https://orcid.org/0000-0003-3207-586X
Abstract
Adsorption of methylene blue from aqueous effluents is a key technique to treat wastewater discharge from chemical industries. Toward methylene blue adsorption, porous metal–organic frameworks have been promising owing to their high surface areas and tunable porosity. The incorporation of polyoxometalates (POMs) into metal–organic frameworks has been found to facilitate methylene blue adsorption due to electrostatic interactions between the anionic polyoxometalate and the cationic methylene blue. However, it remains unclear how the customizable pore of POM@MOF hybrids affects the capture of methylene blue. In this work, we study the effect of the size and environment of metal–organic frameworks on the property of methylene blue capture for [Zr6O4(OH)4(1,4-benzenedicarboxylate)6] (UiO-66) embedding an α-Keggin-type polyoxometalate [α-SiW12O40]4− (SiW12). Tuning pore size and environment based on ligand engineering of the metal–organic frameworks suggests that (i) lengthening the organic linkers and (ii) functionalization of the linker with −NO2 groups could enhance the methylene blue uptakes. Notably, an increase in the loading amounts of SiW12 on UiO-66 functionalized with −NO2 groups led to outstanding methylene blue capture, comparable to that of representative zeolites such as ZSM-5 and zeolite Y.
1. Introduction
The discharge of wastewater containing toxic organic dyes from chemical industries is a serious pollution problem in the ecological system.1,2 Methylene blue (MB, C16H18N3SCl, 17 × 8 × 3 Å) is a representative organic dye with cationic properties and is widely used as a stain reagent. Toward the removal of MB from aqueous effluents, various techniques such as electrochemical,3 sonochemical,4 and photocatalytic5 degradation have been investigated. Adsorption is well known as an equilibrium separation process and has been found to be superior to other techniques in terms of initial cost, simplicity of design, and ease of operation.6 Much research effort has been devoted to developing MB adsorbents by utilizing organic polymers,7,8 metal oxides,9–11 and organic–inorganic composites.12,13 In particular, porous materials including activated carbons14 and zeolites15,16 have been used for practical application for MB adsorption owing to their high uptake capacity and fast adsorption rate.
Among porous materials, metal–organic frameworks (MOFs) consisting of metal nodes and organic linkers have attracted considerable attention for MB adsorption due to their high surface areas and tunable porosity.17–24 MOF-based composite materials embedding ionic components such as ionic liquids25 and polyoxometalates (POMs),26–28 which are anionic metal–oxo clusters, have been investigated as MB adsorbents. As a remarkable example, the incorporation of POMs into MIL-101 has significantly facilitated the MB uptakes due to electrostatic interactions between POM anions and the cationic dyes.26 However, it remains unclear how the customizable pore of POM@MOF composites affects the capture of MB.
Thus, we envisioned clarifying the relation between MB uptake property and the size and environment of MOF pores in POM@MOF composites. For the MOF component of the composite, we focused on thermally and chemically stable Zr-based MOF, [Zr6O4(OH)4(1,4-benzenedicarboxylate)6] (UiO-66), in which Zr6O4(OH)4 clusters are linked by 1,4-benzenedicarboxylate (BDC) linkers into a 3-dimensional network with small tetrahedral (ca. 8 Å) and large octahedral (ca. 11 Å) pores (Fig. 1a).29–33 A key feature of UiO-66 is the high designability of the pores: the pore size and environment are tunable by changing the lengths of organic linkers (BDC, fumarate, and biphenyl-4,4′-dicarboxylate) and the functional groups of BDC linkers (−H, −NH2, and −NO2), respectively.34–39 The POM selected for this study is a representative α-Keggin-type POM, [α-SiW12O40]4− (SiW12, Fig. 1b).40–42 In this report, we synthesized a series of SiW12-embedded UiO-66 analogs with different organic linkers and investigated the impact of the pore characteristics on MB sorption (Fig. 1c). The systematic study revealed that (i) lengthening the organic linkers and (ii) functionalization of the linker with −NO2 groups could enhance the MB uptakes. Besides, an increase in the loading amounts of SiW12 on UiO-66 functionalized with −NO2 groups leads to an efficient MB capture, which is comparable to those of representative zeolites such as ZSM-5 and zeolite Y.
a) Crystal structure of UiO-66. Tetrahedral and octahedral pores are represented in pink and yellow balls, respectively. b) Molecular structure of SiW12. c) Schematic illustration of the design of pore size and environment in POM@MOF composites.
2. Results and discussion
2.1 Synthesis and characterization of SiW12@UiO-66
We synthesized SiW12-embedded UiO-66 (SiW12@UiO-66) by a facile one-pot solvothermal method.43 In a typical synthesis, ZrCl4, H2BDC, and acetic acid were dissolved in N,N-dimethylformamide by vigorous stirring, followed by the addition of acid salts of SiW12. Subsequently, the mixture was sealed in a screw vial and heated at 120 °C for 24 h in an oven to obtain the white powder sample of SiW12@UiO-66 (see Supplementary data for the detail).44 As shown in Fig. 1a, SiW12@UiO-66 exhibited an identical powder X-ray diffraction (PXRD) pattern to pristine UiO-66. Besides, the lattice constant of UiO-66 of SiW12@UiO-66 calculated by the Le Bail fitting of the PXRD pattern was fairly consistent with that of pristine UiO-66 (Supplementary Figs. S1 and S2). These results indicate that the structural periodicity of UiO-66 was basically the same even after the incorporation of the SiW12. The IR spectrum of SiW12@UiO-66 contains absorption bands attributing to both SiW12 and UiO-66 (Fig. 2b), which suggests that the molecular structures of SiW12 and UiO-66 are intact after their hybridization. The 29Si solid-state magic angle spinning NMR spectra of SiW12@UiO-66 contained characteristic peaks corresponding to SiW12 (Supplementary Fig. S3), thereby confirming the preservation of the molecular structure of SiW12. The molar ratio of SiW12/Zr6 in SiW12@UiO-66 was estimated to be 0.10 by inductively coupled plasma optical emission spectroscopy (ICP-OES) measurements, which corresponds to ca. 10% of the large octahedral pores in UiO-66 being filled with SiW12. The N2 sorption isotherm of SiW12@UiO-66 was collected at 77 K to address the accessible porosity (Fig. 2c; Supplementary Fig. S4). The isotherm displays a typical type I sorption behavior, suggesting the existence of microporosity. The Brunauer–Emmett–Teller (BET) surface area for SiW12@UiO-66 (1,080 m2 g−1), which was estimated by the N2 sorption, was smaller than that for pristine UiO-66 (1,487 m2 g−1). To reveal the composite states of SiW12@UiO-66, scanning electron microscopy (SEM) images and energy-dispersive X-ray (EDX) elemental mappings of the constituent elements were collected. Figure 2d to 2f show the SEM image and elemental phase mapping of W (in SiW12) and Zr (in UiO-66), suggesting that the SiW12 and UiO-66 were homogeneously distributed in SiW12@UiO-66 (Supplementary Fig. S5). This observation is consistent with the incorporation of SiW12 into UiO-66.
Characterization of SiW12@UiO-66: a) PXRD patterns, b) IR spectra, c) N2 adsorption isotherms at 77 K, d) SEM image, and corresponding color-coded EDX spectral maps of e) Zr in the framework of UiO-66 represented in orange and f) W in SiW12 represented in blue. In c), adsorption is represented by closed circles and desorption by open circles (P/P0, relative pressure).
2.2 MB capture of SiW12@UiO-66
SiW12@UiO-66 was applied to MB capture at room temperature from water (initial MB concentration: 1 mg L−1) (see Supplementary data for the detail). MB exhibits the π → π* type electronic transition of MB+ (605 nm) and (MB+)2 (664 nm),45 resulting in its characteristic blue coloration (Fig. 3a, b). When the MB solution was treated with powder samples of SiW12@UiO-66 for 1 h and subsequently removed by filtration, the blue solution of MB became almost colorless and clear (Fig. 3c). In contrast, the color of the MB solution treated with pristine UiO-66 under similar conditions faded but retained its blue color (Fig. 3d). This observation highlights that SiW12@UiO-66 shows superior MB capture property to pristine UiO-66. We note that there is a commensurate increase in blue hue of SiW12@UiO-66 after the uptake of MB (Supplementary Fig. S6).
a) Molecular structure of MB. b) A photograph of the aqueous solution containing MB with an initial concentration of 1.0 mg L−1. A photograph of the solution after the addition of 10 mg of c) SiW12@UiO-66 and d) UiO-66, followed by filtration of SiW12@UiO-66 after 1 h. e) The UV–Vis spectra of the initial solution (red) and the solution after treating with SiW12@UiO-66 (blue) and UiO-66 (black) for 1 h.
To quantify the adsorption ratio of MB, we performed UV–Vis spectroscopy for the MB solution before and after the treatment (Fig. 3e). The intensity of the adsorption bands at 550 to 700 nm corresponding to MB decreased by the addition of adsorbents (SiW12@UiO-66 or pristine UiO-66) into the solution, and the decreasing rate was used to calculate the adsorption ratio of MB. The adsorption ratio for SiW12@UiO-66 reached 80% after 1 h, which was higher than that of pristine UiO-66 (16%). SiW12@UiO-66 (1,080 m2 g−1) had a smaller BET surface area compared to pristine UiO-66 (1487 m2 g−1); nevertheless, SiW12@UiO-66 demonstrated a higher adsorption rate. Figure 4a shows the time course of MB uptake (initial concentration = 1 mg L−1) by SiW12@UiO-66, indicating that the adsorption equilibrium was reached before ca. 4 h, whereas the MB uptake for pristine UiO-66 did not reach the adsorption equilibrium even after 16 h. These results suggest that embedding of POM in the MOF enhances MB capture despite the decrease in surface area, probably because the incorporation of negatively charged POM increased Coulomb interaction with MB cations. Then, the initial concentration of MB was changed from 1 to 5 mg L−1 to study the effect on MB adsorption. As shown in Supplementary Fig. S7, the adsorption ratio after 1 h (initial concentration = 5 mg L−1) for SiW12@UiO-66 (63%) was higher than that for pristine UiO-66 (11%), suggesting that the incorporation of POMs into MOFs is a versatile approach to improve MB capture.
a) Time course of MB uptake of SiW12@UiO-66 and UiO-66 (MB = 1 mg L−1) and b) the fitting of MB uptake of SiW12@UiO-66 by a FD, FO, and PSO model.
To quantify the adsorption kinetics of SiW12@UiO-66, the experimental time course data were fitted by 3 models: Fickian diffusion (FD), first-order (FO), and pseudo-second-order (PSO) models (Fig. 4b). The experimental data were well reproduced by a PSO model rather than FD and FO models. This result suggests that the kinetic of MB adsorption by SiW12@UiO-66 was determined by the chemical adsorption of MB on SiW12@UiO-66 as well as the diffusion of MB. The rate constants of the MB adsorption from 1 to 5 mg L−1 solution by a PSO mode for SiW12@UiO-66 were calculated to be 2.2 × 10−2 and 3.0 × 10−3 g mg−1 min−1, respectively (Supplementary Tables S1 and S2). We note that the PXRD patterns and IR spectra of SiW12@UiO-66 showed no significant changes after the MB adsorption (Supplementary Fig. S8), confirming the structural stability.
2.3 The pore size effect of MOFs on MB capture
Taking advantage of the high designability of MOFs, we envisioned studying the pore size effect of MOFs on MB capture. Thus, we focus on [Zr6O4(OH)4(fumarate)6] (MOF-801),34 in which BDC linkers of UiO-66 were replaced with fumarate linkers and a smaller pore size, to incorporate SiW12 (see Supplementary data for the detail of the synthesis). The pore size of MOF-801 is ca. 7 and ca. 10 Å, while UiO-66 possesses pores of ca. 8 and ca. 11 Å. The obtained composite was named SiW12@MOF-801, and the molar ratio SiW12/Zr6 was 0.13, which was determined by ICP-OES. PXRD and IR measurement confirms that the incorporation of SiW12 into the MOF-801 causes no significant change in the crystal structure of MOF-801 (Supplementary Figs. S9 to S11). The N2 sorption measurement at 77 K indicates that SiW12@MOF-801 has a lower BET surface area (589 m2 g−1) than pristine MOF-801 (926 m2 g−1), suggesting the hybridization with SiW12 (Supplementary Fig. S12). SEM-EDX suggests SiW12 and MOF-801 were uniformly located in SiW12@MOF-801 (Supplementary Fig. S13).
Under similar MB capture conditions (initial concentration = 1 mg L−1, 1 h), SiW12@MOF-801 exhibits an adsorption ratio of 39%, which is smaller than that of SiW12@UiO-66 (80%). This result demonstrates that UiO-66 with longer BDC linkers is preferable for MB adsorption rather than MOF-801 with shorter fumarate linkers. This observation is in line with previous studies on MB adsorption using SiW12-free UiO-66 and MOF-801.46 As shown in Fig. 5a and Supplementary Fig. S14, the time course of MB adsorption (initial concentration = 1 or 5 mg L−1) indicated that the trends of the adsorption ratio (SiW12@MOF-801 < SiW12@UiO-66) were observed in a wide range of timescales, suggesting that the expansion of linker length of MOFs is promising to facilitate an efficient MB adsorption. We note that PXRD patterns and IR spectra of SiW12@MOF-801 showed no significant changes after the MB adsorption (Supplementary Fig. S15).
Time course of MB uptake (MB = 1 mg L−1) of a) MOF-801, SiW12@MOF-801, and SiW12@UiO-66; b) SiW12@UiO-66-NH2, SiW12@UiO-66-NO2, and SiW12@UiO-66; and c) SiW12@UiO-66-L, SiW12@UiO-66, and SiW12@UiO-66-H. In b) and c), the time course of MB adsorption was fitted by a PSO model.
2.4 The influence linker functionalization of MOFs on MB capture
In addition to the tuning of the pore size of MOFs, modulation of the pore environment of MOFs is expected to effectively control the MB capture ability. To investigate the influence of linker functionalization of UiO-66, we also synthesized SiW12-embedded UiO-66 analogs by use of amino- or nitro-functionalized BDC linkers (see Supplementary data for the detail of the syntheses). The samples functionalized with −NH2 and −NO2 groups were named SiW12@UiO-66-NH2 and SiW12@UiO-66-NO2, respectively. The ICP-OES indicated that the molar ratio SiW12/Zr6 of SiW12@UiO-66-NH2 and SiW12@UiO-66-NO2 was 0.10 and 0.09, respectively, which is fairly consistent with SiW12@UiO-66 (SiW12/Zr6 = 0.10). The PXRD patterns of the SiW12@UiO-66 analogs are almost identical (Supplementary Figs. S16 to S18), indicating that the linker functionalization of UiO-66 causes a minimal change in the crystal structure of UiO-66. The IR spectra of SiW12@UiO-66 analogs contain absorption bands corresponding to both SiW12 and UiO-66 analogs (Supplementary Fig. S19), which suggests that the molecular structure of UiO-66 analogs is unchanged after hybridization with SiW12. As shown in Supplementary Fig. S20, the BET surface area calculated by N2 sorption isotherms at 77 K was 740 m2 g−1 for SiW12@UiO-66-NH2 and 930 m2 g−1 for SiW12@UiO-66-NO2 with decrease compared with SiW12@UiO-66 (1,080 m2 g−1), suggesting that the introduction of functional groups decreased the pores volume of UiO-66. SEM-EDX revealed SiW12 and UiO-66 analogs uniformly existed in the composites even after ligand functionalization (Supplementary Figs. S21 and S22).
The SiW12@UiO-66 analogs were applied to the MB adsorption from 1 mg L−1 MB aqueous solution. The adsorption ratio of SiW12@UiO-66-NH2 and SiW12@UiO-66-NO2 was 90% and 98% after 1 h, respectively. These values are higher than that of SiW12@UiO-66 (80%), demonstrating that proper linker functionalization improves the MB adsorption property, as observed in SiW12-free UiO-66.47 We postulate that the increase in MB uptake is attributed to the host–guest dipole–ion interactions between functional groups and MB cations. In particular, SiW12@UiO-66-NO2 exhibited excellent MB adsorption among the composites probably because negatively charged O atoms of the −NO2 groups attract MB cations via Coulomb interactions.48 The outstanding MB uptake of SiW12@UiO-66-NO2 is observable during time course measurements (initial concentration = 1 or 5 mg L−1; Fig. 5b; Supplementary Fig. S23), suggesting that the MB adsorption is systematically modulated by ligand functionalization. The rate constants of the MB adsorption of SiW12@UiO-66-NO2 from 1 mg L−1 MB solution by a PSO mode were calculated to be 3.2 × 10−1 g mg−1 min−1, which is larger than that of SiW12@UiO-66-NH2 (7.2 × 10−2 g mg−1) and SiW12@UiO-66 (2.2 × 10−2 g mg−1 min−1). This result demonstrates that the functionalization of SiW12@UiO-66 with −NO2 groups exhibited rapid kinetics for MB capture. The structures of SiW12@UiO-66-NH2 and SiW12@UiO-66-NO2 after the MB adsorption were characterized by PXRD and IR (Supplementary Figs. S24 and S25), suggesting their structural stability.
2.5 The impact of SiW12 loading in the SiW12@UiO-66 on MB capture properties
To study the effect of SiW12 loading in the SiW12@UiO-66 on MB adsorption, we prepared 2 SiW12@UiO-66 analogs by changing the loading amount of SiW12, SiW12@UiO-66-L and SiW12@UiO-66-H (L = low loading, H = high loading) (see Supplementary data for the detail). ICP-OES measurements indicated that the molar ratio SiW12/Zr6 of SiW12@UiO-66-L and SiW12@UiO-66-H was 0.05 and 0.17, respectively (SiW12/Zr6 of SiW12@UiO-66 = 0.10). The PXRD patterns of SiW12@UiO-66-L and SiW12@UiO-66-H fairly match with that of SiW12@UiO-66 (Supplementary Figs. S26 to S28), and the IR spectra of SiW12@UiO-66 analogs exhibit nearly identical absorption bands corresponding to UiO-66 (Supplementary Fig. S29). These results suggest that the change in the loading amount of SiW12 minimally affects both the molecular and crystal structure of UiO-66. The IR spectra indicate that the peak intensities corresponding to SiW12 increase in the order of SiW12@UiO-66-L < SiW12@UiO-66 < SiW12@UiO-66-H. In contrast, the N2 sorption measurement at 77 K shown in Supplementary Fig. S30 revealed that the BET surface area decreases in the order of SiW12@UiO-66-L (1,279 m2 g−1) > SiW12@UiO-66 (1,080 m2 g−1) > SiW12@UiO-66-H (899 m2 g−1). These observations are consistent with the fact that the loading amount of SiW12 increases in the order of SiW12@UiO-66-L < SiW12@UiO-66 < SiW12@UiO-66-H. The element mapping by SEM-EDX indicated a homogeneous distribution of SiW12 and UiO-66 in the SiW12@UiO-66 analogs (Supplementary Figs. S31 and S32).
The adsorption ratio of MB from 1 mg L−1 MB solution for SiW12@UiO-66-L and SiW12@UiO-66-H was 49% and 97%, respectively after 1 h (the adsorption ratio of MB for SiW12@UiO-66: 80%), demonstrating that an increased POM loading in UiO-66 is preferable to an efficient MB adsorption. As shown in Fig. 5c, the fitting of time course of MB adsorption by a PSO mode indicated that the rate constants increase in the order SiW12@UiO-66-L (1.9 × 10−2 g mg−1 min−1) < SiW12@UiO-66 (2.2 × 10−2 g mg−1 min−1) < SiW12@UiO-66-H (1.5 × 10−1 g mg−1 min−1). Besides, the same trends were reproduced in the MB adsorption from 5 mg L−1 MB solution (Supplementary Fig. S33). We note that the crystal structure was maintained during the MB uptake process for the SiW12@UiO-66 analogs, which were characterized by PXRD measurements (Supplementary Figs. S34 and S35).
To maximize the MB uptake property, we optimized the pore size and environment of MOFs as well as the loading amount of POMs. Thus, we synthesized highly SiW12-loaded UiO-66 functionalized with −NO2 groups, SiW12@UiO-66-NO2-H, with a molar ratio SiW12/Zr6 = 0.16, and the compound was fully characterized by PXRD, IR, N2 adsorption, and SEM-EDX measurements (Supplementary Figs. S36 to S40).
The MB adsorption ratio of SiW12@UiO-66-NO2-H was >99% and 98% from 1 and 5 mg L−1 MB solutions after 1 h, respectively (Fig. 6a; Supplementary Fig. S41). These values are higher than those of SiW12@UiO-66-NO2 (98% and 65% for 1 and 5 mg L−1 MB solution, respectively) and SiW12@UiO-66-H (97% and 77% for 1 and 5 mg L−1 MB solution, respectively). Besides, the rate constants of the MB adsorption of SiW12@UiO-66-NO2 by a PSO mode were calculated to be 2.6 and 2.8 × 10−2 g mg−1 min−1 for 1 and 5 mg L−1 MB solution, which is larger than those of SiW12@UiO-66-NO2 (3.2 × 10−1 and 1.4 × 10−3 g mg−1 min−1 for 1 and 5 mg L−1 MB solution, respectively) and SiW12@UiO-66-H (1.5 × 10−1 and 3.2 × 10−3 g mg−1 min−1 for 1 and 5 mg L−1 MB solution, respectively). These results demonstrate that increasing the loading amounts of SiW12 on UiO-66 functionalized with −NO2 groups results in effective capture of MB. We note that the estimated particle size of SiW12@UiO-66-NO2-H is nearly identical to that of SiW12@UiO-66 analogs (Supplementary Fig. S40), minimizing the effect of particle size on MB uptake. Notably, the MB uptakes observed in SiW12@UiO-66-NO2-H (>99% and 98% for 1 and 5 mg L−1 MB solution, respectively) are comparable to that of representative zeolite such as ZSM-5 (53% and 11% for 1 and 5 mg L−1 MB solution, respectively) and zeolite Y (>99% and 95% for 1 and 5 mg L−1 MB solution, respectively) after 1 h under identical conditions (Fig. 6b; Supplementary Fig. S42), highlighting the outstanding MB uptake capacities of SiW12@UiO-66-NO2-H among porous materials.
Time course of MB uptake (MB = 5 mg L−1) of a) SiW12@UiO-66-NO2-H, SiW12@UiO-66-H, and SiW12@UiO-66-NO2 and b) SiW12@UiO-66-NO2-H, ZSM-5, and zeolite Y. The time course of MB adsorption was fitted by a PSO model.
3. Conclusion
In this work, we investigated a series of SiW12 and UiO-66 composite materials for their uptake of MB. The incorporation of SiW12 into UiO-66 resulted in a significantly improved MB removal rate compared to pristine UiO-66, indicating the effectiveness of SiW12@UiO-66 composites for MB sorption. This enhancement in MB sorption can be attributed to the Coulombic interaction between SiW12 anions and MB cations. Reducing the pore size of the SiW12@UiO-66 composite by substituting UiO-66 with MOF-801 led to decreased MB uptake, suggesting that larger pore sizes are more favorable for MB sorption. Furthermore, modifying the pore environment of SiW12@UiO-66 composites by functionalizing with −NH2 and −NO2 groups increased the MB uptake, likely due to host–guest dipole–ion interactions between the functional groups and MB cations, and SiW12@UiO-66-NO2 showed a higher efficiency. Increasing SiW12 loading into UiO-66 resulted in an increased MB uptake, and SiW12@UiO-66-NO2-H led to an outstanding MB capture comparable to that of representative zeolites such as ZSM-5 and zeolite Y. This work establishes a comprehensive relationship among pore size, pore environment, and MB uptake properties for the SiW12@UiO-66 analogs. SiW12@UiO-66 analogs are non-regenerable and cannot be reused, thus highlighting the need for improvement in their recyclability. We anticipate that optimizing the combination of POM and MOF for POM@MOF would enhance the MB uptake capability. Incorporation of redox-active POMs may represent a promising approach for effective MB sorption; the reduction of POMs increases the overall negative charge, thereby facilitating the uptake of cationic MB to compensate for the charge imbalance.49 In addition, the fabrication of POM@MOF-based membrane would represent a step toward practical application.50–53
Acknowledgments
Center of Advanced Instrumental Analysis, Kyushu University, is acknowledged for the SEM measurements. We thank Mr. R. Nakamura and Mr. K. Iwakiri for SEM observation.
Supplementary data
Supplementary material is available at Bulletin of the Chemical Society of Japan online.
Funding
This work was supported by the JSPS Grants-in-Aid for Scientific Research from MEXT of Japan (JP22H04914, JP22H05145, JP23H04613, and JP23K13759); the International Network on Polyoxometalate Science at Hiroshima University, JSPS Core-to-Core program (International Network on Polyoxometalate Science for Advanced Functional Energy Materials); and a research grant from Daiichi Kigenso Kagaku Kogyo Co., Ltd.
Data availability
The data supporting this article have been included as part of the Supplementary Information.
References
Naoki Ogiwara
Naoki Ogiwara received his Ph.D. degree from Kyoto University in 2019 under the supervision of Professor Hiroshi Kitagawa. He spent 1 year as a postdoctoral fellow at Tokyo Institute of Technology. In 2020, he became an assistant professor at the University of Tokyo working with Professor Sayaka Uchida. His research interests include porous coordination polymers and nanosized metal–oxo clusters for functional applications such as sorption, conductivity, and catalysis.
Sayaka Uchida
Sayaka Uchida received her Ph.D. degree from the Department of Applied Chemistry, School of Engineering, The University of Tokyo, in 2002, under the supervision of Professor Noritaka Mizuno. She became an assistant professor in the Mizuno group and was promoted to associate professor in 2009 in the School of Arts and Sciences at the same university. In 2023, she was appointed professor. Her research interests include synthesis of functional solid-state compounds based on metal–oxo clusters.
Author notes
Conflict of interest statement. None declared.
