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Theoretical investigation of 27Al chemical shifts dependence on water amount and temperature in zeolite MFI
Willimetz, Daniel ; Grajciar, Lukáš (advisor) ; Blahut, Jan (referee)
Title: Theoretical investigation of 27 Al chemical shifts dependence on water amount and temperature in zeolite MFI Abstract: This bachelor thesis explores 27 Al NMR spectra in zeolite MFI and investigates the impact of several factors on the chemical shielding values, including water loading, temperature, and the relative positions of aluminum pairs. Various machine learning-based methods for calculating chemical shift are evaluated. Molecular dynamics simulations with neural network potentials are used to simulate experimental conditions. Neural network potentials offer a highly efficient means of calculating energy with a significant speed-up of approximately 1000 times faster than density functional theory, while maintaining a high level of accuracy. This study is the first to examine 27 Al NMR under operando conditions, with a focus on the experimentally relevant amount of water. Keywords: 27 Al NMR, machine learning, zeolites, MFI, operando conditions, neural network potentials
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The role of boron oxide in oxidative dehydrogenation of hydrocarbons
Stoklasa, Jan ; Rubeš, Miroslav (advisor) ; Grajciar, Lukáš (referee)
Hexagonal boron nitride appears to be a suitable catalyst for the oxidative dehydrogenation of hydrocarbons. With its high selectivity towards alkenes and low production of carbon monoxide and dioxide, this catalyst emerges as an ideal candidate for this process. Literature references indicates that a radical mechanism seems probable, as evidenced by the dependence on the reactor shape and catalytic volume. The exact mechanism is still under investigation, and the focus of this study is to describe the initial steps of the reaction and investigate poorly defined boron oxide (BOx) species. It has been found that during the reaction of the catalyst surface model with the O2H radical, two types of complexes are observed - dispersive and covalently bound. By monitoring the interaction energies, it was discovered that both types are energetically very close to each other, with a low barrier for transition between them. When the surface interacts with the propyl radical, only dispersive complexes were observed.
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The role of topology and chemical composition of zeolites on their biomedical characteristics
Nováková, Denisa ; Grajciar, Lukáš (advisor) ; Rubeš, Miroslav (referee)
7 Abstract Zeolites are traditionally used materials in the petrochemical and chemical industry that have recently drawn attention for their possible utilization in biomedicine as drug and gene carriers, materials in bone tissue engineering, or adsorbents for detoxifying human organisms. Zeolites offer a possible solution for removing protein-binding uremic toxins, such as p-cresol, that are unremovable during classical hemodialysis. This bachelor's thesis focuses on analyzing the motion of water within zeolite with CAN framework topology using molecular dynamics simulations with neural network potentials. The obtained data is used to calculate the self-diffusion coefficients and analyze the effect of water loading, aluminum content, and the distribution of aluminum atoms on water diffusion and degree of deprotonation of Brønsted acid sites (BASs). In addition, the thoroughly tested water-loaded CAN models are used for evaluation of the interaction between the zeolite framework and water molecules and p-cresol in the context of potential application of zeolites for dialysis. Key words zeolites, machine learning, biomedicine, p-cresol, CAN
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Theoretical Investigation of the Zeolite Hydrolysis under Realistic Conditions
Jin, Mengting ; Grajciar, Lukáš (advisor) ; Chizallet, Céline (referee) ; Piskorz, Witold (referee)
Zeolites are one of the most widely used and one of the most industrially important materials. They are utilized in a variety of commercial applications, particularly in heterogeneous catalysis, adsorption/separations applications and as ion exchangers. Zeolites are stable when exposed to water or water vapor at ambient conditions and exhibit high thermal stability. However, zeolites can be partially or even fully hydrolyzed under certain conditions. This hydrolytic instability of zeolites can be considered both a major problem in some applications and a useful means to tune catalyst properties in the others. However, the thorough studies of the hydrolytic mechanisms under realistic conditions (high water loading, alkaline conditions, high heteroatom concentrations, etc.) and employing realistic models (dynamical simulation of ab initio accuracy) is mostly lacking. This dissertation uses the ab initio calculations to systematically study the hydrolysis mechanism of zeolites under realistic conditions, including: (1) hydrolysis mechanism of germanosilicate zeolite UTL, (2) hydrolysis of siliceous zeolite CHA under alkaline conditions, and (3) identification of the nature of framework-associated Al(Oh) species in aluminosilicate zeolite CHA under wet conditions. The main results obtained are as...
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Theoretical Investigation of Low-dimensional Magnetic Materials
Li, Shuo ; Grajciar, Lukáš (advisor) ; Frauenheim, Thomas (referee) ; Jelínek, Pavel (referee)
Low-dimensional (D) materials, such as graphene, transition metal dichalcogenides and chalcogenide nanowires, are attractive for spintronics and valleytronics due to their unique physical and chemical properties resulting from low dimensionality. Emerging concepts of spintronics devices will greatly benefit from using 1D and 2D materials, which opens up new ways to manipulate spin. A majority of 1D and 2D materials is non-magnetic, thus their applications in spintronics are limited. The exploration, design and synthesis of new 1D and 2D materials with intrinsic magnetism and high spin-polarization remains a challenge. In addition, the valley polarization and spin-valley coupling properties of 2D materials have attracted great attention for valleytronics, which not only manipulates the extra degree of freedom of electrons in the momentum space of crystals but also proposes a new way to store the information. The computational investigation of magnetic and electronic properties of low-dimensional materials is the subject of this thesis. We have systematically investigated geometric, electronic, magnetic and valleytronic properties of several 2D and 1D materials by using the density functional theory. These investigations not only theoretically show rich and adjustable magnetic properties of...
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Teoretické studium kyselých zeolitů
Grajciar, Lukáš
The acidic zeolite H-FER with Si:Al ratios of 71, 35 and 8 was investigated employing a periodic DFT model, as well as cluster models and the CCSD(T) level of theory. The computational study of the H- FER in a high silica form (Si:Al = 71) accompanied by the investigation of the interaction of the CO and N2 probe molecules with the H-FER sample was supplemented with the experimental data obtained from the variable temperature infrared spectroscopy (VTIR). The H-FER samples with different Si:Al ratios were characterized by the structure, location and relative stabilities of the Brønsted acid groups. Relative stability of these sites was found to be dependent on Si:Al ratio, which is the main factor dictating the relative concentration of Brønsted acid sites having different types of local configuration. The number of AlO4 tetrahedra sharing an oxygen with the SiO4 tetrahedron involved in the Brønsted acid site determines the Si-O(H)-Al angle, O-H stretching frequency and deprotonation energy (and hence acid strength). Furthermore, the theoretical results obtained for the CO and N2 interaction with the H-FER were found to be in a good agreement with the experimental VTIR data. Several types of hydrogen-bonded OH···CO and OH···N2 complexes were characterized, formed by interaction between the adsorbed...
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Theoretical Study of Influence of Silanol Nest Defects on Hydrolysis of Zeolite Chabazite
Vacek, Jaroslav ; Grajciar, Lukáš (advisor) ; Fišer, Jiří (referee)
This thesis is focused on theoretical study of influence of the silanol nest defects on the hydrolysis of zeolite Chabazite under harsh steaming conditions. The motivation of the thesis was a recent experiment proving that the silanol nest defect enhances the hydrolysis of a zeolite. The harsh steaming conditions have been chosen as some important technological processes involving zeolites require high temperatures and have water vapour present. The study was performed by using density functional theory calculations. To investigate the influence of the defect two models were used a reference pristine model and a defected model containing the silanol nest defect. The two models were pure siliceous Chabazite periodical models with supercell containing 36 and 35 Si tetrahedra respectively. A multi-step hydrolysis leading to detachment of a Si(OH)4 cluster from the zeolite, known as total desilication, was calculated for the two models. Multiple possible paths of the hydrolysis were discovered, compared and discussed on both models. Both the most favourable hydrolysis paths of the two model as well as their arithmetic means were compared. The experimentally set expectations that a silanol nest defect enhances the hydrolysis of the zeolite have been met.
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Quantum-chemical study of adsorption in microporous materials
Grajciar, Lukáš ; Nachtigall, Petr (advisor) ; Bučko, Tomáš (referee) ; Sierka, Marek (referee)
Microporous materials play a crucial role in a wide range of applications in chemical engineering, chemistry, material science or lately even in medicine. Zeolites and metal- organic frameworks (MOFs) take a prominent place among them. The most important fields of applications include gas separation, purification or gas storage. A detailed understanding of adsorption properties of these materials represents a long-standing effort from experimental as well as computational chemistry community. However, ac- curate computational description of adsorption in microporous materials represents a significant challenge for computational chemists as: (i) unit cells of the crystalline mi- croporous materials are typically large, (ii) dispersion interactions are of importance, and (iii) there are metal cations, often with open-shell electronic structure, present in the framework interacting strongly and specifically with adsorbing molecules. Despite a significant progress made in theoretical description of adsorption mechanisms in both zeolites and MOFs in last decade, there is a number of applications and systems for which the commonly used computational approaches fail to provide a needed accuracy. A whole class of such systems is represented, for example, by MOFs containing tran- sition metal coordinatively...
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Teoretické studium kyselých zeolitů
Grajciar, Lukáš
The acidic zeolite H-FER with Si:Al ratios of 71, 35 and 8 was investigated employing a periodic DFT model, as well as cluster models and the CCSD(T) level of theory. The computational study of the H- FER in a high silica form (Si:Al = 71) accompanied by the investigation of the interaction of the CO and N2 probe molecules with the H-FER sample was supplemented with the experimental data obtained from the variable temperature infrared spectroscopy (VTIR). The H-FER samples with different Si:Al ratios were characterized by the structure, location and relative stabilities of the Brønsted acid groups. Relative stability of these sites was found to be dependent on Si:Al ratio, which is the main factor dictating the relative concentration of Brønsted acid sites having different types of local configuration. The number of AlO4 tetrahedra sharing an oxygen with the SiO4 tetrahedron involved in the Brønsted acid site determines the Si-O(H)-Al angle, O-H stretching frequency and deprotonation energy (and hence acid strength). Furthermore, the theoretical results obtained for the CO and N2 interaction with the H-FER were found to be in a good agreement with the experimental VTIR data. Several types of hydrogen-bonded OH···CO and OH···N2 complexes were characterized, formed by interaction between the adsorbed...
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