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

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 (H₃O⁺(H₂O)_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 X-ray 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 H₂O ligands.