HK1112610A - Process for the production of toluene diisocyanate - Google Patents

Description

Method for producing toluene diisocyanate

Technical Field

The invention relates to a process for the production of tolylene diisocyanate, in which crude tolylenediamine obtained by hydrogenation of dinitrotoluene is purified and then subjected to phosgenation.

Background

It is known that diisocyanates and polyisocyanates are produced by phosgenation of the corresponding diamines and polyamines. In the case of distillable diisocyanates and polyisocyanates, the isocyanate is distilled after the phosgenation and the separation of the solvent used. For example, Toluene Diisocyanate (TDI) is obtained in the form of a distillation product, whereas a mixture of polyisocyanates of the diphenylmethane series with high molecular weights is obtained in undistilled form as a bottom product.

In the processing of Toluene Diisocyanate (TDI), the crude product from which a large amount of solvent has been removed is subjected to distillation to remove high-boiling components and low-boiling components which remain after the solvent separation removal. Thus, multistage distillation is carried out to separate off the low-boiling and high-boiling residues remaining in the crude product [ see, e.g., industrille aromatic chemie, H. -G.Franck, J.Stadelh  fer, Springer-Verlag Berlin, 1987, page 253 ], or the use of more energy-efficient integrated distribution columns as described in U.S. published application 2004/118672 (which is believed to correspond to EP-A1-1413571) or DE-A1-1260092.

In the distillation of TDI, the crude TDI product, which is clearly coloured, gives a clear (water-clear) almost colourless end product. However, even finely distilled TDI is not completely free of color-imparting components which sometimes result in undesirable coloration of the TDI.

The causes of the disadvantageous coloration of the distilled TDI are numerous and need to be avoided by different methods.

For example, U.S. Pat. No. 6900348 (believed to correspond to EP-A1-1187808) describes that the use of phosgene having a bromine content of less than 50ppm in the production of isocyanates of the diphenylmethane series makes it possible to obtain off-white isocyanates after the phosgenation.

Likewise, EP-A1-816333 describes a process for reducing the color of TDI, wherein the crude TDI solution is treated with hydrogen before the solvent is removed by separation.

In addition to treating the crude isocyanate product after the phosgenation, treating the amine stream can also reduce the color of the isocyanate produced by the phosgenation. U.S. Pat. No. 5872278 (believed to correspond to EP-A1-866057) describes a process in which an amine is treated with a solid substance containing Lewis acid and/or Bronsted acid centers prior to its reaction with phosgene. The color of the resulting isocyanate is lighter than that obtained using the untreated amine.

Disclosure of Invention

It has been found that the color of the distilled TDI can be significantly reduced if the total content of cyclic ketones in the Toluenediamine (TDA) used for the phosgenation is less than 0.1% by weight, based on 100% by weight of TDA used.

These cyclic ketones are derived from TDA, based on six-membered structures with zero, one or two double bonds. The double bonds may be conjugated to a ketone functionality or may be independent of each other. The cycloalkane structure or cycloalkene structure having zero, one or two double bonds may be substituted with one or two amino functional groups. The structure may also be substituted with methyl groups.

For example, in the hydrogenation of Dinitrotoluene (DNT), cyclic ketones or their precursors in TDA are formed as a result of the substitution of amino functions by water in the aqueous medium. It is also possible to oxidatively introduce ketone functions during nitration by oxidative attack, with the formation of nitrocresols.

In addition to Tolylenediamine (TDA), commercial TDA typically contains byproducts of the hydrogenation reaction, e.g., TDA in which the benzene rings have been fully or partially hydrogenated. For example, if one of the amino functions in TDA, whose benzene ring has been completely or partially hydrogenated, is replaced by water, aminomethylcyclohexanone or aminomethylcyclohexenone is formed. For example, if one of the amino functions in TDA, which has been fully or partially hydrogenated in the benzene ring and subjected to an additional deamination treatment, is replaced by water, methylcyclohexanone or methylcyclohexenone is formed. For example, if both amino functional groups are substituted with water, methylcyclohexanedione is formed. If the nitrocresols resulting from the production of dinitrotoluene are subjected to a hydrogenation process, they likewise react, with partial hydrogenation of the benzene ring to form cyclic ketones.

In a subsequent phosgenation step, the cyclic ketones present in the TDA can then be phosgenated and optionally chlorinated, to form in the first case isocyanatocycloalkenones or isocyanatocycloalkanones or in the second case chloroisocyanatocycloalkenes.

Since cyclic isocyanatocycloalkenones, isocyanatocycloalkanones and/or chloroisocyanatocycloalkenes are active substances, they are capable of forming adducts with TDI, themselves or with other TDI by-products. For example, isocyanatocycloalkenones can undergo Diels-Alder reactions with dienes or with aromatics (Angell, E.Charles; Fringelli, France sco; Guo, Ming; Minuti, Lucio; Taticchi, Aldo; Wenkert, Ernes. Diels-Alder reactions of cyclodekeenones.14. Endociastereoselect of 2-cycloheonein reactions with cyclopropentadine. journal of Organic Chemistry (1998), 53, (18), 4325-8). Likewise, dimers (Anenon-Dienol Tautomerim and an Iron (III) -catalysis of Cycloalkenone-2-carboxylates. Christoffset, Jens. Institutes f ü r Organische Chemie, Technische Universet Berlin, Berlin, Germany. Journal of Organic Chemistry (1998), 63(13), 4539 (4540)) may be formed. In the course of the single-stage or multistage treatment of the crude TDI product by distillation, the cyclic isocyanatocycloalkenones, isocyanatocycloalkanones and/or chloroisocyanatocycloalkenes as low-boiling substances can be concentrated in the circulating stream in a disadvantageous manner, further increasing the activity to form dimers or Diels-Alder adducts.

The dimer or Diels-Alder adduct formed by the reaction of isocyanatocycloalkenone, isocyanatocycloalkanone and/or chloroisocyanatocycloalkene with TDI, themselves or other by-products imparts more or less color to the product. The large amount of the colored adduct or dimer described above formed in the distillation operation from cyclic isocyanatocycloalkenones, isocyanatocycloalkanones and/or chloroisocyanatocycloalkenes is subsequently discharged from the TDI distillation process along with the TDI stream, resulting in colored TDI.

If a TDA stream having a total cyclic ketone content of less than 0.1 wt.%, based on 100 wt.% of TDA, is used in the phosgenation, the concentration of cyclic isocyanatocycloalkenone, isocyanatocycloalkanone and/or chloroisocyanatocycloalkene in the production of TDI (e.g., in the recycle stream of the distillation of the TDI production process) can be reduced to such an extent that only small amounts of color-imparting adducts or dimers are formed. If very small amounts of color-imparting adduct or dimer leave the reaction process together with the TDI product, the color of the TDI product obtained is significantly reduced.

Detailed Description

The present invention relates to a process for the production of toluene diisocyanate by phosgenation of toluene diamine. The method comprises the following steps:

a) hydrogenating dinitrotoluene in the presence of a catalyst to obtain a crude toluenediamine mixture,

b) purifying the crude toluene diamine mixture to obtain toluene diamine containing less than 0.1 wt.% total cyclic ketones, based on 100 wt.% toluene diamine, and

c) phosgenating the toluenediamine formed in b) containing less than 0.1 wt.% of cyclic ketones in total, based on 100 wt.% of the toluenediamine, to obtain toluenediisocyanate.

The toluenediamine obtained from the purification step b) contains less than 0.1% by weight, in total, of cyclic ketones, based on 100% by weight of the toluenediamine. a) The crude toluenediamine mixture formed in (a) contains various TDA isomers such as 2, 4-TDA, 2, 6-TDA, 3, 5-TDA (m-TDA), 2, 5-TDA (p-TDA) and 2, 3-TDA, 3, 4-TDA (o-TDA) and cyclic ketones, as well as varying amounts of water, hexahydrotda, hexahydrotoluidine, nitrotoluidine, toluidine and high boiling point materials (e.g., diphenylamine, diphenylmethane and/or phenazine derivatives). The crude TDA mixture typically comprises at least about 98 wt% TDA.

The toluenediamine obtained in purification step b) contains less than 0.1% by weight, preferably less than 0.07% by weight, more preferably less than 0.04% by weight, in total, of cyclic ketones, based on 100% by weight of the toluenediamine. Preferably, the toluenediamine obtained from purification step b) contains alkylated cyclic ketones in a concentration of less than 0.1 wt.%, preferably less than 0.07 wt.%, based on 100 wt.% of the toluenediamine.

It is understood that the total amount (i.e. weight%) of cyclic ketones present refers to the total weight of all cyclic ketones remaining in the purified toluene diamine after step b). These cyclic ketones include, for example, methyl-substituted, ethyl-substituted or unsubstituted cyclic alkanones, cyclic enones, aminocycloalkanones, aminocyclocycloalkenones and cyclic alkanediones, preferably methylcyclohexanone, methylcyclohexenone, aminomethylcyclohexanone, aminomethylcyclohexenone and methylcyclohexanedione, more preferably aminomethylcyclohexanone and aminomethylcyclohexenone.

According to the invention, in step a), the hydrogenation of dinitrotoluene is carried out in a conventional manner in the presence of a catalyst, for example a nickel catalyst or a noble metal catalyst, in the form of a suspension, a fluidized bed or a fixed bed, in a reaction vessel containing the catalyst. Such processes are known in the art, for example, such processes are described in DE 2135154, DE 3734344, U.S. Pat. No. 5563296 (believed to correspond to EP-634391A), U.S. Pat. No. 5779995 (believed to correspond to DE 4435839A), U.S. published applications 2003/049185, 2003/050510 and 2005/129594 (believed to correspond to EP-1287884A), U.S. Pat. No. 6005143 (believed to correspond to EP-978505A) and U.S. Pat. No. 6140539 (believed to correspond to EP-1033361A), the contents of which are incorporated herein by reference. The hydrogenation reaction may be carried out in the gas phase or in the liquid phase. The mixing power required for the liquid phase reaction can be introduced via stirrers or nozzles into internal and/or external circulation pumps. Heat removal can be carried out by a heat exchanger in the reactor or by external circulation. If the catalyst is a fixed bed catalyst, the catalyst may be present in the form of a trickle bed or monolith. In the case of liquid phase hydrogenation, the hydrogenation reaction may be carried out with or without the use of additional solvents. In the case of gas phase hydrogenation, the hydrogenation reaction may be carried out with or without the use of a carrier gas.

After the hydrogenation, the crude TDA mixture is purified in step b). Purification can be carried out by distillation, crystallization and/or thermal work-up, as well as by chemical oxidation or reduction methods.

In a preferred embodiment of the process of the invention, the purification process of step b) is carried out by distillation, whereby reaction water and low-boiling by-products such as hexahydrotoluidine, hexahydrotda and optionally solvent are partially or completely removed. This separation of water, low boilers and optionally solvent can be carried out in one or more stages. Preferably, this operation is followed by the removal of one or more ortho-TDA isomers by distillation, and this separation of ortho-TDA by distillation can be carried out in one or more stages. After this, the high boilers are preferably separated off from the remaining meta-TDA by distillation.

According to the prior art, high boiling materials are typically separated from meta-TDA by using a simple combination of evaporator and condenser, as described, for example, in U.S. patent 6359177, the contents of which are incorporated herein by reference. However, it has been found that this process does not necessarily result in a TDA having a total cyclic ketone content of less than 0.1 wt.%.

Thus, in a preferred embodiment according to the invention, the reaction water, the low boilers and optionally the solvent and the ortho-TDA are first partially or completely removed from the crude toluenediamine mixture, and the high boilers and cyclic ketones are then partially or completely separated from the meta-TDA. The separation of the high boilers and of the cyclic ketones from the meta-TDA is preferably carried out by means of a distillation column having an evaporator and a condenser. The separation is preferably carried out at an absolute head pressure of 50 to 2000 mbar, more preferably 60 to 500 mbar, most preferably 70 to 200 mbar. Thereby obtaining a top temperature of preferably 140-. The distillation column preferably has at least 3, more preferably at least 5, most preferably at least 7 to 20 theoretical plates in the stripping section. Any insert element known to the person skilled in the art may be used as a separation aid, such as a sieve, a blister or a valve plate, or an ordered or unordered filler material. The pressure loss through the separation aid should be kept low, preferably less than 150 mbar, more preferably less than 100 mbar. The specific surface area of the bulk filler material and the ordered filler material is preferably 100-500m²/m³More preferably 200-350m²/m³. The bottom temperature is determined by the content of high boilers and the pressure loss in the column; the operating conditions of the column are preferably such that the bottom temperature is less than 260 c, more preferably less than 240 c. The number of theoretical plates and the reflux ratio in the rectification section are controlled by the desired cyclic ketone content in the product. It has been found, however, that the process according to the invention can also be carried out without a rectification section. This is advantageous from the cost aspect of the process.

In a further preferred embodiment of the process according to the invention, the reaction water, low boilers and optionally solvent are first partially or completely removed from the crude toluenediamine, and the ortho-TDA, high boilers and cyclic ketone are then partially or completely separated from the meta-TDA. The separation of ortho-TDA, high boilers and cyclic ketones by distillation can be carried out in a single-stage or multistage distillation sequence using a distributor column, which advantageously combines the separation of ortho-TDA and high boilers. As feed to the distributor column, preference is given to using a crude tolylenediamine mixture which has been partially or completely freed of reaction water, low boilers and optionally solvent and which contains less than 0.5% by weight, more preferably less than 0.3% by weight, of cyclic ketones, based on 100% by weight of tolylenediamine.

For example, U.S. published application 2003/0230476A 1, the contents of which are incorporated herein by reference, has been described several times with respect to a distribution tower suitable for use in the present invention. In this preferred variant of the process according to the invention, the meta-TDA freed from the cyclic ketone is preferably discharged as a side stream. The side stream is preferably discharged in the region of the dispensing position (at the level of the oil dispensing position), that is to say laterally of the oil dispensing position. Thus, the cyclic ketone is partly or completely discharged together with the high boilers from the bottom of the distributor column. Preferably, the separation is carried out at an absolute head pressure of 50-2000 mbar, more preferably 60-500 mbar, most preferably 70-200 mbarAnd (6) rows. In the stripping section, below the TDA between the side stream withdrawals, the column preferably has at least 5, more preferably at least 6, most preferably from 7 to 20 theoretical plates. Any insert element known to those skilled in the art may be used as a separation aid, such as a screen, a blister or a valve plate, or an ordered or disordered filling material. The pressure loss through the separation aid should be kept low, preferably less than 150 mbar, more preferably less than 100 mbar. The specific surface area of the stacking filling material and the ordered filling material is preferably 100-500m²/m³More preferably 200-350m²/m³. The bottom temperature is determined by the content of high boilers and the pressure loss in the column; the operating conditions of the column are preferably such that the bottom temperature is less than 260 c, more preferably less than 240 c.

During the purification in step b), a significant reduction in the total weight content of cyclic ketones in the TDA is preferably achieved. The total weight content of cyclic ketones present in the TDA is preferably reduced in step b) by more than 25 wt.%, more preferably by more than 40 wt.%.

In the purification process of step b), purified toluenediamine is obtained. The purified toluene diamine contains less than 0.1 weight percent total cyclic ketones, based on 100 weight percent toluene diamine.

By appropriate selection of the conditions of the hydrogenation of step a), the cyclic ketone content in the crude TDA mixture has been made lower. Different catalysts, different reactors and different temperatures and pressures can be used for producing TDA according to the prior art. Examples of these different variants known from the prior art are mentioned in the following documents: for example, DE 2135154, DE 3734344, U.S. Pat. No. 5563296 (believed to correspond to EP-634391A), U.S. Pat. No. 5779995 (believed to correspond to DE 4435839A), U.S. published applications 2003/049185, 2003/050510 and 2005/129594 (believed to correspond to EP-1287884A), U.S. Pat. No. 6005143 (believed to correspond to EP-978505A) and U.S. Pat. No. 6140539 (believed to correspond to EP-1033361A), which are incorporated herein by reference. However, the processes described in these references for the hydrogenation of dinitrotoluene to TDA do not give crude TDA mixtures having a total cyclic ketone content of less than 0.1% by weight.

However, by optimizing the conditions of the hydrogenation (e.g.the hydrogenation catalyst used, the basic reaction parameters such as pressure, temperature, residence time), the content of cyclic ketones in the crude TDA mixture produced in the hydrogenation can be reduced. For example, crude TDA mixtures having a lower cyclic ketone content can be produced by selecting a combination of a high selectivity catalyst, low temperature and short residence time of the TDA product in the reaction system. Suitable catalysts are, for example, highly selectively doped or undoped Raney nickel catalysts, doped or undoped nickel catalysts which are immobilized on a support, noble metal catalysts which are immobilized on a support and are loaded with one or more noble metals. Suitable hydrogenation conditions are, for example, 120 ℃ and 180 ℃, a hydrogen pressure of from 20 to 40 bar and a residence time in the reaction system of from 0.5 to 4 hours.

In addition, the content of cyclic ketones in the crude TDA mixture produced in step a) can be further reduced by using dinitrotoluene having a relatively low content of nitrocresols (for example, less than 0.05% by weight) for the hydrogenation step.

The purified toluenediamine obtained in step b), which contains less than 0.1% by weight of cyclic ketones in total (based on 100% by weight of the toluenediamine), is subsequently subjected to a phosgenation reaction in step c). The phosgenation reaction can be carried out in the liquid phase. Suitable solvents are preferably unsubstituted and appropriately substituted aromatic or aliphatic solvents having from 5 to 12 carbon atoms, such as toluene, chlorobenzene, benzene, dichlorobenzene, trichlorobenzene, decalin, 1, 2, 3, 4-tetrahydronaphthalene, cyclohexane, hexane, cycloheptane, heptane, octane, nonane, decane, xylene or mixtures of these solvents.

Particularly preferably used solvents are benzene, toluene, chlorobenzene, dichlorobenzene.

The phosgenation of toluenediamine can also be carried out in the gas phase, with the toluenediamine being used at a temperature above or below the boiling point at a suitable pressure.

The following examples further illustrate the process of the present invention in detail. The invention, which is set forth in the foregoing disclosure, is not to be limited either in spirit or scope by these examples. Those skilled in the art will readily appreciate that known variations of the conditions of the following steps may be used. Unless otherwise indicated, all temperatures are degrees Celsius and all percentages are percentages by weight.

Examples

Examples 1 to 3:

5.4 tons/h of Toluenediamine (TDA) containing the cyclic ketone in an amount shown in Table 1 were phosgenated according to an industrial TDI production method. The phosgenation reaction was carried out in a perforated-base column (perforated-base column) at a temperature of 70.125 ℃. The solvent was removed by separation and the crude TDI was fractionally distilled at a temperature of 140 ℃ and 160 ℃ to give pure TDI. The last distillation step described above is carried out in a packed distributor column in which the high boilers are concentrated and separated off at the bottom and the major part of the low boilers contained in the feed stream is concentrated and separated off at the top. The high-boiling substances are discharged from the reaction process through subsequent evaporation and concentration; the low boilers are returned again to the solvent circuit.

The color of the pure TDI thus obtained was analyzed using a LICO 200 measurement device. The measured color indices of examples 1-3 are all shown in table 1.

The TDA used in examples 1 to 3 was obtained by mixing the following two TDA components (fractions):

and (2) component A: obtained by hydrogenation of dinitrotoluene in a stirred vessel at a pressure of 25 bar. The catalyst used is a nickel catalyst. The downward stream of the TDA reaction is separated by sequential distillation to remove water, low boilers and ortho-TDA from the crude product.

And (B) component: obtained by hydrogenation of dinitrotoluene in a stirred vessel at a pressure of 15-30 bar and a temperature of 120-140 ℃. The catalyst used is a nickel catalyst. The downward stream of the TDA reaction is separated by distillation in succession, removing water, low boilers, ortho-TDA, high-boiling residues and a portion of the cyclic ketone present from the crude product.

TABLE 1

	Total cyclic ketone content in TDA [ wt.%]	Color index [ APHA ]]
			Example 1	0.04	15
Example 2	0.09	22
			Example 3	0.15	48

Examples 4 to 5

10.5 tons/h of Toluenediamine (TDA) containing the cyclic ketone in an amount shown in Table 2 were phosgenated according to an industrial TDI production method. The phosgenation reaction is carried out in a perforated bottom tower at a temperature of 70-125 ℃. The solvent was removed by separation to fractionally distill the crude TDI at a temperature of 140 ℃ and 180 ℃ to obtain pure TDI. The last distillation step described above is carried out in a packed column in which the high boilers are concentrated and separated off at the bottom and some of the low boilers contained in the feed stream are concentrated and separated off at the top. The high-boiling substances are discharged from the reaction process through subsequent evaporation and concentration; the low boilers separated off at the top are returned again to the solvent circuit.

The color of the pure TDI thus obtained was analyzed using a LICO 200 measurement device. The measured color index is shown in the table.

The TDA used in example 5 was obtained by hydrogenation of dinitrotoluene in a stirred vessel at a pressure of 25 bar. The catalyst used is a nickel catalyst. The downward stream of the TDA reaction is separated by sequential distillation to remove water, low boilers, ortho-TDA, high boiling residues, and more than 25% of the cyclic ketone present from the crude product.

In the case of example 4 using TDA, compared with example 5, 65% of the cyclic ketone present can be removed again by additional separation by distillation by increasing the separation efficiency of the last distillation step.

Table 2:

	total cyclic ketone content in TDA [ wt.%]	Color index [ APHA ]]
			Example 4	0.03	12
Example 5	0.09	20

Although the invention has been described in detail in the foregoing for the purpose of illustration, it is to be understood that such detail is solely for that purpose and that variations can be made therein by those skilled in the art without departing from the spirit and scope of the invention except as it may be limited by the claims.

Claims (5)

1. A method of producing toluene diisocyanate, comprising:

a) hydrogenating dinitrotoluene in the presence of a catalyst to obtain a crude toluenediamine mixture,

b) purifying the crude toluene diamine mixture to obtain toluene diamine containing less than 0.1 wt.% total cyclic ketones, based on 100 wt.% toluene diamine, and

c) toluene diamine containing less than 0.1 wt% total cyclic ketones based on 100 wt% toluene diamine is phosgenated to obtain toluene diisocyanate.

2. The process of claim 1, wherein the toluene diamine obtained in step b) has a total concentration of alkylated cyclic ketones of less than 0.1 weight percent, based on 100 weight percent of toluene diamine.

3. The method of claim 1, wherein the cyclic ketone is a methyl-substituted, ethyl-substituted or unsubstituted cyclic alkanone, cyclic alkenone, amino cyclic alkanone, amino cyclic alkenone, and/or cyclic alkanedione.

4. The method according to claim 1, wherein b) the purification step is carried out according to a multi-stage distillation comprising a stage of separating off high boilers.

5. The process of claim 1, wherein the total weight of cyclic ketones present in the toluene diamine mixture formed in step a) is reduced by more than 25% in step b).