Synthesis Of Meso-porous Alumina Via Ionic Liquid-assisted Routes

Alumina are important industrial materials which are used widely as catalyst supports, catalyst, absorbents, ceramics, abrasives and filters due to their unique characteristics. The property of Alumina is mainly depending on its shape and structure. The common method for synthesis of Alumina is to dehydrate its precursor via a calcining process with the morphology and textural structure preserved. Among these precursors of Alumina, y-AlOOH is the common and widely used precursor for preparation of Alumina. Hence, controlling the size and morphology of y-AlOOH nanostructure is the vital factor to control and improve the morphology, structure and property of the final Alumina material. In this dissertation, a series of well-defined3D nanomaterials were hydrothermal synthsized used green reagents-ionic liquids as tempate or structure-directed reagent. Furthermore, we investigated detailedly the effect of structure and dosages alteration of ionic liquid on morphology, textural property, crystal structure, and thermal transition process of y-AlOOH and surface acidity of its calcined product γ-Al2O3in order to discover the nature reason and formation mechanism of the effect of ionic liquid on the shape and structure of y-AlOOH and fabricate the different property of γ-Al2O3in the future works.Firstly, Hierarchically organized y-AlOOH and γ-Al2O3hollow spheres have been successfully synthesized via an ionic liquid [C4mim]-Cl--assisted hydrothermal synthesis method, and it was found that [C4mim]+Cl-played an important role in the morphology of the product. Without the addition of [C4mim]+Cl-, there was no hollow spheres produced, only bunched messy nanoflakes. According to the results in the experiments, an "aggregation-solution-recrystallization" had been proposed.Secondly, we investigated the effect of alkyl chain length ([C4mim]+Cl-and [C8mim]+Cl-and [C16mim]+Cl-) and the type of anion (hydrophilic [C4mim]+Cl-and hydrophobic [C4mim]+PF6-) in ionic liquid on texture structure (morphology and textural property) of y-AlOOH. The results show that (1) the type of anion not only affect the morphology but also affect the phase structure of the product. γ-AlOOH was the only product in the hydrophilic Cl-based ionic liquids. But in the hydrophobic BF6"-ionic liquid, it produced the massy morphological AlF1.96(OH)1.04due to the decomposition of BF6-anion.(2) The length of alkyl chain length had significant effect on texture structure of γ-AlOOH sample. Using the short alkyl chain length-based [C4mim]+Cl-as the structure-directed reagent, the morphology of γ-AlOOH was not changed and preserved as the hollow sphere structure in all experiments. The specific area and the number of small meso-pores of γ-AlOOH increased with the increase of [C4mim]+Cl-dosage. While, using the larger alkyl chain length-based ionic liquids as the soft-template, such as [C8mim]+Cl-and [C16mim]+Cl-, the morphologies of γ-AlOOH were changed from initiative hollow spheres into the final microflowers. The specific areas of γ-AlOOH firstly increased then decreased with the increase of their dosage. According to the results in the experiments, an "aggregation-solution-recrystallization" formation mechanism is proposed for hollow spheres structure, and a "viscosity controlled" formation mechanism is involved in microflower architecture.Then, we further investigated ionic liquid [Cnmim]+Cl-(n=4,8,16) on crystal structure and surface properties (thermostability and surface acidity) of γ-AlOOH and γ-Al2O3. The results show:(1) the molecules of ionic liquid were easily adsorbed on the surface of (020) crystal face of γ-AlOOH and would induce nanopartiles to form cone-like flake based structure.(2) The alkyl chain length had significantly affected the thermal transition process of y-AlOOH, especially, to the number of OH on the surface of y-AlOOH. In the case of [C4mim]+Cl-, the number of surface OH, namely molar dehydration rate of γ-AlOOH at temperature range from200℃to600℃, was increasing with the increase of [C4mim]+Cl-dosage. While in the case of the large alkyl chain length-based ionic liquid [C8mim]+Cl-or [C16mim]+Cl-, the number of surface OH was changed according to ionic liquid content in the system. Under the low concentration, the variation trend of the number of surface OH was similar with that of [C4mim]+Cl-. However, in high concentration, the variation trend of the number of surface OH was opposite with that of [C4mim]+Cl-, that is the number of surface OH was decreasing with the increase of ionic liquid dosage. The possible formation mechanism and reason was analyzed and proposed in this work.(3) The variation trend of surface acidity sites of its calcined product γ-Al2O3was similar with that of the number of surface OH of y-AlOOH. However, whatever the alkyl length and the dosage of ionic liquid were, L acidity sites was the only acidity site on the surface of γ-Al2O3Furthermore, most of the L acidity site intensity was weak acidity site (120-300℃).Finally, we simplified and popularized this reaction system, and confirmed the formation mechanism of3D-nanomaterials via ionic liquid-assisted routes proposed in the work via synthesis of flower-like y-AlOOH andγ-Al2O3.Furthermore, we investigated and proposed the route to recovery and reusability of ionic liquid to synthesis of3D-nanomaterials.