Acrolein dimerisation

The most notable Diels-Alder reaction in which the heteroatom is part of the diene system is the dimerisation of acrolein 6.178 giving the adduct 6.179. As in the reaction of butadienecarboxylic acid 6.154 with acrylic acid 6.155 [see (Section 6.5.2.2) page 229], the two electrophilic carbon atoms are the ones which have become bonded. 

Several multicomponent metal oxide catalysts, developed for this process, have achieved excellent product selectivity with a high conversion of propene Mo-Bi-Fe-Co-M-K-O (M = V or W) used for the first step can attain >90% acrolein yields [6,7] while for the second step Mo-V-Cu-based oxides can lead to >97% acrylic acid yields [8,9], giving, in theory, an overall acrylic acid yield from propene of 87%. In addition to the compositional differences in fhe catalysis for the two-step process, there is also a difference in the optimal reaction temperatures 320-330°C for the first step and 210-255°C for the second step. One has to keep in mind that propene and oxygen can form an explosive mixture and therefore, certain limitations in the feed composition (propene oxygen (air) steam) exist. In addition, the acrylic acid easily dimerises at temperatures above 90 C, meaning that the reactor effluent should be quickly quenched after the second catalyst bed to temperatures below this critical value. 

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