English

Solid Frustrated Lewis Pairs Constructed on CeO₂ for Small-Molecule Activation

• Zhang Sai ,
• Zhang Mingkai ,
• Qu Yongquan ^,

• School of Chemical Engineering and Technology, Frontier Institute of Science and Technology, Xi'an Jiaotong University, Xi'an, 710049, P. R. China

Author Bio: Dr Yongquan Qu received his BS in Materials Science and Engineering from Nanjing University in 2001, MS in Chemistry from the Dalian Institute of Chemical Physics in 2004, and PhD in Chemistry from the University of California, Davis, in 2009. He worked as a postdoctoral research fellow in the University of California, Los Angeles, from 2009 to 2011. He became a faculty member of the Center for Applied Chemical Research, Frontier Institute of Science and Technology, Xi'an Jiaotong University, China, in 2012. His research interests focus on heterogeneous catalysis in the areas of organic synthesis, clean energy production and environmental remediation. Details can be found at: http://gr.xjtu.edu.cn/web/yongquan;

Corresponding author: Qu Yongquan, yongquan@mail.xjtu.edu.cn

• Received Date: 12 November 2019
  Accepted Date: 13 January 2020
  Revised Date: 13 January 2020
  Available Online: 15 September 2020
• Fund Project:

  The project was supported by the National Natural Science Foundation of China (21872109) and the China Postdoctoral Science Foundation (2018T111034). Y. Qu is supported by the Cyrus Tang Foundation through the Tang Scholar program. S. Zhang is supported by the Fundamental Research Funds for the Central Universities, China (xjj2018033) and the Natural Science Foundation of Shaanxi Province, China (2019JQ-039)

Abstract: Solid materials containing frustrated Lewis pairs (FLPs) as active sites have attracted much attention due to their ability to activate and transform small molecules. However, it is still highly challenging to precisely construct FLP sites on the surfaces of nanomaterials, thereby limiting the applications of these materials. Nanostructured ceria (CeO₂) is commonly employed as a catalyst or functional support, and exhibits both Lewis acid and basic properties as well as abundant and easily regulated surface defects, which originate from the reversible Ce³⁺/Ce⁴⁺ redox pair. When the Lewis acid and base sites of CeO₂ are independent of each other, the combined Lewis acid-base sites play a similar role to that of homogeneous FLP sites. Thus, the rich surface properties of nanostructured CeO₂ provide significant potential for the construction of solid FLPs.Herein, we demonstrate that solid FLP sites can be successfully constructed on the surface of CeO₂(110) via the regulation of surface defect clusters, which can be used to create new Lewis acid sites composed of two adjacent Ce³⁺ atoms on the surface. Novel interfacial FLP sites can then be formed by combining these Lewis acid sites with neighboring surface lattice oxygens, which act as Lewis base sites. Porous CeO₂ nanorods (PN-CeO₂) with boundary surface defects were prepared by a special two-step hydrothermal process, and exhibited remarkable catalytic FLP properties. Hydrogen molecules could be effectively activated on the surface of PN-CeO₂ with a low activation energy of 0.17 eV via a heterolytic cleavage process. Hydrogenation of alkenes and alkynes to alkanes could then be realized by the activated hydrogen under mild reaction conditions.PN-CeO₂ nanorods with FLP active sites were also able to activate CO₂ molecules effectively. Unlike in other solid FLP sites, CO₂ molecule activation was realized via a Lewis base site binding with the C atom while two Lewis acid sites bound the two O atoms, owing to the unique configuration of the FLP sites in PN-CeO₂. When combined with the epoxidation of olefins by "isolated" Ce³⁺ sites in PN-CeO₂, the FLP-inspired activated CO₂ could be used to transform olefins and CO₂ to cyclic carbonates through a selective tandem transformation route. In addition, density functional theory studies indicate that the FLP sites on CeO₂(110) can activate the C―H bond of CH₄ with activation energies as low as 0.63 eV, which can be attributed to the enhanced acidity and basicity of the FLP sites.With this improved understanding of solid FLP sites constructed on ceria, we have also been able to summarize the challenges and prospects in this field, including their construction, characterization, and mechanism analysis.

Key words:
• Frustrated Lewis pairs
•  /  CeO₂
•  /  Surface defect
•  /  H₂ activation
•  /  CO₂ activation
•  /  CH₄ activation

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