Elucidation of the local structure of active titanium(iv) sites on silica-based phase-boundary catalysts for alkeneepoxidation 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 (T_tet) to that of a 6-coordinated one (T_oct) was determined. Monotonic increments of catalytic activity for epoxide production and efficiency of H₂O₂ utilization with the ratio T_tet/(T_tet + T_oct) revealed that the highly active catalysts mainly include T_tet but not T_oct. UV-Vis-DR spectra of samples with external surfaces partially covered with alkylsilyl groups indicated that there are at least two kinds of T_tet sites with different ligands. One site anchors an acidic hydroxyl (T_tet(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 (T_tet(OSiR)) formed via reaction of an acidic hydroxy of T_tet(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 (T_tet(OH,w)) and hydrophobic (T_tet(OH,o)) surfaces and the latter, T_tet(OSiR), exists only on the hydrophobic surface. From analyses of catalytic activities of several catalysts with different distributions of these T_tet sites, both T_tet(OH,o) and T_tet(OSiR) sites on the hydrophobic surface were proved to be active, while T_tet(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 T_tet(OSiR) site acted as a more effective site for selective epoxidation when compared with T_tet(OH,o), which induced ring-opening of epoxide to give a by-product, 1,2-octanediol.