Copper-Based Catalysts For Acetylene Selective Hydrogenation

Ethylene is one of the most basic raw materials in petrochemical industry.The ethylene feedstock produced from steam cracking often contains a trace amount of acetylene,which will poison the downstream ethylene polymerization catalysts and lowers the quality of polyethylene products by crosslinking.As a result,it is indispensable to remove acetylene impurity completely for the production of polymer grade ethylene.The preferred process of removing acetylene from ethylene stream in industry is selective hydrogenation by the supported Ag-promoted Pd catalyst,which is of high cost.Therefore,it is of significance in application and theory to design non-precious metal catalysts for selective hydrogenation of acetylene.Excellent ethylene selectivity could be achieved over Cu catalysts in alkyne hydrogenation.Nevertheless,due to the poor ability of H2 dissociation on Cu surface,hydrogenation is always carried out at high temperatures.Oligomerization and polymerization of acetylene are generally enhanced over the Cu surface at high temperatures,resulting in rapid deactivation of the catalyst.If the hydrogenation activity of Cu at low temperatures can be improved,the polymerization of acetylene could be avoided,resulting in the enhanced activity and stability of the Cu catalyst.In the present thesis,a series of catalysts with excellent performance in acetylene hydrogenation were prepared from Cu2O,Cu(OH)2,and Cu2(OH)2CO3 via thermal treatment with low-concentration acetylene and reduction with H2(AT-HR method).In addition,Zn was introduced as a promoter to modify the structure of the catalyst and enhance the catalytic performance of the catalyst.The structure-performance relationship of the catalysts was investigated by means of XRD,SEM,TEM,XPS,Raman,TGDSC,and H2-TPR.The main results are as follows.The Cu2O nanocubes with the size of 50-100 nm were synthesized in a microreactor.The acetylene hydrogenation catalyst Cu2O(T160-R1 80)was fabricated from Cu2O nanocubes via the AT-HR method involving a surface reaction with an acetylene-containing gas at 160℃ and subsequent H2 reduction at 180℃.The Cu2O(T160-R180)catalyst was in a core-shell structure with a core of Cu metal and a shell of Cu and CuxC particles covered by an amorphous carbon layer.The CuxC phase is generated from hydrogen reduction of Cu2C2,Cu paritcles and amorphous carbon are obtained from thermal decomposition of Cu2C2 simultaneously.The Cu2O(T160-R180)catalyst displays an excellent performance in acetylene hydrogenation.In a simulated industrial gas(0.72%CH4/0.45%C2H2/10.00%H2/88.83%C2H4),complete acetylene conversion with～10%ethane selectivity was achieved at atmospheric pressure and 100℃.In addition,when the H2/C2H2 ratio increased from 8 to 22,acetylene is completely converted and the ethane selectivity increased from 8%to 31%.The ethane selectivity is insensitive to the H2/C2H2 ratio,avoiding a net ethylene loss.The DFT calculation implied the structure of CuxC was in accordance with CU3C.The dissociation energy of H2 on Cu3C(0001)(48.2 kJ/mol)is markedly lower than that on Cu(111)(109.4 kJ/mol),indicating that the high hydrogenation activity of Cu2O(T160-R1 80)is from CuxC phase.Furthermore,the activity and product selectivity was stable in the 350-h run and no obvious polymer was formed from the SEM image of the used catalyst,indicating the excellent stability of the catalyst.The favorable stability might be associated with the lower reaction temperature and the amorphous carbon shell,which poses a steric hindrance to the chain growth of linear hydrocarbons,thereby inhibiting the oligermerization and polymerization of acetylene.Cu(OH)2 and Cu2(OH)2CO3 were taken to prepare the Cu(OH)2(Tx-Ry)and Cu2(OH)2CO3(Tx-Ry)catalysts through the AT-HR method.Similarly,both the two catalysts were composed of Cu and CuxC nanoparticles which were covered by an amorphous carbon layer.The CuxC was prepared from H2 reduction of CuC2,accompanying with the CuC2 decomposition to Cu particles and amorphous carbon.Both the catalysts displayed an excellent acetylene hydrogenation performance with high activity,low ethane selectivity and excellent stability at low temperatures.Complete acetylene conversion was achieved at atmospheric pressure and 110℃.And the ethan e selectivity is insensitive to the H2/C2H2 ratio.In addition,the excellent stability is associated with the low reaction temperature and the amorphous carbon layer.The acetylene hydrogenation catalyst Cu0.8Zn0.2(OH)(T140-R150)was prepared from Cu1xZnx(OH)through the AT-HR method.After the addition of Zn,the morphology of precursor evolved from nano wires to nanosheets,and the intensity of the diffraction peaks of Cu(OH)2 in XRD pattern decreased,indicating the decrease of the crystalline size of Cu(OH)2.In addition,the hydrogenation activity of the catalyst was enhanced with the addition of Zn.On the one hand,the introduction of Zn reduced the crystalline size of Cu(OH)2,resulting in more CuC2 in the acetylene treatment step,on the other hand,the thermal stability of CuC2 is improved with the Zn addition,which is conducive to the formation of CuxC phase in the hydrogen reduction step.