R. Maurel

Platinum-rhenium-alumina catalysts: III. Catalytic properties

Abstract The activity of PtRe catalysts on α- or γ-alumina has been determined for various reactions. In no case is the activity of bimetallic catalysts the sum of the activity of platinum and rhenium of which the catalyst is composed. Curves of activity as a function of the composition of the catalyst exhibit one or two maxima in benzene hydrogenation, benzene-deuterium exchange, cyclopentane, and butane hydrogenolysis, those maxima being more or less pronounced according to the reaction and the support. Conversely, the rate of 1,1,3-trimethyl cyclohexane dehydrogenation decreases when the percentage of rhenium is increased. A pronounced analogy has been established between, on the one hand, the changes of activity with bimetallic composition, and, on the other hand, with the position of the metal in the Periodic Table for the monometallic period Pt, Ir, Os, Re deposited on alumina.

Platinum-rhenium-alumina catalysts: II. Study of the metallic phase after reduction

Abstract Two percent ( Pt + Re ) Al 2 O 3 catalysts prepared by co-impregnation of Al2O3 with H2PtCl6 and Re2O7 and generally reduced by hydrogen at 500 °C were mainly investigated by thermogravimetry (H2O2 cycles at 25 °C), electron microscopy, and infrared spectroscopy (chemisorption of CO at 25 °C). The greater reducibility in the presence of Pt of oxygen chemisorbed by Re, and the vCO frequency shifts of the PtCO and ReCO species against the percentage of Re indicated a strong interaction between both metals. Most probably Pt and Re are alloyed. The H2O2 cycles at 25 °C allowed the determination of the overall dispersion of the (Pt + Re) phase while the optical density of the infrared band of CO adsorbed on platinum gave some information about the surface concentration of Pt in the (Pt + Re) phase.

Platinum-rhenium/alumina catalysts: I. Investigation of reduction by hydrogen

Abstract Pure Re2O7 when heated in a hydrogen atmosphere, sublimes before reduction. However, when Re2O7 is mixed with Pt or Pd or Re powder, instead of volatilization complete reduction to metallic Re occurs below 200 °C. A strong activation by Pt of the reduction of Re2O7 also takes place on Al2O3 samples coimpregnated with H2PtCl6 and Re2O7. With such catalysts, the reduction of Re2O7 to metallic Re under 1 atm pressure of hydrogen seems to be complete at a rather low temperature.

Steam dealkylation of aromatic hydrocarbons: II. Role of the support and kinetic pathway of oxygenated species in toluene steam dealkylation over group VIII metal catalysts

Abstract The role of the support in steam dealkylation (SDA) is studied on a series of Group VIII metal catalysts supported on alumina, silica, and titania. When possible turnover frequencies are given on the basis of the free metal fraction during the reaction. The values are generally constant with time-on-stream and represent the actual turnover frequency of the catalyst. Metals can be classified into two groups, namely, support-sensitive metals (Pt, Rh, Pd) and support-insensitive metals (Ni, Co, Ru, and to a certain extent Ir). Support sensitivity is related to the oxidizability of the metallic surface. For metals of the first group, the reaction is probably governed by a noncompetitive mechanism in which the metal coverage by the oxygenated species is negligible. Kinetic derivation leads to a rate law where there is at once intervention of the support site concentration and of the specific perimeter of the metal/support interface. One can thus explain the support effect for this metal group and the slight sensitivity to the crystallite size observed in the Rh Al 2 O 3 series. For metals of the second group, a competitive mechanism probably takes place on the metal. Kinetic derivation leads to a rate law independent of the support site concentration and accounting for the slight negative order with respect to toluene as previously reported. The conspicuous parallelism between the selectivities of the various metals in SDA, in hydrodealkylation, and in hydrogenolysis is also discussed. In addition to the metal, the support and the crystallite size are determining factors of the selectivity to benzene in SDA.

Activity of metallic catalysts: IV. Influence of the nature of the support and effect of sulfur-containing poisons on two examples of “demanding reactions”☆

Platinum catalysts were prepared from two transition aluminas with respective surface areas of 120 and 180 m2g and two α-aluminas. Their metallic surface areas were varied by thermal treatment at various temperatures. Their activities with respect to the hydrogenation of benzene (“facile reaction”), the hydrogenolysis of cyclopentane (“demanding reaction” of the first type) and the exchange between benzene and deuterium (“demanding reaction” of the second type) were measured. It is shown that the effect caused by the support does not arise from a difference in crystallinity, but rather from a selective poisoning brought about by the reduction of sulfate, in certain aluminas. Moreover, it is shown that certain poisons such as H2S and SO2 are “nonselective,” i.e., they have the same effect on the three reactions in the same way. On the contrary, the mixture H2S + SO2, which produces atomic sulfur, is a selective poison which reduces the rate of the hydrogenolysis of cyclopentane more than that of the hydrogenation of benzene, whereas the exchange between benzene and deuterium is far less affected than the hydrogenation. The cause of this selective poisoning is discussed.

Hydrogenolysis of saturated hydrocarbons: III. Selectivity in hydrogenolysis of various aliphatic hydrocarbons on platinum/alumina

The reactions with hydrogen of various aliphatic hydrocarbons ranging from C4 to C8 have been studied over PtAl2O3 as catalyst. The main reaction is hydrogenolysis, parallel isomerization and dehydrocyclization being very slow if the hydrocarbon chain is less than four carbon atoms long. Whenever 1,5-diadsorbed species can be formed, the rates of isomerization and dehydrocyclization are increased. The rates of hydrogenolysis of a variety of different carbon-carbon bonds have been measured and tabulated. The reactivity of a bond depends not only on the substitution of the two carbon atoms involved, but also on the substitution of the neighboring atoms. Bonds in the β position to a tertiary carbon atom are extensively broken. On the contrary, with a quaternary carbon atom, the hydrogenolysis in the α position to this atom is more rapid than in the β position. The results cannot all be rationalized by a single reaction mechanism, but support the view that 1,2-, 1,3-, 1,4-, and 1,5-diadsorbed species can all play a role in reaction between hydrogen and hydrocarbons on platinum.

Hydrogenolysis of saturated hydrocarbons: II. Comparative hydrogenolysis of some aliphatic light hydrocarbons on platinum-alumina

Abstract Kinetics of catalytic hydrogenolysis of ethane, propane, butane, isobutane, neopentane and isopentane on PtAl2O3 have been investigated in a flow reactor at atmospheric pressure. These hydrocarbons are arranged according to increased rates of hydrogenolysis at 300 °C in the series: ethane, propane, neopentane, isobutane, butane and isopentane. The kinetic orders and apparent activation energies of the breaking of different types of CC bonds have been determined. The exponents of hydrocarbon partial pressure are always positive, while the exponent of hydrogen pressure can be positive or negative, and in some cases the rate is maximum for a certain hydrogen pressure. All these observations are consistent with the kinetic scheme proposed by Cimino, Boudart and Taylor [J. Phys. Chem.58, 596 (1954)]. In terms of these hypotheses an extensively dehydrogenated surface species has been found for all hydrocarbons investigated. The rate constant of the rupture of the carbon-carbon bond is not much changed with the hydrocarbon. On the other hand, the adsorption equilibrium is greatly influenced by the structure of the hydrocarbons: the adsorption equilibrium constant increases with the molecular weight in a homologous series, but for an equal number of carbon atoms, it decreases when branching increases.

Selective steam reforming of aromatic hydrocarbons: IV. Steam conversion and hydroconversion of selected monoalkyl- and dialkyl-benzenes on Rh catalysts

Abstract Steam conversion and hydroconversion of a series of monoalkylbenzenes (C 6 H 5 R, R = C 2 H 5 , n -C 3 H 7 , i -C 3 H 7 , tert -C 4 H 9 ) and of dialkylbenzenes ( o - and p -xylenes) are studied at 713 K and atmospheric pressure on supported rhodium catalysts ( Rh Al 2 O 3 , Rh SiO 2 and Rh TiO 2 ), and compared to the toluene steam dealkylation previously studied on the same catalysts. Three types of reaction, namely dealkylation (CC rupture on the side chain), dehydrogenation (on the side chain), and degradation (i.e., ring opening) account for virtually the whole product spectrum. Isomerization, transalkylation, and dehydrocyclization reactions may, in general, be discounted. In the presence of steam, the main initial product of monoalkylbenzene dealkylation is invariably benzene, but the splitting of the CC bonds in the middle or end of the side chain always increases with conversion. As a rule, the specific activities (per metal site) in dealkylation decrease with the degree of substitution in the alkyl group (primary > secondary > tertiary > quaternary carbon). On the other hand, the specific activities in ring opening remain constant for all the hydrocarbons and even for the benzene. In the presence of hydrogen, multiple CC bond splittings are invariably observed and benzene is no longer, in general, the principal initial product. The activities in ring opening are equally constant, but at a lower level than in steam conversion. These results are in overall agreement with the model of the dual active sites: sites I appear operative for dealkylation and dehydrogenation, whereas ring opening takes place at sites II with a high probability, independent of the alkyl group size. Possible adsorbed species on each type of site are described. An attempt is made to rationalize the effects of assorted selectivity-determining factors (metal particle size, support effects, selective poisons such as S and CO) in terms of electronic or geometric effects.

Selective Poisoning by Coke Formation on Pt/AL2O3

Several Platinum catalysts of differing metal dispersion have been coked by cyclohexane at 450°C and characterized by carbon analysis and by their activities in respect to the hydrogenation of benzene, isotopic exchange between benzene and deuterium and hydrogenolysis of eyelopentane. (change-para-here) The thermal programmed oxidation of coke by oxygen was studied in the 0-500°C range. Two peaks were observed. The first at 200°C could be ascribed to metal deactivation when carbon deposited on alumina could bring about a second peak at 380°C. (change-para-here) On the other hand, deactivation by coke formation is a selective poisoning which reduces the rate of the hydrogenolysis of cyclopentane more than that of the hydrogenation of benzene, whereas the exchange between benzene and deuterium is far less affected than the hydrogenation. Thus coke formation on platinum would preferably occur on some special sites.

Isotope effects in the hydrogenation and exchange of benzene on platinum and nickel

Abstract The rates of hydrogenation and deuteration of benzene and perdeuteriobenzene have been measured at 85 °C and atmospheric pressure in a circulation flow system over a number of platinum/alumina and nickel/silica catalysts. At the same time the degree of exchange of the hydrogen or deuterium atoms in the benzene molecule has been measured mass spectrometrically. Surprisingly no isotope effects were found for hydrogenation and deuteration of benzene on platinum. On nickel only a slight effect was measured. So far no explanation can be given for this absence of isotope effects. On platinum and nickel the rate of the exchange reaction between C 6 H 6 and D 2 appears to be six times faster than the exchange reaction between C 6 D 6 and H 2 . With statistical-thermodynamical calculations it is shown that the dissociative π-complex substitution mechanism is able to explain the kinetic isotope effect, whereas the associative mechanism is not.