Development of electrochemical steps for glucose electrooxidation to value-added products
Carbohydrates are renewable, inexpensive and available organic raw materials. Only 3–5% of carbohydrates have industrial use, the rest decays and recycles along natural pathways. One interesting finding in this field has been the recognition that aldonic and aldaric acids, sugar acids, have potential uses in fine chemistry. The carboxylic group is, in fact, able to react selectively with different amines, alcohols and vinyl derivatives forming products with new application profiles like aldonolactones, which are used in the preparation of N-alkyl aldonamide surfactants. However, the reasons for the limited use of carbohydrates as raw materials in fine chemistry are related to their over functionalization and also their poor solubility in most of the commonly used organic solvents. The challenge is to achieve a direct and region-selective oxidation of saccharides in aqueous media, which is difficult by classical chemical methods without a preliminary protection strategy. Electroorganic approaches have currently fascinated academicians and industrial researchers because of their high potential prospects for industrial ventures. Electrocatalytic organic synthesis provides a powerful tool to control the reaction rate and selectivity through electrode potential and current. Furthermore, electrosynthesis is naturally suited to obey the principles of Green Chemistry, owning to several environmentally favorable features: i.e., reduced energy consumption, use of renewable raw materials, decreased emission of pollutants or toxic raw materials. For this reason, the use of electrochemical organic synthesis represents a promising alternative to the traditional industrial methods. Despite its sustainable nature and its potential to electrify the industry, as such replacing traditional, non-sustainable production processes of a broad range of fine chemicals, electrochemical synthesis methods are still very underdeveloped as compared to their traditional alternatives. More research is needed to better understand electrochemical processes and address the main challenges that prevent their application at industrial scale: i.e., the still unsatisfactory selectivity and/or productivity, the electrodes’ limited lifetime and the insufficient know-how on up-scaling towards industrial scale. This PhD thesis is specifically dedicated to the study of electrocatalytic routes for the selective oxidation of glucose to gluconic and glucaric acid (both of which are commercially relevant carbohydrates). To achieve this, it is of crucial importance that the exact reaction mechanism is understood. The aim here is thus to investigate the factors that determine the selectivity of the reaction towards the two products of interest, including the choice of the catalyst and the reaction conditions, and, as such, unravel the reaction mechanism beyond it. To this end, a combination of electrochemical and analytical techniques is used where microscopical surface analysis, used for the morphological characterization, is linked to its electrocatalytic performance. This research starts with the investigation of the electrocatalytic activity of MnO2-based catalysts towards glucose oxidation, to understand the underlying mechanism and its potential application for the production of gluconic and glucaric acid. MnO2 films are first synthesized by electrodeposition and chemical impregnation methods on various supports and then tested in a batch cell for glucose electrooxidation at various reaction conditions. Electrochemical evaluation reveals that the catalyst with the smallest porosity (average pore size 45 nm), also results in the highest electrocatalytic activity in presence of glucose (~10.5 mA cm-2), but it does not yield the desired products in significant quantities, as shown by chromatographic analysis, and is therefore discarded for further studies. Next, the mechanism of three noble metals (Cu, Pt and Au) towards glucose electrooxidation is investigated using rotating disk electrodes in a batch cell. For all three, a strong relationship is found between potential and reactivity of the functional groups in the glucose molecule. The Au electrode shows the highest activity (2 to 4 mA cm-2) and selectivity to gluconic (86.6 %) and glucaric acid (13.5 %), owning to two distinct oxidation peaks, one, at low potential (0.55 VRHE), for the oxidation of the aldehyde group on C1, the other, at higher potential (1.34 VRHE), for the oxidation of the hydroxymethyl group on C6. These experiments thus suggest that the reaction takes place in two separate steps: first, the oxidation of glucose to gluconic acid and, then, the further oxidation to glucaric acid. Given its promising electrochemical performance, Au is selected for further studies. In the second part of this work, to help unravel the glucose electrooxidation mechanism on Au, we systematically investigate the influence of different operational parameters, i.e., pH, initial substrate concentration, applied potential, reaction temperature and time, for both oxidation steps. To this purpose long-term electrolysis experiments are conducted in a batch cell at different reaction conditions after which the products are analyzed by liquid chromatography. Results show that, in the first oxidation step, glucose to gluconic acid, the maximum selectivity (97.6 %) is achieved at moderately high pH, low temperature and low initial concentration of glucose, and is mainly limited due to the presence of competitive, base-catalyzed, chemical reactions. In the second electrooxidation, gluconic to glucaric acid, these parameters do not have a significant impact on the selectivity, that reaches a maximum of 89.5 % at 1.1 VRHE, but only limited concentrations of glucaric acid are obtained due to early deactivation of the Au electrode possibly caused by fouling by glucaric acid itself. This is established as the most likely cause for deactivation after discarding all potential other options including e.g., Au leaching, which is investigated by analyzing the reaction solution after operation. In the final section of this work, a novel electrocatalyst is developed with a higher electroactive surface area, 39.5 cm2 compared to the 0.702 cm2 of the bulk electrode, and a gold loading of 9.3 %, to be feasible for larger scale applications. To this aim, an Au-based catalyst consisting of Au nanoparticles deposited over a porous activated carbon is synthesized using a straightforward chemical impregnation method. The final material is characterized electrochemically and analyzed by various physical characterization techniques. Its electrocatalytic activity is determined by the EASA and tested for gluconic acid oxidation by electrolysis in batch and flow cell. While further optimization of the catalyst is needed for its effective application for glucose and gluconic acid electrooxidation, results suggest that this catalyst could have a potential use for a broad range of applications, including reactions of small gas molecules such as O2, CO2, CH4 and N2. In conclusion, in this work, a promising electrocatalytic route for the selective oxidation of glucose to glucaric acid is developed and the electrochemical process explained. Much research is still needed to make this process possible at a large scale.
Antwerp : University of Antwerp, Faculty of Applied Engineering , 2024
xiii, 127 p.
Supervisor: Breugelmans, Tom [Supervisor]
Supervisor: Helsen, Joost [Supervisor]
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Creation 22.04.2024
Last edited 23.04.2024
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