The effect of phenoxy oxygen atoms on extremely high extraction ability and less separation efficiency of trivalent rare earth elements with tetraphosphonic acid derivative of calix[4]arene

Article abstract of DOI:10.1007/s10847-012-0255-0

A cone confomational p-t-octylcalix[4]arene with four methylenephosphonic acids at the lower rim as well as its monomeric analog have been synthesized as a new extraction reagent to investigate the extraction behavior of the nine trivalent rare earth elements: Y, La, Pr, Nd, Sm, Eu, Gd, Ho and Er. The new calix[4]arene exhibited significantly higher extraction capacity than not only the monomeric derivative and the commercial extraction reagent, 2-ethylhexyl hydrogen 2-ethylhexylphosphonate, but also the cone conformational calix[4]arene extractants employed in our previous work, composed of a tetrapropylenephosphonic acid with a longer spacer, a tetraphosphonic acid at the upper rim, and tetraacetic acid at the lower rim. A dependence on the pH was observed for the new extractant in the highly acidic region, and the extraction took place via a simple ion-exchange mechanism. Using slope analysis, the stoichiometries of tetrameric and monomeric extractants to rare earth metal ions were determined to be 2:1 and 3:1, respectively. Using the proposed extraction equations, extraction equilibrium constants and separation factors were estimated. The extremely high extraction ability and moderate separation efficiency were attributed to the chelating effect of the phosphonic acid and the phenoxy oxygen atom. The effect of the phenoxy oxygen atom on extraction ability and separation efficiency of calix[4]arene derivatives was found to be significant. Springer Science+Business Media Dordrecht 2012.

Full text of DOI:10.1007/s10847-012-0255-0

J Incl Phenom Macrocycl Chem
DOI 10.1007/s10847-012-0255-0
ORIGINAL ARTICLE
The effect of phenoxy oxygen atoms on extremely high extraction
ability and less separation efficiency of trivalent rare earth
elements with tetraphosphonic acid derivative of calix[4]arene
•
Keisuke Ohto Akihito Takedomi
•
•
Anup Basnet Chetry Shintaro Morisada
•
•
Hidetaka Kawakita Tatsuya Oshima
Received: 4 July 2012 / Accepted: 10 October 2012
Ó Springer Science+Business Media Dordrecht 2012
Abstract A cone confomational p-t-octylcalix[4]arene
with four methylenephosphonic acids at the lower rim as
well as its monomeric analog have been synthesized as a
new extraction reagent to investigate the extraction
behavior of the nine trivalent rare earth elements: Y, La, Pr,
Nd, Sm, Eu, Gd, Ho and Er. The new calix[4]arene
exhibited significantly higher extraction capacity than not
only the monomeric derivative and the commercial
extraction reagent, 2-ethylhexyl hydrogen 2-ethyl-
hexylphosphonate, but also the cone conformational
calix[4]arene extractants employed in our previous work,
composed of a tetrapropylenephosphonic acid with a
longer spacer, a tetraphosphonic acid at the upper rim, and
tetraacetic acid at the lower rim. A dependence on the pH
was observed for the new extractant in the highly acidic
region, and the extraction took place via a simple ion-
exchange mechanism. Using slope analysis, the stoichi-
ometries of tetrameric and monomeric extractants to rare
earth metal ions were determined to be 2:1 and 3:1,
respectively. Using the proposed extraction equations,
extraction equilibrium constants and separation factors
were estimated. The extremely high extraction ability and
moderate separation efficiency were attributed to the che-
lating effect of the phosphonic acid and the phenoxy
oxygen atom. The effect of the phenoxy oxygen atom on
extraction ability and separation efficiency of calix[4]arene
derivatives was found to be significant.
Keywords Calix[4]arene ꢀ Phosphonic acid ꢀ Solvent
extraction ꢀ Rare earth elements ꢀ Extraction ability ꢀ
Separation efficiency
Introduction
Rare earth metals are difficult to mutually separate due to
their similar chemical properties and ionic size. These
metals are key elements in a number of advanced industrial
applications, including magnetic materials, luminescent
materials, superconductors, catalysts, polishers, and lasers.
Separation techniques are necessary because high purity is
generally required for these applications. Among the many
techniques for recovery and separation of metals, solvent
extraction has been used extensively due to its high sepa-
ration efficiency, excellent versatility, and mass disposal
potential. The role of the extraction reagents is very
important in the solvent extraction method. Various types
of extractants have been prepared, and organophosphorus
acid materials were found to be effective for rare earth
extractions [1]. Peppard et al. [2–6] performed extensive
investigations on the solvent extraction of rare earth ele-
ments using organophosphorus extractants for the appli-
cation of separation from nuclear waste and actinides.
Yuan et al. [7–10] prepared a series of acidic phosphorus
esters to study the relationship between structure and
extraction behavior. Furthermore, novel organophosphorus
K. Ohto (&) ꢀ A. Takedomi ꢀ A. B. Chetry ꢀ S. Morisada ꢀ
H. Kawakita
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
URL: http://saga-univ-chemical-engineering.jp/ohto/index.html
T. Oshima
Department of Applied Chemistry, Faculty of Engineering,
University of Miyazaki, 1-1 Gakuen Kibanadai-nishi,
Miyazaki 889-2192, Japan
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J Incl Phenom Macrocycl Chem
acid extraction reagents for rare earth elements have also
been prepared [11–14].
chelation of the phosphonic acid and the phenoxy oxygen
atom. Although the chelation contributes to the high
extraction ability, it also results in poor separation
efficiency.
Calixarenes are macrocyclic oligomers that provide
three-dimensional coordination sites for a variety of inter-
esting compounds [15, 16]. The ability to recognize and
discriminate metal ions is a remarkable feature of these
site-specific receptors. Several review articles have been
published on these interesting ionophores [17–25]. Acidic
organophosphorus compounds based on a calixarene
framework for rare earth elements have been reported.
Oshima et al. [26] prepared hexaphosphorylated calix[6]-
arene to investigate the extraction behavior of nine rare
earth elements and reported high extraction ability and
selectivity for heavy rare earth metals. Jurecka et al. [27]
prepared three calix[4]arene derivatives containing two
dibasic phosphonic acids with different spacer lengths in a
cone conformation and also calix[4]arene derivative with
four dibasic phosphonic acids and a propylene spacer
length in a cone conformation [28]. They found that the
extraction efficiencies decrease with increasing length of
the CH2 spacer. The tetraphosphonic acid with a propylene
spacer length exhibited selectivity for light rare earth
metals, whereas the three diphosphonic acid derivatives
demonstrated selectivity for heavy rare earth metals. In
addition, calix[4]arene derivatives with four phosphonic
acids at upper rim were prepared in our previous work [29,
30]. Several phosphorus-containing calixarenes have been
prepared [31–35]. Since the binding site at the upper rim is
much larger than the ion diameter of rare earth metals, the
separation efficiency for phosphorus-containing calixare-
nes at upper rim may be poorer than that of the corre-
sponding monomeric analogue, because acidic phosphorus
groups tend to smaller heavier rare earth selectivity. Thus,
the effect of the cyclic structure should well multiply by
the effect based on the functional group affinity [29].
More recently, calix[4]arene derivatives containing four
mono-basic propylenephosphonic acid groups at the lower
rim were prepared to investigate the extraction behavior of
rare earth metals including both the extraction ability and the
mutual separation efficiency [36]. These reagents exhibited
extremely high extraction efficiency for trivalent rare earth
metals and significant heavy rare earth selectivity.
Experimental
Reagents
The chemical structures of extractant 5 and the corre-
sponding monomeric analog 7 together with the calix[4]-
arene phosphonic acid derivative with a longer spacer 8, its
monomeric acid derivative 9, carboxylic acid derivatives of
calix[4]arene 10, and the commercial extractant 11 (PC-
88A is its commercial name), are shown in Fig. 1.
The commercial extractant 11, 2-ethylhexyl 2-ethyl-
hexylphosphonic acid was kindly supplied by the Daihachi
Chemical Industry Co., Ltd. and was used without further
purification. The other chemicals were purchased and also
used without further purification. The extractant 5 has been
prepared in three steps, which include cyclization, tetra-
phosphonation, and basic hydrolysis from the corresponding
phenol. 5,11,17,23-Tetrakis(1,1,3,3-tetramethylbutyl)calix
[4]arene-25,26,27,28-tetol 3 was synthesized in a similar
manner to a previously described procedure [37]. The syn-
thetic schemes for extractants 5 and 7 are shown in Fig. 2.
Dibutyl hydroxymethylphosphonate(1) [38]
Under
a nitrogen stream, dibutylphosphite (49.7 g,
256 mmol) was added to paraformaldehyde (9.28 g,
309 mmol) and dry potassium carbonate (1.80 g,
13.0 mmol) in 100 cm3 1-butanol. The mixture was stirred
and heated at 135 °C for 30 h. The solvent was removed in
vacuo. Chloroform was added to the residue to extract the
desired compound. The organic solution was washed with
1 M (M = mol dm-3) hydrochloric acid, saturated brine
and distilled water each three times. After drying over
anhydrous magnesium sulfate, the solvent was removed in
vacuo and a colorless liquid was obtained. 42.3 g (yield
73.7 %), 1H-NMR (300 MHz, CDCl3, TMS, 30 °C) d 0.93
(6H, t, 2CH3), 1.40 (4H, m, 2CH2CH3), 1.67 (4H, m,
2CH2CH2CH3), 3.92 (2H, d, J = 6.0 Hz, CH2OH), 4.08
(4H, t, OCH2), 4.67 (1H, br s, OH).
In this contribution, we have synthesized calix[4]arene
derivatives containing four mono-basic methylenephosph-
onate groups at the lower rim to investigate the extraction
behavior of rare earth metals (Y, La, Pr, Nd, Sm, Eu, Gd,
Ho, and Er), including both the extraction ability and the
mutual separation efficiency. By comparing the extraction
efficiency of two monomers, that is, the corresponding
monomer and a phosphonic acid derivative with its similar
lipophilicity and without phenoxy oxygen atoms, it is
indirectly interpreted that the extremely high extraction
ability of the calix[4]arene extractant is attributed to the
Dibutyl phosphonomethyl tosylate (2) [39]
Under a nitrogen stream, 1 (30.0 g, 133 mmol) was added
to 150 cm3 dry pyridine in an ice bath. To this mixture,
p-tosyl chloride (32.6 g, 171 mmol) was added and stirred
for 24 h. The reaction was monitored by TLC, and upon
completion, pyridine was removed in vacuo. Chloroform
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J Incl Phenom Macrocycl Chem
Fig. 1 Chemical structures of
extractants
Fig. 2 Synthetic schemes for
extractants 5 and 7
25,26,27,28-Tetrakis(butyl hydrogen phosphoryloxy)
methoxy-5,11,17,23-tetrakis(1,1,3,3-tetramethylbutyl)
calix[4]arene (5) [27]
was added to the residue to extract the desired compound.
The organic solution was washed with 1 M hydrochloric
acid, saturated brine and distilled water each three times.
After drying over anhydrous magnesium sulfate, the sol-
vent was removed in vacuo and the residue was purified by
column chromatography (SiO2, chloroform:hexane = 5:
1 v/v and chloroform); yellow viscous liquid; 26.3 g (yield
51.8 %); 1H-NMR (300 MHz, CDCl3, TMS, 30 °C) d 0.92
(6H, t, 2 CH2CH3), 1.37 (4H, m, 2CH2CH3), 1.65 (4H, m,
2CH2CH2CH3), 2.46 (3H, t, CH3), 4.06 (4H, t, POCH2),
4.19 (2H, d, J = 10.2 Hz, PCH2O), 7.36 (2H, d, ArH-
CH3), 7.80 (2H, d, ArH-S).
Under a nitrogen stream, the parent calix[4]arene 3 (10.0 g,
11.5 mmol), potassium carbonate (31.3 g, 226 mmol), and 2
(31.4 g, 82.9 mmol) were added to 100 cm3 dry acetonitrile
and the mixture was refluxed for 144 h. The reaction was
monitored by TLC, and upon completion, the solvent was
removed in vacuo. Chloroform was added to the residue to
extract the desired compound. The organic solution was
washed with 1 M hydrochloric acid and distilled water each
123

J Incl Phenom Macrocycl Chem
three times. After drying over anhydrous magnesium sulfate,
the solvent was removed in vacuo and the residue was
purified by column chromatography (SiO2, chloroform and
chloroform:butanol = 5:1 v/v); yellow viscous liquid;
7.02 g. Although the crude compound was mainly composed
of the desired compound 4, 25,26,27,28-tetrakis(dibutyl-
phosphoryloxy)methoxy-5,11,17,23-tetrakis(1,1,3,3-tetra-
methylbutyl)calix[4]arene, it also contained the reactant 2
as an impurity. Nevertheless, hydrolysis was performed on
the crude material as described below.
hydroxide (14.1 g, 249 mmol) in water (50.3 g, 2.79 mol)
was added. The mixture was refluxed for 36 h. The reac-
tion was monitored by TLC, and upon completion, the
solvent was removed in vacuo. Chloroform was added to
the residue to extract the desired compound. The organic
solution was washed with 1 M hydrochloric acid and dis-
tilled water each three times. After drying over anhydrous
magnesium sulfate, the solvent was removed in vacuo;
yellow viscous liquid; 4.12 g (yield 54.6 % from p-t-oc-
tylphenol); IR (neat) mP(O)OH 2,627, 2,278 and 1,678 cm-1
;
Under a nitrogen stream, the crude 4 (2.03 g) was dis-
solved in 50 cm3 ethanol. To the mixture, potassium
hydroxide (2.08 g, 37.1 mmol) in water (28.9 g, 1.61 mol)
was added. The mixture was refluxed for 120 h. The
reaction was monitored by TLC, and upon completion, the
solvent was removed in vacuo. Chloroform was added to
the residue to extract the desired compound. The organic
solution was washed once with 6 M and then with 1 M
hydrochloric acid, followed by four washes with distilled
water. After drying over anhydrous magnesium sulfate, the
solvent was removed in vacuo. The desired compound was
recrystallized from acetonitrile and chloroform; white
powder; 0.792 g (yield 44.8 % from 3); IR (KBr) disap-
peared mO–H 3,150 cm-1 (br), mP(O)OH 2,671, 2,285 and
1,671 cm-1; 1H-NMR (300 MHz, CDCl3, TMS, 30 °C) 0.69
(36H, s, C(CH3)3), 0.90 (12H, t, CH2CH3), 1.10 (24H, s,
C(CH3)2), 1.36 (8H, m, CH2CH3), 1.58 (8H ? 8H, s ? m,
CCH2C ? OCH2CH2), 3.24 (4H, d, exo-CH2), 3.92 (8H,
m, OCH2CH2), 4.50 (8H, d, J = 12.3 Hz, CH2P), 4.63 (4H,
d, endo-CH2), 6.80 (8H, s, ArH), 10.50 (4H, s(br), OH),
Found: C, 64.93; H, 8.97 %, Calcd for C80H132O16P4: C,
65.20, H, 9.03 %.
1H-NMR (300 MHz, CDCl3, TMS, 30 °C) 0.70 (9H, s,
C(CH3)3), 0.91 (3H, t, CH2CH3), 1.41 (6H ? 2H, s ? m,
C(CH3)2 ? CH2CH3), 1.64 (2H ? 2H, t ? s, OCH2
CH2 ? CCH2C), 4.15 (8H, m, OCH2CH2), 4.27 (8H, d,
J = 10.8 Hz, CH2P), 6.85 (2H, d, O-ArH), 7.26 (2H, d,
C-ArH), 11.82(1H, s(br), OH), Found: C, 63.43; H, 9.27 %,
Calcd for C19H33O4P: C, 64.02, H, 9.33 %.
Distribution study of rare earth metals
The extraction was performed in a similar manner as previ-
ously described [36, 37]. An organic solution was prepared by
diluting each extractant with analytical grade of chloroform
to the concentration of 5.0 mM for 5 and 20 mM for 7. The
three types of aqueous solutions, each containing three types
of rare earth ions, were prepared by dissolving analytical
grade rare earth chloride in 0.01, 0.1, 1.0 or 5.0 M hydro-
chloric acid and then diluting to a concentration of 0.1 mM.
Among 17 rare earth elements, some pairs of adjacent ele-
ments are remarkably difficult to be mutually separated such
as Pr/Nd, Sm/Eu, Eu/Gd, Ho/Er, Ho/(Y)/Er in using orga-
nophosphorus acid reagents. Nine elements containing above
pairs were selected for this experiment in similar manner as
the previous one. The first solution contained La, Pr, and Nd;
the second Sm, Eu, and Gd; and the last Ho, Er, and Y. In the
case of the monomeric extractant, aqueous solutions were
prepared by dissolving analytical grade rare earth chloride in
0.1 M hydrochloric acid and 0.1 M HEPES (2-[4-(2-
hydroxyethyl)-1-piperazinyl]-ethanesulfonic acid) buffer.
The hydrochloric acid concentration or the initial pH value of
the aqueoussolutionwas adjusted byarbitrarilymixing above
stock solutions. Equal volumes (5 cm3) of both phases were
mixed and vigorously shaken at 303 K for more than 3 h.
After phase separation, the metal concentrations in the
aqueous solutions were measured by ICP-AES (Shimadzu
ICPS-8100). The extracted metal ion concentrations were
calculated from the differences in metal concentrations of the
aqueous phase before and after equilibration. Proton con-
centrations were calculated from the mixing ratio of hydro-
chloric acid solutions, and the proton activities were
estimated usingactivitycoefficients[40]. The equilibriumpH
was assumed to be the same as the initial pH because the pH
was low enough to ignore the change before and after the
Butyl hydrogen[4-(1,1,3,3-
tetramethylbutyl)phenoxymethyl]phosphonate (7)
Under
a
nitrogen stream, p-t-octylphenol (9.99 g,
48.0 mmol), potassium carbonate (35.1 g, 254 mmol), and 2
(26.0 g, 69.0 mmol) were added to 100 cm3 dry acetonitrile
and the mixture was refluxed for 90 h. The reaction was
monitored by TLC, and upon completion, the solvent was
removed in vacuo. Chloroform was added to the residue to
extract the desired compound. The organic solution was
washed with 1 M hydrochloric acid and distilled water each
three times. After drying over anhydrous magnesium sulfate,
the solvent was removed in vacuo; yellow viscous liquid;
28.1 g. Although the crude compound was mainly composed
of the desired compound 6, dibutyl[4-(1,1,3,3-tetramethyl-
butyl)phenoxymethyl]phosphonate, it also contained the
reactant 2 as an impurity. Nevertheless, hydrolysis was
performed on the crude material as described below.
Under a nitrogen stream, the crude 6 (12.0 g) was dis-
solved in 100 cm3 ethanol. To this mixture, potassium
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J Incl Phenom Macrocycl Chem
extraction. When using the continuous variation method
(Job’s method), the total concentration of europium ion and
extractant was maintained at 1 mM.
representative Job’s plots for Eu(III) with 5 and 7 are shown
in Fig. 5a, b, respectively. The maximum ratio of extractant
concentration to total concentration of extractant and euro-
pium ion is 0.53 for 5 and 0.75 for 7. These results indi-
cated that the trivalent europium ion formed a 1:1
(metal:extraction reagent) complex with 5, and a 1:3 com-
plex with 7.
Results and discussion
First, the extraction time was first investigated. The effects
of shaking time on the extraction percentage of trivalent
samarium, europium, and gadolinium ions with 5 and 7 are
shown in Fig. 3a, b. Although the extraction samples were
shaken mildly at 130 rpm, the extraction of rare earth metal
ions was fast and reached equilibrium within an hour for 5
and 15 min for 7. The equilibrium time was longer for the
calix[4]arene compound than the corresponding mono-
meric acid, perhaps because of the strong intramolecular
hydrogen bonding of the former compound due to its
localized polarity and cone conformation.
It should be noted that the stoichiometries determined by
the slope analysis of the extractant concentration depen-
dence are different from those derived by Job’s method. The
effects of extractant concentration on the distribution ratio of
samarium and europium ions with 5 and 7 are shown in
Fig. 6a, b. All points for 5 in Fig. 6a can be fit by straight
lines with the slope of 2, whereas those of 7 in Fig. 6b exhibit
slopes of 3. These results indicate that the stoichiometries of
samarium and europium ions with 5 and 7 are 1:2 and 1:3
(metal:extraction reagent), respectively, which is inconsis-
tent with the result obtained using Job’s method. The dif-
ference in the stoichiometry values for 5 determined by the
two methods is caused by the different experimental con-
ditions. That is, the extractant concentration in Job’s method
is equivalent to the metal concentration at maximum point,
whereas in the slope analysis method, it is much higher than
that of the metal. Accordingly, extractant 5 forms 1:2 com-
plexes with rare earth metals when the extractant concen-
tration exceeds that of the metal. Thus, the stoichiometry for
the pH dependence experiments was presumed to be 1:2. In
this case, some of oxygen atoms of the functional groups
may be related to the complexation so as to saturate the
coordination number of rare earth elements.
Next, the pH dependence of trivalent rare earths with the
extractants was investigated. The effects of pH on the distri-
bution ratio of rare earth metals with 5 and 7 are shown in
Fig. 4a, b. The pH region for both extraction reagents was
remarkably low, meaning that both extraction reagents exhib-
ited high extraction ability for rare earths, and the extraction
occurred despite a high proton concentration. All points in
Fig. 4 can be fit by straight lines with a slope of 3, which is
equivalent to the charge of rare earth metals. This result indi-
cates that all trivalent metal ions are ion-exchanged with the
three protons from the extractants, even in such a low pH
environment. The following extraction selectivity was
observed: Er [Y[Ho [Gd [Eu [Sm [Nd [Pr [La.
Heavier rare earth metals are preferentially extracted over
lighter ones. The selectivity trends are similar to those of con-
ventional acidic phosphorus extractants, although the selectivity
is relatively poor. For acidic phosphorus extractants, yttrium is
placed between holmium and erbium [41].
Based on the above results and presumptions, the stoi-
chiometric relationship in the extraction reaction of triva-
lent rare earths with 5 is proposed by the Eq. (1);
Kex
RE3þ þ 2H4L ꢀ REꢁðH2LÞðH3LÞ þ 3Hþ
where RE and H4L represent the rare earth metal and
ð1Þ
Another experiment was performed using Job’s method to
confirm the stoichiometry of the extraction reaction. A
tetrameric phosphonic acid 5, respectively, and Kex is the
Fig. 3 Effects of shaking time
on the extraction percentage of
trivalent samarium, europium,
and gadolinium ions.
a [5] = 5 mM, initial
pH = 0.31; b [7] = 20 mM,
initial pH = 1.53
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J Incl Phenom Macrocycl Chem
Fig. 4 Effects of pH on the
distribution ratio of rare earth
metals with a 5 and b 7
Fig. 5 Job’s plots for Eu(III)
with a 5 and b 7.
[Extractant]initial ? [Eu ion]
= 1 mM, initial
initial
pH = 5.30
Fig. 6 Effects of the extractant
concentration on the distribution
ratio of Sm and Eu ions with a 5
(initial pH = 0.08) and b 7
(initial pH = 1.17)
extraction equilibrium constant. This stoichiometric
relationship yields the following equilibrium relationship:
log D ¼ 3 pH þ 2 log½H4Lꢂ þ logKex
where D value is the distribution ratio defined by Eq. (4).
ð3Þ
ꢀ
The Eq. (2) can be rewritten as Eq. (3):
ꢁ
Â
Ã
3
2
Kex ¼ ½REꢁðH2LÞðH3LÞꢂ½Hþꢂ = RE3þ ½H4Lꢂ
ð2Þ
Â
Ã
Â
Ã
Â
Ã
D ¼ RE3þ org= RE3þ ¼ ½REꢁðH2LÞðH3LÞꢂ= RE3þ
aq
ð4Þ
123

J Incl Phenom Macrocycl Chem
separation efficiency can be compared because the number
of functional groups for all experimental conditions is the
same. The extraction ability of extractant 5 is much greater
than that of 7, 8, 10 and 11, because its plots are clearly
higher than those of other extractants, which is attributed to
the highly pre-organized structure for the rare earth metals
due to the size-fitting effect, the complimentary effect and
the converging effect of the four phosphonic acids arranged
in a cone conformation. The difference between the plots
for each rare earth ion corresponds to the separation effi-
ciency. Although the plots for the extractants do not
exactly form straight lines and appear to organize as
‘tetrads’ [42], the slope between the two plots roughly
indicates the metal selectivity and separation efficiency. As
described earlier, 5 exhibits high heavy rare earth metal
selectivity with an extraction selectivity of Er [ Y [ Ho [
Gd [ Eu [ Sm [ Nd [ Pr [ La.
The separation efficiency of 5 is comparable to that of
the monomeric analog 7 lower than that of longer spacer
derivative 8 and the commercial extractant 11, and higher
than that of the tetraacetic acid 10. Calix[4]arene deriva-
tives and its corresponding monomers exhibited higher
extraction ability and separation efficiency compared with
the calix[4]arene tetraacetic acid materials. Molecular
design, including the nature of the functional group, is
shown to be truly significant.
where subscripts
and
represent the species in the
aq
org
organic and aqueous phases, respectively.
The slope of the pH dependence in Fig. 4a is 3, which is
derived from the charge of the rare earth elements. Because
the initial extractant concentration is significantly higher
than the total metal concentration, the difference in the
extractant concentration before and after the extraction is
negligible. Consequently, the results of the pH dependence
study are in agreement with the extraction reaction (1).
The slope of the extractant concentration dependence in
Fig. 6a is 2, which is consistent with the stoichiometry of
the complex. The results of the extractant concentration
dependence are also in agreement with the Eq. (1).
Similarly, the extraction relation of trivalent rare earths
with 7 is proposed by the Eq. (5);
Kex
RE3þ þ 3HL ꢀ REꢁL3 þ 3Hþ
where HL represents the monomeric phosphonic acid 7.
The Eq. (5) can be rewritten as Eq. (6):
ð5Þ
log D ¼ 3 pH þ 3 log½HLꢂ þ logKex
The results shown in Figs. 4b and 6b are also consistent
with the extraction Eq. (5).
ð6Þ
Using the above data and equations, the half pH values
(pH1/2), the difference between the half pH values (-DpH1/
2), the extraction equilibrium constants (Kex) and the sep-
aration factors (b), were estimated for each extractant. The
half pH value, pH1/2, is defined as the pH value at which
half of the metal ions are extracted under the experimental
conditions used in this study. The difference between the
half pH values, -DpH1/2, is calculated from the difference
in the pH1/2 values between the heavier and lighter metal
ions (a negative DpH1/2 value means that a heavier metal
ion is more easily extracted in comparison with a lighter
one). The separation factor, b, is defined by Eq. (7):
Extractant 5 exhibited extremely high extraction ability
among the extractants introduced in this work, but only
moderate separation efficiency. Why did it exhibit much
higher extraction ability and lower separation efficiency
than derivative 8 with a longer spacer and the commercial
extractant 11? To explain this phenomenon, the results for
the monomeric derivative 7 must be considered. The
extraction efficiency of 7 is much higher than that of tetr-
apropylenephosphonic acid calix[4]arene 8. Although
the monomeric derivative 7 does not correspond to the
calix[4]arene derivative 8, the result is unusual because
calix[4]arene derivatives with a cone conformation typi-
cally possess a higher extraction efficiency than the cor-
responding monomers due to the size-fitting effect, the
complimentary effect and the converging effect of multi-
functionality based on their pre-organization, as described
in our previous studies [32–45]. In contrast, a calix[4]arene
derivative with tetraphosphonic acid at the upper rim
exhibited comparable extraction ability to that of its cor-
responding monomer [29]. The key for obtaining high
extraction ability is related to the chelation capacity of the
phosphonic acid and phenoxy oxygen atom as functional
groups. Propylenephosphonic acid derivative 8 forms a
7-membered chelate ring during the extraction, which is
not a preferable chelate form, resulting in low extraction
ability. In contrast, methylenephosphonic acid derivative 5
forms a more preferable 5-membered chelate ring, resulting
b ¼ DHeavier metal=DLighter metal
¼ Kex;Heavier metal=Kex;Lighter metal
:
ð7Þ
The obtained values are provided in Table 1, together with
the values for tetraphosphonic acid calix[4]arene with a
longer spacer 8 [36], and the commercial extractant 11
[36]. The Kex values for the extractants are not directly
comparable due to the difference in stoichiometries,
whereas the pH1/2, -DpH1/2, and b values are directly
comparable.
Figure 7 provides a visual comparison of the relation-
ship between the pH1/2 values and the reciprocal number of
ionic radii for trivalent rare earth metals with a coordina-
tion number of six, r-1. The y-axis is inverted such that the
plots for the extractants with high extraction efficiency
appear upside down. Although the stoichiometry for each
extractant is not the same, the extraction ability and
123

J Incl Phenom Macrocycl Chem
Table 1 Half pH values (pH1/2), difference between half pH values (-DpH1/2), the extraction equilibrium constants (Kex) and the separation
factors (b)
Extractant No
Concentration
Stoichiometry
Rare earths
Extractant 5
5 mM
Extractant 7
20 mM
2:1 (Extractant: RE)
3:1 (Extractant: RE)
pH1/2
0.48
-DpH1/2
Kex
b
pH1/2
1.55
-DpH1/2
Kex
b
La
Pr
1.48E?03
2.21E?01
0.23
0.01
4.90
1.06
3.68
1.21
0.90
1.19
0.83
1.53
0.19
0.02
0.10
0.05
0.00
0.36
0.09
0.06
3.77
0.25
0.24
0.05
0.02
0.04
0.01
0.04
7.26E?03
7.73E?03
2.85E?04
3.43E?04
3.10E?04
3.70E?04
3.08E?04
4.72E?04
1.36
1.34
1.24
1.19
1.19
0.83
0.75
0.69
8.32E?01
9.35E?01
1.88E?02
2.67E?02
2.67E?02
3.15E?02
5.66E?02
8.57E?02
1.12
2.01
1.42
1.00
11.78
1.80
1.51
Nd
Sm
Eu
Gd
Ho
Y
0.19
0.03
-0.01
0.03
-0.03
0.06
Er
-0.02
Extractant No
Concentration
Stoichiometry
Rare earths
Extractant 8
5 mM
Extractant 11(PC-88A)
20 mM
2:1 (Extractant: RE)
3:1 (Extractant: RE)
pH1/2
1.46
-DpH1/2
Kex
b
pH1/2
3.91
-DpH1/2
Kex
b
La
Pr
1.67E?00
1.80E-06
0.24
0.04
0.23
0.10
0.09
0.35
0.10
0.05
5.25
1.32
4.90
2.00
1.86
11.22
2.00
1.41
0.47
0.05
0.34
0.15
0.05
0.47
0.11
0.02
26.28
1.42
1.22
1.18
0.95
0.85
0.76
0.41
0.31
0.26
8.75E?00
1.15E?01
5.65E?0.1
1.13E?02
2.10E?02
2.36E?03
4.70E?03
6.64E?03
3.44
3.39
3.05
2.90
2.85
2.39
2.28
2.25
4.73E-05
6.74E-05
6.98E-04
1.98E-03
2.79E-03
6.98E-02
1.48E-01
1.73E-01
Nd
Sm
Eu
Gd
Ho
Y
10.36
2.83
1.41
25.00
2.13
1.17
Er
in high extraction ability. Calix[4]arene derivatives with
tetraphosphonic acid at the upper rim do not possess
phenoxy oxygen atoms for the chelation, and thus exhibit
ordinary extraction ability. As support for this explanation,
the relationship between pH1/2 values for the monomeric
extractants 7 and 9 and the reciprocal number of ionic radii
123

J Incl Phenom Macrocycl Chem
rare earth metals, especially in the case of the smaller
heavier rare earths that are selectively complexed with
phosphonic acid, as well as the presence of 1:2 stoichi-
ometry and the absence of a single molecular extraction. In
fact, the slope of 7 in Fig. 8 is lower than that of 9,
meaning that the separation efficiency of 7 is lower than
that of 9. In our previous works, the extraction behavior of
acetic acid containing calix[4]arenes with different spacers
was investigate [46, 47]. The extraction ability and the
separation efficiency were found to be strongly related to
the length of the spacer. In this study, the effect of the
phenoxy oxygen atom on the extraction behavior is sig-
nificant, and its presence in the molecular design of
calix[4]arene derivatives should be evaluated.
Conclusions
Fig. 7 Relationship between pH1/2 values and the reciprocal number
of ionic radii for trivalent rare earth metals, r-1
A cone conformational p-t-octylcalix[4]arene with four
methylenephosphonic acid at the lower rim have been syn-
thesized to investigate the extraction behavior of the nine
trivalent rare earth elements. The extractant exhibited sig-
nificantly higher extraction ability than the corresponding
monomer, as well as calix[4]arenes employed in our previ-
ous work, containing tetrapropylenephosphonic acid with a
longer spacer, tetraphosphonic acid at the upper rim, and
tetraacetic acid at the lower rim. Using slope analysis, the
stoichiometries of the tetrameric and the monomeric ex-
tractants to the rare earth metal ions were determined to be
2:1 and 3:1, respectively. From the proposed extraction
equations, the extraction equilibrium constants and separa-
tion factors were estimated. The extremely high extraction
ability and moderate separation efficiency were attributed to
the chelating effect of phosphonic acid and phenoxy oxygen
atoms. The effect of the phenoxy oxygen atom on the
extraction ability and separation efficiency in calix[4]arene
derivatives is significant, and its inclusion in the molecular
design of calix[4]arene derivatives should be evaluated.
Fig. 8 Relationship between pH1/2 values of the monomeric extract-
ants 7 and 9 and the reciprocal number of ionic radii for rare earth
metals
Acknowledgments This work was financially supported in part by
‘‘The Environment Research and Technology Development Fund,’’ No.
K123022, from the Ministry of the Environment, Government of Japan.
for the rare earth metals is illustrated in Fig. 8. As the
chemical formulas and the functionalities of both extract-
ants are nearly the same, their lipophilicities must also be
similar. The extraction abilities are, however, completely
different. The difference is clearly due to the absence or
presence of the phenoxy oxygen atoms, which participate
in the chelation of the extractant. The chelation of the
functional group and the phenoxy oxygen atom contributes
the high extraction ability, as well as the low separation
efficiency. In addition to the strong extraction efficiency
caused by the phenoxy oxygen atom, the main reason for
the poor separation efficiency is the mismatch in the dis-
tance between the chelating atoms and the ionic radii of the
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