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.
Highlights
Chemical reactivity and sorption in zeolites are coupled to confinement and—to a lesser extent—to the acid strength of Brønsted acid sites (BAS)
There have been several prior studies of water in zeolites at the classical molecular dynamics[49,50,51,52] (CMD) and AIMD53–57 levels of theory. The latter studies, which account for both reactivity and diffusion, reveal a complex behavior of the protonation state as a function of the number of adsorbed water molecules and the nature of the BAS ranging from sharing the proton between water and the BAS, to complete solvation of the proton within water clusters
ab initio molecular dynamics (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 surrounding the catalytic site weakens with increasing water loading and plateaus upon adsorption of 7–8 water molecules per BAS44,45
Summary
Chemical reactivity and sorption in zeolites are coupled to confinement and—to a lesser extent—to the acid strength of Brønsted acid sites (BAS). It was shown that the hydrated hydronium ions sorbed water are limited in size by the difference between the standard free energy of the water cluster in the pores compared to one in the aqueous phase[44,45].
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