Relation Between Coordination and Lewis‐Acid Property of MOF‐Derived Mononuclear Zn(II) Catalyst Toward Epoxide Hydroxylation

• Published in: ChemCatChem 2021-12, Vol.13 (24), p.5236-5242
• Main Authors: Xiao, Juan‐Ding, Lu, Sheng‐Mei, Jia, Guo‐Qing, Wang, Qing‐Nan, Li, Can
• Format: Article
• Language: English
• Subjects: Acids; Catalysis; Catalysts; Coordination numbers; epoxide hydroxylation; heterogeneous catalysis; Hydroxylation; Lewis acid; Metal-organic frameworks; mononuclear catalyst
• ISSN: 1867-3880; 1867-3899
• DOI: 10.1002/cctc.202101340

Lewis‐acid properties of the coordinatively unsaturated metal ions are highly relevant to the coordination environment; however, it remains a challenge to establish the coordination structure‐activity relationship, especially in heterogeneous catalysis. Here, a typical metal‐organic framework, ZIF‐8, is selected as the precursor to prepare Lewis‐acid based Zn−N/P−C mononuclear catalysts, with well‐designed external structures but difference in the local coordination. The obtained mononuclear Zn(II) catalysts can not only inherit the advantages of MOF in coordination, but also simulate the homogeneous environment during the catalysis owing to the monodispersed metal sites in ZIF‐8. Eventually, relation between coordination and Lewis‐acid property toward epoxide hydroxylation has been illustrated. Results show the substituted coordination atom of P and the decrease of coordination number strongly enhance the Lewis‐acidity of Zn2+, causing the TOF values changes from 8.7 to 80.1 h−1 based on Zn content. The work affords an understanding and inspiration of coordination environment matters toward Lewis‐acid catalysis in organic transformations. Metal‐organic frameworks: Zn‐centered mononuclear catalysts with tuned N/Zn and P/Zn molar ratios have been obtained through calcination in the presence of defect modulators. The samples possess local structure of ZnNxPy. The decrease of coordination numbers in Nx, and the substitution by coordination atom of P, synergistically promote the Lewis acidity of Zn2+, leading to the enhancement of catalytic performance from 8.7 h−1 for ZIF‐8‐700 to 80.1 h−1 for ZIF‐8‐P‐900.