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-tra nsform infrared spectroscopy and temperature-programmed desorption and reac tion (TPD and TPR). The decomposition of ethane and its reaction with CO2 h ave been studied in a fixed-bed continuous-flow reactor. IR measurements sh owed 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 decompositi on of ethane produces hydrogen, methane, and propane. Above 623 K ethylene became the main product, but benzene and toluene were also detected. Indepe ndent 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. Car bon exhibited the highest reactivity with O-2 and less reactivity with CO2. The peak temperatures of its reaction in TPR shifted to a higher temperatu re with the temperature of its production in all the three cases. Carbon fo rmed 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 catal yst occurred at the stoichiometric CO2/C2H6 ratio. This feature is attribut ed to the low reactivity of hydrocarbon fragments formed by the decompositi on of ethane compared to those produced by CH4 dissociation. Deactivation c an be decreased or almost ceased by using a large excess of CO2. (C) 1999 A cademic Press.