Aerobic Thiols\nOxidative Coupling to Disulfides over\nRobust CoO<sub><i>x</i></sub> Nanoclusters Confined within\nHierarchical Silicalite-1 Zeolite

Disulfide is an important organic reagent and synthetic\nintermediate\nthat is widely used in organic synthesis, polymers, and other fields,\nbut its synthesis still suffers from many environmental pollution\nand economic problems. Here, we present an environmentally friendly\nand efficient base-free aerobic oxidative thiol coupling catalyzed\nby heterogeneous CoO<sub><i>x</i></sub> nanoclusters entrapped\nin hierarchical silicalite-1 zeolite, synthesized by combining silane\npore expansion and metal coordination methods under hydrothermal conditions.\nIt is confirmed that open hierarchical channels favor mass diffusion,\nand the chemical valence of Co species in CoO<sub><i>x</i></sub>/<i>h</i>-S-1-H is +2, which is different from that\nof Co<sub>3</sub>O<sub>4</sub> particles in CoO<sub><i>x</i></sub>/<i>h</i>-S-1-I. CoO<sub><i>x</i></sub> nanoclusters, are strongly fixed in the channels of silicalite-1\nzeolite via Co−O−Si bonds, which is of great importance\nfor the high catalytic activity in both symmetrical and unsymmetrical\noxidative thiol coupling reactions. After recycling experiments four\ntimes, the CoO<sub><i>x</i></sub>/<i>h</i>-S-1-H\nused has almost the same chemical state and the same distribution\nof Co(II) species as the fresh catalysts. Based on DFT calculations\nand inhibition experiments, the oxidative coupling of thiols undergoes\na free radical mechanism in which Co(III) causes RS−H cleavage\ninto RS· and H· species. Subsequently, two RS· radicals\nare coupled to disulfides, while H· radicals react with the O\nspecies to form H<sub>2</sub>O molecules. This work not only provides\nguidance on catalyst design and parameter optimization for oxidative\nthiol coupling but also advances the understanding of the aerobic\noxidation mechanism.