Maleimide compounds are compounds useful as resin raw materials, raw materials for medicines and agricultural chemicals, and particularly, used as one of copolymerization components for improving the heat resistance of styrene-based resins such as ABS resins, AS resins, AB resins, ACS resins, AES resins and AAS resins, polyvinyl chloride resins, polymethyl methacrylate resins, phenol resins, or the like. Among them, N-phenyl maleimides (hereinafter, will be also referred to as PMI) are excellent regarding reactivity or heat resistance and are particularly widely used.
As the methods for preparing maleimide compounds, various methods are known since before, such as 1) a producing method via dehydration reaction of maleic anhydride (hereinafter, will be also referred to as MAH) and primary amines in one-step, 2) a producing method by producing maleamic acids from maleic anhydrides and primary amines and performing the dehydration ring-closure imidization reaction of the maleamic acids, and 3) a producing method via the ring-closure imidization reaction of corresponding maleamic acid monoesters.
Among the methods, in the 1) producing method from maleic anhydride and primary amines in one-step, defects of low productivity due to still low yields arise, and in the 3) producing method from maleamic acid monoesters, defects of remaining and inclusion of alcohols produced by the ring-closure imidization reaction in products arise. Accordingly, industrially, the 2) producing method via the dehydration ring-closure imidization reaction of maleamic acids is generally conducted.
Meanwhile, the primary amine during preparing N-phenyl maleimides is aniline (hereinafter, will be also referred to as ANL), and the maleamic acid is N-phenyl maleamic acid (hereinafter, will be also referred to as PMA).
Most of the catalysts used in the conventional 2) synthesis method of N-phenyl maleimides directly use a liquid phase catalyst which is a homogeneous system, or a catalyst carrying an active component with a homogeneous system in a carrier. However, in this case, the active component may be lost due to water which is a by-product produced during the synthesizing N-phenyl maleimides, and defects of difficult supplement and regeneration of the lost active component may arise. Also, complicated problems may arise regarding the separation process of catalysts including the limitation of solvents used for separating catalysts, the generation of a large amount of waste water in the removing process of remaining catalysts and impurities, and requirements on energy input due to the execution of cooling and heating.
Therefore, the present inventors studied to solve the above-described defects and found that the above-described defects could be solved by using a zirconium(IV) hydrogen phosphate which is a solid acid catalyst in the synthesis method of N-phenylmaleimides of 2) and completed the present invention.