WO2020048965A1

WO2020048965A1 - Method for the production of alkane sulfonic acids

Info

Publication number: WO2020048965A1
Application number: PCT/EP2019/073444
Authority: WO
Inventors: Timo Ott; Ingo Biertuempel
Original assignee: Basf Se
Priority date: 2018-09-04
Filing date: 2019-09-03
Publication date: 2020-03-12

Abstract

The present application refers to a method to produce alkane sulfonic acids as well as a device for carrying out said method.

Description

Method for the production of alkane sulfonic acids

Alkane sulfonic acids are organic acids that can reach a similar acid strength as that of inorgan- ic mineral acids, for example, sulfuric acid. However, in contrast to common mineral acids such as sulfuric and nitric acids, sulfonic acids are non-oxidizing and do not produce vapors that are harmful, as can be observed with hydrochloric or nitric acids. Furthermore, many sulfonic acids are biologically degradable. The applications of sulfonic acids are many, for example, in clean- ing agents, surfactants, galvanic and electronic industry, as catalysts, and in organic synthesis, pharmaceutical chemistry, for example, as protective groups, and others. Also the salts of sul- fonic acids are employed, for example, as surfactants, for example, sodium dodecyl sulfonate, or in the electroplating industry, especially as tin, zinc, silver, lead and indium, but also other metal, alkyl sulfonates.

The reaction conditions in conventional processes of alkane sulfonic acid production can result in undesirable side products, which can even manifest themselves as disturbing inhibitors in the production of alkane sulfonic acids. WO 2007/136425 A2 discloses the use of DMSP, which must be prepared by complex electrolysis and, in addition, as a crystallizable highly explosive solid, as an initiator reaction in which sulfonic acid is formed from sulfur trioxide and methane.

It is an object of the present application to provide a method to produce alkane sulfonic acids ALK-SO₃H enabling the production of the pure sulfonic acids. Preferably, the water content is less than 1 wt.-%, and the amount of sulfide in the alkane sulfonic acid is 100 ppm or less, pref- erably 50 ppm or less.

The object of the present invention is solved by a method as claimed in claim 1 and a device as claimed in claim 10. Preferred embodiments are mentioned in the dependent claims. In a first embodiment, the object of the present invention is therefore achieved by a method to produce alkane sulfonic acid ALK-SO₃H comprising the following steps: a. providing an alkane,

b. providing SO₃, and

c. providing a solution comprising an initiator or an initiator precursor enabling the reaction of the alkane with SO₃ to the alkane sulfonic acid,

d. bringing into contact the alkane, SO₃ and the initiator or the initiator precursor in a reaction chamber at a temperature T₁ and a pressure pi,

e. allowing the reaction mixture to react in the reaction chamber to obtain the alkane sulfonic acid in a mixture with sulfuric acid and potentially SO₃,

f. transferring the mixture of alkane sulfonic acid, sulfuric acid, and potentially SO₃ to a distillation means,

g. removing the SO₃ of the mixture, if necessary,

h. distilling the mixture of g) in a distillation means at a temperature T₂ and a pres- sure P₂ to obtain the pure alkane sulfonic acid and, as distillation residue, a mix- ture of alkane sulfonic acid and sulfuric acid,

i. if necessary diluting the pure alkane sulfonic acid of step h) with water from a concentration ci to a concentration c₂ and filling it in suitable packing units.

In another embodiment the present invention provides for a device for carrying out the process according to the present invention. Said device comprises: i. independently from each other devices to provide the educts: alkane, SO₃, and a so- lution comprising an initiator or an initiator precursor;

ii. at least one reaction chamber to allow the alkane, SO₃, and a solution comprising an initiator or an initiator precursor to come into contact and to react with each other;

iii. at least one distillation means;

iv. if the alkane is gaseous, at least one gas entrainment impeller inside the at least on reaction chamber mentioned under ii.;

v. where appropriate, at least one tank for providing a coolant;

vi. where appropriate, at least one tank for providing water;

vii. where appropriate, at least one tank for thermal oil; and

viii. where appropriate, at least one exhaust gas purification device. These embodiments as well as preferred embodiments are disclosed in the following in more detail. Any feature disclosed for one embodiment is also meant to be disclosed for any other embodiment. Each feature can be combined with any other feature without limitation.

The present invention therefore provides for a method to produce alkane sulfonic acids. Said alkane sulfonic acids are abbreviated with ALK-SO3H in the present application. The alkane ALK can be a branched or unbranched alkyl group with 1 -20 C-atoms. Preferably, ALK is a me- thyl, ethyl, propyl, butyl, isobutyl, or higher alkyl group. Higher alkyl group within the meaning of the present application refers to an alkyl group with 1-15 C-atoms, preferably with 1-12 C- atoms, especially preferred with 1-10 C-atoms, preferred with 1-5 C-atoms. If the alkane is sub- stituted, one or more H-atoms of the alkyl group are substituted with a halogen. Especially pre- ferred within the meaning of the present application, ALK is methyl so that the present applica- tion in a preferred embodiment refers to a method to produce methane sulfonic acid.

In the following the inventive method as well as the device according to the present invention as well as its preferred embodiments are described in more detail and particularly with respect to the functionalization of methane to methane sulfonic acid. As far as the description relates to methane and methane sulfonic acid, it is not meant to limit the scope of the invention to me- thane. The same considerations apply mutatis mutandis to the employment of other alkanes, which are also within the scope of the invention. Methane is merely chosen as an illustrative example, although it is also preferred as alkane. By replacing hydrogen atoms with alkyl substi- tutes, methane can be converted to other alkanes.

The alkane which is provided in step a) of the method according to the present invention, is one of the necessary educts of the reaction; the other one is SO₃. Said SO₃ might be provided as pure SO₃ (SO₃ content: 100%). This avoids the preparation of sulfur trioxide solutions. The reac- tion conditions are here without added solvents. Furthermore, non-reacted sulfur trioxide can be evaporated, avoiding the necessity of quenching. Alternatively, sulfur trioxide can be used in a solution or as oleum (optionally with a sulfur trioxide content of 50 % (w/w) or less, or 65 %

(w/w) or more). Surprisingly, it has been found that contrary to the prior art for the processes of the present invention, 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. Moreover, pure sulfur trioxide (100 % (w/w) sulfur trioxide) may be used. The sulfur trioxide content in solu- tion of oleum is preferably within the range from 15 % (w/w) to 99 % (w/w), preferably from 25 % (w/w) to 95 % (w/w), especially preferred from 35 % (w/w) to 90 % (w/w). SO₃ contents below 15 % (w/w) will also result in the formation of alkane sulfonic acids but the reaction time will be very long and the reaction will become uninteresting due to economic reasons. Especially in cases where pure SO₃ is used as one of the educts, it may be advisable to use a solvent. Said solvent is preferably H2SO4.

To initiate the reaction between the alkane and SO3, there is a need of an initiator or an initiator precursor to enable the said reaction. Said initiator is selected from the group of Hydrogen Per- oxide, Caro’s acid HS03-0-0-H, Marshall’s acid HS03-0-0-S03H. It is preferably selected from ALK-SO2-O-O-SO2-OX or ALK-S02-0-0-S02-ALK (ALK is a branched or unbranched alkyl group, especially a methyl, ethyl, propyl, butyl, isopropyl, isobutyl group, or a higher alkyl group as defined above, and X = hydrogen, zinc, aluminium, an alkali or alkaline earth metal; with ALK = methyl and X=H being preferred), or a compound comprising at least one inorganic peroxoacid or a salt thereof. Using an asymmetric precursor with ALK = methyl and X = H the reaction is sur- prisingly improved compared to the known symmetric DMSP as initiator with respect to selectivi- ty and rates. Alternatively, a mixture of two or more of the initiators named above can be used.

The initiator may be provided in pure form or solved in a suitable solvent. Preferably, the initial molar ratio between the initiator and SO3 is in the range of 1 :50 to 1 :10000, more preferably 1 :100 to 1 :500, particularly 1 :150. The initiator may be provided in a solvent, particularly in sul- furic acid, Alkanesulfonic acid, e.g. methanesulfonic acid or a mixture of Alkanesulfonic acid and sulfuric acid.

The initiator ALK-SO2-O-O-SO2-OX may be prepared by reacting an alkanesulfonic acid R- SO3H, i.e. the desired product, with hydrogen peroxide to form an initiator-precursor R-SO2-O- OH. Said initiator-precursor can further react with SO3, for example in the reaction mixture, yielding initiator compounds such as R-SO2-O-O-SO3H. Hence, some amount of the desired product to form an initiator is required.

Alternatively, a compound comprising at least one inorganic peroxoacid or a salt, wherein the peroxoacid is optionally stable at room temperature, may be used as precursor. (Stability at room temperature is particularly to be understood as stability in a reaction solvent comprising sulfur trioxide and an alkane, especially methane.) The inorganic peroxide may be provided in pure form or solved in a suitable solvent This solvent may be sulfuric acid, akanesulfonic acid, e.g. methanesulfonic acid or a mixture of Alkanesulfonic acid and sulfuric acid. The peroxoacid according to the invention must be stable enough to act as initiator in the production of al- kanesulfonic acids and not to decompose. According to the invention, the peroxoacid is used as an initiator in a homogeneous condensed- phase process. The peroxoacid initiator is solved in the same phase as the reactants, i.e., an alkane and sulfur trioxide.

In the following, the assumed reaction cycle is exemplary described for the employment of me- thane as alkane. The same reaction cycle is assumed to apply to other alkanes. In general, the peroxoacid according 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 first step, the peroxoacid reacts with sulfur trioxide upon which an activated form of sulfur trioxide is formed:

R-O-O-H + SOs— > R-O-O-SOsH (R1 )

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

R-O-O-SOsH + CH₄— > H3C-SO3H + R-O-O-H (R2)

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 EO_x(OH)^OH used as an initiator or catalyst according to the inven- tion is obtainable by a reaction 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 being bound by theory, the reaction of an oxoacid with hydrogen perox- ide can for example be described by

EOx(OH)^OH + H2O2— > E0x(0H)^0-0H + H₂0. (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 (H₃PO4) with hydrogen peroxide, the reaction product of boric acid (H₃BO₃) with hydrogen peroxide and/or potassium peroxomonosulfate (KHSOs) is used as stable inorganic peroxoacid according to the invention. Surprisingly, it has been found that said preferred peroxoacids are particularly suita- ble as catalysts in the preparation of alkanesulfonic acids from alkanes and sulfur trioxide. Cata- lyst and initiator are used as equivalent within the meaning of the present invention.

Alternatively, and also within the meaning of the present invention, a precursor of the initiator (initiator precursor) can be added together with further substances which then react in the reac- tion chamber with the initiator which afterwards enables the reaction of the alkane with the SO₃ to the alkane sulfonic acid. Thus, adding an initiator precursor in step d) inside the reaction chamber enables the in-situ formation of the precursor directly in the reaction chamber. This is an advantage, as there is no need to synthesize the initiator in advance. The initiator precursor can be any substance from which an initiator as defined above can be formed in-situ in the re- action chamber. The concentration of the initiator precursor is preferably 5 mol-% or less, espe- cially 3 mol-% or less, especially preferred 2 mol-% or less or 1 mol-% or less - based on the amount of SO₃.

Preferably, the initiator precursor is a compound of the 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 as defined above, and X = hydrogen, zinc, alumin- ium, an alkali or alkaline earth metal, as an initiator-precursor for preparing alkanesulfonic acids, especially methanesulfonic acids from alkane, especially methane, and sulfur trioxide.

Accordingly, the alkane ALK can be a branched or unbranched alkyl group with 1 -20 C-atoms. Preferably, ALK is a methyl, ethyl, propyl, butyl, isobutyl, or higher alkyl group. Higher alkyl group within the meaning of the present application means an alkyl group with 1 -15 C-atoms, preferably with 1 -12 C-atoms, especially preferred with 1 -10 C-atoms or 1 -5 C-atoms. Especially preferred within the meaning of the present application, ALK is methyl so that the present appli cation in a preferred embodiment refers to a method to produce methane sulfonic acid.

The initiator-precursor (e.g. alkane sulfonic hydroperoxide) that reacts“in situ” to a suitable initi- ator is added to this solution. The initiator-precursor is preferably prepared by reacting an al- kane sulfonic acid or a solution of such alkane sulfonic acid with hydrogen peroxide to the al- kane sulfonic hydroperoxide according to the reaction scheme 1 and can optionally be isolated but isolation is not necessary:

Reaction scheme 1 : ALK-SO2-OH + H2O2 ® ALK-SO2-O-O-H + H2O.

The alkane sulfonic hydroperoxide (initiator-precursor) reacts“in situ” during the addition to the reactor to an alkane sulfonyl peroxo sulfate according to reaction scheme 2:

Reaction scheme 2: ALK-SO₂-O-OH + SO₃ ® ALK-SO₂-O-O-SO₂-OH.

(“in situ”-reaction)

In such a case the concentration of the hydrogen peroxide may be 20 to 100% (w/w). Subsequent- ly, the reaction is completed at 0 to 100 °C. The raw product can be processed by extraction, crys- tallization, distillation or chromatography.

In a further preferred embodiment, the initiator precursor is a mixture comprising hydrogen perox- ide and an inorganic oxoacid as a mixture. Optionally, the mixture may comprise a solvent.

The oxoacid used in the mixture according to the invention must be suitable to yield a stable peroxoacid upon the reaction with hydrogen peroxide. In this alternative embodiment, the initia tor compound is produced in situ in the reaction mixture. Particularly suitable oxoacids comprise oxoacids of boron, silicon, phosphorus and/or sulfur.

The oxoacid may be a monoprotic or a polyprotic acid. Particularly, if a polyprotic acid is used, only a part of the hydroxyl groups may be replaced by peroxy groups upon the reaction with hydrogen peroxide. Preferably, boric acid (H₃BO₃) and/or phosphoric acid (H₃PO₄) are used in a mixture according to the invention.

Preferably, the oxoacid and hydrogen peroxide are added in a molar ratio of 1 :5 to 5:1 , more preferably in a molar ratio of 1 :2 to 2:1 , most preferably in a molar ratio of 1 :1. The initial molar ratio between the oxoacid and the SO₃ is preferably in the range of 1 :50 to 1 :10000, more preferably in the range of 1 :100 to 1 :500.

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, especially 50 to 150 bar, preferably 80 to 125 bar gas pressure is set.

Selectivity and reactivity of the reaction are dependent on the initiator or initiator precursor used in the process. Therefore, in one embodiment, the process according to the present invention further includes the step of providing a regulator between steps c) and d). In such an embodi- ment step d) reads:

d. bringing into contact the alkane, SO₃ and the initiator or the initiator precursor to- gether with a regulator in a reaction chamber at a temperature T1 and a pressure Pi.

In such a preferred embodiment, the process according to the present invention comprises thus the following steps:

a. providing an alkane,

b. providing SO₃, and

c. providing a solution comprising an initiator or an initiator precursor enabling the reaction of the alkane with SO3 to the alkane sulfonic acid,

e. allowing the reaction mixture to react in the reaction chamber to obtain the alkane sulfonic acid in a mixture with sulfuric acid and potentially SO3,

g. removing the SO₃ of the mixture, where appropriate,

h. distilling the mixture of g) in a distillation means at a temperature T₂ and a pres- sure P2 to obtain the pure alkane sulfonic acid and, as distillation residue, a mix- ture of alkane sulfonic acid and sulfuric acid,

j. if necessary diluting the pure alkane sulfonic acid of step h) with water from a concentration ci to a concentration c₂ and filling it in suitable packing units. The regulator enables to regulate the reaction between an alkane and SO₃ to be either faster or more selective in view of the formation of alkane sulfonic acids compared to reactions without the presence of a regulator, depending on the needs of the producer. For the reaction, if the same initiator is used, only the addition of the regulator helps to either enhance the reactivity or the selectivity or even both.

The regulator of the present invention might be any transition metal, metal or metalloid of any of groups 3 to 14 of the periodic table. Such transition metal, metal or metalloid might be used in its elemental form. It might also be used as salt or as oxide. Furthermore, mixtures of two or more of the transition metals or metals or metalloids might be used. Within the meaning of the present application is that two transition metals or metals or metalloids might be mixed with each other. Also, they can be present in their elemental form or as salts or as oxides. Thus, for example a metal oxide might be mixed with a metal, or a transition metal might be mixed with a metal, or a metalloid oxide might be mixed with a metal, or a metalloid might be mixed with a metal salt, or any other combination is within the meaning of the present application.

Mixtures within the meaning of the present application also encompass that two or more transi- tion metals or metals or metalloids might form another regulator compound. For example, zeo- lites or any other aluminumsilicates can act as regulators within the meaning of the present ap- plication.

Preferably, the regulator is selected from an element of any of groups 7 to 12 of the periodic table, preferably of any of groups 8 to 11. Especially preferred are elements of groups 9, 10 or 1 1 of the periodic table. Especially preferred are Pt, Pd, Ir, Rh, Ru, Ag or mixture of these. On each of these preferred embodiments, the mentioned elements can be present in their ele- mental form or as salts or as oxides. Suitable salts are alkaline or earth alkaline metal salts. Thus, the salts and/or oxides of Pt, Pd, Ir, Rh, Ru, Ag are especially preferred.

Surprisingly, it has been found that a regulator carrying free hydroxyl groups on its surface is especially preferred. Without being bound to theory such hydroxyl groups on the surface of a regulator might be able to activate SO₃ or stabilize any intermediates which are formed during the reaction between the alkane and SO₃. The hydroxy functionalization on the surface of the regulator thus enables a higher reactivity and faster reactions to obtain alkane sulfonic acids, especially methane sulfonic acid. Suitable regulators are for example alumina, especially y-AhCh, silica (S1O2), zirconia, zeolites, Pt, Pd, Ir, Rh, Ru, Ag, as well as mixtures of those. Especially alumina, silica, zirconia, zeolites might provide free hydroxyl groups on the surfaces. Preferably, they also provide large surfaces for the reaction to take place. The surface might be provided in terms of the specific surface area according to the BET theory. The BET theory aims to explain the physical adsorption of gas molecules on a solid surface and serves as the basis for the measurement of the specific surface area.

Alumina, especially y-AhCh, silica (S1O2), zirconia or zeolites might be used alone or together with another metal, especially together with any of Pt, Pd, Ir, Rh, Ru and/or Ag.

For example, y-AhCh might be used together with Rh, whereas the Rh is located on the y-AhCh. Instead of Rh, Pt and/or Ir and/or Ru and/or Ag might be used together with y-AhCh. Rh or any of the other metals might, without being bound to that theory, activate the alkane, especially methane, for a faster reaction with SO₃.

Surprisingly, it has been found that the addition of a transition metal, or metal or metalloid in its elemental form enhances the yield of the reaction by means of improvement of selectivity. Addi- tion of the salts or the oxides of the transition metal or metal or metalloid leads to an enhance- ment of the reactivity resulting in a faster reaction.

The initiator or the initiator precursor is then added into a reaction chamber together with the alkane and SO₃ and, if needed, the regulator, so that all these components are in contact with each other. The temperature inside the reaction chamber is the temperature T1 and the pres- sure within the reaction chamber is pressure pi.

The temperature T1 is preferred within a range from 0°C to 100°C, preferably from 10 °C to 75 °C, especially from 20 °C to 60 °C, preferred from 25 C to 55 °C, especially preferred from 30 °C to 50 °C.

The pressure pi is preferably within a range from 1 to 200 bar, preferably within a range from 10 or from 20 to 150 bar, preferably from 50 to 100 bar.

Said temperature T1 and said pressure pi enable the reaction between the components of the reaction mixture. Higher pressures are not necessary and would only increase the energy con- sumption. The same is true for higher temperatures which would then also lead to the formation of side reactions. The reaction time is usually within a range of 30 min to 30 h, preferably from 1 h to 20 h, espe- cially from 2 h to 10 h. (The reaction time may also be within a range of 30 min to 16 h.)

The educts, being the alkane, SO₃, the initiator or the initiator precursor and, if needed, the reg- ulator remain in the reaction chamber or set of reaction chambers until the reaction is complet- ed. During said reaction, the alkane sulfonic acid is formed. As a further product, sulfuric acid may be formed and usually residual SO₃ is also part of a mixture obtained from the reaction.

Said mixture, comprising alkane sulfonic acid, sulfuric acid and SO₃, is afterwards transferred to a distillation means. Prior to distilling the mixture, SO₃ must be removed.

If SO₃ is within the mixture and is not removed, there will be a reaction between SO₃ and the alkane sulfonic acid forming the anhydride of the sulfonic acid. For example, if methane sulfonic acid is formed, methane sulfonic acid will react with SO₃ to methane sulfonic anhydride and sulfuric acid. The anhydride decomposes under the influence of heat and/or UV light to SO₂ (gaseous) and H₃C-SO₂-O-CH₃, which is a toxic component. Said reaction is thus of course to be avoided.

Therefore, it is necessary to remove SO₃ from the mixture prior to the distillation. The removal is preferably obtained by the addition of water to the mixture. It is important to do not add large amounts of water as this will complicate the distillation of the pure methane sulfonic acid. Thus, a large amount of water has a negative influence on the method of the present invention. On the other side, if the amount of water is not enough, SO₃ will remain in the mixture leading to the unwanted side reaction shown above. The addition of water can be done at reaction pressure or at normal pressure, or at any pressure in between. Particularly if done at pressures above nor- mal pressure, methane leaving the reaction mixture upon decompression after quenching of residual S03 with water does not contain any S03 then. Such a methane stream could be used as such, or undergo further treatment, but the treatment effort is significantly reduced if no or just minimum amounts of S03 are left, e.g. < 500 ppm, preferred < 200 ppm, more preferred < 100 ppm.

To control the amount of water to be added, the conductivity of the mixture can be measured during the addition of water. An increase in conductivity shows that an excess of water is in the mixture. At this point, the addition of water can be stopped. Alternatively, IR, RAMAN, UVA/is, NMR, speed of sound or other analytical methods can be used to determine the amount of wa- ter necessary to be added into the reaction mixture. In a preferred embodiment, not only the conductivity but also the density and the sound velocity of the mixture are measured either during the transfer in step f) or prior to the distillation in step h) of the method according to the present invention. The measure of the conductivity enables to control the removal of SO3 of the mixture, as shown above. Measuring density and sound veloc- ity enable to define the composition of the mixture alkane sulfonic acid/sulfuric acid/SOs/water.

Measuring conductivity, density and sound velocity thus enables the gathering of information about the reaction which took place in the reaction chamber and helps to define the conditions for the distillation. With the method according to the present invention, the amount of water to be added to destroy the excess SO₃ can be controlled in detail. Adding water to SO₃ usually results in an explosive reaction. Due to the small amount of SO₃ being present in the mixture the heat coming from the reaction is distributed within all the mixture, so that potentially explo- sive or other dangerous side reactions can be avoided.

After SO₃ is removed from the mixture, the mixture is distilled at a temperature T₂ and a pres- sure P2 to obtain the pure alkane sulfonic acid. The temperature T₂ is preferably within a range of from 160°C to 220°C, preferably from 175°C to 215°C. The pressure p₂ is preferably within a range of from 1 to 30 mbar, preferably from 3 to 20 mbar and most preferably from 5 to 15 mbar.

These temperature and pressure values enable the production of pure alkane sulfonic acid. If the temperature is above 220°C and especially if it is above 215°C, the methane sulfonic acid will dehydrate and the anhydrate of the alkane sulfonic acid will formed which might then de- grade to a toxic composition as discussed above for cases in which SO₃ is still present. Fur- thermore, the anhydrates are usually solid and should therefore be avoided in the distillation process.

The mixture to be distillated and the residues from the distillation are preferably circulated dur- ing the distillation process to maintain the temperature T₂. Preferably, the mixture is passed to a heat exchanger during the circulation. Said heat exchanger preferably is a SiC heat exchanger.

Distilling the mixture obtained from the reactor is a crucial step in the method according to the present invention. There are several conditions under which an alkane sulfonic alkyl ester is formed. Said ester being in most of the cases toxic. Thus, this reaction has to be avoided. The inventors found that the ester is formed if - the temperature inside the reactor is too high, even if it is only at a local spot due to in- homogeneous heating of the reactor,

- the concentration of SO₃ of the solution to be distilled is too high,

- the concentration of alkane sulfonic acid of the solution to be distilled is too high,

- the temperature during distillation is too high, and/or

- certain catalysts, such as Lewis acids or transition metals, catalyze the reaction to- wards the ester.

Therefore, it is necessary to have a homogenous temperature distribution during all of the reac- tion, starting from the temperature of the educts, the temperature during the reaction in the re- actor as well as during distillation. To obtain said homogenous temperature, all liquid compo- nents are pumped in circles during the process and pass through the heat exchanger. This avoids the need of heating elements inducing local temperatures above a certain threshold.

The method of the present invention enables to obtain a mixture of alkane sulfonic acid and sulfuric acid which is essentially free of any side products. Excess of SO₃ is destroyed prefera- bly by the addition of water resulting in the formation of sulfuric acid. The mass ratio between alkane sulfonic acid and sulfuric acid is such that it can be distilled without any problems in view of the formation of alkane sulfonic alkyl esters, as long as temperature and pressure are in a certain range and as long as no catalyst is present.

Catalysts inducing the formation of alkane sulfonic alkyl esters are e.g., Lewis acids or transition metals. Thus, the mixture to be distilled as well as all devices being part of the distillation means is free of Lewis acids and transition metals. One Lewis acid promoting the formation of alkane sulfonic alkyl esters is for example AI₂O₃. Transition metals to be avoided are mainly the transi- tion metals of groups 4 to 6 of the periodic table, namely Ti, Zr, Hf, Rf, V, Nb, Ta, Db, Cr, Mo, W and Sg. Especially the transition metals of group 5 and here especially Ta has to be avoided so that the distillation means is free of Tantalum in a preferred embodiment. The amount of transi- tions metals entering the distillation column should be below 1000 ppm, preferred below 500 ppm and most preferred below 100 ppm or even below 10 ppm.

When providing the educts, namely alkane, SO₃ and the initiator or initiator precursor, they also are preferably already at the temperature T . To keep them at the temperature T , they are also preferably circulated and pass through a heat exchanger during the circulation. Also, a respec- tive heat exchanger is preferably a SiC heat exchanger. The method according to the present invention can be a continuous method or a non- continuous method, meaning a batch process. In cases where the method is a continuous method, the reaction time in the reaction chamber is controlled and the educts, especially the alkane and SO₃, are added continuously. In a batch process after completion of the reaction and removal of the products, the reaction chamber can be filled again with the educts.

Also within the scope of the present application the methodology shown in Fig. 1 correspond to a continuous process. SO₃ (in the figure pure SO₃ as exemplary embodiment), CFU (exemplary for any alkane within the scope of the present invention) and the initiator or initiator precursor (pre-cat) are fed into the first reaction chamber (1 ) and then passing the reaction mixture to the next reaction chamber (2) and to further reactors up to n reaction chambers. With the number of reaction chambers, the concentration of MSA increases. The final step is the distillation taking place in distillation column (D).

As shown schematically in Fig. 1 , the sump stream of the distillation comprising H₂SO₄ and the alkane sulfonic acid are both recycled back to the first reaction chamber (1 ). This recycling can take place also in a continuous process or a batch-process and is a preferred step of the pro- cess according to the invention. The recycling enables an increase in the yield and thus a re- duced amount of waste. Additionally, the recycle stream can be used as the solvent described above to solve the respective initiator according to the invention and then fed back into the reac- tion chamber

Said reaction chamber (1 , 2, ..., n) is preferably made of stainless steel, glass, ceramics, SiC, enameled steel, and/or Teflon. The reaction chamber must be made of a material being stable against the components used for the reaction. Surprisingly, it has been found that stainless steel is sufficient, as the mixture of alkane sulfonic acid with sulfuric acid is not as corrosive as one of the acids alone. Thus, the reaction chamber is preferably made of stainless steel.

The distillation means is necessary for the distillation of the mixture in step h) of the method of the present invention comprises a distillation column. Said distillation column is preferably made of glass, ceramics, SiC and enameled steel and/or Teflon. The Teflon might be filled with fibers such as glass fibers or carbon fibers. Especially preferred are ceramic materials for the distilla tion column. Optionally, the surface may be a smooth surface.

In another embodiment, the present application provides for a device for carrying out the pro- cess of the present invention. The device comprises:

i. independently from each other devices to provide the educts: alkane, SO3, and a so- lution comprising an initiator or an initiator precursor;

ii. at least one reaction chamber to allow the alkane, SO3, and a solution comprising an initiator or an initiator precursor to come into contact and to react with each other; iii. at least one distillation means;

v. where appropriate, at least one tank for providing a coolant;

vi. where appropriate, at least one tank for providing water;

vii. where appropriate, at least one tank for thermal oil; and

viii. where appropriate, at least one exhaust gas purification device.

The device further comprises in a preferred embodiment heat exchangers, valves and pipes, enabling the circulation of the educts, components and mixtures and at the same time maintain- ing the necessary temperatures T1 and T₂ respectively. Reaction chamber, devices to provide the educts, distillation means, heat exchangers, valves and/or pipes are preferably made of stainless steel, glass, ceramics, SiC, enamelled steel, and/or Teflon, where Teflon can be filled with fibers, especially glass fibers or carbon fibers.

The device according to the present invention comprises devices to provide the educts, namely alkane, SO₃ and a solution comprising the initiator or initiator precursor (not shown in Fig. 1 ). In cases where an additive is used, the device further comprises a device to provide the additive.

In case the educts are liquid, the device is preferably a tank. In case the educt is gaseous, the device may be a tank or a pipeline.

The device according to the invention comprises at least one reaction chamber (1 , 2, .., n), in which the process according to the present invention takes place. In a batch process, the device can comprise only one reaction chamber. It is preferred that the device comprises two or more reaction chambers, as in such cases the concentration of alkane sulfonic acid, being the final product, increases with each chamber. Such a configuration, as schematically depicted in Fig.

1 , allows a quasi-continuous production with high yield and high selectivity.

The reaction chamber may be a high pressure reactor but may also be a tubular reactor.

In cases where the alkane is a gas, the device further comprises gas entrainment impellers for mixing the alkane into the solution when of the other educts. The inventive device further comprises at least one distillation means, depicted in Fig. 1 as col- umn D. One distillation means is usually sufficient. But for economically reasons or due to lim- ited space it may be advisable to have two or more distillation columns as part of the device.

The at least one distillation column is preferably made of glass, ceramics, SiC and enameled steel and/or Teflon. Teflon might be filled with fibers such as glass fibers or carbon fibers. Espe- cially preferred are ceramic materials for the distillation column. To obtain the needed pressure P2 during distillation, a vacuum pump may be present as well.

The device to produce alkane sulfonic acid, especially methane sulfonic acid, according to the present invention may additionally comprise at least one tank for providing a coolant, at least one tank for providing water, at least one tank for thermal oil and/or at least one exhaust gas purification device.

During the reaction in the at least one reaction chamber and/or the elimination of SO₃, exother- mic reactions take place. The generated heat must be removed or dissipated. This may be car- ried out with a coolant being provided in a suitable tank. Said at least one tank for a coolant is connected over suitable pipes with the at least one reaction chamber and, where appropriate, the distillation means.

The distillation means usually comprises a distillation column or set of columns. The energy required to operate said distillation column or set of distillation columns can be provided as common for a distillation, e.g. via a suitable thermal oil or via steam. The energy is introduced into the distillation through a heat exchanger or a set of heat exchangers, through which the liquid in the bottom of the column or set of columns is circulated as described earlier. According- ly, in cases where a thermal oil is used, the device according to the present invention additional- ly comprises a tank for a thermal oil. Said tank is connected over suitable pipes with the distilla tion means and especially with the distillation column. The distillation means may further corn- prise a distillation splitter in a preferred embodiment.

The device may further comprise a tank for providing water. The result of the distillation means is a pure alkane sulfonic acid, especially pure methane sulfonic acid. For some applications it may be needed to dilute the acid with water prior to filling it. This water is clean water which is provided in a tank. In case where a dilution is needed, the device according to the present invention may further comprise

ix. at least one tank for diluting the alkane sulfonic acid, especially methane sul- fonic acid, from the concentration ci to the concentration C2.

The tank for providing water is connected with the at least one tank for diluting the alkane sul- fonic.

After distillation, the alkane sulfonic acid, especially methane sulfonic acid, has a purity of more than 98.0 wt.%, preferred more than 98.5 or 99.0 wt.%, and most preferred more than 99.5 wt.% or 99.8 wt.% or 99.9 wt.%. Thus, Ci is about 100 wt.% . This pure methane sulfonic acid might be used as such or it might be diluted to a concentration C2. Said C2 is usually within a range from 0.5 wt.% to 95 wt.% While methane sulfonic acid (MSA) is available on the market with a concentration of 70 wt.% MSA in water or 94 wt.% MSA in water, for final applications named earlier in this invention MSA is typically diluted further, e.g. down to between 0.5 wt.% to 50 wt.%.

The tanks under items v., vi., vii., and ix., as well as the at least one reaction chamber and the devices to provide the educts may comprise a stirrer. Especially the at least one reaction cham- ber comprises a stirrer to enable a thorough mixture of all educts inside the reaction chamber. The stirrer may comprise gas entrainment impellers to allow, where applicable, the mixture of volatile alkanes into the sulfuric acid and SO₃ in the reaction chamber. The reaction chamber(s) may contain baffles or flow brakers.

Finally, the alkane sulfonic acid may be filled in a container or other packing material, either as pure acid or after dilution to a specific concentration.

During all the method to produce alkane sulfonic acid, exhaust gases may occur. These ex haust gases may be cleaned. For example, the exhaust gas may comprise excess of SO₃ or methane which should be removed. In such cases, at least one exhaust gas purification device is present. The at least one exhaust gas purification device is connected over suitable pipelines with the at least one reaction chamber.

The device according to the present invention may also comprise pumps to transport the educts and/or reaction mixture or other liquids or gases from tanks to other tanks or reaction devices. The pumps may be suitable to transport also corrosive media such as sulfuric acid, alkane sul- tonic acid, hydrogen peroxide, and others being present for the method according to the present invention.

The device according to the present invention may also comprise one or more waste containers to collect the waste occurring during the method according to the present invention.

Claims

1. Method to produce alkane sulfonic acid ALK-SO₃H comprising

a. providing an alkane,

b. providing SO₃, and

d. bringing into contact the alkane, SO₃ and the initiator or the initiator precursor in a reaction chamber at a temperature T1 and a pressure pi,

e. allowing the reaction mixture to react in the reaction chamber to obtain the al- kane sulfonic acid in a mixture with sulfuric acid and potentially SO3, f. transferring the mixture of alkane sulfonic acid, sulfuric acid, and where appropri- ate SO₃ to a distillation means,

g. removing the SO₃ of the mixture, if necessary,

2. Method according to claim 1 , wherein ALK is a branched or unbranched, substituted or un- substituted alkyl group with 1 to 20 C-Atoms, especially a methyl, ethyl, propyl, butyl, isobu- tyl, or higher alkyl group, preferably ALK is methyl.

3. Method according to claims 1 or 2, wherein the temperature T1 is from 0 °C to 100 °C, pref- erably from 10 °C to 70 C, more preferably from 20 °C to 60 °C, and/or the pressure pi is within a range of from 1 to 200 bar, preferably from 10 to 150 bar, especially from 50 to 100 bar and/or the reaction time is within a range of 30 min to 30 h, preferably from 1 h to 20 h, especially from 2 h to 10 h.

4. Method according to one or more of claims 1 to 3, wherein the removal of SO₃ in step g. is performed by addition of water to the mixture.

5. Method according to claim 4, wherein the amount of water is controlled by measuring the conductivity of the mixture.

6. Method according to one or more of claims 1 to 5, further comprising measuring the density and / or the speed of sound of the mixture prior to the distillation.

7. Method according to one or more of claims 1 to 6, wherein the temperature T₂ is within a range of from 160 °C to 220 °C, preferably from 175 °C and 215 °C and/or the pressure p₂ is within a range of from 1 to 30 mbar, preferably from 3 to 20 mbar and most preferably from 5 to 15 mbar..

8. Method according to one or more of claims 1 to 7, wherein SO3 is provided as oleum or as pure SO3 and the concentration of SO3 is optionally 65% by weight or less or 85% by weight or more.

9. Method according to one or more of claims 1 to 8, wherein the reaction chamber(s) is/are made of stainless steel.

10. Device for carrying out the process according to any of claims 1 to 9 comprising:

i. independently from each other devices to provide the educts: alkane, SO3, and a so- lution comprising an initiator or an initiator precursor,

ii.at least one reaction chamber to allow the alkane, SO3, and a solution comprising an initiator or an initiator precursor to come into contact and to react with each other; iii.at least one distillation means;

iv.if the alkane is gaseous, at least one gas entrainment impeller inside the at least on reaction chamber mentioned under ii;

v.where appropriate, at least one tank for providing a coolant;

vi.where appropriate, at least one tank for providing water;

vii.where appropriate, at least one tank for thermal oil;

viii.where appropriate, at least one exhaust gas purification device.

1 1. Device according to claim 10 further comprising heat exchangers, valves and pipes.

12. Device according to claims 10 or 11 , wherein the reaction chamber, devices to provide the educts, distillation means, heat exchangers, valves and/or pipes are made of stainless steel, glass, ceramics, SiC, enameled steel, and/or Teflon.

13. Device according to one or more of claims 10 to 12 further comprising one or more vacu- um pumps and/or waste containers.

14. Device according to one or more of claims 10 to 13 wherein the distillation means is free of Lewis acids and especially free of Tantalum.