Verdedigde doctoraten

Roy Aerts

• 22/11/2022
• 5 p.m.
• Venue: Campus Drie Eiken, Q0.02
• ​Online PhD defence
• Supervisors: Christian Johannessen & Wouter Herrebout
• Department of Chemistry

Abstract

Fundamental spectroscopic research revolves around comprehending the subject technique on a theoretical level and discovering in what chemical queries it is deployable. This dissertation attempts to do this for the technique called Raman optical activity (ROA).

ROA is the detection of a differential Raman intensity between sending in right- and left circular polarized light or the differential detection of right- or left circular polarized Raman scattered intensities. The preference for one of the circular polarization states finds its origin in the chirality of a system. The main application of ROA were found to be absolute configuration determination of small organic compounds and the secondary structure elucidation of proteins. Only from the unlocking of ROA spectral calculations onwards more sophisticated elucidations took place. Various polypeptides and (poly)carbohydrates became subjects of investigation by combining empirical work with spectral simulations. By 2018, when this thesis was initiated, ROA calculations and analyses were possible for quite sophisticated systems, finally permitting the investigation of molecules that are actually being used in pharmacy, an application area where ROA was believed to be a valuable future contributor. The question that remained, however, was: how exactly can ROA contribute and how should this be done? The main goal of this dissertation is to respond hereon, as such bringing together ROA and pharmacy.

The antibiotic glycopeptide molecular class was carefully selected as subject of investigation, the aim being the determination of the conformational ensemble of this molecular class by means of ROA. Besides ROA, also the closely related vibrational circular dichroism (VCD) technique is added to the study. Subsequently, derivatives of vancomycin -oritavancin, dalbavancin, teicoplanin- were recorded, so as to experimentally observe the spectral effect of changing the chemical structure and/or conformation of the glycopeptide. Finally, the vancomycin binding to its biological target, Lipid II -a bacterial cell-wall precursor- was investigated using ROA. The principal conclusion that can be derived from all the above studies is that the aromatic systems that adopt a fixed conformation define the spectral responses, whereas the carbohydrates barely contribute.

All combined, the obtained results provide important insights in the applicability of the VOA techniques in the conformational elucidation of challenging, real-world molecular systems. Both the strategies and the type of measurements contained herein represent a step in connecting the ROA field and pharmaceutical world.

Nick Gys

• 19/09/2022
• 4 p.m.
• Venue: Campus Drie Eiken, Building Q, Promotiezaal Q0.02
• Promotoren: Vera Meynen, Steven Mullens & Bart Michielsen
• Departement Chemie

Abstract

The recovery of Platinum-Group-Metals (PGMs) from secondary sources such as spent industrial catalysts is of increasing concern from both economic and environmental point of view. On industrial scale, PGMs are recycled by means of pyrometallurgical processes, followed by hydrometallurgical refining to acidic aqueous leachates. Given the low concentrations (ppm-ppb) of PGMs and complexity of these feed streams, selective adsorption is considered a promising recovery method.

Mesoporous SiO2 materials grafted with organosilanes containing N, S functional groups, are by far the best documented in literature. However, these materials require strict synthetic control to prevent the formation of a disordered organic layer, and under prolonged exposure to acidic conditions, leaching of the grafted groups becomes prominent. Ensuring a full monolayer coverage increases the stability, but reduces to some extent the tailoring of the surface properties. Grafting transition metal oxides with organophosphonic acids (PAs) has been reported as an interesting alternative, offering more stable bonds. Depending on the applied modification conditions, the grafting can be tuned towards different surface properties such as the surface coverage of functional groups, their bonding states and their distribution.

To adjust the surfaces to the application needs, the synthesis-properties-performance correlation needs to be understood. However, to date, in-depth insights into the synthesis-properties correlation are lacking for PAs containing hetero-groups. This know-how is crucial for a rationalized design of materials with clear insights into their applicability and which specific surface properties govern the performance. Therefore, the goal of this PhD is to unravel the synthesis-properties correlation on the grafting of mesoporous TiO2 powder with amino- and mercapto-alkylphosphonic acids, using a combination of experimental and computational approaches to strengthen our conclusions during in-depth surface characterization of these materials. Furthermore, palladium adsorption is envisioned as application domain to study the impact of the synthesis conditions and type of PA on the material’s performance.

In a second step, the knowledge transfer is made from powder to 3D-structured TiO2 materials. 3D shaping and the development of structured materials is a key step towards their successful implementation under industrial conditions. Recent advances in additive manufacturing has resulted in the formation of hierarchically 3D-structured materials with a high geometric flexibility and controllable meso-and macrostructures. As knowledge on the PA grafting of such materials is currently lacking, the impact of varying macroscopic properties of 3D printed titania structures on the extent of grafting throughout these materials is also studied in this PhD.

Kevin van 't Veer

• 10/05/2022
• 2 p.m.
• Venue: Campus Drie Eiken, O.01
• Supervisors: Annemie Bogaerts & François Reniers
• Department of Chemistry

Abstract

Ammonia (NH3) synthesis is crucial for the production of artificial fertilizer and is carried out through the Haber-Bosch process. With an energy consumption of 30 GJ/t-NH3 and the emission of 2 kg-CO2/kg-NH3, ammonia is the chemical with the largest environmental footprint. Haber-Bosch operates under high pressure and high temperature conditions. Plasma technology potentially allows greener ammonia production. Dielectric barrier discharges are a popular plasma source in which a catalyst is easily incorporated. The combination of plasma and catalyst can circumvent the harsh reaction conditions of the Haber-Bosch process.

Plasma kinetics modelling is used to gain insight into the mechanisms of such plasma-catalytic systems. Special attention is given to the instantaneous power absorbed by the electrons, the relevant fraction of the microdischarges and the discharge volumes. The importance of vibrational excitation is investigated. Depending on the exact discharge conditions, it was found that both the strong microdischarges and vibrational excitation can be simultaneously important for the ammonia yield.

The temporal behavior of filamentary dielectric barrier discharges was explicitly taken into account. Ammonia was found to decompose during the microdischarges due to electron impact dissociation. At the same time atomic nitrogen and other excited species are created. Those reactive species recombine to ammonia in the afterglow through various elementary Eley-Rideal and Langmuir-Hinshelwood surface reaction steps with a net ammonia gain.

Finally, the concept of the fraction of microdischarges was generalized. It directly represents the efficiency with which the applied electric power is transferred to each individual particle in the plasma reactor. It is argued that any type of spatial or temporal non-uniformity of the plasma will cause unequal treatment of the gas molecules in the reactor, corresponding to a lower efficiency at which the power is transferred to the gas molecules.

All of those insights aid in an increased understanding of plasma-catalytic ammonia synthesis as a potential green chemistry solution to the synthesis of ammonia on small scale.

Liselotte Neven

• 30/03/2022
• 10 a.m.
• Venue: Campus Drie Eiken, O.01
• ​Online PhD defence
• Supervisors: Karolien De Wael & Sabine Van Doorslaer
• Department of Chemistry

Abstract

Phenolic compounds can be found everywhere in our daily lives but exhibit high toxicity, low (bio)degradability and hormone-disrupting effects when they are released in the environment. It is for this reason imperative to develop detection strategies for these pollutants. A promising approach involves the use of a photoelectrochemical (PEC) sensor. In this sensor, a photosensitiser (PS) type II, which generates 1O2 under illumination, is used to oxidise phenolic compounds present in the sample. The oxidised phenols are reduced at the electrode surface leading to the generation of an electrocatalytic redox cycle.

In this thesis, an in-depth understanding, through the identification of the reactive oxygen species (ROS) in the PEC sensing mechanism, is obtained. The detection strategy is optimised by choosing the PS with the highest 1O2 production and by optimising the detection parameters so that the PEC sensor can be successfully applied for the detection of phenols in industrial samples.

First, it was determined that the use of highly fluorinated zinc phthalocyanine derivatives, F52PcZn and F64PcZn, as photocatalysts was optimal for the sensing of phenol due to their high 1O2 production and improved single-site isolation. However, next to 1O2, it was shown that the ROS O2•- and H2O2 were also generated in the PEC sensor. Their contribution to the photocurrent response was studied by rotating disk electrode measurements in function of the pH and applied potential. After this, the PEC detection strategy was optimised in terms of pH and applied potential for the detection of doxycycline, cefadroxil, and phenol. It was found that the use of alkaline pH-levels led to nmol L-1-level detection limits. The combination with square wave voltammetry (SWV) was, also, proposed to allow the quantification and identification of phenolic compounds in a specific sample. At last, the developed PEC and SWV sensors were applied for the measurement of phenolic compounds in industrial water samples. The PEC sensor could follow the decrease of the phenolic concentration throughout the wastewater treatment process while the SWV sensor provided the electrochemical fingerprints of these samples. The thesis concluded that the use of the PEC sensor was advantageous in the measurement of lower concentrated phenolic samples due to its high sensitivity and fast measurement time in comparison to commercial test kits.

Tim Van Everbroeck 

• 28/03/2022
• 2 p.m.
• Venue: Campus Drie Eiken, R1
• ​Online PhD defence
• Supervisor: Pegie Cool
• Department of Chemistry

Abstract

One of the big problems with gasoline-powered cars is that the exhaust gas contains pollutants which are detrimental for human health and the environment. For this reason cars are equipped with a three-way catalytic converter which has the function to convert carbon monoxide (CO), hydrocarbons and nitrogen oxides (NOx) to molecules that are not harmful for human health. The materials that are used to do this are the platinum group metals (PGMs), platinum, palladium and rhodium, which are very rare, expensive, critical raw materials. The increasing demand for new cars and progressively stricter emission standards drives the price even higher. So, there is a need to reduce the amount of PGMs in three-way catalytic converters and replace them by materials that are more common and cheap. A good candidate to do this is copper oxide (CuO) because it is common and shows some catalytic activity. While it is not as active as the PGMs it can be used in much larger quantities for compensation. To keep the CuO particles small, which is more efficient, they need to be supported on a material with a large surface area. Furthermore, the catalytic activity can be altered by interactions with other materials. For this thesis CuO is deposited on different support materials such as alumina, titania and ceria. The catalytic performance of the final materials are evaluated against its characteristics. Another part of the research focuses on the precipitation of copper with other elements is to obtain intimate mixtures of metal oxides and mixed metal oxides. The spinel-type materials are evaluated against pure CuO to find synergies between the different elements. A final study makes use of layered double hydroxides to obtain mixtures of metal oxides and mixed metal oxides. Again, we evaluate how certain properties and elements in the composition affect the catalytic performance. At the end of the thesis the conclusions are presented and the developed materials are compared to a commercial three-way catalyst. Furthermore, the limitations of this work are discussed and an outlook on the future of the automotive industry is presented and how the developed materials can play a role in this.

Karthik Gadde

• 02/02/2022
• 3.30 p.m.
• ​Online PhD defence
• Supervisors: Bert Maes & Kourosch Abbaspour Tehrani
• Department of Chemistry

Abstract

A major issue within chemistry is the use of expensive, non-renewable and toxic reagents/reactants, catalysts or solvents to carry out organic reactions. The replacement of these by inexpensive, abundant and benign alternatives is an ongoing challenge for the chemical community. The research of this doctoral thesis is focused on the development of novel sustainable methodologies for the synthesis of α-substituted homoallylamines via 2-aza-Cope rearrangement and subsequent functionalization of its alkene moiety via visible light photocatalysis.

α-Substituted homoallylamines are valuable synthetic building blocks and precursors for the synthesis of a variety of nitrogen-containing heterocycles, natural products and pharmaceutical compounds in organic synthesis and medicinal chemistry. In this doctoral thesis, a metal-free or base-metal-catalyzed 2-aza-Cope rearrangement strategy has been developed for the synthesis of α-substituted homoallylamines by using easily accessible aldehydes and 1,1-diphenylhomoallylamines.

Alkenes are one of the most fundamental functional groups in organic chemistry and are often derived from simple chemical feedstock. In the past few decades, the functionalization of alkenes has caught tremendous attention, especially strategies to achieve functionalization in an anti-Markovnikov manner were the focus of fundamental research. The installation of sulfonyl (R1SO2-) and sulfenyl (R2S−) moieties specifically is of application interest as they appear in natural products, bioactive molecules, and pharmaceuticals. Besides, they are attractive functionalities in organic synthesis as they are easily transformed into other functional groups. Direct functionalization methods of alkenes, which allow to introduce two different groups in a single reaction step such as the thiosulfonylation are therefore highly desired. In this doctoral thesis, a metal-free methodology for the vicinal thiosulfonylation of unactivated alkenes has been developed by using thiosulfonates as a reactant and the acridinium salt 9-mesityl-10-methylacridinium perchlorate as a photo-organocatalyst with visible-light irradiation. To illustrate the synthetic potential, the method was applied for homoallylamines and late stage functionalization of olefins in active pharmaceutical ingredients.