TECHNICAL FIELD

The invention relates the field of isotope separation in the field of chemical industry, in particular to a process for extraction and enriching lithium isotopes.

More particularly, the invention relates to a liquid-solid separation process of lithium-6 and lithium-7 isotopes from a solid alkali lithium salt, where an organic solution of a macrocycle in a water non-miscible solvent is contacted with a solid lithium salt, said macrocycle being chosen among phosphorylated crown-ethers in which the macrocycle comprises from 12 to 16 intracyclic atoms, at least one phosphorus atom, at least one oxygen atom and at least three additional heteroatoms selected in the group consisting of oxygen, sulphur and nitrogen. The invention also relates to the use of such a macrocycle for the selective trapping of lithium-7 isotope from a suspension of a solid lithium salt in a water non-miscible organic solvent.

BACKGROUND

Naturally occurring lithium (Li) is composed of two stable isotopes, lithium- 6 ( ⁶ Li) and lithium-7 ( ⁷ Li), with the latter being far more abundant on Earth (natural relative abundance: ⁶ Li at 7.5% and ⁷ Li at 92.5%).

Lithium isotopes separation technologies are very challenging and it is of great importance to develop cost-effective and environmentally-friendly processes.

Both lithium isotopes have important industrial applications in the field of nuclear industry:

Lithium-7 has a very low neutron cross section, making it transparent to neutrons. This property is used in lithium highly enriched in lithium-7 (more than 99%) as a coolant in molten salt reactors, but also as a pH stabilizer in pressurized water reactors. In particular, ⁷ LiOH of high isotopic purity (99.95%) is required as pH adjuster for coolant fluids in nuclear plants (Pressurize Water Reactors: PWR). The presence of the lighter isotope ⁶ Li for this application is very deleterious because it could produce radioactive tritium which the concentration is strictly regulated. In fluoride salt-cooled high-temperature reactors (FHR), in which the salt (also called FLIBE) is a mixture of lithium (14 w/w%), fluoride and beryllium, higher enrichment is required. For FHRs, the acceptable ⁷ Li purity requirement is at least 99.995%.

In the meantime, isotopically pure ⁶ Li can be used as a mean of tritium production which is crucial for nuclear fusion technologies.

By the past, the most important industrial method for lithium isotopes separation was the mercury amalgam process also known under the acronym "COLEX" for COLumn Exchange process. Indeed, mercury amalgamates with many metals, including lithium. Lithium-6 has a greater chemical affinity for mercury than lithium-7. When an amalgam of lithium and mercury is added to an aqueous lithium hydroxide solution, lithium-6 becomes more concentrated in the amalgam and lithium-7 more concentrated in the hydroxide solution. The COLEX process exploits this phenomenon by passing a downward flow of lithium-mercury amalgam against an upward flow of aqueous lithium hydroxide in a cascade. Lithium-6 is preferentially carried away by the amalgam, while lithium-7 is preferentially drained by the hydroxide. At the bottom of the column, the amalgam is recovered and the lithium-6 is extracted. At the top of the column, the lithium hydroxide solution is electrolyzed to extract lithium-7. From a technical and economic point of view, the COLEX process is so far the only method allowing the production of enriched lithium on an industrial scale, at affordable costs. The technology has reached a certain maturity allowing it to be adopted by many countries with a nuclear industry. However, large-scale exploitation of this technology would have disastrous consequences for the environment. Thousands tons of mercury are needed and the risks of leaks into the environment are high. Maintenance and cleaning operations are also difficult and expensive.

To date, many researches are conducted in order to find a cost effective and green process: solvent extraction, chromatography, membrane separation or electrochemical depositions are some of the technologies evaluated.

Among those technics, the use of crown-ethers is widely studied. As an example, it has already been proposed lithium isotopes separation by the liquid-liquid extraction technic involving the use of benzo-15-crown-5 ether (B15C5) in chloroform (CN108854536). However, this process is highly selective for lithium-6 isotope and not for lithium-7 isotope. This process thus does not allow reaching aqueous solutions of lithium salts with a high level of ⁷ Li isotope and does not allow high lithium concentration in aqueous phase.

Therefore, there is a need for a process allowing reaching aqueous solutions of lithium salts with a high level of ⁷ Li isotope, such a process being easy to carry out and to implement continuously, environmentally friendly and economic.

Description of the invention:

A first object of the present invention is a liquid-solid separation multi-stage process of lithium-6 and lithium-7 isotopes from a solid alkali lithium salt, said multi-stage process comprising at least the following steps: i) contacting an organic solution of a macrocycle in a water non-miscible solvent with a solid lithium salt to obtain a suspension of said solid alkali lithium salt in said organic solution, ii) stirring the suspension obtained in step i) for a period of time sufficient to form a complex of lithium-7 enriched isotope with said macrocycle, iii) mechanically separating said solid lithium salt from said organic solution, said organic solution comprising lithium-7 enriched isotope complexed with said macrocycle, iv) contacting said organic solution with an aqueous phase to extract said lithium-7 enriched isotope from the macrocycle present in the organic solution, and obtain an aqueous phase comprising said lithium-7 enriched isotope and an organic solution comprising said macrocycle freed from said lithium; v) evaporating said lithium-7 enriched isotope aqueous phase to produce solid lithium-7 enriched isotope and optionally reprocessing said solid lithium-7 enriched isotope according to step i) to v) until production of lithium-7 salt with the desired purity; wherein said macrocycle is a phosphorylated crown-ether in which the macrocycle comprises from 12 to 16 intracyclic atoms, at least one phosphorus atom, at least one oxygen atom and at least three additional heteroatoms selected in the group consisting of oxygen, sulphur and nitrogen.

The process according to the present invention has the following advantages: - Selectivity for the ⁷ Li isotope (all other processes are selective for the other isotope, i.e. ⁶ Li isotope)

- Easy to implement (solid/liquid extraction) allowing a continuous process,

- Environmentally friendly (no use of toxic products, can operate in closed circuit).

- Ease of preparation and industrial cost price of phosphorylated macrocycles.

Indeed, according to the process of the invention, a solution of a phosphorylated crown-ether in a water non-miscible solvent is stirred in presence of solid lithium salt. After allowing a period of contact the lithium cations distribute into the organic phase and form complexes with the phosphorylated crown-ether. The complex [ ⁷ Li/phosphorylated crown-ether] is more stable than the complex [ ⁶ Li/phosphorylated crown-ether] allowing the ⁷ Li / ⁶ Li ratio increases in the organic solution. The system is very advantageous due to:

- very high extraction efficiency (almost quantitative)

- high single-stage isotopes separation factor (1.016-1.027)

- no needs of additive

- extracting agent (phosphorylated macrocycle) cost

- process can be automated relatively easily

- preferred extraction of ⁷ Li.

In the sense of the invention, the terms "intracyclic atoms" in reference to the macrocycle, refer to the number of atoms engaged in the cycle constituting said macrocycle. It thus excludes any atom that would belong to a substituent of said macrocycle.

According to a preferred embodiment of the invention, the phosphorylated crown-ether is selected among phosphorylated crown-ethers of formulae (I) to (III) below:

Wherein:

- X is O, NR ¹ or S,

- Y represents a non-substituted aryl group; an aryl group bearing at least one substituent selected among a halogen atom chosen among Cl, Br and F, a linear or branched alkyl group having from 1 to 18 carbon atoms, and a linear or branched alkenyl or alkynyl group having from 2 to 18 carbon atoms; a linear or branched alkyl group having from 2 to 18 carbon atoms, a linear or branched alkyl group bearing at least one substituent selected among a halogen atom chosen among Cl, Br, F, mesyle and tosyle; or a linear or branched alkenyl or alkynyl group having from 2 to 18 carbon atoms;

- R ¹ represents a hydrogen atom, a linear or branched alkyl group having from 1 to 18 carbon atoms, or a linear or branched alkyl group having from 1 to 18 carbon atoms and substituted by one substituent chosen among an alcoxy group having from 1 to 6 carbon atoms and a trialkyl(C ₁ -C ₆ )siloxane;

- n is an integer ranging from 0 to 4;

- Z ₁ , Z ₂ and Z ₃ , identical or different, represent (i) a linear or branched alkenyl or alkynyl group having from 2 to 18 carbon atoms, (ii) a 1,2 or 1,3 or 1,4- O-disubstituted aryl group, or a (iii) a linear alkyl chain having 2 ou 3 carbon atoms, said alkyl group being optionally substituted by:

* an OR ² group in which R ² represents hydrogen, an alkyl group having from 1 to 6 carbon atoms or a phosphate group;

* an alkyl group having from 1 to 8 carbon atoms, said alkyl group being optionally substituted by a thioalkoxy(C ₁ -C ₈ ) group. According to a particular and most preferred embodiment of the present invention, the phosphorylated crown-ether is a compound of formula (I) in which Z ₁ = Z ₂ = Z ₃ = -CH ₂ -CH ₂ -, n = 1, 2, 3 or 4, X = O and Y = a phenyl group or a compound of formula (II) in which Z ₁ = Z ₂ = Z ₃ = -CH ₂ -CH ₂ -, n = 1, 2, 3 or 4, X = O and Y = a phenyl group.

Among these particular phosphorylated crown-ethers, the compound of formula (I) in which Z ₁ = Z ₂ = Z ₃ = -CH ₂ -CH ₂ -, n = 2, X = O and Y = a phenyl group and the compound of formula (II) in which Z ₁ = Z ₂ = Z ₃ = -CH ₂ -CH ₂ -, n = 1, X = O and Y is a phenyl group are particularly preferred.

These compounds can be respectively represented by the following formulae MCI and MC2:

The phosphorylated crown-ethers of above formulae (I), (II) and (III) can be prepared as described in international application PCT/CL2020/050133.

According to a preferred embodiment of the present invention, the amount of phosphorylated crown-ether in the organic solution ranges from about 0.01 M and about 5, and more preferably from about 0.1 to about 1.0 M.

According to the invention, the water non-miscible solvent is preferably selected in the group comprising dichloromethane (DCM), chloroform, heptane, cyclohexane, toluene, xylene, ethyl acetate, butyl acetate, chlorobenzene, dichlorobenzene, anisole, methyl tert-butyl ether (MTBE), and nitrobenzene. Among these particular solvents, dichloromethane, chloroform and chlorobenzene are particularly preferred.

The solid lithium salt may be chosen among LiCI, LiBr, Lil, Li ₂ CO ₃ , lithium thiocyanate (LiSCN), Li ₃ PO ₄ , lithium acetate, lithium trifluoroacetate and LiOH. Among these particular lithium salts, LiCI is particularly preferred. According to a preferred embodiment of the present invention, the solid lithium salt is in the form of particles whose size ranges from about 40 to about 500 pm and more preferably from about 40 to 100 pm.

The molar ratio [solid lithium salt/phosphorylated crown-ether] is not critical. However, this molar ratio preferably ranges from about 0.1 to 2000 and more preferably from about 10 to 1000, and even more preferably from about 100 to 500.

According to a preferred embodiment of the present invention, the period of time sufficient to form a complex of lithium-7 isotope with said macrocycle at step ii) may range from about 1 to 48 hours, preferably from about 1 to 2 hours.

The temperature of the suspension of said solid alkali lithium salt in said organic solution during step ii) is not critical. However, according to a particular embodiment of the present invention, the temperature of the suspension of said solid alkali lithium salt in said organic solution during step ii) ranges from about - 20°C and 100°C, and even more preferably from about 20 to 40°C.

At step iii), the mechanical separation of said solid alkali lithium salt from said organic solution may be carried out by any appropriate mean, such as for example by filtration or by centrifugation.

According to a particular and preferred embodiment of the present invention, the solid lithium salt recovered from the said organic solution at the end of step (iii) and the organic solution recovered at the end of step iv), i.e. the organic solution comprising said macrocycle freed from said lithium-7 enriched isotope, may both be re-engaged at least once in step (i) of the process, preferably from 1 to 2500 times and even more preferably from 50 to 500 times. According to this embodiment, the process of the invention may thus be carried out continuously and in a closed circuit.

The aqueous phase suitable for back lithium extraction, i.e. for extracting said lithium-7 enriched isotope from the organic solution, can be pure water or water comprising at least one additive selected from hydrochloric acid, sulphuric acid, phosphoric acid, ethanoic acid, and dimethyl sulfoxide (DMSO).

As above-explained, the process according to the first object of the present invention allows the separation of lithium-7 isotope from a solid lithium salt. Therefore, a second objet of the present invention is the use of a macrocycle selected among phosphorylated crown-ethers in which the macrocycle comprises from 12 to 16 intracyclic atoms, at least one phosphorus atom, at least one oxygen atom and at least three additional heteroatoms selected in the group consisting of oxygen, sulphur and nitrogen for the selective trapping of lithium-7 isotope from a suspension of a solid lithium salt in a water non-miscible organic solvent.

According to a preferred embodiment of this use, the phosphorylated crown- ethers are selected among phosphorylated crown-ethers of formulae (I) to (III) as defined here-above according to the first object of the present invention, in particular among compounds of formula (I) in which Z ₁ = Z ₂ = Z ₃ = -CH ₂ -CH ₂ -, n = 1, 2, 3 or 4, X = P and Y = a phenyl group and a compound of formula (II) in which Z ₁ = Z ₂ = Z ₃ = -CH ₂ -CH ₂ -, n = 1, X = O and Y = a phenyl group, and even more particularly among the compound of formula (I) in which Z ₁ = Z ₂ = Z ₃ = - CH ₂ -CH ₂ -, n = 1, X = O and Y = a phenyl group and the compound of formula (II) in which Z ₁ = Z ₂ = Z ₃ = -CH ₂ -CH ₂ -, n = 2, X = O and Y = a phenyl group.

Other advantages and embodiments of the present invention are given by the following examples.

EXAMPLES:

Chemicals

Phosphorylated crown-ether were prepared as generally described in PCT/CL2020/050133. In particular, phosphorylated crown-ethers of formula MCI and MC2 were respectfully prepared as described in preparation examples 1 and 2 below.

Benzo-15-crown-5 (CAS RN: 14098-44-3, Supplier: TCI Europe N.V.) has been used as received.

Anhydrous LiCI (99%, Supplier: Acros) has been used as received and stored in a glove-box.

Solvents, HPLC grade, were used as received. Ultra-pure Water (<lpS/cm) from Chem-Lab has been used for extraction. Analytical equipment:

Lithium isotopic ratio was measured by inductively coupled plasma mass spectrometer (ICPMS -).

Lithium concentration was measured by ionic chromatography (IC).

Extraction procedure:

In a vial lithium chloride and a 0.5 M solution of macrocycle were added. The mixture was stirred at room temperature with a magnetic stirring for a definite time. The mixture was then allowed for decantation during 2 h. After filtration (0.22mm filter) the solution was extracted with ultrapure water. After phase separation the aqueous phase was washed with chloroform in order to remove any traces of macrocycle. The later aqueous phase was analysed by ICPMS and IC.

The separation factor a (alpha,) measures the ability of a single-stage process to selectively transfer one isotope to one of the two phases a _{lpha =}

[ ⁷ Li/ ⁶ Li] _initial is the lithium isotope ratio of the commercial natural LiCI salt,

[ ⁷ Li/ ⁶ Li] _aqueous is the lithium isotope ratio in the aqueous phase after extraction by the macrocycle.

EXAMPLE 1: Preparation of a phosphorylated crown-ether of formula MCI

A phosphorylated crown-ether of the following formula MCI has been prepared according to the process represented below on Scheme 1:

SCHEME 1 The following procedure has been applied:

A dichloromethane (DCM) solution (20 mL) of Compound 1 (X = 0, Hal = Cl, Y = Phenyl, 10.0 mmoles, 1.95 g) was added drop-wise (3.5h) to a DCM solution (250 mL) of Compound 2 ( Z ₁ = Z ₂ = Z ₃ = -CH ₂ -CH ₂ -, n=2, 10.0 mmoles, 1.94 g, 0.04M) at 35°C under argon. The reaction mixture was stirred overnight. The solvent was then evaporated until a crude oil was obtained. The crude oil was purified by flash chromatography using a mixture of DCM and methanol (DCM- MeOH : 98/2 v/v) as eluent to yield a colorless oil (1,28 g, 40 %).

¹ H NMR (300 MHz, Chloroform-d) 7.84-7.78 (m, 2H), 7.56-7.41 (m, 3H), 4.52-4.42 (m, 2H), 4.13-4.08 (m, 2H), 3.87-3.75 (m, 4H), 3.70 (m, 8H).

³¹ P NMR (121 MHz, Chloroform-d) δ 19.03.

¹³ C NMR (75 MHz, Chloroform-d) δ 132.31 (d, J - 3.1 Hz), 131.41 (d, J = 10.0 Hz), 128.33 (d, J = 15.3 Hz), 128.07 (d, J = 193.2 Hz), 70.62 (d, J = 12.7 Hz), 70.19 (d, J = 5.4 Hz), 65.16 (d, J = 6.4 Hz).

HRMS [M+H ⁺ ]: Calculated: 317.1154, Experimental: 317.1144 for C ₁₄ H ₂₂ O ₆ P

EXAMPLE 2: Preparation of a phosphorylated crown-ether of formula MC2

A phosphorylated crown-ether of the following formula MC2 has been prepared according to the process represented below on Scheme 2:

SCHEME 2

In formula MC2 above, Ph represents a phenyl cycle. K ₂ CO ₃ (1.38 g, 10 mmoles) was added to a solution of Compound 4 in which ( Z ₁ = Z ₂ = Z ₃ = -CH ₂ -CH ₂ - and n = l (310 mg, 1 equivalent) and Compound 3 (X = O and Y = phenyl, 310 mg, 1 equivalent) in dry dimethylformamide (DMF) (30 ml) under argon atmosphere. The reaction mixture was stirred at 80°C until completion (48 hours). DMF was removed under reduced pressure. The residue was diluted with ethyl acetate (EtOAc) and washed with water, brine and dried over sodium sulfate. The crude was purified by flash chromatography (acetone/EtOAc 40/60 v/v) to yield a whit-off powder.

NMR (300 MHz, Chloroform-d) 8.14-8.07 (m, 2H), 77.67-7.60 (m, 2H), 7.47-7.37 (m, 5H), 7.03-6.90 (m, 4H), 4.14-3.91 (m, 4H), 3.44-3.18 (m, 4H), 3.19-3.00 (m, 4H).

³¹ P NMR (121 MHz, Chloroform-d) 5 24.72

¹³ C NMR (75 MHz, Chloroform-d) 6 159.86, 133.81 (d, J = 7.8 Hz), 134.15 (d, J = 110.1 Hz), 132.71 (d, J = 2.0 Hz), 132.13 (d, J = 10.6 Hz), 130.87 (d, J = 2.9 Hz), 127.75 (d, J = 12.8 Hz), 122.01 (d, J = 107.9 Hz), 120.11 (d, J = 12.2 Hz), 112.23 (d, J = 6.6 Hz), 70.82, 70.57, 66.11.

HRMS [M + H ⁺ ] : Calculated: 425.1518, Experimental: 425.1525 for C ₂₄ H ₂₆ O ₅ P

Melting point: 195-198°C

EXAMPLE 3: Extraction of lithium 7 according to the process of the invention

In this example different extraction experiments have been performed according to the process of the invention, using phosphorylated crown-ethers of formulae MCI or MC2, and as a comparative example not forming part of the present invention, using a crown-ether not useful according to the process of the invention, namely Benzo-15-crown-5.

3.1 Evaluation of the influence of the nature of the solvents

The extraction process has been carried out according to the "Extraction procedure" detailed above using MCI, in different solvents (DCM, Methyl tert-butyl ether (MTBE), n-heptane, toluene and chloroform. The initial molar ratio ⁷ Li/ ⁶ Li before the start of the extraction process was 12.1773 corresponding to an initial amount of ⁷ Li of 92.41 %. The results are presented in the following Table 1 :

TABLE 1

Here the efficiency (%) is described by the ratio of the concentration of LiCI in the aqueous phase to the maximum theoretical concentration of LiCI (assuming that one equivalent of macrocycle could extract one equivalent of lithium cation).

These results demonstrate that all the tested solvent are efficient to extract lithium-7 according to the process of the invention. However, the data suggest that the best compromise is to use chloroform as solvent (the separation factor is lower than MTBE but the efficiency is by far much better).

3.2. Evaluation of the influence of the number of LiCI equivalents

In this experiment, the influence of the number of LiCI equivalents compare to the extracting agent was evaluated. For this experiment, phosphorylated crown- ether of formula MCI as prepared according to example 1 was engaged in the extraction procedure detailed above; solvent was chloroform, and the contact time between LiCI and MCI under stirring was set to 60 hours.

The results are reported in the following Table 2:

TABLE 2 As it appears in Table 2, the number of LiCI equivalents is not critical according to the process of the invention. However, a very large excess compare to the extracting agent is favourable to reach higher single-stage separation factor.

3.3 Evaluation of the influence of the time of contact

In this experiment, the influence of the contact time between the LiCI salt and the phosphorylated macrocycle was evaluated (stirring time). For this experiment, phosphorylated crown-ether of formula MCI as prepared according to example 1 was engaged in the extraction procedure detailed above; solvent was chloroform, 5 equivalents of LiCI was used and the contact time between LiCI and the phosphorylated crown-ether of formula MCI varied from 1 to 24 hours.

The corresponding results are presented in the following Table 3:

TABLE 3

These results show that the maximum lithium concentration is reached within 1 hour, the equilibrium exchange reaching a plateau after 24 h.

3.4, Evaluation of the influence of the temperature

In this experiment, the influence of the temperature was evaluated. For this experiment, phosphorylated crown-ether of formula MCI as prepared according to example 1 was engaged in the extraction procedure detailed above; solvent was dichlorobenzene, 500 equivalents of LiCI were used and the contact time between LiCI and the phosphorylated crown-ether of formula MCI was 7 hours. The temperatures were 0, 20 or 40°C. The corresponding results are given in the following Table 4:

TABLE 4

Although the choice of the temperature is not critical according to the process of the invention, these results show that the formation of the complex [MCl- ⁷ Li]CI is favoured with the rise of temperature.

3.5 Extraction of lithium-7 according to a two-stage process

Stage 1 : in a 50 mL Erlenmeyer flask was weighed lithium chloride (420 mg, 10 equivalents) and a solution of phosphorylated crown-ether of formula MCI as prepared according of example 1 (330 mg, 1 mmole, 1 equivalent) in 25 mL chloroform was added. The resulting mixture was stirred for 2 hours and allowed to decantation for further 2 hours.

20 mL of the organic solution were withdrawn and LiCI was extracted with 30.0 mL of deionized water. Resulting solid LiCI salt was then washed with 5 mL of chloroform.

A 0.5 mL aliquot of aqueous solution was diluted to 4.0mL with deionized water for analysis by ICPMS

The whole aqueous solution was evaporated to dryness to yield 26 mg of solid LiCI (62 % of maximum mass).

Stage 2: To the solid LiCI salt recovered from stage 1, a solution of phosphorylated crown-ether of formula MCI as prepared according of example 1 (30 mg, 0.01 mmole) in 5 mL de chloroform was added.

The resulting mixture was stirred for 2 hours and allowed to decantation for further 2 hours and then filtrated (22 pm).

LiCI was extracted with 2.0 mL of deionized water. The corresponding results are presented in following Table 5:

TABLE 5

These results show that ⁷ Li enrichment process is still operating during the second stage.

3.6 Concentration process of LICI in the aqueous phase

It is advantageous to concentrate the aqueous phase at each stage by repeating the procedure with the same organic and aqueous solutions.

After a determined numbers of cycles, the concentrated aqueous phase can be dried for a second stage of isotope separation.

In the example, the extraction procedure of example 3.5 step 1 has been repeated from 1 to 4 four times with the same salt, organic and aqueous solutions,

The corresponding results are reported in the following Table 6:

TABLE 6

This batch process clearly indicates the increasing amount of LiCI in aqueous phase after 4 cycles. A continuous flow extraction process could be advantageous to reach higher salt concentration in aqueous phase.

The batch process described here can be automated by a continuous extraction process with in-line phase separation and thus enables high concentrations of lithium salts in the aqueous phase to be achieved (a favourable factor for the upper stages).

3.7 Evaluation of the nature of macrocycle

In the example, the performance of the phosphorylated crown-ether of formula MCI has been compared to that of the phosphorylated crown-ether of formula MC2 as prepared according to example 2 above, and also as a comparative example, to a non-phosphorylated crown-ether not forming part of the present invention, i.e. Benzo-15-crown-5 (abbreviated B15C5).

B15C5 can be represented by the following formula :

For this experiment, each tested crown-ether (MCI, MC2 or B15C5) was engaged in the extraction procedure described in example 3.5. step 1.

The results are presented in Table 7 below:

TABLE 7

In numerous papers B15C5 was described as a potent candidate that can separate lithium isotopes in a liquid/liquid process. These results show that in the solid/liquid lithium isotopes separation process of the invention only phosphorylated macrocycles (for example of formulae MCI or MC2) are efficient; B15C5 is not active at all.