Influence of the Preparative Method on the Activity of Highly Acidic WOx/ZrO2and the Relative Acid Activity Compared with Zeolites

Abstract

The acid activity of the solid tungsten/zirconia can be affected by the catalyst preparation method. Reflux of the hydrous zirconia prior to tungsten impregnation gives a catalyst with higher surface area than the nonrefluxed material, 62 vs 32 m2/g, but with the same strong acid site density, as determined byn-pentane isomerization activity. It is also shown that the simultaneous coprecipitation of tungsten with the formation of hydrous zirconia yields a catalyst with about two times the strong acid site density of WOx/ZrO2catalysts prepared by impregnation. Catalytic titration experiments using 2,6-dimethylpyridine reveal a distribution of strength of Brønsted acid sites, with a strong acid site density responsible for pentane isomerization activity of these materials of approximately 0.002 meq H+/g catalyst for materials prepared by impregnation versus 0.004 meq H+/g catalyst for catalysts prepared by coprecipitation. These strongly acidic sites are estimated to be about four orders of magnitude more active for pentane isomerization than the sites present in zeolite β. XPS of chemisorbed 2,6-dimethylpyridine and pyridine on the WOx/ZrO2prepared by coprecipitation also indicates the presence of strong and weak Brønsted sites and Lewis acid sites. Weak Brønsted acid sites are predominant on the surface of the catalyst. The concentration of the different acid sites, as determined by XPS, is 0.013 meq Lewis acid sites/g catalyst, 0.011 meq strong Brønsted sites and 0.043 meq weak Brønsted sites. Further experimental work combining various characterization techniques with kinetic studies is needed to elucidate whether the Lewis acid sites play any role in the remarkable catalytic activity exhibited by these materials. The acid activity of WOx/ZrO2catalysts has a strong dependence on the tungsten loading. The acid activity, as measured byn-pentane isomerization activity, passes through a maximum at approximately 16 wt% W loading. This amount represents about two monolayers coverage, assuming tungstate oxyanions, WO4=, on the surface of zirconia and is equivalent to the optimum atomic level of SO4=needed for high acid activity suggesting that the site configuration may also be similar.