Molybdenum-doped α-MnO 2 as an efficient reusable heterogeneous catalyst for aerobic sulfide oxygenation

• Published in: Catalysis science & technology 2016-01, Vol.6 (1), p.222-233
• Main Authors: Uematsu, Tsubasa, Miyamoto, Yumi, Ogasawara, Yoshiyuki, Suzuki, Kosuke, Yamaguchi, Kazuya, Mizuno, Noritaka
• Format: Article
• Language: English
• ISSN: 2044-4753; 2044-4761
• DOI: 10.1039/C5CY01552A

Oxygenation of sulfides to sulfoxides and/or sulfones is an important transformation, and the development of efficient heterogeneous catalysts for oxygenation, which can utilize O 2 as the terminal oxidant, is highly desired. In this study, we have successfully developed manganese oxide-based efficient heterogeneous catalysts for aerobic oxygenation of sulfides. Firstly, we prepared four kinds of manganese oxides possessing different crystal structures, such as α-MnO 2 , β-MnO 2 , γ-MnO 2 , and δ-MnO 2 , and their structure–activity relationships were examined for the aerobic oxygenation of thioanisole. Amongst them, α-MnO 2 showed the best catalytic performance for the oxygenation. Moreover, α-MnO 2 was highly stable during the catalytic oxygenation possibly due to the tunnel K + ions. In order to further improve the catalytic performance of α-MnO 2 , substitutional doping of transition metal cations, such as Mo 6+ , V 5+ , Cr 3+ , and Cu 2+ , into the framework was carried out. Undoped α-MnO 2 possessed a fibrous morphology. When high-valent transition metal cations were doped, especially Mo 6+ , the lengths of the fibers drastically shortened to form grain-like aggregates of ultrafine nanocrystals, resulting in an increase in specific surface areas and the numbers of catalytically active surface sites. In the presence of Mo 6+ -doped α-MnO 2 (Mo–MnO 2 ), various kinds of sulfides could efficiently be oxidized to the corresponding sulfoxides as the major products. The observed catalysis was truly heterogeneous, and Mo–MnO 2 could repeatedly be reused while keeping its high catalytic performance. Besides sulfide oxygenation, Mo–MnO 2 could efficiently catalyze several aerobic oxidative functional group transformations through single-electron transfer oxidation processes, namely, oxygenation of alkylarenes, oxidative α-cyanation of trialkylamines, and oxidative S-cyanation of benzenethiols.