Yu. S. Demidova

Controlled synthesis of PVP-based carbon-supported Ru nanoparticles: synthesis approaches, characterization, capping agent removal and catalytic behavior

Polyvinylpyrrolidone (PVP) stabilized Ru nanoparticles (NPs) with a mean size of ca. 2.4 nm were synthesized and immobilized on mesoporous Sibunit, macroporous carbon nanofibers of platelet structure (CNF-Pl) and micro-/mesoporous TiC carbide-derived carbon (CDC), providing 1.7–2.9 wt% Ru/C catalysts with a mean Ru size of 2.1–2.7 nm. The presence of PVP on the catalyst surface drastically diminished the activity in structure-sensitive hydrogenation of galactose to galactitol. Different PVP removal and support modification methods were tested to elucidate the effect of support hydrophilicity/hydrophobicity, preliminary support functionalization, and additional Ru NPs washing prior to immobilization as well as degree of PVP removal on catalytic behavior. For characterization of the carbon supports and synthesized catalysts TEM, XPS, XRD, XRF, water adsorption/desorption experiments and N2 physisorption were applied. Carbon supports were functionalized before Ru NPs immobilization by treatment with 5 wt% HNO3, conc. HNO3, Ar (700 °C), air (350 °C), and H2 (700 °C) without visible support microstructure alterations. Several protocols for PVP removal were applied such as solvothermal (with water and acetic acid aqueous solutions at 220 °C) and thermal treatment (in air, argon, hydrogen or nitrogen at different temperatures) not resulting in noticeable changes in Ru NPs size. The thermal treatment in air at 180 °C followed by reduction at 250 °C was found to be more effective in the case of Sibunit and CNF-Pl, while solvothermal treatment in water at 220 °C (PN2 25 bar) improved significantly the activity of TiC-CDC-based catalysts. Untreated Sibunit and CNF-Pl carbon supports provided higher activity in galactose hydrogenation with Ru/Sibunit exhibiting the best catalytic activity and being also the most hydrophilic according to water sorption isotherms. The catalytic activity of untreated micro-/mesoporous Ru/TiC-CDC increased noticeably depending on the support pretreatment in the order: untreated < 5% HNO3 < conc. HNO3. Additional TiC-CDC support functionalization was proposed to be required because of a relatively low amount of oxygen-containing groups on the surface compared to Sibunit and CNF.

Hydrogenation of (–)-Carvone in Presence of Gold Catalysts: Role of the Support

The liquid phase hydrogenation of biomass derived (–)-carvone into industrially valuable dihydrocarvone was studied over monometallic Au catalysts supported on alumina, titania and zirconia, as well as on the mesoporous carbon support Sibunit in methanol as a solvent (100°C, hydrogen pressure 9 bar). It was shown that among the three types of functional groups present in carvone, which can be hydrogenated, namely C=O, conjugated and isolated C=C groups, hydrogenation of the latter was predominant. The catalytic activity was found to depend on the catalyst support type. Under comparative reaction conditions, the carvone conversion increased in the following sequence: Au/C ≪Au/ZrO2 < Au/Al2O3 ≪Au/TiO2. A higher activity of Au catalysts over metal oxides as compared to Au/C can be caused by the presence of acid sites as well as oxygen vacancies in their structure allowing strong adsorption of carvone through its carbonyl moiety. All catalysts supported on oxides showed similar selectivity towards trans- and cis-dihydrocarvone with the ratio between isomers (trans-/cis-isomer) being about 1.8, while this value for Au/C was close to 3.9, which can be related to a much lower carvone conversion in the latter case.