Nitrogen influence on aromatic selectivity during n-heptane reforming on fresh and deactivating platinum/alumina and platinum–rhenium/alumina catalysts

The aromatization of n-heptane on Pt-containing reforming catalysts was significantly enhanced in either nitrogen alone or nitrogen-containing diluent gas. This study was carried out on fresh Pt– and Pt–Re/Al2O3 catalysts in a microcatalytic reactor at temperatures between 420 and 500°C, W/F between 0.83 and 3.75 mg min−1 ml−1 and at a total pressure of 3.92 bar. These studies were complimented with others on a decaying Pt/Al2O3 catalyst. Results obtained suggest fundamental differences between the two catalysts and also with the choice of carrier gas. On Pt/Al2O3 catalyst in N2 carrier, aromatization with complete demethylation is obtained at all temperatures, except at 420°C where demethylation is found to be incomplete. In contrast, in H2 carrier, demethylation is negligible on the Pt/Al2O3 catalyst. With the Pt–Re/Al2O3 catalyst, reaction in H2 predominantly produces methane while reaction in N2 appears to impart intrinsic aromaticity to the bimetallic catalyst albeit with an attenuation of the overall activity. In H2–N2 mixtures, demethylation activity seems to decrease with increasing H2 content on Pt/Al2O3 catalyst and found to be quite low at 50% H2. The threshold of the introduction of intrinsic aromaticity on the Pt–Re/Al2O3 was found to occur at 50% N2; below this value, the bimetallic catalyst reverts to a complete cracking catalyst. The aromatization on a deactivating Pt/Al2O3 catalyst was successfully modeled with first order reaction and deactivation rates.