M. Bocus

Unraveling the nature of adsorbed olefins in zeolites is crucial to understand numerous zeolite-catalyzed processes. A well-grounded theoretical description critically depends on both an accurate determination of the potential energy surface (PES) and a reliable account of entropic effects at operating conditions. Herein, we propose a transfer learning approach to perform random phase approximation (RPA) quality enhanced sampling molecular dynamics simulations, thereby approaching chemical accuracy on both the determination and exploration of the PES. The proposed methodology is used to investigate isobutene adsorption in H−SSZ−13 as prototypical system to estimate the relative stability of physisorbed olefins, carbenium ions and surface alkoxide species (SAS) in Brønsted-acidic zeolites. We show that the tert-butyl carbenium ion formation is highly endothermic and no entropic stabilization is observed compared to the physisorbed complex within H−SSZ−13. Hence, its predicted concentration and lifetime are negligible, making a direct experimental observation unlikely. Yet, it remains a shallow minimum on the free energy surface over the whole considered temperature range (273–873 K), being therefore a short-lived reaction intermediate rather than a transition state species.

Biorefinery technologies that convert lignin into platform chemicals are essential to reduce our future dependence on fossil resources. In these technologies, a key process is the acid-catalysed O-demethylation of guaiacol derivatives in hot-pressurized water using Brønsted mineral acids or microporous zeolites. The fundamental understanding of how hydronium ions behave in a confined environment versus bulk is still limited. Here we investigate the O-demethylation of guaiacol in hot-pressurized water with HCl or H-BEA zeolite catalysts to elucidate the impact of zeolite microporosity on reaction mechanisms and rates. Operando molecular simulations combined with experimental kinetic studies reveal that, regardless of the catalyst type, O-demethylation follows a concerted O-activated S_N2 mechanism. The reaction rate is higher in the zeolite due to more active, under-coordinated hydronium ions. Additionally, the molecular organization of solvent and reactants around the confined active site plays a crucial role in modulating the association of the reacting species and the reaction kinetics.

In the stride toward the production of low-carbon-footprint commodity chemicals, the development of a complete wood biorefinery plays a pivotal role. The lignin fraction of wood can be depolymerized and demethoxylated mainly into 4-alkylphenols. These phenolic compounds can further catalytically be C-dealkylated within the H-ZSM-5 zeolite at relatively high temperatures and in the presence of steam, producing phenol and olefins. Experimentally, the dealkylation reaction was found to have two striking features: first, different reactants possess very different reactivity. 4-Ethylphenol (4-EP) is somehow less reactive than 4-n-propylphenol (4-n-PP), which is in turn much less reactive than 4-isopropylphenol (4-iso-PP). Second, cofeeding of steam in the reaction mixture was necessary to prevent rapid and reversible catalyst deactivation. Herein, a combination of static and dynamic density functional theory (DFT) simulations is used to unravel the molecular and mechanistic origin of these observations. Free-energy profiles obtained from static calculations confirm the experimentally observed reactivity sequence, where our computations show that the secondary nature of the alkyl carbon involved in 4-iso-PP dealkylation strongly stabilizes the respective transition states. To investigate the effect of water on the mobility of the reactive species and their interaction with the active site, we investigated the diffusion of phenol along the H-ZSM-5 straight channel in the presence of water loadings from 0 to 3 molecules per zeolite unit cell. We show that water has a strongly beneficial effect in promoting desorption and diffusion of phenol away from the Brønsted acid site through competitive adsorption and by the formation of hydrogen bond chains with the diffusing phenol. This effect could lead to a shorter residence time inside the zeolite, preventing active site poisoning and condensation to bulkier biphenylether moieties.

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.