National Repository of Grey Literature 29 records found  previous10 - 19next  jump to record: Search took 0.00 seconds. 
The effect of reaction conditions and type of active sites on the kinetics of heterogeneously catalysed formylation of amines
Olšovská, Adéla ; Veselý, Ondřej (advisor) ; Kubička, David (referee)
Formamides are valuable compounds in pharmaceutical production and organic synthesis; however, their common synthesis methods suffer from number of drawbacks including low reaction rate and contamination with heavy metals leached from a catalyst. According to several articles, these problems could be overcome by using zeolite-based catalysts. Nevertheless, the mentioned articles provide somewhat conflicting results and also lack a clear description of which centres in zeolites are catalytic active. Therefore, we prepared a set of zeolite-based catalysts commonly described in the literature (zeolite Beta ion exchanged to H+ and Cu2+ form) in order to test them as catalysts and clarify their catalytic behaviour and nature of the active sites. We chose formylation of N-methylaniline by formic acid as the model reaction for our research and performed a series of experiments under varying conditions (solvents, concentration of reactants, temperature). In contrast to the published articles, none of the investigated materials demonstrated significant catalytic activity in our experiments. Moreover, we found that the often pronounced catalyst: zeolite Beta ion-exchanged into the copper form, is unstable in the presence of formic acid, which causes leaching of copper into the reaction mixture. At the same...
Dynamic Processes in Porous Nanomaterials: Adsorption and Diffusion
Trachta, Michal ; Bludský, Ota (advisor) ; Heard, Christopher James (referee) ; Čičmanec, Pavel (referee)
Dynamical processes in porous nanomaterials (adsorption, diffusion, separation) play a significant role in many industrial applications. Modeling of these applications theoretically provides atomistic details of adsorption and diffusion phenomena - information that is often difficult to obtain using contemporary experimental methods. Theory may thus provide a deeper insight into these processes, e.g., explaining the mechanisms and assisting in the search for new materials suited for a specific application through material screening. In this work, the dynamical processes in porous materials were investigated by means of force field and ab initio modeling. On a set of relevant simulation data, two particular problems related to the modeling of such processes are thoroughly addressed: (i) the sensitivity of the results to material geometry, and (ii) the accuracy of the potential energy surface description. Regarding the structure description, the reliability of theoretically-predicted structures is still limited, despite the significant increase in computational power and algorithm development. The drawbacks of individual methods are identified; even state-of-the-art density functional theory approaches suffer from inaccuracies, e.g., due to an imbalance between the semi-local (GGA) functional and...
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...
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...
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...
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...
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...
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.
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...
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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