To Be or Not to Be: The Elusive Nature of Wheland-type Intermediates in Zeolite-Catalyzed Aromatic Alkylation Revealed by CCSD(T)-quality Metadynamics

The sustainable utilization of biomass feedstocks to produce value-added chemicals is a central challenge in heterogeneous catalysis. Cyclic alcohols constitute a major fraction of biomass-derived compounds, and their catalytic upgrading via zeolite-catalyzed alkylation provides an efficient route toward fuels and fine chemicals. In particular, benzene alkylation enables the synthesis of industrially relevant alkylated aromatics, while phenol alkylation is crucial for the valorization of lignin-derived feedstocks. Here, we employ machine-learning interatomic potentials (MLIPs) combined with well-tempered Metadynamics (WTMetaD) to investigate the alkylation of benzene and phenol using cyclohexene---the dehydrated form of cyclohexanol---as the alkylating agent within a zeolite framework. Free-energy surfaces (FES) obtained from enhanced sampling simulations are refined beyond standard generalized gradient approximation (GGA) density functional theory (DFT) using free-energy perturbation (FEP) to achieve MP2 and CCSD(T) accuracy. Our results reveal that it is essential to move beyond standard GGA-based DFT to accurately assess the stability of charged intermediates. The arenium ion (or Wheland intermediate), a key $\sigma$-complex in electrophilic aromatic substitution, appears relatively stable at the GGA level of theory. However, higher-level CCSD(T) calculations show that it corresponds to only a weakly stabilized, shallow minimum, indicating a highly transient character. The presence of an activating group such as the hydroxyl substituent in phenol significantly stabilizes both the arenium intermediate and the corresponding transition state, thereby lowering the overall alkylation activation barrier.