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/SiO2-I and Cu/Al2O3-I), precipitation–deposition (Cu/SiO2-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/SiO2-I, while none crystalline phase was observed in the case of Cu/SiO2-PD. Instead, a unique spinel-like phase was detected with Cu/Al2O3-I, CuMgAl and CuZnAl. Combining XRD and TPR results, we concluded that Cu2+ is highly dispersed in the Cu/SiO2-PD, Cu/Al2O3-I, CuMgAl and CuZnAl calcined precursors. As a consequence, after reduction in H2 flow, the metal dispersion and hydrogen chemisorption capacity of these four samples were one order higher than for Cu/SiO2-I. The catalytic tests showed that Cu/SiO2-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 SiO2. Additional catalytic tests, varying fructose initial concentration (0.028–0.220 M) and hydrogen pressure (20–40 bar), were carried out with Cu/SiO2-PD. A zero reaction order respect to D-fructose and a second reaction order respect to H2 were estimated. In addition, it was found that d-mannitol selectivity is not dependent on reactant initial concentration and hydrogen pressure.