Microscopic description of acid–base equilibrium

Grifoni, Emanuele; Piccini, GiovanniMaria; Parrinello, Michele

doi:10.1073/pnas.1819771116

Cited by 41 publications

(61 citation statements)

References 32 publications

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“…Recently, AIMD combined with IR and high-resolution solid-state NMR on H-ZSM5 zeolites, showed that the interaction between the proton and the water cluster formed becomes weaker with increasing water loading, but plateaus out at a maximum solvation of 7-8 water molecules per BAS 44,45 . A preliminary computational study performed with AIMD simulations and static DFT calculations supported these conclusions 58 , and showed that the bond distance between the acid proton and the releasing framework oxygen is proportional to the water concentration. Here, we ask if this formation of water clusters for proton solvation is an attribute of the MFI structure or if it is a generic property of all zeolites and their cavities?…”

Section: Introductionmentioning

confidence: 73%

“…In order to further probe this hypothesis, reaction free energies have been computed and decomposed to their enthalpic and entropic components. For this purpose, we adopted two recently developed descriptors 58 , known as Collective Variables (CVs), as are a measure of the protonation state of our systems (s p ) and the distance between the charge carrier and the BAS site (s d ), see section S1 in SI for details. As can be seen from Figure 5, free energy surfaces (FESs) projected along the two CVs have roughly the same shape for all these systems with a deep and narrow minimum for and equal to zero and an elongated branch for equal to one.…”

Section: Structure Of Water Clusters Within the Zeolite Frameworkmentioning

confidence: 99%

“…In order to overcome these limitations, we performed metadynamics simulations [59][60][61] to assess the relative free energies of potential protonation sites. We adapted a set of generalized collective variables, which enabled us to assess the relative acid-base properties of multiple sites and allowed to compute relative pKa values 58 . We use this method to investigate the protonation state of several zeolites as a function of water content to probe changes in acid strength and to determine if general behaviors exist to guide our understanding of the surface chemistry of protons in zeolites.…”

Section: Introductionmentioning

confidence: 99%

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Confinement Effects and Acid Strength in Zeolites

Grifoni

Piccini

Lercher³

et al. 2020

Preprint

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Chemical reactivity and sorption in zeolites is coupled to confinement and - to a lesser extent- to the acid strength of Brønsted acid sites (BAS). In presence of water the zeolite Brønsted acid sites eventually convert into hydronium ions. The gradual transition from zeolite Brønsted acid sites to hydronium ions conversion in zeolites of varying pore size is examined by ab initio molecular dynamics combined with enhanced sampling based on well-tempered metadynamics and a recently developed set of collective variables. While at low water content (1-2 water/BAS) the acidic protons prefer to be shared between zeolites and water, higher water contents (n>2) invariably lead to solvation of the protons within a localized water cluster adjacent to the BAS. At low water loadings the standard free energy of the formed complexes is dominated by enthalpy and is associated with the acid strength of the BAS and the space around the site. Conversely, the entropy increases linearly with the concentration of waters in the pores, favors proton solvation and is independent of the pore size/shape.

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Section: Introductionmentioning

confidence: 73%

Section: Structure Of Water Clusters Within the Zeolite Frameworkmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Confinement Effects and Acid Strength in Zeolites

Grifoni

Piccini

Lercher³

et al. 2020

Preprint

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“…The 1-dimensional free energy profile was expanded in terms of two new collective variables, namely the distance between the ethylbenzene para carbon and the atom carrying the extra proton in the system and the minimum distance between the oxygen atoms in the first coordination sphere of the Al defect and, again, the atom carrying the extra proton. The latter is defined using a combination of the definitions proposed by Pérez de Alba Ortíz et al 47 and Grifoni et al 69 (more detailed information can be found in the Supporting Information, Section S5.2 ). The two-dimensional FESs for the various water loadings can be seen in Figure 5 g–i.…”

Section: Results and Discussionmentioning

confidence: 99%

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

Bocus

Vanduyfhuys

Proft

et al. 2022

JACS Au

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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.

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“…To learn more about this potential activation mode in atomistic detail, we performed a QM/MM enhanced sampling molecular dynamics (metadynamics) 51,52 simulation (see SI for details). Figure 7 reports three snapshots of a reactive event during approx.…”

Section: Mechanistic Investigationsmentioning

confidence: 99%

Hydrogen Bond Synchronized Dual Activation Enables the Unified β-Selective O-Glycosylation Inside a Molecular Capsule

Li¹,

Huck²,

Piccini³

et al. 2021

Preprint

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<div>Carbohydrates are of central importance in biology. The selective chemical synthesis of carbohydrates, however, still poses a challenge; particularly, the selective formation of the</div><div>thermodynamically labile b-glycosidic bond is difficult and depends on the substrate’s substitution pattern. We here demonstrate that a molecular capsule catalyzes the highly</div><div>challenging selective formation of b-glycosides independent of the substrate’s substitution pattern and configuration. We demonstrate the versatility of the catalyst by synthesizing small to medium sized 1,2-cis, 2-deoxy, and 1,2-trans b-glycosides in very high selectivity and good yield. The confined space inside the molecular capsule naturally limits the scope concerning the size of reactants. Interestingly, the proposed mechanism involves the synchronized activation of the glycosyl donor and acceptor inside the supramolecular capsule via a relay involving seven hydrogen bonds. Such an activation is known for enzymes, however, to our knowledge, is unprecedented for man-made catalysts.</div>

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Microscopic description of acid–base equilibrium

Cited by 41 publications

References 32 publications

Confinement Effects and Acid Strength in Zeolites

Confinement Effects and Acid Strength in Zeolites

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

Hydrogen Bond Synchronized Dual Activation Enables the Unified β-Selective O-Glycosylation Inside a Molecular Capsule

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