Kinetic study of cyclooctene epoxidation with aqueous hydrogen peroxide over silica-supported calixarene-Ta(V)

Natalia Morlanes, Justin M Notestein

Research output: Contribution to journalArticle

23 Citations (Scopus)

Abstract

Replacing traditional TaCl5 precursors, calixarene-Ta(V) complexes are grafted on SiO2 at self-limiting loadings of 0.2 mmol·g-1 (0.25 Ta·nm-2) to synthesize an active and selective catalyst for epoxidation with aqueous H2O 2. The catalyst is synthesized in a one-pot procedure and has turnover rates that are a very weak function of Ta surface density, in contrast with catalysts that are subsequently calcined, following traditional practice. Cyclooctene epoxidation turnover rates exceed 240 h-1 and epoxide hydrolysis to the cyclooctanediol is extremely low, 2O2 decomposition is also decreased by 50% when using the calixarene capping ligand as compared to the bare oxide. A kinetic study of this system indicates a near first order dependence of the epoxidation rate on Ta content and H 2O2 concentrations; cyclooctene is weak positive order indicating some inhibition. H2O, product epoxide, and co-product cyclooctanediol are inhibitors, with cyclooctanediol being the strongest inhibitor on a molar basis, underscoring the importance of the reduced selectivity towards this species for the capped catalyst. A kinetic expression is proposed and describes initial rates and epoxide yields with time in good agreement with experimental data. The proposed catalytic cycle includes equilibrated formation of six-coordinate Ta(OH)X surface species as the most abundant intermediates. The rate limiting step is proposed to be the reaction between an activated Ta-hydroperoxide and an alkene in free solution, involving an external nucleophilic attack of the pi-system of the olefin on the relatively electropositive oxygen of the Ta-hydroperoxide species. The bulky and hydrophobic phenolate ligands pay a role in maintaining high Ta oxide dispersion, decreasing inhibition by polar species, and creating intrinsically more Lewis acidic sites, leading to increased epoxidation rates and decreased rates of undesired H2O2 decomposition and epoxide hydrolysis. This new class of ligand-capped, supported Ta oxide catalyst is not only more active and selective than its bare oxide analogue, but also provides a well-behaved catalyst for kinetic modeling.

Original languageEnglish
Pages (from-to)45-54
Number of pages10
JournalApplied Catalysis A: General
Volume387
Issue number1-2
DOIs
Publication statusPublished - Oct 20 2010

Fingerprint

Calixarenes
Epoxidation
Hydrogen peroxide
Silicon Dioxide
Hydrogen Peroxide
Epoxy Compounds
Silica
Oxides
Catalysts
Kinetics
Ligands
Alkenes
Olefins
Hydrolysis
Decomposition
Catalyst selectivity
Oxygen

Keywords

  • Epoxidation
  • Hydrogen peroxide
  • Organic-inorganic hybrid
  • Tantalum

ASJC Scopus subject areas

  • Catalysis
  • Process Chemistry and Technology

Cite this

Kinetic study of cyclooctene epoxidation with aqueous hydrogen peroxide over silica-supported calixarene-Ta(V). / Morlanes, Natalia; Notestein, Justin M.

In: Applied Catalysis A: General, Vol. 387, No. 1-2, 20.10.2010, p. 45-54.

Research output: Contribution to journalArticle

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N2 - Replacing traditional TaCl5 precursors, calixarene-Ta(V) complexes are grafted on SiO2 at self-limiting loadings of 0.2 mmol·g-1 (0.25 Ta·nm-2) to synthesize an active and selective catalyst for epoxidation with aqueous H2O 2. The catalyst is synthesized in a one-pot procedure and has turnover rates that are a very weak function of Ta surface density, in contrast with catalysts that are subsequently calcined, following traditional practice. Cyclooctene epoxidation turnover rates exceed 240 h-1 and epoxide hydrolysis to the cyclooctanediol is extremely low, 2O2 decomposition is also decreased by 50% when using the calixarene capping ligand as compared to the bare oxide. A kinetic study of this system indicates a near first order dependence of the epoxidation rate on Ta content and H 2O2 concentrations; cyclooctene is weak positive order indicating some inhibition. H2O, product epoxide, and co-product cyclooctanediol are inhibitors, with cyclooctanediol being the strongest inhibitor on a molar basis, underscoring the importance of the reduced selectivity towards this species for the capped catalyst. A kinetic expression is proposed and describes initial rates and epoxide yields with time in good agreement with experimental data. The proposed catalytic cycle includes equilibrated formation of six-coordinate Ta(OH)X surface species as the most abundant intermediates. The rate limiting step is proposed to be the reaction between an activated Ta-hydroperoxide and an alkene in free solution, involving an external nucleophilic attack of the pi-system of the olefin on the relatively electropositive oxygen of the Ta-hydroperoxide species. The bulky and hydrophobic phenolate ligands pay a role in maintaining high Ta oxide dispersion, decreasing inhibition by polar species, and creating intrinsically more Lewis acidic sites, leading to increased epoxidation rates and decreased rates of undesired H2O2 decomposition and epoxide hydrolysis. This new class of ligand-capped, supported Ta oxide catalyst is not only more active and selective than its bare oxide analogue, but also provides a well-behaved catalyst for kinetic modeling.

AB - Replacing traditional TaCl5 precursors, calixarene-Ta(V) complexes are grafted on SiO2 at self-limiting loadings of 0.2 mmol·g-1 (0.25 Ta·nm-2) to synthesize an active and selective catalyst for epoxidation with aqueous H2O 2. The catalyst is synthesized in a one-pot procedure and has turnover rates that are a very weak function of Ta surface density, in contrast with catalysts that are subsequently calcined, following traditional practice. Cyclooctene epoxidation turnover rates exceed 240 h-1 and epoxide hydrolysis to the cyclooctanediol is extremely low, 2O2 decomposition is also decreased by 50% when using the calixarene capping ligand as compared to the bare oxide. A kinetic study of this system indicates a near first order dependence of the epoxidation rate on Ta content and H 2O2 concentrations; cyclooctene is weak positive order indicating some inhibition. H2O, product epoxide, and co-product cyclooctanediol are inhibitors, with cyclooctanediol being the strongest inhibitor on a molar basis, underscoring the importance of the reduced selectivity towards this species for the capped catalyst. A kinetic expression is proposed and describes initial rates and epoxide yields with time in good agreement with experimental data. The proposed catalytic cycle includes equilibrated formation of six-coordinate Ta(OH)X surface species as the most abundant intermediates. The rate limiting step is proposed to be the reaction between an activated Ta-hydroperoxide and an alkene in free solution, involving an external nucleophilic attack of the pi-system of the olefin on the relatively electropositive oxygen of the Ta-hydroperoxide species. The bulky and hydrophobic phenolate ligands pay a role in maintaining high Ta oxide dispersion, decreasing inhibition by polar species, and creating intrinsically more Lewis acidic sites, leading to increased epoxidation rates and decreased rates of undesired H2O2 decomposition and epoxide hydrolysis. This new class of ligand-capped, supported Ta oxide catalyst is not only more active and selective than its bare oxide analogue, but also provides a well-behaved catalyst for kinetic modeling.

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