Effect of metals on the catalytic activity of sulfated zirconia for light naphtha isomerization

We studied on the function of the metal in the sulfated zirconia(SO ²⁻₄ /ZrO₂) catalyst for the isomerization reaction of light paraffins. The addition of Pt to the SO ²⁻₄ /ZrO₂ carrier could keep the high catalytic activity. The improvement in this isomerization activity is because Pt promotes removal of the coke precursor deposited on the catalyst surface. Though this catalytic function was observed in other transition metals, such as Pd, Ru, Ni, Rh and W, Pt exhibited the highest effect among them. It was further found that the Pd/SO ²⁻₄ /ZrO₂–Al₂O₃ catalyst possessed a catalytic function for desulfurization of sulfur-containing light naphtha in addition to the skeletal isomerization. The sulfur tolerance of catalyst depended on the method of adding Pd, and the catalyst prepared by impregnation of the SO ²⁻₄ /ZrO₂–Al₂O₃ with an aqueous solution of Pd exhibited the highest sulfur tolerance.

Further, we investigated the improvement in sulfur tolerance of the Pt/SO ²⁻₄ /ZrO₂–Al₂O₃ catalyst by impregnation of Pd. The results of EPMA analysis indicated that this catalyst was a hybrid-type one (Pt/SO ²⁻₄ /ZrO₂–Pd/Al₂O₃) in which Pt/SO ²⁻₄ /ZrO₂ particles and Pd/Al₂O₃ particles adjoined closely. This hybrid catalyst possessed a very high sulfur tolerance to the raw light naphtha that was obtained from the atmospheric distillation apparatus, although this light naphtha contained much sulfur. We assume that such a high sulfur tolerance in the hybrid catalyst is brought about by the isomerization function of Pt/SO ²⁻₄ /ZrO₂ particles and the hydrodesulfurization function of Pd/Al₂O₃ particles. Besides, since the hybrid catalyst also provides high catalytic activity in the isomerization of HDS light naphtha, we suggest that the Pd/Al₂O₃ particles supply atomic hydrogen to the Pt/SO ²⁻₄ /ZrO₂ particles by homolytic dissociation of gaseous hydrogen and also enhance the sulfur tolerance of Pt/SO ²⁻₄ /ZrO₂ particles. Finally, we also propose the most suitable location of Pd and Pt in the metal-supported SO ²⁻₄ /ZrO₂–Al₂O₃ catalyst.