1. Modelling-assisted optimisation of hemicellulose-derived monosaccharide valorization : dissertationAna Jakob, 2025, doctoral dissertation Abstract: Hemicellulose, one of the key components of lignocellulosic biomass, and its sugar monomers offer significant potential as a substrate for producing value-added platform chemicals. With both pentoses (xylose and arabinose) and hexoses (glucose, galactose, and mannose), the conversion of hemicellulose can yield various products including furfural, hydroxymethylfurfural (HMF), and levulinic acid. This doctoral thesis investigates the dehydration of various hemicellulose-derived monosaccharides, utilizing catalyst-free systems, as well as homogeneous and heterogeneous catalysts, in both aqueous and organic solvents. Monosaccharide conversion was evaluated in a catalyst-free aqueous solution at temperatures ranging between 130 – 190 °C. Conducting dehydration reactions under simple, hydrothermal conditions highlighted the differences in the reactivity of individual monosaccharides. Experimental data from extensive activity testing enabled the development of accurate kinetic models. Aiming at the optimization of reaction parameters relevant to the post-Organosolv production of furanics and levulinic acid, sulfuric and formic acid were introduced into the reaction system as homogeneous acid catalysts. Reaction kinetics of acid catalysed conversion using sulfuric and formic acid indicated a significant reduction in the activation energy of ketose dehydration. In addition, the addition of both acids enhanced the dehydration rates and promoted the conversion of HMF towards levulinic acid due to its low activation energies 86 – 91 kJ mol−1 and high reaction rate constants. The established kinetic model for individual saccharides was proven to be robust and able to accurately predict optimal reaction conditions for two distinct industrially relevant hemicellulose streams. In pursuit of a more sustainable and recyclable catalyst, various zeolites, specifically H-BEA, were tested. Although H-BEA exhibited excellent catalytic activity, its selectivity towards furanics remained limited in aqueous media. The low activation energy for HMF rehydration facilitated the formation of levulinic acid even at a moderate reaction temperature of 165 °C. Notably, the addition of THF as an organic solvent in combination with H-BEA, drastically improved the product selectivity towards furanics, particularly furfural. In addition to H-BEA zeolite, various types of zeolites with different Si/Al ratios were studied. Catalyst characterization revealed a strong correlation between catalytic activity and acidity. The addition of THF resulted in 90 mol % of furfural after only 30 min, demonstrating an exponential increase in dehydration rates and a decrease in activation energies. This zeolite-THF system was therefore applied for tandem reactions involving both xylose dehydration and furfural hydrogenation. The addition of metallic active sites via a Ni/H-BEA catalyst, and a hydrogen atmosphere, resulted in the catalytic hydrogenation of xylose to tetrahydrofurfuryl alcohol. Adjusting reaction temperatures allowed the production of secondary hydrogenated (2-methylfuran and 2-methyltetrahydrofuran) and dehydration products (tetrahydropyran), while the addition of water as a co-solvent with THF resulted in the complete inhibition of dehydration reactions and the selective formation of xylitol. Through a systematic study of reaction conditions and the development of comprehensive kinetic models, this work provides an in-depth investigation of monosaccharide dehydration. This doctoral thesis represents a comprehensive study to systematically investigate both catalytic and non-catalytic dehydration of five relevant monosaccharides, providing a methodical kinetic modelling approach that addresses the challenges associated with the complexity related to hemicellulose conversion.
Keywords: hemicellulose, monosaccharides, kinetic modelling, furanics, catalysis, dissertations
Published in RUNG: 19.02.2025; Views: 798; Downloads: 18
 Full text (11,80 MB) |
2. Reaction mechanism and microkinetics of heterogeneously catalysed lignin depolymerisation and (de)functionalization : dissertationTina Ročnik, 2024, doctoral dissertation Abstract: Lignin, a complex aromatic polymer derived from lignocellulosic biomass, is a renewable resource for the production of aromatic-like chemicals and materials. However, its complex nature, depolymerisation and valorisation remain a major challenge for the bio-based community within biorefinery concepts. For this reason, lignin model compounds have been used to understand and design lignin depolymerisation, but insufficient attention has been paid to linking knowledge between simpler systems and applying it to a more complex problem. The objectives of this thesis were formulated accordingly to address the aforementioned gap in the literature. The objectives included a systematic approach correlating studies of lignin model compounds with lignin. Hydrodeoxygenation, cleavage of the β-O-4 bond and depolymerisation were investigated to evaluate the process- and structure-dependent correlations, effects on product distribution and kinetic parameters.
The catalytic reactions were carried out in batch reactor and the conditions were applied and intensified according to the knowledge gained during the experimental work. The liquid samples for monomer yield evaluation were analysed by GC-MS, while the structural characteristics of lignin and oligomeric fragments, e.g. molecular weight, functionality/reactivity and structural features, were examined by SEC, quantitative 31P and 2D-HSQC NMR. Kinetic modelling was performed to determine the kinetic parameters (e.g. kinetic constants and activation energies) describing defunctionalisation, β-O-4 bond cleavage and depolymerisation.
The study with monomeric lignin compounds contributed to the understanding of the key parameters leading to condensation during hydrotreatment. The unsaturated alkyl side-chain of eugenol and the reactive hydroxyl groups bound irreversibly and formed the carbonaceous species, while 4-propylphenol and 4-propylguaiacol provided important insights into the contribution of steric hindrances to a favourable reaction mechanism. An initial assessment of β-O-4 cleavage was performed with β-O-4-model compound, 2-phenoxy-1-phenylethanol, and linked to the lignin macromolecule by process- and structure-dependent correlations. Although the lignin model compound lacks the structural complexity of lignin, important insights into possible reaction processes were gained and accurate kinetic parameters were determined. Furthermore, lignin isolated in 50 vol% EtOH/H2O solutions was depolymerised and the optimal
reaction conditions were defined at a temperature of 275 °C and a pressure of 1.5 MPa with regard to the product distribution and the changes in the structural characteristics of the corresponding oligomeric fragments. Depolymerisation of lignins isolated in different EtOH/H2O solutions showed the importance of the structural features, especially ethoxylation degree and the content of β-O-4 or α-ethoxylated β-O-4 bonds. A higher ethoxylation degree of lignin reduced its potential to depolymerise and achieve the theoretical monomer yield. Therefore, the structural characteristics of lignin play the main role in designing and prediction of lignin depolymerisation.
The thesis represents a systematic approach of increasing the complexity of structures exposed to hydrodeoxygenation and depolymerisation. The approach contributed to incorporating the knowledge gained from less complex model compounds to real lignin samples by tailoring and designing lignin depolymerisation to exploit the potential of lignin for biorefinery concepts.
Keywords: model compounds, hydrodeoxygenation, organosolv lignin, depolymerisation, structural characteristics, kinetic modelling, dissertations
Published in RUNG: 08.11.2024; Views: 1208; Downloads: 17
Full text (5,04 MB) |
3. Source apportionment of fine and ultrafine particle number concentrations in a major city of the Eastern MediterraneanPanayiotis Kalkavouras, Georgios Grivas, Iasonas Stavroulas, Kalliopi Petrinoli, Aikaterini Bougiatioti, Eleni Liakakou, Evangelos Gerasopoulos, Nikolaos Mihalopoulos, 2024, original scientific article Keywords: UFP, PNSD, receptor modelling, lockdown, Athens
Published in RUNG: 10.05.2024; Views: 1658; Downloads: 0
This document has many files! More... |
4. Time-series analysis of oxygen as an important environmental parameter for monitoring diversity hotspot ecosystems : an example of a river sinking into the karst undergroundSaptashwa Bhattacharyya, Janez Mulec, Andreea Oarga-Mulec, 2023, original scientific article Abstract: Predicting variations in dissolved oxygen concentration (DO) is important for management and environmental monitoring of aquatic ecosystems. Regression analyses and univariate and multivariate time-series analyses based on autoregressive methods were performed to investigate oxygen conditions in the Pivka River, Slovenia. The monitoring site was established upstream where the river sinks into the karst cave Postojnska jama, which hosts one of the richest subterranean faunas yet studied worldwide. It was found that abnormal variations of DO started to be noticeable at values of DO < 3 mg/L and became more pronounced until the ecosystem reached fully anoxic conditions. The abnormal fluctuations during the critical summer period were due to environmental conditions, organic load and resident biota. Predictions for future detection of anomalies in DO values were made from stable residuals of the measured data, and it was demonstrated that the model could be used to obtain a reliable estimate for a short period, such as one day. The example presented an analysis pipeline based on specific and established threshold DO values, and it is particularly important for ecosystems with diversity hotspots where prolonged low DO values can pose a threat to their biota.
Keywords: karst (geology), aquatic ecosystems, dissolved oxygen, modelling, prediction
Published in RUNG: 05.09.2023; Views: 2347; Downloads: 7
Full text (15,17 MB)
This document has many files! More... |
5. RELATIVISTIC TIDAL DISRUPTIONS OF REALISTIC STARS BY SUPERMASSIVE BLACK HOLESTaj Jankovič, 2023, doctoral dissertation Abstract: Stellar tidal disruption events (TDEs), where a star gets disrupted by strong tidal forces of a supermassive black hole (SMBH), offer a unique opportunity for studies of SMBHs and stellar dynamics in galactic nuclei and provide insights into accretion physics. Currently, there are ≈ 100 observed TDEs, however, this number is expected to increase significantly with the start of new wide-field optical surveys, e.g. with the Vera Rubin Observatory.
We focus on hydrodynamic simulations of TDEs with the smoothed particle hydrodynamics code Phantom. To begin with, we simulate TDEs in a general relativistic and Newtonian description of an SMBH’s gravity. Stars, which are placed on parabolic orbits with different parameters β (to be defined here), are constructed with the stellar evolution code MESA and therefore have realistic stellar density profiles. We study the mass fallback rate of the debris Ṁ, a quantity often assumed to determine the TDE light curves, and its dependence on the β, stellar mass and age as well as the black hole’s spin and the choice of the gravity’s description. We find that relativistic disruptions at the same pericenter distance are stronger than disruptions in a Newtonian description of the SMBH’s gravity. We also determine the differences between Ṁ of realistic stars with various ages and masses. In addition, we characterize the effect of SMBH’s rotation on the Ṁ and find that it depends on the orientation of SMBH’s spin vector relative to the stellar orbital angular momentum. Encounters on prograde orbits result in narrower Ṁ curves with higher peak values, while the opposite occurs for retrograde orbits.
Stellar disruption results in an elongated stream of gas that partly falls back to the pericenter. Due to apsidal precession, the returning stream may collide with itself, leading to a self-crossing shock that launches an outflow. If the black hole spins, this collision may additionally be affected by Lense-Thirring precession which can cause an offset between the two stream components. We study the impact of this effect on the outflow properties by carrying out local simulations of collisions between offset streams. As the offset increases, we find that the geometry of the outflow becomes less spherical and more collimated along the directions of the incoming streams, with less gas getting unbound by the interaction. However, even the most grazing collisions we consider significantly affect the trajectories of the colliding gas, likely promoting subsequent strong interactions near the black hole and rapid disc formation. We analytically compute the offset to stream width ratio, finding that even slowly spinning black holes can cause both strong and grazing collisions. We propose that the deviation from outflow sphericity may enhance the self-crossing shock luminosity due to a reduction of adiabatic losses, and cause significant variations of the efficiency at which X-ray radiation from the disc is reprocessed to the optical band depending on the viewing angle. These potentially observable features hold the promise of constraining the black hole spin from tidal disruption events.
Keywords: Computer modelling and simulation, hydrodynamics, black holes, infall
Published in RUNG: 29.08.2023; Views: 2588; Downloads: 38
Full text (16,29 MB) |
6. |
7. |
8. An anti-HER2 nanobody binds to its antigen HER2 via two independent paratopesDaniele Ubbiali, Marco Orlando, Matic Kovačič, Claudio Iacobucci, Marta S. Semrau, Gregor Bajc, Sara Fortuna, Gregor Ilc, Barbara Medagli, Sandra Oloketuyi, Ario De Marco, 2021, original scientific article Keywords: NMR, cross-linking, mass spectrometry, nanobodies, modelling, rational mutagenesis
Published in RUNG: 15.04.2021; Views: 3858; Downloads: 0
This document has many files! More... |
9. Converting apprehensive customers to willing customers : building trust in online shopping arenaSaibal K. Pal, 2017, original scientific article Abstract: This study focuses on knowing about the factors that can affect the trust levels of the consumers involved in online shopping. Trust has been identified as a prominent factor that ultimately leads to the transformation of the apprehensive customers into willing customers. The role of website design features and portal affiliation is tested regarding their impact in affecting the trusting beliefs of the customers. Structural Equation Modeling (SEM) was put to use to do the statistical testing on the data collected through students of tier III city of India. The results
showed that the website design features have an important role to play in affecting the trusting beliefs of the people whereas portal affiliation didn’t have much of a role. The study
implies that the website managers must focus on their design features if they want to gain the trust of their customers. The portal affiliation wouldn’t be fruitful in the case of a student
at least. Better shoppers amongst students must be attracted towards portals through innovative designs and features.
Keywords: online shopping, building trust, website design features, portal affiliation, structural equation modelling, theory of reasoned action
Published in RUNG: 01.04.2021; Views: 3184; Downloads: 0
This document has many files! More... |
10. A Bottom-Up Building Stock Model for Tracking Regional Energy Targets—A Case Study of KočevjeMarjana Šijanec-Zavrl, Gašper Stegnar, Andraž Rakušček, Henrik Gjerkeš, 2016, original scientific article Abstract: The paper addresses the development of a bottom-up building stock energy model (BuilS) for identification of the building stock renovation potential by considering energy performance of individual buildings through cross-linked data from various public available databases. The model enables integration of various EE and RES measures on the building stock to demonstrate long-term economic and environmental effects of different building stock refurbishment strategies. In the presented case study, the BuilS model was applied in the Kočevje city area and validated using the measured energy consumption of the buildings connected to the city district heating system. Three strategies for improving the building stock in Kočevje towards a more sustainable one are presented with their impact on energy use and CO2 emission projections up to 2030. It is demonstrated that the BuilS bottom-up model enables the setting of a correct baseline regarding energy use of the existing building stock and that such a model is a powerful tool for design and validation of the building stock renovation strategies. It is also shown that the accuracy of the model depends on available information on local resources and local needs, therefore acceleration of the building stock monitoring on the level of each building and continually upgrading of databases with building renovation information is of the utmost importance.
Keywords: bottom-up modelling, renovation scenarios, building stock, regional energy policy
Published in RUNG: 25.10.2016; Views: 5986; Downloads: 239
Full text (5,27 MB) |
