Synthesis Of(Rh,Ru,Pd)-Based Catalysts And Their Catalytic Performance Of Selective Hydrogenation Of Nitroaromatics

Arylamines are an important class of organic intermediates that can be widely used in various fields of life,such as agrochemicals,drugs,dyes and pigments.At present,the main method of industrial production of arylamines is to reduce nitroaromatics using traditional reducing agents such as:iron powder,Raney nickel,etc.This method has low yield and serious environmental problems.In addition,when there are two or more reducible groups in the nitroaromatics molecule,it is difficult to obtain the desired product by conventional reduction methods,and the catalyst cost is high and the economic efficiency is poor.Currently,catalytic hydrogenation of aromatic nitro compounds is the preferred method for the preparation of aromatic amines and an important class of hydrogenation reactions in chemical production,and the design and construction of catalysts with high selectivity for nitro hydrogenation and low catalyst cost,high activity and stability are essential.In this paper,a series of highly disperse catalysts with different noble metal loadings were synthesized using different catalyst supports and catalyst preparation methods,and the catalytic performance of the catalysts for the selective catalytic hydrogenation of nitroaromatics was investigated to further investigate the reasons for the differences in catalytic performance.The specific studies are mainly as follows:（1）Co（OH）₂ supports were synthesized by hydrothermal method,and a series of 0.54wt%（Pt,Rh,Pd,Ru）/Co（OH）₂ and 1.23 wt%Rh/Co（OH）₂ catalysts were successfully synthesized for the selective hydrogenation of nitrobenzene and its derivatives using the impregnation method with Co（OH）₂ as the support,and the research showed that 0.54wt%Rh/Co（OH）₂ outperformed the other catalysts:it showed excellent catalytic performance（97.6%conversion and 100%selectivity）for the catalytic hydrogenation of2-chloronitrobenzene to 2-chloroaniline in 0.66 h at 3.0 MPa H₂,100°C.In addition,the turnover frequency（TOF）of 0.54 wt%Rh/Co（OH）₂-575 h^-1 was much higher than that of 1.23wt%Rh/Co（OH）₂-398 h^-1.XRD,TEM,HRTEM,AC-STEM（spherical aberration corrected scanning transmission electron microscopy）,AC-STEM-EDX elemental surface and line sweeps,H₂-TPD and other characterization means were used to demonstrate that Rh in the0.54 wt%Rh/Co（OH）₂ catalyst was dispersed on Co（OH）₂ nanoplates as single atoms and ultrasmall nanoclusters.Meanwhile,0.54 wt%Rh/Co（OH）₂ also exhibited high selectivity for the hydrogenation of other nitroaromatic substrates by-NO₂.（the yields of nitrobenzene at 1 h were 97.5%,1,3-dinitrobenzene at 1 h was 97.5%,2-nitrobenzaldehyde at 2 h,p-nitroacetophenone at 0.5 h,and p-nitrobenzonitrile at 7 h were 96.4%,96.3%,and 98.2%,respectively）The possible reason is that the synergistic effect of Rh and the support Co（OH）₂jointly promotes the hydrogenation of nitrobenzene and its derivatives to the corresponding aromatic amine products.（2）Ru/MoS₂ catalysts with different Ru loadings（0.37 wt%,0.88 wt%,2.45 wt%）were synthesized for the ethylene hydrogenation of 3-nitrobenzene by impregnation method using Mo S₂ as the support.The results showed that the catalytic performance of the 0.88wt%Ru/Mo S₂ catalyst at 3.0 MPa H₂ and 100°C was better than the remaining two catalysts（96.8%conversion with 100%selectivity in 1.33 h）.In addition,the 0.88 wt%Ru/Mo S₂catalyst exhibited the highest catalytic activity at this condition with a TOF of 928 h^-1 higher than 0.37 wt%Ru/Mo S₂-692 h^-1 and 2.45 wt%Ru/Mo S₂-902 h^-1.Ru/Mo S₂ also showed the best catalytic performance when different noble metal-loaded M/Mo S₂（M=Pt,Rh,Pd,Ru）catalysts were used for the selective hydrogenation of p-nitroacetophenone.Also,the 0.88wt%Ru/Mo S₂ catalysts showed high-NO₂ hydrogenation selectivity for other selected nitrobenzene derivatives（the yields of nitrobenzene at 2.5 h were 94.5%,2-chloronitrobenzene at 3 h was 96.6%,1,3-dinitrobenzene at 9 h was 75.7%,p-nitroacetophenone at 4 h and p-nitrobenzonitrile at 2 h were 98.5%and 98.2%,respectively）.The catalyst characterization results showed that Ru in the 0.88 wt%Ru/Mo S₂catalyst was dispersed on the support as single atoms or nanoclusters,and the high dispersion of Ru atoms led to an increase in the number of active sites on the catalyst surface,while the special structure of the Mo S₂ surface of the support facilitated the adsorption of-NO₂ on Mo⁴⁺,which improved its catalytic activity and selectivity.In addition,the 0.88 wt%Ru/Mo S₂catalyst was used for five cycles of experiments,and it was found that the catalytic performance did not change significantly,indicating that the catalyst has good stability.（3）The g-C₃N₄ was prepared by high-temperature roasting method,and four different noble metal loaded catalysts of M/g-C₃N₄（M=Pt,Rh,Pd,Ru）were synthesized by Na BH₄reduction method using g-C₃N₄ as the support.The catalytic performance of the Pd/g-C₃N₄catalysts was found to be superior to the other three catalysts in catalyzing the selective hydrogenation of p-nitroacetophenone（96.5%conversion with 100%selectivity at 3.0 MPa H₂,20°C,0.33 h）.In addition,the Pd/g-C₃N₄ catalysts with different Pd loadings（0.38 wt%,0.76 wt%,2.51 wt%）catalyzed the hydrogenation of 1,3-dinitrobenzene,and the 2.51wt%Pd/g-C₃N₄ catalyst obtained 97.6%conversion and 100%selectivity at 3.0 MPa H₂,20°C for 1 min.Comparison of TOF shows that 2.51 wt%Pd/g-C₃N₄-101 min^-1>0.76wt%Pd/g-C₃N₄-32 min^-1>0.38 wt%Pd/g-C₃N₄-28 min^-1.Characterization of the prepared catalysts（XRD,SEM,TEM,HRTEM,BET and H₂-TPD,etc.）demonstrates that that the Pd nanoparticles in the Pd/g-C₃N₄ catalyst had good dispersion on the support.Also,the 2.51wt%Pd/g-C₃N₄ catalyst showed high-NO₂ hydrogenation selectivity for other selected substituted nitro substrates（the yields of nitrobenzene at 1 min were 97.5%,2-chloronitrobenzene at 1 min was 96.4%,2-nitrobenzaldehyde at 120 min was 92.6%,p-nitroacetophenone at 20 min and p-nitrobenzonitrile at 120 min were 96.5%and 92.4%,respectively）.This may be attributed to the high dispersion of Pd nanoparticles in the catalyst and the synergistic interaction between Pd nanoparticles and the support g-C₃N₄ jointly promoting the catalytic activity and selectivity of the catalyst.In addition,the catalytic performance of the catalyst remained stable after five cycles of the reaction,indicating that the catalyst has good stability.