(694g) Transient Kinetics Analysis of Ethane Aromatization over Metal Functionalized ZSM-5 Catalyst

The light alkanes aromatization to valuable petrochemical feedstocks (benzene, toluene, and xylene (BTX)) is one of the most challenging reactions in catalysis. While C₃₊ alkanes aromatization have been demonstrated in industry by UOP/BP’s Cyclar^TM, IFP/SALUTEC’s Aroforming, and Chevron’s Aromax, heretofore the effort has failed to provide a continuous process for ethane (the largest volume percent of natural gas liquids per barrel) aromatization. Although, several papers and patents have demonstrated the successful conversion of ethane to BTX using Zeolites modified with Pt, Zn, Ga and Re. The fundamental knowledge underlying the catalytic mechanisms and structure/kinetics relationship of mono and bifunctional zeolites has rarely been discussed in the literature. Here we present preliminary results of our recent observations through the combination of transient kinetics analysis and advanced in-situ spectroscopy characterization.

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 (550-650^oC) 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 550^oC 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 ⁓80 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.