Al K-edge XAFS, including both extended x-ray absorption fine structure (EXAFS) and x-ray absorption near edge (XANES) regions, are used to probe the chemical and structural state of the Al T-site in independent ways. A parallel effort involves calculating the MD-XAFS spectra for a particular Al T-site within the zeolite. Using this method an XAFS spectrum is calculated from an ensemble of molecular configurations generated from the molecular dynamics trajectory. In this way a direct comparison can be made between the theoretical and the experimental structures. We report in situ structural measurements of the Brønsted acidic proton localization on the Al tetrahedral (T) sites for an MFI-type zeolite H-ZSM5 as a function of temperature. The formation of this (-O3)-Al-(OH-) 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. Further, the presence of water or organic species near the T-site alters the Al K-edge XANES spectrum thereby providing sensitivity for the local chemical environment.

Al K-edge XAFS, molecular dynamics-XAFS and the chemistry of zeolite Brønsted acid sites

253rd National Meeting (2017)

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