Thermochemical adsorptive and catalytic events occurring during isopropanol decomposition over MnOx-modified aluminas. In situ infrared spectroscopic studies

Adsorptive and catalytic events occurring at gas/solid interfaces established during isopropanol decomposition over pure and MnOx-modified aluminas were monitored and characterized by in situ infrared spectroscopic measurements. On pure alumina the alcohol adsorbs dissociatively and non-dissociatively at room temperature, giving rise respectively to isopropoxides coordinated to Lewis acid sites and isopropanol molecules hydrogen-bonded to hydroxyl (and oxide) sites. Those adsorbed species withstand outgassing at room temperature, but are completely eliminated near 200 degrees C. Meanwhile, the alcohol catalytic dehydration is commenced at 150 degrees C and gas-phase propene is thus released. Quantitative conversion into the alkene is effected near 225 degrees C. The modification with MnOx stabilizes the isopropoxide species to thermo evacuation at well above 200 degrees C, but activates their conversion into surface carboxylates at > 300 degrees C. Moreover, the isopropanol dehydration selectivity of alumina is critically suppressed and acetone becomes the dominant gas-phase product. Thus, a strong dehydrogenation selectivity is generated. These and other MnOx-influenced alterations to the adsorptive and catalytic behaviours of alumina are discussed on the basis of available characterization results for the catalysts and mechanistic aspects established in the field.