Ag-Promoted Dehydroaromatization of Ethylene over ZSM-5 Catalysts
Southwest Process Technology Conference
2017
9th Southwest Process Technology Conference
Southwest Process Technology Conference
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Ag-ZSM-5 was chosen for the ethylene DHA reaction, in part because Ag-ZSM-5 was reported to promote aromatic selectivity in the isobutane aromatization,[2] and to facilitate non-oxidative methane upgrading at moderate reaction temperatures.[3] Here, we will report the influence of Ag species in Ag-ZSM-5 on ethylene DHA without and with co-feeding methane. We observe that Ag-ZSM-5 greatly promotes aromatic selectivities by ~3-fold. In contrast, H-ZSM-5 tends to produce light olefins and other aliphatic hydrocarbons under otherwise identical conditions.[4] With methane co-feed, Ag- and H-ZSM-5 both exhibit very similar catalytic behavior compared to the conditions without methane. Interestingly, methane is activated over Ag-ZSM-5 (≤ 5% conversion), but H-ZSM-5 is inactive for methane conversion.
To rationalize these observations, we use periodic and van der Waals corrected density functional theory (DFT) to investigate the adsorption and activation of methane and ethylene over H- and Ag-ZSM-5. The results reveal that ethylene reacts with a moderate activation barrier over H-ZSM-5, whereas the energy barrier for methane can be considered practically inert. In the case of Ag-ZSM-5, methane activation requires a lower barrier than ethylene; however, ethylene adsorbs strongly to Ag sites and blocks site accessibility for methane, consistent with experimental observations. Through systematic studies of Ag/H-ZSM-5 at different reaction conditions, we have been able to differentiate the role of Ag species in methane/ethylene activation and product selectivities. This work provides insights for designing metal-containing zeolite catalysts for non-oxidative natural gas upgrading.
[1] aH.-G. Franck, J. W. Stadelhofer, Industrial Aromatic Chemistry, Spring, 1988; bH.-J.Arpe, S. Hawkins, Industrial Organic Chemistry, 5 ed., 2010.
[2] Y. Ono, Catalysis Reviews 1992, 34, 179-226.
[3] Y. Ono, T. Baba, Physical chemistry chemical physics : PCCP 2015, 17, 15637-15654.
[4] M.-F. Hsieh, Y. Zhou, H. Thirumalai, L. C. Grabow, J. D. Rimer, ChemCatChem 2017, 9, 1675-1682.