Modification Of Metal-Organic Frameworks ZIF-8 And Their Application In CO₂ Capture And Conversion

Due to the transitional exploitation of fossil fuels and the destruction of large quantities of vegetation,the concentration of CO₂ in the atmosphere has increased dramatically since the beginning of the Industrial Revolution.On one hand,a lot of CO₂ and other greenhouse gases were released into the atmosphere as the burning of fossil,leading to the growing global climate warming and other environmental problems.On the other hand,CO₂ is a renewable C1 resource,which has been widely used as an environmentally friendly raw material in the production of fuels,materials and chemical products.Undoubtedly,CO₂ utilization is one of the best ways to reduce its emission.But,the CO₂ has the characteristics of good thermal stability and chemical stability.So,It is very important to develop efficient capture agents and catalysts to realize the efficient capture and transformation of CO₂ under mild conditions.The metal-organic frameworks?MOFs?which are made up of organic ligands and metal ions or metal ion clusters have the advantages of large specific surface area,high thermal stability,easy recovery,diverse structure and adjustable property.However,the existing MOF-based catalysts still suffer from some problems such as low catalytic activities and/or harsh reaction conditions.Therefore,in order to improve the efficiency of the capture and conversion of CO₂,great efforts have to be done to modify MOFs.The main contents of this paper are as follows:?1?Firstly,the physical hybrid catalytic system of ionic liquids?ILs?and MOFs was studied.It was found that ZIF-8 and tetrabutylammonium bromide?TBAB?had a good synergistic effect in the conversion of CO₂.When the molar ratio of ZIF-8?based on Zn?/TBAB was 0.58 mmol/1.80 mmol,the yield of PC reached 98% at 90 °C and 0.35 MPa for 48 h.However,it is difficult to recover the co-catalyst TBAB.Therefore,it is necessary to combine ionic liquids and MOFs to develop a heterogeneous catalyst.?2?We make use of the coordination effect?-NH2 with Zn2+?to graft amino ionic liquids onto the MOFs to obtain a series of ZIF-8-AFIL-x composites.AFILs are distributed homogeneously in the framework of ZIF-8,thus bringing the novel nanocomposites multiple active sites,large specific surface areas,open porous channels,and good thermostability.The nanocomposites exhibit superior ability to efficiently capture CO₂ and convert it into cyclic carbonate under mild conditions?1 atm,? 50 oC?.The CO₂ adsorption capacity of ZIF-8-AFIL-3 was 2.38 mmol·g-1?25 °C,1 atm?and 2.68 mmol·g-1?100 °C,1 atm?.Meanwhile,the adsorption rate is large,and can be regenerated by simple operation?140 °C,purge N2 for 1 hour?.When ZIF-8-AFIL-3 was used as catalyst,the reaction could be substantially completed after 6 h at 100 oC without co-catalyst and solvent.Moreover,ZIF-8-AFIL-3 shows superior stability and recyclability in CO₂ adsorption and catalytic process.?3?Three kinds of organic molecules with different sizes were synthesized by imitating the ligand?2-methylimidazole?structure of ZIF-8.In the synthesis of ZIF-8,these organic ligands were doped as a second ligand to obtain a series of mixed ligand metal-organic frameworks?m-MOFs?.The pore structure and specific surface area of m-MOFs were adjusted to obtain m-MOFs with high adsorption capacity to CO₂ by adjusting the structure and content of the second ligand.It was found that the adsorption capacity of several m-MOFs can reach up to 6 mmol·g-1 under mild conditions,and the desorption method is simple?140 °C,purge N2 for 1 hour?.The relationship between CO₂ adsorption and its structure was studied by XRD,TGA,N2 adsorption/desorption,SEM and ¹H NMR.This work advances a new way for the development of efficient CO₂ adsorbents.