Mechanistic Characterization of Zeolite-Catalyzed Aromatic Electrophilic Substitution at Realistic Operating Conditions

Zeolite-catalyzed
benzene ethylation is an important industrial
reaction, as it is the first step in the production of styrene for
polymer manufacturing. Furthermore, it is a prototypical example of
aromatic electrophilic substitution, a key reaction in the synthesis
of many bulk and fine chemicals. Despite extensive research, the reaction
mechanism and the nature of elusive intermediates at realistic operating
conditions is not properly understood. More in detail, the existence
of the elusive arenium ion (better known as Wheland complex) formed
upon electrophilic attack on the aromatic ring is still a matter of
debate. Temperature effects and the presence of protic guest molecules
such as water are expected to impact the reaction mechanism and lifetime
of the reaction intermediates. Herein, we used enhanced sampling ab
initio molecular dynamics simulations to investigate the complete
mechanism of benzene ethylation with ethene and ethanol in the H-ZSM-5
zeolite. We show that both the stepwise and concerted mechanisms are
active at reaction conditions and that the Wheland intermediate spontaneously
appears as a shallow minimum in the free energy surface after the
electrophilic attack on the benzene ring. Addition of water enhances
the protonation kinetics by about 1 order of magnitude at coverages
of one water molecule per Brønsted acidic site. In the fully
solvated regime, an overstabilization of the BAS as hydronium ion
occurs and the rate enhancement disappears. The obtained results give
critical atomistic insights in the role of water to selectively tune
the kinetics of protonation reactions in zeolites.