Layered Double Hydroxides:Concept, Controlled Synthesis, And Applications

ZnAl-LDHs microspheres, MgCoAl-LDHs nanosheets and LDH-APP nanoshells have been preparated by hydrothermal method, coprecipitation method and one-pot in situ method, respectively. Their adsorption capacity and flame-retardant performance have been investigated. The main points are summarized as follows:1. Hierarchical ZnAl-LDHs with uniform, self-assembled and 3D sphere-like nanostructures have been synthesized through a simple solution route. The whole process involved the use of a simple hydrothermal method, in which a precursor mixture was prepared and calcined to obtain ZnAl-layered double oxides (LDOs) at 250℃ for 4h, which were then transformed into ZnAl-LDHs by reaction with carbonate solution. The morphology of ZnAl-LDHs can be tuned from hollow flower-like spheres to porous microspheres by simply varying the molar ratio of urea to metallic cations. Adsorption properties of methyl orange (MO) on ZnAl-LDO and ZnAl-LDHs were studied. The influences of initial solution pH and temperature on the adsorption of MO were investigated. The results of adsorption experiments indicated that the maximum capacity of MO at equilibrium (qe) and percentage of adsorption with a fixed adsorbent dose of 0.2 g·L-1 were 497 mg·g-1 and 99.4%, when concentration of MO, temperature and pH were 100 mg·L-1，25℃ and 3, respectively. The adsorption isotherms and the adsorption kinetics of MO on the ZnAl-LDO can be described by the Langmuir isotherm and pseudo-second order kinetic, respectively.2. MgCoAl layered double hydroxides (MgCoAl-LDHs) which have different Mg2+/Co2+ mole ratio was synthesized by co-precipitation method. The as-prepared samples were characterized by X-ray diffraction (XRD), thermogravimetric (TG) analysis and scanning electron microscopy (SEM), respectively. The 4 genus received MgCoAl-LDHs were added respectively into polypropylene to prepare MgCoAl-LDHs/PP composites. The influence of cobalt content of LDHs on fire retardancy was investigated by UL-94 test and limiting oxygen index (LOI), and the influence of MgCoAl-LDHs addition on the mechanical properties were evaluated using flexural strength and tensile strength as main index. It was found that the limiting oxygen index of MgCoAl-LDHs/PP composites was improved from 17.8% to more than 21.8% when MgCoAl-LDHs addition was 20%. The mechanical property of the MgCoAl-LDHs/PP composite was better than pure PP. And the possible reasons for its influence were analyzed.3. One-pot in situ approach has been employed to prepare hierarchical Layered double hydroxides-ammonium polyphosphate (LDH-APP) hollow nanostructures through controlling the mass ratio of NaAlO2/APP. Their formation mechanism and flame-retardant performance for PP have been investigated. The results indicate that the suitable mass ratio of NaAlO2 to APP is crucial to generate this kind of nanoshell structure; Compared with LDHs and APP, LDH-APP has advantages in at least two aspects:(1) the particlesize of LDH-APP is much smaller than that of traditional LDHs, which is conducive to the high dispersion of inorganic matrices in organic polymers and dramatically enhance the gas permeability, flame retardancy, and thermal stability ofthe base polymer; (2) LDH-APP used as flame retardants cannot migrate to the surface of the polymer, which overcomes the problem that APP tend to migrate to the surface of the polymer and prematurelyrelease the acid and carbon sources at low temperatures.