Study On Chemical Clipping Of The Melting Properties Of Zeolitic Imidazolate Frameworks ZIF-62

In recent years,some researchers have found that several metal organic frameworks?MOFs?can be melted and quenched to form a new kind of glass material.This finding has improved the problems of difficult processing and difficult molding,and it also links the melting and glass transition processes with chemistry and topology,which gives us a deeper understanding of glass formation.In this paper,one kind of zeolitic imidazolate framework,ZIF-62,has been used as the research object.In view of the current lack of research on the melting properties of MOFs,we systematically studied the relationship between synthetic factors and the melting behavior of ZIF-62 and the effect of desolvation process on the melting properties of ZIF-62.Furthermore,through modern structural characterization methods such as Raman and FT-IR,the structural origin of the change of melting behavior was explored.In addition,a series of bimetallic ZIF-62 samples were synthesized by postsynthetic cation exchange.Then their melting properties,thermal stability,microstructure and sodium storage electrochemical properties were explored.The main research results are as follows:?1?A series of ZIF-62 samples were prepared by solvothermal synthesis.It was found that ZIF-62 crystals were always obtained by changing the ratio of the Im and bIm ligands in a wide range.The melting point?T_m?and glass transition point?T_g?of ZIF-62 both rose when increasing the content of benzimidazole,possibly due to the increased“steric hindrance”caused by the addition of excessive benzimidazole.This finding indicates that the melting properties of ZIF-62 can be regulated by chemical and structural tailoring.?2?The ZIF-62 crystals were desolvated in two different atomosphere to explore the sensitivity of ZIF-62 crystals to the atmosphere.It was found that in air condition,the ZIF-62 crystals tended to be oxidized to from C-O and C=O during the desolvation process,affecting the melting behavior of crystals.Furthermore,we have proposed the mechanism of ZIFs oxidation for the first time,which has some guiding significance for the subsequent study of ZIFs.?3?The bimetallic ZIF-62 crystals were prepared by postsynthesis cation exchange,and the color of the samples changed significantly after the replacement.The SEM-EDX results confirmed successful exchange of metal ions.At the same time,the cation exchange efficiency is related to the type of metal ion and solvent.The Mn²⁺substitution efficiency is higher than that of Co²⁺,and the order of different solvent substitution efficiencies is:EtOH>MeOH>DMF>H₂O.Compared with the original ZIF-62,the T_m and T_g of bimetallic ZIF-62?Zn/Mn?and ZIF-62?Zn/Co?samples are significantly reduced,and the thermal stability is also reduced.?4?Compared with the original ZIF-62 sample,the morphology of bimetallic ZIF-62 samples changed significantly from regular octahedron to porous network or fish scale structure,forming many connected tunnels.Using this unique morphology,we prepared bimetallic ZIF-62 samples as the anode for a sodium ion battery.But the first cycle discharge capacity was only 42 mAh/g and the electrochemical performance was not satisfactory.We think it may be due to the poor conductivity of the sample or the damaging of the porous structure in the grinding process.