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41003-AC5
Synthesis and Application of Alumina with Hierarchical Nanoporous Structure
Project Overview:
The goal of the proposed research in this project is to elucidate the formation mechanism of hierarchically structured aluminas that were discovered in our laboratory. These unique aluminas consist of particles having macroscopic pores that are interconnected with mesoscale pores. Ultimately, we would like to have the capability of systematically controlling, to the extent possible, the structure of the meso- and macro-scale pores since control of the structure formation will allow them to fulfill their excellent potential for use in catalysis and adsorption applications.
Technical Status:
Our previous study focused on optimizing and understanding the formation of macropores in these materials. A unified synthesis approach was adopted where a variety of metal oxide materials, aluminum oxide, titanium oxide and zirconium oxide, with macropores in their final powders were synthesized using a common methodology under similar laboratory environment. Such a study was unique in providing a consistent set of data to form a basis for systematic interpretation and is a very important contribution to the hierarchical material literature by providing an essential guideline for future research involving these unique materials. The results revealed that by carefully adjusting parameters such as the central metal atom of the precursor alkoxide, the alkyl group of the alkoxide and the starting pH of the reaction mixture, the macroporosity in the final powder can be greatly influenced both in terms of extent and size for specific applications. Therefore, key parameters that facilitate tailoring of the macroporous structure were identified.
A sol-gel type of mechanism seems to be applicable for the formation of these materials. The above parameters operate by influencing the solution chemistry of the alkoxide. Hence, an attempt was made to associate the observations from the above parametric study with sol-gel chemistry for the formation of oxides from alkoxide precursors to explain the particular macroporous structure in the final powders. The results clearly indicated that although the starting alkoxides must have rapid hydrolysis and condensation rates for materials with the desired pattern to form, it is the relative contribution of these two reactions in concert that dictates the macropore formation in the resulting powders and that these processes cannot be viewed separately. Depending upon the type of material and reaction environment a balance between the hydrolysis and condensation processes is required to obtain highly macroporous powders. This balance can be approached by adjusting the relative contributions of these two processes with the help of parameters discussed above. Since the speed of the process makes the determination of the exact underlying mechanism of pattern formation in these materials difficult, an indirect approach was adopted to gain mechanistic insights. A manuscript on the above results is nearing completion for submission to Chemistry of Materials.
After identifying the optimum conditions for formation of macropores in various materials and that these conditions vary for different materials because of their characteristic chemistries, the techniques for enhancing the meso-structure of the materials were investigated. The current results have demonstrated that hydrothermal treatment of the materials at elevated temperatures for extended periods of time leads to the opening of pores that were collapsed due to rapid condensation under highly alkaline conditions. Moreover, higher temperatures during hydrothermal treatment resulted in atomic rearrangement leading to particle growth and appearance of crystallinity in the system. Such results have been confirmed for titania and alumina materials and investigations exploring zirconia materials are underway. A manuscript discussing these results will be prepared soon.
Having synthesized materials with optimal structural properties, we are currently evaluating the performance of such potential materials by making active catalyst out of these materials. We plan to design and synthesize meso-macroporous aluminosilicate materials, which will display Brönsted acidity, and perform catalytic testing for esterification of fatty acids. The challenge is to obtain homogeneity in mixing of both components while at the same time keeping the hydrolysis-condensation process rapid enough to obtain macropores in the resulting materials. We believe such a catalyst will likely have enhanced performance owing to reduced mass transfer restrictions.
This project provided initial support for the PI's work in mesoporous metal oxides for use in catalytic applications, which has subsequently led to additional grant support in this area for the PI. The project has formed the basis for a thesis for one PhD student and has led to an undergraduate research project for one additional student.
