Showing papers in "Advances in Catalysis in 1969"

Catalysis by Supported Metals

Abstract: Publisher Summary This chapter describes the interaction between metal and support, which may complicate the interpretation of data obtained with supported metals, the critical problem of determining the surface area of the metal, and illustrates the use of supported metals for the elucidation of the problem of catalyst specificity. Work on the series of platinum catalysts suggests that many reactions of hydrogenation, dehydrogenation, and hydrogenolysis are structure-insensitive. If, these same catalysts are now used in reactions involving oxygen—decomposition of hydrogen peroxide, oxidation of methanol and ethanol, oxidation of acetaldehyde—the picture is substantially altered: while specific rates are again almost the same for highly dispersed samples with percentages of dispersion between 80 and 100%, these specific rates change by at least one order of magnitude on samples with a smaller dispersion (50%) and on platinum black.

Modern State of the Multiplet Theor of Heterogeneous Catalysis

Abstract: Publisher Summary This chapter reviews the modern state of the multiplet theory of heterogeneous catalysis The multiplet theory deals with numerical values of bond lengths and bond energies, as well as with the geometrical form of reacting molecules and the crystal lattices of catalysts This allows definite results to be obtained for many reactions on an atomic level This is how the multiplet theory differs from a number of other theories on catalysis The theory of the structure of matter is based on both electronic theory and quantum mechanics—the basis for the multiplet theory Bond lengths and energies represent a stable complex of electronic properties essential for catalysis The multiplet theory proceeds from the premise that catalysis is a chemical phenomenon and that covalent bonds require catalytic activation

Chemisorptive and Catalytic Behavior of Chromia

Abstract: Publisher Summary The catalytic properties of chromia have received considerable attention, probably more than those of any other catalysts except Group VIII metals, silica, and alumina. This chapter describes the unsupported chromia as a catalyst for relatively low temperature reactions under reducing conditions, and provides an outline of the wide variety of reactions catalyzed by chromia or supported chromia. A chromia gel activated only at 150° has no detectable catalytic activity for the hydrogenation of olefins at room temperatures nor does it chemisorb oxygen or carbon monoxide. Catalytic and adsorptive capacity develops upon pretreating the gel at higher temperatures. After heating to 400°, chromia rapidly chemisorbs oxygen and carbon monoxide at −78° and it leads to rapid hydrogenation of ethylene at −78°. The surface of chromia appears to be an ideal case for study at the present. By activation at increasing temperatures, one can vary the number of active sites from none to some maximum number. At a low density of sites, one can hope that the sites are well separated and non-interacting. One can compare chemisorption of various gases with specific catalytic activities at various levels of site densities and hope to gain information about site heterogeneity.

Catalytic activities of thermal polyanhydro- alpha-amino acids

Abstract: Catalytic activities of thermal polyanhydro-alpha- amino acids for modeling enzymes and prebiotic protein

Molecular Orbital Symmetry Conservation in Transition Metal Catalysis

Abstract: Publisher Summary This chapter presents the specific catalytic functions of the transition metal in the various types of transformations, and discusses the associated chemistry. Molecular orbital symmetry conservation constrains all molecular systems to specific paths of transformation. Symmetry conservation principles have proven to be powerful tools for understanding a large body of complex organic chemistry. These concepts further bear on molecular stability. A molecule in one bonding configuration transforms into other configurations primarily through allowed paths. The thermal stability enjoyed by simple olefins to a certain extent rests on orbital symmetry restraints. Both olefin cyclobutanation and double-bond isomerization (through a 1,3-hydrogen shift), involving forbidden passages, are not observed at moderate temperatures. Simple olefins are fixed in their bonding configurations and cannot interconvert through the sterically-preferred paths. The thermal interconversion of olefins is necessarily a high-temperature process involving predominantly the higher energy, allowed transformations incorporating free radical intermediates.

Carbon Monoxide Oxidation and Related Reactions on a Highly Divided Nickel Oxide

Abstract: Publisher Summary This chapter describes the preparation and properties of a highly divided nickel oxide, chemisorptions on pure nickel oxide, surface interactions between cases and adsorbed species, room-temperature oxidation of carbon monoxide, room-temperature oxidation of carbon monoxide on doped nickel oxides, high-temperature (200°) oxidation of carbon monoxide on nickel oxide, and decomposition of nitrous oxide on a highly divided nickel oxide at 250°C. At low temperatures, where the surface ionic mobility is restricted the catalytic activity of a divided oxide for oxidation or reduction processes is determined primarily by the nature and the concentration of lattice defects in the surface layer and by the strength of the bond between oxygen and these defects. The nature and concentration of the defects depend upon the chemical nature of the catalyst, its previous history, and on the course of the catalytic reaction itself.

Catalysis and Inhibition in Solutions of Synthetic Polymers and in Micellar Solutions

Abstract: Publisher Summary This chapter describes some characteristics of enzymic catalysis, reaction rates in solutions of long-chain macromolecules, catalysis and inhibition in micellar solutions, and critique of the use of synthetic polymers and micelles as enzyme models. The catalysis of various reactions in the solutions exhibits a number of characteristics normally associated with heterogeneous catalysis. An inhibition of the catalytic effect may result, when the reagent has to compete with an inert species for the available catalytic sites. The analogy between the high reactivity of the enzyme-substrate complex and the reactivity of substrates bound to polymers or micelles is even less satisfactory. Kinetic effects based on long-range electrostatic effects, particularly the concentration of catalytic hydrogen or hydroxyl ions in the vicinity of a polyion or the micellar surface, have no counterpart in enzyme chemistry. An inhibition of the catalytic effect may result, when the reagent has to compete with an inert species for the available catalytic sites.

Enhanced Reactivity at Dislocations in Solids

Abstract: Publisher Summary This chapter discusses the properties of dislocations and describes the geometrical description and energetics of isolated and aligned dislocations. Plastic deformation in a crystal occurs by the movement or sliding of one plane of atoms over another. The movement is known as slip and it takes place on the so-called slip plane. It is much easier for the crystal to be deformed under stress if part of the atoms above the slip plane move by one lattice spacing at a time. A multiple-strength dislocation has a Burgers vector several times that of the lattice vector. The chapter describes the experimental methods of identifying dislocations. A dislocation represents the boundary of a slipped area and it cannot be discontinuous. It must either extend from one crystal surface to another or make a closed loop (a dislocation loop). Any particular dislocation may possess both edge and screw character. The symbols used to define a dislocation convey both the magnitude and the direction of the Burgers vector as well as the slip plane in which the dislocation moves. A perfect dislocation lying in the basal plane of close-packed hexagonal solids—zinc, magnesium, or graphite—may dissociate to yield two partial “Shockley” dislocations.

Catalysis by Electron Donor-Acceptor Complexes

Abstract: Publisher Summary This chapter provides an overview of the mechanism of hydrogen exchange or hydrogenation over the electron donor–acceptor (EDA) complexes of alkali metals, and of organic electron donor molecules, such as phenothiazine. The hydrogen exchange reaction takes place between D2 or C2D2 and various EDA complexes, and that the H2–D2 exchange reaction to form HD proceeds at a considerable rate over the complexes, while no reaction takes place in the absence of alkali metals even at 200°. A closed circulating system was employed to follow the reaction rate. The reactor was a U-shaped glass tube equipped with two side-arms, each containing electron acceptor (phthalocyanine) and donor (sodium), respectively. When, C2D2 (15 cm Hg) was admitted to the EDA complexes of various phthalocyanines with organic electron acceptors such as 2,3-dicyanoquinone at temperatures between 25 and 90°, the hydrogen exchange reactions of acetylene proceeded at a negligible rate and the components of the complexes were separated from each other above 100° as a result of sublimation.

Correlation Among Methods of Preparation of Solid Catalysts, Their Structures, and Catalytic Activities*

Abstract: Publisher Summary This chapter describes the traditional methods of preparation of supported nickel catalyst, such as superhomogeneous coprecipitation (SHCP) method, cation exchange method, palladium on aluminosilicate by complex-ion exchange, palladium on active charcoal, and nickel-phosphorus alloy. The catalytic activity of a solid catalyst, including its selectivity and life, is one of the attributes inherent to this solid substance itself and, therefore, depends on its physical and chemical structures, which are, in turn, governed by the method of preparation of this solid substance. The catalytic reaction on the solid catalyst is a kind of reaction that occurs between the reactant and the catalyst surface and, therefore, the physical and chemical structures of the surface must be among the main controlling factors of the surface reaction. The surface of the solid catalyst is heterogeneous in the geometrical composition of atoms and also in the distribution of surface energy.

Catalytic Research on Zeolites

Abstract: Publisher Summary This chapter provides an overview of the work on zeolites carried out at Princeton University, and presents some recent gas chromatographic results on the stability of the pore structure of the zeolite as the material undergoes these transformations. The Princeton work is divided into three programs: (1) the synthesis of zeolites and their derivatives, (2) the study of catalytic properties, and (3) the characterization of the active centers particularly using magnetic resonance techniques. The catalytic work on the zeolites has been carried out using the pulse microreactor technique on the following reactions: cracking of cumene, isomerization of 1-butene to 2-butene, polymerization of ethylene, equilibration of hydrogen–deuterium gas, and the ortho-para hydrogen conversion. Zeolites, crystalline alumina silicates with open regular structure, offer unusual opportunities for carrying out catalytic studies. Their well-defined crystalline structure and their regular pore distribution permit a better description of the surface than that offered by alumina–silica gel catalysts.

The Polymerization of Olefins by Ziegler Catalysts

Abstract: Publisher Summary This chapter discusses the mechanism by which Ziegler polymerizations proceed. The numerous theories that have been proposed fall in two classes: (1) theories in which the customary use of catalyst combinations containing two metals is essential—bimetallic mechanism and (2) theories according to which only one metal (the transition metal) is necessary—monometallic mechanism. The monometallic mechanism proposes that the active initiation site resides at the transition metal atom and does not directly involve the second metal. The Ziegler–Natta polymerizations are designated as “anionic” or “anionic coordinated.” Any acceptable mechanisms suggested for Ziegler polymerizations go some way to explain the stereospecificity of these reactions. The degree of specificity of a polymer depends on conditions of preparation and choice of catalyst combination. There exist isolated reaction sites on the surface of the TiCl 3 crystals that show different capacities to coordinate electron donor compounds such as amines and olefins. Only exposed titaniums along the edges of the crystals favor stereospecific growth.

Dynamic Methods for Characterization of Adsorptive Properties of Solid Catalysts

Abstract: Publisher Summary The frequency response technique for studying catalytic adsorption phenomena is an important experimental method The method is illustrated by actual data from a hydrogen-on-nickel system The amount of adsorbed gas on a catalyst that is part of an isothermal system varies with time when the pressure changes This variation depends on the adsorption kinetics and the heterogeneity of the surface During chemical adsorption on a catalyst surface, several processes occur simultaneously This may result from the heterogeneous nature of the surface or to the existence of different adsorbed states The tools of process control theory can be used to separate the phenomena and yield information on their nature To determine the dynamic characteristics of an unknown system, the control engineer uses or induces certain forms of disturbances or “inputs” and observes or interprets their effects or “outputs” Two of the most useful types of inputs for the study of process dynamics are the step function and the sine wave

Acid-Catalyzed Isomerization of Bicyclic Olefins

Abstract: Publisher Summary This chapter describes the acid-catalyzed isomerization of bicyclic olefins including experimental and isomerizations in the C 8 H 12 series, C 9 H 14 , and C 10 H 16 . The low-temperature stability of bicyclo[2.2.1]heptane derivatives is well documented by the huge literature on liquid-phase norbornyl carbonium chemistry; under such conditions, no ring expansion is observed, but instead, multiple Wagner–Mecrwein rearrangements take place, preserving the bicyclo[2.2.l]heptane structure in the course of nucleophilic substitution or elimination reactions. However, if the norbornyl carbonium is brought to the proper temperature, passage to the more stable bicyclo[3.2.1]octane structure becomes possible. Lack of evidence for the reverse passage points to the lack of thermodynamic stability of the norbornyl structure even at low temperatures. The evolution of the systems with increasing temperature or reaction time indicates that bicyclo[3.2.1]octane structures, rather stable in the medium range of temperatures (160–200°) are irreversibly converted to bicyclo[3.3.0]octane isomers at higher temperatures (250°).