H-spillover and SMSI effects in paraffin hydroisomerization over Pt/WOx/ZrO2 bifunctional catalysts

The Pt-modified WOx/ZrO2 catalyst system is found to undergo various transformations in the presence of hydrogen, These transformations include the generation of spillover hydrogen at room temperature, the development of a strong Pt-W interaction at elevated temperatures, and tungsten reduction. We have characterized these effects using hydrogen chemisorption and temperature-programmed reduction techniques. We have also employed pentane isomerization as a test reaction to relate these changes to the acid catalysis of these materials. Our findings indicate that all of the above transformations are sensitive to tungsten loading. Room temperature hydrogen spillover is observed at 15.9 wt% tungsten, but is absent at tungsten loadings of less than or equal to 8.4 wt%, The higher tungsten loading catalyst exhibits a Pt-W SMSI effect in hydrogen at >200 degrees C; at lower tungsten loadings much higher temperatures are required to induce this effect, This result agrees with the increasing reducibility of tungsten oxospecies at higher loadings. Platinum greatly facilitates the tungsten reduction, is involved in a reversible Pt-W strong metal interaction, and likely enhances hydrogen spillover during pentane isomerization. By facilitating tungsten reduction, Pt can have a deleterious effect on the high acid activity of these WOx/ZrO2 catalysts. (C) 1999 Academic Press.