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 5.9 wo3/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 3650 cm⁻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 2930 cm⁻¹, indicating the presence of bulk crystalline wo3 for wo3 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) 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.