Preparation And Characterization Of Ordered Mesoporous Alumina With High Thermostability

Ordered mesoporous alumina materials have received extensive attention due to their uniform pore structures, high BET surface areas and large pore sizes, tunable pore size in the range of 1.5-30 nm, and adjustable acid sites, that make them potentially important in many fields such as ion-exchange, bulky molecule catalysis, adsorption and separation, host-guest chemistry and nanoreactor ect. Compared to mesoporous silica materials, however, the synthesis of ordered mesoporous alumina materials is really difficult, because they often show unstable mesoscopic phase, and the collapse of the pore structure always happen when they are heated to high temperature to remove the template, and will lead to the decrease of surface area and pore volume significantly in the end. Thus, poor thermal stability, relatively low acidity, and high synthesis cost all severely hinder their practical applications as catalyst, catalyst carrier and adsorbent, and become the key problems which would restrict their development ultimately.A large number of studies have shown that, the introduction of additives can effectively remove the hydroxyl and ion cavity of the alumina surface, and thus can significantly increase its thermal stability. Moreover, the research on the introduction of metal atoms improved its physical and chemical modification, thermal stability, specific surface area, and increase its surface properties of aluminum oxide is almost invisible. Herein, we present a feasible approach to synthesize environmental friendly, high thermal stability and ordered mesoporous alumina on the base of the impact of evaporation induced self-assembly and solvent heat treatment process on the growth of distributed wall and aggregation. Further study has been carried out to discuss the introduction of metal atoms and their impact on the skeleton structure and relative amount of acid/alkali active sites. Through synthesis and characterizations, we have achived the following results:(1) In the process of self-assembly, by introducing Zr ions moderatly can significantly reduce the electronegativity of aluminum, and thus can effectively inhibit affinity reaction. By optimizing the assembly conditions and selectively introduced organic carboxylic acid can effectively control the hydrolysis and polymerization rate of aluminum precursor and metal ions, therefore, aluminum hydroxyl and metal heteroatomic can bind on the interface of the template effectively, then can improve the degree of order and increase the specific surface area eventually. In addition, the introduction of Zr atoms and the formation of skeketon metal impurity atoms Zr-O-Al will increase the content of aluminum species of the wall and the coordination four or five coordination aluminum in the framework, it can also adjust the acid/alkaline of the sample at the same time, besides that, it can effectively restrain the sinter and phase transformation of aluminum, and enhance the thermal stability eventually. Compared with the methods repored before, we have put forward a simple and high efficient approach to synthesis mesoporous alumina, and can promote the highly dispered of Zr and alumina in the wall at the atomic level eventually.(2)In the process of solvent evaporation induced self-assembly, the solvent evaporation rate will significantly affect the structure and texture of mesoporous alumina. Thus, we made the temperature and relative humidity under control by use our home-made oven, and focused on the influence of the solvent evaporation conditions on the synthesis of mesoporous aluminum. XRD and nitrogen adsorption have shown that, compared with the relative humidity, the temperature will determine the solvent evaporation rate directly, and then decide the successful synthesis of mesoporous aluminum. It has been proven that, mesoporous alumina with highly uniform mesoporous structure, large specific area and pore volume can be obtained in 45 ℃, and large pore diameter, high surface area and large pore volume are always obtained in the same relative humidity of 50%.(3) To reduce the cost of the synthesis of mesoporous aluminum, we try to use inorganic aluminum source to replace the organic aluminum as the precursor in the synthesis of mesoporous aluminum. By introducing citric acid moderately and via an evaporation-induced self-assembly pathway associated with thermal treatment, the hydrolysis of Al(H2O)63+ can be promoted towards the positive, and then more Al-OH would be obtained, which can bond with the hydrophilic polyethylene oxide of the surfactant molecules, and highly ordered mesoporous aluminum materials were prepared finally. More importantly, the synthesized mesoporous alumina exhibits high thermal stability even up to 900℃, and possesses high surface area, narrow pore-size distribution, and large pore volume simultaneously. Compared with the methods repored before, we have put forward a simple way to synthesis mesoporous alumina without adding any mineral acid (hydrochloric acid or nitric acid). Our contribution provides a green approach to reduce the synthetic cost of ordered mesoporous alumina with high thermal stability.