Geoffrey Webb

The formation and role of carbonaceous residues in metal-catalysed reactions of hydrocarbons.

Abstract In the course of this review some of the evidence for the formation of surface carbonaceous residues resulting from the interaction of hydrocarbons with metal surfaces, the mechanism of formation of such residues and, finally, the effects of the presence of the carbonaceous deposits on the catalytic behaviour of the surface is examined. Particular attention is paid to the adsorption and hydrogenation of unsaturated hydrocarbons and to the reforming of hydrocarbons in the context of the refining of petroleum naphthas. A brief mention is also made of the formation and role of carbonaceous residues in reactions of carbon monoxide in the presence of hydrogen from the standpoint of hydrocarbon synthesis and the production of oxygenated products.

Carbonaceous deposition associated with the catalytic steam-reforming of hydrocarbons over nickel alumina catalysts

The reactions of n-pentane, n-hexane, n-heptane, 1-heptene, cyclohexane, benzene, toluene, ethylbenzene, and the xylenes with steam have been investigated using 75% w/w nickel/alumina catalysts at 475 °C, with particular reference to the formation of surface carbonaceous deposits. Using steam/hydrocarbon ratios of between 0 and 15, up to four distinct types of surface carbonaceous deposit have been identified. Extensive carbon filament formation is observed when no steam or low steam concentrations are used. A hydrocarbonaceous polymeric species, extractable from the used catalysts with tetrachloromethane, is formed with each hydrocarbon under all reaction conditions. The chemical identity of the polymer, established by infrared spectroscopy and mass spectrometry, is apparently independent of the reactant hydrocarbon. Evidence for a reactive and a nonreactive surface carbon has also been obtained. Admission of [14C]CO or [14C]CO2 with the reactant hydrocarbon leads to scrambling of the [14C]label among the reaction products and the surface deposit. A mechanism for the formation of both the gaseous reaction products and the various surface residues is discussed in terms of the adsorbed intermediates. The use of carbon monoxide adsorption as a probe for exposed surface nickel shows that, although the exposure of the nickel surface is markedly reduced by the formation of surface deposits, the activity of the catalyst for the gasification of the hydrocarbons is virtually unaffected. The possibility of reaction occurring on a carbonaceous overlayer on the nickel surface is discussed.

Deactivation of bi- and multimetallic reforming catalysts: influence of alloy formation on catalyst activity

Abstract A number of bi- and multimetallic reforming catalysts including Pt–Re, Pt–Ir, Pt–Sn, Pt–Ge and Pt–Ir–Ge were prepared and studied in this work. The alloy particles formed in these systems were analysed using transmission electron microscopy (TEM) and energy dispersive X-ray analysis (EDX). n-Octane reforming experiments were also performed using a pressurised microreactor system. Tin and germanium were found to modify the properties of these catalysts mainly via a geometric effect. However, formation of bulk Pt–Sn and Pt–Ge alloys contributed to the overall rate of deactivation of these systems. In the case of Pt–Ir and Pt–Re, metallic rhenium and iridium provided sites for the hydrogenolysis of coke deposits resulting in improved resistance to deactivation.

The enantioselective hydrogenation of methyl acetoacetate over supported nickel catalysts: I. The modification procedure

Enantioselective Ni/SiO2 catalysts have been prepared by modification with (R)-(+)-tartaric acid (TA) and used in the asymmetric hydrogenation of methyl acetoacetate (MAA) to (R)-(−)-methyl-3-hydroxybutyrate (MHB). The effects on the overall asymmetric activity of systematically varying modification conditions, namely time, temperature, TA concentration, and pH, are discussed. The amounts of TA adsorbed have been correlated with conversion and selectivity and an optimum surface coverage of 0.2 was identified. The nature of the interaction of TA with the surface nickel metal is considered and data on the corrosive leaching and the changes in metal dispersion are presented. The consequences of adding NaBr as a subcomponent of the modification solution are examined and the results are discussed in terms of surface coverage and nonselective site poisoning. The results obtained reveal the interrelated effects of each modification variable on surface TA coverage and ultimately on enantiomeric selectivity.

Hydrogenation of acetylene over supported metal catalysts. Part 1.—Adsorption of [14C]acetylene and [14C]ethylene on silica supported rhodium, iridium and palladium and alumina supported palladium

The adsorption of [14C]ethylene and [14C]acetylene on supported palladium, rhodium and iridium catalysts occurs irreversibly at 298 K in two distinct stages; a non-linear primary region, in which the species are predominantly dissociatively adsorbed, and a linear secondary region. Hydrogenation catalysis is associated with the hydrocarbon species adsorbed on the secondary region. From [14C]carbon monoxide adsorptions it is concluded that the hydrocarbon primary region is associated with the metal, whilst the secondary region probably involves the formation of overlayers on the primary adsorbed species. The co-adsorption of ethylene and acetylene shows that, under acetylene hydrogenation conditions, both are adsorbed at separate sites and undergo hydrogenation independently of each other. The relevance of these observations to the selective hydrogenation of acetylene is discussed.

Halogen-promoted γ-alumina. I, The interactions of 36Cl- and 14C-labelled carbon tetrachloride and of 36Cl- and 2H-labelled hydrogen chloride with γ-alumina

Abstract The behaviour of [ 36 Cl]-chlorine and [ 14 C]-carbon labelled carbon tetrachloride at 500 K and of [ 36 Cl]-chlorine and [ 2 H]-hydrogen labelled anhydrous hydrogen chloride at 293 K towards calcined γ-alumina has been investigated under static conditions. Two types of surface chlorine species are formed using CCl 4 ; one type is inert with respect to [ 36 Cl]-chlorine exchange with anhydrous HCl at 293 K, the other is labile and behaves identically to the surface species resulting from treatment with HCl. Radiotracer and neutron activation analyses indicate that more chlorine is retained by samples treated with CCl 4 under the conditions used. The results of isotopic exchange reactions together with an examination of the CCl 4 reaction using IR spectroscopy and 27 Al MAS NMR spectra of chlorinated γ-alumina are used to make proposals about the nature of the acidic sites formed.

The adsorption and hydrogenation of benzene and toluene on alumina- and silica- supported palladium and platinum catalysts

Abstract The adsorption and hydrogenation of benzene and toluene has been studied in the temperature range 378–608 K over alumina- and silica-supported platinum and palladium catalysts, using a microreactor technique. Over each of the catalysts, in the absence of hydrogen, adsorption of both hydrocarbons leads to the formation of retained species, which, as shown by [ 14 c]labelling experiments, are not active in either hydrogenation or molecular exchange. Retention is also observed when the aromatic hydrocarbon is hydrogenated over the platinum catalysts, but not when palladium catalysts are used. In the hydrogenation of benzene and toluene over both the palladium and platinum catalysts, the activity of the catalyst passes through a maximum as the temperature is increased, the position of the maximum depending upon the catalyst and the hydrocarbon. Hydrogenation of [ 14 c]benzene in the presence of cyclohexane shows that this behaviour is a kinetic, rather than a thermodynamic, effect. No hydrogenolysis products are observed with either catalyst. Platinum has a higher specific activity than palladium for both benzene and toulene hydrogenation. The results are interpreted in terms of dissociative adsorption leading to the formation of retained secies, whilst the species active in hydro-genation is associatively adsorbed. Hydrogenation of mixtures of benzene and toluene shows that the latter is more readily hydrogenated over both metals. It is suggested that the sites for the adsorption and hydrogenation of benzene are probably distinct from those involved in toluene adsorption and hydrogenation.

Radiotracers in fluorine chemistry: Part XVHalogen exchange and isomerization reactions involving chlorofluoroethanes on fluorinated chromia surfaces labelled with anhydrous hydrogen [18F]-fluoride, hydrogen [36Cl]-chloride or 1,1-di[36Cl]chlorotetrafluoroethane

Abstract Series of fluorination and chlorination reactions are observed when CCl2FCClF2 or C2Cl2F4 isomers are flowed at ca. 700 K over fluorinated chromia catalysts that have been labelled by prior treatment with H18F or H36Cl. These reactions result in the incorporation of the radiolabel in each component of the mixtures of C2Cl6-nFn that are formed. The probability of a radiolabel, [18F] or [36Cl], in a given component correlates directly with the degree to which that component has been fluorinated or chlorinated on the surface. The isomerization of CCl2FCClF2 to CCl3CF3 is an important reaction on fluorinated chromia at 700 K; [36Cl] tracer experiments indicate that the process is intramolecular. Isomerization of CClF2CClF2 probably occurs also but is less dominant in the overall behaviour observed.

Hydrogenation of acetylene over supported metal catalysts. Part 2.—[14C]tracer study of deactivation phenomena

In the hydrogenation of acetylene over Pd + SiO2, Rh + SiO2, Ir + SiO2 and Pd + Al2O3, the activities of the catalysts progressively decrease to a steady state constant activity. This deactivation is irreversible at 298 K and can only be effected by acetylene + hydrogen reaction mixtures; acetylene alone produces no significant deactivation. The selectivity and overall reaction orders are independent of the catalytic activity. Adsorptions of [14C]C2H2 and [14C]C2H4 show that the deactivation is associated with the progressive build up of permanently retained acetylenic species on the catalyst surface. Three types of adsorbed acetylene are recognised. The results are interpreted in terms of the hydrogenation reaction occurring by a hydrogen-transfer mechanism between a dissociatively adsorbed C2Hx species and associatively adsorbed acetylene, which forms an overlayer on the dissociatively adsorbed acetylene. The permanent retention of acetylenic species may be due to surface polymer formation.

Halogen-promoted γ-alumina Part II. The catalytic dehydrochlorination of 1,1,1-trichloroethane at room temperature on chlorinated γ-alumina

Abstract The dehydrochlorination of 1,1,1-trichloroethane vapour to give 1,1-dichloroethene and hydrogen chloride is catalysed by γ-alumina which has been chlorinated with carbon tetrachloride or carbonyl chloride at 500 K. Chlorination of γ-alumina with hydrogen chloride does not result in a catalyst for this reaction. 1,1-Dichloroethene reacts further to produce a purple, oligomeric material on the surface of chlorinated γ-alumina. The overall reaction of CH 3 CCl 3 has been followed using IR spectroscopy and [ 36 Cl]-chlorine as a radiotracer. The results obtained are consistent with dehydrochlorination occurring at strong Lewis acid surface sites, although the chlorine associated with these sites is not involved directly in the reaction.