Overcoming the deleterious effect of hafnium in tungsten–zirconia catalysts: The use of doping and thermal treatments

Abstract

The effect of hafnium and aluminum addition to Ni/WO 3/ZrO 2 was examined as a function of calcination temperature for the isomerization of a 5 wt% cyclohexane in n-hexane feed. Hafnium, a common and difficult to separate impurity in zirconia, was found to decrease catalytic activity, primarily through the generation of monoclinic and cubic phases that are considered inactive for acid-catalyzed reactions. A hafnium induced monoclinic phase is formed at relatively low calcination temperatures and is fundamentally different from the thermodynamically stable monoclinic phase generated at higher temperatures. The addition of aluminum as well as higher calcination temperatures limited the formation of this monoclinic phase resulting in very active catalysts without the use of more expensive, hafnium-free catalyst precursors. Catalysts with tetragonal zirconia contents less than 40 wt% had initial isomerization activities that scaled linearly with tetragonal zirconia content. Catalysts with a tetragonal zirconia contents greater than 40% had initial n-hexane isomerization activities that were unrelated to zirconia morphology; a limitation that was a result of the hydrogenation function. The lack of correlation between zirconia crystal phases and catalytic activity for high tetragonal zirconia content catalysts implies that the measured isomerization activity may better represent the hydrogen spillover rate of nickel and not necessarily the intrinsic acid-catalyzed rate. Improved control over hydrogenation activity would allow for the preparation of more active catalysts.