PNNL

Abstract

REQUIRED: Short description/abstract (appropriate acknowledgment to be on submission): A holistic approach to analyze the structure of adsorbates and their impact on the zeolite combining experimental and computational analysis of X-ray absorption spectra around the Al tetrahedral atoms that form zeolite Brønsted acid sites (BAS) is presented. Adsorbed 1-propanol on acidic zeolite H-MFI as an example occurs as monomers, dimers, and trimers dependent upon the pressure. The tetrahedrally coordinated lattice aluminum (Al T-sites) interacting with 1-propanol was investigated by in situ Al K-edge EXAFS and simulated MD-EXAFS spectra, which allowed speciate adsorbate structures. The decreasing intensity of the pre-edge signal of in situ Al K-edge XANES spectra as 1-propanol partial pressure increases is attributed to the Al T-sites becoming more symmetrical when the alcohol transitions from monomers to dimers and trimers. The excellent agreement between experimental XANES and simulated MD-XANES, utilizing a time-dependent density functional theory, demonstrates the robustness of the approach. We conclude that adsorption of monomeric 1-propanol on BAS diminishes the distortion of the symmetry of the associated Al T-site, shortening the Al-O distance thereby causing the formation of a Zundel-like structure. With dimeric and trimeric alcohol clusters the zeolite proton is fully transferred to the alcohols. The differential interaction energy between the alcohol molecule and BAS decreases from 101 kJ/mol for the first 1-propanol molecule to approximately 83 and 51 kJ/mol by adding the second and third. The approach allows elucidation of the local structures of adsorbed species and their dynamics, which provides accurate models to predict the population and reactivity of alcohols within zeolite pores.