Pierre Pareja

Low temperature catalytic homologation of methane on platinum, ruthenium and cobalt

Abstract Adsorption of methane on metallic surfaces can proceed with hydrogen evolution if the temperature is sufficiently high (above approximately 100°C). Under flowing methane the desorbed hydrogen is continuously removed and the surface becomes increasingly covered with H- deficient CHx species. On Pt, Ru and Co, interruption of the methane flow followed by hydrogen flush causes the release of CH4 as well as higher hydrocarbons ranging from C2 to C7. On this basis a cyclic procedure is described for the selective homologation of methane at low temperature.

The steady state catalytic activity of nickel in ethylene hydrogenation as essentially due to the promoting effect of oxygen traces

Abstract The catalytic activity of nickel in ethylene hydrogenation is shown to result quasi-completely from active sites created by the partial or entire simultaneous conversion into water of the oxygen traces carried by the reacting mixture. The ethylene hydrogenation and the water synthesis appear then to be coupled upon the catalyst surface, the former proceeding on the active centers generated by the latter. As the rate of disappearance of active centers increases with temperature whereas the total oxygen conversion is limited, a maximum occurs in the steady-state activity versus temperature. This scheme is proved by the disappearance of this optimum, with a considerable lowering of activity, following a drastic purification of the reacting mixture. The maximum and activity both reappear when the purification becomes less rigorous.

Isothermal conversion of methane into higher hydrocarbons and hydrogen by a two-step reaction sequence involving a rhodium catalyst

Abstract A two-step reaction sequence for converting methane to higher hydrocarbons and hydrogen was investigated on a 5%Rh–5%CeO 2 /SiO 2 catalyst. Isothermal cycles were carried out from room temperature to 275°C and produced alkanes up to hexanes. In contrast with previous observations on platinum and ruthenium, the conversion was possible even at room temperature and two maxima versus temperature were evidenced. The first maximum occurred at about 100°C while the second one took place beyond 200°C. A noticeable selectivity to n -pentane was also observed. This work includes a study of the phenomena occurring during the exposure of the catalyst to flowing methane at atmospheric pressure (evolution of hydrogen at T ≥100°C and of ethane at T ≥150°C). A determination of the amounts of methane chemisorbed under various conditions of exposure as well as the evaluation of the H/C ratios of the corresponding surface species was achieved. These quantities were derived from temperature programmed desorption (up to 300°C) followed by reaction with hydrogen at room temperature and further temperature programmed surface reaction with hydrogen. Interpretations in line with those put forward in the case of platinum and ruthenium are proposed.

Periodic operation of a catalyst as a means of overcoming a thermodynamic constraint. The case of methane homologation on metals

Abstract Periodic operation of a catalyst can be a way of overcoming a thermodynamic constraint. Homologation of methane is thermodynamically disfavored. However, a two-step procedure using metal catalysts under non-oxidative conditions allows the thermodynamic limitations to be circumvented. Metal catalysts, such as Pt, Co and Ru are exposed first to methane and then to hydrogen. In their dual-temperature procedure, van Santen et al. carry out the first step with dilute methane at a high temperature (usually 725 K), which allows the endothermic decomposition of methane to take place. Part of the C deposits may yield higher alkanes up to C4–C5 through the following hydrogenation at a much lower temperature (368 K). In contrast, we carry out these two steps at atmospheric pressure and at the same but moderate temperature (usually less than 570 K). In this case, chemisorption of methane is accompanied by release of hydrogen whereas coupling of H-deficient CHx adspecies may take place. Numerous higher alkanes up to C7-C8 are then removed by supplying hydrogen at ordinary pressure and at the same temperature as that of the first step. The driving force can be found in the energy which has to be supplied in order to compress part of the dilute hydrogen removed in the first step to make it usable in the second one. The influence of some key factors is studied.

Increasing the yield in methane homologation through an isothermal two-reaction sequence at 250°c on platinum

Abstract Appreciable yields of conversion ( >40%) of methane to higher alkanes can be obtained at 250°C when, after exposure of a Pt/SiO 2 catalyst to methane in a stirred batch reactor including a hydrogen trap, the reactor is switched into a hydrogen circuit.

Possibility of obtaining appreciable yields in methane homologation through a two-step reaction at 250 °C on a platinum catalyst

Methane is converted (>40% yield) to higher alkanes at 250 °C on exposure to a Pt/SiO2 catalyst and subsequent treatment with hydrogen.

Cyclohexane, methyl- and 1,2-dimethyl-cyclohexane as the major C2+ products of an oxygen-free CH4 conversion

Abstract Methane can be mostly converted to cyclohexane and its derivatives upon feeding a silica supported Ni and Ni–Cu catalysts with CH4 and H2 successively at temperatures around 275–300°C and under pressures equal to or higher than atmospheric pressure. Increasing the H2 pressure very strongly enhances the formation of heavier products. Increasing the CH4 pressure allows the first step of the sequence to be shortened but excessive shortening of the exposure prevents the adspecies from undergoing H2 loss to the extent required for the formation of heavier products.