The photo-induced oxidation of acetylene

Abstract

The oxidation of acetylene catalysed by the photolysis of acetone between 190° and 250°C has been studied using a static system by following the small pressure changes occurring and by analysis. The uncatalysed oxidation has also been investigated, but less extensively. With excess acetylene the pressure decreased to a minimum and then rose to a maximum, which was very sharp in the induced oxidation. The products were carbon monoxide, carbon dioxide and water with less formaldehyde, formic acid, glyoxal and acetaldehyde. An unidentified polar compound containing probably more than two atoms of carbon was also formed and, in the induced oxidation, methanol was produced. The rates of consumption of reactants and of formation of the main products increased slightly at first and then fell sharply to zero at a time, in the induced oxidation, coinciding with the sharp pressure maximum. The concentration of glyoxal rose to a sharp maximum early in the reaction, the amounts presents in the latter stages being very small. The rate of formation of acetaldehyde was very low at first but increased markedly just before the end of the oxidation. The activation energy of the uncatalysed oxidation was about the same (30 to 34 kcal/mole), whether it referred to the consumption of reactants or to the formation of carbon monoxide, carbon dioxide or water. The kinetics of the photo-induced oxidation were investigated more fully. The average rates of consumption of acetylene and oxygen and of formation of carbon monoxide were all proportional to I0.5[CH3COCH3]0.25[C2II2], where I is the intensity of the incident u.v. radiation, and independent of oxygen concentration. The kinetics of the formation of carbon dioxide and methanol were somewhat different, however. The activation energies for the consumption of reactants and the formation of products were less than the value for the uncatalysed oxidation, although most of the Arrhenius plots were curved. The mechanisms of both catalysed and uncatalysed oxidations are probably similar. It is like y that degenerate chain branching due to glyoxal is of minor importance only, and that products such as carbon monoxide, glycxal, formaldehyde and acetaldehyde are formed in elementary steps in the main chain propagation process subsequent to the reaction of OH and HO2 radicals with acetylene, whereas methanol is the result of the addition of CH3O2 and CH3O to acetylene.