Porous inorganic solids have found great utility as catalysts and separation media for industrial applications. The openness of the microstructure allows molecules access to the surface area of the materials that enhance their catalytic and sorption activity. The porous materials in use today can be sorted into several categories based on their microstructure, molecular sieves being one of these.
Molecular sieves are structurally defined materials with a pore size distribution that is typically very narrow because of the crystalline nature of the material's microstructure. Examples of molecular sieves are zeolites and mesoporous materials. Zeolites are generally aluminosilicate materials with pore sizes in the microporous range which is between two to twenty Angstroms.
Zeolites have been demonstrated to exhibit catalytic properties. Zeolites are porous crystalline aluminosilicates which have a definite crystalline structure within which a large number of smaller cavities may be interconnected by a number of still smaller channels or pores. Relatively little advance has been achieved in fine chemical synthesis with zeolite-based catalysis due, at least in part, to the limitations of redox activity in currently available molecular sieves (B. Notari, Stud. Surf. Sci. Catal. 37 (1988) 413; P. Roftia, Stud. Surf. Sci. Catal. 55 (1990) 43; N. Herron, et al. J. Am. Chem. Soc. 109 (1987) 2837; R. F. Parton, et al. Nature 370 (1994) 541; P. T. Tanev, et al. Nature 368 (1994) 321; M. Iwamoto et al. Chem. Lett. (1989) 213; Y. Li et al. J. Phys. Chem. 94 (1990) 9971; and Y. Li et al. Appl. Catal. B. 1 (1992) L31). In most redox reactions, a catalyst with variable oxidation states is required to assist in charge or electron transfer between reactant molecules. To use inert aluminosilicate-based zeolites as redox catalysts, metal cations need to be introduced into the zeolitic matrices. This can be achieved only in very limited concentration in the form of dopants without affecting the crystallinity of the zeolitic structure. More commonly, metal cations are introduced into the zeolitic cage structure by cation exchange or metal salt impregnation. The metal cations introduced into zeolites have been found capable of catalyzing some redox reactions. The turnover frequency (TOF) of the catalyst is, however, very restricted by the number of catalytically active sites that can be introduced, which is in turn limited by the Si/Al ratio in the zeolite framework structure. Furthermore, the catalytic activity of the zeolite materials can be severely reduced due to aggregation of metal cations caused by hydration of the metal cations and/or dealumination of the zeolite framework in the presence of water vapor at temperatures of 500-800.degree. C.
Mesoporous materials, however, generally have larger pore sizes. Mesoporous materials have a pore size from about 10 to 500 Angstroms. Examples of conventional mesoporous solids include silicas and modified layered materials, but these are amorphous or 2-dimensional crystalline structures, with pores that are irregularly spaced and broadly distributed in size. Pore size has been controlled by intercalation of layered clays with a surfactant species, but the final products have typically retained the layered nature of the precursor material.
Porous transition metal oxides have been the subject of increasing interest as materials which can be utilized in partial oxidation, complete combustion, NO.sub.x decomposition, hydrodesulfurization, photocatalytic decomposition of organic compounds and solid acid catalysis. Most attempts, however, to prepare mesoporous materials suitable for such purposes have typically led to lamellar phases where surfactant and metal oxide phases are layered.
Efforts to synthesize hexagonally-packed mesoporous oxides have focused on an inverse-micelle template mechanism. Inorganic precursors, pH and surfactant head groups have been adjusted to achieve optimal electrostatic charge balance between the organic and inorganic phases during the self assembly process. This led to the synthesis of silica-based mesoporous materials as disclosed in U.S. Pat. No. 5,098,684. Efforts to extend this approach to non-silica or alumina-based systems have mostly led to formation of layered phases. The few hexagonally-packed mesostructures derived by such approaches, composed of tungsten, antimony, lead and iron oxides, were not found to be thermally stable upon surfactant removal or thermal treatment (Huo et al. Chem. Mater. 6:1176 (1994)). High surface areas and well-defined porosities for transition metal products could not be achieved in these systems upon surfactant removal in contrast to alumina- and silicate-based systems mentioned above.
One reported example of a transition metal-containing hexagonally packed mesoporous material has only a small percentage of titanium dioxide incorporated into a silica structure. (Tanev et al. Nature 368:321 (1994)). The method used to form the hexagonal mesoporous materials utilized a primary, rather than a quaternary ammonium ion surfactant as the templating reagent. The titanium-doped silicate-based hexagonal mesoporous materials were found to be more active in the catalytic oxidation of arenes than the conventional microporous titanium silicate zeolites.
Therefore, a need exists for a thermally stable mesoporous transition metal material and a method for forming mesoporous transition metal materials which overcome or minimize the above mentioned problems. A need also exists for a crystalline microporous metal oxide having a dimensionally consistent pore structure and a method for forming crystalline microporous metal oxide materials which overcome or minimize the above mentioned problems.