Platinum- and iron-doubly promoted tungstated zirconia catalyst for n-butane isomerization reaction

摘要

The catalysis of n-butane isomerization over iron-promoted tungstated zirconia (F(1.2)WZ) and platinum-iron-promoted tungstated zirconia (P/F(1.2)WZ) catalysts was studied and correlated to the catalyst characterization results. In the singly promoted F(1.2)WZ catalyst, we suggest that the iron species promotes the n-butane isomerization reaction through a redox effect. The formation of W-O-Fe linkages optimized the surface reduction of WO, thereby facilitating the formation of Bronsted acid sites necessary for the generation of carbenium ion. In addition, Fe3+ itself can also act as a redox site. In the case of the doubly promoted P/F(1.2)WZ catalyst, its catalytic activity is much higher than the sum of the singly promoted P/WZ and F(1.2)WZ catalysts. The exceptionally high activity of P/F(1.2)WZ catalyst in n-butane isomerization reaction is explained by a cooperative effect. The diffusion of activated n-butane species stabilized by iron promoter to the Bronsted acid sites created by platinum promoter will be improved as both promoters are in close proximity to each other. The interaction between iron promoter and nbutane or its activated species is shown by the influence of iron on the propane selectivity of the reaction. In addition, the close proximity of platinum and iron promoters in P/F(1.2)WZ catalyst is shown by TPR. The location and nature of iron promoter on WZ were also characterized. Results revealed that the iron promoter is located on the surface of F(1.2)WZ catalyst. XANES, EXAFS, and EPR studies suggest that the iron promoter exists as highly dispersed Fe3+ species either bound to WO, surface or located at the surface vacant sites of zirconia. In addition, EPR study indicates the presence of fine alpha-Fe2O3 Clusters on the surface of catalyst; the TPR peak at 421 degrees C is probably due to Fe-O-Fe of this species. (c) 2005 Elsevier B.V. All rights reserved.