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Direct synthesis of H2O2 at ambient conditions and its applicability for water treatment with Fenton’s catalysts

Santos Hernandez, Alba 2022. Direct synthesis of H2O2 at ambient conditions and its applicability for water treatment with Fenton’s catalysts. PhD Thesis, Cardiff University.
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Abstract

Hydrogen peroxide (H2O2) is a colourless liquid oxidant widely applied in several industrial activities such as bleaching, chemical synthesis, mining and disinfection. Currently, over 4 million tons of H2O2 were reported in 2021 to be annually produced to meet the demands of the general public. In 1832 a variant of Thenard’s process became the first manufacturing route for the production of H2O2. Nowadays, there are known three industrial routes to obtain H2O2; i) the oxidation of primary and secondary alcohols, ii) the electrochemical synthesis iii) anthraquinone autoxidation process. The latter is the main industrial route accounting for the 95 % of the annual H2O2 production. This is a multistep process that involve the hydrogenation of the anthraquinone to 2-alkylanthrahydroquinone and its subsequent oxidation to re-generate the anthraquinone and give H2O2. However, the H2O2 generated needs to be separated from the organic solvents that were employed as the working carrier for the anthraquinone molecule, and subsequently, the solution containing the H2O2 needs to be purified to remove any impurities generated as a result of undesired side reactions and residues of organic solvents that remain after the separation. These two steps, separation and purification, lead to an aqueous solution of 70 wt.% of H2O2. However, there are many drawbacks associated with this industrial route. In addition to the high energy consumption associated with the separation and purification of the H2O2, the limited stability of the H2O2 in the aqueous solution (70 wt.%) requires the inclusion of stabilisers to prohibit its degradation that lead to form H2O. Also, this highly concentrated solution of H2O2 (70 wt.%) needs to be diluted to concentrations more suitable for a safety transportation (2-8 wt.%), requiring further water consumption that leads to an increase of the overall cost of the process. However, many applications require the use of relatively low concentrations H2O2 including bleaching, chemical synthesis and cosmetics (< 9 wt.%) and < 0.1 wt.% for water treatment, therefore the high energy demand of the separation and purification steps of the initial phases of H2O2 production are effectively wasted. Hence, there is an interest in developing technologies to decentralise the production of H2O2 to reduce the cost and make the process more environmentally friendly as well as reduce the risk in transportation. The direct synthesis of H2O2 from H2 and O2 would be a more atomically and straight forward route for small scale and on-site production. The direct synthesis of H2O2 is commonly undertaken at sub-ambient temperatures (2 °C) and usually employs a mixture of organic solvents such as methanol and water. These conditions are chosen to avoid the degradation of H2O2 which leads to the formation of H2O, reducing the overall selectivity of the process. However, these conditions, are not considered environmentally friendly or industrially applicable due to the high energy demand associated with the process of cooling. In addition, for some applications where H2O2 is generated and consumed in-situ, the presence of organic solvents can be an inconvenience. For instance, for water treatment VII purposes, the organic solvent must be removed during the treatment process to make the treated water suitable for consumption or suitable to pour into the water effluents. Bi-metallic AuPd supported catalysts have been widely reported in the literature to be more selective than mono-metallic Pd systems towards the direct synthesis of H2O2. It has been documented that Au can control the dispersion of Pd, modifying by this way its nanoparticle size, shape and surface configuration so as to make it more selective towards the direct synthesis of H2O2. With these prior studies and the need to reduce the cost of the process associated with H2O2 production in mind, the aim of Chapter 3 was to study the activity towards the direct synthesis and degradation of H2O2 with different AuPd bi-metallic supported catalysts (1 wt.% AuPd/TiO2), made by modified impregnation, at conditions more suitable for industrial processes (ambient temperature (25 °C) and water-only in the reaction medium). The results showed that the catalyst’s activity and selectivity towards the direct synthesis of H2O2 was driven by the reaction conditions used. However, the addition of Au to the mono-metallic Pd system was seen to be key to improve the activity and selectivity towards the direct synthesis of H2O2 –in comparison to the mono-metallic Pd and Au analogue- under conditions considered to be more favourable on industrial scale (25 °C and H2O-only as solvent). Thus, this lead to the conclusion that Au is key in controlling the selectivity of Pd catalysts towards the direct synthesis of H2O2, although the utilisation of other secondary metals in recent years has also shown promise in controlling its selectivity. Water remediation employing green oxidants such as H2O2 would be highly convenient since H2O2 produces only H2O as a result of the oxidation reactions. Another positive aspect of employing H2O2 as water remediation, is that H2O2 does not produce disinfection byproducts (DBPs) which are detrimental to human health. These DBPs are formed due to the reaction of dissolved organic matter (DOM) with some oxidants such as chlorine (Cl2) that is still applied for water remediation. The main drawback is that H2O2 is not effective to oxidise aromatics, which may be present in polluted waters such as antibiotics. However, reactive oxygen species (ROS), that refers to any specie with one or more unpaired electrons (superoxide (O2 •- ), hydroxyl (HO• ), peroxyl (RO2 • ) and hydroperoxyl (HOO• ) radicals), can be generated through the cleavage of H2O2, offering greater oxidation potential than the H2O2 itself. Thus, catalyst that could synthesise in-situ H2O2 and its subsequent conversion to ROS at ambient temperature and water-only in the reaction medium would be highly desirable for water treatment process. Chapter 4 aimed to identify key bi-metallic and tri-metallic combinations of supported metals which are active for both H2O2 synthesis and its conversion to ROS (1 wt.% XPd/TiO2; X: Fe, Cu, Co, Au). These catalysts prepared by modified impregnation were investigated towards the direct synthesis and degradation of H2O2 and towards the degradation of phenol, which was used as a model pollutant commonly found in industrial waste waters. Among the four bi-metallic VIII combinations tested (1 wt.% XPd/TiO2, X: Fe, Cu, Co, Au), the bi-metallic 1 wt.% AuPd/TiO2 showed the lower activity towards the degradation of phenol, revealing the inefficient activity of H2O2 in oxidising aromatics. On the other hand, the bi-metallic 1 wt.% FePd /TiO2 supported catalyst showed better performance towards the degradation of phenol in comparison to the other two bi-metallic CoPd/TiO2 and CuPd/TiO2 catalysts. It was suspected that the low activity towards the degradation of phenol that the bi-metallic 1 wt.% CoPd/TiO2 and 1 wt.% CuPd/TiO2 catalysts presented could have been related to the Co and Cu poisoning the Pd active sites, avoiding the in-situ formation of H2O2 and enhancing its degradation. The impregnation of the Fe on the surface of the support, alongside Pd, led to the generation of FexOy species which were easily leached by the generation of the diacids which were synthesised as a result of the oxidation of phenol. As such, despite 1 wt.% FePd/TiO2 showed promising activity, they are not suitable for application in in-situ oxidative treatment of waste streams. Subsequently, further synthetic routes were investigated in order to stabilise Fe during the oxidation of phenol, which was the scope of Chapter 5. In this chapter, Pd impregnated on HZSM-5 support containing Fe immobilised in the zeolitic channels (0.5 wt.% Pd/ X wt.% Fe-HZSM-5, X: 3-0.06), were studied for phenol degradation. It was hypothesised that through the attachment of Fe species within the HZSM-5 channels, the stability of Fe against leaching would be improved. While the leaching of Fe was significantly reduced, and the resulting catalyst were found to be more active and stable than those developed in Chapter 4 (1 wt.% FePd/TiO2), the loading of Fe had to be significantly increased to see relevant activity. The crystallinity of the HZSM-5 got compromised when the amount of Fe increased, which led to produce instable Fe species as those found for the bi-metallic 1 wt.% FePd/TiO2 catalysts. In addition to this, Pd was found to leach during the direct synthesis of H2O2 and during the degradation of phenol, which it was not the case for the bi-metallic 1 wt.% FePd/TiO2 catalysts. Therefore, it was concluded that further investigation about how to improve the activity and stability of the bimetallic 0.5 wt.% Pd/ X wt.% Fe-HZSM-5 (X: 3-0.06) catalysts needed to be developed to make them suitable for commercialization.

Item Type: Thesis (PhD)
Date Type: Completion
Status: Unpublished
Schools: Chemistry
Date of First Compliant Deposit: 30 January 2023
Last Modified: 05 Jan 2024 05:38
URI: https://orca.cardiff.ac.uk/id/eprint/156360

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