WOx/ZrO2 catalysts Part 2.: Isomerization of n-butane

The activity of WOx/ZrO2 (ZW) catalysts for n-butane isomerization was investigated in the temperature range 423-673 K. Reference samples of crystalline WO3 and ZrO2 displayed low catalytic activity. When ZW catalysts were calcined at 873 or 1073 K, samples prepared from amorphous hydrous zirconia showed activity levels-both on areal rate and turnover frequency per total tungsten bases-that were I order of magnitude higher than those of the samples prepared from crystalline zirconia. The higher the calcination temperature in the range 873-1073 K the higher was the activity. The deactivation due to coking was fast, but treatment with flowing oxygen at 773 K completely restored the catalytic activity for several cycles. The activity decrease with time on stream was satisfactorily described up to 623 K by the empirical equation r = at(-n). Active isomerization sites in the catalysts produced coke (average composition H/C = 1.5). The presence of hydrogen in the reactant stream substantially decreased the deactivation rate, most probably by hindering the dehydrogenation reactions which ultimately lead to coke formation. Treating catalysts in hydrogen at 673 or at 443 K before the reaction shortened the induction period, suggesting that hydrogen adsorption had a role in generating hydroxyls active for the isomerization (Bronsted acid sites). The activity trend with surface density of tungsten indicated a polynuclear isomerization site. (C) 2002 Elsevier Science B.V. All rights reserved.