Nobuki Oka

Molecular sieve effect of chemically modified Na-A type zeolite and its molecular dynamic simulation

Summary In this study, the precise window shrinkage of Na-A type zeolite (Na-A) in the order of 0.1by calcination after rehydration, a partial K exchange and low temperature adsorption was evaluated[1],[2],[3]. It was understood that the precise window shrinkage of thermally and chemically modified Na-A was related to the behavior of Na located in the center of 8 oxygen members ring. Following the adsorption evaluation of oxygen selectivity based on an oxygen/nitrogen binary system and CO2 selectivity based on CO2/nitrogen binary system, a more precise relationship between the window shrinkage and the behavior of Na located at the center of 8 oxygen members ring was studied with an MAS-NMR, a single crystal X-ray diffraction (SCXD) and a molecular dynamic simulation (MD).

Mechanism of Oxygen Adsorption on Partially K Exchanged Na-A Type Zeolite

Accordingly, the observation of oxygen selectivity on Na-K-A would indicate that nitrogen has a greater steric hindrance at the window than does oxygen. Using single crystal X-ray diffraction (SCXD), it was confirmed that K at 8-member rings was relocated to 6-member rings and that the location of Na at 8-member rings became uncertain with calcination. It was also observed that Na-K-A was thermally more stable than Na-A. Using magic angle spin nuclear magnetic resonance (MAS-NMR), it was confirmed that Na at 8-member rings irreversibly changed location from a symmetric position to an asymmetric position with calcination.Using the atomic positions determined by SCXD, the mean square displacements (MSD) of oxygen and nitrogen inside the crystal of Na-A and Na-K-A were estimated by means of molecular dynamic simulation (MD). When temperature was decreased, the MSD of nitrogen was reduced to a greater extent than that of oxygen and the MSD of nitrogen inside Na-K-A was reduced by a greater margin than that inside Na-A.