Highly stable Pd2+ species anchoring on ethylenediamine-grafted-MIL-101(Cr) as a robust oxidation catalyst

Abstract

Highly stable Pd²⁺ species were anchored on ethylenediamine-grafted MIL-101(Cr). Ethylenediamine (0.3–1.2 mmol g⁻¹) was first grafted onto MIL-101(Cr), then Pd²⁺ (0.03–0.2 mmol Pd per g) was incorporated by double-solvent adsorption. Fourier transform infrared, inductively coupled plasma-optical emission spectroscopy, transmission electron microscopy and CHN analysis confirmed the incorporation of ethylenediamine and Pd²⁺ in MIL-101(Cr). X-ray photoelectron and Raman spectroscopy suggested that one amino moiety of ethylenediamine coordinated with the Cr³⁺ nodes of MIL-101(Cr). The other served as an anchoring site for the incorporated Pd²⁺ species. In situ X-ray absorption near-edge structure analysis showed that the strong interaction between Pd²⁺ and ethylenediamine, within the confinement of the MIL-101(Cr) structure, effectively prevented reduction to Pd⁰, even in the presence of H₂ at 150 °C. At relatively low temperatures, the catalysts with ethylenediamine : Pd²⁺ molar ratios of 10 provided a higher activity for styrene oxidation (TOF ∼30 h⁻¹), as compared to previous reports. The presence of ethylenediamine as an anchoring ligand also inhibited the reduction of Pd²⁺ by the feed (styrene) and minimized leaching of the active Pd²⁺ species under oxidizing and acidic conditions. The Pd²⁺ species anchoring on ethylenediamine-grafted-MIL-101(Cr) exhibited an improved catalytic activity and stability, as compared to typical liquid-phase oxidation catalysts.