Compound, Synthesis Method Thereof, and Separation and Recovery Agent Thereof

Abstract

To provide a new compound with pores finely tunable in size so as to take up a specific element and release the specific element taken up in the pores as necessary, a synthesis method of the new compound, and a separation and recovery agent. The new compound represented by the following molecular formula:

(NH.sub.4)[Ln(C.sub.2O.sub.4).sub.2(H.sub.2O)]

wherein Ln represents a lanthanide selected from Sm, Eu, Tb, Dy, Ho, Er, Tm, Yb, and Lu.

Claims

1.-5. (canceled)

6. A method for separating ions from water, comprising adding a separation and recovery agent comprising a compound of formula (1) to water,
(NH.sub.4)[Ln(C.sub.2O.sub.4).sub.2(H.sub.2O)]  (1), wherein Ln is selected from the group consisting of Sm, Eu, Tb, Dy, Ho, Er, Tm, Yb, and Lu, wherein the water comprises ions, wherein the separation and recovery agent adsorbs the ions present in the water to form a coordination polymer with adsorbed ions.

7. The method of claim 6, wherein the ions are selected from the group consisting of strontium ions, barium ions, and cesium ions.

8. The method of claim 6, further comprising removing the coordination polymer with adsorbed ions from the water.

9. The method of claim 8, further comprising recovering the ions from the coordination polymer with adsorbed ions.

10. The method of claim 8, wherein the coordination polymer with adsorbed ions is removed from the water by filtration.

11. The method of claim 9, wherein the ions are recovered from the coordination polymer with adsorbed ions by placing the coordination polymer with adsorbed ions in an acidic condition.

12. The method of claim 6, wherein Ln is selected from the group consisting of Tb, Er, and Tm.

13. The method of claim 6, wherein the ions are strontium ions and Ln is selected from the group consisting of Er and Tm.

14. The method of claim 6, wherein the ions are barium ions and Ln is Tb.

15. The method of claim 6, wherein the water is seawater.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

[0018] FIG. 1 is a schematic diagram illustrating the relationship between metal ions and ligands in a coordination polymer;

[0019] FIG. 2 is a schematic diagram illustrating lanthanide contraction;

[0020] FIG. 3 is a schematic diagram illustrating the relationship between pore size and ionic size;

[0021] FIGS. 4A and 4B are a table and a diagram, respectively, showing results of single crystal X-ray structure analysis to illustrate the structure of an element separating and recovering agent according to an embodiment of the present invention;

[0022] FIG. 5 shows comparison of distribution constants (Kd) among combinations of adsorbents and their target ions;

[0023] FIGS. 6A and 6B are graphs showing the relationship between Sr.sup.2+ ion adsorption and Ba.sup.2+ ion adsorption, respectively, and the reaction time;

[0024] FIG. 7 is a graph showing Sr.sup.2+ ion adsorption and Ba.sup.2+ ion adsorption, respectively, from seawater and the reaction time;

[0025] FIG. 8 is a composition table of artificial seawater Marine Art SF-1;

[0026] FIGS. 9A and 9B are graphs showing the relationship between the log (Kd) for barium uptake and the log for strontium uptake, respectively, and lanthanide ions;

[0027] FIG. 10 is a graph showing the relationship between the affinity for barium and strontium and central metals; and

[0028] FIGS. 11A to 11D show changes in powder X-ray pattern caused by cesium uptake, cesium release, and a regeneration reaction.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0029] First, a method for synthesizing a new compound that can be used as an element separating and recovering agent will be described referring to FIGS. 1 and 2. The new compound according to an embodiment of the present invention as described above was synthesized by a reaction expressed by the following formula:

##STR00002## [0030] wherein Ln is Sm (samarium), Eu (europium), Tb (terbium), Dy (dysprosium), Ho (holmium), Er (erbium), Tm (thulium), Yb (ytterbium), or Lu (lutetium).

[0031] As shown in Formula 2 above, the new compound according to an embodiment of the present invention was obtained by first immersing dimethyl oxalate and terbium chloride hexahydrate in an ammonium chloride-containing aqueous solution. The solution was then subjected to heating at 130° C. for 24 hours and filtering to separate colorless block-like crystals, which were cleansed.

[0032] FIG. 1 is a schematic diagram illustrating the relationship between metal ions and ligands in the new compound, namely a coordination polymer, according to an embodiment of the present invention. The coordination polymer is synthesized by self-assembly of ligands and metal ions as shown in FIG. 1. In this self-assembly, portions that look like the interstices of a jungle gym are formed to become pores. Since a typical coordination polymer is obtained as a single crystal, the pores formed are highly ordered in size. Also, as suggested by FIG. 1, the pores of the coordination polymer are tunable in size by the ligand size and the metal ionic size. Normally, the pore size is tuned by changing the length of the ligands by several angstroms. In the present invention, however, the pore size is more finely tunable by changing the size of the metal ions finely.

[0033] FIG. 2 is a schematic diagram illustrating lanthanide contraction. The lanthanide ions that configure the coordination polymer synthesized according to an embodiment of the present invention, Sm, Eu, Tb, Dy, Ho, Er, Tm, Yb, and Lu, are known to reduce in radius as the atomic number becomes larger by a phenomenon called “lanthanide contraction” as schematically illustrated in FIG. 2. This means that metals with a relatively small atomic number such as Sm and Eu have relatively large pores, while elements with a relatively large atomic number such as Yb and Lu have relatively small pores. Normally, the variation in ionic radius among the 10 metal elements from Sm to Lu ranges from 0.2 Å to 0.3 Å. This fine variation allows fine pore size tuning.

[0034] Next, how to recover a specific metal selectively by using the synthesized new compound as an element separating and recovering agent will be described referring to FIG. 3.

[0035] FIG. 3 is a schematic diagram illustrating the principle why conventional microporous materials such as zeolites and Prussian blue are capable of separating cesium and strontium. As understood from FIG. 3, ions that can be separated with conventional microporous materials are limited to those with an ionic size that matches the pore size of conventional microporous materials such as zeolites and Prussian blue. Metals with an ionic size smaller than the pore size as shown at the lower right of the figure cannot be separated, while metals with an ionic size larger than the pore size as shown at the upper left of the figure cannot be separated either.

[0036] In other words, only the ions with a size that matches the size of the pore size can be separated. Actually, the difference in radius size among elements is extremely minute, so it would be difficult to recognize only specific metal ions by changing the pore size by 0.1 Å or larger. In the present invention, in order to recognize this minute difference in ionic radius, the pore size of the coordination polymer can be finely tuned by 0.1 Å or smaller by using lanthanide ions, which exhibit slight variation in ionic radius, as a component of the coordination polymer, to separate only specific ions.

[0037] As an exemplary result, separation of strontium from seawater, which would be difficult with conventional zeolites, is made possible by tuning the pore size of this coordination polymer to meet the specific purpose. Also, based on this finding, it is now possible to select an ion separation agent that is best suited to a specific situation, for example, where small ions need to be removed from relatively large ions.

[0038] Next, how to separate the recovered metal from the element separating and recovering agent will be described. By the scheme described below, the element is separated in a neutral state and then placed under an acidic condition to be separated and recovered from the coordination polymer. The coordination polymer itself becomes Ln.sub.2(C.sub.2O.sub.4).sub.3, to which ligands and ammonium salt is added so that it is restored to the original coordination polymer, namely (NH.sub.4)[Ln(C.sub.2O.sub.4).sub.2(H.sub.2O)], by a synthesis reaction. Through the series of treatments as described above, the original coordination polymer can be regenerated while separating and recovering the element.

##STR00003##

<Experiment 1> Synthesis of Element Separating and Recovering Agent (NH.SUB.4.)[Tb(C.SUB.2.O.SUB.4.).SUB.2.(H.SUB.2.O)]

[0039] FIGS. 4 (A) and 4 (B) show results of single crystal X-ray structure analysis to illustrate the structure of an element separating and recovering agent according to an embodiment of the present invention. FIG. 4 (A) is a table showing crystallographic parameters, and FIG. 4 (B) is a structural diagram.

[0040] The element separating and recovering agent (NH.sub.4)[Tb(C.sub.2O.sub.4).sub.2(H.sub.2O)] as shown in FIGS. 4 (A) and 4 (B) was synthesized as follows. First, 3.0 g of dimethyl oxalate, 3.0 g of terbium chloride hexahydrate, 6.0 g of ammonium chloride, and 50 ml of water were heated at 130° C. for 24 hours. The solution was then subjected to filtering to separate colorless block-like crystals. The obtained crystals were cleansed with water (5 ml×3), ethanol (5 ml×3), and acetone (5 ml×3) and dried to obtain the element separating and recovering agent [yield: 2.58 g, percentage yield: 87% (in terms of terbium), elemental analysis: C.sub.4H.sub.6NO.sub.9Tb: C, 12.95 (12.78); H, 1.63 (1.70); N, 3.78 (3.52)].

<Experiment 2> Uptake of Strontium and Barium in Pure Water

[0041] The results of strontium and barium uptake testing will be described referring to FIG. 5 and FIGS. 6 (a) and 6 (b). FIG. 5 is a comparison table of distribution constants (Kd) among combinations of adsorbents and their target ions. FIG. 6 (a) is a graph showing the relationship between Sr.sup.2+ ion adsorption and the reaction time, and FIG. 6 (b) is a graph showing the relationship between Ba.sup.2+ ion adsorption and the reaction time.

[0042] 20 mg of (NH.sub.4)[Tb(C.sub.2O.sub.4).sub.2(H.sub.2O)] was added to a solution containing 162 ppm of strontium or 200 ppm of barium and stirred at 500 rpm for 10 minutes. The solution was then subjected to filtering, and the concentration of strontium or barium in the solution was measured to obtain the distribution constant (Kd) for strontium or barium. The results showed that the distribution constant for strontium and the distribution constant for barium of (NH.sub.4)[Tb(C.sub.2O.sub.4).sub.2(H.sub.2O)] were 3.1×10.sup.4 and 1.4×10.sup.5 (ml/g), respectively. This distribution constant (Kd) for strontium was equivalent to those with zeolites, which are considered to be particularly effective in cleanup of strontium (see FIG. 5). Also, as seen from FIGS. 6 (a) and 6 (b), showing the testing results of adsorption behavior, it was found that this adsorption agent or element separating and recovering agent is capable of removing 99% or more ions in a solution in 10 minutes, which means it is a material that exhibits a much faster adsorption speed than those of zeolites and is capable of efficiently separating specific ions.

[0043] The distribution constants (Kd) were obtained by the following formula:

[00001]$\begin{array}{cc}{K}_d\left(\mathrm{mL}{g}^{-1}\right)=\frac{C_i-C_e}{C_e}\times \frac{V}{M}& \left[\mathrm{Formula}4\right]\end{array}$

[0044] wherein Ci represents the initial concentration of the ions in the solution (ppm), Ce represents the concentration of the ions in the solution at equilibrium (ppm), V represents the amount of the solution (2 ml), and M represents the amount of the element separating and recovering agent, (NH.sub.4)[Tb(C.sub.2O.sub.4).sub.2(H.sub.2O)], as an adsorbent (0.02 g).

<Experiment 3> Ion Separation from Seawater

[0045] 20 mg of (NH.sub.4)[Tb(C.sub.2O.sub.4).sub.2(H.sub.2O)] was added to an artificial seawater Marine Art SF-1 (Osaka Yakken, Osaka, Japan, see the table of FIG. 8) containing 200 ppm of strontium or 200 ppm of barium and stirred at 500 rpm for 10 minutes. The solution was then subjected to filtering, and the concentration of strontium or barium in the solution was measured. The results showed that the distribution constant for strontium and the distribution constant for barium of (NH.sub.4)[Tb(C.sub.2O.sub.4).sub.2(H.sub.2O)] were 8.2×10.sup.3 or higher and 2.5×10.sup.3 or higher (ml/g), respectively. Considering the fact that in the efforts to search for adsorbents in Fukushima, distribution constants for strontium in seawater have been reported to be around 1,000 (ml/g).sup.1, 2, this distribution constant (Kd) for strontium in seawater proved to be much higher than those with adsorbents reported so far. Also, since it was capable of separating target elements from an environment full of other ions like seawater, it was revealed to be a separating agent highly capable of selectively separating elements (see FIG. 7 (a) and (b)).

<Experiment 4> Selective Ion Uptake with Different Lanthanide Ions

[0046] 20 mg of (NH.sub.4)[Ln(C.sub.2O.sub.4).sub.2(H.sub.2O)] was added to a solution containing 20 ppm of strontium or 20 ppm of barium and stirred at 500 rpm for 10 minutes. The solution was then subjected to filtering, and the concentration of strontium or barium in the solution was measured to obtain the logarithm (log (K.sub.d)) of the distribution constant with different metal elements. The results are shown in FIGS. 9A and 9B. As seen from FIGS. 9A and 9B, central metals with a higher log (K.sub.d) exhibit a higher adsorption capability.

[0047] For comparison between affinity for barium and affinity for strontium, the difference between the log for Ba, Log (Kd (Ba)), and the log for Strontium, Log (Kd (Sr)), for each metal is shown in FIG. 10. Herein, each log (Kd) is a chemically significant value that indicates a free energy difference when the target element is taken up. FIG. 10 shows that, in general, the larger the pore size is, the higher the affinity for barium is, and the smaller the pore size is, the higher the affinity for strontium is. It also shows that with the lanthanide metals between Tb and Er, there is no difference between barium uptake and strontium uptake. These results indicate that since barium has a larger ionic size than that of strontium.sup.1, barium is taken up by coordination polymers with larger pores more easily than strontium. Also, the affinity for barium is particularly high with the central metal being Tb, and the affinity for strontium is particularly high with the central metal being Er or Tm. Based on these results, it was successfully proved and found that there exists a material that has a pore size that is suited for the ionic radius of each metal.

<Experiment 5> Release of Radioactive Element from Coordination Polymer

[0048] (1) 100 mg of (NH.sub.4)[Yb(C.sub.2O.sub.4).sub.2(H.sub.2O)] was soaked in 10 ml of a cesium chloride solution (cesium concentration: 1,000 ppm) for four hours (298K, pH 7±1). It was observed that the cesium concentration in the solution had reduced by 61%. In this process, the powder X-ray pattern changed from a) to b) in FIG. 11. Since the change in powder X-ray pattern was small, and the change in chemical structure and state was little, the cesium uptake is considered to have been caused mainly by the reaction expressed by the following formula:

(NH.sub.4)[Yb(C.sub.2O.sub.4).sub.2(H.sub.2O)]+CsCl.fwdarw.Cs[Yb(C.sub.2O.sub.4).sub.2(H.sub.2O)]NH.sub.4Cl  [Formula 5]

[0049] (2) Next, the solution was filtered to obtain the powder, which was then suspended in 10 ml of pure water. To the obtained suspension, 0.001 ml of NHCl was added by an automatic titrator to keep its pH at 4, which allowed release of 95% of the cesium taken up in the polymer into the solvent. In this process, the powder X-ray pattern changed from b) to c) in FIG. 11. Since the powder X-ray pattern is for Yb.sub.2(C.sub.2O.sub.4).sub.3.n(H.sub.2O), the cesium release is considered to have been caused by the acidolysis reaction expressed by the following formula:

2(Cs[Yb(C.sub.2O.sub.4).sub.2(H.sub.2O)])+HCl.fwdarw.Yb.sub.2(C.sub.2O.sub.4).sub.3+2Cs.sup.++H(C.sub.2O.sub.4).sup.−+Cl.sup.−  [Formula 6]

[0050] (3) Next, the suspension was filtered to obtain the powder, or Yb.sub.2(C.sub.2O.sub.4).sub.3.n(H.sub.2O), to which dimethyl oxalate and ammonium chloride were added, and the mixture was heated at 130° C. for 24 hours. As a result, the powder X-ray pattern changed from c) to d) in FIG. 11. Since the powder X-ray pattern is for (NH.sub.4)[Yb(C.sub.2O.sub.4).sub.2(H.sub.2O)], the original coordination polymer is considered to have been regenerated by the reaction expressed by the following formula:

Yb.sub.2(C.sub.2O.sub.4).sub.3+(CH.sub.3).sub.2(C.sub.2O.sub.4)+NH.sub.4Cl.fwdarw.(NH.sub.4)[Yb(C.sub.2O.sub.4).sub.2(H.sub.2O)]+2CH.sub.3OH  [Formula 7]

[0051] It was proved that through the reactions as described above, this coordination polymer is capable of taking up cesium, releasing the cesium that it has taken up, and being restored to the original coordination polymer after the release and the use.