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 Ce³⁺ 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 Ce⁴⁺/Ce³⁺ 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.

Keywords: Ce-MOF; ligand-to-metal charge transfer; oxidase activity; oxidative halogenation; subnanometric CeO2−x clusters.

Figure 1

Curve fitting for the Ce 3d XPS spectrum of (a) nanoceria and (b) UiO-66(Ce) samples. Continuous lines correspond to Ce⁴⁺ and dotted ones to Ce³⁺.

Figure 2

IR spectra of CO adsorption at −165 °C at saturation coverage (2 mbar) on nanoceria (red) and UiO-66 (Ce) (blue). The inset IR spectrum of CO adsorption at 0.5 mbar on the UiO-66 (Ce) sample.

Figure 3

Cyclic voltammograms recorded at the indicated scan rates in a solution containing 0.1 M [Bu₄N]PF₆ in N,N-dimethylformamide at 25 °C of UiO-66(Ce)@Nafion (top panel) or nanoCeO₂@Nafion composites deposited on a pyrolytic graphite electrode. The inset plot: Comparison of the cyclic voltammograms of both composites recorded at 0.1 V s^–1.

Figure 4

Kinetic profiles for 3,3′,5,5′-tetramethylbenzidine aerobic oxidation employing nanoceria (red squares) and UiO-66(Ce) (blue squares) as nanozymes.

Scheme 1. Oxidative Halogenation of TMB using 1,3-Dibromopropane as a Bromination Agent and O₂ as a Green Oxidant

Figure 5

Kinetic profiles for 1,3,5-trimethoxybenzene conversion employing nanoceria (red circles) and UiO-66(Ce) (blue squares) as catalysts. Reaction conditions: TMB (126 μmol) in 1,3-dibromopropane (1.5 mL), fixing a TMB:Ce molar ratio of 1 (64 and 23 mg for UiO-66(Ce) and nanoceria, respectively).

Figure 6

(a) Reusability of UiO-66(Ce). The feeds for the consecutive runs were adjusted to the Ce catalyst. Soxhlet extraction with methanol was performed in the solid catalyst after run 3. (b) Kinetic profiles for 1,3,5-trimethoxybenzene (TMB) conversion employing UiO-66(Ce) as the catalyst (blue line and squares) and after being removed from the reaction mixture after 1 h (red line and triangles). Reaction conditions: TMB (126 μmol) in 1,3-dibromopropane (1.5 mL), fixing a TMB:Ce molar ratio of 1.

Figure 7

Reaction mechanisms proposed for the generation of hypohalite active species for the halogenation of arenes catalyzed by metalloenzymes (a) and UiO-66(Ce) (b) (Oxygen: red circles, hydrogen: white circles, halide: green circles).