“In reality, the light hydrogen molecules even tunnel with a higher probability,” says Thomas Heine. “Our calculations show however that it is not the hydrogen molecules, but the deuterium molecules that sit directly at the openings.” The deuterium molecules therefore have better starting conditions for the tunnelling and are more likely to arrive in the cavities of the framework.
The fact that deuterium manages to get the better starting position is again down to a quantum effect: due to the characteristics of the quantum world, there is no complete standstill, even at absolute zero; this would mean that position and momentum, i.e. the velocity of the particles, are accurately known. This is forbidden by the laws of the quantum world, however, as it contains no absolute certainties, only probabilities. All particles thus have a zero point energy, and this is smaller for heavier particles than for lighter ones. Since heavy particles, figuratively speaking, do not move as much as light ones, they adhere better to surfaces at low temperatures. This means that more deuterium molecules than hydrogen molecules also sit at the openings of the metal-organic frameworks, and they are more likely to tunnel into them.
Helium-3 and helium-4 could also be separated with quantum sieves
“We were able to use this experimentally to show, for the first time, that quantum sieving is a very effective method to separate gas mixtures and extract deuterium,” says Michael Hirscher. Previous methods, which utilised the different boiling points of the two isotopes, for example, enrich deuterium by a factor of a mere two and a half per separation cycle. The new method is thus two to three times more effective.
“Technical separation methods like quantum sieving require that porous materials with cavities of a desired size can be specifically produced,” says Dirk Volkmer. “It is precisely this condition which the MFU-4 family fulfils.” The materials could only be tested because the research team, headed by Michael Hirscher, had designed an apparatus in which they can analyse the stored quantities of different gases directly with the aid of a mass spectrometer.
They now want to use this instrument to investigate whether other metal-organic frameworks whose cavities and openings are larger or smaller would perhaps be even more suitable as quantum sieves for deuterium. “We also want to attempt to enrich helium-3 in this way,” explains Michael Hirscher. Helium-3 is the light sibling of helium-4; however, it is so rare that there are only 1.4 parts of helium-3 for one million parts of helium-4. It is sought after as a coolant mainly in science. It could also serve as the fuel for nuclear fusion and would then produce significantly less radioactive material in the future than nuclear fusion with deuterium. “I expect that we will also see a separation effect for the helium isotopes,” says Michael Hirscher. “We are not yet sure whether it will also be worthwhile to produce helium-3 industrially in this way, of course.” For the time being, this issue remains unresolved for hydrogen, as well as its heavier brother deuterium.