Hydrogenation Properties Of Preparation And Fe (Ⅲ) MWCNTs Noble Metal Catalysts For Pt / MWCNTs Catalytic Citral

Palladium and platinum supported on multiwalled carbon nanotubes(MWCNTs) catalysts are highly efficient catalysts that widely used in the fields of fine chemical and petrochemical industries. They possess high specific areas and show high activity, good stability and selectivity when used in catalytic reactions. Liquid reduction is a method commonly used in the preparation of supported catalysts because this method is easily operated and the toxicity of the solvent used is quite low.Citral is a special kind of a, β-unsaturated aldehyde, besides a pair of conjugated C=O bond and C=C bond, there is also an isolate C=C bond in its molecule structure. The special structure and lots kinds of different hydrogenated products make its hydrogenation a complex reaction. Among the numerous hydrogenated products, the two kinds of unsaturated alcohols, nerol and geraniol, are important intermediates for perfumes and pharmaceuticals industries. Compared with other products, they value more and get a wider range of applications. However, it is difficult to obtain unsaturated alcohols from the view point of thermodynamics, because the bond energy of C=C bond is lower than that of C=O bond. So how to get a high selectivity for the hydeogenation product of C=O bond becomes more and more popular in the field of catalysis. One of the solutions is to develop catalysts with high selectivities to unsaturated alcohols. Supported catalysts play an important part in hydrogenation reactions due to its catalytic effects.According to reports in literatures, catalysts of Pt supported on carbon materials do better in selective hydrogenation of C=O bond. Due to the high cost of Pt researchers use promoters to make the catalysts cheaper. It is very common to use MWCNTs as supported materials because of the particular physical and chemical properties of MWCNTs. So Pt/MWCNTs can be of better catalytic performances after some pretreatments. Methods of changing reaction conditions can also be taken to control the distribution of hydrogenation productions. Conditions such as reaction time, pressure, temperature etc. can be decisive factors for chemical reaction products.In the present thesis, palladium and platinum granules were supported on MWCNTs via the method of liquid reduction using ethylene glycol as reductant. The influence of the kind and amount of complexing agents on the mean size and dispersity of palladium granules were investigated. The influence of Fe(III) on the hydrogenation of citral was investigated after it was introduced into Pt/MWCNTs by the methods of impregnation. The mechanism of the catalytic performance for the hydrogenation of citral over Pt-Fe(III)/MWCNTs catalyst was proposed based on the results of some characterizations and the wavenuber changes of C=O bond in citral during the reaction process. The main results are given below:1. Pd/MWCNTs catalysts were prepared by the method of liquid reduction using ethylene glycol as a reductant. The structures and morphologies of Pd/MWCNTs were characterized by XRD and TEM. The influence of the kind and amount of complexing agents on the mean size and dispersity of palladium granules was investigated. Sodium citrate was found to be the best complexing agents and the optimal molar ratio for sodium citrate and palladium was40:1.2. Pt/MWCNTs catalysts were prepared by the method of liquid reduction using ethylene glycol as a reductant. The structures, morphologies and pore structural behaviours of Pt/MWCNTs were characterized by XRD, TEM and N2physisorption. The catalytic performance of Pt/MWCNTs for citral hydrogenation was investigated by changing reaction time, H2pressure and temperature using isopropanol as a solvent. The optimal result was found under the reaction conditions of T=80℃, P(H2)=2MPa, t=3h. The influence of Fe(III) on the performance of citral hydrogenation was investigated after it was introduced into Pt/MWCNTs by the methods of impregnation. A higher selectivity for unsaturated alcohols and lower conversion for citral were found after Fe(III) was introduced into Pt/MWCNTs. It was found that when Fe(III) was added with a fraction of0.25wt.%the conversion for citral makes no change but the selectivity to unsaturated alcohols increased greatly from33.7%to90.7%.3. Different methods for introducing Fe(III) into Pt/MWCNTs were used. The catalytic performances of different catalysts were investigated under the optimal reaction conditions. Characterizations for the catalysts, such as XRD, TEM, TPR, XPS and N2physisorption were carried out. The wavenumber changes for the C=O bond in citral were monitored via a high-pressure in situ infrared spectrometer. The mechanism of the catalytic performance for the hydrogenation of citral over Fe(III) modified Pt/MWCNTs catalyst was proposed as follows: The Fe(III) combined with the catalyst with forms of Fe2O3and FeOOH after it was introduced into Pt/MWCNTs, making the specific surface area, mean pore size and pore volume of Pt/MWCNTs changed, which decreased the conversion of citral. The C=O bond of citral was polarized by Fe(Ⅲ) during the reaction process, making itself easier to be adsorbed by active sites, so as to increase the selectivity to unsaturated alcohols.