MX2008007412A - A process for preparing high purity terephthalic acid - Google Patents

Abstract

The present invention relates to a process for preparing high purity terephthalic acid by catalytic, liquid phase oxidation of p-xylene utilizing a partially homogeneous reaction in the presence of a very specific catalyst. The process produces a high purity terephthalic acid without the secondary purification step currently practiced.

Description

PROCEDURE TO PREPARE HIGH PURITY TEREFT ACID BACKGROUND OF THE INVENTION Pure terephthalic acid (PTA), an important raw material used in the production of poly (ethylene terephthalate) (PET) for conversion into fibers, films and containers, is produced commercially by purifying raw or technical grade terephthalic acid produced by catalytic oxidation. with air in liquid phase of p-xylene (PX). Virtually all technical grade PTA is produced by catalytic oxidation with p-xylene liquid phase air. Commercial processes use acetic acid as a solvent and a multivalent heavy metal catalyst, more broadly based on cobalt and manganese compounds, and a promoter, with bromine or bromide ions as the renewable source of free radicals. The acetic acid, air, p-xylene and catalyst are continuously fed into an oxidation reactor which is maintained from 175 ° C to 225 ° C under a pressure of 1.5-3.0 MPa. The feed weight ratio of acetic acid to p-xylene is usually less than 5: 1. The air is added in amounts that exceed the stoichiometric requirements to minimize the formation of side products. The oxidation reaction is exothermic, and the heat is typically removed allowing it to boil the acetic acid solvent The corresponding vapor condenses and most of the condensate is refluxed in the reactor. The residence time is usually 30 minutes to 2 hours, depending on the procedure. The reactor effluent is a suspension of crystals. of raw terephthalic acid which are recovered by filtration, washed, dried and transported to storage. Subsequently, the crystals are fed to separate purification steps (see US Patent No. 5,350, 133) Although the main impurity is 4-carboxy? benzaldehyde (4-CBA), p-toluic acid (pTA) is also present in a significant amount Although the purity of crude grade PTA is generally greater than 99%, it is not pure enough for the PET made from it achieve the required degree of polymerization From U.S. Patent No. 6,034,269, a process for the production of high purity terephthalic acid by oxidation is known. catalytic ion of p-xylene liquid phase carried out in a piston flow reaction zone, where a high weight ratio between the solvent (acetic acid) and p-xylene is used and sufficient temperature and pressure to maintain the PTA in solution as it is formed The acid is crystallized from the resulting reaction medium and recovered without the need for separate purification Although the purity can be as high as 99 95%, the pTA is present in an amount greater than 80 -90 ppm In US Patent No. 6,307,099, a process for homogeneous liquid phase oxidation of p-xylene in PTA is described, wherein the 4-CBA content of the terephthalic acid preferably recovered is not more than about 50 ppm, for example , 20 to 300 ppm No data are reported regarding pTA, color parameters and catalytic residues The procedure requires circulating a large volume of the solvent and uses a very high concentration of oxidation catalyst, calculated for oxidized p-xylene The procedure operates under reaction conditions, where substantially all of the terephthalic acid produced in the oxidation reaction remains in the solution during the reaction. The detailed description of this patent specifies the possibility of some precipitation during the reaction, for example up to 10% but preferably no more than about 2% by weight of the terephthalic acid produced can be precipitated during the reaction. The patents (US Patent No. 6,034,269 and US Patent No. 6,037,099) do not specify the oxidation catalyst, its concentration, and its important influence on the quality of terephthalic acid. According to these patents, the selection of the catalyst and oxidation promoter is within the conventional practice BRIEF DESCRIPTION OF THE INVENTION The present invention relates to a process for preparing highly pure terephthalic acid comprising the steps of A) Oxidising para-xylene in terephthalic acid with air in the presence of a liquid reaction phase maintained at a temperature between 180 ° C and 230 ° C, wherein the liquid reaction phase comprises para-xylene, acetic acid, water, and a catalytic composition, wherein the water has 5 to 12 weight percent of the acetic acid, the weight ratio of para-xylene to acetic acid is such that to 50% of the terephthalic acid in reaction is present as a solid at the oxidation temperature, and the catalytic composition comprises cobalt, manganese and bromine in combination with at least one element selected from the group consisting of zirconium and hafnium, wherein the Atomic ratio of C Mn Br. is on the scale of 1 0 2 - 1 0 1 1 - 2.7 and the atomic ratio of cobalt to the elements selected from the group consisting of zirconium and hafnium is 1 0 03 - 3 0, where the total weight of Co and Mn is 100-500 mg per 1 Kg of the liquid reaction phase; B) recover terephthalic acid by crystallization at a temperature in scale of 150 ° C to 80 ° C DETAILED DESCRIPTION OF THE INVENTION It has now been unexpectedly discovered that it is possible to produce, by catalytic oxidation in the liquid phase of p-xylene, a highly pure PTA without the purification step. The process described herein produces, without a purification step, a PTA containing small amounts of impurities, preferably less than 40 ppm by weight in total of 4-CBA and pTA, and less than 20 ppm of pTA, and having excellent amounts of impurities. color parameters. Such a high quality of PTA can be obtained under very specific reaction and concentration conditions described in this specification. The catalytic oxidation in liquid phase of p-xylene must be carried out in acetic acid as solvent, at a temperature, pressure and with a weight ratio of acetic acid to p-xylene such that 15 to 50% of the PTA formed does not is maintained in solution, and operating under specific temperature conditions in the presence of a very specific composition and concentration of a catalyst. The catalyst must contain compounds comprising Co and Mn compounds with Zr and / or Hf, and as a promoter, bromine, and / or bromine compounds; where the atomic ratio Co: Mn: Br is on the scale of 1: 0.2-1.0: 1.1 -2.7, preferably 1: 0.3-0-0.8: 1.1 -1.8 and the atomic ratio of Co: Zr and / or Hf is 1: 0.03-0.3. The oxidation is carried out in a stirred reactor under suitable stirring conditions to homogenize the liquid phase and provide similar temperature conditions at all points of the reactor, at a temperature comprised in the range of 195 ° C to 220 ° C or, preferably, according to a temperature profile that starts from 180 ° to 200 ° C and then up to 230 ° C Preferably, the temperature in the last period is 205 ° -215 ° C Temperatures above 230 ° C are not recommended because acetic acid reacts and is lost However, in some cases it is convenient after finishing the oxidation reaction, heat the reaction mixture for a short time at the temperature of 240 ° C in the absence of an air flow (for example, for 10-30 min) The concentration of the catalyst, expressed as total weight of Co and Mn per 1 kg of liquid reaction phase, is 100-500 mg The PTA is recovered from the reaction phase by crystallization at a temperature of 80 ° to 150 ° C, and then washed with acetic acid and / or water The mother solution is partly recycled to the oxidation reactor and partly regenerated. The color L * determined according to the standard method CIÉ in pulverized PTA having an average particle size of less than 50 μm has a minimum of 95 5 impurities The derivatives of the catalyst expressed as Co metal and Mn were in total less than 8 ppm by weight, in particular, Co was less than 4 ppm and Mn less than 2 ppm The PTA crystallized from the reaction solution, at least in part, is in the form of clearly angular crystals, for example, which have a rhombus structure and therefore are different from the PTA produced according to the commonly used prior art processes of catalytic oxidation in heterogeneous liquid phase of p-xylene, where the crystals tend to be round agglomerates of crystals more small The acetic acid used as the solvent contains from 1 to 15% by weight of water, preferably 5 to 10% The weight ratio of acetic acid / p-xylene is not less than 30 1, but must be such that the at 50%, particularly 20-30% by weight of the PTA after the oxidation reaction is present as a solid phase. The oxidation reaction is e > Typically, in known processes, the heat is removed by allowing the acetic acid solvent to boil, condensing the resulting vapor and condensate, in a variable amount being refluxed in the reactor. Typically, in the process of the present invention the reaction temperature and pressure are maintained at the level necessary to maintain the preferred temperature profile and to reach the preferred temperature of the last heating step. The reaction can also be carried out in a plug flow reactor. Piston flow reactor, molecular oxygen is dissolved in the feed stream to reach a concentration of dissolved oxygen that exceeds the stoichiometric value Using a stirred reactor, it is passed air through the liquid phase with a sufficient flow velocity to remove heat and not exceed the flammability limit at the top of the reactor. The oxygen source can be pure oxygen, air, or any convenient gas containing oxygen. Examples of compounds of cobalt, manganese, zirconium and hafnium which can be used as the catalyst component are acetates, carbonates, hydroxides and oxides. Examples of bromine or bromine-containing compounds are bromine, HBr, NaBr, KBr, and organic bromides known to provide ions of bromide at the oxidation temperature, such as bromobenzenes, benzyl bromide and tetrabromoethane. The reaction time in a stirred reactor depends on the reaction conditions and is generally 13 to 45 minutes. The PTA obtained with the process of the present invention, thanks to its very low content of 4-CBA and pTA, is particularly suitable for the production of high molecular weight alkylene terephthalates and copolymers thereof The following examples are presented for Polish and not to limit the scope of the present invention EXAMPLES Oxidation conditions The oxidation of p-xylene with air is carried out in a reactor of 250 ml of a titanium alloy. The reactor is equipped with a magnetic stirring system operating at 100-3000 rpm, has an air inlet in the reactor. bottom and an outlet through a condenser equipped with a phase separator, pressure and temperature regulator, electric heating mantle and outlet for a product probe The reactor is charged with p-xylene, catalyst and solvent (acetic acid added with water) and it is presupposed with nitrogen at 2 5 MPa Then the temperature rises to the desired temperature for 15-20 minutes When the temperature of the liquid medium inside the reactor reaches the desired temperature, the air flow in the liquid phase stabilizes at 0 5 l / rnin and the agitated speed at 2200 rpm The exit gas is continuously monitored using an oxygen analyzer When the oxygen consumption is stopped, the agitator speed decreases at 200-300 rpm and heating of the reactor is completed. The temperature of the reactor contents is cooled to approximately 85 ° C in 10-60 minutes. During this cooling period, the PTA is crystallized from the stirred liquid medium. isolate at about 80 ° C, wash (3 times with 10 ml of acetic acid and 1 time with 20 ml of H20), dry 80 ° C for 5 hours, weigh, pulverize and analyze. The content of 4-CBA and pTA is determined by HPLC.

Analytical determinations pTA and 4-CBA are measured through liquid chromatography (HPLC) using a column of NH2"Zorbax" from Du Pont, a buffer solution of pH of ammonium phosphate (adjusted pH 4.25 with concentrated NH4OH if it has to be high , with H3P04 if it has to be reduced) and an absorbance detector of 254 nm. A dry PTA sample of 0.2 + 0.005 g is dissolved with 20 ml of an ammonium hydroxide solution of 3.7% by weight; 20 ml of distilled water and 10 ml of benzyl alcohol are added to the PTA sample. The pH is adjusted to 7 (6.8-7.2) with concentrated H3P04. Before use, the column is initially rinsed with CH3CN, and after 15 minutes with distilled water. The pH buffer runs through the column for 16 hours at a rate of 1.3 ml / min to stabilize the column. A Sigma 10 Perkin-Elmer data processor and a Spectra-Physics computation integrator are used for the analysis.

EXAMPLE 1 The reactor was charged with 2.5 g p-xylene, 150 grams of acetic acid containing 10% by weight of H20, 0.100 g of Co (CH3COO) 2. 4H20 and Mn (II) acetate, Zr (IV) acetate, HBr (47% H20 solution) in the atomic ratio Co: Mn: Zr: Br = 1: 0.6: 0.05: 1.7. The mixture was oxidized with air (flow 0.5 l / min) at 195 ° C for 20 minutes, then for 3 minutes, the temperature increased to 210 ° C and at this temperature the oxidation proceeded for 22 minutes to reach the complete conversion of p-xylene. HPLC analysis of terephthalic acid determined that it contained 11 ppm of 4-CBA and 4 ppm of p-toluic acid. The color parameter L * was 95.8.

EXAMPLE 2 The procedure of Example 1 was repeated, except that hafnium was used instead of zirconium in the amount corresponding to the atomic ratio Co: Mn: Hf: Br = 1: 0.6: 0.1: 1.7. The terephthalic acid contained 14 ppm of 4-CBA and 5 ppm of p-toluic acid.

EXAMPLE 3 (COMPARATIVE) The procedure of Example 1 was repeated, except that no zirconium was added. The quality of terephthalic acid produced in the absence of Zr or Hf is lower. The PTA contained 89 ppm of 4-CBA and 6 ppm of p-toluic acid.

EXAMPLES 4 TO 7 The procedure of Example 1 was repeated, except that the mixture was oxidized with air at 195 ° C for 15 minutes and at 210 ° C for 7 minutes. The composition of the catalysts in the experiments was modified as illustrated in table 1.

TABLE 1 The results clearly demonstrate the high influence of the catalytic composition on the purity of the formed terephthalic acid. Only very high PTA purity is achieved if the manganese concentration varies by a certain scale. In the absence of manganese (Exp. No. 7), the oxidation rate is very low and PTA is practically not formed.

EXAMPLES 8 TO 11 The procedure of Example 4 was repeated, except that the composition of the catalysts in the experiment was changed as illustrated in Table 2.

TABLE 2 Exp. No Ratio Purity of PTA, atomic ppm 4-CBA p-toluic acid Co: Mn: Zr: Br 8 1 1 .2: 0: 2.3 90 5 4 1 1 .2: 0.1: 2.3 85 9 9 1 0.6: 0: 2.3 104 4 10 1 0.6: 0.1: 2.3 6 4 1 1 1 0.2: 0: 2.3 1 18 8 6 1 0.2: 0.1: 2.3 14 4 These results clearly show that the synergistic effect of zirconium is strongly influenced by the atomic ratio of Co: Mn in the oxidation catalysts. When the ratio of Co: Mn is 1: 1 .2, the synergistic effect of zirconium on the purity of terephthalic acid is negligible.

EXAMPLE 12 The reactor was charged with 2.5 g of p-xylene, 150 g of acetic acid containing 5% by weight of H20, 0.120 g of cobalt acetate tetrahydrate and Mn (II) acetate, Zr (IV) acetate, HBr ( solution in H20 to 47%) in the atomic ratio Co: Mn: Zr: Br = 1: 0.2: 0.1: 1.35. The air flow used for oxidation of p-xylene was modified during oxidation in the range of 1.5 to 0.05 l / min depending on the oxygen content in the gas emitted. The mixture was oxidized at 195 ° C for 7 minutes, then for 2 min, the temperature increased to 205 ° C and at this temperature the oxidation proceeded for 9 minutes (total reaction time 18 min.). The obtained terephthalic acid contained 24 ppm of 4-CBA and 6 ppm of p-toluic acid. The experiment shows that under specific reaction conditions and catalyst composition it is possible to obtain high purity PTA at the temperature of 205 ° C in the last oxidation step.

EXAMPLE 13 (COMPARATIVE) The procedure of Example 12 was repeated, except that the oxidation of p-xylene was carried out isothermally at 205 ° C for 19 minutes. The terephthalic acid contained 42 ppm of 4-CBA and 6 ppm of p-toluic acid. The color parameter L * is 93.1. The comparison of the results of experiments 12 and 13 confirms that isothermal oxidation of p-xylene produces terephthalic acid with lower purity than stepwise oxidation at different temperatures.

EXAMPLES 14 TO 17 The procedure of Example 4 was repeated, except that the composition of the catalysts in the experiments was Co: Mn: Zr = 1: 0.6: 0.1 and the atomic ratio Co: Br was modified as described in table 3.

TABLE 3 As can be seen from the results, the concentration of bromine in the reaction system must be optimal in order to obtain PTA of high purity.

EXAMPLE 18 The reactor was charged with 5.0 g p-xylene, 150 g of acetic acid containing 5% by weight of water, 0.20 g of cobalt acetate tetrahydrate and Mn acetate (ll), Zr (IV) acetate, HBr (solution in H20 at 47%) in the Atomic ratio Co: Mn: Zr: Br = 1: 0.6: 0.1: 2.3. The air flow was modified during the oxidation in the scale from 1.5 to 0.05 l / min depending on the oxygen content in the gas emitted. The mixture was oxidized at 190 ° C for 10 minutes, then for 5 minutes the temperature increased to 220 ° C and at this temperature the oxidation proceeded for 5 minutes. In the next step, the air flow inlet was stopped and for 8 minutes the reaction mixture was heated to 235 ° C and at this temperature it was stirred for 20 minutes. The formed terephthalic acid contained 33 ppm of 4-CBA and 5 ppm of p-toluic acid.

EXAMPLE 19 (COMPARATIVE) The procedure of Example 18 was repeated, except that the reaction mixture after the oxidation reaction was not heated to 235 ° C. The formed terephthalic acid contains 117 ppm of 4-CBA and 27 ppm of p-toluic acid. The results of examples 18 and 19 confirm that the subsequent heating of the reaction mixture in the absence of air increases its purity.

Claims (3)

NOVELTY OF THE INVENTION CLAIMS

1. - A process for preparing highly pure terephthalic acid, comprising the steps of A) oxidizing para-xylene in terephthalic acid with air in the presence of a liquid reaction phase maintained at a temperature between 180 ° C and 230 ° C, wherein the liquid reaction phase comprises para-xylene, acetic acid, water, and a catalytic composition, wherein the water has 5 to 12 weight percent of the acetic acid, the weight ratio of para-xylene to acetic acid is such that 50% of the terephthalic acid in reaction is present as a solid at the oxidation temperature, and the catalytic composition comprises cobalt, manganese and bromine in combination with at least one element selected from the group consisting of zirconium and hafnium, wherein the ratio atomic of C: Mn: Br: is on the scale of 1: 0.2 - 1.0: 1.1 - 2.7 and the atomic ratio of cobalt to the elements selected from the group consisting of zirconium and hafnium is 1: 0.03 - 3.0, where e the total weight of Co and Mn is 100-500 mg per 1 kg of the liquid reaction phase; B) recover the terephthalic acid by crystallization at a temperature in scale of 150 ° C to 80 ° C.

2. The process according to claim 1, further characterized in that the oxidation temperature is on the scale of 180X to 200 ° C in a first oxidation stage and is on the scale of 200 ° C to 225 ° C in a final oxidation step, while the degree of conversion of p-xylene to acid derivatives in the first oxidation step is within the range of 50 to 80 percent.

3. The process according to claim 2, further characterized in that after finishing the oxidation step and before the recovery step, the reaction mixture is heated for 10-30 min on the temperature scale of 230-240. ° C in the absence of air.