Preparation And Thermal Stability Of Silicon-doped Ordered Mesoporous Alumina

Ordered mesoporous alumina has big surface area and uniform pore structure, which makes it a good candidate for catalyst support. However, the phase transformation and sintering in the framework happened at high-temperature will lead to the collapse of the pore structure and the reduction of the surface area, so it’s necessary to improve the thermal stability of the ordered mesoporous alumina. Nanocasting method and sol-gel method are commonly used nowadays to synthesize ordered mesoporous alumina. In nanocasting pathway, the synthesis procedure is complex and the resultant samples can hardly retain the ordered pore structure at1000. But for the sol-gel method, the procedure is easy to operate, and the as-synthesized sample can keep the ordered pore structure and γ-Al2O3phase at1000℃, however, the surface area is low (116m2g-1). Considering that silicon is effective in suppressing the phase transformation of γ-Al2O3and keep the high surface area, we will study the thermal stability of silicon-doped ordered mesoporous alumina synthesized in a sol-gel method. The main work is list as follows.1. Silicon-doped ordered mesoporous alumina is prepared with P123as the template, ethanol (EtOH) and2-propanol (IPO) as the solvent, aluminum isopropanol (Al(OiP)3) as the aluminum source, tetraethylorthosilicate (TEOS) as the silicon source and hydrochloric acid as the pH adjuster. Study the effect of HCl, water, IPO and silicon content on the thermal stability of the sample to optimize the synthesis condition.2. Investigate the effect of carboxylic chelating agents on the thermal stability of the samples by adding citric acid (CA), salicylic acid (SA) and acetic acid (AA) in the synthetic system.3. By introducing cerium acetate (Ce(Ac)3) or zirconium acetate (Zr(Ac)4) into the synthetic system to optimize the doping parameters to improve the thermal stability of silicon-doped ordered alumina. Evaluate the potentiality of the samples synthesized under optimized synthesis condition and doping parameters to be as a catalyst support by study the thermal stability of them after support palladium. Some meaningful results are list as follow:In a sol-gel system with the pH of ca. 0.85, the synthesis condition are optimized with the composition n(Al(OiP)3):n(TEOS): n(P123):n(HCl):n(H2O):n(EtOH):n(IPO)=100:6-11:1.7:119.5:412.5:1716:523, and the surface area and pore volume of the samples at1000℃are tailored as235-255m2g-1and0.60-0.66cm3g-1respectively with the ordered pore structure and γ-Al2O3phase, and the sample synthesized with n(TEOS):n(Al(OiP)3)=0.07shows the highest surface area while the sample synthesized with n(TEOS):n(Al(OiP)3)=0.11shows the highest thermal stability. The interaction between carboxylic chelating agents (CA, SA and AA) and Al3+will disturb the uniform binding between Si and Al atoms and may also lead to the formation of Al-rich and Si-rich parts. And for this reason, the interaction and charge matching at the organic-inorganic interface are changed leading a change in the pore structure. In addition, the reaction between Al-rich and Si-rich parts will form mullite which will destroy the pore structure. For the volatile acetic acid (AA), when n(AA): n(Al(OiP)3)≤0.15, the pore structure stability and surface area increase with increasing the AA content. The sample synthesized with n(AA):n(Al(OiP)3)=0.15show a surface area of263m2g-1and γ-Al2O3framework upon calcination at1000℃. The proper content of Ce and Zr can disperse in the alumina matrix homogeneously and inhibit the phase transition of γ-Al2O3thus contributes to an enhancement of the pore structure. The sample synthesized with n(Ce(Ac)3):n(Zr(Ac)4):n(Al(OiP)3)=0.04:0.06:1shows higher structural order than others with the formation of Ceo.5Zro.5O2which can be more effective in inhibiting the phase transition of γ-Al2O3. All the samples synthesized under optimized doping parameters can still show high mesoscopic order of their pore structure at1000℃after supported palladium, which suggests that these samples can be good candidatea as catalyst support.