Structural Characterization of Protonated Water Clusters Confined in HZSM-5 Zeolites

Abstract

A molecular description of the structure and behavior of water confined in aluminosilicate zeolite pores is a crucial component for understanding zeolite acid chemistry under hydrous conditions. In this study, we use a combination of ultrafast two-dimensional infrared (2D IR) spectroscopy and ab initio molecular dynamics (AIMD) to study H2O confined in the pores of highly hydrated zeolite HZSM-5 (∼13 and ∼6 equivalents of H2O per Al atom). The 2D IR spectrum reveals correlations between the vibrations of both terminal and H-bonded O–H groups and the continuum absorption of the excess proton. These data are used to characterize the hydrogen-bonding network within the cluster by quantifying single-, double-, and non-hydrogen-bond donor water molecules. These results are found to be in good agreement with the statistics calculated from an AIMD simulation of an H+(H2O)8 cluster in HZSM-5. Furthermore, IR spectral assignments to local O–H environments are validated with DFT calculations on clusters drawn from AIMD simulations. The simulations reveal that the excess charge is detached from the zeolite and resides near the more highly coordinated water molecules in the cluster. When they are taken together, these results unambiguously assign the complex IR spectrum of highly hydrated HZSM-5, providing quantitative information on the molecular environments and hydrogen-bonding topology of protonated water clusters under extreme confinement.

Reactive Neural Network Potential for Aluminosilicate Zeolites and Water: Quantifying the effect of Si/Al ratio on proton solvation and water diffusion in H-FAU

“…6,7 Another interesting branch of research focused on how, typically acidic, zeolites effect the water mobility through the thin zeolite membranes. 8 Both of these research branches stimulated a lot of computational efforts aimed at understanding the mechanism of proton solvation and/or the water dynamics in acidic zeolites, typically focused on MFI, [9][10][11] BEA 12,13 and FAU 14 topologies. In particular the work by Grifoni et al 15 employing extensive biased ab initio molecular dynamics (AIMD) simulations for four topologies (GIS, FAU, MFI, CHA) seemed to settle the case about the thermodynamics of the proton solvation in high-silica zeolites showing that regardless of the zeolite topology/confinement not more than two to three waters are needed to solvate the Brønsted acid site (BAS) proton and that these protonated water clusters stay close to the BAS.…”

Unusual water-assisted NO adsorption over Pd/FER calcined at high temperatures: The effect of cation migration

“…18 Clustered water molecules are always present adjacent to Brønsted acid sites under ambient conditions and solvate charge balancing cations and reacting molecules. [19][20] These confined water molecules have different characteristics from bulk water depending on the shape of the cage and the Al distribution in the framework. A variety of Al ions and ion pairs in zeolite structures provide negative charges to the framework that are charge balanced by mono-or divalent cations.…”

HZSM-5 zeolite modification and catalytic reaction mechanism in the reaction of cyclohexene hydration

“…Carbocation was confined to near the oxygen atom, and the central carbon atom was connected with the oxygen atom on the zeolite framework by covalent bonds. 25,26…”