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    Rational Synthesis of Activated Carbons for Enhanced CO2 Uptake and Methane Storage
    (University of Nottingham, 2024) Albeladi, Nawaf Sweilem; Mokaya, Robert
    The atmospheric concentration of CO2 has reached alarming levels, primarily due to the combustion of fossil fuels. This surge in CO2 concentrations has led to rising global temperatures, increased frequency of extreme weather events, and threats to all forms of life on Earth. This scenario necessitates the urgent need to develop CO2 capture and removal technologies to ensure a more sustainable future. Moreover, the use of fossil fuels not only contributes significantly to CO2 emissions but reserves and supply are also limited. This Thesis covers themes on the rational preparation of porous carbons that are targeted at the capture of CO2 or the storage of gases (e.g. methane) that may be used as transitional energy sources towards a fossil-free economy. In this regard, ‎Chapter 1 explores the state-of-the art on potential solutions, including CO2 capture and storage via physisorption on porous materials, with a focus on readily synthesised activated carbons. Additionally, the chapter highlights the potential of methane-based natural gas as an alternative to conventional fossil fuels for vehicular transport, given its abundant reserves and lower carbon emissions. The storage of methane for vehicular use may be achieved using porous materials. However, the preparation of porous materials that meet set targets, such as those outlined by the US Department of Energy (DOE), remains a challenge. The considerations that must be carefully considered in seeking suitable materials for such gas storage applications include precise control over porosity parameters such as surface area, pore volume, pore size, and packing density. Therefore, both the ability to accurately modulate the textural properties and a comprehensive understanding of the relationship between gas (CO2 or CH4) uptake capacity and pore size are essential. This thesis is aimed to address these issues. ‎Chapter 2 concentrates on material characterisation to gain a comprehensive understanding of the targeted porous carbon nanostructures. To fully assess these materials, a combination of chemical, structural, and surface characterisation techniques have been employed. This chapter provides a concise overview of the equipment and techniques used for the characterisation of the materials prepared during this research programme. Regarding preparation routes to activated carbons, this work investigates two primary synthesis methods. Firstly, ‎Chapter 3 delves into the effect of the air-carbonisation treatment. This study aimed to investigate the impact of air-carbonisation temperature followed by potassium hydroxide (KOH) activation on the structure and gas adsorption performance of activated carbons. It primarily focused on the effect of air-carbonisation treatment on the O/C ratio (a measure of a precursor’s susceptibility to activation) in carbonaceous matter. The study found a role of air-carbonisation temperature in tailoring the porosity, surface area, and packing density of the activated carbons. It revealed a very notable trend in the O/C ratio of the carbonaceous matter, which decreases as the temperature increases. This knowledge enables the optimal design of the pore structure of the activated carbons, resulting in porous carbons with high CO2 uptake, at 25 °C, of 2.0 and 5.2 mmol g-1 at 0.15 and 1 bar, respectively. As a result of the ability to adjust the O/C ratio of the precursor, activated carbons with a high packing density of up to 0.79 g cm-3 were obtained, resulting in high CH4 uptake, wherein the gravimetric and volumetric uptake reach of 0.38 g g-1 and 278 cm3 (STP) cm-3, respectively, at 25 °C and 100 bar. The second approach centres on the effect of the activation technique when utilising N-rich precursors generated via varying pathways. Thus in ‎Chapter 4, a N source was introduced to biomass-derived carbonaceous matter through the use of additives, resulting in a incorporation of N in the precursor. This process involves adding melamine or urea as a nitrogen source to an activation mixture containing biomass-derived carbonaceous matter of low O/C ratio (air-carbonised date seed, Phoenix dactylifera, ACDS), and KOH as an activating agent. These carbons exhibit a broad range of surface areas and porosity characteristics, controlled by varying the amount of melamine or urea, the KOH/ACDS ratio, and the activation temperature. The N added to the activation mix serves as both an N-dopant and porogen, with the later effect enabling the formation of larger pores, extending the pore size distribution into the mesopore region, and increasing the surface area. This results in carbons with tunable porosity and variable packing density, suitable for enhanced CO2 and methane uptake. The carbons exhibited excellent low-pressure CO2 capture at 25 °C, of 1.7 mmol g-1 at 0.15 bar and 4.7 mmol g-1 at 1 bar. The porosity and packing density of the carbons also were directed and modulated towards methane storage, showing a gravimetric uptake of up to 0.42 g g-1 at 25 °C and 100 bar, and volumetric storage capacity of up to 266 cm3 (STP) cm-3 at 25 °C and 100 bar. Expanding on the knowledge gained in ‎Chapter 4 from KOH activation of precursors with nonhomogeneous incorporation of N additives and low O/C ratio precursors, ‎Chapter 5 explores activated carbons derived from precursors with homogeneous incorporation of N. ‎Chapter 5 explores the potential of N-rich crosslinkable imidazolium-based ionic liquids as a new class of carbon precursors for activated carbons. Upon carbonisation, the ionic liquids yield carbonaceous matter (designated as IL-C) with the unusual combination of a high N content and low O/C atomic ratio. During activation, the resulting ionic liquid-derived carbonaceous matter (IL-C) generates activated carbons with a mix of micro- and mesoporosity, having ultra-high surface area of up to ~4000 m² g-1 and a pore volume of up to 3.3 cm3 g-1. These carbons, due to their porosity and packing density, exhibit outstanding gravimetric and volumetric methane uptake of up to 0.53 g g-1 and 289 cm3 (STP) cm-3, respectively, at 25 °C and 100 bar. This surpasses both gravimetric and volumetric methane storage targets, making these ionic liquid-derived activated carbons the first porous materials (carbon or MOF) to meet both gravimetric and volumetric storage targets as set by the US DoE. To further enhance understanding of the role of N in shaping the porosity of activated carbons, ‎Chapter 6 goes beyond KOH activation to explore non-hydroxide activation of IL-C. Activated carbons were generated via chemical activation of IL-C using a non-hydroxide activating agent, potassium oxalate (PO). Due to the very low atomic ratio O/C of 0.116 of IL-C, and gentler activating nature of PO, the generated activated carbons display moderate to high surface area of 447 to 2202 m2 g-1, depending on the activation temperature. The porosity of the activated carbon could be tailored to exhibit a highly microporous structure with porosity suitable for post-combustion CO2 uptake, having CO2 capacities of 1.6 and 4.2 mmol g-1 At 25 °C and pressures of 0.15 and 1 bar, respectively. As a benefit of PO being a milder and greener activating agent, the activated carbons have excellent properties in terms of high packing density (0.98 cm3 g-1) combined with a high surface area density (2039 m2 cm-3), which translates to excellent volumetric methane gas uptake. At 100 bar and 25 °C, the carbons achieve total volumetric uptake of 282 cm3 (STP) cm-3 along with excellent deliverable capacity of 200 cm3 (STP) cm-3 (for a pressure swing of 100 bar → 5 bar). ‎Chapter 7 provides a summary and conclusions of this thesis, along with a future look at specific aspects. As an overview, this thesis delves into the rational and intentional synthesis of activated carbons for which various precursors and activation techniques are explored. The outcome has yielded exceptional activated carbons distinguished by the possibility of deliberately tailoring the porosity towards high surface area and pore volume. The motivation of the thesis is achieving porosity in the activated carbons that is suitable for addressing challenges associated with CO2 uptake and optimising methane storage. This research also opens up new possibilities in terms of applications for the synthesised activated carbons. A particularly promising application for future exploration lies in the utilisation of these activated carbons for hydrogen storage, presenting an exciting prospect for further investigation.
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    The Dissolution of Regenerated Cellulose Multifilament Bundles in Ionic Liquid of 1- ethyl-3-methyl- imidazolium acetate [C2mim]+ [OAc]-
    (University of Leeds, 2023-04-23) Alanazi, Maer; Ries, Mike; Hine, Peter
    Regenerated cellulose fibres like Cordenka and Lyocell have been studied for their potential use as reinforcement in polymer composites. These fibres are attractive candidates for improving the mechanical and environmental characteristics of various polymer materials. In our research group we have devolved the idea of manufacturing ‘all-cellulose’ composites from a single cellulosic source. The idea is to create the ‘matrix’ of all cellulose composite by selectively dissolving the surface of each fibre or filament, which on coagulation forms the matrix. Being also cellulose, this should give excellent compatibility/adhesion between the phases. This thesis has studied the dissolution of two commercial regenerated cellulose yarns, namely Cordenka™ and Lyocell™. Optical microscopy, Wide angle X-ray diffraction (WAXS) and mechanical testing techniques have been used to track the dissolution of these multifilament bundles in the ionic liquid 1-ethyl-3-methyl-imidazolium acetate [C2mim]+ [OAc]- for different times and temperatures. This allowed both the speed of the dissolution to be determined at different temperatures as well as the dissolution activation energy Ea from time-temperature superposition. The different nature of the multifilament bundles (Cordenka™, which was untwisted and Lyocell™ where the bundles were twisted) resulted in different techniques being most suitable for their study. For Cordenka, WAXS and mechanical measurements on partially dissolved composite filaments proved most successful. In the dissolution process, the oriented cellulose II crystals in the regenerated cellulose fibres dissolve and then reform into randomly oriented crystals to form a matrix phase. This change in orientation enabled the dissolution process to be followed and hence determine the growth of the dissolved matrix fraction of 𝑣𝑚 with time and the dissolution activation energy. On the other hand, optical microscopy was found to work very well with the Lyocell multifilament bundles to directly determine the dissolved matrix volume fraction 𝑣𝑚. Mechanical measurements of Young’s Modulus and ultimate tensile strength on partially dissolved composites proved successful for both Cordenka and Lyocell multifilament yarns. The change in the average molecular orientation 𝑃2 determined from an azimuthal (𝛼) X-ray scan, allowed the growth of the matrix volume fraction 𝑣𝑚to be calculated with time and temperature. This is an indirect measurement and relies on using a rule of mixtures approach. The optical microscopic method offered a direct method to measure the growing area of the dissolved and coagulated fraction for the Lyocell multifilament bundle with increasing time and temperature. The twisted fibres meant that the dissolved fraction formed a ring on the outside of the multifilament, allowing a measurement of the decrease of the inner core (the undissolved original fibre fraction) and the increase in the area and thickness of the dissolved and coagulated outer ring. The decrease of the inner core and the growth of coagulation fraction C.F. and the thickness and area of the dissolved and coagulated outer ring was found to follow time temperature superposition, with an Arrhenius behaviour, giving consistent values for the activation energy of Ea= 141 ± 15, Ea= 141 ± 16 and Ea= 127 ± 14 respectively. Young’s modulus and ultimate tensile stringth was measured on all the resulting processed composites for Cordenka and Lyocell multifilament bundles. The fall of Young’s modulus and ultimate tensile strength with dissolution time and temperature was found to follow time-temperature superposition for the Cordenka multifilament bundle, with an Arrhenius behaviour giving a value for Ea= 198± 29 kJ/mol. The Young’s Modulus and ultimate tensile strength results were plotted against 𝑣𝑚 determined from the WAXS measurements and were found to agree well to the Voigt upper bound parallel Rule of Mixtures. This suggests that the resulting composites are well bonded and that the dissolved Cordenka material (which has a higher molecular weight compared to the Lyocell material) is a suitable matrix material for to make all a cellulose composite. For the Lyocell multifilament bundle, the Young’s modulus of the processed composites was found to be quite scattered and so it could not be ascertained if this followed time-temperature superposition. However, the fall of the ultimate tensile strength of the composites with dissolution time and temperature was found to follow time-temperature superposition, with an Arrhenius behaviour giving a value for Ea= 144± 27 kJ/mol. The ultimate tensile strength results plotted against 𝑣𝑚 determined from the optical microscopic method was found to lie significantly below the Voigt rule of mixtures. This suggests that either the dissolved Lyocell material is less successful as a matrix, or that the twisted nature of the Lyocell multifilaments does not allow dissolution to happen in the interior of the bundle as the ionic liquid cannot penetrate. In terms of the difference between the Cordenka and Lyocell multifilament bundles, it was found from the Optical microscopic results, that the geometry of the Cordenka multifilament bundle is untwisted with a few hundred individual multifilaments, which appeared as a loose microstructure with significant inner spaces in between. On the other hand, the geometry of Lyocell multifilament bundle is twisted with few hundred individual fibres that are close to each other without significant inner spaces. The Cordenka multifilament bundle has higher average orientation, and a higher Young’s modulus, ultimate tensile strength, and activation energy compared to the Lyocell multifilament bundle, which we attribute to the fibres being untwisted. The Lyocell bundle has lower average orientation, which was shown to be due to the significant twist of the bundle. These findings, especially the geometry and molecular weight lead to the Cordenka multifilament bundle having a faster dissolution rate than the Lyocell multifilament bundle. The comparative geometry (untwisted fibres), the speed of dissolution and the higher molecular weight, lead to the important result that the Cordenka multifilament bundle would make an excellent basis for an all cellulose regenerated fibre composite (ACC). However, it is appreciated that if woven cloth is to be used to manufacture all-cellulose composites (ACC) then some degree of twist will be required to stop the individual fibres from breaking during the weaving process, so there is maybe an optimum bundle twist to be discovered in any future work.
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    Modelling and Sustainability Assessment of Industrial Ionic Liquid Production and Applications using Life Cycle Thinking
    (Saudi Digital Library, 2023-02-02) Baaqel, Husain; Hallett, Jason; Chachuat, Benoit; Guillen-Gosalbez, Gonzalo
    The main goal of this dissertation is to build on the current state-of-the-art research regarding ionic liquid (IL) sustainability assessment to help answer the research question, "how sustainable are ILs as alternatives to conventional technologies?" This is accomplished by developing a systematic computer-aided framework that integrates life cycle assessment (LCA) and life cycle costing (LCC) with process modelling and simulation for the consistent and complete economic and environmental assessment of ILs. The work further explores the use of monetization and an advanced framework that couples uncertainty analysis and global sensitivity analysis to enhance decision-making. The main novelties of the thesis are the methodological components and the case studies, in which the production of different ILs are evaluated in the context of relevant applications including their use. This thesis has contributed to the existing body of research by developing the following aspects. First, an integrated framework that combines LCA, LCC with process modelling and simulation was applied to evaluate the production of 1-butyl-3-methylimidazolium tetrafluoroborate using two synthesis routes and compare them with two conventional solvents in terms of their application in fuel desulfurization. Second, the developed framework was enhanced by incorporating factors of monetization, and this was applied to a case study involving hydrogen sulfate-based ILs to quantify their externalities and compare the true cost of these ILs with that of conventional solvents in biomass pretreatment applications. Finally, LCA uncertainty and global sensitivity analysis (GSA) was included in the framework to improve uncertainty analysis by accounting for process model uncertainties and identifying key parameters in non-linear systems, which was demonstrated in a case study involving the production of dialkylimidazolium ILs. The results show that the use of data from detailed process models, as highlighted in the holistic framework, makes a big difference compared to the use of simplified methods. Unlike short-cut methods, the framework accounts for process efficiency, emissions, and waste and covers a wide range of environmental impact categories for a more consistent and complete assessment. Additionally, coupling monetization with LCA can improve the assessment by turning a multiobjective problem into a single-objective problem, and hence, facilitating decision-making. The environmental externalities quantified through monetization reveal hidden costs that are usually overlooked when conducting a conventional economic assessment. Moreover, the importance of including foreground uncertainties in the uncertainty analysis was demonstrated by the results obtained from applying uncertainty-GSA analysis. In particular, foreground uncertainties can significantly overlap with the background uncertainties because of the multiplicative effect, which impacts decision-making. Furthermore, using GSA can help correctly identify uncertain parameters by accounting for collaborative effects in non-linear systems. Finally, case studies were used to test the efficiency of the developed framework and its methodological components. The contributions of this thesis build on the state-of-the-art economic assessment and LCA for ILs and support research on evaluating the sustainability of ILs and similar novel chemicals. This in turn will help us better understand the potential of such chemicals in terms of their sustainability performance as decision-making in most industries today is driven by policies pursuing sustainable development.
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