D.L. Trimm

Carbon formation from light hydrocarbons on nickel

Abstract Preliminary studies of the formation of carbon from several light hydrocarbons on nickel foils over the temperature range 400–600 °C have been completed. Deposition from acetylene was found to be rapid, while deposition from olefins is autocatalytic and is accelerated by hydrogen. Carbon formation from paraffins is comparatively slow. The mode of pretreatment of the nickel was found to influence the early stages of carbon deposition. Factors such as temperature, pressure of gas, annealing, and order of admission of reagents had a marked effect on the initial rate of deposition. The kinetics of the carbon deposition at later stages of the reaction appear to depend on nickel crystallites transported from the foil surface by the growing carbon.

Studies of the partial oxidation of methane over heterogeneous catalysts

Abstract Studies have been made of the effects of certain gaseous additives on the oxidation of methane over palladium catalysts. It has been shown that, whereas higher alkanes and partial oxidation products of methane simply retard overall oxidation of the hydrocarbon, certain organic halogen compounds not only reduce the rate of oxidation of methane but also result in the production of isolatable quantities of formaldehyde in high selectivity. In the light of a proposed mechanism for the oxidation of methane in the absence of additives, the action of the halogen compounds is interpreted in terms of the ability of such additives to modify the catalyst surface and to inhibit the further oxidation of the initial products of methane oxidation.

Gasification of carbon deposits on nickel catalysts

Abstract The kinetics of the reaction of steam and of hydrogen with carbon deposited on nickel foil and on supported nickel catalysts are reported for the temperature range 820–1020 K. The rate of reaction with steam has been shown to be controlled by the diffusion of carbon through nickel. Gasification of carbon by hydrogen is governed by the rate of the surface reaction.

A study of nickel-molybdate coal-hydrogenation catalysts using model feedstocks

Abstract A model feedstock consisting of dibenzothiophene, dibenzofuran, quinoline, and phenanthrene dissolved in heptane has been hydroprocessed in a flow reactor at 6.89 MPa and 400 °C over a series of nickel-molybdate catalysts. Under simultaneous processing, the main product of dibenzothiophene hydrodesulphurization was diphenyl, the main product of quinoline hydrodenitrogenation was propylcyclohexane, and that of dibenzofuran hydrodeoxygenation was cyclohexyl benzene. Sulphur was removed without the need for saturation of the sulphur-containing ring. For both nitrogen and oxygen removal, the ring containing the hetero atom was saturated before scission of the CN or CO bond. The main products of the hydroprocessing of phenanthrene were hydroderivatives of varying degrees of saturation. Very little hydrocracking took place under our conditions, and this occurred predominantly by successive saturation and cracking of terminal rings. With increasing catalyst acidity, the rates of all reactions studied increased, as did the degree of hydrocracking relative to hydrogenation. Coke formation on the catalyst also increased markedly with acidity.

Particle size effects during the sintering of silver oxidation catalysts

The activity of evaporated silver films used as model supported silver catalysts has been examined as a function of catalyst life during the oxidation of ethylene and of hydrogen. Electron optical examination of the surface shows that significant sintering of the metal occurs when the catalyst is exposed to oxygen at the temperatures of the reaction, and changes in catalytic activity are shown to be related to the reorganization of the metal. Particularly large changes in activity were noted as small particles of silver (<500 A diameter) were removed by aggregation.

The influence of electron directing effects on the catalytic oxidation of toluenes and xylenes

Abstract The relationship between the catalytic activity of an oxidation system and the electron directing properties of hydrocarbons has been investigated using the oxidation of unsubstituted and substituted toluenes over molybdenum trioxide. This catalyst is found to be selective but rather inactive, with a maximum activity in the temperature range 450–460 °C. Kinetic studies of the oxidation of toluene show that the reaction is controlled by the rate of reduction of the catalyst. Comparison with the results for xylenes shows that the formation of the hydrocarbon-molybdenum complex is the slow step. The oxidation of substituted toluenes is found to be controlled by the rate of the surface reaction between adsorbed oxygen and toluene. The electron directing properties of the organic fuel are found to be affected by substituents in the molecule and by the field effects of the catalyst. Catalytic activity is found to be proportional to these modified electron directing properties, and the system obeys a linear free energy relationship similar to the Hammett equation.

Propylene hydrogenation over platinum/carbon molecular sieve catalysts

Abstract Studies have been made of the kinetics of hydrogenation of propylene over a catalyst consisting of platinum suspended in a carbon molecular sieve matrix. Rates of adsorption and diffusion of reactants and products in the catalyst have been found to have a controlling influence on the kinetics. Two processes can be identified, one involving fast irreversible adsorption of propylene on metal and one involving slow reversible adsorption in the carbon molecular sieve. Hydrogenation has been found to be dependent on the latter, which has been suggested to be an adsorption/surface diffusion process involving migration through the pores to metal sites.

The catalytic oxidative dimerization of propylene

Studies have been made of the catalytic oxidation of propylene to hexadiene over thallium oxides. The reaction is selective to the 1,5-diene, the only other product being carbon dioxide which originates from the overoxidation of the hexadiene. The active component has been found to be the thallic oxide, which is reduced during the course of the reaction. Permanent deactivation of the catalyst, which occurs if thallous salts distil out of the reactor, may be avoided if sufficient oxygen is present to rapidly reoxidize the catalyst. The production of hexadiene is favored at high temperatures, although the catalyst may deactivate faster under these conditions. A reaction mechanism is proposed in which the high selectivity to the 1,5-dimer is explained in terms of the simultaneous donation of two electrons to a Tl3+ ion.

The catalytic activity of supported silver for the oxidation of ethylene: Activity patterns on different supports

Abstract The catalytic activity of supported silver for the oxidation of ethylene to ethylene oxide and to carbon dioxide has been investigated as a function of support. Significant changes in activity with time of use of catalyst have been observed: these changes depend on the nature of the support. It is shown that the initial changes in activity are not connected with catalyst sintering nor with crystallite size changes. Electron spectroscopy reveals that impurities, originating from the support and from adsorption of organic material, concentrate at the surface of the silver. A tentative explanation of the changes in activity pattern is proposed in terms of the modification of the surface by these impurities.

The catalytic oxidation of propylene to benzene

Abstract The oxidation of propylene to benzene has been found to be catalyzed by indium oxide. 1,5-Hexadiene and acrolein are produced in the early stages of the reaction, the diene oxidizing further to produce benzene. The kinetics of the reaction have been examined in some detail, and a tentative mechanism has been advanced involving the removal of hydrogen from the allylic carbons of propylene π-bonded to an In3+ ion as the rate-determining step.