The Synthesis Mechanism,Structural Regulation And Properties Study On The Layered Double Hydroxides Through An Atomic Economic Reaction

Layered double hydroxides(LDHs)are a class of layered functional materials with tunable structure and composition,which has been widely applied as PVC thermal stabilizer,smoke suppression agent and UV blocking materials.The conventional synthesis of LDHs was mainly based on the coprecipitation method.During the reaction,many byproducts were formed,which needs considerable water to purify the products.The waste of resources and energy disobeyed the development of green chemistry.To solve the problems,MgAl LDHs were prepared through an atomic economic reaction employing metal hydroxide as raw materials.However,the method was failed to control the size and morphologies of LDHs,and the impurity phases usually appeared for the LDHs with different constitutions.The unsatisfactory performances of such LDHs have greatly affected the industrial implementation.In our thesis,we have presented a mechanism study on the atomic economic preparation for LDHs through both experimental approaches and kinetics calculations.The results demonstrated that the size of raw materials showed great effect on the growth of LDHs,and the control on the size of MgAl LDHs was realized.Based on these facts,the preparation of ZnAl LDHs was accomplished by regulating the size of raw materials and reaction conditions.The specific thermal stability of ZnAl LDHs was studied,which explained the reason for the abnormal structural memory effect.The LDHs were further doped in PVC,EVA and PP,the LDHs particles showed good dispersibility,and their performance was same as LDHs synthesized by coprecipitation method.Therefore,we have realized the finely regulation on the composition and morphologies on LDHs through the mechanism study,which may laid foundation for the fundamental research and industrialization through atomic economic reaction.The main contents are described as follows:1.The synthesis mechanism and size regulation on MgAl-CO3 LDHs through atomic economic reactionThe MgAl-CO3 LDHs was prepared through an atomic economic reaction by regulating the size of pseudo-boehmite.The reaction mechanism and the effect of the reactant size on the nucleation and growth of LDHs was studied by experimental approaches and kinetics calculations:Mg2+ and Al(OH)4-were dissociated under hydrothermal treatment,leading to the nucleation and growth of MgAl LDHs on the surface of Mg(OH)2.The size of pseudo-boehmite severely affected the reaction mechanism.The reaction is determined by one-dimensional diffusion when the pseudo-boehmite is in micrometer scale,while it changed to two-dimensional interface reaction when the size of pseudo-boehmite decreased to nanometer.Furthermore,the size of Mg(OH)2 was further tuned to prepare MgAl LDHs with different sizes based on the reaction mechanism.2.The synthesis and mechanism study of ZnAl-CO3 LDHs through atomic economic reactionBased on the studies on reaction mechanism and particle size control of MgAl LDHs.ZnAl LDHs was firstly synthesized through the atomic economic reaction by regulating the reaction temperature and pressure,and ZnO(30 nm),pseudo-boehmite and CO2 was taken as raw materials.Experimental approaches and kinetics calculations demonstrated the reaction progress through dissolution,nucleation and diffusion,which is rate determined by the one-dimensional diffusion reaction.The reaction progress can be described as follows.Firstly,the raw materials were dissolved to Zn2+,OH-,Al(OH)4-and CO32-in the aqueous solution.Secondly,these ions nucleated on the surface of ZnO nanoparticles to form LDHs crystal seeds.The LDHs crystal seeds grew to a complete ZnAl-CO3 LDHs crystal along the interface between LDHs and ZnO,which is determined by one-dimensional diffusion.Moreover,we have investigated the growth of ZnAl LDHs under 120℃ by ZnO with different sizes.With the increased sizes of ZnO,the diffusion rates at the interfaces between ZnO and LDHs decreased,leading to the slowed growth of ZnAl LDHs,incomplete reaction of ZnO and formation of impurities.These results laid foundation for the synthesis of LDHs with different compositions through the atomic economic reaction.3.Thermal stability and structural memory effect of ZnAl LDHs under hydrothermal treatmentOur results showed that the ZnAl LDHs exhibited poor thermal stability,and it would be decomposed above 150℃.Experimental data demonstrated the appearance of ZnO and ZnAl-MMO when the temperature is above 170℃.The decomposed ZnO nanoparticle is active and it can form ZnAl LDHs again with the addition of aluminium.In contrast,the decomposed ZnAl-MMO showed a similar structure of spinel which is inactive and cannot reconstruct LDHs.These results explained the reason that the ZnAl LDHs decomposition product under hydrothermal treatment can not be reconstructed through structural memory effect,which is beneficial for the understanding of the structures for LDHs.4.The performance and application studies on LDHs prepared by atomic economic reactionTo explore the application of the MgAl-CO3 and ZnAl-CO3 LDHs prepared by atomic economic reaction,the LDHs with different compositions and sizes were doped into PVC,EVA and PP.The dispersion of the LDHs in PVC were analyzed through a Aggregation-Induce emission(AIE)fluorescent labelling technique,and the performance in thermal stabilization,smoke suppression and UV blocking were evaluated.The MgAl-CO3 LDHs with the size of 420 nm showed uniform dispersion in PVC by AIE fluorescent labelling technique,improving the thermal stability.The promoted performance of LDHs by atomic economic reaction is comparable with the LDHs prepared by coprecipitation method.In addition,the MgAl-CO3 and ZnAl-CO3 LDHs showed superior smoke suppression performance in EVA and PVC matrices.Specifically,the maximum smoke density of MgAl-CO3 LDHs-EVA has decreased more than 70%of the original value,which showed great advantages than the LDHs by coprecipitation method.The ZnAl-CO3 LDHs also demonstrated promoted UV blocking properties due to the existence of abundant defects.Therefore,the synthesis of LDHs through atomic economic reaction has been realized,and the structures,morphologies and compositions of LDHs can be finely tuned.The as-prepared LDHs exhibited good dispersibility,and the superior performances of LDHs enabled great potential in the polymer products.Our work may provide theoretical and technical support for the industrialization of LDHs through atomic economic reaction.