The oxidative cleavage of bio-based unsaturated fatty acids with hydrogen peroxide offers pathways to create unique monomers (e.g., C9 diacids), which are difficult to obtain from petroleum-based precursors, for high-performance polymers, plasticizers, and lubricants. Tungstates facilitate this chemistry, however, the mechanism and identity of reactive species over these solid catalysts remain debated. Here, we present a combination of kinetic, spectroscopic, and calorimetric results that reveal the H2O2-derived intermediates, the reaction mechanism by which 4-octene and oleic acid undergo oxidative cleavage, and the differences between kinetics and active site structures for WOx complexes on γ-Al2O3 (WOx-Al2O3) and WO3 nanoparticles. A combination of in situ spectroscopy, the measured turnover rates for epoxide and diol conversions, and the dependence of these rates on catalytic functions reveal a complex reaction network and steps with distinct site requirements. W-(η2-O2) complexes react with 4-C8H16 to form 4,5-C8H16O, followed by the ring-opening to form 4,5-C8H16(OH)2. The diol deprotonates by reaction with the CH3CN solvent to form the alpha hydroxy ketone, which reacts with H2O2 in the liquid phase to form aldehydes and water. WOx-Al2O3 shows the lower apparent activation enthalpies (36 ± 3 kJ mol-1) than WO3 (60 ± 6 kJ mol-1) due to the greater electrophilicity of W-(η2-O2) sites on γ-Al2O3. Adsorption enthalpies of 1,2-epoxyoctane in CH3CN were obtained by isothermal titration calorimetry and the values are 45 kJ mol-1 more exothermic on the WOx-Al2O3. This result indicates WOx-Al2O3 stabilizes kinetically relevant transition states more effectively following linear free energy relationships. The oxidative cleavage of oleic acid with H2O2 over WOx-Al2O3 forms aldehydes and acids, however, WO3 forms only the epoxide. This difference is because limited numbers of Brønsted acid sites and high coverages of oleic acid on the WO3 surface inhibit epoxide adsorption and ring-opening of epoxide.

Oxidative cleavage of alkenes and unsaturated fatty acids with H2O2 over tungstates: Mechanisms, site requirements, and support effects

ACS Spring Meeting 2021

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