Harnessing Pickering Emulsion Interfaces to Modulate Singlet Oxygen Generation for Highly Selective Oxidation of Cyclohexene

The selective oxidation of cyclohexene to high-value cyclohex–2–enone using singlet oxygen holds significant importance in chemical production. However, conventional methods for generating singlet oxygen are often constrained by harsh conditions, typically requiring high-energy input. This study develops a new methodology that leverages the Pickering emulsion droplet interfaces to facilitate the generation of singlet oxygen for the efficient oxidation of cyclohexene. First, an amphiphilic catalyst SiO2/CNx was synthesized by growing hydrophobic nitrogen-doped carbon on the surface of hydrophilic sheet-like silica through a combined surface-polymerization and carbonization process. Such amphiphilic catalyst can then self-assemble at the interfaces of the oil phase (cyclohexene in ethyl acetate) and water phase (tert-butyl hydroperoxide aqueous solution), thereby stabilizing a water–in–oil Pickering emulsion. The graphitic nitrogen sites on the nitrogen-doped carbon promote the cleavage of tert-butyl hydroperoxide to form superoxide radicals, while the pyridinic nitrogen sites can adsorb and enrich the resulting radicals. In addition, the unique proton-rich microenvironment at the Pickering emulsion droplet interfaces promotes the disproportionation of adsorbed superoxide radicals, generating the desired singlet oxygen for the selective oxidation of cyclohexene. As a result, this Pickering emulsion system achieved a cyclohexene conversion of >99.0% with a 95.0% selectivity toward cyclohex–2–enone, significantly outperforming the results reported in prior studies. This strategy of harnessing the Pickering emulsion droplet interfaces to regulate the generation of highly reactive singlet oxygen opens up a promising route for the efficient synthesis of oxygenated fine chemicals.