Study On SiC-based Catalysts For Selective Hydrogenation Of Phenylacetylene

Styrene is an important monomer of polystyrene and other polymers,as well as an important raw material for synthesizing coatings and medicines.There usually exists trace phenylacetylene in the styrene raw material,which easily causes the poisoning of the catalyst in the polymerization process of styrene,thereby reducing the catalyst activity.In addition,phenylacetylene can also deteriorate the quality of polystyrene,such as odor,discoloration,degradation,etc.Therefore,it is very important to eliminate the trace phenylacetylene impurity in styrene.It is well known that alkynes are easily hydrogenated to olefins.However,the hydrogenation usually is a continuous process,and the intermediate product olefins are easily hydrogenated to alkanes.So,it is necessary to develop new catalysts that can completely convert phenylacetylene and maintain high selectivity for styrene.Supported catalysts have unique catalytic performance and are widely used in selective hydrogenation.The catalytic activity of the catalysts is closely related to the dispersion of active components and the properties of support.Therefore,changing the support and adding a second metal are usually effective to promote the performances of catalyst.At present,commonly used Pd/C catalysts have some problems such as low activity or low selectivity.In this work,high specific surface area silicon carbide（SiC）was used as the support of Pd.The electron transfer between the active component Pd and the support and the introduction of the second component metal Cu were studied.The main results are summarized as follows:（1）Pd/SiC catalyst with a Pd loading of 3wt.%was prepared,and named as Pd₃/SiC.Under mild conditions,phenylacetylene can be selectively hydrogenated to styrene,and the turnover frequency（TOF）can reach to 1342h^-1.Influences of different supports and solvents on the reaction and the stability of the Pd₃/SiC catalyst were studied.The catalyst was characterized by X-ray diffraction（XRD）,X-ray photoelectron spectroscopy（XPS）,high-resolution transmission electron microscopy（HRTEM）,and in-situ Fourier transform infrared spectroscopy（FTIR）.According to our results,the high activity of Pd₃/SiC for the selective hydrogenation of phenylacetylene is due to the formation of the Mott-Schottky contacts between Pd and SiC.Because of the Mott-Schottky effect,part electrons in the conduction band of SiC were transferred to Pd,forming an electron-rich Pd and positively charged carrier surface.H₂ was dissociated on the surface of Pd particles,and the dissociated hydrogen spillovered to the SiC surface to form surface hydrogen species.These surface hydrogen species migrated on the SiC surface and took part in the hydrogenation reaction with the phenylacetylene activated by the SiC surface to form styrene.Compared with other supports,Pd₃/SiC catalyst has higher catalytic activity for selective hydrogenation of phenylacetylene.（2）Bimetallic PdCu/SiC catalysts were prepared.Under mild conditions,phenylacetylene can be selectively hydrogenated to styrene on the PdCu/SiC catalyst.The effects of Pd/Cu ratio,catalyst support,solvent,etc.on the catalytic activity and the stability of the catalyst were studied.HRTEM,XRD and other methods were employed to characterize the catalyst.