Selective liquid-phase hydrogenation of fructose to d-mannitol over copper-supported metallic nanoparticles

Abstract The selective liquid-phase hydrogenation of d -fructose was studied on Cu-based catalyst, using an ethanol-water (70:30) mixture as solvent. The catalysts were prepared by three different methods: incipient wetness impregnation (Cu/SiO 2 -I and Cu/Al 2 O 3 -I), precipitation–deposition (Cu/SiO 2 -PD) and co-precipitation (CuMgAl and CuZnAl). After the thermal treatment, the samples were characterized by X-ray diffraction (XRD) and temperature-programmed reduction (TPR). Only a tenorite-like polycrystalline phase, formed by large CuO crystallites, was identified in Cu/SiO 2 -I, while none crystalline phase was observed in the case of Cu/SiO 2 -PD. Instead, a unique spinel-like phase was detected with Cu/Al 2 O 3 -I, CuMgAl and CuZnAl. Combining XRD and TPR results, we concluded that Cu 2+ is highly dispersed in the Cu/SiO 2 -PD, Cu/Al 2 O 3 -I, CuMgAl and CuZnAl calcined precursors. As a consequence, after reduction in H 2 flow, the metal dispersion and hydrogen chemisorption capacity of these four samples were one order higher than for Cu/SiO 2 -I. The catalytic tests showed that Cu/SiO 2 -PD was not only the most active but also the most selective and stable catalyst of these series: a D-fructose conversion of around 100% was reached after 6 h reaction, with a selectivity to d -mannitol of around 78–80%. These results show that selective hydrogenation of fructose to d -mannitol is favoured over metal Cu nanoparticles dispersed on the surface of a neutral support as SiO 2 . Additional catalytic tests, varying fructose initial concentration (0.028–0.220 M) and hydrogen pressure (20–40 bar), were carried out with Cu/SiO 2 -PD. A zero reaction order respect to D-fructose and a second reaction order respect to H 2 were estimated. In addition, it was found that d -mannitol selectivity is not dependent on reactant initial concentration and hydrogen pressure.