Browsing by Subject "Zeolites"
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Item Aqueous solution and vapor phase adsorption of oxygenates onto zeolites(2012-11) Mallon, Elizabeth EmmaThe ability of zeolites to discriminate between molecules on the basis of size and functionality gives them the potential to be effective adsorbents and membrane materials for purification of biomass-derived chemicals and fuels. Since molecules from biomass are polyfuntional and non-volatile, it is necessary to decouple the interactions that drive aqueous adsorption of oxygenates onto zeolites in order to develop efficient zeolite-based separations for biomass processing. In this dissertation, the roles of adsorbent structural and chemical composition and adsorbate functionality are explored through the systematic development of aqueous and vapor adsorption isotherms of C2-C6 oxygenates on small (FER), medium (MWW, MFI, BEA), and large (MOR, FAU) pore zeolites as well as on hierarchical microporous-mesoporous materials (MCM-36, 3DOm-MFI, and SBA-15). Ambient temperature Henry’s constants (Kads) for aqueous diol and triol adsorption on silicalite-1 (aluminum-free MFI) increase exponentially with carbon number demonstrating that confinement of the adsorbate in the zeolite pores is a primary driving force for adsorption. This conclusion is supported by a monotonic decrease in propylene glycol Kads values with an increase in adsorbent pore size, and by a comparison of propylene glycol Kads values on MWW and MFI and their hierarchical counterparts (MCM-36 and 3DOm-MFI, respectively) that shows that propylene glycol preferentially adsorbs in the micropores of hierarchical materials. A comparison of diol and triol adsorption on silicalite-1 demonstrates that increasing the number of hydroxyl groups causes a decrease in adsorption affinity, and this phenomenon is probed by comparing Henry’s constants for aqueous adsorption of C3 polyfunctional molecules onto zeolites with their octanol-water partition coefficients, Kow, which were calculated using the prevalent ClogP group contribution method. It was found that Kads increases linearly with Kow for these adsorbates on H-ZSM-5 (aluminum-containing MFI), FAU, BEA, and ITQ-1 (MWW) at 278 K regardless of interactions in the bulk phase as measured by the solution activity coefficient. Exceptions to the correlation established between Kads and Kow are the adsorption of 1,2,!-triols with carbon number greater than three on H-ZSM-5 and adsorption of all oxygenates studied on FER, which we postulate is due to a shift in the adsorption configuration with adsorbate/zeolite structure which cannot be captured by Kow alone. The effect of zeolite defects on oxygenate adsorption was isolated through the development of vapor and aqueous adsorption isotherms on silicalite-1 materials that vary in structural and surface properties. Silicalite-1 crystals prepared through alkaline-synthesis, alkaline synthesis with steaming post-treatment, and fluoride synthesis routes are confirmed as crystalline MFI by SEM and XRD and are shown to contain ∼8.5 to 0 silanol defects per unit cell by 29Si MAS, 1H MAS, and 1H-29Si CPMAS NMR. A hysteresis in the Ar 87 K adsorption isotherm at 10−3 P/P0 evolves with a decrease in silanol defects, and, through features in the XRD and 29Si MAS NMR spectra, it is postulated that the hysteresis is the result of an orthorhombic-monoclinic symmetry shift with decreasing silanol defect density. Gravimetric and aqueous solution measurements reveal that propylene glycol adsorption at 333 K is promoted by silanol defects, with a maximum 20-fold increase observed for aqueous adsorption in the Henry’s Law regime with an increase from ∼0 to 8.5 silanols per unit cell. A comparison of vapor and aqueous propylene glycol adsorption on defect-free silicalite-1 at 333 K, both of which exhibit the Type V character, indicates that water enhances adsorption by a factor of 2 in the Henry’s Law regime, which is in agreement with simulations reported in the literature. Kads values for aqueous C2-C4 polyol adsorption at 298 K are shown to have a linear dependence on the silanol defect density, which indicates that these molecules preferentially interact with silanol defects. iItem Early stages of zeolite growth.(2010-08) Kumar, SandeepZeolites are crystalline nonporous aluminosilicates with important applications in separation, purification, and adsorption of liquid and gaseous molecules. However, an ability to tailor the zeolite microstructure, such as particle size/shape and pore-size, to make it benign for specific application requires control over nucleation and particle growth processes. But, the nucleation and crystallization mechanisms of zeolites are not fully understood. In this context, the synthesis of an all-silica zeolite with MFI-type framework has been studied extensively as a model system. Throughout chapters 2, 4 and 5, MFI growth process has been investigated by small-angle x-ray scattering (SAXS) and transmission electron microscopy (TEM). Of fundamental importance is the role of nanoparticles (~5 nm), which are present in the precursor sol, in MFI nucleation and crystallization. Formation of amorphous aggregates and their internal restructuring are concluded as essential steps in MFI nucleation. Early stage zeolite particles have disordered and less crystalline regions within, which indicates the role of structurally distributed population of nanoparticles in growth. Faceting occurs after the depletion of nanoparticles. The chapter 6 presents growth studies in silica sols prepared by using a dimer of tertaprpylammonium (TPA) and reports that MFI nucleation and crystallization are delayed with a more pronounced delay in crystal growth.Item Enabling the selective conversion of biomass-derived oxygenates to C4-C5 dienes(2021-05) Kumar, GauravThe catalytic conversion of biomass-derived saturated furans over zeotype solid acids affords a potentially renewable route to access conjugated C4-C5 dienes — commodity monomers in tires, plastics, adhesives, and resins. A lack of fundamental understanding of reaction mechanisms and pathways coupled with existing trial-and-error catalyst design approaches have limited diene yields to <60%. Poor catalyst lifetimes, attributed to rapid coking typical for oxygenate conversion reactions, have also remained a challenge. Improving the diene yields and mitigating catalyst deactivation are the first key steps to engender industrial interest in the resulting process technology. In this dissertation,we first highlight the mechanistic details of the tandem-ring opening and dehydration of tetrahydrofuran (THF) to butadiene on the aluminosilicate H-ZSM-5, which enable the formulation of the relative ratio of C-O to C-C scission rates as the diene selectivity descriptor. By considering aluminum-, and boron-substituted zeolites in 2-methyltetrahydrofuran (2-MTHF) dehydration to pentadienes, we demonstrate the weakening of solid acid strength as a strategy to tune this descriptor towards dienes’ production. By exploiting the thermodynamic stability of the desirable C5 conjugated diene (1,3-pentadiene), we further explicate strategies harnessing diffusional hurdles to suppress the production of its non-conjugated isomer (1,4-pentadiene). Combined, these insights lead to ~30% improvement in 1,3-pentadiene yield. Having discovered the utility of mild solid acids, we focus the rest of the dissertation on investigating the broad implications of weak surface binding in dehydration catalysis. Using two distinct classes of solid acid zeotype materials with weak Brønsted acidity (namely, borosilicates, and phosphorous-modified zeosils), we detail how these materials can potentially improve dehydration selectivity and stability, albeit often at a cost of lower overall turnover rates. Tying this discussion back to renewable dienes production on these materials, we conclude this work by underscoring the technological and economic improvements still required to achieve competitive diene prices from this process technology.Item Hydrothermal stability of hierarchically-structured two-dimensional MFI zeolite nanosheets(2019-06) Guefrachi, YasmineTwo-dimensional (< two-unit-cell-thick) zeolites are an important class of two-dimensional nanoporous materials. They possess the intrinsic properties of the conventional three-dimensional crystalline silicate/aluminosilicate zeolites and when synthesized in a pillared or intergrown form they exhibit a hierarchical porosity. Despite the tremendous effort directed towards their synthesis, very limited knowledge is available on their hydrothermal stability under industrially-relevant conditions. The hydrothermal stability of the thin crystalline domains governs their enhanced use as selective catalysts, adsorbents and in thin membranes’ preparation. This dissertation focuses on developing a fundamental understanding of the response of these novel thin crystallites under hydrothermal environments towards an advanced design and engineering of their synthesis and industrial applications. Self-Pillared Pentasil zeolite was utilized as a model system of hierarchically-structured two-dimensional MFI zeolite nanosheets in the structural and catalytic stability investigations explored in this dissertation. The nanostructural evolution of purely-siliceous nanosheets in presence of water under different atmospheres was studied via a thorough structural characterization analysis which involved the use of high-resolution and three-dimensional tomography bright-field transmission electron microscopy (the tilt series acquired in this analysis are enclosed as supplementary media files in this dissertation). The kinetic and thermodynamic driving forces behind the captured structural changes were investigated. The acidity modifications and the catalytic consequences resultant from the hydrotreatment structural transformation were explored using the aluminosilicate nanosheets counterparts.Item Molecular Simulation of Adsorption in Zeolites(2014-08) Bai, PengZeolites are a class of crystalline nanoporous materials that are widely used as catalysts, sorbents, and ion-exchangers. Zeolites have revolutionized the petroleum industry and have fueled the 20th-century automobile culture, by enabling numerous highly-efficient transformations and separations in oil refineries. They are also posed to play an important role in many processes of biomass conversion. One of the fundamental principles in the field of zeolites involves the understanding and tuning of the selectivity for different guest molecules that results from the wide variety of pore architectures. The primary goal of my dissertation research is to gain such understanding via computer simulations and eventually to reach the level of predictive modeling. The dissertation starts with a brief introduction of the applications of zeolites and computer modeling techniques useful for the study of zeolitic systems. Chapter 2 then describes an effort to improve simulation efficiency, which is essential for many challenging adsorption systems. Chapter 3 studies a model system to demonstrate the applicability and capability of the method used for the majority of this work, configurational-bias Monte Carlo simulations in the Gibbs ensemble (CBMC-GE). After these methodological developments, Chapter 4 and 5 report a systematic parametrization of a new transferable force field for all-silica zeolites, TraPPE-zeo, and a subsequent, relatively ad-hoc extension to cation-exchanged aluminosilicates. The CBMC-GE method and the TraPPE-zeo force field are then combined to investigate some complex adsorption systems, such as linear and branched C6--C9 alkanes in a hierarchical microporous/mesoporous material (Chapter 6), the multi-component adsorption of aqueous alcohol solutions (Chapter 7) and glucose solutions (Chapter 8). Finally, Chapter 9 describes an endeavor to screen a large number of zeolites with the purpose of finding better materials for two energy-related applications, ethanol/water separation and hydrocarbon iso-dewaxing.Item Morphology control of porous materials and molecular sieve membrane applications.(2010-12) Yoo, Won CheolFor advanced applications of porous materials, morphology control of porous materials is desired. Simultaneous dissolution and surfactant-induced re-assembly via micelle formation with dissolved species were performed to introduce mesoporosity in the amorphous silica spheres and zeolite crystals. Textural features of mesoporous silica spheres were controlled from corrugated to smooth. Mesoporous zeolite catalysts were synthesized via surfactant-induced re-assembly with dissolved crystal fragments of zeolite crystals. In addition, the concept of confined synthesis was used for control of morphologies of zeolite crystals. Using 3-D ordered macroporous (3DOM) carbon as a template, growth patterns and shape development of zeolite crystals were studied. Various shaped zeolite crystals, e.g., hollow interior (geode-like structure), corrugated/smooth surface of polycrystalline crystals, needle-shaped crystals and 3DOM imprinted single crystals, were produced by careful choice of reaction parameters during the confined synthesis. Polycrystalline aggregates produced by the confined synthesis were used as seed particles for zeolite membrane fabrication. Novel seeding techniques, rubbing and leveling methods, were applied to deposit the confinement product directly on the surface of porous alumina supports, and zeolite membranes were grown by secondary hydrothermal growth. Moreover, rapid thermal processing and conventional calcination were used to investigate the effect of thermal treatments on the overall membrane quality. To evaluate the membrane quality, permeation measurements to separate p-/o-xylene isomers based on the zeolite's intrinsic capability of size-selective molecular separations were conducted. A high separation factor and high permeance were observed.Item Tailoring the Microstructure of 2D Molecular Sieve Materials for Thin Film Applications(2018-05) Shete, MeeraZeolites and metal organic frameworks (MOFs) are microporous materials, with pores of molecular dimensions, that are of interest in a variety of applications including catalysis, adsorption, ion-exchange, separation membranes etc. With a global need of developing clean energy resources and reducing the carbon footprint of existing processes, they are being increasingly sought after as catalysts for the conversion of biomass to chemicals and fuels, in separation membranes to replace the existing energy intensive industrial separations with clean energy-efficient processes and for capture and storage of carbon dioxide. Their performance in these applications depends mainly on their pore size but also on our ability to tune their microstructure (crystal morphology and size, orientation, phase purity, defect densities etc.) as desired for an optimum performance. Recent advances in synthesis of molecular sieve materials have resulted in the development of advanced morphologies such as hierarchical materials, core-shell catalysts, two-dimensional nanosheets and thin films. However, a lot of the reports in the literature focus on conventional crystals and studies focusing on nanoscale crystal growth control are still in their infancy. This dissertation focuses on developing synthetic methods that will enable us to tailor the microstructure of 2D molecular sieve materials at a nanoscale approaching single-unit-cell dimensions with a goal of optimizing their performance in thin film applications. A novel coating technique was applied to isolate 2D MFI zeolite nanosheets and form monolayer coatings on versatile supports such as Si wafers. Using this as a prototype, growth conditions were developed that lead to unprecedented control of zeolite MFI growth at a scale approaching single-unit-cell dimensions. It was demonstrated that these growth conditions are robust enough and can be used to grow zeolite MFI crystals of varied sizes and morphology. It also enabled us to precisely control the microstructure of MFI thin films leading to the development of a material that had one of the lowest reported dielectric constant. Furthermore, the nanoscale growth control also allowed us to tailor the design of hierarchical catalysts by controllably thickening the zeolite domains in them and open opportunities to design multifunctional catalysts. A scalable and direct synthesis of Cu(BDC) MOF nanosheets was developed. Hybrid nanocomposites incorporating the MOF nanosheets in polymer matrices were fabricated which demonstrated significantly improved performance for CO2/CH4 separation.Item Ultra-thin MFI zeolite films: Synthesis, Characterization and Progress Toward Industrial Applications(2017-05) Rangnekar, NeelSeparation processes account for 10-15% of US energy consumption. A large fraction of that energy is consumed by energy-inefficient thermal separation processes like distillation. If membranes could perform these separations, up to 90% of that energy could be saved. Zeolites have ideal properties for separations, which include their high thermal and chemical stability. However, there are currently very few examples of industrial zeolite membrane separation processes. This is due to the high cost associated with their manufacture, industrially unattractive throughput and lack of membrane separation experiments at industrially relevant conditions. This dissertation aims to make progress on some of these fronts. The recent advances in zeolite membranes are reviewed, with an emphasis on industrial applications. A membrane fabrication procedure using 3.2 nm-thick MFI zeolite “nanosheets” is reported, resulting in high-flux and high separation efficiency membranes. High performance membrane separations at industrially relevant conditions have also been achieved for the first time. Moreover, further progress towards synthesis of even thinner films and membranes has been made. The discovery of a novel deposition technique enables the transfer of monolayers of nanosheets to silicon wafers. By intergrowing them, the thinnest-ever MFI films have been synthesized. In future, this technique could be extended to fabricate even higher-flux membranes. An application of zeolite films on silicon wafers as a low-dielectric constant material is also described. Superior insulating properties and mechanical strength compared to previously reported MFI films is achieved. Such a film could save energy and promote the development of the next generation of computer chips.