Organozirconium complexes are chemisorbed on Brønsted acidic sulfated ZrO2 (ZrS), sulfated Al2O3 (AlS), and ZrO2-WO3 (ZrW). Under mild conditions (25 °C, 1 atm H2), the supported Cp∗ZrMe3, Cp∗ZrBz3, and Cp∗ZrPh3 catalysts are very active for benzene hydrogenation with activities declining with decreasing acidity, ZrS ' AlS ' ZrW, arguing that more Brønsted acidic oxides (those having weaker corresponding conjugate bases) yield stronger surface organometallic electrophiles and for this reason have higher benzene hydrogenation activity. Benzene selective hydrogenation, a potential approach for carcinogenic benzene removal from gasoline, is probed using benzene/toluene mixtures, and selectivities for benzene hydrogenation vary with catalyst as ZrBz3+/ZrS-, 83% > Cp∗ZrMe2+/ZrS-, 80% > Cp∗ZrBz2+/ZrS-, 67% > Cp∗ZrPh2+/ZrS-, 57%. For Cp∗ZrBz2+/ZrS-, which displays the highest benzene hydrogenation activity with moderate selectivity in benzene/toluene mixtures. Other benzene/arene mixtures are examined, and benzene selectivities vary with arene as mesitylene, 99%, > ethylbenzene, 86% > toluene, 67%. Structural and computational studies by solid-state NMR spectroscopy, XAS, and periodic DFT methods applied to supported Cp∗ZrMe3 and Cp∗ZrBz3 indicate that larger Zr···surface distances are present in more sterically encumbered Cp∗ZrBz2+/AlS- vs Cp∗ZrMe2+/AlS-. The combined XAS, solid state NMR, and DFT data argue that the bulky catalyst benzyl groups expand the 'cationic' metal center-anionic sulfated oxide surface distances, and this separation/weakened ion-pairing enables the activation/insertion of more sterically encumbered arenes and influences hydrogenation rates and selectivity patterns.
ASJC Scopus subject areas
- Colloid and Surface Chemistry