Preparation Of Magnetic Catalysts Based On Mofs And Their Performance In Liquid-phase Organic Catalytic Reaction

The catalytic transformation is an effective and ecofriendly strategy to covert small C1 molecules(CO2 and methanol)into valuable chemicals that minimize the consumption of rapidly depleting fossil fuel resources.Compared with the gas-phase reaction,the liquid-phase catalytic process is easy to operate,large in processing capacity,and easier to control the selectivity of the products.Among them,the design of efficient catalysts is the key.However,traditional homogeneous catalytic systems have some problems,such as catalyst recovery,product separation and environmental pollution.The separation process of common heterogeneous catalysts is complicated and the energy consumption is high.External field(magnetism)efficiently separate the magnetic catalysts,which can simplify the separation operation and avoid the loss of catalyst.So,it’s enormously beneficial in liquid-phase reactions.In this paper,four kinds of catalysts including Pd/MIL-53(Al),MIL88B(Fex,Co1-x),magnetic MIL-53(Fe)@SiO2@Fe3O4 and magnetic Au/MIL101(Cr)@SiO2@Fe3O4 were prepared based on metal-organic frameworks(MOFs).The structure and magnetic properties of the catalysts were characterized by XRD,SEM,HRTEM,FT-IR,XPS,N2 adsorption-desorption,NH3/CO2-TPD and VSM.The catalytic performance of the catalysts for liquidphase oxidation of methanol and the N-methylation reaction of CO2 with amine and H2 were evaluated in a batch reactor,and the reaction mechanisms of different catalyst systems were studied based on the experimental data.The research results were obtained as follows:(1)The synthesized MIL-53(Al)with coordinated unsaturated Al3+ site was used as the support.Pd nanoparticles were loaded on MIL-53(Al),and a series of Pd/MIL-53(Al)catalysts with different Pd contents were prepared.All Pd/MIL-53(Al)catalysts maintain a complete MIL-53(Al)nanostructure.Pd nanoparticles(size～4.6 nm)are homogeneously distributed on the MIL～53(Al).Using O2 as the oxidant,we measured the reaction activity(liquid-phase oxidation)of methanol over Pd/MIL-53(Al)surface with different Pd loadings.The results showed that 5wt%Pd/MIL-53(Al)had the best reaction performance.The MeOH conversion and MF selectivity can be obtained 60.3%and 62.2%,respectively,when reaction prolonged for 5 h at 2 MPa,150℃.Mechanism studies have shown that methanol is first adsorbed on the surface of Pd to form adsorbed methoxy intermediates.O2 is activated on the surface of Pd to produce atomic oxygen.The methoxy intermediates undergo β-H elimination to generate formaldehyde.The removed H atoms combine with O atoms to form H2O.Formaldehyde reacts with H2O to form methyl glycol and then further dehydrogenates to form formic acid.Formic acid is adsorbed on the coordinated unsaturated A13+Lewis acid sites and undergoes esterification with methanol to form MF.(2)MIL-88B(Fe)with coordinated unsaturated Fe3+ site was used as the matrix.A series of MIL-88B(Fex,Co1-x)bimetallic MOFs catalysts with different Fe/Co molar ratios were synthesized by partially replacing Fe3+ with Co2+.Structural characterization shows that MIL-88B(Fex,Co1-x)has needle-like morphology with an average length of 400-600 nm and a width of 100-150 nm.Fe and Co elements are uniformly distributed.Co doping does not change the topological structure of MIL-88B(Fex,Co1-x).The introduction of Co can adjust the electronic environment of the Fe center,and Fe and Co have synergistic catalytic effects.Using H2O2 as the oxidant,the performance test results of the methanol liquid-phase oxidation reaction of MIL-88B(Fex,Co1-x)with different Fe/Co molar ratios showed that MIL-88B(Fe0.7,Co0.3)exhibited the best catalytic performance.The MeOH conversion was 34.8%after reacting at 80℃and 0.5 equivalent H2O2 for 60 min,and the MF selectivity increased from 50.7%(monometallic Fe)to 67.6%.Mechanism studies have shown that H2O2 is first adsorbed and activated on Fe3+ to generate·OH radicals.Methanol is adsorbed on the O atoms of MOF through hydrogen bonds and is gradually oxidized by·OH to formic acid.Then the formic acid reacts with the residual methanol on the Lewis acidic sites Fe3+ and Co2+ to generate MF.(3)With Fe3O4 as the magnetic core and SiO2 as the protective shell layer,SiO2@Fe3O4 magnetic microspheres were synthesized;then MIL-53(Fe)was grown in-situ on the surface of SiO2@Fe3O4,and MIL-53(Fe)@SiO2@Fe3O4 magnetic catalysts with different MIL-53(Fe)contents were prepared.The catalyst has both redox activity and acidity and exhibits good catalytic performance in the methanol liquid-phase H2O2 oxidation reaction.With the increase of MIL-53(Fe)content,the methanol conversion and MF selectivity are continuously improved.Taking the 40%MIL-53(Fe)@SiO2@Fe3O4 magnetic catalyst as an example,the MeOH conversion reached 45.2%when reaction prolonged for 60 min at 0.75 eq.H2O2,80℃ and the MF selectivity was 50.7%.The catalyst exhibits good recyclability,and the recovery rate was as high as 98%.After 5 cycles,the MeOH conversion and MF selectivity did not decline significantly.(4)MIL-101(Cr)@SiO2@Fe3O4 magnetic composite support was synthesized by the in-situ growth of MIL-101(Cr)with coordinated unsaturated Cr3+site on the surface of SiO2@Fe3O4 magnetic micro spheres;then Au nanoparticles were loaded on MIL-101(Cr),and a series of Au/MIL101(Cr)@SiO2@Fe3O4 magnetic catalysts with different MIL-101(Cr)and Au contents were prepared.MIL-101(Cr)with a size of 100-120 nm was grown around SiO2@Fe3O4.Au nanoparticles were highly dispersed on the surface and pores of MIL-101(Cr)with an average particle size of 3.0 nm.The test results of the reaction performance between CO2 and aniline/H2 of the catalysts showed that 2%Au/50%MIL-101(Cr)@SiO2@Fe3O4 had the best performance.After reacting at 200℃ for 10 h,the aniline conversion could reach 44.0%,and the selectivity of N-methylaniline and N,N-dimethylaniline was 71.8%and 27.6%,respectively.The catalyst exhibits good recyclability,and after five times of reuse,it still maintains stable catalytic performance.Besides,different types of amines can be successfully converted to the corresponding N-methylamine or N-formamide.Mechanism studies have shown that H2 is dissociated into H atoms on the surface of Au,and reacts with CO2 adsorbed on the O2-Lewis base sites in the Cr-O cluster of MIL-101(Cr)to form formic acid.Aniline is adsorbed on the Cr3+Lewis acid sites and reacts with the adsorbed formic acid to form N-formanilide.N-formanilide is further hydrogenated to Nmethylaniline.