PALLADIUM(2)-CATALYZED OXIDATION OF FORMIC ACID IN ACETIC ACID SOLUTION

The oxidation of formic acid to carbon dioxide and water in acetic acid-lithium acetate solution with palladium(II) catalysts was studied. The reaction was carried out in the presence of cupric salts and oxygen to reoxidize elementary palladium: (1) HCOOH + Pd(II) → C02 + Pd0 + 2H+; (2) Pd0 + Cu(II) → Cu(I) + Pd(II); (3) 2Cu(I) + 2H+ + 1 2O2 → 2Cu(II) + H2O. Under conditions in which the reaction represented by Eq. (1) was rate-determining the reaction rate was directly proportional to formic acid, palladium(II) concentration and approximately directly proportional to lithium acetate concentration. With excess lithium acetate the rate expression became zero order in acetate: -d[HCOOH] dt = k [Pd(II)] [HCOOH] and kwas found to be 0.12 1 mole(su-1) sec(-1) at 112 °C. From the variation of k with temperature, values of ΔH‡ = 22 kcal mole-1 and ΔSDG = -6 cal mole-1 deg-1 were calculated. The deuterium isotope effect for DCOOH was determined to be k[H] k[D] = 2.0 at 112 °C. A mechanism is proposed in which the rate-limiting step is the decomposition of a palladium(II)-formate complex. This complex decomposes either via a two-electron transfer from the coordinated formate ion coupled with proton loss, or via a palladium(II) hydride complex which then decomposes rapidly. Similar mechanisms had been proposed by other authors for the oxidation of formic acid with thallium(III), mercury(II), and mercury(I). © 1969.