WO2020217001A1

WO2020217001A1 - Process for obtaining tetraethylammonium bromide and tetraethylammonium tetrafluoroborate, and corresponding products and uses thereof

Info

Publication number: WO2020217001A1
Application number: PCT/FR2020/000112
Authority: WO
Inventors: Pascal Dufour; Cyrille LE TOULLEC
Original assignee: Arkema France
Priority date: 2019-04-25
Filing date: 2020-04-10
Publication date: 2020-10-29
Also published as: FR3095437A1; FR3095437B1

Abstract

A process for producing tetraethylammonium bromide, and also a process for producing tetraethylammonium tetrafluoroborate from tetraethylammonium bromide, are obtained. These processes make it possible to obtain tetraethylammonium tetrafluoroborate in anhydrous and purified form, and to obtain an organic electrolyte solution comprising same. The tetraethylammonium tetrafluoroborate obtained can be used as a supercapacitor electrolyte.

Description

Process for obtaining tetraethylammonium bromide and tetraethylammonium tetrafluoroborate, corresponding products and uses

Technical area

The present invention relates to a process for the preparation of tetraethylammonium bromide, a process for the preparation of tetraethylammonium tetrafluoroborate from tetraethylammonium bromide, tetraethylammonium tetrafluoroborate in anhydrous and purified form thus obtained, an organic electrolyte solution comprising it, as well as its use as electrolyte for supercapacitor. The present invention makes it possible in particular to obtain tetraethylammonium bromide suitable for the preparation of tetraethylammonium tetrafluoroborate of sufficient quality for use as a supercapacitor electrolyte.

Technical background

The field of the invention is that of ammonium salts suitable for use as electrolytes, in particular as supercapacitor electrolytes, as well as the intermediate compounds necessary for their production. These ammonium salts must in particular be of sufficient quality and purity for such use.

Various processes for preparing ammonium salts, in particular tetraalkylammonium tetrafluoroborate, have been documented.

It is known from Japanese application JP2000212132 the preparation of tetraalkylammonium tetrafluoroborate from tetraethylammonium chloride. It is exemplified in particular the preparation of tetraethylammonium chloride under overpressure (0.75 MPa), at high temperature (130 ° C), for an average reaction time (6 h), in the presence of acetonitrile. It is also known from American application US6444846 the preparation of tetraalkylammonium tetrafluoroborate from tetraalkylammonium halide. It is exemplified in particular the preparation of tetraethylammonium chloride under overpressure (1, 17 MPa or 1, 21 MPa), at high temperature (140 ° C or 145 ° C), during a medium or high reaction time (5:30 or 10 h), in the presence of acetonitrile.

In addition, the preparation of tetraalkylammonium bromide is also known. The American application US3965178 relates to the preparation of tetra (/ 7-butyl) ammonium bromide under pressure atmospheric, at room temperature, in the presence of acetonitrile, but with a high reaction time between 12 h and 24 h, preferably between 18 h and 24 h. The tetra (/ 7-butyl) ammonium bromide obtained is mixed with water, before its extraction with an organic solvent, in particular benzene, cyclohexane and hexane. Czechoslovak application CS1976-8246A also relates to the preparation of tetra (/ 7-butyl) ammonium bromide at atmospheric pressure, with an average reaction time of 8 h, but in the presence of acetone. Finally, Chinese application CN102020569 relates to the preparation of tetraethylammonium bromide at atmospheric pressure, at high temperature (100 ° C.), for a long reaction time (between 22 h and 24 h), in the presence of benzene. None of these applications discloses the preparation of tetraalkylammonium tetrafluoroborate, in particular of tetraalkylammonium tetrafluoroborate suitable for use as an electrolyte, from the tetraalkylammonium halides obtained, the latter not necessarily being suitable for the preparation of quality tetraalkylammonium tetrafluoroborate sufficient.

The techniques of the prior art have the drawbacks of carrying out complex processes for preparing tetraalkylammonium tetrafluoroborate and their intermediate tetraalkylammonium halide compounds. These techniques require in particular the implementation of processes under overpressure, at high temperatures and / or during long reaction times, which then require the provision of suitable complex and expensive equipment. In addition, these techniques do not necessarily make it possible to obtain tetraalkylammonium tetrafluoroborate of sufficient quality for use as an electrolyte, in particular as an electrolyte for a supercapacitor. In fact, when the reaction is carried out in the presence of solvents other than water or alcohols having C1-C4 carbon chains, a tetraethylammonium tetrafluoroborate is obtained having an insufficient degree of purity, traces of solvent being always present.

There is therefore a real need to provide a process for preparing tetraethylammonium bromide which is simple to implement and suitable for the preparation of tetraethylammonium tetrafluoroborate. In addition, there is a real need to provide a process for preparing tetraethylammonium tetrafluoroborate from tetraethylammonium bromide, but being of sufficient quality to allow its use as an electrolyte, in particular as an electrolyte for a supercapacitor. Thus, there is a real need to provide tetraethylammonium tetrafluoroborate in suspension. in purified, or even ultrapure, and anhydrous form, such properties being necessary for its use as an electrolyte.

Summary of the invention

The invention relates firstly to a process for preparing tetraethylammonium bromide comprising the following steps:

the mixture of ethyl bromide and triethylamine; and

reacting ethyl bromide and triethylamine at atmospheric pressure and in the presence of acetonitrile.

In some embodiments, the process for preparing tetraethylammonium bromide comprises mixing excess ethyl bromide with respect to the triethylamine.

In some embodiments, the process for preparing tetraethylammonium bromide comprises reacting ethyl bromide and triethylamine for a reaction time of between 15 min and 3 h and at a temperature between 45 ° C and 70 ° C. .

In some embodiments, the process for preparing tetraethylammonium bromide comprises reacting ethyl bromide and triethylamine with refluxing ethyl bromide in acetonitrile.

In some embodiments, the mixing and reaction of ethyl bromide and triethylamine are carried out under inert gas.

In some embodiments, the process for preparing tetraethylammonium bromide further comprises the following steps: cooling the resulting tetraethylammonium bromide solution to maximize the crystalline tetraethylammonium bromide;

optionally filtration to remove the excess reaction medium, in particular acetonitrile.

In some embodiments, the process for preparing tetraethylammonium bromide is devoid of any additional step of removing traces of acetonitrile. The preparation process can be devoid of an evaporation step. The preparation process may in particular be devoid of washing and / or ultimate purification steps.

The invention also relates to a process for the preparation of tetraethylammonium tetrafluoroborate, comprising the following stages: obtaining tetraethylammonium bromide by the process defined opposite; then mixing and reacting tetraethylammonium bromide and an aqueous solution of fluoroboric acid, to obtain a suspension of tetraethylammonium tetrafluoroborate in hydrobromic acid.

In some embodiments, the process for preparing tetraethylammonium tetrafluoroborate further comprises the following step: heating for evaporation of the suspension of tetraethylammonium tetrafluoroborate in hydrobromic acid, to remove hydrobromic acid and traces acetonitrile.

In some embodiments, the process for preparing tetraethylammonium tetrafluoroborate further comprises the following steps:

cooling the unpurified suspension of tetraethylammonium tetrafluoroborate; and

filtration.

In some embodiments, the process for preparing tetraethylammonium tetrafluoroborate includes at least one alcohol washing step.

In some embodiments, the process for preparing tetraethylammonium tetrafluoroborate further comprises admixing tetraethylammonium bromide and fluoroboric acid in a substantially stoichiometric ratio or in a substantially excess ratio of fluoroboric acid.

The invention also relates to tetraethylammonium tetrafluoroborate in anhydrous and purified form, which can be obtained by the preparation process opposite.

The invention also relates to tetraethylammonium tetrafluoroborate in anhydrous and purified form, comprising between 10 and 5000 ppm of acetonitrile; 50 ppm or less of water; 30 ppm or less of the element chlorine; 30 ppm or less of hydrofluoric acid; 30 ppm or less of fluoroboric acid; and 30 ppm or less of boric acid.

The invention also relates to an electrolyte for a supercapacitor comprising tetraethylammonium tetrafluoroborate, capable of being obtained by the preparation process opposite, in an acetonitrile solution.

The invention finally relates to the use of tetraethylammonium tetrafluoroborate, obtainable by the preparation process opposite, as electrolyte for supercapacitors. The present invention overcomes the drawbacks of the state of the art. It relates more particularly to a process for preparing tetraethylammonium bromide, the implementation of which is simple, which does not require the use of complex and expensive devices. This preparation process also has a very high yield of tetraethylammonium bromide. This preparation process finally allows the supply of tetraethylammonium bromide of sufficient quality and necessary for the preparation of tetraalkylammonium tetrafluoroborate.

In addition, the invention also relates to a process for preparing tetraethylammonium tetrafluoroborate from tetraethylammonium bromide, the implementation of which is simplified. This process also allows the preparation of a tetraethylammonium tetrafluoroborate of sufficient quality for its use as an electrolyte, in particular as a supercapacitor electrolyte. Thus, the invention also relates to tetraethylammonium tetrafluoroborate in anhydrous and purified (ultrapure) form, an electrolyte for a supercapacitor comprising it in a solution of acetonitrile and therefore its use as an electrolyte for a supercapacitor.

This is accomplished by virtue of the particular selection of the starting compounds, of the solvent and of the conditions for carrying out the processes according to the invention. Indeed, the inventors have surprisingly and unexpectedly demonstrated that the particular selection of bromide salts from halide salts, the selection of the ethyl group from the alkyl groups, and the implementation of a preparation process in mild pressure conditions and in the presence of acetonitrile nevertheless made it possible to obtain a tetraalkylammonium halide - namely tetraethylammonium bromide - with a very high yield and without the need to resort to a long reaction time and / or to high temperatures. This is all the more surprising since it was expected, by using an alkyl halide having an alkyl group of shorter chain compared to the n-butyl group for example, that the conditions of use were more severe, in particular in terms of pressure and temperature. For the manufacture of tertiary amines, it is commonly accepted that the reactivities between alcohols and ammonia or alkyl halides and ammonia are quite similar if the alcohols or alkyl halides have linear alkyl chains. If the alkyl chains are branched and hindered, the manufacture of tertiary amines may require more restrictive operating conditions in terms of duration, temperature and pressure. The production of tertiary amines may even be impossible with certain alkyl groups, for example the isopropyl group. In addition, it is commonly believed that under moderate operating conditions of temperature and pressure, tetra (/ 7-butyl) ammonium bromide is easier to prepare than tetraalkylammonium bromides having shorter alkyl groups, especially bromide. of tetraethylammonium. The n-butyl group - of formula -CH2CH2CH2CH3 - is a linear chain which, associated with the bromide, forms a linear alkyl bromide. Its small steric hindrance and its high boiling point (101.4 ° C) are favorable to a rapid reaction. By comparison, ethyl bromide and methyl bromide have lower boiling points of 38 ° C and 3.56 ° C, respectively. In addition, these boiling points are higher than those of the corresponding alkyl chlorides. Thus, by using alkyl halides having a high boiling point, for example n-butyl bromide, atmospheric phase volatilization of the reactor is avoided, which increases the reaction rate and therefore promotes a faster reaction. Surprisingly, the inventors prepared tetraethylammonium bromide, from ethyl bromide, with a reaction rate greater than that expected. In addition, the inventors have demonstrated that the tetraethylammonium bromide obtained, even if it contained traces of acetonitrile, was particularly suitable for the supply of tetraethylammonium tetrafluoroborate. Thus, the inventors have demonstrated that tetraethylammonium tetrafluoroborate could be obtained in anhydrous and purified form, and be used as a supercapacitor electrolyte in an acetonitrile solution.

detailed description

The invention is now described in more detail and in a non-limiting manner in the description which follows.

Process for obtaining tetraethylammonium bromide

The invention relates to a process for the preparation of tetraethylammonium bromide - of formula Br (C2H5) 4N - by reacting ethyl bromide with triethylamine. This reaction makes it possible to obtain a solution of tetraethylammonium bromide. By “solution of tetraethylammonium bromide” is meant a solution with partial suspension of crystals of tetraethylammonium bromide. Ethyl bromide - of formula (BrCaHs) - and triethylamine - of formula (C2H5) 3N - are mixed in the presence of acetonitrile. Acetonitrile - of formula CH ₃ CN - is an organic solvent. Different types of organic solvents can be used in the preparation of tetraalkylammonium halides. However, at present, it has been found that acetonitrile was a solvent of choice, with a view to obtaining tetraethylammonium bromide suitable for the preparation of tetraethylammonium tetrafluoroborate of sufficient quality to be used as an electrolyte, in particular for supercapacitors. Indeed, acetonitrile is a solvent commonly used in the design of electrolytes. Thus, by using a reaction solvent identical to the application solvent (electrolyte), the inventors have developed a process for preparing tetraethylammonium bromide, and by extension a process for preparing tetraethylammonium tetrafluoroborate, which may contain traces of the reaction solvent, without the need to resort to an ultimate purification step to remove any possible trace of solvent. In fact, at present, any trace of acetonitrile in the final product will not be considered as an impurity potentially detrimental to the use of tetraethylammonium tetrafluoroborate as an electrolyte. This specific choice of reaction solvent simplifies the process for preparing tetraethylammonium bromide.

The reaction mixture can comprise between 12 and 26%, preferably between 15 and 25%, very preferably between 18 and 24%, of triethylamine relative to the total weight of the reaction mixture.

The mixture can comprise a mass ratio (corresponding to the mass of triethylamine divided by the mass of ethyl bromide) of between 0.4 and 1.0, preferably between 0.5 and 0.9, very preferably between 0.5 and 0.7.

Preferably, the ethyl bromide is present in excess by weight relative to the triethylamine. The mixture can comprise an excess of ethyl bromide of at least 5%, preferably between 10 and 165%, very preferably between 50 and 80%, relative to the triethylamine. By mixing in excess of ethyl bromide, it is possible to aim for a yield of about 100% of transformed triethylamine.

The reaction mixture can comprise between 30 and 70%, preferably between 35 and 60%, very preferably between 35 and 45%, of acetonitrile relative to the total weight of the reaction mixture. The reaction between ethyl bromide and triethylamine is carried out at atmospheric pressure. By “atmospheric pressure” is meant carrying out the reaction at a pressure of approximately 0.1013.10 ⁶ Pa (0.1013 MPa). Thus, no overpressure, even minimal, is applied during the implementation of this reaction. As a corollary, this reaction is not carried out in a closed chamber resistant to high pressures, such as for example an autoclave.

The reaction between ethyl bromide and triethylamine can be carried out at a temperature between 45 and 70 ° C, preferably at a temperature between 50 and 65 ° C, very preferably at a temperature between 51 and 62 ° vs. This moderate temperature range was found to be sufficient to achieve a high rate of reaction rate and high yield after reasonable and rapid reaction time.

The reaction between ethyl bromide and triethylamine can be carried out for a reaction time of between 15 min and 3 h, preferably for a reaction time of between 15 min and 2 h, very preferably for a reaction time of between 30 min and 1 h 30. The present conditions were found to cause a rapid reaction between ethyl bromide and triethylamine and a high yield, making this process even easier to carry out.

The reaction between ethyl bromide and triethylamine can be carried out by refluxing ethyl bromide in acetonitrile. It can be used conventional reflux techniques. Reaction at reflux is possible by applying a temperature between 45 and 70 ° C. This reaction at reflux will lead to a moderate rise in the temperature of the reaction medium, for example a rise in temperature of between 5 and 25 ° C, preferably between 5 and 15 ° C.

The mixing and reaction between ethyl bromide and triethylamine can be carried out in the presence of air (oxygen) or in the absence of oxygen. In particular, when carried out in the absence of air, these steps can be carried out under inert gas, in particular nitrogen.

The process for preparing tetraethylammonium bromide can further comprise a step of cooling the tetraethylammonium bromide solution. This cooling step allows maximum precipitation of tetraethylammonium bromide in the form of crystals. The tetraethylammonium bromide solution can be cooled to a temperature between -5 and 20 ° C, preferably at a temperature between -5 and 5 ° C, very preferably at a temperature of about 0 ° C. In addition, the tetraethylammonium bromide solution can be cooled for a period of time between 30 min and 3 h, preferably between 1 h and 2 h.

The process for preparing tetraethylammonium bromide can further comprise a filtration step. This filtration step makes it possible to remove the excess reaction medium, in particular the excess acetonitrile which has not reacted with the triethylamine. The excess reaction medium is commonly referred to as "mother liquors". This filtration step can be implemented by means of different filtration devices, preferably by means of devices chosen from the group consisting of filter presses, centrifugal dryers, centrifugal decanters, pressure filters or rotary filters; preferably pressure filters or centrifugal dryers. Pressure filters can be pressure hood filters sold under the name GUEDU. Rotary filters can be rotary filters covered to capture flammable vapors. This step is sufficient, in that it is neither useful nor necessary to remove all traces of acetonitrile. The presence of traces of acetonitrile has no effect on the subsequent preparation of tetraethylammonium tetrafluoroborate, and on its use as an electrolyte for a supercapacitor.

If the filtrate obtained during the filtration step still contains dissolved tetraethylammonium bromide (not precipitated in the form of crystals), this filtrate can be recycled by mixing with ethyl bromide and triethylamine. A reaction step is then implemented, then a cooling step, then a filtration step, as detailed opposite.

Considering the non-need to remove all traces of acetonitrile in the product obtained, the process for preparing tetraethylammonium bromide can be devoid of any additional step of removing these traces of acetonitrile, in particular by evaporation (for example evaporation dry), by washing and / or by ultimate purification.

This process makes it possible to obtain tetraethylammonium bromide at a high yield, that is to say at a yield greater than 90%, preferably at a yield greater than 95%, very preferably at a yield greater than 97%, when ethyl bromide is in excess relative to triethylamine. The recycling of the filtrate contributes in particular to obtaining a high yield. This process also makes it possible to obtain tetraethylammonium bromide, in a precipitated form (crystals) not free from traces of acetonitrile, which is an intermediate product suitable for the preparation of tetraethylammonium tetrafluoroborate, which can be used as an electrolyte, in particular for supercapacitors.

Process for preparing tetraethylammonium tetrafluoroborate

The invention also relates to a process for preparing tetraethylammonium tetrafluoroborate - of formula BF4 (C2H5) 4N - from tetraethylammonium bromide obtained according to the process opposite.

This process involves mixing the tetraethylammonium bromide with an aqueous solution of fluoroboric acid. Fluoroboric acid - with the chemical formula HBF4 - has the molar formula HBF4, xx with the decimal "xx" varying from -0.1 to +0.5, when used in the form of an aqueous solution also comprising hydrofluoric acid - of the formula HF. This aqueous solution may include, for example, hydrofluoric acid slightly in excess of fluoroboric acid. The excess of hydrofluoric acid relative to fluoroboric acid (or stoichiometric excess with decimal "xx" of more than 0 to +0.5) is preferred to the lack of hydrofluoric acid relative to fluoroboric acid (or stoichiometric decimal defect "xx" from -0.1 to less than 0). Indeed, the inventors have advantageously observed that the hydrofluoric acid released at the end of the reaction evaporates more easily with the hydrobromic acid. This makes it possible to obtain higher concentrations of evaporated hydrobromic acid than those obtained with a stoichiometric defect. Fluoroboric acid can be used in aqueous solution, in particular between 10 and 55%, preferably between 40 to 55%, relative to the weight of the aqueous solution. For example, aqueous solutions containing approximately 48% fluoroboric acid are known. This makes it possible in particular to limit the cost of the evaporation of hydrobromic acid, a reaction by-product.

The reaction mixture can comprise between 20 and 60% of tetraethylammonium bromide relative to the total weight of the reaction mixture, preferably between 49 and 55% of tetraethylammonium bromide relative to the total weight of the reaction mixture.

The mixture can comprise a relative proportion by mass of fluoroboric acid between 40 and 55%, preferably between 41 and 53%, very preferably between 41 and 52%, relative to the tetraethylammonium bromide. The relative mass proportion of fluoroboric acid corresponds to the proportion of the chemical fluoroboric acid HBF4 independently of its aqueous dilution, that is to say the weight ratio HBF / BrN (C2H5) _.

Tetraethylammonium bromide and fluoroboric acid are preferably mixed in a substantially stoichiometric ratio. By “substantially stoichiometric ratio” is meant a ratio close to 1. Such a ratio helps to minimize excess tetraethylammonium bromide or excess fluoroboric acid. Minimizing the excess of tetraethylammonium bromide or fluoroboric acid further facilitates the subsequent purification step of the tetraethylammonium tetrafluoroborate suspension. If needed, it is better to have a tetraethylammonium bromide defect to fully convert the tetraethylammonium bromide. This results in leaving a slight excess of fluoroboric acid unconverted. The inventors have observed that, advantageously, excess fluoroboric acid or unreacted hydrofluoric acid is better removed by washes with ethyl alcohol. This is because fluoroboric acid and hydrofluoric acid are more soluble in alcohol than tetraethylammonium tetrafluoroborate.

Reacting tetraethylammonium bromide with an aqueous solution of fluoroboric acid provides a suspension of tetraethylammonium tetrafluoroborate and hydrobromic acid. This reaction is carried out at atmospheric pressure and at room temperature. By "room temperature" is meant a temperature between 15 and 25 ° C, in particular a temperature of about 18 ° C. This reaction is carried out for a period of time between 15 min and 2 h, preferably between 30 min and 1 h. This reaction is exothermic, causing a rise in temperature. At the end of this reaction, a suspension of tetraethylammonium tetrafluoroborate is obtained.

This method can further comprise a heating step. This heating step can be carried out concomitantly or consecutively to the reaction step. This heating step can make it possible to remove the hydrobromic acid by-product and the major part of the traces of acetonitrile by evaporation. This heating step can be carried out at a temperature between 30 and 100 ° C, preferably at a temperature between 40 and 90 ° C. In addition, this heating step can be carried out for a period of time of between 1 h and 4 h, preferably for a period of between 1 h and 2 h. Finally, this heating step can be carried out at a pressure of between 1000 Pa and atmospheric pressure, preferably at a pressure of between 2000 Pa and atmospheric pressure, very preferably at a pressure of about 2000 Pa. This step of heating can therefore be carried out under partial vacuum. This heating step makes it possible to obtain a suspension - viscous and pasty - of tetraethylammonium tetrafluoroborate, freed of a large part of the hydrobromic acid and of traces of hydrofluoric acid.

This method may further comprise a step of cooling the suspension of tetraethylammonium tetrafluoroborate. The tetraethylammonium tetrafluoroborate suspension can be cooled to a temperature of between -15 and 5 ° C, preferably at a temperature of between 0 and 5 ° C, for example at about 0 ° C. In addition, the suspension of tetraethylammonium tetrafluoroborate can be cooled for a period of time between 1 h and 4 h, preferably between 1 h and 2 h. This cooling can lead to crystallization of the tetraethylammonium tetrafluoroborate salts. This crystallization is not, however, systematic and compulsory.

This process may further comprise a step of filtering the cooled suspension of tetraethylammonium tetrafluoroborate. This filtration step is useful, especially when the tetraethylammonium tetrafluoroborate from the suspension has crystallized - at least partially - during the cooling step. This filtration step can be implemented by means of different filtration devices, preferably by means of devices chosen from the group consisting of filter presses, centrifugal dryers, centrifugal decanters, pressure filters or rotary filters; preferably pressure filters or centrifugal dryers. Pressure filters can be pressure hood filters sold under the name GUEDU. Rotary filters can be rotary filters covered to capture flammable vapors.

This process may further comprise at least one alcoholic washing step. These washing steps can be implemented by using ethanol. These washing steps can be implemented in particular by using the so-called “repulping” technique. The repulping technique consists of successive passages of the product in alcohol baths with stirring, with filtration and dewatering between each bath. Depending on the needs, it can be carried out between 2 and 10 washes. Indeed, these steps allow easily remove excess fluoroboric acid and / or any traces of boric acid and unreacted tetraethylammonium bromide.

This process can further include a final drying step. This drying step can be carried out at a temperature between 80 and 100 ° C, for example in an oven.

Thus, tetraethylammonium tetrafluoroborate is obtained in anhydrous and purified form. This tetraethylammonium tetrafluoroborate may contain traces of acetonitrile of the order of a few tens of ppm, which are irrelevant considering the intended use. Conversely, this tetraethylammonium tetrafluoroborate is free from all traces of alcohol and water.

Tetraethylammonium tetrafluoroborate obtained and corresponding use as electrolyte for supercapacitor

The invention also relates to tetraethylammonium tetrafluoroborate in anhydrous and purified form, which can be obtained by the preparation process described below. By "purified" or "ultrapure" is meant a product substantially free of the elements chlorine, hydrofluoric acid, boric acid - of formula B (OH) 3, fluoroboric acid and water. By "anhydrous" is meant a product containing a small amount of water measured by the Karl Fischer method. Due to its quality, this tetraethylammonium tetrafluoroborate can be used as an electrolyte for supercapacitors. A particularly suitable supercapacitor electrolyte comprises tetraethylammonium tetrafluoroborate in acetonitrile solution.

Tetraethylammonium tetrafluoroborate is characterized in particular in that it contains 50 ppm or less preferably, between 1 and 25 ppm, very preferably between 1 and 10 ppm, of water.

Tetraethylammonium tetrafluoroborate is characterized in particular in that it contains 30 ppm or less, preferably between 1 and 20 ppm, very preferably between 1 and 10 ppm, of the element chlorine.

Tetraethylammonium tetrafluoroborate is characterized in particular in that it contains between 30 ppm or less, preferably between 1 and 20 ppm, very preferably between 1 and 10 ppm, of hydrofluoric acid.

Tetraethylammonium tetrafluoroborate is characterized in particular in that it contains between 30 ppm or less, preferably between 1 and 20 ppm, very preferably between 1 and 10 ppm, of fluoroboric acid. Tetraethylammonium tetrafluoroborate is characterized in particular in that it contains 30 ppm or less, preferably between 1 and 20 ppm, very preferably between 1 and 10 ppm, of boric acid.

Impurities of chlorides, fluorides of hydrofluoric acid and boric acid are quantifiable by ion chromatography. Fluoroboric acid can be identified by a specific electrode for fluoborate BF4. The fluorides of hydrofluoric acid can be identified by a specific F-fluoride electrode. Chlorides can be identified by a potentiometric method with silver nitrate.

Tetraethylammonium tetrafluoroborate is characterized in particular in that it contains between 10 and 5000 ppm, preferably between 10 and 1000 ppm, very preferably between 10 and 50 ppm, of acetonitrile. The inventors have observed that the residual presence of acetonitrile is a specific marker of tetraethylammonium tetrafluoroborate obtained from tetraethylammonium bromide according to the process described opposite. In addition, the inventors have observed that the residual presence of acetonitrile was irrelevant for use as an electrolyte, in particular for a supercapacitor. Indeed, acetonitrile, as a reaction solvent, contributes to the simplified implementation of the process for preparing tetraethylammonium bromide. In addition, acetonitrile, as an application solvent, is particularly suited to the use of tetraethylammonium tetrafluoroborate as an electrolyte. In addition, the use of acetonitrile as a reaction solvent and as an application solvent makes the residual presence of acetonitrile in the final product - namely tetraethylammonium tetrafluoroborate in anhydrous and purified form - indifferent, which therefore makes it possible to remove any unnecessary intermediate step of ultimate purification, and therefore further facilitate the implementation of the methods described opposite. Finally, the residual presence of acetonitrile is a specific marker for the tetraethylammonium tetrafluoroborate obtained.

Examples

The following examples illustrate the invention without limiting it.

Example 1 - Process for preparing tetraethylamine bromide

A 500ml three-necked flask is supplied, equipped with a stirrer, reflux condenser and introduction plug.

120 g of ethyl bromide is introduced with 69.9 g of triethylamine in 150 ml of acetonitrile (quality for HPLC liquid chromatography) of density 0.786 g / cm ³ . The mixture is heated to reflux with a starting temperature of 60 ° C. After only 1 hour of reflux, the temperature rose to about 62 ° C. The reaction mixture (solution with partial suspension of solids) changes in color from colorless to orange, and crystals are formed rapidly. The solution is then cooled to 0 ° C. in ice for 2 h. The crystallized salts obtained are filtered off. Approximately 142 g of solid is obtained, which is drained in air on a sintered glass filter, which then forms a slightly orange cake, as well as approximately 169 g of filtrate. The filtrate comprises acetonitrile and dissolved (non-crystallized) tetraethylammonium bromide. With an impregnation of the mother liquors of 5.7%, a yield of tetraethylammonium bromide (crystallized) of 92% is calculated.

The 169 g of filtrate comprising acetonitrile and dissolved tetraethylammonium bromide are reintroduced into the three-necked flask. 69.9 g of trimethylamine are added with 120 g of ethyl bromide. The mixture is heated to reflux with a starting temperature of 51 ° C. After only 20 min of reflux, the temperature rose to about 57 ° C. The solution is then cooled to 0 ° C. in ice for 1 hour 30 minutes. The crystallized salts obtained are then filtered. 164 g of solid are obtained, still containing 10% of mother liquor, essentially acetonitrile and ethyl bromide. The yield is 101% taking into account in the calculation of the elimination of mother liquors contained in the cake.

An NMR analysis on carbon and hydrogen confirms that the product is overwhelmingly 98% tetraethylammonium, ie tetraethylammonium bromide (no NMR signals obtained for the halide). Increasing the residence time after reaction improves crystal size and speeds up filtration. However, it is not necessary to exceed a reflux reaction time beyond 3 hours in total.

Comparative Example 2 - Process for preparing tetra (n) -butyl ammonium bromide according to a known technique

A 500 ml three-necked flask is supplied, equipped with a stirrer, reflux condenser and introducer cap.

60.3 g of n-butyl bromide (unbranched) is introduced with 74.1 g of tributylamine in 100 ml of acetonitrile. The mixture is heated to reflux with a starting temperature of 81 ° C. After 22 hours of reflux, the temperature rose to about 87 ° C. The reaction mixture changes color from colorless to orange. An evaporation step is carried out using a rotary evaporator at a starting temperature of 23 ° C under a pressure of 80 mbar, to finish at a temperature of 52 ° C under a pressure of 50 mbar. 115 g of a moist solid containing 3.8% mother liquor are obtained, which corresponds to a yield of 87%. The tetra (n) -butyl ammonium bromide thus obtained has a slight orange tint.

To purify it, according to the technique disclosed in US 3965178, the solid obtained is dissolved in water, then the solution obtained is washed with three successive extractions in benzene solvent. Tetra (nj-butyl ammonium bromide thus remains in aqueous solution, without the aim of obtaining it solid.

Example 3 - Process for the preparation of tetraalkylammonium tetrafluoroborate

829 g of boron trifluoride trihydrate (BF3.3H20 concentrated at 55% in BF3) are mixed with 371 g of hydrofluoric acid assaying 34.02% without leading to a strong exothermicity. 1200 g of fluoroboric acid at 49% HBF4 are thus produced. There is therefore a stoichiometric excess of hydrofluoric acid of about 0.04 mol% (HBF4.04). 122 g of concentrated fluoroboric acid are taken for further processing. It is possible to start with molar boric acid and 4 times molar hydrofluoric acid. As the reaction is very exothermic, cooling media must be used to prevent the fluoroboric acid from boiling.

It is mixed in a beaker 122 g of fluoroboric acid HBF4.04 with 142 g of crude filtered tetraethylammonium bromide (obtained according to the first part of Example 1 above), i.e. 133 g of tetraethylammonium bromide as as such, during a period of 20 min. Tetraethylammonium bromide has not been subjected to any prior washing and / or purification step of the acetonitrile mother water. It is introduced at a rate of 8 mol% so that the stoichiometric molar ratio BrN (C2H5) 4 / HBF4 is 100%. After mixing, neither heating nor gas evolution is observed. Despite a color change to orange, no precipitate formation is observed. Tetraethylammonium fluoborate is extremely soluble in the hydrobromic acid by-product.

The suspension of tetraethylammonium fluoborate (not crystallized) and hydrobromic acid is evaporated by means of a rotary evaporator, under partial vacuum. This step allows the evaporation of hydrobromic acid, as well as traces of acetonitrile. This evaporation step begins at 40 ° C under a pressure of 2000Pa (20 mbar), with a rise in temperature to 60 ° C then at 90 ° C. 135 g of a viscous orange solid are then obtained at the limit of the pasty appearance. The solid thus obtained is cooled for 2 h at 0 ° C. The cooled solid is filtered off and washed with 20 ml of 100% absolute ethanol. A first washing with ethanol is carried out. The solid is resuspended with 250 g of absolute ethanol. The solid is stirred by rotation in a flask, at atmospheric pressure and for 1 h, without evaporation, in order to extract the impurities by repulping. 146 g of a white solid with orange iridescence is obtained by filtration. The crude solid thus obtained is larger than that containing hydrobromic acid, the latter compound having been replaced by ethanol. A second washing with ethanol is carried out by suspending with 250 g of absolute ethanol and repulping for 1 h. 123 g of a perfectly white solid are obtained without orange iridescence. A third washing with ethanol is carried out again by suspension with 250 g of absolute ethanol and repulping for 1 h. 120 g of a perfectly white solid are obtained by a filtration step. Finally, a fourth washing with ethanol is carried out by suspending with 250 g of absolute ethanol and repulping for 1 h. 110 g of a perfectly white solid are obtained after a filtration step. The solid is dried in an oven below 120 ° C. to remove the residual ethanol. A dry and purified white powder of 90 g is obtained which is free from ethanol and contains only minute traces of water. This powder is packaged in an airtight bottle with wide opening closed in an oven, then finished in an anhydrous glove box fitted with a vacuum airlock, then checked in contact with the glove box, under anhydrous argon, equipped with a humidity sensor measuring ppm water volume. If the tetrafluoroborate is dry, there is no increase in humidity.

The B, C, H, F NMR spectrum by calibration with trifluorotoluene demonstrates a purity of tetraethylammonium tetrafluoroborate of the order of 100.00%. In addition, the water content is less than 10 ppm by mass (Karl Fischer method) and the bromide content is less than 10 ppm by mass (potentiometry with silver nitrate or by ion chromatography).

However, by ¹³ C carbon NMR, acetonitrile is still detectable at a maximum threshold of 50 ppm by mass, in particular between 20 to 40 ppm by mass. Ethanol is no longer detectable, it being easily removable during drying in an oven.

The process for preparing tetraethylammonium bromide, and the process for tetraethylammonium tetrafluoroborate, according to the present invention, therefore make it possible to obtain tetrafluoroborate ultrapure and anhydrous tetraethylammonium, that is to say in a sufficient grade for use as an electrolyte, in particular as an electrolyte for a supercapacitor, for example in an acetonitrile solution.

Claims

1. A process for preparing tetraethylammonium bromide, comprising the following steps: mixing ethyl bromide and triethylamine; and reacting ethyl bromide and triethylamine at atmospheric pressure and in the presence of acetonitrile.

2. A process for preparing tetraethylammonium bromide, according to claim 1, comprising mixing excess ethyl bromide relative to the triethylamine.

3. A process for preparing tetraethylammonium bromide, according to any one of the preceding claims, comprising reacting ethyl bromide and triethylamine for a reaction time of between 15 min and 3 h and at a temperature between 45 and 70 ° C.

4. A process for preparing tetraethylammonium bromide, according to any one of claims, comprising reacting ethyl bromide and triethylamine with refluxing ethyl bromide in acetonitrile.

5. Process for preparing tetraethylammonium bromide, according to any one of claims, wherein the mixing and reaction of ethyl bromide and triethylamine are carried out under inert gas.

6. A process for preparing tetraethylammonium bromide, according to any one of claims, further comprising the following steps: cooling the obtained tetraethylammonium bromide solution, to precipitate as much as possible the tetraethylammonium bromide in crystals; optionally, filtration to remove the excess reaction medium, in particular acetonitrile.

7. Process for preparing tetraethylammonium bromide, according to any one of claims, said process being devoid of any additional step of removing traces of acetonitrile.

8. A process for preparing tetraethylammonium tetrafluoroborate, comprising the following steps: obtaining tetraethylammonium bromide by the process according to any one of the preceding claims; then mixing and reacting tetraethylammonium bromide and an aqueous solution of fluoroboric acid, to obtain a suspension of tetraethylammonium tetrafluoroborate in hydrobromic acid.

9. A process for preparing tetraethylammonium tetrafluoroborate, according to claim 8, further comprising the step of: heating for evaporation of the suspension of tetraethylammonium tetrafluoroborate in hydrobromic acid, to remove hydrobromic acid and traces of acetonitrile.

10. A process for preparing tetraethylammonium tetrafluoroborate, according to claim 9, further comprising the steps of: cooling the unpurified suspension of tetraethylammonium tetrafluoroborate; and filtration.

11. A process for preparing tetraethylammonium tetrafluoroborate, according to claim 10, further comprising at least one alcoholic washing step.

12. Process for preparing tetraethylammonium tetrafluoroborate, according to any one of claims 8 to 11, comprising the mixture of tetraethylammonium bromide and fluoroboric acid in a ratio substantially stoichiometric or in a substantially slight excess ratio of fluoroboric acid.

13. Tetraethylammonium tetrafluoroborate in anhydrous and purified form, obtainable by the preparation process according to any one of claims 8 to 12.

14. Tetraethylammonium tetrafluoroborate in anhydrous and purified form, comprising between 10 and 5000 ppm of acetonitrile; 50 ppm or less of water; 30 ppm or less of the element chlorine; 30 ppm or less of hydrofluoric acid; 30 ppm or less of fluoroboric acid; and 30 ppm or less of boric acid.

15. Electrolyte for a supercapacitor comprising tetraethylammonium tetrafluoroborate, obtainable by the preparation process according to any one of claims 8 to 12, in a solution of acetonitrile.

16. Use of tetraethylammonium tetrafluoroborate, obtainable by the preparation process according to any one of claims 8 to 12, as electrolyte for supercapacitors.