Immobilization of Keggin-type polyoxometalates in cyclodextrin-based polymers for oxidation catalysis

Abstract

Immobilization of polyoxometalates (POMs) in organic materials by electrostatic attraction has emerged as one of the most effective methods to design robust and efficient heterogeneous catalysts. However, alternative strategies based on other weak supramolecular interactions, such as molecular recognition, remain less explored. In the present work, we investigate the adsorption capacity of insoluble γ-cyclodextrin (γ-CD)-based polymers towards Keggin-type POMs driven by the chaotropic effect, to design supramolecular hybrid composites for oxidative catalysis. First, the polymeric host materials were synthesized by crosslinking γ-CD with epichlorohydrin (EPI) and fully characterized by infrared spectroscopy, solid-state NMR and thermogravimetric analysis (TGA). Four POMs, [PW₁₂O₄₀]³⁻, [PV^VW₁₁O₄₀]⁴⁻, [PV^IVW₁₁O₄₀]⁵⁻, and [H₂W₁₂O₄₀]⁶⁻, with different ionic charges and chemical compositions, were studied to evaluate and compare their adsorption kinetics and equilibrium isotherms. The results show a fast and efficient adsorption process for all POMs, characterized by pseudo-second-order kinetics and a Langmuir isotherm model. The obtained POM@CD–EPI hybrids were then tested as heterogeneous catalysts in the oxidation reaction of benzyl alcohol. Excellent catalytic performances in terms of conversion (up to 100%), selectivity towards benzoic acid (up to 100%), and recyclability (up to 5 cycles) were achieved under mild conditions (60 °C/24 h) using tert-butyl hydroperoxide as the oxidizing agent. However, the use of H₂O₂ as an oxidant led to lower catalytic performance due to the rapid degradation of the POM–polymer composites under these conditions. It is then proposed that the POM–CD host–guest supramolecular arrangement plays a key role in both catalyst stability and the selective oxidation mechanism.