The effect of ultrasonic treatment on iron–chromium pillared bentonite synthesis and catalytic wet peroxide oxidation of phenol

• Fe/Cr-pillared bentonites were prepared by considerably reducing the water consumption and synthesis time.
• Catalytic activities were tested for the catalytic wet peroxide oxidation of phenol with ultrasonic heating.
• Temperature was found to have a more pronounced effect, increasing the phenol conversion and mineralization.
• Aromatic and aliphatic oxidation intermediates are identified.

In this study, compared to traditional methods, Fe/Cr-pillared bentonite synthesis whose Fe/Cr mol ratio is 1/9 is carried out by considerably reducing the water consumption and synthesis time. Fe/Cr-pillared bentonites were characterized by scanning electron microscope with energy dispersive system (SEM–EDS), powder X-ray diffraction (XRD) and N2-adsorption/desorption isotherm. The characterization results indicated that the Fe/Cr-DC sample prepared by adding dry clay to the pillaring solution directly exhibits comparable surface properties to those of the sample (Fe/Cr-SS) prepared by adding the pillaring solution on clay dispersion slowly. Catalytic behavior of Fe/Cr-pillared bentonites was tested for the catalytic wet peroxide oxidation (CWPO) of phenol with ultrasonic heating. No significant difference is observed among the phenol conversions for all of the synthesized samples. Phenol conversion increased with increasing temperature until 45 °C, but further increasing the temperature in higher than 45 °C was not very effective. The phenol conversion at pH 5 was higher than at pH 7. Additionally, a nearly complete removal of phenol was obtained over 2 h using Fe/Cr-DC catalysts, at the experimental conditions of T = 45 °C, pH 5, mcat = 5 g/L, [H2O2/phenol] = 16, during reaction peroxide addition (2 × 10− 4 mol/h) and ultrasonic heating. It is obtained that, aromatic intermediate products as catechol, hydroquinone and benzoquinones which are obtained in the beginning of oxidation oxidated into carboxylic acids. However, carboxylic acids such as oxalic and formic acid showing resistance to oxidation results by the level of total organic carbon (TOC) conversion being low.

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