Metal-organic framework for removing nitrates from aqueous solutions

Abstract

A metal-organic framework containing [[Cu2(Hbtc)2(bpe)2]] and / or [[Cu2(Hbtc)2(b-py)2(H2O)2]], wherein bpe represents 1,2-bis(4-pyridyl)ethane, Hbtc represents 1,3,5-benzenetricarboxylic acid, and bpy represents 4,4'-bipyridine; a method for adsorbing and removing nitrates from an aqueous solution using the metal-organic framework; regeneration of the nitrate-containing adsorbent; and use of these compounds as adsorbents for nitrates.

Description

Technical Field

[0001] The present invention relates to a metal-organic framework. The present invention further relates to a method for adsorbing nitrate from an aqueous solution using an adsorbent. Furthermore, the present invention relates to a method for producing a metal-organic framework. Background of the Invention

[0002] Nitrate is sometimes a very problematic environmental pollutant because it is widely used in agriculture. Furthermore, since nitrate is highly water-soluble, it is easy for nitrate ions to disperse in soil and thus in groundwater. High nitrate concentrations cause eutrophication of water areas and have an adverse effect on the quality of drinking water.

[0003] According to Directive 2006 / 118 / EC of the European Parliament and of the Council of 12 December 2006 for the protection of groundwater against pollution and deterioration, the limit value of NO3 in drinking water is set at a maximum of 50 mg / l. Therefore, in areas with high nitrate pollution in soil and groundwater, it is necessary to remove nitrate. - In order to protect groundwater from pollution and deterioration, according to Directive 2006 / 118 / EC of the European Parliament and of the Council of 12 December 2006, the limit value of NO3 in drinking water is set at a maximum of 50 mg / l. Therefore, in areas with high nitrate pollution in soil and groundwater, it is necessary to remove nitrate.

[0004] To remove dissolved nitrate ions from an aqueous solution, various methods according to the prior art are used, such as, for example, reverse osmosis, ion exchange, biological denitrification, catalytic denitrification, electrodialysis, treatment with magnesium, chemical denitrification with iron, etc.

[0005] However, these methods are partly complex and partly not very efficient. Therefore, in order to reduce environmental pollution, an improved method for removing nitrate ions from an aqueous solution is needed.

[0006] (Brief Description of the Invention) Therefore, an object of the present invention is to provide a method for adsorbing nitrate (NO3) from an aqueous solution having a higher adsorption capacity of NO3 than the prior art. Furthermore, an object of the present invention is to provide a selective adsorbent for NO3.

[0007] This object is, on the one hand, [Cu2(Hbtc)2(bpe)2](bpe) and / or [Cu2(Hbtc)2(bpy)2(H2O)2](bpy) A metal-organic framework comprising bpe represents 1,2-bis(4-pyridyl)ethane, btc represents 1,3,5-benzenetricarboxylic acid (trimesic acid), bpy represents 4,4'-bipyridine is solved by a metal-organic framework.

[0008] One of the two metal-organic frameworks is [Cu2(Hbtc)2(bpe)2](bpe), where bpe represents 1,2-bis(4-pyridyl)ethane and Hbtc represents 1,3,5-benzenetricarboxylic acid, hereinafter referred to as Compound 1, MOF-1 or 11.

[0009] The other of the two metal-organic frameworks is [Cu2(Hbtc)2(bpy)2(H2O)2](bpy), where Hbtc represents 1,3,5-benzenetricarboxylic acid and bpy represents 4,4'-bipyridine, hereinafter referred to as Compound 2, MOF-2 or 2.

[0010] Compounds 1 and 2 are metal-organic frameworks (MOFs).

[0011] Compounds 1 and 2 are suitable for use in or as an adsorbent, and thus an adsorbent comprising Compound 1 and / or Compound 2 is provided.

[0012] On the other hand, the above problem is solved by a method for removing nitrate (NO3 - ) from an aqueous solution, in which the aqueous solution is brought into contact with an adsorbent, characterized in that the adsorbent contains Compound 1 and / or Compound 2.

[0013] The inventors have found that a special metal-organic framework can remove nitrate ions (NO3 -) It has been discovered that it can adsorb highly selectively and in high yield. Generally, the adsorption of nitrate ions by an adsorbent is considered attractive compared to chemical methods because the adsorbent is environmentally friendly. Furthermore, such a method becomes cheaper and faster when using an adsorbent with easy regeneration and high cost-effectiveness.

[0014] A metal-organic framework (MOF), abbreviated, is a type of organic-inorganic hybrid functional material composed of inorganic units and organic molecules as linkers between the inorganic units, and has a microporous structure.

[0015] Basically, most MOFs are highly porous and thus have a large surface area, so it is known to use MOFs as non-specific adsorbents. Although their use for the adsorption of various impurities has been described, their application in aqueous systems has not yet been successful because MOFs have relatively low stability in aqueous environments.

[0016] Water can cause structural collapse, crystal phase transition, morphological change, and defect formation in MOFs.

[0017] Surprisingly, a specific copper-based MOF having two different ligands according to the present invention disclosed herein is not only stable in aqueous solution but also specific for the adsorption of NO3 - .

[0018] Copper is inexpensive and non-toxic, and together with the above-mentioned ligands 1,2-bis(4-pyridyl)ethane, 1,3,5-benzenetricarboxylic acid, and 4,4'-bipyridine, forms a strong and selective complexing agent for NO3 - . Compounds 1 and 2 have been shown to be stable over a wide pH range and a wide temperature range.

[0019] In contrast to the known MOFs, Compound 2 has a complex structure of a distorted square pyramid around the copper atom. In Compound 2, the copper(II) atom forms unique bonds with two carboxylic acid oxygen atoms of trimesic acid and two nitrogen atoms of the bpy ligand and the aqua ligand.

[0020] Compound 2 has a complex crystal structure of "zigzag chains" that are connected to each other to form layers. These layers are further interconnected, thereby forming a three-dimensional structure. Furthermore, Compound 2 exhibits a unique connectivity pattern in which both the btc ligand and the bpy ligand play a decisive role in the bonding of adjacent copper(II) atoms.

[0021] (Detailed Description of the Invention) The further details and advantages of the present invention will be described below with reference to examples and figures.

Brief Description of the Drawings

[0022]

Fig. 1a

[0010] of the crystal structure of Compound 1 (Figure 1a) and the projection views along

[0001] of the crystal structure of Compound 2 (Figure 1b) are shown as "capsticks", respectively.

Fig. 1b

[0010] of the crystal structure of Compound 1 (Figure 1a) and the projection views along

[0001] of the crystal structure of Compound 2 (Figure 1b) are shown as "capsticks", respectively.

Fig. 2

Fig. 3

Fig. 4

Fig. 5

[0023] The structural and physicochemical properties of copper-based MOF-1 and MOF-2 were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FTIR), total reflection X-ray fluorescence analysis (TRFA), and N2 physical adsorption. The effects of various factors on the nitrate removal efficiency and adsorption capacity were examined, including the dosage of adsorbent, exposure time, competing ions, and recyclability.

[0024] Synthesis of Compounds 1 and 2 Synthesis of Compound 1: [Cu2(Hbtc)2(bpe)2](bpe) In an exemplary experiment, copper(II) acetate monohydrate (0.199 g, 1 mmol) was dissolved in 15 mL of H2O with gentle stirring. This first solution was added to a second solution. The second solution consisted of 1,3,5-benzenetricarboxylic acid (Hbtc; 0.141 g; 0.67 mmol), 1,2-bis(4-pyridyl)ethane (bpe; 0.184 g; 1 mmol), and NaOH (0.080 g, 2 mmol) in 15 mL of H2O.

[0025] Subsequently, this mixture was placed in a 40 mL stainless-steel autoclave lined with Teflon, sealed, and heated at 120 °C for about 24 hours. After cooling to room temperature, the resulting product was filtered and washed with 100 mL of distilled water. The resulting solid was dried overnight in a vacuum oven at 50 °C.

[0026] Synthesis of Compound 2: [Cu2(Hbtc)2(bpy)2(H2O)2]·(bpy) In a method similar to the synthesis of [Cu2(Hbtc)2(bpe)2](bpe), for the synthesis of [Cu2(Hbtc)2(bpy)2(H2O)2]·(bpy), first 4,4'-bipyridine (0.156 g = 1 mmol) was dissolved in 15 mL of H2O and added to 15 mL of H2O together with 1,3,5-benzenetricarboxylic acid to a solution of copper(II) acetate monohydrate (0.199 g, 1 mmol).

[0027] Subsequently, this mixture was placed in a 40 mL stainless-steel autoclave lined with Teflon, sealed, and heated at 120 °C for about 24 hours. The resulting dark blue solid was filtered through a dense filter paper. The blue crude product was washed with distilled water, filtered off, and dried overnight in a vacuum oven at 50 °C.

[0028] In Figure 1a, for Compound 1, the copper(II) atom has a coordination number of 5 with four short bonds of similar length (about 2.0 Å) to two N atoms (N1, N2) of the trans-positioned bpe ligand and two carboxylic acid O atoms (O3, O5) of two Hbtc ligands. The fifth ligating atom is derived from a third carboxylic acid O atom (O4) at a relatively large distance of 2.23 Å, defining a distorted square pyramid (τ5 = 0.285, where τ5 = 0 for an ideal square pyramid and τ5 = 1 for an ideal trigonal bipyramid).

[0029] In Figure 1b, for Compound 2, it can be recognized that the structure has a coordination number of 5 with a distorted square pyramid (τ5 = 0.306) and a copper(II) atom with a bond length distribution similar to that of Compound 1. Two carboxylic acid O atoms (O2, O5) and two trans-positioned N atoms (N1, N2) define the base of the pyramid at short distances of about 2.0 Å. The apex of the pyramid is derived from an aqua ligand (O1W) at a longer distance of 2.32 Å. Similarly, the nearest O atom is at a rather large distance of 2.69 Å from the central copper(II) atom. Adjacent chains are extended parallel to

[0100] by medium-strength hydrogen-bonding between the aqua ligand and the non-coordinating carboxylic acid O atoms of the Hbtc ligand (O1, O6; O···O distance is 2.69 Å - 2.74 Å and the OH···O angle is 160° - 177°). The parallel layers are connected by μ2-bridging bpy ligands, resulting in a three-dimensional structure that defines channels parallel to

[0001] . As in 1, the channels are filled here with non-coordinating solvent molecules, which are also bonded via strong OH···N hydrogen bonds (O4···N3 = 2.61 Å; O4 - H4···N3 = 166°) between the carboxylic acid group and the pyridine N atom.

[0030] Synthesis of HKUST-1 (Cu-BTC) MOF Cu(OAc)2·H2O (0.359 g, 1.8 mmol) and 1,3,5-benzenetricarboxylic acid (H3btc; 0.210 g; 1.0 mmol) were ground by hand for 15 minutes. The resulting powder was washed with a small amount of DMF to remove unreacted starting materials. Subsequently, the powder was dried in air overnight to isolate HKUST-1.

[0031] The N2 physical adsorption isotherm was measured at 77 K using a Micromeritics ASAP2020 instrument. Before measurement, the sample was degassed at 150 °C under vacuum for 12 hours. The total pore volume of all samples was estimated from the amount of nitrogen adsorbed at P / P0 = 0.95. The apparent surface area was calculated using the Brunauer-Emmet-Teller (BET) equation. The related pore size distribution was calculated from the adsorption branch of the isotherm by applying the theory of Barrett, Joyner, and Halenda (BJH).

[0032] The structural properties such as the pore volume, average pore diameter, or BET surface area of the metal-organic framework are summarized in Table 1.

[0033] [Table 1]

[0034] Stability test Figure 2 summarizes the results of the stability tests of two compounds 1 and 2 in an aqueous solution after immersion in water. X-ray diffraction tests (XRD) were carried out immediately after production (0), and after 10, 30, 50, 100, 150, and 200 days. The X-ray diffraction test of compound 1 is summarized in graph a, and that of compound 2 is summarized in graph b, where the intensity is in arbitrary units. The X-ray diffraction test shows recognizable structural changes over time, and as a result, it can be assumed that there is no change in the crystal structure, from which it can be inferred that both MOF1 and 2 with mixed ligands are unexpectedly stable in water at room temperature. Graph C (Figure 2) shows total X-ray fluorescence analysis (TXRF) over 20, 30, 40, 50, 130, and 200 days, which proves that the proportion of metal ions (Cu ions) entering the solution can be ignored (the tolerance is about 10%). Graph (d) shows the spectra of TXRF measurements of two MOF1 and 2 after immersion in water for 200 days, and the peaks are shown for silicon, Ar, Ca (mainly from water), Cr (reference), or Kα and Kβ of Cu.

[0035] The stability of the skeletal structure in an aqueous environment belongs to the most important factors to be considered when using MOF for the treatment of aqueous hazardous substances. Excessively low stability has limited the use of MOF in aqueous solutions so far. In hydrolysis, the reaction with water molecules causes the breakage of the ligand-metal bond.

[0036] In the case of ligands replaced by water, as in Equation 1, the exchange is caused by water molecules embedded in the metal-ligand bond of the metal-organic framework. Equation 1: M-L+H2O→M-(H2O)...L Equation 1

[0037] In hydrolysis, the interaction between the metal and the ligand begins. Water molecules are decomposed into HO and H as shown in Equation 2, where HO interacts with the metal and H binds to the ligand. M-L+H2O→M-(OH)+LH Equation 2

[0038] The inventors evaluated the water stability of the MOF by two different measurements hereinafter referred to as "short-term" experiment and "long-term" experiment. In the long-term experiment, the MOF according to the present invention was immersed in distilled water (pH: 6.8) and stored at room temperature for up to 120 days. This is a method that reflects most of the stability tests in the literature. Using XRD, changes in the framework structure were observed throughout the immersion period. Both 1 and 2 were stable in water for 200 days at room temperature and showed no significant structural changes (Figure 2a). The proportion of metal ions entering the solution was also negligibly small, thus enabling excellent reproducibility of adsorption measurements (Figure 5).

[0039] In the short-term experiment, the stability of the MOF was observed in water at 100 °C. The XRD tests of 1 and 2 showed no changes up to 16 hours, indicating a remarkable level of stability.

[0040] In summary, both MOFs have extraordinary high stability in water at both room temperature and 100 °C, making these MOFs the most stable ones among those known so far. The high water stability is due to the strong coordination of two different ligands of Cu. However, the much lower stability of the MOF having only one ligand (HKUST-1, also called MOF-199) for only 3 days suggests a synergistic contribution of the two ligands.

[0041] Adsorption of Nitrate Figure 3 shows the adsorption capacity (graph a) of Compounds 1 and 2 compared with the adsorbent HKUST-1. This graph shows the effect of the dosage of each adsorbent (Compound 1 and Compound 2 in graph a, HKUST-1 in graph b) related to the nitrate removal efficiency. The initial concentration of nitrate was 15 mg L -1 at pH value 6.8 and temperature 25 °C, respectively.

[0042] In Figure 4, to show the specificity for nitrates, the adsorption capacities of Compound 1 (graph a, left) and Compound 2 (graph a, right) in the presence of various competing ions are shown (◆ represents nitrate solution, ● represents nitrate solution in the presence of sulfate, ▲ represents nitrate solution in the presence of chloride, ■ represents nitrate solution in the presence of phosphate). The initial concentration of nitrates was 10 - 200 mg L -1 -1. The concentration of the adsorbent was 12 mg L -1 -1 in a volume of 0.05 L (pH value: 6.8; temperature: 25 °C). It has significantly higher specificity compared to nitrates, sulfates, chlorides, and phosphates. Graph b (left: Langmuir isotherm, right: Freundlich isotherm) shows the adjusted results of nitrate adsorption. In actual groundwater samples (graph c), both Compound 1 and 2 showed similar results. The groundwater samples were collected from the western part of the Mashhad urban aquifer (graph c, bottom), with an initial nitrate concentration = 8.46 mg L -1 -1, or from the southern part of the Mashhad urban aquifer (graph c, top), with an initial nitrate concentration = 70.56 mg L -1 (adsorbent dosage, 12 mg; volume = 0.05 l; pH value: 6.8; temperature: 25 °C) 12 mg L -1 -1.

[0043] The adsorption capacities of Compound 1 and Compound 2 for nitrate adsorption were examined using aqueous solutions containing different adsorbent concentrations varying from 2 - 20 mg L -1 -1. Figure 3a shows that the nitrate removal efficiency (mol% of nitrate removal relative to the total amount added) increased and increased almost linearly at low concentrations. Immediately beyond an adsorbent dosage of 12 mg L -1 -1, the nitrate removal efficiency reached saturation. Therefore, in all subsequent adsorption experiments, the adsorbent dosage was set at 12 mg L -1 -1. The nitrate removal efficiency of 1 was 90.02 mol%, which was slightly higher than that of 2 (86.2%, 2.07 mg L -1 -1 nitrate). For other NO3 -1 -1 -The adsorption efficiencies at 1 and 2 were far better than those of the remover (see Table 2). Considering that these values were obtained at pH 7, this is even more remarkable. That is, in the prior art, the adsorption capacity for nitrate removal in water is lower at pH values of 6.5 - 8.5 compared to pH 1.

[0044]

Table 2

[0045] The adsorption kinetic characteristics were further investigated using the Langmuir model and the Freundlich model. Figure 4b shows the correlation of nitrate adsorption as a function of contact time. The adsorption capacity increased rapidly within the first 20 minutes and reached saturation at 58.54 mg g -1 and 55.66 mg g -1 after 30 minutes at values of 1 or 2. Compared with the single - ligand MOF adsorption capacity, HKUST - 1 reached saturation at a value of 14 mg g -1 after 18 hours.

[0046] The maximum capacities of nitrate adsorption for 1 and 2 were calculated to be 119.42 mg g -1 or 105.93 mg g -1 . These q max values are higher than the values of MOFs including HKUST - 1 with a single - ligand of 9.69 mg g -1 and 1.22 mg g-1 Significantly higher than commercially available activated carbon containing

[0047] Testing of natural water samples The above tests were carried out in water containing a specific concentration of nitrate in the absence of other ions. However, natural water systems contain many different inorganic ions (in addition to organic impurities) that can compete with nitrate in the adsorption process. Therefore, 1 and 2 were tested in an aqueous solution containing a mixture of chloride, sulfate, and phosphate in equimolar amounts with NO3 - The adsorption of nitrate by 1 and 2 was followed as a function of time and is shown in Figure 4a. Figure 5 shows that the introduction of competing anions decreases the amount of nitrate adsorbed, while the amounts of chloride, sulfate, and phosphate adsorbed remain low (variations are < ±5 mg -1 ).

[0048] Upon closer observation, the decrease in nitrate removal efficiency appears to be affected by competing ions in the order sulfate > chloride > phosphate.

[0049] The risks to human health and the problem of eutrophication of reservoirs can be considerably minimized by removing nitrate from water. Therefore, both MOFs were also evaluated from actual groundwater samples for nitrate extraction (Figure 4c). Both MOFs were able to completely remove nitrate from water with an initial nitrate concentration (c0) of 8.465 mg L -1 within just 20 minutes. This demonstrates that the MOF according to the present invention is an efficient adsorbent that can rapidly and successfully remove low concentrations of nitrate from actual water samples. In water samples with a high concentration (i.e., c0 = 70.561 mg L-1), the extraction rate was approximately 50%, whereby the amount of nitrate was still significantly reduced to less than 50 mg L -1 below.

[0050] Regeneration of the compound Figure 5 shows the reproducibility and repeatability regarding the adsorption characteristics of both Compound 1 (Graph a) and Compound 2 (Graph b), where the adsorption capacity is maintained even after 6 regenerations. XRD data are shown in Graphs c and d, and electron microscope images are shown in Graphs e and f, which indicate the stability of the compounds even after 6 adsorption experiments.

[0051] The reusability of MOF is of utmost importance when the performance of the material is a concern. Therefore, the MOF according to the present invention was collected up to 6 times from various water samples, the structural characteristics were analyzed each time, and their performance regarding nitrate removal was examined. Figure 5a shows the same values of q max and adsorption efficiency in each of the 6 reusability tests. The results of XRD and TEM (Figure 5b) also reveal the high structural stability of the two compounds during adsorption measurements.

[0052] For regeneration, after adsorbing nitrate, the metal-organic structure contaminated with nitrate was centrifuged, collected, and dried. The dried powder was immersed in a saturated sodium chloride solution with pH = 7.22 (without pH adjustment). After about 5 minutes, the solution was centrifuged, washed 3 times with water and ethanol, and then dried overnight in a vacuum oven at 60 °C. The metal-organic structure regenerated in this way could be used again for the next nitrate adsorption. When the recyclability of the metal-organic structure was examined 6 times in the same way, no significant loss in nitrate adsorption was observed.

Claims

1. [Cu ２ (Hbtc)( ２ (bpe)( ２ )·(bpe) and / or [Cu ２ (Hbtc) ２ (bpy) ２ (H ２ (O) ２ ·(bpy) A metal-organic framework comprising bpe represents 1,2-bis(4-pyridyl)ethane, Hbtc represents 1,3,5-benzenetricarboxylic acid, bpy represents 4,4'-bipyridine, and the metal-organic framework.

2. Metal-organic structure [Cu ２ (Hbtc) ２ (bpe) ２ (bpe), where bpe represents 1,2-bis(4-pyridyl)ethane and Hbtc represents 1,3,5-benzenetricarboxylic acid, the metal-organic structure [Cu ２ (Hbtc) ２ (bpe) ２ (bpe).

3. Metal-organic structure [Cu ２ (Hbtc) ２ (bpy) ２ (H ２ O) ２ ] (bpy), where Hbtc represents 1,3,5-benzenetricarboxylic acid and 4bpy represents 4,4'-bipyridine. Metal-organic structure [Cu ２ (Hbtc) ２ (bpy) ２ (H ２ O) ２ ]·(bpy).

4. An adsorbent comprising the metal-organic framework according to any one of Claims 1 to 3.

5. A method for removing nitrate (NO ３ ) from an aqueous solution, in which the aqueous solution is brought into contact with an adsorbent, wherein the adsorbent contains the metal-organic framework according to any one of claims 1 to 3.

6. The amount of the adsorbent is 2 to 20 mg / L －１ , preferably 12 mg / L －１ The method according to claim 5, characterized in that it is so.

7. A method for regenerating an adsorbent obtained from the method according to Claim 5 or Claim 6, the method comprising contacting a nitrate-containing adsorbent with a saturated solution of NaCl, subsequently washing with water, and isolating.

8. In the method for producing the compound according to claims 1 to 3, an aqueous solution of a copper (II) salt, preferably an aqueous solution of Cu(OAc) ２ is reacted at high temperature and high pressure with 1,3,5-benzenetricarboxylic acid or a salt thereof and an alkaline aqueous solution containing 1,2-bis(4-pyridyl)ethane or 4,4'-bipyridine, and the resulting solid crude product is subsequently washed with water and / or ethanol. A method characterized by this.

9. Use of the compound according to any one of claims 1 to 3, as an adsorbent for NO ３ － .

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