WO2003078369A1 - Oxidation of o-xylol to form o-toluic acid and other oxidation products by the staggered supply of an oxygen-containing gas - Google Patents

Abstract

The invention relates to a method for producing oxidation products of o-xylol, which can be oxidized further to form phthalic acid. According to the invention, a liquid stream containing o-xylol is guided through at least one reaction zone, and an oxygen-containing gas is turbulently bubble injected into the liquid stream at at least two locations that are interspaced in the direction of flow of the liquid stream.

Description

Oxidation of o-xylene to o-tolylic acid and other oxidation products by staggered supply of an oxygen-containing gas

description

The present invention relates to a process for the preparation of oxidation products of o-xylene which can be further oxidized to phthalic anhydride by oxidation of o-xylene in the liquid phase.

Phthalic anhydride is one of the technically most important aromatic compounds. It is e.g. B. used in the synthesis of alkyd resins, unsaturated polyester resins, paints, plastics, plasticizers, phthalocyanine dyes, insect repellents, denaturing agents and for the production of numerous fine chemicals.

Phthalic anhydride is produced industrially by the oxidation of o-xylene or naphthalene. Two oxidation processes are common. On the one hand, the oxidation in the gas phase is carried out on a heterogeneous catalyst, generally a vanadium-containing catalyst. In this process, however, about 20 mol% of the starting material is converted to CO _x and is therefore lost as a yield.

Because of the better selectivity of the oxidation reaction, a two-stage reaction has been proposed as an alternative, in which o-xylene is initially in the liquid phase with an oxygen-containing gas, mostly air, with homogeneous catalysis to intermediates on the way to phthalic anhydride, predominantly o- Tolylic acid, 2-methylbenzaldehyde, 2-methylbenzyl alcohol and phthalide, is oxidized. These intermediates are isolated, then evaporated and oxidized to phthalic anhydride in the gas phase in a second stage.

Such a two-stage process is described in EP-A 0 256 352. The implementation of the o-xylene oxidation in the liquid phase is illustrated on a stirred tank reactor. Compressed air, which is introduced via an immersion tube, serves as the oxidant. Wang Y., Xie Y. and Zhang J. report in the Chinese scientific journal "Petroleum Refining and Chemical Industry" 2000, Volume 31, about "Investigation of the oxidation of o-xylene in the liquid phase for the production of o-tolylic acid". The influences of the reaction conditions, such as. B. reaction temperature, reaction pressure, flow rate of air and reaction time, examined for the yield of the product o-tolylic acid. The authors come to the conclusion that there is an ideal reaction temperature, an optimal pressure of the reaction system and an ideal reaction time. As the amount of air increases, the yield of o-tolylic acid increases.

Cheng Z., Li G., Li W., Dai L., Niu Y., Hu J., Gu J., Mao X. and Li M. report in the Chinese journal "Journal of Fuel Chemistry" (Ranliao Huaxue Xuebao) , 1981 Volume 9, Issue 2, pp. 152-162 on a "Study on the Kinetics of the Oxidation Reaction of o-Xylene".

DE 100 02 807 and DE 100 02 810 also relate to processes for the oxidation of o-xylene with oxygen.

The two-stage reaction of the oxidation of o-xylene to phthalic anhydride generally has a better selectivity than the one-stage oxidation in the gas phase. Nevertheless, there is also a risk of over-oxidation of the o-xylene in the liquid phase oxidation with the formation of CO _x , acetic acid, formic acid or other products which arise from the loss of one or more carbon atoms from the o-xylene and can no longer be converted into phthalic anhydride by further oxidation ,

The invention is therefore based on the object of specifying a process for the liquid phase oxidation of o-xylene which leads as selectively as possible to those oxidation products which can be further oxidized to phthalic anhydride.

According to the invention, this object is achieved by a process in which an o-xylene-containing liquid stream is passed through at least one reaction zone and an oxygen-containing gas is bubbled into the liquid stream at at least two points spaced apart in the direction of flow of the liquid stream. '

If the liquid stream is successively through more than one reaction zone, eg. B. passed through two or more reactors connected in series, there is no gas / liquid phase separation between the reaction zones. The last reaction zone becomes a Oxidation products of the o-xylene-containing liquid reaction mixture and an oxygen-depleted gas stream (hereinafter also referred to as "exhaust gas").

In addition to oxygen, the oxygen-containing gas usually contains one or more inert gases, such as, for. B. nitrogen, argon or carbon dioxide. The oxygen volume fraction of the oxygen-containing gas is preferably 10 to 90% by volume, preferably 15 to 35% by volume; the rest is made up of inert gas. The use of air as an oxygen-containing gas is particularly preferred. The oxygen-containing gas is preferably bubbled in under conditions which allow intimate mixing of the gas and liquid phases.

According to the process of the invention, in addition to a small amount of phthalic acid, those oxidation products of o-xylene which can be further oxidized to phthalic anhydride are obtained with high selectivity, in particular o-tolylic acid, methylbenzaldehyde, methylbenzyl alcohol, phthalide, di (methylbenzyl) phthalate, methylbenzylmethylbenzoate, mono (methylbenzyl) phthalate, methylbenzylformylbenzoate, formylbenzylmethylbenzoate, monomethylphthalate, methylbenzyl formate and methylbenzyl acetate, of which o-tolylic acid forms the main part. As far as the "selectivity" of the oxidation reaction is addressed in the following, the selectivity is meant with reference to the sum of the above-mentioned compounds.

There is a risk of undesired over-oxidation of the o-xylene if local oxygen concentration maxima occur. Therefore, in the process according to the invention, the oxygen-containing gas is supplied in a manner which largely avoids local oxygen concentration maxima. At the first dossier, the bubbled-in oxygen-containing gas reacts with xylene while consuming oxygen, the dispersed gas phase becoming depleted of oxygen. At the second and further metering points, the bubbled-in oxygen-containing gas can coalesce with the dispersed low-oxygen gas phase to form gas bubbles with an average oxygen content.

In order that there is sufficient mass transfer of oxygen from the gas phase to the liquid phase, the oxygen content of the gas phase should not drop below a critical value. This is preferably achieved by monitoring the oxygen volume fraction of the exhaust gas. The oxygen volume fraction of the oxygen-depleted gas is preferably 0.01 to 7.5% by volume, in particular 0.05 to 6% by volume, particularly preferably 1 to 6% by volume. Any reaction apparatus in which a gas phase is dispersed into a liquid phase by suitable organs, such as gas distributors or nozzles, is suitable for carrying out the process according to the invention. Suitable examples of this are stirred tank reactors and bubble columns.

In a preferred embodiment, baffles are provided in the reaction zone which extend essentially in a plane perpendicular to the direction of flow of the liquid stream and are spaced apart from one another. The internals divide the reaction zone into individual chambers, which the liquid stream passes through in succession. The internals prevent backmixing in the direction of flow. The oxygen-containing gas is preferably bubbled in at least at one point between two successive internals. The reaction zone is preferably elongated and in particular has a ratio of length to diameter of more than 5, in particular more than 10. The number of sections separated from one another by internals can, for. B. 2 to 15 or 5 to 10. Suitable internals such. B. perforated plates or valve bottoms.

The reaction zones or the sections of a reaction zone which are separated from one another by internals are advantageously provided with a packed bed, at least in partial areas. The packed bed causes (re) dispersion of the gas phase and good mixing transversely to the direction of flow.

The oxidation of the o-xylene is exothermic. The heat of reaction is preferably removed. For this purpose, heat exchangers which are arranged in the reaction zones or are attached to the outer reactor jacket or are therefore in a heat-conducting connection and through which a suitable cooling medium flows can be provided. So it is z. B. possible to provide cooling coils or cooling fingers arranged in the reaction zones. In embodiments of the process according to the invention in which a partial stream of the liquid reaction mixture removed from the last reaction zone is returned to the first reaction zone, a preferred way of dissipating the heat of reaction is to cool the recycle stream by z. B. conducts through a heat exchanger. Of course, different types of heat dissipation can be combined.

The pressure is preferably 10 to 30 bar, in particular 15 to 25 bar. If several reaction zones connected in series are used, the pressure in the individual reaction zones is preferably essentially the same (apart from a small pressure difference which is necessary for the transport of the mixture of liquid Stream and dispersed gas phase from one reaction zone to the next is required). The reaction temperature is preferably 120 to 210 ° C, in particular 138 to 180 ° C, particularly preferably 140 to 165 ° C. The residence time of the liquid stream in the reaction zone or the accumulated residence time in the reaction zones when using reaction zones connected in series is preferably 0.5 to 4.5 hours.

The process according to the invention is preferably carried out in the presence of a transition metal-containing catalyst.

Suitable catalysts are, on the one hand, transition metal salts or complexes soluble in o-xylene, such as, in particular, cobalt and / or manganese salts or complexes. Cobalt salts of aliphatic, alicyclic or aromatic carboxylic acids are preferred. Examples include naphthenates, acetates, propionates, butanoates, pentanoates, hexanoates, heptanoates, octanoates, 2-ethylhexanoates, nonanoates, isononanoates, decanoates, laurinoates, palmitates, stearates, benzoates, tolylates, phthalates and citrates. Examples of other molecules which bring about a solubility of the transition metals are acetylacetonate, porphyrins, salcomine, halls or phthalocyanines. A suitable catalyst is e.g. B. Cobalt naphtenate. The catalyst which is suitably metered in in the form of a catalyst stock solution is preferably added to the feed containing o-xylene outside the reaction zone. This avoids the coincidence of locally high catalyst concentrations with the reactants o-xylene and oxygen, which could otherwise lead to undesired overoxidation. In general, the catalyst is used in an amount of 2 to 500 ppm, preferably 5 to 50 ppm, calculated as metal, based on the feed containing o-xylene. Higher catalyst concentrations can lead to undesirable precipitation in the reactor.

Alternatively, heterogeneous catalysts, i.e. unsuitable or hardly soluble transition metal salts or complexes in the liquid stream. They can be in the form of a suspension in the liquid stream or can be arranged firmly in the reaction zones, for. B. as a bed or coating of the inner walls. As a heterogeneous catalyst z. B. Cobalt (II) oxalate is suitable.

After leaving the reaction zone, the reaction discharge is suitably fed from a gas-liquid separator in which the liquid reaction mixture and the oxygen-depleted gas are separated from one another. As a gas-liquid separator, for. B. a gravity separator, ie a vessel with a low flow velocity, in which the gas phase and the liquid phase due to their different Separate density from each other, suitable. Alternatively, a swirl separator, ie a cylindrical container, into which the mixed-phase reaction discharge is introduced tangentially to the container wall can be used.

In preferred embodiments, the separated liquid reaction mixture is at least partially returned to the reaction zone. The recycling of part of the liquid reaction mixture allows a sufficiently large liquid volume flow to be maintained in the reaction zones. At the same time as the recycled liquid reaction mixture, a fresh o-xylene-containing feed is preferably fed into the reaction zone. The ratio of feed to recycled liquid reaction mixture is generally 1:10 to 1: 200, preferably 1:50 to 1: 100. A quantity of the liquid reaction mixture discharged from the reaction zone is expediently discharged as a product stream.

The oxidation products of o-xylene, in particular o-tolylic acid, methylbenzaldehyde, methylbenzyl alcohol, phthalide, methylbenzylmethylbenzoate and monomethylbenzyl phthalate, can be isolated from the product stream by customary processes, in particular by distillation. The separated, unreacted o-xylene is expediently returned to the oxidation reaction.

To produce phthalic anhydride, the oxidation products separated off are evaporated and, in a second step, oxidized to phthalic anhydride in the gas phase in the gas phase in the presence of an oxygen-containing gas. One is generally used to carry out the second stage

Fixed bed reactor, preferably a tube bundle reactor. Suitable catalysts are those with an active mass of titanium dioxide in the form of its anatase modification and / or vanadium pentoxide.

The invention is explained in more detail by the attached figures and the examples below.

Fig. 1 shows a schematic representation of a device suitable for carrying out the method according to the invention with a reaction zone divided by internals.

2 shows a schematic representation of a device suitable for carrying out the method according to the invention with two reaction zones connected in series. 1, o-xylene 3 is premixed with a catalyst stock solution 2 and the o-xylene-containing feed is fed to reactor 1. The reactor 1 is divided into chambers by perforated plates 10a-d arranged transversely to the direction of flow. Via the lines 4a-4e, an oxygen-containing gas, eg. B. air introduced. After leaving the reactor 1, the reaction discharge is fed to the gas-liquid separator 6. The liquid phase is cooled in the heat exchanger 8 and fed to the pump 9. A partial stream of the liquid reaction mixture is discharged via line 5. The oxidation products of o-xylene formed are isolated therefrom by methods known per se. The other partial stream of the liquid reaction mixture is introduced again into the reactor 1 via line 11.

2, o-xylene 3 is premixed with a catalyst stock solution 2 and the o-xylene-containing feed is fed to the reactor la. The discharge from the reactor la is fed to the reactor 1b via line 12 without phase separation. Via the lines 4a, 4b, an oxygen-containing gas, for. B. air introduced. After leaving the reactor 1b, the reaction discharge is fed to the gas-liquid separator 6. The liquid phase is cooled in the heat exchanger 8 and fed to the pump 9. A partial stream of the liquid reaction mixture is discharged via line 5. From this, the oxidation products of o-xylene formed are isolated by methods known per se. The other partial stream of the liquid reaction mixture is introduced again into the reactor la via line 11.

Examples 1 and 2

A device with two reactors and a circulating pump connected in series, as shown in FIG. 2, was used. The two reactors are vertically arranged high-pressure reaction tubes with a length of 2 m and a diameter of 30 mm. A metering point for air was arranged near the lower end of the reaction tubes. A continuous liquid oxidation of o-xylene was carried out at a pressure of 20 bar. The supplied o-xylene was mixed with cobalt naphtenate before being introduced into the first reactor. An amount of the reaction mixture corresponding to the o-xylene fed was continuously discharged from the second reactor. The composition of the oxidation products was analyzed by high performance liquid chromatography. The operating parameters and analysis results are summarized in the following table. Example 3 (comparison)

A device was used as in the previous examples, but comprising only one reactor.

It can be seen that the overall selectivity is higher if the air is introduced at two metering points.

Claims

claims 1. A process for the preparation of oxidation products of o-xylene which can be further oxidized to phthalic acid, in which an o-xylene-containing liquid stream is passed through at least one reaction zone and an oxygen-containing gas in at least two points spaced apart in the direction of flow of the liquid stream bubbled in the liquid stream. A method according to claim 1, wherein baffles are provided in the reaction zone which extend essentially in a plane perpendicular to the direction of flow of the liquid stream and are spaced apart from one another. 3. The method according to claim 2, wherein the oxygen-containing gas is bubbled in at least one location between two successive internals. 4. The method according to any one of the preceding claims, wherein the oxygen volume fraction of the oxygen-depleted gas stream discharged from the reaction zone is 0.01 to 7.5 vol .-%. 5. The method according to any one of the preceding claims, wherein one uses a transition metal salt or complex as a catalyst. 6. The method according to claim 5, wherein the catalyst used is a cobalt salt of an aliphatic, alicyclic or aromatic carboxylic acid which is soluble in the liquid stream. 7. The method according to any one of the preceding claims, wherein the liquid reaction mixture withdrawn from the reaction zone is partially returned to the reaction zone. 8. The method according to claim 7, wherein the volume ratio of o-xylene feed to recycled liquid reaction mixture is 1:10 to 1: 200.