(574a) Citral Hydrogenation over Nipt and Cupt Dilute Alloy Catalysts

The selective hydrogenation of α,β-unsaturated aldehydes is used in fine chemical production, including a variety of pharmaceuticals and flavorings. However, achieving selective formation of unsaturated alcohols is challenging, since hydrogenation of the C=C bond is thermodynamically favored over hydrogenation of the C=O bond. Here, we investigate the selective hydrogenation of citral, which contains both isolated and conjugated C=C bonds in addition to the C=O bond. To elucidate the role that different alloy catalysts and Pt structures have on citral hydrogenation performance, we have synthesized NiPt and CuPt dilute alloy catalysts.

These alloy catalysts were characterized using in situ and ex situ techniques and it was determined that Pt is well diluted in both metal systems. CuPt catalysts have Pt preferentially near the nanoparticle surface, while the NiPt catalysts have Pt more evenly distributed across the nanoparticles. The dilute nature of these materials suggests that the Pt does not electronically influence the Ni or Cu, and the alloy primarily influences the structural nature of the two metals.

Citral hydrogenation reactions were carried out over the host and alloy catalysts. The addition of Pt to the Cu/SiO₂ and Ni/SiO₂ catalysts increases the rate of citral conversion due to an additive, rather than synergistic effect. The alloy catalysts, however, significantly change the observed selectivity. Ni/SiO₂ and NiPt/SiO₂ catalysts are primarily selective to citronellal, an undesired product, due to the predominantly Ni surface of the nanoparticles. The Cu-containing catalysts increase the selectivity to the desired unsaturated alcohols, with 29% selectivity over Cu/SiO₂ increasing to 47% over CuPt/SiO₂.

We observe rate enhancements and selectivity changes for citral hydrogenation over Pt containing alloy catalysts compared to the monometallic Cu and Ni catalysts. These catalysts offer uniform active site structures that enable us to quantify the influence of the alloy structure on the observed activity and selectivity.