Carbon-carbon double bond versus carbonyl group hydrogenation: Controlling the intramolecular selectivity with polyaniline-supported platinum catalysts

The use of polyaniline (PANI) as catalyst support for heterogeneous catalysts and their application in chemical catalysis is hitherto rather poorly known. We report the successful synthesis of highly dispersed PANI-supported platinum catalysts (particle sizes between 1.7 and 3.7 nm as revealed by transmission electron microscopy, TEM) choosing two different approaches, namely (i) deposition-precipitation of H2PtCl6 onto polyaniline, suspended in basic medium (DP method) and, (ii) immobilization of a preformed nanoscale platinum colloid on polyaniline (sol-method). The PANI-supported platinum catalysts were applied in the selective hydrogenation of the a,p-unsaturated aldehyde citral. In order to benchmark their catalytic performance, citral hydrogenation was also carried out by using platinum supported on the classical support materials silica (SiO2), alumina (Al2O3), active carbon and graphite. The relations of the structural characteristics and surface state of the catalysts with respect to their hydrogenation properties have been probed by EXAFS and XPS. It is found that the DP method yields chemically prepared PtO2 on polyaniline and, thus, produces a highly dispersed and immobilized Adams catalyst (in the beta-PtO2 form) which is able to efficiently hydrogenate the conjugated C=C bond of citral (selectivity to citronellal=87%), whereas reduction of the C=O group occurs with polyaniline-supported platinum (selectivity to geraniol/nerol=78%) prepared via the sol-method. The complete reversal of the selectivity between the preferred hydrogenation of the conjugated C=C or C=O group is not only particularly useful for the selective hydrogenation of cc,p-unsaturated aldehydes but also unveils the great potential of conducting polymer-supported precious metals in the field of hitherto barely investigated chemical catalysis.