Construction And Adsorption Properties Of Metal-Organic Frameworks Based On Tetrathiafulvalene Derivative Ligands

Tetrathiafulvalene(TTF)and its derivatives are a siginificant class of organic sulfurcontaining compounds.Because TTF is redox-active and electron-donating,with a nearly rigid planar structure,it is widely used in the construction of coordination compounds/polymers to give the product unique porous structure or electrical conductivity.In this work,with tetrathiafulvalene derivatives as ligands,two types of metal-organic frameworks(MOFs)with three-dimensional penetrating pores had been constructed.Based on their unique pore structures and chemical properties,they were applied in two fields,which were adsorptive separation and adsorption-enhanced electrical conductivity,and their adsorption performance and mechanism had been studied.Firstly,M2TTFTB(M=Zn,Mn,Cd)were constructed based on tetrathiafulvalenetetrabenzoate(H4TTFTB)and divalent metal cations,and their crystal structures,pore characteristics and stability were characterized.The results showed that the pore size distribution of Zn2TTFTB,Mn2TTFTB and Cd2TTFTB ranged from 4.2 (?)-7.2 (?),4.8 (?)-6.3 (?) and 4.4 (?)-7.0(?),respectively.The degree of distortion of internal pores differed a lot.The order of distortion was Zn2TTFTB<Mn2TTFTB<Cd2TTFTB.By changing the metal center,we successed in controlling the size of the channels on sub-angstrom-level,which would provide new opportunities for the selective separation of structurally similar substances.In this work,the adsorption equilibriums and adsorption kinetic curves of n-hexane(nHEX),3-methylpentane(3MP)and 2,2-dimethylbutane(22DMB)on M2TTFTB(M=Zn,Mn,Cd)were determined.And column breakthrough tests for mixture were also done to explore the relationship between the shape of the pores and separation performance of hexane isomers.The adsorption isotherms of M2TTFTB(M=Zn,Mn,Cd)to nHEX and 3MP were all extremely steep,and none of them excluded 22DMB.Among them,the adsorption capacity of Mn2TTFTB for 3MP at 393 K was up to 156 mg/g,which was the highest value of MOFs materials so far.The kinetic diffusion experiment showed that the diffusion of 22DMB in the pores of Mn2TTFTB and Cd2TTFTB was limited,and the kinetic selectivity of Mn2TTFTB for 3MP/22DMB could be 88.0.The threecomponent fixed bed breakthrough experiments showed that the breakthrough time intervals of Mn2TTFTB and Cd2TTFTB to 22DMB and 3MP were 190 and 155 min/g,respectively.It was confirmed that 22DMB was excluded due to slow diffusion in the pores,leading to excellent separation performance based on kinetic control.In addition,density functional theory calculation and single crystal X-ray diffraction test showed that hexane molecules and the material framework formed multiple interactions such as hydrogen bonds,and the order of adsorption energy was 22DMB<3MP<nHEX,which was consistent with the order of the adsorption amount in the low pressure zone of the static adsorption experiments.Inspired by the strong polarity of the pore surface of M2TTFTB,the adsorption and separation performance of xenon(Xe)/krypton(Kr)was further studied.The static adsorption test showed that the IAST selectivity of Mn2TTFTB and Cd2TTFTB to Xe/Kr(20/80)at 273 K was 12.9 and 17.8,respectively.The column breakthrough tests further confirmed that Mn2TTFTB and Cd2TTFTB had good dynamic separation performance for Xe/Kr(20/80)mixed gas,with the yield of pure krypton(>99.9%)reached up to 79 and 74 mL/g,respectively.Based on tetra(4-pyridyl)-tetrathiafulvalene(TTF(py)4),two novel anionic pillared metalorganic frameworks named TiFSIX-TTF(py)4-Zn and SiFSIX-TTF(py)4-Zn with threedimensional pillared structure were synthesized.And their crystal structures,stability and redox adsorption properties were characterized.By changing the types of fluorine-containing anions,the distance between layers could be adjusted.After the adsorption of TTF,the content of TTF·+radical cations increased,the optical band gap narrowed,and the electrical conductivity increased.After the adsorption of iodine,the above phenomenon became more obvious.All indicated that the electrochemical performance of the material had been effectively improved after adsorbing guest molecules.Single crystal X-ray diffraction experiments showed that the adsorbed TTF guest molecules induced adjacent TTFs of the materials to approach each other,thereby constructing a charge transport path.