The role of nitrogen in facilitating methylcyclohexane aromatization on Pt/Al2O3 and Pt-Re/Al2O3 catalysts

The kinetics of methyleyelohexane aromatization on commercial Pt/ Al2O3 and Pt-Re/Al2O3 catalysts was investigated in a micro-reactor using N-2 and/or H-2 as carrier gases at temperatures ranging between 300-500degreesC, W/F values ranging between 0.83-3.75 mg min/mL and at a total pressure of 4.0 kg/cm(2). On both catalysts in N-2 atmosphere, aromatization accompanied by demethylation was observed with the formation of cracked products, benzene and toluene. However, in H-2 methane was the predominant product of methylcyclohexane reforming on PtAl2O3 and Pt-Re/Al2O3 at 500degreesC and 400-500degreesC respectively, whereas at 350degreesC, aromatization was predominant on Pt/Al2O3 but on Pt-Re/Al2O3, aromatization was accompanied by fragmentation to methane. In N-2-H-2 mixtures, demethylation activity was observed to decrease with H-2 content of the mixture on Pt-Re/Al2O3. A preliminary test of the kinetic data using Sica's method of pulse kinetic analysis suggests a first order rate in methylcyclohexane with activation energies of 3.21 kcal/gmol in N-2 and 19.70 kcal/gmol in H-2 for the Pt/Al2O3 catalyst and 16.66 kcal./gmol in N-2 and 34.94 kcal/gmol in H-2 for the Pt-Re/ Al2O3 catalyst. However, a more comprehensive kinetic analysis suggested an aromatization mechanism for Pt-Re/Al2O3, where adsorbed H-2 was a participant. A different aromatization mechanism for the reaction in N-2 where hydrogen was not needed explained the data on Pt/Al2O3. In both cases, the desorption of toluene was determined as the rate determining step.