Direct propylene epoxidation via water activation over Pd-Pt electrocatalysts

Abstract

Direct electrochemical propylene epoxidation by means of water-oxidation intermediates presents a sustainable alternative to existing routes that involve hazardous chlorine or peroxide reagents. We report an oxidized palladium-platinum alloy catalyst (PdPtO x /C), which reaches a Faradaic efficiency of 66 ± 5% toward propylene epoxidation at 50 milliamperes per square centimeter at ambient temperature and pressure. Embedding platinum into the palladium oxide crystal structure stabilized oxidized platinum species, resulting in improved catalyst performance. The reaction kinetics suggest that epoxidation on PdPtO x /C proceeds through electrophilic attack by metal-bound peroxo intermediates. This work demonstrates an effective strategy for selective electrochemical oxygen-atom transfer from water, without mediators, for diverse oxygenation reactions.

Highly Selective Electrochemical Baeyer–Villiger Oxidation through Oxygen Atom Transfer from Water

“…At 1.71 V, an apparent reaction order of 1.33 was determined, whereas at 1.42 V, it substantially decreased to <1. These results provided additional support for the surface-initiated reaction mechanism . At high applied potentials, the water oxidation kinetics is fast and the surface is expected to be saturated with *OOH species.…”

“…By comparison, at low applied potentials, a relatively slow H 2 O oxidation is expected, implying a relatively low coverage of surface *OOH species . In this case, the overall reaction rate would be largely determined by the availability of surface *>OOH species (Figure c) . As such, the availability of cyclohexanone molecules is not the only limiting factor to the overall reaction rates.…”

“…In the pursuit of developing sustainable alternatives for catalysis, − oxygen atom transfer (OAT) reactions are of great importance for chemical synthesis . Traditionally, this class of reactions relies on strong oxidants such as peroxyacids, peroxides, or molecular oxygen (O 2 ), which are often difficult to handle. , As a result, it presents a challenge for large-scale implementations, such as the preparation of monomers for polymeric materials. , Recent advances have inspired researchers to address these challenges by exploiting in situ generated oxidants. − Promising results have been obtained by electrochemically (or photoelectrochemically) oxidizing H 2 O to produce intermediates, including oxo species, , peroxo species, hydroperoxo species, , hydroxyl radicals, and reactive atomic surface oxygen species, which can be directly utilized for OAT. For instance, it was recently reported that manganese oxo species generated through an electrochemical approach can efficiently oxidize thioethers to sulfoxides .…”

“…10,11 Recent advances have inspired researchers to address these challenges by exploiting in situ generated oxidants. 12−15 Promising results have been obtained by electrochemically (or photoelectrochemically) oxidizing H 2 O to produce intermediates, including oxo species, 16,17 peroxo species, 18 hydroperoxo species, 19,20 hydroxyl radicals, 21 and reactive atomic surface oxygen species, 22 which can be directly utilized for OAT. For instance, it was recently reported that manganese oxo species generated through an electrochemical approach can efficiently oxidize thioethers to sulfoxides.…”

“…Indeed, surface-adsorbed peroxo ([M − -OO + ]) intermediates generated from water on the PdPtO x /C catalyst were recently shown to facilitate propylene epoxidation with excellent selectivity. 18 To achieve Baeyer− Villiger oxidation, we targeted water oxidation catalysts that can produce critical M-OOH intermediates, 19,28−30 nucleophilic attacks on the organic ketone substrate, as shown in Figure 1. We report in this work the application of a known water oxidation catalyst, iron oxide (Fe 2 O 3 ), 31−34 for the synthesis of ε-caprolactone with ca.…”