Molybdenum-doped α-MnO₂ as an efficient reusable heterogeneous catalyst for aerobic sulfide oxygenation

Abstract

In the presence of Mo6+-doped α-MnO2 (Mo–MnO2), various sulfides could efficiently be oxidized to the corresponding sulfoxides as the major products. In addition, Mo–MnO2 could repeatedly be reused.

Aerobic Oxygenation of Alkylarenes over Ultrafine Transition‐Metal‐Containing Manganese‐Based Oxides

“…Several manganese oxides are known to function as “stoichiometric oxidants” for oxidation reactions accompanied by structural change. Once the structure changes into a redox inactive phase during the reaction, the generation of the product is suppressed significantly, and severe deactivation is observed if we use the recovered catalyst for reuse experiments . To investigate whether the formation of 2 a in the presence of Ni‐MnO x was caused by “aerobic oxidation catalysis” or “stoichiometric oxidation”, the reaction was performed under nonaerobic conditions (Ar atmosphere).…”

“…Once the structure changes into ar edox inactive phase during the reaction, the generation of the product is suppressed significantly,a nd severe deactivation is observed if we use the recovered catalyst for reuse experiments. [22,41] To investigate whether the formation of 2a in the presenceo fN i-MnO x was caused by "aerobic oxidation catalysis" or "stoichiometric oxidation", the reaction was performed under nonaerobic conditions (Ar atmosphere). Under nonaerobic conditions, the conversion of 1a was 11 %, the yield of 2a was only 3% under standard conditions (c.f., > 99 %y ield of 2a under aerobic conditions, Ta ble2,e ntry 4), and the formation of 9,9'-bifluorene,w hich is likely produced through radical-radical homocoupling, wasa lso observed albeit in as mall amount (3 %y ield;T able 2, entry 8).…”

“…Several manganese oxides are susceptible to reduction; once such manganese oxides are reduced by substrates, their reoxidation by O 2 is quite difficult, which leads to the formation of redox inactive species . Recently, we reported that manganese oxides with additional metals in their structures, such as K‐containing tunnel manganese oxide (α‐MnO 2 , K‐OMS‐2) and K‐containing layered manganese oxide (δ‐MnO 2 ), exhibit a high catalytic performance for several aerobic oxidation reactions and maintain their crystal structures after their use for oxidation reactions even under nonaerobic conditions, unlike manganese oxides that contain no additional metals such as β‐MnO 2 and γ‐MnO 2 . (3) Selection of the appropriate additional metals is also important.…”

“…[16][17][18][19][20][21][22][23][24] In particular,t heir oxidation catalysis is of great interest, and thus numerousa erobic oxidation systemst hat use catalysts based on manganese oxide have been developed. [25][26][27][28][29][30][31][32][33][34][35][36][37][38][39] As fora lkylarene oxygenation, several catalysts based on manganese oxide have also been reported to be highly effective (Table S2); [40][41][42][43][44] for example, Hara and colleagues reported recently that nanosized hexagonal SrMnO 3 perovskite exhibits ah igh catalytic performance for alkylarene oxygenation under relatively mild conditions (60-120 8C, 1atm O 2 ,n oa dditives). [43,44] In this study,w ep repared heterogeneous catalysts based on manganese oxide for alkylarene oxygenation and we considered the following factors.…”

“…K-containing layered manganese oxide (d-MnO 2 ), exhibit a high catalytic performance for several aerobic oxidationr eactions and maintain their crystal structures after their use for oxidation reactions even under nonaerobic conditions, unlike manganese oxidesthat contain no additional metalssuch as b-MnO 2 and g-MnO 2 . [41] (3) Selection of the appropriate additional metals is also important.T he catalytic activity of manganese oxide is improved frequently by the incorporation of 3d metals;h owever, the role of the additional metal is often unclear. [47][48][49] As there has been little insight into the detailedc atalytic function of manganese oxides for oxidation reactions thus far,a nu nderstanding of the reaction mechanismf or this MnO x -catalyzed oxidation is also very important.…”