Aerobic oxidation and oxidative esterification of alcohols through cooperative catalysis under metal-free conditions

Heterogeneous solvent-metal-free aerobic oxidation of alcohol under ambient conditions catalyzed by TEMPO-functionalized porous poly(ionic liquid)s

Heterogeneous solvent-metal-free aerobic oxidation of alcohols under ambient conditions is interesting but remains a significant challenge. Herein, a series of porous TEMPO-functionalized poly(ionic liquid)s (TEMPO-PILs) featuring a pure polycationic framework were successfully developed through the free radical polymerization of the ionic liquid 3-(2-chloroacetic acid-2,2,6,6-tetramethyl-1-oxo-4-piperidyl)-1-vinylimidazolium chloride and bis-vinylimidazolium bromide salt. Characterizations revealed that the obtained TEMPO-PILs possessed a high TEMPO density, abundant bromide ions, and a tunable porous structure, which enabled them to serve as solvent-free heterogeneous organocatalysts for the metal-free aerobic oxidation of benzyl alcohol under ambient conditions, exhibiting high catalytic activity and stable recyclability. A high yield of 99% coupled with a turnover frequency (TOF) of 13.3 h-1 was obtainable, which is higher than most of the reported TEMPO-based heterogeneous catalysts, even superior to homogeneous TEMPO-functionalized ionic liquids. Furthermore, a broad range of alcohols were effectively converted into their corresponding ketones and aldehydes. A possible reaction mechanism is proposed for understanding the catalytic oxidation behavior, indicative of the synergistic effect of TEMPO moieties and bromide ions.

Plasmon-Induced Radical-Radical Heterocoupling Boosts Photodriven Oxidative Esterification of Benzyl Alcohol over Nitrogen-Doped Carbon-Encapsulated Cobalt Nanoparticles

Selective oxidation of alcohols under mild conditions remains a long-standing challenge in the bulk and fine chemical industry, which usually requires environmentally unfriendly oxidants and bases that are difficult to separate. Here, a plasmonic catalyst of nitrogen-doped carbon-encapsulated metallic Co nanoparticles (Co@NC) with an excellent catalytic activity towards selective oxidation of alcohols is demonstrated. With light as only energy input, the plasmonic Co@NC catalyst effectively operates via combining action of the localized surface-plasmon resonance (LSPR) and the photothermal effects to achieve a factor of 7.8 times improvement compared with the activity of thermocatalysis. A high turnover frequency (TOF) of 15.6 h-1 is obtained under base-free conditions, which surpasses all the reported catalytic performances of thermocatalytic analogues in the literature. Detailed characterization reveals that the d states of metallic Co gain the absorbed light energy, so the excitation of interband d-to-s transitions generates energetic electrons. LSPR-mediated charge injection to the Co@NC surface activates molecular oxygen and alcohol molecules adsorbed on its surface to generate the corresponding radical species (e.g., ⋅O2 - , CH3 O⋅ and R-⋅CH-OH). The formation of multi-type radical species creates a direct and forward pathway of oxidative esterification of benzyl alcohol to speed up the production of esters.

ABNO-Functionalized Silica as an Efficient Catalyst for Enhancing Permanganate Oxidation of Emerging Contaminants

Recent reports indicate that some soluble electron-shuttling compounds (or organic mediators) can accelerate reactions between permanganate (Mn(VII)) and contaminants of emerging concern. However, practical application is limited to homogeneous electron-shuttling compounds. This study reports on the development and application of a heterogeneous electron-shuttling catalyst for Mn(VII) reactions with bisphenol A (BPA). First, we screened a series of poly/monocyclic nitroxides, finding that 9-azabicyclo[3.3.1]nonane N-oxyl (ABNO) provides the most significant enhancement of Mn(VII)/BPA reaction kinetics, where Mn(VII) oxidizes ABNO to BPA-reactive ABNO⁺. Next, we immobilized ABNO onto silica (SiO₂) by covalent bonding of 9-azabicyclo[3,3,1]nonan-3-one-9-oxyl (keto-ABNO) via a 3-aminopropyltriethoxysilane bridge to yield an ABNO@SiO₂ heterogeneous catalyst. The performance of ABNO@SiO₂ in catalyzing Mn(VII)/BPA reactions is demonstrated, with BPA reaction kinetics being highly dependent on catalyst dosage and pH conditions. The stability of ABNO@SiO₂ was retained at pH 5.0 and decreased slightly at pH 7.0 over five successive Mn(VII)/BPA reaction cycles. Kinetics modeling shows that BPA reacts with immobilized ABNO⁺, Mn(VII), and in situ formed MnO₂. Moreover, ABNO⁺ can form via ABNO reactions with both Mn(VII) and the in situ formed MnO₂. These results indicate a promising strategy for developing practical heterogeneous catalysts for enhancing Mn(VII) reactivity and treatment applications.