Decomposition of ethane and its reaction with CO2 over Rh/ZSM-5 catalyst

The interaction of ethane with Rh/ZSM-5 and its decomposition and reactions with CO2 on Rh/ZSM-5 have been investigated. Methods used were Fourier-transform infrared spectroscopy and temperature-programmed desorption and reaction (TPD and TPR). The decomposition of ethane and its reaction with CO2 have been studied in a fixed-bed continuous-flow reactor. IR measurements showed that ethane interacted strongly with the highly dispersed Rh above 206 K and gave rise to the formation of ethylidyne surface species very likely through the transient formation of ethylene. At 523-573 K, the decomposition of ethane produces hydrogen, methane, and propane. Above 623 K ethylene became the main product, but benzene and toluene were also detected. Independent of the temperature, the rate of the decomposition decayed after 5-10 min to a very low level (1-2% conversion), but it did not cease completely even after several hours (673 K). The reactivities of surface carbon formed at different temperatures toward H-2, O-2, and CO2 have been examined. Carbon exhibited the highest reactivity with O-2 and less reactivity with CO2. The peak temperatures of its reaction in TPR shifted to a higher temperature with the temperature of its production in all the three cases. Carbon formed at 773 K in the ethane decomposition reacted with CO2 at maximum rate at 973 K. The reaction between C2H6 and CO2 occurred rapidly above 700 K to give mainly H-2 and CO with a ratio of 0.3-0.6. In contrast with the CH4 + CO2 reaction on the same catalyst, a significant deactivation of the catalyst occurred at the stoichiometric CO2/C2H6 ratio. This feature is attributed to the low reactivity of hydrocarbon fragments formed by the decomposition of ethane compared to those produced by CH4 dissociation. Deactivation can be decreased or almost ceased by using a large excess of CO2. (C) 1999 Academic Press.