The Effect On The Electronic Property And Adsorption Ability Of MOF Modification By Applying A First Principles Study

Metal-organic frameworks (MOFs) are a class of nanoporous materials containing three dimensional (3D) and periodic networks of metals, metal clusters, or metal oxide clusters held together by bridging organic linkers.It have good application prospects in gas adsorption, storage, separation, catalysis and sensor because of their extremely large surface area and tunable porous structures. Recently, the research on the modification of metal sites and organic chains has received extensive attention. The modification can not only affect the electronic structure distribution of MOF in order to improve the gas separation or change the magnetic state, but also is an important approach to introduce the new functional groups.The major target of this dissertation is to study the effect on the electronic property and adsorption ability of MOF modification by using the first principle method based on density functional theory (DFT). Through the transition metal doping on the metal sites of Fe-MOF-74, the influence of the dopant on the electronic structure and magnetic properties of Fe-MOF-74 was studied. Simultaneously, salt-chelated and active groups on unsaturated NN organic chain of MOF-253 were adopted to study the enhanced adsorption of CO2 and CO2 catalytic internal electronic changes.The details include below:(1) The electronic properties of different number density Ni and Co doped on the center metal of Fe-MOF-74 were computed using a hybrid exchange-correlation functional (HSE06). Through the different settings on the initial magnetization of the metal atom, it had found that the ground state energy difference between the antiferromagnetic (AFM) and ferromagnetic (FM) was marginal, with several meVs. This shows that the FM can be kept as metastable magnetic state. We have calculated the electron density and energy band of states effects for different number density and sites of Ni doping on the Fe-MOF-74. It had shown that in the two coupling structures of AFM and FM, the band gap of the spin down electrons significantly decrease with the Ni doping from 1 to 3 atoms. In particular, when the 2 Ni atoms are used to carry out the inter chain doping, the FM states reduced from 1.38eV to 0.68eV. Local density of state analysis indicated that the Ni-3d orbital caused the downshift of Fermi level, leading to a narrower band gap between VBM and CBM, which the metal-doped MOF makes favors the occurrence of a single spin electron flow, and further improves the prospect application of MOF in the field of spin electronics.(2) The basic electron variation in salt-chelated MOF-253 was investiaged through the DFT-D2 method, and results demonstrated that the CO2 adsorption was enhanced by chelating PdCl2 and Cu (Bf4)2. The two N atoms preferentially oriented to the opposite side of the 2,2-bipyridine (bpy) in bare MOF-253, and the adsorption energy of 0.15 eV was obtained CO2. In the next step, salt molecules were incorporated into the MOF structure, and the optimized structure for the hybrid suggested that the two pyridine rings were adjusted to the same side, with the salt molecule adhering to NN atoms. The PdCl2 insertion hardly enhanced any CO2 adsorption, however, the dipole polarization leaded to multiple adsorption sites by adding a Pd-Cl bond and co-linker site, and thereby a higher adsorption capacity can be predicted. Cu (BF4)2 chelation greatly increased the CO2 adsorption energy to 0.49 eV due to the large positive charge of the Cu ion, which induced significant electrostatic interactions with the O atom of CO2. Our calculation results demonstrated the effect of inherent electron distribution during salt insertion on CO2 adsorption, which clearly predicated a 7 kJ/mol enhancement in CO2 absorption upon loading of Cu (BF4)2 on MOF-253.