microreactor-based methanol dehydration studies, analysis of infrared spectra of adsorbed pyridine, and ammonia saturation and temperature-programmed desorption. As expected, the dehydration activity of the samples was strongly correlated to their potassium loading and, hence, their Lewis acidity. More intriguing, however, are the insights gained in this analysis concerning the nature of the interactions between Mo, Rh, and the Lewis acid sites on the alumina surface. Our results are consistent with the principle, suggested in the Mo/γ-Al2O3 literature, that our Mo(CO)6 precursor interacts primarily with nonacidic hydroxyl groups on the dehydrated alumina surface. Furthermore, we provide strong evidence for the diminution of acidity brought on by the adsorption of Rh on Lewis acid sites of Mo/γ-Al2O3. |
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