Investigation Of The Ni-Fe Catalysts For Dinitrotoluene Hydrogenation

Toluendiamine (TDA) is also known as diaminotoluene, mainly used for the synthesis of tolylenediisocyanate (TDI). In industry, the production of TDA is mainly via a single step catalytic hydrogenation of DNT using both noble metal catalyst mainly included Pd/C and Pt/C or Raney-Ni catalyst. Supported noble metal catalysts usually have excellent catalytic performance, but are very expensive and cost a lot. Raney-Ni catalysts have better catalytic activity as well, but Raney-Ni pyrophoric nature and the consequent serious safety hazards have limited its further application. Urushibara nickel catalyst has a considerable catalytic hydrogenation activity as Raney-Ni catalyst and can be used for the same hydrogenation reaction. In addition, the preparation process of urushibara nickel catalyst is relatively simple. However, the strong reduction of elemental metal Zn is easy to react with water to generate hydrogen and release a large amount of heat in the substitution process of Ni, making the sample expand and solidify.In this paper, Ni-Fe nickel catalysts were prepared by substitution reaction using Fe substituting Zn and applied in the hydrogenation reaction of dinitrotoluene (DNT) to toluenediamine (TDA). It is mainly focused on the influence of catalyst preparation conditions on the catalytic performance. The relationship between catalyst microstructure and hydrogenation properties were characterized by XRD, H2-TPD, XPS, BET and TEM technology. The main conclusions were summarized as follows:(1) The Fe powder displaced Ni2+in the solution to prepare Ni-Fe catalysts. In this process, the displacement of elemental metals Ni firstly deposited in Fe grain surface, then gradually transferred into the Fe grain formed Ni-Fe alloy and Ni grain. Using acetic acid solution for activation, it could dissolve the iron and iron oxide and remove unreacted elemental metal Fe, which was made the catalyst forming certain channel and exposed more of the active species of Ni. As a result, the catalyst had a certain catalytic properties;(2) The effects of Ni/Fe molar ratio were studied on the microstructure and catalytic hydrogenation properties of Ni-Fe catalysts. The results showed that when Ni/Fe molar ratio was1:4, Ni-Fe catalyst had a smaller grain size, adsorbed a large amount of chemical hydrogen, exposed a lot active species, exhibited a better catalytic performance. DNT conversion rate reached98.1%, TDA selectivity reached86.4%, TOF value reached3.126mol/(g·h), the catalyst owned higher catalytic efficiency;(3) The effects of activation temperature and activation time were studied on the microstructure and catalytic hydrogenation properties of Ni-Fe catalyst. The results showed that the Ni-Fe catalyst had a higher activity when the activation temperature was70℃, the activation time was2h. The catalyst chain presented dendritic shape, the grain size of the catalyst was11.38nm, the DNT conversion and the TDA selectivity were98.9%and98.8%, respectively;(4) The effects of reaction temperature and reaction time were studied on the micro structure and catalytic hydrogenation properties of Ni-Fe catalyst. The results showed that when the reaction temperature was70℃and the reaction time was7h, Ni-Fe catalyst had the maximum amount of chemical hydrogen absorption and more active species of Ni, the catalyst exhibited higher catalytic activity. The conversion of DNT and selectivity of TDA were99.2%and98.7%, respectively;(5) The effect of acetic acid concentration were studied on the microstructure and catalytic hydrogenation properties of Ni-Fe catalyst. The results showed that when the concentration of acetic acid was3mol/L, the grain of the catalyst was only15.3nm. The catalyst exhibited excellent catalytic performance and uniform distribution.(6) By optimizing the preparation conditions of catalyst, such as the Ni/Fe molar ratio, activation temperature, activation time, reaction temperature, reaction time and concentration of acetic acid. The analysis results showed that when the Ni/Fe molar ratio was1:4, the activation time was2h, the activation temperature was70℃, the reaction time was7h, the reaction temperature was70℃and the concentration of acetic acid was3mol/L, the Ni-Fe catalyst owed optimal hydrogenation performance. Under the conditions of2.0MPa and110℃, the conversion of DNT and the selectivity of TDA was99.8%and 99.9%, respectively.(7) Synthesized toluene diamine through DNT hydrogenation was a complex solid-liquid-gas phase reaction. By continuous sampling from Urushibara nickel catalyst hydrogenation process and Ni-Fe catalyst hydrogenation process, we analysed how the content of intermediate products change with time. The results showed that Urushibara nickel catalyst and the Ni-Fe catalyst were both produced four kinds of intermediate products during the hydrogenation process. Urushibara nickel catalyst could promote DNT converted to intermediate products within a short time, while Ni-Fe catalyst could fast prompting intermediate products converted to TDA.