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 esta blished during isopropanol decomposition over pure and MnOx-modified a luminas were monitored and characterized by in situ infrared spectrosc opic measurements. On pure alumina the alcohol adsorbs dissociatively and non-dissociatively at room temperature, giving rise respectively t o isopropoxides coordinated to Lewis acid sites and isopropanol molecu les hydrogen-bonded to hydroxyl (and oxide) sites. Those adsorbed spec ies withstand outgassing at room temperature, but are completely elimi nated 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 t hermo evacuation at well above 200 degrees C, but activates their conv ersion into surface carboxylates at > 300 degrees C. Moreover, the iso propanol dehydration selectivity of alumina is critically suppressed a nd acetone becomes the dominant gas-phase product. Thus, a strong dehy drogenation 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 c atalysts and mechanistic aspects established in the field.