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

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°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.