FR2471358A1 - Sulphonation of organic cpds. using sulphur tri:oxide - with membranes of tetra:fluoroethylene and per:fluorovinyl:ether mixed polymers - Google Patents

Abstract

The sulphonating agent and the organic compound to be sulphonated are maintained basically separate from each other, by a membrane which is near enough chemically inert relatively to the partners in the reaction, in the sulphonation of organic compounds. Pref. the membrane is of a perhalogen-carbon polymer which has terminal carbon acid-, phosphonic acid or sulphonic acid radicals or mixtures of these. Used for the production of cationic exchange materials, anionic wetting agents (surfactants), as well as additives to lubricating oil, such as sodium dodecylbenzolsulphonic acid, a main constituent of detergents. Certain types of membranes can be used in non-aqueous environments under conditions which permit a selective contact between partners in a reaction, so that reactions which were previously difficult to control can be carried out easily. The invention overcomes the problems of known processes and yields a sulphonated product which is free from undesired byproducts.

Description

 The present invention relates to an improved process for sulfonating organic compounds using a sulfonating agent.

The sulfonation of aromatic hydrocarbons is an industrially important process for the production of materials for cation exchange, anionic detergents and additives for lubricating oils. An example of these products is sodium dodecylbenzene sulfonate which is one of the main ingredients in laundry detergent formulations.

The sulfonation of dodecylbenzene is carried out on an industrial scale by heating this dodecylbenzene with oleum to 20 Ó or by bringing it into contact with vaporized sulfuric anhydride (or sulfur trioxide).

 The oleum process generally involves mixing the hydrocarbon with the oleum at 20 to 35-50DC for two hours.

After adding a small amount of water to separate the layers, neutralization of the organic layer with sodium hydroxide gives a product comprising 88 to 90 Ó of sodium dodecylbenzene sulfonate and 10 to 12 Ó of sulfate sodium. Separation and use or disposal of sodium sulfate are problematic.

 The processes using sulfuric anhydride use less expensive materials and give a product containing less sodium sulfate. However, the reaction is very exothermic and somewhat difficult to control. Liquid sulfuric anhydride cannot be added directly to the aromatic hydrocarbon without causing extensive charcoal and dealkylation of the 1-aromatic alkyl. Normally, sulfuric anhydride is used at a concentration of 5 Ó in an inert gas, such as dry air or dry nitrogen. The viscous mass must be stirred effectively to promote gas-liquid contact throughout the reaction mass. This is particularly difficult at the end of a discontinuous reaction due to the extreme viscosity of the product obtained. Although the reaction occurs easily at low temperatures, a temperature of about 500C is used to decrease the viscosity. Neutralization of the reaction mass gives a product which is essentially sodium dodecylbenzene sulfonate in aqueous solution.

Membranes of various types are used to effect certain separations of aqueous solutions.

An example of this type of separation is the desalination of seawater which is discharged under pressure through a membrane.

 It has just been found that certain membranes can be used in non-aqueous media and under conditions allowing selective contacting of the bodies which have to react, in order to allow easy carrying out of reactions which until now have been difficult to control. The present discovery provides a simple and effective method for sulfonating organic compounds while avoiding the problems associated with known methods, while providing a sulfonated product which is relatively free of unwanted by-products.

 The present invention relates to a process for sulfonating organic processes and in particular aromatic organic compounds. The contact between the compound to be sulfonated and the sulfonating agent is carried out using a membrane which however essentially keeps the organic compound separated from the sulfonating agent.

The process of the present invention makes it possible to easily sulfonate any organic compound capable of being sulfonated. For example, it is generally preferred to use aromatic compounds in the present invention, since they readily undergo sulfonation to form very useful products. The aromatic or aliphatic compounds which can be sulfonated by application of the process of the present invention can be hydrocarbons or their substitution derivatives. The aliphatic compounds must contain at least two carbon atoms, and preferably from three to twenty carbon atoms. The aromatic compounds preferably contain from 6 to 40 carbon atoms. The substituted derivatives of the aliphatic or aromatic hydrocarbons can be substituted by groups containing nitrogen, oxygen, silicon, phosphorus, sulfur and / or halogen, without affecting the process of the present invention. For example, this process can be used to sulfonate aromatic compounds containing one or more atoms or groups, such as amino, nitroso groups, chlorine, fluorine or bromine atoms, silano, ether-oxide, hydroxyl and carboxyl groups.

 Because of their industrial importance, aromatic hydrocarbons are particularly advantageous to use in this sulfonation process. For example, alkylbenzenes having a linear alkyl group, in particular those in which this alkyl group contains 10 to 16 carbon atoms, are sulphonated for use in laundry detergents.

Aromatic petroleum fractions, containing alkylbenzenes, naphthalenes and condensed higher aromatic systems, are sulfonated to serve as a detergent in lubricating oils.

In the present process, the organic compound is sulfonated using a sulfonating agent. This agent can be sulfuric anhydride, sulfuric acid or chlorosulfonic acid. If desired, the sulfonating agent can be used in solution in a diluent.

The diluent can be used to adjust the strength of the sulfonation reaction, which is extremely exothermic.

Suitable diluents are those which do not prevent sulfonation. For example, sulfuric anhydride can be diluted with sulfuric acid, sulfur dioxide, hydrochloric acid or boron trifluoride. If the sulfonating agent is used with a diluent, it is preferably used at a concentration of 1 to 99 ÓX even better 20 to 80 PÓS of the sulfonating agent in the diluent.

The advantages of the process of the present invention result from the use of a membrane to isolate essen
tially the sulfonating agent with the organic compound to undergo the sulfonation reaction, while allowing contact of these two substances through the membrane.

The membrane must allow the sulfonating agent and the organic compound to come into contact at the interface formed by this membrane. The membrane must also be essentially inert to the chemical action of the organic compound and the sulfonating agent, in order to remain intact and to preserve its role as a separating organ. Materials that are chemically inert with respect to the organic compound and the sulfonating agent must be used in the membrane at the temperatures at which the reaction will occur.

These materials must be able to be transformed into membranes with minimum thickness, in order to obtain the desired transformation without long reaction times. Due to their inertia, fluorocarbon polymers have been found to be very suitable for the present process. These fluorocarbon polymers are perhalogen-carbon polymers that do not contain hydrogen atoms on the polymer framework.

The carbon framework only has as substituents (1) chlorine and / or fluorine atoms with a maximum of 25 Ó by weight for chlorine and (2) lateral or pendant acid groups, which are chosen from groups of the type sulfonic acid, carboxylic acid and phosphonic acid or are selected mixtures of at least two of these types. The preparation of such materials, for example "Kel-F" modified by the above-mentioned acid groups, is well known in practice.

The membrane is preferably made of a polymer of perfluorocarbon-sulfonic acid, as described in US Patents NO 3,041,317, NO 3,282,875 and NO 3,624,053, to which reference may be made. These materials are preferred because of their inertness to the bodies reacted under the conditions under which the reaction occurs, and as a result of the ease of their transformation into suitable membranes. A particularly preferred material for the manufacture of membranes is a copolymer of tetrafluoroethylene and a perfluorovinyl ether oxide having side groups of the sulfonic acid type. An example of such a membrane is the "Nafion" resin, available from E.I.

DuPont and Co.

 The perfluorocarbon-sulfonic membrane can be used in the form of sheets, tubes and hollow fibers.

The material can be laminated by association with gauze to give its dimensions more stability. The material can be incorporated into the diffusion cell using a member which may or may not have a support. For example, when using hollow fibers, they generally do not need support at points other than the entry of the hollow fibers into the diffusion cell and the exit of these fibers. When the material forming the membrane is present in the form of sheets, it can be placed on a support formed by a stainless steel cloth, a porous metallic surface or a fabric of inert material allowing the bodies to pass freely. reacted and the products obtained.

It has been found that when the preferred perfluorinated sulfonic acid polymer is used as the membrane, the weights of the polymer equivalents have a significant effect on the rate of transformation. That is to say, we have found that, all the other variables remaining equal,
higher reaction rates are obtained by using polymer with low equivalent weight. The equivalent weight is defined as the weight, in grams, of the membrane polymer per mole of sulfonic acid group.

It has been found that weights as low as 1060 give optimal results, while weights greater, i.e., equal to or greater than 1340, result in lower processing rates. The practical minimum usable for the weight of the equivalent of these polymers is approximately 900. For weights below this value, difficulties arise during the production of films, hollow fibers, tubes, etc., in a polymer d 'fluorinated sulfonic acid the mechanical properties of these objects decrease.

The process of the present invention can be conveniently carried out in a diffusion cell which includes a chamber which a membrane divides into separate compartments.

Each compartment has organs allowing its contents to be removed. The process can be carried out continuously or discontinuously, but it is preferably carried out continuously.

The membrane can be used in various shapes and thicknesses and it is possible, for example, to place hollow fibers, tubes, planar membranes, etc., in a diffusion cell, in order to form the separate compartments. The thickness of the membrane can vary between 2.5 microns and 1 mm, and preferably between 25 microns and 2.5 mm. The thickness of the membrane will be chosen to facilitate its manufacture and allow it to be kept intact during the implementation of the present invention.

The organic compound is present in the form of a liquid, which is in solution or without solvents and is brought into contact with the sulphonating agent by circulating in the same direction as it or against the current.

 Means well known to experts in this field make it possible to vary the variables characterizing the contacting of the organic compound with the sulfonating agent in order to obtain various degrees of transformation by passage. In general, longer contact times and smaller thicknesses of the membrane favor a greater degree of transformation. The temperature at which contact takes place essentially influences the degree of transformation. That is, as the temperature increases, the degree of transformation also increases. The contact time of the sulphonation medium with the organic compound can be varied by decreasing the flow rate or by increasing the size of the diffusion cell or the area of the contact surface in this cell.

 The sulfonation material is, if liquid, preferably used without solvent, so as to minimize any unwanted side reactions that may occur with a solvent. If it is solid at room temperature, the material can be heated to a temperature at which it can be sulfonated as a melt. For example, the sulfonation process can be carried out at temperatures up to a maximum of 2000C. Thus, molten and essentially fluid materials can be used at temperatures up to this maximum limit.

 Materials which cannot be fluidized by raising the temperature can be dissolved in a suitable solvent and used in this way. The use of a solvent is generally better than heating at high temperatures to fluidize a solid body intended for the reaction. It is, of course, obvious to those skilled in the art that the solvent should be chosen so as to minimize any discomfort brought to the action of the sulfonating agent and / or any effect on the material of the membrane. reaction temperatures. The solvent must also not be miscible with the sulfonating agent, so as to maintain the phase separation which constitutes one of the advantages of the present process. Examples of suitable solvents include nitrobenzene, dichlorobenzene, cyclohexane, CCl4, etc.

The process of the present invention can be carried out under reaction conditions similar to those of the previous processes. That is to say that the temperature at which the sulfonation process can be carried out varies between the freezing point of the organic compound, of the sulfonation medium or of the mixture of the organic compound and the solvent optionally used, and the point of boiling of any of the constituents at the pressures applied.

Thus, the process can be carried out over wide ranges of temperature and pressure. It has been found that by increasing the temperature, the rate of transformation is increased. However, the upper limit depends on the appearance of competitive side reactions and the discomfort they bring, as well as the inertia of the membrane material with regard to temperature. It has been found that when using the process of the present invention, side reactions or competing reactions are minimized, contrary to what the prior art provides. Thus, the process of the present invention makes it possible to operate at higher temperatures and to obtain higher rates of transformation than with the prior processes.

 The following examples are presented to illustrate and in no way limit the invention.

"Nafion" (perfluorosulfonic acid) membranes are obtained in the form of unreinforced sheets and tubes at EI DuPont of Nemours and Co. The weights of the membrane equivalents (expressed in grams of the polymer per sulfonic acid group when the polymer is balanced with atmospheric humidity) are indicated by Du
Bridge. The tubes used as the membrane have an inside diameter of 0.61 mm and an outside diameter of 0.09 mm.

The sheet membranes have a thickness of 0.1 mm. The membranes are cleaned and regenerated by bringing them into contact with nitric acid at 70 to 80 ° -10 ° C., then washing them thoroughly in distilled water and drying them at 80 "-100 C in a stream of dry nitrogen.

Static sulfonations are carried out in 316 stainless steel cells with two compartments, comprising two corresponding circular halves, the cavity of which has identical dimensions (0.6 x 3.7 cm in diameter) separated by a membrane clamped between the two halves. and which consists of an unreinforced sheet. After assembly, the cells are placed on a shaking platform, immersed in a water bath and allowed to come to the desired temperature with the dodecylbenzene in a compartment. The reaction begins when the liquid sulfur dioxide is introduced into the other compartment.

Take samples at intervals and measure the detergent content
Sulfonations with continuous flow are carried out by operating in an annular reactor comprising an internal tube of the membrane and an external tube of "FEP-Teflon" (external diameter: 3.2 mm; internal diameter: 1.58 mm) with fittings. extremes in "Swagelok" arranged so as to allow heat exchange. Dodecylbenzene or sulfuric anhydride is introduced into the internal tube (membrane) and the other substance is introduced into the annular envelope between the two tubes, and this other substance is removed therefrom. coil is immersed in a distilled water boiler with an aqitation device and maintained at the desired temperature by a thermistor temperature control device. A back pressure is exerted on the flow of the hydrocarbon using a check valve with adjustable pressure. A metering valve is placed at the outlet of the sulfuric anhydride flow. The flow of dodecylbenzene is ensured by a calibrated pump with constant stroke of the piston and the outlet of which includes a check valve allowing passage for a pressure of 1.75 bar. A pressure gauge is placed on a T between the pump and the inlet to the reactor. The flow of sulfuric anhydride is ensured by a pressure tank containing the liquid and which comprises a pressure gauge above the liquid and a metering valve to the exit. The pressure tank and the tubes leading to the reactor are contained inside an oven maintained at 350-400C, in order to keep the sulfuric anhydride melted.

Dodecylbenzene ('Nalkylene 500 "), tribenzene (" Nalkylene 600 ")," C560 "(suspension at 50 Ó of sodium dodecylbenzene sulfonate) and dodecylbenzene sulfonic acid at 97 Ó are obtained from Conoco Chemicals.

The fixing product of a linear alkyl group for obtaining a detergent consists of a mixture of isomers in which the benzene nucleus can be fixed on any of the carbon atoms inside the linear chain in C12 or C13. Sulfuric anhydride ("Sulfan B", with stabilizer) and oleum at 20 Ó (fuming sulfuric acid) are products from
Baker and Adamson.

The detergent content is measured using a modified methylene blue phase transfer method (Longwell and Manviece, Chem. Abstr. 78, 26907b; Kolthoff,
Elving, Strauss, Treatise on Anal. Chemistry (Treatise on Analytical Chemistry) III, volume I, page 439)
Samples (typically 30 to 100 mg) from sulfonation are weighed into test tubes and diluted with 5.0 ml of 95. ethanol. An aliquot (0.002 to 0.005 ml) of this solution is further diluted in a 13 x 100 mm test tube with 2.0 ml of an aqueous solution of methylene blue (H2PO4 (10
2 4 (10 g),
H2SO4 to 10 (14 ml), 0.0203 g of methylene blue, with dissolution to 1 liter) and 3.0 ml of reagent grade chlorofome. The contents of the tube are vigorously mixed on a "vortex mixer" and centrifuged to clarify the layers. The absorption coefficient of the detergent-methylene blue complex in chloroform is read at 650 nm and compared with a blank test and with controls prepared from dodecylbenzene sulfonic acid (97 Ó) and sodium dodecyl sulfate.

Example 1
Static sulfonation of dodecylbenzene with 50
Effect of equivalent weight of the membrane
We obtain sheet membranes with different weights of equivalent, but of similar manufacture, which we assemble in three identical cells of stainless steel 316. The cells are filled on one side with dodecylbenzene (Nalkylene 500 ") and place them on a shaking platform in a water bath at 40 ° C. The reaction begins when liquid sulfur dioxide is introduced on the other side. It is withdrawn at intervals, using a syringe, samples from the hydrocarbon side and analyzed for detergent content by the "methylene blue" process The results obtained appear in Table I. A higher equivalent weight appears to lead to more processing slow, although it has little effect on the final percentage of processing.

The higher proportion of sulfonic acid groups in the lower equivalent weight polymer probably increases the permeation rate of sulfuric anhydride. The decrease in the detergent content over long reaction times (which is particularly evident in the case of the sample corresponding to an equivalent weight of 1100) is attributed to the formation of more highly sulfonated products in the presence, here, of '' an excess of sulfuric anhydride. These products probably do not undergo phase transfer with methylene blue as easily as the mono-anions.

Table I shows the results of a static sulfonation in cells with two cavities separated by a membrane, the membranes (0.1 mm thick) having different equivalent weights. The temperature (T) is 400C. The weight of the equivalent (P.Eq.) is the weight of the equivalent of the membrane polymer. Analysis of the weight percentage of detergent is carried out in duplicate (using a 97 u alkylbenzene sulfonic acid as a control).

TABLE I
Time (hours) P.Eq. 1100 P.Eq. 1200 P Eq. 1340
0-0.1 0.2 0.1 0
0.5 12.6 4.5 0.5
0.6 12.4
1.0 33.9 19.1 12.3
1.2 37.5
2.0 46.7 33.5 24.1
2.2 48.4
4.0 - 41.9 41.8
4.5 62.6
5.9 63.0 47.9
6.2 74.0
22.5 67.6 85.6 86.9
Example 2
Sulfonation of dodecylbenzene in a tubular reactor
Sulfuric anhydride is introduced into the internal tube (membrane) and dodecylbenzene ("Nalkylene 500") is introduced into the annular envelope of a tubular reactor (6.7 meters; internal diameter: 0.61 mm; thickness of wall: 0.114 mm; equivalent weight of the membrane 1340) maintained AT 6O0C. The flow of dodecylbenzene is kept constant and, after the steady state has been established, samples are collected at intervals, the detergent content being analyzed. In a test (Table IIA) the pressure of the sulfuric anhydride (from the tank) is maintained .r <Pns the tube formed by the membrane, but sulfuric anhydride is not allowed to leave the reactor. In a second test (Table IIB), a fairly constant leak of 2.3 ml of sulfuric anhydride per hour is allowed to leave the reactor.

Table IIA shows the sulfonation of dodecylbenzene by SO 3 in a tubular reactor (6.7 meters; weight of equivalent: 1340), without leakage of 503. The temperature is 600C, the flow of dodecylbenzene of 0, 20 ml / min at a pressure of 2.45 - 2.66 bars the pressure of the SO 3 is 2.66 bars.

TABLE IIA
Sample NO Time of sampling v of detergent
(hours)
1 (O-) 63.1
2 1.1 57.9
3 2.0 12.4
4 2.5 4.7
5 3.0 6.4
6 3.5 14.0
7 4.0 27.8
Table IIB is presented as Table IIa, except that there was a leak of 2.3 ml of SO 3 per hour (gauge pressure 2.31 bars); flow of dodecylbenzene: 0.10 ml per minute (gauge pressure 2.66-2.80 bars).

TABLE 118
Sample N Time of sampling Dó detergent
(hours)
1 (O) 60.3
2 0.5 75.3
3 1.0 64.9
5 .2.0 51.7
6 2.5 69.9
7 3.0 68.2
8 3.5 66.8
9 4.0 53.7
Although the results vary somewhat, they indicate that the reaction slows considerably over time in the absence of a continuous 503 leak, suggesting the accumulation of an inhibitor. The inhibition shown in Table IIA is completely removed when the reactor is rapidly swept with 503. The cessation of the flow of 503 again causes increasing inhibition of sulfonation over time. The lack of reaction of the system with oleum at 20 Ó as a sulfonating agent at this temperature (see below) suggests that sulfuric acid may be the inhibitor, formed from traces of water in the system and side reactions (for example, formation of anhydride or sulfur), producing water.

Example 3
Sulfonation of dodecylbenzene with oleum
20 m in a tubular reactor
0.095 ml (per minute) of tridecylbenzene ("NALKYLENE 600") is introduced into the internal membrane tube) and 0.10 al (per minute) of oleum (fuming sulfuric acid containing 20 Ó sulfuric anhydride) is introduced in the annular duct of a 6.7-meter tubular reactor (equivalent weight: 1310 for the membrane). The reactor is kept at constant temperature using an oven with forced air circulation. Samples are collected on the two effluents after the passage of at least the equivalent of a volume of the reactor at the desired temperature.

Neutralization and analysis of the oleum samples revealed no detectable detergent (less than 0.1 m).

The results obtained on the tridecylbenzene effluent are presented in Table III, which shows the sulfonation of tridecylbenzene ("Nalkylene 600) with oleum at 20 in the tubular reactor.

TABLE III
Sample NO Temperature OC S detergent
1 61 0
2 81 0
3 101-102 0.1
4 110-111 6.5
5 119-120 21.2
To sulfonate with 20% oleum. temperatures much higher than those required for sulfuric anhydride are required, which suggests that the presence of sulfuric acid may inhibit the reaction with sulfuric anhydride. However, this sulfonation process gives reasonable sulfonation of tridecylbenzene without introducing large quantities of sulfuric acid into the hydrocarbon product. Higher concentrations of sulfuric anhydride compared to sulfuric acid (e.g. 50% oleum ) can thus be useful as sulfonating agents whose activity is intermediate between those of sulfuric anhydride and sulfuric acid, when a lesser degree of activity is required to moderate the reaction.

Example 4
Sulphonation of dodecylbenzene with 503 in a reactor
tubular; effect of temperature and pressure
different from 503
The reactor of Example 2 is used with a pressure difference of about 0.7 bar between the two faces of the membrane to increase the diffusion rate of SO 3 and with a leakage flow of 503 through the reactor ( T = 600C; gauge pressure of 503: 3.64-3.70 bars; gauge pressure of dodecylbenzene 2.8-2.94 bars at 0.10 ml / min). Detergent concentration measured in hydrocarbon effluent increases to 90.1
By increasing the temperature up to 700C under these conditions, the concentration of the detergent in the hydrocarbon effluent increases to 95.3 Ó.

Example 5
Preparation test
Sulfuric anhydride is introduced (at a pressure of 3.29 bars) into the internal tube (polymer whose equivalent weight is 1340) and 0.12 ml (per minute) of dodecylbenzene is introduced ( at a gauge pressure of 2.45-2.80 bars) in the annular duct of a 6-meter tubular reactor at 600C, with a slow leak of 503. Samples of 4 to 5g are collected, the content of which is analyzed. detergent. Pool samples 3 to 8 (25.8 g; Table V) and carry out a new analysis of the detergent content (75.0%), then neutralize with sodium hydroxide at 45 Ó until at pH 10 and extracted with two portions of 75 ml each of n-pentane to remove the unaltered dodecylbenzene.

Evaporation of the solvent gives 3.2 g of a material whose infrared spectrum is identical to that of the starting dodecylbenzene. The aqueous layer (which contains sodium dodecylbenzene sulfonate) is evaporated until a moist paste is obtained which is extracted with three portions (125 ml each) of absolute ethanol, which leaves a light tan precipitate that dried and weighed (10.1 g). The ethanolic extracts are combined and dried under vacuum to obtain 14.05 g of slightly wet sodium dodecylbenzene sulfonate (whose infrared spectra are identical to those of a sample prepared in an analogous manner from dodecylbenzene sulfonic acid. at 97 Ó (Conoco)).

The detergent content of the ethanol insoluble precipitate (19.1 Ó) and the isolated sodium dodecylbenzene sulfonate (94.5%) was analyzed, indicating a high degree of separation during isolation. The infrared spectrum of the precipitate indicates a low content of organic matter.

It is identical to the infrared spectrum of a precipitate isolated by the same procedure from commercial sulfonic acid at 97 Ó.

TABLE V
Example N Time (hours) Weight / gram Ó deter
gent start
collecting
fraction
1 0 4.6 41.2
2 0.7 5.1 68.0
3 1.3 5.1 83.4
4 2.0 5.2 77.7
5 2.7 4.9 68.8
6 3.3 4.9 70.6
7 4.0 3.5 72.8
8 4.7 5.7 73.7
9 5.3 4.3 58.3
10 6.0 3.8 58.5
11 6.7 4.3 54.9
12 7.3 2.6 61.1
It goes without saying that, without departing from the scope of the invention, numerous modifications can be made to the sulphonation process described.

Claims (9)

 1. A process for sulfonating organic compounds by bringing the organic compound into contact with a sulfonating agent, this process being characterized in that the sulfonating agent and the organic compound are essentially separated using a membrane essentially inert to the chemical action of the organic compound and the sulfonating agent. 2. Method according to claim 1, characterized in that this membrane is made of a perhalogenocarbon polymer comprising pendant or lateral groups chosen from the carboxylic acid, phosphonic acid, or sulfonic acid groups, or mixtures thereof.  3. Method according to claim 2, characterized in that the organic compound is an alkylbenzene. 4. Method according to claim 3, characterized in that the sulfonating agent is sulfuric anhydride or a solution of sulfuric anhydride in sulfuric acid.  5. Method according to claim 4, characterized in that the alkyl group of the alkylbenzene contained from 10 to 16 carbon atoms.  6. Method according to claim 2, characterized in that the organic compound is an aromatic hydrocarbon soluble in oils.  7. Method according to claim 2, characterized in that the polymer of perfluorocarbon-sulfonic acid has an equivalent weight between 900 and 1700. 8. Method according to claim 7, characterized in that the membrane is made of a copolymer of tetrafluoroethylene and perfluorovinyl ether oxide, comprising lateral or pendant sulfonic acid groups.  9. Method according to claim 8, characterized in that the organic compound is dodecylbenzene or tridecylbenzene.