A TAP REACTOR INVESTIGATION OF C6 REFORMING ON NONACIDIC AND ACIDIC SUPPORTED METAL-CATALYSTS

The reforming of C6 hydrocarbons on Pt/Mg(Al)O, Pt/K-L, Pd/Mg(Al)O, an d Pt-Re/Al2O3 (sulfided and unsulfided forms) has been investigated us ing the temporal analysis of products (TAP) reactor. Pulse experiments with several different pure C6 feeds were performed in a continuous f low of H-2 at a pressure of 1 bar and temperatures between 400 and 510 -degrees-C. Under these conditions the reaction network for all of the catalysts appeared to occur by a monofunctional metal pathway: methyl pentane --> methylcyclopentane --> n-hexane --> benzene. Evidence of p artially dehydrogenated linear C6 molecules as intermediates between n -hexane and benzene was also obtained. Turnover frequencies and benzen e selectivities for the Pt/Mg(Al)O and the Pt/KL catalysts were compar able. For the Pd/Mg(Al)O catalyst the selectivity to benzene was highe r and the turnover frequency was lower. Because the TAP technique refl ects only the activity of truly active catalytic sites, the turnover f requencies from this study (10(-12) sec-1 at 425-degrees-C on Pt) are higher than generally reported in the literature. Previous investigati ons of the Pt-Re/Al2O3 catalyst under industrial conditions have shown that the bifunctional mechanism was dominant. The monofunctional mech anism found here implies the existence of two dehydrocyclization pathw ays on these catalysts: a monofunctional metal pathway favored at low pressures, and a bifunctional pathway which is dominant at higher pres sures. (C) 1994 Academic Press, Inc.