Density Functional Theory Study On The Microscopic Structure Of Layered Double Hydroxides And The Hydrdesulfurization Of Thiophene

In the field of theoretical chemistry, the density functional theory(DFT) is regarded as one of the most popular and convenient methodologies. Because of its high accuracy and low cost, the DFT has been widely used in different disciplines such as Physics, Chemistry, Biology, Materials Science and so on. Based on the software of Material Studio, we discussed the locations of metals in the layers, the distribution of anions in the interlayer and the supra-molecular interaction between the host layer and the guest anions of CuZnMgAl-X-LDHs. Furthermore, we investigated the arrangements of interlayer waters and its stepwise hydration energy of CuFe-LDHs. Meanwhile, we discussed the adsorption and hydrodesulfurization of thiophene on different metal surfaces and determined the optimal path and the rate-limiting step by using DMol3 program in detail.(1) A periodic interaction model with Cu, Zn, Mg, Al and different intercalated anions(X=F-, Cl-, Br-, I-, NO3-, OH-) was proposed for the CuZnMgAl quaternary hydrotalcites(Cu ZnMgAl-X). Based on the binding energies, the geometric parameters, mulliken charge populations, hydrogen-bonding, we found that the Cu located at the vertices and the Al located at the center of unit cell was stability. In addition, the Jahn-Teller effect does not only exist in Cu2+ when its d orbital is partially filled but it also exists in Zn2+ when its d orbital is full as well as in Mg2+ when its p orbital is partially filled. With the decrease of electronegativity of interlayer anions, the charges have transferred from the guest anions to the host layer, the strength of electrostatic interaction and hydrogen bonding decreases gradually while the band gap of the systems gets narrow, electrons transit to higher energy level easily and the stability of the systems gets worse. Besides, the doped Cu makes the valence band of CuZn MgAl-X system deviate to the position of high energy. Compared with the traditional layered double hydroxide, the band gap is more narrow and the stability is lower, that further explain the reasons why it is hard to synthesize copper-containing hydrotalcite.(2) A periodic interaction model was proposed for the copper-iron layered double hydroxides with different number of water molecules. The distribution of H2 O in the interlayer and the supermolecular interaction between host and guest was investigated by analyzing the hydration energies, hydrogen-bonding, charge populations. Between the host layer and the guest, there was strong supramolecular interaction which including hydrogen-bonding and electrostatic interaction. Electrostatic interaction was weaker to the hydrogen-bonding in process of hydration. The strength of hydrogen bonding was Layer-Anion type hydrogen bonding > Anion-Water type hydrogen bonding > L-A type hydrogen bonding > Layer-Water hydrogen bonding > Water–Water type hydrogen bonding. With an increase in the number of interlayer water molecules, the interlayer distance decreased slightly and then increased significantly. The Cu-O octahedral forms were stretched gradually, because the Jahn–Teller effect of Cu2+ increased. The absolute value for the hydration energy decreased gradually with an increase in the number of water molecules. This suggested that the hydration of Cu3Fe-LDHs reached a definite saturation state.(3) The periodic on Pd(111), Pt(111) and Au(111) surfaces have been designed. The stability adsorption of thiophene was discussed by analyzing the adsorption configurations, the adsorption energies, the charge density difference, the mulliken populations and the density of states analysis. The adsorption energies for thiophene on different surfaces following the order of Pd(111) > Pt(111) > Au(111). The adsorption structure on Au(111) surface showed almost no change and the most stable adsorption structure was the tilt adsorption on the top site through the S atom of thiophene. For Pd(111) and Pt(111) surface, the most stable adsorption structure was the parallel adsorption for the hollow site through the ring plane of thiophene. After adsorption, the H atom of thiophene moved upward and the structure of which was distortional and folding. The aromaticity of thiophene was damaged and the C atoms were characteristic of sp3 hybridization. Furthermore, the electrons of M(111) surfaces and thiophene were redistributed after adsorption. The electrons transfer from thiophene to M(111) surface and the order is Pd(111), Pt(111) then Au(111). Meanwhile, the electrons of M(111) surface were also back-denoted to the empty orbitals of thiophene molecule. This collaborative process will eventually lead to the adsorption of thiophene on M(111) surface.(4) Based on the optimal adsorption configurations, the mechanism of hydrodesulfurization of thiophene at Pd(111) and Au(111) surface were elucidated by using method of Complete LST/QST. The activation energy and reaction energy of each step in different mechanisms have been calculated. The direct hydrodesulfurization mechanism had a low energy of activation, but it is difficult to control the products. For Pd(111) surface, The mechanism of indirect hydrodesulfurization was best fit for the 1,2-cis-hydrogenation process. The most feasible reaction pathway is(1) C4H4S+H2â†'α,β-C4H6S;(2) α,β-C4H6S+H2â†'C4H8S;(3) C4H8S+H2â†'C4H10+S. Among different steps, the formation of C4H8 S is the rate-determining step. For Au(111) surface, the mechanism of indirect hydrodesulfurization was best fit for the hydroisomerization process. During the process, the bond length of C-S in thiophene increases gradually when the bond energy of C-S decreases. Among in different steps, the removal of S is the rate-determining step.