A study on pyrrolidone derivatives as selective precipitant for uranyl ion in HNO3

Article abstract of DOI:10.1016/j.jallcom.2003.11.050

We have found out that N-cyclohexyl-2-pyrrolidone (NCP) can selectively precipitate UO₂²⁺ ions in HNO₃ solutions. In order to investigate factors of such a specific property of NCP, we have examined reactions of NCP with UO₂²⁺ ions in HCl, HClO₄, or H₂SO₄, and the precipitation abilities of pyrrolidone derivatives other than NCP for UO₂ ²⁺ in HNO₃ solutions. As a result, it was found that UO₂²⁺ ions in HCl, HClO₄, or H ₂SO₄ are not precipitated by NCP, that N-methyl-2-pyrrolidone (NMP) and N-ethyl-2-pyrrolidone (NEP) with lower hydrophobicity than NCP do not precipitate UO₂²⁺, and that hydrophobic N-dodecyl-2-pyrrolidone can precipitate UO₂ ²⁺. Furthermore, we have investigated the crystal structures of UO₂(NO₃)₂(L)₂ (L = NMP, NEP) complexes to compare with that of UO₂(NO₃) ₂(NCP)₂. The bond distance between uranium and carbonyl oxygen of NCP in UO₂(NO₃)₂(NCP)₂ was found to be shorter than those in UO₂(NO₃) ₂(L)₂ (L = NMP, NEP). From these results, it is proposed that the specific property of NCP is ascribed to its relatively high hydrophobicity owing to cyclohexyl group, to its strong coordination ability to UO₂²⁺ for forming the symmetrical complex accompanied by two bidentate NO₃^-, and to the surface of UO ₂(NO₃)₂(NCP)₂ which is surrounded by the hydrophobic cyclohexyl groups of coordinated NCP.

Full text of DOI:10.1016/j.jallcom.2003.11.050

Journal of Alloys and Compounds 374 (2004) 420–425
A study on pyrrolidone derivatives as selective precipitant
for uranyl ion in HNO3
Yasuhisa Ikedaa,∗, Emiko Wadaa, Masayuki Haradaa, Takahiro Chikazawab,
Toshiaki Kikuchib, Hideaki Mineoc, Yasuji Moritac,
Masanobu Nogamid, Kazunori Suzukid
a
Research Laboratory for Nuclear Reactors, Tokyo Institute of Technology, 2-12-1 O-okayama, Meguro-ku, Tokyo 152-8550, Japan
b
Mitsubishi Materials Co., 1-3-25 Koishikawa, Bunkyo-ku, Tokyo 112-0002, Japan
Japan Atomic Energy Research Institute, Tokai-mura, Naka-gun, Ibaraki 319-1195, Japan
c
d
Institute of Research and Innovation, 1201 Takada, Kashiwa, Chiba 277-0861, Japan
Abstract
We have found out that N-cyclohexyl-2-pyrrolidone (NCP) can selectively precipitate UO22+ ions in HNO3 solutions. In order to investigate
factors of such a specific property of NCP, we have examined reactions of NCP with UO22+ ions in HCl, HClO4, or H2SO4, and the precipitation
abilities of pyrrolidone derivatives other than NCP for UO22+ in HNO3 solutions. As a result, it was found that UO22+ ions in HCl, HClO4, or
H2SO4 are not precipitated by NCP, that N-methyl-2-pyrrolidone (NMP) and N-ethyl-2-pyrrolidone (NEP) with lower hydrophobicity than
NCP do not precipitate UO22+, and that hydrophobic N-dodecyl-2-pyrrolidone can precipitate UO22+. Furthermore, we have investigated the
crystal structures of UO2(NO3)2(L)2 (L = NMP, NEP) complexes to compare with that of UO2(NO3)2(NCP)2. The bond distance between
uranium and carbonyl oxygen of NCP in UO2(NO3)2(NCP)2 was found to be shorter than those in UO2(NO3)2(L)2 (L = NMP, NEP). From
these results, it is proposed that the specific property of NCP is ascribed to its relatively high hydrophobicity owing to cyclohexyl group, to
2+
its strong coordination ability to UO2 for forming the symmetrical complex accompanied by two bidentate NO3−, and to the surface of
UO2(NO3)2(NCP)2 which is surrounded by the hydrophobic cyclohexyl groups of coordinated NCP.
© 2003 Elsevier B.V. All rights reserved.
Keywords: Uranyl complexes; Pyrrolidone; Precipitation; Crystal structure; Hydrophobicity
1. Introduction
On the basis of such a viewpoint, we have inves-
tigated simple reprocessing processes based on pre-
cipitation methods [2–4]. Recently, we found out that
N-cyclohexyl-2-pyrrolidone (NCP) can selectively precipi-
In most of commercial reprocessing plants, spent nu-
clear fuels have been treated by Purex method [1], in
which U and Pu are extracted from HNO3 solutions of
spent nuclear fuels by using the extractant (30% tributyl
phosphate/n-dodecane), and separated from most of fission
products and other transuranium elements. Hence, facilities
of extraction processes and amounts of extractant become
relatively large. This results in complexity of processes
and an increase in amounts of radioactive wastes. If a
tate UO22+ as UO2(NO3)2(NCP)2 in 1–7 M (M = mol/dm3)
2+
HNO3 solutions containing UO2
and other metal ions
[5], and proposed a simple reprocessing process which used
the specific property of NCP [6].
On the other hand, from aspects of coordination
chemistries of UO22+, it is interesting to examine factors
2+
inducing selective precipitation ability of NCP for UO2
2+
large portion of UO2 can be selectively separated from
in HNO3 sol−utions. In our previous study [5], we found
HNO3 solutions of spent nuclear fuels using simple method,
subsequent extraction processes are expected to become
extremely simple or useless.
out that NO3 and cyclohexyl group of NCP play impor-
−
tant roles in the selective precipitation, because CF3CO2
−
and CH3SO3 ions do not give precipitates under analo-
gous conditions, and N-methyl-2-pyrrolidone (NMP) does
2+
not precipitate UO2
in HNO3 solutions. Furthermore,
∗
Corresponding author. Tel.: +81-3-5734-3061;
in more recent study [7], we clarified that the structure of
UO2(NO3)2(NCP)2 is similar to those of UO2(NO3)2(L)2
fax: +81-3-5734-2959.
E-mail address: yikeda@nr.titech.ac.jp (Y. Ikeda).
0925-8388/$ – see front matter © 2003 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2003.11.050

Y. Ikeda et al. / Journal of Alloys and Compounds 374 (2004) 420–425
421
(L = unidentate oxygen donor ligands) [8–12], i.e., the
Dichloromethane-d2 (CD2Cl2) and tetramethylsilane were
used as solvent and a reference for chemical shifts, respec-
tively. IR spectra (KBr pellet) were recorded on a Shimadzu
FT-IR-8400S spectrophotometer. Elemental analyses were
carried out by using CE INSTRUMENTS Flash EA1112.
2+
−
equatorial site of UO2 is occupied by two trans NO3
and L ligands. However, the factors inducing unique pre-
cipitation ability of NCP are still ambiguous.
In order to investigate the origins of specific property
of NCP in more detail, we have examined whether NCP
shows such a property in acid solutions other than HNO3
and whether the pyrrolidone derivatives other than NCP can
2.2. Syntheses of UO2(NO3)2(NMP)2 and
UO2(NO3)2(NEP)2 complexes
2+
precipitate UO2 in HNO3 solutions. Moreover, we have
analyzed the crystal structures of UO2(NO3)2(NMP)2 (1)
and UO2(NO3)2(NEP)2 (2) (NEP = N-ethyl-2-pyrrolidone)
complexes to compare with that of UO2(NO3)2(NCP)2
(3).
UO2(NO3)2·6H2O (2.517 g, 5.014 mmol) was dissolved
in ethanol (30 ml) and then NMP (1.971 g, 19.88 mmol) was
added. The solution was stirred for 1.5 h. Yellow precipitates
were separated, washed with hexane, and dried. This product
was recrystallized from warm ethanol solution containing a
small amount of NMP. The resulting yellow crystals were
filtered off and dried in vacuo for one night. Anal. Calcd. for
UO2(NO3)2(NMP)2: C, 20.8; H, 3.28; N, 9.71. Found: −C1,
2. Experimental
2.1. Materials and methods
=
20.3; H, 3.06; N, 9.71. IR (KBr): 1631 ν(C O) and 932 cm
1
= =
ν(O U O). H NMR (CD2Cl2): δ = 2.24 (4H, m), 2.86 (4H,
t), 3.35 (6H, s, –CH3), and 3.82 (4H, t). 13C NMR (CD2Cl2):
δ = 17.8 (s), 31.3 (s), 31.6 (s), 51.5 (s), and 182.0 (s). In a
similar manner, the crystals of UO2(NO3)2(NEP)2 were also
prepared. Anal. Calcd. for UO2(NO3)2(NEP)2: C, 23.0; H,
As pyrrolidone derivatives, NCP (Aldrich, 99%),
NMP (Kanto Kagaku, 99%), NEP (Aldrich, 98%),
NOP (N-octyl-2-pyrrolidone, Aldrich, 98%), and NDP
(N-dodecyl-2-pyrrolidone, Aldrich, 99%) were used with-
out further purification. All other chemicals used in the
present study are reagent grade and were used without fur-
3.75; N, 9.18. Found: C,−213.2, H, 3.57; N, 9.03. IR (KBr):
1
=
= =
1626 ν(C O) and 926 cm ν(O U O). H NMR (CD2Cl2):
δ = 1.38 (6H, t, –CH3), 2.24 (4H, m), 2.86 (4H, t), 3.83
(4H, m, –CH2–), and 3.89 (4H, m). 13C NMR (CD2Cl2):
1
ther purification. H and 13C NMR spectra were measured
using a JEOL JNM LA 300 WB FT-NMR spectrometer.
Table 1
Crystallographic data for UO2(NO3)2(NMP)2 (1), UO2(NO3)2(NEP)2 (2), and UO2(NO3)2(NCP)2 (3) complexes
Compound
1
2
3
Empirical formula
Formula weight
Crystal system
Space group
a (Å)
C
10H18N4O10
U
C12
620.36
H
22N4O10U
C20H34N4O10U
728.54
Triclinic
¯
P1
592.30
Monoclinic
P21/n
7.602 (2)
6.813 (2)
16.642 (6)
Monoclinic
P21/n
7.321 (1)
7.299 (1)
18.252 (3)
8.627 (1)
8.748 (1)
9.707 (1)
113.61 (1)
93.73 (1)
108.74 (1)
619.6 (1)
1
b (Å)
c (Å)
α (◦)
β (◦)
90.72 (3)
96.45 (1)
γ (◦)
V (Å3)
861.8 (5)
2
969.2 (3)
2
Z
Temperature (K)
93
2.282
556
1
93
2.126
588
1
113
1.952
354
1
Dcalc (g cm−3
F(0 0 0)
)
Radiation (λ (Å))
µ (cm−1
Mo K␣ (0.71075)
94.78
Mo K␣ (0.71075)
84.33
Mo K␣ (0.71075)
66.11
)
Crystal size (mm)
Crystal color/shape
2θ range max (◦)
Observed data [I > −10.00 σ(I)]
R1a
0.15 × 0.08 × 0.05
Yellow/block
60.1
2506
0.021
0.15 × 0.10 × 0.05
Yellow/block
60.1
2840
0.019
0.40 × 0.50 × 0.30
Yellow/block
55.0
2822
0.014
wR2b
0.071
0.045
0.038
ꢀ
ꢀ
a
R1 = ||Fo| − |Fc||/ |Fo|.
ꢁ
ꢂ1/2
ꢀ
ꢀ
[w(Fo2 − Fc2)2]/ w(Fo2)2
.
b
wR2 =

422
Y. Ikeda et al. / Journal of Alloys and Compounds 374 (2004) 420–425
δ = 12.5 (s), 17.9 (s), 32.0 (s), 39.4 (s), 48.7 (s), and 181.6
3. Results and discussion
(s).
−
2+
3.1. Role of NO3 ions in the precipitation of UO2 by
NCP
2.3. X-ray crystal structure determination of
UO2(NO3)2(NMP)2 and UO2(NO3)2(NEP)2 complexes
−
In our previous paper [5], we proposed that NO3 2+ions
play an important role in the precipitation of UO2
by
Yellow block crystals of UO2(NO3)2(NMP)2 (1) and
UO2(NO3)2(NEP)2 (2) were mounted in loops. Inten-
sity measurements were carried out at 93 K on a Rigaku
RAXIS RAPID diffractometer with Mo K␣ radiation
(λ = 0.71075 Å). Crystal data and data collection param-
eters are listed in Table 1. The structures were solved by
using heavy-atom Patterson method [13] and expanded
using DIRDIF-99 program [14]. Non-hydrogen atoms
were refined anisotropically. Hydrogen atoms were re-
fined using the riding model. The final cycle of full-matrix
least-squares refinements on F2 were based on 2506 and
2840 reflections and 124 and 135 variable parameters for
UO2(NO3)2(NMP)2 and UO2(NO3)2(NEP)2 crystals, re-
spectively, and converged with unweighted and weighted
agreement factors (R1 = 0.021 and 0.019, wR2 = 0.071
and 0.045) for UO2(NO3)2(NMP)2 and UO2(NO3)2(NEP)2
crystals, respectively. All calculations were carried out
by using Crystal Structure 3.10 crystallographic software
package [15,16]. Furthermore, the crystal structure of
UO2(NO3)2(NCP)2 (3) was determined again with higher
precision than the previous measurement [7].
NCP. In order to confirm the validity of our proposal, ac-
tions of NCP on UO2 were examined by adding 0.2 ml of
NCP to 2.9 M HCl, HClO4, or H2SO4 (4 ml) solution con-
taining 0.1 M UO22+. As expected, precipitation phenom-
ena were not observed in these solutions. Furthermore, the
UO2(NO3)2(NCP)2 complex was found to be soluble in di-
2+
lute HNO3 solutions. This is considered to be due to phe-
−
nomenon that the equilibrium reaction, UO22+ + 2NO3
+
2NCP = UO2(NO3)2(NCP)2, is shifted to left in the dilute
HNO3 solution. From these results, it is concluded that the
−
existence of a large excess of NO3 is essential for the pre-
2+
cipitation of UO2 by NCP.
−
In addition, the trans coordination of two bidentate NO3
to UO2 seems to be an important factor, because this co-
2+
ordination accompanied by two NCP molecules contributes
to the formation of non-charged and symmetrical uranyl
complex with coordination number of 6 in the equatori−al
plane [7]. On the other hand, anions such as Cl−, ClO4
,
2−
2+
and SO4 should not coordinate to UO2 more strongly
than NO3−. Even if the former anions react with UO2
to form the non-charged uranyl complexes with NCP, their
2+
Fig. 1. ORTEP drawing of the molecular structure of UO2(NO3)2(NMP)2 showing atomic numbering. The thermal ellipsoids are drawn at 50% probability
level.

Y. Ikeda et al. / Journal of Alloys and Compounds 374 (2004) 420–425
423
Fig. 2. ORTEP drawing of the molecular structure of UO2(NO3)2(NEP)2 showing atomic numbering. The thermal ellipsoids are drawn at 50% probability
level.
Fig. 3. ORTEP drawing of the molecular structure of UO2(NO3)2(NCP)2 showing atomic numbering. The thermal ellipsoids are drawn at 50% probability
level.

424
Y. Ikeda et al. / Journal of Alloys and Compounds 374 (2004) 420–425
structures should not be symmetrical. These may be also
reasons why UO2 ions are precipitated by NCP in only
From IR measurements, the shifts to lower frequencies in
2+
=
the C O stretching modes of coordinated NMP, NEP, and
HNO3 system. However, some amide complexes of uranyl
nitrate, which are not precipitated in aqueous solutions and
synthesized using non-aqueous solvent, are known to have
similar structure to that of UO2(NO3)2(NCP)2 [17–19].
Hence, it is considered that the formation of non-charged
and symmetrical uranyl complex is not necessarily the con-
NCP were found to be 56, 60, 76 cm−1, respectively, and are
comparable with the shifts observed in uranyl nitrate com-
plexes with unidentate amides [17–19]. These results sug-
gest that UO2(NO3)2(L)2 (L = NMP, NEP, NCP) are typi-
cal amide complexes of uranyl nitrate, and that NCP of three
pyrrolidone derivatives coordinates most strongly to ura-
2+
=
ditions for the selective precipitation of UO2 in HNO3
nium atom via C O oxygen. Furthermore, the uranyl asym-
= =
by NCP.
metric stretching bands [ν(O U O)] of UO2(NO3)2(L)2
(L = NMP, NEP, NCP) are observed at 932, 926, and
927 cm−1, respectively. This is consistent with the general
tendency that the larger is the ligand donation, the lower
3.2. Precipitation abilities of pyrrolidone derivatives for
UO2 in HNO3 solutions
2+
= =
is the asymmetric stretching frequency of O U O bond
As one of factors for the specific property of NCP, the
hydrophobicity of NCP should be proposed, because NMP
[23,24].
ORTEP views of UO2(NO3)2(NMP)2 and UO2(NO3)2
(NEP)2 complexes are shown in Figs. 1 and 2. These
complexes are found to have the same symmetrical struc-
tures as that of UO2(NO3)2(NCP)2 (see Fig. 3). Selected
bond distances and angles for three uranyl complexes
are listed in Table 2. As seen from this table, significant
differences are observed in the U1–O2–C1 moiety, i.e.,
the U1–O2 bond distance and the bend of U1–O2–C1 in
UO2(NO3)2(NCP)2 are the smallest in three complexes.
being lower hydrophobic compound than NCP does not pre-
2+
cipitate UO2
in HNO3 solutions [5]. Hence, we inves-
tigated the precipitation abilities of pyrrolidone derivatives
(NEP, NOP, and NDP) with different hydrophobicity. We
used the octanol–water partition constants (log P) [20,21] as
the measure of hydrophobicity of pyrrolidone derivatives.
The calculated values of log P for NMP, NEP, NCP, NOP, and
NDP are −0.11 (experimental value: −0.38), 0.38 (−0.04),
2.16, 3.33, and 5.30 (4.20), respectively. The former three
compounds are miscible with water, while NOP and NDP
are not. As expected from log P values, NEP was found to
This is similar to results observed in UO2(NO3)2(LP
)
O 2
=
(LP O: triphenylphosphine oxide, hexaethylphosphoric tri-
=
amide, and diphenyl-N-ethylphosphine amide) complexes,
where the U–O(P) distances shorten with an increase in
linearity of U–O–P angle [12,25,26]. Furthermore, the
2+
have no precipitation ability for UO2 in HNO3. Further-
more, it was found that NOP and NDP can extract and pre-
2+
cipitate UO2 in HNO3 solutions, respectively. As an ex-
ample of other pyrrolidone derivatives, Doyle et al. have re-
ported that N,Nꢀ-ethylenebis(2-pyrrolidone) (NEBP) reacts
with uranyl nitrate in non-aqueous solvents to form solid
product, i.e., helical chain polymer [UO2(NO3)2(NEBP)]n
[22]. The log P value of NEBP is estimated to be −0.55,
which is comparable with those of NMP and NEP. These
results suggest that the pyrrolidone derivatives with low
Table 2
Selected bond distances (Å) and angles (◦) of UO2(NO3)2(NMP)2 (1),
UO2(NO3)2(NEP)2 (2), and UO2(NO3)2(NCP)2 (3) complexes
Compound
1
2
3
2+
hydrophobicity cannot precipitate UO2 in aqueous solu-
U1–O1
U1–O2
U1–O3
U1–O4
O2–C1
O3–N2
O4–N2
O5–N2
O1–U1–O2
O1–U1–O3
O1–U1–O4
O2–U1–O3
O2–U1–O4
O3–U1–O4
U1–O2–C1
U1–O3–N2
U1–O4–N2
O2–C1–C2
O2–C1–N1
O3–N2–O4
O3–N2–O5
O4–N2–O5
1.769 (4)
2.368 (4)
2.510 (3)
2.546 (4)
1.255 (6)
1.283 (6)
1.273 (6)
1.211 (6)
90.2 (2)
1.763 (3)
2.371 (3)
2.519 (3)
2.535 (2)
1.251 (4)
1.279 (4)
1.274 (4)
1.216 (4)
91.4 (1)
1.771 (2)
2.348 (2)
2.525 (2)
2.536 (2)
1.253 (3)
1.273 (3)
1.274 (3)
1.218 (3)
91.8 (1)
tions, and indicate that the unique precipitation ability of
NCP is due to its miscibility with water and relatively high
hydrophobicity.
3.3. Comparison of precipitation abilities of pyrrolidone
2+
derivatives for UO2 based on their coordination abilities
and structural aspects of complexes
89.9 (2)
88.1 (1)
88.6 (1)
89.2 (2)
92.5 (1)
93.8 (1)
As mentioned above, only NCP of three pyrrolidone
derivatives (NMP, NEP, and NCP), which are miscible with
65.7 (1)
65.8 (1)
65.1 (1)
116.0 (1)
50.3 (1)
115.9 (1)
50.4 (1)
114.8 (1)
50.2 (1)
2+
water, was found to be able to precipitate UO2 in HNO3
136.1 (3)
98.3 (3)
140.5 (2)
97.6 (2)
147.7 (2)
97.8 (1)
solutions. Hence, in order to examine the differences in
precipitation abilities of pyrrolidone derivatives from the
viewpoint of their coordination abilities and structural as-
pect of their uranyl complexes, IR measurements and crystal
structural analyses of UO2(NO3)2(L)2 (L = NMP, NEP)
were carried out, and their results were compared with that
of UO2(NO3)2(NCP)2.
96.8 (3)
97.0 (2)
97.3 (1)
127.9 (5)
123.3 (5)
114.5 (4)
121.7 (5)
123.8 (5)
126.9 (3)
122.7 (3)
115.0 (3)
122.3 (3)
122.8 (3)
124.3 (2)
125.1 (2)
114.8 (2)
122.8 (2)
122.4 (2)

Y. Ikeda et al. / Journal of Alloys and Compounds 374 (2004) 420–425
425
distance of U1–O1 in UO2(NO3)2(NCP)2 is longer than
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Technical report of the crystallography laboratory, University of
Nijmegan, The Netherlands, 1999.
4. Summary
The results of present study are summarized as follows.
−
• Existence of a large excess of NO3 ions is essential for
2+
2+
the precipitation of UO2 by NCP, because UO2 ions
in HCl, HClO4, and H2SO4 are not precipitated by NCP,
and the resulting precipitates are soluble in dilute HNO3
solutions.
• The pyrrolidone derivatives with lower hydrophobicity
than NCP, such as NMP and NEP, do not precipitate
2+
UO2 ions in HNO3 solutions, while NDP being more
highly hydrophobic than NCP can precipitate. This indi-
cates that the specific property of NCP is attributed to
its relatively high hydrophobicity owing to cyclohexyl
group.
• NCP seems to coordinate to UO22+ more strongly through
=
the C O oxygen than other pyrrolidone derivatives, such
[15] Crystal Structure 3.10, Crystal structure analysis package, Rigaku
and Rigaku/MSC, 2000–2002.
as NMP and NEP, because the bond distance between ura-
nium and C O oxygen in UO2(NO3)2(NCP)2 is shorter
=
[16] D.J. Watkin, C.K. Prout, J.R. Carruthers, P.W. Betteridge, CRYS-
TALS Issue 10, Chemical Crystallography Laboratory, Oxford, UK.
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Soc., Chem. Commun. (1993) 1170.
than those of UO2(NO3)2(L)2 (L = NMP, NEP).
• The UO2(NO3)2(NCP)2 complex has symmetrical struc-
ture and its surface is surrounded by the hydrophobic cy-
clohexyl groups of two NCP ligands.
• Factors mentioned above contribute to the unique prop-
erty that NCP can precipitate selectively UO22+ in HNO3
solution.
[23] E. Forsellini, G. Bombieri, R. Graziani, B. Zarli, Inorg. Nucl. Chem.
Lett. 8 (1972) 461.
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491.
Acknowledgements
This work was partly supported by the Ministry of Edu-
cation, Culture, Sports, Science and Technology of Japan.
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