Fabrication And Exploratory Application Of Layered Double Hydroxides With Controllable Morphology

With the development of materials science and technologies, scientists are exploring some methods involving chemistry or physics to construct materials with specific morphology and pore architecture and realizing the design and adjust and control of some functional materials such as new types of catalyst supports,membranes for the separation of large polymers,biomedical materials with macroporosity and drug carriers.Layered double hydroxides(LDHs)are a large family of synthetic anion type of layered materials with two-dimensional nanostructures consisting of positively charged layer with charge balancing anions and water moleculars between the layers.As a result,a large calss of functional isostructural materials with widely varied physicochemical properties can be obtained by changing the nature and molar ratios of the metal elements as well as the type of intercalated anions.Thus,LDHs are promising functional materials for a large number of applications in catalysis,adsorption,separation,functional additives,pharmaceutics and biomaterials.So far,LDHs involved in the above fields such as catalysis,adsorption and separation are usually in powder form,which inevitablly give rise to problems such as high pressure drops,poor mass/heat transfer,poor contact efficiency and difficulties in separation.Thus,it is essential both in science and practice to undertake the morphology control study related to LDHs materials.In the present thesis,we take the morphology control of synthetic LDHs as target and put emphasis onto the preparation of microspherical LDHs with porous architecture and macrospherical composites consisting of magnesia and magnesium aluminate(MgO/MgAl₂O₄),which was used to catalyze the transesterification of methanol and soybean oil.First,we investigated the preparation of microspherical MgAl-CO₃-LDHs by using spray drying technique without the help of templates.Scanning electron microscopy(SEM),low-angle laser light scattering and microscopic particle size analysis system(MPSAS) techniques were adopted to characterize the products.The resulting products are composed of nanosized LDH particles aggregated into solid microspheres with an diameter of 10-50μm,a sphericity of 0.84. Mercury porosimetry was further consolidated that the microspheres were porous with modal pore access diameter of 87.9 nm,surface area of 43 m²/g and total pore volume of 1.29 cc/g.When calcined for 8 h at 500℃in N₂ atmosphere or calcined and rehydrated in decarbonized dioxide water,the spherical morphology was well restained,which demonstrated that these microspheres have a good structural stability.Besides,in order to demonstrate the common utility of the spray drying method,a variety of LDHs with different compositions such as NiAl-LDHs,ZnAl-LDHs and CuZnAl-LDHs with interlayer carbonate anions were successfully prepared and fabricated into microspheres using the same procedure as for MgAl-CO₃-LDHs microspheres.This confirms that the spray drying method is a very simple yet effective common process for the preparation of solid LDHs microspheres with macroporous.At last,macrospherical composites(MgO/MgAl₂O₄)consisting of magnesia and magnesium aluminate were prepared by using hard template method.We thoroughly characterize the composition, morphology,structure and architecture and choose the methanolysis of soybean oil as probe reaction to investigate its catalysis performance. The process involved in situ growth of magnesium-aluminum layered double hydroxides into the channels of theγ-Al₂O₃ (MgAl-CO₃-LDHs/γ-Al₂O₃)macrospheres(0.5-1.0 mm in diameter)by using urea hydrolysis method,followed by calcination,tuning of the base strength through etching of excess aluminium with aqueous alkali and a final calcination step.X-ray diffraction(XRD),scanning electron microscopy(SEM),transmission electron microscopy(TEM), high-resolution transmission electron microscopy(HRTEM),elemental analysis and low temperature N₂ adsorption-desorption studies demonstrate that the composite materials(MgO/MgAl₂O₄)are composed of nanosized rod-like particles which aggregated into a spherical framework with specific surface area of 92.5 m²/g,total pore volume of 0.65 cc/g and the most optimization pore radius of 17.4 nm.At the same time,temperature programmed desorption with carbon dioxide as probe molecular(CO₂-TPD)was applied to characterize the base strength for MgO/MgAl₂O₄ prepared by a conventional impregnation method and MgO/MgAl₂O₄/γ-Al₂O₃.The general shift to higher desorption temperatures for the MgO/MgAl₂O₄ framework catalysts compared with those of MgO/MgAl₂O₄/γ-Al₂O₃ can be attributed to the increase of base strength resulting from the leaching of acidic Al³⁺ions from the materials. Catalytic reactivity was investigated by using methanolysis of soybean oil as probe reaction.The MgO/MgAl₂O₄ composite showed a biodiesel yield 57%,which was increased by 20%compared to MgO/MgAl₂O₄/γ-Al₂O₃ with the same loading of magnesium.The enhanced catalytic activity of MgO/MgAl₂O₄ can be ascribed to its higher base strength,specific surface area,pore volume and pore size.