Research On The Application Of MOFs Materials For The Removal Of Gaseous Mercury

As the main source of human power,coal-fired power plants make human life more convenient,but meanwhile they cause pollution to the environment.The heavy metal mercury in emissions has received high attention worldwide owing to its stability,high toxicity,and bioaccumulation.At present,adsorption and catalytic oxidation are the mainstream technologies in the treatment of mercury pollution resulted from coal-fired power plants.As a new type of porous material,metal organic framework materials show excellent properties such as stable structure,high porosity,and unsaturated metal sites.It has been applied in many fields.The goal of the presenwet paper is to explore the application possibilities of MOFs in the field of mercury removal.By combining the excellent characteristics of MOFs with the current mercury removal technology,the improvement of mercury adsorbents and catalysts can be achieved.Therefore,a series of MOFs and their derived metal oxides were synthesized in the present dissertation.Their adsorption or catalytic performance for heavy metal mercury was discussed and the corresponding mercury pollution removal mechanisms were proposed.The main research contents and results of the present article are as follows:(1)Effect of MOFs as a carrier on Hg0 removal performance of MnO2:The transition metal oxide MnO2 has excellent Hgo catalytic oxidation performance.MIL-96 has excellent structural stability.It can avoid decomposition during loading,so it was selected as the carrier.Potassium permanganate and manganese sulfate were used as raw materials to synthesize α-MnO2 on the surface of MIL-96 by hydrothermal method.In this paper,we synthesized three αMnO2-MIL-96 catalysts with different MnO2 loadings:5%α-MnO2MIL-96,10%α-MnO2-MIL-96,and 15%α-MnO2-MIL-96.15%α-MnO2-MIL-96 exhibited the best mercury removal performance,and its Hg0 removal capacity reached 214.15 μg·g-1,which is much higher than that of pure α-MnO2(146.95 μg·g-1).It can be seen that as MIL96 was used as a carrier to disperse α-MnO2 and reduce the particle size of the catalyst to nanoscale,the utilization efficiency of the catalyst has also been greatly improved.It was found that α-MnO2 on the surface of MIL-96 catalyzed the oxidation of Hg0 to Hg2+through the Mn4+ active center in its structure.After Mn4+ was converted to Mn3+,it can be restored by O2 oxidation.(2)Hg0 removal performance of transition metal oxide Co3O4 prepared with MOFs as a template:The sacrificial template method is an effective means to improve the performance of the catalyst.It is difficult for manganese ions to form a metal-organic framework with organic ligands.In this study,Co3O4 was selected as the research object.The metal ion centers in MOFs are independent of each other and separated by organic ligands.In this paper,Co-BDC was used as a sacrificial template to synthesize nano-sized Co3O4@C particles.The research results showed that the organic ligands(terephthalic acid)in the Co-BDC structure were carbonized during the calcination process,which hindered the agglomeration of metal oxides.The performance of the catalyst was improved after its particle size was reduced.The Co-O bond in Co3O4@C was weaker than that in the commercial Co3O4 structure.The oxygen atoms were more likely to leave the surface of Co3O4@C.It made the Hg0 removal temperature window of the catalyst shift to a low temperature range.Commercial Co3O4 showed poor Hgo removal performance at low temperatures.Meanwhile,the Hg0 catalytic oxidation efficiency of Co3O4@C could reach nearly 100%at room temperature.The flue gas composition has little effect on the mercury removal performance of Co3O4@C.SO2 was toxic to C03O4,but Co3O4@C exhibited excellent sulfur resistance at low temperatures,and the mercury removal capacity of Co3O4@C has increased in SO2 atmosphere at room temperature.It was found that SO2 did not drive Hg0 away from the surface of Co3O4@C at room temperature,but participated in the oxidation of Hg0 through Co3O4 surface activation.(3)Removal of Hg0 removal by magnetic Co3O4-CoFe2O4 prepared by MOFs template:Both the carrier and template methods have successfully improved the performance of the catalyst.In order to further enhance the activity of Co3O4,this study selected MIL-101(Fe/Co)as a sacrificial template,and synthesized Co3O4-CoFe2O4 by calcination.Compared with Co3O4@C,the particle size of Co3O4-CoFe2O4 was further reduced to about 20 nm due to different MOFs templates.At the same time,the Fe and part of the Co in MIL-101(Fe/Co)generated magnetic carrier CoFe2O4 during the calcination process,which further dispersed the Co3O4 particles and improved the utilization rate of Co3O4.It was found that Co3+and oxygen defects on the surface of Co3O4 are the reaction centers for the catalytic oxidation of Hg0.The experiment results showed that Co3O4-CoFe2O4 had good adaptability to complex gas environment.It showed excellent sulfur resistance at low temperature.Co3O4-CoFe2O4 has great regeneration performance.Its mercury removal activity can be restored by heating.(4)The Hg2+ removal perforLance of MOFs materials in wastewater:During the flue gas treatment process in coal-fired power plants,mercury-containing wastewater was continued to be produced,and the Hg0 in the flue gas was oxidized to Hg2+ and entered the water body.In order to avoid secondary pollution,the wastewater containing mercury needs to be purified.MOFs materials can be used not only as carriers and sacrificial templates,but also as adsorbents.We have discovered a kind of MOFs material-ZIF-67 has an excellent separation effect on the heavy metal Hg2+ in the water body.It was found that UiO-66,MIL101,HKUST-1 and MIL-96 had almost no adsorption effect on Hg2+ in water,but ZIF-67 showed high separation capacity for Hg2+.Its Hg0 removal capacity could reach 1740 mg·g1.ZIF-67 has great reproducibility.It was found that ZIF-67 separated Hg2+ from water through the competitive coordination between Co2+ in its structure and Hg2+ in the solution.