ABSTRACT
Butenes, like propene, contain hydrogen atoms in the a position with respect to the double bond which are more reactive than other hydrogen atoms and are readily abstracted to form an allylic moiety. However, at variance with propene, this allylic moiety may react with oxygen along several different reaction pathways. The majority of oxide surfaces, particularly those used as supports, contain acid centers. In their presence isomerization of but-l-ene to but-2-ene takes place. Indeed, on many oxidation catalysts this is the reaction pathway prevailing at lower temperatures. At higher temperatures, the surface oxide ions which exhibit basic properties abstract hydrogen atoms from the a position in both but-l-ene and but-2-ene to fornl allylic species and OH-groups, which are then removed from the surface in the course of its dehydroxylation. The allyl species, thereby generated at the surface, have several possibilities of further transformations. The simplest is the repetition of the hydrogen abstraction from the a position, but now in respect to the allylic bond, i.e., at the C4 atom. This results in the formation of butadiene, which may be desorbed or react further. The allyl species may also undergo a nucleophilic attack by lattice oxygen ions when the surface of the catalyst has the ability to perform such an attack. Depending on the properties of the catalyst, the nucleophilic addition of the 0 2-ion may take place either at the C3 position to form methyl vinyl ketone, or at the Cl position, leading to the appearance of crotonaldehyde. When electrophilic oxygen species are simultaneously present at the catalyst surface, they may react with butadiene to form furan, reacting further to give maleic anhydride. When only electrophilic oxygen species are present at the surface, an electrophilic attack on the double bond of but-2-ene may take place, resulting in the oxygenolysis of this bond and formation of acetaldehyde or acetic acid. The possible reaction pathways are shown in Fig. 7. 34. In the presence of electrophilic oxygen further consecutive reactions can take place, essentially all the intermediate compounds being liable to undergo combustion to CO, C02, and H20. The steps leading to total oxidation were omitted from Fig. 7.34 to avoid unnecessary complication of the reaction network. In the case of isobutene the properties of the allyl species resemble those observed in the case of propene. The nucleophilic 0 2-addition may be performed only at two equivalent terminal carbon atoms of the three-atom chain, giving methacrolein as the final product. Thus, the mechanism of the reaction is similar to that described in the case of propylene oxidation.
