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

Abstract

Tungsten based catalysts supported on silica (zWO_X/SiO₂) and silica promoted by titania (zWO_X/yTiO_X/SiO₂) were studied for their catalytic activity towards propylene metathesis. The catalysts were prepared by a simple incipient wetness impregnation method using a large pore SiO₂ of intermediate surface area (∼50 m²/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 zWOx/SiO₂ catalysts (z = 0.5−6 W/nm²). It was shown that the catalytic activity of a poorly WO_X dispersed 6WO_X/SiO₂ catalyst could be enhanced and maximized by an optimum titania promotion of 2 wt% TiO₂ (∼3 Ti/nm²). 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 TiO₂ nanocrystals. The UV-Vis results also evidenced the existence of highly dispersed isolated WO_X species, WO_X clusters, and WO₃ nanoparticles in the 6WO_X/yTiO_X/SiO₂ (y = 0.5−6 wt% TiO₂ or ∼0.7–9 Ti/nm²) 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/SiO₂. 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 TiO₂ loadings) due to the trimeric TiO_X cluster poor reactivity towards coke formation.