Selective Hydrogenation Of Aromatic Nitro-compounds Over Supported Metal Catalysts

Aromatic haloamines are used as important chemicals and intermediates for the production of many fine chemicals such as dyes, herbicides, pesticides, rubber and drugs. It seems to be an attractive choice to selectively hydrogenate of haloaromatic nitro-compounds to the corresponding aniline catalyzed by supported noble metal catalysts in liquid phase. In this research, a variety of supported catalysts containing noble metal Ru, Pt, Pd and Ir, prepared through different methods, were used in the selective hydrogenation of haloaromatic nitro-compounds to the corresponding anilines. Different methods to prepare the Ru-Pt/γ-Al2O3 bimetallic catalyst and corresponding catalytic performance in the selective hydrogenation of 2,5-dichloronitrobenzene was investigated. It was found that polyvinylpyrrolide(PVP)-stabilized Ru-Pt/ γ-Al₂O₃ (n(Ru)/n(Pt)=4) bimetallic catalyst â…¢1 exhibited the best catalytic properties. There was a synergistic effect between Ru and Pt in the hydrogenation of 2,5-dichloronitrobenzene. The addition of Ru to Pt/γ-Al2O3 not only improved the catalytic activity, but also inhibitd the dechlorination of 2,5-dichloronitrobenzene. Adding different metal cations directly into the reaction solution exerted different influences on both the activity and selectivity of the catalyst. The most favorable influence was obtained when Sn⁴⁺ was used as modifier for the bimetallic catalyst Ru-Pt/γ-Al2O3 â…¢1. The conversion of 2,5-dichloronitrobenzene and the selectivity for 2,5-dichloroaniline increased from 41.4% and 63.5% to 100% and 77.6%, respectively, when Sn⁴⁺ was added in the molar ratio of Sn4+/(Ru+Pt) = 1 under the reaction conditions: 50 ℃, 1.0 MPa, 1 h and n(substrate)/n(Ru+Pt) =1000 in absolute ethanol solution. The conversion and selectivity reached 100% and 99.7%, respectively, when the reaction time was prolonged to 4 h under above mentioned reaction conditions. No dechlorinated aromatic amines were determined during the reaction process. Meanwhile, it was found that the modification of Sn4+ to the catalyst â…¢1 was outstanding in the hydrogenation of other haloaromatic nitro-compounds. The selective hydrogenation of different dichloronitrobenzenes to form the corresponding anilines was accomplished over (PVP)-stabilized Pd/γ-Al2O3, which exhibited considerable dehalogenation activity. To improve the selectivity for dichloroaniline, the influence of different metal cations and the influence of different adding methods of Sn4+was investigated. It was found that different metal cations exerted different influence on the activity and selectivity. On the other hand, although adding Sn4+ decreased the activity to some extent, it obviously inhibited the dechlorination after SnCl4 was directly added into the reaction system. When the reaction temperature was increased from 80 ℃to 110 ℃, the conversions of 2,5-, 2,4-, 2,3-dichloronitrobenzenes reached 100% over Sn4++Pd/γ-Al2O3, and the selectivities of 2,5-, 2,4-, 2,3-dichloroaniline enhanced from 84.5%,76.1%,86.8%, over Pd/γ-Al2O3, to　98.7%,100%,100% over Sn4++Pd/γ-Al2O3, respectively. In the selective hydrogenation of mono-chloronitrobenzenes, with prolonging the reaction time, the selectivities of p-, m-, o-CAN enhanced from 12.7%,33.0%,42.0%, over Pd/γ-Al2O3, to 95.5%,95.9%,98.5%over Sn4++Pd/γ-Al2O3, with 100% conversion of substrates. The effects of reduction temperature, reduction method, differences in reductants and supports in the preparation of supported Ir catalyst on the catalytic and selectivity were studied. The results showed that the catalyst Ir/γ-Al2O3 (â… 3), prepared by conventional impregnation and reduced with hydrogen under the reduction conditions: 120 ℃, 2.0 MPa, 1 h, ethanol as solvent, exhibited excellent catalytic performance in the selective hydrogenation of p-Chloronitrobenzene. The conversion of p-Chloronitrobenzene and selectivity for p-Chloroaniline were both100% under reaction conditions: 50 ℃, 0 .5 MPa, 2.5 h and n(substrate)/n(Ir)=3000. Despite that the conversion and selectivity remained high after the second recycle of the catalystâ… 3, they gradually declined as the recycle went on. The catalyst exhibited excellent activity and selectivity in the hydrogenation of other halonitroaromatics.