Citral hydrogenation over alumina supported Rh-Ge catalysts - Effects of the reduction temperature

International audienceTwo series of alumina supported Rh-Ge catalysts were prepared either by surface redox reaction (catalytic reduction (CR)) or by a simple impregnation of germanium on rhodium/alumina (SI samples). They were characterized by temperature-programmed reduction (TPR) and by their activity for the liquid phase hydrogenation of citral used as model reaction. The addition of germanium to rhodium by CR allows to preferentially hydrogenate citral to the unsaturated alcohols (geraniol and nerol), whereas the saturated aldehyde (citronellal) is the main product on the monometallic rhodium catalyst. The citral conversion and the selectivity to unsaturated alcohols increase with the germanium content and go through a maximum as a function of the reduction temperature of catalysts. At higher germanium loadings, the activity decrease observed after a reduction at 500degreesC is explained by the reduction of germanium species deposited on the support in the course of the CR preparation procedure. For reduction temperatures in the 150-350degreesC range, bimetallic catalysts exhibit better hydrogenating properties in the solvent isopropanol in comparison with n-heptane while the reverse occurs after a reduction at 500degreesC. The CR catalysts are more active and selective to unsaturated alcohols than bimetallic catalysts prepared by the SI method. However, the exposure of CR bimetallic catalysts to ambient air deteriorates their catalytic properties for citral hydrogenation

Bimetallic Rh-Ge and Pt-Ge catalysts supported on TiO2 for citral hydrogenation I. Preparation and characterization of the catalysts

International audienceBimetallic TiO2-supported Rh-Ge and Pt-Ge catalysts were prepared by surface redox reaction between hydrogen activated on a parent monometallic rhodium or platinum catalyst and a germanium salt dissolved in water (catalytic reduction method). They were characterized by elemental analysis, transmission electronic microscopy (TEM), temperature-programmed reduction (TPR) and by their activity for the gas phase dehydrogenation of cyclohexane. Elemental analysis of the bimetallic catalysts showed that germanium can effectively be deposited by the catalytic reduction method on titania-supported Rh and Pt catalysts. Moreover, the different characterization methods (TEM, TPR and cyclohexane dehydrogenation) proved that germanium is in great interaction with rhodiurn and platinum. Nevertheless, some germanium deposition occurred also separately on the titania support. TEM and cyclohexane dehydrogenation results revealed that both rhodium and platinum particles were stable on titania support under the conditions of bimetallic catalyst preparation contrary to previous results obtained with silica or alumina supports. Effectively, no sintering has been observed when they were immersed in an aqueous solution under hydrogen bubbling (catalytic reduction protocol). Their catalytic performances for the cyclohexane dehydrogenation reaction indicate that all the catalysts reduced at high temperature (500 degrees C versus 300 degrees C developed the. strong metal-support interaction (SMSI) effect, which implied the formation of TiO(2-x) species. Whatever the nature of the parent metal (Rh and Pt), this effect was totally destroyed by air exposure of the samples at ambient temperature whereas one part of the TiO(2-x) moieties remained after immersion of the catalysts in an aqueous medium

Bimetallic Rh-Ge and Pt-Ge catalysts supported on TiO2 for citral hydrogenation II. Catalytic properties

International audienceBimetallic Rh-Ge/TiO2 and Pt-Ge/TiO2 catalysts prepared by surface redox reaction (catalytic reduction method) were tested for the selective hydrogenation of citral. Samples were reduced either at 300 degrees C or at 500 degrees C (SMSI effect). For both Rh and Pt series, the unsaturated alcohols (UA) selectivity goes through an optimum as a function of the germanium content whatever the reduction temperature. For low Ge loadings, it was possible to combine the bimetallic and SMSI effects, inducing a high UA selectivity on bimetallic catalysts supported on TiO2. On the other hand, at high Ge loadings the bimetallic effect was predominant and led also to a high UA selectivity. Moreover, TiO2 support contributed to stabilize the bimetallic Rh-Ge and Pt-Ge particles prepared by catalytic reduction toward an air exposure at ambient temperature

Influence of the nature of the precursor salts on the properties of Rh-Ge/TiO2 catalysts for citral hydrogenation

International audienceFour sets of Rh-Ge/TiO2 bimetallic catalysts were prepared by surface redox reaction (i.e., catalytic reduction method) between hydrogen activated on a Rh parent catalyst and a Ge salt dissolved in aqueous solution. The four sets of catalysts differ depending on the presence or absence of chloride ions in the Rh and Ge precursor salts used (i.e., RhCl3 vs. Rh(NO3)(3), GeCl4 vs. GeO2). Samples were reduced either at a lower temperature (300 degrees C) or at a temperature chosen to induce a strong metal-support interaction (SMSI) effect (500 degrees C). Catalysts were characterized by elemental analysis, transmission electronic microscopy (TEM), and Fourier transform infrared (FTIR) spectroscopy of adsorbed CO and evaluated for their activity for the gas phase dehydrogenation of cyclohexane and selective hydrogenation of citral. Regardless of the nature of the Rh and Ge precursor salts, the catalytic reduction method causes the Ge to be in intimate contact with the Rh particles, favoring the citral hydrogenation toward unsaturated alcohols (UA: nerol and geraniol). For low Ge loadings, the bimetallic effect can be combined with the SMSI effect. It was observed that the UA selectivity is directly correlated to the ratio R (R = Sigma A(COads on oxidized Rh >= 1+ species)/Sigma A(COads on total exposed Rh species)) determined by FTIR. A better UA selectivity is obtained when bimetallic catalysts possess a surface in a predominantly oxidized state, a situation that is enhanced when chlorinated rhodium and germanium precursors are used. (C) 2010 Elsevier Inc. All rights reserved