Dynamic apertures with diffusion-regulatory functionality in soft porous crystals: A key to solving the century puzzle on isotopologues separation

The separation of water isotopologues has been a “century-old” challenge, also known as “the Holy Grail” in separation science. Heavy water (D₂O) is a stable non-radioactive water isotopologue that plays a critical role in scientific research, military, nuclear energy, and medical fields. Heavy water was first reported in 1931 when H. C. Urey discovered D₂ and D₂O, for which he was awarded the Nobel Prize in Chemistry in 1934. Then in 1933, G. N. Lewis et al. obtained 0.5 µL of heavy water at a concentration of around 65.7% when electrolyzing 10 liters of water. In 1935, the world's first heavy water plant was built in Norway, producing 2 tons of heavy water by electrolysis, but the separation factor was merely 1.05, which was extremely energy-consuming. In 1943, the first heavy water plant employing the distillation method was built in the United States, but the separation factor was as low as 1.02. In the early 1950s, the Geib-Spevack exchange method was developed, by which 540–1,600 tons of heavy water are produced currently per year, with a separation factor of again, only 1.2. To date, this exchange method remains to be the prevalent approach to acquiring heavy water on a large scale. At the laboratory level, there are no materials available to separate water and heavy water, and even the most basic separation mechanisms have not yet been well proposed. With that being said, in recent days, it is gratifying to see a great leap in this tricky area, thanks to porous coordination polymers (PCPs) or metal-organic frameworks (MOFs), which are proven to be effective separators by subtly designing and controlling their guest-diffusion functions based on their temperature-dependent capability.

 

Despite substantial progress in conventional gas storage and separation, rational design and control of gas diffusion in these materials remain to be challenging, because diffusion control requires narrow and contracted pores with dynamic functions. To this end, local and/or global framework flexibility, which serves to regulate guest traffic in channels and guest recognition for separation, is first and foremost when designing these materials. It is generally acknowledged that some PCPs/MOFs exhibit distinctive temperature-regulated adsorption behavior, where gas diffusion is significantly hindered at low temperatures, leading to a dramatically lower gas uptake depending on the actual pore volume. On the contrary, when the temperature is elevated, the gas-diffusion process will be greatly boosted. This “pore-blocking effect” originates from mobile constituents (such as guest cations/molecules) trapped in cavities and/or the vibrational motion of the framework, in which local thermodynamic motions or perturbations in the crystalline framework affect the guest-diffusion process.

 

In a recently released work published in the journal Nature, a novel proposal for designing flip-flop dynamic crystals (FDCs) with diffusion-regulatory apertures has been developed by S. Kitagawa and C. Gu et al. for the separation of water isotopologues at room temperature. Researchers Fangli Yuan , Yunfa Chen and Mingshui Yao from the Institute of Process Engineering, Chinese Academy of Sciences commented on its Highlights in the journal Nano Research on November 14, 2022.

 

The newly designed isostructural FDCs contain two locally flexible components in NbO topology, namely FDC-1 and FDC-2, in which the gate-admission dragonfly-type ligands were constructed by connecting dibenzo[b,f]azepine (DBAP) and 2,2'-iminodibenzyl (IDB) with dynamic flip-flop molecular motions and isophthalic acid. In particular, the presented FDCs exhibited temperature-regulated aperture-expanding behaviors in which the amplitude of flipping motion of DBAP and IDB increased with the gate-admission temperature, facilitating the control of the diffusion rates and amplification of their slight differences for separation of water isotopologues, and exhibiting excellent separation factor of 123.5 and 212.1. As far as this distinct diffusion-regulatory system is concerned, it can be traced back to their first observation of the Cu-based PCP with a butterfly-type encoded for diffusion-regulatory gate functionality, which was also achieved by the same group. The described PCP enabled the gas separation of O₂/Ar and C₂H₄/C₂H₆ with high selectivity of ~ 350 and 75, respectively, due to the thermally adjustable kinetic gate functionality of organic constituents that distinguishes adsorbate molecules according to the gate-admission temperature.

 

In this work, S. Kitagawa and C. Gu et al. proposed a new mechanism for the separation of water isotopologues, which consists of three principles: 1) constructing porous materials that can maintain structural and phase stability in water sorption; 2) designing pore apertures with pore diameters slightly smaller than the kinetic diameters of water isotopologue molecules at low temperatures; 3) expanding diffusion pores slightly at relatively high temperatures through temperature-responsive "local flexibility" gate-admission apertures, allowing single water isotopologue molecule to pass through the gate, and thus controlling diffusion process and amplifying the slight differences in diffusion rates to achieve the kinetic-based separation of water isotopologues.

 

The innovative FDCs-based separation mechanism for water isotopologues proposed by S. Kitagawa, C. Gu, and co-workers is a remarkable milestone in the field of isotopologues separation. It overcomes the scientific bottleneck of water isotopologues separation by adsorption mode and also advances the separation industry of water isotopologues. The most outstanding feature of this mechanism is the perfect match of fundamental temperature-regulated local flexibility with the applicable solution to the century puzzle of isotopologues separation, which not only confirms the feasibility and effectiveness of predesigning flexible organic entities at molecular-level precision but also provides an excellent proof-of-concept as to how an effective combination of foundation and application can effectively advance the progress of human sciences. Inspired by this specific perspective, more dynamic constituents can be exploited for the design of new stimuli-responsive systems, thus broadening their working scenarios to effectively address the growing threats to human society.

 

The paper is also available on SciOpen (https://www.sciopen.com/article/10.1007/s12274-022-5258-6) by Tsinghua University Press.

 

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About Nano Research 

 

Nano Research is a peer-reviewed, international and interdisciplinary research journal, publishes all aspects of nano science and technology, featured in rapid review and fast publishing, sponsored by Tsinghua University and the Chinese Chemical Society. It offers readers an attractive mix of authoritative and comprehensive reviews and original cutting-edge research papers. After 15 years of development, it has become one of the most influential academic journals in the nano field. In 2022 InCites Journal Citation Reports, Nano Research has an Impact Factor of 10.269 (9.136, 5 years), the total cites reached 29620, ranking first in China's international academic journals, and the number of highly cited papers reached 120, ranked among the top 2.8% of over 9000 academic journals.

 

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