Preparation Of Platinum Colloid And Their Catalytic Properties For Hydrogenation Of Chlorobenzene

Metal nanoclusters in the range of 1-10 nm are very important in academia and industry, especially in the field of catalysis. However, the surfactant-stabilized metal clusters are not stable and intend to agglomerate during reaction even under very mild conditions. Thus, the polymer-stabilized metal clusters are extremely important for their catalytic use.Chlorinated organic compounds have been used on a large scale in chemical, petrochemical, and electronic industries. Those compounds are widespread environmental contaminants. They are of great health and environmental concern besaused of their acute toxicity and strong bioaccumulation potential. Among all, the chlorinated aromatic compounds are most toxic and thermally stable. Once released into environment, they will accumulate in the surrounding and endanger human as well as its ecological environment over a long period of time. In this paper, chlorobenzene was chosen as model compounds for the study of hydrogenation over polymer-stabilized platinum colloidal catalysts. Methanol was chosen as the solvent since it allows us to investigate the HDC reaction at a high concentration of the water-insoluble halogenated compound.Firstly, poly (N-vinyl-2-pyrrolidone)-stabilized platinum colloids (PVP-Pt) were prepared by varying the amount of H₂PtCl₆·6H₂O, the amount of PVP and stirring stopping time after discoloring solution for 5 min in methanol and water mixed sol-vents. The metal colloids were characterized by TEM. No significant effect on parti-cle size was observed by varying the amount of H₂PtCl₆·6H₂O and the amount of PVP. But the Pt nanoparticles have a tendency to aggregate because of the small amount of the stabilizing polymer. The longer stirring stopping time was in the preparation of colloidal platinum, the larger the particle size and size distribution were. When the time of stopping stirring was 30s, the largest particle size (4.39 nm) and the widwst size distribution (σ= 0.78 nm) could be obtained.Secondly, hydrogenation of monochlorobenzene (MCB) was carried out in a batch mode using hydrogen over PVP-Pt at 298 K and atmospheric pressure. The product consisted of benzene and cyclohexane during the reaction, and nearly 100 % selectivity to cyclohexane can be obtained at 100 % conversion of MCB. The catalytic performance of the PVP-Pt colloids is dependent on the preparation conditions. The small amount of the stabilizing polymer (PVP) in the preparation of colloidal platinum could not protect the platinum colloid commendably, but the large amount of PVP hindered the contact of reactant with catalyst surface and desorption of product. Extra PVP added in the reaction system has some inhibiting effect on the reaction activity, and also resulting in some decrease in selectivity to cyclohexane. The performance of catalyst was proportional to the concentration of H2PtCl6?6H2O. The catalytic properties of catalysts were affected remarkably by the time of stopping stirring in the preparation of catalysts. The conversion and the selectivity could be down to 79.06 % and 74.40 %, respectively, by prolonging the time of stopping stirring to 30 s. The reaction was verified to be first order to the concentration of MCB. The reaction is kinetic control at lower catalyst concentration (< 1.7×10-6 mol/mL), and the diffusion controls the reaction when the catalyst concentration beyond 1.7×10-6 mol/mL. The reaction took place on the metal surface in a pseudo-first-order reaction, and the rate constant was 0.5026 h-1.Thirdly, effect of metal ions on hydrogenation of chlorobenzene (MCB) over poly-vinylpyrrolidone stabilized platinum colloid (PVP-Pt) was studied. It was shown that the catalytic properties of platinum clusters for the hydrogenation of chlorobenzene to cyclohexane were remarkably affected by the metal cations added. Among these cations, the most favorable influence on the activity and selectivity was obtained when Al3+ was used as modifier. The conversion of chlorobenzene was enhanced remarkably from 77.49 % to 90.64 %, and the selectivity for cyclohexane was increasd from 65.58 % to 72.24 %.Finally, effect of metal complex on hydrogenation of chlorobenzene (MCB) over PVP-Pt has been studied. It was shown that the addition of metal complexes and ligands to the catalytic system had great effect on activity and selectivity of the catalyst. When the neat platinum cluster was served as the catalyst, only 77.49 % conversion of chlorobenzene and moderate selectivity for the cyclohexane (65.58 %) were obtained. On the addition of the ligands acetylacetone, the conversion and the selectivity were increased together (the conversion was 90.85 %, the selectivity was 68.45 %). It was indicated that the effect resulted from the incorporation of metal complexed was not the simple sum of those of the corresponding metal central ions and the ligands. The complex effect is related to the interaction of metal complex with reaction substrate, reaction product or the metallic catalyst, etc. Owing to the equilibrium of step dissociation of metal complexes, there is a mixture of multistep complexes at an integer molar ratio of ligand to metal. This causes the mechanism of metal complexes effect very complicated.