Confinement effects and acid strength in zeolites

Abstract

Chemical reactivity and sorption in zeolites are 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 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.

Dealumination of the H-BEA Zeolite via the SN2 Mechanism: A Theoretical Investigation

“…Hence, one additional water molecule is needed for the complete dealumination by generating the more stable Al(OH) 3 (H 2 O) complex. 17 A spontaneous S N2 mechanism, which is previously proposed in the alkaline environment, 21 The underlying mechanism of the proton transfer process between the BAS and the surrounding water cluster has been explored in detail 38,39 in the previous work. Four proton transfer processes are needed to activate four Al−O Al bonds in the dealumination process.…”

“…The proton can be provided by either the adsorbed water molecule at the Al atom or water molecules nearby. Therefore, the proton transfer is a prerequisite step for the S N2 mechanism.The underlying mechanism of the proton transfer process between the BAS and the surrounding water cluster has been explored in detail38,39 in the previous work. Four proton transfer processes are needed to activate four Al−O Al bonds in the dealumination process.…”

Brønsted acidity in zeolites measured by deprotonation energy

“…On the other hand, there are several simple reactions such as H/D exchange, adsorbate protonation or water adsorption, which in principle should reflect the intrinsic acidity of BAS unless zeolite topology (i.e. confinement) starts to play an important role 20 , 21 , 27 , 29 .…”

Dynamic Pt–OH−·H2O–Ag species mediate coupled electron and proton transfer for catalytic hydride reduction of 4-nitrophenol at the confined nanoscale interface

“…4a) was closely related to the electron transfer process between proton of water and hydride of NaBH 4 as an electron donor by surface dark states even without the involvement of d 8 -d 10 group metals. Very recently, in the confined subnanopores of microporous zeolites, by 1 H NMR, Lercher's group captured similar H species, [77][78][79][80][81][82][83][84][85] which was called hydronium ions [H + (H 2 O) n ], but their formation was just simply attributed to the physical confinement of the nanocavity. The capturing of activated intermediate hydrogen species not only answered the origin of non-metal catalysts for hydride catalytic reductivity of 4-NP, 50,51 but also shed new light on the H + activation of water by non-metal catalysts in the electrocatalytic reactions.…”