Activity, yield patterns, and coking behavior of Pt and PtRe catalysts during dehydrogenation of methylcyclohexane: I. In the absence of sulfur

Abstract

Starting with freshly reduced catalysts in a flow reactor, conversion of methylcyclohexane decreased less rapidly during a 20-h period while selectivity and yield of aromatics increased more rapidly with PtRe Al 2O 3 than with Pt Al 2O 3 . During this initial 20-h period, coke deposition was retarded on the PtRe as compared to the Pt catalysts. When the coked catalysts were then exposed to pure flowing hydrogen at reaction temperatures, the coke was partially removed from both types of catalyst. When, after such hydrogen treatment, the catalysts were again exposed to MCH feed, the different catalysts behaved quite differently. Immediately after the hydrogen treatment the selectivity and yield for aromatic products was lower for the PtRe and higher with the Pt catalyst. After shifting from pure H 2 to the hydrocarbon feed, however, the PtRe catalyst began to gain aromatic selectivity whereas the Pt catalyst began to lose such selectivity. Several hours after changing from pure H 2 to HC the selectivity of the PtRe catalyst had again become significantly greater than that of the Pt catalyst, although the situation was the converse just after the change. Treatment with pure hydrogen also increased hydrogenolysis to C 6-products in the case of PtRe but decreased this cracking in the case of Pt. As exposure to hydrocarbon continued, however, the effect of the pure hydrogen treatment was lost as the cracking increased with Pt but decreased with the PtRe catalysts. These results are interpreted in terms of a carbonaceous overlayer which does not retard the dehydroaromatization reaction in its early stages but becomes site blocking in later stages. Early deposition may be primarily at kinks at the edges of exposed crystal planes with later deposition on terraces. The catalytic action of Re may preserve only certain sites free of deactivating deposits during exposure to HC + H 2, and may catalyze removal of such deposits from certain sites during treatment in pure H 2. In pure H 2, cracking sites seem to respond more to hydrogenative reaction when Re is present but this trend seems to reverse when MCH is also present.