Enhanced metathesis of ethylene and 2-butene on tungsten incorporated ordered mesoporous silicates

Abstract

Tungsten-incorporated 3D mesoporous silicates, W-KIT-6, W-KIT-5, and W-SBA-16 catalysts, outperform supported tungsten oxide catalysts (WO 3 /SiO 2 and WO 3 /KIT-6) for the metathesis of ethylene and 2-butene to propene at 450°C. All catalysts exhibit steady activity and stability during 7 h runs in a fixed-bed reactor. Furthermore, on all mesoporous catalysts, a maximum propylene yield was obtained at an optimum W loading at which the catalyst acidity is also maximum. Slightly delayed addition of the W source during the one-pot synthesis protocol markedly improves the propene yield on W-KIT-6 catalyst. XPS results conclusively show that this enhancement is due to preferential enrichment of active W species on the catalyst surface. Extended runs lasting five days reveal very little loss of activity even which is easily recovered by calcination in air.

Enhanced Olefin Metathesis Performance of Tungsten and Niobium Incorporated Bimetallic Silicates: Evidence of Synergistic Effects

“…Clearly, the W 10 Nb 1 ‐KIT‐6 catalyst shows the best apparent activity value of 9.92 mmol g −1 h −1 . Interestingly, as seen from Figure , the propene yield and the total acidity follow an identical trend, as also reported in previous studies …”

“…Consequently, the optimum Nb loading that maximizes the population of the active sites (and the propene yield) is relatively low compared to the W loading. The propene yield of 69.6±0.6 %, observed with W 10 Nb 1 ‐KIT‐6, exceeds those reported with other W‐incorporated mesoporous silicates [59.2±0.5 % for W‐KIT‐6 (Na) and 60.1±1.4 % for W‐EISA] at the same reaction conditions. The apparent activities of various catalysts from our work and those reported by others are compared in Table .…”

“…We find that the catalyst performance is indeed enhanced at certain bimetal loadings, with the W 10 Nb 1 ‐KIT‐6 catalyst (∼20 wt% W loading and ∼1 wt% Nb loading) yielding up to ∼70 % propene yield at steady state. This substantially exceeds the performances of W‐KIT‐6 and W‐EISA catalysts (∼60 % propene yield) at identical reaction conditions. The enhanced performance is attributed to the traditional W dioxo species and the formation of new active site precursors (O=) 2 W(O−Si)(O−Nb), which were confirmed by H 2 ‐TPR, XPS, EXAFS and ToF‐SIMS techniques.…”

“…The foregoing results suggest that if the dopant and W form active site precursors as shown in Scheme , the O=W=O bond angle in such W‐based bimetallic catalysts can be used to predict the relative metathesis activity of the doped catalysts. Given that the metathesis activity follows the same trend as acidity, establishing a definitive link between bond angle and acidity in future investigations will shed further fundamental insights into the underlying mechanism and guide the rational design of metathesis catalysts.…”

“…The metathesis activity of Nb 1 ‐KIT‐6, W 10 ‐KIT‐6 (Na), W 10 ‐KIT‐6, and W 10 Nb x ‐KIT‐6 catalysts (x=0.25–10) were compared at the following conditions: T =450 °C, P =1 atm, WHSV (ethylene and 2‐butene)=2.0 h −1 , n(ethylene)/n(2‐butene)=3/1 (Figures , S8 and S9). Compared with the W 10 ‐KIT‐6 (Na) catalyst, the W 10 ‐KIT‐6 catalyst (prepared with ammonium metatungstate as source) showed a remarkable increase in 2‐butene conversion from 46.6±0.7 % to 71.7±0.6 % and propene yield from 36.6±0.5 % to 60.7±0.7 %. This enhancement is consistent with better W dispersion based on wide‐angle XRD results (Figure ).…”