LiOH修饰的Ru-Rh/γ-Al_2O_3催化2,4-甲苯二胺加氢合成1-甲基-2,4-环己二胺的研究

1-Methyl-2,4-Cyclohexanediamine Synthesis via 2,4-Toluenediamine Hydrogenation Catalyzed by LiOH Modified Ru-Rh/γ-Al_2O_3

2,4-甲苯二胺(2,4-TDA)催化加氢合成1-甲基-2,4-环己二胺(2,4-MCHD)具有对环境友好、原子经济性高等优点,但是易发生脱氨基副反应。研究采用Li OH修饰的Ru-Rh/γ-Al_2O_3双金属催化剂同时提高了2,4-TDA的转化率和2,4-MCHD选择性,并考察了Rh/Ru比、LiOH浓度对催化剂性能的影响。研究表明,在温度为180℃、压力8.0 MPa和催化剂与2,4-TDA质量比为0.1的条件下反应8 h,随Rh/Ru比增大,即Ru含量固定为5%(wt),Rh含量由0增加至5%(wt),2,4-TDA转化率逐渐由45.62%增加至99.55%,2,4-MCHD选择性稳定在84%以上;浸渍Li OH对Ru-Rh/γ-Al_2O_3的2,4-TDA转化率没有影响,但当Li OH水溶液浓度为40%(W/W)时,2,4-MCHD选择性进一步提升至95.36%,收率为94.92%;催化剂经10次循环使用后仍保持较高的活性和选择性。通过BET、TEM、EDX、XRD、H2-TPR、XPS和CO2-TPD等方法对催化剂表征分析发现,加入Rh可形成Ru-Rh合金并通过双金属协同作用,抑制Ru纳米颗粒的聚集,减小金属颗粒粒径,同时Ru-Rh合金的电子转移促使Ru处于缺电子价态,降低了对苯环的吸附活化能垒,富电子态Rh将吸附的氢活化后再经过氢溢流作用传递至Ru上参与苯环加氢反应,使Ru-Rh/γ-Al_2O_3加氢活性显著提升。Li OH在室温下与γ-Al_2O_3形成Li Al2(OH)7·2H2O碱式盐,增强了γ-Al_2O_3表面碱性,抑制氨基在催化剂表面的吸附,促进2,4-MCHD选择性的提高。

Synthesis of 1-methyl-2,4-cyclohexanediamine （2,4-MCHD） via catalytic hydrogenation of 2,4-toluenediamine （2,4-TDA） has advantages of environmental friendliness and high atom economy. However, this reaction is easy to have a deamination side reaction. LiOH-modified Ru-Rh/γ-Al2O3 was used to enhance reaction conversion and selectivity in this study. Effects of Rh/Ru ratios and LiOH concentrations on hydrogenation performance of the catalyst were investigated. The results show that under condition of 180℃, 8.0 MPa, catalyst/2,4-TDA（mass ratio）=0.1 and 8 h, 2,4-TDA conversion gradually increases from 45.62% to 99.55% with the increase of Rh/Ru ratios （Rh content increased from 0 to 5%（wt） when keeping Ru content at 5% （wt））, and the selectivity of 2,4-MCHD is stable of over 84%. LiOH has no obvious effects on 2,4-TDA conversion, but 2,4-MCHD selectivity can be improved to 95.36% and the yield is 94.92% when 40%（W/W） LiOH solution is added. The catalyst remains high activity and selectivity after recycling for 10 times. BET, TEM,EDX, XRD, H2-TPR, XPS and CO2-TPD characterization results show that Ru-Rh alloy is formed after Rh addition. Ru NPs aggregation is suppressed and the metal particle size is reduced by bimetallic synergic effects. Electron transfer in Ru-Rh alloy prompts Ru to electron-deficient state and the activation energy of benzene adsorption is reduced. Meanwhile, hydrogen activated by rich electronic state of Rh participates in benzene hydrogenation reaction by overflow effects, which makes hydrogenation activity of Ru-Rh/γ-Al2O3 significantly increase. The alkalinity of γ-Al2O3 is enhanced owing to the formation of LiAl2（OH）7·2H2O basic salt through LiOH treatment in γ-Al2O3 at room temperature, which improves 2,4-MCHD selectivity by inhibiting amino adsorption on catalyst surface.