Conventional methods for H2 isotope separation, such as cryogenic distillation or thermal diffusion, are complex and energy intensive. From a classical viewpoint,

Fig. 2.9 QENS self — 10-7

diffusivities for H2 in ZIF-8 at

77 K (circles), compared with

values computed with a rigid

(squares) and flexible

(triangles) framework model 10

‘со

CM ‘

E,

CO

10-9 10-10

H2 and D2 are similar in terms of size and shape as well as energetic considerations. However, H2 and its isotopes can no longer be treated as classical molecules at low temperatures, as quantum effects become important due to their low mass. Quantum separation should be facilitated by the larger de Broglie wavelength of the lighter isotope. Kinetic molecular sieving, based on differences in diffusivity of the two isotopes, is attractive for porous materials with pore sizes comparable to the de Broglie wavelength.

MD simulations, with quantum corrections incorporated via the Feynman-Hibbs approach, showed that D2 should diffuse faster than H2 below 150 K [27], in a hypothetical zeolite rho having a window diameter of 5.43 A (this corresponding to the distance between the O centres, upon subtraction of the O diameter a free diameter of 2.73 A is obtained). QENS experiments were performed to test this prediction. However the zeolite rho used for the measurements had a different chemical composition, so that the free diameter of the windows was approximately 3.26 A [28]. A new set of potential parameters was defined for the MD simulations, which yielded excellent agreement with the QENS diffusion coefficients of H2 and D2, but no quantum effect was observed. This was attributed to the larger pore size in the rho sample used experimentally.

Another series of QENS experiments performed with a carbon molecular sieve (CMS) confirmed the predictions made by the simulations [29]. A CMS can be obtained with different pore sizes, and the Takeda 3 A CMS has pores below 3 A. In this material, it was indeed found that D2 diffuses faster than H2, below 100 K (Fig. 2.10). This shows the extreme sensitivity of the reverse kinetic-selectivity on the window dimension. Transition-state theory and MD calculations indicated that to achieve high flux the cages interconnected by these windows must be small.

Fig. 2.10 Diffusion 10-7

coefficients of H2 (squares)

and D2 (rounds) in Takeda 3

A CMS, as a function of

temperature (loading

0. 5 mmol/g)

10-8