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

The titanium species in four kinds of titanium-containing MFI zeolites have been studied by ultraviolet (UV)-Raman and ultraviolet visible (UV-Vis) ab sorption spectroscopies and by the epoxidation of propylene with diluted H2 O2 solution (30%). UV-Raman spectroscopy is proved to be a suitable means t o estimate qualitatively the framework titanium in TS-l zeolites. Based on the comparison of the relative intensity ratio I-1125/I-380 of UV-Raman spe ctra, the TS-1(conv.) sample synthesized hydrothermally by the conventional procedure shows the highest amount of framework titanium. UV-Vis spectrosc opy reveals that besides minor anatase. titanium species are mainly tetrahy drally coordinated into the framework for TS-l(conv.) or the Ti-ZSM-5 sampl e prepared by gas-solid reaction between deboronated B-ZSM-5 and TiCl4 vapo r at elevated temperatures. For the TS-1(org.) and TS-1(inorg.) samples syn thesized hydrothermally using tetrapropylammonium bromide (TPABr) as templa te and tetrabutylorthotitanite (TBOT) and TiCl3 as titanium source, respect ively, the presence of mononuclear and isolated TiOx species which are prop osed to bond to the zeolite extraframework is observed. In addition to the framework titanium species, these isolated TiOx species are assumed to be a lso active for propylene epoxidation.