Hierarchically Ordered Oxides

Abstract

Porous silica, niobia, and titania with three-dimensional structures patterned over multiple length scales were prepared by combining micromolding, polystyrene sphere templating, and cooperative assembly of inorganic sol-gel species with amphiphilic triblock copolymers. The resulting materials show hierarchical ordering over several discrete and tunable length scales ranging from 10 nanometers to several micrometers. The respective ordered structures can be independently modified by choosing different mold patterns, latex spheres, and block copolymers. The examples presented demonstrate the compositional and structural diversities that are possible with this simple approach.Several approaches are currently available for the preparation of ordered structures at different length scales. For example, organic molecular templates can be used to form crystalline zeolite-type structures with ordering lengths less than 3 nm (1); mesoporous materials with ordering lengths of 3 to 30 nm can be obtained using surfactants or amphiphilic block copolymers as structure-directing agents (2-7); the use of latex spheres yields macroporous materials with ordering lengths of 100 nm to 1 m (8 -13); and soft lithography can be used to make high-quality patterns and structures with lateral dimensions of about 30 nm to 500 m (14 -16 ). Despite all of these efforts in nanostructuring materials, the fabrication of hierarchically ordered structures at multiple length scales, such as seen in nature in diatoms (17), has remained an experimental challenge. Such materials are important both for the systematic fundamental study of structure-property relations and for their technological promise in applications such as catalysis, selective separations, sensor arrays, wave guides, miniaturized electronic and magnetic devices, and photonic crystals with tunable band gaps.Previously, micromolding has been used to form patterned mesoporous materials (18,19). These studies, however, used acidic aqueous conditions to carry out the cooperative self-assembly (20), which is disadvantageous because of the limited processibility of the aqueous solutions. Either noncontinuous films were formed (18) or an electric field was needed to guide pattern formation, which requires a nonconducting substrate (19). Latex spheres have also been used to make disordered macro-and mesoporous silica (9). We have developed a simple procedure for preparing hierarchically ordered structures by concurrently or sequentially combining micromolding, latex sphere templating, and cooperative assembly of hydrolyzed inorganic species (metal alkoxides, metal chlorides) and amphiphilic block copolymers. The materials generated from this process exhibit structural ordering at multiple discrete length scales (in this case, 10, 100, and 1000 nm). Patterned macro-and mesoporous materials of various compositions, including silica, niobia, and titania, were synthesized. Such multiple-scale structural organization makes it possible to tune the physical properties of the materials over a wide ...

Hierarchically structured meso-macroporous aluminosilicates with high tetrahedral aluminium content in acid catalysed esterification of fatty acids

“…Hierarchically structured meso-macroporous aluminosilicates containing high content of tetrahedral aluminium featuring high number of active sites were successfully synthesised via the mixing of single-source molecular alkoxide precursor (sec-OBu) 2 -Al-O-Si-(OEt) 3 and an inorganic silica co-reactant (TMOS). By the modification of the (sec-OBu) 2 -Al-O-Si-(OEt) 3 /alkoxysilane molar ratios ranging to 1/1-4/1, this strategy leads to the design of a new generation of stabilized alkoxy-bridged single molecular precursors with modified aqueous reactivities.…”

“…By the modification of the (sec-OBu) 2 -Al-O-Si-(OEt) 3 /alkoxysilane molar ratios ranging to 1/1-4/1, this strategy leads to the design of a new generation of stabilized alkoxy-bridged single molecular precursors with modified aqueous reactivities. This results in a significant increased amount of tetrahedral aluminium atoms incorporated into the aluminosilicate framework.…”

“…The Al-Si ester, di-s-butoxyaluminoxytriethoxysilane ((Bus O) 2 Al-O-Si(OEt) 3 ), was purchased from Gelest, and other chemicals from Aldrich. They were used without any further purification.…”

“…Recent studies highlight the promising superiority of hierarchical meso-macrostructured materials in large series of catalytic reactions [1][2][3][4][5][6][7][8][9][10][11], thanks to improved diffusion and mass transfer properties as well as limitation of poisoning and coking, especially when large molecules are used and reactions are performed in viscous liquid phase [12,13]. A large panel of original strategies, often quite tedious to achieve, have been developed to allow the incorporation of desirable porosities at different length scales into one single body.…”

“…These molecules, constructed from a preformed M-O-Si linkages, are able, under controlled hydrolysis and condensation reactions, to form unequalled chemically homogeneous metallosilicate materials. An Al-Si ester type precursor which already contains an Al-O-Si linkage, the di-sec-butoxyaluminoxytriethoxysilane (sec-BuO) 2 Al-O-Si(OEt) 3 has been used in pioneer works [60][61][62][63]. More importantly, this singlesource molecular precursor possesses, in addition to the preformed Al-O-Si bond, two alkoxide functionalities that are able to undergo fast hydrolysis and polycondensation reactions, rendering possible the auto-generation of a hierarchical meso-macroporosity with unmatched chemical homogeneity, thus, introducing unprecedented acid-base properties with a totally controlled stoechiometry and improved diffusion properties.…”