No description available.

The conversion of methane into valuable chemicals remains one of the major challenges in catalysis science. Both academia and industry are showing keen interest in developing direct one-step catalytic processes in contrast to the existing indirect technologies based on syngas (i.e., Fischer-Tropsch, Methanol-to-Hydrocarbons) that are highly energy-intensive and require high capital investment for the syngas preparation-compression units. Therefore, methane dehydroaromatization (MDA) catalysis on transition metal (i.e., Mo, W, Fe, V) loaded zeolites (i.e., ZSM-5, MCM-41, TNU-9) is still being studied by many researchers, as this process is considered to be one of the most promising alternatives despite the thermodynamic limitations and rapid catalyst deactivation. To develop stable catalysts and commercialize this process, one needs to understand the fundamentals of the reaction and investigate the possible process enhancement options. For instance, although several studies proposed the bifunctional pathway (CH4 coupling on activated Mo sites and oligomerization on Brønsted acids), the details of the mechanism are still ambiguous. Besides, there are options like H2O co-addition to enhance the performance and stability of catalysts against high coke deposition rates. However, a proper structure-function relationship during co-feeding was lacking in previously reported works. Furthermore, there can be alternative metals that may replace Mo. For example, tungsten oxides having similar chemical features with molybdenum oxides are thermally more stable; they can persist even during high-temperature air regeneration. However, W-supported catalysts cannot reach the activity levels that those based on Mo display. This performance difference between W and Mo should be investigated to improve the catalytic performance of W/ZSM-5. Also, the utilization of hierarchical zeolites in MDA catalysis has received a great deal of attention in the last two decades, since they have a great potential to help in improving catalysis performance and overall lifetime. However, when literature survey is done regarding this topic, it would be seen that there is a great inconsistency in many aspects (type of hierarchy, process performance, catalyst deactivation etc.) among the previous studies. Therefore, the hierarchical zeolite applications in MDA reactions should be revisited, and a more detailed discussions should be presented to catalysis community. Considering all these, we have developed new strategies to study MDA. First, we investigated the initial C-C bond formation mechanism during the early stages of MDA by applying “mobility-dependent” advanced ssNMR techniques on labeled methane (13CH4) treated Mo/ZSM-5 catalyst. We identified two mechanisms (mono- and bi-functional) leading to an initial C-C bond based on the detected species. Moreover, we elucidated the effects of H2O co-feeding over Mo/ZSM-5 by employing a battery of advanced characterization techniques. It has been found that water does not change the initial C-C bond formation mechanism but results in steam reforming reaction proceeding parallel to MDA. Also, we investigated W/ZSM-5 catalyst for MDA with a different perspective. The experiments conducted indicate that enhanced catalytic performance might be achieved if the dispersion and distribution of W sites on ZSM-5 can be tuned. Lastly, we revisited the hierarchical zeolite application in MDA catalysis. After analyzing our experimental results and the previous studies, a detailed discussion was presented to give some directions to those interested in hierarchy in zeolites.

No description available.

This item was digitized as part of the Million Books Project led by Carnegie Mellon University and supported by grants from the National Science Foundation (NSF). Cornell University coordinated the participation of land-grant and agricultural libraries in providing historical agricultural information for the digitization project; the University of Arizona Libraries, the College of Agriculture and Life Sciences, and the Office of Arid Lands Studies collaborated in the selection and provision of material for the digitization project.

No description available.

No description available.

A dissertation submitted to the Faculty of Science, University of the Witwatersrand in fulfilment of the requirements for the award of Master of Science degree, 2017. / Mining is an integral sector of most developing countries and it is a highly lucrative industry that has been in existence for centuries, and assumes an essential part in their economies. However, the legacy of mining in these countries has posed a threat to underground and surface water as a result of contamination arising from Acid Mine Drainage (AMD). Bearing in mind the environmental and ecological impairment posed by AMD there is a need for innovation in the treatment of AMD, to enable financially savvy treatment of the contaminated waters. This research is focused on the extraction of U(VI), As(III) and As(V) from synthetic metal solutions as well as field removal of these metal ions by application of iron hydroxide/oxide-modified zeolite. Batch experiments were performed to evaluate the effectiveness of iron hydroxide/oxide-modified zeolite as a potential low-cost sorbent for extracting As(III), U(VI) and As(V) from AMD. The research approach was based on the possible changes that can occur to a zeolite surface that has been in contact with an iron-laden solution. Zeolite is a commonly used adsorbent, but fewer studies have explored changes that it undergoes as an adsorbent on contact with iron solutions. Thus, the study involved modifying zeolite with iron hydroxide/oxide, which are the main precipitates of iron in the environment and which can possibly alter the adsorption properties of zeolite. Batch extraction studies were performed using the modified zeolite. In paper I, the synthesis of iron (hydr) oxide modified zeolite was achieved through precipitation of iron on the zeolite. The kinetic data for As(V) adsorption by iron (hydr) oxide-modified zeolite model fit well into pseudo second-order and the adsorption capacity was obtained as 0.080 mg g-1. The application of iron (hydr) oxide modified zeolite on AMD for As(V) recovery showed that > 99% of As(V) was extracted from the solution. The high removal efficiency of oxyanionic arsenic species was attributed to arsenic forming complexes with iron oxyhydroxide surface on the surface of the sorbent. Paper II dealt with adsorption of U(VI) from aqueous solution by application of iron hydro (oxide)-modified zeolite in a single-component system. Parameters such as: solution pH, contact time, adsorbent dosage, initial concentration and temperature were optimized before field application to real acid mine drainage. The optimum parameters for U(VI) adsorption were: adsorbent dosage (3.0 g), solution pH (6 ±0.1) and contact time (30 min). Optimum parameters where then applied to acid mine drainage were the effluent was found to be cleaner than the influent. In Paper III, iron oxide-coated zeolite (IOCZ) nanocomposite was prepared and fully characterized. This sorbent was then used for extraction of U(VI) and As(III) from aqueous solutions by application of batch techniques. Batch study results were modelled best by the pseudo second-order kinetic model and Freundlich isotherm. The adsorption capacity of both U(VI) and As(II) was dependent on the temperature. The presence of Cd2+, Co2+ and Cr3+ ions enhance the adsorption of As(III) whereas the opposite trend was observed for U(VI) sorption onto IOCZ nanocomposite. / XL2018

The presented PhD thesis is focused on the synthesis, characterization, and modifications of zeolites and zeolitic materials. The main interests are two-dimensional (2D) zeolites and modification of their interlamellar space. Presented work was performed at the Department of Synthesis and Catalysis at J. Heyrovský Institute of Physical Chemistry in Prague, Czech Republic under the supervision of Prof. Jiří Čejka. Zeolites are inorganic crystalline solids with a microporous framework structure. They are widely used as catalysts, sorbents, and ion-exchangers. Conventional zeolites have been recognized as three-dimensional (3D) tetrahedrally-connected frameworks. However, some of them are also known to exist in various layered forms (2D zeolites). Recently, the transformation of 3D germanosilicate UTL into layers (IPC-1P) has started a new branch in 2D zeolites chemistry. This chemically selective degradation of UTL framework was performed via acid hydrolysis. In the structure of this germanosilicate, Ge atoms are preferentially located in specific building units, double-four-rings (D4R), which connect dense silica layers. Modifications of the layered precursor IPC-1P led to discovery of the two novel 3D zeolites: IPC-4 (PCR) and IPC-2 (OKO). This novel approach in the zeolite synthesis, called ADOR...

Improvement of the properties of zeolites for application in the nitrogen andoxygen separation process and in acid catalysisLes zeolites són aluminosilicats cristal·lins formats per un esquelet aniònic rígid amb una microporositat en forma de cavitats i/o canals. La seva fórmula és (Mn+)x/n[(AlO2)x(SiO2)y]· mH2O, on M representa els cations situats fora de l'estructura que contraresten la càrrega de l'esquelet. Presenten una elevada àrea superficial interna, elevada cristal·linitat i estabilitat química i tèrmica, així com capacitat de bescanvi catiònic.Industrialment s'utilitzen principalment com intercanviadors iònics, adsorbents altament selectius i com a catalitzadors.L'objectiu d'aquesta tesi és modificar zeolites i utilitzar-les per millorar els processos de separació denitrogen i oxigen de l'aire i el d'isomerització de l'òxid d'estirè al fenilacetaldehid, així com caracteritzar les mostres per correlacionar-ho amb el seu comportament.El primer capítol resumeix la bibliografia trobada per als processos de separació de nitrogen i oxigen del'aire i d'isomerització de l'òxid d'estirè, les variables que influencien i els millors resultats trobats.El segon capítol descriu les tècniques de caracterització utilitzades i els procediments de preparació demostres i caracterització emprats.El capítol 3 fa referència a l'aplicació de zeolites modificades en la separació de nitrogen i oxigen. S'hautilitzar la mordenita, una zeolita natural amb una porositat en forma de canals, de manera que els cations, responsables de la interacció, es trobarien en posicions força accessibles. L'adsorció de N2 i O2 a 298 K indica que la mordenita dóna uns volums d'adsorció lleugerament superiors a la faujasita mantenint-se la selectivitat d'adsorció, degut a la major accessibilitat dels cations situats en els canals.La primera modificació ha estat introduir àtoms de fluor i mirar l'efecte en l'adsorció de N2 i O2. Els resultats indiquen que la introducció de petites quantitats de fluor millora la separació, ja que disminueix la polarització de la càrrega dels cations i augmenta la interacció amb el moment quadrupolar del N2 i O2, augmentant la selectivitat d'adsorció a baixes pressions.També s'ha provat per a la separació mordenita intercanviada amb cations Ag+, capaços d'interaccionar mitjançant -complexació amb el N2 i O2 quan es formen espècies Agm n+. Amb la mordenita no s'haaconseguit formar aquestes espècies probablement per la major distància entre cations, mentre que sí s'ha aconseguit amb la zeolita A, on la seva formació depèn de les condicions de preparació, de manera que els intercanivis amb cations Ag+ només tenen lloc quan s'utilitzen concentracions de Ag+ elevades.El quart capítol estudia l'aplicació d'algunes mordenites modificades amb fluor en la isomerització de l'òxid d'estirè al corresponent aldehid, reacció de gran importància en la indústria dels perfums. Els resultats obtinguts indiquen que la introducció de petites quantitats de fluor augmenta l'acidesa de Brönsted i dóna lloc a majors conversions quan s'utilitzen mètodes convencionals d'escalfament, però a la desactivació del catalitzador quan s'escalfa per microones. En canvi, amb quantitats de fluor elevades, es formen centres àcids de Lewis, que són menys actius en aquesta reacció. Així doncs, l'escalfament per microones per aquesta reacció disminueix molt els temps de reacció, però, no obstant, desactiva elscatalitzadors més àcids.Per últim s'ha realitzat un seguiment per espectroscòpia Infra-Roja de l'adsorció de diferents molèculessonda en diverses mordenites i altres zeolites. Els resultats han permès conèixer millor la posició, accessibilitat, així com el tipus d'interaccions favorables que donen els cations de la mordenita. També es posa de manifest la formació d'interaccions inusuals, de tipus múltiple. L'últim capítol mostra les conclusions més rellevants, destacant la importància en la modificació de les zeolites per a la millora de les seves propietats, aconseguint-se els objectius proposats.Improvement of the properties of zeolites for application in thenitrogen and oxygen separation process and in acid catalysisZeolites are crystalline aluminosilicates formed by an anionic skeleton and by a microporous system disposed in cavities and/or channels. Present the molecular formula (Mn+)x/n[(AlO2)x(SiO2)y]· mH2O, where M represents the extraframework cations that compensate the negative charge of the framework. They are characterized by high internal surface area, high cristallinity and also high chemical and thermal stabilities as well as cation exchange capacity. At industrial level they are basically used as ion exchangers,as highly selective adsorbents and also as catalysts.The main objective of this thesis was to modify zeolites and use them to improve two industrial processes, which are the nitrogen and oxygen separation process and the styrene oxide isomerisation reaction to phenylacetaldehyde. It was also and other scope to characterise deep the samples andcorrelate the results with their behaviour on both processes in order to optimise them.The first chapter describes the literature found related to the nitrogen and oxygen separation process and to the isomerisation of styrene oxide, specifically, the influencing variables and themost outstanding results.In chapter two, the characterization techniques and the samples preparation and characterizationprocedures are described.Chapter 3 refers to the application of several modified mordenites to the nitrogen and oxygen separation. Mordenite was used because the porosity, based on channels, should favour the accessibility to cations by the adsorbate molecules. The nitrogen and oxygen adsorption results indicate that mordenite can adsorb slightly higher volumes than faujasite, fact that evidences the role of cations accessibility in this process.The first modification was the introduction of fluorine ions on mordenite structure in order to see the effect on the N2 and O2 adsorption properties. The results show that the introduction of low quantities of fluorine improves the separation, since the presence of fluorine decreases the shielding on cations and therefore improve the interaction between those and the quadrupolar moment of N2 and O2 molecules, increasing also the N2/O2 selectivity at low pressures.It was also studied the interaction of silver exchanged zeolites with nitrogen and oxygen molecules by -complexation with Agm n+ species formed during the activation. We were not able to form these species inside mordenite, probably due to the bigger distances between cations, while these species were formed inside A zeolite. The adsorption results indicate that the formation of these species depends on the preparation and activation conditions and good adsorption/separation results are obtained when suitable conditions are used. In chapter four, the application of modified mordenites in the styrene oxide isomerisation reactionto phenyl acetaldehyde using microwaves as a new heating method has been studied. The obtained results show that the introduction of low amounts of fluorine increases the Brönsted acidity, which improve the conversion results when using conventional heating but deactivates the catalysts when microwaves are used. Otherwise, when higher amounts of fluorine are used, Lewis acid sites are formed, less active in this isomerisation reaction. Therefore, the use of microwaves improves reaction times on this reaction, but deactivates the more acidic catalysts.In chapter five, the nature and position of several mordenites was studied by means of infrared spectroscopy by studying the adsorption desorption of several prove molecules. These studies allowed us to know better the position, accessibility and the existing interactions between the cations and the prove molecules. It was also evidenced the presence of a new interaction, unusual and probably atributed to a multiple interaction. The last chapter expose the most important conclusions, pointing out the importance of some modifications to improve their properties for application in those processes.

No description available.