| The selective hydrogenation of aromatic halonitro compounds to the corresponding aromatic haloamines is one of the most essential transformations in the synthesis of fine chemicals and intermediates. Although many efforts have been made to minimize dehalogenation, up to now, it remains a big challenge to completely avoid the hydrodehalogenation as a result of the electron donating effect of amino groups in the aromatic ring. In addition, most of the hydrogenation processes entail the use of organic solvents including methanol or alcohol etc, which plays an important part in dissolution, mass and heat transfer, solvation effect and so forth. However, organic solvents are hazardous in terms of toxicity, waste generation and flammability, especially in the presence of ignitable Raney Ni and noble metal catalysts. Thus removal of solvents and trace impurities is highly significant to achieve green production of aromatic haloamines.On the base of the existing problem and deficiency aforementioned, we reported the catalytic performance of Ni catalyst modified with minute quantity of organic amine compounds in the selective hydrogenation of chloronitrobenzenes to chloroanilines. There is a strong interaction between the electron-deficient Lewis acid site characterized as Ni-H+ specie on the surface of Raney Ni catalyst and the nitrogen atom in organic amine molecule with high electronegativity. And then C-Cl bond would not be polarized and activated. The hydrodechlorination side reaction would be inhibited. Over RND-4 catalyst, the selectivity to 3-chloro-2-methylaniline reached up to 99.96% when the 6-chloro-2-nitrotoluene hydrogenation was fully accomplished within 64 min. The amount of hydrodechlorination by-product in the 6-chloro-2-nitrotoluene hydrogenation over modified Ni catalyst was 10 times lower than that over unmodified Raney Ni catalyst. Furthermore, the selectivity to chloroanilines was, to a large extent, depended on the hydrogen diffusion and dissociative adsorption of hydrogen on the catalyst surface efficiently. We think that the superior selectivity for the selective hydrogenation of chloronitrobenzenes to chloroanilines could not be obtained, unless the catalyst possesses excellent dissociative adsorption hydrogen and the catalytic sites in favor of the hydrodehalogenation are covered or deactivated.Selective hydrogenation of aromatic halonitro compounds to aromatic haloamines is usually carried out in methanol solvent, which is hazardous in terms of toxicity, flammability and waste generation. Aromatic haloamines and the corresponding aromatic halonitro compounds have the relatively low melting points, which provide a possibility to carry out catalytic hydrogenation under solvent-free conditions. Whereas much less effort have been paid to the study on the catalytic hydrogenation in solvent-free. In the present work, we prepared a series of Pd/C catalysts with different Pd particles sizes, through the pretreatment of activated carbon with halogen ions solution. The sizes of Pd particles were dependent on the electrical double layer built up with halogen ions and the activated carbon. The Pd/C catalyst with the average Pd particles size of 35 nm and a narrow particle size distribution could be prepared after the activated carbon was pretreated with KI aqueous solution (2.5 mol/l). Compared with the hydrogenation in organic solvent, the selectivity to the desired product of catalytic hydrogeantion of 6-chloro-2-nitrotoluene and some other aromatic halonitro compounds over Pd/C(â… ) catalyst could reach up to as high as 99.9% under solvent-free conditions. Furthermore, Pd/C catalyst could be recovered and reused more than 10 times without any notable loss of catalytic activity and selectivity. The results infer that the larger Pd particles with narrow particle size distribution, hydrogen diffusion, and the competitive adsorption of water and aromatic haloamines on the catalyst surface could be responsible for the almost complete suppression hydrodehalogenation.
|
PDF Full Text Request