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

摘要

The partial oxidation of cinnamyl alcohol to cinnamaldehyde with air in an aqueous solution has been studied over a series of Bi-Pt/alumina catalysts. The bimetallic catalysts were prepared by selective Bi deposition onto supported Pt particles of 3-4 nm. XPS and electrochemical measurements indicated that an increase in Bi/Pt-s overall ratio results in higher Bi coverage and suppresses the hydrogen sorption on Pt, without any detectable influence on the electronic state of the noble metal, The bimetallic catalysts proved to be superior to 5 wt% Pt/alumina: an enhancement in initial rate by a factor of more than 26 was observed and the cinnamaldehyde yield increased by Pi promotion from 9 to 94-96% under otherwise identical conditions, Monitoring of catalyst potential during reaction, combined with cyclic voltammetric measurements, revealed that the low activity of Pt/alumina was due to the initial, destructive adsorption of the reactant (''self-poisoning''). The formation 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 coverage of active sites enhanced the oxidative removal of surface impurities and accelerated the desired reaction. The main role of Bi promotion is a geometric (blocking) effect which decreases the size of Pt ensembles. In addition, at higher conversions Pi adatoms might behave as new active centers which adsorb OH better than Pt (bifunctional catalysis), 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 further oxidation of cinnamaldehyde. (C) 1995 Academic Press, Inc. [References: 73]