HK1048108A - A continuous process for the production of carboxylic acid diaryl esters - Google Patents

Description

Continuous process for producing carboxylic acid diaryl esters

Technical Field

The present invention relates to a process for the production of diaryl carboxylates by a continuous two-step process wherein the diaryl carboxylate is produced by reacting a monohydric phenol with phosgene in an inert solvent in the presence of a base at phase boundaries and low reaction temperatures, the pH of the reaction being maintained within a narrow range during each step.

Summary of The Invention

An improvement in an interfacial boundary process for the continuous production of diaryl carbonates is disclosed. The process for reacting a monohydric phenol in an inert solvent in the presence of an alkaline solution and a nitrogen base catalyst is carried out in two steps. In a first step phosgene, inert solvent and phenol are combined in solution to form a material system and in a second step the diaryl carbonate forming reaction is completed. The improvement comprises maintaining a pH of 11.0 to 12.0 at a temperature of < 40 ℃ in a first step and a pH of 7.5 to 10.5 at a temperature of < 50 ℃ in a second step.

Background

In the literature, for example, the chemical and physical properties of polycarbonates, Polymer Reviews (Polymer Reviews), volume 9, John Wiley and Sons, Inc. (1964), page 50/51, by H.Schnell, essentially describe the production of diaryl carboxylates by means of the phase boundary process, U.S. Pat. No. 4016190 describes a process for producing diaryl carboxylates at temperatures > 65 ℃. In this method, the pH is first adjusted to a low value (pH 8-9) and then to a high value (10-11).

However, in this known process, the purity of the product obtained is still to be improved, which requires costly purification operations. Another problem is that the residual phenol content in the wastewater of this method is high, which pollutes the environment and causes a problem that a sewage treatment plant increases sewage treatment. Furthermore, the yield of products such as diaryl carbonates produced on an industrial scale is still in need of improvement.

It is therefore an object of the present invention, starting from the above-mentioned prior art, to provide a process for producing a product of high purity in high yield, in order to reduce the purification costs of the product. It is also an object of the present invention to reduce the organic content of wastewater in order to reduce environmental pollution and reduce wastewater treatment problems in wastewater treatment plants.

Detailed description of the invention

It has now been found, surprisingly, that in the continuous production of diaryl carboxylates by reacting monophenols with phosgene in inert solvents in the presence of a base at phase boundaries and low reaction temperatures, the reaction pH in the first step is from 11 to 12 and the reaction pH in the second step is from 7.5 to 10.5, whereby high phosgene yields, high conversions and high product purities are obtained, while the residual phenol in the waste water also reaches low levels. Thus, unlike the prior art, a high purity product can be obtained directly from the reaction. Thus greatly simplifying the subsequent purification process.

In addition, the residual phenol content is low, which contributes to reducing environmental pollution and reducing wastewater treatment problems of wastewater treatment plants.

The base used may be an aqueous base (Na, K, Li or Ca hydroxide). Aqueous solutions of sodium hydroxide are preferred, and in the process according to the invention, preferably 20 to 55% by weight aqueous solutions are used, most preferably 30 to 50% by weight.

Phosgene may be used in liquid, gaseous form, or in dissolved form in an inert solvent.

Monohydric phenols suitable for the reaction are phenols of the general formula (I):

(I)wherein R is hydrogen, tert-butyl, halogen, or branched or unbranched C₈And/or C₉An alkyl group.

Examples of compounds of formula (I) thus include phenol itself, alkylphenols such as cresol, p-tert-butylphenol, p-cumylphenol, p-n-octylphenol, p-isooctylphenol, p-nonylphenol and p-isononylphenol, and halogenated phenols such as p-chlorophenol, 2, 4-dichlorophenol, p-bromophenol, and 2, 4, 6-tribromophenol. Phenol is preferred.

Examples of inert organic solvents employed in the process include methylene chloride, toluene, various dichloroethane and chloropropane compounds, chlorobenzene, and chlorotoluene. Dichloromethane is preferably used.

The reaction is carried out continuously, preferably in plug flow mode without significant back mixing. For example, the reaction may be carried out in a tubular reactor. For example, the two phases (aqueous and organic) can be thoroughly mixed by means of installed baffles in the pipe, static mixers, and/or pumps.

In the first step of the continuous process according to the invention, the reaction between the reaction components is initiated by the starting materials comprising phosgene, inert solvent and phenol, preferably first using inert solvent as solvent for the phosgene and preferably dissolving the phenol in advance in an aqueous solution of a base. In the continuous process according to the invention, the residence time in the first step is from 2 to 300 seconds, preferably from 4 to 200 seconds. The pH of the first step is adjusted by the ratio aqueous alkali/phenol/phosgene to a pH of 11.0-12.0, preferably 11.2-11.8, most preferably 11.4-11.6. The reaction temperature of the first step is maintained by cooling at < 40 ℃ and preferably < 35 ℃.

In the second step of the continuous process of the present invention, the reaction to form the diaryl carboxylate is completed. In the second step of the process according to the invention, the residence time is from 1 minute to 2 hours, preferably from 2 minutes to 1 hour, most preferably from 3 to 30 minutes. The second step of the process according to the invention is controlled by continuously monitoring the pH (the pH of the continuous process is preferably monitored on-line by known methods) and by suitably adjusting the pH by adding an aqueous solution of a base. The amount of aqueous alkaline solution added is adjusted so that the pH in the second step of the process is from 7.5 to 10.5, preferably from 8 to 9.5, most preferably from 8.2 to 9.3. The reaction temperature of the second step is maintained by cooling at < 50 ℃, preferably < 40 ℃ and most preferably < 35 ℃.

However, the general or preferred parameters given in the present application can be arbitrarily combined with each other, that is, the general and preferred parameters can be combined.

Secondly, the aqueous phase according to the process of the invention comprises < 250ppm of phenol, preferably < 150ppm, more preferably < 100ppm, most preferably < 50 ppm.

In the process according to the invention, the ratio of phosgene to phenol is from 1.01 to 1.15 mol%, preferably from 1.05 to 1.12 mol%, calculated on the basis of phenol. The solvent is mixed in such an amount that the concentration of the solution of diphenyl carboxylate after the reaction is 5 to 60% by weight, preferably 20 to 45% by weight, in terms of% by weight of the solution.

The reaction can be accelerated by catalysts such as tertiary amines, N-alkylpiperidines or onium salts. Tributylamine, triethylamine and N-ethylpiperidine are preferably employed. The catalyst concentration is from 0.0001 to 0.1mol, calculated on the phenol employed.

In the present application, an onium salt is understood to mean, for example, NR₄X, wherein R may be an alkyl and/or aryl group, and/or a H atom, and X is an anion.

After the reaction, the organic phase comprising the diaryl carboxylate is generally washed with an aqueous liquid and, after each washing operation, the organic phase is separated from the aqueous phase as far as possible. Preferably, the washing is carried out using deionized water. After washing and separation of the wash liquor, the diaryl carboxylate solution is typically opaque. Using aqueous liquids, e.g. using HCl or H₃PO₄And (3) using dilute inorganic acid as a washing liquid for separating the catalyst, and using deionized water as a washing liquid for further purification. For example, HCl or H in the wash liquor₃PO₄The concentration of (A) can be up to 0.5-1.0 wt%. For example, the secondary organic phase is preferably washed.

It is possible to use basically known separation vessels, phase separators, centrifuges or coalescers, and even combinations of these, as phase separation devices for separating the washing liquid from the organic phase.

In this way, it is surprisingly possible to obtain diaryl carboxylates of high purity (determined by GC) of > 99.85%, irrespective of the solvent to be separated.

The solvent is evaporated off to obtain the carboxylic acid diester in high purity. The evaporation may be carried out in a plurality of stages, for example in a series of one or more distillation columns, in which the solvent is separated from the diaryl carboxylate.

These purification steps can be carried out continuously, for example in a distillation process, with bottoms temperatures of > 150 to 310 ℃ and preferably > 160 to 230 ℃. The pressure required to carry out the distillation is from 1 to 1000mbar, preferably from 5 to 100 mbar.

These carboxylic acid diesters are distinguished by their particularly high purity (> 99.95% by GC) and by their extremely good transesterification behavior, so that polycarbonates of good quality can be produced.

The use of diaryl carboxylates for the production of aromatic oligo/polymeric carbonates by the melt transesterification process is known from the literature and has been described previously, for example, in Encyclopedia of Polymer Science, Vol.10 (1969), in the chemistry and physics of polycarbonates by H.Schnell, in the reviews of polymers, Vol.9, John Wiley and Sons, Inc. (1964), and in U.S. Pat. No. 5340905.

The following examples are given to illustrate but not to limit the invention.

Examples

Example 1

In a vertically cooled tubular reactor, a mixture comprising 103kg/h of deionized water, 42.2kg/h of 50% NaOH and 48.3kg/h of phenol was continuously mixed with a solution comprising 86.2kg/h of methylene chloride and 27.5kg/h of phosgene (8 mol% excess relative to phenol). The reaction mixture was cooled to a temperature of 33 ℃ and after an average residence time of 15 seconds, the pH was determined to be 11.5. Then in the second step of the process, 5.4kg/h of 50% NaOH were added to the reaction mixture so that after a further 5 minutes residence time, the pH in the second reaction step was 8.5. In this continuously carried out reaction, any fluctuations in the rate of addition are compensated in each case by adjusting the rate of addition of NaOH. In the second step of the process, the reaction mixture is continuously mixed by passing it through a tube equipped with a constriction. After the renewed addition of NaOH, the reaction temperature was adjusted to 30 ℃ by cooling. 20ppm phenol were contained in the slightly basic aqueous phase. The yield of DPC was quite high, i.e., 99.998% calculated on phenol. After separation of the organic phase from the aqueous phase, the product was washed with 0.6% HCl and water and after the final phase separation the methylene chloride was removed by evaporation to give 99.9% (according to determination with GC) of diphenyl carbonate. Comparative example 1

The same as in example 1, except that no cooling was used in the second step of the process, the reaction temperature was 60 ℃. The slightly alkaline aqueous phase then contained 2400ppm of phenol. Thus, the yield of DPC was only 99.76% based on phenol. Comparative example 2

As in example 1, except that 7.8kg/h of 50% NaOH were added instead of 5.4kg/h, the pH in the second stage of the reaction was 11.7 after a further 5 minutes residence time. The slightly alkaline aqueous phase then contained 900ppm of phenol. The yield of DPC is therefore only 99.91% calculated on phenol.

Although the invention has been described in detail in the foregoing for the purpose of illustration only, 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. In an interfacial boundary process for the continuous production of diaryl carbonates from phosgene and monohydric phenol in an inert solvent in the presence of an alkaline solution and a nitrogen base catalyst, which process comprises a first step in which phosgene, inert solvent and phenol are mixed in solution to form a material system and a second step in which the diaryl carbonate formation reaction is completed, the improvement which comprises maintaining a pH of from 11.0 to 12.0 in the first step at a temperature of < 40 ℃, and a pH of from 7.5 to 10.5 in the second step at a temperature of < 50 ℃.

2. The process of claim 1, wherein in the first step of the process the pH is between 11.2 and 11.8 and in the second step the pH is between 8 and 9.5.

3. The process of claim 1, wherein in the first step of the process the pH is between 11.4 and 11.6 and in the second step the pH is between 8.2 and 9.3.

4. The process of claim 1, wherein the temperature of the first step of the process is maintained at < 35 ℃ and the temperature of the second step is maintained at < 40 ℃.

5. The process of claim 1 wherein the temperature of the second step is maintained at < 35 ℃.