A microwave assisted ionic liquid route to prepare bivalent Mn 5 O 8 nanoplates for 5-hydroxymethylfurfural oxidation.

In order to develop highly active non-precious metal catalysts for the selective oxidation of the platform compound 5-hydroxymethylfurfural (HMF) to the value-added bio-chemical 2,5-diformylfuran (DFF), we prepared high purity bivalent Mn ₅ O ₈ nanoplates by a microwave-assisted ionic liquid route. The precursor of bivalent Mn ₅ O ₈ nanoplates was formed through π-π stacking between imidazolium rings of the ionic liquid 1-butyl-3-methyl-imidazolium chloride and extending hydrogen bonds between Cl anions and hydrohausmannite. An oriented aggregation growth occurred on the basis of the Ostwald ripening under microwave heating. The high purity bivalent Mn ₅ O ₈ nanoplates obtained through calcination at 550 °C for 2 h exhibited high HMF conversion (51%) and DFF selectivity (94%) at 5 bar of oxygen pressure in 2 h. The high concentration of Mn ⁴⁺ on the exterior surfaces of Mn ₅ O ₈ nanoplates as active sites coupled with good crystallinity played key roles for desirable mass and heat transfer, and for fast desorption avoiding over-oxidation. The reaction process over the Mn ₅ O ₈ nanoplates was proposed based on the understanding of Mn ⁴⁺ active centers and lattice oxygen via a Mn ⁴⁺ /Mn ²⁺ two-electron cycle to enhance their catalytic performance. Furthermore, the Mn ₅ O ₈ nanoplates could be readily recovered and reused without loss of catalytic activity. Thus, the high purity Mn ₅ O ₈ nanoplates with good catalytic performance raises the prospect of using the type of sole metal oxide for practical applications.