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.

No abstract

No abstract

The utilization of CO 2 as a non-toxic and cheap feedstock for C1 is a desirable route to achieve high valueadded chemicals. In this context, we report a highly efficient ruthenium-catalyzed semi-hydrogenation reaction of CO 2derived ureas. Various alkyl and aryl urea derivatives were successfully hydrogenated to obtain the corresponding recyclable amines and formamides (up to 97 % yield), highlighting the good substrate applicability of this method, which makes this method a sustainable alternative for the hydrogenation of CO 2 to formamides in the presence of amines. In the meantime, we have discovered a new pathway that enables rapid hydrogenation of urea derivatives even at lower H 2 pressure (< 5 bar). This methodology might provide a new insight into the reduction functionalization of CO 2 under mild pressure to form new CÀ N bond. Based on the control experiments and the observed intermediate products, we clarify the mechanism for selective semi-hydrogenation of ureas.

No abstract

Kinetic studies on the catalytic oxidation of 4‐methyl guaiacol using an ether intermediate towards vanillin in an agitator bubbling reactor were presented. The reaction was catalyzed by N,N′‐ethylenebis(acetylacetoniminato)‐cobalt(II) hexafluorophosphoric pyridinium (Co‐[Salen‐Py][PF6]2) under strong alkaline conditions with molecular oxygen as the terminal oxidant. The reaction scheme was demonstrated by a detailed analysis of the products distribution and control experiments. The intrinsic kinetics were explored based on our proposed p‐benzoquinone methide mechanism, which revealed that the oxidation rate was determined by the oxidation of phenoxyl anion to phenoxyl radical catalyzed by LCo3+(O2·) (L = [Salen‐Py][PF6]2). The kinetic experiments were performed under a mass transfer‐reaction condition due to the inevitable mass transfer resistance involved. Pseudo‐first order fast reactions were observed for both steps of the consecutive reactions based on the calculated physical and mass transfer parameters. The intrinsic kinetic equations for the first and second steps can be written as RA=5.98×105exp(−31950RT)CCcat.CO2A and RF=1.02×106exp(−29770RT)CCcat.CO2E, respectively, which are well in accordance with the mechanism. These conclusions are expected to give a clue for the mechanistic study, kinetic investigation, reaction optimization, and catalyst design for the oxidation of various p‐cresols.

No abstract

No abstract