Metalloenzyme-Inspired Ce-MOF Catalyst for Oxidative Halogenation Reactions

Abstract

The structure of UiO-66(Ce) is formed by CeO 2– x defective nanoclusters connected by terephthalate ligands. The initial presence of accessible Ce 3+ sites in the as-synthesized UiO-66(Ce) has been determined by X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FTIR)-CO analyses. Moreover, linear scan voltammetric measurements reveal a reversible Ce 4+ /Ce 3+ interconversion within the UiO-66(Ce) material, while nanocrystalline ceria shows an irreversible voltammetric response. This suggests that terephthalic acid ligands facilitate charge transfer between subnanometric metallic nodes, explaining the higher oxidase-like activity of UiO-66(Ce) compared to nanoceria for the mild oxidation of organic dyes under aerobic conditions. Based on these results, we propose the use of Ce-based metal–organic frameworks (MOFs) as efficient catalysts for the halogenation of activated arenes, as 1,3,5-trimethoxybenzene (TMB), using oxygen as a green oxidant. Kinetic studies demonstrate that UiO-66(Ce) is at least three times more active than nanoceria under the same reaction conditions. In addition, the UiO-66(Ce) catalyst shows an excellent stability and can be reused after proper washing treatments. Finally, a general mechanism for the oxidative halogenation reaction is proposed when using Ce-MOF as a catalyst, which mimics the mechanistic pathway described for metalloenzymes. The superb control in the generation of subnanometric CeO 2– x defective clusters connected by adequate organic ligands in MOFs offers exciting opportunities in the design of Ce-based redox catalysts.

MOF-Derived CeO2 and CeZrOx Solid Solutions: Exploring Ce Reduction through FTIR and NEXAFS Spectroscopy

“…Furthermore, the natural separation of the oxide-based clusters by organic ligands prevents crystals from sintering even at high calcination temperatures, thus preserving the surface defects of the catalysts. It is noteworthy that Ce-based MOFs were recently studied as replacements for oxides in several chemical reactions [ 31 , 32 , 33 ]. However, the MOFs’ stability still limits their practical applications [ 34 , 35 ].…”

Enhanced transformation of CO2 over microporous Ce-doped Zr metal–organic frameworks

“…33 Owing to its high oxygen mobility, easy and reversible transition between Ce 4+ and Ce 3+ , introducing Ce into MOF materials has also been implemented. 34,35 Nouar et al partially substituted Zr in UiO-66 by Ce, leading to easy catalytic decomposition of the methanol, which can be attributed to structural defects and redox activity generated from the introduction of Ce. 35 Besides, Stawowy's research found that missing linker molecules in UiO-66(Ce) results in enhanced CO 2 adsorption.…”

Metal–organic framework (MOF)-, covalent-organic framework (COF)-, and porous-organic polymers (POP)-catalyzed selective C–H bond activation and functionalization reactions

“…Recently, Corma et al reported a Ce-based MOF (UiO-66(Ce)) as a robust and efficient catalyst for the oxidative halogenation of 1,3,5-trimethoxybenzene (TMB) (Scheme 7). 56 The as-synthesized UiO-66(Ce) possessed Ce 4+ /Ce 3+ redox sites as revealed by XPS and cyclic voltammetry. The FTIR-CO spectra confirmed that a large proportion of Ce 3+ sites are accessible by TMB.…”

“…55 Scheme 7 Oxidative halogenation of TMB using 1,3-dibromopropane as the bromine source and O 2 as the oxidant. 56 This journal is © The Royal Society of Chemistry 2022 UiO-66(Ce) was still 2-fold more active than nanocrystalline CeO 2 . The higher activity of Ce 3+ present in UiO-66(Ce) was attributed to the influence of the terephthalate ligands in the MOF structure, which can facilitate ligand-to-metal charge transfer by their highly delocalized p-electrons.…”