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 homog eneous solution containing aluminum tri-sec-butoxide (TEA). After gela tion, the solids were dried, calcined and then impregnated with an aqu eous solution of hexachloroplatinic acid. Catalysts prepared by this m ethod show specific BET areas around 270 m(2)/g and platinum dispersio n 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 impre gnation of a commercial alumina. Moreover, selectivity to benzene form ation was found 54% lower with sol-gel catalysts than with commercial based catalysts. A high tolerance to thiophene poisoning during n-hept ane dehydrocyclization was also found. Additional catalytic tests for cyclohexane dehydrogenation, isopropanol dehydration and toluene dealk ylation were performed. High selectivity to acetone in the dehydration reaction and very low activity for toluene dealkylation were found fo r sol-gel preparations. The high activity in n-heptane dehydrocyclizat ion and the high selectivity to acetone in Pt-Sn-alumina sol-gel catal ysts was interpreted as a result of the modification by tin of the sup port acidity. Nevertheless, the tolerance to thiophene poisoning and t he low dealkylation rate observed for sol-gel preparations can only be explained by an important tin-alumina support effect on platinum acti vity.