| In recent years,a large number of MOFs materials with proton conduction properties have been reported.Structurally,these materials are mostly one-and two-dimensional structures containing abundant guest water molecules.In addition to the excellent proton conductivity,the complex hydrogen bond of these materials makes the conduction mechanism of the materials unclear and the hydrogen bond transfer path fuzzy.To clarify the proton transfer pathway,and improve the proton conductivity of MOFs materials is the most important problem to be solved at present.Based on this,this thesis focuses on the synthesis of three-dimensional microporous MOFs containing simple hydrogen bonds by hydrothermal method using multi-carboxyl functional ligands containing N.The relationship between structure and proton conduction performance is studied,and the conduction pathway is simulated to provide ideas for the research of high performance proton conductor.We selected four kinds of polycarboxylic acid ligands containing nitrogen heterocyclic substituents to react with different rare earth metal ions and successfully obtained 32 kinds of crystalline Ln-MOFs materials.The structure and proton conductivity of the material were studied.The proton donor and acceptor were determined by theoretical calculation,and the proton conduction path was simulated.The effect of open circuit voltage stimulation on the proton conductivity of the material was studied.The specific research contents are as follows:(1)Synthesis,structure and electrochemical properties of MOFs based on 2-(4'-pyridyl)-2H-imidazole-4,5-dicarboxylic acid(H2L1)Using 2-(4'-pyridyl)-2h-imidazole-4,5-dicarboxylic acid(H2L1)and different rare earth metal salts under hydrothermal reaction conditions,12 isomorphic L1-LN MOFs were obtained:{[Ln2(L1)2(ox)(H2O)2]·4H2O}n(Ln=La,Pr,Nd,Sm,Eu,Gd,Tb,Dy,Er,Tm,Yb,Y).The structure study showed that L1 ligand connects rare earth metal ions to form three-dimensional porous MOFs by a nitrogen atom attached to carboxyl and imidazole groups.The imidazolimine and pyridine nitrogen atoms of the L1 ligand do not coordinate with the metal,but form hydrogen bonds with the water of crystallization and coordination water molecules filled in the pores.The electrochemical test results showed that L1-Ln MOFs exhibited different degree of proton conduction behavior,and the proton conductivity increased with the increase of voltage.Theoretical calculation of electron cloud distribution shows that uncoordinated imidazolium N can provide protons and pyridine N can act as proton receptor.The proton conduction path of the L1 ligand was simulated.It was found that the L1 ligand isomerized under the stimulation of external voltage,and the proton was transferred from imidazolimide to pyridine nitrogen atom.The intramolecular proton transfer caused by isomerization of L1 ligand guarantees the continuity of proton conduction path.(2)Synthesis,structure and electrochemical properties of rare earth metal MOFs based on 2-(3'-pyridyl)-2H-imidazole-4,5-dicarboxylic acid(H2L1)Nine L2-Ln microporous MOFs were constructed by 2-(3'-pyridine)-2h-imidazole-4,5-dicarboxylic acid(H2L2)and rare earth metal ions at different positions of pyridine nitrogen atoms.In the{[Ln(L2)(ox)0.5(H2O)]·2H2O}n(L2=La,Pr,Nd,Sm),L2 ligand uses carboxyl and pyridine nitrogen atoms to connect metal ions to form a three-dimensional skeleton,and neither of the two nitrogen atoms of imidazolium group has coordination.Electron cloud distribution analysis shows that imidazolium N can be used as both proton donor and proton acceptor.Kinetic simulation results show that two nitrogen atoms of imidazolium group act as proton donor and acceptor respectively to form hydrogen bond with water molecules.Imidazolium group takes place intramolecular proton transfer,which forms continuous proton conduction path,and proton conduction can proceed smoothly.When the rare earth metal ions are heavy Eu(?)and Gd(?),the results are{[Ln(L2)(ox)0.5(H2O)]·2H2O}n(Ln-Eu,Gd).The main skeleton structure of L2-Eu and L2-Gd is similar to that of L1-Ln except that the position of pyridine nitrogen atom is different.Due to the interposition of pyridine nitrogen atoms,L2ligand cannot be isomerized,protons cannot transfer in the ligand and proton conduction path is discontinuous,so L2-Eu and Gd have low proton conductivity when the open circuit voltage is 0V.Through electron cloud density analysis,it is still imidazolium N as proton donor to provide protons,and pyridine N as proton acceptor to receive protons.Kinetic simulations show that the proton conduction process provides protons for the uncoordinated imidazolium N to the water of crystal O8,the H of O8 can be transferred to O9,and the H of O9 can be transferred to pyridine N in another unit.After a while,imidazolium N loses protons,pyridine N gains protons,and the ligand is in valence equilibrium.The H of pyridine N is unstable,and proton transfer to imidazole N by diffusion is a spontaneous process.Under the excitation of external voltage,the spontaneous proton transfer process provides a continuous path for proton transfer.In[Ln(L2)(ox)0.5]n(L2-Er,L2-Tm,L2-Yb),L2 ligand connects metal ions through two nitrogen atoms of carboxyl group and imidazolium group to form a three-dimensional skeleton,in which there is no water molecule and no continuous hydrogen bond.However,when imidazolimine nitrogen atoms give lone electron pair and coordinate with metal ions,the bond energy of N-H bond will be reduced and free protons will be released,which makes the material show good proton conduction performance.(3)Synthesis,structure and electrochemical properties of rare earth metal MOFs based on 2-(3'-pyridyl)-2H-imidazole-4,5-dicarboxylic acid(H2L3)Six L3-Ln MOFs were prepared by hydrothermal reaction of4-(pyridine-3-methoxy)m-phthalic acid(H2L3)with different rare earth metal salts.In{[Ln2(L3)3(H2O)2]n·4H2O}n(Ln=Dy,Ho,Er,Tm,Yb,Y),L3 ligand still uses its carboxyl group to connect metal ions to form three-dimensional skeleton.There are crystal and coordination water molecules in the structure.Water molecules are connected to each other through hydrogen bonds to form continuous helical water chains.Through electron cloud distribution analysis,it is coordination water molecules that provide protons in L3-Ln MOFS.The proton transfer intermediate H3O+can be clearly seen through kinetic simulation,thus proving that the proton is transferred through the water chain.(4)Synthesis,structure and electrochemical properties of rare earth metal MOFs based on 4-(3'-pyridyl)-phthalic acid(H2L4)Five L4-Ln MOFs with two structures were prepared by hydrothermal reaction of 4-(3'-pyridyl)-m-phthalic acid(H2L4)with different rare earth metal salts.The crystal structure of{[Ln2(L4)3(H2O)4]·2H2O}n(Ln=La,Pr,Nd,Gd)MOFs shows that L4 ligand uses its carboxyl group to connect metal ions to form a three-dimensional skeleton,and the pyridine nitrogen atom does not coordinate with metal ions and its position is disordered.It can be seen from electron cloud distribution calculation that in L4-Ln(Ln=La,Pr,Nd,Gd)structure,coordination water molecules give protons,which are transferred to water molecules of another unit through disordered N atoms,thus completing proton transfer.In the structure of[Y2(L4)3(H2O)2]n MOF,L4 ligand still uses its carboxyl group to connect Y(?)to form a three-dimensional skeleton,and the pyridine nitrogen atom does not coordinate with metal ions.Due to the free rotation of pyridine groups during crystallization,the position disorder of pyridine nitrogen atoms is serious.There is no crystal water molecule in the structure,and the coordination water molecule and disordered pyridine nitrogen atom are connected by hydrogen bond to form hydrogen bond network.L4-Y MOF showed significantly higher proton conductivity than L4-La,Pr,Nd and Gd MOFs,apparently due to the existence of continuous proton conduction pathways assisted by pyridine rotation.When the open circuit voltage is increased,the probability of pyridine rotation is higher,the continuity of proton conduction path is increased,and the proton conductivity of L4-Y MOF is increased.In conclusion,two new forms of proton conduction assisted by intramolecular proton transfer caused by isomerization of nitrogenous substituents and nitrogen atom position disorder caused by substituent rotation are proposed for the first time.It is found that when the open circuit voltage increases to a certain value,the proton conductivity of the material increases obviously.It was found that when imidazolium N was used as the proton donor,the proton conductivity distribution of Ln MOFs was the same as that of light,medium and heavy rare earth,and the proton conductivity decreased with the increase of atomic radius in the same interval.When the coordination water molecule was used as the proton donor,the proton conductivity of Ln MOFs was related to M3+/M potential. |
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