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

The catalysis of n-butane isomerization over iron-promoted tungstated zirconia (F_1.2WZ) and platinum-iron-promoted tungstated zirconia (P/F_1.2WZ) catalysts was studied and correlated to the catalyst characterization results. In the singly promoted F_1.2WZ 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_x, thereby facilitating the formation of Brønsted 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_1.2WZ catalyst, its catalytic activity is much higher than the sum of the singly promoted P/WZ and F_1.2WZ catalysts. The exceptionally high activity of P/F_1.2WZ 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 Brønsted 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 n-butane 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_1.2WZ 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_1.2WZ catalyst. XANES, EXAFS, and EPR studies suggest that the iron promoter exists as highly dispersed Fe³⁺ species either bound to WO_x surface or located at the surface vacant sites of zirconia. In addition, EPR study indicates the presence of fine α-Fe₂O₃ clusters on the surface of catalyst; the TPR peak at 421 °C is probably due to Fe-O-Fe of this species.