P. Tétényi

The mechanism of aromatization on platinum black catalyst; dehydrocyclization of hexadienes and hexatrienes

Abstract The aromatization of hexadiene isomers as well as trans - and cis -1,3,5-hexatriene on Pt-black catalyst has been studied in the presence of helium and hydrogenhelium mixtures. The extent of benzene formation from all of the hydrocarbons was of the same order of magnitude, and the activity of the catalyst rapidly decreased when subsequent pulses of hydrocarbons were introduced without regeneration. The only exception was cis -triene where the benzene yield was much higher and deactivation slower. The presence of hydrogen even at small partial pressure considerably slowed down the deactivation. The role of hexatrienes as intermediates of aromatization has been evaluated in the case of different hexadiene isomers. A stepwise scheme of aromatization involving steps of dehydrogenation and cis-trans isomerization has been proposed. The importance of these two processes is considered to be about the same in the overall reaction rate, whereas the ring closure step is regarded as very fast. The precursors of coke formation on the catalyst surface are supposedly trans -polyenes. The relative importance of benzene and coke formation are determined by the presence of hydrogen in the gas phase.

Comparative study of various platinum catalysts in skeletal reactions of C6-hydrocarbons

Four platinum catalysts, Pt-black, PtC, PtSiO2, and PtAl2O3, have been compared in skeletal reactions of 3-methylpentane (3MP) and methylcyclopentane (MCP), in the presence of various partial pressures of hydrogen. Analogous hydrogen effects (defined as primary and secondary) have been observed for each catalyst. The selectivities could be explained almost exclusively by the influence of hydrogen. The selectivity of isomerization plus C5-cyclization vs fragmentation increased with increasing hydrogen pressure as did the ratio of saturated vs unsaturated C6 products. The 2-methylpentane vs n-hexane ratio (2MP/n-H) from both 3MP and MCP exhibited a strong hydrogen dependence (permitting separation of bond shift and C5-cyclic isomerization as a function of the hydrogen pressure), and significant crystallite size effects were also observed here.

Kinetics and mechanism of ethane hydrogenolysis on silica-supported platinum and platinum-iron catalysts

A kinetic analysis of ethane hydrogenolysis has been carried out on PtSiO2 and PtFeSiO2 catalysts with a wide range of concentration of the components, and with an excess of ethane. With the help of the theory of stationary reactions on heterogeneous surfaces, kinetic equations were obtained describing the reaction rate both in excess hydrogen and excess ethane which are in full agreement with the experimental observations. On platinum in excess hydrogen the rate of hydrogenolysis is determined by the CC bond rupture of ethane adsorbed in a mildly dissociated C2Hx form, while in excess hydrocarbon it is determined by the CC bond rupture of ethane adsorbed in the deeply dissociated form of C2H2. The reaction rate passes through a maximum vs ethane pressure at constant hydrogen pressure and vice versa. The formal reaction order in hydrogen and hydrocarbon can be either positive or negative depending on the conditions; the formal power-rate equations may be considered as approximations of the more complicated equations presented in this work.

Catalytic transformations of cyclohexanol on Group VIII metal catalysts

Abstract Transformations of cyclohexanol and cyclohexanone have been studied over various Group VIII metals as catalysts. For each metal the predominant reaction of cyclohexanol was dehydrogenation to cyclohexanone. Two main groups of metals can be distinguished. Selectively dehydrogenating metals are those where dehydrogenation stops at the stage of cyclohexanone (Os, Co, Fe, Re, Ru). Aromatizing metals catalyze also the further dehydrogenation of cyclohexanone to aromatics (Pd, Pt, Ni). Rh and Ir occupy an intermediate position: they dehydrogenate in nitrogen and aromatize in hydrogen. Radiotracer methods show that cyclohexanone is the intermediate of phenol formation, except for Pt and Pd where there is a “direct” route of phenol formation from cyclohexanol. Benzene is the product of the hydrogenolytic splitting of the phenolic OH group. Dehydration of cyclohexanol to cyclohexene is not important, although it occurs over some dehydrogenating metals. Ru is the only metal where there is considerable additional formation of benzene via cyclohexene. Hydrogenolysis of the alcoholic OH group was not observed. Hydrogenolysis of the CC bond of the ring is favoured by hydrogen carrier gas; it is considerable over dehydrogenating metals as well as Rh and Ir. The enhanced reactivity of cyclohexanol as compared with cyclohexane is due to the presence of the OH group facilitating the interaction of the molecule with the surface. Knors model of localized/free-electron interplay as well as the number of unpaired d electrons could be used to interpret the different activity of various metals.

Thermodynamics and kinetic effects in platinum-catalyzed n-hexane transformations

Abstract Equilibrium concentrations of n -hexane, isohexanes, methylcyclopentane, and benzene have been calculated and compared with experimental conversion data in the presence of a platinum catalyst. The results can be interpreted in terms of equilibration between n -hexane and each of two types of intermediates leading to saturated C 6 products and benzene, respectively, at a hypothetical “effective surface hydrogen pressure” which is higher than that measured in the gas phase.

The mechanism of catalytic hydrogenolysis of ethane over nickel

Hydrogenolysis of ethane was investigated on nickel powder catalyst from 170 to 320 °C. The reaction was first order with respect to ethane and it was inhibited by hydrogen at sufficiently low temperature whereas at high temperature it was independent of hydrogen pressure. This behaviour as well as the relation among the energies of activation of ethane exchange, methane exchange and ethane hydrogenolysis (20, 30 and 39 kcal mole−1, respectively) give further evidence for the validity of the kinetic picture described by Sinfelt and Taylor. The application of deuterium and tritium in place of hydrogen offers new information for the mechanism of hydrogenolysis. The distribution of deuterated methane shows that the initial step for ethane exchange and hydrogenolysis is probably common and the adsorption with CC bond rupture may be ruled out. In comparison with methane-deuterium exchange under the same condition, clear evidence was obtained that methane desorption is fast and may not be a ratelimiting step. For this direct proof has not been found in literature. An explanation is given for the change in distribution of deuterated methane formed in the hydrogenolysis taking place with variation of temperature and activity of catalyst.

Conversion of thiophene on molybdena–alumina catalysts containing Group 8–10 metals: effect of H2S uptake

Ageing and the effect of H2S on the activity and selectivity of Co-, Ru-, Pd-, Ir-and Pt-promoted and unpromoted MoOx–Al2O3 catalysts have been studied in relation to decomposition of thiophene. The overall activity increased on unpromoted and Pd-promoted molybdena–alumina, whereas on other catalysts it decreased substantially. The selectivity of production of butane and of fragmented hydrocarbons decreased on all catalysts. The changes in catalytic behaviour were caused by the uptake of H2S added to the system and produced in hydrodesulfurization (HDS) of thiophene. The results indicate no direct correlation between the propensity to interact with H2S and the HDS activity. It follows from data on changes in selectivity, that metal and molybdena act predominantly independently at the early stages of sulfidation.

Investigations on the chemisorption of benzene on nickel and platinum

Abstract The chemisorption of 14C-labeled benzene on nickel, platinum, and copper catalysts at atmospheric pressure in the temperature range of 100–300 °C was investigated. The amounts of benzene chemisorbed were determined by radioactivity. In the case of different nickel catalysts the coverage with chemisorbed benzene was 0.2–0.6, in case of platinum 0.08. Chemisorption was not observed in the case of copper. The chemisorbed benzene could be fully removed only with hydrogen, benzene removed one part of it. It was found by special measurements that a small part of the compound desorbed with hydrogen was in the form of benzene, and the other part in the form of cyclohexane. New evidence of the dissociative adsorption of benzene was given by the obtained data. It was shown that there is no cause for metallic catalysts to be poisoned by chemisorbed benzene in the conditions of hydrogenation-dehydrogenation of six-membered rings.

Preparation and characterization of candidate catalysts for deep hydrodesulfurization of gasoils. Sulfidation and acidity characteristics of supported Ni/W and Ni/Mo catalysts

Abstract The uptake of 35 S -labelled H 2 S, the thiophene hydrodesulfurization (HDS) activity in pulse and flow system and the acidity measured by infrared (IR) band intensities of adsorbed pyridine were studied after various treatments over NiW/amorphous silica–alumina (ASA), NiW/Al 2 O 3 and NiMo/Al 2 O 3 catalysts. The Fourier Transformation Infrared (FTIR) measurements indicated that the NiW/ASA catalyst contained some Bronsted and much more Lewis acid sites, whereas less Bronsted acid sites were observed on the alumina supported samples. The concentration of Bronsted and Lewis acid sites decreased after sulfidation. The amount of total sulfur uptake of NiMo/Al 2 O 3 catalyst was about twofold of the one of NiW samples. The sulfidation level of the catalysts was much lower in the pulse system than in the flow one and also the thiophene conversion was much lower on NiW catalysts in the former setup than in the latter system. A band was detected in the radiochromatogram of NiW/ASA catalyst during desorption of H 2 S in the pulse system which was attributed to weakly adsorbed H 2 S on the Bronsted acidic sites. This weakly adsorbed H 2 S hindered the isomerization of n -butane in the flow system but not in the pulse system.

Hydrogenolysis of some saturated hydrocarbons over a platinum black catalyst

Deuterium exchange reactions of methane, ethane, propane and neopentane, and the hydrogenolysis and isomerization of ethane, propane, n-butane and isobutane on platinum black have been studied in the temperature range 363–743 K. There is a gap of 150 K between the temperature of deuterium exchange and that of hydrogenolysis and isomerization. Only at high hydrogen pressure (about 50 kN m–2) and at low catalytic activity is the hydrogenolysis characterized by selective loss of a terminal methyl group, except for n-butane in which there is an equal chance of any of the C—C bond rupturing. The selectivity for isomerization has a maximum at high hydrogen pressure. These values together with kinetic data suggest that the ability of platinum in an exchange reaction to convert weakly bonded intermediates into strongly bonded ones is much less than that of nickel. The phenomenon of π-olefin and π-allyl bond conversion is applied in part to explain the results obtained. Brief comparison is given between the catalytic properties of platinum and those of nickel for the reactions investigated.