Influence of the nature of support on Ru-supported catalysts for selective hydrogenation of citral

Ruthenium catalysts supported on KL zeolite, ZrO2 and graphite, prepared by incipient wetness impregnation, were characterized by N-2 adsorption, H-2 chemisorption, TEM and CO adsorption microcalorimetry and tested in the selective hydrogenation of citral in the liquid phase, at 5 MPa and 323 K. Characterization studies reveal that graphite promotes formation of electron-rich metal species (Run-) that difficult hydrogenation of the conjugated C=C double bond of citral and indirectly favor the production of geraniol and nerol. For Ru/ZrO2 catalyst, the Ru <-> ZrO2 interaction at the interface, with formation of Ru-0-Zrn+ species as evidenced by the TEM and CO adsorption microcalorimetry measurements, activates the carbonyl group of citral and enhances the selectivity towards unsaturated alcohols. Characterization measurements of Ru/KL evidence that metal nanoparticles placed inside of the zeolite channels block part of ruthenium loading and hinder the free transit of citral through the channels, thus reducing the overall hydrogenation activity of the catalyst. Furthermore, this steric hindrance impedes adsorption the conjugated C=C double bond of citral and forces hydrogenation of the carbonyl group in terminal position. On the other hand, the fact that active sites on occluded particles are more resistant to poisoning than those easily accessible to citral enhances the selectivity towards geraniol and nerol, as the reaction time increases. The comparative analysis of the results indicates that, in order to increase the selectivity towards unsaturated alcohols, geometrical effects derived from the size, shape and location of ruthenium particles in the KL structure are more effective than the electronic modifications induced by graphite or zirconia on the ruthenium particles. (C) 2012 Elsevier B.V. All rights reserved.