EP0621352A2 - Process for manufacturing terephthalaldehydetetraalkylacetals - Google Patents

Abstract

Electrochemical preparation of terephthalaldehyde tetraalkyl acetals I <IMAGE> in which R is C1-C6-alkyl, by the process of compounds (II) or their mixtures, which are formed, in addition to I and p-tolylaldehyde dialkyl acetal (III), during the electrochemical oxidation of p-xylene in the presence of a neutral supporting electrolyte or an acid in alkanolic solution, after I and, if required, part of the oxidation products II and III have largely been separated, being subjected to further electrochemical oxidation.

Description

in der die Reste R C₁-C₆-Alkylgruppen bedeuten.The present invention relates to an improved process for the electrochemical production of terephthalaldehyde tetraalkylacetals of the formula I.

in which the radicals R are C₁-C₆-alkyl groups.

Terephthalaldehyde tetraalkylacetals are valuable intermediates for organic syntheses and, after their hydrolysis, can be used in the form of terephthalaldehyde for the production of optical brighteners, fiber precursors and polymers.

From JP-A 85/39 183 it is known to produce terephthalaldehyde tetraalkylacetals by electrooxidation of p-tolylaldehyde dialkyl acetals in the presence of an aliphatic alcohol or an alcohol / acetic acid mixture and a neutral conductive salt. Furthermore, this document mentions without further explanation that p-xylene can be oxidized electrochemically to the corresponding terephthalaldehyde tetraalkylacetal in the presence of an aliphatic alcohol and acetic acid.

EP-A 12 240 describes the electrochemical oxidation of p-xylene in alcoholic solution in the presence of a neutral conductive salt to give the corresponding p-tolylaldehyde dialkyl acetals. However, the direct oxidation to the respective tetraalkylacetals is not mentioned.

DE-A 33 22 399 describes the anodic oxidation of p-xylene to p-tolylaldehyde dimethyl acetal, the electrochemical reaction taking place in methanol and sulfuric acid or a sulfonic acid. The direct oxidation to terephthalaldehyde tetramethyl acetal is also not observed in this process.

The invention was therefore based on the object of making the terephthalaldehyde tetraalkylacetals (I), which are valuable for organic syntheses, electrochemically accessible in a simple and economical manner.

in der die Reste R C₁-C₆-Alkylgruppen bedeuten, gefunden, welches dadurch gekennzeichnet ist, daß man Verbindungen (II) oder deren Gemische, die bei der elektrochemischen Oxidation von p-Xylol neben I und p-Tolylaldehyddialkylacetal (III) in Gegenwart eines neutralen Leitsalzes oder einer Säure in alkanolischer Lösung entstehen, nach weitgehender Abtrennung von I und gewünschtenfalls eines Teiles der Oxidationsprodukte II und III der weiteren elektrochemischen Oxidation unterwirft.Accordingly, a process for the electrochemical preparation of terephthalaldehyde tetraalkylacetals of the formula I

in which the radicals R are C₁-C₆-alkyl groups, which is characterized in that compounds (II) or mixtures thereof which are used in the electrochemical oxidation of p-xylene in addition to I and p-tolylaldehyde dialkyl acetal (III) in the presence of a neutral conductive salt or an acid in alkanolic solution, after extensive separation of I and, if desired, a part of the oxidation products II and III subject to further electrochemical oxidation.

noch die folgenden intermediären methylalkoxylierten Oxidationsprodukte des p-Xylols, welche als Verbindungen (II) bezeichnet werden:

Bei der elektrochemischen Herstellung von I werden als Ausgangsverbindungen die Verbindungen (II) entweder einzeln oder als Gemisch eingesetzt. Sie sind üblicherweise zu ca. 1 bis 50 Gew.-%, vorzugsweise zu ca. 5 bis 30 Gew.-% Bestandteil der alkanolischen Elektrolyselösung.The electrochemical oxidation of p-xylene in alkanolic solution produces not only the valuable products terephthalaldehyde tetraalkylacetal (I) and p-tolylaldehyde dialkyl acetal (III)

the following intermediate methylalkoxylated oxidation products of p-xylene, which are referred to as compounds (II):

In the electrochemical production of I, the compounds (II) are used either individually or as a mixture as starting compounds. They are usually about 1 to 50% by weight, preferably about 5 to 30% by weight, part of the alkanolic electrolysis solution.

In addition, p-xylene and p-tolylaldehyde dialkyl acetal (III) in a mixture with II are oxidized electrochemically. They are contained in the electrolysis solution in an amount corresponding to the compounds (II).

In the event that mixtures are electrochemically oxidized, the amounts of the various starting compounds can vary. As a rule, the mixture to be oxidized, after recycling the components separated from I and optionally from III and adding fresh p-xylene, has the following composition:
50 to 90 wt% p-xylene
0.1 to 2% by weight of I
0.1 to 5% by weight IIa
0.1 to 5 wt% IIb
5 to 40 wt% IIc and
0 to 30% by weight III
The following alkanols, for example, are suitable as an alkanol component of the electrolysis solution:
Methanol, ethanol, n-propanol, n-butanol, n-pentanol and n-hexanol, the isomers of the four last-mentioned compounds also being possible. Particularly preferred alkanols are methanol and ethanol.

Not only do they serve as a reaction medium for electrochemical oxidation, they also act as an alkoxylating agent. Expediently, they are contained in the electrolysis solution to 50 to 99% by weight, preferably 70 to 95% by weight.

Another component of the electrolysis solution are neutral conductive salts or acids, which can also be referred to as auxiliary electrolytes and serve to improve conductivity.

The neutral conductive salts are e.g. Halides, sulfonates, sulfates, phosphonates, phosphates, hexafluorophosphates, tetrafluoroborates and perchlorates. Its cationic part can be an alkali metal, e.g. lithium, sodium, potassium and rubidium, preferably sodium and potassium, as well as a quaternary ammonium residue, e.g. Tetramethylammonium, tetraethylammonium, tetra-n-propylammonium, dimethyldiethylammonium, dimethyldi-n-propylammonium and diethyldi-n-butylammonium.

The relevant conductive salts are, for example, halides such as sodium and potassium fluoride, sulfonates such as sodium and potassium benzene sulfonate, sulfates such as tetramethylammonium methylsulfate, phosphate such as tetramethylammonium methylphosphate, hexafluorophosphates such as sodium and potassium hexate fluorate such as tetrafluorotoro fluorate such as tetrafluorotoro fluorate such as tetrafluorotoro fluorate, such as tetra fluorine, such as tetra fluorine, such as tetra fluorine, such as tetra fluorine, such as tetra fluorine, as well as tetramethyl fluorate, such as tetra fluoride, such as tetrafluoride, and tetrafluoride, such as tetrafluoride, such as tetrafluoride, and tetrafluorotetramate, such as tetrafluoride, and tetrafluorotetramate, such as tetrafluorobluorophorate, and tetrafluorotetramate, such as tetrafluorotetramorate and tetrafluorotetramate Sodium benzenesulfonate is particularly preferred.

In addition, mixtures, for example of sodium and potassium benzene sulfonate, can also be used as conductive salts.

The process according to the invention can also be carried out in the presence of an acidic auxiliary electrolyte. The following acids are suitable: sulfuric acid, half esters of sulfuric acid such as methyl sulfate, alkane sulfonic acids such as methyl and ethyl sulfonic acid and aromatic sulfonic acids such as benzene and p-toluenesulfonic acid, with sulfuric acid and benzenesulfonic acid being preferred.

The auxiliary electrolytes are preferably used in the electrolysis solution in an amount of 0.1 to 5% by weight, in particular 0.2 to 3% by weight.

The production of terephthalaldehyde tetraalkylacetals (I) can be carried out in the electrolysis cells customary for electrochemical oxidation. As a rule, are suitable for this particularly undivided electrolysis cells. The reaction can also be carried out in divided electrolysis cells.

For example, noble metals such as platinum, metal oxides such as lead dioxide, ruthenium dioxide, chromium-III-oxide and metals such as titanium, which are coated with a metal oxide layer such as ruthenium dioxide, and graphite can be used as the anode material. Suitable cathode materials are e.g. Metals such as lead, iron, copper, nickel and zinc, metal alloys such as steel, precious metals such as platinum and graphite. Graphite is preferably used for both electrodes.

According to previous observations, the higher the temperatures, the better the electrochemical oxidation. For this reason, a range of 20 to 200, preferably 80 to 140 ° C is recommended.

The electrochemical oxidation can be carried out at 0.5 to 20 and preferably at 1 to 10 bar.

In the process according to the invention, electrolysis can be carried out at a current density of preferably 0.1 to 30 A / dm², particularly 1 to 10 A / dm².

The theoretically necessary amount of charge is 8 F. In practice, 8.5 to 12 F per mole of starting compounds used is expediently used.

The electrolysis can be carried out batchwise or continuously.

The electrolyzed solution can be worked up using customary methods such as distillation, extraction, filtration and crystallization. The process products are preferably distilled off.

After extensive separation of I and, if desired, some of the oxidation products II and III, the compounds (II) or their mixtures are subjected to further electrochemical oxidation. In particular, the complete or partial removal of the likewise desired p-tolylaldehyde dialkyl acetal (III) is recommended when the ratio of I to III is in a range from about 0.03 to 1 to 0.33 to 1, in particular 0.2 to 1 to 0.3 to 1 falls.

In the event that predominantly terephthalaldehyde tetraalkylacetal (I) is desired as the product, the corresponding dialkylacetal (III) can be subjected to the electrochemical oxidation either individually or together with II.

The process according to the invention is of particular importance if the compounds (II) or their mixtures are returned to the stage of the electrochemical oxidation of p-xylene.

In two preferred embodiments of the method according to the invention, the reaction solution obtained in the electrochemical oxidation in the electrolysis cell is expanded to a pressure which is 10 mbar to 10 bar lower than the pressure in the electrolysis cell. The electrolysis cell preferably has an overpressure, based on normal pressure, of 0.1 to 6 bar. This pressure can preferably be built up in the electrolysis cell by means of a pump, but can also be generated by an inert gas such as nitrogen. After the oxidation step, the reaction solution is preferably let down to normal pressure.

The first of these preferred embodiments also provides the following continuous mode of operation: After the reaction solution has been let down, a partial stream of the reaction solution is separated off and worked up. This partial stream is generally less than 5% by weight, preferably 0.01 to 1% by weight, of the total stream. Part of the gas dissolved in the reaction solution is discharged from the electrolysis circuit through this partial flow. A separate degassing of the entire reaction solution is not necessary, but can be advantageous in the case of small partial flows and relatively large amounts of gas. The partial flow is worked up as described above. Solvents, auxiliary electrolyte, starting compounds and, if appropriate, incompletely oxidized intermediates can be added to the reaction solution which is returned to the electrolytic cell. The recycled reaction solution is further replaced by the amount of starting compounds which corresponds to the amount of the separated product. After recycling and oxidation, the cycle described is repeated as often as required.

In the second of these preferred embodiments of the invention, in the case of batchwise work, the gas released during the expansion of the reaction solution after the electrolysis is also separated off, which is predominantly hydrogen discharged from the electrolysis cell. The reaction solution is then returned to the electrolysis cell, electrolyzed and then expanded. This sequence of process steps is referred to below as cycles. It has proven to be beneficial exposing the reaction solution to a large number of cycles, as a result of which a higher yield can be achieved economically than in just two cycles. 50 to 5000 cycles are preferred, particularly preferably 200 to 3000 cycles. The oxidation of p-xylene in one cycle is generally not carried out until conversion is complete. Depending on the number of cycles, the turnover is generally 0.01 to 5% of the theoretical turnover. When the desired degree of oxidation of p-xylene has been reached, the reaction solution is worked up on the product. This is done in a manner known per se, predominantly by distillation. If a solvent is present in the reaction solution, it is distilled off. If neutral salts are used as auxiliary electrolyte, these can then be filtered off before the acetal I is distilled. Solvents, electrolyte and unreacted starting compound can be used again in further process approaches.

In a further preferred embodiment of the process according to the invention, preferably 60 to 95, in particular 70 to 90,% by weight of the p-xylene to be reacted are initially introduced into the electrolysis cell in a batchwise procedure, and the remaining amount is added, preferably continuously, during the electrolysis.

The process according to the invention has the advantage over the prior art that terephthalaldehyde tetraalkylacetals (I) are selectively accessible directly from p-xylene by electrooxidation. It is also advantageous that the corresponding p-tolylaldehyde dialkyl acetals (III) can also be obtained here as valuable products.

Examples

The electrolysis apparatus used in the following examples consisted of an undivided flow cell with 11 bipolar graphite electrode plates. The electrode plates each had an area of 150 cm². The distance between the electrodes was 1 mm in each case.

The individual electrolyte solutions were each passed through the cell at a rate of 200 l / h.

example 1 Batch production of terephthalaldehyde tetramethyl acetal (I) from p-methoxymethylbenzaldehyde dimethyl acetal (IIc)

285 g (1.45 mol) of p-methoxymethylbenzaldehyde dimethyl acetal (IIc), dissolved in 2,670 g of methanol, were anodized in the presence of 30 g of sodium benzene sulfonate at 60 ° C. and at a current density of 3.4 A / dm². A charge of 5 F was consumed per mole of p-methoxymethylbenzaldehyde dimethyl acetal used.

After methanol and sodium benzene sulfonate had been separated off, the crude product was worked up by distillation. 246 g of terephthalaldehyde tetramethyl acetal (I) were obtained.
Yield, based on IIc used: 75%
Yield, based on converted IIc (selectivity) 93%;
Turnover: 80%
The starting compound (IIc) was prepared in repeated runs as follows:
450 g (4.2 mol) of p-xylene, dissolved in 2520 g of methanol, were anodized in the presence of 30 g of sodium benzenesulfonate at 65 ° C. and at a current density of 3.4 A / dm². A charge of 9 F was consumed per mole of p-xylene used.

• a) 25 g Terephthalaldehydtetramethylacetal I (Ausbeute 3 %)
• b) 104 g p-Methoxymethylbenzaldehyddimethylacetal IIc (Ausbeute 13 %)
• c) 448 g p-Tolylaldehyddimethylacetal III (Ausbeute 64 %)

Methanol and sodium benzene sulfonate were separated and the crude product was worked up by distillation for the following three fractions:

• a) 25 g of terephthalaldehyde tetramethyl acetal I (yield 3%)
• b) 104 g of p-methoxymethylbenzaldehyde dimethyl acetal IIc (yield 13%)
• c) 448 g of p-tolylaldehyde dimethyl acetal III (yield 64%)

Example 2 Batch production of terephthalaldehyde tetramethyl acetal (I) from p-methoxymethylbenzaldehyde dimethyl acetal (IIc)

127.7 g (0.65 mol) of p-methoxymethylbenzaldehyde dimethyl acetal, dissolved in 1200 g of methanol, were anodized in the presence of 13 g of sodium benzene sulfonate at 60 ° C. and a current density of 3.4 A / dm². A charge of 5 F was consumed.

The electrolysis solution was worked up in accordance with Example 1. 111.2 g of terephthalaldehyde tetramethyl acetal (I) were obtained.
Turnover: 80%
Yield: 75%

Example 3 Batch production of terephthalaldehyde tetramethyl acetal (I) from a mixture of p-xylene and p-methoxymethylbenzaldehyde dimethyl acetal (IIc)

A mixture of 450 g (4.24 mol) of p-xylene and 110 g (0.56 mol) of p-methoxymethylbenzaldehyde dimethyl acetal (IIc), dissolved in 2,410 g of methanol, was in the presence of 30 g of sodium benzene sulfonate at 65 ° C. and one Current density of 3.4 A / dm² anodized. A charge of 8.5 F was consumed per mole of the starting substances used.

Working up was carried out as in Example 1. In addition to 172 g IIc, the following compounds were isolated:
75.8 g terephthalaldehyde tetramethyl acetal (I); Yield based on p-xylene and IIc: 7%
440.7 g of p-tolylaldehyde dimethyl acetal (III), yield, based on p-xylene: 63%

Example 4 Batch production of terephthalaldehyde tetramethyl acetal (I) from a mixture of p-xylene and p-methoxymethylbenzaldehyde dimethyl acetal (IIc)

A mixture of 450 g (4.24 mol) of p-xylene and 170 g (0.87 mol) of IIc, dissolved in 2 350 g of methanol, was in the presence of 30 g of sodium benzenesulfonate at 50 ° C and a current density of 3.4 A / dm² anodized. A charge of 12.5 F was consumed per mole of the starting substances used.

Working up was carried out as in Example 1. In addition to 218 g IIc, the following compounds were isolated:
138 g terephthalaldehyde tetramethyl acetal (I); Yield based on p-xylene and IIc: 12%
409 g p-tolylaldehyde dimethyl acetal (III), yield, based on p-xylene: 58%

Example 5 Continuous production of terephthalaldehyde tetramethyl acetal (I) from a mixture of p-xylene and p-methoxymethylbenzaldehyde dimethyl acetal

110 g of an electrolysis solution consisting of 15% by weight of p-xylene, 2.7% by weight of p-methoxymethylbenzaldehyde dimethyl acetal, 80.3% by weight of methanol and 2% by weight of sodium benzenesulfonate were added continuously in an hourly flow cell 60 ° C anodized, ie 396 g (3.74 mol) of p-xylene and 71.3 g (0.36 mol) of p-methoxymethylbenzaldehyde dimethyl acetal over a total of 24 hours. A charge of 10.9 F was consumed per mole of the starting substances used.

The electrolysis discharges (2,650 g) collected in 24 hours were worked up in accordance with Example 1. In addition to 4 g of p-xylene, 8 g of α-methoxy-p-xylene and 130 g of p-methoxymethylbenzaldehyde dimethyl acetal, the following compounds were isolated:
101 g terephthalaldehyde tetramethyl acetal (I); Yield based on p-xylene and IIc initiated in 24 hours: 11%
357 g of p-tolylaldehyde dimethyl acetal (III), yield, based on p-xylene initiated in 24 hours: 57.5%

Example 6 Continuous production of terephthalaldehyde tetramethyl acetal (I) from a mixture of p-xylene and p-methoxymethylbenzaldehyde dimethyl acetal (IIc)

The procedure was carried out in accordance with Example 5. 132 g of electrolysis solution consisting of 15% by weight of p-xylene, 2.7% by weight of IIc, 80.3% by weight of methanol and 2% by weight of sodium benzene sulfonate were continuously added per hour the flow-through electrolysis cell anodically oxidized, ie over a total period of 24 hours 475.2 g (4.48 mol) of p-xylene and 86.6 g (0.44 mol) of IIc. A charge of 9.1 F was consumed per mole of the starting substances used.

The electrolysis discharges (3 168 g) collected in 24 hours were worked up in accordance with Example 1. In addition to 6 g p-xylene, 12 g α-methoxy-p-xylene (IIa) and 135 g IIc, the following compounds were isolated:
71 g of terephthalaldehyde tetramethyl acetal (I); Yield, based on p-xylene and IIc initiated in 24 hours: 6%
490 g p-tolylaldehyde dimethyl acetal (III), yield, based on p-xylene initiated in 24 hours: 66%

Example 7 Preparation of terephthalaldehyde tetramethyl acetal (I) from p-xylene

An electrolyte solution of 300 g (2.8 mol) of p-xylene, 2,700 g of methanol and 60 g of sodium benzenesulfonate was anodized at 50 ° C. and at a current density of 3.4 A / dm². A charge of 5.5 F was used per mole of p-xylene. Then 5 g (47 mmol) of p-xylene were introduced per hour. A total of 45 g (0.42 mol) of p-xylene were introduced over a period of 9 hours, a charge of 10 F being consumed per mole of p-xylene.

Working up was carried out in accordance with Example 1. In addition to 1 g of p-xylene, 1.7 g of α-methoxy-p-xylene (IIa) and 78 g of IIc, the following compounds were isolated:
60 g terephthalaldehyde tetramethyl acetal (I); Yield based on p-xylene 8%
305 g of p-tolylaldehyde dimethyl acetal (III), yield, based on p-xylene: 57%

Claims (5)

Process for the electrochemical production of terephthalaldehyde tetraalkylacetals of the formula I.

in which the radicals R are C₁-C₆-alkyl groups, characterized in that compounds (II) or mixtures thereof which are used in the electrochemical oxidation of p-xylene in addition to I and p-tolylaldehyde dialkyl acetal (III) in the presence of a neutral conductive salt or one Acid in alkanolic solution is formed, after extensive separation of I and, if desired, part of the oxidation products II and III, subject to further electrochemical oxidation. Process for the electrochemical production of terephthalaldehyde tetraalkylacetals (I), characterized in that the compounds (II) or their mixtures with III are returned to the stage of the electrochemical oxidation of p-xylene. Process according to Claim 1 or 2, characterized in that, in a continuous procedure, the reaction solution obtained is expanded outside the electrolysis cell to a pressure which is 10 mbar to 10 bar lower than the pressure in the electrolysis cell, and part of the reaction solution is worked up to give the product, the remaining part of the reaction solution is mixed with an amount corresponding to the removed part of the reaction solution originally used, returned to the electrolytic cell, electrolyzed and relaxed. Process according to Claim 1 or 2, characterized in that 70 to 90% by weight of the p-xylene to be reacted is initially charged in the electrolysis cell in the case of discontinuous oxidation of the p-xylene and the remaining amount is added during the electrolysis. Process according to Claims 1, 2 or 4, characterized in that the reaction solution obtained is operated outside the electrolysis cell in a batchwise procedure depressurized a pressure that is 10 mbar to 10 bar lower than the pressure in the electrolysis cell, which separates released gas from the reaction solution during the expansion, returns the reaction solution to the electrolysis cell at least once, electrolyses, decompresses and separates from released gas and then onto the gas Refurbished product.