Method for production of maleimides

Claim 1

A method for the production of maleimides, which comprises subjecting maleinamic acids to ring-closure imidation in an organic solvent capable of forming an azeotrope with water in the presence of an acid catalyst at a temperature in the range of 120° to 250° C. while removing the formed water in the form of an azeotrope with said organic solvent and thereafter purifying the produced maleimides, wherein said purification comprises adding to crude maleimides an organic acid or inorganic acid selected from the group consisting of ortho-phosphoric acid, pyrophosphoric acid, sulfuric acid, sulfuric acid anhydride, fuming sulfuric scid, methane sulfonic acid, chlorosulfonic acid, chloromaleic acid, para-toluenesulfonic acid and trifluoroacetic acid in an amount of from about 5 to about 30% by weight based on the maleinamic acids used as the raw material, heating the resultant mixture at a temperature in the range of 20° to 100° C., removing the resinous impurities formed, separating the organic product containing phase from the aqueous acid phase and washing the resultant organic layer with a sufficient amount of water to remove the water-soluble impurities from the organic layer and removing the solvent portion of the organic phase under reduced pressure to yield essentially pure maleimide.

Claim 2

A method for the production of maleimides, which comprises subjecting maleinamic acids to ring-closure imidation in an organic solvent capable of forming an azeotrope with water in the presence of an acid catalyst at a temperature in the range of 120° to 250° C. while removing the formed water in the form of an azeotrope with said organic solvent and thereafter purifying the produced maleimides, wherein said purification comprises adding to crude maleimides an organic acid or inorganic acid selected from the group consisting of ortho-phosphoric acid, pyrophosphoric acid, sulfuric acid, sulfuric acid anhydride, fuming sulfuric scid, methane sulfonic acid, chlorosulfonic acid, chloromaleic acid, para-toluenesulfonic acid and trifluoroacetic acid in an amount of from about 5 to about 30% by weight based on the maleinamic acids used as the raw material, heating the resultant mixture at a temperature in the range of 20° to 100° C., removing the resinous impurities formed, separating the organic product containing phase from the aqueous acid phase and washing the resultant organic layer with a sufficient amount of water to remove the water-soluble impurities from the organic layer and removing the solvent portion of the organic phase under reduced pressure to yield essentially pure maleimide.