Preparation, Characterization And Performance Of Ordered Mesoporous Zirconia-Alumina Material

Ordered mesoporous alumina with large surface area and pore volume, narrow pore-size distribution, tunable pore sizes and moderate Lewis acidity should possess much more excellent properties as catalysts or catalyst supports employed in petroleum refinement, automobile emission control and others. However, Compared to silica materials, its further applications are restricted due to the relatively low thermal stability. The hydrothermal stability of alumina with amorphous framework is another critical challenge in practical applications.Herein, we present an easily accessible, reproducible evaporation-induced self-assembly (EISA) method and employ zirconium oxide chloride as the pH a-djustor and hydrolysis-condensation controller to synthesize highly ordered mesoporous zirconia-alumina materials with extremely high thermal and hydrot-hermal stability. Furthermore, to the best of our knowledge, no systematic, high-throughput synthesis of ZrO2-Al2O3 mesoporous materials with wide Al/Zr ratios, ordered mesostructures, high thermal stability and hydrothermal stability based on the EISA method, we have carried out comprehensive studies and achieved the following results:(1) A systematic study of ZrO2-Al2O3 mesoporous material are prepared at various molar fractions of aluminum to zirconia by X-ray powder diffraction, transmission electron microscopy(TEM), N2 adsorption-desorption method, which clearly shows the ordered mesostructure improved with the addition of suitable Zr.In the case of the results, the sample of Al/Zr=5 (S-5) is the best one with highly ordered 2D hexagonal mesostructure, narrow pore-size distribution, and BET analysis for the sample treated at 550℃ give a surface area of 212 m2/g, a pore volume of 0.51 m3/g and the average pore size of 9.8 nm. Furthermore, the corresponding 27Al MAS NMR spectras of all the samples treated at 550℃ show the incorporation of Zr can effectively change the coordination states of Al. The S-5 sample loaded with 14.7% tetrahedrally and 24.5% pentahedrally coordinated aluminum, compare with the pure mesoporous alumina, which the Al content of the two coordination states were increased 4.7 %,5.5%, respectively.(2) The results of the samples treated at high tempurture convince us that the thermal stability of ZrO2-Al2O3 mesostructures is obviously enhanced with the introduction of suitable Zr. For example, the S-5 sample calcined at 1000℃ for 1h indicates three strong diffraction peaks around 1° and 1.7°, which, according to the TEM observation, can be attributed to p6mm hexagonal symmetry. This is further confirmed by TEM-SAED observations, which clearly show the mesostructure possessed amorphous wall, no γ-Al2O3 phase or t-ZrO2 phase are observed. Moreover, these results are consistent with the wide-angle XRD patterns without any phase diffraction peak appearing. Thus it can be seen that the homogeneous distribution of Zr of the mesoporous wall can prevent Al atomic diffusion and sinter and further suppresses the change of Al phase at high temperature.(3) In the case of hydrothermal stability, the results indicate that the ordered mesostructure of pure alumina has collapsed during the hydrothermal treatment in boiling weater for 2h. In contrast, the incorporation of a required amount of Zr and the formation of Zr-O-Al bonding can act as a support for the inorganic alumina framework and effectively protect the highly ordered mesoporous alumina from the hydrolysis of Al-O-Al and collapsing during the hydrothermal treatment process, which lead to the improvement of hydrothermal stability of Al2O3 nanomaterials with amorphous wall. For example, ordered mesostructure domains and narrow pore-size distributions are also observed for the S-5 sample treated in boiling water for 6 h, accompanying large surface area and pore volume.(4)The thermal stability of ZrO2 is also improved by the incorporation of a small amount of Al similar to that described aboved, uising EISA method to systhesize the ordered mesoporous ZrO2-Al2O3 nanomaterials with high thermal stability. An important finding in our study is that the added Al can effectively prevent the phase transition of Zr and enhance the thermal stability of mesoporous ZrO2. The Al-promoted sulfated zirconia catalysts were prepared via the impregnation method with (NH4)2SO4 solution and calcination at high tempreture. Catalytic tests show that the Al-promoted sulfated mesoporous zirconia catalysts, compared with sulfated ZrO2 without using Al as a promoter, exhibit much higher activity in catalytic esterification of cyclohexanol with acetic acid, which indicate that the small amount of Al not only improve the thermal stability of the soild acid catalysts, but also reduce the loss of sulfer during reaction and increase the acid active sites.