WO2008154716A1

WO2008154716A1 - Alkaline and terrous-alkaline transition metal oxide micro- and submicro-materials, and hydrothermic process for obtaining the same

Info

Publication number: WO2008154716A1
Application number: PCT/BR2008/000175
Authority: WO
Inventors: Bojan Marinkovic; Lidija Mancic; Fernando COSME RIZZO ASSUNÇÃO; Paula Mendes Jardim
Original assignee: FACULDADES CATÓLICAS, SOCIEDADE CIVIL MANTENEDORA DA PUC Rio
Priority date: 2007-06-21
Filing date: 2008-06-23
Publication date: 2008-12-24
Also published as: BRPI0721806A2

Abstract

This invention provides micro and submicro materials of metal transition oxides from crystalline structure and CaFe2O4 formula RxFexTi2-xO4, where R is an alkali or alkaline-earth. The materials of the invention, alkali and alkaline-earth metal transition oxides submicro and micro materials, are useful in various applications. It is also provided a low-cost process for the obtainment of micro and submicromaterials through an hydrothermal treatment of raw materials as mineral sands.

Description

Specification of Patent of Invention

Alkaline and terrous-alkaline transition metal oxide micro- and submicro-materials, and Hydrothermic process for obtaining the same

Field of the Invention

This invention relates to micro and submicro materials of transition metal oxides, of crystalline structure identical to that of compound CaFe₂O₄, and produced from mineral sands, for example, ilmenite sands. More specifically, this invention relates to micro and submicro materials of alkali and alkaline-earth transition metal oxides obtained from natural mineral sands obtained through a low-temperature and low-cost hydrothermal process. The products of the invention present morphology, crystalline structure, thermal stability, dimensions and other physical properties that render them useful in various applications. The process of the present invention presents substantial advantages over those known in the art, notably in terms of low-cost and high production versatility of micro and submicro materials of transition metal oxides.

Background of the Invention

The materials based on alkali and alkaline-earth transition metal oxides have attracted special attention due to their potential applications in ionic conductors, lithium batteries, among others.

Metal titanium is not freely found in nature, but it ranks ninth in terms of abundance on the Earth's crust and is present in most igneous and sediments derived from these rocks. It is mainly found in the minerals anatase (TiO₂), brookite (TiO₂), ilmenite (FeTiO₃), leucoxene, perovskite (CaTiO₃), rutile (TiO₂) and titanite (CaTiSiO₅). Of these minerals, only ilmenite, leucoxene and rutile present economic importance. Major deposits can be found in Australia, South Africa, Scandinavia, USA, Canada, Malaysia and Brazil. In Brazil, the ilmenite sands reserves were estimated at 30 million tonnes in 2000, where the deposit of Mataraca in Paraiba State concentrated around 64% of these reserves. Approximately 95% of all titanium is consumed in the titanium dioxide form (TiO₂), a permanent intensely white pigment. Paints prepared with titanium dioxide are excellent reflectors of infrared radiation, and accordingly are much used in astronomy.

Ilmenite is a weakly magnetic mineral found in metamorphic rocks and geological intrusions of igneous rocks. Most of the mined ilmenite is obtained from secondary sources such as beach sands. It is an iron titanium oxide in crystalline form.

Most ilmenite is used as a material for producing pigments. The product, in this case, is titanium dioxide, which is a white substance used as a high quality paint base.

AB₂X₄ stoichiometry is instead distributed among complex oxides and sulfides. The spinel has the most important and well-known crystalline structure in this stoichiometry. However, there is another type of crystalline structure, though less known and exploited, which belongs to AB₂X₄ stoichiometry. This is the CaFe₂O₄ type structure, called in accordance with the prototype compound, whose parameters of the unit cell were first reported by Malquori and Cirilli [1], followed by determination of crystalline structure made by Hill et al. and Bertaut et al. [2,3]. The basis of this structure is a rutile double chain composed of octahedra connected by edges. The double chain is connected to four other double chains through the corners, thus forming, subnanometric tunnels oriented along the shortest axis in the unit cell. In order to offset the negative charge of the chains, these tunnels are more often occupied by Na⁺ and Ca²⁺ or more rarely by Sr²⁺ or Ba²⁺. Reid et al. [4] showed that the CaFe₂O₄ structure has a chemistry relatively high flexibility, limited basically by factors such as the ionic radius similarity between ions occupying B crystallographic sites and ionic radius of the cation A. These authors, also were the first to report the synthesis of Na_xFe_xTi_2-x0₄ phase, in this case with x = 1, establishing that it crystallizes within the CaFe₂O₄ type structure [4]. Mumme et al. and Kuhn et al. [5-7] reported two others ferrititanates formulas, Na_xFe_xTi_2-x0₄ (x < 1) and Na_x-δFe_xTi_2-xO₄. In the last one, iron can appear in a highly oxidized form Fe⁴⁺. The crystalline structure of these two phases are similar and differ slightly on CaFe₂O₄ type structure.

The crystalline phases of the CaFe₂O₄ type structure are usually obtained through a synthesis route at a high temperature. For example, a phase of CaFe₂O₄ type, in which octahedric crystallographic site are completely occupied by some trivalent element (Fe³⁺, for example), as SrFe₂O₄, or where octahedric sites are equaly occupied by a trivalent and a quadrivalent element, such as NaFe³⁺Ti⁴⁺O₄ or NaSc³⁺Ti⁴⁺O₄ (a general formula for these stages may be writing as NaBlB2O₄) are being prepared by the classic method of reaction in solid form by the appropriate heating of reagents at temperatures above 950°C [2,4]. Sometimes, high pressure must be applied simultaneously with the high temperatures in order to obtain components such as Naj. _xCa_xRh₂0₄ [8]. The NaTi₂O₄ phase of mixed valence and CaFe₂O₄ structure is also difficult to synthesize. This phase was first obtained by heating a mixture of TiO, Ti₂O₃ and Na₂O under argon flow at 1200°C [9]. Geselbracht et al. [10] has recently decreased, significantly, the temperature of synthesis to 770°C, and also eliminated Na₂O as reagent, which is air-sensitive. They reduced Na₈Ti₅O₁₄ with metal Ti at 770⁰C under a mixed flow NaCl / KCl, obtaining NaTi₂O₄. More recently, NaTi₂O₄ was obtained by Takahashi et al. [11], by reacting metal Na with TiO₂ powder at 1200°C under an argon atmosphere. From the obtained NaTi₂O₄ material, a deficient phase in sodium Na_xTi₂O₄ was prepared by a topotatic chemical oxidation using an acid solution [H].

Only one reference in literature of hydrothermal synthesis phase of CaFe₂O₄ type is known. This was the Na₃Mn₄Te₂Oj₂ phase, which adopts a superstructure closely related to CaFe₂O₄ type structure, with Mn and Te atoms ordered along the shortest axis in the CaFe₂O₄ structure (b-axis for Pnma (62) or c-axis for Pnam (62)). Thus, the shortest axis is triplicated in this stage. Feger and Kolis [12] reacted MnO₂ and Te(OH)₆ with IM NaOH solution at 375°C for 5 days and obtained Na₃Mn₄Te₂O₁₂ with an yield of 30% (estimated by volume).

Patent literature comprises some documents related to this kind of materials and their production processes. Although neither one of the found documents fully anticipates or suggests, even indirectly, the inventive concept of the present invention, certain documents are cited herein as reference.

The Japanese patent application JP2004196555 from Akimoto and Takahashi published on July 15, 2004, reveals a complex transition metal oxide for the use in lithium cell solid electrolyte containing sodium, iron and titanium complex oxide. Said oxide, whose method of synthesis is being revealed; presents crystalline structure with tunnel format and that is also an excellent conductor of ions. The complex oxide is represented by the general formula Na_2+xFe_xTi_4-xθ₉ (O < x <1) and whose crystalline structure presents similarities with the phase obtained by this invention, but does not belong to the CaFe₂O₄ structure.

The Japanese patent application JP2005263583 from Akimoto, Takahashi and Kijima published on September 29, 2005, refers to the obtention of crystalline material useful as positive electrode material for a secondary lithium batteries. Said crystalline material is composed of a sodium transition metal oxide and represented by the chemical formula Na_xMn i_-yTi_y0₂. The crystalline structure presents tunnel format of orthorombic system and differs from the structure of this invention, which is synthesized hydrothermally.

The scientific publications listed below do not anticipate or suggest, even indirectly, the scope and spirit of this invention and are detailed below only for reference.

[1] G.L.Malquori and V.Cirilli, Third International Symposium on the Chemistry of Cement. London: Cement and Concrete Association. (1952).

[2] P.M. Hill, H.S.Peiser, J.G.Rait, Acta Cryst, 9 (1956) 981.

[3] E.F. Bertaut, P. Blum, G. Magnano, Bull. Soc. Franc. Mineral. Crist., 129 (1956)

536.

[4] A.F. Reid, A.D. Wadsley, MJ. Sienko, Inorg. Chem., 7 (1968) 112. [5] W.G. Mumme and A.F. Reid, Acta Cryst., B24 (1968) 625.

[6] A. Kuhn, F. Garcia-Alvarado, E. Morin, M.A. Alario-France, U. Amador, Solid

State Ion., 86-88 (1996) 811.

[7] A. Kuhn, N. Menendez, F. Garcia-Alvarado, E. Morin, J.D. Tornero, M.A. Alario- France, J. Sol. State Chem., 130 (1997) 184. [8] K. Yamaura, Q. Huang, M. Moldovan, D.P. Young, A. Sato, Y. Baba, T. Nagai, Y.

Matsui, and E. Takayama-Muromachi, Chem. Mater., 17 (2005) 359.

[9] J. Akimoto, H. Takei, J. Sol. State Chem., 79 (1989) 212.

[10] MJ. Geselbracht, L.D. Noailles, L.T. Ngo, J.H. Pikul, R.I. Walton, E.S. Cowell, F.

Millange, D. O'Hare, Chem. Mater., 16 (2004) 1153. [11] Y. Takahashi, K. Kataoka, K. Ohshima, N. Kijima, J. Awaka, K. Kawaguchi, J.

Akimoto, J. Sol. State Chem., 180 (2007) 1030. [12] CR. Feger and J.W. Kolis, Acta Cryst, C54 (1998) 1055. [13] A.A. Coelho, Topas- Academic, 2004.

[14] Transmisson Electron Microscopy IV, Spectrometry, David B. Williams and C. Barry Carter, Plenum Press, New York, 1996 p. 600.

Summary of the Invention

It is one of the objects of the present invention to provide micro and submicro materials of transition metal oxides of R_xFe_xTi_2-xθ₄ formula, where R is an alkali or an alkaline-earth element with a CaFe₂O₄ crystalline structure. In one aspect of the present invention, and therefore another of its objects, submicro materials of alkali and alkaline-earth transition metal oxides of CaFe₂O₄ structure obtained from mineral sands, such as sand of ilmenite, are provided. The obtainment of these materials is carried out at a low-temperature hydrothermal synthesis, and exemplified by obtaining the Na_xFe_xTi_2-χ0₄ phase, in a reaction of ilmenite natural sand with a 1OM NaOH solution in temperatures of about 200°C. The typical product is a Na_xFe_xTi_2-xθ₄ phase deficient in sodium, with x approximately equal to 0.76, and with a small amount of magnetite (Fe₃O₄). The products of the invention include micro and submicro materials of alkali and alkaline-earth transition metal oxides useful in various applications, such as semiconductor devices, ionic conductors, secondary lithium batteries, and others.

It is another object of the present invention to provide a process for producing micro and submicro materials of alkali and alkaline-earth transition metal oxides which uses mineral sands as low-cost precursor materials.

These objects, and other objects, features and advantages of the present invention will become more apparent to those of ordinary skill in the art when the following detailed description is read in conjunction with appended figures and claims.

Brief Description of the Figures

Figure 1 presents an X-ray diffraction pattern of the precursor (ilmenite natural sands), illustrating the corresponding composition of crystalline phases. Figure 2 illustrate the Rietveld refinement on X-ray Diffraction Patterns of a typical product from a hydrothermal synthesis at 190°C. M - diffraction lines of magnetite; i - diffraction lines of ilmenite; All other diffraction lines corresponds to the main phase Na_{0 J6}Fe_Oj₉Ti_{1 >2}i O₄. Figure 3 a) and b) illustrate images of Transmission electron microscopy

(TEM) of a typical Na_xFe_xTi_2-x0₄ crystal, c) represents SEAD of Na_xFe_xTi_2-x0₄ crystals of Figure 3 b).

Figure 4 a) and b) illustrate the magnetite crystal in a cube form (marked with an arrow) with a [-114] zone axis SEAD. Figure 5 illustrates the Energy Dispersive X-ray Spectrometry (EDS) of the largest Na_xFe_xTi_2-x0₄ crystal in Figure 3 a). The EDS spectrum indicates that there is also a small amount of Mn in Na_xFe_xTi_2-x0₄ crystals. Probably Mn cations occupy the octahedric sites.

Detailed Description of the Invention

The details and preferential examples reported below are intended to facilitate the reproduction of the invention and should therefore be understood as merely illustrative, without restricting the scope of the invention.

The natural sand ilmenite is a precursor of low-cost (-0.08 US$/kg). Its pattern of X-ray diffraction, Figure 1, illustrated that it is not a monophase sand and that addition of ilmenite (FeTiO₃) includes other minerals such as rutile, pseudorutile and quartz.

The present invention provides micro and submicro materials of transition metal oxides of formula R_xFe_xTi_2-x0₄, where R is an alkali or an alkaline-earth element with a CaFe₂O₄ crystalline structure.

In one of the preferred embodiments of this invention, as described below in more details, R is sodium (Na). In a typical synthesis process, 0.5g of ilmenite natural sand are added to 25ml of 5-15M NaOH aqueous solution, preferably 1OM. The reaction mixture obtained is transferred to a steel-coated autoclave with 30 ml of Teflon (using different degrees of filling, preferably 83%) and kept at different temperatures (170-210⁰C, preferably at 190°C) for 60-80 hours, preferably 70 hours under intense agitation. The resulting powder are firstly dispersed in 200ml of distilled water and then filtered. Next, the material retained in the filter is washed with distilled water up to a pH value of approximately 7. The products obtained after washing are dried in an oven for 5 hours. Maintaining the proportions described in the prior paragraph, or by increasing the content of the solid phase (ilmenite natural sand) in an aqueous solution of NaOH, this synthesis may be repeated on a larger (industrial) scale, providing the same end products.

Example - Characterization of the products obtained

The Rietveld refinement is performed using the software TOPAS [13], as Figure 2, where you can see the Rietveld refinement for standard X-ray diffraction of a typical product in a hydrothermal synthesis at 190°C.

The images of transmission electron microscopy (TEM) and patterns of selected area electron diffraction (SAED) are recorded using a Gatan CCD camera installed in JEOL-201 microscope, as Figure 3 a) and b), where you can see images of transmission electron microscopy (TEM) of a typical crystal of Na_xFe_xTi_2-xθ₄, where the Figure 3 c) represents SAED of Na_xFe_xTi_2-XO₄ crystals from Figure 2 b).

Samples for observation in TEM are prepared by powder dispersion into alcohol under ultra-sonic treatment, and then dropped into a perforated carbon film supported by a grid of copper. The chemical composition of the crystals is examined by energy dispersive spectroscopy (TEM / EDS).

Table 1 lists atomic coordinates and factors of occupation, while Table 2 summarizes details of refinement and factors of refinement with parameters of the unit cell phase Na_xFe_yTi_2-y0₄.

Table 1 - Refined atomic positions and occupancy factors (Nj) from Na_xFe_xTi_2-xθ₄ at 25°C.

Atom Site X Y Z Nj

NaI 4c 0.24444(99) 0.34783(82) 0.75 0.76

Bl (Til) 4c 0.05986(35) 0.11390(35) 0.75 0.47

Bl (FeI) 4c 0.05986(35) 0.11390(35) 0.75 0.53 B2 (TiI) 4c 0.08608(37) 0.60468(31) 0.75 0.74

B2 (Fe2) 4c 0.08608(37) 0.60468(31) 0.75 0.26

Ol 4c 0.3108(10) 0.64888(90) 0.75 1

02 4c 0.39026(82) 0.99072(95) 0.75 1

03 4c 0.4688(12) 0.21033(84) 0.75 1

04 4c 0.0571(11) 0.92440(72) 0.75 1

Table 2 - Details of Rietveld refinement, confidence factors corresponding to Nao ₇₆Feo_,79Tii_;2i0₄ and results of quantitative analysis of crystalline phases.

Molecular Formula Nao_,76Feo,79Tii_,2i0₄

Z 4

Molecular Weight (g/mol) 734.5

Volume (A³) 289.8

Dcaic (g/cm³) 4.207

Space group Pnam (62) a (A) 9.1230 b (A) 10.7469 c (A) 2.9562

RB 3.86%

*MVp 10.81%

S (Adjustment facilities) 1.76

Quantitative Analysis

Na₀₁₇₆Fe₀J₉Ti_I52IO₄ 92 wt%

Magnetite (Fe₃O₄) 7 wt%

Ilmenite (FeTiO₃) 1 wt%

The quantitative analysis by Rietveld method indicates that the magnetite

Fe₃O₄ is the minority phase, whose percentage of weight is not higher than 7%. Residues of ilmenite that did not react also appear, however their percentage of weight never exceeds 1% and can be removed by increasing the time or the temperature of synthesis. Additionally, Na_xFe_xTi_2-x0₄ hydrothermally prepared has a typical CaFe₂O₄ structure and there is not this variety of structure described by Mumme and Reid [5] to Na_xFe_xTi_2-x0₄ (0.9 > x > 0.75) obtained by high-temperature process (1000°C). It can be concluded from the Rietveld refinement (Figure 2 and Table 1), that the ferrititanate prepared is sodium deficient and that its occupation factor is approximately 0.76. This corresponds well with the total occupation Of Fe³⁺ on Bl and B2 sites, estimated at 0.79. This could mean that all trivalent cations on Bl and B2 sites, which are needed to compensate the load in the composition of Na_xFe_xTi_2-x0₄, have been due to Fe³⁺. The values of x that determine the levels of sodium and Fe³⁺ should be equal, resulting in a small discrepancy, 0.76 for sodium and 0.79 for Fe³⁺, being therefore, within the range of error for the technique of X-ray diffraction of powder.

The Na_xFe_xTi_2-xθ₄ appears in crystals from submicron to micron, with features and sense (direction) of growth well defined. The zone axis for Na_xFe_xTi_2-x0₄ crystals is [100], as the direction of growth is [001] assuming space group Pnam (62). This means that the tunnels are oriented along the direction of growth of the crystals. The crystals are thinner in the direction [100]. The crystallographic direction [010] is perpendicular to the direction of growth. The features of the crystals are composed of plans (100), (010) and (031), and the more extensive plan (100).

Through the reason of Cliff-Lorimer method [14], a 1.41 reason was obtained for the Ti / Fe in Na_xFe_xTi_2-x0₄, estimating a relative error of no more than 1%, if considering the standard deviation of the peak intensity of the lines of Ti-Ka and Fe- Ka. The reason Ti / Fe calculated from the factors of occupation of Fe and Ti by Rietveld refinement (Table 1) is 1.53. However, the reason Ti / Fe estimated by EDS deviated approximately 8% of the value calculated from X-ray diffraction patterns, which suggest that the real reason Ti / Fe may be around 1.5, as indicated independently by Rietveld method and by the reason of Cliff-Lorimer method.

The present invention enables a new route for the synthesis at low temperature of alkali and alkaline-earth transition metal oxides of CaFe₂O₄ structure, and allows the formation of new metastable phases that can not be obtained by methods at high temperatures. These phases would have large differences in ionic radius of the elements located in on Bl and B2 octahedric sites, such as, for example, Zr⁴⁺ and Fe³⁺. The process of this invention also provides the obtainment of metastable phases, in which cations on the octahedric sites have considerable differences in oxidation states. Additionally, the process of this invention also provides the control of sodium deficiency in Na_xFe_xTi_2-JcO₄, through the choice of different concentrations of NaOH. Finally, the low-cost and low-temperature of the hydrothermal route are widespread for the group of alkali and alkaline-earth transition metal oxides of CaFe₂O₄ structure, using suitable bases and mineral resources as precursors.

The weight yield of Na_xFe_xTi_2-x0₄ phase is higher than 92%, having the magnetite as the main impurity. Thus, this invention deals with the synthesis of alkali and alkaline-earth transition metal oxides of CaFe₂O₄ structure, obtained from hydrothermal synthesis at temperatures lower than 200°C. The example of alkali transition metal oxide (Na_xFe_xTi_2-XO₄) presented, is merely illustrative and does not restrict the scope of this patent, since it includes all other alkali and alkaline-earth transition metal oxides of CaFe₂O₄ structure .

The skilled in the art will immediately understand the significant benefits of the use of this invention. Variations on the way of perfoming the inventive concept exemplified here should be understood as within the spirit of invention and the claims attached.

Claims

ClaimsAlkaline and terrous-alkaline transition metal oxide micro- and submicro-materials, and Hydrothermic process for obtaining the same

1. Micro materials and/or submicro materials of metals transition oxides characterized by having the general formula of R_xFe_xTi_2-XO₄ phase, where R is an alkali and/or alkaline-earth, these micro and/or submicro materials presenting CaFe₂O₄ structure.

2. Micro materials and/or submicro materials, according to claim 1, characterized by comprinsing small amount of magnetite (Fe₃O₄).

3. Micro materials and/or submicro materials, according to claim 1 or 2, characterized by being deficient in the alkali or alkaline-earth element.

4. Micro materials and/or submicro materials, according to claims 1-3, characterized by the fact that this alkali element is sodium.

5. Micro materials and/or submicromateriais of alkali and/or alkaline-earth transition metal oxides characterized by the fact of exhibiting CaFe₂O₄ structure and being obtained by a low-temperature hydrothermal processing of mineral sands.

6. Production process of micro and/or submicro materials of alkali and/or alkaline-earth transition metal oxides of CaFe₂O₄ structure characterized by comprising the following steps: a) addition of mineral sand to an alkali or alkaline-earth aqueous solution, following of agitation of the obtained mixture, and b) heating of said mixture at temperatures below 200°C, until the material is dried.

7. Process according to claim 6, characterized by further comprising the steps of: c) adding distilled water to the resulting powder and filtering; d) washing the material retained in the filter with distilled water, until a neutral pH is achieved, and e) drying at a temperature within the range of 60 to 100⁰C.

8. Process, according to claims 6-7, characterized by additionally comprising an initial step of milling of mineral sand.

9. Process, according to claims 7-8, characterized by the fact that step c) is substituted by the addition of an acid, preferably HCl.

10. Process, according to claims 6-9, characterized by the fact that the temperature of hydrothermal treatment influences the size and/or the morphology of micro and/or submicro obtained materials.

11. Process according to the claims 6-10, characterized by the fact that the deficiency of alkali or alkaline-earth element is controlled by the concentration of alkali or alkaline- earth agent used.