The role of outer-sphere surface acidity in alkene epoxidation catalyzed by calixarene-Ti(IV) complexes

Justin M Notestein, Andrew Solovyov, Leandro R. Andrini, Felix G. Requejo, Alexander Katz, Enrique Iglesia

Research output: Contribution to journalArticle

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Abstract

Cooperativity between Brønsted acidic defect sites on oxide surfaces and Lewis acid catalyst sites consisting of grafted calixarene-Ti(IV) complexes is investigated for controlling epoxidation catalysis. Materials are synthesized that, regardless of the surface or calixarene substituent, demonstrate nearly identical UV-visible ligand-to-metal charge-transfer bands and Ti K-edge X-ray absorption near edge spectral features consistent with site-isolated, coordinatively unsaturated Ti(IV) atoms. Despite similar Ti frontier orbital energies demonstrated by these spectra, replacing a homogeneous triphenylsilanol ligand with a silanol on a SiO2 surface increases cyclohexene epoxidation rates with tert-butyl hydroperoxide 20-fold per Ti site. Supporting calixarene-Ti active sites on fully hydroxylated Al 2O3 or TiO2, which possess lower average surface hydroxyl pKa than that of SiO2, reduces catalytic rates 50-fold relative to SiO2. These effects are consistent with SiO2 surfaces balancing two competing factors that control epoxidation rates-equilibrated hydroperoxide binding at Ti, disfavored by stronger surface Brønsted acidity, and rate-limiting oxygen transfer from this intermediate to alkenes, favored by strongly H-bonding intermediates. These observations also imply that Ti-OSi rather than Ti-OCalix bonds are broken upon hydroperoxide binding to Ti in kinetically relevant steps, which is verified by the lack of a calixarene upper-rim substituent effect on epoxidation rate. The pronounced sensitivity of observed epoxidation rates to the support oxide, in the absence of changes to the Ti coordination environment, provides experimental evidence for the importance of outer-sphere H-bonding interactions for the exceptional epoxidation reactivity of titanium silicalite and related catalysts.

Original languageEnglish
Pages (from-to)15585-15595
Number of pages11
JournalJournal of the American Chemical Society
Volume129
Issue number50
DOIs
Publication statusPublished - Dec 19 2007

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Calixarenes
Epoxidation
Alkenes
Acidity
Olefins
Oxides
Hydrogen Peroxide
Ligands
tert-Butylhydroperoxide
Lewis Acids
Titanium
Catalysis
Hydroxyl Radical
Catalytic Domain
Catalysts
Metals
X ray absorption
X-Rays
Oxygen
Charge transfer

ASJC Scopus subject areas

  • Chemistry(all)

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The role of outer-sphere surface acidity in alkene epoxidation catalyzed by calixarene-Ti(IV) complexes. / Notestein, Justin M; Solovyov, Andrew; Andrini, Leandro R.; Requejo, Felix G.; Katz, Alexander; Iglesia, Enrique.

In: Journal of the American Chemical Society, Vol. 129, No. 50, 19.12.2007, p. 15585-15595.

Research output: Contribution to journalArticle

Notestein, Justin M ; Solovyov, Andrew ; Andrini, Leandro R. ; Requejo, Felix G. ; Katz, Alexander ; Iglesia, Enrique. / The role of outer-sphere surface acidity in alkene epoxidation catalyzed by calixarene-Ti(IV) complexes. In: Journal of the American Chemical Society. 2007 ; Vol. 129, No. 50. pp. 15585-15595.
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