Preparation of metal alkoxides

Details Preparation of metal alkoxides Preparation of metal alkoxides

2000-07-05

2002-03-12

Nazario-Gonzalez, Porfirio (Department: 1621)

Organic compounds -- part of the class 532-570 series

Organic compounds

Heavy metal containing

C556S001000, C556S042000, C556S045000, C556S054000, C556S076000, C556S081000, C556S113000, C556S130000, C556S146000, C556S182000, C556S400000, C423S592100, C423S604000, C423S606000, C423S612000, C423S617000, C423S618000, C427S108000, C427S126300, C252S182110

Reexamination Certificate

active

06355821

ABSTRACT:

TECHNICAL FIELD
This invention primarily relates to metal alkoxides and metal alkoxide solutions suitable for use as precursors in the production of metal oxide coatings on glass and similar substrates, to methods for the preparation of such metal alkoxide precursors and to the coatings and devices employing such coatings. The glass substrates of particular interest are those employed in large-area electrochromic ‘smart windows’ that require a variety of nanocrystalline metal oxide coatings.
Unless otherwise made clear by the context:
‘Metal’ refers to any of the conventional metals and also those ‘metals’ or ‘metalloids’ of Groups III-A, IV-A and V-A of the Periodic Table of the elements, such as silicon, boron, aluminum, tin, antimony and the like.
‘Metal alkoxide’ refers to any metal compound derived from an alcohol or alcohol-containing organic moiety wherein the hydroxylic hydrogen of at least one hydroxyl group has been replaced by a metal, and thus includes alkali and alkali earth metal alkoxides.
‘Metal halides’ includes oxymetal halides.
The metal alkoxides and alkoxide precursor solutions of this invention have particular use in the production of nanocrystalline metal oxide films on large-area glass panels. The films are of high quality, transparent and suitable for use in electrochromic devices. They have valuable application in other areas of industry and technology, such as in catalysts and electronic devices.
BACKGROUND TO THE INVENTION
Metal alkoxides are important in the formation of thin nanocrystalline transparent layers of metal oxides on glass substrates by sol-gel techniques. Typically, the metal alkoxide is applied in a liquid solvent (precursor solution) to a substrate such as glass by dip-coating or spin-coating. The solvent is removed by evaporation and the metal alkoxide is exposed to water vapour to enable hydrolysis and condensation to produce a metal oxide and/or hydroxide sol-gel and alcohol. The coated substrate is then baked at moderate temperatures to remove residual alcohol and form the desired nanocrystalline metal oxide thin layer.
The production of alkoxides from metal halides—especially metal chlorides—for use in electrochromic devices is well known, but the methods are generally tedious and can result in low yields because of the problem posed by the removal of the hydrogen halide by-product. Whereas the reaction of metal chlorides with any of the common alcohols is mildly exothermic and proceeds without difficulty, the standard method of removing by-product HCl (Bradley, D. C.; Mehrotra, R. C.; Gaur, D. P.; Metal Alkoxides, Academic Press, London, 1978) is laborious and involves the use of anhydrous agents in non-aqueous solvents. The HCl by-product is usually allowed to react with a base such as ammonia, alkyl amines, pyridine or sodium alkoxide to produce chloride salts that are insoluble and precipitate from the solvent used. The above textbook reports the use of ammonia for the preparation of alkoxides from the metal halides of Si, Ge, Ti, Zr, Hf, Nb, Ta, Fe, Sb, V, Ce, U and Th. It also reports the use of sodium alkoxide for the preparation of the corresponding metal alkoxides from the metal halides of Ga, ln, Si, Ge, Sn, Fe, As, Sb, Bi, Ti, Th, U, Se, Te, W, lanthanides, Ni and Cr.
These methods, however, are cumbersome and suffer from several disadvantages. The fine precipitates (e.g. NH
4
Cl, NaCl) are impractical to filter and the products are obtained only after several prolonged steps of settling, decantation and washing with excess solvent to obtain the maximum yield. Further, in some cases (e.g. Sn, W) the products are often contaminated by the presence of varying amounts of chloride and sodium ions and also NH
3
or its derivatives (Bradley, D. C., Caldwell, E. V., and Wardlaw W., Journal of Chemical Society, 1957, 4775; also, vide infra, Example 1). Washing the non-aqueous solution with water to remove chlorides is unacceptable because the metal alkoxides would be rapidly transformed to metal hydroxides and alcohol.
In Japanese patent [Sho 61-36292 (1986)] the following reaction between tungsten hexachloride and n-butanol was described in which part of the HCl was removed under reflux in CCl
4
and the remainder with gaseous NH
3
as a precipitate of NH
4
Cl after the addition of benzene:
WCl
6
+ROH+NH
3
→O═W(OR)
4
+RCl+
n
NH
4
Cl+(5−
n
)HCl
After three extractions with benzene a yield of 85% of tungsten (Vl) oxo-tetra-n-butoxide [O═W(OBu
n
)
4
] was obtained. The method, however, has the same drawbacks as noted above.
In particular, we have found that, during the preparation of tungsten (Vl) oxo-tetra-alkoxide, [WO(OR)
4
], from WOCl
4
, alcohol and ammonia in n-pentane, an insoluble tungsten-containing compound often coprecipitates with ammonium chloride, making the extraction of the product extremely difficult. Reasons for this behaviour are unclear, but insoluble tungsten material could be due to dimer formation or some other incompletely understood interaction of NH
3
and/or NH
4
Cl with tungsten alkoxide in hydrocarbon solvent. Excess ammonia can be added to dissolve the precipitated tungsten compound, but we have found that the final tungsten oxide obtained from such precursors is unsuitable as a film for electrochromic applications since the reversibility of the colouration-bleaching cycle is inadequate. This is still the case even when no significant amounts of chloride ions and ammonia-derived impurities were detectable in the alkoxide. We believe that the behaviour of WO
3
in the film prepared by this route is heavily dependent upon the structure of the precursor tungsten alkoxide, which in turn is influenced by the NH
3
concentration during its preparation. Furthermore, we have found that alcoholic solutions of metal alkoxides prepared by the NH
3
route are often unstable, resulting in the precipitation of insoluble metal-containing material over time. This means that a single batch of alkoxide produces variable quality coatings.
Similar difficulties with the removal of chloride are encountered in the production of tin alkoxides for use in glass coatings and are addressed in U.S. Pat. No. 4,731,461 (1988). This patent teaches the use of ammonia as described by Bradley et al. as the first step of a two step process in which the product of the first step is treated with a metal amide or a metal alkoxide and additional alcohol. This results in the precipitation of a metal halide salt that can then be removed by filtration. With other metals of interest, particularly tungsten, the two step process taught by U.S. Pat. No. 4,731,461 is not satisfactory. As explained above, the first step is cumbersome, has a poor yield and generates little understood side products that render the resultant alkoxide material unsuitable for electrochromic purposes.
Electrochromic Coatings from Solutions of Metal Chlorides and Alcohols
A number of prior-art patents disclose the production and use of metal alkoxides for use in electrochromic coatings but do not address the problem of chloride removal. For example, in U.S. Pat. No. 4,347,265 (1982) tungsten (Vl) chloride (WCl
6
) is dissolved in an organic solvent such as methanol, isobutanol, ethanol, or acetic anhydride and it is implied that the resultant solution is applied to the substrate without further treatment and then baked to form the desired electrochromic layer. Similarly, U.S. Pat. Nos. 4,996,083 (1991) and 4,855,161 (1989) disclose the preparation of electrochromic coating solutions from anhydrous transition metal halides, preferably chlorides, such as tungsten chloride, and lower carbon, anhydrous alcohols, but no manner of removing contaminating chloride is disclosed. While U.S. Pat. No. 5,659,417 (1997) discloses the use of tungsten and molybdenum alkoxides in alcohol solutions, the method of preparing the alkoxides from metal chlorides, is that of U.S. Pat. No. 4,996,083 outlined above.
Our experience has shown that satisfactory coatings can be made in rare instances by the methods of the

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