Synthesis process of methyl methacrylate (MMA)

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Currently, in an industrialized world, the MMA production technologies in use mainly include the acetoncyanohydrin method (ACH method), the isobutylene method (including the traditional i-C4 method, the tert-butanol method, and the ASAHI method), the ethylene method (BASF method), and the improved acetoncyanohydrin method (MGC method). 2.1 Acetonitrile hydroxyl method (ACH method): This process starts with acetonitrile and hydrocyanic acid (or pre-made ACH) as raw materials, followed by dehydration, hydrolysis, and esterification to produce MMA; it is known as the ACH process. A large amount of the highly toxic substance HCN must be used in the production process, and 1.2 tons of sulfuric acid an byproduct are generated for every ton of MMA produced, posing difficulties in handling. However, the ACH process accounts for about 80% of global MMA production at present. This process still requires further improvement, especially in the dehydration/hydrolysis step. The reaction is shown in Equation (1). Reaction equation (1) 2.2 Isobutylene method: This process involves two steps of gas-phase oxidation of isobutylene (or tert-butanol) to produce methacrylic acid, followed by esterification to yield MMA. This process is called the i-C4 route. Industrial production facilities already exist in the Far East. The reaction is shown in Equation (2). CH3 CH3 | | 2CH2=C-CH3 + 3O2 → 2CH2=CCOOH + 2H2O ↓ CH3OH CH3 | CH2=CCOOCH2 Reaction (2) A new process has also been developed for the gas-phase oxidation of isobutylene to methacraldehyde. The resulting liquid methacraldehyde is first mixed with methanol, and then oxidized in the liquid phase using air as a catalyst along with Pd/Pb to produce methacrylic acid, while esterification occurs to yield MMA. This process is called the Asahi Kasei direct method or the Asahi (D) route. Recently, Asahi Kasei has put the Asahi (D) process into production in Japan, replacing its other unique technology based on acrylonitrile. 2.3 Ethylene carbonylation method: In this approach, ethylene is first subjected to carbonyl synthesis (aldolation) to produce propionaldehyde, which is then condensed with formaldehyde to yield methacraldehyde; subsequently, oxidation and esterification are carried out to obtain MMA. Since BASF was the first and only company to use this route, this process is also known as the BASF route. The reaction is shown in Equation (3). There is a factory in Ludwigshafen, Germany, that uses the BASF process to produce MMA on an industrial scale. The drawback of this route is the presence of the intermediate methylacraldehyde in the production process, and the oxidation of methylacraldehyde is very costly. H2C=CH2+CO+H2→CH3CH2CHO ↓+H2CO CH2=C-COOH←(O2)CH2=C-CHO | | CH3 CH3 ↓CH3OH CH2=C-COOCH3 | CH3 Reaction (3) 2.4 Improvement of the propiononitrile method (MGC method): Mitsubishi Gas Company has developed a recyclable ACH route. In this route, ACH is first produced using propionone and hydrocyanic acid as raw materials by conventional methods; thereafter, ACH is hydrolyzed to α-hydroxyisobutyramide. This α-hydroxyisobutyramide then reacts with carbon monoxide and methanol under pressure to produce formamide and methyl α-hydroxyisobutyrate. Dehydration of methyl-α-hydroxyisobutyrate yields MMA, while the by-product formamide can be dehydrated to produce HCN, which can then be recycled. This process is called the MGC(R-HNC) route. The reaction is shown in Equation (4). Japan has already established an industrialized facility. 2.5 Summary At present, there are only the four methods mentioned above for producing MMA worldwide. Although new processes for MMA production have been developed, they are all aimed at industrial-scale application; the reasonable economic scale for MMA production is above 40,000 tons per year. According to publicly available foreign data, the investment cost for medium-scale plants (40,000–60,000 t/year) using the isobutylene method is lower than that of the propylene cyanohydrin method, resulting in better economic benefits ； The advantages of the propyl cyanohydrin method will become apparent in larger-scale plants (100,000 t/year and above), as the capital investment per unit will decrease significantly, giving it strong competitiveness; however, this will be limited by the availability of hydrogen cyanide as a raw material.