Unravelling the Redox-catalytic Behavior of Ce4+ Metal-Organic Frameworks by X-ray Absorption Spectroscopy

Abstract

The introduction of Ce⁴⁺ as a structural cation has been shown to be a promising route to redox active metal-organic frameworks (MOFs). However, the mechanism by which these MOFs act as redox catalysts remains unclear. Herein, we present a detailed study of the active site in [Ce₆ O₄ (OH)₄ ]-based MOFs such as Ce-UiO-66, involved in the aerobic oxidation of benzyl alcohol, chosen as a model redox reaction. X-ray absorption spectroscopy (XAS) data confirm the reduction of up to one Ce⁴⁺ ion per Ce₆ cluster with a corresponding outwards radial shift due to the larger radius of the Ce³⁺ cation, while not compromising the structural integrity of the framework, as evidenced by powder X-ray diffraction. This unambiguously demonstrates the involvement of the metal node in the catalytic cycle and explains the need for 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) as a redox mediator to bridge the gap between the one-electron oxidation of the Ce⁴⁺ /Ce³⁺ couple and the two-electron alcohol oxidation. Finally, an improved catalytic system with Ce-MOF-808 and TEMPO was developed which outperformed all other tested Ce⁴⁺ -MOFs.

Keywords: X-ray spectroscopy; catalysis; cerium; metal-organic frameworks; redox chemistry.