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

Weixing Gu, Madelyn Marie Stalzer, Christopher P. Nicholas, Alak Bhattacharyya, Alessandro Motta, James R. Gallagher, Guanghui Zhang, Jeffrey T. Miller, Takeshi Kobayashi, Marek Pruski, Massimiliano Delferro*, Tobin J. Marks

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

43 Scopus citations

Abstract

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.

Original languageEnglish (US)
Pages (from-to)6770-6780
Number of pages11
JournalJournal of the American Chemical Society
Volume137
Issue number21
DOIs
StatePublished - Jun 3 2015

ASJC Scopus subject areas

  • Catalysis
  • Chemistry(all)
  • Biochemistry
  • Colloid and Surface Chemistry

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