A Robust Zn-Hydroxamate Metal–Organic Framework Constructed from an Unsymmetrical Ligand for Iodine Capture

Abstract

To qualify as competent sorbents for airborne contaminants such as iodine vapor, permanent porosity and chemical stability are key criteria for the selection of candidate metal-organic frameworks (MOFs). To ensure these characteristics, in the present study, an unsymmetrical bifunctional ligand incorporating both carboxylic acid and hydroxamic acid groups was employed for MOF [Zn(CBHA)](DMF) [SUM-13; CPHA = 4-carboxyphenylhydroxamate, DMF = N,N-dimethylformamide] design and synthesis. Though coupled with Zn²⁺, which does not typically yield kinetically robust MOFs with hard acids, the SUM-13 featuring differentiated coordination modes of chelating, bridging and monodentate bonding exhibited exceptional chemical stability and permanent porosity, with a Brunauer–Emmett–Teller (BET) surface area of 296.9 m²/g and a total pore volume of 0.1196 cm³/g. Additionally, with porosity and open metal sites at the five-coordinate Zn²⁺ centers, SUM-13 was demonstrated to be an eligible iodine adsorbent, reaching a maximum uptake of 796 mg/g. These findings underscore the validity and potential of the design strategy in constructing stable metal–organic frameworks.

1. Introduction

As a form of clean energy, nuclear energy plays an essential role to meet global environmental sustainability commitments and ensure global energy security [1,2]. On the other hand, nuclear energy has its own system of waste management and disposal to live up to the “clean” label and minimize environmental impacts [3,4]. For example, iodine isotopes make up approximately 0.69% (by mass) of the fission products from Uranium-235 in the nuclear fuel cycle. This constitutes a significant portion of nuclear waste, especially considering its gaseous nature [5]. During the dissolution of spent nuclear fuel in concentrated HNO₃ at reprocessing facilities, a substantial amount of radioactive iodine is released into the dissolver off-gas (DOG) stream. The primary chemical species of concern in the DOG include highly volatile I₂ (90–100%) and small amounts of organic iodides (e.g., CH₃I) (0–10%) [6,7]. Within these inorganic/organic iodine species, key radioactive isotopes are ¹²⁹I and ¹³¹I, which have half-lives of 1.57 × 10⁷ years and 8.02 days, respectively [5]. Radioactive iodine is highly mobile in the environment and can directly interfere with human metabolic processes, causing acute, chronic, and delayed health issues, given that they can accumulate in the thyroid gland. Therefore, in order to remove radioactive iodine isotopes from DOG and other sources, porous solid materials with accessible internal cavities, such as zeolites, microporous polymers, covalent–organic frameworks, and metal-organic frameworks (MOFs) have been tested for iodine capture [5,8,9,10,11,12,13,14,15]. These solid adsorbents offer clear advantages over traditional liquid scrubbing technique in terms of industrial application [16]. Among the various categories of promising adsorbents, highly crystalline MOFs feature long-range structural order and possess high density of adsorption sites, constituting viable high-capacity candidates as well as facilitating the study of host–guest interactions to advance our understanding of the adsorptive mechanisms [17,18,19]. Hence, continuous iterations of development regarding MOF-based iodine adsorbents provide great opportunities toward industrial implementation and can shed light on other research fronts for MOFs, including guest adsorption in gas and liquid phases. In fact, gas adsorption (including storage and separation) is arguably the most active and fruitful field in MOF research, including toxic and corrosive gases [20,21,22]. MOFs have also been extensively studied toxic waste removal/control, including corrosive gases, and heavy metal recovery/enrichment [23,24,25]. Particularly, MOFs have been shown to adsorb various radionuclides, including uranium and iodine [17,26,27,28]. No matter what the specific research subjects entail, chemical stability is a prerequisite for all MOF materials developed for downstream utilization, as it determines their performance in real scenarios and cycle life [29,30]. Consequently, the design and construction of stable MOFs have garnered significant attention and resources, as underscored by recent review articles [31,32,33,34,35].

To develop MOFs for iodine adsorption or to develop materials that simultaneously possess permanent porosity and robustness, the key chemical handle is to enhance metal–ligand interactions, i.e., coordination bonds. Taking the most common category of MOFs that are constructed from carboxylate ligands as an example, based on the hard–soft acid-base (HSAB) theory, one should employ high-valent metals to increase the coordinative stability [31]. The successful application of the guideline has given us some of the most robust and widely studied MOFs (e.g., MIL-101, UiO-66, PCN-222, MOF-525, NU-1000) [36,37,38,39,40,41,42]. The secondary building units (SBUs) in these renowned MOFs are typically connected to multiple carboxylate groups from different linker molecules, further increasing the overall connectivity and stability in the MOF structures. Another promising strategy to improve coordinative strengths in MOFs is using chelating linkers, featuring functional groups like hydroxamic acid [43,44,45,46,47,48,49,50,51,52,53,54] and pyrazole [55,56]. Combined with primary building units (PBUs) or SBUs, these chelating groups/linkers can stabilize the metal–ligand interactions from an entropic perspective, in addition to HSAB principles that are essentially solely enthalpic considerations [57,58,59].

As part of the ongoing research effort in our group, studying and utilizing the coordinative properties of hydroxamic acid linkers in the context of MOFs, here a novel bifunctional ligand with both carboxylic acid and hydroxamic acid groups is proposed. Common linkers utilized in MOF synthesis are usually symmetrical in terms of positioning of coordinating groups, for example terephthalic acid and benzene-1,4-dihydroxamic acid. In concept, the design of the proposed unsymmetrical bifunctional ligand could introduce distinct coordination modes in the same MOF structure, thereby differentiating the decomposition conditions at different coordination sites. We hypothesize that this strategy may improve the structural stability of the MOF from a kinetic perspective. Given that the combination of Zn²⁺ and hard acid linkers (e.g., carboxylic acid) typically yields MOFs with poor kinetic stability [60,61], here we were able to prepare a robust Zn-MOF with permanent porosity by using bifunctional linker 4-carboxyphenylhydroxamic acid. Further, the MOF was demonstrated as a promising adsorbent for iodine vapor capture.

2. Materials and Methods

Monomethyl terephthalate (>98%), oxalyl chloride (>98%), tetrahydrofuran (THF, 99.9%), N,N-dimethylformamide (DMF, >99%), sodium bicarbonate (>99%), hydroxylamine hydrochloride (>99%), sodium hydroxide (>99%), and Zn(NO₃)₂·6H₂O (>99%) were purchased from Adamas Co., Ltd. (Shanghai, China) and used without further treatment unless otherwise noted. Ultrapure water was obtained from a Millipak^® Express 40 system (Merk-Millipore, Darmstadt, Germany). The synthetic route for 4-carboxyl-phenylhydroxamic acid is illustrated in Scheme S1 in the Supporting Information file.

2.1. Synthesis of Methyl 4-(Chlorocarbonyl)benzoate

Monomethyl terephthalate (5.00 g, 27.75 mmol) was added to a 250 mL round-bottom flask containing dry THF (100 mL) and a catalytic amount of DMF. The mixture was cooled to 0 °C and stirred for 5 min. Then, oxalyl chloride (7 mL, 3 equiv.) was added dropwise to the mixture, and stirring was continued at room temperature for 4 h. The resulting mixture was concentrated under reduced pressure to quantitatively afford methyl 4-(chlorocarbonyl)benzoate, which was used directly without further purification for the subsequent step.

2.2. Synthesis of 4-Methoxycarbonylphenylhydroxamic Acid

Sodium bicarbonate (9.32 g, 111 mmol) and water (50 mL) were added to a 250 mL round-bottom flask, followed by the addition of hydroxylamine hydrochloride (3.90 g, 55.5 mmol). The mixture was stirred for 10 min under N₂ protection. Then, the acyl chloride from Section 2.1 was added to the mixture, and the resulting solution was stirred at room temperature for 20 h, during which time a precipitate formed. The precipitate was filtered and washed with water to obtain a white solid, which was used directly without further purification for the subsequent reaction. ¹H NMR (DMSO-d₆):  δ 3.85 (s, 3 H), 7.96 (d broad, 2 H), 7.98 (d, 2 H), 9.16 (s, 1 H), 11.38 (s, 1 H).

2.3. Synthesis of 4-Carboxyphenylhydroxamic Acid

4-Methoxycarbonylphenylhydroxamic acid (3.90 g, 20 mmol) was refluxed with 30 mL of 4 M aqueous sodium hydroxide solution for 1 h. While still hot, the mixture was acidified with concentrated hydrochloric acid solution to precipitate 4-carboxybenzohydroxamic acid. The suspension was cooled to room temperature, and the colorless solid was filtered off. The crude product was then dried under reduced pressure in a desiccator to obtain a light pink powder (yield 95%). ¹H NMR (DMSO-d₆):  7.80 (d broad, 2 H), 7.96 (d, 2 H), 10.24 (s, 1 H), 11.33 (s, 1 H).

2.4. Synthesis of SUM-13

4-carboxyphenylhydroxamic acid (5.43 mg, 0.03 mmol) and Zn(NO₃)₂·6H₂O (17.85 mg, 0.06 mmol) were mixed with DMF (1 mL) and ethanol (EtOH) (0.250 mL) in a 4 mL glass vial, which was then tightly capped and briefly sonicated. The mixture was heated at 80 °C in an isothermal oven for 24 h. Before being dried in air, the crystals were washed with dry DMF (3 × 20 mL) and then with acetone (3 × 20 mL).

2.5. Iodine Vapor Adsorption Measurement

An open vial (4 mL) with 50 mg of the MOF sample was placed in a 150 mL glass vessel which contained 1 g of iodine. The glass vessel was sealed and subsequently placed in an 80 °C isothermal oven. The vial containing the iodine adsorbed sample was weighed periodically until the mass of it remained unchanged [62,63,64].

3. Results

As described in Section 2.4, the solvothermal reaction of Zn(NO₃)₂·6H₂O and 4-carboxyphenylhydroxamic acid (CPHA) in DMF and EtOH yielded large block-shaped single crystals (Figure 1a). The crystal structure was determined to be a MOF by single-crystal X-ray diffraction (SC-XRD), hereafter denoted as SUM-13 (SUM = Sichuan University Materials; CCDC depository number 2368077), with refinement details provided in the Supporting Information (Table S1). SUM-13 crystallizes in the hexagonal P6₅ space group, and the asymmetric unit consists of one Zn²⁺, one ligand, and one uncoordinated DMF molecule (Figure S3). As shown in Figure 1b, every Zn²⁺ is coordinated to five oxygen atoms, adopting a square pyramidal configuration, including two O atoms of a chelating hydroxamate group, two O atoms from two separate monodentate carboxylate groups, and one O atom from an adjacent hydroxamate. Within the polyhedron, the Zn–O bond lengths range from 1.985 Å to 2.0687 Å, which are fairly large values compared to typical Zn-O bonds in corresponding MOFs [65,66,67]. The phase purity of SUM-13 samples was confirmed via powder X-ray diffraction (PXRD) by comparing the experimental diffractogram of as-synthesized crystals with the simulated pattern from the SC-XRD structure (Figure 1c). The square-pyramidal Zn²⁺ cations then form one-dimensional (1D) chains by sharing the aforementioned hydroxamate O atoms and bridging carboxylate groups. Overall, SUM-13 is constructed based on these parallel 1D SBU chains being linked together by bifunctional ligand molecules, generating a 3D network with channels running in the [001] and [100] directions (Figure 1d,e). To visualize the channels in SUM-13, a Connolly representation using a virtual N₂ probe with a radius of 1.86 Å is depicted in Figure 1f,g. The estimated surface area was 336.6 m²/g. To the best of our knowledge, SUM-13 is the first example of Zn-based MOFs constructed using carboxylate/hydroxamate bifunctional ligands.

As previously established in the literature [68,69,70,71,72], coordination bond lengths were closely related to the binding strengths and the robustness of the overall MOF structure. However, in the case of SUM-13, the relatively long bonds did not compromise the MOF’s chemical stability, possibly due to the chelating nature of hydroxamate groups and the high degrees of connectivity of 1D SBUs with differentiated coordinating modes (vide supra). Specifically, PXRD analysis confirmed that SUM-13 not only showed excellent stability in various organic solvents (Figure 2a) but also maintained its crystallinity when exposed to a wide pH range (2–11) of aqueous solutions, after soaking in respective media for 24 h. Though in the latter case, most peaks on the diffractogram corresponding to the as-synthesized sample shifted when exposed to aqueous solutions, indicating that the SUM-13 structure (unit cell) was swollen in water, while retaining the overall crystalline order. We reiterate that the almost unusual chemical robustness of this Zn-MOF could be attributed to the differentiated coordinating modes of carboxylates and hydroxamates which co-exist in the structure. Compared to conventional carboxylate-based MOFs, the use of an unsymmetrical ligand introduces a second coordinating group, providing structural restraints and stabilizing the bond dissociation and repair processes for SUM-13 under harsh chemical environments.

As shown in the SC-XRD structure, SUM-13’s internal cavities were occupied by solvent molecules. Therefore, to activate the porous structure for porosity test, a solvent exchange procedure (with acetone) was necessary. Consequently, N₂ sorption experiments for SUM-13 were conducted at 77 K. Based on the isotherms (Figure 3a), the Brunauer–Emmett–Teller (BET) surface area and total pore volume of SUM-13 was calculated to be 296.9 m²/g and 0.1196 cm³/g, respectively. Furthermore, the pore size distribution was modelled using non-local density functional theory (NLDFT), showing the presence of micropores with a diameter of 0.61 nm (Figure 3b), consistent with the crystal structure determined by SC-XRD (vide supra). Additionally, as shown in Figure 3c, SUM-13 maintained excellent crystallinity after activation and N₂ sorption, further demonstrating its structural robustness. On the other hand, the thermal decomposition profile of solvent-exchanged SUM-13 was determined by thermogravimetric analysis (TGA). As shown in Figure 3d, the TGA curve indicated a two-stage weight loss profile of SUM-13. The initial decomposition temperature was approximately 190 °C, corresponding to the thermal decomposition of hydroxamate-based ligands, similar to what was observed in previously reported hydroxamate-based MOFs [47,52,53]. The second weight loss phase started at approximately 404 °C, indicating the continuous decomposition of remaining organic compounds in the sample.

Finally, considering its surface area and accessible pores large enough for I₂ molecules (diameter = circa. 0.42 nm) [73], we investigated the iodine vapor adsorption capacity of SUM-13. Based on our previous findings analyzing MOF big data for I₂ capture [27], SUM-13 indeed possess several characteristics favorable for this purpose, for example, the high density of accessible aromatic rings and the ubiquitous presence of coordinatively unsaturated Zn²⁺ sites. The experiments were conducted by exposing SUM-13 to iodine vapor at 80 °C [62,63,64]. Iodine uptake data were obtained by weighing the samples at different time intervals, as shown in Figure 4. The amount of iodine adsorption gradually increased over time, reaching a maximum capacity of 796 mg/g after 72 h. Therefore, SUM-13 exhibits an impressive iodine vapor adsorption capacity among reported MOFs [17].

4. Conclusions

In summary, as part of the continuing effort to create and diversify hydroxamate-based MOF structures in our group, starting from an unsymmetrical bifunctional CPHA ligand with both carboxylate and hydroxamate groups and Zn²⁺, a novel 3D MOF was prepared, namely SUM-13. In the MOF structure, the differentiated coordination modes of monodentate, bidentate, and bridging were responsible for the overall kinetic stability and structural robustness of SUM-13 whose crystallinity persists across a wide pH range of 2–11 and in various organic solvents. The successfully activated SUM-13 has a BET surface area of 296.9 m²/g and total pore volume of 0.1196 cm³/g, constituting a promising porous matrix that provides numerous adsorption sites for iodine. Experimentally, the porous SUM-13 showed an I₂ uptake of 796 mg/g at 72 h. These findings represent yet another avenue to design and construct metal–organic framework materials with desirable stability, which is the cornerstone for downstream applications.