Browsing by Author "Wills, George B."

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    Aromatization of n-hexane by platinum containing molecular sieves and distribution and motion of organic guest molecules in zeolites
    Hong, Suk Bong (Virginia Tech, 1992-04-05)
    A vapor phase impregnation technique with Pt(acac)₂ has been developed and used to load Pt into aluminosilicate (KL, BaKL, KBaKL, NaY, CsNaY, FAU, EMT, ZSM-12 and SSZ-24) and aluminophosphate (AIP0₄-5 and VPI-5) molecular sieves. ¹³C MAS NMR, TEM and H₂ chemisorption measurements reveal that Pt can be loaded into the micropores of molecular sieves with both charged and neutral frameworks. Pt containing molecular sieves were tested as catalysts for the aromatization of n-hexane at 460 - 510°C and atmospheric total pressure in order to study the influence of Pt cluster size and support acidity/basicity, microstructure and chemical composition on activity and selectivity. High selectivity to benzene over most of the zeolite samples demonstrates that support acidity/basicity and microstructure do not contribute directly to the aromatization selectivity over Pt catalysts. A clear trend of increasing benzene selectivity with decreasing Pt cluster size is found. These observations suggest that the exceptional reactivity of Pt/KL for the aromatization of n-hexane results from the lack of any acidity in the support and the ability of zeolite L to stabilize the formation of extremely small Pt clusters. Pt/AIP0₄-5 and Pt/VPI-5 show high selectivity to n-hexane with little formation of benzene while opposite is observed for Pt/SSZ-24. The differences in catalytic behavior are attributed to variations in the environment of Pt clusters which are situated in either an aluminophosphate or silicate micropore. See document for rest of abstract.
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    Bed dynamics and heat transfer in shallow vibrated particulate beds
    Mason, Mark Olin (Virginia Polytechnic Institute and State University, 1990)
    A vibrated bed is a mobile layer of solid particles contained in a vessel that is vibrated vertically. This study investigates bed dynamics and heat transfer from a vertical surface in shallow vibrated beds in absence of aeration. In general, "shallow" means a depth-to-width ratio less than one. In this study, bed depth is 30 mm, and this ratio is about 0.2. All experiments are at 25 hertz and at vibrational amplitudes affording peak accelerations between 2 and 7 times gravity. The study uses spherical glass beads of two densities and "Master Beads," nearly spherical particles of a crude, dense alumina, in size fractions from 63 to 707 micrometers. A disc embedded in the vessel floor, vibrated at 4.5 kilohertz, gives data on bed-vessel separation, showing it to occur later than predicted by plastic, single-mass models. The delay is attributed to bed expansion, monitored by piezoelectric force gauges mounted on floor and wall of the vessel. In large-particle beds, bed-vessel collision occurs simultaneously everywhere. In small-particle beds, exhibiting an uneven top surface, collision occurs first at the side walls and moves toward the center. In small-particle beds, pressure gradients appearing during the bed's free flight drive a horizontal component of particle circulation from the vessel's side walls toward its center. An apparent viscosity of the bed, estimated crudely by pulling a rod through it, influences this component's velocity. In beds of large particles, circulation is almost entirely vertical, a layer of two or three particles moving downward at a wall, and a slow return flow moving upward elsewhere. The study confirms the downward wall motion to be driven by friction. Heat transfer closely follows trends in rate of circulation. Greater dependence upon vibrational intensity is seen in small-particle beds. Values as high as 578 W/m²-K are measured. Comparison of vertical-surface heater geometry with an earlier horizontal tube shows the former to be generally superior for surface-to-bed heat transfer.
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    Break-in behavior of a tungsten oxide on silica catalyst during propylene disproportionation
    Fathi-Kalajahi, Jamshid (Virginia Tech, 1978-04-05)
    This investigation consisted of a study of the break-in behavior of a tungsten oxide on silica catalyst during propylene disproportionation. A catalyst of 10 percent WO3 on silica gel (223 square meters per gram B.E.T. surface area) was used in a microcatalytic reactor. During the initial contacting of freshly activated samples of this catalyst with propylene, significant increases in disproportionation activity were observed for periods of up to 20 hours. The object of this study was to investigate the phenomena responsible for this break-in. The rate of approach to steady-state activity data were obtained using catalyst samples which were first saturated with each of the three gases involved in this reaction (propylene, ethylene, and 2-butene) by pulsing at .94 and 2.7 atmospheres before.starting the propylene flow. Effects of each gas on the break-in behavior of the catalyst were determined. A material balance around the reactor was made for each gas by pulse reactor techniques.
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    Element substituted aluminophosphates
    Correa, Maria del Consuela Montes de (Virginia Polytechnic Institute and State University, 1989)
    This dissertation reports the findings of an investigation aiming to the incorporation of the transition elements: cobalt, vanadium, and molybdenum into AlPO₄-5 molecular sieves, to the study of the redox properties of the resulting materials and to the potential application of these compounds in the partial oxidation of methane to methanol and formaldehyde. AlPO₄-5 molecular sieves containing Co, V, and Mo were synthesized by hydrothermal crystallization of typical aluminophosphate gels with the adequate metal substrates at temperatures of 200°C, 150°C, and 150°C respectively. Samples of each material were calcined in oxidant, reducing and inert atmospheres and the properties of the treated solids studied by different characterization techniques. The significant feature of the Co and V containing AlPO₄-5, CoAPO-5 and VAPO-5, is that they exhibit different colors depending on the treatment atmosphere. Evidence for cobalt contained in framework positions of CoAPO-5 is obtained by the change in its unit cell volume relative to AlPO₄-5, and by diffuse reflectance spectroscopy. CoAPO-5 also contains an appreciable quantity of extra-framework cobalt occluded in the pores, and/or as balancing cations. Evidence for this was obtained from ion exchange, and from oxygen and argon adsorption. Pentavalent vanadium incorporation is inferred from the change in the unit cell volume of VAPO-5 relative to AlPO₄-5, from pH measurements of the reaction vessel contents before and after heating, from oxygen and argon adsorption, chemical analysis, SEM, XPS, NMR, and diffuse reflectance spectroscopy. A scheme for the substitution of V into AlPO₄-5 is postulated based on the acidity observed in VAPO-5 by ion exchange, and potentiometrlc titration. Only a very small amount of Mo was found in the molybdenum containing AlPO₄-5. Evidence for Mo incorporation into the framework was not obtained. The use of CoAPO-5 and VAPO-5 as redox catalysts for the partial oxidation of methane to methanol and formaldehyde lead mostly to oxides of carbon.
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    Hydrodynamics and heat transfer in shallow fluidized beds
    Yang, Jyh-Shing (Virginia Polytechnic Institute and State University, 1986)
    The use of shallow fluidized beds for heat exchange has been suggested because they give high bed-to-surface heat transfer rate and require very low bed pressure. However, in comparison with research on deep fluidized beds, only relatively few studies have been devoted to heat transfer in shallow beds, and results from the available literature are often inconsistent. This study represents an integrated research on the hydrodynamics and bed-to-surface heat transfer in shallow beds. The results from this study provide the quantitative basis for the design and efficient operation of shallow fluidized-bed heat-recovery systems. Based upon their physical appearance, shallow fluidized beds have been categorized into nine different types. A "phase diagram" (plot of superficial gas velocity versus static bed height) can be used to delineate the ranges of fluidization variables within which each type of shallow beds will be seen. Pressure-drop data in gas flowing upward through a shallow bed reflect pressure recovery in jets formed immediately above a gas distributor at the bottom of the bed. Pressure-recovery data provide an effective means of distinguishing a shallow bed from a deep one, and suggest that the power consumption across a fluidized bed can be reduced dramatically by dividing a single deep bed into many multi-staged shallow beds. A computerized light probe has been developed for measurements of particle volume-fraction distribution and its statical fluctuation (standard deviation). These data have been shown to quantitatively define: (1) different types of shallow beds; (2) relative magnitude of solid mixing; (3) bed surface and bed height; and (4) jet penetration depth. Based upon observations of the hydrodynamic behavior of shallow fluidized beds, three regions can be identified for heat-transfer applications: a jet-affected region at the bottom, a free-board region at the top, and, sandwiched between theses, a homogeneous region. Only heat-transfer data in the homogeneous region are sufficiently well-behaved to be subjected to quantitative correlation in terms of fluidization variables. For relatively coarse particles (Geldart's Group B particles) the vigor of solid mixing can be the most important factor in affecting the heat-transfer performance. Bed voidage and static electricity effects are found to be important for smaller and/or lighter particles (i.e., Geldart's Group A particles).
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    Multicomponent mass transport in aqueous and membrane systems
    Wills, George B. (Water Resources Research Center, Virginia Polytechnic Institute, 1968)
    Project Objectives The basic objective of this study has been to develop and critically test models for multicomponent mass transport in aqueous and membrane systems. More specifically, this basic aim has been pursued through the following set of sub-objectives:
    1. Selecting a set of aqueous systems for detailed study followed by a comprehensive literature review to collect all pertinent data for the selected systems.
    2. Measuring multicomponent mass transport data in a selected system over a wide range of compositions.
    3. Developing a general mathematical model for multicomponent mass transport
    4. Obtaining optimum parameters for the mass transport models developed in (3).
    5. Determining whether the effective water of hydration of solutes in aqueous solutions can be reasonably determined as that value giving the best fit of the experimental transport data.
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    Predispersed solvent extraction of copper from dilute aqueous solution
    Rodarte, Alma Isabel Marín (Virginia Tech, 1991-09-20)
    Predispersed Solvent Extraction (POSE) was used to extract copper ions from dilute acidic aqueous solution. POSE is based on the principle that there is no need to comminute both phases. All that is necessary is to comminute the solvent phase prior to contacting it with the feed. This is done by converting the solvent into aphrons. which are micron-sized globules encapsulated in a soapy film. Since the aphrons are so small, it takes a long time for the solvent to rise to the surface under the influence of gravity alone. Therefore, the separation is expedited by piggy-back flotation of the aphrons on especially prepared gas bubbles, which are somewhat larger than aphrons and are called colloidal gas aphrons (CGA). Polyaphrons of various types were studied extensively. The apparatus used to generate the polyaphrons was upgraded. The residence time distribution of a liquid in the polyaphron generator was determined. The particle size distribution of polyaphrons was determined using photo-microscopy and sedimentation among other methods. Batch tests were done using both conventional and POSE. Results showed that POSE approaches equilibrium much faster than conventional extraction. Equilibrium isotherms were drawn and empirical equilibrium relationships were developed. The dynamics of the kinetics of the extraction was modeled using film theory. Equipment for the POSE process was built. Experiments were carried out in continuous mode and the process was optimized. An empirical statistical equation was developed for the extraction process in continuous mode. Depending on the aqueous to solvent ration, more than 99% copper can be extracted.
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    Reaction of meta-diisopropylbenzene on acid molecular sieves and synthesis of zeolites by a vapor phase transport method
    Kim, Man-Hoe (Virginia Tech, 1992-05-15)
    Meta-diisopropylbenzene is reacted with propylene over the acid form of the molecular sieves SAPO-5, mordenite, offretite, beta, hexagonal and cubic faujasite (hex and FAU), L, SAPO-37, and an amorphous silica-alumina at temperatures around 463 K in a flow-type fixed-bed reactor. A small amount of cracking is observed. The main reactions of meta-diisopropylbenzene are isomerization and alkylation. This alkylation is proposed as a new test reaction to characterize the effective size of the voids in larger pore (12 T-atom rings or above) molecular sieves by measuring the amount ratio of formed 1,3,5- to 1,2,4-triisopropylbenzene. In most cases, this ratio increases with the increasing effective void size of the molecular sieves in the order: SAPO-5 < mordenite < offretite < beta < hex ≈ FAU < L < SAPO-37 < amorphous silica-alumina. Since samples with the FAU topology show lower selectivities to 1,3,5-triisopropylbenzene than the mesoporous, amorphous silica-alumina, pore curvature has an influence on alkylation selectivity even for voids of 13 A size.
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    Reverse osmosis transport phenomena in the presence of strong solute-membrane affinity
    Dickson, James Morley (Virginia Polytechnic Institute and State University, 1985)
    The reverse osmosis performance of cellulose acetate membranes has been examined and analyzed for several aqueous systems where there is a strong attraction between the organic solute and the membrane material. The systems investigated included the aromatic hydrocarbons benzene, toluene, and cumene in single-solute aqueous solutions. Six cellulose acetate membranes, modified by annealing at different temperatures, were studied. Experiments were performed at four pressures (690, 1725, 3450, and 6900 kPa) and at several concentrations (in the range 5 to 260 ppm). The results were found to be markedly different than those observed in the absence of strong solute-membrane affinity. In particular, the solute-water separation decreased rather than increased with increasing pressure and the flux decreased with increasing concentration even though low concentrations, with low osmotic pressures, were studied. Qualitatively, the behavior was explained in terms of a porous membrane mechanism with both solute-membrane affinity and solute mobility varying as a function of solute position with respect to the membrane. The observed reduction in flux was expressed by an empirical equation as a function of concentration of solute in the boundary layer. The experimental results were analyzed quantitatively by several transport models. The irreversible thermodynamics phenomenological transport, solution-diffusion imperfection and extended solution-diffusion relationships generated parameters that were inconsistent with the original formulations of the models. The irreversible thermodynamics Kedem-Spiegler model, solution diffusion model, Kimura-Sourirajan analysis, and the three parameter finely-porous model were functionally unable to represent the data. Only the four parameter finely-porous model and the surface force-pore flow model were consistent with experimental results. From the finely-porous model the partition coefficient was found to be different on the high and low pressure sides of the membrane and this difference was a function of both pore size and solute. For the surface force-pore flow model, the agreement between the model and data was excellent. However, the surface force-pore flow model was considerably more difficult to use.
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    Rheological characterizaton, and the development of molecular orientation and texture during flow for a liquid crystalline copolymer of para- hydroxybenzoic acid and polyethylene terephthalate
    Viola, Georg Giuseppe (Virginia Polytechnic Institute and State University, 1985)
    It is generally agreed that the high physical properties arising in as-processed liquid crystalline materials are due to the high degree of molecular orientation which develops during the processing step. In order to more fully understand and predict such behavior, a constitutive equation describing the flow behavior of these materials would be useful. It has been suggested that in order to describe the rheology of liquid crystalline fluids such a constitutive equation would need to include molecular orientation effects. The purpose of part of this study has been to examine the usefulness of several constitutive equations for describing the steady and transient behavior of several liquid crystalline polymers. These include a copolyester of para-hydroxybenzoic acid and polyethylene terephthalate, and an anisotropic solution of 12 weight% Kevlar in 100% sulfuric acid. It was found that in the case of the copolyester system, the steady shear and dynamic viscosities were equal over certain temperature ranges. For this reason, the constitutive equation of Zaremba, Fromm, and DeWhitt (ZFD model) was used to predict the steady state behavior of the system studied. From knowledge of either the steady shear or dynamic viscosity it was possible to predict both the steady state normal stresses (N1) and the storage modulus (G'). The model could not, however, predict the transient behavior of the systems studied. Ericksen’s anisotropic fluid theory has been investigated in detail as it takes molecular orientation effects into account. Ericksen’s theory can partially explain the transient behavior of the systems studied in terms of molecular orientation which develops during shear flow. However, wide angle x-ray scattering (WAXS) and scanning electron microscope (SEM) studies reveal that shear flow has little effect on the development of molecular orientation during flow. In addition, any orientation produced during flow may be lost within thirty seconds at the melt temperature. It appears that a disruption of texture is occurring during flow which may need to be incorporated into the theory of Ericksen.
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    Shallow vibrated particulate beds - bed dynamics and heat transfer
    Thomas, Benku (Virginia Polytechnic Institute and State University, 1988)
    Particulate beds which are mobilized and expanded by the application of mechanical vibrations are called vibrated beds. These beds are generally defined as shallow, if the depth-to-width ratio is less than unity. The dynamics of shallow vibrated beds and the heat transfer from immersed tubes to such beds are investigated using a vibrational frequency of 25 Hz. The vibration equipment is designed to minimize distortions in the applied displacement waveform. Transducers used are of a sufficiently high frequency response to accurately follow the variation of bed properties over a vibrational cycle. An electronic circuit is designed to exactly phase-match data collected by a transducer with the vibrational displacement. The circuit may also be used to trigger a strobe lamp at any phase angle, thus permitting an accurate examination of the evolution of bed characteristics over a cycle. Measurements of floor pressures beneath the bed, indicate cyclic characteristics, caused by the bed motion. Horizontal floor-pressure gradients cause the bed to pile up or bunker within the vessel. In bunkered beds, particle motion is determined by horizontal gas flows, and a compaction wave which propagates diagonally through the bed during the bed-vessel collision. In non-bunkered beds, particle motion is driven largely by wall friction. The observed instant of bed-vessel separation lags the theoretical prediction by several degrees, most likely because of bed expansion associated with the bed lift-off. Different "states" of shallow vibrated beds are identified, each with a unique set of characteristics. One state which exists in ultra-shallow beds of depths between 6 and 15 particle diameters is characterized by a high porosity and good gas-solid interaction, making it potentially useful for studies of reaction kinetics. Surface-to-bed heat-transfer coefficients are measured for Master Beads and glass beads, and found to vary with particle size and vibrational intensity. Heat-transfer coefficients as high as 484 W/m²-K are obtained. Heat transfer depends on particle circulation and the formation of air gaps which periodically surround the heater surface. A simplified theoretical formulation for the heat-transfer coefficient appears to qualitatively predict observed trends in heat transfer.
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    Studies in vibrofluidized beds and synthesis of silica catalysts
    Sprung, Renato (Virginia Polytechnic Institute and State University, 1987)
    The effect of the solid-circulation rate and pattern as well as the air-gap size on heat-transfer coefficients between a horizontal, cylindrical heater and vibrated beds of Master Beads (spherical alumina) and glass spheres was studied. Solid piles were observed to form at specific bed locations. Solid-circulation paths were directed from the shallowest toward the deepest region of the vibrated bed. For beds in which the solid pile formed above the heating surface, local solid-circulation loops were observed above and below the heater. Air gaps developed at the top and bottom of the cylindrical heater. Heat-transfer coefficients of 140-350 W/m²K in beds of glass spheres and 180-480 W/m²K in beds of Master Beads were determined for a temperature difference of 30°C between the heater and vibrated bed. The trends in the behavior of the heat-transfer coefficient could be explained in terms of a model that accounted for the air-gap size and particle renewal in the layer closest to the heater. Increased solid-circulation rates improved the heat-transfer performance until larger air-gap sizes eventually compromised any increase in solid circulation. The expansion of the interlayer spacing of H-Magadiite (a layered silicic acid) by the introduction of pillars containing silicon atoms was investigated. A trisiloxane and two trichloroorganosilane compounds were used as the pillaring agents. The interlayer space of H-Magadiite was successfully expanded by pillaring with trichloroorganosilanes. The minimum dimensions of the pores that access the interlayer space of the pillared compounds were determined as being 6.2 Å and 9.5 Å (dimensions at perpendicular directions). Pillaring of H-Magadiite at low pH and temperatures close to 0 °C yielded the highest surface areas, e.g., increasing the surface area from 35 to 130-200 m²/g. The pillared compounds were found to be thermally stable up to temperatures of 650°C.
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    Studies of an alkali impregnated cobalt-molybdate catalyst for the water-gas shift and the methanation reactions
    Berispek, Vasfi (Virginia Tech, 1975-03-08)
    On the basis of our investigation of the "Aldridge" catalyst, an alkali impregnated cobalt0-molybdate on an A1 203 support, for the water-gas shift, methanation, and ethanol dehydration reactions, we can make the following conclusions: 1. The cesium-impregnated "Aldridge" catalyst is highly active for the water-gas shift reaction under sulfur tolerant conditions. 2. The activity of this catalyst is strongly dependent upon the cesium:molybdenum molar ratio. The normalized first order rate constant increases with this ratio until an optimum is reached for the full strength and half strength catalyst. 3. The transition temperatures appeared only with the cesium-impregnated full and half strength catalysts, but not with the one-fifth catalysts. 4. The potassium-impregnated cobalt-molybdate catalyst is quite active, in Comparison to lithium- and sodium-impregnated versions. 5. The cesium-impregnated zinc-molybdate catalyst is not as active as the unimpregnated cobalt-molybdate. Its activity is approximately half that of catalyst "Z" at 400°C. 6. We don't believe that the "Aldridge" catalyst is a catalytic melt.
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    Supercritical extraction of coal
    Sunol, Aydin Kemal (Virginia Polytechnic Institute and State University, 1982)
    Supercritical extraction of coal is removal of a select fraction of the coal by a solvent which is slightly above its critical temperature and above its critical pressure. The objective of this dissertation was to understand the mechanism of supercritical extraction, to test some promising solvents, and to explore the design implications of the findings. Supercritical extraction of Wyodak coal was studied by passing various solvents upwards through a 15-gram sample of 12-20 mesh coal. For the high temperature experiments, the coal was heated to 375°C and 425°C in a hot fluidized sand bath. The main solvent used was toluene, while extractions with n-pentane, xylene, methanol, and water were also done. The extract was fractionated into oils, asphaltenes, and asphaltols. Supercritical extraction of coal near pyrolysis temperatures affords an opportunity to remove unstable decomposition products from the reaction environment to avoid repolymerization and pore blinding. Stronger aromatic solvents removed the decomposition products as they were formed. However, product degradation even with the strongest solvents was inevitable during the initial few minutes. For the low temperature experiments (below 95°C), the solvent was carbon dioxide. Effects of liquid entrainers (pre-mixed with the coal), and heat-pretreatment of the coal (at 400°C for 1 hour) were also studied. The major difference between the high and low temperature extractions was that coal reactions occurred at high temperatures simultaneously with solubilization. Extraction of raw coal and heat-pretreated coals with carbon dioxide was negligible. However, extractions as high as 12% were possible when small amounts of liquid entrainers such as pyridine, toluene, and tetralin were pre-mixed with the coal. The entrainers were almost completely recovered with the extract. The process design implications of the supercritical extractions of coal were studied using the method developed by ESCOE (Engineering Societies Commision On Energy Inc.). Preliminary design estimates showed that the following supercritical extraction processes were possible alternatives to present commercialization efforts and deserve further attention: 1. Gasification of the extraction residua; 2. Satellite plants operating in parallel with coal-burning utilities; 3. Entrainer-aided extraction.
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    Synthesis, characterization and reactivity of transition metal containing zeolites
    Rossin, Joseph A. (Virginia Polytechnic Institute and State University, 1986)
    Transition metal containing zeolites (zeolite A and ZSM-5) were prepared by addition of various transition metal containing substrates to zeolite synthesis gels. Crystal growth data were recorded in order to determine the influence of the transition metal species on the rate of crystal growth. X-ray diffraction, oxygen adsorption, FTIR and SEM were utilized to evaluate crystal purity. X-ray photoelectron spectroscopy (XPS), chemical analysis and electron microprobe analysis were performed in order to ascertain the position (intrazeolitic versus surface) and homogeneity of the transition metal. It was concluded that intrazeolitic transition metals were produced by the novel procedure presented in this work. 1-Hexene hydroformylation by rhodium zeolite A showed intrazeolitic rhodium to migrate to the external surface of the zeolite. However, in the presence of a solution and surface rhodium poison, intrazeolitic rhodium was found to hydroformylate 1-hexene exclusively to heptanal. Ruthenium containing zeolite A was evaluated under CO-hydrogenation conditions. No migration of intrazeolitic ruthenium to the external surface of the zeolite was observed over the course of the reaction. The product distribution obtained for this catalyst did not follow a log normal behavior. Also, loss of zeolite crystallinity was observed following the reaction. Cobalt ZSM-5 was evaluated under CO-hydrogenation conditions. No migration of cobalt to the external surface of the zeolite occurred. XPS analysis of the catalyst following various stages of the reaction indicated that intrazeolitic cobalt was not reduced to the zero valent state. Consequently, the non-zero valent cobalt was not capable of hydrogenating carbon monoxide.