Kinetics and mechanism of the oxidation of acrylic acid and methyl methacrylate

Abstract

The kinetics of the initiated oxidation of acrylic acid and methyl methacrylate in the liquid phase were studied volumetrically by measuring oxygen uptake during the reaction. Both processes proceed via the chain mechanism with quadratic-law chain termination. The oxidation rate is described by the equation w = k 2/(2k 6)1/2[monomer]w 1/2 , where w i is the initiation rate and k 2 and k 6 are the rate constants of chain propagation and termination. The parameter k 2/(2k 6)1/2 is 7.58 × 10−4 (l mol−1 s−1)1/2 for acrylic acid oxidation and 2.09 × 10−3 (l mol−1 s−1)1/2 for the oxidation of methyl methacrylate (T = 333 K). For the oxidation of acrylic acid, k 2 = 2.84 l mol−1 s−1 (T = 333 K) and the activation energy is E 2 = 54.5 kJ/mol; for methyl methacrylate oxidation, k 2 = 2.96 l mol−1 s−1 (T = 333 K) and E 2 = 54.4 kJ/mol. The enthalpies of the reactions of RO 2 − with acrylic acid and methyl methacrylate were calculated, and their activation energies were determined by the intersecting parabolas method. The contribution from the polar interaction to the activation energy was determined by comparing experimental and calculated E 2 values: ΔE μ = 5.7 kJ/mol for the reaction of RO 2 − with acrylic acid and ΔE μ = 0.9 kJ/mol for the reaction of RO 2 − with methyl methacrylate. Experiments on the spontaneous oxidation of acrylic acid provided an estimate of the rate of chain initiation via the reaction of oxygen with the monomer: w i,0 = (3.51 ± 0.85) × 10−11 mol l−1 s−1 (T = 333 K).