Novel tungsten-incorporated mesoporous silicates synthesized via evaporation-induced self-assembly: Enhanced metathesis performance

Abstract

Tungsten-incorporated mesostructured silicates were successfully synthesized by evaporationinduced self-assembly (EISA) method. Tungsten loadings from 2.9 to 22.2 wt% were achievable, with the corresponding BET surface areas decreasing from 520-284 m 2 /g at increased W loading. The W-EISA samples displayed amorphous ordered pore structure with an average pore size distribution of 2.3-3.3 nm. Hydrated Isolated tetrahedral [WO 4 ] 2species and hydrated octahedral polytungstate species were identified from diffuse reflectance UV/Vis spectra under ambient conditions. Uniform distribution of W species, mostly located near the surface, was evident from XPS W 4f photoelectron image. At 450C and 1 atm, for the reaction of ethylene+2-butene metathesis to form propene, the W-EISA catalysts outperform either impregnated catalysts (WO 3 /SiO 2 and WO 3 /KIT-6) or one-pot synthesized W-KIT-6 catalyst, implying better W utilization in the W-EISA catalyst.

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

“…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.…”

Tungsten-Based Mesoporous Silicates W-MMM-E as Heterogeneous Catalysts for Liquid-Phase Oxidations with Aqueous H2O2

“…The small angle XRD patterns of W-MMM-E samples, shown in Figure 1, exhibit a broad diffraction reflex around 3.0 • (2θ). This indicates a long-range structural order of cylindrical mesopores, which is typical for silicate materials prepared by EISA [47,48,50]. The lattice constant, calculated using the XRD (10) reflection position for W-MMM-E, is close to that reported for Ti-MMM-E (5.0 vs. 5.2 nm) [47].…”

“…Employment of acetylacetone (acac) as the chelating agent for stabilization of metal precursors in the synthesis prevented TiO 2 or Nb 2 O 5 precipitation and assisted in the generation of active centers that were well-dispersed within the silica matrix. Recently, Subramaniam and co-workers employed EISA to synthesize zirconium- [49], tungsten- [50], and molybdenum-containing [51] mesostructured silicates using TEOS as the silica precursor, Pluronic P123 as the templating agent and ZrOCl 2 ·8H 2 O, WCl 6 or MoCl 5 as metal sources. Highly dispersed tungsten species (isolated tetrahedral [WO 4 ] 2− along with octahedral polytungsten species) were generated at W loadings of 2.9-13.8 wt %, while at the higher loading (22.2 wt % W), WO 3 nanocrystallites were also formed [50].…”

“…Recently, Subramaniam and co-workers employed EISA to synthesize zirconium- [49], tungsten- [50], and molybdenum-containing [51] mesostructured silicates using TEOS as the silica precursor, Pluronic P123 as the templating agent and ZrOCl 2 ·8H 2 O, WCl 6 or MoCl 5 as metal sources. Highly dispersed tungsten species (isolated tetrahedral [WO 4 ] 2− along with octahedral polytungsten species) were generated at W loadings of 2.9-13.8 wt %, while at the higher loading (22.2 wt % W), WO 3 nanocrystallites were also formed [50]. Thus, obtained W-EISA materials were examined during the gas-phase reaction of ethylene and 2-butene metathesis to form propene (450 • C, 1 atm).…”