Browsing by Author "Hightower, Joe W."
Now showing 1 - 13 of 13
Results Per Page
Sort Options
Item A study of active sites on fibrillar alumina catalysts(1976) Koyama, Yoshikazu; Hightower, Joe W.Deuterium exchange with benzene and isomerization/ deuterium exchange of n-butenes were used as test reactions to investigate both the concentration and chemical nature of active sites on two kinds of fibrous alumina catalysts. The results of this investigation are comparable to results reported in the literature describing similar reactions studied over more conventional forms of gamma alumina. However, there were a few notable exceptions which make it difficult to draw general conclusions about the catalytic activity of such materials. It has been shown that the surface of alumina is quite complex and consists of several types of independent active sites. Three of these catalytic sites have been isolated in this work, and previous work has pin-pointed at least two more independent strong adsorption sites. Titration of the catalytic activity with chemical poisons that selectively deactivate only one type of site is the technique that was used to discriminate among the various sites. CC2 selectively poisons the deuterium exchange sites, and titration curves indicate that 6 x 1 12 molecules/cm2 is the "lethal dose" required to stop this reaction completely for both benzene and the n-butenes. Actually, the lethal dose increases with increasing reaction temperature, and this is the value at 75°C. If one adsorbed CC molecule poisons one active site, then the E-sites have a concentration of about 6 x 1 /cm , or in other words, they cover less than 1 percent of the catalyst surface. Since both the catalytic activity and the active site concentration differed by a factor of 1 between the two fibrillar catalysts, the activity/site was about the same for the two materials. The sites were thus assumed to be the same. Butene isomerization had to be divided into two types of reactions, one involving double-bond migration (1-butene to 2-butene) and the other involving cis-trans rotation (cis-2-butene to trans-2-butene). is an effective poison for the former (the I-sites), and by titration the site density was estimated to be about 15 x 1 cm. However, does not appear to be an effective poison for the cis-trans rotation reactions which must occur on separate I-sites. The surface intermediates in the double-bond migration reaction may possibly be the anti- and syn-1-methylirallyl species for conversion of 1-butene into cis- or trans-2-butene, respectively. These species can either be formed on the I'-sites. However, a radical ion CHg -A1 species was postulated to account for the intramolecular cis-trans rotation reaction, this species being found through reaction with a surface A1 Lewis acid site. Some minor complications seem to have made these findings somewhat less certain than would be desired. One of these is the fact that the added poisons may undergo slow surface chemical reactions that can alter their poisoning characteristics. Thus, the length of time of the poison has been in contact with the surface, as well as the temperature, can affect the results. Nevertheless, the overall conclusions are clear, and these results have added new insight into the chemical nature and catalytic activity of alumina surfaces.Item Alumina-supported palladium and palladium/copper catalysts: Characterization and use for selective hydrogenation of 1,3-butadiene(1996) Furlong, Brian Keith; Hightower, Joe W.Butadiene is a normal deleterious contaminant that must be removed from C$\sb4$-olefin streams before being used in several industrial applications. While this impurity is substantially eliminated by selective catalytic hydrogenation, there is a considerable economic incentive to improve the currently employed palladium-based catalysts. The purpose of this research has been to investigate the effects of adding copper to alumina-supported palladium to enhance the selectivity of palladium for butadiene hydrogenation. 1,3-Butadiene semi-hydrogenation was studied in the presence of a 10:1 excess of 1-butene over supported bimetallic catalysts containing palladium in a vapor-phase atmospheric plug flow reactor in the temperature range 299-313 K. Copper addition to alumina-supported Pd increases its selectivity for converting butadiene to n-butenes without saturating or isomerizing the n-butenes. For example, with a Cu/Pd atomic ratio of two, the bimetallic catalyst selectively converts $>$99% of the diene to n-butenes while isomerizing or saturating less than 1% of the starting 1-butene. Under similar conditions, Pd alone maintains high n-butene selectivity to only about 50% conversion of the diene. Copper also promotes higher trans-2-butene selectivity at the expense of 1-butene production. The kinetics of 1,3-butadiene hydrogenation over the Pd and PdCu catalysts are first order in hydrogen and zero order in butadiene. Hydrogen sorption measurements show that copper addition decreases both surface adsorption and bulk absorption, suggesting surface decoration of Pd by Cu and possibly bulk Cu-Pd interactions in the supported bimetallic samples. Surface Cu-Pd mixing is confirmed by X-ray photoelectron spectroscopy (XPS) measurements. Extended X-ray absorption fine-structure (EXAFS) results reveal that oxidation of bulk palladium particles is slowed by the presence of copper. Probably copper surface mixing and/or decoration protects bulk palladium particles by suppressing oxygen diffusion. Combined, the results indicate that the bimetallic particles are present as bulk palladium particles with Cu-Pd mixing at the surface. These bimetallic catalysts containing Cu and Pd show promise for commercial use since they are considerably more selective than Pd-alone.Item Catalytic alumina: an isotopic tracer study of active sites for deuterium exchange with benzene(1969) Saunders, Peter Clark; Hightower, Joe W.Catalytic exchange of deuterium with benzene was used as a test reaction to investigate both the number and chemical nature of active sites on alumina. Previous work has involved deuterium exchange with paraffins and olefins on alumina. The results of this work are similar to those reported for the earlier studies. In comparison with hydrogen exchange with perdeuteriobenzene, a primary kinetic isotope effect of 1.8 was found at 20°. However, redistribution of "hydrogen" atoms among the benzene isotopes was much faster than either of the exchange reactions. The addition of a small amount of benzene reduced the rate of the usually fast hydrogen-deuterium exchange by 98%. The strong, dissociative adsorption of benzene to the exclusion of hydrogen is a possible explanation for these results. After catalyst pretreatment at 530°, "titration1" using carbon dioxide as a poison of catalytic activity indicated an active site density of about 15 x 10 to the 12th/cm2 for both the exchange and redistribution reaction. This number is close to that necessary to poison hydrogen-deuterium exchange, which indicates that all three reactions occur on the same or on closely related sites. Because C1402 poisoning demonstrated that other sites besides those active for exchange also adsorb carbon dioxide, the above active site density is only an upper bound. Poisoning with other materials had only a slight effect, which again may indicate that benzene was so strongly adsorbed that it excluded other molecules from the catalyst surface. The similarity of the exchange reaction over a deuterated catalyst ruled out the possibility of exchange with a majority of catalyst hydroxyl groups.Item Characterization of semiconducting catalysts by gas adsorption: Nickel oxide and magnesium ferrite(1973) Allspach, Eugene Robert; Hightower, Joe W.; Leland, Thomas W.In previous work in this laboratory Murphy, Veerkamp, and Leland have derived a model that measures the fraction of a semiconductor catalyst participating in electronic surface effects due to gas adsorption. The parameter, xg, proposed in the model represents the fraction of the catalyst that has undergone significant changes in its electronic properties when compared to the properties that existed before gas adsorption. (The properties at the center of the catalyst particles normally remain unaltered so the comparison can be made between the surface and the center of a particle.) xg is calculated from an equation involving the thermoelectric power, Q, and an electronic heterogenity factor, f. The latter factor is determined using the linear slope of the lncrvs. Q plots obtained from gas adsorption on the catalyst, where a-is the electrical conductivity. In the model the quantitative value of xg for a catalyst is postulated as being a characteristic of that catalyst. If so, the x values for different catalysts can then be used to compare the electronic surface effects which occur an different catalysts. In this work the model was tested with two catalysts: nickel oxide and magnesium ferrite. With nickel oxide reasonable x values were calculated from the model for the carbon monoxide, hydrogen, and oxygen adsorptions which were conducted. The following xg values were found for the various adsorbates: .23 for carbon monoxide, .19 for hydrogen, and .37 and .44 for oxygen. For magnesium ferrite a reasonable xg value Df .65 for carbon monoxide adsorption was calculated from the model but the x value for oxygen adsorption was greater than unity which is meaningless. The failure during oxygen adsorption is attributed to the presence of significant numbers of holes compared to electrons. The number of holes present is assumed negligible in the theory. However, due to the small number of electrons available for conduction in magnesium ferrite, the loss of electrons to oxygen during adsorption significantly alters the relative proportions of electrons and holes and causes comparable contributions to the total conductivity by each.Item Investigation of a polymer concrete composite material for runway applications(1983) Zeilenga, Jay Howard; Armeniades, C. D.; Hightower, Joe W.; Roberts, John M.The use of fiberglass cloth layers as flexural reinforcement for polymer concrete was investigated. The influences of resin systems, cloth, aggregate, fine fillers, moisture content of aggregate, and specimen depth on the properties of the composite material were studied. The laboratory investigation was oriented towards the composite’s considerable potential for panel applications and possible application as a pavement material. The concept of quickly installed runways made of this polymer concrete composite material is discussed. The composite with two bottom layers of a light-weight glass cloth exhibited twice the flexural strength and three times the elongation limit of polymer concrete alone. A silane coupling agent was shown to improve the retention of strength of polyester concrete made with wet aggregate for moisture contents below three percent. The thickness of the composite, and of the polymer concrete alone, only slightly affected the flexural strength. This lack of sensitivity to specimen depth is important when considering production of panels of varying thicknesses. The ability of the composite to deform considerably, and its higher strength in excess of that for polymer concrete and ordinary concrete was noted as being desirable for constructing thin, durable, fast-setting pavements over weak and non-uniform subgrades.Item Isomerization of C₄¿ olefins and alkyl cyclopropanes on transition metal substituted clay mineral catalysts(1978) Bader, Shirine; Hightower, Joe W.The catalytic activity of four types of clay minerals, Amesite, Co-amesite, Synthetic Mica Montmorillonite (SMM) and Ni-SMM, was investigated by studying the isomerization reactions of C4 normal olefins and methyl and dimethyl alkyl cyclopropanes. Since Co-amesite catalyzed the rapid isomerization of 1-butene and cis-2-butene while amesite by itself was inactive, it was concluded that the activity was due to the presence of the cobàlt. Over Co-amesite, cis-trans rotation occurs much more easily than does double bond migration, which requires a higher temperature than the former. Very little activity for saturation was observed over this catalyst when hydrogen was present; however, the rates of the isomerization reactions increased substantially when hydrogen gas was mixed with the reactant. The low activity of cobalt for hydrogenation of n-butenes was reported earlier, and the increased rate of isomerization in the presence of hydrogen is very typical of activity characteristic of metallic catalysts. These two factors both supported the conclusion that cobalt is the only active center in the Co-amesite catalyst for these particular reactions. In the case of synthetic Mica-Montmorillonite and Ni-SMM, it was concluded that SMM catalyzes the isomerization of C5 cyclic compounds while Ni provides active sites for both the hydrogenation and H2-D2 exchange reactions. The product distribution observed over SMM for all these isomerization reactions were identical to those observed over an amorphous silica-alumina cracking catalyst. The mechanism suggested for silica-alumina, proton addition coupled with the elimination of any pathway that passes through an unstable primary carbonium ion, appears to explain the data for SMM. The catalytic activity of Ni for hydrogenation, as well as for its promotion of H2-D2 exchange reaction, is very typical of nickel catalysts. Mass spectral analysis was used to study the relatively small amount of deuterium incorporation into the hydrocarbon molecules.Item Mechanistic tracer studies of a synthetic mica-montmorillonite: n-butene isomerizations(1971) Milliron, Dennis Luke; Hightower, Joe W.This thesis examines the catalytic characteristics of a synthetic mica-montmorillonite clay. The double bond isomerizations (including both double bond migration and rotation, but excluding structural rearrangements of the carbons) of the n-butenes were employed as the probe reactions. Product analyses were performed via gas chromatography and mass spectroscopy. The reaction products distribution was kinetically controlled, which suggests the possibility of a metastable intermediate. The only intermediate compatible with tracer data obtained herein is the secondary butyl carbonium ion. The isotopic data also indicate that both types of isomerization proceed via an intermolecular mechanism rather than intramolecularly. The isomerization reaction appears to follow first order kinetics. The existence of a significant isotope effect intimates that carbon-hydrogen bond cleavage occurs in the rate determining step of the reaction sequence. Bronsted type acid sites appear to be responsible for the catalytic activity of this synthetic clay for the isomerization of n-butenes. Bronsted type acid sites are associated with the hydroxyl groups which occur in this clay. Since up to 20% of the total catalyst hydrogen can occur on 12% of the surface (the circumferential surface area of the layer-lattice crystals), and up to 55% of the hydrogens which lie near the crystal surface can occur on this same 12% of the external surface area, it is hypothesized that the circumferential hydroxyls are sites of high catalytic activity.Item Polymer formation, deactivation, and ethylene selectivity decline in palladium/aluminum oxide catalyzed selective acetylene hydrogenation(1989) LeViness, Stephen Claude; Hightower, Joe W.The removal of acetylene by selective hydrogenation is a critical step in the purification of ethylene streams for industrial polyethylene production. Hydrogenation of 0.35-1.0% acetylene in ethylene over Pd/Al$\sb2$O$\sb3$ catalysts is accompanied by the formation of significant amounts of surface oligomers/polymers. During the initial stages of CSTR operations at 40-120$\sp\circ$C and P = 1 atm these accounted for 8 to 50% of the acetylene consumed. Ethylene selectivities calculated ignoring these polymers are incorrect. Approximately 30% of these products are volatile and soluble in hydrocarbon solvents. They consist of even carbon numbered chains from C$\sb8$ to at least C$\sb{30}$. Normal paraffins are the major species at each carbon number, although branched paraffins, linear and branched mono-and di-olefins, and alkylbenzenes are also produced. The remainder is nonvolatile and insoluble in any known solvent; its degradation products are consistent with those of polyacetylenes of carbon number greater than 24. A mechanism involving polymerization in regions of low surface hydrogen concentrations and terminated by hydrogenation is proposed. The accumulation of the liquids in catalyst pores imposes diffusion limitations on the acetylene reaction; both the rate of acetylene consumption and its selectivity to ethylene decrease. These effects are reversible upon removing the liquid polymers. The decrease in surface polymer selectivity observed during operation is compensated by an increase in gas phase oligomer selectivity; the total oligomer/polymer selectivity does not change appreciably. In conjunction with previous investigations showing nearly constant ethane production only from acetylene, this suggests that the product distribution from acetylene hydrogenation does not change significantly. All catalysts exhibited an induction period where the activity increased and the ethylene selectivity decreased, the latter due only to an increase in the rate of ethylene hydrogenation. The duration of this period increased sharply with decreasing catalyst metal loading, which is proposed to be inversely related to metal dispersion. The increasing duration of activation is therefore a form of structure sensitivity. Activation was only reversible upon moderate temperature hydrogen or oxygen/hydrogen treatments and represents the formation of some active surface species.Item Selective hydrogenation of 1,3-butadiene in 1-butene over alumina supported palladium and palladium/copper catalysts(1994) Furlong, Brian Keith; Hightower, Joe W.Copper addition to palladium increases both activity and selectivity in the selective hydrogenation of 1,3-butadiene to n-butenes. The bimetal catalyst (Cu/Pd:2/1(atomic)) hydrogenates the diene to virtually 100% conversion without significant n-butene isomerization or butane formation. While at moderate conversions monometallic palladium is quite selective for n-butene production, 1-butene is quickly isomerized and saturated at higher conversions resulting from butadiene's inability to monopolize the active surface at lower partial pressures. Copper also promotes higher trans-2-butene selectivity and modifies the rate dependence on 1,3-butadiene from zero to negative order. These results suggest a donor "ligand" effect in which copper changes the palladium's electronic character. The apparent activation energy for 1,3-butadiene hydrogenation over palladium is 14.9 $\pm$ 0.2 kcal/mol.Item Study of trace contaminants co-adsorbtion on activated charcoal(1980) Amladi, Pradeep Manohar; Davis, S. H.; Deans, H. A.; Hightower, Joe W.The object of this research was to study trace contaminants coadsorption phenomena on activated charcoal. Selected compounds were injected in pulses into a stream of nitrogen carrier gas flowing through a packed column of activated charcoal. The effluent concentration of the injected compound was detected by either a flame ionization detector or an electron capture detector. The first and second moments of the effluent peaks of injected compounds were calculated and the observed changes in these moments were then interpreted in terms of changes in the adsorption equilibrium constant (H) and the mass transfer resistance coefficient (1/fl). Adsorption studies of the following compounds were done: (1) methanol at 1°C, (2) 2-propanol at 175°C, (3) ethyl acetate at 25°C, (4) trichloroethylene at 25°C and 29°C, (5) 1,1,1-trichloroethane at 2°C and 22°C, and (6) toluene. For each of the first five compounds, the following cases were considered: adsorption on (a) a clean activated charcoal bed, (b) a bed having the same compound preadsorbed on it, (c) a bed having toluene preadsorbed on it, and (d) a bed having toluene and the same compound, both preadsorbed on it. Methanol, 2-propanol and ethyl acetate were studied with a FID. Each of these three compounds were irreversibly adsorbed to some extent. The presence of the irreversibly adsorbed portion and toluene decreased the first and second moments of subsequent adsorption peaks. Changes in both moments indicate that the adsorption equilibrium constant decreases as the surface coverage by toluene and the irreversibly adsorbed amount increases. In the case of 1,1,1-trichloroethane and trichloroethylene, a substantial amount of irreversibly adsorption was noticed. The irreversibly adsorbed amount in each case showed signs of forming a pseudo-liquid layer on the activated charcoal surface. The effect of this was an increase in both moments. Toluene, however, decreased both moments of these two compounds. Sample doses in the experiment were kept low enough to get an average surface coverage of the order of 1“J to 1-4. However, the actual surface coverage at some sites might have been much higher resulting in a highly non-ideal behavior as was observed with 1,1,1-trichloroethane and trichloroethylene.Item The catalytic cracking of cumene over silica-alumina. A study with stable and radioactive tracer compounds(1981) Azuaje-Fernandez, Alejandro J.; Hightower, Joe W.; Brooks, Phillip R.; Curl, Robert F.The catalytic cracking of cumene (iso-propyl benzene) over a silica-alumina catalyst (Houdry M-46) has been studied using the pulse technique in a microcatalytic reactor. The initial deactivation of the catalyst was found to follow an empirical first order equation with respect to the initial activity towards the formation of products. Cumene is given as the precursor of the coke formed at the surface of the catalyst. As in (18) the deactivation is thought to proceed by a Hinshelwood mechanism in which two sites are lost in every deactivating event or adsorption of the poison on two sites. The rate of the cracking reaction is described by a first order equation in reactant cumene and is proposed to follow a Langmuir-Hinshelwood mechanism in the range of 25° to 4°C and 1.67 atm. The true activation energy of the reaction was found to be 21.4 kcal mole. Experiments with H-D exchange over this catalyst showed that deuterium distribution in either benzene and cumene occurred to statistical scrambling at equilibrium, indicating the lability of the C-H bonds in the hydrocarbons. D-H exchange measurements gave as result 1.8 x 1 sites/nr and is set as an upper limit for the active sites on the catalyst surface if Br^nsted centers are thought to be the active centers for the dealkylation. Results of experiments with radioactive tracer compounds indicated that the reaction proceeded to almost equilibrium without influence of the back reaction. Values, based on a reaction model of a first order reversible process occurring near equilibrium, obtained for thermodynamic functions for the cracking reaction are reported. These values are in good agreement with the theoretical values obtained from the standard functions of formation of the reaction participant compounds.Item The interaction of certain electrical and catalytic properties in cupric and zinc oxides(1973) Petterson, William Charles; Leland, Thomas W.; Hightower, Joe W.During previous work in this laboratory, Murphy, Veerkamp, and Leland developed a relationship to describe the charge transfer process between a semiconducting catalyst and a chemisorbing gas. The expression derived states that for certain pressure ranges of the adsorbing gas, a plot of In a versus Q for a p-type semiconductor will be linear with a slope of -1/1-Xs(e/K). The Xg term represents the fractional extent of electronic inhomogeneity in the catalyst in terms of distinct surface and bulk regions. The first part of this work evaluated the behavior of cupric oxide relative to the theory. The results show cupric oxide to remain virtually homogeneous electronically during both anionic and cationic adsorption. Photodesorption of carbon dioxide from zinc oxide was studied as a possible mechanism for the photocatalytic effect observed for the oxidation of carbon monoxide over zinc oxide. The results of the study indicate that ultraviolet light of energy slightly greater than the ZnO band gap energy (same wavelength as the light which caused the photocatalytic effect) does not enhance the desorption of the CO2. As a consequence it is concluded that the photodesorption of carbon dioxide is not explicitly involved in the photocatalytic mechanism.Item The selective hydrogenation of 1,3-butadiene in the presence of n-butenes over supported palladium catalysts(1989) Riley, Mark G.; Hightower, Joe W.The selective hydrogenation of 1,3-butadiene in the presence of n-butenes over supported palladium catalysts, an industrially important reaction, was investigated at atmospheric pressure between 308K and 393K. The study included measuring the reaction kinetics both in the presence and absence of CO, examining of the differences between palladium supported on six different carriers, testing for the effects of hydrogen spillover, and investigating the catalyst's aging behavior. Isotopic tracers (single $\sp{13}$C labeled 1-butene) provided direct information about the reaction network. The kinetics of butadiene hydrogenation were measured on a 0.02% Pd/alumina catalyst. Reaction orders were one in hydrogen and zero in butadiene. Addition of CO decreased the rate of butadiene hydrogenation; the reaction rate was proportional to the negative one-half power of the CO pressure in the range of 25 to 145 ppm CO. CO affects the hydrogenation rate by reducing the surface concentration of hydrogen. Activation energies for butadiene hydrogenation were 13.3 + 0.8 kcal/mole in the absence of CO and increased to 15.5 + 0.5 kcal/mole with the addition of 40 ppm CO. Six supports were tested to determine what effect the support might have on the catalytic properties of the supported palladium catalyst. The six materials tested were: activated carbon, alumina, barium carbonate, barium sulfate, silica gel, and titania. Alumina, silica, and titania all displayed approximately the same behavior; barium carbonate and sulfate supported catalysts exhibited significantly higher production of butane than those supported on refractory oxides. High palladium loading (all catalysts were 5 weight percent palladium) may have obscured real differences between alumina, silica, and titania. The presence of support activity induced by spiltover hydrogen was also investigated. Blending the supported Pd catalyst with additional support containing no Pd usually caused some increase in both hydrogenation and isomerization activity, but the results were not sufficiently reproducible to provide conclusive evidence for hydrogen spillover effects. The variability in the results, probably caused by the low reduction temperatures used. Through the use of $\sp{13}$C labeled 1-butene, n-butane was observed to come from the hydrogenation of both 1-butene and 2-butene during the selective hydrogenation of 1,3-butadiene in the presence on n-butenes.