Defended Ph. D. Theses

Bart Wanten

• 19/12/2024
• 3 p.m.
• Venue: Campus Drie Eiken, S.01
• Supervisor: Annemie Bogaerts
• Department of Chemistry

Abstract

Several established processes face a significant cost to capture and store CO2, required to avoid its emission. Therefore, processes that are electricity driven, coupled to a renewable energy source, while providing a pathway to valorise CO2 and other greenhouse gases, are required.

In this thesis, I focus on the combined conversion of CO2 and CH4 (the so-called dry reforming of CH4) by means of a plasma reactor, in particular a confined atmospheric pressure glow discharge. Specifically, I conducted experiments for a wide range of conditions and additive gases, with the goal to find the optimal reaction conditions with this specific plasma reactor, that leads to the formation of syngas, a mixture of CO and H2. Syngas can be further processed into a variety of liquid fuels, such as methanol and various long-chain hydrocarbons, depending on the relative amount of CO and H2. An excellent performance compared to the state-of-the-art is achieved. Specifically, a CO2 and CH4 conversion of 64 % and 94 % is reached, respectively, at an energy cost of 3.2-4 eV/molecule. The confinement in the plasma is assumed to play a crucial role with regards to these excellent results. Unfortunately, the product output’s value remains limited, as soot formation limits the use of sufficiently high amounts of CH4 with this setup, necessary to achieve a syngas ratio (H2/CO) that is high enough for further processing towards various liquid chemicals.

A potential solution for this issue is the addition of other gases next to CO2 and CH4 that can avoid the formation of soot particles to some extent. When a small fraction of O2 is added (the so-called oxidative CO2 reforming of CH4), the syngas ratio is only slightly improved. The bi-reforming of CH4, where H2O is added instead of O2, effectively helps in counteracting soot formation. Moreover, a syngas ratio of 2 is reached, which is ideal for further methanol synthesis.

Finally, these processes are compared in terms of environmental impact and economic potential. Oxidative CO2 reforming of CH4 is found to be the most promising process, with a score twice as high as for the bi-reforming of CH4 process on almost all environmental parameters considered. Compared with current syngas production technologies, all three plasma processes lead to an improved environmental performance. These results show the potential of a bright future for plasma-based syngas production.

Yuman Qin

• 22/11/2024
• 4 p.m.
• Venue: Campus Groenenborger, V.008
• Supervisor: Shoubhik Das
• Department of Chemistry

Abstract

The advancement of sustainable technologies is imperative for mitigating global environmental issues. Photoredox catalysis leverages light to facilitate chemical reactions, representing an energy-efficient and environmentally benign strategy. Complementary to this, circular chemistry (CC) provides a guiding framework for chemical researchers, emphasizing resource efficiency and waste minimization, which is crucial for driving a sustainable future. By converting plastic waste into valuable resources, advanced catalytic processes like photoredox catalysis can substantially lower environmental impact. Furthermore, integrating CO2 capture technologies with CC principles is vital for combating climate change by reducing atmospheric CO2 levels. Additionally, the direct oxidation of C(sp3)–H bonds using O2 as an oxidant is a significant process in fine chemical industries, underscoring the importance of efficient and sustainable oxidation processes. This thesis investigates the synergistic potential of photoredox catalysis, circular chemistry, plastic waste valorization, CO2 capture, and efficient oxidation processes, illustrating how these methodologies can effectively reduce environmental impact, optimize resource use, and support sustainable development. The thesis is divided into four chapters:

• Chapter 1: An overview and introductory exposition of the fundamental principles and concepts of photoredox catalysis are provided. 

• Chapter 2: We have developed a metal-free photocatalytic system for the valorization of plastic waste, specifically targeting 13 different polystyrene-based plastics that are commonly found in daily life. These plastics were successfully converted into benzoic acid. Subsequently, benzoic acid was transformed into aromatic building blocks such as benzene, toluene, salicylic acid, and biphenyl. This approach demonstrated significant potential in addressing the challenges posed by plastic pollution and the requirement for the production of aromatic compounds. 

• Chapter 3: We have pioneered a methodology for utilizing CO2 from exhaust gases to synthesize γ-lactam to mitigate atmospheric CO2 accumulation. This approach boasted a wide substrate scope and high functional group tolerance. Furthermore, it enabled the construction of an γ-lactam core between diverse bioactive compounds, highlighting substantial potential for drug design applications. 

• Chapter 4: We have developed an efficient single-atom heterogeneous photocatalyst for the oxygenation of allylic C-H bonds by utilizing O2 as the oxidant which did not require an additional hydrogen atom transfer (HAT) agent. This reaction system has been meticulously engineered, demonstrating excellent substrate compatibility and functional group tolerance, which underscores its industrial applicability and validates the significance of this approach.

Abstract

This thesis explores the development and application of novel heterogeneous photocatalysts for environmentally sustainable production of hydrogen peroxide (H₂O₂) and CO₂ reduction. Utilizing sunlight as an energy source, these photocatalytic processes aim to address the global energy demand and reduce dependence on non-renewable energy sources while mitigating climate change impacts. Conventional methods of H₂O₂ production rely on hazardous chemicals, generating considerable waste. In contrast, photocatalytic H₂O₂ synthesis presents a cleaner alternative, with fewer harmful by-products. Similarly, the rising CO₂ levels, primarily due to fossil fuel combustion, contribute significantly to global warming. Photocatalytic CO₂ reduction, integrating Carbon Capture and Utilization (CCU) strategies, offers a promising solution by capturing CO₂ from exhaust gases and repurposing it for various applications, thus reducing emissions.

The thesis begins with an introduction to semiconductor photocatalysts, with a focus on BiOBr-based, single-atom, and g-C₃N₄-based catalysts, covering their synthesis, photocatalytic applications, and associated reaction mechanisms. Subsequent chapters delve into the specific catalysts developed for H₂O₂ production and CO₂ reduction.

In Chapter 2, a lignin-supported BiOBr photocatalyst (LBOB) is presented, designed for direct H₂O₂ production from seawater. Various characterization techniques (e.g., XRD, SEM, UV-Vis, ssNMR, XPS) were employed to analyze its structural and optical properties. Results indicated that lignin enhanced the photocatalytic performance by providing structural stability, lowering reduction potential, and facilitating electron transfer via functional groups acting as electron sinks.

Chapter 3 introduces a manganese-based single-atom photocatalyst optimized for H₂O₂ production via water oxidation. The catalyst's structural and electronic properties were characterized using UV-Vis, EPR, XANES, EXAFS, HRTEM, XPS, and ssNMR. The Mn centers played a pivotal role in generating hydroxy radicals (•OH), crucial intermediates for H₂O₂ synthesis.

Chapter 4 examines an iron-based photocatalyst for converting CO₂-rich gas streams into valuable products, demonstrating an impressive carbon monoxide (CO) production rate. This catalyst’s efficiency highlights its potential for practical CCU applications, leveraging sunlight for sustainable chemical transformations.

These studies underscore the potential of advanced photocatalysts in achieving efficient, sustainable solutions for H₂O₂ production and CO₂ reduction, aligning with global environmental goals.

Joachim Slaets

• 08/11/2024
• 14.00 uur
• Locatie: Campus Drie Eiken, O.01
• Promotor: Annemie Bogaerts
• Departement Chemie

Abstract

Global CO2 concentrations in the atmosphere have reached unprecedented levels, driven primarily by anthropogenic emissions. This alarming rise in greenhouse gases (GHGs) presents a significant challenge to global climate stability, with CO2 being the primary contributor to climate change. Industrial activities are major sources of these emissions, highlighting the urgent need for innovative and sustainable solutions.

Plasma technology emerges as particularly promising, which creates a highly reactive environment through the presence of high-energy electrons. By leveraging such processes, CO2 can be transformed into useful chemicals, contributing to both emissions reduction and resource circularity. One interesting reaction, which can be carried out in a plasma environment, is the dry reforming of methane (DRM), a process that utilizes CO2 and methane (CH4) to produce a mixture of carbon monoxide (CO) and hydrogen (H2). These are valuable intermediates for further chemical synthesis, which can be used to synthesize a variety of chemicals and fuels.

Through chemical kinetics modelling a wide range of conditions is explored to better understand the core chemical kinetics of DRM in warm plasmas. Thereby examining the performance of the process across a wide temperature range and highlighting the limitations of various gas mixtures. The findings demonstrate where plasma-specific kinetics diverges from thermal gas-phase chemistry, offering new insights into the unique behaviour of plasma-driven reactions. Also, the effect of nitrogen (N2) on plasma-based DRM is investigated through computational modelling to support experimental results and demonstrate the role of N2 in the conversion process within a gliding arc plasmatron (GAP) reactor. Revealing a small fraction of N2 can improve the process.

Furthermore, after the plasma has converted the gas molecules, further chemical changes can still occur, influencing the overall efficiency and product distribution. The model demonstrates that quenching the gas temperature does not generally improve performance, except in CO2-rich mixtures where certain reactions are influenced by the cooling process, leading to notable changes in the product distribution. The benefits of combining the hot plasma effluent with unconverted gas are also explored, as the residual heat from the plasma can be reused to drive additional reactions, thereby improving the overall efficiency of the process.

The findings presented improve the understanding of plasma-based DRM technology, forming a foundation for further experimental and theoretical studies, and contribute to the broader goal of reducing GHG emissions and supporting the transition to a sustainable, low-carbon future.

Shangkun Li

• 10/10/2024
• 1.30 p.m.
• Venue: Campus Drie Eiken, R2
• Online PhD defence
• Supervisors: Annemie Bogaerts & Erik Neyts
• Department of Chemistry

Abstract

The direct conversion of CH4 to value-added chemicals has attracted intensive interest from both academic and industrial communities. Plasma technology is a promising approach to activate gas molecules by electricity instead of heat, and it can be operated at mild conditions and allows easy upscaling. The combination of plasma and catalysis often shows synergy. In order to optimise this syngergistic operation, it is important to understand the plasma and plasma-catalyst interactions at a fundamental level. However, it is not straightforward to reveal the entire mechanism because of the intrinsically highly complex reactions of plasma catalysis. This thesis aims to provide a fundamental understanding of plasma-driven direct CH4 conversion into value-added products using various oxidants (i.e., O2, CO2, and H2O), involving plasma gas-phase reactions and plasma-assisted surface reactions on a catalyst by combining experiments and modeling, which could be of great interest for the application of plasma-based gas conversion at a wider scale, boosting the transition towards a more sustainable energy economy.

Hamid Ahmadi Eshtehardi

• 25/04/2024
• 10 a.m.
• Venue: Campus Drie Eiken, R.R4
• ​Online PhD defence
• Supervisors: Annemie Bogaerts & Marie-Paule Delplancke
• Department of Chemistry

Abstract

Despite the recent increasing interest in plasma technology for nitrogen fixation purposes, industrialization of this technology faces several challenges, including challenges of plasma catalysis for selective production of chemicals, the high energy cost of plasma-based nitrogen fixation compared to current industrial processes, and the design and development of scaled-up and energy-efficient plasma reactors for industrial purposes. In the framework of this thesis, we have tried to tackle these challenges and add to the state-of-the-art in plasma-based nitrogen fixation using a combination of experimental and modelling work.

Fardokht Rezayi

• 22/02/2024
• 4 p.m.
• Venue: Campus Drie Eiken, O.01
• ​Online PhD defence​
• Incoming Joint PhD Cardiff University - Universiteit Antwerpen
• Supervisors: Sabine Van Doorslaer & Damien Murphy
• Department of Chemistry

Abstract

Cu(II) coordination chemistry is of significant importance due to copper's widespread applications, particularly in chemical catalysis. This thesis explores the molecular structure, electronic properties, and variable coordination geometry of trigonal bipyramidal complexes of Cu(II) with tripodal ligands, more specifically different tripodal tetraamines. While square planar and square pyramidal Cu(II) complexes are commonly studied, less attention is given to trigonal bipyramidal Cu(II) centres. A variety of Electron Paramagnetic Resonance (EPR) techniques is used as a unique analytical tool to probe Cu(II) complex chemistry.

While the counter ions had only a negligible effect on coordination through outer sphere interactions, the effect of the type of tetraamine, pH and their concentration was significant, revealing subtle and strong variations in the coordination chemistry upon change of these conditions and thus emphasizing the importance of understanding the solution-based structures when aiming for specific applications.

The performance of different trigonal bipyramidal Cu(II)-tetraamine complexes for the selective oxidation of glycerol was further explored. The interest in glycerol oxidation is growing, since glycerol is a valuable bio-renewable compound formed during biomass conversion. Through a combination of different techniques, the catalytic behaviour could be fit to the faith of the Cu(II) complex during reaction. Attempts were made to heterogenise the Cu(II) complexes into Y zeolites in order to allow easy removal of the catalyst from the reaction mixture after glycerol oxidation. Though the correlation between Cu(II)-complex encapsulation, Si:Al ratio, and proton count in the zeolitic structure was identified, the heterogeneous material proved unsuitable for glycerol oxidation. Nevertheless, it holds promise for exploring alternative catalytic reactions.

Tong Zhang

• 26/01/2024
• 3 p.m.
• Venue: Campus Groenenborger, V.008
• Supervisors: Shoubhik Das & Bert Maes
• Department of Chemistry

Abstract

Due to the climate change, pollutions, energy shortage and other interrelated global crises, there is always an increasing demand for the development of environmentally friendly processes in the chemical industries. In the last two decades, the field of photochemistry has emerged as a potent methodology across diverse domains, enabling the synthesis of numerous intricate compounds through environmentally sustainable means. This thesis elucidates four distinct methodologies concerning the generation of valuable products across diverse domains through the utilization of photoredox and photochemical reactions. The thesis is divided into five chapters:

• Chapter 1: An overview and introductory exposition of the fundamental principles and concepts pertaining to photochemistry are provided.

• Chapter 2: We have enhanced the generation of hydrogen peroxide by introducing an aryl amino group in polymeric carbon nitrides via visible light-mediated photocatalysis. In addition to increasing the efficiency of photocatalytic system, the description of the whole reactive scenario for the polymeric carbon nitrides has been depicted by combining diverse characteristic methods and theoretical calculations. Futhermore, the possible active catalytic sites are identified with the aid of 15N and 19F solid state NMR without using any expensive labeling reagent.

• Chapter 3: We have developed a unique methodology for the generation of α-amino radicals under the irradiation of visible light under a metal-free condition. This strategy is induced by π–π stacking and ion-pairing interactions and facilitated the synthesis of functionalized amines through three-component coupling reactions.

• Chapter 4: We have designed an efficient method for the red light-mediated sulfonyltrifluoromethylation of olefins which provide remarkable regioselectivity. This reaction system has been thoughtfully designed, and excellent substrate compatibility and functional group tolerance exhibits the industrial potential, thus validating the significance of this strategy.

• Chapter 5: We have developed a metal-free photocatalytic system for the transformation of biomass into formic acid. Compared to previous strategies, our method can work efficiently at room temperature and atmospheric pressure. Notably, real biomass and even daily-life-based-materials such as waste papers and oak cork stoppers of wine bottles are also smoothly converted to formic acid.