Preparation Of Noble Metal Colloids And Their Catalytic Properties For Hydrogenation Of Ortho-chloronitrobenzene

Polymer-stabilized metal colloidal catalysts are very important in the field of catalysis because of their fascinating catalytic properties. In this paper, the selective hydrogenation of o-CNB to o-CAN was chosen as model for the study of poly(N-vinyl-2-pyrrolidone)-stabilized Pt and Pt/Ru bimetallic colloidal catalysts.Firstly, poly (N-vinyl-2-pyrrolidone)-stabilized platinum colloids (PVP-Pt) were prepared via the reduction of hexachloroplatinic acid in a refluxing mixture of water and alcohol (methanol, ethanol, n-propanol and n-butanol) by varying the amount of alcohol in the solvent. The metal colloids were characterized by TEM. The nanoparticles with smaller diameter and better size distribution were obtained in alcohol with higher boiling point and higher amount of alcohol in the solvent. Catalytic performance of the PVP-Pt colloids was tested by selective hydrogenation of o-CNB at 298 K and 0.1 MPa hydrogen pressure. The catalytic performance of the PVP-Pt colloids is dependent on the size of the nanoparticles. The selectivity to o-CAN decreased but the activity of the catalyst increased with the decreasing of the nanoparticles size.The effect of Ru³⁺ metal ions on hydrogenation of o-CNB over PVP-Pt was also studied. Extra Ru³⁺ added in the reaction system has some inhibiting effect on the reaction activity, but resulting in some increase in selectivity to o-CAN.Secondly, PVP-stabilized platinum/ruthenium colloids (PVP-Pt/Ru) were prepared via the reduction of hexachloroplatinic acid and ruthenium (III) chloride in a refluxing mixture of water and ethanol or n-propanol. The morphology of bimetallic nanoparticles was characterized by UV–visible absorption spectrophotometry, X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM). The average diameters of the PVP-Pt/Ru bimetallic colloids determined by means of TEM are in a range from 1.9 to 2.8 nm with narrow size distributions. The average diameters tend to decrease and the standard deviations tend to increase with increasing molar content of ruthenium in the PVP–Pt/Ru system, although the tendencies are not so obvious. The PVP–Pt/Ru nanoparticles of smaller diameter and better size distribution were obtained in the alcohol with higher boiling point, and the standard deviations decreases with increasing the heating rate.Thirdly, poly (N-vinyl-2-pyrrolidone)-stabilized platinum/ruthenium colloids (PVP-Pt/Ru) were prepared in methanol-water mixed solvents by varying the molar rate of H₂PtCl₆·6H₂O to RuCl₃·nH₂O and the amount of methanol. The bimetallic colloids were characterized by UV–visible absorption spectrophotometry and TEM. When the molar ratio of Pt to Ru was higher than 1/1, Ru³⁺ was entirely reduced to Ru0. The average diameters tend to decrease and the standard deviations tend to increase with increasing molar content of ruthenium in the PVP–Pt/Ru colloids. When the molar ratio of Pt to Ru was less than 1/1, there are still some Ru³⁺ ions in the colloidal solution after prolonged refluxing.Finally, hydrogenation of ortho-chloronitrobenzene (o-CNB) was carried out in a batch mode using hydrogen over the PVP-Pt/Ru bimetallic colloids at 298 K and atmospheric pressure. The product consisted of ortho-chloroaniline (o-CAN), aniline and nitrobenzene during the reaction and the selectivity to o-CAN was in the range 93.08%-99.00% at 100 % conversion of o-CNB. It was observed that the catalytic performance of the PVP-Pt/Ru colloids is dependent on the compositions of the bimetallic colloids. The selectivity to o-CAN increased but the activity of the catalyst decreased with the increasing proportion of ruthenium in Pt/Ru colloidal catalysts. And a suitable amount of Co2+ ions can improve the selectivity to o-CAN and the activity of the PVP-Pt/Ru bimetallic colloids.