Effect of pretreatment atmosphere of WO_x/SiO₂catalysts on metathesis of ethylene and 2-butene to propylene

Abstract

The effect of a gas pretreatment atmosphere (pure N 2 , pure H 2 and mixed H 2 /N 2 ) on the metathesis reaction between ethylene and 2-butene to propylene over calcined and non-calcined WO 3 /SiO 2 catalysts was investigated. The non-calcined catalysts exhibited higher activity than the calcined catalysts under different gas pretreatment atmospheres. The non-calcined catalyst with the use of pure H 2 pretreatment showed the highest catalytic performances. As revealed by various characterization results from N 2 physisorption, XRD, XPS, TEM, SEM-EDX, UV-Vis, Raman, H 2 -TPR, and NH 3 -TPD techniques, the WO 2.83 phase occurring from the H 2 pretreatment of the non-calcined catalyst played an important role on the high activity of the catalyst. In addition, better tungsten dispersion, higher isolated surface tetrahedral tungsten oxide species, and W 5+ species were obtained on the H 2 -treated non-calcined WO 3 /SiO 2 catalyst.

Effects of calcination and pretreatment temperatures on the catalytic activity and stability of H2-treated WO3/SiO2 catalysts in metathesis of ethylene and 2-butene

“…Two absorption bands at 230 and 270 nm were observed in the spectra for all the catalysts (inset gure), whereas the adsorption bands between 400 and 800 nm were also observed for all the catalysts. According to the literatures, 8,15,17 the absorption bands at 230 nm could be assigned to isolated tetrahedral [WO 4 ] 2À species, while the absorption bands at 270 nm corresponded to octahedral polytungstate species. The adsorption bands between 400 and 800 nm could be assigned to W 4+ and W 5+ species.…”

“…In addition, these bands of the non-calcined catalysts were broader than the calcined catalysts, indicating that higher oxygen vacancies occurring on the WO 3 crystalline. 8 Considering the Raman band at 970 cm À1 of the calcined and non-calcined catalysts, it was found that this band of the non-calcined catalyst showed more obvious peaks than that of the calcined samples, indicating more isolated surface tetrahedral tungsten oxide species as also conrmed by the UV-Vis results. In addition, higher calcination temperature caused low intensity of the Raman band at 970 cm…”

“…8,[10][11][12][13][14][15] Among them, it was reported that the tungsten dispersion, tetrahedral tungsten oxide species, intermediate WO 3Àx , or W 5+ species are the active sites for metathesis, [16][17][18][19] and that WO 3 crystals are not active in metathesis and catalyst sites should be contained in the amorphous surface. 8,20 At present, the activation by pretreatment conditions to improve performance of supported WO x /SiO 2 catalysts in metathesis reaction has received much attention because of pre-activating the active sites of catalysts to form the activated state before running the reaction. 21,22 Pretreatment of supported WO 3 /SiO 2 catalysts with CO 2 , H 2 or N 2 gas at around 600 C could improve the propylene self-metathesis activity.…”

“…The ammonia desorption peaks of the non-calcined WO 3 /SiO 2 catalysts occurred at higher temperature than the calcined catalysts, indicating the higher acid strength as stronger acidity. 8,32 In addition, increasing the pretreatment temperature caused the ammonia desorption peak to slightly shi to lower temperature, indicating lower acid strength.…”

“…9), which was an indicative of the relative amount of isolated surface tetrahedral tungsten oxide species (active sites) to WO 3 crystal (non-active sites). 8,33 The high pretreatment temperature exhibited the higher I 970 /I 805 ratio in both calcined and non-calcined catalysts and the H 2 pretreatment at 650…”