DE2930480A1 - Electrochemical prodn. of benzaldehyde di:alkyl acetal - by oxidn. of toluene deriv. in alcohol soln., converted to corresp. benzaldehyde by acid hydrolysis - Google Patents

Abstract

(A) Prodn. of opt. substd. benzaldehyde dialkylacetals comprises anodic oxidn. of a toluene deriv. (I) dissolved in an alkanol in presence of a conductivity salt (II). (R1 = H, (ar)alkyl, aryl, (ar)alkoxy or aryloxy; R2 and R3 = H, (ar)alkyl or aryl. Y = alkali metal or ammonium, opt. partly or completely substd. by alkyl or benzyl; A = SO2 or -P(O)(R6)-; R5 = OH, OY or is (ar)alkyl, aryl, (ar)alkoxy or aryloxy opt. substd. by alkyl and/or halo; R6 = (ar)alkoxy, aryloxy, OH or OY). Oxidation is at current density 0.1-50 A/dm2 and the by-prods. formed are recycled to the starting material opt. after treatment with catalytically-activated H2 (esp. that generated at the cathode). (B) Also new is prodn. of opt. substd. benzaldehydes (III) by hydrolysis of reaction mixts. contg. the acetals with >=1 equiv. of water in presence of acid or acidic ion exchanger catalyst. (III) are useful in perfumery. The corresp. acetals are now made simply with high selectivity and purity.

Description

Process for the production of optionally substituted

tuted benzaldehyde dialkyl acetals The invention relates to a method for the preparation of optionally substituted benzaldehyde dialkyl acetals.

A process for the preparation of optionally substituted benzaldehyde dialkyl acetals has been found which is characterized in that a toluene of the formula in which R¹, R² and R3 are identical or different and are hydrogen, alkyl, aralkyl, aryl, alkoxy, aralkoxy or aryloxy, dissolved in an alcohol of the formula R4-OH (II), in which R4 is alkyl, in the presence of 0.01 to 2 mol per liter of a conductive salt of the formula in which Y is an alkali metal, unsubstituted ammonium or ammonium partially or completely substituted by alkyl or benzyl and R5 is alkyl, aralkyl, aryl, alkoxy, aralkoxy, aryloxy, hydroxy or OY and R6 is alkoxy, aralkoxy, aryloxy, hydroxyl or OY is, electrochemically oxidized at a current density of 0.1 to 50 A / dm2 and the byproducts formed are returned to the starting material for the electrochemical oxidation, the byproducts optionally being treated with catalytically activated hydrogen prior to recycling.

Examples of alkyl are straight-chain or branched hydrocarbon radicals with 1 to 4 carbon atoms, such as methyl, ethyl, propyl, iscpropyl, butyl or isobutyl. Preferred alkyl is methyl and ethyl.

Aralkyl are, for example, hydrocarbon radicals with 1 or 2 carbon atoms in the aliphatic part and 6 to 14 carbon atoms in the aromatic part Part called such as benzyl, 2-phenyl-ethyl, naphthylmethyl, 2-naphthyl-ethyl, anthrylmethyl or 2-anthryl-ethyl. The preferred aralkyl is benzyl.

Aryl can be, for example, aromatic hydrocarbon radicals 6 to 14 carbon atoms mentioned, such as phenyl, biphenyl, naphthyl or anthryl. The preferred aryl is phenyl.

Examples of alkoxy are methoxy, ethoxy, propoxy, isopropyloxy, Butoxy or isobutyloxy, preferably methoxy and ethoxy, mentioned.

Aralkoxy is, for example, benzyloxy, 1-phenylethoxy, 2-phenylethoxy, p-Biphenylmethoxy, naphthylmethoxy or naphthylethoxy, preferably benzyloxy, mentioned.

Examples of aryloxy are phenoxy, biphenyloxy, naphthyloxy or Anthryloxy, preferably phenoxy, mentioned.

The radicals mentioned, preferably those optionally contained therein aromatic nuclei, in turn, can be substituted by alkyl and / or halogen be. As an alkyl substituent, for example, methyl, ethyl, propyl, isopropyl, Butyl or isobutyl, preferably methyl, mentioned. An example of a halogen substituent is Fluorine, Chlorine or bromine, preferably chlorine, mentioned.

Possible toluenes of the formula (I) for the process according to the invention are named for example: toluene, o-, m- and p-xylene, mesitylene, 1-methyl-4-ethyl-benzene, 1-methyl-4-propyl-benzene, 1-methyl-4-isopropylbenzene, 1-methyl-4-butyl-benzene, 1-methyl-4-isobutylbenzene, 1,2-dimethyl-4-ethylbenzene, 2-methyl-, 3-methyl- and 4-methyl-diphenylmethane, 2-methyl-, 3-methyl- and 4-methyl-diphenyl, 1-methyl-4- (t-butyl) -benzene, p-methoxy-toluene, p-ethoxy-toluene, p-propoxy-toluene, p-isopropyloxytoluene, p-isobutyloxy-toluene, p-phenoxy-toluene, p-benzyloxy-toluene, 2-methyl-4-methoxy-toluene, 2-methyl-4-methoxy-toluene, 2,3-dimethyl-4-methoxy-toluene, 2,5-dimethyl-4-methoxy-toluene, 2,6-dimethyl-4-methoxytoluene, 2-phenyl-4-methoxy-toluene, 3-phenyl-4-methoxytoluene, 2-benzyl-4-methoxy-toluene, 3-benzyl-4-methoxytoluene, 2-phenyl-3-methyl-4-methoxy-toluene, 2-methyl-3-phenyl-4-methoxy-toluene, 2-ethyl-3-benzyl-4-phenoxytoluene, 2-benzyl-4-benzyloxy-4-isopropyl-toluene, 2,6-dibutyl-4-benzyloxy-toluene.

As a conductive salt of the formula (III), for example, a salt-like compound an acid derived from @valent phosphorus called. @ 1 acid that is derived from pentavalent phosphorus, for example from phosphoric acid, an allyl Arallyl or aryl phosphoric acid, a phosphonic acid monoester or diester or a Phosphonic acid monoester called, the ester groups differing from an aliphatic or arali- phatic alcohol or derived from a phenol.

The cation, in the case of polyvalent acids, the cations, can, for example an alkali metal, the unsubstituted ammonium or a partial or complete be ammonium substituted by alkyl or benzyl.

Examples of alkali metals are lithium, sodium, potassium, rubidium or cesium, preferably sodium or potassium.

As partially or fully substituted by alkyl or benzyl Examples of ammonium are: methylammonium, dimethylammonium, trimethylammonium, Tetramethylammonium, ethylammonium, diethylammonium, triethylammonium, tetraethylammonium, Dimethylethylammonium, Propylammonium, Dipropylammonium, Tripropylammonium, Tetrapropylammonium, Butylammonium, dibutylammonium, tributylammonium, tetrabutylammonium, dimethylbutylammonium, Dimethyldibutylammonium, benzylammonium, dibenzylammonium, dimethylbenzylammonium, Diethylbenzylammonium, dibutylbenzylammonium, dimethyldibenzylammonium, diethyldibenzylammonium.

Preferred cations for the conductive salt of the formula <III) are Tetraalkylammonium cations.

The anion for the conductive salt is, for example, the phosphate, the hydrogen phosphate, dihydrogen phosphate, methyl phosphonate, ethyl phosphonate, propyl phosphonate, Butyl phosphonate, phenyl phosphonate, benzyl phosphonate, p-chlorobenzene phosphonate, Methyl phosphonate methyl ester, methyl phosphonate ethyl ester, ethyl phosphonate ethyl ester, Propyl phosphonate methyl ester, phenyl phosphonate methyl ester, benzyl phosphonate methyl ester, Benzyl phosphonate ethyl ester, p-chlorobenzene phosphonate methyl ester, methyl phosphinate, Ethyl phosphinate, propyl phosphinate, methyl ester phosphate, ethyl ester phosphate, propyl ester phosphate, Phenyl ester phosphate, benzyl ester phosphate, dimethyl ester phosphate, diethyl ester phosphate, Methyl ethyl ester phosphate, dipropyl ester phosphate, diphenyl ester phosphate, dibenzyl ester phosphate, Phenyl methyl ester phosphate, di (p-toluyl) esters = phosphate, di (p-chlorobenzene) ester phosphate.

The preferred anions for the conductive salt (III) are derived from the doubly esterified phosphoric acid or singly esterified phosphonic acid, for example diphenyl ester phosphate, di (p-toluyl) ester phosphate, di-fp-chlorobenzol ester) phosphate, Dimethyl ester phosphate, diethyl ester phosphate, methyl phosphonate methyl ester, methyl phosphonate ethyl ester, Ethyl phosphonate ethyl ester, phenyl phosphonate methyl ester, p-chlorophenyl phosphonate methyl ester.

Preferred electrolyte salts for the process according to the invention are those of the formula R7 O = P-ON (alkyl) 4 (IV), '8th R. in which R7 denotes methyl, ethyl, phenyl, methoxy or ethoxy which is optionally substituted once or twice by methyl or chlorine, and R8 denotes methoxy or ethoxy.

Examples of conductive salts of the formula (IV) are: tetramethylammonium dimethyl phosphate, Tetraethylammonium diethyl phosphate, triethylmethylammonium dimethyl phosphate, tetramethylammonium methylphosphonate methyl ester, Tetramethylammonium ethylphosphonate methyl ester.

As alcohol of formula (II) for the process according to the invention for example methanol, ethanol, propanol, isopropanol, butanol or isobutanol, preferably methanol, called.

The concentration of the conductive salt in the process according to the invention is for example 0.01 to 2 mol per 1, preferably 0.05 to 0.5 mol per 1 of the electrolyte.

The inventive method can, for example, in a customary Electrolysis cell known to those skilled in the art can be carried out with conventional electrodes.

The concentration of the optionally substituted toluene in the alcoholic solution in the process according to the invention can, for example, be 1 to 50 % By weight, preferably 10 to 50% by weight.

The current density in the method according to the invention can, for example 0.1 to 50 A / dm2, preferably from 1 to 20 A / dm2, can be selected. To increase the Selective "t and to increase the electricity yield, it can be cheap be to lower the current density in the course of the electrochemical reaction.

The inventive method is at a temperature of -20 to + 600C, preferably at +10 to + 400C.

In the process according to the invention, optionally substituted benzaldehyde dialkyl acetals of the formula in which R1 to R4 have the abovementioned meaning, are prepared, for example p-methoxy-benzaldehyde-dimethylacetal, p-methoxy-benzaldehyde-diethylacetal, p-methoxy-benzaldehyde-dipropylacetal, p-methoxybenzaldehyde-dibutyl acetal, p-ethoxy-benzaldehyde dimethylacetal, p-ethoxy-benzaldehyde-diethylacetal, p-propoxy-benzaldehyde-dimethylacetal, p-butoxy-benzaldehyde-dimethylacetal, p-phenoxy-benzaldehyde-dimethylacetal, p-phenoxy-benzaldehyde-isopropyl acetal, p-benzyloxy-benzaldehyde-dimethylacetal, p-Benzyloxy-benzaldehyde-diisobutyl acetal, 2-methyl-4-methoxy-benzaldehyde dimethyl acetal, 3-methyl-4-methoxy-benzaldehyde-diethylacetal, 2,5-dimethyl-4-methoxy-benzaldehyde-dimethyl acetal, 2,6-dimethyl 4-phenoxy-benzaldehyde-dimethyl-acetal, 2-methyl-4-methoxy-5-phenyl-benzaldehyde-dimethylacetal, 2-benzyl-4-phenoxy-5-isopropyl-benzaldehyde-diethylacetal, 2-ethyl-3-benzyl- 4-methoxy-benzaldehyde dimethyl acetal, benzaldehyde dimethyl acetal, 2-methylbenzaldehyde dimethyl acetal, 3-methylbenzaldehyde dimethyl acetal, 4-methylbenzaldehyde dimethylacetal, 3,5-dimethylbenzaldehyde dimethylacetal, 4-ethylbenzaldehyde dimethylacetal, 4- (n-propyl) -benzaldehyde-dimethylacetal, 4- (i-propyl) -benzaldehyde-dimethylacetal, 4- (n-Butyl) -benzaldehyde-dimethylacetal, - 4- (i-Butyl? -benzaldehyde dimethyl acetal, 4- (t-butyl) -benzaldehyde dimethyl acetal, 4-phenylbenzaldehyde dimethyl acetal, 4- (4'-methyl) -biphenylaldehyde dimethyl acetal, 4- (4'-methoxy) -biphenylaldehyde dimethyl acetal, 4 -Benzylbenzaldehyde dimethyl acetal.

In the process according to the invention, by-products occur which reduce the yield affect the desired product. These by-products can be extracted from the reaction mixture the electrochemical oxidation, for example by distillation, are isolated.

These by-products are according to the invention into the starting product recycled for electrochemical oxidation This can be done as follows: that during the separation of the reaction mixture any that may still be present Starting product, by-products and the desired process product are separated be caught. Then the by-products, as well as the starting product in the electrochemical oxidation can be recycled. It is However also possible during the separation of the reaction mixture, for example by distillation, to collect only a single fraction in addition to the desired process product, the contains the by-products, the unchanged starting product and some end product, and this fraction without further purification and without further separation into the individual components returned to the starting material for the electrochemical oxidation. These the latter process variant is preferred.

The by-products mentioned may contain proportions that are at a mere recycling to the starting material for the electrochemical oxidation without further treatment does not contribute to an increase in the yield, but it does a suitable pretreatment before returning to the starting material to the above-mentioned by-products also cause an increase in yield. Such proportions can arise, for example, when toluenes of the formula (1) are used in which the substituent R1 is hydrogen, alkyl, aralkyl or aryl. A suitable treatment for these proportions of the by-products is, for example Treatment with catalytically excited hydrogen called when such occurs By-product fractions that may require treatment prior to recycling For example, the following variants of the method according to the invention can be used: a) If the proportion of by-products obtained after treatment by hydrogenation Contributing to the increase in yield is very small in relation to those already described above By-products, hydrogenation can be dispensed with, and the total amount of the by-products directly into the starting material for the electrochemical oxidation to be led back. To avoid an accumulation of by-product components, which would require hydrogenation, it can be beneficial to have a part the by-product mixture is discharged before being returned to the starting material and destroy them in an appropriate manner.

b) In a further variant of the procedure, the following can be done. that the by-product mixture is separated into its components, for example by distillation, and the individual components of the by-product mixture are separated be returned to the starting material.

The by-product proportions for which it was found that they contribute to an increase in yield without hydrogenating treatment, directly are returned to the starting mixture, while those by-product fractions, in which it has been found that they only become one after hydrating pretreatment This hydrating pretreatment contribute to an increase in yield.

c) In a further variant of the process according to the invention, in the separation of the reaction mixture, as already described above, in the separation of the reaction mixture by distillation from the electrochemical Reaction only a single forerun removed before the desired main product. This First runnings then contain some of the unreacted starting material, all by-product fractions and optionally some of the desired main product. Since now those by-product shares, which contribute to an increase in yield even without hydrogenating treatment the hydrating treatment are not adversely affected, it is advantageous to use the subject the entire flow to the hydrating treatment and then transfer it to the Feed back starting material for the electrochemical oxidation.

This latter variant is preferred because of the simplicity.

The hydrating treatment of the by-products, both individually and as a mixture with some starting product and some main product, as described according to the invention by treatment with hydrogen in the presence of a hydrogenation catalyst, in an alcohol as a solvent at a temperature of about 0 ° C to the boiling point of the alcohol is carried out at normal pressure or elevated pressure.

The hydrogenation catalysts are customary and known to the person skilled in the art Hydrogenation catalysts in question. Examples include: Precious metal catalysts, such as finely divided platinum or palladium; Noble metal oxide catalysts that are only are reduced in the hydrogenation mixture by the hydrogenation hydrogen; Raney catalysts, such as Raney nickel, Raney nickel iron, Raney nickel copper, Raney nickel cobalt, Oxide or sulfide catalysts, such as copper chromium oxide, zinc chromium oxide, molybdenum sulfide or tungsten sulfide. The catalysts can be used with or without a carrier will. As a carrier substance are, for example, carbon, silicon dioxide, aluminum oxide, Called alkaline earth sulfates, alkaline earth oxides and alkaline earth carbonates.

Particularly suitable hydrogenation catalysts are the noble metal catalysts, for example platinum or palladium, if appropriate on a carrier material. Palladium on magnesium oxide / kieselguhr is particularly preferred as the hydrogenation catalyst.

The inventive hydrogenation of the by-products takes place in the solution a lower alcohol such as methanol, ethanol, propanol or butanol.

The by-products treated by hydrogenation can then, for example separated from the alcohol used in the hydrogenation by distillation.

However, the same alcohol R40H of the formula (II) is preferred for Hydrogenation is used, such as that used to carry out the electrochemical oxidation of the method according to the invention. The by-products treated by hydrogenation can then in more preferred Way in the form of their solution in the alcohol R4OH (II) can be returned to the electrochemical oxidation.

The temperature for carrying out the hydrogenation is such from about 0 ° C to the boiling point of the alcohol used as solvent. A temperature of about 15 to 40 ° C. is preferred.

The hydrogenation according to the invention of the by-products can be carried out at normal pressure or with increased pressure. The increased pressure is a pressure of called up to 200 bar. In a preferred variant, it is used in the electrolysis cathodically accumulating hydrogen is used for the hydrogenation of the by-products. at In this preferred variant, working under normal pressure is preferred.

The process according to the invention can be explained using the example of the preparation of p- (t-butyl) benzaldehyde dimethylacetal by the following equation: The process according to the invention can be carried out, for example, as follows: The electrolyte is prepared by dissolving a toluene of the formula (I) and a conductive salt of the formula (III) in an alcohol of the formula (II). It may be advantageous to protect the acid-sensitive acetals by adding an oxidation-stable base, for example collidine or lutidine, which is known per se. The electrolyte is electrolyzed to the desired degree of conversion, for example 60 to 90% of the theoretical degree of conversion, optionally with a reduction in the current density in the course of the electrochemical reaction. After the electrolysis, the excess alcohol is distilled off and the electrolyte salt which has precipitated in crystalline form is filtered off. The benzaldehyde dialkyl acetal formed can then be obtained from the reaction mixture by customary working methods, for example by distillation. When the desired end product is separated off, the by-products of the electrochemical oxidation are generally obtained in the first run before the end product when a separation by distillation is used.

This preliminary run, the unreacted starting product, the by-products, as well as possibly contains some end product, is then without further separation, optionally with the interposition of a hydrogenating treatment in the starting material recycled for the electrochemical reaction.

When the process according to the invention is carried out continuously the electrolyte is pumped through the electrolysis apparatus. After leaving the electrolysis machine the reaction mixture is wholly or partially by distillation separated into a forerun and into the benzaldehyde dialkyl acetal formed. Of the The flow is then returned to the starting material for the electrochemical oxidation, where appropriate, a hydrogenating treatment is carried out beforehand. The toluene the formula (1) as the starting product is in the starting product according to its consumption added. If only part of the reaction mixture is carried out when the reaction is carried out continuously is removed after leaving the electrolysis apparatus, the remaining reaction mixture guided in a circle and subjected to the flow or a hydrating treatment Forerun, which contains the by-products, and toluene as the starting compound supplemented according to its consumption.

Benzaldehyde dialkyl acetals of the formula (V) are valuable intermediates and can for example in the presence of dilute acids to the corresponding benzaldehydes be hydrolyzed. Substituted benzaldehydes are, for example, valuable fragrance components (Ullmann's Encyclopedia of Technical Chemistry, 3rd Edition, Volume 14, Page 738, Urban and Schwarzenberg, Munich-Berlin, 1968). Taking into account further processing the benzaldehyde dialkyl acetals can form optionally substituted benzaldehydes the electrolyte may also be worked up in such a way that not the substituted benzaldehyde dialkyl acetal, but the free substituted benzaldehyde is isolated, which can be obtained in a known manner by acid hydrolysis.

To do this, it is expedient to proceed in such a way that the electrolyte follows Termination of the oxidation to its content of substituted benzaldehyde dialkyl acetal is checked and this equivalent amount of water is added. At a sour Ion exchanger or by adding an acid, for example p-toluenesulfonic acid, the associated aldehyde is then isolated from the benzaldehyde dialkyl acetal. Man then filtered off from the acidic catalyst or neutralized the added acid and works in such a way that the methanol is distilled off from the precipitated Conductive salt is filtered off and the remaining reaction mixture, for example by distillation, is worked up to the desired benzaldehyde.

In the process according to the invention, the benzaldehyde dialkyl acetals of formula (V) in a technically advanced manner through a single-stage electrochemical Oxidation can be produced with high selectivity and in high purity. By the inventive recycling of the by-products, optionally after hydrogenating Treatment, high material and current yields are achieved. It is known, for example p-methoxytoluene in methanolic solution electrochemically to p-methoxy-benzaldehyde-dimethylacetal to oxidize. It is surprising that the conductive salts used according to the invention, which are derived from an acid of pentavalent phosphorus and which are used as conductive salts for such electrochemical oxidations were previously unknown, to increased yields lead to optionally substituted benzaldehyde dialkyl acetals.

Example 1 A solution of 740 g (5 mol) of p- (t-butyl) -toluene in one Electrolytes consisting of 2000 g of methanol and 130 g of tetramethylammonium dimethyl phosphate, is in an electrolysis cell with bipolar connected graphite electrodes at a Current density of 8 A / dm² and a temperature of 250C up to consumption of 16F electrolyzed.

The methanol is distilled off and the electrolyte salt which has precipitated is suctioned off and isolates a first run of 581 g and a product fraction K: 650C from 271 g mbar (1.3 mol) p- (t-butyl) benzaldehyde dimethyl acetal. There remains a residue from 47 g.

The first run is methanolic on a Pd catalyst in 30 pointer Solution hydrogenated at normal pressure and room temperature and the starting material a following electrolysis added. It results from taking the traceability into account of the first runnings a yield of 92% of the theoretical yield of p- (t-butyl) -benzaldehyde dimethyl acetal.

Example 2 A solution of 610 g (5 mol) of p-methoxytoluene dissolved in 1800 g of methanol containing 100 g of tetramethylammonium dimethyl phosphate is like up to consumption electrolyzed by 16 F. You get one First run of 237 g and 597 g of anisaldehyde dimethyl acetal (boiling point 15 mbar: 123 ° C). The advance is added to the starting material of the next electrolysis. One calculates under Taking this recycling into account, a yield of 95% anisaldehyde dimethyl acetal.

Claims (6)

Process for the preparation of optionally substituted benzaldehyde dialkyl acetals, characterized in that a toluene of the formula in which R¹, R² and R³ are identical or different and represent hydrogen, alkyl, aralkyl, aryl, alkoxy, aralkoxy or aryloxy, dissolved in an alcohol of the formula R4-OH, in which R4 is alkyl, in the presence of 0.01 up to 2 moles per liter of a conductive salt of the formula in which Y is an alkali metal, unsubstituted ammonium or ammonium partially or completely substituted by alkyl or benzyl and R5 and R6, identically or differently, are and are alkyl, aralkyl, aryl, alkoxy, aralkoxy, aryloxy, hydroxyl or OY, at a current density of 0.1 to 50 A / dm2 is electrochemically oxidized and the byproducts formed are returned to the starting material for the electrochemical oxidation, the byproducts optionally being treated with catalytically activated hydrogen before the recycling. 2) Process according to claim 1, characterized in that there is a conductive salt of the formula R7 O = P-ON (alkyl) 4 R8 in which R7 denotes methyl, ethyl, phenyl, methoxy or ethoxy optionally substituted once or twice by methyl or chlorine and R8 denotes methoxy or ethoxy. 3) Method according to claim 1, characterized in that at working up the reaction mixture by distillation in addition to the desired reaction product catches only one fraction which, in addition to the by-products, still contains unchanged starting material and contains some desired reaction product, and this one fraction unchanged returned to the starting material for the electrochemical oxidation. 4) Method according to claim 1 and 3, characterized in that one carries out the electrochemical oxidation continuously, the reaction mixture a continuously operating distillation column and one at the top of the column removed fraction containing the by-products, unreacted starting product, as well as optionally contains some benzaldehyde dialkyl acetal, continuously, optionally after hydrogenating treatment, returned to the electrochemical oxidation. 5) Method according to claim 1, 3 and 4, characterized in that if the electrochemical oxidation is carried out continuously, only one Part of the reaction mixture is withdrawn and recycled in the claimed manner the by-products worked up, while the part of the reaction mixture that was not removed is led in a circle. 6) Method according to claim 1, characterized in that the Treatment of the by-products with catalytically activated material, if necessary, to be carried out Carries out hydrogen in such a way that the resulting from the electrochemical reaction By-products individually or as a mixture in an alcohol R4OH, where R4 is the above Has meaning to be dissolved and in the presence of a hydrogenation catalyst a temperature from OOC to the boiling point of the alcohol R40H and at normal pressure or elevated pressure are treated with hydrogen and then into the starting product for electrochemical oxidation.