PL167122B1 - Method of stabilising unsaturated carboxylic esters - Google Patents

Abstract

This invention relates to a method of inhibiting polymerization of an ester of an unsaturated carboxylic acid in a process stream comprising water and said ester comprising: adding to said process stream, a stabilizer blend comprising (a) a first N,N substituted paraphenylenediamide which is soluble in water at a pH of 2; and (b) a second N,N substituted paraphenylenediamine which is non soluble in water at a pH of 2, preferably one of the N,N substituents of (a) is a C1-C4 alkyl, more preferred (a) compounds have structure (I) and (b) compounds have structure (II), R1 and R2 are independently selected from phenyl or C5-C9 alkyl and R3 is C1-C4 alkyl, R4 is phenyl or C1-C9 alkyl.

Description

The present invention relates to the stabilization of unsaturated carboxylic acid esters with mixtures of N1-substituted p-phenylenediamines (pFDA). In particular, it relates to a method of stabilizing methyl methacrylate or ester derivatives of acrylic acid,

167 122 derived from lower alkyl groups, during the preparation of such esters, by introducing an appropriate mixture of N, N'-substituted p-phenylenediamines which is soluble in the aqueous phase of the process, containing water and an ester.

Methacrylic and acrylic acid alkyl esters are widely used as raw materials for the production of fibers and plastic masses. Since these compounds have reactive unsaturated bonds in their molecules, they tend to polymerize under appropriate conditions during manufacture or storage. Polymerization can occur as a result of the presence of heat, light, oxygen, and other factors. Consequently, it is most desirable to reduce or eliminate the tendency of methacrylic acid or its esters, such as methyl methacrylate, to polymerize during manufacture. Elimination of the tendency to polymerization ensures that the tanks and pipelines used for transporting substances during production are kept clean and the reactors are protected against the accumulation of polymerized material of high viscosity and high molecular weight. In the preparation of methyl methacrylate (MAM) and other unsaturated carboxylic acid esters, it has become customary to add polymerization inhibitors such as hydroquinone and certain p-phenylenediamine or phenothiazine derivatives.

Some N, N'-substituted p-phenylenediamines are known to be used, including the individual substances disclosed in Japanese Patent 49-43920, which uses N-sec-hexyl-N'-phenyl-p-phenylenediamine and N, N'- di-sec-heptylp-phenylenediamine for stabilizing unsaturated carboxylic acids or their esters, including ethyl acrylate, 2-ethylhexyl acrylate, methyl acrylate, acrylic acid, methacrylic acid, in the organic phase. Methyl methacrylate was not mentioned or tested, and the importance of stabilizing the aqueous phase was underestimated. The above publication of the description only exposes the thesis that certain p-phenylenediamine derivatives have good activity as polymerization inhibitors in the pure liquid (organic) phase of an acid or its ester.

Despite such effectiveness demonstrated in the laboratory scale study of inhibition characteristics in pure organic monomer phase, in large-scale commercial production units on a global scale, there were still numerous cases of troublesome polymerized ester or acid formation in various tanks, settlers, distillation columns, pipelines, exchangers heat and reboilers of the technological process. All this indicated that the inhibition of polymerization was still inadequate. The present invention was accomplished when it was realized that the esters (and their precursors) needed to be protected in both the organic and the aqueous phase. This recognition of the hitherto elusive key element of the problem initiated the urgent search for an inhibitory system that could protect both phases. The second significant recognition of the problem was the finding that the protection of the aqueous phase by various p-phenylenediamine derivatives is very dependent on the pH of the aqueous phase. Neither of these phenomena had previously been identified, nor had any of them received due attention.

It is an object of the present invention to provide a stabilization feed that is effective in both the organic and the aqueous phase. A further aim is to try to protect even at the low pH level, characteristic of disturbed production conditions, when various inorganic and organic acids raise the acidity of the aqueous phase to a pH of 4, 3 or even 2. The process according to the invention protects both phases by reducing polymer build-up in the water phase. production plant, thus eliminating the many routine until now interruptions in the operation of various units in the process line to clean the polymerized residue and improve the efficiency of the plant.

The invention relates to a method of preventing the polymerization of an unsaturated carboxylic acid ester in a process stream comprising water and said ester, and having an aqueous phase having a pH ranging from about 1 to about 5, with a small amount of said ester, and an organic phase composed mostly of said ester and consisting of adding to said process stream a stabilizer mixture composed of (a) a first N, N-substituted p-phenylenediamine having a solubility of less than 20% in said aqueous phase at a pH of 2; and with (b) - a second N, N-substituted p-phenylenediamine having a solubility greater than 50% in said aqueous phase at a pH of 2.

The first N, N-substituted p-phenylenediamine (a) is represented by formula (I) wherein R1 is phenyl and R2 is C5-C6alkyl and the second N, N-substituted p-phenylenediamine (b) is represented by formula (II) wherein R3 is C1-C4alkyl and R4 is phenyl or C1-C4alkyl.

Methyl methacrylate (MMA) has been selected for illustrative purposes only, and it will be understood that the method can apply to any of the many unsaturated dicarboxylic acid esters, not just MAM, with appropriate modification that may be made by one skilled in the art. Esters of acrylic acid and methacrylic acid are the preferred substances that can be stabilized by the process of the invention. The methyl, ethyl, propyl, butyl, pentyl, hexyl, heptyl and octyl esters are preferred. Most preferred are the methyl, ethyl and propyl esters of acrylic and methacrylic acid. The most preferred ester is methyl methacrylate (hereinafter sometimes abbreviated as MAM) which is discussed for purposes of example only and should not be taken as limiting the scope of the present invention.

The industrial method of producing methyl methacrylate is based on the basic raw materials: acetone, hydrogen cyanide, methanol and sulfuric acid. Acetone reacts with hydrogen cyanide to form acetone cyanohydrin.

The chemistry of the process involves the reaction of acetone cyanohydrin (ACN) with an excess of concentrated sulfuric acid to form methacrylic amide sulfate. The methacrylic acid amide sulfate stream is reacted with an aqueous methanol solution to form methyl methacrylate in a combined hydrolysis-esterification reaction.

Modifications to the esterification step are changes to the procedures to recover the crude ester and separate the methanol and methacrylic acid for recycling back into the process. In addition, processing conditions differ in terms of methanol and water feed rates of methacrylic acid amide, reactor temperatures, and residence times in the reaction space.

In one embodiment, the methacrylic amide sulfate stream, excess methanol aqueous solution, and recirculated streams are continuously reacted in a series of steam jacketed esterification reactors at temperatures ranging from 80 to 100 ° C with residence times of 2 to 4 hours. The effluent from the reactor is distilled in an acid release column to obtain crude methyl methacrylate, methanol and water. The water-washed crude ester is purified in a multi-column distillation system and the aqueous methanol solution is distilled to recover methanol for recycling. The yield of methyl methacrylate is about 90%, based on ACN, and depending on the technological scheme and reaction conditions, it ranges from 80-90% based on methanol.

In another version of the esterification step, the reaction is carried out at pressures up to 790 kPa (100 psig - pounds per square inch) at 100-150 ° C with residence times of 1 hour or less, depending on the reaction temperature. The product can be recovered as described

167 122 above, or else the reactor effluent may be separated into an organic phase and a waste acid phase.

The light fraction (low boiling fractions) is removed from the organic layer in the flash column. The crude ester is then washed with water or aqueous ammonia to remove methanol and some methacrylic acid; the aqueous raffinate solution is recycled to the esterification reactor. The washed crude ester is purified in the tri? In the first stage column, water and methanol are withdrawn from the overhead and returned to the esterification reactor. The final product is collected from the top of the product column. The distillation residue from the product column is stripped to recover compounds for recycling to the process, and the residue is burned.

Ta b e l a 1

Azeotropic mixtures with methyl methacrylate

Pressure, kPa Boiling point ° C Ester content,% water 101 83 86 27 49 88.4 methanol 101 64.2 15.5 ethanol 27 34.5 18

Ta b e 1 a 2

Mutual solubilities of methyl methacrylate and water

Temperature, ° C Methyl methacrylate in water g / 100 g solvent Water in methyl methacrylate g / 100 g of solvent 0 1.85 0.85 10 1.72 0.99 twenty 1.59 1.15 thirty 1.50 1.34 40 1.43 1.56 50 1.43 1.80 60 1.49 2.07 70 1.60 2.38 80 1.80 2.74

The present invention is based on the finding that MAM (or any other ester) must protect itself in the aqueous phase due to the high solubility of MAM in water and the tendency of MAM to form azeotropic mixtures with water.

Tables 1 and 2 give details of these solubilities. The next required step in implementing the present invention is the need to evaluate whether the potential stabilizer / inhibitor candidate compound is soluble in the aqueous phase at the desired pH levels. The following test system is used to make this determination.

METHODOLOGY OF THE WATER SOLUBILITY TEST EXTRACT TEST GENERAL DESCRIPTION

OBJECTIVE:

Determine the division of polymerization inhibitors between MAM and aqueous compositions having a specific pH

METHOD:

1. Stock samples of pure ItLgM, containing 1 (X) 0 ppm for each inhibitor, were prepared.

16'J 122

2. Aqueous solutions are made up according to the following recipe:

water 96.5% methanol 2.0%

I HAVE 1.5%

3. Samples of the aqueous solutions were adjusted to the desired pH using sulfuric acid.

4.11X) g of each MAM solution was extracted with 100 g of an aqueous solution of the desired pH.

a) The solutions were placed in 453.6 gram (16 oz) jars on a mechanical shaker. The solutions were shaken for 16 hours at low speed.

b) The layers were allowed to stand. If emulsions were present, they were quenched by careful heating with a heat gun.

5. Each layer was analyzed to determine the level of the inhibitor. in each fas according to the pFDA LEVEL ANALYTICAL TEST.

ANALYTICAL TEST AT THE pFDA LEVEL

The method used is based on the oxidation / coupling of quinone with p-phenyldiamine towards the formation of highly colored solutions. Levels were determined colorimetrically using a visible spectrophotometer equipped with either 1 cm or 0.5 cm cuvettes. Standard solutions of p-phenylenediamine and p-benzoquinone are prepared by carefully weighing about 0.1 of each reagent in 100 ml of acetic acid.

Calibration is performed by: (1) preparing a reagent blank by pipetting 10 ml of MAM into a 50 ml volumetric flask, adding 25 ml of acetic acid followed by addition of 1 ml of p-benzoquinone solution, then filling the flask to the mark with distilled water; (2) 1,2 and 3 ml of a standard solution of the appropriate p-phenylenediamine are added to labeled 50 ml volumetric flasks, then 10 ml of MAM (used for blank test) and sufficient acetic acid to make 25 ml are added. total acetic acid in the solution, including the pipetted volume, then add distilled water rapidly to the mark, mix and make a spectrum of the solutions 5 to 15 minutes after the addition of the water.

The test samples are analyzed according to the calibration guidelines, with the difference that 10 ml of the MAM solution is replaced with the solution to be analyzed. A calibration curve is used to obtain the amount of micrograms of p-phenylenediamine in each sample. Conversion to ppm of p-phenyleneSiamine is done by dividing the amount determined in micrograms by the MAM weight per 10 ml (9.34 g / 10 ml).

Methyl methacrylate has been described above only as a representative example, and the other esters with which the process of the invention is concerned can be evaluated analogously. If an ester other than MAM is to be protected, appropriate changes to the above method should be made due to the different weight.

Table of inhibitors

Relationship Composition 1 A mixture of N- (1,4-dimethylpentyl) -N '-phenylenediamine and N- (1-methylpentyl) -N'-phenyl-p-phenylenediamine 2 N- (1,4-dimethylpentyl) -N'-phenyl-p-phenylenediamine 3 N, N′-di (isopropyl) -p-phenylenediamine 4 N-isopropyl-N'-phenyl-p-phenylenediamine

The compounds in Table 3 are given by the formula

R-NH- (C6H4) -NH-R '.

167 122

Table 3

Water extraction data pH = 2

Relationship R R ' water phase% I HAVE % 1 phenyl C6H11 / C7H15 2.3 91.1 2 phenyl C 7H15 2.3 91.1 3 C 3 H 7 C 3 H 7 97.2 2.0 4 phenyl C 3 H7 * 90.0 10.0

* The water phase did not contain methanol.

It can be readily seen from the data in Table 3 that certain p-phenylenediamine derivatives readily enter the aqueous phase at low pH levels. In order for a given compound to be considered a suitable inhibitor in the aqueous phase, it must pass to the aqueous phase by at least 10%, more preferably 30% and most preferably 50% as determined by the test method at a pH of 2. It can be seen that compounds 3 and 4 they are such substances. N, N'-substituted p-phenylenediamines having at least one lower alkyl (C1-C4) substituent are compounds that constitute a preferred group of substances when used as a component of an aqueous phase inhibitor system. Compounds 3 and 4, which are isopropyl substituted p-phenylenediamines, provide exceptionally pronounced protection of the MAM present in the aqueous phase, especially under strongly acidic conditions. Such conditions may be normal technological conditions, or they may be transitory conditions during periods of production disturbances. Without the enhanced protection afforded by the process of the invention, undesirable polymerization can occur to a significant extent under highly acidic processing conditions, causing clogging of the apparatus, reducing heat transfer efficiency as well as causing downtime for plant cleaning.

Compounds of formula (I) do not pass the water solubility test, while compounds of formula (II) pass the test and are considered soluble in the water phase.

Table A below shows the results obtained in the dissolution test carried out by the extraction method described above at the different pH of the aqueous phase. The compounds 12, 3,4 defined previously were used as inhibitors.

Table A Water Extraction Data

Relationship R R ' pH 2 pH 3 pH 4 % H2O % I HAVE % H2O % I HAVE % H 2O % I HAVE 1 phenyl C6H13 / C7H15 16.7 83.3 2.3 97.7 0 100 2 phenyl C 7H15 13.2 86.8 2.6 97.4 0 100 3 C 3 H7 C 3 H7 99.5 0.5 24.8 75.2 11.0 89.0 4 * phenyl C 3 H 7 90 10 NB NB 15 85

* The aqueous phase did not contain methanol NB = not tested

As can be seen from this table, compounds 1 and 2 in the pH range 2 to 4 have little or no transfer to the aqueous phase, while compounds 3 and 4 of formula II show almost 100% solubility in the aqueous phase at pH 2 and they still retain 11 to 15% solubility in the water phase at a high pH of 4.

The results in Table A confirm the graphs in Figures 1, 2 and 3.

167 122

Figures 1 and 2 show the solubility of compounds of formula I over a wider pH range (2-8) than in Table A.

Figure 3 shows the extractability of compound 3 (which is representative of compounds of formula II). Extraction was performed as described above. It is clear from Fig. 3 that the paraphenylenediamines of formula II extract very well with the aqueous phase and are very soluble therein, in contrast to the paraphenylenediamines of formula I (compounds 1 and 2), which are only soluble in the methyl methacrylate phase and insoluble in the water phase.

If the selected substance meets the water-solubility criteria, it must also exhibit suitable polymerization-inhibiting properties. It will be appreciated that the substances do not need to exhibit the best or optimal activity since the aqueous phase inhibitor is accompanied by an organic phase inhibitor which will be a very active inhibitor. It should also be borne in mind that currently there is relatively little ester in the aqueous phase compared to the organic phase in a conventional process stream, and therefore the amount of protection required is less than in the organic phase (which is essentially all of MAM or another ester).

The polymerization test procedure described below was used to evaluate the selected substances that passed the water solubility test method.

POLYMERIZATION TEST PROCEDURE

1. Any inhibitor used for storage of MAM is removed by distillation.

2. Prepare 0.1% stock solutions of test inhibitor in MAM.

3. Solutions containing 10 ppm of the inhibitor to be tested are prepared from the stock solutions and placed in test tubes.

4. The test tubes, fitted with an internal oil tube and thermocouple, are placed in a thermostated oil bath at 80 ° C.

5. The first symptoms of exotherm, read from the recorder, indicate the occurrence of polymerization. The time after which the polymerization has taken place is marked.

Table 4

Polymerization data Methyl methacrylate Conditions: 80 ° C

Inhibitor (10ppm) Time (hours) Attempt 1 Attempt 2 Blind control A 4.75 4.75 Control sample B Phenothiazine 12.75 14.25 Control sample C Hydroquinone 19.25 19.50 Relationship 1 83.50 103.00 Relationship 2 68.75 96.00 Relationship 4 136.50 143.50

The data in Table 4 show the excellent, long-lasting effect of inhibiting polyneurization exerted by the p-phenylenediamine derivatives of formula I and II. Controls A, B and C give no more than 20 hours of protection, while the alkyl-substituted p-phenylenediamines show 60 to 140 hours of protection.

The process according to the invention uses a stabilizer mixture of (a) an organic phase inhibitor, which may be represented non-exhaustively by the compounds of formula I, and (b) an aqueous phase inhibitor, which may be represented non-exhaustively.

167 122 exhaustive through the compounds of formula II. The stabilizer mixture may be used in any effective polymerization-inhibiting amount, preferably from 50 to 10,000 ppm (parts per million) based on the weight of the ester to be stabilized. More preferably, from 100 to 2000 ppm, and most preferably from 200 to 1000 ppm, can be used for line stabilization of the ester and its precursors during the production of the ester. The proportion of components (a) to (b) can vary widely from 5/95 to 95/5, more preferably from 30/70 to 70/30, and most preferably from 40/60 to 60/40.

Preferably, in compounds of formula I and II, R3 and R4 are C3-C4 alkyl substituents, R1 is phenyl and R2 is C6-C7 alkyl substituent. Throughout this specification and claims, the term alkyl includes linear, branched, and cyclic alkyl substituents.

The stabilizer mixture consists of (a) N-alkyl, N-phenyl substituted p-phenylenediamine, having a solubility greater than 50% in the aqueous phase at pH 2, and (b) a second N, N-alkyl or N-alkyl-N-phenyl substituted p-phenylenediamine having a solubility of less than 20% in said aqueous phase at a pH of 2.

Example. Small scale laboratory testing does not adequately capture the variety of conditions in the numerous vessels and streams of the large volume chemical plant for producing esters of the present invention, especially methyl methacrylate. Therefore, in order to demonstrate a significantly better effect, it is most advisable to introduce an experimental batch of the inhibitor into a long-term trial. In the course of such tests, it can be determined whether the inhibitor can in fact eliminate or at least reduce the build-up of polymerized MAM in various production vessels, piping, heat exchangers, etc. Such a test was carried out using a mixture of compound 1 (defined above in the table of inhibitors) of formula I and compound 3 (according to the table of inhibitors) of formula II in a proportion of 50/50% by weight. In the production column, which would normally have to be stopped and cleaned every 2-3 months, the test ran for 9 months without the need for cleaning - showing an unprecedentedly developed inhibition of polymerization. Other episodes showed significantly extended intervals between cleanings. When both aqueous and organic phases are present in the system, the process of the invention gives surprisingly better results compared to using a single N, N-substituted p-phenylenediamine.

Claims (9)

Patent claims 1. Method for the stabilization of uneven esters of poyrhep carboxylic acids. Development of their polymerization in technological streams, at least one of which includes an aqueous phase with a pH of about 1 to about 5, with a low content of the ester mentioned, and an organic phase, which is mostly this ester, characterized in that 50 to about 10,000 ppm, based on ester, of a stabilizer mixture consisting of (a) a first N, N-substituted p-phenylenediamine of formula (I) and (b) a second) are added to said process streams. H. WITH \ R (I) (Π) wherein R 1 is phenyl and R 2 is C 8 -C 4 alkyl, R 3 is C 1 -C alkyl, and R 4 is phenyl or C 1 -C 4 alkyl. 2. The method according to p. The process of claim 1, wherein the process streams used contain lower alkyl esters of acrylic or methacrylic acid. 3. The method according to p. Process according to claim 2, characterized in that the process streams used contain methyl methacrylic acid ester. 4. The method according to p. The process of claim 1, wherein the second p-phenylenediamine is a compound of formula (II) in which R3 and R4 are C3-C4alkyl. 5. The method according to p. The process of claim 1, wherein the first p-phenylenediamine is a compound of formula (I) in which R1 is phenyl and R2 is C6-C6alkyl. 6. The method according to p. The method of claim 1, wherein the stabilizer mixture comprises component (a) and component (b) in a ratio of (a): (b) from about 5/95 to about 95/5. 7. The method according to p. The method of claim 1, wherein the stabilizer mixture comprises component (a) and component (b) in a ratio of (a): (b) from about 30/70 to about 70/30. 8. The method according to p. The method of claim 7, wherein the stabilizer mixture comprises about 100 to about 1000 ppm of component (a) and about 100 to about 1000 ppm of component (b). 9. The method according to p. The process of claim 1, wherein about 200 to about 2000 ppm of the stabilizer mixture are added.