Comparative study of the hydrogenolysis of glycerol over Ru-based catalysts supported on activated carbon, graphite, carbon nanotubes and KL-zeolite

Supported ruthenium catalysts were prepared by incipient wetness impregnation of three different carbon materials: activated carbon (AC), high surface area graphite (HSAG) and multiwalled carbon nanotubes (CNT). Another catalyst was prepared by treating KL zeolite with RuCl3·xH2O in aqueous solution. All these samples were characterized by temperature programmed reduction (TPR), CO chemisorption coupled with microcalorimetry and transmission electron microscopy (TEM). The reduced catalysts were tested in the hydrogenolysis of glycerol in the liquid phase, under a reaction pressure of 8 MPa and isothermally at the reaction temperature of 453 K. The CO microcalorimetry measurements evidenced that electron donor properties of graphite and carbon nanotubes promote formation of electron-rich metal species (Ruδ−) in Ru/HSAG and Ru/CNT catalysts, which not only favors formation of 1,2-propanediol from glycerol but also enhances the successive CC cleavage, with formation of undesired products, mainly methane. For Ru/KL the occurrence of Brönsted acid sites, resulting of the reduction of the chlorinated ruthenium species bonded to the zeolite framework, was verified by temperature programmed desorption (TPD) of NH3. Furthermore, observations by TEM of the Ru/KL catalyst showed an important population of metal nanoparticles lower than 1 nm, part of which exhibits electron deficient character as indicated by the CO microcalorimetry. As a consequence, the transformation of glycerol into 1,2-PDO over Ru/KL seems to be promoted through formation of the intermediate acetol on acid sites, while for the scarcely acid Ru-carbon catalyst conversion of glycerol occurs mainly on metal sites, ethylene glycol being the preferred hydrogenolysis product.E.G.S. would like to thank to UNED of Spain for a scholarship grant. The financial support of the Spanish Government under projects CTQ2011-29272-C04-01 and -03 is recognized.Peer Reviewe