Study on Gas Sorption and Iodine Uptake of a Metal-Organic Framework Based on Curcumin

Medi-MOF-1 is a highly porous Metal-Organic framework (MOF) constructed from Zn(II) and curcumin. The obtained crystal was characterized using powder X-ray diffraction (PXRD) and scanning electron microscopy (SEM). A micrometer-sized crystal with similar morphology was successfully obtained using the solvothermal method. Thanks to its high surface area, good stability, and abound pores, the as-synthesized medi-MOF-1 could be used as a functional porous material to adsorb different gases (H2, CO2, CH4, and N2) and iodine (I2). The activated sample exhibited a high I2 adsorption ability of 1.936 g g–1 at room temperature via vapor diffusion. Meanwhile, the adsorbed I2 could be released slowly in ethanol, confirming the potential application for I2 adsorption.


Introduction
With the rapid growth of global energy demand, the pursuit of energy storage technology has drawn special attention from scientists in recent years [1,2]. Compared to coal and petroleum, gaseous fuels are more friendly to the environment. However, the transportation and storage of gaseous fuels is a major challenge for researchers. Therefore, functional porous materials to capture or separate gas have been investigated, such as zeolite [3][4][5], Metal-Organic frameworks [6][7][8], and porous organic polymers [9,10]. Among these materials, Metal-Organic frameworks (MOFs), constructed from secondary building units (SBUs) and organic linkers, have been widely researched in the past decades. Due to their high porosities, tunable pore sizes, etc., the unique structural advantage of MOFs has been widely pursued for gas storage and separation [11][12][13][14], catalysis [15], chemical sensing [16][17][18], and so on. Hydrogen is considered as one of the alternative energy sources for fossil fuels. With a large surface area and tunable pore structure, MOFs can also provide unsaturated metal sites for hydrogen sorption [19][20][21]. At present, growing efforts to develop MOFs that efficiently capture or separate CO 2 is desired [22][23][24][25]. Compared to traditional methods, MOFs based on adsorption to capture or separate CO 2 can reduce energy consumption, showing great advantages in these technologies. Thus, constructing an economical and preferable tunable pore structure and pores is highly desirable.
Moreover, MOFs also exhibit potential applications for toxic waste elimination, such as heavy metal [26], uranium [27], iodine [28][29][30][31], and so on. Radioactive iodine 129 I and 131 I have a long half-life (1.57 × 10 7 years), which compounds damage the environment and human beings. How to dispose the nuclear waste timely and effectively has become an important issue that needs to be addressed. Although zerovalent iron [32], zeolite [33], and functionalized clays [34] have been used for radioiodine capture, their low absorption capacity and less interactive sites with iodine have limited the applications. Thus, MOFs with designable architecture and excellent properties have been synthesized for iodine capture. There are two kinds of MOFs for iodine adsorption, including non-iodine MOFs capture. There are two kinds of MOFs for iodine adsorption, including non-iodine MOFs and iodine-templated MOFs [35][36][37]. The presence of iodide groups in the framework can affect the iodine uptake [38,39]. For example, [(ZnI2)3(tpt)2] was constructed with ZnI2 nodes with tpt (2,4,6-tris(4-pyridyl)-1,3,5-triazine), which has a good I2 loading amount of 173 wt.% at room temperature, which is similar to reported for Cu-BTC (175 wt.%) [40]. In addition, non-iodine MOFs have been developed and utilized in the field to capture I2.
[Zr6O4(OH)4(edb)6]n can uptake iodine by means of chemi-and physisorption [41]. Although the presence of the iodide group in the framework can affect the iodine uptake, the structure of the organic ligands is too complex, which can make it difficult to obtain in the synthetic reaction. Hence, choosing a simple ligand and reaction process to construct porous MOFs for gas and I2 sorption is highly needed.
In our previous work [42], medi-MOF-1 was successfully synthesized using structurally symmetric ligand curcumin with the Zn(II) ion. In this study, we report on micrometer-sized porous medi-MOF-1 via large-scale reactions (Scheme 1). Subsequently, we studied the adsorption properties of medi-MOF-1 for H2, CO2, CH4, and N2. It is revealed that medi-MOF-1 can adsorb 1.57 wt.% H2 at 77 K and 1 bar, and displayed commendable CO2 adsorption and selectivity for CO2 over CH4 and N2 at 273 K. Furthermore, it is worth noting that medi-MOF-1 exhibits an outstanding I2 adsorption capacity of 1.936 g g −1 . It is also revealed that the I2 sorption process of medi-MOF-1 is reversible. Scheme 1. The synthetic process of medi-MOF-1.

Physicochemical Properties of Medi-MOF-1
As reported previously by us, medi-MOF-1 crystallizes in the trigonal chiral space group P3221. It is constructed by trinuclear clusters and curcumin ligands, leading to a three-dimensional (3D) porous coordination framework (Scheme 1). In this work, we synthesized medi-MOF-1 by expanding the reaction by 10 times. Powder X-ray diffraction (PXRD) of medi-MOF-1 confirmed the phase purity of the bulk crystalline materials due to the same PXRD pattern with the simulated data ( Figure 1b). The pore structure properties of medi-MOF-1 were characterized at 77 K. The N2 adsorption-desorption isotherm of type-I adsorption curves with a capillary in the low P/P0 region. It confirms that the assynthesized medi-MOF-1 is microporous. The adsorption isotherm data were fitted to the Langmuir equation and gave a surface area of 563 m 2 g −1 (BET surface area: 475 m 2 g −1 ). This surface area is similar to nanosized medi-MOF-1 particles but far below the value of 2675 m 2 g −1 reported large size of the crystal [42]. The decreased size and crystallinity may be the main reason for reducing the BET of medi-MOF-1 [43]. Subsequently, scanning electron microscopy (SEM) technologies have been widely used to study the morphology of nanoparticles. Herein, SEM images were recorded for inspecting the morphology and structure of as-synthesized medi-MOF-1. As seen in Figure 1c,d, SEM images show that the prepared solid samples are agglomerated with small nanocrystals which have good uniformity and dimensional consistency. SEM images showed rod-like crystals of medi-MOF-1 with a diameter of 0.2 µm and length of 1 µm which had a similar morphology to Scheme 1. The synthetic process of medi-MOF-1.

Physicochemical Properties of Medi-MOF-1
As reported previously by us, medi-MOF-1 crystallizes in the trigonal chiral space group P3 2 21. It is constructed by trinuclear clusters and curcumin ligands, leading to a three-dimensional (3D) porous coordination framework (Scheme 1). In this work, we synthesized medi-MOF-1 by expanding the reaction by 10 times. Powder X-ray diffraction (PXRD) of medi-MOF-1 confirmed the phase purity of the bulk crystalline materials due to the same PXRD pattern with the simulated data ( Figure 1b). The pore structure properties of medi-MOF-1 were characterized at 77 K. The N 2 adsorption-desorption isotherm of type-I adsorption curves with a capillary in the low P/P 0 region. It confirms that the assynthesized medi-MOF-1 is microporous. The adsorption isotherm data were fitted to the Langmuir equation and gave a surface area of 563 m 2 g −1 (BET surface area: 475 m 2 g −1 ). This surface area is similar to nanosized medi-MOF-1 particles but far below the value of 2675 m 2 g −1 reported large size of the crystal [42]. The decreased size and crystallinity may be the main reason for reducing the BET of medi-MOF-1 [43]. Subsequently, scanning electron microscopy (SEM) technologies have been widely used to study the morphology of nanoparticles. Herein, SEM images were recorded for inspecting the morphology and structure of as-synthesized medi-MOF-1. As seen in Figure 1c,d, SEM images show that the prepared solid samples are agglomerated with small nanocrystals which have good uniformity and dimensional consistency. SEM images showed rod-like crystals of medi-MOF-1 with a diameter of 0.2 µm and length of 1 µm which had a similar morphology to that of larger crystals [42]. According to these results, the as-synthesized medi-MOF-1 can be successfully synthesized as crystalline powder materials.
Molecules 2023, 28, x FOR PEER REVIEW 3 of 10 that of larger crystals [42]. According to these results, the as-synthesized medi-MOF-1 can be successfully synthesized as crystalline powder materials.

Gas Sorption Properties
Much effort has been devoted to hydrogen storage since hydrogen is considered to be an excellent alternative energy source. In order to test the hydrogen uptake of medi-MOF-1, the as-synthesized medi-MOF-1 was activated after soaking in the solvent of CH2Cl2 for 2 days [44] and heating at 100 °C under vacuum for 8 h. The H2 sorption experiments of activated samples were measured, which showed that the H2 uptake of medi-MOF-1 is as high as 1.57 wt.% at 77 K and 1 atm (Figure 2a), which is similar to bio-MOF-11 constructed from biomolecules solely [45]. We calculated the isosteric heats of adsorption (Qst) of H2 using the Clausius equation following the fitting of the isotherm data at 77 and 87 K using a virial equation. The initial Qst value for H2 of medi-MOF-1 is calculated to be −6.25 kJ mol −1 at zero coverage ( Figure 2b). Furthermore, the initial Qst value for H2 of medi-MOF-1 is smaller than that of bio-MOF-11 (−13 kJ mol −1 ) [46,47]. These results may be attributed to the more metal clusters of bio-MOF-11, for the force of metal clusters on hydrogen is greater than that of the benzene ring on hydrogen. Additionally, the M2(olz) materials are bioactive frameworks with similar frameworks exhibiting potential H2 storage capacities [48]. The relevant BET data and H2 adsorption capacities of the selected MOFs constructed with biomolecules or drug molecules are summarized and tabulated in Table S1. The isosteric heat of Zn2(olz) adsorption is similar to medi-MOF-1, which is lower than other M2(olz) frameworks in the series due to fewer open metal sites in the activated materials.

Gas Sorption Properties
Much effort has been devoted to hydrogen storage since hydrogen is considered to be an excellent alternative energy source. In order to test the hydrogen uptake of medi-MOF-1, the as-synthesized medi-MOF-1 was activated after soaking in the solvent of CH 2 Cl 2 for 2 days [44] and heating at 100 • C under vacuum for 8 h. The H 2 sorption experiments of activated samples were measured, which showed that the H 2 uptake of medi-MOF-1 is as high as 1.57 wt.% at 77 K and 1 atm (Figure 2a), which is similar to bio-MOF-11 constructed from biomolecules solely [45]. We calculated the isosteric heats of adsorption (Q st ) of H 2 using the Clausius equation following the fitting of the isotherm data at 77 and 87 K using a virial equation. The initial Q st value for H 2 of medi-MOF-1 is calculated to be −6.25 kJ mol −1 at zero coverage ( Figure 2b). Furthermore, the initial Q st value for H 2 of medi-MOF-1 is smaller than that of bio-MOF-11 (−13 kJ mol −1 ) [46,47]. These results may be attributed to the more metal clusters of bio-MOF-11, for the force of metal clusters on hydrogen is greater than that of the benzene ring on hydrogen. Additionally, the M 2 (olz) materials are bioactive frameworks with similar frameworks exhibiting potential H 2 storage capacities [48]. The relevant BET data and H 2 adsorption capacities of the selected MOFs constructed with biomolecules or drug molecules are summarized and tabulated in Table S1. The isosteric heat of Zn 2 (olz) adsorption is similar to medi-MOF-1, which is lower than other M 2 (olz) frameworks in the series due to fewer open metal sites in the activated materials.
As we know, the separation of CO 2 to CH 4 and N 2 by porous materials is favorable in the environment. In this text, we measured the adsorption ability of different gases in medi-MOF-1. At 273 K and 1 bar, medi-MOF-1 exhibits higher uptake of CO 2 than CH 4 and N 2 -which is 34.7 cm 3 g −1 (1.55 mmol g −1 ). The datapoint is lower than those of bio-MOFs constructed with biomolecules [49]. The reason for this phenomenon may be the fewer adsorption sites in the framework of medi-MOF-1. The relevant BET data and CO 2 adsorption capacities of the selected bio-MOFs are summarized and tabulated in Table 1. The maximum uptakes of CH 4 and N 2 are 13.9 cm 3 g −1 and 5.95 cm 3 g −1 at 273 K and Molecules 2023, 28, 5237 4 of 10 1 bar, respectively (Figure 3a), which are lower than that of CO 2 in the same conditions. Especially, the uptake of CO 2 for medi-MOF-1 is 2.5 times as large as CH 4 and 5.8 times as large as that of N 2 . The adsorption selectivity of CO 2 relative to CH 4 and N 2 was calculated using Henry's law before 0.1 bar. Based on Henry's law, the material shows CO 2 over N 2 or CH 4 adsorption selectivity (Figure 3b), ranking medi-MOF-1 as better porous adsorbents constructed from biomolecules for separating CO 2 from N 2 [50][51][52]. As we know, the separation of CO2 to CH4 and N2 by porous materials is favorabl in the environment. In this text, we measured the adsorption ability of different gases i medi-MOF-1. At 273 K and 1 bar, medi-MOF-1 exhibits higher uptake of CO2 than CH and N2-which is 34.7 cm 3 g −1 (1.55 mmol g −1 ). The datapoint is lower than those of bio MOFs constructed with biomolecules [49]. The reason for this phenomenon may be th fewer adsorption sites in the framework of medi-MOF-1. The relevant BET data and CO adsorption capacities of the selected bio-MOFs are summarized and tabulated in Table  The maximum uptakes of CH4 and N2 are 13.9 cm 3 g −1 and 5.95 cm 3 g −1 at 273 K and 1 ba respectively (Figure 3a), which are lower than that of CO2 in the same conditions. Espe cially, the uptake of CO2 for medi-MOF-1 is 2.5 times as large as CH4 and 5.8 times as larg as that of N2. The adsorption selectivity of CO2 relative to CH4 and N2 was calculated usin Henry's law before 0.1 bar. Based on Henry's law, the material shows CO2 over N2 or CH adsorption selectivity (Figure 3b), ranking medi-MOF-1 as better porous adsorbents con structed from biomolecules for separating CO2 from N2 [50][51][52].  Ref.   As we know, the separation of CO2 to CH4 and N2 by porous materials is favorable in the environment. In this text, we measured the adsorption ability of different gases in medi-MOF-1. At 273 K and 1 bar, medi-MOF-1 exhibits higher uptake of CO2 than CH4 and N2-which is 34.7 cm 3 g −1 (1.55 mmol g −1 ). The datapoint is lower than those of bio-MOFs constructed with biomolecules [49]. The reason for this phenomenon may be the fewer adsorption sites in the framework of medi-MOF-1. The relevant BET data and CO2 adsorption capacities of the selected bio-MOFs are summarized and tabulated in Table 1. The maximum uptakes of CH4 and N2 are 13.9 cm 3 g −1 and 5.95 cm 3 g −1 at 273 K and 1 bar, respectively (Figure 3a), which are lower than that of CO2 in the same conditions. Especially, the uptake of CO2 for medi-MOF-1 is 2.5 times as large as CH4 and 5.8 times as large as that of N2. The adsorption selectivity of CO2 relative to CH4 and N2 was calculated using Henry's law before 0.1 bar. Based on Henry's law, the material shows CO2 over N2 or CH4 adsorption selectivity (Figure 3b), ranking medi-MOF-1 as better porous adsorbents constructed from biomolecules for separating CO2 from N2 [50][51][52].

Iodine Uptake and Release
To prevent the presence of radioactive toxic gases in the environment, researchers have focused on the preparation of selective porous materials. As medi-MOF-1 has developed porosity and a stable crystalline structure, the I 2 uptake experiment was carried out in the vapor phase. Before the adsorption experiments were started, the sample was activated at 60 • C for 6 h under a vacuum. Iodine uptake was measured using the gravimetric method. In the vapor phase, 30 mg of medi-MOF-1 [39] was placed in an I 2 chamber at 298 K. After 12 h, the color of medi-MOF-1 changed from orange-red to dark brown. No further change was observed after 10 h, and the maximum adsorbed amount of I 2 was as high as 1.936 g g −1 (Figure 4a). Compared to other typically porous MOFs usually used for I 2 adsorption via vapor diffusion, medi-MOF-1 also exhibits higher I 2 adsorption. The relevant BET data and iodine adsorption capacities of the selected MOFs are summarized and tabulated in Table 2. This result may be attributed to the frameworks with conjugated π-electrons, which could produce multiple interactions for iodine [54,55]. Iodine templates are introduced into the assembly process of MOFs such as (ZnI 2 ) 3 (tpt) 2 at room temperature in order to affect the uptake capacities of iodine [56]. Owing to medi-MOF-1 can keep its crystal structure unchanged in ethanol, we soaked 100 mg of medi-MOF-1 crystals in 3 mL of a dry ethanol solution of I 2 in a sealed glass vial at room temperature. After 48 h, I 2 molecules were mostly adsorbed by the free active sites in medi-MOF-1, and no more free sites were left. There is almost no N 2 sorption in the low-pressure region after the incorporation of iodine, indicating that I 2 completely fills the pores (Figure 4b). A PXRD study was carried out before and after the I 2 absorption experiment and proved that the framework of medi-MOF-1 retained the host framework crystallinity after loading the I 2 molecules (Figure 4c). Thermogravimetric analysis (TGA) was performed to check the I 2 loading amount. The I 2 @MOF showed a weight loss of~52% from 100 to 500 • C (Figure 4d). Combined with the thermogravimetric curve and the molecular formula of medi-MOF-1, it can be calculated as 500 mg of iodine per gram of MOF. Compared to other porous MOFs usually used for I 2 adsorption via solution-based processes in ethanol, medi-MOF-1 also exhibits higher iodine adsorption than JLU-Liu14 [57,58]. The high loading of I 2 for medi-MOF-1 can be attributed to the different pore sizes of these MOFs and the possible interaction between the porous skeleton and I 2 .
method. In the vapor phase, 30 mg of medi-MOF-1 [39] was placed in an I2 cham K. After 12 h, the color of medi-MOF-1 changed from orange-red to dark brow ther change was observed after 10 h, and the maximum adsorbed amount of I2 w as 1.936 g g −1 (Figure 4a). Compared to other typically porous MOFs usually u adsorption via vapor diffusion, medi-MOF-1 also exhibits higher I2 adsorption vant BET data and iodine adsorption capacities of the selected MOFs are summa tabulated in Table 2. This result may be attributed to the frameworks with conj electrons, which could produce multiple interactions for iodine [54,55]. Iodine are introduced into the assembly process of MOFs such as (ZnI2)3(tpt)2 at room ture in order to affect the uptake capacities of iodine [56]. Owing to medi-MOF-1 its crystal structure unchanged in ethanol, we soaked 100 mg of medi-MOF-1 cr mL of a dry ethanol solution of I2 in a sealed glass vial at room temperature. Af molecules were mostly adsorbed by the free active sites in medi-MOF-1, and no sites were left. There is almost no N2 sorption in the low-pressure region after poration of iodine, indicating that I2 completely fills the pores (Figure 4b). A PX was carried out before and after the I2 absorption experiment and proved that work of medi-MOF-1 retained the host framework crystallinity after loading th cules (Figure 4c). Thermogravimetric analysis (TGA) was performed to check t ing amount. The I2@MOF showed a weight loss of ~52% from 100 to 500 °C (F Combined with the thermogravimetric curve and the molecular formula of med it can be calculated as 500 mg of iodine per gram of MOF. Compared to oth MOFs usually used for I2 adsorption via solution-based processes in ethanol, m 1 also exhibits higher iodine adsorption than JLU-Liu14 [57,58]. The high loadin medi-MOF-1 can be attributed to the different pore sizes of these MOFs and th interaction between the porous skeleton and I2.   From the point of view of recyclability, the I 2 desorption from the porous framework is also essential. The I 2 release process was detected by UV-visible (UV-vis) spectra. The captured I 2 could be easily separated from the frameworks upon immersion in I 2 -loaded medi-MOF-1 in ethanol. A UV-vis spectrophotometer recorded the release of I 2 from the medi-MOF-1 framework at different times at room temperature. When the I 2 @medi-MOF-1 was soaked in dry ethanol, the color of the iodine-loaded sample changed gradually from dark brown to orange-red. Afterwards, the release slows down and subsequently, the color of the ethanol solution changes from colorless to yellow. As illustrated in Figure 5, UV-vis spectra show absorption bands at λ max = 218 and 263 nm, which can be attributed to I 2 . And the band observed at 263 nm may be assigned to polyiodide ions (I 3 − ), established due to the reaction of I 2 with decomposed iodide. The release of iodine increased with time, suggesting that this desorption behavior of iodine is based on host-guest interactions. This adsorption and release of I 2 by medi-MOF-1 reveals that the I 2 sorption process of the MOF is reversible. From the point of view of recyclability, the I2 desorption from the porous fram is also essential. The I2 release process was detected by UV-visible (UV-vis) spect captured I2 could be easily separated from the frameworks upon immersion in I2medi-MOF-1 in ethanol. A UV-vis spectrophotometer recorded the release of I2 fr medi-MOF-1 framework at different times at room temperature. When the I2@med 1 was soaked in dry ethanol, the color of the iodine-loaded sample changed gr from dark brown to orange-red. Afterwards, the release slows down and subseq the color of the ethanol solution changes from colorless to yellow. As illustrated in 5, UV-vis spectra show absorption bands at λmax = 218 and 263 nm, which can be att to I2. And the band observed at 263 nm may be assigned to polyiodide ions (I3 − ), esta due to the reaction of I2 with decomposed iodide. The release of iodine increase time, suggesting that this desorption behavior of iodine is based on host-guest tions. This adsorption and release of I2 by medi-MOF-1 reveals that the I2 sorption of the MOF is reversible.

Materials and General Methods
All reagents were obtained from commercial sources and used as received. other chemical reagents used were of AR grade. PXRD was collected on a Rigaku 2550 X-ray diffractometer with Cu-Kα radiation (λ = 1.5418 Å). FT-IR spectra w tained by a Nicolet Impact 410 Fourier-transform infrared spectrometer in the 40 cm −1 range with KBr pellets. TGA was performed under an air atmosphere in the r 30-800 °C at a heating rate of 10 °C min −1 using a Perkin-Elmer TGA 7 thermograv analyzer. Gas adsorption and desorption isotherms were measured on Quantachr

Materials and General Methods
All reagents were obtained from commercial sources and used as received. All the other chemical reagents used were of AR grade. PXRD was collected on a Rigaku D/Max 2550 X-ray diffractometer with Cu-Kα radiation (λ = 1.5418 Å). FT-IR spectra were obtained by a Nicolet Impact 410 Fourier-transform infrared spectrometer in the 400-4000 cm −1 range with KBr pellets. TGA was performed under an air atmosphere in the range of 30-800 • C at a heating rate of 10 • C min −1 using a Perkin-Elmer TGA 7 thermogravimetric analyzer.
Gas adsorption and desorption isotherms were measured on Quantachrom Autosorb-iQ after degassing the sample for 8 h at 100 • C. H 2 adsorption tests were performed at 77 and 87 K. CO 2 , CH 4 , and N 2 adsorption tests were performed at 273 K. The morphologies of the powder materials were recorded using a JEOL-JSM-6700F field-emission scanning electron microscope (SEM).

Synthesis of Medi-MOF-1 and Iodine-Loaded Medi-MOF-1
Medi-MOF-1 have been produced on a large scale. A mixture of Zn(OAc) 2 ·2H 2 O (200 mg, 0.9112 mmol), curcumin (600 mg, 1.6287 mmol), N,N-dimethylformamide (40 mL), and ethanol (10 mL) was sealed into a 100 mL capped vessel. The vessel was heated at 75 • C. The crystals were obtained after 5 days and washed with DMF. Yield: 55% (based on curcumin). Iodine uptake experiment was carried out via vapor diffusion and solution phase. A 100 mg sample was immersed in an ethanol solution of iodine. The complete absorption experiment was done at room temperature for 48 h.

Conclusions
In summary, we report on large-scale reactions to synthesize medi-MOF-1 toward the capture of I 2 and gas sorption. We utilized medi-MOF-1 based on curcumin, which displays an interesting and important ability to capture I 2 (1.936 g g −1 ) at room temperature. The medi-MOF-1 microcrystals have lower Langmuir specific surface areas than previously published medi-MOF-1. In particular, it is amongst the more efficient porous materials for I 2 adsorption. Furthermore, medi-MOF-1 also exhibits hydrogen adsorption capacities of 1.57 wt.% at 77 K and 1 bar. The adsorption capacity of CO 2 by activated medi-MOF-1 is greater than that of CH 4 and N 2 at 273 K, and the selectivities of CO 2 /N 2 and CO 2 /CH 4 are 5.8 and 2.5, respectively. This study aims to provide material for the potential application of I 2 adsorption as well as gas adsorption and separations.

Data Availability Statement:
No new data were created or analyzed in this study in this study. Data sharing is not applicable to this article.

Conflicts of Interest:
The authors declare no conflict of interest.