Selective Catalytic Hydrogenation Of Dinitroaromatics To Nitroanilines Over Ru-based Catalysts

Nitroanilines are important intermediates widely used in pharmaceuticals,polymers, herbicides,fine chemicals and some other industrial fields;therefore,there is a strong incentive both in theoretical and economic perspective to develop chemoselective catalyst for the partial hydrogenation of dinitrobenzenes to nitroanilines.Although the reductions of various simple nitro compounds have been readily carded out with different commercial catalysts,the selective reduction of a nitro group amoung several reducible groups in the same molecule is more challenging,especially when two NO₂ groups co-exist.In recent years,ruthenium-catalyzed processes have become one of the most preferred methodologies in reduction reactions due to their high efficiency and versatile applications.In this dissertation,hydrogenations of m-dinitrobenzene over four kinds of skeletal catalysts were compared.The hydrogenation of m-dinitrobenzene to m-nitroaniline over skeletal RuNiC was higher than other catalysts,when m-dinitrobenzene converted completely,the selelctity to m-nitroaniline reached 96%.Based on this result,effects of the reaction conditions including temperature,pressure,and substrate concentration on selective hydrogenation of m-dinitrobenzene over Skeletal RuNiC were investigated.Additionally,the hydrogenation of o-,p-dinitrobenzene over Skeletal RuNiC also showed high selectivity to o-, p-nitroaniline.The reaction courses were tracked by gas chromatography,indicating that three substrates converted in a similar rule with different reaction order.In order to extend the applicability of skeletal ruthenium,selective hydrogenations of substituted dinitrobenzenes(2,4-dinitrophenol,2,4-dinitrobenzamine,2,4-dinitrotoluene, 3,5-dinitrobenzoic acid) were studied.The electron-withdrawing groups favored the hydrogenation of substituted dinitrobenzenes than electron-donating group.In addition, different position of the substituted group to two nitro groups in 2,4-dinitrobenzenes could lead to isomers,and the effect of substituted group on the isomer ratio was discussed.The industrial application of Skeletal Ru-based catalysts is limited by the high-cost and high melting point of metal ruthenium.Thus,supported ruthenium catalyst with high dispersion was prepared through surfactant-protection Ru colloid method.Hydrogenation of m-dinitrobenzene was chosen as the model reaction,and it was found that the Ru/C showed the highest selectivity to m-nitroanilines among several supported catalysts.The selective hydrogenation of m-dinitrobenzene over Ru/C was investigated and effects of reaction conditions such as temperature,pressure,substrate concentration,and Ru load amount and catalyst amount as well as catalyst lifetime were studied during circulation use.The reaction course showed that the hydrogenation of m-dinitrobenzene was a typical tandem reaction including m-dinitrobenzene to m-nitroaniline and m-nitroaniline to m-phenyldiamine,and the second step only occurred when the m-dinitrobenzene was converted completely;Ru/C also showed high selectivity to o-,p-nitroaniline in hydrogenation of o,p-dinitrobenzene.Similar to Skeletal RuNiC systermRu/C catalyst was also suitable for chernoselective hydrogenation of other substituted dinitrobenzenes.The selectivity of 2,4-dinitrotoluene,1,3-dinitrotoluene,2,5-dinitro-p-xylene, to partially hydrogenated products was 98%,92%and 100%respectively,when the conversion of substrate was 98%,100%,95%;the selectivity to partially reduced product was 100%,100%and 95%when 2,4-dinitrophenol,2,4-dinitrobenzamine,2,4-dinitroanisole converted completely;the selectivity of partial hydrogenation of 3,5-dimtrobenzoic acid, 3,5-dinitrosalicylic acid and 2,4-dinitrofluorobenzene was 97%,100%and 97%when substrate converted completely.All these results are higher than the selectivity published in literature.Similar to Skeletal RuNIC systerm,substited group played important role in the selective hydrogenation.The dinitrobenzene with electron-withdrawing group can be hydrogenated much easier than the dinitrobenzene with electron-donating group.The relative position of substituted group to two nitro groups can lead to foramation of isomers,when the group was -NH₂,-OH,-OCH₃,the reaction showed strong ortho effect,that is,the -NO₂ at the otho position of substituted group could be reduced easily.The hydrogenation of was not only step-wised,but also a two-stage reaction.At first stage,3,5-dinitrosalicylic acid partially hydrogenated to 3-amino-5-nitrosalicylic acid with high selectivity(100%),and the second-stage reaction,in which 3-amino-5-nitrosalicylic acid hydrogenated to 3,5-diaminosalicylic acid,occurred until the reaction conditions were increased.