The influence of the structural characteristics of the support in the liquid-phase reduction of trans-4-phenyl-3-buten-2-one or benzalacetone (C6H5-CH=CH-CO-CH3) to the corresponding alpha, beta-unsaturated alcohol (UA) on gold-supported iron oxide catalysts was investigated. Commercial goethite [FeO(OH)], maghemite (gamma Fe2O3), and hematite (alpha Fe2O3), along with an iron oxy-hydroxide prepared by precipitation, were used as supports. The catalytic behavior of Au/Fe2O3 reference catalyst (supplied by the World Gold Council) was also investigated. Gold-supported catalysts and the parent supports were extensively characterized by BET, X-ray diffraction, temperature-programmed reduction, and transmission electron microscopy. On all gold catalysts except the Au/Fe2O3 reference, the catalytic activity increased with increasing gold content. The lowest activity of reference with respect to the catalyst with a similar gold loading has been ascribed to the different sample pre-treatments. Samples prepared by us were reduced under very mild conditions (in H-2 at 343 K for 1 h), whereas the Au/Fe2O3 reference was calcined at 773 K. From our data, no correlations can be drawn between activity and selectivity and mean gold particle size. Selectivity toward the hydrogenation of the conjugated C=O bond varied with the structural characteristics of the support, ranking in the order FeO(OH) > iron oxy-hydroxide > gamma Fe2O3 > alpha Fe2O3. On the most selective catalyst-gold supported on goethite-the selectivity toward the formation of UAs was 64%. A correlation between the reducibility of catalysts and selectivity was found. Selectivity toward the formation of UA increased with the reducibility of the support. In accordance with data in the literature, we propose that an electron transfer from the reduced support to the metal creates more electron-enriched gold particles on which the back-bonding with the pi* C=O orbital is favored, so that the hydrogenation of the C=O group increases over that of the C=C group. However, it cannot be ruled out that the enhancement of the selectivity is favored through a cooperative effect of special Fe(III) or Fe(II) sites, and that more negatively charged gold nanoparticles nearby present the reduced support.
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