(721e) Insights into the Dehydroaromatization of Ethylene over Ag-ZSM-5 Catalysts Using Transient Kinetics Techniques and Density Functional Theory

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 useful products such as aromatic
hydrocarbons. Benzene, toluene and xylene (BTX) are important commodity
chemicals that are used as fuel additives and as raw materials in the synthesis
of specialty chemicals.¹ While the functionalization of methane
occupies the minds of industry and academia alike, a process that has only
recently garnered attention is the zeolite-catalyzed conversion of ethylene to
aromatics. The use of Lewis acids expands the applicability of zeolite
catalysts into processes such as ethylene upgrade.² Here, we examine
Lewis acid zeolites (Ag- and Ga-ZSM-5) as catalysts for the
dehydroaromatization (DHA) of ethylene using temporal analysis of products
(TAP) transient kinetics experiments and density functional theory (DFT)
calculations.

Transient kinetics experiments were performed to carefully
probe the initiation phase of the hydrocarbon pool dual-cycle mechanism and to
identify primary reactions that dominate in this phase. The H⁺ Brønsted
acid site in H-ZSM-5 shows high initial oligomerization activity, converting C₂H₄
to C₄H₈. With increased carbon buildup after multiple
pulses, C₄H₈ production decreases at the expense of
aromatics formation. Similar trends in product evolution were observed for CH₄
and C₃H₈. Overall, the observed catalytic function of
H-ZSM-5 conforms to that reported in literature and is consistent with the
hydrocarbon-pool mechanism.^3,4 Exchanging Ag into 8% of acid sites
(Ag/Al=0.08) in the zeolite results in the complete disappearance of C₄H₈
at the reactor outlet, indicating that any C₄H₈ formed at
the Brønsted acid sites is retained in the zeolite. Visualized in Figure 1 a), C₂H₄
pulses exit the reactor in ~6 s as
compared to ~0.3 s for H-ZSM-5 samples,
indicative of strong interaction and slow desorption with Ag⁺ present
in the zeolite. As seen in Figure 1 b), the presence of Lewis acids in the
zeolite rapidly enhances the production of H₂ and C₆H₆,
indicating in an enhanced rate of dehydroaromatization through accelerated
carbon retention. Similarly, qualitative analysis of TAP pulse responses for
Ga-ZSM-5 catalyst samples suggest differences in Lewis acidity of these samples
that is dependent of the synthesis technique and nature of Ga framework or
extraframework species in the zeolite, some of which are depicted in Figure 1
c).

Figure 1: a) C₂H₄ pulse responses
obtained from H-ZSM-5 and Ag-ZSM-5 (Ag/Al = 0.08 and 0.48), b) Total moles of
product formed at the end of transient kinetics experiments, c) Visualization
of Ga-ZSM-5 models - Ga⁺, [GaH₂]⁺ and [GaH]²⁺,
and d) Binding of olefins to different active sites in Ag-ZSM-5.

DFT simulations support the findings from our transient
kinetics studies, with the Lewis acid zeolite Ag-ZSM-5 showing strong binding
towards olefins and also providing facile pathways to dehydrogenation and
cyclization (Figure 1 d)). This approach of using information from precise
transient kinetics experiments in computational modeling helps identifying
centers of reactivity and their functions in complex catalytic materials such
as metal-exchanged zeolites. Such fundamental understanding enables the development
of structure-function relationships to design catalysts for tailored chemical
conversions.

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.     WestgÃ¥rd
Erichsen, M., Svelle, S. & Olsbye, U. Catalysis Today, (2013), 215,
216-223. 

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