Effect of formic acid and formates on the oxidation of hydrocarbons and hydrocarbon polymers

Abstract

AbstractIt is known that the rate of oxygen absorption and the concentration of peroxides in the course of hydrocarbon oxidation passes through a maximum. A similar picture is observed also on oxidation of liquid hydrocarbons. However, the lowering of the oxidation rate and of the concentration of peroxides is evidently due to the accumulation of oxidation inhibitors in the system rather than to a drop in the hydrocarbon concentration. In order to ascertain the nature of the inhibiting action of the products of hydrocarbon oxidation, a kinetic study was made of the oxidation of cetane, a hydrocarbon serving as a model of the polyethylene chain, at 140–150°C. The absorption of oxygen by cetane exhibits an autocatalytic character in the initial stages of the process (to the extent of 25–30%); however, on further oxidation, there is a rapid drop in the reaction rate. When the reaction has proceeded to above 40–50% its rate becomes practically constant. The peroxide accumulation curve passes through a maximum at 25–30% reaction and then falls abruptly. At extents of reaction above 40–50% the concentration of peroxides remains practically constant. At the same time the kinetic chain length of the oxidation reaction diminishes. A study of the effect of the oxidation products on the oxidation process showed that an inhibiting action is exerted by formic acid formed in the course of reaction. Its removal from the reacting system leads to an increase in the rate of oxygen absorption, accompanied by increase in concentration of the peroxides and in the kinetic chain length. The data obtained in the study of cetane oxidation of the hydroperoxide‐induced polymerization of various monomers in the presence of formic acid, and of the decomposition of the hydroperoxides in the presence of the latter give grounds for believing that inhibition of the oxidation of hydrocarbon polymers and of liquid hydrocarbons is associated with interaction between peroxides and the free radicals formed from the formic acid.