Linköping Studies in Science and Technology
Dissertation No. 544

Bonding and Diffusion of Molecules in Zeolites by ESR and ENDOR

Haitao Li

Akademisk Avhandling

som för avläggande av teknologie doktorsexamen vid Tekniska Högskolan i Linköping kommer att offentligt försvaras i sal C3, Universitetet i Linköping, Fredagen 25 September 1998, kl. 13:15. Opponent air Dr. Eli Olaug Hole, Fysisk Institutt, Universitetet i Oslo, Norge.

ABSTRACT

This thesis studies bonding and diffusion of molecules in zeolites by Electron Spin Resonance (ESR) and Electron Nuclear Double Resonance (ENDOR). In the first part an introduction is given. In the second part (chapter 2) the detailed structure of a Cu2+ complex with water molecules in Cu-ZSM-5 zeolite was characterized with ENDOR at 4 K. The complex has been found to have an axial distorted octahedral structure with two water molecules at a longer distance in the axial position and four ones in the equatorial positions.

In the third part (chapter 3) the motion of adsorbed NO2 has been examined with ESR in several zeolite samples and results were analyzed using the slow-motional ESR theory. It was observed that for the diffusion of NO2 in Na-mordenite and Na-ZSM-5 and K-L-type the broadening of the spectra with respect to increased temperature could be better simulated with a Heisenberg spin exchange model. The exchange was attributed to the interaction between NO2 molecules diffusing along the zeolite channels. In Na-ZSM-5 the spin exchange rate increased rapidly with an increase in Si/Al ratio of the zeolite. The effect was attributed to the hindrance against diffusion by Na+, the amount of which increases with a decreased Si/Al ratio. In cation-exchanged mordenites a moderate decrease of the spin exchange rate with increased ionic radius was observed, indicating increased hindrance against diffusion. In the zeolites with different channel structures (e.g. Beta-type, ZSM-5, Ferrierite, L-type and Mordenite), the spin exchange rate of NO2 strongly depended on the structure and the size of the channels. The diffusion of NO2 was faster in multiple-channel zeolites than in single-channel ones. The diffusion increased with an increase in the size of zeolite channel.

In the last part a high-resolution slow-motional analysis method based on the eigenspectra principle is developed. With the analysis method the relation of the distribution of diffusion rates with respect to temperature can be directly visualized. In experiments, we found that two independent dynamic processes of NO2 adsorption and desorption, could be determined with the different activation energies on the Na-Mordenite.

Department of Physics and Measurement Technology
Linköpings universitet, S-581 83, Linköping, Sweden

Linköping 1998

ISBN 91-7219-285-2     ISSN 0345-7524