Selective Oxidation Of 5-hydroxymethylfurfural Into 2,5-diformylfuran Over Manganese Oxides

The catalytic conversion of biomass platform compounds into high value-added chemicals and high-quality fuels has become a research hotspot in the field of catalysis.5-Hydroxymethylfurfural(5-HMF)is a biomass platform compound that can be selectively converted to intermediates for the synthesis of pharmaceuticals,plastics,and liquid fuels.2,5-Diformylfuran(DFF)is one of the high value-added derivatives of 5-HMF,which can be used as a precursor in the synthesis of functional polymers,pharmaceuticals,antifungal agents and furan-based resins.In this paper,two novel manganese oxides catalysts were synthesized by different methods for the selective oxidation reaction of 5-HMF.In the first case,a mixed-form MnOx catalyst was prepared by simple combustion synthesis method.And then,a high-purity Mn5O8 nanoplate catalyst was synthesized by a microwave-assisted ionic liquid route for the first time.The catalysts were characterized by XRD、XPS、FT-IR、TEM、SEM and TGA techniques.The MnOx-500-2.0 catalyst with dominated Mn3O4 crystal phase,prepared under the condition of burning ratio R=2.0 and calcination temperature of 500℃,has higher Mn4+/Mn3+ratio and exposed lattice oxygen concentration on the catalyst surface.The synthesized catalyst exhibits high catalytic activity for selective oxidation of 5-HMF with 23.9%conversion and 85.0%selectivity of DFF using ethanol as solvent at 140℃ and 0.5 MPa O2 for 4 h.In order to obtain highly active single-crystal manganese oxides,MnsOs catalyst was prepared by microwave-assisted ionic liquid synthesis,which was formed through π-π stacking between imidazolium-rings of ionic liquid and extending hydrogen bonds between anions and precursor.Mn5O8 catalyst calcined at 550℃ has the highest relative crystallinity and the best catalytic activity with with a 51%conversion of 5-HMF and a 94%selectivity to DFF at the same reaction condition.The high activity of Mn5O8 is related to the crystallinity and the high concentration of Mn4+ on the exterior surfaces of Mn5O8 nanoplates as active sites.The reaction mechanism over the Mn5O8 nanoplates was proposed based on the understanding of the Mn4+active center and the lattice oxygen via a Mn4+/Mn2+ two electron cycle to enhance catatlytic performance.