Preparation,Characterisation And Evaluation Of Ni/CeO₂ Catalysts For Liquid Phase Hydrogenation Of Maleic Anhydride

Maleic anhydride?MA?is petroleum,coral and biomass derived C₄building block used for the production of many chemicals.Its hydrogenated products,such as succinic anhydride?SA?,?-butyrolactone?GBL?,tetrahydrofuran?THF?and 1,4-butanediol?BDO?,are high value-added fine chemicals,which are widely used in mechanic,battery,pesticide,military industry,and textile manufacturing.As an important derivates from benzene,selective hydrogenation of MA to produce high value-added chemcials can be regarded as effective routine to utilization of coral resource,which promotes the transformation of coral from fuel to feedstock.Due to the significant importance of MA conversion in chemical industry,selective MA hydrogenation attracts tremendous interest in the catalysis.For supported catalysts,the choice of metal species,acid-base active sites on the surface of catalysts,surface defects and metal-support interaction exert an important role in the adsorption and coordination of MA on the catalysts,which leads to different reaction activity and selectivity.As an important reducible oxide,CeO₂ serves as support and/or dopant to create oxygen vacancies,M-CeO₂ interface and strong M-CeO₂ interaction in the CeO₂-based catalysts,which can significantly enhance their catalytic performance in CO,CO₂ and ethylene hydrogenation.However,little knowledge of CeO₂ in the MA hydrogenation is present in the current research.In addition,the M-CeO₂ interaction,support properties and their effects on MA adsorption and coordination and further on catalytic performance are also need to be dug out.In the PhD project,a series of Ni/CeO₂ catalysts are prepared and employed for in-depth understanding of the importance of CeO₂ in MA hydrogenation.Through comparative study on support selection,tuning the Ni loading and support morphology,the correlation of Ni-CeO₂ interaction,oxygen vacancies,M-CeO₂ interface with the catalytic performances will be investigated and established.The main outcomes in this thesis are summarized as following:1.A comparative study between Ni/CeO₂ catalyst and the conventional Ni/Al₂O₃ catalyst was conducted in the liquid hydrogenation of MA.Despite that CeO₂ possessed lower surface area than Al₂O₃,the strong interaction between Ni and CeO₂ support was verified,which facilitated Ni dispersion with small particle size.A portion of Ni species can incorporate into the CeO₂ matrix and promote the Ovac formation on the surface of Ni/CeO₂catalyst.The superior catalytic performance of Ni/CeO₂ catalyst than the Ni/Al₂O₃ catalyst in the MA hydrogenation was attributed to higher dispersion of metallic Ni,oxygen vacancies on the CeO₂ support and the strong Ni-CeO₂ interaction.2.Surface CeO₂ modified 15Ni/SiO₂ catalysts were prepared by two-step impregnation method in order to investigate the effect of Ni-CeO₂interface on the MA hydrogenation.Compared with Ni/SiO₂ catalyst,the CeO₂ covered on surface of Ni species and consequently reduced the amount of Ni exposure and weakened the binding energy of atomic H adsorbed on Ni surface,which gave rise to the low activity of CeO₂-15Ni/SiO₂ catalyst in MA hydrogenation to SA.In contract,the CeO₂-15Ni/SiO₂ catalyst displayed higher GBL selectivity than 15Ni/SiO₂ catalysts under the same condition.The cyclohexane adsorption monitored by IR spectroscopy indicated that CeO₂-15Ni/SiO₂ catalyst strengthened the C=O adsorption on the surface of catalyst.Therefore,it is concluded that the Ni-CeO₂ interface can enhance the C=O adsorption and consequently improve the hydrogenolysis of C=O.3.To investigate the effect of Ni loading on the MA hydrogenation,a series of xNi/CeO₂ catalysts with a wide Ni loading range from 0 to 30 wt%were prepared and evaluated in the MA hydrogenation for in-depth understanding of Ovac,Ni nature and Ni-CeO₂ interaction in MA hydrogenation with the XRD,Raman,H₂-TPR,XPS and H₂-TPD results.It is found that metallic Ni species were active sites for C=C hydrogenation and C=O hydrogenolysis.The Ovac can enhance the H₂ activation and diffusion on the Ni/CeO₂ catalyst,which offered active H atoms for the hydrogenation.The Ni^?+species,induced by strong electronic perturbation of CeO₂ support,can synergistically enhance the C=O adsorption and improve the activity of C=O over the Ni species.4.The different shaped CeO₂ supports,nanorod?CeO₂-R?,nanocube?CeO₂-C?and nanoparticle?CeO₂-P?were synthesized by hydrothermal method and used to support Ni species as catalysts for MA hydrogenation.It is found that the rod-like CeO_2--R support with abundant Ovac can significantly enhance the Ni dispersion and improve the capacity of metallic Ni species in hydrogen adsorption.Compared with the 5Ni/CeO₂-C and5Ni/CeO₂-P catalysts,the 5Ni/CeO₂-R catalyst with the largest amount of Ovac displayed the highest catalytic activity in MA hydrogenation with the lowest apparent activation energy,indicative of an important role of Ovac in MA hydrogenation.5.A comparative study of MA hydrogenation over various TM/CeO₂catalyst reveals that the Ni/CeO₂ catalyst showed the best activity in MA hydrogenation and highest selectivity in GBL yield,followed by the Co/CeO₂ catalyst,while the Cu/CeO₂ catalyst displayed the lowest activity in MA hydrogenation to SA and inert for SA hydrogenolysis to GBL.Furthermore,the MA hydrogenation to SA over Ni/CeO₂ and Co/CeO₂catalysts followed quasi first order kinetic,while hydrogenation over Cu/CeO₂ catalysts displayed a complicated kinetic mechanism.The nature of active metal dominated the activity of MA hydrogenation.The metallic Cu with closed d orbital(d¹⁰)had weak capability of electron-donation to MA molecule.In contrast,the unpaired electrons of Co?3d⁷?and Ni?3d⁸?would favour the coordination and activation of MA,thus led to higher activity.Compared with Co/CeO₂ catalysts,Ni/CeO₂ catalyst possessed higher metal dispersion and more Ovac,which contributed to the high activity in C=C hydrogenation and C=O hydrogenolysis.