Synthesis And Properties Of Layered Double Hydroxide Nanomaterials With High Specific Surface Area

Layered double hydroxides （LDHs） often called as anionic clays, area kind of novel inorganic functional materials. LDHs have uniquestructure and functional characteristics and find application in diverseareas such as catalysts, as adsorption, medicine, etc. However, the LDHshave a small specific surface area, because they have no specific internalsurface, which restricted their promoted application. Therefore, it is avery important and worth exploring topic to increase the specific surfacearea of LDH materials.To increase the specific surface area of LDH materials, reducing thethickness of particles become the essential problem for that the specificsurface area of LDHs is determined by the surface area of LDHs. TheLDHs synthesized by traditional methods such as co-precipitation,hydrothermal process display small specific surface area （20-80m²/g） andlarge particle thickness （20-50nm）.In this article, the MgAl-CO3-LDHs with thin particle thickness （<15nm）, high specific surface area （>165.9m²/g） were synthesized by a simple new method by adding additive. The additive inhibits the crystalgrowth in c direction （the stacking direction, perpendicular to the layers）.The details are shown below:1. The influence of additives （sodium dodecyl benzene sulfonate,sodium dodecyl sulfate and polyethylene glycol） and preparationmethods on specific surface area of LDHs have been studied. Thecomparison of experiment showed that: Synthetize of LDHs usingdouble-drop method with PEG as additive, display thinnest thickness（14.2nm） and highest specific surface area （142.6m²/g）. The LDHssynthetized with sodium dodecyl benzene sulfonate or sodium dodecylsulfate, display the relatively small specific surface area. PEG absorptedon the surface of LDHs and formed absorption layer through thehydrogen bondind force. The stereo-hindrance effect between absorptionlayers inhibited the reunion and polymerization of LDH embryos,inhibited the growth in c direction, and resulted in the generation ofultrathin LDHs.2. The influence of adding time, mixing time, degree ofpolymerization, adding order, concentration of PEG, crystallizationtemperature, and the amount of alkali on particle thickness and specificsurface area of LDHs have been studied. The LDHs with the particlethickness10.8nm and specific surface area160.9m²/g have beensynthesized under the concentration of PEG-10008g/L, and added salt, alkaline, PEG-1000solutions simultaneously. With the increasing of PEGpolymerization degree, the molecular chain of PEG in solution increaseswhich facilitates the occurrence of cross-linking effect and results in theincrease of particle thickness. When the concentration of PEG exceedsthe saturated adsorption value in solution, the free PEG will connect theparticles together and make the LDHs grains grow up together. Addingthe PEG earlier than the salt and alkali solution, the high concentration ofPEG in solution will exceed the saturated adsorption value and connectthe particles. The function of PEG would be small if added the PEG afterthe introducing of the salt and alkali solutions for that the LDHs hadalready grew. The precipitants of NaOH and Na₂CO₃have an importantrole on the growth of LDHs. Experimental purposes can be obtained byadjusting the concentration proportion of of NaOH and Na₂CO₃to metalions.3. The influence of crystallization time, solid content and the ratioof Mg/Al on the particle and specific surface area of LDHs have beensystematic investigated. The results show that the MgAl-CO3-LDHswith high specific surface area can be synthesized under the conditions（crystallization time:6-8h, Mg/Al ratio:2.5-4, solid content:<9wt%）.The condition to synthesis the LDHs with the specific surface area of167.7m²/g is as follows: crystallization time is6h, solid content:1.38%,Mg/Al ratio is3. 4. The device for SO_xpick-up performance was designed on a basisof thermogravimetric instrument. It can provide different reactionatmosphere by multiple sets of gas regulation and hybrid systems, andswitch reaction atmosphere by multiplexing valve. The environment ofregenerator and riser reactor of FCC was simulated, and the SO_xpick-upperformance of MgAl-CO₃-LDHs with high specific surface area hasbeen evaluated successfully.