Nanostructred Hydroxyapatite-Supported Monometallic Or Bimetal Pd Catalysts For Selective Hydrogenation Of Acetylene To Ethylene

As a basic chemical raw material,ethylene is mainly produced though the steam cracking of petroleum hydrocarbon.However,the production of ethylene is the process of cracking accompanied with the production of trace amounts of acetylene,which will affect the activity of polyethylene catalyst in tail-end production and the quality of polyethylene.A best method of removing acetylene so far is selective hydrogenation of acetylene to ethylene industrially.At present,alumina-supported palladium silver alloy catalysts are mainly used industrially.Such catalysts are produced mainly by Beijing Research Institute of Chemical Industry of China Petrochemical Group in our country,the loading of metal palladium is 0.03 wt.%.This catalyst has obtained good applications industrially,but it is made as a classic model for detailed analysis because of its relatively simple composition,including two typical interactions of metal alloying and the interaction between metal and support.Understanding the truth of them will be beneficial to guide further improvement of catalytic performance and affect the gradually in-depth trend of development of other catalyst research.The traditional support loaded monometallic palladium(Pd)catalyst usually has good activity,but acetylene and ethylene simultaneously hydrogenate to ethane and the polymerization of acetylene and ethylene leads to a decrease in catalytic activity and ethylene yield.The researches mainly focus on improving the ethylene selectivity while maintaining activity of the catalyst by adding a second metal with palladium to form an alloy or support effects to adjust the dispersion of palladium and electronic and geometric structure for maximum reduction the content of palladium.Hydroxyapatite(HAP)is a kind of environmentally and bio-friendly new material with weak alkaline surface and a potential good catalyst support,which may be suitable to load metal palladium to catalyze selective hydrogenation of acetylene reaction.Based on the existing work of the research group,the hydroxyapatite nanotubes(HAP-NT)with the mainly exposure(100)plane and the hydroxyapatite nanoparticles(HAP-NP)with irregular morphology were prepared,which were loaded with Pd or Pd-Ag alloy as the catalysts for the selective hydrogenation of acetylene.The effects of the crystal surface and morphology of the hydroxyapatite support on the catalytic performance of the selective hydrogenation were investigated through a series of characterization.The main contents and conclusion of the paper are summarized as follows:(1)Hydroxyapatite nanotubes with the mainly expossure of(100)crystal surface were prepared by hydrothermal method using organic amine mixture as template,while hydroxyapatite nanoparticles were prepared by chemical precipitation method,and the difference of bulk and surface structure over two HAP supports with different morphologies was compared.The catalytic performance of acetylene selective hydrogenation for two catalysts with different Pd loadings was investigated.The results show that compared with Pd/HAP-NP,the Pd/HAP-NT catalyst has lower surface area,but better catalytic activity and higher ethylene selectivity and catalytic performance stability,which mainly due to the stronger interaction between HAP-NT and Pd,making the higher dispersion of Pd,palladium species in a state of electron deficient and the less formation of palladium hydride on HAP-NT under hydrogen condition.(2)The two components of palladium and silver were loaded on two HAP supports with different morphology by sequential impregnation method.The acetylene hydrogenation performance of Pd1Ag1/HAP catalyst with different amounts of palladium-silver alloy loaded was investigated.The results show that with various amount of palladium-silver alloy loaded,two kinds of hydroxyapatite-supported palladium-silver alloy catalysts exhibit different change trend of performance,namely,when the amount of Pd and Ag loaded is higher than 0.1%,with the decreaseing loading amounts of Pd and Ag,the catalytic activity of Pd1Ag1/HAP-NT catalyst decreases more and the selectivity increases,while the catalytic activity of Pd1Ag1/HAP-NP catalyst decreases less and the selectivity decreases.The difference of support morphology and the facet effect,the difference of the alloy effect and the interaction between Pd and Ag and between the support,result in the reduction behavior and the dissociation/activation of hydrogen capacity of the active center.In general,the addition of Ag reduces the reactivity,cooperating with facet effect of the support,further inhibiting the formation of palladium hydride,improving the selectivity of ethylene and the comprehensive performance of the catalyst.