Adsorption of hydrogen isotopes in the zeolite NaX: Experiments and simulations

Among the different methods to separate hydrogen isotopes one is based on the physisorption at low temperature (below 100 K) where quantum effects induce a particular behavior. In the present work, we study the adsorption of single H2 and D2 on the zeolite NaX by combining experiments (manometry) from 30 to 150 K and molecular dynamics simulations at 40 and 77 K. Simulations also include the adsorption analysis for T2. Adsorption on NaX membranes is simulated and quantum corrections are introduced by using the well-known Feynman-Hibbs approach into the interaction potentials. Experimental adsorption isotherms are reproduced by using the Toth equation and it is shown that the adsorption capacity increases with the molecular weight of the isotopes. Isosteric enthalpies evidence a heterogeneous adsorption process with two type of hydrogen isotopes differently linked to the zeolitic structure. The calculated pair distribution functions at high loadings exhibit a liquid-like structuration in the supercages of NaX, which may explain the different adsorption capacities for H2, D2 and T2 and the heterogeneity of the adsorption process.