| dc.description.abstract | The PVP-stabilized NiB catalysts were prepared using the chemical reduction method with NaBH4, dissolving the water-soluble polymer of polyvinylpyrrolidone (PVP) in the precursor salt solution as a protective reagent. The PVP-NiB catalysts were characterized and examined for their catalysis on the hydrogenation of furfural, crotonaldehyde and citral. PVP polymer could adsorb on the nano-particles of NiB via a weak coordination bonding and stabilize it; the molecular weight of PVP about 10,000 was suitable, and the optimal quantity of PVP (PVP/Ni) in the salt solution for preparing catalysts was around 20. The PVP-NiB samples were characterized by XRD as an amorphous structure and by TEM with a particle size distribution in the range of 2.5–7.7 nm. On catalysis, the PVP-NiB catalyst was significantly more active and slightly more selective than NiB for hydrogenating furfural to furfuryl alcohol and crotonaldehyde to butyraldehyde. A good yield of citronellal about 92% could be obtained by reducing citral in cyclohexane at a low reaction temperature of 50ºC over the PVP-NiB catalyst.
Surfactant-stabilized NiB catalysts (ME-NiB) were prepared using the chemical reduction method in the ternary microemulsion system of water/CTAB/n-hexanol. The surfactant molecules could adsorb on the surface of the formed particles; they act as a protective agent and restrict the growth of nano-particles. The size of nano-particles was not completely determined by the size of the microemulsion droplets, but also depended on the composition of the solution. Additionally, the concentration of nickel salt, the amount and speed of addition of NaBH4, and the temperature influenced the sizes of the particles and the reactivity of the ME-NiB nano-particles. The ME-NiB catalyst was characterized and examined for its catalysis on the hydrogenation of furfural, crotonaldehyde and citral. It was thus compared with the NiB and PVP polymer-stabilized NiB catalysts. The ME-NiB sample was characterized by XRD as an amorphous structure and by TEM with a particle size distribution in the range 1.2–5.0 nm. The ME-NiB catalyst was markedly more active and slightly more selective than NiB or PVP polymer-stabilized NiB in the hydrogenation of furfural to furfuryl alcohol and crotonaldehyde to butyraldehyde. A good yield of citronellal, around 88%, was obtained by reducing citral in cyclohexane at a room temperature of 30ºC over the ME-NiB catalyst.
A super-active supported nickel catalyst-NiB/SiO2 could be obtained with chemical reduction method for liquid-phase hydrogenation. The precursor salt of nickel was mounted on SiO2 by impregnating, drying and calcination at an appropriate temperature without being decomposed, and then reduced with aqueous NaBH4 solution. The influential factors for preparation NiB/SiO2 catalysts were examined by the hydrogenation of butyraldehyde. The NiB/SiO2 catalysts were characterized as an ultrafine and amorphous structure, which are much more active than NiB and Ni/SiO2 a conventional supported nickel catalyst reduced by H2. The optimal catalyst of 5%NiB/SiO2 was used for hydrogenating citral to citronellal and cironellol, which was about fourteen times as active as NiB, but less selective than it. Nevertheless, the reaction could be performed at room temperature to promote the selectivity. A good yield of citronellal/cironellol about 90% and a yield of citronellal about 81% over 5%NiB/SiO2 could be obtained at a low temperature of 30ºC. | en_US |