DE4333361C2 - Process for obtaining m-dichlorobenzene from dichlorobenzene isomer mixtures by isolating p-dichlorobenzene - Google Patents

Description

The present invention relates to an advantageous method for the production of m-dichlorobenzene (m-DCB) from dichlorobenzene isomer mixtures.

m-DCB is an important intermediate, for example for the production of Phar pharmaceuticals and pesticides.

Dichlorobenzenes are usually obtained by chlorination of benzene or mono chlorobenzene produced. Depending on the reaction conditions used gene and catalysts are obtained differently composed isomer mix, for example 55-80 wt .-% p-DCB, 1-3 wt .-% m-DCB and 20-25 wt .-% o-DCB can contain (see Ullmanns Encyclopedia of Industrial Chemistry, 5th edition, volume A 6, page 333 ff.).

An increase in the m-DCB content in the isomer mixture is e.g. B. by isomerization o- and p-DCB possible on catalysts, for example on aluminum tri chloride or zeolites. So you can dichlorobenzene isomer mixtures with a m-DCB content of up to 60 wt .-% obtained.

For the separation of dichlorobenzene isomer mixtures, especially m- / p- DCB mixtures into the pure DCB isomers are different processes known.

Due to the very low boiling point differences of the individual DCB isomers with each other (o-DCB: 180 ° C; m-DCB: 173 ° C; p-DCB: 174.1 ° C) can pass through Distillation with reasonable effort only one separation into o-DCB (bottom product) and an m- / p-DCB mixture (top product) are accomplished.  

Extracting distillation processes have also been proposed (see e.g. EP-A 451 720). This often becomes difficult to access and therefore expensive Extractant required (e.g. alkylene carbonates) without being satisfactory Separation between m-DCB and p-DCB can be achieved.

By sulfonation or bromination of m- / p-DCB mixtures one can be right selectively convert m-DCB to sulfo or bromine-m-DCB, which is then easily converted unreacted p-DCB can be separated. The disadvantage here is that then desulfurization or debromination to be carried out again to m-DCB to obtain.

A combination of crystallizations and distillations (see DE-B1 28 55 940) allows m-DCB enrichment up to eutectic mixing (approx. 85% by weight m-DCB / approx. 15 wt .-% p-DCB) but the distillation requires one high energy and time expenditure and is therefore not for industrial use particularly advantageous.

It is also known that p-DCB with 1-bromo-4-chlorobenzene and 1,4-dibromobenzene forms eutectic crystals. It was therefore proposed in DE-A 36 17 137 add these substances to m- / p-DCB mixtures and by crystallization remove p-DCB in the form of eutectic crystals, the p-DCB and 1-bromo-4- Contain chlorobenzene or 1,4-dibromobenzene. Only those are technically interesting Crystallizations based on eutectic mixtures m-DCB in purities deliver over 90 wt .-%.

The separation of the components of the eutectic crystals can be made by distillation. The greater the distance between the The boiling point of the components of the eutectic crystals is the faster and they can be separated more easily. That according to DE-A 36 17 137 1-Bromo-4-chlorobenzene to be used boils at 196 ° C. It is therefore only with considerable effort to separate from p-DCB.

There is therefore still a need for a method by which m-DCB in the purest possible form is easily accessible.

A process for obtaining m-dichlorobenzene from dichlorobenzene Isomer mixtures by crystallization of p-dichlorobenzene as eutectic Crystals found with an auxiliary, which is characterized in that as Auxiliary uses nitrochlorobenzenes or chlorobenzaldehydes.

Any suitable are suitable for use in the method according to the invention Dichlorobenzene isomers. Mixtures are preferably used, such as one after the dichlorination of benzene, subsequent catalytic Isomerization and separation by distillation of the main amount of o-DCB can be obtained. Preferred dichlorobenzene isomer mixtures can, for. B. 80-86% by weight m-DCB, 14-20% by weight p-DCB and optionally up to 2% by weight contain other substances.

The auxiliaries can, for example, in amounts of 20-200 parts by weight per 100 parts by weight Dichlorobenzene isomer mixture can be used. Preferably the amount is 50 to 130 parts by weight per 100 parts by weight of dichlorobenzene Isomers.

For example, the adjuvant to be used with a purity of over 90%, preferably over 95%.

The method according to the invention can be carried out, for example, in such a way that first dissolves the auxiliary in a dichlorobenzene isomer mixture, optionally with gentle heating, and in a known, cools continuously or discontinuously operating crystallization temperature. The crystallization temperature is generally below + 20 ° C, for example in the range +5 to -25 ° C.  

From the slurry present after crystallization, the Liquid portions (= mother liquor) are removed in the usual way, for example by blowing with a gas, filtration, decanting and / or centrifuging. After warming up the remaining crystals to room temperature these are removed from the crystallization apparatus.

From both separated fractions (mother liquor and melted again Crystals) can be removed from the auxiliary by simple distillation of dichlorobenzenes separated and the auxiliary material can be used again. Obtained from the mother liquor m-DCB in a purity of generally more than 90% by weight and from the melted crystals m- / p-DCB mixtures, which again the fiction appropriate procedures can be supplied.

The method according to the invention has a number of advantages: auxiliaries to be used are readily and inexpensively available and they show Boiling points on which are separated from mixtures with dichlorobenzenes simple distillation enables (chlorobenzaldehyde: boiling point 219 ° C, p- Chloro-nitrobenzene: boiling point 242 ° C). They are therefore technically simpler and to perform more economically than known methods.

Examples Example 1 (see also Table 1)

Approx. 500 g were placed in a double-walled glass cylinder (180 mm × 90 mm) a mixture of equal parts by weight of dichlorobenzenes and p-nitrochlorobenzene cooled to temperatures between 0 and -20 ° C with stirring. Within 1 hour the precipitated crystals spread over the entire crystal space. The glass cylinder was then opened down and the mother liquor under Let the nitrogen purge run off. The crystals thus obtained contained one higher proportion of p-DCB, the mother liquor a higher proportion of m-DCB, each in Regarding the dichlorobenzene mixture used. More details are in Table 1 given. The data in% by weight contained therein were gas chromato graphically determined.

Example 2 (see also Table 2)

A mixture of 5 g of dichlorobenzenes was placed in a 50 ml glass cylinder and 5 g of p-chlorobenzaldehyde cooled to -20 ° C. The by stirring with a glass Stab spontaneously formed crystals were sucked off from the mother liquor freed. Further details can be found in Table 2. These data in% by weight were determined by gas chromatography.  

Table 1

Table 2

Claims (8)

1. A process for obtaining m-dichlorobenzene from dichlorobenzene isomers by mixing crystallization of p-dichlorobenzene as eutectic crystals with an auxiliary, characterized in that nitrochlorobenzenes or chlorobenzaldehydes are used as auxiliary. 2. The method according to claim 1, characterized in that 20 to 200 parts by weight of the auxiliary per 100 parts by weight of dichlorobenzene isomers mixed uses. 3. The method according to claims 1-2, characterized in that as Dichlorobenzene isomer mixtures uses those which contain 80-86% by weight of m-DCB, 14-20% by weight of p-DCB and optionally up to 2% by weight of other substances contain. 4. The method according to claims 1-3, characterized in that the crystallisa tion temperature is below + 20 ° C. 5. The method according to claims 1-4, characterized in that the crystals sation temperature is in the range +5 to -25 ° C. 6. The method according to claims 1-5, characterized in that according to the Crystallization collects the mother liquor and separately the molten eutectic crystals. 7. The method according to claims 1-6, characterized in that from the Mother liquor and the molten mixture of eutectic crystals used excipient separated by distillation and into the Invention appropriate procedures.   8. The method according to claims 6 and 7, characterized in that one of the mother liquor and the melted eutectic crystals in it contained portions of dichlorobenzene, separating from the mother liquor an m-DCB in a purity of over 90 wt .-% and from the melted eutectic crystals receives an m- / p-dichlorobenzene mixture, which again corresponds to the Method according to the invention is supplied