The effects of sulfur contained in alumina support were investigated on the catalytic behaviors. Commercial alumina support from Johnson Matthey (JM-alpha) with sulfur below 0.05wt% was chosen. The alumina was pretreated before using; dried at 120℃ for 4 hours (denoted as "JM") and calcined at 800℃ for 12 hours (denoted as "JC"). The platinum catalysts (PtJM and PtJC) were prepared through impregnation on the JM and JC support, and also the platinum catalyst (PtST) was prepared on the sulfur-free support (ST). These catalysts were used for liquid-phase hydrogenation of p-chloronitrobenzene. It was observed that the effect of the reduction temperature on catalytic activity was different with the characteristics of the supports used: The activity of the sulfur-free PtST was almost constant or showed a maximum at 600℃. The activity of PtJM increased in sigmoid shape with temperature, and PtJC showed a minimum at 600℃ and increases with temperature. These phenomena could be explained in terms of the types and the concentration of sulfur on the supports.
The sulfur in the supports was desulfurized under $H_2$ flow at a temperature higher than 600℃, showing good thermal stability. From chemical analysis, it was found that the sulfur in JC was more oxidized than that in JM. Thereto, the weight of JM increased in thermal gravimetric analysis(TGA) under $O_2$ flow at above 300℃ due to the reaction of the unsaturated sulfur with $O_2$, while the saturated sulfur in JC did not show any change in weight.
The capabilities of $H_2$ chemisorption in PtJM and PtJC were suppressed after reduction treatment at high temperature. The sulfur could be removed from PtJM and PtJC in the forms of $SO_2$ at 950℃ and $H_2S$ above 600℃ by flowing He and $H_2$, respectively. These temperatures for desulfurization were too high to postulate possible interactions between the sulfur and Pt. XPS results also confirmed no direct interaction between the sulfur and Pt. According to a theory of redox metal-support interaction (RMSI) proposed by Kunimori, the sulfur is presumed to act as a promoter for RMSI without any direct interaction with platinum. In this study, RMSI was diminished by the calcination or by the contact with air or nitro-compound. A model equation (eq'n (7)) was proposed to explain the behavior of sulfur in catalysts.
The change in the activity of PtJM with reduction temperature might be associated with the effect of sulfur concentration on the catalyst surface. The concentration of sulfur on the surface of catalysts was decreased with reduction temperature. On the other hand, the activity change of PtJC with reduction temperature could not be explained by the effect of sulfur concentration. The sulfur in PtJC was non-poisonous but becoming poisonous by being reduced at high temperature until 600℃.
The suppression was postulated to be caused by the presence of unsaturated sulfur in the support and/or by strong interaction between Pt and alumina in catalysts where the sulfur had been removed. Therefore, the extents of the recovery of the suppressed adsorption in the two situations were quite different. In the latter situation, the suppression was presumed to be recovered more in degree by oxidative treatment.
The liquid-phase hydrogenation of aromatic nitro compounds was explained by the mechanism proposed by Haber. A side reaction, hydrodechlorination occurred at the beginning stage and the reaction rate slowed down with reaction time. Even though the nitro compounds was supplied further, the concentration of side product could be kept constant with time. The selectivity decreased monotonously with the amount of catalysts used. It seems obvious that the site for the side reaction decreases with reaction time. It was confirmed experimentally that this observation is not attributed to chlorine from the side reaction but to the intermediates of the reaction.
