| Proton conductors possess important applications in proton exchange membrane fuel cells,biochemistry,electrochemical devices and other fields.Crystalline metal-organic frameworks(MOFs)have attracted considerable attention in the field of proton conduction due to their high porosity and functional controllability.Recently,we have focused on the design and properties of proton conductive MOFs based on imidazole polycarboxylate ligands.The research of this thesis mainly focuses on the following three aspects:I.Proton conductive behaviors of imidazole polycarboxylate-based MOFs under water vapor;II.The regulation of the proton conductivity of relevant MOFs by the change of test environment atmosphere was studied;III.The impedance recognition performance of proton conducting MOFs for acidic or alkaline small molecules.The details are as follows:Ⅰ.Six 3D lanthanide(Ⅲ)MOFs,{[Ln2(HMIDC)2(μ4-C2O4)(H2O)3]·4H2O}n(LnⅢ=GdⅢ(1),EuⅢ(2),SmⅢ(3),NdⅢ(4),PrⅢ(5)and CeⅢ(6)),based on 2-methyl-4,5-imidazole dicarboxylic acid ligand(H3MIDC)were synthesized through solvothermal method.Both 1-6 exhibit high water,thermal and acid-base stability,and contain a large number of uncoordinated imidazolium-N atoms,coordinated water molecules,crystalline water molecules and abundant H-bonded networks in the frameworks.The AC impedance determination showed that the proton conductivity increased with the increasement of temperature and humidity.At 98%RH-100℃,the optimal proton conductivities are 2.01×10-3(1),1.40×10-3(2),9.31×10-4(3),2.25×10-4(4),1.11×10-4(5)and 9.62×10-5 S·cm-1(6),respectively,which also deliver excellent reproducibility and durability.The conductivity of 1-6 increases with the reduction of the ion radius of lanthanide metal ions,and the conductivity of 1 is nearly 21 times than that of 6.Obviously,this is because the acidity of coordination water molecules increases with the increasement of charge density of metal ions,which promotes the release of protons from coordination water molecules.Ⅱ.By using 2-(3-methoxyphenyl)-1H-imidazole-4,5-dicarboxylic acid(m-H3MOPhIDC),windmill like 3D {[Mn3(m-MOPhIDC)2(H2O)6]·4H2O}n(7)and{[Zn3(m-MOPhIDC)2(H2O)6]·4H2O}n(8)were synthesized.Their structures bear hexagonal one-dimensional channels,which are filled with a large amount of free and coordination water molecules.Both of them show good thermal,water,and acid-base stability and water vapor adsorption capacity.It was found that at 98%RH-100℃,the pproton conductivities of 7 and 8 reached the maximum values being 4.64×10-5 and 5.31×10-6 S·cm-1,respectively.Considering that their optimal conductivities are low,we expect to improve their conductivities by changing the atmosphere of the test environment and introducing guest molecules.Therefore,they were exposed to different concentrations of aqua-ammonia vapor,and the effects of temperature and ammonia concentration on their proton conductivity were explored.The results demonstrated that the proton conductivity increased with the enhancement of temperature and ammonia concentration.At 100℃ and 2 M aqua-ammonia vapor,the conductivities of 7 and 8 attained 3.4×10-3 and 1.38×10-3 S·cm-1,respectively,which are 2-3 orders of magnitude higher than those of 98%RH,indicating that NH3 molecules and NH4+ ions,water molecules and hydrated ions in the environment play a synergistic role for the improvement of proton conduction.3D MOF {[Ba(p-CPhH2IDC)]·H2O}n(9)was synthesized by employing 2-(4-carboxyphenyl)-1H-imidazole-4,5-dicarboxylic acid(p-CPhH4IDC).There are a large number of undeprotonated carboxylic oxygen atoms and imidazolium nitrogen atoms as well as coordination water molecules in the framework forming rich H-bonded networks.The results denoted that the proton conductivity increases with the increasement of temperature and humidity.At 98%RH/100℃,the proton conductivity reaches the maximum value 1.13×10-5 S·cm-1.Exposed to different concentrations of ammonia and organic amine vapor,the effect of temperature and concentration on its conductivity was examined.The conductivity of 9 is 5.26×10-3 S·cm-1 at 100℃ and 2 M ammonia,and is 465 times higher than that at the same temperature and 98%RH(1.13×10-5 S·cm-1).At 70℃ and 2 M methylamine,dimethylamine and ethylamine vapor atmosphere,the proton conductivities of 9 reach 1.3×10-3,9.79×10-4 and 7.53×10-4 S·cm-1,respectively,which are 367,299 and 230 times compared with that at the same temperature and 98%RH(3.27×10-6 S·cm-1).Obviously,under aqua-ammonia and organic amine vapor,ammonia and organic amine composition would exist in a crack in the framework through adsorption,which can not only act as proton carriers to improve proton conduction,but also be used as proton sources.Moreover,they can form a rich H-bonded network with the framework component to facilitate proton transmission.III.The 3D ionic MOF,{Na[Cd(MIDC)]}n(10)was prepared by using H3MIDC.In the framework,two open 1D channels are formed along the c-axis,half of which are occupied by uncoordinated carboxylic oxygen atoms and free sodium ions.This special structural feature provides a new pathway for proton conduction.First,we investigated the proton conduction behavior at different temperatures and humidity.It was observed that the optimal conductivity is 1.04×10-3 S·cm-1 at 98%RH-100℃,which also possess good stability and cyclicity.Considering the porous structure,excellent chemical stability and proton conductivity under water vapor,we further investigated the impedance recognition performance of 10 against ammonia and amine gases at room temperature and at 98%,85%and 68%RHs.At 98%RH,the optimal detection limits of 10 for ammonia and amine gases are 0.05(ammonia),0.1(methylamine),0.5(dimethylamine),1.0(trimethylamine)and 4 ppm(ethylamine),respectively.Two novel 3D Co(II)MOFs,{[Co3(p-CPhHIDC)2(4,4’-bipy)(H2O)]·2H2O}n(4,4’-bipy=4,4’-bipyridine)(11)and {[Co3(p-CPhHIDC)2(bpe)(H2O)]·3H2O}n(bpe=1,2-bis(4-pyridyl)ethylene)(12)were synthesized by using p-CPhH4IDC.The structures of 11 and 12 are similar,except the main difference being the introduction of different lengths of nitrogen-bridged ligands(4,4’-bipy and bpe).At 98%RH-100 ℃,the optimal conductivities of them are 1.04×10-3 and 7.02×10-4 S·cm-1,respectively.Considering that their frameworks contain left helix and right helix water chains formed by hydrogen bonds,which have good thermal stability,water stability,acid-base stability,and good proton conduction behavior.We studied their impedance recognition performance for formic acid gas at 68%and 98%RHs,and room temperature,respectively.The results confirmed 11 and 12 have the best detection limits to formic acid gas at 98%RH,which are 35 and 70 ppm,respectively.Finally,the proton conducting mechanism,regulation mechanism and molecular impedance recognition mechanism of the above-mentioned MOFs are discussed by combining the calculated values of activation energy,crystal structural characteristics,N2 adsorption,water vapor adsorption and ammonia vapor adsorption data. |
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