MOBILITY OF METHANE IN ZEOLITE NAY BETWEEN 100 AND 250 K - A QUASI-ELASTIC NEUTRON-SCATTERING STUDY

The translational and rotational dynamics of methane in NaY zeolite ha ve been studied by quasi-elastic neutron scattering (QENS) at differen t temperatures and loadings. The QENS results reveal a marked temperat ure sensitivity. At 100 K, there is no migration of the methane from c age to cage on the 35-ps time scale of the experiment so that the time spent by a molecule in a supercage is longer than this. At 100 K, 88% of the methane molecules are found to be localized, performing rotati onal diffusion, and 12% of molecules diffuse in a volume limited by th e walls of the supercages. At 150 K, there are no trapped molecules, 4 4% of methane molecules diffuse between the supercages and 56% diffuse locally within the supercages. At 200 and 250 K, the proportion of mo bile molecules and their diffusion coefficients increase. The QENS res ults are in good agreement with theoretical methods which predict a pr ogressive delocalization of the molecules with increasing temperature. The self-diffusion coefficients determined by QENS are in excellent a greement with the NMR results and, to a lesser extent, with molecular dynamics simulations. The activation energy for self-diffusion is of 6 .3 kJ mol(-1).