A robust Zr(IV)-based metal-organic framework featuring high-density free carboxylic groups for efficient uranium … – ScienceDirect.com

The exponential rise in human population and industrial activities has precipitated a marked depletion of global petrochemical fuel reserves, coupled with an uptick in greenhouse gas emissions. This critical scenario underscores the imperative for an immediate investigation into sustainable and environmentally benign alternative energy sources. Nuclear power stands out as a green energy option due to its high efficiency in electricity generation and minimal greenhouse gas emissions [1]. However, the constrained supply of nuclear fuel resources represents a salient impediment to the accelerated deployment of nuclear energy [2]. Uranium, the essential strategic nuclear power resource, is found in terrestrial ores with reserves of only about 6.3 million tons. The ocean contains an enormous amount of uranium, accounting for 99.9 % of the total uranium found on Earth, which adds up to a staggering 4.5 billion tons. Yet, the paltry concentration of uranium in seawater, a mere 3.3 parts per billion (ppb), alongside the presence of copious competing ions, presents a formidable extraction challenge [3]. Additionally, extracting uranium from spent fuel, which contains a significant amount of unreacted uranium, during the reprocessing process is also a challenge. One crucial aspect of tackling these challenges involves the creation of novel adsorbent materials that possess exceptional chemical stability and demonstrate specific and efficient capabilities for capturing U (VI) ions in both seawater and radioactive water [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22].

Over the past two decades, metal–organic frameworks (MOFs), a class of inorganic–organic hybrid materials constructed from the coordination of metal centers and organic linkers, have garnered immense attention for their intriguing architectures as well as their vast potential applications such as gas storage [23], [24], gas separation [25], heterogeneous catalysis [26], [27], proton and/or electron conduction [28], [29], degradation of chemical warfare agents [30], [31], food safety [32], sensing [33], enantioselective separation [34], and so on. Particularly, the remarkable design flexibility of MOFs enables the incorporation of functional groups with precise binding sites for guest molecules within the pores of MOFs, achieved through pre-linker or metal node design, as well as post-synthesis modification approaches [35]. This ability allows MOFs to effectively and specifically capture targeted substances. These methods have been employed to develop MOF-based sorbents that effectively capture U(VI) from seawater or radioactive wastewater [36], [37]. For instance, we successfully incorporated amidoxime (AO), the highly effective functional group for uranium extraction, into the pores of UiO-66 for the first time. The obtained functional MOF, UiO-66-AO, demonstrated selective capture of U(VI) from seawater with a sorption capacity of 2.68 mg/g in real seawater [38]. Following a similar approach, AO was introduced into other MOFs, such as MIL-53 and ZIF-90, to extract uranium from seawater [39], [40]. Despite the promising extraction performance of AO-functionalized MOFs for U(VI), the current methods for synthesizing these materials typically involve a two-step post-synthetic modification process. In this process, the –CN functional group is first introduced to the pores of MOFs and then converted into AO [38]. The density of AO groups achieved through this approach is thus relatively low, which hinders the full potential development of the material. In addition to AO, the introduction of other functional groups such as –OH [41], [42], –NH₂ [43], [44], [45], [46], –COOH [47], and -H₂PO₄ [48], [49], [50], [51] can also enhance the adsorption capacity of the functional MOFs for U(VI). Among these groups, carboxyl group-functionalized MOFs are considered excellent materials for capturing U(VI) due to the relative strong coordination ability of carboxylic group with U(VI). Furthermore, –COOH can be introduced into the ligands of MOFs through rational structural design to maximize their density in the pores, thereby increasing the adsorption capacity for U(VI). Zhao et al. studied the adsorption performance of two reported carboxylated MOFs, UiO-66-COOH and UiO-66-2COOH, for U(VI) sorption. The results showed that UiO-66-2COOH with a higher carboxyl density exhibited superior U(VI) adsorption performance [47]. Therefore, it is reasonable to believe that constructing MOFs with high carboxyl density is an effective strategy to improve U(VI) adsorption capacity. Additionally, the chemical stability of the MOFs is an important prerequisite for the extraction of U(VI) from seawater or radioactive water, and MOFs containing high-valence metal ions including Zr⁴⁺, Al³⁺, and Cr³⁺ usually exhibit excellent chemical stability [52].

In the previous work, we reported an excellent chemically stable Zr(IV)-based MOF, BUT-12, prepared by reacting a tritopic carboxylic acid ligand named H₃CTTA with ZrCl₄ in N’,N-dimethylformamide (DMF, Fig. 1b,d). The distinctive feature of the ligand used in the construction of BUT-12 is the presence of three methyl groups attached to the central benzene ring (Fig. 1b). These methyl groups introduce steric hindrance, causing the three peripheral carboxyl groups to be perpendicular to the central benzene ring. This unique configuration allows the ligand to connect with the 8-connected Zr₆O₄ clusters, resulting in the formation of the first known Zr-MOF with the the-a topology (Fig. 1d) [53]. Interestingly, by employing formic acid as the modulating agent and DMF as the solvent, Bumstead et al. have documented the structure of the doubly interpenetrated phase of BUT-12, STA-26(Zr) [54], [55], [56]. Given the unique positions of these methyl groups, we anticipate that converting them to carboxylic groups will not alter the structure of the MOFs. As a result, a significant number of uncoordinated carboxylic groups can be incorporated into the MOF pores, giving it great potential in U(VI) capture. The ligand, 5′-(4-carboxyphenyl)-[1,1′:3′,1′’-terphenyl]-2′,4,4′,4′’,6′-pentacarboxylic acid (H₃CTTA-3COOH), was successfully synthesized using H₃CTTA as the starting material through a one-step oxidation reaction in a nitric acid aqueous solution at 180 °C, and its reaction with ZrOCl₂ successfully yielded the carboxylated MOF, Zr₆O₄(OH)₈(H₂O)₄(CTTA-3COOH)_8/3 (BUT-12-3COOH). BUT-12-3COOH represents good chemical stability, high surface areas, as well as high density of uncoordinated carboxylic groups (6.93 mmol/g). BUT-12-3COOH demonstrates an impressive maximum sorption capacity of 235 mg/g for UO₂²⁺ ions, achieving equilibrium within a period of 360 min. Moreover, the potential practical applicability of BUT-12-3COOH for the treatment of actual radioactive wastewater has been validated via dynamic sorption experiments. Furthermore, the interaction between UO₂²⁺ and the structural framework of BUT-12-3COOH has been substantiated through comprehensive analyses utilizing techniques such as energy-dispersive X-ray spectroscopy (EDS), Fourier-transform infrared spectroscopy (FT-IR), and X-ray photoelectron spectroscopy (XPS) analysis.