Opposite activation conditions of acid and metal functions of Pt/SO 42−-ZrO 2 catalysts

Abstract

The influence of the thermal treatments of Pt/SO 4 2−-Zr(OH) 4 catalysts on the activity for the metal-catalyzed reaction of cyclohexane dehydrogenation and the acid-catalyzed reaction of n-butane isomerization, were studied in this work. A mutual antagonism between the conditions for optimal activity of the acid and metal functions was found and was seemingly related to the crystallization of the support. In order to be able to isomerize n-butane, SO 4 2−-Zr(OH) 4 had first to be calcined in air at T calc>400 °C. The onset of activity and strong acid properties coincided with the appearance of the tetragonal crystal phase. SO 4 2−-Zr(OH) 4 supported Pt, prepared from chloroplatinic acid, was tried to be converted to the metal state (Pt 0) in order to have full catalytic capacity. When Pt/SO 4 2−-Zr(OH) 4 was first calcined in air at T calc>400 °C, Pt remained in a seemingly oxidized state, with no de/hydrogenation properties even after reduction in H 2 at 300 °C. Under certain conditions, Pt metal properties were improved: (i) calcining Pt/SO 4 2−-Zr(OH) 4 in air at T calc<400 °C; (ii) calcining Zr(OH) 4 at T calc>400 °C before sulfating the support; and (iii) calcining Pt/SO 4 2−-Zr(OH) 4 in N 2 instead of air. In these cases, though Pt dehydrogenation activity increased, the activity of the acid function decreased (iii) or was practically null ((i) and (ii)). The support was amorphous in case (i) and mainly monoclinic in case (ii). Sulfate loss and conversion into the monoclinic phase occurred in case (iii). As compared to sulfate-free Pt/ZrO 2, sulfur poisoning always decreased the metal activity of sulfated catalysts but the decrease was higher for mainly tetragonal sulfate-doped catalysts. The final conclusion is that the optimum activation conditions for the metal and acid functions in Pt/SO 4 2−-Zr(OH) 4 are mutually excluding. The deleterious effect of SO 4 2−-ZrO 2 (SZ) on Pt metal activity is closely related to the growth and/or the presence of the tetragonal phase and cannot be prevented if a high activity of the acid function is demanded by the reacting system.