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/CN_<i>x</i> 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 (<i>tert</i>-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 <i>tert</i>-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.