The Electronic Effect Between Ionic Liquid And Palladium And Its Application In Acetylene Semi-hydrogenation

Catalytic cracking petroleum hydrocarbon gradually becomes the mainstream of ethylene preparation method.In hydrocarbon steam cracking reaction,furnace off-gas often contains trace amounts of impurities such as acetylene,propyne and butadiene.In the production of polyethylene,the downstream products of ethylene,Ziegler-Natta catalyst's channel,which is the catalyst used in industrial production,is easy to be blocked by acetylene polymerization,resulting in a decline in the activity of catalyst,as a result,remove trace acetylene from ethylene becomes a very important part of the ethylene industry.Nowadays industry mainly uses palladium catalysts for hydrogenation acetylene,but palladium catalyst has a problem of low ethylene selectivity,green oil also can result in the deactivation of the catalyst.To modify the single metal palladium catalyst,there are two main methods:firstly,adding a second metal as additives,such as gold,silver,etc.;the second method is adding organic compounds as additives.This paper use the second method,find an agent to improve the selectivity of palladium catalyst.Solid catalysts immobilized ionic liquids have been widely used in hydrogenation,esterification,coupling and many kinds of catalytic reaction system.Such kind of catalyst has both the advantages of ionic liquid and heterogeneous catalyst.At the same time,avoid the disadvantages of ionic liquid such as mass transfer capacity difference and reduces the usage of ionic liquids.So we try to use this method to modify palladium catalyst.Under the reaction condition of 170?C and6000 h^-1,using the reaction gas formed by 0.3 vol%acetylene,0.6 vol%hydrogen,30vol%ethylene,69 vol%nitrogen,the ethylene selectivity of the catalyst increased from-20%?Pd/Al₂O₃?to over 90%.The ionic liquid used in this catalyst is1-propyl-3-methyl imidazole chloride salt?[Prmim][Cl]?,the temperature of 100%acetylene conversion increased after modified ionic liquid,the reason should be that ionic liquids is a larger viscosity liquid,after immobilized the mass transfer rate is reduced.By Soxhlet extraction,the location of palladium in Pd-IL/Al₂O₃ catalyst has been detected.With the strong interaction of ionic liquid and palladium,almost all the palladium atoms are in the ionic liquid layer.In order to further illustrate in ionic liquid catalyst role,a range of different ionic liquids?[Pdmim][Cl],[Prmim][PF₆]?was added as a modifier.The preparation of catalysts was same.After observed the catalyst performance,the results show that the conversion of all the catalysts reached 100%,the ethylene selective of the catalyst immobilized by[Pdmim][Cl]was only 45%,significantly lower,but the selectivity of the catalyst immobilized by[Prmim][PF₆]reached 80%,maintained at a high level.According to the XPS characterization we think imidazole ring and Pd formed the structure of palladium nitrogen heterocyclic carbene?Pd-NHC?,and that is the main reason for improving selectivity of catalyst,and[Pdmim][Cl],with the existence of2-methyl,is impossible to form the structure,so that the catalyst selectivity is low.Using[Prmim][Cl]to prepare the structure of Pd-NHC and loaded on the alumina support was also tried,with the 100%conversion of acetylene,the ethylene selective also has been greatly improved,but only reached about 88%with a long induction period.Apart from the electronic effect,the ionic liquid can also change the atmosphere catalyst contact by the different solubility of ethylene and acetylene,it may be the other way to improve the selectivity of the catalysts.