Ag-Promoted Dehydroaromatization of Ethylene over ZSM-5 Catalysts

Industrial production of aromatics heavily relies on catalytic reforming in refineries.[1] Driven by the limited supply of fossil fuels and the recent shale gas revolution, it is highly desirable to develop methane- and/or light olefin-based processes to meet the growing demand for aromatics. Ethylene dehydroaromatization (DHA) over Ag-ZSM-5 is one of the promising technologies given the availability of ethylene feedstock.

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.