Browsing by Subject "Zeolite"

Now showing 1 - 15 of 15
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    Directed synthesis and characterization of zeolite nanoparticles
    (2013-10) Zhang, Xueyi
    Zeolites are a class of materials with ordered micropores (smaller than 2 nm), that can be used for gas separation, catalysis, and adsorption. Structurally, zeolites are composed of SiO4 tetrahedra sharing corners in an ordered manner. The numerous arrangements of these SiO4 tetrahedra give zeolites micropores in the forms of channels and cages. Although the applications of each zeolite depends on the spatial arrangements and the sizes of these micropores, size and morphology of zeolite particles are equally important. By preparing zeolite nanoparticles, diffusion paths of zeolite particles can be shortened, total surface area of zeolite particles can be increased, which are beneficial to reducing energy consumption in gas separation, reducing deactivation in catalytic reactions, and increasing adsorption capacities. This dissertation introduces various methods to prepare zeolite nanoparticles. Zeolite nanoparticles prepared with the help of mesoporous carbon templates (hard template) are firstly introduced where the shape and size of zeolite particles are imprinted from the templates. In addition to hard templates, the use of bifunctional surfactant (soft templates) to prepare ultra-small zeolite nanoparticles and lamellar zeolite membrane is also introduced. With only one structure-directing agent that is less intuitive than hard or soft templates, the preparation of hierarchical lamellar zeolite with 2 nm lamellae, the self-pillard pentasil (SPP) zeolite, is introduced, where the intrinsic growth patterns of the crystal played an important role. Finally, template-free synthesis of zeolite nanoparticles, where zeolite formation is totally driven by the intrinsic growth patterns, is introduced. In addition to the preparation methods, a series of computational methods to determine and study the structures of zeolite nanoparticles are also introduced.
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    Dispersible exfoliated zeolite nanosheets and their application in high performance zeolite membrane
    (2013-10) Agrawal, Kumar Varoon
    In the wake of the energy crisis, an efficient separation technology such as membrane is required to replace the energy intensive processes like distillation. High performance zeolite membrane can be fabricated by coating of a thin film of high-aspect-ratio zeolite nanosheets on a porous support. However, the synthesis of highly crystalline and morphologically intact zeolite nanosheets by the direct hydrothermal synthesis has been challenging. Successful reports on the synthesis of zeolite nanosheets by the exfoliation of their layered structure exist, but the synthesis routes provided in these reports often lead to significant damages to the structure and the morphology of nanosheets. This dissertation focuses on the development of a scalable method for the synthesis of zeolite nanosheets, while preserving their structure and the morphology. MWW and MFI nanosheets were prepared by polymer melt compounding of their layered precursors with polystyrene. Zeolite nanosheets were extracted out of the polymer matrix by solution processing of the zeolite-polymer nanocomposite. Exfoliated nanosheets and their coatings were then characterized by the scanning electron microscopy (SEM), transmission electron microscopy (TEM), atomic force microscopy (AFM), nuclear magnetic resonance (NMR), and X-ray diffraction (XRD). A compact, oriented, 300-nm thick zeolite film was fabricated on a symmetric alumina support by a one-step filter coating method. This nanosheet film demonstrated molecular sieving capabilities after a mild hydrothermal treatment. Density gradient centrifugation was used to purify the zeolite nanosheets from the polymer matrix, and the large unexfoliated particles, resulting in a two-fold increase in the yield of nanosheets in the final coating suspension. Sub-100 nm thick films of these nanosheets were made on a symmetric alumina supports. Nanosheet films with thickness ranging from 10 nm to 100 nm were prepared on an asymmetric silica supports. In-plane secondary growth of these films by the impregnation growth method led to b-oriented, 100-150 nm thick zeolite film that separated xylene isomers with separation factors of 100-800, while providing a high permeance of p-xylene (4 x 10-7 moles/m2-s-Pa).</
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    Fabrication of Zeolite MFI Membranes on Low Cost Polymer Supports
    (2017-07) Zhang, Han
    About 10~15% of total energy consumption in US is attributed to energy intensive chemical separation processes, such as distillation. The alternative membrane-based separation could save up to 90% energy consumption with outstanding separation performance. Zeolite MFI membranes have been demonstrated for xylene and butane isomer separations with high separation factors and permeances. However, high cost and scale-up difficulty prevent the commercialization of MFI membranes in industries. This dissertation attempts to explore the methods for MFI membranes supported on low cost polymer supports. The major challenge is the stability of polymer support during the detemplation treatment of the MFI membrane after secondary growth. Two mild detemplation methods, thermal treatment at 280 °C and UV/ozone treatment, were identified with sub-100 nm MFI membranes supported on quartz supports. These two methods were then applied to MFI membranes supported on mesh-polyethersulfone (PES) supports and MFI membranes supported on mesh-polybenzimidazole supports. However, cracks formed after the treatments due to the damage of polymer layer by UV light and the mismatch of linear thermal expansion co-efficient, respectively. Another approach, which utilizes the open-pore MFI nanosheets, have been demonstrated. The organic structure directing agents (OSDA) occluded inside the micropores of nanosheets were removed by successive piranha solution treatment, while the crystallinity and morphology were still preserved that confirmed by X-ray diffraction(XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), and gas adsorption. The simple deposition of such open-pore MFI nanosheets on porous PBI support, without the need of secondary growth and detemplation, exhibited n-/iso-butane ideal selectivity of 5.4 with n-butane permeance of 3.5×10-7 mol/m2-s-Pa. In addition, the nanosheet exfoliation yield was significantly improved by an oligomeric polystyrene resin. Ultrafiltration polymer hollow fibers were also prepared as suitable supports for nanosheet coating.
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    First principles simulations of hydrocarbon conversion processes in functionalized zeolitic materials
    (2013-05) Mazar, Mark Nickolaus
    With increasing demand for chemicals and fuels, and finite traditional crude oil resources, there is a growing need to invent, establish, or optimize chemical processes that convert gasifiable carbon-based feedstocks (e.g., coal, natural gas, oil sands, or biomass) into the needed final products. Catalysis is central to almost every industrial chemical process, including alkane metathesis (AM) and the methanol-to-hydrocarbons (MTH) process, which represent final steps in a sequence of hydrocarbon conversion reactions. An in depth understanding of AM and MTH is essential to the selective production of the desired end products. In this dissertation, ab initio density functional theory simulations provide unique mechanistic and thermodynamic insight of specific elementary steps involved in AM and MTH as performed on zeolite supports. Zeolites have been employed throughout the petroleum industry because of their ability to perform acid-catalyzed reactions (e.g., cracking or MTH). The crystalline structure of zeolites imparts regular microporous networks and, in turn, the selective passage of molecules based on shape and functionality. Many different elements can be grafted onto or substituted into zeolites, resulting in a broad range of catalytic behavior. However, due to the variety of competing and secondary reactions that occur at experimental conditions, it is often difficult to extract quantitative information regarding individual elementary steps. ab initio calculations can be particularly useful for this purpose. Alkane metathesis (i.e., the molecular redistribution or chain length averaging of alkanes) is typically performed by transition metal hydrides on amorphous alumina or silica supports. In Chapter 3, the feasibility of AM in zeolites is assessed by using a grafted Ta-hydride complex to explore the full catalytic cycle in the self-metathesis of ethane. The decomposition of a Ta-metallacyclobutane reaction intermediate that forms during olefin metathesis is responsible for the largest activation energy of the catalytic cycle. This assessment is similar to the findings of alkane metathesis studies on alumina/silica supports and indicates that the entire AM cycle can be performed in zeolites by isolated single-atom transition metal hydrides. Performed over acid form zeolites, MTH is used in the conversion of methanol into a broad range of hydrocarbons, including alkenes, alkanes, and aromatics. For reasons that are not yet rigorously quantified, product selectivities vary dramatically based on the choice of catalyst and reaction conditions. The methylation of species containing double bonds (i.e., co-catalysts) is central to the overall process. Distinct structure-function relationships were found with respect to the elementary steps in the methylation and β-scission of olefins. In Chapter 4, the role of zeolite topology in the step-wise methylation of ethene by surface methoxides is investigated. Elementary steps are studied across multiple frameworks (i.e., BEA, CHA, FER, MFI, and MOR) constituting a wide variety of confinement environments. The reaction of surface methoxides with ethene is found to require a transition state containing a primary carbocation. The barrier height is found to decrease nearly monotonically with respect to the degree of dispersion interactions stabilizing the primary carbocationic species in the transition state. In addition, quantification of the ``local'' dispersion energy indicates that confinement effects can not be simply correlated to pore size. The β-scission of olefins plays an important role in the product selectivities of many important chemical processes, including MTH. In Chapter 5, β-scission modes involving C6 and C8 isomers are investigated at a single, isolated Bronsted acid site within H-ZSM-5. We find that the relative enthalpic barriers of β-scission elementary steps can be rationalized by the substitution order of the two different carbocationic carbon atoms that are present in the reactant (C+) and transition states (βC). In fact, the increase in charge required by the βC atom to go from the physi/chemi-sorbed reactant state to the β-scission transition state (+0.23e-0.33e) is found to correlate almost linearly with the intrinsic activation energy (89-233 kJ mol-1). The charge of the βC atom depends, to a large extent, on the substitution order of both the C+ and βC atoms and, therefore, each $beta$-scission mode is a sub-category onto itself. Isomerization reactions, which are fast with respect to β-scission, enable reactant hydrocarbons to explore and find low barrier β-scission pathways. Selectivities predicted on the basis of the relative barrier heights of β-scission modes accessible to C6 and C8 species indicate general agreement with experimental observations.
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    Kinetics and mechanism of deoxygenation reactions over proton-form and molybdenum-modified zeolite catalysts
    (2014-07) Bedard, Jeremy William
    The depletion of fossil fuel resources and the environmental consequences of their use have dictated the development of new sources of energy that are both sustainable and economical. Biomass has emerged as a renewable carbon feedstock that can be used to produce chemicals and fuels traditionally obtained from petroleum. The oxygen content of biomass prohibits its use without modification because oxygenated hydrocarbons are non-volatile and have lower energy content. Chemical processes that eliminate oxygen and keep the carbon backbone intact are required for the development of biomass as a viable chemical feedstock. This dissertation reports on the kinetic and mechanistic studies conducted on high and low temperature catalytic processes for deoxygenation of biomass precursors to produce high-value chemicals and fuels. Low temperature, steady state reaction studies of acetic acid and ethanol were used to identify co-adsorbed acetic acid/ethanol dimers as surface intermediates within specific elementary steps involved in the esterification of acetic acid with ethanol on zeolites. A reaction mechanism involving two dominating surface species, an inactive ethanol dimeric species adsorbed on Brønsted sites inhibiting ester formation and a co-adsorbed complex of acetic acid and ethanol on the active site reacting to produce ethyl acetate, is shown to describe the reaction rate as a function of temperature (323 - 383 K), acetic acid (0.5 - 6.0 kPa), and ethanol (5.0 - 13.0 kPa) partial pressure on proton-form BEA, FER, MFI, and MOR zeolites. Measured differences in rates as a function of zeolite structure and the rigorous interpretation of these differences in terms of esterification rate and equilibrium constants is presented to show that the intrinsic rate constant for the activation of the co-adsorbed complex increases in the order FER < MOR < MFI < BEA. High temperature co-processing of acetic acid, formic acid, or carbon dioxide with methane (CH3COOH/CH4 = 0.04-0.10, HCOOH/CH4 = 0.01-0.03, CO2/CH4 = 0.01-0.03) on Mo/H-ZSM-5 formulations at 950 K and atmospheric pressure in an effort to couple deoxygenation and dehydrogenation reaction sequences results instead in a two-zone, stratified bed reactor configuration consisting of upstream oxygenate/CH4 reforming and downstream CH4 dehydroaromatization. X-ray absorption spectroscopy and chemical transient experiments show that molybdenum carbide is formed inside zeolite micropores during CH4 reactions. The addition of an oxygenate co-feed causes oxidation of the active molybdenum carbide catalyst while producing CO and H2 until completely converted. Forward rates of C6H6 synthesis are unperturbed by the introduction of an oxygenate co-feed after rigorously accounting for the thermodynamic reversibility caused by the H2 produced in oxygenate reforming reactions and the fraction of the active catalyst deemed unavailable for CH4 dehydroaromatization. All effects of co-processing C1-2 oxygenates and molecular H2 with CH4 can be interpreted in terms of an approach to equilibrium. Co-processing H2O, CO2, or light (C1-2, C/Heff < 0.25) oxygenates with CH4 at 950 K over Mo/H-ZSM-5 catalysts results in complete fragmentation of the oxygenate and CO as the sole oxygen-containing product. The C/Heff accounts for removal of O as CO and describes the net C6H6 and total hydrocarbon synthesis rates at varying (0.0-0.10) C1-2 oxygenate and H2 to CH4 co-feed ratios. Co-processing larger (C3-5, C/Heff ≥ 0.25) oxygenates with CH4 results in incomplete fragmentation of the co-fed oxygenate and preferential pathways of C6H6 synthesis that exclude CH4 incorporation. This results in greater net C6H6 synthesis rates than would be predicted from observations made when co-processing C1-2 oxygenates. Catalytic technologies have served a crucial role in processing petroleum feedstocks and are faced with new challenges as the feedstock shifts to chemically diverse but renewable biomass sources. This research addresses these challenges at fundamental and applied levels as it offers the potential to convert readily available biomass to commodity chemicals and fuels while simultaneously examining the elementary concepts of deoxygenation reactions on catalytic surfaces.
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    Kinetics and mechanisms of methanol to hydrocarbons conversion over zeolite catalysts
    (2013-05) Hill, Ian Michael
    The methanol-to-hydrocarbons (MTH) process over zeolite catalysts is the final step in the synthesis of commodity chemicals and fuels from alternative carbon sources via synthesis gas intermediates. Emerging research has shown that olefins and aromatics are critical intermediates, acting as scaffolds for the addition of methyl groups from methanol or dimethyl ether (DME) in an indirect "hydrocarbon pool" mechanism. Outstanding questions in this research pertain to (i) the quantitation of reaction rates for C1 homologation and (ii) the mechanism of activating methanol or DME for the formation of carbon-carbon bonds. This research reports rate constants and activation energies of olefin and aromatic methylation steps over zeolites of different pore sizes and geometries from steady-state methylation reactions, as well as isotopic and post-reaction titration studies to determine mechanistic details regarding the structure of the active zeolite surface species. Specifically, isolated steady-state methylation of C2 to C4 olefins over zeolites at differential conditions have shown that reactions producing higher degrees of substitution of intermediate carbocations have rate constants that are an order of magnitude higher than less substituted intermediates. Benzene and toluene methylation reactions show similar kinetic behavior to propylene and linear butene, respectively, over H-ZSM-5. Pressure-dependent studies show a first-order rate dependence on olefin or aromatic pressures which is invariant of DME partial pressures, indicating a surface saturated in DME-derived species reacting with a gas phase co-reactant in the rate-limiting step. These surface species have been identified as methoxides, as observed using post-reaction titration and isotopic studies. The methylation of para- and ortho-xylene with DME at low conversions showed linear dependence of the reaction rate at low pressures of xylene, but the reaction rate became zero-order at higher xylene pressures over H-ZSM-5. The reaction rate remained zero-order in DME pressure, and when taken in conjunction with results from isotopic studies and surface titrations, indicates that the surface methoxides become saturated in adsorbed xylene isomers. A reduction in the critical diffusion length by a factor of >150 did not increase the reaction rate, indicating that the effect is adsorption and not one of transport limitations. Arguments based on derived rate equations modeling observed trends in kinetic, isotopic, and titration studies set a basis for building a microkinetic model for MTH reactions over H-ZSM-5, which can predict expected product distributions for a given set of reaction conditions.
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    Modeling, film formation, and material synthesis for performance optimization of mixed matrix membranes.
    (2009-01) Sheffel, Joshua Alexander
    Mixed matrix membranes offer the hope of improving the performance of a separating membrane by dispersing a second phase within it. By combining the processability of a continuous phase (the matrix) with the separation characteristics of a dispersed phase (the flake), mixed matrix membranes aim to provide a step-change improvement in membrane performance without dramatically increasing the cost of membrane technology. In this dissertation, a numerical model for the performance of mixed matrix membranes is presented that accounts for effects such as competitive adsorption and concentration-dependent diffusivities. It is shown that these effects are vital for the modeling of a membrane containing zeolite flakes. This insight is then used to formulate a semi-empirical model for mixed matrix membrane performance that does not require extensive numerical calculations. Through a series of case studies on relevant gas and vapor separations, these models are applied to material and process design for mixed matrix membranes. Finally, experimental aspects of mixed matrix membrane formation are presented, including the synthesis of layered aluminophosphate molecular sieves and the fabrication of mesoporous silica/silicalite-1 zeolite films.
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    Probe reactions of alcohols and alkanes for understanding catalytic properties of microporous materials and alumina oxide solid acid catalysts.
    (2012-08) Chiang, Hsu
    Steady-state chemical reactions of alkanol dehydration and alkane hydroisomerization were employed as probe reactions to evaluate the effects of structure and composition of zeolites and gamma alumina (γ-Al2O3) on measured catalytic rates and selectivity to establish the mechanistic cycles and catalytic site requirements for the corresponding reactions. The measured kinetic effects of ethanol and ethylene pressure on diethyl ether (DEE) formation over three zeolite materials (H-MFI, H-FER and H-MOR) show that ethanol dimers are formed during reaction and that these dimeric species are subsequently dehydrated to form DEE. In zeolites, ethylene formation was only observed on zeolites possessing 8-MR channels (H-MOR). The 8-MR channels protect ethanol monomeric species and prevent the formation of ethanol dimeric species due to size restrictions. The results of ethanol dehydration reactions studied in this research imply that the design and selection of microporous catalysts for performing shape-selective reactions of oxygenates requires us to consider the size and stability of the corresponding surface intermediates as well as the location of Brønsted acid sites. Hydroisomerization reactions on bifunctional metal-acid catalysts (Pt/Al2O3 and acid zeolites) can convert n-hexane into 2-methylpentane (2MP) and 3-methylpentane (3MP). The measured rate of n-hexane isomerization was linearly proportional to the molar ratio of H2 to n-C6H14 over three zeolites (FER, MOR, BEA), consistent with a bifunctional mechanism involving the facile dehydrogenation of n-hexane into n-hexene on the metal catalyst and a kinetically-relevant step involving isomerization of n-hexene to 2MP and 3MP on zeolitic acidic sites. Sodium-exchanged MOR was used to study the rate of n-hexane isomerization in 12MR channel (MOR(12MR) ) and 8MR pockets (MOR(8MR)) in MOR. The measured rate of isomerization over zeolites increase in the order of FER < MOR(12MR) < MOR(8MR) < BEA, showing that pore size cannot be used to accurately predict the occurrence or exclusion of a particular reaction within zeolitic solids. The mechanisms and site requirements for unimolecular and bimolecular dehydration reactions of ethanol on γ-Al2O3 were investigated using steady state and isotopic kinetic studies and in situ titration. The rates of ethylene and DEE formation from ethanol dehydration on γ-Al2O3 at 488K were not inhibited by exposure to CO2 (0-47kPa) while pyridine exposure (3kPa) nearly shut down the rates of formation, showing that acid sites rather than basic sites are required for ethylene and DEE formation. The proposed mechanisms for ethylene and DEE formation proceed through desorption of surface-bound ethoxide species and the activation of surface ethanol dimeric species, respectively. The proposed mechanisms are consistent with the measured pressure dependence in ethanol and water, and the measured kinetic isotope effects using isotopic labeling reactants. The rate and equilibrium constants in the kinetic models derived from these mechanisms were estimated using Athena Visual Studio software to assess the stability of surface species formed during ethanol dehydration reactions.
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    Production Of Renewable Aromatic Chemicals From Biomass-Derived Furans Through Brønsted Acid Zeolites
    (2018-07) Vinter, Katherine
    Renewable chemicals remain a major need for society as a whole with the eventual extinction of petroleum sources. Though natural gas resources may curtail our energy demand in the foreseeable future, larger carbon-based chemicals are necessary to replace aromatics used in the production of essential polymers and plastics. The most promising choice stems from biomass, as it is cheap and its monomers are similarly structured to those of plastics. This thesis focuses on thermochemical routes to plastics precursors through the use and understanding of zeolite acid catalysts. Specifically, the conversion of biomass-derived 2,5-dimethylfuran (DMF) and ethylene for the production of p-xylene, and water byproduct, with H-Y and H-BEA zeolite catalysts was investigated. This reaction is completed through a two-step reaction pathway: (1) homogeneous Diels-Alder cycloaddition of ethylene and DMF to form an instable cycloadduct intermediate and (2) an acid-catalyzed dehydration of the cycloadduct to form water and p-xylene. Activation energies and reaction rate orders using an H-BEA catalyst were examined and two regimes were found. The first regime, at low acid site concentration, is dehydration limited and the rate of reaction scales linearly with acid site concentration. The second regime, at high acid site concentration, is cycloaddition limited; the reaction rate is zero order with respect to acid site concentration. The high selectivities achieved to p-xylene in this reaction were puzzling and attempts were made to understand the underlying mechanism. Under these reaction conditions, p-xylene should readily isomerize and disproportionate to form other aromatics products. However, none of these products are formed. Upon further adsorption investigations using techniques such as Solid State MAS NMR, FT-IR, and TGA, as well as computational calculations, it was found that the hydrolysis product of DMF, 2,5-hexanedione, preferentially adsorbed onto the zeolite acid sites, protecting them from p-xylene adsorption and reaction. The dehydration reaction still occurred due to the fact that the cycloadduct adsorbed as strongly as 2,5-hexanedione. With this new observation of the ability for an inert species to inhibit reaction of preferred products, a new chemistry was tested. Cyclohexanol dehydration to cyclohexene using an H-BEA catalyst was investigated with and without the addition of an inert chemical DMF/hexanedione. It was found that the addition of the inert increased the yield of cyclohexene by inhibiting its reaction to oligomers. A more general model and approach was developed to use competitive adsorption with an inert as a tool for increasing reaction yields. A new technique for acid site counting in solid acid catalysts was discovered. Reactive Gas Chromatography (RGC) allows for the automated, sensitive and simple counting of acid sites in zeolites with use of a modified gas chromatograph. Counting acid sites in zeolites is essential for the comparison of two catalysts of differing acid site densities and acid sites. The validity of this new technique, which uses the robust method of alkylamine decomposition, was determined with comparison to literature values and those from in situ pyridine titration experiments. Finally, a new application for the RGC, known as Size Exclusion Reactive Gas Chromatography (SE-RGC) was invented for the purpose of understanding and predicting the size of molecules relative to the size of zeolite pores. It is apparent that the zeolitic community needs better measures for understanding the ability for a molecule to fit into a pore. Here, multiple sizing methods (kinetic diameter, van der Waal diameter and minimal enclosing cylinder diameter) were investigated. Though minimal enclosing cylinder was the most accurate predictor, it was found each of these methods overpredicted molecular size, when compared to results using SE-RGC. FT-IR experiments will next be completed to more fully understand molecule fitting in zeolites.
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    Sour Gas Sweetening and Ethane/Ethylene Separation
    (2018-05) Shah, Mansi S
    Chemical separations are responsible for nearly half of the US industrial energy consumption. The next generation of separation processes will rely on smart materials to greatly relieve this energy expense. This thesis research focuses on two very energy-intensive and large-scale industrial separations: sour gas sweetening and ethane/ethylene separation. Traditionally, gas sweetening has been achieved through amine-based absorption processes to selectively remove H2S and CO2 from CH4. Ethane/ethylene is an even harder mixture since the two molecules have very similar sizes, shapes, and self-interaction strengths. Despite their low relative volatility (1.2-3.0), cryogenic distillation is the most commonly used technique for this separation. Compared to absorption and cryogenic distillation, adsorption allows for better performance control by choosing the right adsorbent. Crystalline materials such as zeolites, that have precisely defined pore structure, exhibit excellent molecular sieving properties. Performance is closely linked to structure; identifying top zeolites from a large pool of available structures (~300) is thus crucial for improving the separation. In this thesis research, molecular modeling is used to identify optimal materials for these two separations. Since the accuracy of predictive molecular simulations is governed by the underlying molecular models, the first objective of this thesis research was to develop improved molecular models for H2S, ethane, and ethylene. A wide variety of properties such as vapor-liquid and solid-vapor equilibria, critical and triple points, vapor pressures, mixture properties, relative permittivities, liquid structure, and diffusion coefficients were studied using molecular simulations to parameterize transferable molecular models for these molecules. These models are designed to strike a very good balance between accuracy of predictions and efficiency of simulations. For some of the zeolites for which experimental data existed in the literature, purely predictive adsorption isotherms agreed quantitatively with the available experiments. A computational screening was then performed for over 300 zeolite structures using tailored molecular simulation protocols and high-performance supercomputers. Optimal zeolites for each of the two applications were identified for a wide range of temperatures, pressures, and mixture compositions. Finally, a brief literature survey of the zeolites that have been synthesized in their all-silica form is presented and syntheses for two of the important target framework types is discussed.
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    Structure determination of zeolite nanosheets
    (2012) Zhang, Xueyi; Tsapatsis, Michael
    MFI and MWW zeolite nanosheets are building units for state-of-the-art zeolite thin films for gas separation. In this study, the structures of exfoliated MFI and MWW zeolite nanosheets were determined using a combination of experimental and simulation methods. Based on characterization results from atomic force microscopy and transmission electron microscopy, the structures and thicknesses of the exfoliated zeolite nanosheets were proposed. After optimization with Car-Parrinello molecular dynamics, X-ray diffraction patterns and electron diffraction patterns are simulated from these structures. The agreement between experimental and simulated characterization data suggested that the proposed structures should represent the actual structures of the exfoliated zeolite nanosheets. The methods used in this study can be extended to determining structures of other zeolite nanostructures.
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    Synthesis, characterization, and applications of porous and hierarchically-porous silica nanostructures
    (2014-10) Swindlehurst, Garrett Richard
    Silicate nanostructures can be broadly defined as any material primarily composed of silicon dioxide and having one or more dimensions smaller than 100 nm. Silica is formed of SiO4 tetrahedra connected at their vertices, and the way in which these tetrahedra can be arranged leads to materials classified as amorphous or crystalline, depending on the degree of long-range order in the structure. Due to the complexity of tetrahedral connectivity that is possible, pores can be formed in silicas with length scales ranging from a few angstroms to tens of nanometers. Some microporous silicates exist in nature, but many other porous silicas of considerable importance to chemical engineering are synthetic. One important class of these synthetic porous silicates is the zeolites, which contain pores on the size of angstroms and therefore can act as molecular sieves. In this dissertation, methods for the synthesis and characterization of "zero-dimensional" silica nanoparticles, "two-dimensional" zeolite nanosheets, and "three-dimensional" mesoporous silicas and zeolites are presented. Applications for these materials in catalytic and adsorption processes are also explored. Many of these nanostructured silicates contain hierarchical pore structure with different characteristic pore sizes existing in the materials. One particularly studied material, the self-pillared pentasil (SPP) zeolite, contains both the microporosity of traditional zeolites and mesoporosity resulting from its crystal growth mechanism. Hierarchical pore networks can significantly improve intraparticle mass transfer for interacting chemical species, offering great performance gain in the considered applications.
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    Synthesis, characterization, and applications of porous and hierarchically-porous silica nanostructures
    (2014-10) Swindlehurst, Garrett Richard
    Silicate nanostructures can be broadly defined as any material primarily composed of silicon dioxide and having one or more dimensions smaller than 100 nm. Silica is formed of SiO4 tetrahedra connected at their vertices, and the way in which these tetrahedra can be arranged leads to materials classified as amorphous or crystalline, depending on the degree of long-range order in the structure. Due to the complexity of tetrahedral connectivity that is possible, pores can be formed in silicas with length scales ranging from a few angstroms to tens of nanometers. Some microporous silicates exist in nature, but many other porous silicas of considerable importance to chemical engineering are synthetic. One important class of these synthetic porous silicates is the zeolites, which contain pores on the size of angstroms and therefore can act as molecular sieves. In this dissertation, methods for the synthesis and characterization of "zero-dimensional" silica nanoparticles, "two-dimensional" zeolite nanosheets, and "three-dimensional" mesoporous silicas and zeolites are presented. Applications for these materials in catalytic and adsorption processes are also explored. Many of these nanostructured silicates contain hierarchical pore structure with different characteristic pore sizes existing in the materials. One particularly studied material, the self-pillared pentasil (SPP) zeolite, contains both the microporosity of traditional zeolites and mesoporosity resulting from its crystal growth mechanism. Hierarchical pore networks can significantly improve intraparticle mass transfer for interacting chemical species, offering great performance gain in the considered applications.
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    Zeolite Incorporated Materials for Targeted Biomass Retention and Pollutant Removal
    (2022-04) Chester, Anndee
    This dissertation describes the assessment and treatment of pollutants, namely nutrients, in waste streams. Nutrients such as nitrogen, are of major and growing concern because nitrogen removal from waste streams is energy and cost intensive; yet, without treatment cause eutrophication in aquatic systems. The aquatic health of the Volta River in Ghana was assessed by monitoring pollutants including water quality parameters, contaminants of emerging concern, antibiotic resistance, and the microbial community. While Ghana is a low- to middle- income country, inadequate sanitation infrastructure and environmental regulations contribute to environmental and human health issues. In this highly collaborative work, common (e.g., nitrogen) and emerging contaminants (e.g., DEET, PFAS) were detected and the microbial community was analyzed from samples collected along the length of the lower Volta River. Spikes in microbial detection (16S rRNA gene) and antibiotic resistant genes were associated with anthropogenic activities indicating adverse effects of human activities on the health of the Volta River. Additionally, novel biofilm technologies were explored to enhance nitrogen removal from waste streams. Specifically, zeolite-coated hollow fiber membranes and zeolite-coated biofilm carriers were designed to facilitate the partial nitritation-anammox (PNA) processes in mainstream wastewater, where significant cost savings and improved treatment could be realized. Zeolite particles and zeolite coated membranes in batch systems fed with mainstream-like synthetic wastewater demonstrated that anammox bacteria could be enriched and total nitrogen removal enhanced when compared to control systems without zeolite. By varying the mass of zeolite in the system it was discovered that a minimum amount of zeolite, or ammonium sorption capacity, was needed to achieve anammox retention. Zeolite-coated materials were further tested in flow-through systems to determine under what wastewater-relevant conditions nitrogen treatment enhanced. Zeolite-coated carriers in reactors under anaerobic conditions significantly retained anaerobic ammonia oxidizing (anammox) bacteria over systems with uncoated carriers; however, identical reactors operated under aerobic conditions did not retain aerobic oxidizing bacteria (AOB) on the carriers themselves. In both anaerobic and aerobic conditions, AOB were preferentially retained in the liquid of the reactors containing zeolite-coated carriers. Unexpectedly, denitrifying genes (specifically nirS, nirK, and nosZ) were also retained in systems with zeolite-coated carriers, indicating the nitrite-shunt process maybe another application. Zeolite-coated membranes were configured in flow-through membrane-aerated reactors and subject to varying operating lengths, inter-lumen oxygen concentrations, and influent nitrite with mixed results. Anammox bacteria were only detected in high quantities on zeolite membranes when operated for two weeks with 100% oxygen with and without nitrite in the influent. AOB were not enriched under any conditions at a 95% confident interval. Further exploration is needed to better understand the lack of AOB retention on both zeolite-carriers and membranes. Finally, zeolite-coated carriers were tested in stormwater-like systems both in the field and in laboratory reactors for retention of anammox, AOB, and feammox bacteria. Anammox bacteria and AOB were detected in increased quantities on zeolite-coated carriers over uncoated carriers when deployed in a raingarden, but not when deployed in a stormwater pond outlet structure. Carriers were also pre-seeded with anammox biofilm prior to field deployment in order to monitor biomass retention, and at the 2.5-month time scale tested, both control and zeolite carriers in both stormwater systems demonstrated excellent retention of biomass. Biomass was also well retained when both carrier types were pre-seeded and tested in laboratory reactors with simulated storm events. When pre-seeded, both reactors also demonstrated high rate of ammonium removal. Systems containing zeolite carriers inoculated with pond-water, however, had much higher rates of ammonium removal over control carriers indicating that under some conditions, zeolite coating did improve reactor performance. Finally, zeolite particles and zeolite-coated carriers were explored to determine if they also would preferentially retain feammox bacteria, the only known microorganism to defluorinate per- and polyfluorinated alkylated compounds. Reactors with zeolite particles and zeolite-coated carriers, had increased feammox bacteria and higher rates of ammonium removal. Overall, this research has demonstrated that zeolite-incorporated technologies are promising solutions to retaining anammox, AOB, and feammox bacteria and enhancing nitrogen removal in waste streams if applied under the right conditions. Treating waste streams to reduce the impacts of excess nutrients and other pollutants from human sources is important to protecting the health of aquatic systems.
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    Zeolite MFI Membranes Towards Industrial Applications
    (2020-11) Duan, Xuekui
    Zeolite membranes have been the interest of research for decades due to their potentials in various separation applications including gas separation, water purification, pervaporation, etc. Among the zeolite materials studied, MFI zeolite (Silicalite-1 and ZSM-5) is one of the major subjects of research, mainly because of its suitability for the separation of hydrocarbons, such as n-butane from iso-butane and para-xylene from its isomers. Besides, all-silica Silicalite-1 and high-silica ZSM-5 have been explored for organic/water pervaporation as well by utilizing their high hydrophobicity. Despite years of research efforts on these applications, the industrialization of MFI membranes has not been achieved. One reason is that the cost associated with the fabrication of these membranes is too high to be commercially attractive. The high-cost, specially engineered silica membrane supports account for a major share of the total cost. Alternative supports such as polymeric supports and low-cost and commercially available alumina supports are possible substitutes to explore. Another problem is the lack of demonstration of high membrane separation performance at industrially relevant conditions (high temperature and high pressure). It is thus the goal of this thesis to address these problems and make progress towards the commercialization of MFI membranes. First, the recent advances of MFI zeolite membranes were reviewed. Then, the fabrication of high-performance MFI membranes using aqueous dispersions of open-pore, two-dimensional MFI zeolite nanosheets on low-cost polymeric substrates was demonstrated. Next, progress towards making MFI membranes on alumina supports has been made. Despite these efforts to use other supports, we failed to make high-performance membranes as comparable to the silica-supported ones. Besides these efforts, ultra-thin MFI membranes fabricated using dc-5 nanosheets as seeds were showed to have high xylene isomer separation performance at industrial conditions and high performance for H2/hydrocarbons separation and ammonia/H2/N2 separation. These works demonstrated the potential of high-performance MFI membranes for energy-efficient separation processes in industrial conditions.