Accelerated selective oxidation of benzyl alcohol to benzaldehyde via a self-catalyzed Pickering emulsion microreactor

Abstract

Selective oxidation of alcohols to aldehyde or ketone is a fundamental reaction in organic synthesis. Herein, a self-catalyzed Pickering emulsion microreactor (PEM) was designed for the selective oxidation of benzyl alcohol (BzOH) to benzaldehyde (BzH) using peroxymonosulfate (PMS) as an oxidant. Featuring inherent pyridinic N and pyridine N-oxide species, Enteromorpha prolifera-derived biochar displayed particular super-amphiphilicity, able to serve as an emulsifier and catalyst simultaneously in the PMS-based PEM (PPEM) system. The results showed that, without extra additives, the PPEM system could 100% selectively oxidize 2.88 mM of BzOH to BzH (yield: 90.4%) within 30 min at room temperature. After systematic and theoretical studies, we found that radical (O₂˙⁻) and non-radical (¹O₂ and mediated electron transfer) oxidation routes were responsible for the selective BzOH oxidation to BzH, where the reaction energy barrier for the oxidation of BzOH by ¹O₂ (18.0 kcal mol⁻¹) is lower compared to that of O₂˙⁻ (25.8 kcal mol⁻¹). Overall, the rationally fabricated biochar with special surface wettability and catalytic activity offers a novel emulsion self-catalysis route to achieve the selective oxidation of alcohol.