INFLUENCE OF METAL-SUPPORT EFFECTS ON ACETOPHENONE HYDROGENATION OVERPLATINUM

The vapor-phase acetophenone (AcPh) hydrogenation reaction was studied over Pt/SiO2, Pt/eta-Al2O3, Pt/SiO2. Al2O3 and Pt/TiO2 catalysts to d etermine if the intramolecular selectivity to phenylethanol (PhEt) cou ld be enhanced by metal-support interactions (MSI). At low conversions ( < 10%), selectivities to PhEt above 95% were obtained over the Pt/T iO2 catalysts compared to 70-80% over Pt/SiO2 and Pt/eta-Al2O3, and at high conversions (> 70%) the selectivity remained above 80% over Pt/T iO2 after a HTR pretreatment. Very high yields of ethylbenzene (90-95% ) were obtained with Pt/SiO2.Al2O3 which are attributed to a bifunctio nal reaction involving PhEt dehydration to styrene on the acidic suppo rt followed by a hydrogenation step on the platinum. Activation energi es for AcPh hydrogenation were typically between 5 and 10 kcal/mol, an d turnover frequencies at 358 K did not vary markedly as they ranged f rom 0.028 to 0.058 s-1 at an AcPh pressure of 3 Torr. Significant deac tivation was observed with platinum powder and Pt/SiO2, especially at a higher AcPh pressure of 10 Torr, but it was much less severe with th e other catalysts. Turnover frequencies for the formation of phenyleth anol, acetylcyclohexane, cyclohexylethanol, ethylbenzene, and ethylcyc lohexane were also determined for these catalysts. This ability to inc rease intramolecular selectivity by favoring hydrogenation of a carbon yl bond is forwarded as additional evidence to support our MSI model i nvoking special sites at the metal-support interface which interact wi th the oxygen end of the carbonyl bond to activate it.