Relating n-Pentane Isomerization Activity to the Tungsten Surface Density of WOx/ZgO2

Zirconia-supported tungsten oxide (WOx/ZrO2) is considered an important supported metal oxide model acid catalyst, for which structure-property relationships have been studied for numerous acid-catalyzed reactions. The catalytic activity for xylene isomerization, alcohol dehydration, and aromatic acylation follows a volcano-shape dependence on tungsten surface density. However, WOx/ZrO2 has not been studied for more acid-demanding reactions, like n-pentane isomerization, with regard to surface density dependence. In this work, WOx/ZrO2 was synthesized using commercially available amorphous ZrOx(OH)(4-2x) and model crystalline ZrO2 as support precursors. They were analyzed for n-pentane isomerization activity and selectivity as a function of tungsten surface density, catalyst support type, and calcination temperature. Amorphous ZrOx(OH)(4-2x) led to WOx/ZrO2 (WZrOH) that exhibited maximum isomerization activity at similar to 5.2 W.nm(-2), and the crystalline ZrO2 led to a material (WZrO2) nearly inactive at all surface densities. Increasing the calcination temperature from 773 to 973 K increased the formation of 0.8-1 nm Zr-WOx clusters detected through direct imaging on an aberration-corrected high-resolution scanning transmission electron microscope (STEM). Calcination temperature further increased catalytic activity by at least two times. Bronsted acidity was not affected but Lewis acidity decreased in number, as quantified via pyridine adsorption infrared spectroscopy. WOx/ZrO2 exhibited isomerization activity that peaked within the first 2 h time-on-stream, which may be due to Zr-WOx clusters undergoing an activation process.