GB2292142A - Process for separating mixtures of m-and p-dichloro-benzene - Google Patents

Description

2292142 Process for separating mixtures of m- and p- dichlorobenzene.

is The present invention describes a process for separating mixtures of mand p-dichlorobenzenes, pdichlorobenzene preferably being separated off from such mixtures. The separation is carried out over pentasil zeolites in the liquid phase using solvents.

In the chlorination of benzene or monochlorobenzene to give dichlorobenzene there generally results a mixture of the three isomeric dichlorobenzenes. The dichlorobenzene fraction can be easily separated off, owing to the different boiling and melting points, from the other products in the chlorination mixture, e.g. the starting materials and higher chlorination products. However, the separation of the individual dichlorobenzene isomers in this fraction is difficult and very complicated. The dichlorobenzene fraction freed of the byproducts is first distilled, the m- and p-dichlorobenzene being obtained as top product and the o-dichlorobenzene as bottom product. Since m- and pdichlorobenzene have identical boiling points, the further separation is not carried out by distillation but via fractional crystallization, in which a large part of the p-dichlorobenzene is recovered in pure form and there remains an m-dichlorobenzene concentrate which, depending on the crystallization effort, contains from 70 to 80 t by weight of m- dichlorobenzene and from 30 to 20 t by weight of p-dichlorobenzene (EP 012 891). Adsorptive processes for separating or - 1 is further separating mixtures of dichlorobenzenes with the aid of molecular sieves are also known in principle. Thus, EP 334 025 describes the separation of halogenoaromatics over any zeolites in the presence of aromatic hydrocarbons. However, the separation of mand p-dichlorobenzene over pentasil zeolites is not mentioned in this description. EP 278 680, JP 62/175433 (1987) and US 4 996 380 describe the separation of mdichlorobenzene from its mixtures with o- and pdichlorobenzene without using eluants. These processes are suitable for separating small amounts of mdichlorobenzene from mixtures which predominantly contain 0- and p- dichlorobenzene, but not for separating off p-dichlorobenzene. Since, however, the workup of industrial dichlorobenzene product mixtures by distillation and a simple crystallization step easily results in an m- dichlorobenzene concentrate from which the residues of p-dichlorobenzene present can only be separated by time- and energy-consuming distillation and crystallization steps, a process making possible an economical separation of the pdichlorobenzene from an m-dichlorobenzene concentrate is extremely desirable.

Such a separation of small amounts of p-dichlorobenzene from mixtures predominantly comprising mdichlorobenzene with the aid of molecular sieves has not been satisfactorily solved by the prior art.

2 is US 4 571 441 and JP 58/131924 (1983) do describe the separation of pdichlorobenzene from mixtures with oand m-dichlorobenzene using faujasite X and faujasite Y, which contain silver, copper, sodium or potassium or a plurality thereof as exchangeable cations. Substituted aromatic hydrocarbons are used as desorbents. The selectivities achieved in these processes are extremely low and do not ensure an economical process.

JP 92/330025 describes the separation of the isomeric dichlorobenzenes over Pb-containing zeolites using 3,4dichlorotoluene as desorbent. The preparation and use of Pb-containing zeolites is disadvantageous because of the toxicity of Pb and its compounds. It is not possible to use the isomers separated over such a leadcontaining material as intermediates for active ingredients, since even traces of Pb cannot be tolerated.

A process has been found for separating mixtures of mand pdichlorobenzene by treatment of such mixtures with a zeolite in the presence of solvents, which is characterized in that the mixture together with a solvent combination of at least two different solvents which originate from two groups A and B respectively, of which the representatives of the group A correspond to the formula 3 Hal I - R 1 (1) R 2 in which Hal represents chlorine or bromine, chlorine, preferably R1 denotes Hal or Cl-C4-alkyl and R 2 represents hydrogen, Hal or Cl-C4-alkyl, where R' and R 2 can together represent the remainder of an indane, tetralin or naphthalene system and where 4-chlorotoluene and the dichlorobenzenes are excepted, and of which the representatives of the group B correspond to the formula Hal 6-R 3 in which Hal has the meaning given above and 4 R 3 denotes hydrogen, Cl-C4-alkyl, Cl-C4-alkoxy, C 1-C4alkylcarbonyl, Cl- C4-alkoxy-carbonyl, phenyl, nitro or the aldehyde group, is is treated at from 20 to 2500C with a pentasil zeolite containing protons, cations of the first or second main group of the Periodic Table (Mendeleev), cations of the rare earth metals or a mixture of a plurality thereof as exchanged cations, the enriched m-dichlorobenzene is taken off as filtrate and p-dichlorobenzene is recovered by extraction from the pentasil zeolite, the above solvent combination being used for the extraction.

This provides a process which allows p-dichlorobenzene to be very selectively separated f rom mixtures with mdichlorobenzene in only one elution step. The process of the invention offers the advantage of being able to be operated both discontinuously in a pulsed process and continuously in a countercurrent process. The way in which these two process variants are carried out is known to those s killed in the art and as such is not a subject of the present invention.

In the process of the invention, p-dichlorobenzene is very selectively separated from its mixtures with mdichlorobenzene by bringing the mixture of the dichlorobenzene isomers into contact with zeolites of the pentasil type in the presence of solvents under adsorption conditions as are known in principle to those skilled in the art.

Zeolites can, in general, be described by the empirical formula M21.0 A1203 XS'02. yH20 (I) in which m denotes an exchangeable cation or a mixture of a plurality thereof, n indicates the charge of the exchangeable cation, X represents integers from 2 to 2000 and indicates the amount of the sorbed water.

The sorbed water phase yH20 is reversibly removabl without the zeolite skeleton losing its structure.

e Among the great number of zeolites known to those skilled in the art, those having a pentasil structure are used according to the invention.

The following pentasil structure types are preferably used: ZSM 5, ZSM 11, ZSM 8, ZSM 5/ZSM 11 intermediates, Zeta 1, Zeta 3, ZBM 10, Ultrasil, Ultrazet, TZ-01, NU4, NU-5, AZ-1. Pentasil zeolites and particularly the individually specified structure types are known to those skilled in the art and have been described many times, for example in EP 54 386, EP 65 401, EP 34 727, EP 57 016 and EP 113 116. Pentasil zeolites which are particularly preferably used are ZSM 5, ZSM 8, ZSM 11 and ZSM 5/ZSM 11 intermediates. Very particularly preferably, pentasil zeolites of the types ZSM 5 and ZSM 11 are used, the SiO,/A1203 ratio being subject to no limitation.

As exchangeable cations M, the pentasil zeolites can contain the abovementioned cations. Examples are cations of the following elements: H, Li, Na, K, Mg, Ca, La, Ce, Pr, Nd, preferably H, Li, Na, K and Mg. The introduction of the exchangeable cations M is known to those skilled in the art; frequently use is here made of ion exchange processes in which an aqueous metal salt solution is brought into contact with the zeolite (D.W. Breck, "Zeolite Molecular Sieves, Structure, Chemistry and Use", J. Wiley/Sons, New York, 1974).

Before use of the pentasil zeolites in the process of the invention, the sorbed water phase yH20 is removed by calcination at from 10 0 to 5000C.

The process of the invention is carried out in such a way that the mixture of the isomeric dichlorobenzenes (DCB) is present as a liquid phase in the solvent combination described. For the economics of the process, it is important that the solvents used can be separated from the DCB by a simple distillation. It is therefore particularly advantageous if, as main component of the solvent combination, use is made of a 7 - solvent whose boiling point is above that of the DCB, so that by f ar the largest amount of the raf f inate and extract does not have to be subjected to distillation; accordingly, only the DCB and any subordinate amounts of a second solvent have to be distilled off, if its boiling point is below that of the DCB.

The proportions of the isomers to be separated can vary within wide limits. Preference is given to mixtures containing at least 50 k by weight of m-DCB, particular preference is given to those mixtures which contain more than 70 % by weight of m-DCB.

The solvent combination used for the process claimed is at least two different solvents which originate from the above groups A and B of the formulae (I) and (II) respectively, of which the representatives of the group A bear a halogen atom and one or two further substituents or at least two halogen atoms and optionally one further substituent, with the exception of 4-chlorotoluene and dichlorobenzenes, and of which the representatives of the group B bear only one halogen atom and optionally one further substituent with the exception of halogen. Typical representatives of the group A are: 1,2,3-trichlorobenzene, 1,2, 4trichlorobenzene, 1,3,5-trichlorobenzene, 2,4dichlorotoluene, 2,6dichlorotoluene, 3,4dichlorotoluene, 2-chloro-p-xylene, 4-chloro-o-xylene, 2-chlorotoluene, 1-chloronaphthalene, chlorodiethylbenzenes and chlorocumene and also the corresponding brominated or mixed chlorinated/brominated representatives compounds are preferred.

is the chlorinated Typical representatives of the group B are: chlorobenzene, 4- chlorotoluene, 4-chloroacetophenone, 4chloroanisole, 4-chlorobenzaldehyde, methyl 4chlorobenzoate, 4-phenylchlorobenzene, 1-chloro-2nitrobenzene and 1-chloro-3-nitrobenzene and also the corresponding brominated or mixed chlorinated/brominated representatives; the chlorinated compounds are preferred.

Preference is given to combinations which contain, on the one hand, the isomers of the trichlorobenzene series and/or the isomers of the dichlorotoluene series as representatives of the group A and, on the other hand, chlorobenzene, 4-chlorotoluene, 4-chloroanisole, methyl 4chlorobenzoate and/or 4-chloroacetophenone as representatives of the group B. Particular preference is given to the isomeric trichlorobenzenes and/or the isomeric dichlorotoluenes in combination with chlorobenzene, pchlorotoluene or p-chloroacetophenone. Very particular preference is given to combinations of 1,2,4-trichlorobenzene with chlorobenzene or pchlorotoluene and 3,4- or 2,6-dichlorotoluene with chlorobenzene or pchlorotoluene.

The combinations contain 50-98 % by weight of solvent of the group A and 50-2 % by weight of solvent of the group B, preferably 80-98 % by weight of solvent of the 9 is group A and 20-2 % by weight of solvent of the group B; particularly preferably they contain 90-98 % by weight of solvent of the group A and 10-2 % by weight of the group B. All percentages by weight are based on the total weight of the combinations.

The amount of the solvent combination is 5-20 times, preferably 5-15 times, particularly preferably 5-10 times, the amount of the DCB mixture to be separated.

The amount of the mixture of the DCB to be separated, based on the amount of the pentasil zeolite, is limited by the intended separation effect which decreases per separation step and stage with increases in the amount of DCB mixture available. In general from 0.05 to 2, preferably from 0.1 to 1, particularly preferably from 0. 2 to 0. 8, amount units of DCB are used per amount unit of pentasil zeolite.

The process of the invention is carried out at a temperature of from 20 to 2500C, preferably from 20 to 2000C. The pressure is non-critical for the process of the invention; therefore, to simplify the reaction apparatus, it is generally carried out at atmospheric pressure. However, it is also possible to carry out the reaction at increased pressure, if the solvent is used at an operating temperature above its boiling point.

The mixture of m- and p-DCB is, dissolved in one of the solvent combinations specified above, brought into contact with the pentasil zeolite in powder or granular form. The preferentially adsorbed component p-DCB is removed f rom the mixture. After the adsorption is complete, the liquid phase is separated from the zeolite. It contains m-DCB in greatly enriched form. This liquid phase (solvent combination and remaining DCB isomer) is worked up separately. The adsorbed p-DCB can be stripped from the pentasil zeolite by desorption known per se, the same solvent combination being used. A special extraction step for the adsorbed p-DCB with, for example, additional auxiliaries is therefore not required. Thus, the regeneration of the zeolite also occurs automatically as a result of the process procedure.

The process can be carried out in a conventional apparatus known to those skilled in the art for separation by means of adsorption. Suitable apparatus are those which make possible continuous or batchwise modes of operation. The shape and dimensions of these adsorption apparatus can be optimized and are, of themselves, not a subject of this invention. The process of the invention is preferably carried out in a facility for continuous countercurrent chromatography.

In the following examples, the quality of the adsorption properties is given by the adsorptive selectivity which is defined as follows:

OCA/B = is (% by weight of A/%- by weight of B) adsorb. phase (%- by weight of A/t by weight of B) non-adsorb. phase If the selectively adsorbed component p-dichlorobenzene among the dichlorobenzene isomers is denoted by A and the less well adsorbed component m-dichlorobenzene by B, then the value aA/B increases with increasingly good separation.

Those skilled in the art will know that separation systems whose a values are only slightly above or below 1 are unsuitable for a separation process of the type described. Separation factors which differ from 1 by orders of magnitude are also unsuitable for processes such as those claimed, which omit an additional desorption step, since in such a case the eluting power of the solvents used is insufficient for the more strongly adsorbed component. The advantage of the process of the invention lies in carrying out adsorption and desorption of the two components in one step using a single eluant which comprises a combination of two solvents.

Examples 1 to 5 To determine the adsorption selectivity, in each case 5 9 of a 10 % strength solution of a mixture of 85 % by weight of m-dichlorobenzene (m- DCB) and 15 % by weight of p-dichlorobenzene (p-DCB) in the solvents or solvent mixtures specified in Table 1 were brought into contact with 2.5 9 of powdered zeolite and the mixture was stirred for 1 hour at 250C. Subsequently the solution was separated from the zeolite and analysed by gas chromatography after addition of undecane as internal standard. The results are shown in Table 1.

Co=arative ExamDles 1-4 The experiments were carried out in accordance with Examples 1-5. The results are shown in Table 1.

Table 1

Ex. I 2 3 4 cl C2 C3 C4 Adsorbent Mg-WM 5 Mg-WM 5 Mg-zSM 5 SE-WM 5 SE-WM 5 Mg-WM 5 Mg-WM 5 SE-WM 5 Mg-WM 5 Desorbent 1,2,4-trichlorobenzene/ 5 k by weight p-chlorotoluene 3,4- dichlorotoluene/ 10 k by weight chlorobenzene 3,4-dichlorotoluene/ 5 k by weight p-chloroacetophenone 1,2,4-trichlorobenzene/ 5 % by weight chlorobenzene 2,4-dichlorotoluene/ 10 W by weight chlorobenzene 1,2,4- trichlorobenzene 3,4-dichlorotoluene 3,4-dichlorotoluene chlorobenzene S/M 6.3 3.7 8.3 4.2 5.5 93.1 133.5 407.0 1

Claims (1)

1. The Comparative Examples Cl-C3, in which only solvents of the group A were

   used as eluants, have separation factors which are an order of magnitude higher than the separation factors achieved with the adsorbent/ solvent combinations of the invention. The affinity for the zeolite of the pure solvents of the group A according to Claim 1 is too small to elute the more strongly adsorbed p-dichlorobenzene isomer to a sufficient extent. The Comparative Example C4 shows that the use of individual solvents of the group B as eluants leads to complete desorption of both isomers and thus not to the desired separation of the isomers.

   - 14 Comparative Examples 5 and 6 An LiX type zeolite prepared according to EP 0 334 025 was subjected to a static experiment in accordance with Examples 1-5 to determine its selectivity in respect of p-DCB. However, in contrast to the examples of the invention, use was made of the solvent toluene employed in the above patent specification. This experiment was carried out at 250C (C5) and at 800C (C6). The results are shown in Table 2. As mentioned above, experiments carried out in this way enable those skilled in the art to determine adsorbent/solvent systems suitable for the separation, which can then be carried out in continuous or batchwise process modes such as, for example, are described in the EP mentioned. If, in such an experiment, a selected adsorbent/solvent system shows no separation effect, then those skilled in the art Can consider this system unsuitable for a continuous or batchwise process.

   is Table 2 Example c 5 C 6 Adsorbent Li-X Li-X T ['>C] ccp/m 1 1 It will of course be understood that the present invention has been described above purely by way of example, and that modifications of detail can be made within the scope of this invention.

   C-Ligms 1.

   is Process for separating mixtures of m- and pdichlorobenzene by treatment of such mixtures with a zeolite in the presence of solvents, characterized in that the mixture together with a solvent combination of at least two different solvents which originate from two groups A and B respectively, of which the representatives of the group A correspond to the formula Hal 11 -R (1) 2 R in which Hal represents chlorine or bromine, preferably chlorine, R1 denotes Hal or Cl-C4-alkyl and R 2 represents hydrogen, Hal or Cl-C4-alkyl, where R1 and R 2 can together represent the remainder of an indane, tetralin or naphthalene system and where 4-chlorotoluene and the dichlorobenzenes are excepted, - 16 in which and of which the representatives of the group B correspond to the formula Hal 6-R 3 (11) Hal has the meaning given above and is R 3 denotes hydrogen, Cl-C4-alkyl, Cl-C,-alkoxy, Cl-C4-alkylcarbonyl, Cl - C4 - alkoxy- carbonyl, phenyl, nitro or the aldehyde group, is treated at from 20 to 25010C with a pentasil zeolite containing protons, cations of the first or second main group of the Periodic Table (Mendeleev), cations of the rare earth metals or a mixture of a plurality thereof as exchanged cations, the enriched m-dichlorobenzene is taken off as filtrate and p-dichlorobenzene is recovered by extraction from the pentasil zeolite, the above solvent combination being used for the extraction.

   2. Process according to Claim 1, characterized in that pentasil zeolites of the following structure types are used: ZSM 5, ZSM 11, ZSM 8, ZSM S/ZSM 11 intermediates, Zeta 1, Zeta 3, ZBM 10, Ultrasil, Ultrazet, TZ-01, NU-4, NU-S, AZ-1, preferably ZSM is 5, ZSM 8, ZSM 11 and ZSM 5/WM 11 intermediates, particularly preferably ZSM 5 and ZSM 11.

   3. Process according to Claim 1, characterized in that mixtures containing at least 50 % by weight of m-dichlorobenzene, preferably mixtures containing more than 70 k by weight of mdichlorobenzene, are used.

   4. Process according to Claim 1, characterized in that the solvent combination used comprises the isomers of the trichlorobenzene series and/or the dichlorotoluene series as representatives of the group A and chlorobenzene, 4-chlorotoluene, 4chloroanisole, methyl 4-chlorobenzoate and/or 4chloroacetophenone as representatives of the group B. B. Process according to Claim 4, characterized in that the solvent combination used comprises the isomeric trichlorobenzenes and/or the isomeric dichlorotoluenes in combination with chlorobenzene, p- chlorotoluene and/or pchloroacetophenone.

   6. Process according to Claim 5, characterized in that the solvent combination used is one comprising 1,2,4-trichlorobenzene with chlorobenzene or p-chlarotoluene or one comprising 18 - is 3,4- or 2,6-dichlorotoluene with chlorobenzene or p-chlorotoluene.

   Process according to Claim 1, characterized in that the solvent combination used comprises 50-98 % by weight of solvent of the group A and 50-2 96 by weight of solvent of the group B, preferably 80-98 t by weight of group A and 20-2 % by weight of group B, particularly preferably 90-98 % by weight of group A and 10-2 % by weight of group B, with all percentages by weight being based on the total weight of the combination.

   8. Process according to Claim 1, characterized in that the amount of the solvent combination is 5-20 times, preferably 5-15 times, the amount of the dichlorobenzene mixture to be separated.

   Process according to Claim 1, characterized in that from 0.05 to 2, preferably from 0.1 to 1, particularly preferably from 0.2 to 0.8, amount units of dichlorobenzene mixture are used per amount unit of pentasil zeolite.

   10. Process according to Claim 1, characterized in that it is carried out at from 20 to 200C.

   Process according to claim 1, substantially as described in any one of Examples 1 to 5.

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