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.