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Mechanistic Insights into Reactive Zeolite-Water Interactions
Benešová, Tereza ; Heard, Christopher James (advisor) ; Maldonado Dominguez, Carlos Mauricio (referee)
Title: Mechanistic Insights into Reactive Zeolite-Water Interactions Author: Tereza Benešová Department: Department of Physical and Macromolecular Chemistry Supervisor: Christopher Heard, PhD. Abstract: An in silico investigation of zeolite-water interactions was undertaken using a combined static and dynamic approach within density functional theory. Two periodic models of zeolite chabazite were used, purely siliceous and aluminosilicate in order to explain the different behaviour of water near Si-O-Si and Al-O-Si bridges. One or two water molecules per supercell of the model were used, corresponding to experimental conditions of steaming. Under such conditions, water can interact with zeolites by non-reactive adsorption but also by reactive interactions such as hydrolysis and oxygen exchange between water and the framework. The main goal of the study was to explain oxygen exchange at an atomistic level. The viable mechanisms of oxygen exchange were established. These mechanisms are different for Si-O-Si and Al-O-Si bridges but for both of them oxygen exchange is initiated by partial hydrolysis of the framework. After this hydrolysis, oxygen exchange heals the framework while incorporating into it the oxygen that originated from the water. Both established mechanisms are competitive with non-exchange...
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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 insights into encapsulated noble metals
Hou, Dianwei ; Heard, Christopher James (advisor) ; Logsdail, Andrew (referee) ; Rubeš, Miroslav (referee)
Noble metals are widely used in industry due to their excellent catalytic and optical properties. Because of limited reserves and high prices, it is desired to make effective use of every atom. It has been possible in recent years to produce subnanometer clusters or even isolated atoms, despite their low resistance against sintering. Furthermore, it has been recently established that the encapsulation of single atoms and subnanometer noble metal clusters can be achieved via trapping at metal oxides, ranging from two-dimensional layered materials to the void space within the pores of zeolites. However, neither the atomistic mechanisms of atom/cluster trapping, nor the means by which they may be optimized are known. Furthermore, the relationship between the physical and electronic state of trapped atoms/clusters and their reactive properties in catalysis are currently only weakly understood. In this thesis, we used theoretical methods ranging from global structure optimization to kinetic Monte Carlo to identify the roles of cluster size, charge state, and the present and type of common adsorbents on the stability, catalytic and optical properties of subnanometre-sized noble metal clusters trapped at metal oxides. This thesis takes the form of three sections: (1) Structure, stability, and migration...
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Study of active centers in zeolite catalysts for cascade Prins-Friedel-Crafts reaction
Golis, Petr ; Shamzhy, Mariya (advisor) ; Pinkas, Jiří (referee)
Conventional zeolites are important shape-selective heterogeneous acid catalysts due to the presence of acid sites accessible through uniform micropores. Recently designed hierarchical zeolites with combined micro-mesoporosity also contain acid sites on the external surface or in mesopores. These highly accessible acid sites are promising active centres for the transformation of bulky molecules, which cannot pass through the micropore openings of conventional zeolites. An example of a reaction that can benefit from the use of zeolites is the one-pot cascade Prins-Friedel-Crafts (PFC) reaction of an aldehyde, homoallylic alcohol, and aromatic compound, which yields valuable heterocyclic compounds containing 4- aryltetrahydropyran moieties. In this work, the acidic characteristics of a series of hierarchical aluminium- and gallium- containing MFI and MWW zeolites were evaluated by FTIR-monitored thermodesorption of probe molecules and further related to the catalytic properties of zeolites in the PFC reaction of butyraldehyde, 3-buten-1-ol, and anisole. The nature, strength, and total concentration of acid sites in the catalysts were evaluated using thermodesorption of pyridine (kinetic diameter 0.54 nm), while the characteristics of the external surface Brønsted acid sites were probed using...
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Nanosponge MFI zeolites functionalized with metal nanoparticles for catalysis
Laštovičková, Anna ; Mazur, Michal (advisor) ; Vyskočilová, Eliška (referee)
Metal nanoparticles immobilized at various supports have outstanding catalytic activity in hydrogenation reactions. Zeolites are used as supports, mainly due to their porous structure. In this work, metal nanoparticles were immobilized on MFI nanosponge zeolites. Zeolite nanosponges (pure silica and degallated) were impregnated with pure platinum and platinum mixed with rare-earth elements (cerium, lanthanum, and yttrium) to form alloys. The main aim of this work was to investigate the activity of bimetallic nanoparticles immobilized on zeolite nanosponges in hydrogenation of cinnamaldehyde. It is a versatile model reaction to investigate catalysts, due to the presence of three different reducible groups (C=C bond, C=O bond and benzene ring) in the cinnamaldehyde molecule. The objective was to focus on selectivity effects of various catalyst towards three possible products of cinnamaldehyde hydrogenation (hydrocinnamaldehyde, cinnamyl alcohol and hydrocinnamyl alcohol). The metal alloys evolution is more feasible on the degallated zeolites due to the defects in the structure called silanol nests. These defects stabilize bimetallic nanoparticles. Moreover, samples prepared with use of degallated support showed larger surface areas. Pure-silica zeolites impregnated with metals were also prepared for...
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Theoretical Investigation of ethanol dehydration catalyzed by acid sites in zeolites
Vacek, Jaroslav ; Nachtigall, Petr (advisor) ; Uhlík, Filip (referee)
Zeolites are a group of aluminosilicate minerals with catalytic properties. They may be used for many industrial applications such as catalytic cracking of oil. Zeolites are also capable of converting ethanol to diethylether and ethylen. This reaction is known as dehydration of ethanol. The reaction is potentially interesting as a way of converting ethanol to more valuable molecules. An experimental study (Shashikant A. Kadam, Mariya V. Shamzhy, 2018) has proven that diethylether is the preferred product when the temperatures are low and the partial pressure of ethanol is high. Ethylen is more significant product with higher temperature and lower partial pressure of ethanol. Aim of this thesis is to determine the mechanism of dehydration of ethanol. Furthermore it was attempted to explain the behavior of the reaction under different circumstances. The research was done in silico using the methods of computational chemistry. Such methods give information on the geometry and the energy of systems of molecules. Thus computational chemistry can be used to investigate the relational path and activation energy of the studied reaction. This thesis is a theoretical study of dehydration of ethanol catalysed by a zeolite.
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Theoretical investigation of water-zeolite interactions under steaming conditions
Benešová, Tereza ; Heard, Christopher James (advisor) ; Fišer, Jiří (referee)
The thesis is focused on theoretical study of hydrolysis and isotopic oxygen exchange in zeolite chabazite under steaming conditions. The theoretical study was performed by the methods of computational chemistry, namely density functional theory. Reactions of water with zeolites were investigated within the periodic model with chabazite supercell consisting of 36 Si/Al tetrahedra. Reactivity was investigated for one or two water molecules corresponding to the conditions relevant to high-temperature steaming. Relevant mechanisms of isotopic oxygen exchange were found and their competitiveness with hydrolysis was discussed.
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Theoretical Investigation of silver clusters in zeolites
Hermann, Jan ; Nachtigall, Petr (advisor) ; Fišer, Jiří (referee)
The structure and stability of Agn+ 3 clusters (n = 1, 2, 3) in zeolite Y and their photochemical properties are investigated. Silver-exchanged zeolites have potential technological applications as UV-to-visible light converters. The structure and stability of silver clusters were examined using the periodic den- sity functional theory (DFT), and employing a developed and programmed geometry optimization driver. Non-reduced silver cations are preferably lo- cated in 6-member rings, far from each other. Upon a partial reduction, trigonal clusters are formed in the sodalite cages. The photochemical prop- erties are examined by various methods including the EOM-CCSD, MRPT2 and TD-DFT. No charge transfer transitions from ligands to silver clusters are found. Instead, transitions involving d and s electrons on silver clusters fall in the visible spectral range. Our results contradict the previous sugges- tion proposing existence of linear silver clusters through a double-6-member ring unit. Both parts of our investigations (structural and photochemical) disprove this previous proposition. A new interpretation of experimental spectra is proposed-the reduced trigonal silver cluster is responsible for photoactivity of Ag-doped zeolites with a low silver loading. Theoretical in- vestigation reported in this...
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Preparation of hierarchical zeolites for fine chemical synthesis
Veselý, Ondřej ; Eliášová, Pavla (advisor) ; Přech, Jan (referee)
Preparation of hierarchical zeolites for fine chemical synthesis Author: Bc. Ondřej Veselý Supervisor: Mgr. Pavla Eliášová, Ph.D. Prague, 2019 Abstract (in english): The thesis is focused on synthesis of hierarchical (micro-mesoporous) zeolites by several different methods and their application in catalytic reactions. Performance of hierarchical materials prepared by different approaches has been investigated, as well as the effect of framework topology and type of acidity on the outcome of the reactions. The work was elaborated in the Department of Physical and Macromolecular Chemistry of Charles University under the supervision of Mgr. Pavla Eliášová, Ph. D. The work is divided into three parts. In the first part several methods of preparation of hierarchical zeolites have been investigated and compared in catalytic reactions. Desilication, selective removal of silicon from the framework, is a post-synthetic method that can be used to introduce additional mesoporosity into a zeolite. The process leads to formation of mesopores by introducing defects into the zeolite structure. The resulting mesopore size is very broad. To partially control the pore size, alkylammonium cations may be added to the solution to protect the crystal surface. The desilication was performed on MTW zeolite which contains...
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Metal nanoparticles in zeolites
Zhang, Yuyan ; Čejka, Jiří (advisor) ; Zima, Vítězslav (referee) ; Kubička, David (referee)
Zeolites with encapsulated metal nanoparticles have attracted a wide attention in heterogeneous catalysis due to their high catalytic activity, selectivity, and stability. The PhD thesis was focused on design and synthesis of metal@zeolite catalysts with small and uniformly distributed metal nanoparticles. The main interests were encapsulation of metal nanoparticles into zeolites by co-crystallization strategy and 2-dimensional to 3-dimensional zeolite transformation approach. The PhD work was performed at the Department of Synthesis and Catalysis at J. Heyrovský Institute of Physical Chemistry and Department of Physical and Macromolecular Chemistry, Faculty of Science at Charles University under the supervision of Prof. Jiří Čejka. Zeolites are inorganic crystalline aluminosilicates with microporous framework structures. The micropores of zeolites provide the ideal microenvironment to accommodate metal nanoparticles. During metal nanoparticles formation in zeolite micropores, they can be limited by a rigid framework, preventing the aggregation and leaching of metal during the reaction process. Furthermore, the diameters of zeolite micropores are usually in the range of 0.3-1.5 nm, which can be used to discriminate molecules depending on their size and shape, thus endowing the metal@zeolite...
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