Synthesis, characterization and applications of metal oxides with hierarchical nanoporous structure
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Hierarchical porous materials incorporate pores at two different length scales in the same body and thus offer multiple benefits from characteristic pore sizes. Smaller mesopores (3-10 nm) provide high surface area for fluid contact while larger macropores (0.5-5 ym) allow facile mass transport. Design of materials with structural hierarchy is of primary interest due to their potential applicability in a variety of fields such as catalysis, adsorbents, storage, and biomaterials. A better understanding of the synthesis factors can yield greater control over the porous properties, which, in turn, can lead to new applications for these materials.
In the current work, hierarchically structured porous alumina, zirconia and titania materials were synthesized via a spontaneous template-free self-assembly in solution. Starting materials are liquid alkoxides, which undergo rapid hydrolysis and condensation in aqueous solution to form a solid metal oxide network. The self-assembly taking place at two different length scales gives rise to bimodal porosity in the system. This study focused separately on the formation of macropores and mesopores. Firstly, the information available in literature regarding the formation of macropores in these materials was unified in a consistent fashion. A systematic comparison of these materials was provided, which were split based on the differences between their characteristic chemistries. The synthesis conditions and parameters producing a maximum extent of macroporosity were identified. The resultant materials were amorphous in nature and in some cases, the conditions leading to maximum macroporosity produced poor meso-structure. Thus, secondly, the as-synthesized materials were subjected to hydrothermal treatment and the influence on textural properties, phase composition and, in turn, on previously formed macropore structure was investigated under the conditions identified earlier for maximum macroporosity. The results indicated that by a selective combination of self-assembly, hydrothermal treatment and thermal treatment, hierarchical structures with crystalline walls can be obtained for all the three materials: alumina, titania and zirconia.
Next, the knowledge from the study of porous structure at the individual length scale was utilized to synthesize a hierarchically porous aluminosilicate material that is also catalytically active. In this mixed metal oxide, alumina was the structure forming component while mixing it with silica created surface acid sites. A hierarchical porous structure with the presence of both Brynsted and Lewis acidity was confirmed by means of a variety of characterization techniques. The activity of the acid catalyst was demonstrated with an esterification reaction of palmitic acid with ethanol.