Benzene selectivity in competitive arene hydrogenation: Effects of single-site catalyst···acidic oxide surface binding geometry

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.