The roles of different titanium species in TS-1 zeolite in propylene epoxidation studied by in situ UV Raman spectroscopy

Abstract

Titanium silicalite (TS-1) zeolites with different titanium species were synthesized and characterized by ultraviolet (UV)-Raman, ultraviolet visible (UV-Vis) diffuse reflectance spectroscopies and by the NH3 temperature programmed desorption (NH3-TPD) method. The roles of different titanium species in TS-1 samples have been investigated by gas chromatography-Raman spectrometry (GC-Raman) during the propylene epoxidation process. For the first time, a positive correlation was found among the concentration of framework Ti species, the amount of active intermediate Ti-OOH (η(2)) and the conversion of propylene by the in situ GC-Raman technique. The results give evidence that the framework titanium species is the active center and Ti-OOH (η(2)) is the active intermediate. The presence of extra-framework Ti species is harmful to propylene epoxidation. Furthermore, the amorphous Ti species has a more negative effect on the yield of propylene oxide (PO) than the anatase TiO2. The NH3-TPD results reveal that the amorphous Ti species are more acidic and thus should be mainly responsible for the further conversion of PO.

The effect of acid treatment on the active sites and reaction intermediates of the low-cost TS-1 in propylene epoxidation

“…It was found that the acidity of the amorphous Ti species is stronger than that of the anatase TiO 2 , which leads to undesired side reaction. 13 For the low-cost TS-1 zeolite, the distribution of the titanium species is more complicated. Therefore, to improve its catalytic properties, a systematic study on the distribution and the roles of the different titanium species, before and aer the post-treatment, are highly required to be known.…”

“…12,13 It was accepted that the isolated titanium species in the framework of the TS-1 zeolite is the active site for the selective oxidation. The non-framework Ti species is harmful for the epoxidation reactions.…”

“…[13][14][15][16] is found to be crucial for propylene epoxidation, whose typical peak is located at 837 cm À1 .…”

“…This result is consistent with that obtained on the classical TS-1 synthesised with TPAOH. 13 In a vapor-phase epoxidation of propene, the absorbed PO may open the ring to form branched alkanes, which are subsequently converted into polyalkenes, propanal and so on. [20][21][22][23] The side reaction of PO lowers the selectivity of the TS-1 catalyst unquestionably.…”

“…In this study, the low-cost TS-1 zeolite containing different titanium species was synthesized by changing the Ti/Si ratio and synthesis conditions. Then, the technique of gas chromatography-Raman spectroscopy (GC-Raman) 13 was used to study the relationship between the amount of active intermediate, the product propylene epoxide (PO) and the conversion of propylene. The roles of different titanium species in low-cost TS-1 for propylene epoxidation are discussed herein.…”