The Preparation And Application Of Ni-B Catalysts For The Hydrogenation Of Nitrobenzene

Aniline is one of the most important chemical products in chemical engineering. It is applied in the production of drags, dyes, paints and detonator. It has great potential market. Aniline could be produced by nitrobenzene selective hydrogenation. Compared with other methods, it can decrease the reaction time, improve the qualities of products, reduce the cost of production and decrease pollution. With the increase of the consumption of aniline, the technology becomes more important.During the process of catalytic hydrogenation, catalyst is one of the most factors that determine the cost and the qualities of products. There are three kind of catalysts used in nitrobenzene selective hydrogenation to aniline, such as Cu, Ni and rare metal catalysts .The Cu catalysts have lower price, but they should work in the environment of high pressure and temperature .And Cr element often acts as promoters in Cu catalysts .As to the rare metal catalysts, they can't find big market in the application of industrial process, due to their high price. So it is important to find a kind of catalysts which are friendly to environment and have considerable catalytic properties, lower price and work in mild reaction condition.Ni-B catalysts are a kind of novel materials with short-range ordering and long-range disordering structure .Ni-B catalysts have some unique properties, such as adjustable structure, high concentration of unsaturated sites and chemical uniformity. The process of preparing Ni-B catalysts is of less pollution and cost and relatively simple. They have been employed in many selective hydrogenation and exhibit excellent catalytic properties. They are considered to be one kind of green efficient catalytic material with the potential ability to replace the traditional catalyst for hydrogenation.In this paper, the model reaction is the liquid-phase nitrobenzene selective hydrogenation to aniline. Firstly, the preparation parameter of catalysts including to the precursor of Ni salt and the mole ration of n(KBH₄)/n(Ni) were investigated .It was found that the Ni-B catalyst was more active when using Ni(CH₃COO)₂·4H₂O as precursor. Concerned with the H₂-TPD, the effects of n(KBH₄)/n(Ni) on the catalytic activity and adsorption-desorption performance of catalysts were discussed .It was found that the catalysts had better activities and the H adsorbed on the surface of catalysts was more active when n(KBH₄)/n(Ni) (mol/mol) was high. During the hydrogenation reaction, their catalytic properties were more stable.Because it was reported that bi-metallic catalyst are of short inducing period and more active, the Ni-B catalysts with a small mount of Fe, Co, Cu and La as promoters were investigated respectively in nitrobenzene hydrogenation .It was found that Fe, Co and Cu decreased the activity of the catalyst to a certain degree and the addition of La can improve the activity. Moreover, when adding suitable amount of La(NO₃)₂ to reaction media ,the conversion of nitrobenzene ,aniline selectivity and yield were improved greatly. But when the mole ration of La /Ni exceeded a certain amount, too much La covered the active center on the surface of catalyst and decreased the activities of catalysts.At present ,the supported Ni-B catalysts attract much attention .In this paper ,γ-Al₂O₃, SiO₂ and active carbon(AC) were employed as supports .The activities of catalysts were as follow: Ni-B/AC> Ni-B/γ-Al₂O₃> Ni-B/SiO₂> Ni-B. Then the support active carbon (AC) was pretreated. It was found that the catalyst Ni-B supported on the active carbon (AC) which was treated under higher temperature. had better activities. As to Ni-B/AC, the TOF increased firstly and then got to a max with increase of the usage of Ni. And then, the TOF decreased slightly. As to Ni-B/AC, the catalytic activity first increased and then decreased with the increase of calcinations temperature .In the end, effect of the time of ultrasonic treatment on the catalysts was investigated .it was found that the activity was first increased and decreased softly with increase of the time of ultrasonic treatment.At last, it was investigated the effect on catalytic reaction of the reaction temperature, reaction pressure ,concentration of catalyst, concentration of nitrobenzene, and reaction time in order to find the optimum reaction condition.. The reaction temperature, reaction pressure and concentration of nitrobenzene had great effect on the reaction. When the reaction temperature increased from 363 K to 383 K, the reaction rate increased abruptly. The increases of reaction pressure can improve the selectivity to aniline and reaction rate .To some degree, the concentration of nitrobenzene also influence the nitrobenzene conversion and aniline selectivity.