Method for producing terephthalic acid from polyethylene terephthalate

Abstract

FIELD: polymer materials.SUBSTANCE: method for producing terephthalic acid, used in polymer technology as a raw material or plasticizer. The method includes alkaline hydrolysis of secondary polyethylene terephthalate in the presence of a cationic surfactant containing nitrogen with a concentration of 10-30 g / l for 60-90 min, followed by the addition of water to the reaction mixture until the sodium terephthalate is completely dissolved. Next, the aqueous solution of sodium terephthalate is separated from water-insoluble compounds, terephthalic acid is precipitated from the aqueous solution by acidification, terephthalic acid is separated from water-soluble compounds, and then washed and dried. In this case, after separating terephthalic acid from water-soluble compounds, ethylene glycol is distilled off.EFFECT: increase in the processability of the method due to a reduction in the time of alkaline hydrolysis of polyethylene terephthalate.2 cl, 1 dwg, 1 tbl, 17 ex

Description

[0001] The invention relates to a method for producing terephthalic acid by alkaline hydrolysis of polyethylene terephthalate. The resulting terephthalic acid can be used in polymer technology as a raw material or plasticizer. Terephthalic acid is the primary raw material in the synthesis of polyethylene terephthalate, which is used to produce plastic products, fibers, and threads.

[0002] A known method for producing terephthalic acid (Patent RU 2337903, IPC C07C 51 / 265, C07B 61 / 00, C07C 63 / 15, C07C 63 / 26, published 12.12.2002) is based on a two-stage oxidation of p-xylene. In the first stage, a dialkylbenzene compound, an acetic acid solvent containing oxidation catalyst components dissolved in it, and an oxygen-containing gas are fed into the reactor. At this stage, liquid-phase, exothermic oxidation of the dialkylbenzene compound occurs, with the temperature and pressure in the first oxidation reactor maintained at elevated pressure at 150-180°C and at 3.5-13.0 bar absolute pressure, respectively. At the second stage, liquid-phase exothermic oxidation of the products of incomplete oxidation of the first stage occurs, with the temperature and pressure in the second oxidation reactor maintained at 185-230°C and at 4.5-18.3 bar.The disadvantage of the method is the formation of by-products 4-carboxybenzenealdehyde and p-toluic acid due to the use of xylene as raw material.

[0003] Patent RU 2664547 (IPC C10G 3 / 00, C07C 1 / 207, C07C 4 / 06, C07C 11 / 02, C07C 15 / 08, C07C 51 / 265, C10G 45 / 02, C10G 49 / 04, C10G 65 / 12, published on 20.08.2018) presents a method in which tall oil resin containing a certain proportion of fatty and resin acids and / or their derivatives is heated to a temperature sufficient to convert it into a liquid. An aromatic hydrocarbon that can be converted into terephthalic acid is separated from the resulting liquid and subjected to oxygenation and rearrangement reactions in a layer of one or more catalysts. The disadvantage of this method is the low yield of terephthalic acid, since tall oil contains predominantly aliphatic rather than aromatic compounds.

[0004] One promising method for producing terephthalic acid is the depolymerization of polyethylene terephthalate waste, for example, by alkaline hydrolysis. Thus, the method for producing terephthalic acid (patent RU 2724893, IPC C07C 63 / 26, C07C 51 / 09, C08F 8 / 12, C08J 11 / 10, published on June 26, 2020) involves grinding PET bottle waste, keeping them in a nitrating atmosphere at 130-190°C for 5-24 hours, treating them with a sodium hydroxide solution to form disodium terephthalic acid, and precipitating terephthalic acid with a monobasic mineral acid, followed by filtration, washing, and drying. The disadvantages of the method are the use of nitric acid with a concentration of 12 mol / l, which causes corrosion of the equipment, and the duration of the process of 5-24 hours.

[0005] The closest in technical essence is the method (Patent RU 2616299, C08J 11 / 16, C08J 11 / 22, C07C 63 / 26, C07C 51 / 347, published on 14.04.2017) for obtaining terephthalic acid by alkaline hydrolysis, which includes hydrolysis of polyethylene terephthalate flakes with sodium hydroxide in a T-92 oxal flotation reagent medium with heating, followed by adding water to the reaction mass until the disodium salt of terephthalic acid is completely dissolved, separating the aqueous layer from the T-92 oxal flotation reagent, precipitating terephthalic acid from the aqueous layer with concentrated hydrochloric acid, filtering, washing and drying. The disadvantages of the method are the duration of the process (3-5 hours), which increases the cost of the process, and the limitation on raw materials - flakes, which does not allow the use of waste fibrous materials for alkaline hydrolysis.Moreover, the proposed flotation reagent oxal T-92 is a mixture of polyhydric alcohols, which complicates the separation of ethylene glycol, which is formed together with terephthalic acid.

[0006] The technical result of the claimed method is the elimination of the said disadvantages, namely, an increase in the processability of the method by reducing the time of alkaline hydrolysis of polyethylene terephthalate by using a cationic surfactant containing nitrogen while simultaneously expanding secondary polyethylene terephthalate as a raw material.

[0007] The claimed method for producing terephthalic acid from polyethylene terephthalate involves carrying out alkaline hydrolysis of the polymer with an aqueous solution of sodium or potassium hydroxide in the presence of a cationic nitrogen-containing surfactant upon heating, followed by the addition of water to the reaction mixture until sodium terephthalate is completely dissolved, separating the aqueous solution of sodium terephthalate from water-insoluble compounds, precipitating terephthalic acid from the aqueous solution by acidification with hydrochloric or sulfuric acid, separating terephthalic acid from water-soluble compounds, washing and drying. The cationic nitrogen-containing surfactant acts as a carrier of the hydroxide anion to the polymer surface. A nitrogen-containing quaternary cation with one or two fatty radicals carries OH ions. - to the surface of the polymer, causing hydroxide anions to attack the polyethylene terephthalate macromolecules, causing its depolymerization.

[0008] The alkaline hydrolysis process is carried out as follows. Crushed polyethylene terephthalate (0.5-1.0 cm) and aqueous solutions of a cationic nitrogen-containing surfactant and alkali are placed in a laboratory reactor equipped with a stirrer and reflux condenser. The reaction mixture is heated to 130-150°C and the process is carried out with constant stirring for 60-90 minutes. Upon completion of the process, the reaction mixture is cooled to 70°C and water is added with constant stirring until the disodium salt of terephthalic acid is completely dissolved. Next, the aqueous layer is filtered and neutralized with a solution of hydrochloric or sulfuric acid to a pH of 4. The precipitated terephthalic acid is filtered, washed with water, and dried in a drying oven at 110-120°C. The final filtrate is distilled, collecting the ethylene glycol fraction.

[0009] Alkali is used in excess of the stoichiometric amount per 1 kg of crushed polyethylene terephthalate, 0.4-0.6 kg of sodium hydroxide or 0.5-0.7 kg of potassium hydroxide. The concentration of the cationic nitrogen-containing surfactant is 10-30 g / L. Optimum reagent concentrations enable the alkaline hydrolysis process to achieve a polyethylene terephthalate conversion rate above 90% and a terephthalic acid yield above 80% (table).

[0010] Examples 1, 2, 3 illustrate the optimal ratios of polyethylene terephthalate: alkali, at which the degree of conversion of polyethylene terephthalate is 100% and the yield of terephthalic acid is above 90%.

[0011] Accepted: recycled polyethylene terephthalate bottle - 5 g, trimethylcetylammonium bromide - 20 g / l, sodium hydroxide - 2-3 g, the ratio of polyethylene terephthalate : alkali is 1: (0.4-0.6). Hydrolysis reaction conditions: 130 ° C, 90 min. Acidification of an aqueous solution of sodium terephthalate is carried out with sulfuric acid. The conversion rate of polyethylene terephthalate is 96-100%, the yield of terephthalic acid is 82-92%.

[0012] Examples 2, 4, 5 illustrate the optimum concentration of the cationic nitrogen-containing surfactant at which the conversion rate of polyethylene terephthalate is 100% and the yield of terephthalic acid is 90%.

[0013] Accepted: recycled polyethylene terephthalate bottle - 5 g, trimethylcetylammonium bromide - 10-30 g / l, sodium hydroxide - 2.5 g, the ratio of polyethylene terephthalate : alkali is 1: 0.5. Hydrolysis reaction conditions: 130 ° C, 90 min. Acidification of an aqueous solution of sodium terephthalate is carried out with sulfuric acid. The degree of conversion of polyethylene terephthalate is 95-100%, the yield of terephthalic acid is 80-90%.

[0014] Examples 6, 7 illustrate the possibility of reducing the duration of the alkaline hydrolysis process from 90 to 60 min by increasing the temperature from 130 to 150°C.

[0015] Accounting materials: recycled polyethylene terephthalate bottle - 5 g, trimethylcetylammonium bromide - 20 g / l, sodium hydroxide - 2.5-3 g, polyethylene terephthalate: alkali ratio - 1: (0.5-0.6). Hydrolysis reaction conditions: 150 ° C, 60 min.

[0016]

[0017]

[0018]

[0019] Acidification of an aqueous solution of sodium terephthalate is carried out with sulfuric acid. The degree of polyethylene terephthalate conversion is 100%, the yield of terephthalic acid is 89%.

[0020] Example 8 illustrates the possibility of acidifying an aqueous solution of sodium terephthalate with hydrochloric acid.

[0021] Accepted: recycled polyethylene terephthalate bottle - 5g, trimethylcetylammonium bromide - 20g / l, sodium hydroxide - 2.5g, the ratio of polyethylene terephthalate to alkali is 1:0.5. Hydrolysis reaction conditions: 130°C, 90 min. Acidification of an aqueous solution of sodium terephthalate is carried out with hydrochloric acid. The degree of conversion of polyethylene terephthalate is 100%, the yield of terephthalic acid is 90%.

[0022] Example 9 illustrates the possibility of using potassium hydroxide in the alkaline hydrolysis reaction of recycled polyethylene terephthalate.

[0023] Accepted: recycled polyethylene terephthalate bottle - 5 g, trimethylcetylammonium bromide - 20 g / l, potassium hydroxide - 3 g, the ratio of polyethylene terephthalate : alkali is 1: 0.6. Hydrolysis reaction conditions: 130 ° C, 90 min. Acidification of an aqueous solution of sodium terephthalate is carried out with sulfuric acid. The degree of conversion of polyethylene terephthalate is 100%, the yield of terephthalic acid is 91%.

[0024] Examples 2, 10-13 illustrate the possibility of using different cationic nitrogen-containing surfactants.

[0025] Accepted: recycled polyethylene terephthalate bottle - 5 g, trimethylcetylammonium bromide or trimethyllaurylammonium chloride or dimethyldilaurylammonium chloride or benzyldimethylcetylammonium chloride or benzyldimethyllaurylammonium bromide - 20 g / l, sodium hydroxide - 2.5 g, the ratio of polyethylene terephthalate : alkali is 1: 0.5. Hydrolysis reaction conditions: 130 ° C, 90 min. Acidification of an aqueous solution of sodium terephthalate is carried out with sulfuric acid. The conversion rate of polyethylene terephthalate is 98%, the yield of terephthalic acid is 82%.

[0026] Examples 14-17 illustrate the possibility of using fibrous materials as secondary polyethylene terephthalate.

[0027] Accepted: secondary polyethylene terephthalate fibrous material - 5 g, trimethylcetylammonium bromide - 20 g / l, sodium hydroxide - 2.5 g, the ratio of polyethylene terephthalate : alkali is 1: 0.5. Hydrolysis reaction conditions: 130 ° C, 90 min. Acidification of an aqueous solution of sodium terephthalate is carried out with sulfuric acid. The conversion rate of polyethylene terephthalate is 100%, the yield of terephthalic acid is 84-88%.

[0028] Thus, the examples in the table illustrate that the claimed method makes it possible to obtain terephthalic acid from secondary polyethylene terephthalate with a yield of the target product of 80-92% and a degree of conversion of the feedstock of 90-100%.

[0029] The terephthalic acid obtained by the claimed method is a white crystalline substance insoluble in water. The acid number is 658-660 mg KOH / g. According to elemental analysis, the acid contains 57-58% C and 3-4% H. The IR spectrum of the obtained terephthalic acid is shown in Fig. 1. Absorption bands in the region of 3500-3300 cm -1 characterizes the stretching vibrations of the O-H bonds of the carboxyl group, in the region of 3000-2800 cm -1 - stretching vibrations of the C-H bonds of the aromatic ring. Stretching vibrations of the C=O bond of aromatic carboxylic acids appear as an intense absorption band in the region of 1720-1680 cm -1 . Area 1600-1400 cm -1 proves the aromaticity of the hydrolysis product. An intense absorption band at 1270-1290 cm -1 corresponds to the planar deformation vibrations of the -OH group bonds. The stretching vibrations of the C-O bond cause an absorption band at 1150-1100 cm -1. Intense absorption in the spectrum below 900 cm -1 refers to out-of-plane deformation vibrations of the C-H bonds of the aromatic ring of the n-substituted arene.

Claims

1. A method for producing terephthalic acid from polyethylene terephthalate, comprising alkaline hydrolysis of secondary polyethylene terephthalate upon heating, followed by adding water to the reaction mixture until sodium terephthalate is completely dissolved, separating an aqueous solution of sodium terephthalate from water-insoluble compounds, precipitating terephthalic acid from the aqueous solution by acidification, separating terephthalic acid from water-soluble compounds, washing and drying, characterized in that the alkaline hydrolysis is carried out in the presence of a cationic surfactant containing nitrogen with a concentration of 10-30 g / l for 60-90 min, and after separating the terephthalic acid from water-soluble compounds, ethylene glycol is distilled off.

2. A method for producing terephthalic acid from polyethylene terephthalate according to claim 1, characterized in that the secondary polyethylene terephthalate contains fibrous materials as raw material.

Images