The principal products of propene oxidation are propene oxide, acetaldehyde and formaldehyde, with lesser amounts of carbon oxides, acrolein, methanol, 1,5-hexadiene and other hydrocarbons. Addition of 3.4% (v/v) acetaldehyde at 549K increases the selectivity to the epoxide by only 2-4%.The overall rate of reaction though increases significantly, with the time for oxygen consumption being reduced from 130 to 4 minutes. The selectivities of carbon oxides and methanol also increase by ca 60% and 5 respectively.At lower temperatures the effect on the epoxide selectivity on adding acetaldehyde is more marked, at 505K increasing from 46% to 61% with addition of the aldehyde (table 1). Due to the large increase in the overall rate of reaction on adding acetaldehyde, the experiments with no added aldehyde were conducted at a higher pressure (4 bar), to give practical overall rates of reaction. It was demonstrated in an earlier paper (Stark and Waddington, 1995) that pressure changes over this range have no significant effect on the epoxide selectivity, and thus the increase in epoxide selectivity can be attributed to the addition of acetaldehyde. Decreasing the temperature in propene:acetaldehyde co-oxidation experiments gives a marked increase in the selectivity of the epoxide and carbon dioxide and a decrease of carbon monoxide selectivity (table 1).

On addition of acetaldehyde to mixtures of propene and oxygen in both the static and CSTR systems, the rate of reaction increases significantly (table 2). To avoid significant cracking of the fuel due to the exhaustion of the oxygen in the CSTR experiments, the temperature of the reactor was adjusted to give ca 75% oxygen consumption. Therefore temperature is not an independent variable in these experiments and the increase in overall rate of reaction obtained by adding the acetaldehyde is manifested by a lower operating temperature for the CSTR reactors. Two sets of results are given for the CSTR system, one at intermediate pressure (16 bar) and the other at high pressure (55 bar), both with and without added acetaldehyde. At both pressures adding acetaldehyde increases the epoxide selectivity. These results demonstrate that reasonable epoxide selectivities (40-50%) can be achieved at the higher pressures necessary for significant epoxide production rates.

Formaldehyde is another significant product in terms of selectivity during the oxidation of propene. In order to see whether it behaved in a similar manner to acetaldehyde and gave an increase in propene oxide selectivity if added, co-oxidation experiments were performed at 549K. If formaldehyde was recycled in a flow system, then its eventual stable mole fraction would be expected to be of the order of 2%. Adding 1.6% of formaldehyde gave no noticeable change in the propene oxide selectivity.