Catalyst Designs And Mechanism Studies For Selective Hydrogenation Of Nitroarenes

Catalytic reduction of nitroarenes is of great importance in chemical industries due to the tremendous value of the products.For example,aromatic amines are important raw materials in synthesis for dyes,drugs and agrochemicals.In addition,aromatic amines can further reduce to corresponding cycloaliphatic amines,an important industrial raw material,especially for the synthesis of isocyanates and polyurethanes.It remains important to develop cheap metal catalysts with excellent performance,in order to reduce the use of noble metals.The present work has synthesized rod-like and spherical silica based on Stober method,and then employed the ammonia evaporation hydrothermal method to fabricate a silica supported copper catalyst.It was found that the morphology of the materials changed from solid structure with clean surface to tube-like hollow structure after hydrothermal treatment;meanwhile the pore size of the materials enlarged and macropore structure appeared.These changes can be derived from the formation of lamellar copper phyllosilicate and different hydrothermal stability of the surface and the interior of synthesized silica.Compared with the catalysts without hydrothermal treatment,the catalysts after hydrothermal treatment exhibited a better performance and the conversion of nitrobenzene increased from 36%to 83%(2.4 MPa H2,90 ?,2h),possibly due to larger pore structure and more copper particles exposed on the surface of the catalyst,not inside of the pore.During the synthesis of cycloaliphatic amines from aromatic amines,secondary amines are inevitably formed,which decreases the selectivity of primary amines.Many reports have found specific additives can suppress the formation of side products,but the mechanism is still not clear.In this work,the effects of various types of alkali additives on selectivity of cyclohexylamine have been investigated over Ru-based catalyst.It was found that modification with the hydroxide ion of alkali metal hydroxides,rather than the cations,svppressed side reactions by a series of controlled experiments.Theoretical calculations further disclosed that the suppression of enamine dissociation and the easier desorption of cyclohexylamine from the active sites by alkali metal hydroxides prevent cyclohexylamine from reacting with imine and then improve the selectivity from 85.4%to 100%.Later,to further enhance the catalytic performance,various bimetallic catalysts were prepared.The resultant RuNi/AC showed very high activity with a yield of cyclohexylamine up to 76.1%,while Ru/AC gave only 35.6%conversion of nitrobenzene and the main product was aniline.