Elucidation of the local structure of active titanium(iv) sites on silica-based phase-boundary catalysts for alkene epoxidation with aqueous hydrogen peroxide

Abstract

Structural and functional aspects of active titanium sites for phase boundary catalytic (PBC) epoxidation of 1-octene with hydrogen peroxide (H2O2) were investigated in detail using X-ray absorption fine structure (XAFS) analysis and ultraviolet and visible-light diffuse reflectance (UV-Vis-DR) spectroscopy. By analysis of the Ti K-edge X-ray absorption near edge fine structure (XANES) spectra of several titanium-loaded silica catalysts, the ratio of the amount of 4-coordinated titanium oxide (Ttet) to that of a 6-coordinated one (Toct) was determined. Monotonic increments of catalytic activity for epoxide production and efficiency of H2O2 utilization with the ratio Ttet/(Ttet + Toct) revealed that the highly active catalysts mainly include Ttet but not Toct. UV-Vis-DR spectra of samples with external surfaces partially covered with alkylsilyl groups indicated that there are at least two kinds of Ttet sites with different ligands. One site anchors an acidic hydroxyl (Ttet(OH)), giving absorption centered at the wavelength of ca. 230 nm and the other, exhibiting an absorption peak at the wavelength of ca. 210 nm, is directly attached to an alkylsilyl group (Ttet(OSiR)) formed via reaction of an acidic hydroxy of Ttet(OH) with an alkylsilane reagent. Since the catalysts have both an alkylsilyl-grafted hydrophobic surface and a hydroxy-terminated hydrophilic surface, it is postulated that the former is located on both hydrophilic (Ttet(OH,w)) and hydrophobic (Ttet(OH,o)) surfaces and the latter, Ttet(OSiR), exists only on the hydrophobic surface. From analyses of catalytic activities of several catalysts with different distributions of these Ttet sites, both Ttet(OH,o) and Ttet(OSiR) sites on the hydrophobic surface were proved to be active, while Ttet(OH,w) on the hydrophilic surface did not work for the present PBC system. Among the sites on the hydrophobic surface, moreover, it was found that a Ttet(OSiR) site acted as a more effective site for selective epoxidation when compared with Ttet(OH,o), which induced ring-opening of epoxide to give a by-product, 1,2-octanediol

Oxidation of hydrophobic alcohols using aqueous hydrogen peroxide over amphiphilic silica particles loaded with titanium(IV) oxide as a liquid–liquid phase-boundary catalyst

“…On the other hand, in the case of w/o-Ti/SiO 2 , most of the particles were assembled at the liquid-liquid phase boundary. As reported previously [5][6][7][8]16], this is a phenomenological indication of amphiphilic character, having both hydrophobic and hydrophilic faces, of oxide particles. Based on these results and observations, w/o-Ti/SiO 2 is thought to be assembled at the phase boundary of the water-toluene dual-phase mixture by facing its hydrophobic octadecylsilylated side to the organic phase and its hydrophilic side to the aqueous phase.…”

“…As previously investigated using Ti K-edge X-ray absorption near-edge fine structure (XANES) analysis [8], the loaded titanium species on w/o-Ti/SiO 2 were mainly those with tetrahedral coordination (T tet ) with a small content of octahedrally coordinated ones (T oct ). The former was attributable to the isolated titanium and the latter included titanium dimers, oligomers, and/or bulk (TiO 2 )-like microclusters [21][22][23][24].…”

“…The former was attributable to the isolated titanium and the latter included titanium dimers, oligomers, and/or bulk (TiO 2 )-like microclusters [21][22][23][24]. [8], and thereby the active species might be ascribed to T tet . It has also been clarified in our previous study using ultraviolet and visible-light diffuse reflectance (UV-Vis-DR) spectroscopy in relation to catalytic activity [8] that there are three types of T tet on w/o-Ti/SiO 2 .…”

“…We previously reported in detail phenomenological aspects of PBC for epoxidation of terminal linear alkenes [5,6], asymmetric surface structures of catalyst particles [7], and local structures of loaded titanium species [8]. Results of these studies lead to predictions to extend PBC to other organic reactions in order to obtain information on availability and limitations and to obtain mechanistic insights.…”

“…We have proposed a new concept of a heterogeneous catalytic system for driving a reaction between a water-immiscible organic compound(s) and aqueous reagent(s) using a solid catalyst without a co-solvent or mechanical agitation [5][6][7][8]. For this system, we have prepared solid particles that could assemble at the liquid-liquid phase boundary by partial modification of external surfaces with alkylsilyl groups.…”