WO2001087775A1

WO2001087775A1 - A method of preparation of porous manganese dioxide

Info

Publication number: WO2001087775A1
Application number: PCT/GB2001/002132
Authority: WO
Inventors: George Simon Attard; Mark James Perkins; Neal Charles White
Original assignee: Eveready Battery Company Inc.; University Of Southampton
Priority date: 2000-05-15
Filing date: 2001-05-15
Publication date: 2001-11-22
Also published as: GB0011684D0; AU5650201A

Abstract

Manganese dioxide having a regular nanoporous structure is prepared by the chemical oxidation of a source of Mn(II), chemical reduction of a source of Mn(VI) or Mn(VII), or chemical dissociation of an Mn(II) salt, characterised in that the oxidation, reduction or dissociation reaction is carried out in the presence of a structure-directing agent in an amount sufficient to form a homogeneous lyotropic liquid crystalline phase in the reaction mixture, and under conditions such as to precipitate the manganese dioxide as a porous solid wherein at least 10 % of the manganese dioxide has a porous structure wherein a recognisable architecture or topological order is present in the spatial arrangement of the pores, wherein at least 50 % of the manganese dioxide has a nanoporous structure, and wherein at least 75 % of the pores have pore diameters to within 40 % of the average pore diameter.

Description

A METHOD OF PREPARATION OF POROUS MANGANESE DIOXIDE

The present invention relates to the preparation of manganese dioxide in the form of a porous, and particularly nanoporous or mesoporous, material, and, in particular, a porous material having a substantially regular structure and uniform pore size.

Manganese dioxide is a widely used material having a wide variety of uses. For example, it is used as an oxidising agent, as a catalyst, as the active cathode material in a range of modern batteries, and as a scavenger and decolouriser. For all of these uses, it is advantageous that the manganese dioxide should have the maximum possible accessible surface area, and preferably a porous structure comprising open, rather than closed, pores.

Manganese dioxide is commonly prepared either chemically, for example by reduction of a permanganate, or electrolytically. Although the electrolytic route can be carried out in such a way as to give a crystalline form of Mnθ2 having the most desirable properties, it is substantially more expensive than the common chemical routes, and so it would be desirable to provide a process for preparing MnU2 which imparts a useful physical structure on the formed Mnθ2, but which is chemical, not electrolytic, in nature, so as to benefit from the cost savings thereby achieved.

We have now discovered that manganese dioxide having the requisite properties can be prepared by carrying out a chemical reaction in the presence of a structure- directing agent which forms a homogeneous liquid crystal phase in the reaction mixture.

Thus, the present invention provides a process for the preparation of manganese dioxide by the chemical oxidation of a source of Mn(π), chemical reduction of a source of Mn(VI) or Mn(VTT), or chemical dissociation of an Mn(π) salt, characterised in that the oxidation, reduction or dissociation reaction is carried out in the presence of a structure-directing agent in an amount sufficient to form an homogeneous lyotropic liquid crystalline phase in the reaction mixture, and under conditions such as to precipitate the manganese dioxide as a porous solid wherein at least 10%, and preferably at least 20%, more preferably at least 25%, of the manganese dioxide has a porous structure wherein a recognisable architecture or topological order is present in the spatial arrangement of the pores, wherein at least 50%, and preferably at least 75%, more preferably at least 90%, of the manganese dioxide has a nanoporous structure, and wherein at least 75% of the pores have pore diameters to within 40%, e.g. within 30%, of the average pore diameter.

In the accompanying drawings:

Figure 1 shows the sodium dodecyl sulphate-water phase diagram; and

Figures 2 to 18 show TEM (transmission electron microscope) images of porous structures obtained by using methods according to the invention.

The porous structure wherein a recognisable architecture or topological order is present in the spatial arrangement of the pores is here referred to as a "nanostructure".

Although the material the subject of the present invention is commonly referred to as manganese dioxide and represented by the formula Mnθ2, it will be understood that most samples of so-called manganese dioxide do not adhere strictly to this formula, and could more properly be considered mixtures of oxides of Mn(IV) and Mn(rfl) in varying proportions, and thus represented by the formula MnO_χ, where x is a number which generally falls within the range of from 2 to 1.8. A discussion of the various non- stoichiometric compounds included in the term "manganese dioxide" appears in "Studies On Mnθ2 - 1. Chemical Composition, Microstructure and Other

Characteristics of Some Synthetic Mnθ2 of Various Crystalline Modifications" by K. M. Parida et al [Electrochimica Acta, Vol. 26, 435 - 443 (1981)]. All such materials are included in the term "manganese dioxide" and the formula "Mnθ2", as used herein. By "nanoporous" as referred to herein is meant a pore diameter within the range from about 1.3 to 20 nm (13 to 200A), and by "macroporous" is meant pore diameters exceeding about 20 nm (200A). Preferably, the films are nanoporous, more preferably having a pore diameter within the range from 1.4 to 10 nm (14 to lOOA), and most preferably within the range from 1.7 to 4 nm (17 to 40 A). Such films may also be referred to as 'mesoporous'.

It is a crucial feature of the present invention that the chemical reaction to prepare the Mnθ2 is carried out in the presence of a structure-directing agent in order to impart an homogeneous lyotropic liquid crystalline phase to the mixture. The liquid crystalline phase is thought to function as a structure-directing medium or template for deposition of the Mnθ2- By controlling the nanostructure of the lyotropic liquid crystalline phase, Mnθ2 may be synthesised having a corresponding nanostructure. For example, Mnθ2 deposited from normal topology hexagonal phases will have a system of pores disposed on an hexagonal lattice, whereas Mnθ2 deposited from normal topology cubic phases will have a system of pores disposed in cubic topology. Similarly, Mnθ2 having a lamellar nanostructure may be deposited from lamellar phases.

Accordingly, by exploiting the rich lyotropic polymorphism exhibited by liquid crystalline phases, the method of the invention allows precise control over the structure of the Mnθ2 and enables the synthesis of well-defined porous Mnθ2 having a long range spatially and orientationally periodic distribution of uniformly sized pores.

Any suitable amphiphilic organic compound or compounds which will not adversely affect the Mnθ2-forming reaction and which is capable of forming an homogeneous lyotropic liquid crystalline phase may be used as the structure-directing agent, either low molar mass or polymeric. These compounds are also sometimes referred to as organic directing agents. In order to provide the necessary homogeneous liquid crystalline phase, the amphiphilic compound will generally be used at an high concentration, although the concentration used will depend on the nature of the compound and other factors, such as temperature, as is well known in the chemical industry. Typically at least about 10% by weight, preferably at least 20% by weight of the amphiphilic compound is used, but preferably no more than 95%, by weight, based on the total weight of the solvent and amphiphilic compound. Most preferably the amount of amphiphilic compound is from 30 to 80%, especially from 40 to 75%, by weight, based on the total weight of the solvent and amphiphilic compound. For example, the sodium dodecyl sulphate- water phase diagram shown in Figure 1 of the accompanying drawings exhibits a hexagonal region from about 40%) to 60% by weight sodium dodecyl sulphate and at temperatures above about °C.

Suitable compounds include organic surfactant compounds capable of forming aggregates, and preferably of the formula RpQ wherein R represents a linear or branched alkyl, aryl, aralkyl, alkylaryl, steroidal or triterpene group having from 6 to about 6000 carbon atoms, preferably from 6 to about 60 carbon atoms, more preferably from 12 to 18 carbon atoms, p represents an integer, preferably from 1 to 5, more preferably from 1 to 3, and Q represents a group selected from: [O(CH₂)_m]_nOH wherein m is an integer from 1 to about 4 and preferably m is 2, and n is an integer from 2 to about 100, preferably from 2 to about 60, and more preferably from 4 to 8; nitrogen bonded to at least one group selected from alkyl having at least 4 carbon atoms, aryl, aralkyl and alkylaryl; phosphorus or sulphur bonded to at least 2 oxygen atoms; and carboxylate (COOM, where M is a cation, or COOH) groups.

General classes of surfactant which may be used in the present invention include: alkyl sulphosuccinamates; alkyl sulphosuccinates; quaternary ammonium surfactants; fatty alcohol ethoxylates; fatty alcohol ethoxysulphates; alkyl phosphates and esters; alkyl phenol ethoxylates; fatty acid soaps; amidobetaines; aminobetaines; alkyl amphodiacetates; and ethylene oxide/propylene oxide block copolymers, e.g. of the type sold under the trade name 'Pluronics' .

Preferred examples include cetyl trimethylammonium bromide, cetyl trimethylammonium chloride, sodium dodecyl sulphate, sodium dodecyl sulphonate, sodium bis(2-ethylhexyl) sulphosuccinate, and sodium soaps, such as sodium laurate or sodium oleate; sodium dodecyl sulphosuccinamate; hexadecyl tetraethylene glycol sulphate; and sodium dodecyl hydrogen phosphate.

Other suitable structure-directing agents include monoglycerides, phospholipids, glycolipids and amphiphilic block copolymers.

Preferably non-ionic surfactants such as octaethylene glycol monododecyl ether

(C₁₂EO₈, wherein EO represents ethylene oxide), octaethylene glycol monohexadecyl ether (C₁₆EO₈) and non-ionic surfactants of the Brij series (trade mark of ICI Americas), are used as structure-directing agents.

In almost all cases, it is expected that the manganese-containing compound will dissolve in the hydrophilic domain of the liquid crystal phase, but it may be possible to arrange that it dissolves in the hydrophobic domain.

The reaction mixture may optionally further include a hydrophobic additive to modify the structure of the phase, as explained more fully below. Suitable additives include n-hexane, n-heptane, n-octane, dodecane, tetradecane, mesitylene, toluene and triethyleneglycol dimethyl ether. The additive may be present in the mixture in a molar ratio to the structure-directing agent in the range of 0.1 to 10, preferably 0.5 to 2, and more preferably 0.5 to 1.

The mixture may optionally further include an additive that acts as a co- surfactant, for the purpose of modifying the structure of the liquid crystalline phase or to participate in the chemical reactions. Suitable additives include n-dodecanol, n- dodecanethiol, perfluorodecanol, compounds of structures similar to the surfactants exemplified above but with a shorter chain length, primary and secondary alcohols (e.g. octanol), pentanoic acid or hexylamine. The additive may be present in the mixture in a molar ratio to the structure-directing agent in the range of 0.01 to 2, and preferably 0.08 to 1.

It has been found that the pore size of the deposited MnU2 can be varied by altering the hydrocarbon chain length of the surfactant used as structure-directing agent, or by supplementing the surfactant by an hydrocarbon additive. For example, shorter- chain surfactants will tend to direct the formation of smaller-sized pores whereas longer- chain surfactants tend to give rise to larger-sized pores. The addition of an hydrophobic hydrocarbon additive such as n-heptane, to supplement the surfactant used as structure- directing agent, will tend to increase the pore size, relative to the pore size achieved by that surfactant in the absence of the additive. Also, the hydrocarbon additive may be used to alter the phase structure of the liquid crystalline phase in order to control the corresponding regular structure of the deposited material.

The Mnθ2 obtained has pores of substantially uniform size. By "substantially uniform" is meant that at least 75% of pores have pore diameters to within 40%, preferably within 30%, more preferably within 10%, and most preferably within 5%, of average pore diameter.

The manganese dioxide in accordance with the invention is of a substantially regular structure. By "substantially regular" as used herein is meant that a recognisable topological pore arrangement is present in the material. Accordingly, this term is not restricted to ideal mathematical topologies, but may include distortions or other modifications of these topologies, provided recognisable architecture or topological order is present in the spatial arrangement of the pores in the material. The regular structure of the material may for example be cubic, lamellar, oblique, centred rectangular, body-centred orthorhombic, body-centred tetragonal, rhombohedral, hexagonal, or distorted modifications of these. Preferably the regular structure is hexagonal.

Most commercial grades of surfactant will contain or will be capable of exhibiting reducing action, and so, where the 11O2 is to be prepared by a reduction reaction, they maybe able to provide both the structure-directing agent and the reducing agent. For example, sodium dodecyl sulphonate contains dodecanol, and, in those cases where there is an intrinsic reducing agent, an extrinsic reducing agent may not be necessary, although, in many cases, it may also be desirable.

A variety of chemical methods is available to prepare manganese dioxide, and these are well known to those skilled in the art. In principle, any known method may be used, although care should be taken that the structure directing agent does not interfere with the reaction or that the reagents do not interfere with the structure directing agent.

Examples of suitable reactions which may be employed in the process of the present invention include the following:

1. Reduction of PermanRanate or Manganate

In this reaction, a permanganate or manganate, normally and preferably in aqueous solution, is reduced with a reducing agent.

There is no particular restriction on the permanganate or manganate to be used, provided that it is at least minimally, and preferably substantially, soluble in the reaction medium. Preferred, and commonly available, permanganates include potassium permanganate, sodium permanganate, lithium permanganate and ammonium permanganate, of which potassium or sodium permanganate is preferred. Preferred, and commonly available, manganates include potassium manganate, sodium manganate, lithium manganate and ammonium manganate, of which potassium or sodium manganate is preferred.

The concentration of the permanganate is preferably from 0.1 to 0.5M with respect to the aqueous component of the reaction mixture. Too low a concentration reduces the yield of the desired product to too low a level, whilst we have found that too high a concentration leads to a loss of the desired structure and of nanoporosity. Within this range, however, the concentration may be chosen freely.

The pH of the mixture would normally be expected to be slightly acid, perhaps around 6, and this is acceptable in the present invention. However, it may, in some cases be desirable to adjust the acidity, by the addition of an acid to achieve a pH in the range of from about 4 to about 5 before beginning the reduction reaction. However, it should be noted that, if the pH is too low, the permanganate may begin to decompose prematurely.

The reaction is normally and preferably effected at atmospheric pressure. However, if desired, it may be carried out under superatmospheric pressure. For example, it may be carried out under hydrothermal conditions, in which the reaction is effected in a sealed vessel under endogenous pressure.

The reaction can take place over a wide range of temperatures, and the precise reaction temperature is not critical to the invention. The preferred reaction temperature will depend upon such factors as the nature of the solvent, and the starting material or reagent used. However, in general, we find it convenient to carry out the reaction at a temperature of from 4°C to below the boiling point of the reaction mixture. Thus, if the reaction is carried out under atmospheric or superatmospheric pressure, a preferred temperature range is from 4° to 200°C more preferably from 10° to 90°C, and most preferably from 40° to 90°C. The time required for the reaction may also vary widely, depending on many factors, notably the reaction temperature and the nature of the reagents and solvent employed.

Since permanganates are generally powerful oxidising agents, it is preferred that the reducing agent used should not be too reactive, since the resulting reaction could be undesirably violent, which could result in damage to the desired nanoporous structure. Within these constraints, however, the reducing agent may be chosen freely from a wide range of readily available materials. Indeed, as discussed below, many commercial surfactants (which may be used as the structure-directing agent) contain or are themselves reducing agents, and so an extrinsic reducing agent may be unnecessary.

Examples of suitable extrinsic reducing agents include various organic compounds, including:

alcohols, which may be aliphatic or aromatic, for example:

1-n-alkanols, such as ethanol, propanol, decanol or dodecanol;

diols, such as ethylene glycol;

triols, such as glycerol;

higher alcohols, such as glucose; and esters of polyhydric alcohols, such as diethylene glycol monomethyl ether or diethylene glycol monomethyl ether;

aromatic alcohols, such as benzyl alcohol or phenol;

aldehydes, which may be aliphatic or aromatic, for example:

formaldehyde, acetaldehyde or benzaldehyde;

certain aliphatic carboxylic acids, for example:

citric acid or tartaric acid;

inorganic compounds, including:

hydrazine hydrate and sodium borohydride.

2. Oxidation of an Mn(IT) salt with an oxidising agent

In this reaction, an Mn(13) salt is oxidised using an oxidising agent such as a permanganate. In this case, where a permanganate is used as the oxidising agent, it is reduced and likewise yields Mnθ2-

Where the oxidising agent is a permanganate, this may be any of the permanganates exemplified above in reaction 1. Examples of other oxidising agents which may be used include: persulphates, for example ammonium, sodium or potassium persulphate; persulphuric acid; chlorates, for example sodium or potassium chlorate; and nitrites, for example sodium or potassium nitrite.

There is no restriction on the nature of the Mn(IT) salt, provided that it is soluble in the reaction medium, and any suitable salt may be employed, for example manganese nitrate or manganese sulphate, of which the nitrate is preferred because of its better solubility. Manganese nitrate also has the advantage that it is capable of giving the gamma crystalline form of Mnθ2 when used in a molar excess with respect to the permanganate. This reaction takes place very fast. It is not, therefore, possible simply to mix the reagents and the structure-directing agent, as the reaction will take place before the liquid crystal phase has a chance to form. Accordingly, in order successfully to carry out this reaction, it is desirable to use a "one pot" approach. One way of achieving this is to prepare a liquid crystal phase containing the Mn(H) salt and a surfactant and add a concentrated solution of permanganate thereto. Because of solubility limitations, we prefer to use sodium permanganate in this case. Alternatively, it is possible to prepare two liquid crystal phases, one containing the Mn(IT) salt and the other the oxidising agent, e.g. permanganate, and each containing surfactant, in approximately equal concentrations and then mix the two phases.

The reaction solvent is normally and preferably aqueous and may be simply water. However, especially where the Mn(π) salt is manganese nitrate, a weak solution of nitric acid is preferred.

The reaction can take place over a wide range of temperatures, and the precise reaction temperature is not critical to the invention. The preferred reaction temperature will depend upon such factors as the nature of the solvent, and the starting material or reagent used. However, in general, we find it convenient to carry out the reaction at a temperature of from 4°C to below the boiling point of the reaction mixture. Thus, if the reaction is carried out under atmospheric or superatmospheric pressure, a preferred temperature range is from 4° to 200°C more preferably from 10° to 95 °C, and most preferably from 40° to 90°C. The time required for the reaction may also vary widely, depending on many factors, notably the reaction temperature and the nature of the reagents and solvent employed.

An especially preferred synthetic procedure is as follows, using the 'one pot' approach. A hexagonal phase of surfactant, preferably sodium dodecyl sulphonate, is formed, using a solution of the manganese salt, e.g. manganese nitrate, preferably a concentration of about 0.25M. The amount of surfactant is preferably about 45%, based on the weight of surfactant and water. To this is added a concentrated (e.g. 1M) solution of the oxidising agent, e.g. sodium permanganate. The mixture is then reacted at a temperature of about 75 °C.

3. Reaction of Ozone with Manganese π salts.

In this reaction, a manganese II salt in solution in a suitable solvent is reacted with ozone. This could be regarded as a sub-class of reaction 2, but, since the preferred operating conditions are different, it is treated separately. Examples of manganese salts which may be used are as given for reaction 2.

The solvent is suitably water. The reaction is preferably effected at an acid pH, for example a pH of from 0.5 to 4, preferably 1.5 to 2.5 and more preferably about 2.

The reaction can take place over a wide range of temperatures, and the precise reaction temperature is not critical to the invention. The preferred reaction temperature will depend upon such factors as the nature of the solvent, and the starting material or reagent used. However, in general, we find it convenient to carry out the reaction at a temperature of from 30° to 80°C, more preferably from 50° to 70°C, and most preferably about 60°C. The time required for the reaction may also vary widely, depending on many factors, notably the reaction temperature and the nature of the reagents and solvent employed.

The ozone may be bubbled gently through the reaction mixture, or the reaction may be simply carried out in an atmosphere of ozone.

4. Hydrothermal Decomposition of Manganese II salts.

In this reaction, a manganese II salt in solution in a suitable solvent is decomposed hydrothermally. Examples of manganese salts which may be used are as given for reaction 2.

In order for the reaction to proceed, the solvent should be aqueous and is preferably simply water.

The reaction is normally and preferably effected in a sealed reaction vessel under autogenous pressure, which will normally be from 3 to 40 bar, more preferably from 3 to 39 bar, and most preferably from 3 to 4 bar.

The reaction can take place over a wide range of temperatures, and the precise reaction temperature is not critical to the invention. The preferred reaction temperature will depend upon such factors as the nature of the solvent, and the starting material or reagent used. However, in general, we find it convenient to carry out the reaction at a temperature of from 100° to 200°C, more preferably from 150° to 200°C. The time required for the reaction may also vary widely, depending on many factors, notably the reaction temperature and the nature of the reagents and solvent employed.

In this case, there are restrictions on the nature of the structure-directing agent which may be used, as it must be stable at temperatures up to about 200°C and must be capable of forming a liquid crystal phase at such temperatures.

Of the routes suggested above, one of the most preferred is the oxidation of an

Mn(π) salt with permanganate, as this is capable of yielding gamma Mnθ2, which is the most preferred crystalline form.

Many other chemical routes are known for the preparation of manganese dioxide, and any of these may be used in the process of the present invention, provided that they are not incompatible with structure-directing agents, such as surfactants, and that they can be, if necessary, modified to take place at such a rate as to permit the formation of a liquid crystal phase.

After completion of the reaction forming the Mnθ2, the desired product may be separated from the reaction mixture by conventional means. For example, where the reaction is effected at elevated temperature, the reaction mixture is allowed to cool, and then the structure-directing agent is removed by washing. Since the structure-directing agent is normally a surfactant, this may be achieved by washing with copious amounts of deionised water, followed by centrifugation. This is repeated several times until no more foaming is observed, indicating absence of the surfactant. The resulting manganese dioxide may then be dried by gentle heating, for example at a temperature from about 40 to about 100°C, more preferably about 60°C.

Since the chemical reactions described above will normally produce the δ form of Mnθ2, it may be desirable to convert this to the γ form, which, as is well known, is the preferred form for the construction of electrochemical cells. This may be carried out by well known methods, for example by treating the δ-Mnθ2 with an acid, e.g. nitric acid. The concentration of acid is preferably the minimum needed to achieve the required conversion of the physical form, as higher concentrations tend to result in a loss of the mesoporous structure. Thus, although the acid concentration may range from 1 to 6M, a concentration towards the lower end of this range is preferred. The reaction is preferably effected at ambient temperature, although gentle heating may be possible. The time required for the reaction may vary, depending upon many factors, but mainly the temperature and the concentration of the acid. A period of from 1 to 72 hours is possible, but, again, it is preferred to carry out the reaction towards the lower end of this range. Providing the concentration of the acid and the time of the reaction are not such as to dissolve the Mnθ2 to any great extent, and 6M nitric acid will dissolve it rather quickly, the mesporous structure is retained during the conversion of the γ-Mnθ2 to the δ-MnO₂.

The invention is further illustrated by the following non-limiting Examples.

EXAMPLES

Examples 1 to 3 (Intrinsic Reduction):

Chemical manganese dioxide (CMD) was prepared, using the intrinsic dodecanol present in technical grade sodium dodecyl sulphate SDS (BDH, 90%) as the reducing agent, as follows: Example 1 :

An aqueous solution of potassium permanganate at 0.25M concentration was added to a flask containing SDS to provide a concentration of 50 weight % SDS. The flask was heated and the mixture stirred until a homogeneous mixture was produced. The mixture was then left overnight at 45°C to allow the reaction to take place. The reaction mixture was allowed to cool to room temperature and the surfactant removed from the solid material by washing in copious amounts of deionised water followed by centrifugation. This procedure was repeated several times until foaming was no longer evident. A dark brown solid material was obtained, which was then dried at 60°C. The nanoporous material obtained is shown in Figure 2.

The dried solid exhibited a BET surface area of 34 m² g^"1.

Example 2 :

CMD was prepared using the same method and conditions as Example 1, except that the SDS concentration was 55 weight % and the KMnO₄ concentration was 0.2M.

A dry solid material was obtained, as shown in Figure 3, which exhibited a BET

9 1 surface area of 32 m g^" .

Example 3 :

CMD was prepared using the same method and conditions as Example 1, except that the SDS concentration was 45 weight % and the KMnO₄ concentration was 0.15M.

A dry solid material was obtained, as shown in Figure 4, which exhibited a BET surface area of 37 m² g^"1.

Examples 4 to 6 (Extrinsic Reduction of Potassium Permanganate):

CMD was prepared by reducing an aqueous solution of potassium permanganate, in the presence of SDS, using triethyleneglycol monomethyl ether (TEGMME) as the reducing agent, as follows: Example 4 :

An aqueous solution of potassium permanganate at a concentration of 0.25M was added to a flask containing SDS to provide a concentration of 40 weight % SDS. Dodecane was added to the mixture, in a mass ratio to SDS 1 :8, prior to reduction to stabilise the phase and increase the likelihood of an ordered, hexagonal array of pores within the material produced. The flask was heated and the mixture stirred until a homogeneous mixture was obtained. The reducing agent TEGMME was added at equimolar concentration with respect to KMnO₄ and the mixture left for three hours at 45°C to allow the reduction to take place. The reaction mixture was allowed to cool to room temperature and the surfactant removed from the solid material by washing with copious amounts of deionised water followed by centrifugation. This procedure was repeated several times until foaming was no longer evident. A dark brown to black solid material was obtained, which was then dried at 60 C. The nanoporous material obtained is shown in Figures 5, 6 and 7.

9 1 The dried solid exhibited a BET surface area of 175 m g^" .

Example 5 :

CMD was prepared using the same method and conditions as Example 4, except that the KMnO₄ concentration was 0.3M.

A dry solid material was obtained, as shown in Figure 8, which exhibited a BET

9 ι surface area of 157 m g^" .

Example 6 :

CMD was prepared using the same method and conditions as Example 4, except that the SDS concentration was 50 weight % and the reaction temperature was 90°C.

A dry solid material was obtained, as shown in Figure 9, which exhibited a BET

1 1 surface area of 180 m g^" . Examples 7 to 9 (Extrinsic Reduction of Sodium Permanganate):

CMD was prepared by reducing an aqueous solution of sodium permanganate, in the presence of SDS, using TEGMME as the reducing agent. Because of the increased solubility of sodium permanganate in water, two reaction procedures are possible, the first procedure being followed in Example 7 and the second procedure (premixing) being followed in Examples 8 and 9:

Example 7 :

An aqueous solution of sodium permanganate at a concentration of 0.25M was added to a flask containing SDS to provide a concentration of 45 weight % SDS. Dodecane was added to the mixture, in a mass ratio to SDS 1 :2, prior to reduction to stabilise the phase and increase the likelihood of an ordered, hexagonal array of pores within the material produced. The flask was heated and the mixture stirred until a homogeneous mixture was obtained. The reducing agent TEGMME was added at equimolar concentration with respect to NaMnO₄ and the mixture left for three hours at 90°C to allow the reduction to take place. The reaction mixture was allowed to cool to room temperature and the surfactant removed from the solid material by washing with copious amounts of deionised water followed by centrifugation. This procedure was repeated several times until foaming was no longer evident. A dark brown to black solid material was obtained, which was then dried at 60°C. The nanoporous material obtained is shown in Figures 10, 11, 12 and 13.

The dried solid exhibited a BET surface area of 184 m² g^"1.

Example 8 :

A premixing procedure was performed as follows. 75% of the water required to prepare a 45 weight % SDS solution was added to a flask containing SDS. Dodecane was added to the mixture, in a mass ratio to SDS 1 :2, prior to reduction to stabilise the phase and increase the likelihood of an ordered, hexagonal array of pores within the material produced. The flask was heated and the mixture stirred until a homogeneous mixture is obtained. An aqueous solution of sodium permanganate was then added and stirred into the mixture, the concentration of the sodium permanganate solution being four times the final concentration of 0.25M required prior to reduction. The reducing agent TEGMME was then added at equimolar concentration with respect to NaMnO₄ and the mixture left for three hours at 90°C to allow reduction to take place. The reaction mixture was allowed to cool to room temperature and the surfactant removed from the solid material by washing with copious amounts of deionised water followed by centrifugation. This procedure was repeated several times until foaming was no longer evident. A dark brown solid material was obtained, which was then dried at 60°C. The nanoporous material obtained is shown in Figure 14.

The dried solid exhibited a BET surface area of 180 m² g^"1.

Example 9 :

CMD was prepared using the same premix method and conditions as Example 8, except that the reducing agent TEGMME was then added at 1.5x equimolar concentration with respect to NaMnO₄.

• 9 A dry solid mateπal was obtained which exhibited a BET surface area of 210 m g ¹.

It may be noted that if the concentration of the added alkane (dodecanol) is high, the solid material will float at the top of the water being used to wash the solid. In this case, cleaning may be carried out be shaking the solid material with copious amounts of deionised water, in a separating funnel, followed by removal of the water via the tap on the funnel. This may be repeated several times until no foaming is evident. The solid material may be dried at 60°C and then washed again in copious amounts of water followed by centrifugation. This washing is repeated several times and the solid dark brown to black material is dried at 60°C. This washing procedure can also be used when high concentrations of alkane are used when potassium permanganate is used as the manganese precursor. Examples 10 to 12 (Permanganate / Manganese nitrate system):

CMD was prepared by reacting an aqueous solution of either potassium permanganate or sodium permanganate with an aqueous solution of manganese nitrate in the presence of SDS. Weak nitric acid solutions of manganese nitrate can also be used. Alkanes have also been added to the mixture prior to reaction to stabilise the phase and increase the likelihood of an ordered, hexagonal array of pores within the material produced. Because of the rapidity of the reaction between the two precursors, two procedures have been developed, the one pot and the two pot reactions.

Example 10 :

A two pot reaction was performed as follows. An aqueous solution of manganese nitrate (or a weak nitric acid solution of manganese nitrate) at a concentration of 0.25M was added to a flask containing SDS. The required quantity of alkane (1 :2 molar with respect to surfactant) was also added, the flask heated and the mixture stirred until a homogeneous mixture was obtained. An aqueous solution of either potassium permanganate with the same concentration as the manganese nitrate solution (0.25M) was added to a separate flask containing SDS. The required quantity of alkane was also added, the flask heated and the mixture stirred until a homogeneous mixture was obtained. The two mixtures were added together and stirred (SDS concentration 50 weight %). A rapid reaction at 45°C took place and the mixture was allowed to cool to room temperature. The surfactant was removed from the solid material by washing with copious amounts of deionised water followed by centrifugation. This was repeated several times until no foaming was evident. A solid dark brown to black material was obtained, which was then dried at 60°C. The nanoporous material obtained is shown in Figure 15.

The dry solid material exhibited a BET surface area of 168 m² g^"1.

If the concentration of added alkane is high, the solid material produced will float on water and the same washing procedure as described for the extrinsic reduction of sodium permanganate should be used. Example 11 :

A one pot reaction was performed as follows. An aqueous solution of manganese nitrate (or a weak nitric acid solution of manganese nitrate) at 0.25M concentration was added to a flask of SDS to provide a concentration of 47 weight % SDS. Dodecane was added to the mixture, in a mass ratio to SDS 1 :2, prior to reduction to stabilise the phase and increase the likelihood of an ordered, hexagonal array of pores within the material produced. The flask was heated and the mixture stirred until a homogeneous mixture was obtained. An aqueous solution of sodium permanganate at 1M concentration, i.e. four times as concentrated as the manganese nitrate solution, was then added. A rapid reaction ensued at 75°C, the mixture was allowed to cool to room temperature and the surfactant removed from the solid material by washing with copious amounts of deionised water followed by centrifugation. This procedure was repeated several times until foaming was no longer evident. A solid dark brown to black material was obtained and then dried at 60°C.

The dry solid material exhibited a BET surface area of 150 m² g^"1.

Example 12 :

CMD was prepared using the same method and conditions as Example 10, except that the SDS concentration was 45 weight %, the sodium permanganate at 0.5M was used instead of potassium permanganate, the manganese nitrate concentration was 0.5M, and the reaction temperature was 75°C.

A dry solid material was obtained which exhibited a BET surface area of 89 m²

Claims

CLAIMS:

1. A process for the preparation of manganese dioxide by the chemical oxidation of a source of Mn(II), chemical reduction of a source of Mn(VI) or Mn(VII), or chemical dissociation of an Mn(π) salt, characterised in that the oxidation, reduction or dissociation reaction is carried out in the presence of a structure-directing agent in an amount sufficient to form an homogeneous lyotropic liquid crystalline phase in the reaction mixture, and under conditions such as to precipitate the manganese dioxide as a porous solid wherein at least 10% of the manganese dioxide has a porous structure wherein a recognisable architecture or topological order is present in the spatial arrangement of the pores, wherein at least 50% of the manganese dioxide has a nanoporous structure, and wherein at least 75 % of the pores have pore diameters to within 40%) of the average pore diameter.

2. A process according to Claim 1, wherein at least 20%> of the manganese dioxide has a porous structure wherein a recognisable architecture or topological order is present in the spatial arrangement of the pores.

3. A process according to Claim 2, wherein at least 25%> of the manganese dioxide has a porous structure wherein a recognisable architecture or topological order is present in the spatial arrangement of the pores.

4. A process according to any one of Claims 1 to 3, wherein at least 75%, more preferably at least 90%, of the manganese dioxide has a nanoporous structure.

5. A process according to any one of Claims 1 to 4, wherein a permanganate or manganate is reduced with a reducing agent to produce said manganese dioxide.

6. A process according to Claim 5, wherein said reducing agent is : a 1-n-alkanol, preferably ethanol, propanol, decanol or dodecanol; a diol, preferably ethylene glycol; a triol, preferably glycerol; a higher alcohol, preferably glucose; an ester of a polyhydric alcohol, preferably diethylene glycol monomethyl ether or diethylene glycol monomethyl ether; an aromatic alcohol, preferably benzyl alcohol or phenol; an aldehyde, preferably formaldehyde, acetaldehyde or benzaldehyde; an aliphatic carboxylic acid, preferably citric acid or tartaric acid; or an inorganic compound, preferably hydrazine hydrate or sodium borohydride.

7. A process according to any one of Claims 1 to 4, wherein an Mn(H) salt is oxidised using an oxidising agent to give said manganese dioxide.

8. A process according to Claim 7, wherein said oxidising agent is : a persulphate, preferably ammonium, sodium or potassium persulphate; persulphuric acid; a chlorate, preferably sodium or potassium chlorate; or a nitrite, preferably sodium or potassium nitrite.

9. A process according to any one of the preceding Claims, wherein the reaction is effected in an aqueous medium.

10. A process according to any one of the preceding Claims, wherein the amount of structure-directing agent is from 30 to 80% by weight, based on the total weight of the solvent and structure-directing agent.

11. A process according to any one of the preceding Claims, wherein the structure- directing agent is an organic surfactant compound capable of forming aggregates.

12. A process according to any one of the preceding Claims, wherein the structure- directing agent is cetyl trimethylammonium bromide, cetyl trimethylammonium chloride, sodium dodecyl sulphate, sodium dodecyl sulphonate, sodium bis(2- ethylhexyl) sulphosuccinate, or a sodium soap, preferably sodium laurate or sodium oleate.