The Study On The Effects Of Metal Ions On Gas Adsorption Properties Of Flexible Metal-organic Frameworks

Flexible metal-organic frameworks(FMOFs)as a branch of metal-organic frameworks(MOFs),have become a hot research topic in the field of MOFs because they show dynamic structural changes,also exhibits gate-opening and breathing effects,have potential applications in gas storage and separation.How to design flexible MOFs and studying the influence of changes in their structural flexibility on gas adsorption and separation are also current questions worthing discussing in this field.Therefore,in this paper,two kinds of isostructural flexible metal-frameworks were synthesized with sulfur-containing dicarboxylic acid H₂DMTDC(H₂DMTDC=3,4-dimethylthieno[2,3-b]thiophehe-2,5-dicarboxylic acid)and nitrogen-containing bipyridine linker DPNDI(DPNDI=N,N?-di-(4-pyridyl)-1,4,5,8-naphthalenediimide)or bpe(bpe=1,2-di(-pyridin-4-yl)ethane)with different metal ions under solvothermal conditions.The gas adsorption properties of two kinds of flexible MOFs are also studied.The specific work is as follows:1.A series of flexible frameworks X-pcu-6-M,namely[Zn₂(DMTDC)₂(bpe)](X-pcu-6-Zn),[Cu₂(DMTDC)₂(bpe)](X-pcu-6-Cu),[Co₂(DMTDC)(bpe)](X-pcu-6-Co),with double interpenetrating pcu networks were successfully synthesized by using sulfur-containing dicarboxylic acid H₂DMTDC and flexible bipyridine ligand bpe with different metal ions,and gas-induced structural flexibility was systematacially studied.The results indicate that they exhibit a reversible structural transition between closed and open phases,and both exhibit type F-IV isotherms and large specific surface areas.The N₂ and CO₂ adsorption results show that the order of the gate-opening pressure is as follows:X-pcu-6-Cu<X-pcu-6-Co<X-pcu-6-Zn,and X-pcu-6-Cu always shows the lowest gate-opening pressure,which proves that metal ions can effectively adjust the gate-opening/gate-closing pressure.At 298 K and 90 bar,only X-pcu-6-Cu can show the CH₄-induce structural change.This work confirms that metal ions can affect the gate-opening/gate-closing of flexible MOFs materials,and provides a method for the design of flexible MOFs.2.A series of flexible frameworks X-pcu-9-M,namely[Zn₂(DMTDC)₂(DPNDI)](X-pcu-9-M),[Cu₂(DMTDC)₂(DPNDI)](X-pcu-9-Cu),[Co₂(DMTDC)₂(DPNDI)](X-pcu-9-Co)and[Ni₂(DMTDC)₂(DPNDI)](X-pcu-9-Ni),were successfully prepared by using sulfur-containing dicarboxylic acid H₂DMTDC and rigid bipyridine ligand DPNDI and different metal ions.PXRD and gas adsorption studies show that the series of X-pcu-9-M display the characteristics of flexible structure and stepwise adsorption/desorption isotherms.X-pcu-9-Cu and X-pcu-9-Ni exhibit three-step CO₂ gas adsorption,whereas X-pcu-9-Zn and X-pcu-9-Co display two-step CO₂ gas adsorption In addition,the tests of adsorption enthalpies of CO₂,CH₄ and C₂H₂ show that X-pcu-9-Cu has the largest gas adsorption enthalpy among the four compounds for the three gases,which indicates that X-pcu-9-Cu has a good affinity for gas molecules.The study also confirmed that metal ions have effect on gas adsorption properties of FMOFs.