Methyl methacrylate (MMA) is an organic compound with the formula CH2=C(CH3)COOCH3. This colorless liquid, the methyl ester of methacrylic acid (MAA), is a monomer produced on a large scale for the production of poly(methyl methacrylate) (PMMA).
History MMA was discovered by Bernhard Tollens and his student W. A. Caspary in 1873, who noticed and described its tendency to change into a clear, hard, transparent substance especially in sunlight. Studies on acrylic esters slowly developed until Staudinger's theory of macromolecules and his research into the nature of polyacrylates allowed control over polymerization. The company Rohm and Haas, founded by German chemist Otto Röhm, who investigated the topic for three decades, was finally able to start its industrial production in 1931.
Production and properties Given the scale of production, many methods have been developed starting from diverse two- to four-carbon precursors. Two principal routes appear to be commonly practiced.
Cyanohydrin route The principal route begins with the condensation of acetone and hydrogen cyanide:
(CH3)2CO + HCN → (CH3)2C(OH)CN Sulfuric acid then hydrolyzes acetone cyanohydrin (ACH) to a sulfate ester-adduct, which is cracked to the ester:
(CH3)2C(OH)CN + 2H2SO4 → ((CH3)2C(OSO3H)C(O)NH2·H2SO4 → (CH3)2C(OSO3H)C(O)NH2 + H2SO4 Methanolysis gives ammonium bisulfate and MMA:
(CH3)2C(OSO3H)C(O)NH2 + CH3OH → CH2=C(CH3)C(O)OCH3 + NH4HSO4 Laboratory scale procedures are available for some of these steps. This technology affords more than 3 billion kilograms per year, and the economics have been optimized. Nevertheless, the ACH route coproduces substantial amounts of ammonium bisulfate: roughly 1.1 kg/(kg MMA). The ammonium bisulfate can be converted to ammonium sulfate, which is a common fruit tree fertilizer.
Methyl propionate routes The first stage involves carboalkoxylation of ethylene to produce methyl propionate (MeP):
C2H4 + CO + CH3OH → CH3CH2CO2CH3 The MeP synthesis is conducted in a continuous-stirred tank reactor at moderate temperature and pressure using proprietary agitation and gas-liquid mixing arrangement. In a second set of reactions, MeP is condensed with formaldehyde in a single heterogeneous reaction step to form MMA:
CH3CH2CO2CH3 + CH2O → CH3(CH2)CCO2CH3 + H2O The reaction of MeP and formaldehyde takes place over a fixed bed of catalyst. This catalyst, caesium oxide on silica, achieves good selectivity to MMA from MeP. The formation of a small amount of heavy, relatively involatile compounds poisons the catalyst. The coke is easily removed and catalyst activity and selectivity restored by controlled, in-situ regeneration. The reactor product stream is separated in a primary distillation so that a crude MMA product stream, free from water, MeP and formaldehyde, is produced. Unreacted MeP and water are recycled via the formaldehyde dehydration process. MMA (>99.9%) is purified by vacuum distillations. The separated streams are returned to the process; there being only a small heavy ester purge stream, which is disposed of in a thermal oxidizer with heat recovered for use in the process.
Other routes to MMA
Via propionaldehyde Ethylene is first hydroformylated to give propanal, which is then condensed with formaldehyde to produce methacrolein. The condensation is catalyzed by a secondary amine. Air oxidation of methacrolein to methacrylic acid completes the synthesis of the acid:
CH3CH2CHO + HCHO → CH2=C(CH3)CHO + H2O CH2=C(CH3)CHO + ½ O2 → CH2=C(CH3)CO2H
From isobutyric acid As developed by Atochem and Röhm, isobutyric acid is produced by hydrocarboxylation of propene, using HF as a catalyst:
CH2=CHCH3 + CO + H2O → (CH3)2CHCO2H Oxidative dehydrogenation of the isobutyric acid yields methacrylic acid. Metal oxides catalyse this process:
(CH3)2CHCO2H + O → CH2=C(CH3)CO2H + H2O
Methyl acetylene (propyne) process Using Reppe chemistry, methyl acetylene is converted to MMA. As developed by Shell, this process produces MMA in one step reaction with 99% yield with a catalyst system derived from palladium acetate, an organophosphine, and Bronsted acids:
CH≡CCH3 + CO + CH3OH → CH2=C(CH3)CO2CH3
Isobutylene routes The reactions by the direct oxidation method consist of two-step oxidation of isobutylene or TBA with air to produce methacrylic acid and esterification by methanol to produce MMA.
CH2=C(CH3)2 or (CH3)3C−OH + O2 → CH2=C(CH3)−CHO + H2O CH2=C(CH3)CHO + ½ O2 → CH2=C(CH3)CO2H CH2=C(CH3)CO2H + CH3OH → CH2=C(CH3)CO2CH3 + H2O A process using isobutylene as a raw material has been commercialized by Escambia Co. Isobutylene is oxidized to provide α-hydroxy isobutyric acid. The conversion uses N2O4 and nitric acid at 5–10 °C in the liquid phase. After esterification and dehydration MMA is obtained. Challenges with this route, aside from yield, involve the handling of large amounts of nitric acid and NOx. This method was discontinued in 1965 after an explosion at an operation plant.
Methacrylonitrile (MAN) process MAN can be produced by ammoxidation from isobutylene:
(CH3)2C=CH2 + NH3 + 3/2 O2 → CH2=C(CH3)CN + 3 H2O This step is analogous to the industrial route to acrylonitrile, a related commodity chemical. MAN can be hydrated by sulfuric acid to methacrylamide:
CH2=C(CH3)CN + H2SO4 + H2O → CH2=C(CH3)−CONH2·H2SO4 CH2=C(CH3)−CONH2·H2SO4 + CH3OH → CH2=C(CH3)COOCH3 + NH4HSO4 Mitsubishi Gas Chemicals proposed that MAN can be hydrated to methacrylamide without using sulfuric acid and is then esterified to obtain MMA by methylformate.
CH2=C(CH3)CN + H2O → CH2=C(CH3)−CONH2 CH2=C(CH3)−CONH2 + HCOOCH3 → CH2=C(CH3)COOCH3 + HCONH2 HCONH2 → NH3 + CO
Esterification of methacrolein Asahi Chemical developed a process based on direct oxidative esterification of methacrolein, which does not produce by-products such as ammonium bisulfate. The raw material is tert-butanol, as in the direct oxidation method. In the first step, methacrolein is produced in the same way as in the direct oxidation process by gas phase catalytic oxidation, is simultaneously oxidized and is esterified in liquid methanol to get MMA directly.
CH2=C(CH3)−CHO + CH3OH + ½ O2 → CH2=C(CH3)−COOCH3 + H2O
Uses
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