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

Abstract

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.

First-Generation Organic Reaction Intermediates in Zeolite Chemistry and Catalysis

“…They evidenced that both stepwise and concerted mechanisms were feasible in this catalysis, where the transient Wheland complex appeared as a shallow minimum in the free energy surface after the surface ethoxy species (SES) and zeolite Brønsted acid site activated ethylene/ethanol undergoes the electrophilic aromatic substitution (S E Ar) reaction to attack the benzene ring, respectively. All these reaction intermediates proposed by Speybroeck et al were earlier experimentally verified by Chowdhury et al, which demonstrates the strength of combining multiple complementary tools (e.g., UV–vis DRS, MS, solid-state NMR, MD simulations) to identify elusive carbocationic or any first generation of highly reactive intermediates.…”

“…Such a partial elucidation/weak signal is understandable as σ-complex should have lower stability than π-complex. Very recently, the existence and impact of the Wheland complex have been theoretically verified by Speybroeck and co-workers . They utilized enhanced sampling ab initio molecular dynamics (MD) simulations to thoroughly investigate benzene alkylation with ethylene and ethanol under realistic conditions, including elevated temperature and presence of water, over H-ZSM-5 zeolite (Figure d).…”

“…(I–III) In the stepwise mechanism, zeolite Brønsted acid site first reacts with ethylene or ethanol (TS0) to form chemisorbed surface ethoxide species (SES), which attacked benzene (TS1) to form the ipso -protonated Wheland complex (III). In a concerted mechanism, benzene and ethene or ethanol could directly react to produce the same Wheland complex (TS2), which led to the final ethylbenzene product (IV) via deprotonation (TS3 i ) . Adapted with permission from ref .…”

Carbocation chemistry confined in zeolites: spectroscopic and theoretical characterizations

“…The zeolite-catalyzed alkylation of alkenes and aromatics is a common reaction to increase the alkyl chain of alkenes and decorate aromatic rings with alkyl groups, widely occurring in different zeolite-catalyzed reactions, [270][271][272] such as dual-cycle MTO, 94,273,274 toluene disproportionation, 275,276 benzene ethylation, [277][278][279][280] and alkylation of phenol and substituted phenols. 281 Alcohols, ethers, and alkenes can act as the alkylated agents by dehydration and protonation, 282 and there are two alkylated pathways in zeolites, i.e., stepwise and concerted mechanisms.…”

“…31c and d) under realistic operating conditions. 280 The temperature effects and additional guest water were found to impact the reaction mechanism and life of the reaction intermediates. Water decreases the barrier for the formation of the Wheland intermediate, however, the Wheland intermediate is rather short lived.…”