Tracking the Chemical Transformations at the Brønsted Acid Site upon Water-Induced Deprotonation in a Zeolite Pore

Abstract

The structural changes induced by reversible formation of Brønsted acidic sites and hydronium ions with water in a zeolite with MFI structure are reported as a function of temperature using a combination of physicochemical methods and theory. In the presence of an ample concentration of water, the protons are present as hydrated hydronium ions (H 3 O + (H 2 O) n ) that are ion-paired to the zeolite. Loss of water molecules hydrating the hydronium ions leads to an unstable free hydronium ion that dissociates to form the hydroxylated T-site. The formation of this SiOHAl species leads to the elongation of one of the four Al−O bonds and causes significant distortion of the tetrahedral symmetry about the Al atom. This distortion leads to the appearance of new pre-edge features in the Al K-edge Xray absorption near edge structure (XANES) spectra. The pre-edge peak assignment is confirmed by time-dependent density functional theory calculation of the XANES spectrum. The XANES spectra are also sensitive to solutes or solvents that are in proximity to the T-site. As temperature increases, the minor fraction of extra-framework Al present in the sample at ambient conditions in octahedral coordination is converted to tetrahedral coordination through the decoordination of H 2 O ligands.

Studying Proton Mobility in Zeolites by Varying Temperature Infrared Spectroscopy

“…The above mechanistic hypotheses are supported by numerous reports in the literature suggesting that zeolites at 348–423 K retain one to three water molecules at their acid site, and thus may still contain H 7 O 3 + trimers, H 5 O 2 + dimers, and H 3 O + cations up to 423 K [30,31]. Alberti and colleagues reported a neutron diffraction study indicating that residual H 2 O participates in the H/D-exchange at the level of the acid sites [32].…”

Fast room temperature lability of aluminosilicate zeolites

“…Such a complex water network is due primarily to the topology of the zeolite void space, which comprises small cages connected by narrow (8-ring) windows. Additionally, the water is protonated to a degree estimated at ~99% 33 , due to the equilibrium between Brønsted proton adsorption at the Al–O–Si group, and solvation in the water 34–36 . At 300 K, we find this equilibrium to lie on the side of solvation (See Supplementary Figs.…”

The Nature and Catalytic Function of Cation Sites in Zeolites: a Computational Perspective

“…The hydrogen bonding between a single water molecule and an isolated BAS is not strong enough to form a H 3 O + hydronium, but these can be stabilized upon more extensive solvation to form hydronium clusters H 3 O + (H 2 O) 2 . It was also shown that the ion‐paired hydronium clusters can expel water molecules to form zeolite BAS upon the decrease of the temperature . In addition, the solvation effect and proton transport efficiency can be induced by dispersion interactions as exemplified by the combined NMR and DFT study of trimethylphosphine oxide (TMPO) adsorption in H‐ZSM‐5 zeolite .…”

“…It was also shown that the ion-paired hydronium clusters can expel water molecules to form zeolite BAS upon the decrease of the temperature. [46] In addition, the solvation effect and proton transport efficiency can be induced by dispersion interactions as exemplified by the combined NMR and DFT study of trimethylphosphine oxide (TMPO) adsorption in H-ZSM-5 zeolite. [47] The proton solvation equilibrium depends on the topology and composition of the zeolite lattice which provide different spatial confinement effects and different adsorption structures.…”