The active sites in different TS-1 zeolites for propylene epoxidation studied by ultraviolet resonance Raman and ultraviolet visible absorption spectroscopies

Abstract

The titanium species in four kinds of titanium-containing MFI zeolites have been studied by ultraviolet (UV)-Raman and ultraviolet visible (UV-Vis) absorption spectroscopies and by the epoxidation of propylene with diluted H₂O₂ solution (30%). UV-Raman spectroscopy is proved to be a suitable means to estimate qualitatively the framework titanium in TS-1 zeolites. Based on the comparison of the relative intensity ratio I₁₁₂₅/I₃₈₀ of UV-Raman spectra, the TS-1(conv.) sample synthesized hydrothermally by the conventional procedure shows the highest amount of framework titanium. UV-Vis spectroscopy reveals that besides minor anatase, titanium species are mainly tetrahydrally coordinated into the framework for TS-1(conv.) or the Ti-ZSM-5 sample prepared by gas–solid reaction between deboronated B-ZSM-5 and TiCl₄ vapor at elevated temperatures. For the TS-1(org.) and TS-1(inorg.) samples synthesized hydrothermally using tetrapropylammonium bromide (TPABr) as template and tetrabutylorthotitanite (TBOT) and TiCl₃ as titanium source, respectively, the presence of mononuclear and isolated TiO_x species which are proposed to bond to the zeolite extraframework is observed. In addition to the framework titanium species, these isolated TiO_x species are assumed to be also active for propylene epoxidation.