Browsing by Subject "catalyst"
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Item Acidity and catalytic activity of zeolite catalysts bound with silica and alumina(Texas A&M University, 2004-09-30) Wu, XianchunZeolites ZSM-5 (SiO2/Al2O3=30~280) and Y(SiO2/Al2O3=5.2~80) are bound with silica gel (Ludox HS-40 and Ludox AS-40) and alumina (?- Al2O3 and boehmite) by different binding methods, namely, gel-mixing, powder-mixing and powder-wet-mixing methods. The acidities of the bound catalysts and the zeolite powder are determined by NH3-TPD and FTIR. The textures of these catalysts are analyzed on a BET machine with nitrogen as a probe molecule. The micropore surface area and micropore volume are determined by t-plot method. Micropore volume distribution is determined by Horvath-Kawazoe approach with a cylindrical pore model. Mesopore volume distribution is determined by BJH method from the nitrogen desorption isotherm. Silica from the binder may react with extra-framework alumina in zeolites to form a new protonic acid. SiO2-bound catalysts have less strong acidity, Bronsted acidity and Lewis acidity than the zeolite powder. Also, the strength of strong acid sites of the zeolites is reduced when silica is embedded. Micropore surface area and micropore volume are reduced by about 19% and 18%, respectively, indicating some micropores of ZSM-5 are blocked on binding with silica. SiO2-bound ZSM-5 catalysts have less catalytic activity for butane transformation (cracking and disproportionation) and ethylene oligomerization than ZSM-5 powder. When alumina is used as a binder, both the total acid sites and Lewis acid sites are increased. Micropore surface area and micropore volume of ZSM-5 powder are reduced by 26% and 23%, respectively, indicating some micropores of ZSM-5 are blocked by the alumina binder. Alumina-bound catalysts showed a lower activity for butane transformation and ethylene oligomerization than ZSM-5 powder. Alkaline metals content in the binder is a crucial factor that influences the acidity of a bound catalyst. The metal cations neutralize more selectively Bronsted acid sites than Lewis acid sites. Alkaline metal cations in the binder and micropore blockage cause the bound catalysts to have a lower catalytic activity than the zeolite powder.Item Applications of hydrogenation and dehydrogenation on noble metal catalysts(2009-05-15) Wang, BoHydrogenation and dehydrogenation on Pd- and Pt- catalysts are encountered in many industrial hydrocarbon processes. The present work considers the development of catalysts and their kinetic modeling along a general and rigorous approach. The first part deals with the kinetics of selective hydrogenation, more particularly of the C3 cut of a thermal cracking unit for olefins production. The kinetics of the gas phase selective hydrogenation of methyl-acetylene (MA) and propadiene (PD) over a Pd/?-alumina catalyst were investigated in a fixed bed tubular reactor at temperatures 60 - 80 oC and a pressure of 20 bara. Hougen-Watson type kinetic equations were derived. The formation of higher oligomers slowly deactivated the catalyst. The effect of the deactivating agent on the rates of the main reactions as well as on the deactivating agent formation itself was expressed in terms of a deactivation function multiplying the corresponding rates at zero deactivation. Then, the kinetic model was plugged into the reactor model to simulate an industrial adiabatic reactor. In the second part the production of hydrogen from hydrocarbons was investigated. In both cyclohexane and decalin dehydrogenations, conversions higher than 98% could be obtained over Pt/?-alumina catalyst at temperature of 320 and 340 oC, respectively, with no apparent deactivation for 30 h and with co-feed of H2 in the feed. Except for H2 and trace amounts of side cracking products, less than 0.01%, benzene was the only dehydrogenated product in cyclohexane dehydrogenation. In the case of decalin dehydrogenation, partially dehydrogenated product, tetralin, was also formed with selectivity lower than 5%, depending on operating conditions. A rigorous Hougen-Watson type kinetic model was derived, which accounted for both the dehydrogenation of cis- and trans- decalin in the feed and also the isomerization of the two isomers. Jet A is the logic fuel in the battlefields. The dehydrogenation of Jet A can produce H2 for military fuel cell application. Although the H2 production is lower than that of steam/autothermal reforming, it eliminates the needs of high temperature and product separation operation.Item Catalysts for the hydrolysis of thiophosphate triesters(Texas A&M University, 2005-02-17) Picot, AlexandreThe degradation of phosphate triesters is efficiently catalyzed by organophosphate hydrolases (OPH). While a number of recent studies have focused on optimizing the rate of hydrolysis observed with the native enzyme, no dinuclear complexes that mimic the function of OPH have been reported or investigated. Our present research focuses on the synthesis of dinuclear metal complexes and on the study of their catalytic abilities. An important aspect of this research concerns the investigation of the coordination chemistry of dinuclear ligands designed to hold two metal cations in well defined positions. The ability of the different complexes to catalyze the degradation of thiophosphate triester is presented. Out of several complexes studied, ortho-metallated Pd (II) complexes have been found to display the highest catalytic activity for the hydrolysis of parathion.Item DEVELOPMENT AND MECHANISTIC STUDIES OF THE CHROMIUM TETRAMETHYLTETRAAZAANNULENE CATALYST SYSTEM FOR THE COPOLYMERIZATION OF CARBON DIOXIDE AND EPOXIDES(2010-07-14) Fitch, ShawnA prominent goal of scientists is to develop products and processes to meet the ever-growing needs of society. Today's needs include products that are economical, specialized, and made through processes with minimal impact on the environment. One such product that serves an important and widespread need is poly(bisphenol A carbonate) for its physical properties and ease of synthesis and processing. However, this polymer does not meet the growing need of being environmentally benign as production involves carcinogenic, chlorinated solvents and toxic monomers that can leach out from the polymer product. An answer to this new demand is the development of a different process for the production of polycarbonate plastics utilizing carbon dioxide and epoxides. Carbon dioxide is an attractive monomer that is cheap and nontoxic, and its utilization signifies an important contribution to counteract global greenhouse emissions. The stability of carbon dioxide has posed a significant and complex challenge towards its utilization. Epoxides are attractive since they are synthesized from a wide variety of olefins, both naturally occurring and those derived from petroleum. The exploration of catalysts to facilitate the coupling of epoxides to carbon dioxide to afford polycarbonates has been under investigation in the Darensbourg lab for fifteen years, and has lead to the development of several successful systems such as zinc bisphenoxides and chromium salens. This dissertation focuses on the development of another successful catalyst system, chromium tetramethyltetraazaannulene, and further elucidation of the mechanism by which polycarbonates are formed. Herein, aspects of the copolymerization process using this system will be discussed in detail, such as cocatalyst and pressure dependence, catalyst derivatization, and kinetic and mechanistic investigations. The end result of these investigations is the development of the most active chromium-based catalyst for the copolymerization of cyclohexene oxide and carbon dioxide and a better understanding of how the copolymer product is produced.Item Elucidating the organic-OMS interface and its implications for heterogeneous catalysts(2012-07-16) Wang, QingqingOrganic ? ordered mesoporous silica (OMS) hybrid materials have attracted great interest due to their potential applications for gas separations, and heterogeneous catalysis. Amine-functionalized OMS materials are active in a variety of base-catalyzed reactions. The key to successfully achieving the desired reactivity is the ability to rationally tether the targeted organic functionality onto the OMS surface. Understanding the organic-inorganic interface is crucial for rational design of heterogeneous catalysts, because the local structure and molecule dynamics are paramount in determining the reactivity of the organic groups attached to the OMS surface. This dissertation focuses on three goals that will lead to a description of the organic-OMS interface and designing hybrid catalysts: 1) Determining the dynamics of organic groups attached to the OMS surface, 2) Catalytic testing to understand how the local structure and dynamics of the organic moiety influence the catalytic properties of organic-OMS catalysts, 3) Designing more active hybrid catalysts by introducing higher loadings of organic group using dendrimer structures. Solid-state NMR is uniquely suited for quantifying dynamics in the milli- to nano-second time scale. Deuterium (2H) NMR is a powerful tool to obtain detailed information about the dynamics or organic molecules. In this study, several simple functional groups isotopically labeled with deuterium have been attached to MCM-41 and SBA-15. The spectra display different molecular motions for different organic moieties. The results have indicated that the interactions between the functional groups and silanol groups on the surface influence the mobility of the organic fragments. Also, the porosity of the solid supports effects dynamics via confinement. The catalytic properties of simple amine groups attached to MCM-41, containing primary, secondary, and tertiary amines have been compared in the Nitroaldol (Henry) reaction. The effects of amine identity, structure, loading, presence of surface silanols, and the substrate topology on the catalytic properties have been investigated. The dramatic decrease of the activity of amine-functionalized MCM-41 by capping the residual silanol groups with hexamethyldisilazane was ascribed to the decrease of the interactions of hydrogen bonding between the amine functional groups and surface silanols. The result was consistent with the changes of the molecular motions shown by 2H NMR measurements. Fabricating OMS hybrid materials with high densities of organic functional groups leads to challenges in realizing uniform, catalytically active sites. Our group has immobilized melamine-based dendrimers on the surfaces of amine-functionalized SBA-15 materials by iterative synthesis procedures. The current studies in this dissertation mainly describe the catalytic properties of these dendrimers on SBA-15 and MCM-41 in the Nitroaldol (Henry) reaction, the transesterification reaction of triglycerides and methanol to synthesize methyl esters, and the cross aldol reaction between acetone and 5-hydroxymethylfurfural. The results indicate that the OMS-dendron materials have potential as solid base catalysts for a range of reactions.Item Laboratory-Scale Burning and Characterizing of Composite Solid Propellant for Studying Novel Nanoparticle Synthesis Methods(2013-04-29) Allen, Tyler WinstonThis thesis examines the effects of nanoparticle, metal-oxide additives on the burning rate of composite solid propellants. Recent advancements in chemical synthesis techniques have allowed for the production of improved solid rocket propellant nano-scale additives. These additives show larger burning rate increases in composite propellants compared to previous additive generations. In addition to improving additive effectiveness, novel synthesis methods can improve manufacturability, reduce safety risks, and maximize energy efficiency of nano-scale burning rate enhancers. Several different nano-sized additives, each titania-based, were tested and compared for the same baseline AP/HTPB formulas and AP size distributions. The various methods demonstrate the evolution in our methods from spray-dried powders to pre-mixing the additive in the HTPB binder, and finally to a method of producing the additive directly in the binder as a nano-assembly. Burning rate increases as high as 80% at additive mass loadings of less than 0.5% were seen in non-aluminized, ammonium perchlorate-based propellants over the pressure spectrum of 500 psi (3.5 MPa) to 2250 psi (15.5 MPa). Increases in burning rate up to 73% were seen in similarly formulated aluminized propellants. During the past several years, the research team has refined laboratory-scale techniques for quickly and reliably assessing the mixing and performance of composite propellants with catalytic nanoparticle additives. This thesis also documents some of the details related to repeatability, accuracy, and realism of the methods used in the team?s recent nano-additive research; it also introduces the latest techniques for producing propellants with nano-sized additives and provides new burning rate results for the entire scope of additives and mixing methods. Details on the propellant characterization methods with regard to physical and combustion properties are provided. Snapshots from atmospheric propellant combustion videos taken with a Photron FASTCAM SA3 high-speed camera are included along with existing pressure and light-emission responses.Item Remedial extraction and catalytic hydrodehalogenation for treatment of soils contaminated by halogenated hydrophobic organic compounds(2009-05-15) Wee, Hun YoungThe overall objective of this research was to develop and assess a new method, named remedial extraction and catalytic hydrodehalogenation (REACH), for removing and destroying soil contaminants. In particular, I considered hydrophobic halogenated organic compounds (HHOCs). In this research, I developed a closed-loop treatment process that catalytically destroys the contaminants of concern, and does not generate a secondary waste stream. Mixtures of water and ethanol appear to be good candidates for the extraction of 1,2,4,5-tetrachlorobenzne (TeCB) or pentachlorophenol (PCP) from contaminated soil. Palladium-catalyzed hydrodehalogenation (HDH) was applied for destroying TeCB or PCP in mixtures of water and ethanol in a batch mode. The experimental results are all consistent with a Langmuir-Hinshelwood model for heterogeneous catalysis. Major findings that can be interpreted within the Langmuir- Hinshelwood framework are as follows: the rate of HDH depends strongly on the solvent composition, increasing as the water fraction of the solvent increases; the kinetics of the HDH reaction are apparently first-order with respect to the concentration of TeCB in the solvent; and the HDH rate increases as the catalyst concentration in the reactor increases. Also, TeCB is converted rapidly and quantitatively to benzene, with only trace concentrations of 1,2,4-trichlorobenzene appearing as a reactive intermediate. PCP is transformed to phenol by sequential reductive dehalogenation to tetrachlorophenols, then to trichlorophenols, then to phenol. The degradation of PCP does not follow firstorder kinetics, probably because of competitive reactions of intermediate products that are generated during PCP degradation. Following the batch studies, the REACH technology was applied in continuous mode under baseline conditions for a span of 7 weeks to treat soils that had been synthetically contaminated by HHOCs in the laboratory. Extraction of TeCB and PCP from soils was almost completed within two days by a 50:50 mixture of water and ethanol. Higher reaction rates were observed for TeCB than for PCP. The activity of the catalyst was slowly lost as contaminant mass was removed from the soil. The deactivated catalyst was successfully regenerated with a dilute sodium hypochlorite solution. The results of this research suggest that REACH could be a viable technology for some contaminated soils.Item Selective catalytic reduction (SCR) of nitric oxide (NO) with ammonia over vanadia-based and pillared interlayer clay-based catalysts(Texas A&M University, 2004-09-30) Oh, Hyuk JinThe selective catalytic reduction (SCR) of nitric oxide (NO) with ammonia over vanadia-based (V2O5-WO3/TiO2) and pillared interlayer clay-based (V2O5/Ti-PILC) monolithic honeycomb catalysts using a laboratory laminar-flow reactor was investigated. The experiments used a number of gas compositions to simulate different combustion gases. A Fourier transform infrared (FTIR) spectrometer was used to determine the concentrations of the product species. The major products were nitric oxide (NO), ammonia (NH3), nitrous oxide (N2O), and nitrogen dioxide (NO2). The aim was to delineate the effect of various parameters including reaction temperature, oxygen concentration, NH3-to-NO ratio, space velocity, heating area, catalyst arrangement, and vanadium coating on the removal of nitric oxide. The investigation showed that the change of the parameters significantly affected the removals of NO and NH3 species, the residual NH3 concentration (or NH3 slip), the temperature of the maximum NO reduction, and the temperature of complete NH3 conversion. The reaction temperature was increased from the ambient temperature (25?C) to 450 ?C. For both catalysts, high NO and NH3 removals were obtained in the presence of a small amount of oxygen, but no significant influence was observed from 0.1 to 3.0% O2. An increase in NH3-to-NO ratio increased NO reduction but decreased NH3 conversions. For V2O5-WO3/TiO2, the decrease of space velocity increased NO and NH3 removals and broadened the active temperature window (based on NO > 88% and NH3 > 87%) about 50?C. An increase in heating area decreased the reaction temperature of the maximum NO reduction from 350 to 300?C, and caused the active reaction temperature window (between 250 and 400?C) to shift toward 50?C lower reaction temperatures (between 200 and 350?C). The change of catalyst arrangements resulted slight improvement for NO and NH3 removals, therefore, the change might contribute to more gas removals. The catalyst with extra vanadium coating showed higher NO reductions and NH3 conversions than the catalyst without the extra vanadium coating.Item Vapor transport techniques for growing macroscopically uniform zinc oxide nanowires(2009-08) Baker, Chad Allan; Hall, Matthew John; Shi, Li, Ph. D.ZnO nanowires were grown using carbothermal reduction and convective vapor phase transport in a tube furnace. Si <100> substrates that were 20 mm x 76.2 mm were sputter coated with 2 nm to 50 nm gold which formed nanoparticles on the order of 50 nm in diameter through a process of Ostwald ripening upon being heated. Growth temperatures were varied from 800ºC to 1000ºC, flow rates were varied from 24 sccm to 3300 sccm, and growth durations were varied from 8 minutes to 5 hours. Vapor phase Zn, CO, and CO2, produced by carbothermal reduction and suspended in an Ar atmosphere, were flowed over the Si substrates. The Au nanoparticles formed an eutectic alloy with Zn, causing them to become liquid nanodroplets which catalyzed vapor-liquid-solid nanowire growth. The nanowires were also synthesized by self-catalyzing vapor-solid growth in some cases. Using the tube furnace never resulted in more than 50% of the substrate being covered by nanowires. It was found that a bench-top furnace could achieve nearly 100% nanowire coverage by placing the 20 mm x 76.2 mm sample face down in a quartz boat less than 2 mm above the source powder. This was because minimizing the distance between the sample and the source powder was critical to achieve macroscopically uniform growth consistently.