Platinum-tungstated zirconia isomerization catalysts -: Part II.: Effect of platinum and tungsten loading on the mechanism of isomerization of n-hexane:: a kinetic study

The effect of platinum (0.1-1.5 wt%) and tungsten (14-34 wt%) concentrations on the catalytic properties of platinum-tungstated zirconia catalysts for n-hexane isomerization was investigated at 523 K. The influence of hydrogen over a wide range of pressures (2-45 bar) was studied to elucidate the mechanism. The tungstated zirconia samples were prepared by bringing a silica-modified hydrous zirconia into contact with metatungstate, followed by extrusion with an alumina binder, and finally calcination at 1023 K. At constant tungsten loading, the activity and stability increase with Pt concentration and then stabilize. Reaction orders were about 1 for n-hexane and strongly negative (-0.8) for hydrogen. Kinetic modeling agrees with a classical bifunctional metal-acid mechanism. The tungsten content increased the isomerization activity up to the tungsten monolayer; the isomerization activity then decreased. This variation was related to the concentration of Bronsted acid sites created by the polytungstate, species. Changes in selectivity for the dibranched isomers were related to the metal-to-acid ratio. (c) 2005 Elsevier Inc. All rights reserved.