Enhanced Basicity of MnOx-Supported Ru for the Selective Oxidation of 5-Hydroxymethylfurfural to 2,5-Furandicarboxylic Acid.

The present study focused on developing a stable basic MnOx support for Ru (RuMn) for the efficient oxidation of 5-hydroxymethylfurfural (HMF) to 2,5-furandicarboxylic acid (FDCA) in water in the absence of an external base. A series of MnOx supports, synthesized via hydrothermal approach using urea as precipitant, was prepared by thermal treatment at various temperatures (300-800 °C) before doping with Ru. The RuMn-2 (1 wt % Ru, MnOx calcined at 400 °C) possessed a large number of basic sites (1.72 mmol g-1 ) based on CO2 temperature-programmed desorption analysis, affording an FDCA yield of 87 % with a turnover frequency of 22 h-1 . Transmission electron microscopy energy-dispersive X-ray spectroscopy elemental mapping of RuMn-2 showed a high dispersion of Ru over the surface of MnOx, contributing to the efficient HMF oxidation. Moreover, X-ray diffraction, X-ray photoelectron spectroscopy, and H2 temperature-programmed reduction indicated that the predominant MnO2 phase (ϵ-MnO2 ) played a vital role in HMF oxidation. RuMn-2 was recyclable for up to four runs without significant loss in the activity and retained its structural integrity.