EP1230433B1 - Method for the electrolytic conversion of furane or furane derivatives - Google Patents

Abstract

The present invention relates to a process for the electrolytic transformation of at least one furan-base starting compound (A) in an electrolysis circuit. This process may be carried out in an electrolysis cell and include the electrolytically oxidizing the furan-based starting compound (A) to produce at least one alkoxylated furan compound (B) which has a C-C double bond in the five-membered heterocyclic ring, and hydrogen followed by hydrogenating the C-C double bond using hydrogen obtained at a cathode in the oxidizing step, hydrogen fed to the electrolysis circuit from outside or by electrocatalytic hydrogenation.

Description

The present invention relates to a method for the electrolytic conversion of Furan or one or more furan derivatives.

One goal of preparative organic electrochemistry is that of one Processes occurring on both electrodes parallel to electrochemical processes use. Of particular interest are those procedures in which the two Electrode processes that take place in an undivided cell for the implementation of chemical compounds can be used.

An example of such a process is the oxidative dimerization of 2,6-dimethylphenol, which is coupled with the dimerization of maleic acid esters (M. M. Baizer, in: H. Lund, M. M. Baizer (ed.), Organic Electrochemistry, Marcel Dekker, New York, 1991, pages 1442 ff.).

Another example is the coupled synthesis of phthalide and t-butylbenzaldehyde (DE 196 18 854).

However, it is also possible to use the cathode and anode processes to produce a single product or to destroy an educt. Examples of such electrochemical processes are, for example, the production of butyric acid (Y. Chen, T. Chou, J. Chin. Inst. Chem. Eng. 27 (1996) pages 337-345), the anodic dissolution of iron, that with the cathodic Formation of ferrocene is coupled (T. Iwasaki et al., J. Org. Chem. 47 (1982) pages 3799 ff.) Or the degradation of phenol (AP Tomilov et al., Elektrokhimiya 10 (1982) page 239).
Regarding the processes in which a furan derivative is reacted in an undivided electrolysis cell and the two electrode processes are used, the oxidation of furan carboxylic acid with subsequent ring opening to 1-carboxymethyl-4,4-dimethoxypropene, which is hydrogenated in a further step to the saturated propane derivative , (T. Iwasaki et al., J. Org. Chem. 47 (1982) pages 3799 ff.). However, this is not a catalytic hydrogenation, but a direct electroreduction. In this case, however, it is not the furan that is converted, but the α, β-unsaturated ester, ie a class of substances whose electrochemical reduction is known. In addition, the ring opening and the subsequent hydrogenation do not take place directly from the anodically produced product but from a fragmented stage which is poorer by one carbon atom and has undergone further oxidation.

An electrochemical oxidation of furan or a furan derivative while maintaining the heterocyclic ring structure and subsequent hydrogenation, one Double bond, which is present in the ring structure after oxidation, is hydrogenated however, is not known in processes in which both electrode processes are used.

An object of the present invention is therefore an electrochemical method To provide, which preferably takes place in an undivided electrolytic cell and in which Furan or a substituted furan in an electrode process while maintaining the heterocyclic ring structure is oxidized and this oxidation product with hydrogen is hydrogenated, the hydrogen being the product in the other electrode process arises or as hydrogen equivalent in the sense of an electrocatalysis on the Furan derivative is transferred.

(i) Elektrolytische Oxidation von Furan oder eines substituierten Furans oder eines Gemisches aus zwei oder mehreren davon unter Erhalt

(a) mindestens eines, im heterocyclischen Fünfring eine C-C-Doppelbindung aufweisenden Furanderivates (B), und

(b) Wasserstoff;

(ii) Hydrierung dieser C-C-Doppelbindung unter Verwendung des in Schritt (i) parallel an der Kathode erhaltenen Wasserstoffs oder von dem Elektrolysekreis von außen zugeführten Wasserstoff oder elektrokatalytische Hydrierung,

wobei das Verfahren in einer Elektrolysezelle durchgeführt wird, die mindestens einen Hydrierkatalysator umfaßt.This object is achieved by a method according to the invention for the electrolytic conversion of at least one furan derivative (A) in an electrolysis circuit, which comprises the two steps (i) and (ii):

(i) Electrolytic oxidation of furan or a substituted furan or a mixture of two or more thereof to give

(a) at least one furan derivative (B) which has a CC double bond in the heterocyclic five-membered ring, and

(b) hydrogen;

(ii) hydrogenation of this CC double bond using the hydrogen obtained in parallel in step (i) on the cathode or hydrogen supplied externally from the electrolysis circuit or electrocatalytic hydrogenation,

the method being carried out in an electrolysis cell which comprises at least one hydrogenation catalyst.

The process preferably takes place in an undivided electrolysis cell.

In addition to furan, the following compounds, for example, are preferred as substituted furans:
Furfural (furan-2-aldehyde), alkyl-substituted furans, furans with -CHO, -COOH, -COOR, in which R represents an alkyl, benzyl or aryl group, in particular a C ₁ - to C ₄ -alkyl group, -CH (OR ₁ ) (OR ₂ ), where R ₁ and R _{2 can be the} same or different and R ₁ and R ₂ each represent an alkyl, benzyl, aryl group, in particular a C ₁ to C ₄ alkyl group, and -CN groups in 2-, 3-, 4- or 5-position.

In the inventive reaction of organic compounds and solvents electrolyte salts can be used, as described in H. Lund, MM Baizer, (ed.) "Organic Electrochemistry", 3 ^rd Edition, Marcel Dekker, New York 1991.

According to the invention, the oxidation is preferably carried out in the presence of methanol or in Presence of ethanol or a mixture thereof, but preferably in the presence of Methanol. These substrates can be reactant and solvent at the same time.

In addition to furan or substituted furan and the In general, all suitable alcohols can be used for the compound used for the oxidation.

In addition to NaBr, the following can be used as conductive salts in the process of the invention, for example also alkali and / or alkaline earth metal halides, with bromides, chlorides as halides and iodides are conceivable. Ammonium halides are also suitable used.

Pressure and temperature can depend on the conditions used in catalytic hydrogenations are customary to be adjusted.

In a preferred embodiment of the method according to the invention Reaction temperature T <50 ° C, preferably T <25 ° C, the pressure p <3bar and the pH in the neutral area.

In a preferred embodiment of the process according to the invention, the starting materials are introduced into the undivided electrolysis cell
intermediate products are supplied. The intermediate product is at least one product which is obtained in step (i) of the process described above by electrolytic oxidation of furan or a substituted furan or a mixture of two or more thereof as a furan derivative (B) and is therefore in the electrolysis cycle. The concentration of the additional intermediates is adjusted by conventional electrochemical and electrocatalytic parameters, such as, for example, current density, type and amount of catalyst, or the intermediate is added to the circuit.

Regarding the special choice of the material of the electrodes there is in the invention Process no limit as long as the electrodes are for the same as above described methods are suitable.

Graphite anodes are preferably used in the undivided cell.

As for the geometry of the electrodes in the undivided electrolysis cell, see above there are essentially none for this in the context of the present invention Restrictions. For example, plane-parallel are preferred geometries Electrode arrangements and ring-shaped electrode arrangements to name.

In a preferred embodiment of the invention, at least one electrode is in Contact with at least one hydrogenation catalyst. In a particularly preferred one The at least one hydrogenation catalyst is part of an embodiment Gas diffusion electrode. In a further preferred embodiment of the invention at least one electrode is a graphite electrode coated with noble metal from sheets, nets or felt. In another preferred embodiment of the Invention is the hydrogenation catalyst in the form of a suspension in the electrolyte constantly brought into contact with at least one electrode. The hydrogenation catalyst, d. H. the catalytically active material, pumped around in the cell or onto one appropriately structured cathode or anode washed up. Such Precoat electrode is described for example in DE 196 20 861.

If a gas diffusion electrode is used for at least one of the electrodes, then basically processed the material from which the gas diffusion electrode is made be that the gas diffusion electrode can be used as an electrode without support material can. In a preferred embodiment, at least one of the used electrodes are a composite body, which is at least one conventional Includes electrode material and at least one material for a gas diffusion electrode.

It is conceivable that this further electrode material from or from there are several electrical conductors.

In principle, it is conceivable that the composite body, the conventional Electrode material and the material of the gas diffusion electrode comprises, as one electrode in the process according to the invention together with one or more suitable Counter electrodes are used.

These one or more suitable counter electrodes are subject to their geometry and their chemical composition no restrictions, as long as that method according to the invention can be carried out with them.

In a preferred embodiment of the method according to the invention further electrode material, one with the gas diffusion electrode material Composite forms, also used as a counter electrode of the gas diffusion electrode. This is achieved in that the electrode arrangement is switched bipolar.

In a preferred embodiment of the method according to the invention, as Gas diffusion electrode base material graphite and / or carbon fiber paper used. The catalyst mass is applied to this.

As a support material on which the gas diffusion electrode material is applied, in Within the scope of the method according to the invention, preferably further electrode materials uses that include carbon.

As described above, a C-C double bond is used in the process according to the invention using the hydrogen obtained in step (i) electrocatalytically or with the corresponding hydrogen equivalents in the sense of a Hydrolysis of electrolysis. This hydrogenation preferably takes place so that the hydrogenation Connection is brought into contact with one or more hydrogenation catalysts.

With regard to the selection of hydrogenation-active catalysts exist within the In principle, the method according to the invention has no restrictions. All from the Catalysts known in the art can be used. Among other things are included to name the metals of subgroups I, II and VIII of the periodic table, in particular Co, Ni, Fe, Ru, Rh, Re, Pd, Pt, Os, Ir, Ag, Cu, Zn and Cd.

For example, it is possible to use the metals in finely divided form, among other things use. Examples include Raney-Ni, Raney-Co, Raney-Ag or Raney-Fe, each of which also contains other elements such as Mo, Cr, Au, Mn, Hg, Sn or S, Se, Te, Ge, Ga, P, Pb, As, Bi or Sb can contain.

The described hydrogenation-active materials can of course also be a mixture of comprise two or more of the hydrogenation metals mentioned, optionally with For example, one or more of the above elements can be mixed.

Of course, it is also conceivable that the hydrogenation material on an inert Carrier is applied. Activated carbon, for example, Graphite, carbon black, silicon carbide, aluminum oxide, silicon dioxide, titanium dioxide, Zirconium dioxide, magnesium oxide, zinc oxide or mixtures of two or more of which, e.g. B. as a suspension or as finely divided granules.

In a preferred embodiment of the present invention, the hydrogenation Material applied to gas diffusion electrode base material.

Accordingly, the present invention also relates to a method as described above, which is characterized in that the gas diffusion electrode base material with a hydrogenated active material is loaded.

As a hydrogenation-active material with which the gas diffusion electrode system is loaded, all hydrogenation catalysts as described above are suitable. It goes without saying it is also possible to use a mixture of two or more of these as the hydrogenation-active material Use hydrogenation catalysts.

Of course, it is conceivable in the context of the inventive method that the Gas diffusion electrode material is loaded with hydrogenation material and in addition hydrogenated material is used, which is the same or different to that with which is loaded with the gas diffusion electrode material.

The method according to the invention, as described above, is particularly noteworthy characterized in that it essentially leaves the choice whether the electrocatalytic effective electrode, d. H. the electrode that is in contact with a hydrogenation catalyst, is used as a cathode or as an anode or as a cathode and anode.

Therefore, the present invention also relates to a method as described above, which is characterized in that the electrocatalytically active electrode, such as for example a gas diffusion electrode, used as a cathode and / or as an anode becomes.

Furthermore, the present invention relates to a method as described above, wherein the prepared furan derivative (B) converted to at least one ring-open butane derivative becomes. The at least one ring-open butane derivative is preferably 1,1,4,4-tetramethoxybutane or a substituted 1,1,4,4-tetramethoxybutane.

The following examples are intended to explain the present invention in more detail.

Examples example 1

An undivided cell with 6 ring-shaped electrodes with a surface per side of 15.7 cm ^{2 was} used. The electrodes were separated from each other by 5 spacer networks 0.7 mm thick.

The top and bottom electrodes were in contact with a power connector. The top one Electrode was connected anodically, the bottom one was cathodic, the middle electrodes bipolar.

The electrodes consisted of graphite disks, each 5 mm thick, which were covered on one side with gas diffusion electrode material. This material in turn was coated with 10 g platinum / m ² .

The gas diffusion electrode was switched as the cathode.

The electrolysis batch consisted of 30 g furan, 57.63 g 2,5-dimethoxydihydrofuran, 2 g NaBr and 112 g methanol.

The electrolysis was carried out at 0.47 A and a temperature of 15 ° C. During the Implementation increased the cell voltage from 13.0 V to 17.4 V. The electrolysis was followed by gas chromatography.

After 1 F / mol furan, the GC area percentage of furan was 22.9% 18.8% reduced, the dimethoxydihydrofuran content increased from 32.2% to 34.5%. At the same time, 1.4% of 2,5-dimethoxytetrahydrofuran was formed.

Example 2

The cell arrangement corresponded to that of Example 1. Instead of a Pt-loaded gas diffusion cathode, a gas diffusion electrode loaded with 5.2 g / m ² Pd was used.

The electrolysis batch consisted of 60 g furan, 126.2 g 2,5-dimethoxydihydrofuran, 2 g NaBr and 234.4 g of methanol.

The electrolysis was carried out at 0.47 A and a temperature of approx. 18 ° C. The cell tension rose from 19.1 V to 26.4 V. The electrolysis was monitored by gas chromatography.

After 1 F / mol furan, the GC area percentage of furan was 22.8% 18.0% reduced, the dimethoxydihydrofuran content increased from 30.7 to 30.9%. At the same time, 0.7% of 2,5-dimethoxytetrahydrofuran was formed.

Example 3

The cell arrangement corresponded to that of Example 1. Instead of a gas diffusion cathode, a gas diffusion electrode loaded with 5.2 g Pd / m ^{2 was used} as the anode.

The electrolysis batch consisted of 30 g furan, 57.4 g 2,5-dimethoxydihydrofuran, 2 g NaBr and 110.6 g of methanol.

The electrolysis was carried out at 0.48 A and a temperature of 17 ° C. The cell tension rose from 16.3 V to 19.5 V. The electrolysis was followed by gas chromatography.

After 1 F / mol furan, the GC area percentage of furan had increased from 22.7 to 16.9% reduced, the GC area percentage of 2,5-dimethoxydihydrofuran remained 30%. At the same time, 3.3% of 2,5-dimethoxytetrahydrofuran was formed.

Example 4

A cell with 5 ring-shaped electrodes with a surface area of 44 cm ^{2 was} used. The electrodes were separated from each other by 2 spacer networks of 1 mm thickness.

The electrodes consisted of graphite disks, each 5 mm thick, which were coated on the sides facing the electrolyte both anodically and cathodically with gas diffusion electrode material. This material was loaded with 0.5 mg Pd / cm ² .

The electrolysis batch consisted of 120 g furan, 229.9 g 2,5-dimethoxydihydrofuran, 8 g NaBr and 542.5 g MeOH.

The electrolysis was carried out at 1.32 A up to a current of 2 F / mol furan Electrolysis temperature was 17 ° C. The electrolysis was followed by gas chromatography.

Furan had decreased from 21.2 area% to 13.4 area%, and 2,5-dimethoxydihydrofuran decreased from 25.2 Fl% to 23.3 Fl%.

At the same time, 3.5 F1% dimethoxytetrahydrofuran were formed. In this attempt there was a ring opening.

2,5-Dimethoxydihydrofuran gave 1,1,4,4-tetramethoxy-cis-butene [1.3 F1%] and from 2,5-dimethoxytetrahydrofuran 4.2 area% resulted 1,1,4,4-tetramethoxybutane.

This means that the amount of furan has decreased by over a third, the level of Methoxylation (ring-closed and ring-open) has remained almost constant and that the Further hydrogenation has taken place on a large scale, in accordance with the Decrease in furan.

Example 5 Reaction of furan in the presence of a hydrogenation catalyst

In a frame cell with three anodes and cathodes each, consisting of a flexible, "Sigrabond CFC 07G" graphite cardboard coated on one side with a graphite mesh The company SGL, Meitingen, the implementation took place. There were one anode and one cathode switched monopolar and served as end plates, which were the other two electrodes switched in pairs and thus represented two bipolar electrodes Electrodes were kept at a distance by conventional spacer networks. The distance was 5 mm. The area of each electrode was 4.8 × 9.5 cm.

An electrolyte consisting of 75 g furan, 222 g methanol, 3 g NaBr and reacted 0.5 g of an activated carbon containing 10% paladium at 26 ° C. At a Current of 1.36 A and an average cell voltage of 24 V was 7.5 hours long electrolyzed, the suspension containing the catalyst constantly in the cell circle was pumped around. After completion, the furan content was 55% of the initial value like. With a selectivity of approx. 95%, 2,5-dimethoxydihydrofuran, 2,5-dimethoxytetrahydrofuran had and 1,1,4,4-tetramethoxybutane in the ratio 1: 0.75: 1.55 educated. The proportion of simultaneously methoxylated and hydrogenated products was therefore 70%.

Claims (11)

1. A process for the electrolytic transformation of at least one furan-based starting compound (A) in an electrolysis circuit which comprises both the steps (i) and (ii):

   (i) electrolytic oxidation of furan or a substituted furan or a mixture of two or more thereof to give

   (a) at least one alkoxylated furan compound (B) which has a C-C double bond in the five-membered heterocyclic ring, and

   (b) hydrogen;

   (ii) hydrogenation of this C-C double bond using the hydrogen obtained in parallel at the cathode in step (i) or hydrogen fed to the electrolysis circuit from outside or by electrocatalytic hydrogenation,

   wherein the process is carried out in an electrolysis cell in which at least one hydrogenation catalyst is present.
2. A process as claimed in claim 1, which proceeds in an undivided electrolysis cell.
3. A process as claimed in claim 1 or 2, wherein at least one electrode is in contact with at least one hydrogenation catalyst, in particular with a noble metal.
4. A process as claimed in claim 3, wherein the hydrogenation catalyst, in particular the noble metal, has been applied to a graphite felt.
5. A process as claimed in claim 3, wherein the hydrogenation catalyst has been washed onto the electrode or electrodes.
6. A process as claimed in claim 3, wherein the hydrogenation catalyst is brought in the form of a suspension into contact with the electrode or electrodes.
7. A process as claimed in any of claims 1 to 6, wherein at least one of the electrodes used is a gas diffusion electrode.
8. A process as claimed in any of claims 1 to 6, wherein at least one of the electrodes used is a composite comprising at least one conventional electrode material and at least one material for a gas diffusion electrode.
9. A process as claimed in any of claims 3 to 8, wherein the electrode or electrodes which is/are in contact with a hydrogenation catalyst is/are used as cathode or as anode or as cathode and anode.
10. A process as claimed in any of claims 1 to 9, wherein the alkoxylated furan compound (B) produced in step (i) is reacted in step (ii) to form at least one ring-opened butane derivative.
11. A process as claimed in claim 10, wherein the ring-opened butane derivative is 1,1,4,4-tetramethoxybutane or a substituted 1,1,4,4-tetramethoxybutane.