Grafted Ta-calixarenes: Tunable, selective catalysts for direct olefin epoxidation with aqueous hydrogen peroxide

Natalia Morlanés, Justin M. Notestein

Research output: Contribution to journalArticle

32 Citations (Scopus)

Abstract

Supported Ta(V) oxide catalysts were prepared by grafting calixarene-Ta(V) and TaX5 complexes on SiO2 at coverages less than 0.25 Ta nm-2. Thermogravimetric and elemental analyses and UV-visible spectroscopy indicate that catalysts consist of isolated, 1:1 Ta:ligand surface sites. Catalysts obtained by this one-pot procedure were studied in cyclohexene and cyclooctene epoxidation with H2O2. In sharp contrast with bare oxides, calixarene-containing catalysts had initial cyclohexene direct epoxidation turnover rates of 3.9 ± 0.1 × 10-2 s -1 unaffected by surface density, demonstrating single-site character. Calixarene-containing catalysts were up to 3× more active than the corresponding TaCl5-based catalysts at high surface densities and were also up to 95% selective to direct (non-radical) cyclohexene epoxidation versus <65% selectivity for TaCl5-based catalysts or only ∼20% selectivity for grafted calixarene-Ti(IV) catalysts. Capping silanols with octanol reduced epoxide hydrolysis from >50% to <30%. These catalysts demonstrate the utility of ligand-protected, supported Ta catalysts for epoxidation with H2O2 and demonstrate that a surface ligand on an otherwise traditionally prepared supported oxide can selectively direct oxidation down mechanistically distinct pathways.

Original languageEnglish
Pages (from-to)191-201
Number of pages11
JournalJournal of Catalysis
Volume275
Issue number2
DOIs
Publication statusPublished - Oct 22 2010

Keywords

  • Allylic hydroperoxide
  • Calixarene
  • Green chemistry
  • Heterogeneous catalyst
  • Hydrogen peroxide
  • Olefin epoxidation
  • Single-site catalyst
  • Supported catalyst
  • Supported oxide
  • Tantalum

ASJC Scopus subject areas

  • Catalysis
  • Physical and Theoretical Chemistry

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