Pt-Sn/Al2O3 sol-gel catalysts: Catalytic properties

Platinum-tin supported catalysts at various tin and platinum contents were prepared by the sol-gel method by adding tetrabutyltin to a homogeneous solution containing aluminum tri-sec-butoxide (TEA). After gelation, the solids were dried, calcined and then impregnated with an aqueous solution of hexachloroplatinic acid. Catalysts prepared by this method show specific BET areas around 270 m(2)/g and platinum dispersion up to 90%. Ammonia adsorption shows that there are less acidic sites in tin-alumina sol-gel preparations than in sol-gel alumina. Activity measurements for n-heptane dehydrocyclization show that Pt-Sn sol-gel catalysts are more active than catalysts prepared by successive impregnation of a commercial alumina. Moreover, selectivity to benzene formation was found 54% lower with sol-gel catalysts than with commercial based catalysts. A high tolerance to thiophene poisoning during n-heptane dehydrocyclization was also found. Additional catalytic tests for cyclohexane dehydrogenation, isopropanol dehydration and toluene dealkylation were performed. High selectivity to acetone in the dehydration reaction and very low activity for toluene dealkylation were found for sol-gel preparations. The high activity in n-heptane dehydrocyclization and the high selectivity to acetone in Pt-Sn-alumina sol-gel catalysts was interpreted as a result of the modification by tin of the support acidity. Nevertheless, the tolerance to thiophene poisoning and the low dealkylation rate observed for sol-gel preparations can only be explained by an important tin-alumina support effect on platinum activity.