JPS5920755B2 - Electrochemical reduction method of terephthalic acid - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a process for the electrochemical reduction of terephthalic acid to p-hydroxymethylbenzoic acid (PHMBA).

In more detail: Improving the electrical efficiency of the reaction, treatment of the cathode to obtain a mercury coating as an amalgam on the cand surface, and addition of soluble salts of mercury to the electrolytic catholyte solvent to regenerate the amalgam coating. It is related to the operation by. Since the cost of electricity utilization in the electrochemical conversion of terephthalic acid to p-hydroxymethylbenzoic acid has a direct bearing on the economics of the process,
It is highly desirable to increase current efficiency and minimize side reactions that do not lead to the desired product. If one series of reactions is taking place in this method, i.e. the p-
Ideally, 100% conversion of starting material to the desired product would be achieved if electrochemical reduction at the cathode to hydroxymethylbenzoic acid was occurring. However, as is the case with many electrochemical reactions, this particular reaction is not so simple and requires an electrolytic cell (hereinafter referred to as an electrolytic cell) containing the electrodes and electrolyte contained within the container in which the electrolysis takes place. Many competitive reactions can occur in a device (used to mean a device that includes devices such as The presence of the resulting 4-carpoxybenzaldehyde (4-CBA), dihydroxymethylbenzene, toluic acid, and other impurities allows the resulting p-hydroxymethylbenzoic acid to be converted into a monomer for polymer applications without further expensive purification. This makes it unsuitable for use. It is well known that cathodic reduction of carboxylic acids can produce two types of products.

2-electron process 2e- (RCOOH-=RCHCOH)2-RCHO2H-
→+H2O) or the human 2H-1-loxymethyl compound in a four-electron transition (RCHO--=RCH2OH) in which the aldehyde is further reduced to an alcohol.

(MBitza, Organik Electrochemistry, Detsuka, New York (1973), 414)
. Alcohols can be further reduced to methyl groups. A further complication in the electrolysis of terephthalic acid to p-hydroxymethylbenzoic acid is the development of a deactivation or poison layer that accumulates on the cand when a solid cathode such as lead is used as the electrolysis progresses.

Current efficiency therefore deteriorates. A continuous mercury cathode cell has been developed for the electrolytic reduction of phthalic acid to overcome the above problems (P. Cond., 1EC, 48, 1252 (1956)). However, the use of solid cathodes has advantages over the use of liquid cathodes in that they are easier to fabricate and are more versatile. It is known in the prior art that the reduction of aromatic carbo/acids in a lead or mercury cathode in a protic solvent (proto-Z donor), i.e. an alcoholic solvent etc., gives excellent yields of the corresponding benzyl alcohol. (Ch.
em. Ber. 38, 1747 (1905), 39,
2933 (1906), Ann. , East 1j69 (192
9);0rg. Syn. 21, 10 (1941)).

Beitzer (0p. cit. 417) states that the mechanism of this process is that in a strongly acidic alcoholic medium, the carboxylic acid is reduced to its protonated form RCOOH2+, or the acid first forms an ester with the alcoholic solvent. , suggesting that this ester may be more easily reduced than the acid. Japan Chemical Journal 75, 1195-9 (19
54) (CA5l:12704b), Ono reported that when phthalic acid and isophthalic acid and their esters are electrolytically reduced using a mercury cathode, two types of reactions occur: side chain reduction and benzene reduction occur, and phthalic acid gives dihydrophthalic acid and dimethyl isophthalate gives m-hydroxymethylbenzoic acid. J. Chem. SOc. Ja
pnPureChem. SectlOl74, 907-
11 (1953) (CA48:8082d), Ono et al.
reported the electrolytic reduction of methylhydroxymethylbenzoate. DE 2428878 describes p-hydroxyl by electrochemical reduction of dimethyl terephthalate using methanol as solvent on a solid electrode cathode (lead, zinc, cadmium, graphite, and amalgamated metal monolead, copper, etc.). He teaches a method for producing esters of methylbenzoic acid. However, poisoning effects plague the application of electroreduction methods to aromatic carboxylic acids. El
ectrOchem. TechnOl. , 2 (5-6)
151-6 (1964) (CA6l:6940c),
When electrolytically reducing benzoic acid to benzyl alcohol using a rotating lead cand, the reaction proceeds with good current efficiency until the electrolyte is saturated with benzyl alcohol, at which point the cathode is saturated with benzyl alcohol. It has been reported that the benzyl alcohol becomes covered with a layer of 30% and effectively prevents the diffusion of benzyl alcohol, resulting in a decrease in current efficiency. German publication No. 2,642,496 uses electrodes such as mercury, lead, cadmium-11 and antimony,
A method for producing p-hydroxymethylbenzoic acid by electrochemical reduction of terephthalic acid in the presence of ammonia (a basic protic solvent) is taught. Lead has been shown to be particularly suitable. Unfortunately, his vitality dropped sharply after a few minutes. The problem of maintaining cathode activity is solved by periodically cutting off the DC current and shorting the cell for periods of 1/2 to 3 minutes. From the physical constants given in this publication for p-hydroxymethylbenzoic acid (melting point 1
(82.52-183.5 °C, 100% conversion, and 91% selectivity), the electrochemical product of terephthalic acid is not a pure product and likely contains 4-carboxybenzaldehyde or toluic acid. be. The large excess current of 15.3 FA, which is said to be used, indicates that it is difficult to maintain the cathodic activity of the lead cathode, since 4.0 FA is 100% of the theoretical value. The object of the present invention was therefore to develop an electrochemical process for the production of p-hydroxymethylbenzoic acid from terephthalic acid which avoids the above-mentioned disadvantages.

It is an object of the present invention to provide a process for producing p-hydroxymethylbenzoic acid that minimizes the formation of by-product impurities, namely 4-carboxybenzaldehyde, dihydroxymethylbenzene, and toluic acid. Furthermore, it is an object of the present invention to increase the current efficiency of the electrochemical reduction process over that of previously known processes for the electrochemical reduction of terephthalic acid to p-hydroxymethylbenzoic acid. It is. Another objective is that no interruption of the reduction process is necessary for the development of a poisoning barrier on the cathode.
Another objective will appear hereinafter, which is to provide an efficient continuous process for the electrochemical production of hydroxymethylbenzoic acid. According to the invention, a) the cathode is solid and metallic and has a coating of mercury as an amalgam surface and has a hydrogen overvoltage greater than the potential for the reduction of terephthalic acid to p-hydroxymethylbenzoic acid; , b) Terephthalic acid is electrochemically reduced to p-hydroxymethylbenzoic acid in an electrolytic cell to which sufficient mercury is added as a mercury compound to sustain the process.

Addition of mercury compounds is essential. For example, in continuous operation the current efficiency decreases and the canned amalgam surface loses amalgam formation without the addition of mercury compounds. The term "current efficiency" is defined as the ratio of the amount of FALADE used to make the product to the total FALADE used, multiplied by 100.

The term "amalgam" is defined to refer only to mercury alloys. The present invention provides a method for electrochemically producing p-hydroxymethylbenzoic acid with improved current efficiency and minimal generation of crude impurities.

The method consists of carrying out cathodic reduction in an electrolytic cell having a cathode section and an anode section. The anode and cathode compartments are separated by a cation exchange membrane, although the presence of a separation membrane is not an essential element of the invention. If a separation membrane is used, the cathode, anode and separation membrane are preferably parallel planes. Conveniently, some of the basic electrolyzers, such as filter presses, can be connected together. In general, any metal having a hydrogen overpotential (as an amalgas) higher than the potential of the reduction of terephthalic acid to p-hydroxymethylbenzoic acid, which alloys with lead and forms an amalgas with mercury, is suitable. ing.

Examples of substances forming the cathode are lead and lead and cadmium;
It is an alloy with antimony, tin or bismuth. The cathode is prepared by polishing the solid cathode surface using a suitable method to remove any metal oxidation, and then contacting the polished metal surface with mercury.
prepared by combining them to form an amalgam. In the case of lead, it is sufficient to polish the surface of the lead solid to remove all forms of lead oxide and all other impurities.
99.9% pure liquid mercury is used in the bath for polished solid lead cathodes.

In the case of lead, if it is brought into contact with a mercury bath at room temperature, a lead amalgam is formed on the lead surface. The anode of an electrolysis cell usually consists of a solid electrically conductive material that is electrochemically stable in the anolyte and under the possible operating conditions.

Examples of such materials are metals or metalloids such as brassina, platinized titanium, graphite, lead and its alloys, especially with silver, antimony/tin. Any known cation exchange membrane can optionally be used to separate the catholyte from the anolyte, although membranes in homogeneous form are preferred.

These membranes can optionally be reinforced with screens. In order to carry out electrolytic operation over a long period of time, it is naturally preferable to use a membrane that does not swell and is stable against the effects of various components of the anolyte and catholyte. An example of such a membrane is that of NafiOn (trademark of IdPondenemore & Co.) perfluorosulfonic acid. The catholyte may consist of a neutral solvent, a weakly basic solvent or an aprotic solvent, ie acetonitrile, to which a proton source has been added.

Examples of neutral solvents are water, methanol and other alcohols that are mixed with water to obtain the required solvent properties. Examples of basic solvents are ammonia, methylamine, and ethylenediamine, which are diluted appropriately to maintain weakly basic conditions. In a suitable method of operation, the catholyte is a solvent, preferably water and terephthalic acid, plus a soluble ammonium salt and ammonia. At the beginning of electrolysis, the catholyte contains sufficient ammonia to form the diammonium salt of terephthalic acid. As electrolysis progresses, less ammonium salt is needed.
The concentration of ammonia as ammonium hydroxide is from about 1 y of ammonium hydroxide to 2 t of terephthalic acid to about 1 t of ammonium hydroxide to 1 t of terephthalic acid, and the pH of the resulting solution is at least 6.5, preferably about 8. PH in the range between 5 and about 9.5. The concentrations of terephthalic acid and ammonium salt are either constant when the reaction is carried out continuously, or can vary when the reaction is carried out discontinuously. In all cases, the concentration of terephthalic acid is below the saturation concentration at the temperature of electrolysis, generally this concentration is greater than 2% by weight, and at high current densities preferably greater than 3%;
These values relate in particular to a constant concentration when the reaction is carried out continuously and to a final concentration when the reaction is carried out batchwise. The concentration of ammonium salts is usually about 0.
．． between 1 and about 10% by weight and preferably between about 0.1 and about 1.0% by weight; It particularly relates to the total solution of the components and, when the reaction is carried out batchwise, to the final solution. The ammonium salt may be any ammonium salt, but salts selected from ammonium chloride, ammonium sulfate, and ammonium carbonate are preferred. The catholyte also contains small amounts of reaction by-products, generally less than 1% by weight.

Although it is preferred to use an aqueous acid solution as the anolyte, any other anolyte that can provide electrical conductivity between the two electrodes can be used.

Aqueous solutions of sulfuric and phosphoric acids are commonly used in concentrations of generally 0.1 to 5 mol/l and preferably 0.5 to 2 mol/l. The current density of the cathode is about 1 to about 200 amperes/
in the range of decimeters squared (A/Dm2), preferably from about 20 to about 100 A/Dm2.

The flow of catholyte in the closed circuit is usually provided by a pump. The circuit may further include mounting equipment such as a heat exchanger or an expansion vessel. The expansion vessel allows terephthalic acid to be added to the catholyte and also allows some catholyte to be withdrawn to extract p-hydroxymethylbenzoic acid. By-product hydrogen is also removed. The anolyte can also be circulated and is preferably carried out in an anolyte circuit similar to that of the catholyte so that the pressure on either side of the separation membrane is substantially the same. If a cation exchange membrane is used, it is preferred that at least one spacer is present in the anode and cathode sections. These spacers prevent deformation of the cation exchange membrane and
This prevents contact between this membrane and the electrode. These spacers also help equalize the electrolyte-containing separation between the membrane and the electrodes. These spacers are generally made from chemically inert, non-electroconducting synthetic polymers, which are made of interwoven, entwined, entangled or cohesive threads (e.g. woven fabrics, grids). or they may be in the form of plates with holes or grooves. In practice, these spacers are oriented along a plane parallel to the planes of the electrode and membrane. Terephthalic acid reduction can be monitored to obtain 100% conversion. Conversions below 100% are preferred.

A conversion of 96% or less is more preferred.

Undesirable by-products are produced at high conversion levels. The amount of impurities such as dihydroxymethylbenzene and toluic acid may increase at terephthalic acid conversion levels greater than 95-96%. Preferably, the percent conversion is balanced to obtain maximum conversion of p-hydroxymethylbenzoic acid and minimum conversion of unwanted by-products. Terephthalic acid is virtually insoluble in water at ambient temperatures and requires a weak salt as a reactant to form a soluble salt in water.

Examples of suitable weak bases are ammonia, methylamine and ethylenediamine, but any similar weak base can be used. In the practice of the electrolytic method invention, a weak base such as ammonia and a salt such as an ammonium salt are first added to the catholyte, the ammonia being used to dissolve the terephthalic acid in a solvent such as water, liquid ammonia, etc., with water being preferred. In sufficient concentration, ammonium salts carry electrical current.

After the initial operating period, Te. The monoammonium salt of phthalic acid is added to maintain conditions sufficiently basic to dissolve the added terephthalic acid, bringing the pH above 6.5, preferably from about 8.5 to about 9.5. Ensure complete dissolution of the terephthalic acid within the range. mercury compound in parallel with the addition of the monoammonium salt of terephthalic acid,
Preferably, a solvent-soluble mercury salt is added to the catholyte in an amount sufficient to maintain a continuous process, i.e., to continuously regenerate the amalgam coating of the cathode, and approximately 5 to 10 parts metal per million.
00 parts (Ppm) gives a minimum concentration of mercury metal ions. Mercury metal io7 concentrations greater than 1000 PIj1 can be used if appropriate.

Examples of mercury salts soluble in aqueous solutions are mercuric acetate, mercuric bromide, mercuric chlorate, mercuric chloride, and mercuric cyanide. Mercury acetate is preferred because of its high solubility and ease of handling. At the end of the electrolysis, p-hydroxymethylbenzoic acid is isolated from the electrolyte by known means, optionally due to the difference in water solubility between terephthalic acid and p-hydroxymethylbenzoic acid. Using this method, the catholyte is made acidic and hot, from about 75°C to
Filtered to remove terephthalic acid within a temperature range of approximately 10°C. Although p-hydroxymethylbenzoic acid can be obtained by cooling the mother liquor, it may optionally be concentrated under reduced pressure. Cooling is carried out below 40°C, preferably below 25°C, and the extent of concentration and cooling temperature will of course vary according to the desired degree of purity of the p-hydroxymethylbenzoic acid. The method of the invention has a number of advantages in addition to continuous operation. It allows the use of a catholyte that facilitates the work-up and recovery of p-hydroxymethylbenzoic acid, allows the production of small and easily removed electrolyzers, and allows for easy removal of the gases produced at the anode, especially oxygen. , which can result in high resistance between the electrodes due to gas bubbles, allowing the use of high current densities and the possibility of electrical connection in series between the various elementary electrolyzers as a collection of several cells (cells). feeding is easily achieved and allows the use of vessels with vertical electrodes. Finally, due to the constant geometry of the preferred electrolytic cell, the anolyte and catholyte can be circulated very quickly;
It allows the use of lower concentrations of terephthalic acid and as a result better degrees of conversion can be obtained in continuous processes. The following examples illustrate the invention.

The chemical yield shown is the yield of p-hydroxymethylbenzoic acid based on the initial amount of terephthalic acid present. The concentration of the solution is expressed in grams of solute per solution 11. Comparative Example 1 Terephthalic acid p-hydroxymethylbenzoic acid (PH
The batch reduction to MBA) was carried out in an electrolytic cell in the following manner.

The electrolytic cell was installed in an oil bath. This oil bath was used to heat the electrolyte to pre-initiation reaction temperature and to cool the electrolyte once the reaction began. The oil bath was equipped with an electric heater, and the cooling source consisted of a corrugated tube filled with cooling water and mechanical stirring means. The vessel was a 6001ne glass beaker with a fluorocarbon rubber stopper. Holes passing through the stopper provided access for the thermometer, anode conductors, and cathode conductors. The anode support was a glass anolyte tube with a fluorocarbon plastic holder to support the anode and semipermeable membrane. The diameter of the anode is approximately 2
．． It was a 5cm circular platinum wire mesh. The membrane was made from a sulfonated fluorocarbon polymer. The glass tube served as the anolyte chamber. The Funoleocarpon plastic holder was tilted at an angle from the horizontal to allow gases rising from the cathode to escape. The cathode was a metal disk about 6 cm in diameter. The cathode was made of 99.9% pure electrolytically pure lead. A magnetic stirring rod was placed on top of the cathode disk at the bottom of a glass beaker, which served as the electrolytic cell. In operation, the catholyte solution was placed in a bath with a cathode and a stirring bar in place.

The anode was inserted into the anolyte chamber, the chamber filled with anolyte, and inserted into the fluorocarbon stopper. The anolyte chamber was then inspected for membrane leaks and a person was placed inside the tank. A thermometer was inserted into the fluorocarbon stopper and the vessel was assembled. Heat can be applied to the completed vessel via an oil bath to reach the required temperature, at which point heat application is stopped.

4. The cooling system is then turned on and as soon as the temperature of the cell begins to drop, the electrolytic reaction begins by applying a direct current source.

Alternatively, the reaction can be started at room temperature and allowed to reach operating temperature without direct heating. Alternatively, the electrolyzer can be operated without the presence of a semipermeable membrane. The current density was maintained to maintain consumption of electricity slightly less than the calculated amount of four furades required for one equivalent of terephthalic acid.

An aqueous solution of 2% sulfuric acid (approximately 0.2 moles per liter of water) was used as the anolyte.

The catholyte is water, terephthalic acid, ammonia, and a soluble ammonium salt, ammonium carbonate (NH4)2C03
It consisted of The cathode was lead. The results are shown in Table 1. Comparative data from German Publication No. 2642496 has been included. The “496 current density (C.E.
．． ) has been calculated. Current efficiency (C.E.) was calculated.

The bath was shorted for 1 minute each time. ) Experiment number (5302
) 24(a) and 28(a) are the same as the experimental conditions of comparative experiment (c) based on the calculated current efficiency, except that the experiment was not interrupted and the bath was not shorted. . It is estimated that application of 15.3 fluorates per mole of terephthalic acid in Run No. (5302) 24(a) and 28(a) would have resulted in 100% terephthalic acid conversion. Experiment using interrupted operation (5302) 26 (5
) produced a lower conversion of terephthalic acid than Run No. (5302) 24 and 28, indicating that interrupted operation did not improve current efficiency under these conditions. Comparative Example The procedure of Example 1 was repeated using ammonium chloride and ammonium acid acid as ammonium salts.

9 The results are shown in the table.

A comparison of the data in Table 1 shows that ammonium chloride is a more suitable electrolyte for lead cathodes than either ammonium carbonate or ammonium sulfate.

Chloride ions are at least twice as effective as carbonate or sulfate ions. Comparative Example The procedure of Comparative Example 1 was repeated using ammonium chloride and ammonium sulfate as the ammonium salts and mercury as the cathode.

Liquid mercury was placed at the bottom of a glass beaker. The glass beaker served as a support for the tank. Electrical contact was made with a suitable conductor to the liquid mercury. The results are in the table. The above data shows that compared to the results obtained with the lead cathode in Comparative Example 1,
Figure 3 shows improved terephthalic acid conversion and increased yield of PHMBA obtained with a mercury cathode at high current efficiency.

The data show that ammonium salts of sulfate or chloride are equally suitable when using a mercury cathode, while ammonium chloride is preferred when using a lead cathode, as shown in the Comparative Examples table. Toluic acid is also given the experiment number (5) in the table.
302) occurs as a product in 144, 162 and 136. Running Run No. (5302) for 136 to 180 minutes resulted in a rapid increase in toluic acid production and p-xylene diol production. Current efficiency has decreased. PHMBA was converted to other products (toluic acid and p-xylene diol). Comparative Example The procedure of Comparative Example 1 was repeated using a lead amalgam cathode.

Lead amalgams were made by abrading the surface of electrolytically pure lead to remove any metal oxidation and then contacting the abraded metal surface with mercury to form an amalgam. 99.9% pure mercury was used as the bath for the rubbed solid lead cathode.

A lead-mercury amalgam formed on the lead surface at room temperature. The results are in the table. The above Zetas do not exhibit higher conversion of terephthalic acid to p-hydroxymethylbenzoic acid (PHMBA) and improved current efficiency when using a lead amalgam cathode versus using a lead cathode. As shown in Table 1, the current efficiency was improved compared to when a lead cathode was used. As shown in the table, less toluic acid was produced than when using a mercury cathode. Comparative example p- of V terephthalic acid
Two continuous reductions to hydroxymethylbenzoic acid were carried out in an electrolytic cell in the following manner in order to compare the current efficiencies obtained using a lead cathode and a lead amalgam cathode in continuous operation. .

- The two cathodes were electrolytically pure lead. The other cathode was electrolytically pure lead amalgamated with 99.9% pure mercury. In construction, the two-compartment cell included an artificial board of polyvinyl chloride (PVC) attached to a second board of the same size as the lead amalgam that comprised the cathode. A PVC insert between the cathode plate and the semipermeable membrane acted as a spacer to separate the cathode and membrane sufficiently to not allow flow of catholyte. The anode was an electrically pure -inch titanium plate coated with 250 microinches of platinum. The anode and membrane are connected to P so that the anolyte can flow through the tank.
Separation was performed using a VC spacer. An external reservoir for the anolyte served as an oxygen gas separator. An external reservoir for the catholyte served as a hydrogen gas separator. During operation, electrolyte was continuously pumped from the reservoir to the electrolytic cell and returned to the reservoir via a heat exchanger. No mercury salts were added to either catholyte. Details are in Table V. Ammonium chloride (NH2Cl) was used as the electrolyte at the lead cathode based on the data in the examples, and NH4C
PHM of terephthalic acid (TA) where l is higher as an electrolyte than ammonium sulfate (NH4)2S04
It was shown that the conversion rate to BA and higher current efficiency were provided.

The data in the table shows that an electrolyte with a lead cathode was more efficient, but an electrolyte with a lead cathode gave poor results, and a lead amalgam cathode gave better results.

However, with continuous operation, the current efficiency decreased to inefficient levels for both cathodes and both electrolytes. Comparative amalgam analyzes were performed before and after successive reductions to determine whether mercury loss from the cathodic lead amalgam surface occurred during the electrochemical reduction of terephthalic acid.

Therefore, to obtain qualitative and quantitative analysis of the amalgam cathode surface, X-line energy dispersive analysis (EDAX) was performed before and after experiment 5593-140 reported in the comparative example.

The mercury content on the lead amalgam surface of the cathode decreased after the experiment. The content of lead and iron on the cathode surface was increasing. After the experiment, the surface was lightly abraded to expose a fresh surface and the surface was again analyzed by EDAX. The results are shown in the table. The above data show that electrochemical reduction of terephthalic acid decreases the mercury surface content and increases the lead content of the amalgamated lead cathode.

Comparative Example The lead amalgam cathode used in Comparative Example V was again amalgamated using the procedure of Comparative Example.

The procedure of Example V was repeated without any mercury addition. The electrolyte was analyzed by mercury atomic absorption (AA) using standard analytical techniques three times: twice before the 5-hour run and once after the 5-hour run. The first analysis was of freshly made catholyte. The second analysis was after the catholyte was pumped through the bath in contact with the lead amalgam cathode. The mercury content of the catholyte was increased by contact with the lead amalgam cathode and decreased with the subsequent reduction of terephthalic acid as shown by the third analysis, details of which are in the table. Example 1 Two consecutive reductions of terephthalic acid were carried out in an electrolytic cell according to the procedure of the comparative example in order to compare the current efficiency obtained with and without the addition of a mercury salt.

The cathode was electrically pure lead amalgamated with 99.9% mercury. Periodic additions of mercuric acetate (HV(Ac)2) were made to the electrolytic cell each hour during the course of the experiment. A control experiment was also conducted in which mercuric acetate was not added to the electrolytic cell. The results of continuous operation using ammonium sulfate as the required salt are shown in the table. The data shows that as mercury additions begin to form above 80 parts per million (80 ppm), the amount of hydrogen generated begins to decrease and the current efficiency (C.E.
) begins to increase.

The above zeta shows that when mercury is present above 100 ppm, the trend in % current efficiency reverses from negative to positive rates under the conditions of Example V. EXAMPLES The procedure of Comparative Example V was repeated in multiple experiments using the addition of mercuric acetate in all experiments.

Claims (1)

[Scope of Claims] 1 a) The cathode is solid and metal, and the solid metal cathode is an alloy of mercury and the cathode metal (
amalgam), the coated surface of the cathode has a hydrogen overvoltage greater than the potential for the reduction of terephthalic acid to p-hydroxymethylbenzoic acid, and b) the amalgam coating is continuous. A process for the production of p-hydroxymethylbenzoic acid, which comprises electrochemically reducing terephthalic acid in an electrolytic cell in which sufficient mercury is added as a mercury compound to regenerate it into p-hydroxymethylbenzoic acid. 2 The catholyte in the electrolytic cell of the above method contains a solvent, terephthalic acid, ammonia, an ammonium salt and a mercury compound,
The temperature of the catholyte in the electrolytic cell is in the range from about 0°C to about 100°C, and the current density applied to the method is in the range from about 1 to 200 A/dm^2. Method in section 1. 3. A patent claim in which the metal of the cathode is selected from the group consisting of lead and an alloy of lead and a metal selected from the group consisting of cadmium, antimony, tin, and bismuth, and the mercury compound is a soluble salt of mercury. The method according to the first term of the scope. 4. The method of claim 3, wherein the soluble salt of mercury is selected from the group consisting of mercuric acetate, mercuric bromide, mercuric chlorate, mercuric chloride, and mercuric cyanide. 5. The method of claim 4, wherein the metal of the cathode is lead, the surface of the cathode is lead amalgam, and the solvent is water. 6 The ammonium salt of the catholyte is ammonium chloride, ammonium sulfate, or ammonium carbonate, and the concentration of the ammonium salt is about 0.1 to about 10% by weight of the total solution.
3. The method of claim 2, wherein the concentration of terephthalic acid is greater than 2% by weight of the total solution. 7. The concentration of said ammonia as ammonium hydroxide is in the range from about 1 g of ammonium hydroxide per 2 g of terephthalic acid to about 1 g of ammonium hydroxide per 1 g of terephthalic acid, and the pH of the resulting solution is at least 6. 5 to about 9.5 and the current density is about 2
3. The method of claim 2, wherein the reduction of terephthalic acid to p-hydroxymethylbenzoic acid is less than 100% in the range of 0 to about 100 A/dm^2. 8 The p-hydroxymethylbenzoic acid acidifies the catholyte, filtering the catholyte to remove terephthalic acid at a temperature within the range of about 75°C to about 100°C, and removing the mother liquor at a temperature below 40°C. 3. The method of claim 2, wherein the catholyte is isolated by cooling. 9. The method of claim 1, wherein said method uses a homogeneous separating diaphragm, which is a membrane of perfluorosulfonic acid. 10. The method of claim 1, wherein said method is a continuous method.