Abstract
In this work, the catalytic performance of cobalt oxide supported on activated carbon was studied during the oxidation reaction of several aliphatic/aromatic primary or secondary alcohols. Good results for benzyl alcohol and benzyl alcohol derivatives conversion and selectivity were obtained, while aliphatic, primary, and secondary alcohols in the study exhibited low conversions. The influence of the use of sonication during the impregnation of cobalt species was also evaluated. The particle size, distribution and dispersion of cobalt oxide (Co3O4) were compared with the catalyst prepared conventionally. These results are essential for understanding more about the activity of this type of catalysts in the synthesis of aldehydes and ketones.
Methods: Two powdered activated carbons, RX3 Extra provided by NORIT and G60 provided by Aldrich, were used as support of the catalysts. Cobalt oxide particles were deposited on the supports by means of the incipient wetness technique. A portion of this preparation was manually stirred (i), while another portion was subjected to sonication by 20 min (ii). The catalysts so synthetized were called: Co/RX and Co/G60 as item (i); while Co/RX-s and Co/G60-s for item (ii). The performance and characterization of synthetized catalysts was studied with N2 physisorption, AA (Atomic Absorption), SEM (Scanning Electron Microscopy) and H2-TPR (Temperature Programmed Reduction). Alcohols oxidation reaction was carried out at 80°C using 0.2 mmol of each alcohol, a mass of catalyst (0.1 g, 0.09 mol%) and 20 mL of toluene as solvent. Different types of alcohols were used: benzyl alcohol, 1-phenylethanol, 4-chlorobenzyl alcohol, cyclohexanol, 2-phenylethanol, 2-octanol, 1-heptanol, 1-octanol, 3-phenyl-1-propanol.
Results: Co3O4 particles supported on activated carbon catalysts were prepared through incipient wetness method. According to the characterization results, the use of sonication is an important factor if used in the impregnation process. G60 and RX supports and Co/G60, Co/RX, Co/G60-s,Co/RX-s catalysts have a very high surface area (500-1900 m2 g-1). No differences were found on surface area between catalysts either using or not the sonication treatment. Cobalt loading by AA technique in each support showed a slight variation with the synthesis method. The cobalt content for Co/G60 and Co/RX catalysts was 9.5 and 9.6 % wt Co, respectively; while Co/G60-s and Co/RX-s was 9 % wt Co, indicating that ultrasound affects interaction between precursor and support. SEM analysis showed uniform distribution on the catalysts, when ultrasound is used during the synthesis. It is evidenced that the synthesized catalysts without the sonication treatment have more agglomerations, and consequently the particles are less distributed on the support surface. Excellent results were obtained in the oxidation of aromatic alcohols such as benzyl alcohol and benzyl alcohol derivatives, showing higher conversion and selectivity with Co/G60-s and Co/RX-s (100 % conversion and 99 % selectivity at 3 hours of reaction). It is believed that their chemical properties permit good dehydrogenation and the subsequent oxidation. On the contrary, the aliphatic primary and secondary alcohols by their structural and chemical characteristics do not demonstrate good conversions.
Conclusion: Cobalt oxide-based catalysts supported on activated-carbon (Co/AC) showed good catalytic activity and selectivity during the alcohol oxidation. The use of sonication is an important factor if used in impregnation step, because it causes physicochemical changes by the cavitation and implosion processes, changing the nucleation and agglomeration of the active phase on support. This favors the catalytic development of the system mainly involved in quantity, dispersion and particle size. Excellent results were obtained in the oxidation of aromatic alcohols such as benzyl alcohol and benzyl alcohol derivatives, showing higher conversion and selectivity with this type of catalyst (Co/AC). The effect of sonication on the preparation of this type of catalysts might contribute to the study of the green oxidation reaction of alcohols with O2 as oxidizing agent.
Keywords: Oxidation of alcohols, activated carbon, molecular oxygen, cobalt oxide, sonication, ketones.
Graphical Abstract
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