WO2007118692A1

WO2007118692A1 - Method for producing metal fluoride sols and gels

Info

Publication number: WO2007118692A1
Application number: PCT/EP2007/003309
Authority: WO
Inventors: Erhard Kemnitz; Udo Gross; Stephan Rüdiger
Original assignee: Humboldt-Universität Zu Berlin
Priority date: 2006-04-13
Filing date: 2007-04-13
Publication date: 2007-10-25
Also published as: DE102006017582A1

Abstract

The invention relates to a method for producing metal fluoride sols and gels, comprising the following steps: a) provision of a metal, b) provision of a dissolution of hydrogen fluoride in a non-aqueous solvent, and c) reaction of the metal with the hydrogen fluoride dissolution in a non-aqueous solvent to form a metal fluoride sol.

Description

Process for the preparation of metal fluoride sols and gels

The present invention relates to a process for producing a metal fluoride sol or a metal fluoride gel and to the use thereof.

Metal fluorides are of great importance to the chemical, pharmaceutical and ceramics industries. In particular, inorganic materials with a large surface area are of central importance in heterogeneous catalysis, for example, where the activity of a catalyst is highly dependent on its surface area.

The preparation of metal fluorides can be carried out via the sol-gel route. Such a method is described for example in European patent application EP 04700458.5. According to the teaching disclosed therein, a metal fluoride precursor compound is typically provided, which is typically an alkoxide, an enolate or the salt of a carboxylic acid having a preferred chain length of 1 to 5 carbon atoms. Such a precursor compound is then reacted with anhydrous hydrogen fluoride in a suitable solvent to give, after removal of the solvent, an intermediate referred to as "precursor." This is reacted in a second step with a fluorinating agent according to EP 04700458.5.

In the process described in the European patent application EP 04700458.5, the production of the metal fluoride precursor compound with a coordinatively bound organic group is associated with a certain expense.

The object of the present invention is thus to provide a process for the preparation of a metal fluoride sol and a metal fluoride gel or a process for the preparation of an amorphous metal fluoride.

It is a further object of the present invention to provide metal fluorides having a high surface area, preferably a surface area of, or greater than, 180 m ² / g, for example, for use as a catalyst. Furthermore, it is the task of to provide a coating based on amorphous metal fluoride and articles coated therewith.

This object is achieved according to the invention in a first aspect by a metal fluoride sol and a metal fluoride gel comprising the steps:

a) providing a metal, b) providing a solution of hydrogen fluoride in a non-aqueous solvent, and c) reacting the metal with the solution of hydrogen fluoride in a nonaqueous solvent to form a metal fluoride sol.

In one embodiment of the first aspect it is provided that the metal is a metal of the second, third or fourth main group or a subgroup of the periodic table, wherein the redox potential of the metal in the respective solvent is more negative than that of H ₂ .

In a preferred embodiment of the first aspect, it is provided that the metal is selected from the group comprising magnesium, calcium, strontium, barium, aluminum, zinc, lead, rare earths.

In a preferred embodiment of the first aspect, it is provided that the metal is selected from the group comprising magnesium and aluminum.

In a further embodiment of the first aspect, it is provided that the metal is reacted with the solution of hydrogen fluoride in a stoichiometric ratio based on hydrogen fluoride or with an excess of hydrogen fluoride.

In yet another embodiment of the first aspect, the non-aqueous solvent is an alcohol selected from the group consisting of methanol, ethanol, propanol, iso-propanol, butanol, iso-butanol, tert-butanol, methoxyethanol, Ethoxyethanol includes. In yet another embodiment of the first aspect, it is contemplated that the alcohol contains up to 4% water.

In yet another embodiment of the first aspect, it is contemplated that the nonaqueous solvent is an ether or a ketone or a sulfoxide.

In yet another embodiment of the first aspect, it is provided that the reaction of step c) takes place at room temperature and standard pressure.

In yet another embodiment of the first aspect, it is contemplated that the metal fluoride sol or metal fluoride gel has a solvent content of from 50 mole% to 99.9 mole%, preferably from 70 mole% to 99 mole%.

In yet another embodiment of the first aspect, it is contemplated that the metal is an alloy or mixture of two or more metals.

In a second aspect of the invention, the object is achieved by a metal fluoride sol or metal fluoride gel obtainable by a process according to the invention.

In a still further embodiment of the first aspect it is provided that it further comprises the step:

d) removal of the volatile constituents from the reaction product obtained in step c).

In a preferred embodiment of the first aspect, it is provided that the volatile constituents are removed by a heat treatment.

In a preferred embodiment of the first aspect it is provided that the heat treatment at a temperature of 30 to 500 ⁰ C, preferably from 70 to 250 ⁰ C takes place. In an alternative embodiment of the first aspect, it is provided that the

Hitzebeha innddlung in vacuo at a pressure of 800 up to 10 ^"3 mbar, preferably takes place ^'' from 100 to 10 mbar.

In one embodiment of the first aspect, it is provided that the method is a method for producing an amorphous metal fluoride.

In another embodiment, the metal is activated prior to its reaction with the solution of hydrogen fluoride. Activation converts the metal to a state that allows or accelerates reaction with the solution of hydrogen fluoride. In a particular embodiment, the activation serves to eliminate the passivation of the metal. Appropriate means are known to those skilled in the art and include, for example, the reaction of the metal with mercury, which is particularly preferred when using Al as the metal.

In a further embodiment, the method of the invention comprises the step of converting the metal to a carbon precursor which is then further reacted with hydrogen fluoride and / or in a non-aqueous solvent containing hydrogen fluoride.

In a third aspect of the invention, the object is achieved by an amorphous metal fluoride obtainable by a method according to the invention, in particular by a method according to the first aspect.

In a fourth aspect of the invention, the object is achieved by a method for coating a surface, preferably for film-coating a surface, comprising the steps:

a) providing a surface,

b) providing a metal fluoride sol or metal fluoride gel c) coating the metal fluoride gel or metal fluoride sol on a surface and

d) optionally removing the solvent from the sol or gel.

In a preferred embodiment of the fourth aspect, it is provided that the metal fluoride sol or the metal fluoride gel is provided by a method according to an embodiment of the invention or is a metal fluoride sol or metal fluoride gel according to an embodiment of the invention.

In a fifth aspect of the invention, the object is achieved by a metal fluoride sol or gel according to the second aspect for the production of a coating, preferably a protective coating, an antireflection coating or a catalytically active layer on a suitable support.

In a sixth aspect of the invention, the object is achieved by the use of a metal fluoride according to the third aspect, for the production of ceramics, preferably transparent ceramics.

In a further aspect of the invention, the object is achieved by the use of a metal fluoride gel or metal fluoride sol according to the second aspect for the production of metal fluoride coated fibers or waveguides.

In a seventh aspect of the invention, the object is achieved by an article comprising a metal fluoride sol or metal fluoride gel according to the second aspect, wherein the article is selected from the group comprising glass sheets, cathode ray tubes, liquid crystal displays, metal surfaces and lenses.

These and other objects underlying the present invention are also achieved by the subject matter of the independent claims. Preferred embodiments will be apparent from the dependent claims. The present inventors have surprisingly found that instead of using metal compounds having a coordinatively bonded organic group, metals can also be used directly as starting materials for the production of a metal fluoride sol or gel, which in turn is used to produce an amorphous metal fluoride according to the present invention Invention can be used. According to the technical teaching provided by the present inventors, preferably, the amount of solvent used in this reaction, based on the amount of metal, should be so large that the product is liquid, ie, a sol and not a solid gel. Otherwise, the reaction could be hindered so much by the resulting gel that it does not run completely. The sol thus obtained can easily be concentrated to obtain a gel, if there is a need for the preparation of a gel.

The present invention is further based on the surprising finding that, in the context of the process according to the invention, the nonaqueous solvent serving as solvent for the hydrogen fluoride acts so strongly solvating that the metal fluoride is converted into solution as sol. This could not be expected on the basis of previous observations and assumptions of the prior art for the following reasons.

In the reaction of a metal with hydrofluoric acid, there is often only a surface reaction of the metal, since the insoluble metal fluoride formed protects the underlying metal from further attack. This process is known as "passivation" and is observed, for example, with aluminum and magnesium or iron, whereas the use of aqueous hydrofluoric acid solution, also called hydrofluoric acid, often leads to a progressive attack, as the product formed is However, when water is replaced by a nonaqueous solvent such as alcohol as taught in the present invention, an analogous behavior was not expected to occur in that the metal fluorides in question, and especially neither aluminum fluoride nor magnesium fluoride, were in a nonaqueous state Further, it should be noted that the sol or gel formed in the reaction of metal alkoxide with HF in alcohol, as taught, for example, in European Patent Application EP 04700458.5, first, is not a true solution as in the case of water, and, second, the sol / gel formation g erade understood that way was that it is bound to the metal-organic starting compound by initially taking partial retention of the structure only a partial substitution of the alkoxide groups by fluoride takes place. On the other hand, according to the previous understanding, the formation of a metal fluoride sol observed in aluminum and magnesium in the reaction with alcoholic HF solution could not be explained by a 2-step reaction in which the metal first reacts to form the respective alkoxide and second step reacts this alkoxide with HF, since alcohol as acid is much weaker than hydrofluoric acid. Against this background, it was therefore unlikely that the alcohol or non-aqueous solvents used as solvents for the hydrogen fluoride would have such a strong solvating effect that the metal fluoride would be converted into solution as sol. The sol, in turn, can solidify into a gel as known to those skilled in the art and, for example, described in CJ Brinker and GW Scherer, Sol-Gel Science, Academic Press, Boston 1990.

As used herein in a preferred embodiment, the term nonaqueous solvent refers to a solvent that is free of water, preferably substantially free of water. However, it is also within the scope of the present invention that, in some embodiments, water is present in the non-aqueous solvent, but the amount of water content is such that the reaction described above, and in particular the formation of the metal fluoride sol as disclosed herein, not or not materially affected. Such amounts of water may, for example, be due to the fact that the required for the reaction of hydrogen fluoride is added as a highly concentrated, for example, 70% aqueous solution to the non-aqueous system, especially when it is in the non-aqueous system to a alcoholic solvent or solvent system.

Not least as a result of this reaction mechanism can be understood as metals in the context of the present invention, all metals and mixtures or alloys of such metals whose redox potential in the respective nonaqueous solvent is less than that of hydrogen, ie, the less noble than hydrogen. On the basis of the method according to the invention for producing a metal fluoride sol and a metal fluoride gel, it is then possible to produce an amorphous metal fluoride having the properties described herein. This is based on the metal fluoride sol whose volatile components such as the nonaqueous solvent is removed, preferably under vacuum, whereby a solid is usually obtained. It is to be noted that typically a certain content of the non-aqueous solvent used in the preparation of the metal fluoride sol remains in the crystal structure and as a result it becomes a disorder the regular crystal structure comes, which ultimately causes the amorphous character of the thus prepared metal fluoride.

The amorphous metal fluoride thus obtained can be post-fluorinated by any of the methods described in EP 04700458.5. Preferably, the reaction with the fluorinating agent is carried out at an elevated temperature, but below the crystallization temperature of the metal fluoride. Preferred temperature ranges thus be 350 ° C or less, 200 ° C or less and 100 ⁰ C or 70 ° C or less. Typically, while still present, the content of organic compounds is reduced quantitatively, wherein preferably the carbon content based on the amorphous metal fluoride is less than 1%. A suitable fluorinating agent is typically CH _g Cl _h F _{4 -gh} , where the sum of g + h is equal to a number, preferably an integer from 1 to 3, or gaseous hydrogen fluoride. CH _g Cl _h F ₄ . _gh is typically gaseous, diluted with an inert gas such as N ₂ , where the CH _g Cl _h F ₄ .g. _h partial pressure 5 to 1500 mbar, preferably 25 to 300 mbar, or in pure, non-diluted form, at a temperature of 25 ° C to 45O ⁰ C, preferably from 5O ⁰ C to 300 ⁰ C, are used. The gaseous hydrogen fluoride is typically an inert gas such as N ₂ or other gases used diluted, or in a pure, non-diluted form in a temperature range between 30 to 300 ⁰ C, preferably 75 is used to 18O ⁰ C the Fluorwasserstoffpartialdruck is 50 to 1500 mbar. Preferably 350 to 600 mbar. Typically, the fluorination reaction is carried out in a metal tube reactor. The catalytically active solid material thus obtained retains the amorphous X-ray structure and has a very high specific surface area and a distorted structure as compared with the crystalline form of the corresponding metal fluoride. This disorder accounts for the observed pronounced Lewis acidity of Lewis acid metals as contained in the amorphous metal fluoride according to the present invention.

In addition to using the methods of the invention for the preparation of magnesium fluoride or aluminum fluoride sols and gels, the present technique can be used especially for those metals selected from the group comprising Cr, Fe, V, Ga, Pb, Ca, Ba and Zn. Accordingly, the processes described in more detail herein for magnesium and aluminum, respectively, are also applicable to corresponding fluorides of these metals.

Moreover, it is within the scope of the present invention that the metal fluoride may also be a mixed metal fluoride.

The amorphous metal fluoride prepared according to the invention is X-ray amorphous. As used herein, the term X-ray amorphous means that the microcrystalline domains of the solid, ie, the amorphous metal fluoride, are less than 20 nm in size. A transmission electron microscopic study confirmed, at a magnification of 10 ⁷ , that the amorphous metal fluoride according to the present invention, such as AlF _{3, is} in the form of very small solid particles that are partially agglomerated to form larger units. Further scanning electron microscopic studies revealed a mesoporous surface of the amorphous metal fluoride according to the present invention. The peculiarities of the morphology and the pore structure of the amorphous metal fluoride according to the invention lead to a considerable increase of the catalytically active surface and thus of the catalytic activity.

BET measurements using N _2, as are known in the art and can be determined for example with a ASAP2001 IPMENT (Micromeritics) at 77 K, leading to surface values of> 250 m ² / g. Solid-state NMR studies of the central Al atom can also be used to determine the proximity order on aluminum and to differentiate between different AI species. The highly disordered order of the AIF ₃ lattice is responsible for the x-ray amorphous state of the solid, ie, the amorphous metal fluoride of the present invention, as well as the desired and intended increase in catalytic activity. The strongly disordered lattice structure of the amorphous metal fluoride according to the present invention is also confirmed by IR spectroscopic studies. The experimental evidence of increased Lewis acidity can be provided by pyridine adsorption and NH ₃ -TPD as described in the prior art.

The catalytically active metal fluorides according to the present invention are applicable to reactions where metal fluorides of the prior art can also be used. In addition, the catalytically active metal fluorides can be used for the catalysis of halogen exchange reactions, with the replacement reactions of chlorinated hydrocarbons, chlorofluorohydrocarbons and fluorohydrocarbons being particularly preferred. Also, the catalytically active metal fluorides can be used in any catalytic reaction where a Lewis acid catalyst is used. Further preferred reactions that can be catalyzed using the catalytically active metal fluorides of the present invention are the isomerization of haloperfluoroalkanes, wherein the haloperfluoroalkanes are preferably selected from the group consisting of chloro-bromo-fluoroethanes and propanes, the isomerization of olefins, wherein the olefin isomerization is preferably selected from the group consisting of alkene-1 to alkene-2, for example butene-1 to butene-2 isomerizations, in the Friedel-Crafts acylation reaction catalysis, the acylation reaction being preferably those selected from the Group consisting of benzoylation / acetylation of activated as well as deactivated aromatic systems, as well as in Friedel-Crafts alkylation reactions, wherein the alkylation reactions are preferably selected from the group consisting of the alkylation of aromatic rings.

The amorphous metal fluoride powders obtained by the process can be pressed into transparent ceramics, for example for applications in the optical field. Methods for producing ceramics are known in the art and, for example in J. Lucas, F. Smektala, JL Adam, Fluorine in Optics, in J. Fluorine Chem. 114 (2002), 113-118.

The sol state of the metal fluorides prepared according to the invention can be used to produce thin amorphous metal fluoride layers on different substrates, such as Si, SiO ₂ , metal or polymer material by dip coating, spin coating or other known methods. The corresponding methods are known in the art and described, for example, in CJ Brinker and GW Scherer, Sol-Gel Science, Academic Press, Boston 1990.

Example 1: Preparation of methanolic aluminum fluoride gel

Aluminum foil (0.27 g, 0.01 mol) was brought into contact with a very small amount of mercury (sphere of about 0.5 mm diameter). To the amalgamated aluminum, 7.5 ml of a 4.4 molar methanolic hydrofluoric acid solution (0.033 mol) were added in a plastic vessel and made up to 25 ml with anhydrous methanol. The reaction was stirred at room temperature; The aluminum dissolved during 48 h. The resulting colorless, slightly viscous liquid could be easily separated from the unreacted mercury ball. The white solid obtained after removal of methanol in vacuo at 70 ° C corresponded in its fluorine and carbon content and its behavior in the thermal analysis completely the technical teaching of EP 04700458.5 prepared precursor molecule, there referred to as "precursor", and by treatment with CCl ₂ F ₂ at a maximum of 280 ° C. in the flow reactor to give a HF-AlF ₃ (high surface aluminum fluoride) having the properties described in EP 04700458.5 are post-fluorinated.

Example 2: Preparation of Aluminum Fluoride Coatings

5.4 g of Al powder (0.2 mol) are added with stirring to 75 mL of 16% HF / CH ₃ OH solution (0.6 mol). To complete the reaction, 30 mL of CH ₃ OH are still after 3 h and 30 mg HgCl ₂ added. After further stirring and subsequent filtration, a clear sol-like solution of solvated AlF _{3 is} obtained which shows in the ¹⁹ F-NMR spectrum the characteristic peaks at -164 ppm and -174 ppm.

a) To produce an A1F ₃ layer on a Si surface, the resulting sol solution is applied by spin coating (at 5000 rpm) and dried at 25 ° C for 3 h and at 300 ° C for 1 h. The surface was characterized by force field microscopy and ellipsometry, whereby a layer thickness of 50-60 nm was determined.

b) Spherical (X-Al ₂ O ₃ pellets (about 3 mm diameter) are impregnated with the thus obtained AIF ₃ - sol.) After drying at temperatures of 25-50 ° C., the pellets are treated as described in EP 04700458.5 N ₂ -verdünnten CHClF ₂ -StTOm at temperatures of 25-280 ⁰ C-treated. The supported A1F thus obtained ₃ catalyst is made in accordance with gravimetric analysis 95% α-Al ₂ θ ₃ and 5% AlF _3, and showed a high catalytic activity for CHC1F ₂ dismutation, comparable to that described in EP 04700458.5 for HS-AIF ₃ .

c) From the thus obtained AIF ₃ -SOI, which can be described by the formula AlF ₃ n (CH ₃ θH), where n ~ 16, by heating in vacuo (120 ° C, 0.1 mbar, 6 h) the solvent is removed to give a white powder of AIF ₃ having a carbon content of 0.9%. The aluminum fluoride thus obtained is X-ray amorphous and has a specific surface of 252 m ² / g.

The release of methanol was also monitored quantitatively by thermal analysis, with a 15.6% weight loss of the sample detected by thermogravimetry and differential thermogravimetry. The thermal effects were endothermic.

Example 3: Preparation of methanolic magnesium fluoride gel

In a plastic vessel, magnesium chips were mixed with 75 ml of a 2 molar methanolic hydrogen fluoride solution and stirred at room temperature. After 2 Days were decanted from the gray sediment. From the colorless supernatant solution, after removal of the excess methanol in vacuo, a nearly white solid was obtained which had a specific surface area (BET / N ₂ ) of 348 m ² / g. Despite the large surface area, the sample showed clear, albeit broad, reflections of MgF ₂ in the X-ray powder diffraction gram.

Example 4: Preparation of barium fluoride

690 mg (5 mmol) of metallic barium were mixed with a methanolic HF solution in excess, based on the HF content. Gas evolution and formation of a milky sol dissolve the barium. Concentrating the sol drops a white, X-ray amorphous powder of barium fluoride. The reaction proceeds quantitatively with respect to the barium used.

Example 5: Preparation of zinc fluoride

1.24 g of zinc powder (activated with a copper salt before use) were mixed with a stoichiometric amount of HF in methanol, forming a cloudy, milky solution with the onset of gas evolution. After one day, the HF solution was supplemented. After completion of the reaction and separation of the solvent, a white amorphous powder of ZnF _{2 was} obtained. The X-ray diffractogram has very broad reflections attributable to zinc fluoride. The carbon content is 0.2%.

The features of the invention disclosed in the foregoing description, the claims and the drawings may be essential both individually and in any combination for realizing the invention in its various embodiments.

Claims

claims

A process for producing a metal fluoride sol and a metal fluoride gel, comprising the steps of:

2. The method according to claim 1, characterized in that the metal is a metal of the second, third or fourth main group or a subgroup of the periodic table, wherein the redox potential of the metal in the respective solvent is more negative than that of H ₂ .

3. The method according to claim 2, characterized in that the metal is selected from the group comprising magnesium, calcium, strontium, barium, aluminum, zinc, lead, rare earths.

4. The method according to claim 3, characterized in that the metal is selected from the group comprising magnesium and aluminum.

5. The method according to any one of claims 1 to 4, characterized in that the metal is reacted with the solution of hydrogen fluoride in a relative to hydrogen fluoride stoichiometric ratio or with an excess of hydrogen fluoride.

6. The method according to any one of claims 1 to 5, characterized in that the non-aqueous solvent is an alcohol which is selected from the group consisting of methanol, ethanol, propanol, iso-propanol, butanol, iso-butanol, tert-butanol , Methoxyethanol, ethoxyethanol.

7. The method according to any one of claims 1 to 6, characterized in that the alcohol contains up to 4% water.

8. The method according to any one of claims 1 to 5, characterized in that the non-aqueous solvent is an ether or a ketone or a sulfoxide.

9. The method according to any one of claims 1 to 8, characterized in that the reaction of step c) takes place at room temperature and standard pressure.

10. The method according to any one of claims 1 to 9, characterized in that the metal fluoride sol or metal fluoride gel has a content of solvent of 50 mol% to 99.9 mol%, preferably from 70 mol% to 99 mol%.

11. The method according to any one of claims 1 to 10, characterized in that the metal is an alloy or mixture of two or more metals.

12. A metal fluoride sol or metal fluoride gel obtainable by a process according to any one of claims 1 to 11.

13. The method according to any one of claims 1 to 11, characterized in that it further comprises the step:

14. The method according to claim 13, characterized in that the volatile constituents are removed by a heat treatment.

15. The method according to claim 14, characterized in that the heat treatment at a temperature of 30 to 500 ⁰ C, preferably from 70 to 250 ⁰ C takes place.

16. The method according to claim 14 or 15, characterized in that the heat treatment is carried out in vacuo at a pressure of 800 to 10 ^"3 mbar, preferably from 100 to Kr 'mbar.

17. The method according to any one of claims 13 to 16, wherein the method is a method for producing an amorphous metal fluoride.

18. Amorphous metal fluoride, characterized in that it is obtainable by a process according to any one of claims 13 to 17.

19. A method for coating a surface, preferably for film coating a surface, comprising the steps of:

a) providing a surface,

b) providing a metal fluoride sol or metal fluoride gel

c) coating the metal fluoride gel or metal fluoride sol on a surface and

d) optionally removing the solvent from the sol or gel.

20. The method according to claim 19, characterized in that the metal fluoride sol or the metal fluoride gel is provided by a method according to any one of claims 1 to 11 or a metal fluoride sol or metal fluoride gel according to claim 12.

21. Use of a metal fluoride sol or gel according to claim 12 for the preparation of a coating, preferably a protective coating, an antireflection coating or a catalytically active layer on a suitable support.

22. Use of a metal fluoride according to claim 18 for the production of ceramics, preferably transparent ceramics.

23. Use of a metal fluoride gel or metal fluoride sol according to claim 12 for the production of metal fluoride coated fibers or waveguides.

An article comprising a metal fluoride sol or metal fluoride gel according to any one of the preceding claims, wherein the article is selected from the group comprising glass sheets, cathode ray tubes, liquid crystal displays, metal surfaces and lenses.