Preparation, Characterization And Performance Of Ordered Mesoporous Magnesia-Alumina Composite Material

Ordered mesoporous alumina with high surface area and large pore volume, narrow pore-size distribution and tunable pore sizes should possess much more excellent properties as catalysts or catalyst supports employed in many fields such as ion-exchange, adsorption and separation, bulky molecule catalysis, and host-gust chemistry. Compared to mesoporous silica materials, however, mesoporous aluminum materials prepared by the conventional methods exhibits poor thermal stability, which severely hinder their further catalytic applications.Herein, we present a feasible approach to synthesize ordered mesoporous Mg-Al composite oxide materials with high thermal stability by using aluminium isopropoxide and magnesium nitrate as the aluminum source and magnesium source, respectively. Hydrochloric acid and organic carboxylic acid will be selectively introduced in the synthesis system as pH adjuster, the solvothermal post-thermal treatment conditions will be further optimized. By controlling the hydrolysis and condensation of Al and Mg precursors to adjust the amount of Al and Mg hydroxyl groups and their hydrogen bonding interaction with the hydrophilic polyethylene oxide of the surfactant F127 molecules, the ordered mesostructure can be assembled at the mesoscale and the coordination status, the basic sites can be controlled at the atomic scale. Through comprehensively investigating the effects of the added amount of Mg species on the synthesis, mesostructure, stability, and catalytic performance of mesoporous Mg-Al composite oxide materials, we have achived the following results:(1) The characteristic results from XRD, TEM and N2 adsorption show that during the synthesis of mesoporous Mg-Al composite oxide materials via EISA pathway, the addition of a required amount of Mg species can remarkably increase the mesostructural ordering of resultant Mg-Al composite oxide materials. When the molar ratio of Al to Mg is 8, the sample OMMA-8 exhibits highly ordered 2D hexagonal mesostrucure, narrower pore-size distribution, higher BET surface area and larger pore volume and more homogenous distribution of Mg species than those of Mg/OMA-8 prepared by the incipient wetness impregnation method with ordered mesoporous alumina as support. For OMMA-8, the surface area, pore volume and pore size are 296 m2/g,0.56 cm3/g and 11.9 nm, respectively.(2) The high temperature thermal stability evaluation further confirms that the addition of a required amount of Mg species can effectively enhance the thermal staility of mesoporous alumina materials. Take sample OMMA-8 for example, even after high temperature thermal treatment at 1000℃ for 1 h, two well-resolved diffraction peaks can be observed in its small-angle XRD pattern, indicating that the ordered mesostructure is well maintained. This is further confirmed by the wide-angle XRD and TEM-SAED observations, which clearly show the appearance of Mg0.388Al2.408O4 nanocrystallin phase in the mesoporous walls of OMMA-8 thermally treated at 1000℃, indicating that the introduction of highly distributed Mg can effectively prevent Al atomic diffusion and sinter at high temperature, and further suppresses the formation of y-alumina from the amorphous phase and the subsequent γ-to a-alumina phase transition at an elevated temperature.(3) The basic properties of resultant mesoporous Mg-Al composite oxide materials(OMMA-x) were validated by the CO2-TPD measurements. Compared with the pure mesoporous alumina(OMA) and the magnesium-based materials prepared by an incipient wetness impregnation method with OMA as support, the resultant Mg-Al mesoporous materials display higher basic intensity and more basic sites. The reason can be understood as that the highly homogeneous dispersions of Mg species in the alumina framework of OMMA-x can effectively decreas the particle size of MgO, and further increase the content of surface basic species, which may find important catalytic applications in the energy industry. (4). The results from transesterification reaction and CO2-TPD show thatthe impregnation of a requried amount of KI on Mg-Al composite oxide can not only increase the base strength, but also enhance the transesterification activities for producing biodiesel from vegetable oil and methanol. In addition, the effects of catalyst carriers (OMA、Mg/OMA-8、OMMA-8), molar ratio of Al to Mg, the amount of loading potassium iodide and the reaction time on transesterification profomance were further investigated. The catalytic results show that the structure of catalysts, the exsiting form and distribution of potassium species will directly determine the conversion rate of raw materials.