Process for purifying methacrylate

Abstract: A process for purifying a methacrylate is herein disclosed which comprises treating the colored methacrylate with an aqueous alkali solution such as an aqueous sodium hydroxide solution, and then separating and collecting the methacrylate from the thus treated solution. According to this process, coloring substances can easily be removed, and as a result, the high-quality methacrylate can efficiently be obtained.

Description: 1. Field of the Invention The present invention relates to a process for purifying a methacrylate which can efficiently, economically and advantageously remove coloring substances, particularly impurities such as biacetyl from the methacrylate. The methacrylates have been widely used as optical materials for organic glasses, optical fibers, various coating materials, contact lens materials and the like. 2. Description of the Related Art As industrial techniques for manufacturing methyl methacrylate, there are known an acetone cyanohydrin method (an ACH method) which involves the production of ammonium sulfate as a by-product, an improved ACH method which does not involve the production of ammonium sulfate, a C 4 oxidation method and the like. Methacrylic acid or methyl methacrylate obtained by any of these methods can be subjected to an esterification reaction with an alcohol or an ester exchange reaction to prepare a desired methacrylate. The methacrylate obtained by this method contains coloring substances in such an amount as not to be negligible, which coloring substances are formed when the methacrylate is exposed to a strong oxidizing agent and a high temperature in steps of its preparation. The presence of the coloring substances has a large influence on the commercial value of the methacrylate, depending upon its use. Particularly in the case that the methacrylate is used for an optical fiber or the like, there occurs a serious problem that the presence of the coloring substances impairs the transmittance of light having a specific wavelength. As techniques for removing the coloring substances from the methacrylate, some methods are known. For example, they are a method which comprises separating the coloring substances by distillation at a high reflux ratio in a high multi-stage tower, a method which comprises an adsorption treatment by the use of active carbon, a method which comprises passing the methacrylate on a solid adsorbent such as alumina (Japanese Patent Publication No. 18964/1985), and a method which comprises treating the methacrylate with an aromatic diamine or the like (EP 206, 230). Prior art document US-A-3,576,848 discloses the purification of products prepared by the oxidative dehydrogenation reaction catalysed by hydrogen iodide. The products obtained according to the reaction contained coloured contaminations as well as iodine-containing impurities. Purification is performed by washing with diluted caustic solutions, in particular diluted solutions of sodium hydroxide or potassium hydroxide. The prior art document does not offer any specific details to the chemical nature of the contaminations but it is a well-known fact, that in this type of reaction no biacetyl is formed. The present inventors have investigated these known methods for removing the coloring substances from various angles. For example, when the above-mentioned method using the distillation is used in order to completely remove biacetyl having a low specific volatility which is the coloring substance from methyl methacrylate, there is a problem that enormous energy is required. In sufficiently removing the coloring substances by the use of the active carbon, a huge amount of the active carbon is necessary owing to an adsorption/desorption equilibrium, and there is a problem that a considerable cost for the recovery of the used active carbon is additionally necessary. The method which comprises passing the methacrylate on alumina also has a drawback that a huge amount of alumina is disadvantageously required for the treatment of the coloring substances. In addition, in the method utilizing an aromatic diamine, a large excess of the reagent is required for the treatment of the coloring substances, and so this method is not considered to be practical. SUMMARY OF THE INVENTION An object of the present invention is to provide a process for easily obtaining a high-quality methacrylate which is scarcely colored. Another object of the present invention is to provide a process for advantageously preparing a high-quality methacrylate on an industrial scale. The present inventors have paid much attention to a fact that most of coloring substances with which a methacrylate is colored are compounds each having a carbonyl group, particularly diketones and unsaturated ketones each having a quinoline structure in which a conjugated double bond is present. Thus, it has been found that the coloring substances can be easily removed from the methacrylate phase by reacting these compounds each having the carbonyl group with an aqueous solution of an alkali metal compound to form water-soluble compounds. The present invention has been completed on the basis of this knowledge . According to the present invention, there is provided a process for purifying a methacrylate which comprises the steps of treating the methacrylate colored by biacetyl with an aqueous alkali solution, and then separating and collecting the methacrylate from the thus treated solution. DESCRIPTION OF PREFERRED EMBODIMENTS A process of the present invention is, as described above, characterized by treating a colored methacrylate with an aqueous alkali solution. This process includes the following preferable embodiments (1) and (2). (1) A process which comprises the steps of treating the colored methacrylate with an aqueous alkali solution, particularly an aqueous solution of an alkali metal compound, and then separating and collecting the methacrylate from the thus treated solution (Process 1). (2) A process which comprises the steps of treating the colored methacrylate with an aqueous alkali solution, further treating the same with a reducing agent or a sulfite, and then separating and collecting the methacrylate from the thus treated solution (Process 2). The process of the present invention is directed to a process for purifying a methacrylate colored with trace amounts of biacetyl, whereby the trace amounts of biacetyl alone are chemically converted, substantially without losing the methacrylate, into substances having largely different physical properties which can easily be separated by a usual distillation operation, thereby obtaining the substantially biacetyl free methacrylate. Using the aqueous alkali solution, the coloring substances alone are reacted with the alkali to be easily converted into separable removable water-soluble compounds, thereby obtaining the substantially coloring substances-free methacrylate. As the aqueous alkali solution which is used in the present invention, aqueous solutions of alkali metal compounds are particularly preferable. These alkali metal compounds can suitably be used in conformations such as hydroxides, oxides, carbonates and silicates. Above all, alkali hydroxides are most recommendable by which an excellent purification effect can be obtained and which are excellent in handling, safety, availability and economy, and among the alkali hydroxides, sodium hydroxide and potassium hydroxide are particularly preferable. The amount of the alkali to be used in the present invention is preferably in the range of 0.1 to 50 mols, more preferably 1.0 to 20 mols in terms of an alkali metal per mol of the coloring substances each having the carbonyl group. If the amount of the alkali is less than this range, the purification effect is not always sufficient, and if it is more than the above-mentioned range, the manufacturing cost increases in vain and a disadvantage such as polymerization might occur. The temperature of the treatment with the aqueous alkali solution in the process of the present invention is usually in the range of 0 to 100°C, preferably from ordinary temperature to 50°C. If the temperature is lower than this range, the purification effect is poor sometimes, and if it is more than the above-mentioned range, a side reaction such as polymerization might be brought about. In the process of the present invention, another treatment with a reducing agent or a sulfite may be carried out together with the treatment with the aqueous alkali solution. In this case, no particular restriction is put on the order of these treatments, but the treatment with the reducing agent or the sulfite is preferably done after the treatment with the aqueous alkali solution. As the reducing agent which can be used herein, various kinds of metal hydrogen complex compounds are suitably used, but sodium borohydride is particularly preferable from the viewpoints of safety, easy handling, easy availability and economy. This reducing agent can suitably be used in the form of an aqueous alkali solution, a powder or tablets. No particular restriction is put on the amount of the reducing agent to be used, but this amount is preferably in the range of 0.1 to 50 mols, more preferably 0.5 to 5 mols per mol of the coloring substances each having the carbonyl group. The temperature of the treatment with the reducing agent is usually in the range of 0 to 100°C, preferably from ordinary temperature to 50°C. Examples of the usable sulfite include various kinds of sulfites and hydrogensulfites, but sodium sulfite and sodium hydrogensulfite are most preferable from the viewpoints of safety, easy handling, easy availability and economy. No particular restriction is put on the amount of the sulfite to be used, but this amount is preferably in the range of 0.5 to 50 mols, more preferably 1.0 to 5 mols per mol of the coloring substances each having the carbonyl group. In addition, the temperature of the treatment with the sulfite is usually in the range of 0 to 100°C, preferably from ordinary temperature to 50°C. No particular restriction is put on the preparation method of the methacrylate, typically, methyl methacrylate, ethyl methacrylate, propyl methacrylate or butyl methacrylate to which the process of the present invention can be applied, and the methacrylate contains coloring substances each having the carbonyl group such as biacetyl. According to the process of the present invention, the coloring substances can be converted into such substances as to be easily separable by phase separation or distillation, whereby the coloring substances can be effectively removed. In consequence, the high-quality methacrylate can easily be obtained. Thus, it is apparent that the process of the present invention has an extremely large industrial merit. Next, the present invention will be described in more detail with reference to examples and comparative example, but the scope of the present invention should not be limited to these examples. Example 1 In a flask equipped with a reflux condenser and a stirrer were placed 100 g of methyl methacrylate containing 0.2% by weight of biacetyl. This solution was colored yellow, and an APHA value representing a color index in comparison with a standard solution was 500 or more. Next, 50 g of a 1.4% by weight aqueous sodium hydroxide solution was added to the solution, followed by stirring at room temperature for 10 minutes. The thus treated solution was divided into a methyl methacrylate phase and an aqueous phase, and the methyl methacrylate phase was then analyzed by GC. As a result, the content of biacetyl decreased to 0.011% by weight. The APHA value of the methyl methacrylate phase obtained herein was 10, but when this phase was distilled, the APHA value of the resultant methyl methacrylate was 5 or less which was a value enough to denote high quality. Example 2 The same procedure as in Example 1 was carried out except that 50 g of a 1.4% by weight aqueous sodium hydroxide solution was replaced with 50 g of a 2.0% by weight aqueous potassium hydroxide solution. As a result, the content of biacetyl in a methyl methacrylate phase was 0.010% by weight. When this phase was distilled, the APHA value of the resultant methyl methacrylate was 5 or less which was a value enough to denote high quality. Example 3 The same procedure as in Example 1 was carried out except that methyl methacrylate was replaced with butyl methacrylate. As a result, the content of biacetyl in a butyl methacrylate phase was 0.015% by weight. When this phase was distilled, the APHA value of the resultant methyl methacrylate was 5 or less which was a value enough to denote high quality. Comparative Example 1 The same procedure as in Example 1 was carried out except that water alone was added in place of an aqueous sodium hydroxide solution. As a result, 0.18% by weight of biacetyl remained in a methyl methacrylate phase, and its APHA was 500 or more. Therefore, a purification effect was judged to be scarcely present.

Claims: A method for reducing biacetyl in methacrylate which comprises the steps of treating the methacrylate colored by biacetyl with an aqueous alkali solution, and then separating and collecting the methacrylate from the thus treated solution. The method according to Claim 1 wherein the aqueous alkali solution is an aqueous solution of an alkali metal compound. The method according to Claim 2 wherein the aqueous solution of the alkali metal compound is an aqueous solution of sodium hydroxide or potassium hydroxide. The method according to claim 1, which comprises the steps of treating the methacrylate colored by biacetyl with an aqueous alkali solution, further treating the same with a reducing agent or a sulfite, and then separating and collecting the methacrylate from the thus treated solution.

Inventors: ABE TAKAFUMI, NAITHO SUSUMU, NISHIKAWA MITSUHIRO, ABE, TAKAFUMI, NAITHO, SUSUMU, NISHIKAWA, MITSUHIRO, ABE, TAKAFUMI, C/O MITSUBISHI GAS CHEM. CO. INC., Naitho, Susumu, c/o Mitsubishi Gas Chem. Co. Inc., Nishikawa, Mitsuhiro c/o Mitsubishi Gas Chem. Co.
Applicants: MITSUBISHI GAS CHEMICAL CO, MITSUBISHI GAS CHEMICAL COMPANY, INC.
Application year: 1995
Main classification: C07C67
IPC codes: C07C67