Quantum effect-mediated hydrogen isotope mixture separation in slit pore nanoporous materials

Abstract

We use path integral simulations to investigate the separation of H₂ and HD, as well as H₂ and D₂, in carbon slit pores of various sizes using a new improved set of carbon-hydrogen interaction parameters determined in our laboratory. As expected, the selectivity of HD over H₂ is lower than that of D₂ over H₂ at the same conditions due to the smaller mass of HD molecules and hence larger quantum effects. In the pressure range of 0.1-10.0 bar, the selectivity is not sensitive to the pressure at the temperature of 77 K. At 40 K the selectivity shows a positive relation to the adsorbed phase density. We also report an unusual crossover effect in which the selectivity in a pore of width 0.85 nm exceeds that in a smaller pore (0.69 nm) at high densities due to enhanced quantum confinement effects when a second layer forms in the larger pore. The optimal pore widths for HD/H₂ separations were identified to be 0.56-0.57 nm, with operating pressures of 10.0 and 0.1 bar for the two pore sizes, respectively. We also simulate equilibrium separation in the commercial Takeda 3 Å carbon molecular sieve, based on a slit-like pore model with a distribution of pore sizes, but find only modest equilibrium selectivity for HD over H₂. It is suggested that while quantum effects are small within the pore bodies, narrow pore entrances must lead to significant quantum effects on the dynamics in order to explain literature data of faster uptake of D₂ compared to H₂ at 77 K in this material. Thus, kinetic molecular sieving at narrow necks, for which this material is well established, maybe a more attractive option than equilibrium separation. Alternatively, materials with controlled smaller pore sizes are needed for more efficient equilibrium HD/H₂ separation. The ideal adsorption solution theory (IAST) is also examined for prediction of the binary hydrogen isotope mixture isotherms in the presence of quantum effects and is found to match simulations at all operating conditions investigated.