Zeolite-catalyzed ethylation of benzene with ethene is an important industrial process, being the first step in the synthesis of styrene monomers for polystyrene production. Despite having been extensively studied in the past, the interest in this reaction has been recently renewed since ethanol can also be used as more sustainable ethylating agent.
Mechanistically, the ethylation can proceed through two paths: a concerted one, in which the alkylating agent directly attacks benzene in an electrophilic aromatic substitution (S_EAr) step, and a stepwise one, with the intermediate formation of a surface ethoxide species. The prevailing mechanism depends on the reaction conditions. A debated topic concerns the formation of a Wheland Intermediate (WI, a protonated arenium ion) in the S_EAr step. Computationally, it has often been considered as having negligible stability, but recent experimental work detected it at reaction conditions.
Previous computational studies have adopted a static approach. This can often fail to properly describe the complex entropic effects occurring in the catalyst at the high reaction temperatures. Therefore, we have investigated the ethylation of benzene with both ethanol and ethene in the H-ZSM-5 zeolite using ab initio molecular dynamics simulations coupled with advance sampling techniques. In this contribution, both the stepwise and concerted mechanisms are tackled, with focus on the stability and existence of the WI.
We used metadynamics to bias the system along the S_EAr step in such a way that the WI formation was possible but not mandatory. Regardless of the electrophile, a shallow minimum appeared in the free energy surface where the WI is expected. This shows that the S_EAr occurs in two steps, with an initial electrophilic attack followed by deprotonation of the WI after rearrangement of the molecule. The very low deprotonation barrier, however, also suggests that a negligible WI lifetime is expected, in disagreement with the experimental observations.
In the experiment, various species will be present in the catalyst, remarkably water if ethanol is the alkylating agent. Hence, we are also gaining insights into its effect on the WI stability and understanding its influence on the proton mobility and reactivity. Moreover, we are computing UV-vis spectra of key reaction intermediates in the time-dependent density functional theory formalism to obtain a direct comparison with the experiment.

Veronique Van Speybroeck

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