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

• Published in: Chemistry of materials Vol. 29; no. 21; pp. 9030 - 9042
• Main Authors: Vjunov, Aleksei, Wang, Meng, Govind, Niranjan, Huthwelker, Thomas, Shi, Hui, Mei, Donghai, Fulton, John L, Lercher, Johannes A
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 14.11.2017; American Chemical Society (ACS)
• Subjects: Bronsted acid sites; Environmental Molecular Sciences Laboratory; hydronium ion; INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; pore dehydration; zeolite MFI
• ISSN: 0897-4756; 1520-5002
• DOI: 10.1021/acs.chemmater.7b02133

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.