WO3 monolayer loaded on zro2: property-activity relationship in n-butane isomerization evidenced by hydrogen adsorption and ir studies

Abstract

The property-activity relationship of WO 3 supported on ZrO 2 (WZ) was evaluated in n-butane isomerization for a series of catalysts with WO 3 loading ranging from 5 to 20 wt% on ZrO 2. The catalysts were prepared by incipient-wetness impregnation of Zr(OH) 4 with an aqueous solution of (NH 4) 6[H 2W 12O 40·nH 2O], followed by drying and calcination at 1093 K. The introduction of WO 3 continuously increased the tetragonal phase of ZrO 2, WO 3 surface density and coverage. The specific surface area and total pore volume passed through a maximum of WO 3 loading at 13 wt%; this loading corresponds to 5.9 WO 3/nm 2 and is near the theoretical monolayer-dispersed limit of WO 3 on ZrO 2. The IR results indicate that the presence of WO 3 eroded the absorbance bands at 3738 and 3650 cm -1 corresponding to bibridged and tribridged hydroxyl groups up to near the monolayer-dispersed limit of WO 3. A new broad and weak band appeared, centered at 2930 cm -1, indicating the presence of bulk crystalline WO 3 for WO 3 coverage exceeding the theoretical monolayer-dispersion limit. In addition to the band at 2930 cm -1, two WO stretching bands were observed at about 1021 and 1014 cm -1 for all WZ catalysts, confirming the existence of WO connected to coordinative unsaturated (cus) Zr 4+ through O and to the other W through O, respectively. Pyridine adsorbed IR and NH 3-TPD revealed that the presence of WO 3 modified the nature and concentration of acidic sites. The highest acidity was observed with 13 wt% loading WO 3. The decrease in the intensity of peaks due to increasing WO 3 loading was much higher on Lewis acid sites than on Bronsted acid sites. Hydrogen adsorption isotherms and the IR results for hydrogen adsorption on preadsorbed pyridine were used to evaluate the formation of active protonic acid sites from molecular hydrogen. The catalyst with 13 wt% WO 3 loading showed the maximum hydrogen uptake capacity and formation of protonic acid sites. These results show a direct correlation with the activity of WZ in n-butane isomerization at 573 K in which 13 wt% WO 3 loading on ZrO 2 yielded the highest amount of isobutane. It is suggested that the presence of strong Lewis acid sites on monolayer-dispersed WO 3 facilitates the formation of protonic acid sites from hydrogen in the gas phase which act as active sites in n-butane isomerization. The presence of permanent Bronsted acid sites could not be directly associated with activity. In fact, no isomerization activity was observed in the absence of hydrogen