Selective Hydrogenation Of Haloaromatics In Biphasic System

Aromatic haloamines are of great importance for the production of many fine chemicals, such as dyes, herbicides and pesticides etc.. The preparation of them are either through the Bechamp s procedure or through selective hydrogenation over heterogeneous catalyst or homogeneous catalyst. In recent years, the use of water-soluble transition metal complexes in biphasic (aqueous/organic) catalysis has attracted much attention because this new approach has largely simplified the separation of catalysts from products. According to the reports, much research work has been done on the hydrogenation of unsaturated compounds catalyzed by water-soluble phosphine-transition metal complexes in biphasic(aqueous/organic) system. However, there are few reports about the hydrogenation of halonitroaromatics with similar catalytic system. Therefore, our research aims at probing a new catalytic system for the hydrogenation of halonitroaromatics, in which some water-soluble Ruthenium phosphine complexes have been synthesized to be used as catalytic precursors in the reaction.In our research, selective hydrogenation of halonitroaromatics catalyzed by water-soluble Ru-TPPTS complex formed in situ from RuCl3 and TPPTS in the biphasic catalytic systemic/toluene) has been studied. With p-ClC6H4NO2 as substrate, the influences of several parameters-such as surfactant concentration, reaction temperature, HI pressure, catalyst concentration, TPPTS concentration, aqueous pH value and the reaction time - on the activity and selectivity have been investigated systematically. It has been found that the reaction rate is obviously promoted by the addition of cationic surfactants, among which cetyltrimethylammonium bromide(CTAB) performs the best. Under the conditions of 90, PH2 =4.0MPa, reaction time 5h, the conversion of p-ClC6H4O2 and theselectivity for p-ClC6H4NH2 are respectively 100% and 99.9%. Meanwhile, only a small amount of dehalogenation product (aniline) is formed. It has also been found that the catalytic system exhibits high activity and selectivity for the hydrogenation of other halonitroaromatics with different substituents and substituted positions. The conversion of p-ClC6H4NO2 remains high after the first recycle of the catalyst, but it gradually declines as the experiment goes on.A water-soluble Ruthenium phosphine complex [RuCl2(TPPTS)2]2 has also been synthesized to be used as a catalyst in the hydrogenation of p-CLC6H4NO2 in H2O/toluene biphasic system. The influences of reaction temperature, H2 pressure, catalyst concentration, TPPTS concentration and aqueous pH value on the activity and selectivity have been investigated. Under the optimum conditions of 100, PH2 =3.0MPa, the conversion of p-ClC6H4NO2 reaches 100% after 6h and the selectivity for p-C1C6H4NH2 is 95.2%. The complex [RuCl2(TPPTS)2]2 also exhibits high activity and selectivity for the hydrogenation of other halonitroaromatics.In the hydrogenation of halonitroaromatics catalyzed by Ru/Pt-TPPTS and Ru/Pd-TPPTS bimetallic catalysts, the catalytic activity is greatly affected by the catalyst composition. But both of the bimetallic catalysts exhibit remarkable synergic effect in a wide range of Ru/Pt(molar ratio) and Ru/Pd(molar ratio). The conversion of p-C1C6H4NO2 catalyzed by 0.50Ru/0.50Pt-TPPTS reaches 100% within 40min under the conditions of 70℃, PH2=1.0MPa. Under the same conditions, p-ClC6H4NO2 catalyzed by 0.50Ru/0.50Pd-TPPTS can also be fully converted within 70min. Both of the catalysts, 0.50Ru/0.50Pt-TPPTS and 0.50Ru/0.50Pd-TPPTS, have exhibited high activity and selectivity in the hydrogenation of other halonitroaromatics. The bimetallic catalysts of Ru/Pt-TPPTS and Ru/Pd-TPPTS have exhibited more desirable activity than monometallic catalyst of Ru-TPPTS under mild reaction conditions.