Methacrolein, manufacture

Methacrolein and Methacrylic Acid. A two-stage technology, essentially the same as the propylene oxidation process for the manufacture of acrolein and acrylic acid, was developed to oxidize isobutylene to methacrolein and methacrylic acid 949-951 Two different molybdenum-based multicomponent catalysts are used. In a typical procedure949 isobutylene is reacted with excess steam and air (5 30 65) at about 350°C to produce a mixture of methacrolein and methacrylic acid with 80-85% selectivity at a conversion of 98%. In the second stage this reaction mixture is oxidized at slightly above 300°C to yield methacrylic acid (80% selectivity at >90% conversion). 

In the past, acrolein was produced by the gas phase condensation of acetaldehyde with formaldehyde on sodium silicate, until it was supplanted by the catalytic oxidation of propylene. Early catalysts based on cuprous oxide were only sufficiently selective at low conversions of propylene. The real breakthrough came with the discovery made by Sohio of bismuth molybdate catalysts, developed into formulations specifically optimized for the manufacture of acrylonitrile, acrolein, and methacrolein. 

It is also worth mentioning that the multiphase approach has been used as a strategy to avoid undesired consecutive reactions [lla-c] or even to segregate two different and incompatible catalysts in one-pot or in tandem syntheses. In a typical example, Chaudhari et al. described the combination of a hydroformylation step -catalyzed by phosphine-modified rhodium complexes - with the Mannich reaction of the 0x0 aldehydes formed, catalyzed by tertiary amines. Thus the manufacture of methacrolein according to Figure 7 proceeds best in two different phases the organic phase for hydroformylation and the aqueous phase in which the Mannich reaction is achieved (Figure 8) [lid]. 

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