Charles Kemball

A comparative investigation of silica-supported Ru-Cu and Ru-Ag catalysts

Abstract A series of silica-supported ruthenium catalysts with varying amounts of either silver or copper have been examined by hydrogen chemisorption, X-ray diffraction, and X-ray photoelectron spectroscopy, and tested for the exchange of methane with deuterium and the hydrogenolysis of ethane and 2,2-dimethylpropane. All catalysts contained 3 wt% of ruthenium which was highly dispersed. The silver was thought to be present on the ruthenium crystallites mainly as islands and not atomically dispersed because its effect on hydrogen chemisorption and catalytic activity were broadly similar. The addition of copper had little influence on hydrogen chemisorption and some spillover from ruthenium to the group IB metal may occur. Even though hydrogen chemisorption suggested that little copper was dispersed on the ruthenium there was a significant reduction in catalytic activity which was believed to result from a combination of electronic and geometric effects. Not all reactions were influenced to the same extent by the addition of copper but the effects on the multiple exchange of methane and the hydrogenolysis of 2,2-dimethylpropane were similar.

Hydrogenation of acetylene in excess ethylene on an alumina-supported palladium catalyst at atmospheric pressure in a spinning basket reactor

The hydrogenation of acetylene in the presence of ethylene and carbon monoxide has been investigated in a spinning-basket reactor on a commercial supported palladium catalyst. The rate of disappearance of acetylene was found to be controlled by pore diffusion at low acetylene concentrations and was independent of acetylene at high acetylene pressures. In the non-diffusion-controlled region the reaction of acetylene was first order in hydrogen, and the rate of ethane formation was independent of both the acetylene and ethylene pressures. This latter observation is consistent with the presence of sites which can hydrogenate ethylene even in the presence of acetylene. The nature of the C4 products has been shown to be dependent on the packing of adsorbed acetylene molecules and on the availability of surface hydrogen. The analysis of a heavy involatile product is also reported.

The isomerization of 3,3-dimethylbut-1-ene, 3-methylcyclopent-1-ene, and methylenecyclopentane over γ-alumina

Abstract Skeletal isomerization of 3,3-dimethylbut-1-ene (33DMB1) and double-bond migration of 3-methylcyclopent-1-ene (3MCP = ) and methylenecyclopentane (MCP) have been studied over γ-alumina. The activity for the isomerization of all three alkenes increases with increasing catalyst activation temperature. Addition of fluoride to the catalyst greatly increases its activity. Treatment of the γ-Al 2 O 3 catalysts with H 2 S results in some enhancement of activity for 33DMB1 and MCP isomerization but reduces activity for 3MCP = isomerization by ~90%. The importance of initial water content of the alumina on subsequent activity is discussed. It is concluded that skeletal isomerization of 33DMB1 and double-bond migration in MCP occur via carbonium ion intermediates on Bronsted acid centers whereas double-bond migration in 3MCP = occurs predominantly via π-allylic intermediates on Lewis acid sites.

Hydrogenation of acetylene in excess ethylene on an alumina supported palladium catalyst in a static system

Studies have been made of the kinetics of the hydrogenation of mixtures containing 2 % of acetylene in ethylene on an alumina supported palladium catalyst in a static system. Information has been obtained about the nature of the selectivity of the reaction of acetylene. Detailed analyses of the kinetics have been made as the ratio of the pressure of acetylene to ethylene decreases and subsequently as the remaining hydrogen is used up.The poisoning action of carbon monoxide, which selectively inhibits the hydrogenation of ethylene, has also been studied.Tracer studies using carbon-13 labelled acetylene, light ethylene and hydrogen with analysis by combined gas chromatography-mass spectrometry, have shown unambiguously that ethane produced from the hydrogenation of a mixture of acetylene in ethylene comes predominantly from the ethylene.The reaction between acetylene, ethylene and deuterium gives a greater understanding of the surface processes involved and produces ethane which is mainly [2H2]ethane.It is postulated that two types of site exist on the surface, type X which hydrogenates both acetylene and ethylene and on which acetylene is adsorbed ∼2200 times more strongly than ethylene at 293 K and type Y which is easily poisoned by carbon monoxide and can hydrogenate ethylene even in the presence of acetylene.

The mechanisms of dimethylbutene isomerization over alumina

Reactions of dimethylbutenes over pure and chlorided γ-alumina have been studied in the presence of selective poisons and/or with various host molecules for deuterium label. The extent of H2S poisoning has been used to determine the relative roles of π-allylic species and alkyi carbonium ions in the isomerization of the 2,3-dimethylbutenes on alumina at 273 K. The absence of poisoning by H2S of skeletal isomerization of 3,3-dimethylbut-l-ene at 352 K is evidence that H2S does not inhibit reaction through carbonium ions whereas it is known to poison activity involving π-allylic species. The isomerization of 3,3-dimethylbut-l-ene in the presence of suitably labeled alkenes showed that vinylic and allylic hydrogen atoms can equilibrate with Bronsted-acid-type hydrogen on alumina at about 273 K. But adsorbed vinyl species have been found to retard skeletal isomerization, suggesting identity, or at least close proximity, of the sites for Bronsted acid reactions and vinylic CH exchange. Catalyst chloriding enhanced isomerization but retarded exchange of CH bonds with D. The influence of activation temperature and of impurities is discussed for reactions of dimethylbutenes over alumina.

Reactions of 3,3-dimethylbut-1-ene with deuterium oxide or deuterium over oxide catalysts

Exchange reactions of 3,3-dimethylbut-1-ene (I) with D2O have been studied over four X-type zeolites, silica-alumina and magnesium oxide, and with D2 over some of the catalysts. The isomerization of I, in the presence of D2O or D2, to form 2,3-dimethylbut-1-ene (II) and 2,3-dimethylbut-2-ene (III) was also followed. These reactions are useful in establishing the properties of oxide catalysts for reactions of olefins. For systems where carbonium ions are formed and where D+ is readily available, e.g., zeolites with D2O, isomerization of I to II and III is observed, accompanied by extensive exchange of the C6 olefins. For systems which do not readily form carbonium ions, e.g., MgO with D2, the main reaction is the exchange of the vinyl hydrogen atoms of I without isomerization. The choice of D2 or D2O as the source of the labeling isotope has a profound effect on the character of the reaction observed with some of the catalysts.

Ammonia synthesis and related reactions over iron-cobalt and iron-nickel alloy catalysts. Part I. Catalysts reduced at 853 K

Abstract Studies have been undertaken of ammonia synthesis, nitrogen chemisorption and nitrogen isotope exchange over a series of iron-cobalt and iron-nickel alloy catalysts containing 3% of alumina as a structural stabiliser. Catalysts were reduced for 18 h at 853 K before use and metal surface areas were measured by adsorption of carbon monoxide. While nitrogen chemisorption could be observed below 673 K the rate decreased with coverage so markedly that temperatures of 773 K or more were required to attain adsorption equilibrium within a reasonable period. Ammonia synthesis was observed from 550 K but at 700 K or over the rate of synthesis was affected by the back reaction becoming significant. Higher temperatures in the range from 773 K to 853 K were required to give measurable rates of nitrogen isotope exchange. Thus the temperatures for ammonia synthesis correspond to the start of nitrogen chemisorption and those for exchange to the higher temperatures needed for reversible adsorption and the establishment of the adsorption equilibrium. The pattern of activity with composition for the cobalt alloys showed a small maximum at a composition of iron (95%) cobalt (5%) for all three processes (adsorption, synthesis and exchange). A similar but less well-defined maximum was found with catalysts containing from 5% to 15% of nickel. Possible reasons for the enhanced activity of these alloys are discussed.

Reactions of n-butane and 2,2-dimethylpropane on silica-supported Rh-Pt bimetallic catalysts

Two series of highly dispersed RhPt bimetallic catalysts supported on high-area silica have been prepared, characterized by adsorption of H2 and CO and used in pulse, flow, or static reactor systems for the hydrogenolysis of n-butane and 2,2-dimethylpropane. Activity decreased markedly with increase of the Pt content of the catalysts by factors of 200 to 600 for the hydrogenolysis of 2,2-dimethylpropane and by factors of 4 × 103 to 104 for the reaction of n-butane. Both series of catalysts showed the same trends in the selectivity of the reactions with metal composition but exhibited some minor differences in absolute activity. The types of selectivity noted were the relative extent of multiple-bond cleavage to single-bond cleavage, the preferred position of bond cleavage with n-butane and the relative rates of isomerization and hydrogenolysis. Some comments are made on probable mechanisms of the reactions.

Hydrogenolysis of saturated hydrocarbons on evaporated platinum films

The reactions of ethane, propane, n-butane, isobutane and neopentane have been examined in the presence of deuterium on platinum films. Details of the products of exchange, hydrogenolysis and, where possible, isomerization are reported. The kinetic behaviour of the hydrogenolysis reaction showed a trend in that reactions of ethane were acceleratory, those of propane maintained a constant rate and those of molecules with four or more carbon atoms decelerated with time. The retardation in rates was much more marked with branched chain than with linear hydrocarbons.The results are discussed in terms of the nature of the adsorbed species accumulating on the surface, species possessing some carbonium ion character being likely precursors to the material responsible for the observed poisoning. Little additional mechanistic information was obtained from an analysis of the deuterium contents of the various molecules as exchange was fast compared to hydrogenolysis or isomerization, leading to rapid isotopic equilibration.A fragmentation correction scheme taking into account the influence of molecular composition on bond rupture has also been developed for analysis of the mass spectral data.

Some catalytic exchange reactions with deuterium oxide on evaporated metal films

Abstract Exchange reactions of hydrogen with deuterium oxide have been studied over metal films. The observed pattern of activity, which is Pt >2; Rh > Pd ⪢ Ni, is inversely related to the strength of adsorption of oxygen on the metals. Exchange reactions of propene with deuterium oxide have been studied on films of the same metals and on alumina-supported nickel. Severe poisoning occurred with platinum films but the other catalysts gave constant activity for the exchange which was predominantly stepwise. All hydrogen atoms in the olefin were replaced at similar rates, and the mechanism was thought to involve the reversible formation of adsorbed alkyl radicals. Similar results were found with ethylene and deuterium oxide on nickel films; both olefins showed some self-hydrogenation in addition to exchange on these films. Methane and propane did not exchange with deuterium oxide on nickel films but underwent reforming reactions at temperatures above 573 K.