The present invention refers to a method for the production of alkane sulfonic acid, in which SO ₃ and an alkane are contacted with each other in the presence of a solvent, said solvent does not constitute a superacid and the combination of said solvent with one or more of the reactants also does not give rise to a superacid.

Different methods for the production of alkane sulfonic acids and especially methane sulfonic acid are known in the prior art. Possible is the production of alkane sulfonic acids by direct sul- fonation of the alkane. This is already disclosed in US 2,493,038, where SO ₃ and ChU are re acted in the presence of a sulfonation catalyst, which is a metal or sulfate of a metal in group II- B of the periodic table, particularly mercury.

Recent publications disclose the reaction of SO3 and ChU with an initiator. In the prior art, e.g. in EP 1 558 353 B1 it is described that ChU and SO3 react with each other in the presence of a radical initiator via a radical pathway to obtain methane sulfonic acid. Marshall's acid and Caro's acid are used as initiators. SO3 is added as oleum.

Recently, it was reported that the reaction between ChU and SO ₃ follows a ionic pathway under super acid conditions (WO 2018/146153 A1). According to the classical definition, a superacid is an acid with an acidity greater than that of 100% pure sulfuric acid, which has a Hammett acidity function (Ho) of -12. According to the modern definition, a superacid is a medium in which the chemical potential of the proton is higher than in pure sulfuric acid. Consequently, the pK _a value is negative.

SO ₃ is usually provided as oleum so that H ₂ SO ₄ is present as solvent in the reaction. The sol vent according to the prior art thus is present to provide one of the educts, namely SO ₃ , in an appropriate manner. But an appropriate solvent may also be able to provide more stable reac tion conditions for the reaction and thus, provide a method for the production with a low rate of side products. The object of the present invention is therefore to provide an improved method for the production of alkane sulfonic acids, especially methane sulfonic acid.

Surprisingly it was now found that stable reaction conditions can be obtained when working un der non-superacid conditions. Thus, the solvent has to be selected extremely carefully. Further, it must not react with alkane, especially methane, and SO ₃ to form side product and thus inhibit the reaction so that no alkane sulfonic acid, especially methane sulfonic acid is formed. Surpris- ingly it was found that selecting an appropriate solvent helps to provide improved reaction con ditions.

In a first embodiment, the object underlying the present invention is therefore solved by a meth od for producing alkane sulfonic acid, in which SO ₃ and an alkane are contacted with each other in the presence of a solvent, wherein said solvent does not constitute a superacid and the com bination of said solvent with SO ₃ and/or the alkane sulfonic acid does not give rise to a superac id.

In a preferred embodiment, the solvent further comprises compounds stabilizing SO ₃ as addi tive.

In a further preferred embodiment, the method is a method for the production of methane sul fonic acid.

In another preferred embodiment, the method is a method as disclosed in WO 2018/096138 A1 with the proviso, that pure SO ₃ is used as one of the educts and one of the solvents according to the present invention is used.

In one preferred embodiment, the present inventions provides for a method for manufacturing alkane sulfonic acids, especially methanesulfonic acid, comprising the following steps:

a. Providing sulfur trioxide SO ₃ ;

b. Providing an alkane, especially methane;

c. Providing a solvent, wherein said solvent does not constitute a superacid and the combi nation of said solvent with SO ₃ and/or the alkane sulfonic acid does not give rise to a su peracid;

d. Bringing into contact SO ₃ , alkane and the solvent in a high-pressure autoclave or laborato ry reactor;

e. Setting a pressure of from 1 to 200 bar;

f. Adding a compound which is able to initialize the reaction between SO ₃ and alkane at the described reaction conditions;

g. Controlling the temperature of the reaction mixture at 0 °C to 100 °C;

h. Letting react the compounds so that the alkane sulfonic acid, especially methane sulfonic acid, is formed.

In the following, the present invention will be described in more detail. Features mentioned for one embodiment can be also used in other embodiments, even if not explicitly mentioned. The present invention for the first time shows that direct sulfonation of methane or any other alkane is possible not only under super acid conditions but also under non-super acid reaction conditions. The solvent according to the present invention is selected such that the solvent alone does not constitute a super acid. It may be, that in the reactor, in which the educts and the solvent react with each other, the solvent and SO ₃ react with each other in any kind or inter act with each other. Also, such a reaction or interaction of the solvent with SO ₃ does not give rise to a super acid according to the present invention. The same is true for the combination of the solvent and the alkane sulfonic acid which is formed during the reaction in the reactor. Thus, the solvent alone does not constitute a super acid. Also, the combination of the solvent and SO ₃ does not give rise to a super acid. Further, the combination of the solvent and the alkane sul fonic acid does not give rise to super acid. Also, the combination of the solvent with SO ₃ and the alkane sulfonic acid does not give rise to super acid within the meaning of the present invention.

A super acid is an acid with an acid strength stronger than that of pure H ₂ SO ₄ . Therefore, in the meaning of the present invention a super acid means an acid with a pK _a value less -3.

The method of the present invention is suitable to produce different kinds of alkane sulfonic ac ids. Preferably, the alkane which is used as educt and from which the respective alkane sulfonic acid is formed, is a short chain methane with 1-10 C atoms which can be linear or non-linear. Preferably, the alkane is selected from methane, ethane, propane, butane, and pentane. Pref erably, the alkane is methane so that the alkane sulfonic acid being formed according to the method of the present invention is methane sulfonic acid.

The reaction between SO ₃ and the alkane in the present solvent usually takes place in a high- pressure autoclave. Thus, the pressure at which SO ₃ and alkane are contacted with each other is preferably in a range of from 1 bar to 200 bar, especially from 50 bar to 150 bar, preferably from 80 bar to 120 bar.

The temperature during the reaction is preferably within a range of from 0 °C to 100° C, espe cially from 15 °C to 80° C, especially preferred from 20 °C to 70° C, preferably from 35 °C to 60° C. Therefore, in a preferred embodiment, the temperature at which SO ₃ and alkane are contact ed with each other is below 70 °C, especially below 65 °C, preferably below 60 °C and especial ly preferred below 55 °C. If the temperature is around 0 °C or 10 °C, the reaction time increases tremendously so that for an economically process the temperature is preferably 20 °C or above, especially 25 °C or above, especially preferred 30 °C or above.

If the preferred reaction conditions are used, the pressure being from 1 bar to 200 bar and the temperature is controlled to be between 0 °C to 100 °C, the solvent used in a method according to the present invention is liquid under the respective conditions. Usually, SO ₃ is provided as liquid so it is homogenously distributed in the solvent. Alternatively, the SO ₃ can also be provid ed as gas. The alkane is added either as a gas or as liquid depending on the length of the C chain. For alkanes with low boiling point, the use of a pressure reactor is therefore usually nec essary. For pentane or higher alkanes, a common laboratory reactor is sufficient. In case of gaseous alkanes, for example methane, a pressure of 1 bar to 100 bar is set.

At the interphase between liquid and gas or in the complete reactor in case of sufficient mixture of the reaction mixture the reaction takes place so that in the reactor there remains a mixture of SO ₃ , alkane, solvent as well as the formed alkane sulfonic acid. According to the present inven tion so that this stage of the reaction, the conditions inside the reactor are non-super acid.

Suitable solvents which may be used in a method according to the present invention are select ed from the group consisting of CO ₂ ; carbonates; linear or non-linear, aromatic or aliphatic, sub stituted or non-substituted C _1-20 alkanes; substituted or non-substituted oligomers of alkanes with a chain length of up to C ₂₀₀ , ionic liquids as well as mixtures of two or more of these.

The advantage of using CO ₂ is that it does not give rise to a super acid together with any of the other compounds beeing present in a reactor. The CO ₂ is liquid under the conditions at which SO ₃ and alkane are contacted with each other. After the reaction is finished, the pressure is usually reduced in the reactor to remove non-reacted alkane, in case the alkane is gaseous. This leads also to a removal of CO ₂ from the reactor, as this will become gaseous as well.

According to the present invention it is possible to remove, in a preferred embodiment where CO ₂ is used as solvent, the CO ₂ completely from the reaction mixture together with a non- reacted alkane. In such a case, only SO ₃ and the formed alkane sulfonic acid remain the reac tor. This reaction mixture can be purified e.g. by distillation. In such a case, SO ₃ will be de stroyed usually with water or an alkohol prior to distillation. The thus obtained mixture of sulfonic acid (where water is used to destroy SO ₃ ) and alkane sulfonic acid can be purified by distillation.

In another embodiment where CO ₂ is used as solvent, CO ₂ is not or not completely removed from the reactor together with the non-reacted alkane but remains in the liquid phase. The liquid phase is afterwards separated so that no more non-reacted alkane is part of this phase. Subse quently, the remaining gas phase - which comprises CO ₂ as well as non-reacted alkane - is set under higher pressure so that the alkane and the CO ₂ can easily be separated. The CO ₂ can then be used again in a reaction according to the present invention as solvent. Also, in cases where the CO2 is removed in a gas phase together with the non-reacted alkane, it is possible to separate the alkane and CO2. This can be done by pressurizing the gas phase as described for the other embodiment above. Alternatively, other separating methods in the gas phase are known in the prior art, which can be used also in the present invention for recycling of C0 ₂ .

Alternatively, carbonates and especially dimethyl carbonate can be used as solvent. Again, this solvent enables non-super acid conditions in a method according to the present invention. Sur prisingly, the reaction is faster if the conditions are non-super acid compared to reaction condi tions described in the prior art where sulfuric acid is used as solvent and thus, super acid condi tions are constituted.

Further suitable solvents according to the present invention are alkanes. Examples of suitable alkanes are short length alkanes with a carbon chain of 1-20 carbon atoms. These could be linear or branched, aromatic or aliphatic, substituted or non-substituted.

Substituted alkanes within the meaning of the present invention means that in a C-H-group hy drogen is replaced by another atom or functional group. Preferably, the substitution is with fluo rine and/or a nitro group and/or sulfonic acid group. It is possible, that not only one hydrogen is replaced by a substituent but that two or more hydrogens are substituted. Also, within the inven tion it is possible that the alkane comprises only one substituent so that a suitable solvent within the meaning is e.g., a fluorinated aliphatic alkane with a C-chain length of 1-20. Alternatively, an also suitable solvent is a linear or branched aliphatic alkane with 1-10 C atoms with NO2 as substituent. Also, alkanes comprising fluorine as well as nitro group and/or sulfonic acid group as substituents are within the scope of the present invention. Suitable example for a nitrated solvent is nitro methane. The alkane can be aliphatic or aromatic. In case the alkane is aro matic, it can be substituted as described above for the aliphatic alkanes either with fluorine or nitro group or sulfonic acid group with two or all the three of them.

Other preferred solvents are oligomers of alkanes with a C-chain length of up to 200. Also, these oligomers can be unsubstituted or substituted. In case they are substituted, the substitu tion is especially with fluorine and/or nitro group as described for the aliphatic short chain al kanes above. The oligomers are usually oily or high viscous liquids and are non-reactive so that the educts do not react with the solvent.

An ionic liquid within the meaning of the present application is a salt in which the ions are poorly coordinated, which results in these solvents being liquid also below 100°C. Examples include compounds based on the 1-Ethyl-3-methylimidazolium (EMIM) cation and include: EMIM:CI, EMIM dicyanamide, (CaHsXChyCsHsN I KCN)^, that melts at -21 °C (-6 °F); and 1-butyl-3,5- dimethylpyridinium bromide which becomes a glass below -24 °C (-11 °F).

The solvent further comprises in a preferred embodiment an additive in an amount of 2 ppm to leg (equivalent), the amount referring to the amount of SO ₃ in the solution. This additive is suit able to stabilize SO ₃ . Such stabilizers are present in a very low concentration but help to handle SO ₃ in the reaction. The additive is preferably selected from the group comprising dimethyl sul phate, dimethyl ether, diethyl ether, amines as well as mixtures of these. Suitable amines are for example trimethyl amine, which is known to help for the handling of SO ₃ . The amount is preferably within the range of from 5 ppm to 1eq, or 10 ppm to 1 eq. If the additive is added to avoid the polymerization (i.e., dimethylsulphate) of SO ₃ , few ppm are sufficient. If the additive forms an adduct with SO ₃ (i.e. amines), the additive should be present in equivalent amounts as the SO ₃ is.

In a very preferred embodiment, the method of the present invention comprises the following steps: a. Providing sulfur trioxide SO ₃ ;

b. Providing an alkane, especially methane;

c. Providing a solvent, wherein said solvent does not constitute a superacid and the combi nation of said solvent with SO ₃ and/or the alkane sulfonic acid does not give rise to a su peracid;

d. Bringing into contact SO ₃ , alkane and the solvent in a high-pressure autoclave or laborato ry reactor;

e. Setting a pressure of from 1 to 200 bar;

f. Adding a compound which is able to initialize the reaction between SO ₃ and alkane at the described reaction conditions;

g. Controlling the temperature of the reaction mixture at 0 °C to 100 °C;

h. Letting react the compounds so that the alkane sulfonic acid, especially methane sulfonic acid, is formed.

Accordingly, a compound is added which initializes the reaction between SO ₃ and the alkane at the described reaction conditions (see step f) above). This compound may be provided in pure form or solved in a suitable solvent with the proviso that this solvent again does not constitute a super acid and the combination of said solvent with SO ₃ and/or the alkane sulfonic acid does not give rise to super acid. Preferably, in cases this compound is added together with the sol vent, this solvent is the same as used in the reaction as a whole. If such a compound is added, the initial molar ratio between this compound and SO3 is in the range of 1 :50 to 1 : 10000, prefer ably 1 : 100 to 1 :500, particularly 1 : 150.

The compound added at step f) of the method according to the above preferred embodiment may be a compound being known to initialize the reaction between an alkane and SO3 to form alkene sulfonic acid under super acid conditions. The compound added during step f) of the inventive procedure is thus a compound comprising a heterolyti cally or homolytically cleavable bond. Suitable compounds with homolytically cleavable bonds are organic or inorganic peroxo- compounds. The compound can be an inorganic peroxoacid or an organic peroxoacid. Organic or inorganic peroxo compounds without acidic function are suitable.

The compound added to initialize the reaction between SO ₃ and alkane may be activated either in the reactor or prior to adding it into the reactor. Such an activation may be possible by irradia tion with UV light or the addition of transition metals. Suitable transition metals are any metals of the d-block of the periodic table. Preferred are Pt, Pd, Rh, Ir, Ag, Au, Fe, etc. Such activation enables a higher reaction speed or the need of lower temperature to obtain the same reactin speed as at higher temperatures.

The compound added at step f) is preferably selected from the group consisting of organic or inorganic peroxides being stable at room temperature, compounds with a heterogeneously or homogenously cleavable bond, as well as mixtures of two or more of them. Suitable compounds are for example disclosed in PCT/EP2017/080495, EP 18157127.4, EP 18196493.3, EP 18196498.2, and EP 18196520.3, the content of which is enclosed herein in its entirety. Fur ther suitable compounds are for example hydrogen peroxide, benzoylperoxide, cumolhydroper- oxide, lauroylperoxide, peroxo acetic acid, tert-butylhydroperoxide, Caro's acid, Marhall's acid, or DMSP (dimethyl sulfoyl peroxide). All of them may be used alone or in mixture. Preferably, only one compound is used in a reaction.

Especially, preferred, a compound is added according to the following formula (I):

ALK-SO2-O-O-X (I) wherein ALK is a branched or unbranched alkyl group, especially a methyl, ethyl, propyl, butyl, isopropyl, isobutyl group, or a higher alkyl group, and X = hydrogen, zinc, aluminium, an alkali or alkaline earth metal. Higher alkyl group within the meaning of the present invention means an alkyl group with 1 to 10 carbon atoms. Preferably, ALK is methyl, ethyl, propyl, butyl, isopropyl or isobu tyl, especially methyl or ethyl, particularly methyl. X is preferably hydrogen, alkali or alkaline earth metal. Particularly, X is hydrogen. Thus, in a very preferred embodiment, the compound added to initialize the reaction between SO ₃ and alkane is CH ₃ -SO ₂ -O-O-H.

Alternatively, the compound added in step f) of the preferred embodiment is an organic peroxo- acid which, where appropriate, comprises functional groups. In general, the peroxoacid accord ing to the invention can be described by the formula R-O-O-H. Without the intention of being bound by theory, it is assumed that the peroxoacid acts by activating sulfur trioxide towards the reaction with an alkane. In a preferred embodiment the peroxoacid comprises at least one or ganic peroxoacid of sulfur, phosphorus, silicon, boron, nitrogen or carbon. Any suitable peroxo acid of said elements can be used. The peroxoacids are typically derived from the correspond ing oxoacid of the respective element.

Preferably, the peroxoacid used as catalyst according to the invention comprises a peroxoacid group corresponding to -E(=X) _m (-YZ) _n -0-0-Z, wherein E is selected from the group consisting of S, P, Si, B, N and C, wherein X and Y may be the same or different and are selected from the group consisting of O and S, wherein m is an integer of from 0 to 2, wherein n is an integer of from 0 to 2, and wherein Z is H, Li, Na and/or K.

In a preferred embodiment of the invention, the peroxoacid group is selected from the group consisting of -SO2-O-O-X, -CO-O-O-X, -P0(0H)-0-0-X, PS(0H)-0-0-X, wherein X is H, Li, Na and/or K. Surprisingly, it has been found that said preferred peroxoacids are particularly suitable as catalyst in the preparation of alkanesulfonic acids from alkanes and sulfur trioxide

According to the invention, the organic peroxoacid comprises at least one additional functional group. The additional functional group may particularly be selected from the group consisting of carbon double bonds, carbon triple bonds, aryl groups, heteroaryl groups and functional groups comprising heteroatoms, especially functional groups comprising O, S, N, P, Si, B, Se, Te, F, Cl, Br, I, Mg or Li atoms.

Particularly preferred are aryl groups, halogen atoms, such as F, Cl, Br, I, and siloxane groups. The functional groups, particularly aryl groups, may be further derivatized and may contain fur ther functional groups. Examples of functional groups according to the invention comprise par ticularly phenyl groups, carbonyl groups, ether groups, thioether groups, thioketone groups and halide groups.

Examples of suitable organic peroxoacids according to the invention are peroxybenzoic acid and trifluoroperacetic acid. Any of the aforementioned examples may be derivatized and/or sub stituted with side chains, particularly with alkyl groups, aryl groups or halogen atoms. In a preferred embodiment, the organic peroxoacid is part of or bound to an organic or inorganic polymer. Any suitable polymer may be chosen. The polymer backbone may constitute the addi tional functional group, particularly in cases where the polymer backbone comprises heteroa toms. Particularly preferred polymers comprise polysiloxanes, polyolefins, vinyl polymers, poly ether, polyester, polyamides and polyurethanes. The peroxoacid group may be bound to the polymeric backbone or may be contained in a polymeric side chain. Particularly preferred are polymers comprising -SO2-OOH groups.

The polymer may have any suitable structure. Particularly, homopolymers, copolymers, block copolymers, graft copolymers or comb copolymers may be employed. The polymers may have a dendrimer structure.

The organic peroxoacid used as a catalyst according to the invention may be obtainable by a reaction of the corresponding oxoacid with a peroxide. More preferably, the peroxoacid may be obtainable by a reaction of the corresponding oxoacid with hydrogen peroxide or a salt thereof. Without the intention of being bound by theory, the reaction of an oxoacid with hydrogen perox ide can for example be described by

-EO _x (OH) _y -OH + H2O2— > -EO _x (OH)y-0-OH + H ₂ 0. (R3)

If such an organic peroxoacid is used as compound, it is only suitable if it does not give rise to superacid conditions in the reactor at which the reaction takes place. This may be controlled by selecting the compound carefully or by using it in only small amounts.

In a further preferred embodiment, the compound is an inorganic peroxoacid or a salt thereof, wherein the peroxoacid is stable at room temperature. Stability at room temperature is particu larly to be understood as stability in a reaction solvent comprising sulfur trioxide and an alkane, especially methane. This solvent may be sulfuric acid. The peroxoacid according to the inven tion must be stable enough in order to act as catalyst in the production of alkanesulfonic acids and not to decompose. Said decomposition may particularly take place by the release of reac tive oxygen species such as superoxide anions (O2-). In this sense, stability of the peroxoacid catalysts of the present invention for example means the absence of the release of reactive ox ygen species such as superoxide anions.

In a preferred embodiment the peroxoacid comprises at least one peroxoacid of boron, silicon, phosphorus, carbon, nitrogen or sulfur. Any suitable peroxoacid of said elements can be used. The peroxoacids are typically derived from the corresponding oxoacid of the respective ele ment.

Preferably, the peroxoacid used as a catalyst according to the invention is obtainable by a reac tion of the corresponding oxoacid with a peroxide. More preferably, the peroxoacid is obtainable by a reaction of the corresponding oxoacid with hydrogen peroxide. Without the intention of be ing bound by theory, the reaction of an oxoacid with hydrogen peroxide can for example be de scribed by

EO _x (OH)y-OH + H2O2— > EO _x (OH)y-0-OH + H ₂ 0. (R3)

In a preferred embodiment, the peroxoacid used according to the invention comprises a poly- protic acid. Particularly, the peroxoacid may consist of one or more polyprotic acids. Said poly- protic peroxoacid comprises one or more peroxy groups, which can be described by -O-O-X, wherein X may be hydrogen and/or an alkaline and/or alkaline-earth metal. More preferably X is hydrogen, lithium, sodium and/or potassium. Most preferably, X is hydrogen.

Preferably, if a polyprotic acid is used, the peroxoacid comprises one or more hydroxyl groups in addition to the one or more peroxy groups. Said hydroxyl groups may be present in form of a salt, i.e., the groups can be described by -O-X, wherein X may be hydrogen, an alkaline metal and/or an alkaline-earth metal. Most preferably X is hydrogen. The replacement of hydrogen with an alkaline-(earth) metal, however, may be particularly necessary to stabilize the peroxo acid as required by the invention.

In a preferred embodiment of the invention, the reaction product of phosphoric acid (H ₃ PO ₄ ) with hydrogen peroxide, the reaction product of boric acid (H ₃ BO ₃ ) with hydrogen peroxide and/or potassium or sodium peroxomonosulfate (KHSO ₅ or NaHSOs) is used as stable inorganic peroxoacid according to the invention. Surprisingly, it has been found that said preferred peroxoacids are particularly suitable as catalyst in the preparation of alkanesulfonic acids from alkanes and sulfur trioxide.

If such an inorganic peroxoacid is used as compound, it is only suitable if it does not give rise to superacid conditions in the reactor at which the reaction takes place. This may be controlled by selecting the compound carefully or by using it in only small amounts. In another embodiment, the compound is a compound comprising a heterolytically cleavable bond between an atom selected from the group consisting of nitrogen, phosphor sulfon oxygen and an atom selected from the group consisting of nitrogen, phosphor and sulfur.

A heterolytically cleavable bond in the sense of the present invention is especially a chemical bond -X-Y- between two atoms X and Y, which may be broken in such a way that the remain ing fragments are not two radicals with unpaired electrons. Particularly, the electrons of the bond are unequally partitioned between atoms X and Y upon cleavage of the bond. The atoms X and Y of the heterolytically cleavable bond may additionally be bound to the same or different radicals. The bond between X and Y may be polarized. Polarization of the bond may enable or favor heterolytical cleavage of the bond. Polarization may, for example, be accomplished by choosing two different elements for atoms X and Y, especially elements with different electro negativities. Polarization of the bond may also be accomplished by choosing different radicals, to which atoms X and Y are additionally bound. These measures may be combined, when X and Y are different and bound to at least two different additional radicals. In principal, any com pound comprising heterolytically cleavable bonds in the aforementioned sense can be em ployed according to the invention. Such compounds are cheaply available from commercial dis tributors.

Heterolytic cleavage of the bond of the inventive catalyst preferably generates a cation and/or an anion. If the cleavage of the bond is catalyzed by an acid, particularly H ⁺ , the anion may for mally react with the acid upon cleavage. In this case, only a cation and a neutral compound are generated.

In yet another embodiment, the compound added at step f) is a cation being stable under acid or super acid conditions as catalyst in the preparation of alkane sulfonic acids from alkanes and sulfur trioxide, especially in the preparation of methane sulfonic acid from methane and sulfur trioxide, said cation being able to react with the alkane to form an alkyl cation. The alkyl cation will afterwards react with sulfur trioxide forming the alkane sulfonic acid. Particularly, methane, ethane, propane, butane, isopropane, isobutane or a higher alkane can be reacted with sulfur trioxide to form the corresponding alkane sulfonic acid. Higher alkane within the meaning of the present application are straight or branched alkanes with 20 C-atoms or less. If the cation is stable, stability in this context means that it is able to react with the alkane but does not decom pose within 24 h at room temperature (20 °C), i.e. the half-life time ti/2 at room temperature is at least 24 h, preferably at least 30 h, especially at least 48 h. Stable cations are formed prior to their use, i.e. prior to their addition into the reactor in which the reaction between alkane and sulfur trioxide takes place. Preferably, one type of cation is used alone and not together with another type of cation.

Alternatively, the cation is produced in situ during the production of the alkane sulfonic acid. In such cases a compound is added to the reaction and the cation is formed according to the above shown reaction (R2). Suitable compounds to be used are halogens, especially and Br2, inter halogen compounds, especially l-Br, or solid elements of the 15 ^th or 16 ^th group of the peri odic table of elements, especially S, Se, Te, P, As, Sb.

The present invention is exemplarily further disclosed in the following example:

100ml methane sulfonic acid were cooled with an ice baths and 20 ml of hydrogen peroxide were added dropwise under extensive cooling to obtain an initiator-solution.

In a 1 gallon (3.75L) high pressure laboratory reactor equipped with a gas-injection-stirrer 400g of pure sulfur trioxide (S03) were dissolved in 2 kg supercritical carbon dioxide as solvent. The reactor was pressurized to 100bar and the temperature of the reaction mixture was set to 50°C. Afterwards the initiator-solution was added dropwise. Everytime the pressure dropped for more than 5 bar the reactor was re-pressurized. The reaction was finished after 12h and was worked up by adding distilled water to quench the unreacted sulfur trioxide.

After releasing the gas, the purification using a distillation-unit yields to the pure product (91 %).