DEACTIVATION MODEL WITH RESIDUAL ACTIVITY TO STUDY THIORESISTANCE AND THIOTOLERANCE OF NAPHTHA REFORMING CATALYSTS

The relative sensitivity to sulfur poisoning of typical commercially employed naphtha reforming catalysts was studied using a cyclohexane dehydrogenation as test reaction and thiophene as the poisoning molecule. Monometallic Pt/Al2O3 and bimetallic Pt-Re/Al2O3, Pt-Ir/Al2O3, Pt-Ge/Al2O3, and Pt-Sn/Al2O3 catalysts were used. A deactivation model with residual activity (DMRA) was employed for determining both the thiotolerance and the thioresistance of the catalysts. The DMRA model was developed by using Langmuir-Hinshelwood kinetics and assuming the rate-determining step in the poisoning mechanism to be reversible. The thiotolerance was in the order Pt-Ge > Pt-Ir congruent-to Pt congruent-to Pt-Sn > Pt-Re. According to DMRA equations, the thiotolerance decreases when K(s) the adsorption equilibrium constant of H2S on the catalysts, increases. This DMRA model prediction was verified by measuring the quantities of total and irreversibly held sulfur following exposure of the catalYSts to H-2/H2S mixtures. The thioresistance decreased in the sequence Pt-Ge > Pt-Ir congruent-to Pt > Pt-Re > Pt-Sn. From DMRA equations it was established that the thioresistance is primarily a function of k(p), the reaction rate constant for the hydrogenolysis of adsorbed thiophene; the higher the k(p) value, the lower the catalyst thioresistance. Bimetallic Pt-Ge/Al2O3 was the most thioresistant and thiotolerant catalyst. This superior performance is explained by assuming that upon reduction at 773 K a fraction of the Ge cations is reduced and forms bimetallic Pt-Ge particles. The formation of Pt-Ge clusters increases the electrophilic character of platinum, thereby weakening the strength of the S-Pt bond and reducing the amount of irreversibly held sulfur on platinum. (C) 1994 Academic Press, Inc.