Continuous Aerobic Alcohol Oxidation in a Micro-Packed Bed Reactor with Nitroxyl-Radical Catalysts: From Homogeneous to Heterogeneous
Jisong Zhang,
Assistant Professor,
Tsinghua University
Selective aerobic oxidation is of crucial importance in organic synthesis, and sustainable oxidation strategies are highly desirable under green chemistry viewpoints. However, green oxidation processes rely not only on effective catalysts but also on the implementation of reactor technologies that enhance reaction performance and overall safety. Recently, nitroxyl radicals, as practical catalysts in oxidation have gradually attracted attention. These radicals can be further divided into stable radicals (such as 2, 2, 6, 6-tetramethylpiperidine nitrox, TEMPO) and active radicals (phthalimide-N-oxyl, PINO) produced by N-hydroxyimide (such as N-hydroxyphthalimide, NHPI). Nitroxyl radicals can form efficient catalytic systems in conjunction with metallic or non-metallic co-catalysts, enabling aerobic oxidations with high selectivity under mild conditions to obtain a variety of carbonyl compounds. Nevertheless, the existing nitroxyl-radical catalytic systems still face the challenges of high cost and limited recyclability. In this study, with a self-constructed micro-packed bed reactor platform, the space-time yields of alcohol oxidation to aldehyde and methylene oxidation to ketone were significantly enhanced by one to two orders of magnitude when compared to batch reactors. Furthermore, several novel strategies for nitroxyl-radical functionalized activated carbon catalysts are proposed, which not only surpass the state-of-art commercial silicon-based catalysts in terms of reactivity but also demonstrate exceptional stability in continuous-flow mode.
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