ADSORPTION AND TRANSPORT OF ETHANE AND ETHENE IN ZEOLITE NAA - H-2 NMR AND MONTE-CARLO LATTICE-DYNAMICS STUDIES

The temperature dependence of deuterium NMR spin-lattice and spin-spin relaxation time data for ethane and ethene adsorbed in zeolite NaA is presented for sorbate loadings in the range 1-4 molecules/cavity. Ana lytical expressions relating these data to the dynamics of the sorbate molecules are derived. Simultaneous fitting of the temperature depend ence of both T-1 and T-2 data for each sorbate loading provides estima tes of the correlation time characterizing translational, rotational, and librational motions of the sorbate within the zeolite. The NMR dat a presented are consistent with ethane having a higher mobility than e thene within the zeolite. Data derived from the NMR study are then use d as input to a Monte Carlo lattice dynamics (MCLD) model, which predi cts adsorption and transport characteristics of the sorbate-zeolite sy stem. Desorption events from adsorption sites are characterized by a c orrelation time of the form tau = tau(o) exp[E/(RT)], and the probabil ity of the sorbate attempting an intercage jump process is assumed to be of the form p(w) P-wo exp[-Delta(RT)], where tau(o) and p(wo) are c onstants and E and Delta are the activation energies characteristic of the respective processes. From the NMR analysis, it is found that eth ane transport is characterized by values of tau(o), E, p(wo), and Delt a of 2.6 x 10(-9) s, 16 kJ/mol, 0.48, and 7.8 kJ/mol, respectively, wh ile the corresponding values for ethene are 2.0 x 10(-11) s, 27 kJ/mol , 0.105, and 8.5 kJ/mol. The extent of sorbate-sorbate interaction is also quantified and included in the simulation. Predictions of the tem perature dependence of ethene adsorption and the concentration depende nce of ethene self-diffusivity are shown to be in excellent agreement with existing experimental data. The MCLD model is also used to predic t the self-diffusion coefficients of ethane and ethene species within a binary ethane-ethene mixture.