(111a) Mechanistic Investigation for Oxidative Cleavage of Alkenes and Unsaturated Fatty Acids with H2O2 on Tungsten Oxide Catalysts

Medium length aldehydes and acids are valuable precursors for polymers and lubricants [1] and can be obtained from renewable unsaturated fatty acids via chemical catalysis. Tungstate catalysts catalyze this reaction effectively [2], however, the mechanism and structural dependence of oxidative cleavage with H₂O₂ on solid tungstates are unclear.

We combine kinetics and in situ spectroscopy to investigate the mechanism for oxidative cleavage of alkenes and unsaturated fatty acids on bulk (WO₃) and supported tungstate (WO_x-Al₂O₃) catalysts [3]. In situ Raman, measured turnover rates, and the dependence of rates on catalytic functions reveal complex reaction networks. 4-Octene reacts with H₂O₂-activated W sites (W-(η²-O₂)) to form 4,5-epoxyoctane, and this step determines rates for oxidative cleavage process. Subsequently, 4,5-epoxyoctane protonates and undergoes to form 4,5-epoxyoctanediol. The diol deprotonates by reaction with CH₃CN solvent to form alpha hydroxy ketone, which reacts with H₂O₂ in the liquid phase to form butanal and water. While mechanisms on both catalysts are similar, WO_x-Al₂O₃ shows the lower apparent activation enthalpies (36 kJ mol^-1) than WO₃ (60 kJ mol^-1), and this difference arises from changes in the electrophilicity of the reactive W-(η²-O₂) intermediates when supported on γ-Al₂O₃. Adsorption enthalpies of 1,2-epoxyoctane in CH₃CN, obtained by isothermal titration calorimetry, show that WO_x-Al₂O₃ binds the epoxide products more strongly (-93 kJ mol^-1) than WO₃ (-48 kJ mol^-1), and therefore, stabilizes kinetically relevant transition states more effectively. The oxidative cleavage of oleic acid with H₂O₂ over WO_x-Al₂O₃ forms aldehydes and acids, however, WO₃ forms only the epoxide, likely because high coverages of oleic acid on the WO₃ surface diminish epoxide adsorption and ring opening of the epoxide.

[1] Spannring et al. Catal. Sci. Technol. 2014, 4(8), 2182.

[2] Cousin et al. Catal. Sci. Technol. 2019, 9(19), 5256.

[3] Yun et al. ACS Catal. 2021 11, 3137.