Preparation And Catalytic Properties Of Sulfur-doped Carbon Supported Pd-based Bimetallic Catalysts

In industry,alkenes are produced by steam cracking different raw materials such as ethane and naphtha,cracking gasoline extractive distillation,and alkane dehydrogenation.However,in these production processes,it is unavoidable that a certain amount of alkyne impurities will be generated.The presence of alkyne will reduce the quality of polymerization products and may even cause the poisoning and deactivation of the polymerization catalyst.Therefore,removing the alkyne in the alkene feedstock as much as possible is the problem faced by the polymerization process.The selective hydrogenation of alkyne is the most widely used method for removal of trace amounts of alkyne impurities.Palladium(Pd)-based catalysts have been extensively studied because their excellent hydrogenation activity and selectivity.For monometallic Pd catalysts,the comparable hydrogenation and desorption energy barriers of alkene lead to the occurrence of overhydrogenation of alkene.Moreover,the formation of the surface hydride phase is conducive to the oligomerization of alkyne,which produces green oils and other by-products to cover the active metal sites and deactivate the catalysts.To address these challenges,the second transition metal is incorporated into the Pd catalyst to form an alloy structure.On the one hand,the electronic and geometric effects induced by the alloying lead to the weakened alkene adsorption on the alloy surface,which results in the desired suppression of alkene overhydrogenation.On the other hand,the dilution or even the isolation of active Pd atoms in the alloy phase can greatly inhibit the formation of subsurface hydrides and therefore the overhydrogenation is inhibited.Although the alloying strategy can effectively improve the catalytic selectivity compared to monometallic Pd catalysts,the overhydrogenation of alkene cannot be entirely eradicated and the selectivity is generally below 95%near the full conversion of alkene.In this thesis,a series of small-size palladium-based bimetallic catalysts were prepared using mesoporous sulfur-doped carbon(meso_S-C)as supports.According to the theory of hard and soft acid and bases,there was a strong interaction between the metal and the the sulfur atoms in the meso_S-C supports.On the one hand,the metal could be firmly anchored on the meso_S-C supports.The growth of the particle size of the bimetallic nanoparticles due to sintering during H2 reduction was avoided.On the other hand,the strong interactions between the metal and the sulfur atoms in the meso S-C supports modulate the electronic effect of the active Pd site,resulting in an much enhanced selective for the semihydrogenation of phenylacetylene to styrene.The main results can be summarized as follows:1.A strong metal-support interaction strategy is developed to prepare a family of Pd-based bimetallic catalysts.With the meso_S-C materials with high sulfur content(14 wt%)and high specific surface area(1265 m2 g-1)as supports,five Pd-M(M=Fe,Co,Ni,Cu,Ga)bimetallic nanocluster catalysts are prepared,which are characterized by the XRD,TEM,HAADF-STEM,EDS-mapping,and H2-TPR techniques.The prepared catalysts exhibit uniform alloy structures;the alloy nanoparticles are homogeneously dispersed on the meso_S-C supports with an average particle size of around 2 nm.The meso_S-C materials exhibit excellent loading capacity for metal particles compared with the commercial carbon black supports(Vulcan XC-72R).The strong interaction between sulfur and Pd can strongly inhibit the migration and agglomeration of the alloy nanoparticles during the H2-reduction process,which is the key to prepare small-sized Pd-based bimetallic catalysts.2.The high catalytic performance of the prepared Pd-based bimetallic catalysts for the selective hydrogenation of phenylacetylene are demonstrated.The meso_S-C supported Pd-M bimetallic catalysts have higher styrene selectivity than the commercial Pd/C catalyst and the Vulcan XC-72R supported catalysts.XPS analyses reveal that the electronic effects induced by the alloying and the strong Pd-S interaction positively affects the selectivity.Specifically,the electrons transfer from Pd to sulfur causes the electron-deficient state of Pd,which significantly improves the activation energy barrier for styrene hydrogenation.Meanwhile,the alloying induces the of the lowered adsorption energy of styrene.These electronic effects suppress the excessive hydrogenation of styrene and therefore improve the selectivity.