Iron Based Metal Organic Frameworks Catalyzed Fenton-like Reaction For Organic Pollutants Degradation

The escalating emission of refractory organic pollutants in water has aroused world-wide concern since it undermines the health of environment and organisms.Fenton reaction is an efficient technology for treatment of wastewater containing refractory organic pollutants.Although the traditional homogeneous Fenton technology is of high efficiency,it is restricted to acidic condition.And the homogeneous catalysts can not be recycled,forming a large amount of iron slugde after reaction.Recently,heterogeneous Fenton-like catalysts,promising alternatives to homogeneous catalysts,have aroused increasing attention.These catalysts,however,generally show low activity.Moreover,the catalytic mechanism remains unclear.In this work,to address the above issues,iron-based metal organic frameworks(MOFs)were used as heterogeneous Fenton-like catalysts to enhance the catalytic performace.Benefiting from their diversity and tunable structures,optimizing the structures of iron-based MOFs,doping transition metals and modifying specific groups on organic ligands in MOFs were emplyed to further enhance the catalytic efficiency of iron-based MOFs.Morevoer,the reaction mechanism and the relationship between the structure of iron-based MOFs and their catalytic performance was explored.This work provides scientific basis and theoretical guidance for developing higly effective heterogeneous Fenton-like catalysts.The main research contents and results are shown as follows:(1)Three iron-based MOFs(MIL-53(Fe),MIL-88B(Fe)and MIL-101(Fe))with different structures were employed as heterogeneous Fenton-like catalysts.Phenol was used to evaluate their catalytic performance.The results showed that under the same reaction conditions,the catalytic activity of MIL-88B(Fe)was about 1-3 orders of magnitude higher than that of the other three conventional catalysts(Fe2O3,a-FeOOH and Fe3O4)and the other two iron-based MOFs(MIL-53(Fe)and MIL-101(Fe)).·OH was the main reactive oxidative species and the coordinative unsaturated irons were the catalytic active sites in MIL-88B-Fe.The superior activity of MIL-88B(Fe)could be originated from the abundant amount of the active sites and facilitated reduction of Fe(Ⅲ)to Fe(Ⅱ)in the catalyst.(2)A series of MIL-88B(Fe)-X with substituents(X = NO2,Br,H,CH3,and NH2)of different electrophilicity were prepared and their Fenton-like catalytic activity were tested.The results showed that catalytic activity of MIL-88B(Fe)-X was improved by increasing the electrophilicity of substituent(MIL-88B(Fe)-NH2<MIL-88B(Fe)-CH3<MIL-88B(Fe)-H<MIL-88B(Fe)-Br<MIL-88B(Fe)-NO2).MIL-88B(Fe)-NO2,with the substituent of highest electrophilicity among these catalysts,displayed the best catalytic performance.The kinetic constant of phenol degradation by MIL-88B(Fe)-NO2 was 0.22 min-1,which was 1.7 times more than that of MIL-88B(Fe)-H and was even comparable to that of the homogeneous catalyst(Fe2+).The enhanced catalytic performance of MIL-88B(Fe)-NO2 could be attributed to the lower electron density of Fe(Ⅲ)compared with the other MIL-88B(Fe)-X,which resulted in the faster redox cycle of Fe(Ⅲ)/Fe(Ⅱ)and then enhanced the catalytic performance.(3)Cu doped MIL-88B(Fe)were prepared by solvethermal methd and were employed as heterogeneous Fenton-like catalysts.It was found that doping Cu into MIL-88B(Fe)can enhance the catalytic performance of MIL-88B-Fe.Among the doping catalysts,MIL-88B(Fe0.6Cu0.4)exhibited the highest catalytic activity,which was 1.5 times more than that of MIL-88B(Fe)and was 3 orders of magnitude higher than those for some traditional hetetrogeneous Fenton-like catalysts(Fe2O3,a-FeOOH and FeBiO3).The XPS and electrochemical characterization results showed that doping Cu could enhance the electron transfer in catalyst.As a result,the catalytic performance was improved.(4)MIL-100(Fe)was employed as heterogeneous Fenton-like catalyst.And it catalytic performance for degrading organic pollutants with different redox potentials(methylene blue,E=-0.09V;phenol,E=1.02 V;p-nitrophenol,E=1.23V)was evaluated to investigate the influence of the redox potential of organic pollutants on the catalytic performance and Fe(Ⅲ)/Fe(Ⅱ)redox cycle in MIL-100(Fe).The results showed that the degradation kinetic constant of methylene blue was 90 times more than those of phenol and p-nitrophenol.Notably,in the presence of methylene blue,both of the degradation rate constants of phenol and p-nitrophenol were significantly enhanced by an order of magnitude.This phenomenon was attributed to the lower redox potential of methylene blue compared with that of Fe(Ⅲ)/Fe(Ⅱ)in MIL-100(Fe)(E=0.33 V),which could effectively reduce Fe(Ⅲ)to Fe(Ⅱ),thus facilitating the redox cycle of Fe(Ⅲ)/Fe(Ⅱ)and improving the Fenton-like catalytic performance of MIL-100(Fe).