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

A two-step reaction sequence for converting methane to higher hydrocarbons and hydrogen was investigated on a 5%Rh-5%CeO2/SiO2 catalyst. Isothermal cycles were carried out from room temperature to 275 degrees 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 degrees C while the second one took place beyond 200 degrees 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 greater than or equal to 100 degrees C and of ethane at T greater than or equal to 150 degrees C). A determination of the amounts of methane chemisorbed under various conditions of exposure as well as the evaluation of the WC ratios of the corresponding surface species was achieved. These quantities were derived from temperature programmed desorption (up to 300 degrees 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. (C) 1998 Elsevier Science B.V.