Structural Regulation Of Layered Double Hydroxides And Studies On Cation-π Interaction

The cation-πinteraction is a common non-covalent interaction in nature,which is formed between various positively charged cations and negatively charged electron clouds ofπsystems.Since its discovery in1981,it has become an important molecular interaction in chemistry,biology and materials science.The overall structure of LDHs is composed of hydromagnesite layers stacked with two or more different kinds of metal cations of hydroxides.The layers are positively charged,and there are exchangable anions between the layers to make the whole electrically neutral.In this paper,the cation-πinteraction between LDHs and benzene with different cationic composition,proportion and hydroxyl concentration in different layers was studied by using the structural controllability of LDHs.In addition to the analysis of the interaction strength,the influencing factors were explored,and the adsorption properties and mechanism were studied.The main research contents of this paper are as follows:1.LDHs with different Mg/Al ratios was synthesized by using the adjustable cationic ratio of LDHs laminae.By combining experiment and theoretical calculation,it was found that there is cation-πinteraction between LDHs and benzene,and the relationship of strength is Mg₂Al-LDH>Mg₃Al-LDH>Mg₄Al-LDH.The adsorption energy calculation results further show that the adsorption energy between Mg metal site and benzene molecules of LDHs laminate is less than that between Al and benzene molecules,and the adsorption energy between LDHs metal site and benzene decreases with the increase of Mg content.The adsorption kinetics study shows that the adsorption rate of benzene by LDHs Mg₂Al-LDH>Mg₃Al-LDH>Mg₄Al-LDH is consistent with the interaction strength of cation-π.The adsorption process of Mg₂Al-LDH for benzene in water conforms to the pseudo-first-order kinetic equation,and the adsorption isotherm conforms to the Freundlich isotherm model,indicating that the adsorption is a physical adsorption process dominated by cation-πinteraction.The equilibrium adsorption capacity of LDHs for benzene is Mg₄Al-LDH>Mg₃Al-LDH>Mg₂Al-LDH,which is consistent with its specific surface area.The larger specific surface area provides more active sites for adsorption and improves the adsorption of benzene.2.LDHs with different cationic compositions was synthesized by using the regulability of cationic composition of LDHs laminae.The relationship of cation-πinteraction strength is Ni₂Al-LDH>Mg₂Al-LDH>Co₂Al-LDH>Zn₂Al-LDH.The theoretical calculation results show that the absolute adsorption energy of trivalent metal cations is slightly higher than that of bivalent metal cations on the whole,and is affected by the chemical environment.The adsorption energy of Al in different compositions of LDHs is different from that of benzene molecules.The adsorption rate of benzene by LDHs Ni₂Al-LDH>Mg₂Al-LDH>Co₂Al-LDH>Zn₂Al-LDH is consistent with the interaction strength of cationic-π.The equilibrium adsorption quantity Ni₂Al-LDH>Mg₂Al-LDH>Zn₂Al-LDH>Co₂Al-LDH is consistent with the specific surface area.3.The hydroxyl concentration of the LDHs laminate was modulated by acid etching.It was shown that the hydroxyl group deletion on the metal cation site of the LDHs laminate produced hydroxyl vacancies,which increased the interaction between the vacant orbitals of the metal and theπ-electron cloud of the benzene molecule,and the theoretical calculations showed that the adsorption energy was significantly enhanced.The relationship between the strength of cation-πinteraction is Ni Al-LDH-30>Ni Al-LDH-20>Ni Al-LDH-10>Ni Al-LDH.Etching increases the active sites on the LDHs surface and the equilibrium adsorption amount of benzene increases with the degree of etching.