Catalytic properties in n-heptane reforming of Pt-Sn and Pt-Ir-Sn/Al2O3 catalysts prepared by surface redox reaction

Multimetallic Pt-Ir-Sn/Al2O3 catalysts, prepared by surface redox reaction, were tested in the conversion of n-heptane, used as model reaction of paraffin dehydrocyclization, under industrial-type conditions. Their catalytic performances were measured after 5 and 65 h of reaction in order to evaluate the stability of the catalysts, and compared to those of a well-known Pt-Sn bimetallic catalyst as a function of the preparation medium, of the tin and iridium contents, of the chorine content and of the resulting acidity, and of the amount of coke deposited. The addition of tin to the monometallic Pt and to the bimetallic Pt-Ir increases the stability of the catalysts and also the selectivity toward toluene. The same toluene yield is obtained with bimetallic and trimetallic catalysts after 65 h of reaction, but less tin is needed in the case of the trimetallic catalyst. Similar evolutions between the toluene yield and the amount of Lewis acid sites were observed: the highest amount of Lewis sites, the highest toluene yield. There is no simple relationship between the amount of coke deposited after 65 h of reaction and the activity in n-heptane conversion. The coke deposition affects the strong Lewis and Brønsted acid sites. Whereas catalysts are strongly deactivated after 65 h of reaction for cyclohexane dehydrogenation at atmospheric pressure, they are still active for the n-heptane transformation at higher pressures that means that only few metallic sites take part in this reaction. It was demonstrated that the addition of an inactive promoter on a bimetallic Pt-Ir catalyst can replace the sulfiding step that is necessary to decrease the hydrogenolysis activity of this type of catalyst. However, the deposition of tin modifies not only the metal function but also the acidity of the catalyst.