APCATA 433–2012-49

WO3 monolayer loaded on ZrO2: Property–activity relationship in n-butane
isomerization evidenced by hydrogen adsorption and IR studies

The property–activity relationship of WO3 supported on ZrO2 (WZ) was evaluated in n-butane isomerization
for a series of catalysts with WO3 loading ranging from 5 to 20 wt% on ZrO2. The
catalysts were prepared by incipient-wetness impregnation of Zr(OH)4 with an aqueous solution of
(NH4)6[H2W12O40·nH2O], followed by drying and calcination at 1093 K. The introduction of WO3 continuously
increased the tetragonal phase of ZrO2, WO3 surface density and coverage. The specific surface
area and total pore volume passed through a maximum of WO3 loading at 13 wt%; this loading corresponds
to 5.9WO3/nm2 and is near the theoretical monolayer-dispersed limit of WO3 on ZrO2. The IR
results indicate that the presence of WO3 eroded the absorbance bands at 3738 and 3650cm−1 corresponding
to bibridged and tribridged hydroxyl groups up to near the monolayer-dispersed limit of WO3.
A new broad and weak band appeared, centered at 2930cm−1, indicating the presence of bulk crystalline
WO3 for WO3 coverage exceeding the theoretical monolayer-dispersion limit. In addition to the band at
2930cm−1, twoW O stretching bands were observed at about 1021 and 1014cm−1 for all WZ catalysts,
confirming the existence of W O connected to coordinative unsaturated (cus) Zr4+ through O and to
the other W through O, respectively. Pyridine adsorbed IR and NH3-TPD revealed that the presence of
WO3 modified the nature and concentration of acidic sites. The highest acidity was observed with 13 wt%
loading WO3. The decrease in the intensity of peaks due to increasing WO3 loading was much higher on
Lewis acid sites than on Brønsted 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%WO3 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 573K in which 13 wt%WO3 loading on ZrO2 yielded the highest amount
of isobutane. It is suggested that the presence of strong Lewis acid sites on monolayer-dispersed WO3
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 Brønsted acid sites could not be directly associated
with activity. In fact, no isomerization activity was observed in the absence of hydrogen.