Kinetics and mechanism of the homogeneous oxidation of <Emphasis Type="Italic">n</Emphasis>-butenes to methyl ethyl ketone in a solution of Mo-V-phosphoric heteropoly acid in the presence of palladium pyridine-2,6-dicarboxylate

In catalytic two-step n-butene oxidation with dioxygen to methyl ethyl ketone, the first step is the oxidation of n-C₄H₈ with an aqueous solution of Mo-V-P heteropoly acid in the presence of Pd(II) complexes. The kinetics of n-butene oxidation with solutions of H₇PV₄Mo₈O₄₀ (HPA-4) in the presence of the Pd(II) dipicolinate complex (H₂O)PdII(dipic) (I), where dipic²⁻ is the tridentate ligand 2,6-NC₅H₃(COO⁻)₂, is studied. Calculation shows that, at the ratio dipic²⁻: Pd(II) = 1: 1, the ligand decreases the redox potential of the Pd(II)/Pd_met system from 0.92 to 0.73–0.77, due to which Pd(II) is stabilized in reduced solutions of HPA-4. The reaction is first-order with respect to n-C₄H₈. Its order with respect to Pd(II) is slightly below unity, and its order with respect to HPA-4 is relatively low (∼0.63). The activation energy of but-1-ene oxidation in the temperature range from 40 to 80°C is 49.0 kJ/mol, and that of the oxidation of but-2-ene is 55.6 kJ/mol. The mechanism of the reaction involving the cis-diaqua complex (H₂O)₂Pd^II(Hdipic)]⁺, which forms reversibly from complex I, is proposed. The reaction rate is shown to increase with an increase in the HPA-4 concentration due to an increase in the acidity of the solution.