Synthesis of a cross phosphonic acid type calix[4]arene with two different spacers and its extractive separation of rare earth metals

Article abstract of DOI:10.1007/s10847-015-0490-2

A phosphonic acid derivative of calix[4]arene in a cone configuration with two different alkylphosphonic acid substituents positioned at distal positions has been synthesized and used to investigate the extraction behavior towards rare earth metals. The new reagent exhibited both heavy rare earth metal extraction selectivity and very high extraction ability that is comparable to the separation efficiency of the commercial phosphonic acid extraction reagent, 2-ethylhexyl hydrogen 2-ethylhexylphosphonate, towards rare earth metals. The latter reagent is currently used as the most effective means for separating the rare earths. The high extraction and separation efficiency are largely attributed to the convergence of the functional groups in the above calix[4]arene derivative providing a preorganized metal binding site that reflects both the cyclic structure and the cone configuration of the calix[4]arene backbone. Any loss of "binding complementarity" due to the presence of different spacer groups connecting the two phosphonic acid groups to the calix[4]arene backbone is proposed to have been compensated by the operation of a chelate effect giving rise to high extraction ability being maintained. Extraction equilibrium constants and separation factors were estimated based on the extraction data. The stripping of the loaded rare earth metal ions with hydrochloric acid was also investigated.

Full text of DOI:10.1007/s10847-015-0490-2

J Incl Phenom Macrocycl Chem
DOI 10.1007/s10847-015-0490-2
ORIGINAL ARTICLE
Synthesis of a cross phosphonic acid type calix[4]arene with two
different spacers and its extractive separation of rare earth metals
Masahiro Tanaka
Hidetaka Kawakita
Keisuke Ohto
•
Shintaro Morisada
Katsutoshi Inoue
•
•
•
Received: 28 January 2015 / Accepted: 23 February 2015
Ó Springer Science+Business Media Dordrecht 2015
Abstract A phosphonic acid derivative of calix[4]arene
in a cone configuration with two different alkylphosphonic
acid substituents positioned at distal positions has been
synthesized and used to investigate the extraction behavior
towards rare earth metals. The new reagent exhibited both
heavy rare earth metal extraction selectivity and very high
extraction ability that is comparable to the separation ef-
ficiency of the commercial phosphonic acid extraction
reagent, 2-ethylhexyl hydrogen 2-ethylhexylphosphonate,
towards rare earth metals. The latter reagent is currently
used as the most effective means for separating the rare
earths. The high extraction and separation efficiency are
largely attributed to the convergence of the functional
groups in the above calix[4]arene derivative providing a
preorganized metal binding site that reflects both the cyclic
structure and the cone configuration of the calix[4]arene
backbone. Any loss of ‘‘binding complementarity’’ due to
the presence of different spacer groups connecting the two
phosphonic acid groups to the calix[4]arene backbone is
proposed to have been compensated by the operation of a
chelate effect giving rise to high extraction ability being
maintained. Extraction equilibrium constants and separa-
tion factors were estimated based on the extraction data.
The stripping of the loaded rare earth metal ions with hy-
drochloric acid was also investigated.
Keywords Calix[4]arene ꢀ Phosphonic acid ꢀ Solvent
extraction ꢀ Rare earth elements ꢀ Separation
Introduction
Rare earth metals consist of scandium, yttrium and a series
of 15 lanthanide elements. They are well known to be
extremely difficult to separate reflecting their similar che-
mical properties due to the 4f orbitals being ‘‘buried’’ in
each respective atom’s electronic shell. On the other hand,
most of the rare earth metals have been extensively em-
ployed as raw materials for advanced devices and effective
separation processes for these metals have hence been re-
quired. Among several hydrometallurgical processes, sol-
vent extraction is one of the most effective techniques for
metal recovery and separation and has been extensively
employed due to its versatility. Many factors influence the
optimization of a solvent extraction reagent for use in a
given application [1, 2]. For this purpose, a large number of
extraction reagents for rare earth elements have been pre-
pared, especially acidic organophosphorus reagents that
tend to give rise to better efficiency [3–5]. However, the
separation efficiency towards the rare earth elements tends
to be low and the need for the preparation of more effective
reagents still exist. There has been a focus on calixarene
derivatives for this purpose. Calixarenes are phenolic
cyclic oligomers obtained by the condensation reaction of
p-alkylphenol and formaldehyde and are attractive host
compounds due to the possibility of employing three-di-
mensionally arranged coordination sites [6, 7]. The ability
to recognize and discriminate for specific metal ions is one
of the features for this receptor class and the behavior of
these interesting ionophores has been reviewed [8–12]. A
series of such extraction reagents, including derivatives of
This paper is dedicated to Prof. Jack Harrowfield and
Dr. Jacques Vicens in celebration of their 70th birthdays.
M. Tanaka ꢀ S. Morisada ꢀ H. Kawakita ꢀ K. Inoue ꢀ
K. Ohto (&)
Department of Chemistry and Applied Chemistry, Faculty of
Science and Engineering, Saga University, 1-Honjo,
Saga 840-8502, Japan
e-mail: ohtok@cc.saga-u.ac.jp
123

J Incl Phenom Macrocycl Chem
the tetra-O-acetic acid type (1) [13] (‘‘O’’ expresses a
phenoxy oxygen atom of the calix[4]arene), tetram-
ethylenephosphonic acid type at the upper rim (2) [14],
tetra-O-propylenephosphonic acid type (3) [15], tetra-O-
methylenephosphonic acid type (4) [16], and phosphonic
and carboxylic acid cross type (5) [17], calixarenes have
also been designed and prepared for the extractive
separation of rare earths by our group. The structures of the
previously reported extraction reagents based on the cal-
ix[4]arene backbone are shown in Fig. 1. All of these
reagents are fixed in their cone conformation in which all
the donor functional groups are oriented in the same di-
rection. They were shown to vary in their extraction abil-
ities and separation efficiencies towards the rare earths.
Among them [14–17], reagent 1 exhibited the poorest ex-
traction ability and separation efficiency, although it was
superior to the corresponding monomer, which consists of
a single phenolic unit with the same functional group.
Although 2 exhibited enhanced extraction ability compared
to 1 due to presence of the phosphonic acid functionality in
assigned to the operation of a chelate effect involving the
phosphonic acid and phenoxy oxygen functions. Reagent 5
exhibited moderate extraction ability but the poorest
separation efficiency perhaps due to the presence of the two
functional groups exhibiting different metal-binding
affinities. In this context it is noted that cross carboxylic
acid type calix[4]arenes with two longer carboxylic acid
and two acetic acid functional groups occupying distal
positions exhibited much lower extraction ability compared
with the more symmetrical tetraacetic acid derivative 1 [18,
19]. The cross type calixarene derivatives may exhibit
suppressed extraction ability due to its unsymmetrical
topology, but conversely this may contribute to the im-
proved separation efficiency [20].
In the present work, we have prepared the new cross
phosphonic acid calix[4]arene derivative (6) incorporating
two propylenephosphonic acid and two methylenephos-
phonic acid groups at the distal positions and have under-
taken a comparative investigation of its extraction behavior
towards trivalent rare earth ions in relation to the behavior
of the calix[4]arene derivative reagents prepared previ-
ously and with the commercially available phosphonic
reagent mentioned earlier.
2
(with its high affinity for rare earths), the separation ef-
ficiency was lower than that of the corresponding monomer
due to the offset of two effect, that is, size-fitting effect
suitable for large lighter rare earths and functionality effect
for small heavier rare earths. The excellent separation ef-
ficiency of the phosphonic acid groups towards smaller
heavy rare earths was suppressed by their location on the
wider upper rim. The extraction reagent 3 with propy-
lenephosphonic acid at the narrow lower rim was prepared
for comparison and was found to exhibit a higher extrac-
tion ability than 1 and 2, with comparable separation effi-
ciency to 2. Reagent 4 exhibited the highest extraction
ability but the lowest separation efficiency among 1–4
Experimental
Reagents
A series of extractants based on the calix[4]arene backbone,
25,26,27,28-tetrakis(carboxymethoxy)-5,11,17,23-tetrakis(1,1,
3,3-tetramethylbutyl)calix[4]arene (1) [13], 5,11,17,23-tetrak-
is(butyl hydrogen phosphonomethyl)-25,26,27,28-
Fig. 1 Structures of the
extraction reagents based on
calix[4]arene backbone,
together with phosphonic acid
type of commercial extraction
reagent
1
23

J Incl Phenom Macrocycl Chem
tetrapropoxycalix[4]arene (2) [14], 25,26,27,28-tetrakis[3-
{
5,11,17,23-Tetrakis(1,1,3,3-tetramethylbutyl)-25,27-
bis(diethyl phosphonopropoxy)-26,28-bis(diethyl
phosphonomethoxy)calix[4]arene (9)
butyl hydrogen phosphoryloxy}propoxy]-5,11,17,23-te-
trakis(1,1,3,3-tetramethylbutyl)calix[4]arene (3) [15], 25,26,
2
5
7,28-tetrakis(butyl hydrogen phosphoryloxy)methoxy-
,11,17,23-tetrakis(1,1,3,3-tetramethylbutyl)calix[4]arene(4)
Under a nitrogen atmosphere, 7 (5.00 g, 4.07 mmol) was
3
added to dry DMF (200 cm ) cooled in a salt ice bath.
[16], 25,27-bis(carboxypropoxy)-26,28-bis[3-{hydroxy(butyl)
phosphoryloxy}propoxy]-5,11,17,23-tetrakis(1,1,3,3-tetram-
ethyl)butyl)calix[4]arene (5) [17] were prepared in a similar
manner to those reported previously. The new reagent (6) was
preparedfrom 7 [13] via8 [15] and 9 bytwo diphosphonations
with diethyl iodopropyl phosphonate and diethyl iodomethyl
phosphonate [21] on7 and final hydrolysis with base as shown
inFig. 2.Theproductwasidentifiedasthedesiredtarget(inits
Sodium hydride (1.24 g, 32.4 mmol, 62.7 % in oil) was
added to the stirred mixture which was then stirred for 1 h at
room temperature. Diethyl iodomethyl phosphonate (4.52 g,
16.3 mmol) was carefully added dropwise to the mixture
cooled in a salt ice bath and the solution was stirred for 1 h at
25 °C and then for 72 h at 100 °C. After checking that the
reaction was complete by TLC, methanol was carefully
added to the mixture to destroy excess sodium hydride. The
solvent was evaporated in vacuo and the residue was dis-
solved in chloroform. The solution was shaken with 1 M (=
1
cone conformation) by its TLC, H-NMR and IR spectra.
Diethyl iodomethyl phosphonate
-
3
3
mol dm ) hydrochloric acid (100 cm ) then washed three
times with distilled water and dried over anhydrous mag-
nesium sulfate. After filtration, the solvent was evaporated in
vacuo to yield a brown viscous liquid. It was found that this
crude compound included reactant diethyl iodomethyl
phosphonate as an impurity, purification by crystallization or
column chromatography proved difficult and the crude
product was employed directly for the next hydrolysis step.
Diethyl iodomethyl phosphonate was synthesized by a
Michaelis–Arbuzov reaction [21]. That is, under a nitrogen
atmosphere, triethyl phosphite (31.0 g, 187 mmol) and
diiodomethane (100 g, 373 mmol, 2.0 eq) were added and
stirred for 48 h at 100 °C. Distillation of the mixture to
remove unreacted diiodomethane yielded a brown liquid.
1
19.3 g, yield 37.2 %), H-NMR (300 MHz, CDCl , TMS,
(
3
2
4
7 °C), d 1.35 (6H, t, (OCH CH ) ), 3.05 (2H, d, ICH P),
TLC:SiO
, chloroform:methanol = 10:1, R = 0.84 for the
2 f
2
3 2
2
.19 (4H, m, (OCH CH ) ).
main product.
2
3 2
Fig. 2 Synthetic scheme of cross phosphonic acid type calix[4]arene 6 with two propylenephosphonic acid and two methylenephosphonic acid
groups at the distal position
123

J Incl Phenom Macrocycl Chem
5
,11,17,23-Tetrakis(1,1,3,3-tetramethylbutyl)-25,27-
Phase distribution study for the rare earth metals
bis(ethyl hydrogen phosphonopropoxy)-26,28-bis(ethyl
hydrogen phosphonomethoxy)calix[4]arene (6)
The extraction experiments were performed in a similar
manner to those described previously [15]. For investigating
the extraction dependency on pH, chloroform organic solu-
tions were prepared incorporating each extractant at
5.0 mM. The following three kinds of aqueous solutions,
each containing three different rare earth ions, were prepared
by dissolving the required rare earth chloride to 0.1 mM with
0.010, 0.10 or 5.0 M hydrochloric acid: the first solution
The crude viscous liquid of 9 (5.00 g, 3.30 mmol) was
3
added to a small portion (20 cm ) of ethanol. An aqueous
solution of potassium hydroxide (1.85 g, 33.0 mmol) was
added to the mixture, and then it was stirred and refluxed
for 24 h. After checking the reaction was complete by
TLC, the solvent was evaporated in vacuo and the residue
3
was dissolved in chloroform (500 cm ). The chloroform
contained La, Pr, and Nd, the second Sm, Eu, and Gd, and the
þ
solution was shaken with 6 M hydrochloric acid
3
100 cm ) then the chloroform layer was washed three
third Ho, Er, and Y. The pH (ꢁloga ) of the aqueous so-
H
(
lutions was adjusted by mixing certain volumes of the above
three hydrochloric acid stock solutions containing same
times with distilled water and dried over anhydrous
magnesium sulfate. After filtration, the solvent was re-
moved and the residue that remained was recrystallized
from an equal volume mixture of ethanol and acetonitrile;
3
three rare earth metal ions. Equal volumes (5 cm ) of both
phases were mixed and shaken at 303 K for more than 3 h;
this was more than sufficient to attain equilibrium. After
phase separation, the metal concentrations in the aqueous
phases were determined by ICP-AES. The extracted metal
ion concentrations were calculated from the differences in
the metal concentrations of the aqueous phase before and
after the attainment of equilibration. Proton concentrations
were calculated from the mixing ratios employed to prepare
the hydrochloric acid solutions; then, the proton activities
were estimated using activity coefficients [22]. The equi-
librium pH was assumed to be the same as the initial pH
because the pH employed was low enough to ignore any
small change before and after the extraction.
white powder 3.89 g (yield 70.6 %), TLC:SiO , chloro-
2
-
1
form:methanol = 10:1, R = 0.00; IR (KBr) m
mP(O)OH 2640 (br), 2290 (br) and 1650 (br) cm ; H-NMR
2960 cm ,
1 1
f
C–H
-
(
300 MHz, CDCl , TMS, 27 °C), d 0.55 and 0.80 (18H, s
3
each, (CH ) ), 0.80 and 1.33 (12H, s each, (CH ) ), 1.33 and
3
3
3 2
1.77 (4H, s each, (CH )), 1.33 (12H, t each, OCH CH ), 1.90
2 2 3
(4H, m, CH CH CH P), 2.54 (4H, m, CH CH CH P), 3.17
2 2 2 2 2 2
(4H, d, ArCH Ar(exo)), 3.87 (4H, d, OCH P), 4.18 (8H, m
2 2
each, OCH CH ), 4.18 (4H, t, OCH CH CH ), 4.50 (4H, d,
2
2
3
2
2
ArCH Ar(endo)), 5.32 (4H, s(br), OH) 6.36 and 7.08 (4H, s
2
each, ArH).
2
-Ethylhexyl hydrogen 2-ethylhexylphosphonate (PC-
The complexation ratio of 6 to erbium ion as a typical rare
earth was investigated using Job’s continuous variation
method. Aqueous solutions were prepared by dissolving er-
bium chloride in 0.1 M HEPES pH 5.2 buffer solution to form
a series of solutions in the concentration range 0.1–0.9 mM.
Organic solutions were prepared by adding 6 to chloroform to
form a series of solutions in the concentration range
8
8A is the commercial name) was kindly supplied from
Daihachi Chemical Industry Co., Ltd. and was used without
further purification. The other chemicals were purchased and
also used without further purification.
3
Partitioning of the extraction reagent into the aqueous
phase
0.9–0.1 mM. Equal volumes (5 cm ) of both phases were
mixedsothatthecombinedconcentrationsoferbiumionand6
were maintained 1.0 mM. The rest of the procedure was
carried out in a similar manner same to that described above.
In the case of stripping experiments, the organic phases
(for which metal ions were quantitatively loaded in the
forward extraction) were treated with 1, 2, and 4 M fresh
aqueous hydrochloric acid and the mixtures were shaken at
303 K for more than 3 h. The remaining experiments were
carried out in a similar manner to that described above.
Reagent 6 was dissolved in chloroform to give a 5.0 mM
solution. An aqueous solution was prepared by mixing
certain volumes of three stock solutions, 5.0 M HCl,
0
.10 M HCl, and 0.1 M HEPES (N-(2-Hydrox-
yethyl)piperazin-N’-(2-ethanesulfonic acid) to adjust the
proton concentrations or the pH values desired. Equal
3
volumes (5 cm ) of both phases were mixed and shaken at
3
03 K for more than 3 h. After phase separation, the pH
values and the phosphorus content of the respective aque-
ous solutions were measured by pH meter (TOA-DKK
HM-30R) and ICP-AES (Shimadzu ICPS-8100), respec-
tively. The phosphorus contents were calculated from the
differences between the initial phosphorus concentration in
the organic phase and the phosphorus concentration present
into aqueous phase.
Results and discussion
Partitioning of the extraction reagent into the aqueous
phase
From a preliminary investigation into the degree of parti-
tioning of 6 into aqueous phase, it was found that less than
1
23

J Incl Phenom Macrocycl Chem
1
.1 % of 6 was partitioned to the aqueous phase whose pH
?
equilibrium within 1 h. The shaking time for subsequent
experiments was set to be more than 3 h.
(
-log a ) values ranged from -1.02 to 4.75 and hence
H
covered all of the extraction region. Reagent 6 was suffi-
ciently hydrophobic that partition into the aqueous phase
under the extraction experimental conditions employed
was negligible. This negligible partitioning was attributed
to several factors such as the large molecular weight of the
calix[4]arene derivative along with the presence of four
stepwise acid dissociation constants and the significant
localization of the polar functional groups due to the cal-
ixarene’s cone conformation. This observation was sup-
ported by the similar result reported for 2 that only less
than 1 % was eluted to the aqueous phase whose pH values
ranged from 1.4 to 4.2 [14].
The effect of pH on the distribution ratio of the rare
earth metal ions between the aqueous and organic phases
with 6 is shown in Fig. 4. The distribution ratio of metal
ion, D, between aqueous and organic phases was calculated
using Eq. (2);
C ꢁ C
i
e
D ¼
ð2Þ
C
e
The plots for all rare earth metals lie on straight lines,
each with a slope of 3. The slope corresponds to the charge
on the trivalent rare earths investigated. It means that the
extraction of the rare earths with 6 takes place by a simple
ion-exchange mechanism. Reagent 6 exhibited heavy rare
earth selectivity that is comparable to that of the phos-
phonic acid derivative commercial extraction reagent 10
coupled with very high extraction ability so that 50 % of
the heaviest ion, erbium, is extracted from 1.0 M hy-
drochloric acid under the present conditions employed.
This high extraction can be attributed to the action of a
bidentate chelate effect, a phosphonic acid group and the
corresponding phenoxy oxygen atom, together with the
convergence of the functional groups resulting from the
cone structure (in spite of the unsymmetrical topology). For
Phase distribution study of the rare earth metals
The time course dependency of extraction of the erbium
ion (as a representative rare earth metal) with 6 was in-
vestigated. The effect of shaking time on the percentage
extraction of erbium is shown in Fig. 3. The percentage
extraction, % E, was calculated using Eq. (1);
Ci ꢁ Ce
%
E ¼
ꢂ 100;
ð1Þ
Ci
where C and C represent the initial and equilibrium metal
i
e
concentrations in the aqueous phase, respectively. The
subscripts i and e refer to the initial and final (equilibrium)
values. Despite the extraction samples being shaken at
1
30 rpm, the extraction of the erbium ion had reached
Fig. 4 Effect of pH on the distribution ratio of the rare earth ions
with 6. [6] = 5 mM in chloroform, [metal ion] = 0.1 mM each in
5 M, 0.1 M or 0.01 HCl, 303 K, theoretical lines were drawn using
each extraction equilibrium constant
Fig. 3 Effect of shaking time on the percentage extraction of erbium
3
with 6. [6] = 5 mM in chloroform, [Er ] = 0.1 mM, 303 K
?
123

J Incl Phenom Macrocycl Chem
Based on the above results, the stoichiometric relation-
ship corresponds to the extraction reaction of trivalent rare
earths by 6 is given by Eq. (3);
3þ
þ
M
þ H4R , MRH þ 3H :Kex
where M and H R are the rare earth element and cal-
ð3Þ
4
ix[4]arene derivative 6, respectively. Kex is thus given by
Eq. (4);
þ 3
½
MRHꢃ½H ꢃ
Kex ¼
Â
Ã
ð4Þ
3
þ
½
M
ꢃ H R
4
Since the distribution ratio D is defined by Eq. (2), the
2 -6
extraction equilibrium constant Kex (mol dm ) is ex-
pressed by Eq. (5);
þ 3
½H ꢃ
Kex ¼ D
ð5Þ
ð6Þ
½
Taking the logarithm of Eq. (5) gives Eq. (6);
H4Rꢃ
log D ¼ 3pH þ log½H Rꢃ þ log K
4
ex
Equation (6) is consistent with the experimental results and
the extraction constants for 6 with the rare earth ions were
estimated using Eq. (6). The values are listed in Table 1,
together with those for the other reagents, 1–5 and 10. The
separation factors between pairs of trivalent rare earth ions
were calculated using Eq. (7).
3
?
3?
i i
Fig. 5 Job’s plots for Er with 6. [6] ? [Er ] = 1 mM, 0.1 M
HEPES, 303 K
reagents 6, the bidentate chelate rings formed are strong
and stable 5-membered compared with weak and unstable
7
-membered ones and hence particularly stable. This fact is
consistent that symmetrical 4 forming only strong
-membered chelate rings exhibited much higher extrac-
DMetal1
Kex;Metal1
5
b ¼ D
¼
ð7Þ
Metal2
Kex;Metal2
tion ability than 6.
Job’s continuous variation method was carried out to
3
determine the stoichiometry of 6 with Er (as a repre-
The separation factors for 6, together with those for other
extraction reagents, 1–5 and 10 are listed in Table 2. It was
found that 6 exhibits high separation efficiency, with sig-
nificantly high extraction ability (even though weaker
affinity can improve the separation efficiency in some
situations). While it was expected that 6 might exhibit
lower extraction ability because it provides less
?
3
?
sentative ion), the Job’s plots are shown in Fig. 5. The Er
concentration extracted into the organic phase reached a
maximum value when the ratio of the reagent concentration
to the combined concentration was 0.5. The result clearly
3
?
showed that the stoichiometry of 6 with Er is 1 to 1.
Table 1 Extraction, equilibrium constants for extraction of divalent rare earth metal ions with extraction reagents
Extraction reagent
Stoichiometry (M:Extractant)
Dimension
1
1:1
mol dm
[13]
2
1:1
mol dm
[14]
3
1:2
mol dm
[15]
4
1:2
mol dm
[16]
5
1:2
mol dm
[17]
6
1:1
mol dm
This work
10
1:3
–
a
2
-6
2
-6
-3
-3
-3
-6
Reference
[15]
-
-
-
-
-
-
-
-
-
8
7
8
8
8
8
9
9
9
-8
-7
-7
-6
-6
-6
-4
-4
-4
3
3
3
4
4
4
4
4
4
-6
-5
-5
-4
-3
-3
-2
-1
-1
La
Pr
1.66 9 10
1.00 9 10
7.60 9 10
8.73 9 10
5.77 9 10
2.70 9 10
8.93 9 10
3.40 9 10
4.80 9 10
4.08 9 10
3.48 9 10
4.28 9 10
2.24 9 10
4.18 9 10
5.14 9 10
1.23 9 10
3.03 9 10
2.00 9 10
1.67
5.75
11.5
56.5
1.48 9 10
7.26 9 10
7.73 9 10
2.85 9 10
3.43 9 10
3.10 9 10
3.70 9 10
3.08 9 10
4.72 9 10
0.080
0.419
0.481
0.895
0.960
0.679
2.198
3.327
3.820
0.0239
0.177
0.227
0.497
1.26
1.80 9 10
4.73 9 10
6.74 9 10
6.98 9 10
1.98 9 10
2.79 9 10
6.98 9 10
1.48 9 10
1.73 9 10
Nd
Sm
Eu
Gd
Ho
Y
2
2
3
3
1.13 9 10
2.10 9 10
2.36 9 10
4.70 9 10
1.91
34.9
109
3
Er
6.649 10
135
a
Extractant as dimeric species
1
23

J Incl Phenom Macrocycl Chem
Table 2 Separation factors between pairs or trivalent rare earth metal ions for extraction reagents
Extraction reagent
Stoichiometry (M:Extractant)
Reference
1
1:1
[13]
2
1:1
[14]
3
1:2
[15]
4
1:2
[16]
5
1:2
[17]
6
1:1
This work
10
1:3
[15]
a
Pr/La
6.03
0.76
1.15
0.66
0.47
0.33
0.38
1.41
8.51
5.25
4.90
1.06
3.68
1.21
0.90
1.19
0.83
1.53
5.25
1.15
1.86
1.07
0.71
3.24
1.51
1.15
7.41
1.28
2.19
2.53
1.52
18.25
3.11
1.24
26.28
1.42
Nd/Pr
Sm/Nd
Eu/Sm
Gd/Eu
Ho/Gd
Y/Ho
1.23
5.25
1.86
1.23
23.99
2.45
0.66
1.32
4.90
2.00
1.86
11.22
2.00
1.41
10.36
2.83
1.41
25.00
2.13
Er/Y
1.17
a
Extractant as dimeric species
operation of the chelate effect as discussed above. The
above rationale is supported by the fact that the extraction
ability for 6 was comparable to that for symmetrical 3,
whose phenoxy oxygen atoms are not available for strong
chelate formation due to the presence of the long spacers.
Since the extraction stoichiometries for the different
reagents were not observed to be the same, the extraction
abilities were not compared with each other using the ex-
traction constants, rather half pH values were employed.
The half pH value is the pH value where 50 % of metal ion
is extracted. Comparison of the relationship between the
pH0.5 values for 6 and the reciprocal of the ionic radii for
the trivalent rare earth metals with a coordination number
-
1
of six, r , together with a similar comparison for other
extraction reagents, 1–5 and 10 is shown in Fig. 6. The
vertical axis was inverted so that the plots for the extraction
reagents with high extraction efficiency appear upside
down. The difference between the plots for each rare earth
ion, that is, the slope between two adjacent metals or two
edge metals, corresponds to the separation efficiency. The
commercial reagent 10 exhibited the highest separation
efficiency (very likely due to the presence of two optimal
2-ethylhexyl groups); however, 6 also exhibited high
separation efficiency as did 3. The extraction ability of the
respective extraction reagents follows the sequence as
follows:
Fig. 6 Relationship between pH0.5 values and the reciprocal of ionic
radii for trivalent rare earth metals, r
-1
4
ꢄ 6 ꢅ 3 [ 5 [ 2 [ 1 ꢆ 10;
complementarity due to its unsymmetrical topology due to
it incorporating two phosphonic acid groups linked by
different spacers. However, contrary to expectation, high
extraction ability was observed that may be attributed to
the presence of a strong 5-membered chelate effect, be-
cause its O-methylenephosphonic acid group is able to act
as a strong bidentate coordinating group. That is, any
suppressed extraction ability due to the unsymmetrical
topology arising from the two phosphonic acid groups with
different spacers is more than compensated by the
The separation efficiency of the extraction reagents is given
by:
1
0 [ 6 ꢅ 3 [ 2 [ 1 [ 4 ꢄ 5:
Finally, the stripping behavior of the rare earth metal
ions loaded on 6 was investigated. The effect of hy-
drochloric acid concentration on the percentage stripping
of loaded on 6 is shown in Fig. 7. Likely reflecting the high
extraction ability of 6, the heavier rare earth metal ions
123

J Incl Phenom Macrocycl Chem
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