INFLUENCE OF METAL-SUPPORT EFFECTS ON ACETOPHENONE HYDROGENATION OVER PLATINUM

The vapor-phase acetophenone (AcPh) hydrogenation reaction was studied over Pt/SiO2, Pt/eta-Al2O3, Pt/SiO2. Al2O3 and Pt/TiO2 catalysts to determine if the intramolecular selectivity to phenylethanol (PhEt) could be enhanced by metal-support interactions (MSI). At low conversions ( < 10%), selectivities to PhEt above 95% were obtained over the Pt/TiO2 catalysts compared to 70-80% over Pt/SiO2 and Pt/eta-Al2O3, and at high conversions (> 70%) the selectivity remained above 80% over Pt/TiO2 after a HTR pretreatment. Very high yields of ethylbenzene (90-95%) were obtained with Pt/SiO2.Al2O3 which are attributed to a bifunctional reaction involving PhEt dehydration to styrene on the acidic support followed by a hydrogenation step on the platinum. Activation energies for AcPh hydrogenation were typically between 5 and 10 kcal/mol, and turnover frequencies at 358 K did not vary markedly as they ranged from 0.028 to 0.058 s-1 at an AcPh pressure of 3 Torr. Significant deactivation was observed with platinum powder and Pt/SiO2, especially at a higher AcPh pressure of 10 Torr, but it was much less severe with the other catalysts. Turnover frequencies for the formation of phenylethanol, acetylcyclohexane, cyclohexylethanol, ethylbenzene, and ethylcyclohexane were also determined for these catalysts. This ability to increase intramolecular selectivity by favoring hydrogenation of a carbonyl bond is forwarded as additional evidence to support our MSI model invoking special sites at the metal-support interface which interact with the oxygen end of the carbonyl bond to activate it.