(694g) Transient Kinetics Analysis of Ethane Aromatization over Metal Functionalized ZSM-5 Catalyst
Our effort on this study suggests that a synergy between metal function (for initial step ethane dehydrogenation) and zeolites acid function (for both dehydrogenation and aromatization) is important. However, the synergy between these two functions varies during the high temperature (550-650oC) reaction process due to the reconstruction of the metal species and coke deposition. Transient kinetics analysis shows a clear induction period in terms of the formation of aromatics, but such induction period was less significant for the formation of olefins. The delay time in terms of benzene formation is ~120 seconds for the four catalysts under consideration. This corresponds well with the transient times for ethylene to reach steady-state yields. Therefore, a stepwise ethane dehydrogenation to ethylene followed by ethylene to benzene formation might be involved during ethane aromatization. The process times for benzene to reach the steady state yields are~610 s for Co/HZSM-5, ~550 s for Zn/HZSM-5, and ~490 s for Cu/ and Re/HZSM-5..
The transient kinetics analysis shows that the specific reaction rate constants for benzene and toluene formation (from ethane aromatization) at 550oC on a Zn/HZSM-5 catalyst are 0.06 s-1 and 0.04 s-1, respectively. The activation energies for benzene and toluene formation are 90 kJ/mol and â80 kJ/mol. This is consistent with the fact that low temperatures favors toluene formation and high temperatures favors benzene. Thus far, the intrinsic kinetics parameters for ethane aromatization are measured and we are in the process of using advanced in-situ spectroscopy characterization to elucidate the catalytic mechanisms and the structure/kinetics relationships of the aforementioned catalytic system.