Photo-Fenton catalysts supported on microporous materials for industrial pollutant oxidation
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
Industrial pollution remains an issue in many parts of the world. While the problem is localised to certain areas with high densities of heavy industries, the worldwide demand for the products produced means that there is a worldwide responsibility for dealing with the pollution. As some compounds in the wastewater are difficult to treat with traditional methods, new processes must be developed to remove or degrade these pollutants. Advanced Oxidation Processes (AOPs) are a promising method of wastewater treatment that utilize in situ generation of the powerful hydroxyl radical to oxidize pollutants with little to no secondary waste. A significant disadvantage to AOPs is cost, which is compounded by the need for specific reaction conditions, especially pH. Therefore, any catalyst that can widen the range of reaction conditions will help to reduce the cost and improve adoption of AOPs for recalcitrant wastewater treatment.
In this work, a novel heterogenous photocatalyst was synthesized by the loading of iron onto the metal-organic framework MIL-47 to create Fe/MIL-47. An iron-zeolite Y catalyst was also synthesized following methods previously reported. These catalysts were then tested for the photo-oxidation of methylene blue, a common model pollutant dye, under UV irradiation. The Fe/MIL-47 catalyst was found to be very effective for both the decolourisation and total organic carbon (TOC) removal at low pH when compared with the iron-zeolite catalyst, with 94% colour removal and 52% TOC removal after 180 minutes for the Fe/MIL-47 catalyst and only 74% colour removal and 8% TOC removal for the Fe-zeolite Y catalyst at the same catalyst loading. The base MIL-47(V) catalyst was also tested and was found to be active for the decolourisation of methylene blue, a result in agreement with previous research that found that other metal-carboxylate frameworks were also photocatalytically active. The Fe/MIL-47 catalyst was found to only be effective over a limited pH range, and therefore a copper MIL-47 catalyst was also synthesized, with a goal of possibly creating a bimetallic catalyst with a wide effective pH range. While the Cu/MIL-47 was indeed found to be active at the higher pH that the Fe/MIL-47 was less active at, the stability of the MIL-47 support was determined to be less than what was necessary for a durable catalyst, and therefore the focus of the remaining studies was shifted to the zeolite based catalyst.
While iron-zeolite Y catalysts have been used as heterogenous photocatalysts before, the potential effect of the BrØnsted acid sites on the reaction rate has not been researched. In this section of work, the effect of NH4+, Na+ and H+ in zeolite Y on photo-Fenton oxidation of formaldehyde in a batch reactor was examined. The model pollutant was changed to formaldehyde, as unlike methylene blue, formaldehyde can diffuse within the pore structure of the zeolite, helping to illuminate any effect on the oxidation reaction by the internal pore environment The catalysts were prepared by partial exchange of Fe3+, in the zeolite Y. The charge balancing cations were found to play a vital role in the photo-Fenton oxidation of formaldehyde, with the Fe/zeolite Y catalyst prepared with BrØnsted acid sites (H+) exhibiting three times the reaction rate of the NH4+ or Na+ containing catalysts at pH 7 (TOF are 10.3, 2.7 and 3.4 μmol [mol Fe s]-1, for Fe/H-Y, Fe/NH4-Y and Fe/Na-Y respectively).
The results of the study into the charge balancing cations in zeolite Y, did yield an effective photocatalyst at high pH, however the effect was only observable at low catalyst loading (0.5 g L-1), and low iron loading on the catalyst (50% exchanged), limiting the maximum concentration of pollutant that could be treated. Therefore, a bimetallic copper-iron catalyst was synthesized by incipient wetness ion impregnation of zeolite Y with iron and copper ions. This FeCu-zeolite Y catalyst was then tested for the photo-oxidation of methylene blue over a range of pHs. It was found to be effective at both low pH, with 98% colour removal and 89% TOC removal, and high pH, with 99% colour removal and 79% TOC removal. The FeCu-zeolite Y catalyst was also tested under recycle and found to remain effective after 3 runs. The XRD results also showed no loss in crystallinity, indicating the catalyst support continued to be stable throughout the recycle.
Collectively, these results present the attempt to create a pH insensitive heterogeneous photo-Fenton catalyst. The MIL-47 based catalyst unfortunately did not have the required stability to fulfil this, however the partially exchanged zeolite Y and the bimetallic zeolite Y catalysts did allow for wider pH range photo-Fenton reactions.