The present invention relates to novel uses of compounds comprising heterolytically cleavable bonds as catalysts in the preparation of alkanesulfonic acids from alkanes and sulfur trioxide, methods for the production of alkanesulfonic acids employing said catalysts as well as reaction mixtures comprising said catalysts. The invention particularly relates to the production of me- thanesulfonic acid from methane and sulfur trioxide employing compounds comprising heterolyt- ically cleavable bonds as catalysts.

Alkanesulfonic acids are organic acids that can reach a similar acid strength as that of inorganic mineral acids, for example, sulfuric acid. However, in contrast to usual mineral acids such as sulfu- ric and nitric acids, the sulfonic acids are non-oxidizing and do not give off vapors that are harmful to health, as can be observed with hydrochloric and nitric acids. Further, many sulfonic acids, for example, methanesulfonic acid, are biologically degradable. The applications of sulfonic acids are many, for example, in cleaning agents, surfactants, galvanic and electronic industry, as catalysts, and in organic synthesis, pharmaceutical chemistry, for example, as protective groups. The salts of sulfonic acids are employed, for example, as surfactants, for example, sodium dodecylsulfonate, or in the electroplating industry, especially as tin, zinc, silver, lead and indium, but also other metal, alkylsulfonates. Furthermore, organic salts are employed in pharmaceutical chemistry. The very high solubility of alkyl sulfonates plays an important role, in particular. Further, no harmful gases are formed in electrolysis, and the use of toxic compounds, for example, cyanide, which is common in many cases, is dispensed with.

The structurally simplest representative of alkanesulfonic acids is methanesulfonic acid. US 2,493,038 describes the preparation of methanesulfonic acid from SO ₃ and methane. US

2005/0070614 describes further methods for preparing methanesulfonic acid, and its application. The methods known in the prior art are in part complicated, cost-intensive, and lead to undesirable products because of the harsh reaction conditions.

The reaction conditions in conventional processes of alkanesulfonic acid production can result in undesirable side products, which even manifest themselves as disturbing inhibitors in the produc- tion of alkanesulfonic acids. This may lead to termination of the actual reaction for preparing the alkanesulfonic acid, but also to impurities, formation of side products and poor yields, based on sulfur trioxide and methane.

WO 2007/136425 A2 discloses the use of the compound di(methanesulfonyl) peroxide (DMSP), which must be prepared by a complex electrolysis and, in addition, is a crystallizable highly explo- sive solid, as an initiator in a reaction in which methanesulfonic acid is formed from sulfur trioxide and methane.

WO 2015/071365 A1 and WO 2015/071455 A1 both describe processes for the sulfonation of alkanes. The main steps are:

1 ) Synthesis of an initiator/initiator-solution. 2) Preparation of a sulfur trioxide-solution (oleum) by dissolving sulfur trioxide in an inert sol- vent (e.g. sulfuric acid)

3) Reaction of oleum with the corresponding alkane after or during addition of the initiator/ initiator- solution in a high-pressure-reactor.

4) Quenching of non-reacted starting material

5) Purification (e.g. distillation, crystallization etc.)

6) Recycling of the inert solvent (e.g. sulfuric acid).

According to said prior art, the initiator is particularly prepared by reacting an alkanesulfonic acid ALK-SO3H, i.e. the desired product, with hydrogen peroxide in order to form an initiator- precursor ALK-SO2-O-OH. Said initiator-precursor is then reacted with SO3 yielding initiator compounds such as ALK-SO2-O-O-SO3H. The cited prior art therefore requires some amount of the desired product to form an initiator.

It is thus the object of the present invention to provide novel catalysts for the preparation of al- kane sulfonic acids, especially methane sulfonic acid (MSA). Particularly, it is the object of the invention to provide catalysts that do not require the desired product itself to be present as an initiator-precursor. Further, requirements for sulfurtrioxide and alkanes should be of no rele- vance, meaning that not only absolute pure raw materials might be used, but that impurities do not affect negatively the reaction.

In a first embodiment, the object of the present invention is solved by the use of a compound comprising a heterolytically cleavable bond between an atom selected from the group consisting of nitrogen, phosphorus, sulphur and oxygen and an atom selected from the group consisting of nitrogen, phosphorus and sulphur, as a 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. Particularly, methane, ethane, propane, butane, isopropane, isobutane or a higher alkane can be reacted with sulfur trioxide to form the corresponding alkane sulfonic acid.

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.

Surprisingly, it has been found that compounds comprising such heterolytically cleavable bonds other than peroxide bonds (-O-O-) show a similar catalytic activity as peroxoacids derived from the desired alkane sulfonic acids. Therefore, not only peroxoacids derived from the desired alkane sulfonic acid may be employed as catalysts in the preparation of alkane sulfonic acids from alkanes and sulfur trioxide and the desired product is not required as a precursor of the catalyst. In principal, any compound comprising heterolytically cleavable bonds in the aforemen- tioned sense can be employed according to the invention. Such compounds are cheaply availa- ble from commercial distributors.

Preferably, the compound comprising a heterolytically cleavable bond is used as a catalyst in a condensed-phase homogeneous process. The catalyst is solved in the same phase as the reac- tants, i.e., an alkane and sulfur trioxide.

In the following, the assumed catalytic cycle is exemplary described for the employment of me- thane as alkane. The same catalytic cycle is assumed to apply to other alkanes. In general, the compound comprising a heterolytically cleavable bond according to the invention can be de- scribed by the formula

R-X-Y-H. Without the intention of being bound by theory, it is assumed that the compound may act by activating sulfur trioxide towards the reaction with an alkane.

In a first step, the compound reacts with sulfur trioxide upon which an activated form of sulfur trioxide is formed:

R-X-Y-H + S0 ₃ — > R-X-Y-SOsH (R1 )

In a second step, said activated form is able to react with methane in order to form methanesul- fonic acid upon which the catalyst compound is regenerated:

R-X-Y-SOsH + CH ₄ — > H ₃ C-SO ₃ H + R-X-Y-H (R2)

It is assumed that the second step follows an ionic pathway and CH ₃ ⁺ cations are produced as intermediates. These CH ₃ ⁺ cations may act as catalysts of their own. In this way, the catalyst according to the invention may act as pre-catalyst.

Generation of CH ₃ ⁺ cations may also occur upon cleavage of the -X-Y- bond without prior reac- tion with SO ₃ . For example and without the intention of being bound by theory, the inventive catalyst may act by forming a cation in a first step:

R-X-Y-R + H ⁺ — > R-X ⁺ + HY-R (R3)

Said cation may react with methane to form a methyl cation in a second step

R-X ⁺ + CH ₄ — > CH ₃ ⁺ + R-XH (R4)

The methyl cation then acts as a catalyst in the production of methanesulfonic acid

CH ₃ ⁺ + S0 ₃ — > H ₃ C-S0 ₃ ⁺ (R5)

H ₃ C-S0 ₃ ⁺ + CH ₄ — > H ₃ C-SO ₃ H + CH ₃ ⁺ (R6)

According to this theory, the catalyst of the present invention may act as an initiator.

In what follows, the present invention is described in its preferred embodiments. The description is meant to be exemplary and not to limit the scope of the invention. In a preferred embodiment the heterolytically cleavable bond is a single bond or a double bond. Alternatively, the heterolytically cleavable bond may also be a triple bond. Particularly preferred are single bonds.

The bond is preferably heterolytically cleavable under acidic conditions. Particurly preferred are bonds, which are heterolytically cleavable under superacid conditions. Alternatively, the bond may be heterolytically cleavable at a pH value of 3 or less, preferably of 0 or less. The pH value is preferably at least -10.

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.

The compound, which is used as catalyst according to the invention, is preferably selected from the group consisting of R ¹ -N-N-R ² , R ¹ -N-0-R ² ,

R ¹ -N-P-R ² , R ¹ -S-N-R ² , R ¹ -P-P-R ² , R ¹ -P-0-R ² , R ¹ -S-P-R ² , R ¹ -S-0-R ² and R ¹ -S-S-R ² , wherein R ¹ and R ² may be the same or different and are selected from the group consisting of hydrogen, organic radicals and inorganic radicals. Particularly if the catalyst is chosen from R ¹ - N-N-R ² , R ¹ -P-P-R ² and R ¹ -S-S-R ¹ , R ¹ and R ² may be different in order to polarize the bond and favor heterolytic cleavage of the bond.

In a preferred embodiment R ¹ and/or R ² are selected from the group consisting of -E(=X) _m (- YZ) _n , -E(=X) _m (-YZ)n-R ³ , -E(=X) _m (-YZ) _n -R ³ R ⁴ and-E(=X) _m (-YZ) _n -R ³ R ⁴ R ⁵ , wherein R ³ , R ⁴ and R ⁵ are each directly bonded to E, may be the same or different and are selected from the group consisting of hydrogen, organic radicals and inorganic radicals, 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 3 and wherein Z is H, Li, Na and/or K.

In these preferred embodiments, the atom X and/or the atom Y, which form the heterolytically cleavable bond, may each be directly bound to an atom E selected from S, P, Si, B, N and C. Depending on the valence of atom E, said atom may additionally be bound to up to three further radicals R ³ , R ⁴ and R ⁵ . The place of the radicals R ³ , R ⁴ and R ⁵ can alternatively be filled with oxygen or sulphur atoms, which are double bonded to E, or groups YZ, which primarly corre- spond to OH and SH groups and their derivates. The inventive catalyst compound may there- fore, for example, be a derivate of an oxoacid of sulfur, phosphorus, silicon, boron, nitrogen or carbon.

In a particularly preferred embodiment of the invention, R ¹ and/or R ² are selected from the group consisting of -S0 ₂ -R ³ , -SO2OH, -CO-R ³ , -PO(OH)-R ³ ,

-PS(OH)-R ³ , Si(OH) ₃ , -Si(OH)2-R ³ , S1H3 and -S1H2-R ³ . Surprisingly, it has been found that such compounds are particularly suitable as catalyst in the preparation of alkanesulfonic acids from alkanes and sulfur trioxide. Each of the aforementioned radicals R ¹ , R ² , R ³ , R ⁴ and R ⁵ may preferably be individually select- ed from the group consisting of alkyl radicals, alkyl radicals substituted with one or more func- tional groups, siloxane radicals or any other suitable inorganic or organic radical.

Preferred alkyl radicals are branched or unbranched alkyl radicals with a carbon number of 1 to 20, especially 1 to 10, particularly methyl, ethyl, propyl, butyl, isopropyl, isobutyl or higher alkyl radicals.

The aforementioned additional functional groups 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.

In a preferred embodiment, the catalyst compound according to the invention is part of or bound to an organic or inorganic polymer. Any suitable polymer may be chosen. Particularly preferred polymers comprise polysiloxanes, polyolefins, vinyl polymers, polyether, polyester, polyamides and polyurethanes. The catalyst compound group may be bound to the polymeric backbone or may be contained in a polymeric side chain.

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.

In an alternative embodiment, the object of the invention is solved by a process for the prepara- tion of alkanesulfonic acids from alkanes and sulfur trioxide comprising the steps of

i. providing sulfur trioxide;

ii. reacting the sulfur trioxide with an alkane, especially methane, in a high-pressure autoclave or laboratory reactor;

iii. setting a pressure of from 1 to 200 bar;

iv. introducing a compound comprising a heterolytically cleavable bond between an atom se- lected from the group consisting of nitrogen, phosphorus, sulphur and oxygen and an atom selected from the group consisting of nitrogen, phosphorus and sulphur; v. controlling the temperature of the reaction mixture at 0 °C to 100 °C;

vi. if necessary purifying the reaction product, for example, by distillation or extraction.

The inventive process differs from similar processes from the prior art in that in step iv) a corn- pound comprising a heterolytically cleavable bond between an atom selected from the group consisting of nitrogen, phosphorus, sulphur and oxygen and an atom selected from the group consisting of nitrogen, phosphorus and sulphur, is added as catalyst. Said compound corre- sponds to the abovementioned compound comprising a heterolytically cleavable bond, which may be used as catalysts.

Sulfur trioxide may be provided in the form of oleum, i.e., a solution of sulfur trioxide in sulfuric acid. Instead of oleum also pure sulfur trioxide can be employed. This avoids the preparation of sulfur trioxide solutions. The reaction conditions are here without added solvents. Further, non- reacted sulfur trioxide can evaporate, avoiding the necessity of quenching it.

In a further embodiment, sulfur trioxide is used in a form of oleum with a trioxide content of 50 % (w/w) or less, or 65 % (w/w) or more. Surprisingly it has been found that for the processes of the present invention also oleum with a sulfur trioxide content of 65 % (w/w) or more, especially of 70 % w/w or more can be used without negatively affecting the inventive process. Even pure sulfur trioxide (100 % (w/w) sulfur trioxide) may be used.

The temperature during the reaction is preferably within a range from above 0 °C to 70 °C, es- pecially from 10 °C to 65 °C, preferably from 20 °C to 60 °C. Surprisingly the formation of side products was lower at lower temperatures. If the temperature is around 0 °C or 10 °C, the reac- tion takes place but needs a longer time so that for an economically process the temperature is preferably 20 °C or above, especially about 40 °C to 55 °C.

The pressure is set to be within a range from 1 to 200 bar, preferably from 50 to 150 bar, espe- cially from 80 to 120 bar.

Due to the advantages being connected with the use of pure sulfur trioxide mentioned above, the use of pure sulfur trioxide is preferred in the process for manufacturing alkane sulfonic acids according to the present invention. As contrary to the prior art, a circulation of solvent is not necessary, alkanes comprising higher amounts of impurities compared to the prior art can be used. Impurities usually are enriched in the solvent leading to a reduced yield of alkane sulfonic acids. By avoiding solvents and thus a circulation of them, impurities originating from the al- kanes are not negatively influencing the production of alkane sulfonic acids when pure sulfur trioxide is employed.

Sulfur trioxide, especially pure sulfur trioxide is reacted with an alkane in a reactor. For alkanes with a low boiling point, the use of a high-pressure reactor is necessary. For pentane and higher alkanes, a common laboratory reactor is sufficient. In the case of gaseous alkanes, for example, methane, a pressure of 1 to 200 bar gas pressure is set.

Subsequently, the catalyst compound according to the present invention is added. The catalyst may be provided in pure form or solved in a suitable solvent. Preferably, the initial molar ratio between the catalyst and SO ₃ is in the range of 1 :50 to 1 : 10000, more preferably 1 :100 to 1 :500, particularly 1 :150. The catalyst may be provided in a solvent, particularly in sulfuric acid.

After the reaction has taken place, the reaction mixture contains essentially of the respective al- kane sulfonic acid, especially methane sulfonic acid, as well as sulfuric acid. This mixture of alkane sulfonic acid, especially methane sulfonic acid (MSA), and H2SO4 might afterwards be used as the respective mixture. The combination of an alkane sulfonic acid, especially methane sulfonic acid, and sulfuric acid provides a strong acid in which even gold might be dissolved enabling different fields of technical applicability.

Alternatively, the alkane sulfonic acid, especially MSA, might be separated i.e. the method of the invention comprises the optional step of the purifying the reaction product, which might be done by distillation or extraction.

But also alkane sulfonic acids, and specially methane sulfonic acids, might be used in different technical fields, i.e. as cleaning agent (cleaning comprising the area of cleaning and caring, home care as well as industrial and institutional cleaning of hard and soft surfaces, i.e. in dishwashing, commercial laundry, cleaning and sanitation, vehicle and transportation care, concrete cleaning, membrane cleaning, and others), for regeneration of ion exchange resins, in galvanic proceedings, in the area of oil, gas, mining, treatment of metals and/or their surfaces, in different areas of phar- maceutical, chemical and agro-chemical industry or in the production of biodiesel. MSA might also be used in galvanization process of plastics, the broad area of batteries, such as lead battery recy- cling and recycling in general, such as metal recycling, as well as borane generation are further possible areas of application.

In an alternative embodiment, the object of the invention is solved by a mixture comprising an alkane, sulfur trioxide, a compound comprising a heterolytically cleavable bond between an at- om selected from the group consisting of nitrogen, phosphorus, sulphur and oxygen and an at- om selected from the group consisting of nitrogen, phosphorus and sulphur, and optionally a solvent. The inventive mixture is capable of producing an alkane sulfonic acid. Particularly, if the mixture is set at a pressure of 1 to 100 bar and held at a temperature of 0 to 100 °C, an alkane sulfonic can be produced in an efficient way. The compound comprising a heterolytically cleav- able bond acts as a catalyst.

In a preferred embodiment, the alkane is methane. Such a preferred mixture is capable of form- ing methane sulfonic acid.