Importance of surface peroxo species in the epoxidation of cyclohexene by Mo-doped TS-1 and O2 under solvent-free conditions

Abstract

The selective oxidation of cyclohexene (Cy) to cyclohexene oxide (Cy-ep) using O₂ remains challenging due to low epoxidation selectivity. In this work, a series of Mo-doped TS-1 (Mo-TS-1) catalysts were successfully synthesized for the epoxidation of Cy under solvent- and initiator-free conditions with O₂ as the oxidant. Among them, 5Mo-TS-1 exhibited high catalytic performance, achieving 43.5% Cy conversion and 50.6% selectivity toward Cy-ep. Additionally, valuable by-products such as 2-cyclohexen-1-ol (Cy-ol) and 2-cyclohexen-1-one (Cy-one) were obtained with yields of 28.0% and 21.4%, respectively. Since both Cy-ol and Cy-one are valuable intermediates in fragrance synthesis, over 43.5% of Cy was effectively converted into high-value products. Quenching experiments and Raman spectroscopy revealed that surface oxygen vacancies (O_v) facilitate the activation of O₂ to form O_v-superoxo species, which abstract hydrogen from the allylic C–H bond of Cy to generate 3-cyclohexenyl radicals (Cy·). These radicals subsequently react with O₂ to form Cy–OO·, followed by hydrogen abstraction from another Cy molecule to yield 2-cyclohexene-1-hydroperoxide (Cy–OOH). A positive correlation between Cy–OOH and Cy-ep formation underscores the critical role of Cy–OOH in the epoxidation process. Furthermore, Raman spectroscopy confirmed the presence of Mo-(η²-O₂) peroxo species on the catalyst surface, which preferentially attack the CC bond of Cy to form Cy-ep. DFT calculations elucidated two distinct O₂ activation pathways: in pathway I, O₂ is activated at O_v sites to form O_v-superoxo, which subsequently reacts with Cy to generate O_v-peroxo, Cy–OOH, and Cy·. In pathway II, Mo(V/VI) sites either directly activate O₂ or react with peroxo intermediates (O_v-peroxo or Cy–OOH) to form Mo-(η²-O₂). This species selectively epoxidizes the alkene bond in Cy to Cy-ep. Notably, the direct activation of O₂ at Mo(V/VI) sites bypasses the allylic oxidation route, thereby enhancing the epoxidation selectivity beyond the theoretical limit of 50.0%. This study provides new insight on the importance of surface superoxo and peroxo mediated by O_v and Mo(V/VI) in the epoxidation processes.