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An investigation of aerobic oxidation of 2-methoxy-4-methylphenol to vanillin catalyzed by a number of cobalt salts, complexes, oxides, and hydroxides in alkaline solutions is reported. A good yield of vanillin (90%) was obtained when CoCl2 was used as the catalyst, while a poor yield (6%) was obtained with Co3O4. Unexpectedly, however, the Co3O4 catalytic activity was restored after heating in an alcoholic solution of NaOH for 24 h, producing a good yield of vanillin (85%). The recovered catalytic activity of Co3O4 indicated a conversion of inactivated heterogeneous particles to activated particles. In addition, precipitates were generated when “homogeneous” CoCl2 was employed as a catalyst, making us wonder about the true nature of the catalyst in this reaction. A series of experiments are described, including kinetic studies, filtration tests, centrifugation tests, dynamic light scattering, transmission electron microscopy, X-ray photoelectron spectroscopy, and powder X-ray diffraction, for the investigation of the true catalyst under catalytic conditions. The key observations led to the conclusion that the cobalt salts and complexes act as precatalysts that transform to cobalt oxide/hydroxide [CoO x (OH) y ] nanoparticles, which is responsible for the benzylic C­(sp3)–H oxyfunctionalization of 2-methoxy-4-methylphenol. Additionally, the size of the particles is decisive for catalyst performance. The nature of the active nanoparticles was also studied. With regard to the identification of homogeneous or heterogeneous catalysis, this study provides a complement to existing catalytic reaction types.

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