Fe-based heterogeneous catalysts for the catalytic wet peroxide oxidation of phenolic compounds for wastewater treatment

Water shortages worldwide are exacerbated by wastewater discharge into the environment without adequate treatment, thus requiring developments in water treatment technologies, an area of research that has gained increasing attention in recent years and it is one of the sustainable energy development goals (SDG6) to be achieved by 2030. One of the most promising approaches to tackle and remedy this fundamental environmental issue, we will apply catalysis and the development of new materials to the abatement of toxic pollutants. In this work, we will exploit an emerging catalytic technology known as catalytic wet peroxide oxidation (CWPO) based on Fenton and Fenton-like reactions, to degrade aromatic pollutants in water under mild conditions by targeting phenolic compounds as the representative of pollutants. The scope to develop new iron-activated carbons and zeolites capable of degrading phenol-like compounds to CO2 and water is to have materials that are applicable for large-scale applications. Also, to be durable as expected for the treatment of large volumes of water, by having materials with a diminished amount of metal leaching in solution and thus to increase the catalyst reusability and diminish the environmental impact. Fe-supported activated carbon catalysts were prepared by using a wetness impregnation protocol with a Fe loading fixed at 12wt%. in all our catalytic tests the amount of phenol to consume was set at 1 gL1 and reaction times for 1 to 4 h. A peculiarity of this study was the pre-treatment of the carbon matrices with HCl and HNO3 to induce structural changes on Fe centres and in turn on the catalytic activity of these materials. A comparative analysis was done to study the effect of these pre-acid treatments (using HCl and HNO3). It was observed that that Fe/AC catalysts can show high phenol conversion (100%), but the high Fe leaching (up to 50%) affects the stability of the catalysts. Elemental analysis, XPS and XRD methods were employed to provide ground for structure/activity correlations. We found that the active species was Fe2O3, and most active catalysts were those with a Fe2O3 diameter less than 40 nm whereas those least affected by leaching were those with a diameter greater than 120 nm. Fe leaching though could be reduced to about 12% by the activated carbons doping with S and N. Catalytic activity results show that, the Fe-S-N/AC, prepared by AC pre-treated with HCl and HNO3, catalysts are the most active and stable catalysts when they were applied for phenolic compound oxidation. In view of these results, we then extended our investigation in the use of Fe-doped zeolites. Fe-ZSM-5 was prepared by wetness impregnation method and shows a high phenol conversion (100%), 12% residual intermediates%, and H2O2 consumption (100%) under the reaction conditions However, the Fe-ZSM-5 catalyst suffers poor stability due to high metal leaching losses (> 50%). Fe-ZSM-5 was also synthesized by an innovative wetness impregnation under vacuum method. However, no difference in the catalytic activity between the two catalysts for phenol oxidation by the CWPO reaction at an array of reaction temperatures from 40 to 80 °C. Based on the extreme catalytic activity of Fe-S-N/AC catalysts, we have applied the same doping protocols for the preparation of zeolites, a novelty in this area. The Fe-S-N-Zeolite-Y catalyst was identified as the most efficient catalyst for this reaction in terms of complete phenol oxidation and mineralization (100% and 0% for phenol conversion and residual intermediates, respectively). The Si:Al molar ratio though was able to influence the final Fe-S-N-Zeolite catalyst performance. However, the materials despite being very active, were affected by high Fe-leaching (ca 50%) Fe leaching at a high level affects the stability of these catalysts. The activity of species besides Fe, like Ag, was also considered and Ag as well as Ag/Fe-doped were synthetised and investigated. A strong synergistic effect from these two metals were identified, and this was irrespective of Si:Al ratios or Zeolite types. Then this study concluded by expanding the application of the selected heterogeneous catalysts prepared in this project to oxidise phenolic compounds further than phenol. The catalysts: Fe-S-N-ZSM-5, Fe-Ag-ZSM-5, and Fe-S-N/AC derived from pre-acid-treatment of AC by HCl-HNO3 were identified as the most active when used for the abatement of substrates like: 4-chlorophenol (4CP), 4-bromophenol (4BrP), 3-methoxyphenol (3MOP), 4-cresol (4MP) and 2,4 dimethylphenol (DMP), thus showing the applicability of our methods and materials to an array of compounds which abatement is at the centre of environment application for water purification.