Theoretical Study On MgAl-LDHs Crystal Structure Construction And PXRD Computer Simulation

In this work,the microstructures of chlorine anion,carbonate ion and mordant yellow 10 intercalated MgAl-LDHs have been studied by using density functional theory(DFT)based on the moleculer cluster models.Stable structures of the different intercalated products are obtained through optimizing geometries of moleculer cluster models of LDHs.The calculation results of the geometries are used to build the crystal structure.The main results are as following:(1)The geometry of hydrotalcite slab with Mg/Al=3 and the adjusting and controlling of interlayer spacing by anionThe slab microstructure of layered double hydroxide(LDH)with Mg/Al ratio of three is studied by using density functional theory at the B3PWP91/Lanl2dz level.The optimized geometric parameters and the simulated XRD patterns of hydrotalcite are in agreement with experimental results.The structural simulations of hydrotalcite show that the host layers stack along the c-axis according to space group R-3m,a=1.2552 nm,c=2.3400 nm.Based on the stacking multilayer model,the carbonate anion is introduced into the interlayer space and one special cluster structure is obtained through optimization under the condition of keeping space group R-3m.The computation indicates that the interlayer spacing of hydrotalcite with removed crystal water is 0.7260 nm.The changes of bond parameters of cluster suggest that there are strong supramolecular interactions between the slabs and anion.(2)Theoretical studies on the geometrical and crystal structure of layered double hydroxides:From point group to space groupTwo types of perfect crystals of Mg3Al-Cl^--LDHs with different space groups are built according to the optimized geometries at B3PW91/LANL2DZ level.The simulated powder X-Ray diffraction patterns are used to analyze the theoretical and experimental results.Comparing the diffractions with or without Cl^- for the known P63/mmc crystal structure,it is found that Bragg peaks at 16.60 and 20.29°are created by Cl^- ordered arrangements,and the Cl^-and Al³⁺in LDHs are random or short-range order.Furthermore,an unreported crystal structure with space group P-3ml is predicted by using the crystal-building method from point group to space group.(3)Synthesis and characterization of mordant yellow GG-pillared Mg-Al layered double hydroxide inorganic lamellaMordant yellow 10-pillared magnesium-aluminum layered double hydroxide(MY10-LDH)is prepared by anion exchange method.The intercalated structures and the chemical compositions of MY10-LDH are characterized by XRD and FT-IR,respectively.The experimental data prove that MY10 anion is successfully intercalated into the interlayer space of the LDHs.XRD patterns indicate that the MY10-LDH has the special intercalated structure of LDHs with an interlayer distance of 1.58 nm.IR spectra show that the product has the characteristic absorption bands of azo form of MY10 anion, as well as the Mg-O and Al-O vibrations of LDH layers.Furthermore,the microscopic structure of MY10-LDH is investigated by the quantum chemical method.The stable geometries of MY10 and MY10-LDH model molecules are optimized by using density functional theory of quantum chemistry at the level of B3LYP/6-31G(d,p).Through comparing the molecular size of MY10 anion in MY10-LDH with the interlayer distance of MY10-LDH from XRD data,it is determined that the MY10 anions are inclined arranging in the interlayer space of LDHs,because the gallery height is less than the size of MY10 anion.(4)Theoretical studies on the crystal structure of mordant yellow 10-pillared magnesium-aluminum layered double hydroxidesThree kinds of perfect crystals of MgAl-MY10^2--LDHs with different Mg/Al ratio and MY10 spatial orientation are constructed according to the optimized geometries at B3PW91/LANL2DZ level.The simulated PXRD patterns are used to analyze crystal structure.Through comparing the differences between the theoretical and experimental data,it is found that Bragg peaks at 18.15°and 20.40°are created by the horizontal arrangements and the inclined orderly arrangements of MY 10^2-anion in the MgAl-hydroxide sub-lattice,respectively.