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

Abstract

Two 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 500 °C 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 150–350 °C range, bimetallic catalysts exhibit better hydrogenating properties in the solvent isopropanol in comparison with n-heptane while the reverse occurs after a reduction at 500 °C. 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.