Adv. Mater. http://doi.org/fz9fzh (2012)

Deuterium accounts for only a minute fraction of naturally occurring hydrogen and is used in a variety of applications from nuclear fusion to isotope tracing. It can be separated from isotope mixtures with techniques such as cryogenic distillation and thermal diffusion, but these methods can be expensive and have low selectivity. A technique known as 'quantum sieving' — in which heavier isotopes undergo preferential adsorption in nanoporous structures at low temperatures due to quantum effects — has been proposed as an alternative, and has been tested with materials such as zeolites and metal-organic frameworks (MOFs). However, the D2/H2 uptake ratios have typically been too low to be competitive with conventional methods.

Michael Hirscher and colleagues at the Max Planck Institute for Intelligent Systems, Jacobs University and Augsburg University have now shown that a zinc-based MOF called MFU-4 can offer significantly improved uptake ratios. The MOF has alternating large and small cavities connected by a narrow square-shaped aperture, which is formed from four chlorine atoms and whose size is similar to the kinetic diameter of hydrogen. For a molecule to reach the large pore and to be adsorbed in the framework, it must pass the small cavity and the aperture, which act as separation gates.

Using low-temperature adsorption isotherm measurements at 50 K, a D2/H2 uptake ratio of 4.1 was determined for the material, which the German team suggests is the highest value reported so far.