63 research outputs found
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Tuning Catalytic and Electronic Properties of Supported Ni Catalysts by Altering Zeolite Beta Elemental Composition
No full textZeolites are commonly used as a support for transition metal catalysts due to their high surface area, ability to stabilize isolated cations, and porous structure. The basic building block of a zeolite is the TO4 tetrahedra where T is usually Si4+ but can be substituted with other elements such as Al3+, Fe3+, and B3+. The charge disparity that results from these substitutions creates the need for an extraframework cation that can be a transition metal. The compositional flexibility of certain zeolite frameworks, such as BEA and MFI, make them a good candidate to use a modular support whose elemental composition can be modified without changing the crystallographic structure of the support. The research in this dissertation was focused on using the flexibility in elemental composition of the BEA zeolite framework to tune the properties of a supported metal. The electron density, as well as reactivity, of a supported metal catalyst can be influenced by changes in zeolite elemental composition. This approach resulted in an increased understanding of how zeolite composition influences highly dispersed, supported metals. Zeolite Beta (BEA framework) supported Ni catalysts were synthesized and tested for catalytic activity using ethylene dimerization. The naming convention that will be used is M-[X]-Beta, where M is the extra-framework cation and X is the heteroatom composition. The catalysts were characterized using Fourier Transform Infrared Spectroscopy (FTIR) with various probe molecules, N2 adsorption, x-ray diffraction (XRD), catalysis, and thermogravimetric analysis (TGA). Ni was dispersed onto H-[X]-Beta (X = Al, Ga, and Fe) via anhydrous deposition by using n-pentane solvent and Ni(acac)2 as the metal source. N2 adsorption, as well as XRD, demonstrated that there is no significant change to the crystallinity or pore structure of the zeolite after Ni deposition. CO and NO adsorption onto the cationic Ni sites showed the presence of Ni in extraframework exchange positions and the relative electron density of the Ni cation increases in the order Ni-[Fe]-Beta > Ni-[Ga]-Beta > Ni-[Al]-Beta. C2H4 adsorption shows that the Ni cations on Ni-[Ga]-Beta and Ni-[Al]-Beta have alkyl ligands bonded to them of various length, while after exposure to C2H4 the adsorbed species on Ni-[Fe]-Beta are butenes. As C2H4 dimerization catalysts, operating at 180 °C and 2.16 kPa C2H4, the activity towards the formation of butene is in the order: Ni-[Fe]-Beta > Ni-[Ga]-Beta > Ni-[Al]-Beta. The activation energy was measured to be 44.8 kJ/mol and 32.4 kJ/mol for Ni-[Al]-Beta and Ni-[Fe]-Beta respectively, indicating that Ni-[Fe]-Beta is more effectively stabilizing the TOF determining transition state.
Ni was atomically dispersed into the vacant silanol nests of dealuminated Beta zeolite and used as a C2H4 hydrogenation catalyst. NH4-[Al]-Beta was dealuminated with HNO3 to create silanol nests, then Ni was deposited using Ni(acac)2 in n-pentane. After evacuation of the solvent the sample was calcined in air to remove the acac ligands. The Ni sites were probed by solid-state FTIR with CO and NO adsorption, which confirmed the presence of cationic Ni2+ in silanol nests. X-ray Absorption Spectroscopy (XAS) of the oxidized Ni-[DeAl]-Beta was used to determine the geometry and average local environment of the Ni sites. Wavelet analysis of Ni-[DeAl]-Beta and NiO was used to show the absence of Ni – Ni scattering in the oxidized Ni-[DeAl]-Beta extended x-ray absorption fine spectrum (EXAFS). The pre-edge feature of the XANES region suggested that Ni is in a tetrahedral geometry. EXAFS fitting found a Ni – O coordination number of 4 and Ni – Si coordination number of 4, consistent with Ni dispersed into silanol nests. Ni-[DeAl]-Beta catalysts were activated by reduction in 10% H2 at 300°C, then used for C2H4 hydrogenation. XANES after reduction shows approximately 50% of Ni sites are reduced to a metallic state. EXAFS analysis shows Ni metallic clusters of approximate 1 nm in size based on Ni – Ni coordination numbers. STEM images also indicate that there is a lack of large Ni clusters (> 1 nm) after reduction of Ni-[DeAl]-Beta. Ni-[DeAl]-Beta was found to be 20-fold more active than Ni-[Al]-Beta and 2.9-fold more active than NiO-SiO2 as a C2H4 hydrogenation catalyst. The work in this dissertation furthers knowledge in the field of heterogenous catalysis forward by demonstrating how zeolites can be used as a modular support for transition metals. The compositional flexibility in zeolite Beta is leveraged to influence the active metal site without changing the crystallographic structure of the support. The fine control over the electronic properties and geometry of active Ni sites represents progress towards more creating more tunable sites for a heterogenous catalyst
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Wine Stabilization of Potassium Bitartrate in a Fluidized Bed Crystallizer
No full textCold stabilization of potassium bitartrate is a common practice in wine production, however it is time and energy intensive due to the low temperatures required to facilitate crystallization. It has been demonstrated that a fluidized bed crystallizer could perform the same function as cold stabilization while minimizing drawbacks from batch operation. Two bench scale fluidized beds were constructed and tested with several size fractions of potassium bitartrate crystals in a model wine solution to isolate the parameters to determine bed height expansion. It was found that tube diameter and mass of loading showed little difference between scales, and that the crystal shape and size played a larger role. A pilot-scale fluidized bed crystallizer was designed and tested on an unstable wine to remove potassium bitartrate. The crystallizer selectively removed potassium bitartrate, confirmed by a decrease in conductivity, chemical analysis using HPLC, and particle analysis before and after fluidization. These results provide a positive step in designing a more efficient semi-continuous approach to remove potassium bitartrate analogous to cold stabilization
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Exploring the use of redox potential to predict fermentation outcomes in relation to initial juice conditions
No full textEach year in the wine industry, economic loss occurs due to stuck or sluggish fermentations and the corresponding off-flavors produced. While monitoring using standard methods such as °Brix levels may reveal a problem, often the indication is after the quality of the wine has already been impacted and remediation techniques are less effective or intrusive and costly. The use of redox potential, also called Oxidation Reduction Potential (ORP), as a process parameter is being explored in order to predict fermentation outcomes early in fermentation, theoretically before measurable changes in °Brix levels occurs. ORP reports on the tendency for molecules or ions to gain or lose electrons in relation to the chemical makeup of a solution being measured. Consequently, ORP values are sensitive to the fermentative activity of the yeast as metabolic products are released and alter the chemical conditions of the solution. This makes ORP a sensitive tool in understanding the state of the fermenting yeast in a must, even before sugar consumption can be measured. This study aimed to monitor ORP under varying nutrient conditions or with different yeast strains to better understand the relationship between ORP and fermentation outcomes. Wine strains of Saccharomyces cerevisiae – EC1118, Elixir, CY3079, Montrachet, and RC212 – were observed, as well as varying pH and nutrient conditions used. ORP values showed repeatable patterns based on fermentation conditions which could be used to assist winemakers in monitoring and decision-making
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Evaluating the Impacts of Dispersed Metals’ Local Environments on Catalytically Relevant Outcomes for Chromium- or Platinum-Containing Zeolite Catalysts
No full textWhen supported metal catalysts contain metal components existing at or near atomic dispersion, the support surface largely controls the properties of the highly dispersed metal species by determining the local chemical bonding environments of the metals. The framework structures of zeolites afford unique bonding environments for supported metals which can result in catalysts having unusual properties. The research described in this dissertation was aimed at identifying potential advantages of using zeolites as supports for catalysts containing chromium or platinum ‒ two industrially applied catalyst metals whose intra-zeolite chemistries are not fully resolved in the literature. Chromium was dispersed on HZSM-5. Samples were characterized using X-ray absorption near edge structure (XANES) and infrared (IR) spectroscopies and evaluated for ethane dehydrogenation. At low chromium loadings, chromium was located at zeolite aluminum sites and Cr/HZSM-5 samples displayed stable ethane dehydrogenation activity with time on stream. Higher chromium loadings resulted in catalysts with higher dehydrogenation activity per chromium atom but that deactivated quickly, and this was correlated to higher fractions of electron-rich or multinuclear chromium present in these samples. The results represent an attempt to assess the potential for catalytic application of Cr/ZSM-5, taking into account the speciation of chromium among various anchoring sites on the zeolite surface.
Platinum was dispersed onto HZSM-5 and characterized using X-ray absorption and IR spectroscopies. During exposure of Pt/ZSM-5 to high-temperature, oxidizing conditions, Pt2+ ions were stabilized at six-membered rings in the zeolite that contained paired-aluminum sites. This interpretation was informed by a theory-guided analysis of X-ray absorption fine structure spectroscopy (EXAFS) data. These Pt2+ ions formed highly uniform platinum gem-dicarbonyls, and the steps leading toward formation of platinum clusters were monitored through the evolution of IR spectra during exposure of platinum carbonyls to reducing conditions. Platinum clusters in HZSM-5 were redispersed into Pt2+ cations under high-temperature, oxidizing conditions, with the Pt2+ cations returning to paired-aluminum, six-membered ring sites. Similar platinum gem-dicarbonyl complexes formed in several commercially used zeolites (ZSM-5, Beta, mordenite, and Y), demonstrating the generality of the chemistry across zeolite frameworks. The findings connect catalyst structural properties to critical performance outcomes for an industrially-relevant catalyst material system.
Chromium was dispersed onto a series of MFI zeolites with various support compositions. The catalysts were characterized by IR or X-ray absorption spectroscopies and evaluated for ethane dehydrogenation with or without CO2. The copresence of Cr2+ and Cr3+ in siliceous or borosilicate MFI zeolites was correlated with significant enhancements in the rates ethane dehydrogenation when CO2 was added to the reaction mixture. The aluminosilicate MFI zeolite, in contrast, stabilized chromium in the +2 oxidation state during reaction, resulting in a catalyst that exhibited low rates of CO2 reduction to CO by H2 and no enhancement of ethane dehydrogenation by CO2. The mechanistic role of CO2 is discussed in the context of ethane dehydrogenation with Cr/MFI catalysts.
Additional experiments characterizing Cr/zeolite or Pt/zeolite samples provided insights into the local environments of the supported metals. Platinum carbonyl complexes in HZSM-5 and Y zeolite were characterized by XAS in order to complement the results of IR spectroscopy. Similar chromium or platinum species existed in zeolites of identical framework structure but different heteroatom identity. Supported chromium or platinum species similar to those found in HZSM-5 were found to exist in zeolites other than HZSM-5
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Catalytic Active Site Structure for Soft Oxidant-Assisted Methane Coupling
No full textGlobal climate change is a major motivation for the mitigation of greenhouse gas emissions. The most significant greenhouse gases contributing to US emissions, reported by the EPA, are carbon dioxide, methane, and nitrous oxide – all products of oil production and combustion. Catalytic co-conversion of these gases via CO2- or N2O-assisted methane coupling to high-value commodity chemicals, such as ethylene, has been demonstrated to be a highly selective process over some metal oxide catalysts. The research in this dissertation focused on identifying physical and electronic properties of selective metal oxide catalysts to develop structure-activity relationships between this class of catalysts and this class of oxidative coupling reactions. To study this reaction system, a series of CaO/ZnO catalysts were developed as a platform to study the mechanistic cooperation of binary metal oxide catalyst systems. Calcium oxide, a highly basic metal oxide, was deposited on zinc oxide, a reducible oxide. CaO/ZnO binary metal oxide catalyst is comprised of cheaply abundant materials and has been demonstrated to be highly selective toward C2 products during CO2-assisted methane coupling (CO2-OCM). A series of Ca/ZnO catalysts with varying Ca composition were characterized by microscopy (TEM), X-ray spectroscopies (L-edge XANES, XPS), and CO2 adsorption infrared spectroscopy-temperature programmed desorption (IR-TPD). Catalysts with less than 2 mol% Ca contained highly disperse Ca sites that had lower Lewis basicity compared to bulk CaO. The CO2-OCM performance of these catalysts with low-Ca-loading exhibited a strong dependence on Ca loading, where minor additions of Ca drastically increased C2 product selectivity. These results coupled with further catalytic tests report the medium strength basicity of the interface between dispersed Ca and ZnO present in low-Ca-loading catalysts is optimal for C2 product selectivity.
X-ray absorption spectroscopy (XAS) and complementary theoretical simulations characterized the extent of Ca dispersion in the low-Ca-loading Ca/ZnO catalysts. For catalysts with less than 2 mol% Ca, the Ca most probably exists as linear one-dimensional and planar two-dimensional CaO clusters roughly 7 to 26 Å in length. A pre-edge feature of the XANES spectra unique to the low-Ca-loading catalysts was attributed to the presence of some under-coordinated Ca surface atoms by analysis of the local densities of states. The N2O-OCM performance of these catalysts was evaluated. The presence of the CaO clusters and under-coordinated surface atoms corresponded to higher C2-4 product selectivities than over high-Ca-loading catalysts. These Ca sites are highly dispersed on ZnO, creating many selective Ca/ZnO interfacial sites, which can lead to enhanced methane coupling performance.
Additional experiments comparing kinetic and mechanistic information across various oxidants during methane coupling reveal a strong effect of oxidant partial pressure on reactivity. Co-feeding oxidants at varying partial pressures should be further explored as a route of optimizing product yields. To optimize this system for ethylene production, catalytic oxidant-assisted ethane dehydrogenation should also be further investigated. Preliminary results suggest that Ca/ZnO can effectively catalyze ethane dehydrogenation on the catalyst surface during CO2-OCM. Various morphologies of ZnO were tested with Ca impregnation for their CO2-OCM performance. Surface-area-normalized C2 product yields did show preliminary morphology-dependence, where rod-like structures with a dominant (100) facet had poorer product yields than a commercial ZnO
Understanding the Impact of Key Wine Components on the Use of a Non-Swelling Ion-Exchange Resin for Wine Protein Fining Treatment
No full textThe impact of key classes of compounds found in wine on protein removal by the ion-exchange resin, Macro-Prep® High S, was examined by adsorption isotherm experiments. A model wine system, which contained a prototypical protein Bovine Serum Albumin (BSA), was used. We systematically changed concentrations of individual chemical components to generate and compare adsorption isotherm plots and to quantify adsorption affinity or capacity parameters of Macro-Prep® High S ion-exchange resin. The pH (hydronium ion concentration), ethanol concentration, and prototypical phenolics and polysaccharide compounds are known to impact interactions with proteins and thus could alter the adsorption affinity and capacity of Macro-Prep® High S ion-exchange resin. At low equilibrium protein concentrations (® High S resin). With the addition of ethanol, catechin, caffeic acid, and polysaccharides, the protein adsorption behavior was observed to differ at higher equilibrium protein concentrations (> ~0.3 (g BSA)/L), likely as a result of Macro-Prep® acting as an unrestricted multilayer adsorbent at these conditions. These data can be used to inform the design and scale-up of ion-exchange columns for removing proteins from wines
Understanding the Impact of Key Wine Components on the Use of a Non-Swelling Ion-Exchange Resin for Wine Protein Fining Treatment
No full textThe Future of Potassium Bitartrate Stabilization: Minimizing Energy, Wine Loss, and Treatment Time
No full textSurface basicity controls C–C coupling rates during carbon dioxide-assisted methane coupling over bifunctional Ca/ZnO catalysts
No full textCarbon dioxide-assisted coupling of methane offers an approach to chemically upgrade two greenhouse gases and components of natural gas to produce ethylene and syngas. Prior research on this reaction has concentrated efforts on catalyst discovery, which has indicated that composites comprised of both reducible and basic oxides are especially promising. There is a need for detailed characterization of these bifunctional oxide systems to provide a more fundamental understanding of the active sites and their roles in the reaction. We studied the dependence of physical and electronic properties of Ca-modified ZnO materials on Ca content via X-ray photoelectron and absorption spectroscopies, electron microscopy, and infrared spectroscopic temperature-programmed desorption (IR-TPD). It was found that introduction of only 0.6 mol% Ca onto a ZnO surface is necessary to induce significant improvement in the catalytic production of C2 species: C2 selectivity increases from 5% on un-modified ZnO to 58%, at similar conversions. Evidence presented shows that this selectivity increase results from the formation of an interface between the basic CaO and reducible ZnO phases. The basicity of these interface sites correlates directly with catalytic activity over a wide composition range, and this relationship indicates that moderate CO2 adsorption strength is optimal for CH4 coupling. These results demonstrate, for the first time to our knowledge, a volcano-type relationship between CO2-assisted CH4 coupling activity and catalyst surface basicity, which can inform further catalyst development
Understanding Smoke Exposure Results: Pinot noir Baseline Concentrations of Smoke Impact Markers across Five Vintages
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