In Situ DDR-UV-Vis and Raman spectroscopic study of titania promoting effect on silica supported tungsten catalysts for enhanced propylene metathesis

Tungsten based catalysts supported on silica ( z WO X /SiO 2 ) and silica promoted by titania ( z WO X / y TiO X /SiO 2 ) were studied for their catalytic activity towards propylene metathesis. The catalysts were prepared by a simple incipient wetness impregnation method using a large pore SiO 2 of intermediate surface area (∼50 m 2 /g). Catalytic activity studies carried out in a fixed-bed reactor (723 K, 101 kPa propylene) indicated that propylene conversion increased with increasing W loading in z WOx/SiO 2 catalysts ( z = 0.5−6 W/nm 2 ). It was shown that the catalytic activity of a poorly WO X dispersed 6WO X /SiO 2 catalyst could be enhanced and maximized by an optimum titania promotion of 2 wt% TiO 2 (∼3 Ti/nm 2 ). In situ differential diffuse reflectance (DDR) UV-Vis spectroscopy at reaction conditions showed that TiO X domain size increased with increases in titania loading from isolated TiO X to TiO X clusters to TiO 2 nanocrystals. The UV-Vis results also evidenced the existence of highly dispersed isolated WO X species, WO X clusters, and WO 3 nanoparticles in the 6WO X / y TiO X /SiO 2 ( y = 0.5−6 wt% TiO 2 or ∼0.7–9 Ti/nm 2 ) catalysts. In situ DDR-UV-Vis, Raman, and mass spectrometry during propylene metathesis, and catalyst oxidation and reduction revealed the reasons for an optimum amount of titania promoter in 6WO X /2TiO X /SiO 2 . They were the result of a balanced interplay between two factors: (1) enhanced WOx species dispersion due to the presence of a trimeric TiO X cluster and (2) absence of catalyst deactivation (present at high TiO 2 loadings) due to the trimeric TiO X cluster poor reactivity towards coke formation. • ∼3 Ti/nm 2 loading resulted in optimum 6WO X /TiO X /SiO 2 propylene metathesis catalyst. • In situ DDR UV-Vis identified dispersed WO X and TiO X species in 6WO X /TiO X /SiO 2 . • In situ Raman identified WO X species, supports, and coke in 6WO X /TiO X /SiO 2 . • Optimum TiO X loading led to trimeric TiO 5 clusters which increased WO X dispersion. • TiO X increased dispersion of dioxo WO X species in 6WO X /TiO X /SiO 2 .