Selective hydrogenation of citral over Pt/KL type catalysts doped with Sr, La, Nd and Sm Original Research Article

The effect of Sr and some lanthanides (La, Nd, Sm) as promoters of Pt/KL supported catalysts is analyzed in the selective hydrogenation of citral in the liquid phase. Characterization of the catalysts by XRD, N2 adsorption, H2 chemisorption, TPD of NH3, XPS and competitive hydrogenation of benzene/toluene mixtures shows that impregnation of KL zeolite with Sr(NO3)2 in aqueous solution, followed by calcination prior to the incorporation of Pt, increases the surface basicity of the zeolite, which hinders the dispersion of Pt and promotes the formation of electron-rich platinum nanoparticles (Ptδ−). Ion exchange of K+ by rare earth cations (La3+, Nd3+ and Sm3+) increases the surface acidity of the zeolite and favors the dispersion of Pt, but with preferential location at the external surface of the zeolite. Furthermore, acidity promotes the formation of electron-deficient metal species (Ptδ+). Catalytic performances for the citral hydrogenation at 323 K and 5 MPa show that strontium addition enhances the hydrogenation activity of Pt, thus favoring the formation of citronellol as main reaction product (S = 80%) and increasing the yield to geraniol and nerol. Lanthanides as countercations diminish the overall hydrogenation activity of Pt/KL catalysts, but improve their selectivity towards unsaturated alcohols. Selectivity towards the reaction products is related to the mode and strength of the electronic interactions of the conjugated unsaturated bonds of citral reactant molecules with the metal surface species Pt0, Ptδ+ or Ptδ− on the zeolite.