MOFs Derived Porous Metal Oxides For Selective Catalytic Oxidation Of HMF

Bimetal oxide composites are the very promising catalysts for heterogeneous catalytic reactions.However,the traditional preparation processes are difficult to achieve precise regulation of metal proportions and sequences at atomic level.Herein,we use 1-imidazoleacetate as the connecting ligand,Co ^II and Mn ^II ions as the metal nodes and polyethylene glycol（PEG）as the capping agent for the synthesis of a MOF material under solvothermal synthesis conditions,resulting a MOF-derived Co Mn bimetal oxide composite（denoted as IA-Co Mn O_x）via calcination treatment under air atmosphere.Because 1-imidazoleacetate has different coordination donor atoms（O and N）that could coordinate to metal ions(Co²⁺and Mn²⁺)with different Lewis acidity in similar coordination geometry,resulting a metal-mixed oxide with random distribution and location of the metal sites.Attributed to the synergistic effect from atomic Co/Mn bimetal active sites and the Lewis acid sites on catalyst surface,the prepared IA-Co Mn O_x catalyst demonstrates>99%conversion and 96%selectivity for aerobic oxidation of 5-hydroxymethylfurfural（HMF）to produce highly value-added 2,5-furandicarboxylic acid（FDCA）in the absence of alkali at 90°C.1.Firstly,the effect of hydrothermal process,pyrolysis temperature and reaction parameters on the reaction activity was investigated.The MOF precursors with appropriate size and uniform metal distribution could be prepared by hydrothermal at 100°C for 24 h,with the ratio of ligand to metal of 2:1,the molar ratio of Co to Mn of 1:1,metal ion concentration of 0.2 M,and 0.2 g of surfactant PEG2000.The porous metal oxides（denoted as IA-Co Mn O_x）were prepared via calcination treatment under air atmosphere at 250°C for 4 h,which exhibited high efficiency for base-free aerobic oxidation of 5-hydroxymethylfurfural to 2,5-furandicarboxylic acid with full HMF conversion and 96%FDCA yield under mild conditions.2.Secondly,the porous metal oxides derived from MOFs were systematically characterized by SEM,TEM,PXRD,FT-IR,BET,XPS,Py-IR and DRIFTS spectroscopy to clarify the structure-activity correlation at atomic level.It was found that the prepared IA-Co Mn O_x bimetallic oxides present high porosity,large specific surface area and uniform dispersion of metal active centers,which is favorable to improve the redox activity of catalysts.In addition,IA-Co Mn O_x with moderate Lewis acidity on the surface accelerated adsorption of5-formyl-2-furancarboxylic acid（FFCA）and desorption of FDCA,which avoided the occupation of active centers by acid products,demonstrating high efficiency for base-free aerobic oxidation of HMF to produce FDCA.3.Thirdly,the possible reaction mechanism for base-free oxidation conversion of HMF to FDCA over IA-Co Mn O_x catalyst was proposed.It was found that the Co active center exhibited higher selectivity for 2,5-diformylfuran（DFF）produced after the first step,and the Mn active center presented higher selectivity for FFCA produced after the second step and the final product FDCA,indicating that the Co active center is favorable to accelerate oxidation of hydroxyl to aldehyde,while the Mn active center is favorable to accelerate oxidation of aldehyde to carboxyl.