Studies On Catalysts For Hydrogenations Of Some Important Aromatic Nitro Compounds

Aromatic amines are important organic materials and chemical intermediates widely used for the productions of dyes,pharmaceuticals,pesticides,and polymers.For example,aniline is an important raw material for the manufacture of diphenylmethane diisocyanate and dye intermediate.p-Aminoanisole is an important intermediate for dyes and medicines.2,4-Diamino-hexadecyloxybenzene(16PDA)is one of the main raw materials for the production of high-grade super twisted nematic liquid crystal displays.In industry,most aromatic amines are obtained via the reduction of aromatic nitro compounds.Among others,the catalytic hydrogenation method conducted in a fixed bed reactor has the advantages of mild conditions,simple operation,high product quality,high yield,low waste and easy separation of catalysts from reactants/products,and thus becomes a research hotspot for the industrial synthesis of aromatic amines.In this paper,several supported iron-base and nickel-base catalysts were prepared and tested for the fixed-bed hydrogenations of three aromatic nitro compounds to aniline,p-aminoanisole and 16PDA,respectively.The fresh and used catalysts were characterized by X-ray diffraction(XRD),N2 adsorption-desorption(BET),X-ray fluorescence spectrometers(XRF)and transmission electron microscopy(TEM),and the results were correlated with the catalytic performances of catalysts.The main results are summarized as follows:(1)A 60wt%Fe/Al2O3-C catalyst was prepared by co-precipitation method.It was reduced by H2 to produce metallic Fe,which was then phosphided by triphenylphosphine(PPh3)in liquid phases to Fe2P and FeP.It was found that the activity of the Fe/Al2O3-C catalyst for the hydrogenation of nitrobenzene to aniline decreased rapidly,while that of the FeP/Al2O3-C and Fe2P/Al2O3-C decreased slower,indicating that the iron phosphides were more stable than Fe for the hydrogenation of nitrobenzene.Moreover,the FeP/Al2O3-C was more active and stable than the Fe2P/Al2O3-C for the reaction.In addition,the solvent was found to affect the selectivity significantly.Specifically,both the conversion of nitrobezene and selectivity to aniline were significantly higher in DMF than in ethanol.(2)A Ni/Al2O3 catalyst was used for the hydrogenation of p-nitroanisole to p-aminoanisole.The p-nitroanisole was completely converted to p-aminoanisole with a selectivity of?100%under 40-70? and 1-3 MPa in a time-on-stream test carried out for up to 4656 hours on the Ni/Al2O3 catalyst After the test,the phase,pore volume and pore size of the catalyst did not change significantly,and the particles were still highly dispersed(though grew slightly).These results indicated that the Ni/Al2O3 was highly active,selective and stable for the hydrogenation of p-nitroanisole to p-aminoanisole.(3)The continuous hydrogenations of 2,4-dimethylnitrobenzene,p-nitrobenzoic acid and 2,4-dinitro-hexadecyloxybenzene(16PDN)were also carried out over the Ni/Al2O3 catalyst under 50-125 ?,1-3 MPa and 0.1-1 h-1.The test lasted for 3024 h(2496 h for the hydrogenation of 16PDN),indicating that the Ni/Al2O3 could also be applied for the hydrogenations of long-chain aromatic nitro compounds with relatively large molecular weight.Thus,the results obtained in this work appeared promising for the future applications of the Ni/Al2O3 for the hydrogenations of nitro compounds in industry.