Effect of High‐Temperature Calcination on the Generation of Brønsted Acid Sites on WO₃/Al₂O₃

Abstract

The acid properties of a series of alumina‐supported tungsten oxide (WO3/Al2O3) catalysts with loadings of 5–50 wt % WO3 calcined at various temperatures were investigated by acid‐catalyzed reactions (benzylation of anisole and isomerization of α‐pinene) and FTIR spectroscopy. The relationships between acid properties, structures, and catalytic performances were evaluated. Both the catalytic activity and amount of Brønsted acid sites depend on the calcination temperature and WO3 loading. High‐temperature calcination (1123 K) generated Brønsted acid properties, and 20 wt % WO3/Al2O3 calcined at 1123 K exhibited the highest activity among the catalysts tested. The activities for the benzylation of anisole and α‐pinene isomerization over WO3/Al2O3 calcined at 1123 K were proportional to the Brønsted acidity, which indicates that these reactions occurred on the Brønsted acid sites. Tungsten oxide, which has distorted octahedral symmetry, was loaded as 2 D monolayer domains below 20 wt %, and these domains covered most of the alumina surface at 20 wt %. If the WO3 loading was sufficient to form 2 D tungsten oxide monolayer sheets (>20 wt %), some of the Brønsted acid sites on WO3/Al2O3 were obscured by monoclinic WO3 that has no Brønsted acid sites, which resulted in a decrease of the catalytic activity. This suggests that Brønsted acid sites are generated at the boundaries between tungsten oxide monolayer domains.

Facilitating Pt−WOx Species Interaction for Efficient Glycerol Hydrogenolysis to 1,3‐Propanediol

“…The signal of CO adsorption was not observed on WO x / α ‐Al 2 O 3 , suggesting that the appeared peaks should be originated from CO adsorbed on two different Pt species. For Pt/ α ‐Al 2 O 3 , the dominant band is located at 2082 cm −1 , corresponding to CO linearly absorbed on well‐coordinated Pt nanoparticles [39,42,43] . While the weak bands at 2131 and 1835 cm −1 are ascribed to CO linearly absorbed on positively charged Pt species and the bridge‐bonded CO on larger platinum nanoparticles, respectively [42,44,45] .…”

Highly active and durable WO3/Al2O3catalysts for gas-phase dehydration of polyols

“…A series of WO 3 -loaded catalysts were prepared by impregnation of supports with an aqueous solution of (NH 4 ) 10 W 12 O 42 , dried at 353 K for 6 h, and then calcined at 1123 K for 3 h in owing air. 55,56 The surface coverages of Al 2 O 3 , ZrO 2 and TiO 2 with a WO 3 monolayer were estimated by using the cross-sectional area of a WO 6 octahedral unit (0.22 nm À2 ) 61 and were almost 100% at 20, 10 and 2.5 wt% WO 3 loading, respectively. 60…”

“…In the course of our investigation of the relationship between the structure of a two-dimensional tungsten oxide monolayer supported on metal oxides and their performance as acid catalysts, 55,56 we reported that supported WO 3 catalysts showed high activity for selective biomass conversion, such as the hydrogenolysis of glycerol and tetrahydrofurfuryl alcohol. [57][58][59][60] These results encouraged us to further explore the application of selective conversion of biomass-derived chemicals over supported WO 3 catalysts, in particular polyols to give value-added chemicals.…”

Local structure investigation of WO<i>_x</i> cluster modified on titanium-substituted hydroxyapatite for promoting charge separation under UV illumination

“…From these results, details of the WO x cluster structure can be discussed. Kitano et al 9) reported that the WO x cluster modified on Al 2 O 3 surface has a twodimensional monolayer structure. For this reason, we assume that WO x monolayer is modified on the Ti-HAp surface.…”