Perrhenate recognition within a superphane cavity

In the May issue of Cell Reports Physical Science, Gale and co-workers document a lantern-like receptor (superphane) containing a tailor-made cavity for selective binding of perrhenate anion, leading to an extraordinary capability to separate perrhenate from complex aqueous solutions by either liquid-liquid extraction or solid-phase extraction. In the May issue of Cell Reports Physical Science, Gale and co-workers document a lantern-like receptor (superphane) containing a tailor-made cavity for selective binding of perrhenate anion, leading to an extraordinary capability to separate perrhenate from complex aqueous solutions by either liquid-liquid extraction or solid-phase extraction. Recent calls for further development of nuclear energy raise a compulsive demand for the safe disposal of nuclear waste. Within the used nuclear fuel, 99TcO4− is a high-yield fission product with a long-term radiation hazard and high environmental mobility. It is problematic in the nuclear fuel cycle not only because of its ability to interfere with actinide separation in the stage of used fuel reprocessing but also given its high tendency to escape into the environment as a radioactive concomitant during waste management. Because there are only a handful of chemical research laboratories in the world that can handle the isotope of 99Tc, the perrhenate anion ReO4− with almost identical geometry and charge density is often utilized as a reasonable surrogate for the separation investigations of 99TcO4−. Additionally, Earth’s small rhenium inventory also calls for the feasible reclamation of rhenium from the industrial waste stream containing the generally low-content (ppm-level) perrhenate anion. Hence, selective separation of perrhenate or pertechnetate anions is of high significance for both resource extraction and environmental protection. The challenge of such a task primarily originates from the inherent nature of perrhenate or pertechnetate anions with relatively high symmetry and low charge density.1Katayev E.A. Kolesnikov G.V. Sessler J.L. Molecular recognition of pertechnetate and perrhenate.Chem. Soc. Rev. 2009; 38: 1572-1586Crossref PubMed Scopus (137) Google Scholar Moreover, TcO4− and ReO4− are often located in extremely complex separation environments featuring a low concentration of targeted anions, a large excess of competing anions, harsh acidic or alkaline conditions, and/or intense ionizing radiation fields. In the past decade, significant progress has been achieved on solid-phase extraction materials aimed at separating perrhenate or pertechnetate anions from aqueous solution, including but not limited to cationic inorganic frameworks, organic anion-exchange resins, metal-organic frameworks (MOFs), covalent organic frameworks (COFs), etc.2Banerjee D. Kim D. Schweiger M.J. Kruger A.A. Thallapally P.K. Removal of TcO4‒ ions from solution: Materials and future outlook.Chem. Soc. Rev. 2016; 45: 2724-2739Crossref PubMed Google Scholar,3Xiao C. Khayambashi A. Wang S. Separation and remediation of 99TcO4– from aqueous solutions.Chem. Mater. 2019; 31: 3863-3877Crossref Scopus (79) Google Scholar Some of them feature fast sorption kinetics, high sorption capacity, and great radiation resistance.4Sun Q. Zhu L. Aguila B. Thallapally P.K. Xu C. Chen J. Wang S. Rogers D. Ma S. Optimizing radionuclide sequestration in anion nanotraps with record pertechnetate sorption.Nat. Commun. 2019; 10: 1646Crossref PubMed Scopus (107) Google Scholar,5Li J. Chen L. Shen N. Xie R. Sheridan M.V. Chen X. Sheng D. Zhang D. Chai Z. Wang S. Rational design of a cationic polymer network towards record high uptake of 99TcO4− in nuclear waste.Sci. China Chem. 2021; 64: 1251-1260Crossref Scopus (47) Google Scholar However, there remains a notable amount of room for the improvement of uptake selectivity to bind perrhenate or pertechnetate in the presence of a large excess of co-existing anions, presumably because most of these materials are still driven by the relatively weak electrostatic interaction via the ion-exchange or metal-coordination mechanism. To cope with the complicated situation of real waste, host-guest chemistry is prone to designing a supramolecular receptor6He Q. Vargas-Zúñiga G.I. Kim S.H. Kim S.K. Sessler J.L. Macrocycles as ion pair receptors.Chem. Rev. 2019; 119: 9753-9835Crossref PubMed Scopus (172) Google Scholar,7Zhang D. Ronson T.K. Mosquera J. Martinez A. Nitschke J.R. Selective anion extraction and recovery using a FeII4L4 cage.Angew. Chem. Int. Ed. 2018; 57: 3717-3721Crossref PubMed Scopus (96) Google Scholar with a well-defined cavity for perrhenate or pertechnetate, which could hopefully boost recognition-based selectivity during the separation process. The aimed functional cavity ought to be capable of distinguishing perrhenate or pertechnetate from other competing anions in light of the discrepancy of anions in shape, size, hydration energies, etc. In the May issue of Cell Reports Physical Science, Gale and co-workers employ dynamic imine chemistry to construct a unique superphane (2) with an internal binding pocket for perrhenate recognition (Figure 1).8Zhou W. Li A. Gale P.A. He Q. A highly selective superphane for ReO4− recognition and extraction.Cell Rep. Phys. Sci. 2022; 3: 100875https://doi.org/10.1016/j.xcrp.2022.100875Abstract Full Text Full Text PDF Scopus (7) Google Scholar The binding constant of superphane 2 toward ReO4− is up to 5,620 M−1 for a 1:1 complex in CDCl3/CD3OD (1:1, v/v), principally stemming from 18 hydrogen bonds between this tetrahedral oxoanion and the inward-pointing hydrogen atoms of the cage-like receptor. Additionally, the nearly isolated microenvironment with defined shape and size determines the binding selectivity toward anions in the sequence ReO4− > H2PO4− (2,030 M−1) > SO42− (1,070 M−1) ≫ Cl−, NO3−, or ClO4− (almost no binding), providing the foundation for further separation applications. For these reasons, superphane 2 first serves as a supramolecular extractant in solid-liquid extraction (SLE), indicating that it can selectively extract the extremely low-content (as low as 200 ppb) ReO4− into chloroform solution from the solid mixture containing a large amount of seven other competing anions: F−, Cl−, NO3−, ClO4−, H2PO4−, SO42−, and MoO4−. More importantly, the perrhenate extraction yield of over 99.99% achieved by superphane 2 is also verified in liquid-liquid extraction (LLE). The consecutive multiple extractions promote the rapid removal of ReO4− (initial content: 820 ppm) from complex aqueous solution, ending with less than 10 ppb of ReO4− and a large excess of interfering anions. Moreover, unchanged contents of other competing anions in the aqueous phase further highlight the unprecedented selectivity of this supramolecular extractant. It is noteworthy that the good extraction yield (>80% with one extraction) can maintain itself over a wide pH range from 1 to 10. All these superb characteristics qualify superphane 2 as one of the highest-performing solvent extractants to date. Apart from traditional separation in SLE and LLE, superphane 2 can be loaded as the stationary phase to absorb ReO4− from aqueous eluent via the easy-to-operate column separation. Compared with the former two separation scenarios, the most remarkable advantages of dynamic column absorption are the rapid absorption kinetics and the near-complete removal efficiency, reaching 96.74% and 99.99% with one and two adsorption cycles, respectively. Notwithstanding its saturated adsorption capacity of 267 mg ReO4− g−1 (inferior to that of some burgeoning cationic MOFs and COFs9Wang Y. Xie M. Lan J. Yuan L. Yu J. Li J. Peng J. Chai Z. Gibson J.K. Zhai M. et al.Radiation controllable synthesis of robust covalent organic framework conjugates for efficient dynamic column extraction of 99TcO4−.Chem. 2020; 6: 2796-2809Abstract Full Text Full Text PDF Scopus (72) Google Scholar,10Li J. Li B. Shen N. Chen L. Guo Q. Chen L. He L. Dai X. Chai Z. Wang S. Task-specific tailored cationic polymeric network with high base-resistance for unprecedented 99TcO4− cleanup from alkaline nuclear waste.ACS Cent. Sci. 2021; 7: 1441-1450Crossref PubMed Scopus (66) Google Scholar), the ultra-high removal selectivity of superphane 2 toward ReO4− against SO42− and NO3− is superior to that of all known materials. The absorption selectivity for ReO4− is not affected even in the presence of 500–12,000 times higher concentrations of co-existing SO42− or NO3−, demonstrating that this system is conducive to dealing with efficiency problems hindered by the extreme excess of competing anions (over 6,000 times) in real nuclear waste streams. Likewise, an excellent removal performance (near 100% with two cycles) is also confirmed by the simulated Hanford waste streams through column separation. Furthermore, the good reusability of superphane 2 can be easily realized by elution with 5% NaHCO3 aqueous solution through the spent column once. The perfect separation efficiency is unchanged over a wide pH range (from 1 to 10), particularly under a strong acidic condition, indicating the wide applicability in real wastewater. Nevertheless, the radiation resistance of this organic receptor needs to be further tested considering its great potential for efficient separation of TcO4− in the nuclear industry. Overall, this superphane-based extractant makes clear progress in the recognition and extraction of ReO4− with astonishing efficiency and selectivity in multiple scenarios. The recognition-based separation certainly advances our understanding of the design criteria for desirable ion extractants and adsorbents with high efficiency, thus boosting their applications in the real world. Financial support from the National Natural Science Foundation of China (22006107, 21825601, and 21790374) is acknowledged. The authors declare no competing interests. A highly selective superphane for ReO4− recognition and extractionZhou et al.Cell Reports Physical ScienceApril 29, 2022In BriefExtraction of low concentrations of rhenium anions from waste streams is important industrially. Zhou et al. present the facile and gram-scale synthesis of a superphane via dynamic imine chemistry coupled with NaBH4 reduction, which can separate perrhenate from low-content mixtures and simulated Hanford waste streams with exceptionally high efficiency and selectivity, along with good recyclability and reusability. Full-Text PDF Open Access