HUT64292A - Method for stabilizing esters of unsaturated carboxylic acids - 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

It is stabilized by special mixtures of N, N'-substituted p-phenylene diamides (PPDA). More particularly, the present invention relates to a process for stabilizing lower alkyl esters of methyl methacrylate or acrylic acid by introducing a special mixture of N-Ν'-substituted p-phenylene diamines in an aqueous and ester-containing aqueous process. phase is characterized by solubility.

Alkyl esters of methacrylic acid and acrylic acid are widely used as raw materials in the manufacture of synthetic fibers and plastics. Because these compounds contain reactive unsaturated bonds in their molecules, they also tend to polymerize under normal conditions of manufacture or storage. This polymerization can occur as a result of the presence of heat, light, oxygen or other conditions. Therefore, it is highly desirable to minimize or eliminate the polymerization tendency of methacrylic acid or its corresponding esters, such as methyl methacrylate, during production. The elimination of the polymerization tendency ensures that the vessel and tube system used to transport the material during the process is kept clean and the reactors are free from the accumulation of high viscosity, high molecular weight polymerized materials. Addition of polymerization inhibitors such as hydroquinone and certain derivatives of p-phenylene diamine or phenothiazine has become common in the preparation of methyl methacrylate (MMA) and other esters of unsaturated carboxylic acids.

It is known to use certain N, N'-substituted p-phenylene diamines, including special materials disclosed in Japanese Patent Application JP 49-43920, wherein N-sechexyl-N'-phenyl-p-phenylene diamine and N, N '-his (sec-heptyl) -p-phenylene diamine is used to stabilize unsaturated carboxylic acids or their esters, including ethyl acrylate, (2-ethylhexyl) acrylate, methyl acrylate, acrylic acid, methacrylic acid in the organic phase. Methyl methacrylate is neither mentioned nor tested nor appreciated for the importance of stabilizing the aqueous phase. This publication merely confirms that certain p-phenylene diamine derivatives exhibit good activity as a polymerization inhibitor in the pure liquid (organic) phase of the acid or its esters.

In spite of such potency of the inhibition characteristics of pure organic monomeric phases during laboratory scale testing, large scale production units have been severely affected globally by the inconvenient formation of polymerized ester or acid in various process vessels, decanters, distillation columns, distillation columns. This indicates that the polymerization inhibition was not satisfactory. This invention was made when it was discovered that esters (and their precursors) should be protected in both the organic and aqueous phases. This discovery of the key element of the problem, which is difficult to grasp, has led to a diligent search for an inhibitor system that protects both phases. The second problem recognition was that the protection of the aqueous phase with various p-phenylenediamine derivatives is strongly dependent on the pH of the aqueous phase. None of these phenomena have been recognized or reported so far.

The present invention relates to a stabilization package which is effective in both the organic and aqueous phases. It is also an object of the present invention to provide protection, even at low pH values, which are typical of production disturbances in which various inorganic or organic acids lower the pH of the aqueous phase to 4, 3 or even

2 away. The invention provides protection in both phases, thereby minimizing the formation of polymers in production rents, thus eliminating the frequent forced downtime of various units of the process due to the removal of polymerized residues and the improvement of plant performance.

The present invention relates to a process for inhibiting the polymerization of an ester of an unsaturated carboxylic acid in a manufacturing process comprising a stream comprising water and said ester and comprising an aqueous phase at a pH of about 1 to about 5 in which said ester has only a small amount. and an organic phase in which a greater amount of said ester is present; the method comprising adding to said process stream comprising said stream a stabilizer mixture comprising (a) an N, Ν'alkyl or phenyl substituted p-phenylene diamine having a solubility in said aqueous phase at pH 2 of 50; and (b) a second N, Ν'-alkyl- or phenyl-substituted p-phenylene diamine having a solubility of less than 20% at pH 2 in said aqueous phase.

The invention further relates to a process for the preparation of an ester of an unsaturated carboxylic acid comprising a plurality of streams of water containing water and said ester, wherein at least one of said streams comprises an aqueous phase comprising water and said ester and at least one of the streams said ester being substantially in the anhydrous organic phase; this manufacturing process inhibits the polymerization of said ester by an improved process by adding a stabilizing mixture comprising (a) an N, Ν'-substituted p-phenylene of formula (I) diamine, and (b) a second N, p'-substituted p-phenylene diamine of formula (II) wherein r? and is independently phenyl or

C5-C9 alkyl;

R 1 is C 1 -C 4 alkyl;

R ⁴ is phenyl or C 1-9 alkyl.

With respect to the structures of formulas I and II, it is preferred that:

R ¹ is phenyl;

η is C 5 -C 9 alkyl;

C 1-4 alkyl;

R ⁴ is phenyl.

According to a more preferred combination

R 4 and R ⁴ are C 3 -C 4 alkyl;

R ¹ is phenyl;

η is C 6 -C 7 alkyl.

As used herein, alkyl refers to a linear, branched or cyclic alkyl group.

The present invention further provides a stabilizing mixture consisting essentially of (a) a first N, Ν'-substituted p-phenylene diamine of formula (I); (b) a second N, Ν'-substituted p-phenylene diamine of the formula (II) and (c) an alcohol miscible with (a) and (b).

In the next discussion of the topic, we chose Methyl Methacrylate (MMA) for illustrative purposes only and it should be noted that any of several esters of unsaturated dicarboxylic acids may be replaced by appropriate modifications by one of ordinary skill in the art. Esters of acrylic acid and methacrylic acid are preferred substances to be stabilized by this method. Among the esters, alkyl esters such as methyl, ethyl, propyl, butyl, pentyl, hexyl, heptyl and octyl are preferred. Most preferred are methyl, ethyl and propyl, lower alkyl esters of acrylic acid and methacrylic acid. The most preferred ester, methyl methacrylate (hereinafter sometimes referred to as "MMA"), is provided by way of example only, and should not be construed as limiting the scope of the invention.

An industrial process for the production of methyl methacrylate is based on the basic raw materials of acetone, hydrogen cyanide, methanol and sulfuric acid. The acetone is reacted with hydrogen cyanide to give 2-hydroxy-2-methylpropionitrile.

The chemistry of the process involves the reaction of 2-hydroxy-2-methylpropionitrile with an excess of concentrated sulfuric acid to give methacrylamide sulfate. The methacrylamide sulfate stream is reacted with aqueous methanol to give methyl methacrylate in a combined hydrolytic esterification reaction.

Modifications of the esterification step include variations in the methods for recovering the crude ester and separating it for recycle methanol and methacrylic acid. In addition, the feed conditions for methanol and water relative to methacrylamide are changes in process conditions! ratios, reactor temperature and residence times.

In one embodiment, the methacrylamide sulfate stream, the excess aqueous methanol, and the recycled stream are continuously reacting in a series of vapor-jacketed esterification reactors at temperatures between 80 ° C and 100 ° C with a residence time of 2-4 hours. The reactor effluent was distilled through an acid evaporation column to give crude methyl methacrylate, methanol and water. The crude ester washed with water was purified in a multiple column distillation system and the aqueous methanol was distilled to recover the methanol for recycling. Calculated for ACN, the yield of methyl methacrylate is about 90%; based on methanol and depending on the reaction scheme and reaction conditions, 80 to 90%.

In another embodiment of the esterification step, the reaction is carried out under a pressure of 100 psig (790 kPa) at a temperature of 100 ° C to 150 ° C and a residence time of 1 hour depending on the reaction temperature. The product may be recovered as described above or the reactor effluent may be separated into an organic and a waste acid phase.

The low-boiling fractions were removed from the organic phase in a flash column. The crude ester is then washed with water or aqueous ammonia to remove methanol and some methacrylic acid; recirculating the aqueous raffinate to the esterification reactor. The washed crude ester was purified in a three column distillation system. In the first stage column, water and methanol are collected as head distillates and recycled to the esterification reactor. The final product is perceived as the product column head distillate. The product column bottom distillate was recovered to recover compounds for recirculation purposes; the rest is incinerated.

Table 1

Azeotropic mixtures with methyl methacrylate.

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

Table 2

Relative solubilities of methyl methacrylate and water.

Temperature

Methyl methacrylate in water

Water in methyl methacrylate

g / 100 g solvent g / 100 g solvent

0 1.85 0.85 10 1.72 0.99 20 1.59 1.15 30 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 recognition that methyl methacrylate (or any other ester) must be protected in the aqueous phase due to the high water solubility of MMA and the tendency to form azeotropic mixtures with water. Tables 1 and 2 show azeotropic mixture formation and solubilities. The next necessary step in carrying out the invention is to evaluate whether a particular stabilizer / inhibitor candidate is soluble in the aqueous phase at the desired pH levels. This is determined by the following testing.

TEST METHOD FOR TESTING SOLUBILITY IN WATER.

EXTRACTION TESTING

general description

Purpose: To determine the distribution of polymerization inhibitors between MMA and an aqueous solution at a given pH.

Method:

1. Prepare pure MMA standard samples containing approximately 1000 parts per million (ppm) of each inhibitor.

2. Prepare aqueous solutions of the following composition:

water 96.5 & Ό methanol 2.0 9- MMA 1.5 9 • Ό

3. Aliquots of the aqueous solutions are adjusted to the desired pH with sulfuric acid.

4. Extract 100 g of each MMA solution with 100 g of an aqueous solution of the desired pH.

(a) Place the solutions in a 453.6 g (16 oz) flask on a mechanical shaking table and shake slowly for 16 hours at low speed.

b) Allow the phases to separate. If an emulsion is formed, decompose it by gentle heating with a blowing gun (hot air rifle).

5. In each phase, the concentration of inhibitor is determined by an analytical assay for PPDA concentration.

ANALYTICAL TEST OF THE PPDA CONCENTRATION

The method is based on the oxidation / coupling reaction of quinone with p-phenylene diamine to give highly colored solutions. Concentrations are determined colorimetrically using a spectrophotometer operating in the visible range with 1 cm or 0.5 cm cuvettes. Standard solutions of p-Phenylenediamine and p-benzoquinone are prepared by accurately weighing about 0.1 of each compound and dissolving it in 100 ml of acetic acid.

Calibration is carried out as follows: (1) Prepare a blank solution containing only the reagents by pipetting 10 ml of MMA solution into a 50 ml volumetric flask. Add 25 ml of acetic acid, add 1 ml of p-benzoquinone solution and finally make up to 50 ml with distilled water; (2) Pipette 1, 2, and 3 ml of the appropriate p-phenylenediamine standard solution into 50 ml volumetric flasks; add 10 to 10 ml of the MMA solution (as in the blank containing reagents only) and enough acetic acid to make up to 25 ml of acetic acid in the flask, including the pipetted volume, and then quickly make up to 50 ml with distilled water. After shaking, record the spectrum of the solution between 5 and 15 minutes after the addition of water.

The test solutions are analyzed according to the calibration protocol, except that 10 ml of the test solution is measured instead of 10 ml of MMA solution. The number of micrograms of p-phenylene diamine in each sample is obtained from a calibration curve. The micrograms thus determined are converted to ppm of p-Phenylenediamine (ppm) by dividing by the mass of MMA (9.34 g / 10 ml) in 10 ml.

In the above, methyl methacrylate is given as a representative example only; other esters within the scope of the invention can be evaluated in a similar manner. When it comes to the protection of an ester other than MMA, the former must undergo appropriate changes due to weight differences.

TABLE OF INHIBITORS

Serial Number of Compound Composition

N- (1,4-dimethylpentyl) -? - phenyl-p-phenylene diamine and

Mixture of N- (1-methylpentyl) -Ν'-phenyl-p-phenylene diamine.

N- (1,4-dimethylpentyl) -Ν'-phenyl-p-phenylenediamine

N, N'-diisopropyl-p-phenylenediamine

N-isopropyl-N'-phenyl-p-phenylenediamine

N, N'-bis (1,4-dimethylpentyl) -p-phenylenediamine

The compounds listed in Table 3 have the formula: R-NH- (C ₆ H ₄ ) -NH-R '.

Table 3

AQUATIC EXTRACTION DATA at pH 2.

The compound % of the water number R R solutions % in MMA 1 Phenyl hexyl / heptyl 2.3 97.7 2 phenyl heptyl 2.3 97.7 3 isopropyl isopropyl 97.2 2.0 4 phenyl isopropyl 90.0 * 10.0 *THE aqueous phase contained methanol. Clearly visible the 3. table data, that the p- phenylene slide min derivatives distribution of some shifts the aqueous phase

at low pH values. For purposes of the present invention, to be considered as an inhibitor of the aqueous phase, the compound should be divided so that at least 10%, preferably 30%, most preferably 50%, is present in the aqueous phase at pH 2. measured by the test method. It can be seen that 3rd and

Compound # 4 is such. N, N'-substituted p-phenylene diamines having at least one lower (C 1 -C 4) alkyl group as substituents are preferred materials for the aqueous phase component of the inhibitor system of the invention. The p-Phenylenediamine derivative of the Nos. 3 and 4 containing the isopropyl substituent provides particularly enhanced protection for the MMA monomer present in the aqueous phase, especially under strongly acidic conditions. Such circumstances may be the normal circumstances of the proceedings or may be temporary in periods of disruption of production. The present invention provides an increase * · * ** 9 · * · * »♦ *« »* · *

Unprotected polymerization can lead to substantial undesirable polymerization under highly acidic conditions, resulting in device contamination, reduced heat exchange efficiency, and shutdown due to cleaning.

Compounds of the formula (I) do not meet the requirements of the water solubility test, while compounds of the formula (II) do not satisfy the test; these are considered soluble in the aqueous phase.

If a material selected for this purpose meets the water solubility specifications, it must also have appropriate polymerization inhibitory properties. It should be noted that these substances do not necessarily need to exhibit the best or optimal activity since the aqueous phase inhibitor is accompanied by an organic phase inhibitor which is a very active inhibitor. It should also be borne in mind that in the manufacturing process with a typical material stream, the aqueous phase actually has relatively few esters compared to the organic phase, so that the desired degree of protection is lower than in the organic phase (where substantially all MMA or other ester is present) . The following assay for determining the onset of polymerization is used to further evaluate candidate inhibitors of water solubility.

TEST FOR STARTING POLYMERIZATION

1. MMA is distilled to remove canning! to the end, any inhibitors it may contain.

2. Prepare 0.1% stock solutions of the inhibitor to be tested

f ** · ♦ »» «

We are in MMA.

3. Prepare stock solutions containing 10 parts per million (10 ppm) of the inhibitor to be tested and place them in test tubes.

4. Immerse the test tubes with the inner oil tube and thermocouple in an oil bath at a constant temperature of 80 ° C.

5. The first sign of the exothermic reaction read from a recorder is considered to be the onset of polymerization in hours.

Table 4

DATA ON START OF POLYMERIZATION

FOR METHYL METHACRYLATE AT 80 ° C

inhibitor

Time (in hours) (10 ppm)

1st measurement

Measurement 2

THE control Blind 4.75 4 7 5 B control phenothiazines 12.75 14.25 C control hydroquinone 19.25 19.50 1.sorszámú compound 83.50 103.00 Second 11 68.75 96,00 4th 11 136.50 143.50 5. II 60.00 42.00

The data in Table 4 show the excellent long-lasting polymerization inhibitory activity of the p-Phenylenediamine derivatives of Formulas I and II. Controls A, B, and C do not provide protection for more than 20 hours, alkyl-substituted p-phenylenediine derivatives show protection for 60 to 140 hours.

The present invention utilizes a stabilizer mixture comprising (a) an aqueous phase effective inhibitor which may not be presently represented exclusively by compounds of formula (I) and (b) an organic phase active inhibitor which is not currently only compounds having the structure represented by formula (II) may be represented. The stabilizer mixture may be present in any concentration effective to inhibit polymerization, preferably in the range of 50 to 10,000 parts per million (ppm) by weight of the ester to be stabilized. More preferably, it can be used in the concentration of 100 to 2000 ppm, most preferably 200 to 1000 ppm to stabilize the ester and its precursors in the process. The ratio of components (a) and (b) can vary within a wide range of 5: 95 to 95: 5; this ratio is preferably between 30:70 and 70:30, most preferably between 40:60 and 60:40. With respect to the structures of formulas I and II, it is preferred that R ¹ is phenyl, R ² is C 5 -C 9 alkyl, R ³ is C 1 -C 4 alkyl, and R ⁴ is phenyl. In a more preferred combination, R ³ and R ^{4 are C} 3 -C ⁴ alkyl, R ^{1 is} phenyl, and R ^{2 is C} 6 -C 7 alkyl. Throughout this patent and throughout the claims, the term alkyl refers to linear, branched and cyclic alkyl groups.

The invention is believed to be effective in the general application of (I) and (II) 4 * 4 ··· · · 4 * 4 ···· · ♦ »· ··· ·« · «4«

- also when using materials outside the structural limits represented by the formula 16, if any of the p-phenylene diamines used is soluble in the aqueous phase. Since compounds of this class have not been extensively evaluated, the following alternative formulation of the invention uses the above Solubility Test to assess the utility of the compounds in the present invention. The stabilizing mixture may be (a) an N, N'-alkyl- or phenyl-substituted p-phenylene diamine having a solubility in said aqueous phase of greater than 50% at pH 2; and (b) a second N, Ν'alkyl or phenyl-substituted p-phenylene diamine having a solubility in said aqueous phase of less than 20% pH.

Low-volume laboratory tests do not adequately demonstrate the variety of conditions in the various vessels and equipment of the esters of the invention, particularly a global chemical company producing methyl methacrylate. Therefore, a truly improved performance is indicated by subjecting a trial inhibitor pack to a long-term trial. In such a trial, it can be determined whether the inhibitor is actually capable of eliminating or at least minimizing the formation of polymerized MMA in the various vessels, pipelines, heat exchangers, etc. of the production. Such a test was carried out with a 50:50 mixture of the compounds represented by the formulas I and II. A production column that usually required a higher shutdown and cleaning every 2-3 months, thus operating for 9 months without the need for cleaning - thus demonstrating dramatically improved polymerization inhibition. For other departments

• · · ·

4 ·· ·· · ·

V · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · Longer intervals between the required cleaning. Where both the aqueous and organic phases are present, the present invention provides unexpectedly improved results over single N, N'-substituted p-phenylene diamine.

Another object of the present invention is to provide a novel and useful stabilizer package which is unknown to date. It employs an alcohol compatible with the ester production process and the inhibitor compounds to give a completely soluble liquid form, even if some of the compounds of the formula I are solids. Specifically, the patent discloses: a stabilizing mixture comprising (a) a first N, N'-substituted-p-phenylene diamine of formula (I); (b) a second N, Ν'-substituted p-phenylene diamine of formula II; and (c) an alcohol miscible with (a) and (b).

The alcohol may be present in any amount up to 75% by weight of both (a) and (b). Methanol is preferred for the preparation of methyl methacrylate. The ratio (a) to (b) of the stabilizer mixture may be within the range (5:95 to 95: 5) above.

In view of the many changes and modifications which can be made without departing from the principles of the invention, reference should be made to the appended claims in order to understand the scope of the invention.

Claims (15)

PATENT CLAIMS A process for inhibiting the polymerization of unsaturated carboxylic acid esters in a manufacturing process comprising a stream comprising water and said ester comprising an aqueous phase in which only a small amount of said ester is present and an organic phase in which said ester is larger characterized in that a stabilizing mixture is added to the production process comprising said stream, comprising (a) a first N, N'-substituted p-phenylenediamine of structure (I), and (b) A second N, N'-substituted p-phenylene diamine having the structure (II) wherein ^R1 and ^R2 independently represent phenyl, or C5-C9 alkyl; R ² is C 1 -C 4 alkyl; R1 is phenyl or C1-C9 alkyl. Process according to claim 1, characterized in that said unsaturated carboxylic acid is acrylic acid or methacrylic acid and said ester is a lower alkyl ester thereof. 3. The process of claim 2 wherein said unsaturated carboxylic acid is methacrylic acid and said ester is a methyl ester thereof. The process of claim 1, wherein the stabilizer mixture is present in an amount of about 50 to about 10,000 parts per million (ppm) based on the amount of ester. The process of claim 1, wherein the stabilizer mixture comprises component (a) and (b) in a ratio of from about 5:95 to about 95: 5. The process of claim 1, wherein the stabilizer mixture comprises component (a) and (b) in a ratio of about 30:70 to about 70:30. The process of claim 1, wherein the stabilizer mixture is present in an amount of about 200 to about 2000 parts per million (ppm) based on the amount of ester. The process of claim 7, wherein said stabilizer mixture comprises from about 100 to about 1000 parts per million (ppm) of component (a) and from about 100 to about 1000 parts per million (ppm) of component (a). (b). The process of claim 1, wherein R ¹ is phenyl and R ² is C 5 -C 9 alkyl. The process of claim 9, wherein R ³ is C 1 -C 4 alkyl and R ⁴ is phenyl. The process of claim 1, wherein R ³ and R ⁴ are C 3 -C 4 alkyl. The process of claim 9, wherein R ¹ is phenyl and R ² is C 6 -C 7 alkyl. 13. A process for inhibiting the polymerization of unsaturated carboxylic acid esters in a production process comprising a stream comprising water and said ester and comprising an aqueous phase having a pH of about 1 to about 5 in which only a small amount of said ester is present; an organic phase in which a greater amount of said ester is present, characterized in that a stream of stabilizer mixture is added to the production process comprising said stream comprising (a) an N, Ν'alkyl- or phenyl-substituted p-phenylene a diamine derivative having a solubility of greater than 30% at pH 2 in said aqueous phase; and (b) a second N, Ν'alkyl or phenyl substituted p-phenylene diamine derivative having a solubility of less than 20% at pH 2 in said aqueous phase. A stabilizing mixture consisting essentially of (a) a first N, N'-substituted p-phenylene diamine derivative having the structure represented by formula (I); (b) a second N, N'-substituted p-phenylene diamine derivative having the structure (II); and (c) an alcohol miscible with (a) and (b), wherein 1 7 R and R are independently phenyl or C5-C9 alkyl; R 1 is C 1 -C 4 alkyl; R ⁴ is phenyl or C 1-9 alkyl. An improved process for inhibiting the polymerization of an ester in a process for preparing an unsaturated carboxylic acid ester comprising a plurality of material streams, wherein at least one of said material streams comprises an aqueous phase comprising water and said ester, adding to the stream process a stabilizer mixture consisting of (a) a first N, ,'-substituted p-phenylene diamine derivative of the structure represented by the general formula (I), and (b) a compound of the formula (II). a second N, N'-substituted p-phenylene diamine derivative having the structure: ^R1 and ^R2 independently represent phenyl, or C5-C9 alkyl; R ² is C 1 -C 4 alkyl; R1 is phenyl or C1-C9 alkyl.