Application Of ¹²⁹Xe NMR In Covalent Organic Frameworks

As one of the powerful techniques for characterizing porous materials,129 Xe NMR technique has been widely used to study the structure of porous materials.Compared with X-ray diffraction,N2 gas adsorption,and electron microscopy,129 Xe NMR can provide the information on pore structure of the materials that can not be obtained by other techniques.As a new type of porous organic crystalline material,covalent organic frameworks(COFs)have the advantages of large specific surface area,long range ordered structure,facilely-tailored functionality and relatively stable structure.These advantages render the COFs attractive in numerous applications,including gas adsorption,photoelectric,fluorescence recognition and heterogeneous catalysis.Nevertheless,most studies on COFs are focused on the functionalization of materials and their applications,and there is relatively less research related to the pore structure of COFs.Accordingly,in this dissertation,129 Xe NMR was utilized to study the pore structure of 2D and 3D COFs.In chapter 1,we reviewed the 129 Xe NMR technique in details.Firstly,the basic principle of 129 Xe NMR is introduced,including the generation of 129 Xe NMR signals and the influencing factors of chemical shifts.Secondly,we reviewed the applications of 129 Xe NMR in porous materials,such as,molecular sieve,metal organic frameworks,and mesoporous silicon materials.In chapter 3,we utilized 129 Xe NMR to study the pore structures of 2D COFs.2D COFs are formed by the stacking of lamellar macromolecules through weak ?-? interactions.Different stacking structures of 2D COFs may result in the AA and AB stacking structure.Due to the limitation of resolution,the powder X-ray diffraction(PXRD)technique can not distinguish these two different stacking structures.However,the obvious difference of AA and AB stacking structures is the pore size.Therefore,the difference of two pore structures were considered to distinguish the different stacking structure.Herein,we utilized 129 Xe NMR to study the stacking structure of 2D COFs.Firstly,to explore the feasibility of 129 Xe NMR in characterizing the pore structure of 2D COFs,we applied 129 Xe NMR to study five 2D COFs with different pore structures(including COF-LZU1,TPA,BPDA,TPDA,HP-COF-1).Secondly,the stacking structure of COF-LZU15 was studied by means of variable temperature 129 Xe NMR and 2D EXSY experiments,and the co-crystalline structure of COF-LZU15 with AA and AB stacking structure was verified.In chapter 4,we studied the dynamic structure of 3D COFs(LZU-301)by 129 Xe NMR.Some of metal organic frameworks(MOFs)possess structural flexibility,such as MIL-53,DUT-8 and so on.However,there is no report on the flexible structure of 3D COFs so far.We synthesized a 3D COFs possessing the structural flexibility,labeled LZU-301.129 Xe NMR,PXRD and solvent adsorption analysis were applied to study the structural flexibility of LZU-301.In chapter 5,we used 129 Xe NMR to study the distribution of Pd(OAc)2 in COF-LZU1 material.The Pd/COF-LZU1 material was the first application of a new COF material for heterogeneous catalysis.However,the distribution of the catalyst Pd(OAc)2 in the material is still unclear.In this work,we used 129 Xe NMR,13 C and 15 N solid state NMR to study the distribution of catalyst Pd(OAc)2 in COF-LZU1 material.