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Further Search for Hydroxyl Nests in Acid Dealuminated Zeolite Y

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posted on 23.04.2015, 00:00 by Istvan Halasz, Eric Senderov, David H. Olson, Jian-Jie Liang
The removal of Al with acid solution from a zeolite framework is customarily associated with formation of framework defects known as hydroxyl nests, but their existence has not been unambiguously confirmed thus far. In a recent study on acid dealuminated Y zeolite, pre-exchanged predominantly with Cs in order to ensure maximized elimination of molecular water that can conceal hydrogen bonded OH groups, no indication has been found for the presence of hydroxyl nests. We present here temperature-programmed desorption (TPD) and Fourier transform infra-red (FTIR) evidence that such hydroxyl nests cannot be identified in an approximately 20% acid dealuminated and solely sodium re-exchanged zeolite NaY­(-Al) that has not been exposed to temperatures above 25 °C during and after dealumination. These experimental conclusions were matched by results of combined density functional theory (DFT)-based spectroscopic study and reactive-force field molecular dynamics calculations on full periodic model zeolites. They showed that (1) in contrast to the general view, the four hydroxyls that would form a hydroxyl nest are energetically different from each other as attested by their computed vibrational spectra; (2) the most intense vibrations of hypothetical hydroxyl nests are missing from the experimental FTIR spectra of the dealuminated NaY zeolite; and (3) the Si–OH, O–H, and O–Na bonds dynamically break and interact with each other already at 25 °C. Thus, we conclude that even if hydroxyl nest formation would follow the Al-removal from the Y zeolite lattice by acid leaching, its existence may be ephemeral on a picosecond time scale.

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