Hydrogen Transfer Catalysis by Platinum on Zeolites

Platinum catalysts supported on gamma-alumina and on a variety of zeolites have been characterized and evaluated for activity in the transfer hydrogenation of propylene using butanes as the hydrogen source. Zeolite supports evaluated include both aluminosilicates and borosilicates having the ZSM-5 and beta structures. Platinum on acid zeolites shows enhanced activity for the hydrogen transfer, compared to platinum supported on alumina. However, with platinum on acidic zeolites, propylene hydrogenation is accompanied by a number of competing, acid-catalyzed reactions which lower the selectivity to propane and butenes. Of the acid zeolites studied, low aluminum, borosilicates were found to be more selective than the corresponding aluminosilicates of similar structure. Partial neutralization of the zeolitic acidity with alkali cations lowered the nonselective acid catalysis. Metal-catalyzed hydrogen transfer activity was not diminished by this partial neutralization so that hydrogen transfer selectivity increased. In the case of the borosilicate-supported catalysts, maximum activity near 90% conversion of propylene to propane and selectivity for hydrogen transfer greater than 95% were obtained when sufficient alkali was added to neutralize the acidity associated with framework aluminum while maintaining the acidity associated with boron. Feed studies with an optimized catalyst provide additional mechanistic insights.