The results from co-oxidation of propene and acetaldehyde in the temperature region 505-549K can be explained by a mechanism in which propene oxide is formed from propene by a series of reactions with peroxy radicals, the three most important being HOCH2CHO2CH3, CH3CO3 and HO2. The first species is produced by the successive addition of hydroxy radicals and oxygen to the parent fuel molecule (Tully and Goldsmith, 1985, Wilk et al, 1989). Under the low temperature conditions of these experiments (less than 550K), this adduct has a long enough lifetime to undergo a reaction with the alkene to form propene oxide (Stark and Waddington, 1995):
HOCH2CHO2CH3 + C3H6 = HOCH2CHOCH3 + C3H6O (107)
The resulting hydropropyloxy radicals predominantly decompose to acetaldehyde and hydroxymethyl radicals, the latter
species reacting with oxygen to give formaldehyde and hydroperoxy radicals:
HOCH2CHOCH3 = CH3CHO + HOCH2 = HO2 + HCHO (108)
HOCH2 + O2 = HO2 + HCHO (109)
Most of the acetaldehyde and formaldehyde observed is produced by these two reactions. Recycling acetaldehyde ensures its complete consumption, predominantly by hydrogen abstraction by peroxy radicals:
CH3CHO + RO2 = CH3CO + RO2H (eg. 10)
(where, for example R = H, HOC3H6, etc.) The increase in propene oxide selectivity with added acetaldehyde and the temperature dependence of the effect can be understood in terms of the two main reactions for the acetyl radical in the system, either to decompose or add an oxygen molecule:
CH3CO = CH3 + CO (14)
CH3CO + O2 = CH3CO3 (15)
Acetylperoxy radicals react very rapidly with alkenes (Ruiz Diaz et al, 1975) and for the conditions used in these experiments virtually all of them will react with propene, giving the increased epoxide yield with added acetaldehyde.
CH3CO3 + C3H6 = CH3 + CO2 + C3H6O (22)
The rate constant for addition of oxygen to acetyl is approximately temperature independent (Baulch et al, 1984), whereas the decomposition route has a large activation energy (Baulch et al, 1990). Therefore, increasing the temperature favours decomposition and hence reduces the CO2/CO ratio and propene oxide selectivity, as observed in table I and the corresponding simulated results.
In the static glass system, the species produced by the further reaction of acetaldehyde also increase, ie. methanol and oxides of carbon. Formaldehyde is a product of the oxidation of acetaldehyde (via reaction 14 and subsequent oxidation of methyl). However, it is also formed by other reactions, for example via reaction 109, and so there is no clear trend in its yield on addition of acetaldehyde.
Evzerikhin and Artsis (1969) examined the effect of adding acetaldehyde to fuel-rich (O2 = 3-20%) propene:oxygen mixtures at 623-773K and found no significant effect on the propene oxide yield, and concluded that therefore under these conditions the epoxide was not formed by acetylperoxy radicals. However, it is evident from this work (table I) that at lower temperatures, acetaldehyde does indeed increase the yield, but that this effect is diminished as the temperature is raised.
The lack of effect on epoxide yields on addition of formaldehyde is probably because if a peroxy radical abstracts a hydrogen atom from formaldehyde, the formyl fragment reacts with oxygen to give mostly carbon monoxide and hydroperoxy radicals (Bozzelli and Dean, 1993), ie. the further reaction of formaldehyde gives no net increase in the number of peroxy radicals available to epoxidise propene. This is not the case for acetaldehyde where hydrogen atom abstraction leads to more than one peroxy radical being formed, allowing the further oxidation of acetaldehyde to increase the propene oxide selectivity.