EP0370920B1 - Process for the production of quinone from hydroquinone by electrolysis - Google Patents

Abstract

Quinones are produced by electrolysis of corresponding hydroquinones, in the anode compartment of an electrolytic cell, by electrolyzing a dispersion or emulsion including a conductive aqueous solution of hydroquinone and at least one stable cosolvent which is poorly soluble in water but which is a good solvent for the quinone produced and a poor solvent for the hydroquinone.

Description

The present invention relates to a process for the preparation of quinone from hydroquinone by electrolysis.

It is known that the electrolysis of an aqueous solution of a hydroquinone can lead to the formation of the corresponding quinone. However, this is particularly the case when the electrolysis of an aqueous solution of hydroquinone is carried out, it is found that the yield of this reaction is very low due to the precipitation of a compound resulting from the addition of a hydroquinone molecule on a quinone molecule, said compound being called quinhydrone.

We tried to reduce the risks of formation of this addition molecule (quinhydrone) by operating either in very dilute solutions, which leads to very low faradic yields, or at high temperature but there is then a risk of degradation of the quinone obtained.

The present invention presents a new electrolytic process making it possible to prepare, with industrial yields, a quinone by electrolysis of an aqueous medium containing a hydroquinone; this new process is characterized in that one carries out the electrolysis of an aqueous dispersion or emulsion containing, in addition to said aqueous solution of hydroquinone, a stable co-solvent poorly soluble in water which is a good solvent for quinone and a poor solvent for hydroquinone.

• ledit cosolvant doit être stable ; il s'agit bien entendu d'une stabilité chimique vis-à-vis de l'ensemble des produits présents et d'une stabilité électrochimique dans les conditions opératoires utilisées. Cette stabilité du cosolvant est importante dans la mesure où toute instabilité de ce produit se traduira par l'apparition d'impuretés (qu'il faudra ultérieurement éliminer) et par une baisse du rendement,
• ledit cosolvant doit être peu soluble dans l'eau et réciproquement ; ledit cosolvant est en effet employé pour constituer une phase indépendante de la phase aqueuse, la solubilité de ce cosolvant dans l'eau doit donc être aussi faible que possible. Cette faible solubilité du cosolvant dans l'eau implique que l'électrolyse s'effectuera sur un milieu présentant deux phases (émulsion),
• ledit cosolvant doit être un mauvais solvant pour l'hydroquinone, et un bon solvant pour la quinone. En effet, un des rôles de ce cosolvant sera d'assurer, au cours de l'électrolyse, une séparation effective de l'hydroquinone d'avec la quinone de façon à éviter la formation, connue, d'un produit d'addition du type quinhydrone.

To select the co-solvent to be used, the following requirements should therefore be taken into account:

• said co-solvent must be stable; it is of course a chemical stability with respect to all of the products present and an electrochemical stability under the operating conditions used. This stability of the co-solvent is important insofar as any instability of this product will result in the appearance of impurities (which will have to be removed later) and in a drop in yield,
• said co-solvent must be sparingly soluble in water and vice versa; said co-solvent is in fact used to constitute a phase independent of the aqueous phase, the solubility of this co-solvent in water must therefore be as low as possible. This low solubility of the cosolvent in water implies that the electrolysis will be carried out on a medium having two phases (emulsion),
• said cosolvent must be a bad solvent for hydroquinone, and a good solvent for quinone. Indeed, one of the roles of this cosolvent will be to ensure, during electrolysis, an effective separation of hydroquinone from quinone so as to avoid the known formation of an adduct of the quinhydrone type.

In addition to these essential properties for the proper functioning of the process according to the invention, the cosolvent must allow easy recovery of the quinone produced. Although relatively high-boiling cosolvents may be of interest, since electrolysis will be possible at high temperatures, it is generally desirable to choose a relatively low-boiling product as co-solvent as this may facilitate the subsequent recovery of the quinone by simple evaporation of the cosolvent.

alors que, dans ces mêmes solvants, l'hydroquinone n'est soluble qu'à raison de quelques g/l. On pourra également utiliser des mélanges de ces cosolvantsAmong the co-solvents having such properties, mention may be made of aromatic carbides (in particular toluene and benzene), cycloalkanes, alkanes and halogenated aliphatic carbides (such as methylene chloride and 1,2-dichloroethane). These halogenated aliphatic carbides seem to be the most interesting solvents. The solubilities of para-benzoquinone in various solvents will be mentioned below, by way of examples:

whereas, in these same solvents, hydroquinone is only soluble at a rate of a few g / l. Mixtures of these cosolvents may also be used

In the process according to the invention, the relative amounts of water and of cosolvent can vary with the nature of the cosolvent and possibly of the reactants (hydroquinone and quinone). For given reagents (for example the electrolysis of hydroquinone giving rise to p-benzoquinone), these relative quantities should be adjusted to take into account, on the one hand, the conductivity of the emulsion (which would require a high proportion of aqueous phase) and, on the other hand, the amount of p-benzoquinone to be extracted (which would require a high proportion of organic phase). In practice, the ratio, by volume, of the aqueous and organic phases is from 0.1 to 50 and preferably from 0.5 to 10. When said ratio is less than 0.1, the aqueous phase being in very low proportion, the conductivity of the mixture is poor. When said ratio is greater than 50, the possibilities of solubilization of the quinone are insufficient.

It will also be noted that the "quality" (that is to say in fact the fineness and the stability) of the dispersion of the cosolvent in the aqueous phase can play a role in the yield of the reaction. It will be easy for the specialist, taking into account the preparation processes used to produce the dispersion or the emulsion (for example using a pump or a static mixer), to optimize said dispersion, possibly in it. adding inert emulsifiers or surfactants, to obtain maximum yield.

The temperature at which electrolysis takes place has a known influence (the increase in temperature improving the conductivity of the emulsion, improving the solubility of the reagents in their media and improving the kinetics of the reaction); however, if for quinone recovery problems a cosolvent with a relatively low boiling point is used, one will be limited by the boiling point of this cosolvent. In practice, temperatures of 10 to 80 ° C will be used.

The concentration of hydroquinone in water does not seem to be a decisive factor concerning the rate of conversion of hydroquinone to quinone with equal electrical yield, but any increase in said concentration (within the limit of the solubility of hydroquinone ) will promote volume yield.

As regards the electric current density, it is generally of the order of 5 to 40 A / dm².

The reaction is carried out in a conventional electrolysis cell preferably comprising a separator. When said cell comprises a separator, the latter is preferably of the cationic type such as for example a membrane of registered trademark NAFION. In the cathode compartment, as known, reduction of water made conductive by an acid such as sulfuric acid is carried out (but it is also possible to carry out, in this cathode compartment any other electrochemical reduction reaction); the cathode must be non-corrodible and with as low an overvoltage as possible. In the anode compartment, the dispersion or emulsion according to the invention is therefore admitted, comprising therefore an aqueous phase, the conductivity of which has been improved by the addition of an inert acid with respect to the reactants (such as the acid sulfuric, phosphoric acid or nitric acid) and / or a salt and an organic phase dispersed or emulsified in said aqueous phase. The anode is made of a stable material (that is to say non-corrodible) which is advantageously an oxide or an alloy of lead or, preferably, a stop metal such as, for example, titanium whose surface is covered with metals or metal oxides, one or less of which belongs to the platinum family. The structure of the anode can be very diverse; we will use anodes deployed or perforated, or full.

One can operate, of course, discontinuously or continuously, this latter mode of implementation being preferred; it is also possible, to optimize the yields, to use several reactors connected in series in each of which the operating conditions may be adapted to the mixtures to be treated.

The hydroquinones which can be used according to the invention can be defined as all those which, in an aqueous medium give rise, in the presence of the corresponding quinone, to a quinhydrone.

The typical special case is that where para-benzoquinone is prepared from hydroquinone. It is essentially this particular case which will be illustrated below.

The nonlimiting examples illustrate the invention. Examples 1 to 16 were carried out batchwise, that is to say by carrying out the electrolysis of a certain volume of dispersion (or emulsion), this dispersion being either contained in the suitably agitated anode compartment of the electrolyser , or put into circulation in a closed loop on said anode compartment. Example 17 was carried out continuously.

EXAMPLE 1 TO 13

In all the examples, a cell was used comprising a NAFION 423 separation membrane, a catholyte consisting of an aqueous solution of H₂SO₄ at 0.5 N, an INCOLOY 825 cathode an anode which is either made of coated titanium or of lead ; an amount of cosolvent has always been used in the anolyte such that the ratio, by volume, of the organic phase to the aqueous phase is 0.5, said aqueous phase being at 0.1 N of sulfuric acid.

In all the tests, it was found that the chemical yield, that is to say the percentage of transformed hydroquinone which is found in the form of quinone (in moles), is very high from 91 to 100%. The faradic yield, which is very high (80 to 100%) at the start of the reaction, decreases after a certain time due to the decrease in the concentration of hydroquinone and the development of parasitic reactions (oxidation of the water).

It would therefore logically be necessary to consider the yields of the reaction at each instant of the duration of this reaction; such a study, even if it is interesting for optimizing the reaction on an industrial level, is not currently completed, we will therefore be content to provide results of overall yields at the end of the reaction, said reaction having been stopped after about 70 to 95% of the starting hydroquinone was consumed, that is to say after reaction times of the order of 50 to 80 minutes.

The results obtained in the transformation of hydroquinone into para-benzoquinone are collated in Table 1; Example 1 was carried out with a titanium anode coated with solid platinum; Examples 2 to 7 were carried out using an anode in expanded titanium covered with platinum; Examples 8 to 11 were carried out using a perforated titanium anode on which oxides of iridium, cobalt and tantalum were simultaneously deposited; Example 12 was carried out with a perforated lead electrode; Example 13 was carried out with an aperture in palladium titanium covered with platinum-iridium.

In Examples 2, 3, 5, 6, and 7, the voltage ΔV varied during the test from approximately 6 to approximately 8 V; this voltage remained constant and equal to 4.5 V in Example 4; at 4.25 V in Example 8; at 5 V in Example 9; at 2.8 V in Examples 10 and 11; at 4.9 V in Example 12 and at 3.2 V in Example 13.

EXAMPLE 14

The conditions of Example 2 were reproduced using, in place of hydroquinone, toluhydroquinone in a concentration of 10 g / l. The corresponding toluquinone was obtained with a faradic yield of 84% and a chemical yield of 88%.

EXAMPLE 15

In this example, a palladium titanium anode coated with platinum and iridium was used and an anolyte whose aqueous phase has an acidity of 0.4 N in H₂SO₄.

The other experimental conditions are the following:

The faradic yield was 85%.

EXAMPLE 16

In this example, a titanium anode coated with platinum and an anolyte were used, the aqueous phase of which has an acidity of 0.1 N in sulfuric acid.

Rapport en volume de la phase organique à la phase aqueuse : 1,2.The other experimental conditions are the following:

Volume ratio of the organic phase to the aqueous phase: 1.2.

The faradic yield of the reaction was 68.5% and the chemical yield 100%.

EXAMPLE 17

This example was done "continuously".

The installation includes an electrolyser with two compartments separated by a cationic type separator (NAFION brand membrane).

In the cathode compartment is circulated an aqueous solution of sulfuric acid at 0.5 N.

Into the anode compartment, a mixture of 0.1 N sulfuric acid, dichloromethane (ratio of the organic phase to the aqueous phase of 0.5) and hydroquinone (hydroquinone concentration 20 g / l) is sent. At the exit of this compartment, the mixture is decanted, the organic phase is evacuated in order to recover the quinone produced there, and the aqueous phase is recycled (and supplemented by a supply of water, hydroquinone and dichloromethane).

The anode is made of titanium coated with platinum and iridium.

The temperature is 35 ° C, the current density of 10A / dm² and the potential difference of 4.25 V.

A faradic yield of 100% and a conversion rate of 78% were obtained.

Claims (5)

1. Process for the preparation of quinone from hydroquinone by electrolysis, characterized by performing the electrolysis, in the anode compartment of an electrolyser, of a dispersion or emulsion comprising a conductive aqueous solution of the hydroquinone and at least one stable cosolvent which is poorly soluble in water and which is a good solvent for the quinone produced and a poor solvent for the hydroquinone.
2. Process according to claim 1, characterized in that a solution of hydroquinone is electrolysed with a view to obtaining para-benzoquinone.
3. Process according to either of claims 1 and 2, characterized in that the said cosolvent is chosen from aromatic hydrocarbons, cycloalkanes, alkanes and halogenated aliphatic hydrocarbons and mixtures thereof.
4. Process according to one of claims 1 to 3, characterized in that the electrolyser comprises a separator, preferably of a cationic type.
5. Process according to one of claims 1 to 4, characterized in that the anode consists of a stable metal which is preferably a stop metal such as titanium whose surface is coated with metals or metal oxides, at least one of which belongs to the platinum group.