Design of ceria supported nickel-based catalysts for selective hydrogenation of maleic anhydride

Liao, Xin (2017) Design of ceria supported nickel-based catalysts for selective hydrogenation of maleic anhydride. University of Southampton, Doctoral Thesis, 225pp.

Record type: Thesis (Doctoral)

Abstract

This thesis presents my efforts in the design and development of novel ceria (CeO₂)-supported Ni-based catalytic materials to realise effective and highly selective liquid phase hydrogenation of maleic anhydride (MA), an important process in producing crucial platform chemicals. In my research, metallic Ni and defective ceria (CeO_2-δ) are defined as an efficient metal catalyst and active support, respectively, and tuning the properties of the Ni/CeO_2-δ catalysts through different strategies is verified to be effective in controlling selectivity of succinic anhydride (SA) via preferential hydrogenation of C=C or gamma butyrolactone (GBL) via C=O hydrogenolysis. We made great efforts to correlate the improved performance with various influential properties of the novel catalysts.

A thorough survey of recent advances in MA hydrogenation processes and catalysts (Chapter 1) clarified that MA conversion and selectivity on the catalysts depended on the active metal species and their interaction with support. The wellexplored impacts of crystalline and textural structures of catalyst supports are also important. We therefore proposed the Ni/CeO_2-δ catalysts to be novel, low-cost and green catalysts in order to address the great challenges in the industrial MA hydrogenation process. The rational experimental methodologies and the theoretical background were presented and justified in Chapter 2.

Firstly, through the comparative investigation into the liquid phase MA hydrogenation on Ni/CeO_2-δ and Ni/Al₂O₃ catalysts (Chapter 3), the defective ceria (CeO_2-δ) support was defined as an active support, which offered desirable oxygen vacancies (Ovac) and interaction with Ni responsible for the promoted activity and selectivity. Meanwhile, the outstanding performance of Ni and its unique interaction with CeO_2-δ were specified via a comparative study of MA hydrogenation on CeO_2-δ-supported transition metals (Chapter 4), where the electronic interaction between Ni and CeO_2-δ was stressed.

The fine-tuning of Ni loading on CeO_2-δ support allowed me not only to optimise the catalysts but also to define the active sites, underlining the contributions of Ovac, metallic Ni species, and the Ni-CeO₂ interface to MA hydrogenation on the xNi/CeO₂ catalysts (Chapter 5). Through quasi-quantitative analyses, we established that Ovac played pivotal roles in SA selectivity, while the Ni species and the Ni-CeO₂ interface dominated the yield of GBL.

The Ni/CeO_2-δ catalysts with 5 wt% Ni loading (in avoidance of the influence of significant surface coverage) were prepared using different-shaped CeO₂ supports which terminated with different percentage of predominant crystal surfaces and possessed different Ovac levels. We found that their catalytic performance of MA hydrogenation was dependent on the specific morphology. Peculiarly, the CeO_2-δ nanorod-supported Ni catalyst showed the highest activity due to the highest Ni dispersion and Ovac concentration (Chapter 6).

In order to further tune the performance of MA hydrogenation, CeO_2-δ supported bimetallic Ni-based catalysts were prepared through the incorporation of a second transition metal, including Fe, Co, Cu and Zn, with a different number of outer shell d electrons (Chapter 7). It was found that only Cu can effectively improve MA conversion and the selectivity to SA. Qualitative and quantitative characterisation revealed that Cu perturbed the valance electronic structure of Ni, thus altering the hydrogenation activation ability. The formation of the Ni-Cu alloy also modulated the metal-support interaction. Thus, the CeO_2-δ-supported bimetallic Ni-Cu alloy catalysts displayed enhanced MA hydrogenation performance, particularly SA selectivity.

CeO₂ was applied as a promoter to modify the surface of 15 wt% Ni/SiO₂ catalysts, in order to further validate the roles of CeO₂ (Chapter 8). MA hydrogenation was affected by the coverage of CeO₂, with higher activity and selectivity on the 3CeO₂@15Ni/SiO₂ catalyst with 3 wt% CeO_2-δ loading (versus Ni/SiO₂ weight). The surface CeO₂ species can improve C=O adsorption and activation on the CeO₂-decorated Ni/SiO2 catalyst, which contributed to the excellent catalytic activity in C=O hydrogenolysis.

Overall, the thesis reports a systematic study of the design and application of CeO_2-δ support, the Ni-CeO_2-δ interaction, and the optimisation of active metal for developing green and robust MA hydrogenation catalysts. The roles of Ovac, the Ni-CeO_2-δ electronic interaction and Ni species in MA hydrogenation have been summarised in Chapter 9. The research will be facilitating the future development of novel and effective catalysts for industrial MA hydrogenation processes.