Carbon‐Carbon Double Bond versus Carbonyl Group Hydrogenation: Controlling the Intramolecular Selectivity with Polyaniline‐Supported Platinum Catalysts

Abstract

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 H 2 PtCl 6 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,b-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 (SiO 2 ), alumina (Al 2 O 3 ), 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 PtO 2 on polyaniline and, thus, produces a highly dispersed and immobilized Adams catalyst (in the b-PtO 2 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 polyanilinesupported 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 a,b-unsaturated aldehydes but also unveils the great potential of conducting polymer-supported precious metals in the field of hitherto barely investigated chemical catalysis.

Einfluss ionischer Flüssigkeiten mit funktionalisierten Kationen auf die palladiumkatalysierte Flüssigphasenhydrierung von Citral

“…So können beispielsweise neben der Wahl des Aktivmetalls [1][2][3] ggf. auch Zweitmetalle [4], Trägermaterialien [5] oder Lösungsmittel [6,7] das Selektivitäts-muster beeinflussen. Studien zur Citralhydrierung an Palladium/Träger-Kontakten zeigten die bevorzugte Citronellalbildung mit anschließender Folgereaktion zu Dihydrocitronellal als wesentliche Reaktionsfolge auf (Schema 1) [1,8,9].…”

Application of Conjugated Polymer–Platinum Group Metal Composites as Heterogeneous Catalysts

“…Of the two reactions possible in this process, i.e., carbonyl groups and carbon-carbon double bonds hydrogenation, this catalyst has shown enhanced selectivity towards carbonyl groups hydrogenation products when compared with PtO 2 dispersed on PANI [17].…”

“…This proves that PPyRh, PPy-Pt and PANI-Pt composites are very selective catalyst. It is worth noting that the selectivity of the composites in isopropyl alcohol oxidation exceeds 90% in most cases ( Table 2) which is more than that found in ,e.g., hydrogenation of citral carbonyl groups over PANI-Pt (75% or 78% depending on the metal load in the catalyst) [17].…”

“…Although, as has been already mentioned, application of chemically prepared PANI-Pt or PPy-Pt composites as heterogeneous catalysts has been scarcely studied, catalytic properties of PANI-Pt system in hydrogenation of a,b-unsaturated aldehyde (citral) have been reported recently [17]. Of the two reactions possible in this process, i.e., carbonyl groups and carbon-carbon double bonds hydrogenation, this catalyst has shown enhanced selectivity towards carbonyl groups hydrogenation products when compared with PtO 2 dispersed on PANI [17].…”

Chemoselective hydrogenation of cinnamaldehyde at atmospheric pressure over combustion synthesized Pd catalysts

“…HCAL has been reported to be an important intermediate in the preparation of pharmaceuticals used in the treatment of AIDS [8]. The selectivity and catalytic activity are influenced by several factors including reaction conditions and operation mode (gas or liquid phase hydrogenation), electronic and geometric structures of the metal catalysts, type of catalyst supports, catalyst preparation and activation procedures [9][10][11][12][13] and the polarity of solvent [14,15].…”