Phenol oxidation by air using a Co (II) Salen complex catalyst supported on nanoporous materials: synthesis, characterization and kinetic analysis

• Phenol oxidation by air and Co Salen heterogeneous catalysts in a batch reactor.
• Kinetic analysis of phenol oxidation with Co Salen complex supported on SBA-15 as catalyst.
• Selective oxidation of phenol to catechol.
• Production of catechol under mild conditions.

Cobalt (II) Salen complex catalysts were synthesized and immobilized through covalent bonds on synthetic silica (SBA-15) and commercial supports, including silica (SiO2), alumina (Al2O3) and granular activated carbon (C). Fourier transform infrared spectroscopy (FTIR), ultraviolet-visible (UV–vis), diffuse reflectance ultraviolet-visible spectroscopy (DR–UV–vis) and X-ray diffraction (XRD) were used to characterize and follow the synthesis of the cobalt (II) Salen complex The UV–vis absorption spectra confirmed the formation of the Salen ligand and its coordination with the Co (II). FTIR spectra showed the incorporation of the Co Salen complex and its anchoring to the support was confirmed by DR–UV–vis. The catalytic activity was evaluated measuring the oxidation of phenol by air in a buffer solution using micellar electrokinetic chromatography (MEKC). The effects of support, initial substrate concentration, pH, oxidant (O2 vs H2O2) and temperature were investigated. The best catalytic performance was obtained when the Co Salen complex was immobilized on SBA-15 (Co Salen/SBA-15). Conversely, the results suggest that the activated carbon promoted the irreversible adsorption of the substrate. The Co Salen support was also characterized by scanning electron microscopy (SEM) to elucidate the morphology. Elemental analysis using energy dispersive X-ray spectroscopy (EDX) exhibited better Co Salen dispersion on the SiO2 and SBA-15 supports. The cobalt loading calculated from atomic absorption spectroscopy (AAS) was 0.22, 0.20, 0.14 and 0.09 mmol/g for the Co Salen supported on SiO2, SBA-15, Al2O3 and C, respectively. A specific surface area of 736.13 m2/g and a nanoporosity of 3.62 nm were obtained for SBA-15 using N2 physisorption analysis; this high specific surface area is characteristic of SBA-15 and plays a critical role in the reported activity. According to these results and the corresponding data analysis, the catalyst promoted the oxidation of phenol with a reversible first-order kinetic reaction in air, as observed from the data obtained at 3 different temperatures (25, 38 and 50 °C). These results enabled the calculation of activation energy values of 63 and 57 kJ/mol. In addition, the Co Salen/SBA-15 showed very good selectivity toward the production of catechol, which is one of the most important precursors in the chemical industry.

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