EP1608797B1 - Method for the anodic alkoxylation of organic compounds - Google Patents

Description

The invention is directed to a process for the anodic alkoxylation of organic compounds, in particular cyclic ethers, N-substituted amides, carbonyl compounds, alkylaromatics and heteroaromatics. The anodic alkoxylation, in particular it is a methoxylation, is carried out in an electrolysis cell divided by a membrane electrode unit (MEA) in the absence of a mediator.

Alkoxylation reactions of saturated and unsaturated cyclic ethers and of N-alkylamides and alkylaromatics and alkyl heteroaromatics are of industrial importance, since the resulting products or their hydrolysis products are valuable raw materials for pharmaceuticals and pesticides. Various methods for the anodic alkoxylation of organic compounds are known.

U.S. Patent 2,714,576 teaches the electrolytic production of 2,5-dialkoxy-2,5-dihydrofurans wherein furan or a substituted furan is electrolyzed in an aliphatic alcohol of 1 to 5 carbon atoms in the presence of a soluble electrolyte. The electrolyte used is ammonium bromide, the effect of which is that it acts as a mediator. The substrate to be alkoxylated is thus not alkoxylated directly but indirectly, namely via the intermediate step of a bromination. A major disadvantage of anodic alkoxylation in the presence of a mediator, such as in particular a halogen compound, is that the mediator itself can lead to the increased formation of by-products and thus complicates the work-up and purification of the alkoxylated substrate.

Although it is also known, furan derivatives in the presence of conductive salts, which do not act as a mediator, For example, alcoholates to alkoxylate anodically, but in such methods, the current yields and product yields are usually very low.

In an effort to further improve the alkoxylation of organic substrates, such as furans and N-alkylamides, processes have been developed which do not require any conductivity-increasing additives, but in which solid electrolyte (SPE) is used instead. The solid electrolyte is expediently present in the form of a membrane, wherein the two membrane sides are in close contact with the electrodes. Membrane, anode and cathode thus form a so-called membrane electrode unit (MEA).

Fabiunke et al. describe in the Dechema monograph, Vol. 112, 299-315 (1988) organochemical syntheses, including the methoxylation of furan, in perfused cells with a membrane electrode assembly of an ion exchange membrane with porous catalytic electrodes on both sides of the membrane. The electrode reaction takes place here on catalyst layers on the membrane surface. The current is supplied by suitable current collectors; the swollen ion exchange membrane works as an ionic conductor. According to the teaching of this document, the electrocatalytically active layers can be applied directly to the membrane (attached porous electrode layer) or else porous, possibly coated electrodes can be pressed onto the membrane without a gap (zero gap). The electrode layers for the alkoxylation in this document were porous platinum layers electrochemically applied to a polyfluorinated cation exchange membrane.

While in the previously acknowledged document the current yields and the stability of the porous platinum layers were described as good, Ogumi et al., Nippon Kagaku Kaishi 11, (1984) 1788-1793 in the alkoxylation of furan using a similar membrane electrode assembly to a different result, since only low current efficiencies were obtained. By adding a small amount of bromine, the current efficiency could be increased and the voltage significantly lowered. Since bromine is a typical mediator, the known disadvantages can not be overcome with this method. Since Fabiunke et al. on the one hand and Ogumi et al. On the other hand, if completely different results were obtained, it must be assumed that the structure of the membrane electrode assembly and / or the way in which the electrode layers are applied have a significant influence on the alkoxylation of furan.

Jörissen et al. Report in the Dechema monograph Volume 125 (1992), 993-706 about the use of fuel cells with a membrane electrode assembly for performing organic reactions, including the methoxylation of furan. The membrane electrode unit is a Nafion® membrane (sulfonated polyfluorinated polymer or copolymer from E.I Du Pont) with platinum deposited chemically or electrochemically on the surfaces. As collectors platinum / iridium nets or graphite felt are used. In the alkoxylation of furan, high current yields can only be obtained when very large cell voltages are applied, which is very disadvantageous in view of a larger plant.

In his dissertation (University of Dortmund, 14.10.1999), D. Klein carried out investigations on the use of conduction electrolyte-free SPE electrosynthesis in nonaqueous systems, using SPE fuel cell technology for the methoxylation of carboxylic acid amides and furan. The electric catalyst layer The membrane electrode assembly was located on or within the surface of the Nafion® membrane. The electrocatalyst layer was deposited here either by means of a chemical / electrochemical process as a porous layer on the Nafion® membrane or produced by means of an indirect printing process and pressed onto the membrane. In the indirect process, the electrocatalyst was dispersed in a Nafion® solution and this mixture was applied to a Teflon® film; after evaporation of the solvent mixture at elevated temperature, the catalyst layer including the carrier film was pressed onto the membrane by means of a hot press; then the carrier film was peeled off. The addition-free furan methoxylation led to an increase in the cell tension to indiscussable high values within a very short time. The said electrosynthesis could be improved by the addition of various co-solvents, but this makes it difficult to work up the reaction mixture. The statements in this document suggest that the nature of the electrocatalyst layer is a cause of the unsatisfactory behavior of electrosynthesis in the absence of cosolvents.

In the process according to DE 195 33 773 A1 and EP 0 965 658 A1, EP 0 965 659 A1 and EP 0 965 660 A1, a plate-stack cell with series-connected stacking electrodes is used for the electrolytic oxidation, including an anodic alkoxylation of alkylaromatics, ethers and carbonic acid amides , wherein at least one stacking electrode consists of a graphite felt plate, a carbon felt plate or a fabric of carbon-covered Eduktkontaktfläche. Conveniently, the electrolyte phase contacting the carbonaceous stack electrode is a solid state electrolyte. The technical complexity of the plate stacking cell is significant because the cell has a specific structure and a suitable periphery required. Although partially high selectivities are achievable, the current yields leave something to be desired. There is thus a potential for further improvements.

In GDCH monograph, Vol. 23 (2001), 241-249, Reufer et al. on the methoxylation of furan in a fuel cell as a synthesis reactor. A membrane electrode unit based on a Nafion® membrane was used, which was coated on both sides with soot. Partly, a membrane coated with platinum-modified carbon black was also used. Commercially available graphite paper was used as current collector. In the methoxylation of furan using a bilayer carbon black coated membrane, a rapid increase in voltage was observed. By applying platinum to the soot-coated membrane, although a more favorable voltage profile could be achieved over the electrolysis period, the conversion of furan and product formation were adversely affected by platinum. Although it was found in this document that the internal structure of the electrolysis appears to have an influence on the efficiency of the electrolysis, neither information was given on the composition of the carbon black coating nor on the method of how it was applied to the membrane. Thus, there is still much interest in further improving the alkoxylation of substrates known from many documents.

WO 97/13006 teaches a membrane electrode assembly having on one side of a polymeric perfluorosulfonic acid membrane, an oxidizing catalyst and on the other side a reducing catalyst comprising at least one of the following elements in elemental form or in the form of compounds, namely Zn , La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lo, Bi and In. The oxidizing catalyst suitably contains a Element of the series palladium, platinum and iridium. The membrane electrode assembly is made by directly coating with a suspension containing a catalyst-containing carbon black and an ionomeric binder in a liquid medium such as propylene carbonate. Except for said membrane electrode assembly and a reactor containing it, this document is directed to a method for producing hydrogen peroxide from hydrogen and oxygen. Further, by using a membrane electrode assembly with the aforementioned reducing catalyst, alkenes can be epoxidized and sulfur dioxide oxidized to sulfuric acid. Other possible applications include the conversion of organic nitro compounds into aminic dyes and the recovery of phenol from benzene. Advice to carry out reactions other than those mentioned and / or to use a membrane electrode assembly which has a reducing catalyst other than that mentioned can not be found in this document.

The present invention accordingly provides an improved process for the alkoxylation of organic compounds, in particular those from the series of cyclic ethers, N-substituted amides, carbonyl compounds, in particular ketones, alkylaromatics and alkyl heteroaromatics, wherein the anodic alkoxylation in an electrolysis cell containing a membrane electrode unit in FIG Absence of a mediator can be performed with high current efficiency. In a preferred embodiment, the anodic alkoxylation should be able to be performed under practical operating conditions at a cell voltage below 25 volts.

It has surprisingly been found that the stated objects and further objects, as they are derived from the description below, are thereby achieved can be used as the membrane electrode unit one which on a fluorinated cation exchange membrane or a non-ionomeric microporous polypropylene membrane on both sides of a soot and / or graphite-containing coating which contains in addition to the carbon black or graphite and optionally a heavy metal catalyst additionally an ionomer ,

Accordingly, a process has been found for the anodic alkoxylation of an organic compound by passing a mixture containing the organic compound and an alcohol having 1 to 4 carbon atoms, in particular methanol and ethanol, through the anode compartment of an anode chamber and a membrane electrode assembly (MEA) And the MEA comprising a membrane having both sides provided with an electrode layer characterized by using a reactor with an MEA having a cation exchange membrane or a microporous polypropylene membrane having one or both electrode layers formed thereon using a carbon black and / or graphite, which may be heavy metal doped, and a suspension containing a sulfonated polyfluorinated polymer or copolymer in a liquid suspension medium.

The subclaims are directed to preferred embodiments of the method according to the invention, in particular to embodiments of the coating and to the organic substrates which are preferably to be alkoxylated. When alkoxylating with isopropanol, it should be noted that the stability of a Nafion® membrane in this medium is limited.

The construction of a reactor with a membrane electrode assembly (MEA) is well known to those skilled in the art: The reactor comprises a container which, through an MEA disposed therein, into a cathode compartment and a Anode space is shared. On both sides of the MEA are microporous current collectors, which as well as the actual electrode layers are permeable to material. The current collectors consist of a highly electrically conductive porous material, such as a graphite paper, graphite felt or a network of a noble metal or a metal alloy. The layer of the current collector lying opposite the electrode layer adjoins the cathode space or the anode space. Conveniently, these spaces are in the form of a structured flow field, which may be parallel channels, meandering channels or a checkerboard-like structure enabling cross-mixing. The reactor further comprises in each case an inlet and a discharge into / from the cathode space, and into / from the anode space.

In the alkoxylation of an organic compound, the compound to be alkoxylated is passed through the anode space in a solution of the alcohol used for the alkoxylation. If necessary, the solution can be added to the solution known stability stability of the voltage in effective, the selectivity substantially not lowering amount. Examples are water, H ₂ SO ₄ . It is also possible to use solutions with a cosolvent, such as sulfolane, alkylamides. For removal of the hydrogen formed at the cathode, the reaction mixture to be alkoxylated or an already alkoxylated reaction mixture can be used. Alternatively, in particular in continuous processes, other liquid media or a gaseous medium containing constituents, whereby the effectiveness of the membrane is not adversely affected, can be used as the cathode space medium. According to a particularly preferred embodiment, the mixture to be alkoxylated is first passed through the anode compartment and then also the cathode compartment. After the separation of the hydrogen from the at least partially alkoxylated reaction mixture, the latter can be redirected through the anode compartment. This cycle is repeated until the desired conversion of the compound to be alkoxylated or the desired charge conversion are achieved. The work-up of the alkoxylated reaction mixture depends on the substance data of the reaction components contained therein. Usually, the workup includes steps from the series of distillation and extraction.

Surprisingly, the alkoxylated target products proved to be suitable cosolvents for increasing the selectivity. Accordingly, it may be advantageous to add up to 35 mol% of the alkoxylated product to the feed mixture already at the beginning of the electrosynthesis.

The anodic alkoxylation, in particular a methoxylation or ethoxylation, is expediently carried out at a current density in the range from 1 to 500 mA / cm ² , preferably 10 to 50 mA / cm ² . The operation of the reactor is carried out at a voltage in the range of 1 to 50 volts, preferably 5 to 25 volts. The use concentration of the compound to be alkoxylated in the alcohol used for the alkoxylation is less critical; a use concentration in the range of 0.1 to 5 mol / l, in particular 0.5 to 3 mol / l is preferred.

The membrane (MEA) is preferably an ionomeric membrane having cation exchange properties. In view of the required chemical stability of the membrane, fluorinated membranes which contain sulfonic acid groups as a cation exchanger group have proven to be useful. Preferred polymers and copolymers may have, in addition to a carbon chain forming a polymer chain, also those chain elements or branches containing ether bridges. Such polymers and copolymers are commercially available in the form of films, for example under the name Nafion® (EI

Du Pont) and Gore Asselect® (W.L. Gore and Sociates). The MEA formed as a membrane solid electrolyte may consist of one or more layers and preferably has a thickness in the range of 25 to 300 microns.

Surprisingly, microporous non-ionic membranes, in particular microporous polyolefin membranes, such as preferably a polypropylene membrane, are also suitable. Although the selectivity of the alkoxylation using the microporous polypropylene membrane used by the inventors is somewhat lower than when using an ionomeric membrane, the chemical stability of the membrane is much higher than that of the Nafion® membranes.

For the production of the electrode layers, it is possible to use any sufficiently conductive carbon black known per se for such purposes, as well as graphite or any mixtures of carbon black and graphite. The carbon black or graphite to be used can also be doped with a catalytically active heavy metal, in particular a metal from the series gold, platinum, palladium and iridium, in an effective amount. The suspension used to produce the electrode layers contains, in addition to the carbon black or the doped carbon black, an ionomeric, in particular a polyfluorinated, sulfonated polymer or copolymer in dissolved form or in the form of swollen very small particles. Solvents or swelling agents can be used in pure form or in the form of mixtures. Suitable agents are, for example, alcohols, such as isopropanol, isobutanol and tert-butanol, and esters, in particular cyclic esters, such as Proplencarbonat. Dissolved binders based on perfluorinated sulfonated polymers and copolymers, which are further dilutable with the solvents mentioned, are commercially available. Ionomers in the Na + form are available in aqueous solvent systems. The polymer or copolymer in dissolved form usually not in the form of the free Sulfonic acid, but in the form of a salt, for example a sodium salt or preferably a tetrabutylammonium salt. The solution of the polymer or copolymer may additionally contain water.

The suspension is used in a manner known per se using conventional coating techniques such as brushing, printing, dipping and spraying to form the porous electrode layers. As an alternative to these techniques, the indirect printing method is also suitable, in which case an inert carrier is first coated and then the layer is transferred to the ionomeric carrier. Particularly suitably, the coating is carried out using the screen printing. After coating the membrane with the suspension, the solvent contained in the suspension is evaporated at elevated temperature, and then the membrane is heat-treated together with the one or both electrode layers at a temperature in the range of 75 ° C to about 85 ° C subjected. After the temperature treatment, the electrode layer, if present in salt form, is converted in a manner known per se into the protonated form. The method of making the membrane electrode assembly with the generic electrode layers is disclosed in U.S. Patent No. 5,211,984, which is incorporated herein by reference.

In particular, organic compounds from the series of cyclic ethers, N-substituted amides, carbonyl compounds, in particular ketones, alkylaromatics and alkylheteramines are obtainable from the anodic alkylation according to the invention.

A first class of well-alkoxylated substrates are cyclic ethers which may be saturated, unsaturated or heteroaromatic. The oxygen-containing ring system expediently has 5 to 7 ring members, preferably 5 or 6 ring members with an O atom, but further saturated or unsaturated ring systems, in particular benzene nuclei can be fused to this ring system. Examples of substances from the classes mentioned are furan, as well as mono- to tetra-substituted furans, as well as the dihydro and tetrahydro compounds derived therefrom, such as tetrahydrofuran. Other cyclic ethers are 1,2- and 1,4-pyrans and their di- and tetrahydro derivatives; Finally, 1,4-pyrones and their di- and tetrahydro derivatives of anodic alkoxylation are accessible. Alkoxylated are also 1,2-pyrones, which are, however, lactams. The substituents are in particular alkyl groups, which in turn may have a functional group such as hydroxyl, acetoxy, alkoxycarbonyl, amidocarbonyl, carboxyalkyl, nitrile and amino. Conveniently, such a functional group is bonded to the heterocyclic ring via a methylene or ethylene bridge. Further substituents are alkoxy, halogen, carboxyl, acyl and the aldehyde group. If non-aromatic cyclic ethers are alkoxylated, they must have at least one abstractable H atom on a C atom adjacent to the ether oxygen.

By using furan or a substituted furan, the corresponding 2,5-dihydro-2,5-dialkoxyfurans are formed by the anodic alkoxylation according to the invention with generally high material yield and very high current efficiency. Starting from the hydrogenated furans or other cyclic ethers, such as pyrans, pyrones, dioxane and morpholine, the corresponding mono- and / or dialkoxy derivatives are formed, wherein the alkoxy groups are adjacent to the carbon atom (s) adjacent to the ether oxygen.

According to a further embodiment of the process according to the invention, it is possible to alkoxylate linear and cyclic N-substituted amides. The amide nitrogen atom has one or two alkyl substituents which can also form a saturated or unsaturated, optionally heteroaromatic ring with the N atom. In this case, at least one carbon atom bonded to the nitrogen has at least one abstractable hydrogen atom, or the nitrogen atom is a ring member of a heteroaromatic ring.

Examples of such amides are lactams having 5 to 7 ring members, wherein the amide nitrogen may additionally be alkylated.

The lactams are, for example, N-alkylpyrrolidone, where the heterocyclic ring may additionally contain one or more substituents. The alkyl group bonded to the nitrogen is particularly preferably methyl. Further examples are N-alkyl valerolactam and N-alkyl caprolactam.

Another class of substances is N-acylated saturated and unsaturated N-heterocycles which have at least one abstractable hydrogen atom on at least one of the carbon atoms adjacent to the nitrogen or are heteroaromatic. Examples of the abovementioned classes are: N-acylated, optionally mono- or polysubstituted pyrroles on the ring, pyrrolines and pyrrolidines. The acyl group is, for example, formyl, acetyl, propionyl, benzoyl. The substituents attached to one or more carbon atoms of the N-heterocyclic ring are those substituents listed above in connection with the cyclic ethers. The substituents are particularly preferably an alkyl group having 1 to 4 C atoms, in particular methyl or ethyl, hydroxymethyl, acetoxymethyl and carboxymethyl.

Finally, it is also possible to alkoxylate open-chain N-alkyl or N, N-dialkyl fatty acid amides, in particular amides, of fatty acids having 1 to 6 carbon atoms. It is also possible to use those substrates which have two N-alkylamide structural elements in one molecule.

According to a further embodiment, ketones are alkoxylated with a methyl group or methylene group bonded to the carbonyl carbon atom, in particular methoxylated or ethoxylated. Examples are aliphatic ketones having 3 to 12 carbon atoms, aromatic-aliphatic ketones, such as acetophenone, and methyl benzyl ketone. Usually, the resulting alkoxy ketones are converted directly into the corresponding ketal.

According to a further embodiment, alkylated aromatic and heteroaromatic compounds are alkoxylated, wherein the carbon atom of an aromatic group or heteroaromatic bound alkyl group must have at least one abstractable hydrogen atom. The substrates may additionally have substituents other than alkyl. Conveniently, the aromatic or heteroaromatic contains one or more alkyl groups from the series methyl, ethyl and n-propyl. The alkoxylation according to the invention gives rise to the corresponding alkoxyalkylaromatics or heteroaromatics.

Compared to the present closest prior art, the current efficiency could be significantly increased by the use of the membrane electrode unit according to the invention. It was found that a membrane coated with carbon black on both sides in the described manner gave the best results. Under the chosen test conditions, there was only a slight increase in the operating voltage. In addition, no water and no co-solvent had to be added to lower the tension to a practical level.

example 1

The reactor used in the following example had a fuel cell analogue construction. A membrane electrode unit with an electrode area of 50 cm ² per electrode was used. The MEA included a cation exchange membrane, namely Nafion®117 and carbon black particles embedded in Nafion® on both sides. In further experiments, carbon black particles doped with platinum or with platinum-ruthenium particles were used. The preparation of the MEA is carried out in the manner described above. The membrane was contacted on both sides with graphite paper as a current collector. The electrolyte was successively circulated in the described discontinuous process, ie first pumped into the anode compartment and from there directly into the cathode compartment and back again into the anode compartment, namely, until the desired conversion was achieved.

17.25 g of furan in 70.7 g of methanol were electrolyzed at an electrolyte temperature of 10 ° C galvanostatically at a current density of 50 mA / cm ² to a charge conversion of 60% of the theoretically necessary charge amount. At this time, the furan was almost completely reacted, or by the open system, for the purpose of H ₂ separation, evaporated. The composition of the product mixture was determined by calibrated GC and calibrated HPLC. The following table shows the results. connection Current efficiency (%) 2,5-dihydro-2,5-dimethoxyfuran 84 1,1,4,4-tetramethoxy-cis-butene-2 12 1,1,4,4-tetramethoxy-trans-butene-2 2 4,4-dimethoxy-formylcrotonate 1-2 1,1,4,4-tetramethoxy approx. 1 about 99

Claims (10)

1. Process for the anodic alkoxylation of an organic compound, in which a mixture containing the organic compound and an alcohol having from 1 to 4 carbon atoms is passed through the anode space of a reactor which is separated by means of a membrane electrode assembly (MEA) into an anode space and a cathode space, with the MEA comprising a membrane whose two sides are each provided with an electrode layer, characterized in that a reactor having an MEA having a cation-exchange membrane or a microporous polypropylene membrane whose one or two electrode layers have been produced using a suspension containing carbon black and/or graphite, which can be doped with heavy metal, and a sulphonated polyflorinated polymer or copolymer in a liquid suspension medium is used.
2. Process according to Claim 1, characterized in that an MEA whose two electrode layers have been produced using a suspension containing carbon black, graphite or platinum-doped carbon black is used.
3. Process according to Claim 1 or 2, characterized in that a reactor having an MEA whose electrode layers have been produced using a suspension according to Claim 1 by means of direct or indirect printing of the cation-exchange membrane and removal of solvents present in the liquid medium and thermal treatment of the membrane coated on both sides is used.
4. Process according to any of Claims 1 to 3, characterized in that an organic compound selected from the group consisting of cyclic ethers, N-substituted amides, carbonyl compounds, in particular ketones, alkylaromatics and alkylheteroaromatics is anodically alkoxylated.
5. Process according to any of Claims 1 to 4, characterized in that a cyclic ether selected from the group consisting of furans, dihydrofurans and tetrahydrofurans, 1,2-pyrans and 1,4-pyrans and their dihydro and tetrahydro compounds and also 1,4-pyrones and their dihydro and tetrahydro compounds, with at least one carbon atom bound to the ether oxygen atom in the hydrogenated furans, pyrans and pyrones bearing a hydrogen atom, is methoxylated or ethoxylated, in particular methoxylated.
6. Process according to any of Claims 1 to 4, wherein an amide selected from the group consisting of lactams having from 5 to 7 ring atoms, N-acylated saturated and unsaturated N-heterocycles and also open-chain N-alkylamides or N,N-dialkylamides of fatty acids, with a carbon atom bound to the nitrogen bearing at least one hydrogen atom, is methoxylated or ethoxylated, in particular methoxylated.
7. Process according to any of Claims 1 to 4, characterized in that a ketone having a methyl group or methylene group bound to the carbonyl carbon is methoxylated or ethoxylated, in particular methoxylated.
8. Process according to any of Claims 1 to 4, characterized in that a methyl-substituted aromatic or heteroaromatic is methoxylated or ethoxylated, in particular methoxylated.
9. Process according to any of Claims 1 to 8, characterized in that the alkoxylation is carried out in the alcohol corresponding to the alkoxy group as solvent at a voltage in the range from 1 to 50 volt, in particular from 1 to 25 volt.
10. Process according to any of Claims 1 to 10, characterized in that the alcoholic mixture to be alkoxylated is passed through the anode space and then through the cathode space.