Nature of WO_x Sites on SiO₂ and Their Molecular Structure–Reactivity/Selectivity Relationships for Propylene Metathesis

Abstract

Supported WO x /SiO2 catalysts were investigated for propylene metathesis as a function of tungsten oxide loading and temperature. The catalysts were synthesized by incipient-wetness impregnation of an aqueous ammonium metatungstate solution onto the silica support and calcined at elevated temperatures to form the supported tungsten oxide phase. In situ Raman spectroscopy under dehydrated conditions revealed that below 8% WO x /SiO2, only surface WO x sites are present on the silica support: dioxo (O)2WO2 and mono-oxo OWO4. The in situ XANES analysis showed that dioxo surface WO4 sites were the dominant surface WO x sites on SiO2 (>90%). The isolated nature of the surface WO x sites was confirmed with in situ UV–vis spectroscopy. The surface WO x sites are activated by exposure to propylene at elevated temperature that removes oxygen from these sites. The activation process produces a highly active surface WO x site that can perform olefin metathesis at ∼150–250 °C. For 8% WO x /SiO2 and higher tungsten oxide loading, crystalline WO3 nanoparticles (NPs) are also present, and their amount increases with greater tungsten oxide loading. WO3 NPs, however, are not active for propylene metathesis. The acid character of the surface WO x sites (Lewis) and WO3 NPs (Brønsted) is responsible for formation of undesirable reaction products (C4–C6 alkanes and dimerization of C2 = to C4 =). This study represents the first time that molecular level structure–activity/selectivity relationships have been established for propylene metathesis by conventionally impregnated supported WO x /SiO2 catalysts.

Olefin Metathesis by Group VI (Mo, W) Metal Compounds

“…only 5% of tungsten atoms are active in olein metathesis. This value is similar to what had been reported for MoO 3 supported on silica [53] and that proposed by Wachs et al for WO 3 on silica [51]. Unfortunately, no mechanism was proposed to explain the transformation of the mono-oxo tungsten (IV) species into the bisiloxy-oxo-carbene (≡SiO) 2 (O)W═CHR (R = H, CH 3 ).…”

“…The bands characteristics of the mono-oxo and di-oxo species (which are the sole species on the solid) decrease simultaneously with time and disappear after 100 minutes [51]. This proves that the two species were activated by propene and led to the formation of carbene species with elimination of oxygen from the coordination sphere of tungsten (Figure 13).…”

“…These results were explained as follows: At low coverage (WO 3 < 8 wt.%) the catalytic activity is proportional to the amount of isolated mono-oxo and di-oxo species (which are all assumed to be active in olein metathesis). At high coverage, the reaction rate is not dependent on the tungsten loading, due to the formation of WO 3 crystallites which are inactive [51]. There is also an efect of the WO 3 loading on the amount to ethylene and butenes.…”

“…1 as expected while at high coverage it decreases strongly, due to the formation of C 4 ─C 6 alkanes. This was interpreted as due to the presence of Brönsted acid sites on the WO 3 crystallites (or nanoparticles), which led to oligomerization and cracking [51].…”

“…Wachs et al studied by Raman the efect of the pretreatment in air on a WO 3 /SiO 2 catalyst as a function of the oxide loading [51]. For low coverages, the Raman spectrum shows new bands at 1016 and 958 cm −1 , which were atributed to the symmetric vibrations of di-oxo and mono-oxo species, respectively.…”