SELECTIVE OXIDATION OF CINNAMYL ALCOHOL TO CINNAMALDEHYDE WITH AIR OVER BI-PT ALUMINA CATALYSTS/

The partial oxidation of cinnamyl alcohol to cinnamaldehyde with air i n an aqueous solution has been studied over a series of Bi-Pt/alumina catalysts. The bimetallic catalysts were prepared by selective Bi depo sition onto supported Pt particles of 3-4 nm. XPS and electrochemical measurements indicated that an increase in Bi/Pt-s overall ratio resul ts in higher Bi coverage and suppresses the hydrogen sorption on Pt, w ithout any detectable influence on the electronic state of the noble m etal, The bimetallic catalysts proved to be superior to 5 wt% Pt/alumi na: an enhancement in initial rate by a factor of more than 26 was obs erved and the cinnamaldehyde yield increased by Pi promotion from 9 to 94-96% under otherwise identical conditions, Monitoring of catalyst p otential during reaction, combined with cyclic voltammetric measuremen ts, revealed that the low activity of Pt/alumina was due to the initia l, destructive adsorption of the reactant (''self-poisoning''). The fo rmation and irreversible adsorption of byproducts were suppressed but not eliminated by Bi promotion. Even the bimetallic catalysts were in a partially oxidized state when conversion exceeded 10-15%. There was no sign of ''oxygen poisoning.'' On the contrary, a partial oxygen cov erage of active sites enhanced the oxidative removal of surface impuri ties and accelerated the desired reaction. The main role of Bi promoti on is a geometric (blocking) effect which decreases the size of Pt ens embles. In addition, at higher conversions Pi adatoms might behave as new active centers which adsorb OH better than Pt (bifunctional cataly sis), The good selectivity (up to 98.5%) is due to (i) the negligible hydration of cinnamaldehyde, and to (ii) Pi promotion which decreases the catalyst potential during alcohol oxidation and hinders the furthe r oxidation of cinnamaldehyde. (C) 1995 Academic Press, Inc.