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Study On Oxide-modified Ruthenium-based Catalysts Toward The Selective Hydrogenation Of Benzene

Posted on:2018-10-08Degree:MasterType:Thesis
Country:ChinaCandidate:X G XueFull Text:PDF
GTID:2321330518993700Subject:Chemical Engineering and Technology
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Cyclohexene as an important organic chemical raw materials,can be used to produce polycycloylene resin,rubber chemicals,high octane gasoline stabilizer and other chemical products.Traditional preparation methods of cyclohexene are based on dehydration of cyclohexanol and dehydrogenation of cyclohexane,but these methods generally show the disadvantages of high cost and low yield of cyclohexene.The production of cyclohexene by selective hydrogenation of benzene has attracted more attention due to its advantages in atomic economy and less by-products.Ruthenium-based catalyst is one of the most promising candidates towards selective hydrogenation of benzene,which exhibits high catalytic activity and stability.However,two main problems in ruthenium-based catalysts still remain unsolved:1)how to effectively control the active structure of ruthenium-based catalysts so as to inhibit the over-hydrogenation behavior of benzene and improve the selectivity of cyclohexene;2)how to deepen the regulation of active structure as well as the catalytic mechanism of the ruthenium-based catalysts,which can optimize the catalyst structure,strengthen the catalyst performance and realize the efficient utilization of the precious metal ruthenium.In this dissertation,porous oxides?SiO2,CeO2,ZrO2?coated ruthenium-based catalysts were designed and prepared.The effect of synergistic catalysis between Ru active site and support was enhanced through controlling the variety,structure and acidity of the porous oxide coating,which can effectively inhibit the dehydrogenation behavior of benzene and greatly improve the activity/selectivity of ruthenium towards selective hydrogenation of benzene.In addition,based on experimental studies and DFT calculations,the influences of structure and acidity of the porous oxide coating on the active site structure of ruthenium-based catalysts as well as the synergistic effect between Ru and porous oxide coating have been investigated systematically.This work provides an exploration for the design and preparation of new Ru-based catalyst,which would show potentially application in selective hydrogenation of benzene.The detailed contents are listed as follows:1.Study on the Ru/ZrO2@ZrO2-B catalyst and its catalytic performance for hydrogenation of benzeneIn order to avoid the deep hydrogenation of cyclohexene on the highly-active Ru,a novel Ru/ZrO2 catalyst encapsulated by a porous ZrO2 coating?denoted as Ru/ZrO2@ZrO2?was prepared.Furthermore,the synergistic effect between Ru active site and porous oxide coating was enhanced by controlling the acid sites of the ZrO2 coating.We synthesised a series of Ru-based catalysts by coating the porous B-doped ZrO2 on the RLu/ZrO2?Ru/ZrO2@ZrO2-B?5%?,Ru/ZrO2@ZrO2-B?10%?,Ru/ZrO2@ZrO2-B?15%??.The catalytic evaluations show that the Ru/ZrO2@ZrO2 catalyst exhibits better selectivity than Ru/ZrO2,and Ru/ZrO2@ZrO2-B?5%?gives the best selectivity to cyclohexene?58.8%?.Based on a variety of characterization and DFT calculations,a doule-active site reaction mechanism is verified:hydrogen molecule dissociates on the surface of ruthenium nanoparticles followed by spillover of hydrogen atom to the surface of porous ZrO2;benzene can be hydrogenated to cyclohexene with spillover hydrogen on the surface of ZrO2.A certain amount of B-doping into ZrO2 significantly increases the weak Lewis acid concentration on the surface of ZrO2 coating,which effectively enhances the adsorption and activation of benzene as well as promotes the desorption of cyclohexene.Therefore,an enhanced yield of of cyclohexene is obtaiend over Ru/ZrO2@ZrO2-B?5%?catalyst.2.Study on the Ru/MO2@MO2?M=Zr,Si,Ce?catalysts and their catalytic performance for hydrogenation of benzeneIn order to extend the availability of the above mentioned resolution,three hydrophilic oxide supports,ZrO2,CeO2 and SiO2,were chosen for the prepartion of ruthenium-based catalysts?Ru/ZrO2@ZrO2,Ru/CeO2@CeO2 and Ru/SiO2@SiO2?,via a similar synthetic strategy mentioned above.The effect of synergistic catalysis between Ru active site and three supports for benzene selective hydrogenation was studied.HRTEM images show Ru/ZrO2@ZrO2,Ru/CeO2@CeO2 and Ru/SiO2@SiO2 have a core-shell structure and the Ru nanoparticles are uniformly coated by the oxide layer.H2-TPD and CO-TPD demonstrate that H2 can enter the interior surface of Ru;while benzene molecule is restricted to remain on the surface of porous layer,which avoids the deep hydrogenation of cyclohexene and ultimately improves the selectivity of cyclohexene.The catalytic evaluation results show that Ru/CeO2@CeO2 exhibits the best catalytic performance with a selectivity of 52.9%at a conversion of 58.6%.Furthermore,the TPD results of benzene and cyclohexene demonstrate that the porous CeO2 coating in the Ru/CeO2@CeO2 catalyst promotes the adsorption of benzene and desorption of cyclohexene simultaneously,which optimizes the selective hydrogenation behavior of benzene and thus enhances the yield of cyclohexene.
Keywords/Search Tags:selective hydrogenation of benzene, ruthenium-based catalysts, cyclohexene, acid sites, double-active site separation
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