Enhancement of the Catalytic Activity of Titanium-Based Terminal Olefin Epoxidation Catalysts via Surface Modification with Functionalized Protic Molecules

Abstract

Site-isolated Ti(IV) centers were introduced onto the surface of a mesoporous SBA-15 support via the thermolytic molecular precursor method. Prior to thermal treatment to generate Ti−OH sites, residual silanol groups were capped via reaction with Me2N-SiMe3 to give TiMe cap SBA15. After low temperature treatment in oxygen, the resulting Ti−OH sites of TiMe cap SBA15-O 2 were modified by reaction with a series of protic reagents: phenol, pentafluorophenol, acetic acid, and trifluoroacetic acid. The structure of the resulting TiSBA15 catalysts and the Ti(IV) epoxidation intermediates (formed upon treatment of Ti(IV) materials with TBHP or H2O2) were probed using diffuse-reflectance UV−visible spectroscopy and infrared spectroscopy. A titanium-hydroperoxo species similar to that found in TS-1 is proposed for all catalysts. Samples modified with phenol and pentafluorophenol exhibited conversions of 1-octene that are 20 to 50% higher than those for TiMe cap SBA15-O 2 , without a significant drop in selectivity for the epoxide product, 1,2-epoxyoctane, when TBHP was used as the oxidant. With aqueous H2O2 as the oxidant, the phenol-treated materials exhibited 1-octene conversions that are 15 to 50% greater than those observed for TiMe cap SBA15-O 2 , and an increased selectivity for 1,2-epoxyoctane of 10 to 30%. Additionally, the efficiency of H2O2 usage, as monitored via 1H NMR spectroscopy, increased by a factor of 2 to three for catalysts modified with phenol and pentafluorophenol, with respect to the efficiency observed over TiMe cap SBA15-O 2 . Catalysts modified with acetic acid and trifluoroacetic acid displayed decreased catalytic turnover numbers and epoxide selectivities when TBHP was used as the oxidant, but exhibited catalytic turnover numbers and epoxide selectivities similar to TiMe cap SBA15-O 2 when H2O2 was used as the oxidant. After treatment of TiMe cap SBA15-O 2 with acetic acid, the H2O2 efficiency decreased by a factor of 2 for the epoxidation of 1-octene with H2O2.

Counting Active Sites on Titanium Oxide–Silica Catalysts for Hydrogen Peroxide Activation through In Situ Poisoning with Phenylphosphonic Acid

“…This indicates that the electron density of the Ti site is distorted by the formation of a TiOP bond. The increase in the electron density around the Ti center helps to explain the decrease in reactivity upon poisoning with PPA, as others have reported previously that capping Ti sites with electron‐donating moieties led to decreased reaction rates 23. The negligible shift in the energy of the pre‐edge feature suggests that the binding of PPA does not involve a significant change in the TiO coordination number, consistent with the structures shown in Scheme .…”

Physico-chemical Properties, Biological and Environmental Impact of Nb-saponites Catalysts for the Oxidative Degradation of Chemical Warfare Agents

“…Indeed, niobium centers confer oxidizing properties to the clay and the presence of protonic sites helps to increase the abatement activity through acid‐promoted degradation. In addition, protonic sites may also take part, in a synergistic way, in the formation of niobium η 1 ‐hydroperoxospecies which are the active sites where the selective heterolytic oxygen transfer from hydrogen peroxide to the sulfide takes place ,…”

From single-site tantalum complexes to nanoparticles of TaxNy and TaOxNy supported on silica: elucidation of synthesis chemistry by dynamic nuclear polarization surface enhanced NMR spectroscopy and X-ray absorption spectroscopy

“…On the basis of the literature, we suggest a tentative mechanism for the cyclooctene epoxidation catalyzed by Ta x N y /TaO x N y NPs on silica, as summarized in Scheme S3. † 17 , 68 – 73 …”