Self-Organization of 1-Propanol at H-ZSM-5 Brønsted Acid Sites.

ABSTRACT

In situ Al K-edge X-ray absorption near edge structure (XANES) and Extended X-ray absorption fine structure (EXAFS) spectroscopy in conjunction with ab initio molecular dynamics (AIMD) simulations show that adsorption of 1-propanol alters the structure of the Brønsted acid site through changes in the associated aluminum-oxygen tetrahedron in zeolite H-MFI. The decreasing intensity of the pre-edge signal of the in situ Al K-edge XANES spectra with increasing 1-propanol coverage shows that Al T-sites become more symmetric as the sorbed alcohol molecules form monomers, dimers, and trimers. The adsorption of monomeric 1-propanol on Brønsted acid sites reduces the distortion of the associated Al T-site, shortens the Al-O distance, and causes the formation of a Zundel-like structure. With dimeric and trimeric alcohol clusters, the zeolite proton is fully transferred to the alcohols and the aluminum-oxygen tetrahedron becomes fully symmetric. The subtle changes in Al-K-edge XANES in the presence of sorbate structures, with the use of theory, are used to probe the local zeolite structures and provide a basis to predict the population and chemical state of the sorbed species.