CS211142B1 - A method of producing terephthalic acid dimethyl ester - Google Patents

Abstract

The esterification of the crude p-xylene oxidate with methanol is carried out at a temperature of 170 to 260°C (190 to 240°C) and a pressure of 0.4 to 4.5 KPa under the catalytic effect of molybdenum in an amount of 0.001 to 0.5, preferably 0.01 to 0.1% by weight, calculated on the oxidate.

Description

The invention relates to a process for the preparation of dimethyl terephthalic acid ester from a crude p-xylene oxidation product.

For esterification of organic acids resp. Acid anhydrides are often used as catalysts such as strong inorganic acids, sulfuric acid, phosphoric acid, polyphosphoric acid, hydrochloric acid, but also relatively weak acids such as boric acid and sulfur dioxide, sulfuric acid, and most commonly p-toluenesulfonic acid.

Then benzenesulfonic acid, sulfosalicylic acid, naphthalene-beta-sulfonic acid and ethenesulfonic acid. Furthermore, the use of acid salts as esterification catalysts, e.g. sodium sulphate (Filler R., Sneed R. W., J. Am. Chem. Soc. 75.

765 (1953). Ion exchangers offer some advantages over esterification compared to mineral acids.

They have also been used as catalysts as acid halides, Lewis acids (e.g. boron trifluoride and complexes thereof), some alkali metal halides, soils etc., such as lithium chloride, mercury chloride, ferric chloride (Hauske W., Z. anorg. N. Allgem) Chem.

216 (1951). In addition to acidic catalysts, many works describe processes in which basic esterification catalysts (carbonates, soaps, alcoholates, organic and inorganic bases) are proposed, also the use of powdered metals #, e.g. zinc, tin and magnesium. However, these catalysts are often unsuitable for their esterification at temperatures above 170 ° C due to their side effects.

Of note is the continuous esterification of pure terephthalic acid with methanol, catalyzed by molybdenum metal, molybdenum trioxide, molybdenum sulfides and halides and heteromolybdates at 230 ° C, at a pressure of up to 9.0 MPa, conc. 0.005 to 10% molybdenum and a 4: 1 methanol to terephthalic acid weight ratio (Hol. Pat. 126 637 (1969)).

However, the relatively high esterification temperature and the working pressure, even with a high methanol content, practically nullify the advantages of the catalysed esterification over the non-catalysed so-called esterification. autocatalytic. They are also carried out at temperatures of 150 to 330 ° C and relatively long reaction times. In the case of non-catalytic terephthalic acid esterification, temperatures of 250 to 330 ° C and a pressure of about 2 MPa are employed (U.S. Pat. No. 3,839,414, U.S. Pat. No. 3,845,100).

However, undesirable side reactions such as disproportionation and decarboxylate of terephthalic acid may occur at such high temperatures and long reaction times, which in addition to reducing yields and quality of the resulting dimethyl terephthalate.

Terephthalic acid esterification catalyst systems, e.g. in the vapor phase, they start from silica gel onto which aluminum, sodium, titanium, tin or zinc compounds are deposited (U.S. Pat. No. 4,009,199). In the case of a fixed catalyst, alumina or alkali-treated phosphates are also used as carrier (NSR Pat. 2,261,333 (1975)).

The oxidizing mixture containing p-methyltoluylic acid can be artificulated in the presence of antimony compounds at a concentration of 0.01 to 1.0% antimony (Jap. Pat. 69-04479: Chem.

Abstr. 71 3,157 (1969)). however, the disadvantage is a relatively lower catalytic activity for esterification and higher for side reactions. In some cases also the fact that they are heterogeneous catalysts that can assist in clogging of reactors and fittings. Furthermore, the esterification of terephthalic acid with methanol is important in technical practice not only for the esterification of the pure acid itself, but mainly for the esterification of terephthalic acid in the oxidate which, in addition to the main product, also contains by-acids (benzoic acid, p-methylbenzoic acid, trimellitic acid, isophthalic acid and the like), and at least monomethyl esters thereof, unconverted p-xylene, and understandable oxidation catalysts (cobalt, manganese, nickel) salts, as well as corrosion-producing compounds (chromium, nickel, iron salts and complexes).

Therefore, when selecting catalysts or catalyst systems, it is necessary to consider the entire array of p-xylene oxidate components in order to achieve the desired catalytic effect for the esterification of terephthalic acid with methanol, without any notable undesirable side effects. Thus, the advantages of the known esterification catalyst systems are enhanced and technical problems are solved by the method of the present invention.

The essence of the process for the production of dimethyl ester of terephthalate by treating methanol at elevated temperature in the presence of molybdenum or molybdenum-containing compounds is to ease the oxidation product of p-xylene after removal of the oxygen or oxygen-containing gas at 170 to 260 ° C, preferably 190 to 240 ° C. The amount of molybdenum is 0.001 to 0.5, preferably 0.01 to 0.1% by weight, calculated on the oxidate.

An advantage of the invention; which can be particularly useful in the continuous process for the production of dimethyl terephthalate is the fact that the purity of the esterification of mainly terephthalic acid in the crude oxidate with methanol is substantially increased, which makes it possible to increase the plant production or reduce the esterification plant. The violet advantage is the possibility of esterification at temperatures below 240 ° C, which mainly results in an increase in selectivity of esterification to dimethyl terephthalate, a reduction in side reactions and thus an increase in yields and quality of the main product.

Another advantage is the possibility of recirculating molybdenum compounds as well as the fact that as components of the oxidation catalyst system (e.g., cobalt, manganese compounds) contained in the oxidate, they do not prevent molybdenum and molybdenum compounds in their function! of an active esterification catalyst, and molybdenum, and especially its compounds soluble in the p-xylene oxidation environment, do not retard the effect of the oxidation catalysts. This means that the esterification catalyst can also be added to the process at the oxidation stage. In this way, both catalyst systems can be recirculated simultaneously.

In accordance with the present invention, esterification of a mixture of substances formed by oxidation of p-xylene is most commonly accomplished by batchwise, semi-continuously or preferably continuously metered esterified product, methanol and the catalyst used, if it is no longer mixed with the oxidate. Per 1 wt. 0.5 to 4 wt. parts of methanol. With low methanol contents in the esterification mixture, the reaction rate is slower, with larger methanol contents a larger mass needs to be processed at a similar production rate! reaction apparatuses.

The p-xylene oxidation product, i. j. the oxidate most often has such an approximate composition (in% by weight); p-xylene 0 to 4; methyl benzoate 1 to 4; methyl p-formylbenzoate 0.4 to 0.8; methyl p-methylbenzoate 18 to 21%, dimethyl terephthalate 6.5 to 7.0; benzoic acid 0.8 to 1.5; p-methylbenzoic acid 16-20; monomethyl terephthalic acid 18 to 22 and terephthalic acid 12 to 15.

The composition can be varied beyond the concentration limits, depending on the process technology and the oxidation reaction conditions. The esterification temperature is 170 to 260 ° C. The reaction time or contact time of the esterification is 1 to 4 h. The continuous esterification is most often carried out in cap or sieve columns, generally in such a way that the contact between the oxidate and methanol is often as perfect as possible, often in the form of vapors and an understandable catalyst. Therefore, thinly the oxidate and methanol enter the esterifier (s) countercurrently. In the case of esterification, the pressure in the apparatus depends on the weight of the apparatus. the proportion of oxidate to methanol from 1.2 to 4.5 MPa.

The esterification catalysts used according to the invention contain molybdenum. Molybdenum can be used in the form of inorganic salts, such as ammonium molybdate, preferably phosphomolybdic acid, which also acts as acid catalysts for esterification.

Salts of the molybdenum compound, such as hydroxemates, complexes containing various other ligands, such as hexamethylphosphoric triamide, are also suitable. US Pat. No. 3,956,180 (1976) by reaction of hydroxyl compounds, primary or secondary alcohols, respectively. glycols, phenols with molybdates. However, these homogeneous catalysts are prepared for epoxidation of olefins. In the case of p-xylan oxidation, these types of hydrocarbon-soluble molybdenum compounds can be used together with β-xylan oxidation catalysts, which are mainly cobalt and manganese compounds in the form of a salt of organic carboxylic acids.

Finally, non-catalytic esterification with catalytic can also be combined according to the invention. Dimethyl terephthalate and, where appropriate, the dimeric dimethyl esters are isolated from the esterification product in a known manner.

Further details of the method and advantages are apparent from the examples and the accompanying drawings.

Figure 1 shows the acid number of the esterified oxidate (in mg KOH / g) versus the esterification temperature in ° C.

Curve 1 refers to esterification in the absence of catalyst, curve 2 refers to esterification with 0.01 wt. molybdenum, curve X to 0.015% molybdenum, curve X to 0.02 56 molybdenum. In Figure 1, a molybdenum catalyst in the form of dioxo-bis-benzohydroxamate was used.

Figure 2 shows the dependence of acid numbers on the esterification temperature of the oxidate using the peroxomolybdenum complex with hexamethylphosphoric triamide. Curve X is for uncatalyzed esterification, Curve 2 for molybdenum content 0.01 56, Curve X for molybdenum content 0.015 56, Curve X for molybdenum content 0.02% by weight.

Figure 3 shows the dependence of acid numbers on the temperature of esterification of the oxidate using molybdenum, in the form of phosphomolybdenoic acid as a catalyst. Curve χ refers to esterification without catalyst, curve 2 - molybdenum content 0.01 wt%, curve X - molybdenum content 0.015 wt%, curve X - content 0.02 wt%.

Figure 4 depicts the temperature dependence of the oxidate numbers of the oxidate on esterification using molybdenum trioxide as a catalyst. Curve X refers to the course of non-catalysed esterification, curve 2 to esterification in the presence of 0.01% molybdenum, curve X-course to 0.015% molybdenum, curve X, course of esterification with 0.02 56 wt. molybdenum.

In all 4 figures, the acid value of the esterification product in mg KOH / g is plotted on the y-axis, and the reaction temperature in ° C on the x-axis.

Example l

Catalytic oxidation product of p-xylene containing 0.2 56 p-xylene, 6.4 16 methyl benzoate, 12.2% isobutyl p-methylbenzoate, 0.5 56 p-formylbenzoate methyl, 12.3 56 dimethyl terephthalate, 2.4 56 dimethylisophthalate, 0.9 56 benzoic acid, 17.1 16 p-toluic acid, 29.9 16 mono-terephthalic acid terephthalic acid and 16.2 16 terephthalic acid are esterified in a stainless steel autoclave at 240 ° C. As esterification catalyst, molybdenum trioxide (11% by weight per oxidation product) is used. Mass ratio of oxidate to methanol = 1: 3. The esterification is carried out for 7.5 h and its progress is monitored by an acid number which is a measure of esterification:

Time (h) 1 2 3 5 7,5 Acid number (mg KOH / g) 55,6 7,9 7,3 7,0 5,6

For the non-catalysed esterification at 3 h, the acid number is 25.7 mg KOH / g.

Example 2

The crude product of oxidation of p-xylene and methanol in a wt. ratio 1: 0.8. The acid number of the starting oxidate is 270.2 mg KOH / g. The esterification catalyst used is 0.01 wt. % of finely divided molybdenum per oxidate and 0.005 wt. in the form of a complex compound dioxo-bls-benzohydrooxamate. After a 2 hour esterification time at an average temperature of 235.5 ° C, the product is cooled and analyzed, the acid number of the esterification product reaching 26.7 mg KOH / g. In the case of esterification in the absence of this catalyst, a product with an acid number of 37.9 mg KOH / g is obtained.

Example 3

The procedure is similar to that in Example 2, except that molybdenum naphthene is used as the esterification catalyst in an amount of 0.03% by weight. to the crude oxidation product of p-xylene. After a reaction time of 2 h, the product has an acid number of 18.3 mg KOH / g. In the case of non-catalysed esterification, the acid number of the product is 37.9 mg KOH / g.

Example 4

It is esterified under similar conditions to Example 2. However, molybdenum propylene glycol (34.2% Mo) is used as esterification catalyst in an amount of 0.03% per oxidation product. After 3 hours of esterification, the product has an acid number of 13.1 mg KOH / g. In the case of non-catalysed esterification, the acid number of the product after 3 h is 37.9 mg KOH / g.

Example 5

The esterification of the oxidation product is carried out at a temperature of 180 DEG to 240 DEG C. and with molybdenum dioxo-bis-benzohydroxamate containing molybdenum, calculated on the starting product from 0.01 to 0.02% by weight. with, for comparison, also without esterification catalyst, always during 3 has wt. a ratio of crude oxidation to methanol of 1: 0.8. The initial carboxylic acid content, expressed as an acid number, was 270.2 mg KOH / g. The process of esterification is shown in FIG. Curve J. shows esterification without catalyst (reference experiment), curve ^{23 with} esterification catalyst in an amount of 0.010 wt%. Mo, curve J 0.015 wt. Mo and curve J with 0.020 wt. Mo.

Example 6

The esterification of the crude p-xylene oxidation product is carried out at a temperature of 180 to 240 ° C under similar conditions to that of Example 5. MoO (O ₂ ) HMFT (peroxomolybdenum complex with hexamethylphosphoric triamide) is used as the esterification catalyst. The degree of esterification after 3 h is shown in FIG. 2. Curve J. shows the course of non-catalysed esterification (reference experiment): Curve 2 with 0.010 wt. Mo, curve 3. ^with 0.015 wt. and curve J with 0.020 wt. Mo. The difference in the acid number of the product for the catalyzed and non-catalyzed esterification decreases as the temperature of the reaction increases.

Example 7

The esterification of the p-xylene oxidation product is carried out under similar conditions to Example 6, except that molybdenum is used in the form of phosphomolybdenic acid in an amount of 0 to 0.015% by weight. Mo to oxidation product. The process of esterification at temperatures of 180 to 240 ° C is shown in FIG. 3. Curve J. shows esterification without catalyst; curve 2 with 0.010 wt. Mo and curve J with $ 6,015 wt. Mo.

Example. 8

The esterification of the p-xylene oxidation product is carried out at a temperature of 180 to 240 ° C with methanol (mass ratio oxidate: methanol = 1: 3) »in the presence of molybdenum oxide as the esterification catalyst. The results of esterification after 3h reaction time are shown graphically in FIG. 4. Curve X illustrates the course of the uncatalyzed esterification; curve 2 in the presence of 0.010 wt. Mo; curve J 0, Oi »mass. Mo and curve X in the presence of 0.020 wt. Mo.

Example 9

The oxidate from the oxidation of p-nine is esterified with methanol (w / v oxidate: methanol = > 1: 3) in a two-step esterification with a catalytic effect of 0.015% by weight. Mo in the form ΜοΟ ^. In the first step it is esterified at 195 ° C for 90 min and in the second step at 240 ° C for 30 min. An acid number of 5 + 2 mg KOH / g is obtained, practically the same as for the one-step esterification at 240 ° C, but for 180 min, but the yields of dimethyl terephthalate are 3% higher.

Claims (1)

A process for the preparation of dimethyl terephthalic acid ester, by esterification with methanol at elevated temperature, in the presence of molybdenum or molybdenum-containing compounds, characterized in that the p-xylene oxidation product is esterified after removal of oxygen or oxygen-containing gas at 170 to 260 ° C. preferably 190 to 240 ° C and a pressure of 0.4 to 4.5 MPa, the amount of molybdenum being 0.001 to 0.5, preferably 0.01 to 0.1% by weight, calculated on the oxidate.