Theoretical Investigation on the Spectroscopic Properties and Catalytic Activities of the Ti-Hydroperoxo Intermediates in Titanosilicate Zeolites

Abstract The spectroscopic features of the Ti-hydroperoxo intermediates in TS-1, Ti-BEA, and Ti-YNU-1 zeolites were studied by using theoretical calculations. The Ti-hydroperoxo intermediates, Ti-η1(OOH), Ti-η1(OOH)-H2O, Ti-η2(OOH), and Ti-η2(OOH)-H2O species, as well as Ti-η1(OOH)-CH3OH, Ti-η1(OOH)-CH3CN, Ti-η2(OOH)-CH3OH, and Ti-η2(OOH)-CH3CN species were constructed and their IR, Raman, and UV-vis spectra were calculated. The geometry optimization and vibrational frequency calculations were carried out with DFT method at the theoretical level of B3LYP/6-31G(d, p), and the UV-vis spectra were obtained with TD-DFT method at the theoretical level of PBEPBE/6-311++G(d, p). The calculated structures show the average Ti–O bond distances of 2.01±0.02 A, consistent well with the experimental EXAFS data reported in the literatures. The calculated IR and Raman spectra of Ti-hydroperoxo intermediates show that the Oα–Oβ stretching vibrations are in the range of 870-910 cm-1 for Ti-η1(OOH) and Ti-η1(OOH)-R (R = H2O, CH3OH, CH3CN) species, and 834-855 cm-1 for Ti-η2(OOH) and Ti-η2(OOH)-R species. TD-DFT calculations indicate that for Ti-η1(OOH) and Ti-η1(OOH)-R species, the transition wavelengths are in the range of 350-365 nm and 370-400 nm, respectively, which are ascribed to the electron transition from the p orbital of the active oxygen Oα and Oβ to the unoccupied d* orbital of framework Ti atom, being responsible for the 385 nm band observed in the experimental UV-vis spectrum of TS-1 zeolite as contacting with H2O2 aqueous solution. Whereas, for the Ti-η2(OOH) and Ti-η2(OOH)-R species, all the corresponding transitions are below 350 nm. The catalytic performances of the Ti-hydroperoxo intermediates for olefin epoxidation were also evaluated. In combination of the calculated geometric parameters, the stabilities, the spectroscopic properties, and the catalytic performances, it is demonstrated that the initially formed Ti-hydroperoxo intermediates in titanosilicate/H2O2/H2O system should be Ti-η1(OOH)-H2O or Ti-η1(OOH) species, which then converts into the more stable Ti-η2(OOH)-H2O species that works as the actual catalytic active center for epoxidation reactions. If the solvent is methanol or acetonitrile, Ti-η2(OOH)-CH3OH or Ti-η2(OOH)-CH3CN species should be the predominant active center.