CA1319696C - Method for production of maleimides - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

A method for the production of a maleimide by dehydration and ring-closure of maleinamic acid obtained by the reaction of maleic anhydride with an amine, which method is characterized by effecting ring-closure imidation of said maleinamic acid by heating said maleinamic acid in a water-insoluble or water-immiscible inert organic solvent in the presence of a supported catalyst on a solid carrier an acid, an amine salt, or a mixture thereof.

Description

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METHOD FOR PRODUCTION OF MALEIMIDES
This invention relates to a method for the production of maleimides. More particularly, this invention relates to a method for the production of maleimides by ring-closure imidation of maleinamic acids.
Description of the Prior Art:
Maleimides are compounds useful as raw materials for synthetic resins, medicines, and agricultural chemicals. Researches after methods for their production have long been under way. The most popular method of them all effects the production of maleimides by the dehydration cyclization of maleinamic acids with a dehydrating agent such as acetic anhydride. One version of this method is disclosed in U.s. Patent No.

2,444,536. This method effects the production of maleimides by causing maleic anhydride to react upon amines thereby forming maleinamic acids and dehydration cyclizing and, at the same time, imidating the maleinamic acids in the presence of acetic anhydride and sodium acetate. ~his method, however, has the disadvantage that the imidation requires expensive acetlc anhydride to be used in at least an equivalent relative to the maleinamic acid and the separation and recovery of the formed maleimide from the imidation reaation aolution necessitates use of a large volume of water and, as the result, entails disposal of a large amount of an acetic acid-containing effluent at great expense. Thue, this method may well be called a too expensive method for commercial production of maleimides.
A method which has no use for such a chemical dehydration agent as acetic anhydride is disclosed in Briti~h Patent No. 1,041,027 and U.S. Patent No.

3,431,276. ~his method ef~ects the production of maleimides by thermally dehydrating and cyclizing maleinamic acids in conjunction with a solvent 8UC}l as, for example, toluene, xylene, or /

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chlorobenzene having a boiling point exceeding 80C and serving as a diluent and an acid catalyst such as sulfur trioxide, sulfuric acid, or ortho-phosphoric acid, and distilling the system thereby causing azeotropic expulsion of 5 the consequently formed water in conjunction with the solvent. AS compared with the method which uses acetic anhdyride, this method proves advantageous in that it does not require use of a large amount of such an expensive dehydrating agent as acetic anhydride and further that the 10 formed maleimides are separated and recovered with ease.
This method nevertheless has the disadvantage that the yield of the imidation is low as compared with that obtainable by the method using acetic anhdyride. This disadvantage is logically explained by a postulate that compared with the 15 method which effects the imidation by the use of acetic anhydride, the method which effects the imidation by performing thermal dehydration in the specific solvent as described above involves a high reaction temperature and, therefore, tends to induce side reactions and inevitably 20 manages to produce maleimides abounding with impurities and further that since maleimdies are thermally unstable, the maleimides produced at all are degenerated during the course of the reaction. Purther, as a commercial process, this method i5 not ecconomically satisfactory, because it requires 25 to use an expensive acid catalyst in a relatively large amount and, moreover, produces the maleimides in a low yield.
~ here is another method which, as disclosed in Japanese Patent Laid-Open SHO 53(1978)-68,700 and Japanese Patent Publication SHO 57(1978)-42,043, comprises causing 30 maleic anhydride to react on amines in the presence of an organic solvent thereby forming maleinamic acids and subjecting the maleinamic acids as held in a state not isolated from the reaction system to dehydration and cyclization in the presence of such an aprotic polar solvent 35 as dimethyl formamide or dimethyl sulfoxide and an acid catalyst. By this method, there is offered recognizable 1 3 1~

improvement in yield as compared with the second method described above. This method, however, has these disadvantages, that the cost of production of maleimides is high because expensive and highly toxic aprotic polar solvent 5 such as dimethyl formamide is used in a large amount, that the solvent such as dimethyl formamide is degenerated by the action of an acid catalyst used in the reaction and, therefore, the solvent is lost greatly, and that since the aprotic polar solvent used in the reaction has a high boiling 10 point, the solvent is removed from the produced malimides with great difficulty.
Japanese Patent Laid-Open SHO 54(1979)-30,155 discloses a method for producing an oligoimide by using, as a catalyst, a mixturé of an - inorganic or organic 15 acid-containing acid with a quaternary ammonium salt of the acid. The quaternary ammonium salt which is used as mixed with an acid catalyst in this method, however, is an ammonium salt of the nitrogen atom of which has been at least disubstituted. Specifically, this is an expensive interphase 20 catalyst such as dimethyldialkyl ammonium methane sulfonate or tetraoctyl ammonium methane sulfonate. The method, thus necessitating use of such a compound as indicated aobve, is inevitably judged to be an expensive approach. For this method to maintain a highly satisfactory yield of imidation, 25 however, it is essential that the reaction should be continued with the ratio of the acid catalyst to the quaternary ammonium salt rigidly controlled within a certain range. When the catalyst which has been used once in the reaction is used again, the imidation cannot be obtained in a 30 highly satisfactory yield because the ratio is varied in the presence of the used catalyst. An effort to attain efficient reuse of the used catalyst, therefore, entails as a problem the fact that the management for maintenance of catalytic activity as by subjecting the used catalyst to purifying and 35 readjusting treatments calls for immense labor.

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Japanese Patent Laid-Open SHO 60(1985)-109,562 discloses a method for the production of monomaleimide by the cyclizing imidation of maleinamic acid in a mixed solvent containing a nonpolar solvent such as toluene or xylene and a polar solvent such as dimethyl sulfoxide or N-methyl pyrrolidone in a specific ratio in the presence of an acid catalyst such as p-toluenesulfonic acid or m-toluenesulfonic acid and a mixed catalyst containing the acid catalyst and an ammonium salt such as, for example, the salt thereof with maleinamic acid. In this method, however, since the acid catalyst and the polar solvent in the mixed solvent react with each other to form a complicate complex (which is widely variable with the rat$o of the amounts of the two compounds and the temperature, for example), it is the complicate catalyst system composed of the complex just mentioned, the acid, and the salt that substantially produces a catalytic activity. Thus, the yield of the imidation is affected to a great extent by the composition of the three components mentioned above. When the reaction is carried out batchwise, it does not entail any appreciable disadvantage. When the reaction is carried out in ~uch operation system as require the catalyst and the solvent to be used in a recycling manner, however, it entails various drawbacks. To be specific, this method renders the selection of reaction conditions complicate because the amount of the complicate complex produced owing to the use of the polar solvent is varied and the catalyst is varied in quality from one batch to another. Thls is eguivalent to a statement that the method under discussion has the disadvantage that it is unfit for a continuous reaction.
Thls invention is directed towards the provision of an improved method for the production of maleimides of high purity in a high yield by a safe and simple 13~9~

procedure, which may be effected easily by a continuous reaction.
In accordance with the present invention, there is provided a method for the production of a maleimide by dehydration and ring-closure of maleinamic acid obtained by the reaction of maleic anhydride with an amine, which method is characterized by effecting ring-closure imidation of the maleinamic acid by heating the maleinamic acid in a water-insoluble or water-immiscible inert organic solvent in the presence of a catalyst supported on a solid carrier at least one catalyst inqredient selected from the group consisting of (a) an acid, (b) an amine salt produced from the amine and the acid, and (c) a mixture of the amine salt and the acid.
We have long been continuing a study on the reactions for synthesis of maleimides. Particularly, we have devoted our study to development of a catalyst for use in the ring-closure imidation reaction. The study has resulted in a finding that an aid, an amine salt produced by the neutralization of the amine used as the raw material for the maleimide with the acid or a mixture of the amine ~alt with an acid supported on a solid carrier manlfests a catalytic activity of unusually high selectivity on the cyclizing imidation reaction. This invention has been perfected as the result.
In the light of the long cherished theory that the presence o~ an oxygen-containing acid is indispensable to the reaction of ring-closure imidation, it is liter-ally an amazing fact that the use of the amine saltproduced by the neutralization brlngs about an unusually high catalytic activity in the reaction o~ ring-closure imidation.
The method for the production of an maleimide according to this invention resides in causing dehydra-tion and ring-closure imidation of a maleinamic acid 131969~
obtained by the reaction of maleic anhydride with an amine to be carried 6 13i9~9~
out in a water-insoluble or water-immiscible inactive organic solvent in the presence of a catalyst produced by supporting on a solid carrier (a) an acid (b) or the amine salt (c) or a mixture of the amine salt with the acid.
The maleinamic acids to be used in this invention are easily obtained generally by the reaction of primary amines with maleic anhydride. They are desired to be compounds represented by the following general formula I.
o CH -- ~ - OH
CH - C -- NH -- R (I) o wherein R denotes a member selected from the class consisting of alkyl of l to 20 carbon atoms, phenyl, benzyl, cyclohexyl, pyridyl, and quinolyl groups, and the same groups as mentioned above and possessed of halogen, carboxyl, or nitro substituents; providing that said alkyl groups or phenyl groups are more desirable than the other groups mentioned.
Examples of the primary amine particularly useful as the raw material for the maleinamic acid in this invention include methylamine, ethylamine, n-propylamine, isopropylamine, n-butylamine, sec-butylamine, isobutylamine, tert-butylamine, n-hexylamine, n-dodecylamine, allylamine, benzylamine, cyclohexylamine, aniline, nitroaniline, aminomono-chloroaniline, dichloroaniline, toluidines, xylidines, and ethylanilines.
Synthesis of a maleinamic acid proceeds virtually stoichiometrically. For example, the maleinamic acid can be synthesized by causing the amine in an amount of 0.8 to 1.5 mols, preferably 0.9 to 1.2 mols, to react upon each mol of maleic anhydride.

B' 131~6~6 The organic solvent to be used in the present invention is desired to be capable of permitting the water formed by the reaction of dehydration and cyclization to be expelled from the reaction system through azeotropic distillation therewith, insoluble or immiscible in water, inert, and incapable of partici-pating in the reaction. Examples of the organic solvent meeting this description are benzene, toluene, oil fractions boiling at temperatures in the range of 50~ to 120C, xylenes, ethyl benzene, isopropyl benzene, cumene, mesitylene, tert-butyl benzene, pseudo-cumene, trimethyl hexane, octane, tetrachloroethane, nonane, chlorobenzene, ethyl cyclohexane, oil fractions boiling at temperatures in the range of 120 to 170C, m-dicyclobenzene, sec-butyl benzene, p-dichlorobenzene, decane, p-cymene, o-dichlorobenzene, butyl benzene, decahydronaphthalene, tetrahydronaphthalene, dodecane, naphthalene, cyclohexyl benzene, and oil fractions boiling at temperatures in the range of 170~ to 250C.
From the standpoint of enabling this reaction to proceed smoothly under satisfactorily economic conditions, the amount o~ this solvent to be used in the reaction is in the range of 1 to 20 times, preferably 3 to 7 times (by volume), the amount of maleinamic acid.
Further, the solvent is selected on the condition that it should possess a boiling point suiting the prevalent reaction conditions in due consideration of the solubility of the maleimide, price, and ease of handling. When the separation of the maleimide and the solvent after completion of the reaction demands an important consideration, there are times when the reaction performed by the use of a solvent of a low boiling point under application o~ pressure may prove to be more advantageous.
As a catalyst, there i8 used an inorganic or organic monobasic or polybasic acid such as p-7 13l9~96 toluenesulfonic acid, orthophosphoric acid, metaphosphoric acid, pyrophosphoric acid, benzene sulfonic acid, or trichloroacetic acid. As the catalyst, there is used an amine salt which is obtained by subjecting the acid with an amine as a raw material for the production of maleimide.

~3~9~9g This amine salt is preferred to be such that at least one of the protons of the monobasic acid or polybasic acid is substituted with an amine.
As the catalyst, a mixture of the amine salt with 5 the inorganic or organic acid can also be used. This mixed catalyst produces desirable results when the amine salt content thereof at least exceeds 40 mol%, preferably falls in the range of 50 to 80 mol~.
Further the catalyst is obtained by supporting on a 10 solid carrier either the amine salt or a mixture of the amine salt just mentioned with the inorganic or organic acid can be effectively used. Furthermore, when the catalyst is supported on the solid carrier, the amine content may be zero percent. That is to say, sole acid catalyst may be used.
~he amount of the catalyst to be used falls in the range of 2 to 400 mol~, preferably 20 to 200 mol%, as an acid component contained in the catalyst, based on the amount of the maleinamic acid, wherein the acid component contained in the catalyst means both an acid component which constitutes 20 the amine salt and free acid.
Examples of the solid carrier to be used advantageously herein include natural minerals such as kaolins, clay, talc, chalk, quartz, bentonite, montmorillonite, and diatomaceous earth; synthetic minerals 25 such as highly dispersed silicic acid, alumina, silicates, activated carbon, gypsum, iron oxide red, titanium dioxide, silica, silica-alumina, and zirconium oxide; and natural rocks such as calcite, marble, pumice, sepiolite, and dolomite.
Such an inorganic carrier is used in the form of powder, in the form of granules obtained by pelletizing and classifying the relevant substance, or in the form of a honeycomb.
It is also permissible to use an organic carrier.
35 A granular carrier of polyfluorocarbon, polystyrene, or phenol resin can be effectively used. The catalysis is 1 31 ~

obtained with particularly desirable results when the carrier is made of such a porous substance as diatomaceous earth, silica gel or activated carbon. As typical examples of the cmmercially available carrier usable effectively herein 5 include a product of diatomaceous earth (marketed by Showa Chemical Industry Co., ~td. under trademark designation of "Radiolight") and products of silica gel (marketed by Fuji-Davison Chemical Co., Ltd. under trademark designations of "Carriact" "SYLOID," and "Microbead Silica Gel"), a 1~ product of silica gel (marketed by Wako-Junyaku Industry Co., Ltd. under trademark designation of "Wakogel"), and a product of activated carbon (marketed by Taiyo Kaken Co., Ltd. under trademark designation of "BAC".
Though the amount of the carrier-supported catalyst 15 to be used is variable with the physical properties of the carrier used therein, it is generally in the range of 0.5 to 500 ~ by weight, desirably 5 to 200 ~ by weight, and particularly desirably 10 to 100 % by weight, based on the amount of the carrier.
As means for supporting the catalyst on the carrier, any of the conventional methods such as the immersion method and the spray can be adopted. The catalyst may be supported directly on the carrier or it may be supported in an organic solvent or an aqeous solution.
When the amine salt obtained by the reaction of neutralization of the amine as the raw material with an acid is used as the catalyst either independently or in a form mixed with an acid, the acid catalyst may be supported first on the carrier and then caused to react with the amine or the 30 amine salt or the mixture of the amine salt with an acid may be prepared in advance and subsequently supported on the carrier. It is provided, however, that when the amine salt is used by itself, since the amine salt is solid at normal room temperature, it must be supported on the carrier in the 35 form of an agueous solution.

13~6~6 The amount of the catalyst o~ the foregoing description to be used is in the range of 2 to 400 mol%, preferably 20 to 200 mol%, based on the amount of the maleinamic acid to be contained as an acid component, part or S the whole of the acid destined to serve as the catalyst may be neutralized with an amine.
In the reaction of neutralization, the reaction may be carried out in the presence of a metal-containing compound and a stabilizer when occasion demands.
The catalyst produced as described above is insoluble in the organic solvent to be used in the present invention. In the reaction system, therefore, this catalyst assumes a state separated into the two layers, an organic layer and an layer. This state remains intact during and 15 after the reaction. Moreover, the catalyst remains substantially unchanged before and after the reaction. The catalyst system of this nature itself, therefore, can be utilzied in situ in the next cycle of reaction without being recovered and refined in the meantime.
When this catalyst layer is to be used in the next cycle of reaction, the organic layer and the catalyst layer exi~ting at the end of the reaction may be separated one from the other at a temperature in the range of 120 to 250C, and the catalyst layer consequently recovered may be put to use 25 directly in the next cycle of reaction.
As concerns the manner of use o the supported catalyst of the foregoing description, the catalyst may be used as added in the form of powdered catalyst to a stirring type reaction kettle or it may be used as granulated and 30 packed in the form of a fixed bed in a flow type reaction tube.
There are times when the reaction can be carried out, as disclosed in U.S. Patent No. 4,623,734, in the presence of a metal-containing compound and a stabilizer.
35 The metal-containing compound to be used in this case is selected from among oxides, acetates, maleates, succinates, 69~

nitrates, phosphates, chlorides, and sulfates of at least one metal selected from the group consisting of zinc, chromium, palladium, cobalt, nickel, iron, and aluminum. Among other compounds enumerated above, zinc acetate proves to be 5 particularly effective. The amount of the metal-containing compound to be used is in the range of 0.005 to 0.5 mol%, preferably 0.01 to 0.1 mol~, as metal, based on 1 mol of the maleinamic acid.
Examples of the stabilizer to be used 10 advantageously herein include methoxy benzoquinone, p-methoxyphenol, phenothiazine, hydro~uinone, alkylated diphenyl amines, methylene blue, tert-butyl catechol, tert-butyl hydroquinone, zinc dimethyldithiocarbamate, copper dimethyldithiocarbamate, copper dibutyldithiocarbamate, 15 copper salicylate, thiodipropionic esters, mercaptobenzimidazole, triphenyl phosphite, alkylphenols, and alkylbisphenols.
The stabilizer plays the part of enabling the maleimide which is produced by the reaction of imidation to 20 retain stably during the course of the imidation without being degenerated at the elevated temperature of the reaction.
Concerning the amount of the stabilizer to be added, the addition of the ~tabilizer in a minute amount is 25 not suffici0ntly effective and the addition thereof in an unduly large amount is undesirable because it entails the drawback that the excess of stabilizer finds its way into the final product. The amount of the stabilizer to be effectively used is in the range of 0.005 to 0.5 mol%, 30 preferably 0.05 to 0.3 mol%, based on 1 mol of the maleinamic acid.
Regarding the manner of working out the present invention, first maleic anhydride prepared as a solution in an organic solvent and an amine compound added thereto are 35 allowed to react with each other at a temperature not exceeding 150C, preferably falling in the range of 30 to 13~9~

120C, for a period of 15 to 120 minutes, preferabl~ 30 to 60 minutes to produce maleinamic acid. Then, the reaction system in which the maleinamic acid is left unisolated, the catalyst or the catalyst layer separated from the reaction 5 system of the preceding cycle of reaction, and optionally the metal-containing compound and/or the stabilizer are combined and heated at a temperature in the range of 120 to 250C, preferably 130 to 220C, for a period in the range of one hour to 15 hours, preferably 3 to 7 hours to effect the 10 reaction in a continuous pattern with the formed water expelled from the system through azeotropic distillation or to effect the reaction in a batchwise pattern with the expulsion of the formed water carried out at the end of the reaction. As the result, the maleimide is produced in a high 15 yield.
The maleimide which is consequently obtained is a compound represented by the general formula II, for example.

CH - C~ (II) CH - C /
o wherein R has the same meaning as defined above. Typical examples of the maleimides include N-methyl maleimide, 25 N-ethyl maleimide, N-n-propyl maleimide, N-isopropyl maleimide, N-n-butyl maleimide, N-sec-butyl maleimide, N-tert-butyl maleimide, N-n-hexyl maleimide, N-n-dodecyl maleimide, N-allyl maleimide, N-benzyl maleimide, N-cyclohexyl maleimide, N-phenyl maleimide, N-nitrophenyl 30 maleimide, N-hydroxyphenyl maleimide, N-methoxyphenyl maleimide, N-ethoxyphenyl maleimide, N-monochlorophenyl maleimide, N-dichlorophenyl maleimide, N-monomethylphenyl maleimide, N-dimethylphenyl maleimide, and N-ethylphenyl maleimide. Of course, the maleimides which this invention is 35 intended to embrace are not limited to the examples cited above.

1319~36 This invention which has been described above brings about the following advantages.
(1) This invention permits a maleimide of high purity to be produced in a high yield because the catalyst system produced by supporting on a solid carrier the amine salt obtained from the acid and the amine as the raw material and/or the acid possesses a catalytic activity of high selectivity on the reaction of ring-closure imidation.
(2) Since the catalyst is supported on the solid carrier and, therefore, the separation of the reaction solution and the catalyst can be easily effected, the reaction can be easily carried out in a continuous pattern and, what is more, the productivity of the process can be enhanced to a great extent.
(3) Since the catalyst is lost only nominally, the cost of the catalyst can be substantially disregarded.
As described in (1) through (3), this invention permits a maleimide to be easily produced inexpensively and safely.
Now, the present invention will be described more specifically below with re~erence to working examples.
Example 1 In a beaker having an inner volume of 200 ml, 20g of orthophosphoric acid was placed and then 30 g of diatomaceous earth (product of Showa Chemical Industry Co., Ltd. marketed under trademark designation of "Radiolight #200") was added thereto to effect deposition of the orthophosphoric acid on the diatomaceous earth.
A flask provided with a thermometer, a condenser incorporating therein a water separator, a dropping funnel, and a stirrer was charged with a solution of 55 g of maleic anhydride in 50 g of xylene. Then, the inner temperature of the flask was ad~usted to 80~C and a solution of 50 g of aniline in 400 g of xylene was .~

~3~9~

13a added piecemeal thereto over a period of 30 minutes, to synthesize a slurry solution of N-phenyl maleinamic acid in xylene.

B~

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The slurry solution thus obtained and the catalyst prepared in the beaker in advànce were left reacting at 140C
for three hours. After the reaction was completed, the reaction solution was separated from the catalyst layer. It 5 was then cooled to 80C, washed with water, and thereafter distilled under a vacuum to expel xylene and obtain 83 g of crystals of N-phenyl maleimide. The purity of the crystals, on analysis by liquid chromatography, was found to be 87.4 %
by weight. The yield of this product was 78.0 % based on 10 aniline used as aniline.
Example 2 The procedure of Example 1 was faithfully repeated, excepting 0.034 g of zinc acetate and 0.01 g of copper dibutyl dithiocarbamate were added instead during the course 15 of imidation. As the result, there were obtained 93 g of crystals of N-phenyl maleimide. The purity of the crystals, on analysis by liquid chromatography, was found to be 94.3 %
by weight. The yield thereof was 94.3 mol% based on aniline used as the raw material.
20 Example 3 In a Meyer's flask having an inner volume of 300 ml, 100 g of xylene and 20 g of orthophosphoric acid were dispersed Then, the same diatomaceous earth as used in Example 1 was introduced therein to effect deposition of the 25 orthophosphoric acid on the diatomaceous earth. Then it was reacted with 9.5g of aniline. The procedure of Example 2 was repeated, excepting the deposited catalyst mentioned above was used as a catalyst for the imidation. Consequently, there were obtained g2 g of crystals. The purity of the 30 crystals, on analysis by liquid chromatography, was found to be 98.5 % by weight. The yield thereof was 97~5 mol% based on aniline used as the raw material.
After the reaction was completed, in the reactor containing the used catalyst intact, a total of 20 cycles of 35 the imidation were carried out. The yield of reaction in the .

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20th cycle was 98.1 mol%. The catalyst after the 20th cycle of reaction showed the same a~tributes as the catalyst after the first cycle of reaction.
Control 1 The procedure of Example 1 was repeated, excepting the orthophosphoric acid was not deposited on diatomaceous earth. As the result, there were obtained 82 g of crystals of N-phenyl maleimide. The purity of the crystals, on analysis by liquid chromatography, was found to be 74.8 % by 10 weight. The yield of the product was 66.0 mol~, based on aniline used as the raw material.
Control 2 The procedure of Example 2 was repeated, excepting the orthophosphoric acid was not deposited on diatomaceous 15 earth. Consequently, there were obtained 85 g of crystals of N-phenyl maleimide. The purity of the crystals, on analysis by liquid chromatography, was found to be 90.2 % by weight.
The yield of this product was 80.5 mol% based on aniline used as the raw material.
20 Control 3 The procedure of Example 3 was repeated, excepting the orthophosphoric acid was not deposited on diatomaceous earth. Consequently, there were obtained 90 g of crystals of N-phenyl maleimide. The purity of the crystals, on analysis 25 by liquid chromatography, was found to be 91.7 % by weight.
The yield of this product was 88.8 mol% based on aniline used a8 the raw material.
Example 4 In a meyer's flask having an inner volume of 300 30 ml, 60 g of orthophosphoric acid was placed and stirred with g of a granular silica gel carrier (product of Fuji-Davison chemical Co., Ltd. mar]seted under trademark designation of "Carriact-30") to effect deposition of the acid on the carrier.

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Subsequently, 100 g of orthoxylene was added thereto and the Meyer's flask was kept cooled in a water bath and 37 g of cyclohexylamine was added dropwise thereto, to effect partial conversion of the carried acid into an amine salt.
Separately, a flask provided with a thermometer, a condenser incorporating therein a water separator, a dropping funnel, and a stirrer was charged first with 100 g of orthoxylene and then with loo g of maleic anhydride and heated to an inner temperature of 100C, to effect dissolution of the maleic anhydride.
Subsequently, a solution of 100 g of cyclo-hexylamine in 600 g of orthoxylene was added dropwise in a stirred state over a period of one hour, to synthesize a slurry solution of N-cyclohexyl maleinamic acid in orthoxylene.
Then, the slurry solution and the carried catalyst and 0.1 g of copper dibutyl dithiocarbamate added thereto were left reaction for seven hours by being heated and stirred at 143C, with the water formed by the reaction continuously expelled by distillation in combination with the oroxylene ~rom the reaction system in the meantime.
A~ter the reaction was completed, the reaction solution was analyzed for N-cyclohexyl maleimide by gas chromatography. Thus, the yield of N-cyclohexyl maleimide was found to be 97.8 mol% based on cyclohexyl amine used as the raw material.
Example 5 In a beaker having an inner volume of 500 ml, 100 g of sul~uric acid and 200 g of silica gel (product o~
Wako Junyaku marketed under trademark designation o~
"Wako Gel C100") were stirred to e~ect support o~ the acid on the silica gel. Separately, a reactor wa~
prepared by furnishing a glass flask having an inner volume of 1 liter with a thermometer, a stirrer, and a ~. ~ ' 13î~
16a water separator. Then, a solution of 53 g of powdered maleic anhydride in 50 g of ~ la;~J

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p-cymene was placed in the reactor. Subsequently, the inner temperature of the reactor -was adjusted to 130C and a solution of 40 g of n-butylamine in 400 g of p-cymene was added piecemeal thereto in a dropwise manner over a period of 5 30 minutes, to synthesize a solution of N-(n-butyl) maleinamic acid.
The solution thus obtained and the supported catalyst, 0.034 g of zinc acetate, and 0.065 g of p-methoxyphenol added thereto were left reacting at 180C, lO with the formed water continuously expelled in combination with the p-cymene from the reaction system in the meantime.
After the reaction was completed, the reaction solution was analyzed for the concentration of N-(n-butyl) maleimide. As the result, the yield of the product was found 15 to be 80.4 mol~O.

Claims (8)

1. A method for the production of a maleimide having the formula:

wherein R denotes a member selected from the class consisting of alkyl of 1 to 20 carbon atoms, phenyl, benzyl, cyclohexyl, pyridyl and quinolyl, unsubstituted or substituted by halogen, carboxyl or nitro substituents, by dehydration and ring-closure of a maleinamic acid of the formula:

wherein R is as defined above, obtained by the reaction of maleic anhydride with an amine, which method is characterized by effecting ring-closure imidation of said maleinamic acid by heating said maleinamic acid in a water-insoluble or water-immiscible inert organic solvent at a temperature in the range of 120° to 250°C
in the presence of a catalyst supported on a solid carrier at least one catalyst ingredient selected from the group consisting of (a) an acid, (b) an amine salt produced from said amine and said acid, and (c) a mixture of said amine salt and said acid, wherein the amount of said solvent to be used is in the range of 1 to 20 times by volume, based on the amount of said maleinamic acid, and the amount of said catalyst as an acid component contained in said catalyst is in the range of 2 to 400 mol% as acid, based on the amount of said maleinamic acid.

2. A method according to claim 1, wherein the amount of said catalyst ingredient to be supported is in the range of 0.5 to 500 % by weight based on said carrier.

3. A method according to claim 1, wherein the said solid carrier is at least one member selected from the group consisting of diatomaceous earth, silica gel, and activated carbon.

4. A method according to claim 1, wherein the imidation of said maleinamic acid by dehydration and ring-closure is carried out in the presence of a metal-containing compound.

5. A method according to claim 4, wherein said metal-containing compound is a zinc-containing compound.

6. A method according to claim 4, wherein said metal-containing compound is an acetate.

7. A method according to claim 1, wherein said catalyst ingredient is said acid.

8. A method according to claim 1, wherein said catalyst ingredient is at least one member selected from the group consisting of said amine salt and said mixture of said amine salt and said acid.