(653d) Dehydroaromatization of Ethylene over Bifunctional Lewis-Brønsted Acid Pairs in Ag-ZSM-5

Aromatic
hydrocarbons such as benzene, toluene and xylene are commercially used as fuel
additives and as raw materials in the synthesis of desirable aromatics-derived
products, amongst other uses.¹ Synthesis of these cyclic compounds
are primarily dependent on catalytic reforming processes using catalysts with
limited scope in alternate production routes. The surge in natural gas
production across the world has incentivized the search for processes that can
utilize methane and light olefin derivatives in the manufacture of aromatic
products. The conversion methanol to hydrocarbons over zeolite catalysts such
as H-ZSM-5 has evolved into a mature field of research occupying the minds of
many researchers across the world. Another such process that has garnered only
little attention is the conversion of ethylene to aromatics.²

Figure
1:
Comparison of the UV-vis spectra obtain from simulations (in green) with that
obtained experimentally (in red). The inset figure shows the isolated Ag⁺
Lewis acid site coordinated to two ethylene molecules.

In
this work, we use a combination of density functional theory (DFT)
calculations, flow catalytic experiments and the transient temporal analysis of
products (TAP) technique to shed light on the enhanced aromatic production
exhibited by Ag exchanged ZSM-5 from ethylene.³ The identity of the
Ag exchanged sites was confirmed through the simulation of spectroscopic
techniques such as UV-vis spectroscopy and EXAFS. As seen in Figure 1, our
UV-vis simulations on representative Ag-zeolite models show good agreement with
the experimental spectra consistent with the presence of isolated Ag⁺
as the Lewis acid site.DFT calculations and TAP experiments were
used to decouple to role of the Brønsted (H⁺) and Lewis (Ag⁺)
acid sites in the commonly accepted hydrocarbon pool mechanism.⁴ The
Brønsted acid sites were found to facilitate the alkene cycle i.e. the
initiation of ethylene oligomerization forming longer hydrocarbon chains and
other cracking reactions. Ring closing mechanisms for unsaturated C₅
and C₆ species adsorbed to Ag⁺ Lewis acid site in the
zeolite showed exceptionally low activation barriers, thus highlighting the
role of Ag in the arene cycle of the hydrocarbon pool
mechanism. The distinct functions of the Brønsted and Lewis acid sites in the
ethylene conversion process highlight the bifunctional nature of metal
exchanged zeolites and their immense potential in hydrocarbon upgrade
reactions.

References

1.     Franck,
H.-G., & Stadelhofer, J. W. Industrial Aromatic
Chemistry (1988)

2.     Dufresne,
L. A., & Le Van Mao, R. Cat. Lett.
(1994) 25(3-4) 371–383

3.     Hsieh,
M. F., Zhou, Y., Thirumalai, H., Grabow, L. C. & Rimer, J. D. ChemCatChem (2017) 9(9) 1675-1682

4.     Pappas,
D. K. et al. J. Am. Chem. Soc. (2017) 139 (42)
14961–14975.