Research On The Electron Effect Of Functional Groups In The Rh Catalyzed Hydrogenation Reaction

Industrial production of aromatic amines is mainly derived from the direct reduction of nitroarenes,while conventional metal reduction method has some problems such as environmental contamination,low selectivity,etc.Catalytic hydrogenation method is cleaner and more efficient than metal reduction method,and it can also be applied in manufacture in a large scale.Thus,the catalytic hydrogenation method has receive extensive attention in recent years.It is clear that a suitable catalyst is vital for catalytic hydrogenation of nitroarenes,while modifying the surface of metal catalyst is a crucial subject for improving the heterogeneous metal catalysts.Nevertheless,it is still a great challenge to deeply understand the structure-activity relationship(SAR)between the surface supporting groups and the metal particles and to achieve directional adjustment of the structure and function of heterogeneous catalysts.Herein,we have synthesized a series of Rh NPs based magnetic catalysts with different functional groups.These catalysts are applied to the catalytic hydrogenation of nitroarenes under ambient temperature and pressure conditions,and the SAR of them are investigated in-depth.The Rh NPs based core-shell structure catalysts are initially synthesized,and the structure and properties of the catalysts are characterized by high resolution transmission electron microscopy(HR-TEM),X-ray powder diffraction(XRD),nitrogen adsorption/desorption(BET),X-ray photoelectron spectroscopy(XPS)and vibration sample magnetometer(VSM).The Fe3O4@SiO2-NH2-RhNPs@mSiO2 catalyst can be separated easily by an external magnet after the reaction was completed due to its superparamagnetic property.The catalysts can be used up to 10 times without losing its activi ty,which can be attributed to the cooperation of amino functional groups and porous silica shell.When the Fe3O4@SiO2-NH2-RhNPs@mSiO2 catalyst is applied in the catalytic hydrogenation system to reduce the 2-chloro-3-nitropyridine under mild conditions,no dechlorination is detected,and both the selectivity and conversion reach up to 99%.Furthermore,even various nitro-compounds with different functional groups are introduced in this reaction system,the catalyst can still exhibit excellent catalytic activity and selectivity.To further explore the explanation for the outstanding performance of the catalyst,a series of Rh NPs based catalysts with various functional groups have been prepared for catalytic hydrogenation of 2-chloro-3-nitropyridine under ambient temperature and pressure conditions.We discover that the catalysts with electron-donating groups exhibit better catalytic performance.Based on the result of XPS,it is noted that the chemical state of Rh NPs depends on the supporting groups intensely:The electrondonating functionalities can effectively elevate the Rh0/Rh3+ ratio,which increase the catalytic performance both in conversion and selectivity.