Transient Kinetic Study of Ethane and Ethylene Aromatization over Zinc-Exchanged HZSM-5 Catalyst

The conversion of ethane and ethylene to aromatics over a zinc-exchanged HZSM-5 catalyst has been studied by chemical transient kinetic analysis in a flow reactor under atmospheric pressure. Time-dependent early stage/build-up catalytic behavior and the kinetic parameters of products decay during the back-transient have been discussed extensively at different reaction temperatures. For both ethane and ethylene aromatization, the delay times (relevant to the initial appearance of the reactant in the gas phase) of the formation of aromatics are 10-15 s during the build-up, which decreases with increasing temperature. On the basis of quantitative back-transient analysis, the rate constants for the formation of benzene, toluene, and/or xylene range from 5 to 30 X 10(-3) s(-1) dependent upon the reaction temperatures. According to the Arrhenius equation, the activation energies for the formation of benzene and toluene during ethane aromatization are 57 and 73 kJ/mol, respectively, which are relatively higher than the activation energies from ethylene aromatization.