RU2194034C1 - Method of synthesis of benzenepolycarboxylic acids - Google Patents

Abstract

FIELD: organic chemistry, chemical technology. SUBSTANCE: invention relates to the improved method of synthesis of benzenepolycarboxylic acids, namely, pyromellitic and mellitic acids. Method is performed by electrochemical oxidation of graphite or coal in suspended state and oxidation is carried out in all volume of electrolyte with electrochemically generating "active chloride" as oxidizing agent forming in electrolysis of 10-20% sodium chloride solution, at currant density 0.3-0.5 A/cm² and temperature 50-90 C, from initial pH value of solution 7 using oxide-ruthenium-titanium anode and following isolation of the end product. Proposed ecologically pure method ensures to intensify electrochemical process of synthesis of benzenepolycarboxylic acids by 6-8.5-fold and to simplify process of extraction of the end product from electrolyte. End products are used in industry for production of polyesters, in part, alkyd resins and fibers, chelating agents, drugs, polyimides and pyrrones, electroinsulting covers. EFFECT: improved method of synthesis. 2 cl, 2 tbl, 3 ex

Description

 The invention relates to a method for producing benzenepolycarboxylic acids: pyromellitic and mellitic, used in industry for the production of polyesters, in particular alkyd resins and fibers, as well as complexing agents, drugs, polyimides and pyrrons, electrical insulating coatings.

A known method of producing mellitic (benzenehexacarboxylic C ₆ (COOH) ₆ ) acid by two-stage oxidation of pre-ground fossil fuels or products of their heat treatment (AS USSR 734189, publ. 18.05.80). For this, fossilized coal or heat-treated coal at 600-650 ^o C in the presence of 1.5-2% AlCl ₃ , first oxidized with concentrated nitric acid in a ratio of 1:15 at a temperature of 108-110 ^o C for 18 hours Then nitric acid is distilled off, 25% aqueous NaOH solution is added to the dry residue. In the second stage of oxidation, a stream of chlorine gas is supplied to the reaction mixture until the solution colored yellow-green is neutral. Mellitic acid is isolated on polystyrene cation exchange resin, the eluate is evaporated in a water bath, the dry residue is recrystallized from nitric acid. The yield of mellitic acid is 4.5-33.9% with a content of 0.3% nitrogen impurity and 0.2% mineral impurities in it.

The main disadvantages of this method are:
- multi-stage oxidation process that occurs in acidic, alkaline and neutral environments;
- high corrosivity of the nitric acid oxidizer and the need to work with toxic gases - chlorine and nitrogen oxides;
- the need for additional heat treatment of coal at 600-650 ^o With to obtain the outputs of mellitic acid more than 4.5%;
- the presence in the target product of impurities.

Closest to the claimed technical solution according to the technical essence and the achieved result is a method consisting in the electrochemical anodic oxidation of carbon graphite materials to mellitic acid (ZhPKh, 1990, v. 63, 10, pp. 2301-2307). The process is carried out with direct oxidative degradation of the carbon-graphite anode in a 0.05-1 M solution of H ₂ SO ₄ , a temperature of 75-95 ^o C, a potential of 1.05-1.3 V. In this case, the formation rate of the target product is on average 0.17 g / h in a liter of solution. After completion of the oxidation process, the solution is filtered off from the sludge, evaporated to 1 / 10th volume, centrifuged, the precipitated graphite oxide is filtered off, treated with an ammonia solution to obtain insoluble ammonium salt of mellitic acid, from which pure mellitic acid is obtained in the form of colorless acidified with concentrated nitric acid needle crystals with a yield of 20-30%.

The main disadvantages of this method are:
- the need to use a preformed carbon graphite anode in the process, since the target product is obtained when it is directly destroyed in the electrolyte;
- the impossibility of using fossil coals and similar materials having low electrical conductivity as raw materials for the electrochemical oxidation, since the anodes formed from them practically do not undergo electrochemical oxidation;
- the duration of the electrochemical process, since only the open surface of the carbon-graphite anode in the electrolyte is oxidized;
- multi-stage and the complexity of the selection of the target product;
- the use of a corrosive solution of sulfuric acid as an electrolyte.

To eliminate these drawbacks, a method is proposed whose task is to obtain pyromellitic (1,2,4,5-benzenetetracarboxylic C ₆ H ₂ (COOH) ₄ ) and mellitic acids in the process of electrochemical oxidation of fossil coal or graphite.

The problem is solved by oxidizing a suspension of coal or graphite with an electrochemically generated oxidizer "active chlorine" formed during the electrolysis of a 10-20% solution of sodium chloride, current density 0.3-0.5 A / cm ² , temperature 50-90 ^o C and constantly stirring the coal suspension. The term "active chlorine" means the total content in the solution of hypochlorite ions ClO ^- , hypochlorous acid HclO and free chlorine Cl ₂ .

 The method is as follows. In a diaphragmless electrochemical cell, 0.5 l of a 10-20% sodium chloride solution and 50 g of fossilized coal or graphite crushed to a grain size <0.1 mm are placed, the characteristics of which are given in table 1.

Electrochemical oxidation of a suspension of fossil coal or graphite is carried out in the entire volume of the electrolyte with constant stirring, a current density of 0.3-0.5 A / cm ² , a temperature of 50-90 ^o C. Anode oxide-titanium ruthenium (ORTA), titanium cathode (VT10) . The process of electrochemical oxidation is carried out for 15-17 hours from an initial pH of 7 to obtain a light yellow solution having a pH of 11-12. The rate of formation of the target product is on average 0.86-1.20 g / h in a liter of solution, which is 6-8.5 times more intense compared to the prototype. The resulting solution was filtered off from the sludge, the filtrate was acidified with hydrochloric acid to pH 1. The resulting benzene polycarboxylic acids were recovered by extraction with ethyl acetate, which was carried out three times with a volume of 100 ml portion of ethyl acetate. The solvent is distilled off, the dry residue is recrystallized from water and the desired product is obtained in the form of colorless needle crystals. The remaining electrolyte after neutralization is again used for the next cycle of coal electrochemical oxidation. Table 2 shows the data on the dependence of the yields of benzene polycarboxylic acids on the conditions of electrochemical oxidation of the feedstock.

 From table 2 it can be seen that during oxidation of fossil coals of the Lipovetsky and Partizansky coal sections, pyromellitic acid is formed, while graphite is oxidized, mellitic acid is formed, which is explained by the different chemical structure of these materials. Fossil coals, in particular liptobiolites and humites, are a natural macromolecular copolymer of an inhomogeneous structure and composition, in which various condensed aromatic structures predominate, during the oxidation of which pyromellitic acid is formed. Graphite has a hexagonal layered package, which is an analogue of aromatic compounds, during the oxidation of which mellitic acid is formed.

 In both cases, the concomitant polycarboxylic acids and various impurities formed in small quantities are separated from the target product during recrystallization from water.

When analyzing the data of table 2 it is seen that the highest yield of benzenepolycarboxylic acids is achieved at a current density of 0.4 A / cm ² , a temperature of 90 ^o C and a concentration of sodium chloride solution of 15% (example 3). Depending on the raw material, the yield of pyromellitic acid at the indicated parameters of electrochemical oxidation is from 26.7 to 30.1% of the organic part of coal, the yield of mellitic acid is 37.2% of the organic part of graphite. Increasing the current density to 0.5 A / cm ² (example 5) slightly reduces the yield of the target product due to its partial destruction, which begins to occur when this electrochemical parameter is increased. When the current density equal to 0.7 A / cm ² (example 6), there is a sharp drop in the yield of the target product, which is due to corrosion of the anode that occurs in this heterogeneous system at high current densities. When the current density is below 0.4 A / cm ² (example 7), the yield of the target product decreases due to incomplete interaction of the feedstock with the oxidizing agent generated at this current density.

The data presented in examples 3, 5, 6, 7 of Table 2 indicate that the optimal range of current density to achieve a technical result are the declared values of 0.3-0.5 A / cm ² .

A decrease in temperature from 90 to 70 and 50 ^o C (example 2 and 1, respectively) also reduces the yield of the target product, which is due to the lower reactivity at this temperature of the oxidizer "active chlorine". The increase in temperature to 95 ^o C and higher (example 4) dramatically reduces the yield of the target product, which is due to local boiling of the electrolyte in the electrode space, as a result of which the basic electrochemical processes are inhibited.

Therefore, the optimal temperature range of the electrochemical oxidation of fossil coals or graphite to obtain benzene-polycarboxylic acids is the temperature range claimed in the method, namely 50-90 ^o C.

 The optimal concentration of sodium chloride solution is 15%. A decrease in concentration to 10% slightly reduces the yield of the target product (example 8), which is explained by a decrease in the yield of the generated “active chlorine” oxidizing agent, an increase in the concentration of sodium chloride solution to 20% (example 9) also reduces the yield of the target product due to the salting out (precipitation) effect ) intermediate products of the oxidation of humic and polyfunctional acids.

The identification and purity of the obtained pyromellitic and mellitic acids is confirmed by several physicochemical and analytical methods:
- according to published data for pyromellitic and mellitic acids, the melting points are respectively t _PL = 275 and t _PL = 288 ^o C, for the obtained oxidation products, respectively, T _PL = 274.5 and T _PL = 287 ^o C;
- IR spectra of standard bhp the samples and IR spectra of the obtained oxidation products taken on a Perktron Elmer Spektrum BX FT-IR System instrument in KBr pellets have similar absorption bands;
- elemental analysis for carbon. For pyromellitic acid, C 47% was calculated; found for the oxidation product C C 46.2%. For mellitic acid, C 42% was calculated; found for the oxidation product C 41.1%;
- performed chromatography-mass spectrometric analysis on an HP 5972 MSD / HP 5890 series 2 GS system from Hewlett Packard company shows that the main products according to the mass spectrum are pyromellitic dianhydride, molecular ion 218 and mellitic acid trianhydride, molecular ion 288.

Salient features of the claimed invention in relation to the prototype are:
- electrochemical oxidation of fossil coals or graphite in a suspended state in the entire volume of the electrolyte, which eliminates the need for preliminary formation of the anode for its oxidation according to the prototype;
- the possibility of using as readily available raw materials for the oxidation of fossil coals with low electrical conductivity, in particular natural liptobiolites and humites;
- use as an oxidizing agent of “active chlorine” formed during electrolysis of a 10-20% solution of sodium chloride on an oxide-ruthenium-titanium anode at a current density of 0.3-0.5 A / cm ² and a temperature of 50-90 ^o C ;
- carrying out the process from the initial pH of the solution equal to 7, followed by the selection of the target product.

 Distinctive features, together with the known ones, provide the solution with a new technical property, which consists in intensifying the electrochemical process for producing benzene-polycarboxylic acids by oxidizing suspended graphite or fossil coal with an electrochemically generated oxidizing agent 6-8.5 times.

 An advantage of the method is that the process of extracting the target product from the electrolyte after the completion of the oxidation process is greatly simplified, and the electrolyte can be reused after neutralization for the next cycle of electrochemical oxidation. The method is environmentally friendly, since it does not use toxic oxidizing agents, such as nitric acid. The oxidizing agent "active chlorine" electrochemically generated in a sodium chloride solution, oxidizing fossil coal or graphite, is converted to the same salt. Thus, the process of coal oxidation is cyclical.

 Based on the foregoing, we can conclude that the set of essential features of the claimed invention has a causal relationship with the achieved technical result. The above distinguishing features are not found in available sources of information. Therefore, the claimed technical solution has novelty and has an inventive step, since it does not explicitly follow from the prior art.

 The possibility of carrying out the invention is confirmed by the following examples.

Example 1. In a glass diaphragm-free electrochemical cell, 50 g of crushed by a ball mill is placed to a grain size of <0.1 mm graphite. 0.5 liters of a 15% sodium chloride solution are poured into it. The solution is heated to 50 ^o With constant stirring. After the formation of the suspension, its electrolysis is carried out at a current density of 0.4 A / cm ² . Anode ORTA, cathode VT10 titanium. The oxidation of graphite during the entire electrolysis is carried out in the entire volume of the electrolyte with constant stirring of its suspension. After 17 hours of electrolysis, a light yellow solution having a pH of 12 is filtered off from the sludge, the filtrate is acidified with hydrochloric acid to pH 1. Next, the formed mellitric acid is extracted by extraction with ethyl acetate, which is carried out three times with a volume of ethyl acetate in each portion of 100 ml. Then the solvent is distilled off, the dry residue is recrystallized from distilled water. The result is pure mellitic acid in the form of colorless needle crystals. Yield 27.1% in terms of the organic part of graphite. The remaining electrolyte (filtrate after its neutralization and adjustment of the concentration of sodium chloride solution) can be used for the next cycle of electrochemical oxidation of coal or graphite.

Example 2. The example is carried out analogously to example 1 except that the Partisan coal mine humites are taken as the raw material for oxidation (table 1) and the solution temperature is maintained at 70 ^° C during electrolysis ^. The yield of pyromellitic acid is 24.6%, calculated on the organic part coal.

Example 3. The example is carried out analogously to example 1, only as a raw material for oxidation take lipobiolites of the Lipovetsky coal mine (table 1) and maintain the temperature of the solution during electrolysis 90 ^o C. The yield of pyromellitic acid 30.1% in terms of the organic part of coal.

Claims (2)

1. The method of producing benzenepolycarboxylic acids - pyromellitic and mellitic by electrochemical oxidation, characterized in that graphite or fossil coals in suspended state are subjected to electrochemical oxidation and oxidation is carried out in the entire electrolyte volume by the "active chlorine" electrochemically generated oxidizer formed during electrolysis 10 -20% solution of sodium chloride current density of 0.3-0.5 a / cm ² and a temperature of 50-90 ^o C, the initial pH of 7, using the ruthenium oxide- in the titanium anode with subsequent isolation of the desired product.  2. The method according to p. 1, characterized in that as fossil coals use liptobiolites, humites.

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