Researchers in the catalysis and materials fields have focused considerable efforts on the design of new porous materials, either synthetic or natural with enhanced textural properties, through innovating synthesis procedures as the molecular molding. Generally the porous structure of such solids is formed during its crystallization or during further treatments.
The porous materials are classified, depending on their predominating pore size, as: 1) microporous, with pore sizes <1.0 nm; 2) mesoporous, with pore sizes between 1.0 and 50.0 nm, and 3) macroporous, with pore sizes surpassing 50.0 nm. Of all of them, the macroporous solids have a limited use as adsorbers or catalysts due to the fact that they generally present a low surface area and their large pores are not uniform. On the other hand, the microporous and mesoporous solids are widely used in the technologies of adsorption, separation and catalysis, particularly for the processing and refining of oil. For such applications, nowadays, there is an increase in the demand of new materials with a well defined and homogeneous pore distribution, thermally stable, with a high specific area and large pore volumes; in order to make more efficient the physical and/or chemical processes in which these materials are used.
The porous materials can have an amorphous or nanocrystalline structure. The amorphous materials, such as silica gel or alumina gel, do not have any crystallographic order, while nanocrystalline solids such as transition alumina, gamma or eta, present a partially ordered structured. Generally, these two kinds of materials display a very wide pore distribution, which limits their effectiveness as catalysts, adsorbents and/or ionic exchange systems. The wide pore distribution limits mainly the use of these materials in oil refining processes.
Zeolites and the molecular sieves are a clear example of uniformity in the pore sizes that have to be rigorously established. However, the pore size distribution is limited to the microporous region, due to the fact that the pores are formed from the cavities and/or channels that form the structure itself; therefore, molecules of big dimensions cannot be processed in this type of materials. On the other hand, these materials are generally synthesized under hydrothermal conditions in the presence of a porogen agent that engineers the porous structure.
The need to expand the uniformity and the homogeneity of the pore sizes from the microporous region to the mesoporous region, thus allowing the adsorption and processing of bigger molecules, has led to the search of new organic agents capable of engineering new structures. This has given origin to molecular sieves with bigger pore size as the aluminophosphates, galliophosphates, etc. (Nature, vol. 352, 320-323 (1991); J. Chem. Soc. Chem. Commun., 875-876 (1991)). However, these structures are not thermally stable.
With the discovery of the mesoporous silicates and aluminosilicates in 1992 (U.S. Pat. Nos. 5,098,684 and 5,102,643), a new stage in the development of ordered mesoporous materials started. This type of materials, called M41S, have a uniform pore size, which is adjustable to an interval between 1.3 and 10.0 nm. Such materials display a pore wall with a thickness between 0.8 and 1.2 nm and a crystal size over 50.0 nm. On the other hand, depending on the general conditions of synthesis, in particular on the concentration of the organic porogen agent, the M41S materials can have an hexagonal morphology (MCM-41), a cubic morphology (MCM-48), or a laminar structure (J. Am, Chem. Soc., vol. 114, 834-843 (1992)). This implies a formation mechanism based on strong electrostatic interactions and/or the ionic pairing between the oligomer silicate precursor and the structure engineering agent, making the removal of the later difficult.
The discovery of the carbon fullerene structure (C60) during the 80s, which consists of a hollow sphere whose wall is made up of sixty carbon atoms (Nature, vol. 318, 162-163 (1985)), led to a new materials era of great discoveries as, for example, the carbon nanotubes (Nature, vol. 354, 56-58 (1991)). These structures and/or the nanotubular morphologies present interesting physical and chemical properties, making them suitable for the construction of nanoelectronic innovating devices, among other applications. Due to this, the synthesis of nanomaterials of carbon and inorganic materials has boomed in the past few years. In 1992 the first nanotubes and/or structures fullerene type of MoS2 and WS2 (Nature, vol. 360, 444-446 (1992)), were obtained. Since then a great variety of nanomaterials includes: inorganic oxides such as: VO2, ZrO2, TiO2, SiO2, Al2O3, ZnO and TeO2, sulphides, selenides, telurides, nitrates and transition metal carbides; among others (Dalton Trans., 1-24 (2003)).
On the other hand, a series of studies in confined fluids (M. Lozada-Cassou et al. J. Chem. Phys., vol. 80, 3344-3349 (1984); J. Chem. Phys., vol. 92, 1194-1210 (1990); J. Chem. Phys., vol. 98, 1436-1450 (1993); Mol. Phys., vol. 86, 759-768 (1995); Phys., Rev. E., vol. 53, 522-539 (1996); Phys. Rev. Letts., vol. 77, 4019-4022 (1996); Phys. Rev. E., vol. 56, 2958-2965 (1997); Phys. Rev. Letts., vol. 79, 3656-3659 (1997)), showed that the confinement and curvature at nano-scale produces electric fields and molecular strengths of outstanding intensity. These studies show, for example, that in nano-confinement a separation of charge in the ionic fluid can occur (Phys. Rev. Letts., vol. 79 656-659 (1997)), which implies confinement pressures in the order of 1,250 atm and intermolecular repulsion forces of 5×10−9 N. This result highlights the importance of the confinement for the molecular separation and it oriented the present invention towards the search of tubular structures at nanometric scale and to the development of new materials with enhanced catalytic properties, semiconductive properties, etc.
The nanotubes are materials that are applied, for example, in processes involving adsorption phenomena, as they increase the contact area by exposing the internal surface, the external surface, the surface in the vertex and the surface in the interlayer regions that compose the walls. This together with the increase of the intensity in the force fields, due to curvature and confinement of the nanotubes, enhance the catalytic activity of catalysts or of active phase materials supported on nanotubes. According to the porous materials classification, the nanotubes present mesopores which are homogeneous with a size between 1 to 50 nm and with a high pore volume. These characteristics make the nanotubes potentially useful as catalytic supports or as catalysts.
In the past it has been possible to synthesize nanotubes with walls composed of zirconium oxide, alumina, titania with anatase structure, and transition metal sulfides among others, by means of methods involving the addition of a structure engineering agent, consisting of a cationic, anionic and/or neutral tensoactive agent. However, the tensoactive elimination through calcinations, leads in most cases to the collapse of the nanotubular structure.
Other procedures in the nanotubes synthesis consist in the application of porous membranes, organic or inorganic, to guide the nanotube formation; however, they are generally applied for the case of materials whose structure is compact or tridimensional (3D). The materials with bidimensional structures (2D), like plates and/or sheets, can form unidimensional materials (1D) of the nanotube type and/or nanofibers, by the direct bending and/or rolling of its structure, due to temperature effect, to pressure or to the application of an electric potential, etc.
Titanium oxide is commonly presented as a tridimensional structure material (3D) and it is basically used as a semiconductor material in the construction of electronic and optoelectronic devices, in the manufacturing of pigments and coatings, as catalyst and/or catalyst support in several processes, as photocatalyst in the degradation of organic compounds during environmental protection processes, as photosensitive material in the construction of fuel cells and solar cells, etc.
Titanium dioxide is known to exist in three crystalline phases, anatase, rutile and brookite, as well as an amorphous phase. There are other phases but these ones are the most common. The anatase and rutile phases have a tetragonal crystal lattice, and the brookite phase has an orthorhombic crystal lattice or structure. This information is well known in the area. The anatase and rutile phases which have a tetragonal crystal lattice are different even though they both have a tetragonal crystal lattice. The differences stem from the position of the atoms, the surroundings of the atoms, the lattice parameters, and the space group inside the tetragonal crystal lattice, and because these parameters are different these two phases are differentiated with different names (anatase and rutile). Each phase presents different properties and among all of the phases anatase is the one that has most applications, due to the fact that it can be obtained easily through a conventional chloride or sulfide process.
On the other hand, nanotubes and/or titanium oxide nanofibers with the anatase structure have been obtained, improving in this way the textural properties of the titanium oxide. In this direction, published U.S. Application No. 2004/0265587 describes a procedure to obtain tubular TiO2 particles with the anatase structure, with an external diameter of 5 to 40 nm, with lengths of 50 to 1,000 nm and a specific area of 450 m2/g if only one hydrothermal treatment is carried out and a specific area in the range of 400 m2/g to 500 m2/g, if two hydrothermal treatments are carried out; thus in general the synthesis requires two stages of hydrothermal treatment which involves an alkaline metal and an organic alkaline base. The inventors apply such tubular particles as photocatalysts and/or materials for the construction of photoelectric cells showing good results.
U.S. Pat. No. 6,537,517 refers to a process for titanium oxide production with tubular morphology and anatase structure, with or without the presence of silicon oxide, by means of a hydrothermic treatment involving an alkaline metal hydroxide. The TiO2 nanotubes with anatase structure present specific surface areas between 200 and 500 m2/g. It has been published in the literature (Ma, R.; Bando, Y.; Sasaki, T. Chemical Physics Letters, 2003, 380, 577-582) that it is possible that the so claimed anatase nanotubes in the aforementioned patent might have a lepidocrocite-type structure instead of the so claimed anatase structure. The lepidocrocite structure is defined for an iron oxide compound (iron (III) oxide hydroxide, also known as γ-(FeOOH)). A lepidocrocite-type structure would mean that the so claimed anatase-TiO2 nanotubes would have the same structure as the iron (III) oxide hydroxide and the same space group; however, the cell parameters cannot be exactly the same neither the atoms positions because in one case the titanium atom is involved and in the other case the iron atom is involved. Thus it is clear that the state of the art is that the crystalline structure, space group(s) and atomic positions in the unit cell that composes the so claimed nanotubular anatase-TiO2 structure is not known.
A synthesized nanostructure with a phase different to anatasa is given in the Korean laid-open patent application No. P2003-0026268 where the synthesis of nanoparticles (balls or spherical crystal with a nanometric size), mostly with the brookite phase, which is known to have an orthorhombic crystal lattice, and some rutile phase, with tetragonal crystal lattice, is reported. The starting materials for the synthesis are TiCl4 and HNO3.
In the case of the U.S. Pat. No. 6,537,517 the starting material is a powder of crystalline titanium oxide (crystalline titania powder with the anatase or rutile phase) with an average particle size between 2 to 100 nm, preferably from 2 to 30 nm (the size of the crystallites that compose the particles is not provided). The starting material is subjected to a hydrothermal treatment, in the presence of an alkali metal hydroxide, that comprises one step. However, as it is already mentioned in the published U.S. Application No. 2004/0265587, the use of a titania powder as starting material does not produce a high yield of the titania nanotubes with anatase phase. On the contrary spherical particles are synthesized in a higher yield than the nanotubes and the final product presents a large residual amount of sodium that hinders the efficiency of the nanotubes as possible catalyst. Also in U.S. Pat. No. 6,537,517 it is mentioned that the nanotube titania obtained from the alkali hydrothermal treatment may further be heat-treated at from 200 to 1,200° C. to improve the crystallinity of TiO2 and to increase the catalytic activity and that the nanotube does not collapse through this heat treatment. It is not mentioned how the heat treatment is performed. It is assumed that it was done as a regular well known heat treatment which would involve a static, non-dynamic air atmosphere by placing the product in an oven. It is claimed that the heat treatment is expected to improve the crystallinity and activity of the nanotubes. However, there is no table or data comparing the properties of the nanotubes before and after such heat treatment. Very recently it has appeared in the literature a paper entitled “Regulation of the Physical Characteristics of Titania Nanotube Aggregates Synthesized from Hydrothermal Treatment” (Chien-Cheng Tsai and Hsisheng Teng, Chemistry of Materials 2004, 16, 4352-4358) where the precursor used is a commercial TiO2 powder with a composition 70% anatase and 30% rutile, and a primary particle size of 21 nm (same method of synthesis as the reported in U.S. Pat. No. 6,537,517). In this paper the authors study how the stability and pore structure (surface area) of the obtained nanotubes vary with subsequent calcination at different temperatures (they do not give any specifics about the calcination procedure, thus it is assumed to be static air in an oven). The authors of the mentioned literature paper found that the as-synthesized anatase nanotubes remain tube-like at 400° C. but these nanotubes have a sharp surface area decrease with the calcination temperature sintering (collapsing) at 600° C. to form anatase rodlike structures. Subsequently the rodlike structure agglomerates at 800° C., forming anatase cylindrical particles, and at 900° C. these particles go through a phase transformation to the rutile phase. These results contradict use of thermal treatment over the interval (200° C. to 1200° C.) in U.S. Pat. No. 6,537,517 to improve crystallinity without collapse or phase transformation at the high temperature.
In published U.S. Application No. 2004/0265587, titanium oxide sol is used as a starting material in which particles (no powders) with specific average particle diameters (2 to 100 nm, preferably 5 to 80 nm) are dispersed in water to prepare a water dispersion sol which is used as starting material. The synthesis method outlined in published U.S. Application No. 2004/0265587 to obtain tubular titanium oxide particles involves preparing the water dispersion sol (this step requires heating and many steps). Then the water dispersion sol of titanium oxide particles is subjected to a one step of hydrothermal treatment followed by washing and calcining, or the water dispersion sol of titanium oxide particles is subjected to a two hydrothermal treatments instead of one. The first hydrothermal treatment is carried out in the presence of an alkali metal hydroxide together with ammonium hydroxide and/or an organic base. The presence of the ammonium hydroxide and/or an organic base is claimed to reduce the alkali metal impurities in the tubular titanium oxide particles. The hydrothermal treatment is carried out at temperatures between 80 to 250° C. (which is a higher temperature than the required in the U.S. Pat. No. 6,537,517). While in the second hydrothermal treatment the presence of a cation is required and the temperature is the same as in the first hydrothermal treatment. The synthesis presented in published U.S. Application No. 2004/0265587 involves many steps, many reactants, high temperatures and possibly a second hydrothermal treatment, and consequently the method becomes industrially of high cost; also high temperatures are required in both hydrothermal treatments. The method in published U.S. Application No. 2004/0265587 also involves a heating treatment (named as “reduction treatment”) in an inert gas atmosphere, under reduced pressure or in a reducing gas atmosphere. It is not said if this so called reduction treatment is done in a dynamic flow or static flow of the gas that composes the atmosphere, thus it is assumed that it is carried out in a static way. The formula of the final product given in published U.S. Application No. 2004/0265587 includes nitrogen and another transition metal different to titanium in the case of preparing mixed metal compounds or M=Ti if not mixed metal synthesis is carried out. The given formula of the claimed synthesized tubular titanium oxide particles with anatase, or rutile, or brookite phase is TiaMbOxNy. Experimental evidence on all the cell parameters to support the indication that the tubular titanium oxide particles have anatase, or rutile, or brookite phase is not provided in published U.S. Application No. 2004/0265587.
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