Selective recovery of rare earth elements using chelating ligands grafted on mesoporous surfaces

Article abstract of DOI:10.1039/c5ra21027e

Nowadays, rare earth elements (REEs) and their compounds are critical for the rapidly growing advanced technology sectors and clean energy demands. However, their separation and purification still remain challenging. Among different extracting agents used for REE separation, the diglycolamide (DGA)-based materials have attracted increasing attention as one of the most effective extracting agents. In this contribution, a series of new and element-selective sorbents were generated through derivatisation of the diglycolamide ligand (DGA), grafted to mesoporous silica and tested for the separation of rare earth elements. It is shown that, by tuning the ligand bite angle and its environment, it is possible to improve the selectivity towards specific rare earth elements.

Full text of DOI:10.1039/c5ra21027e

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PAPER
Selective recovery of rare earth elements using
chelating ligands grafted on mesoporous surfaces†
Cite this: RSC Adv., 2015, 5, 103782
ac
Justyna Florek,‡ab Ambreen Mushtaq,‡ac Dominic Lariviere,
Gabrielle Cantin,ac
*
`
ac
ab
*
*
Frederic-Georges Fontaine
´ ´
and Freddy Kleitz
Nowadays, rare earth elements (REEs) and their compounds are critical for the rapidly growing advanced
technology sectors and clean energy demands. However, their separation and purification still remain
challenging. Among different extracting agents used for REE separation, the diglycolamide (DGA)-based
materials have attracted increasing attention as one of the most effective extracting agents. In this
contribution, a series of new and element-selective sorbents were generated through derivatisation of
the diglycolamide ligand (DGA), grafted to mesoporous silica and tested for the separation of rare earth
elements. It is shown that, by tuning the ligand bite angle and its environment, it is possible to improve
the selectivity towards specific rare earth elements.
Received 10th October 2015
Accepted 23rd November 2015
DOI: 10.1039/c5ra21027e
www.rsc.org/advances
REEs.2 However, this type of extraction utilizes large volume of
solvents during the repeated cycles of extraction while gener-
Introduction
ating a signicant portion of undesired and radioactive waste.
In comparison, SLE and SPE are greener approaches for REE
extraction and purication as one of the liquid phase is
reduced/eliminated. Recently, SLE resins containing impreg-
nated ligands have been used for REE purication.3 Among
them, the diglycolamide (DGA) resin which consists of a DGA
derivative (tetraoctyl DGA) impregnated on a solid support is
nowadays one of the most commonly used systems for separa-
tion of REEs from other elements.4 Regrettably, as the ligand is
only supported on the stationary phase, leaching into the
aqueous phase is frequently observed, limiting its reusability.
The challenge in separating REEs from environmental and
industrial samples resides in the fact that REEs, but more
specically the lanthanides (Ln), have very similar physical and
chemical properties. Indeed, while other oxidation states are
synthetically available for the REEs, they tend to exist in envi-
ronmental conditions as trivalent ions.5 In addition, all of the
REEs have similar radii size ranging from 74.5 (Sc3+) to 103.2
pm (La3+).6 They have coordination numbers ranging from 6 to
9, and sometimes more. In order to satisfy their coordination
sphere, they tend to coordinate different solvent molecules and
have a predilection for chelating ligands.7 This affects the
solubility of REE complexes in various solvents and thus makes
their purication by LLE less efficient. However, the Ln3+
cations are highly Lewis acidic and easily coordinate nucleo-
philes to form stable complexes and they show a clear tendency
towards oxygen donors, both in acidic and basic conditions,
which can be advantageous for their efficient extraction.8 It is
documented that the best ligands used for extraction of REEs
are mostly oxygen donors (e.g., carboxylic acids, ketones,
phosphoric acids).9 In particular, the derivatives of DGA are
The rare earth elements (REEs) and their alloys are used in
many modern technologies including notably the production of
consumer electronics, computers, cell phones or network and
communication devices.1 Their unique electrochemical, lumi-
nescent and magnetic properties help to make these technolo-
gies perform with reduced weight and energy consumption, and
give the devices greater efficiency, speed, performance, dura-
bility and thermal stability. As such, the global consumption
rate and hence, the demand for pure REEs have dramatically
increased over the past two decades. Since industrially useful
ores contain several of the REEs among many other contami-
nants, including radioactive uranium and thorium, the sepa-
ration and purication of these mixtures are needed for further
industrial processes. However, the chemical properties of all
REEs are quite similar which complicate their separation.
Industrial purication involves either liquid–liquid (LLE), sup-
ported liquid (SLE) or solid phase (SPE) extraction procedures,
which rely on association constants between ligands and REEs
based essentially on the lanthanide contraction effect. In LLE,
cation- (carboxylic acids, phosphoric acids) and anion-
exchangers (amines, diketones) have been used to recover the
a
´
Department of Chemistry, Universite Laval, Quebec City, G1V 0A6, QC, Canada.
E-mail: freddy.kleitz@chm.ulaval.ca; dominic.lariviere@chm.ulaval.ca; frederic.
fontaine@chm.ulaval.ca; Fax: +1-418-656-7916; Tel: +1-418-656-7812, 7250, 5140
b
´
´
´
Centre de Recherche sur les Materiaux Avances (CERMA), Universite Laval, Quebec
City, G1V 0A6, QC, Canada
c
´
Centre en Catalyse et Chimie Verte (C3V), Universite Laval, Quebec City, G1V 0A6, QC,
Canada
† Electronic supplementary information (ESI) available: Experimental details and
additional data. See DOI: 10.1039/c5ra21027e
‡ Contributed equally to this work.
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used industrially for the selective extraction of REEs from acid mixture while stirring. The reaction was reuxed for 4 h at
aqueous solutions, as they are commonly recognised as effective 80 ꢁC. The reaction was cooled down and put under vacuum for
and size selective binders of trivalent f-elements. They show 1 h to remove the excess of SOCl2. The yellow oil le behind was
high and rapid extraction even in strong acidic conditions (10ꢀ4 dissolved in 2 ꢂ 10 ml of benzene and the solution was ltered.
to 100 M in HNO3).10 Notably, the DIAMEX industrial process The ltrate was evaporated to give a light yellow oil that was
uses DGA derivatives to selectively extract lanthanides during crystallized from a mixture of pentane and ether at ꢀ30 ꢁC (6.30
reprocessing of spent nuclear fuels.11 The structural analysis of g, yield ¼ 87%). 1H NMR (benzene-d6). d 3.76 (s, 4H, CH2–C]O),
the complexes bearing the DGA ligands shows that REEs bind 3.09 (s, 4H, CH2–O). 13C NMR (benzene-d6). d 171.7 (s, C]O),
traditionally in a tridentate fashion, where each metal ion is 76.1 (s, CH2–C]O), 70.9 (s, CH2–O).
coordinated by the three oxygen atoms of DGA-type ligands. The
Synthesis of 3,6-dioxaoctanediamido-propyltriethoxysilane
lanthanide ions (e.g., La, Yb, Ce) are sterically saturated in their (DOODA-APTS). 2.00 g (9.30 mmol) of DOODACl was dissolved
nine-coordinate geometry with three DGA ligands.12 The angle in 25 ml of toluene and cooled down to 0 ꢁC. The solution was
formed by chelate ligands, known as the bite angle, can greatly slowly added to a mixture of 4.57 ml (4.32 g, 0.195 mol) of APTS
affect the binding properties of these ligands in the complexes and 12.6 ml (9.10 g, 0.893 mol) of triethylamine in 20 ml of
(Fig. 1D).13 Ligands with large bite angles will have a higher toluene. The reaction was allowed to warm to room temperature
affinity for larger ions whereas smaller bite angles will favour overnight. The reaction was ltered and the ltrate was evapo-
coordination to smaller ions. However, it is expected that rated and dried under vacuum to give 3.00 g of thick yellow oil
1
a chelating ligand bound to a solid surface by more than one (yield ¼ 96%). H NMR (benzene-d6). d 6.61 (br, 2H, NHAPTS),
anchor point will possess a rigid conformation and will exhibit 3.77 (m, 16H, CH2–O–SiAPTS, CH2–C]O), 3.32 (q, 4H, CH2–
less exibility to adapt itself to the metal electronic require- NHAPTS), 3.00 (s, 4H, CH2–O), 1.72 (m, 4H, CH2–CH2APTS), 1.16 (t,
ments than in solution. Therefore, by the right tailoring of 18H, CHA3PTS), 0.65 (t, 4H, CH2–SiAPTS). 13C NMR (benzene-d6).
chemically similar chelating ligands and their graing on solid d 168.6 (s, C]O), 71.1 (s, CH2–C]O), 70.4 (s, CH2–O), 58.5 (s,
supports, it should be possible for functional materials to CH2–O–SiAPTS), 41.5 (s, CH2–NHAPTS), 23.8 (s, CH2–CH2APTS), 18.5
exhibit selective affinities for smaller or larger ions.
(s, CH3APTS), 8.2 (s, CH2–SiAPTS).
Synthesis of 2,5-furandioylchloride (FDGACl).15 To 2.50 g
(16.0 mmol) of 2,5-furandicarboxylic acid was added 50 ml of
DMF. 5 ml (8.19 g, 68.8 mmol) of SOCl2 was added slowly to the
acid mixture while stirring. The reaction was reuxed for 6 h at
80 ꢁC. The reaction was cooled down and put under vacuum for
1 h to remove excess of SOCl2. A white cꢁrystalline solid was
obtained. The solid was sublimated at 75 C to give 2.74 g of
product (yield ¼ 89%). 1H NMR (benzene-d6). d 6.22 (s, 2H, CH).
13C NMR (benzene-d6). d 155.4 (s, C]O), 148.7 (s, C–O), 122.7 (s,
CH).
Experimental
Ligands synthesis
Synthesis of 3,6-dioxaoctaneꢁdioic acid (DOODA acid).14 20 ml
of HNO3 was heated to 40–45 C, under ambient atmosphere,
and 7.0 g (0.046 mol) of triethylene glycol was added over 1 h.
The temperature of the reaction was raised to 60–70 ꢁC releasing
orange fumes in the process. Aer the addition was complete,
the reaction was cooled down to 40–45 ꢁC then heated at 80 ꢁC
forꢁ1 h. The acid formed was evacuated under vacuum for 1 h at
80 C to get rid of all the nitrogen oxides. The light yellow oil
solidied into a white sticky solid overnight (6.95 g, yield ¼
Synthesis of furan-2,4-diamido-propyltriethoxysilane (FDGA-
APTS). 2.00 g (10.4 mmol) of FDGACl was dissolved in 25 ml of
toluene and to the solution was slowly added a mixture of 5.07
ml (4.79 g, 21.8 mmol) of APTS and 14.4 ml (10.4 g, 103 mmol)
of triethylamine in 20 ml of toluene. The reaction was put under
1
85%). H NMR (acetone-d6). d (ppm) 4.15 (s, 4H, CH2–C]O),
3.73 (s, 4H, CH2–O). 13C NMR (acetone-d6). d 170.9 (s, C]O),
70.4 (s, CH2–C]O), 67.6 (s, CH2–O).
ꢁ
reux at 90 C overnight. The reaction was cooled, ltered and
the ltrate was evaporated and dried under vacuum to give 5.65
Synthesis of 3,6-dioxaoctanedioylchloride (DOODACl). To
6.0 g (33 mmol) of 3,6-dioxaoctanedioic acid was added 60 ml of
DMF. 16 ml (26.2 g, 0.22 mol) of SOCl2 was added slowly to the
1
g of a white solid (yield ¼ 96%). H NMR (benzene-d6). d 7.26
(br, 2H, NHAPTS), 6.95 (s, 2H, CH), 3.74 (q, 12H, CH2–O–SiAPTS),
3.40 (q, 4H, CH2–NHAPTS), 1.76 (m, 4H, CH2–CHA2PTS), 1.14 (t,
18H, CH3APTS), 0.65 (t, 4H, CH2–SiAPTS). 13C NMR (benzene-d6).
d 158.1 (s, C]O), 149.4 (s, C–O), 114.9 (s, CH), 58.5 (s, CH2–O–
SiAPTS), 42.0 (s, CH2–NHAPTS), 23.8 (s, CH2–CHA2PTS), 18.5 (s,
CHA3PTS), 8.3 (s, CH2–SiAPTS).
Synthesis of diglycolyl chloride (DGACl). To 5.00 g (0.037
mol) of diglycolic acid was added 15 ml of CH2Cl2 and 40 ml of
DMF. 17.4 ml (25.8 g, 0.200 mol) of C2O2Cl2 was added slowly to
the acid mixture while stirring in an ice bath. The reaction was
allowed to warm up overnight. The reaction mixture was then
distilled under nitrogen at 100 ꢁC to distil off the excess of
C2O2Cl2 and CH2Cl2. A yellow oil was recovered (5.50 g, yield ¼
Fig. 1 (A) DGA, (B) DOODA and (C) FDGA ligands grafted on silica (KIT-
6). (D) Bite angle between the metal and the coordinating atoms of the
ligand.
1
88%). H NMR (benzene-d6). d 3.47 (s, 4H, CH2–O). 13C NMR
(benzene-d6). d 170.5 (s, C]O), 74.7 (s, CH2–O).
This journal is © The Royal Society of Chemistry 2015
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the homogenous clear solution. The molar composition of the
Synthesis of diglycol-2,4-diamido-propyltriethoxysilane
starting mixture was TEOS/P123/HCl/H2O/BuOH ¼ 1.0/0.017/
(DGA-APTS). 600 mg (3.51 mmol) of DGACl was dissolved in
15 ml of toluene and was cooled down to 0 C. To the solution
ꢁ
ꢁ
1.83/195/1.31. This mixture was le under stirring at 35 C for
ꢁ
24 h, followed by an aging step at 100 C for 24 h under static
was slowly added a mixture of 1.72 ml (1.63 g, 7.36 mmol) of
APTS and 4.89 ml (3.55 g, 35.1 mmol) of triethylamine in 10 ml
of toluene. The reaction was allowed to warm to room temper-
ature overnight. The reaction was ltered and the ltrate was
evaporated and dried under vacuum to give 1.37 g of a thick
conditions. Aer, the resulting solid product was ltered and
dried for 24 h at 100 ꢁC. From the as-synthesized KIT-6 material,
the template was removed by extraction in ethanol–HCl
mixture, followed by calcination in air at 550 ꢁC. Prior to the
graing procedure, the KIT-6 support (1.0 g) was activated
overnight at 150 ꢁC under vacuum and then dispersed in 100 ml
of dry toluene, under N2 ow. Aer 2 h, the solution of the
chosen silane-modied ligand (600 mg of DGA-APTS or 600 mg
of DOODA-APTS or 300 mg of FDGA-APTS) and 1.2 ml of trie-
thylamine (Alfa Aesar, 99%) was added at once to this suspen-
sion, under inert atmosphere. The resulting mixture was le
under reux conditions for 24 h. In case of the DOODA ligand,
the graing procedure was performed at room temperature for
an extended period of the time (48 h). Aer the modication
procedure, the functionalized silica product was ltered,
1
yellow oil (yield ¼ 89%). H NMR (benzene-d6). d 7.52 (br, 2H,
NHAPTS), 3.89 (4H, CH2–O), 3.75 (q, 12H, CH2–OSiAPTS), 3.31 (q,
4H, CH2–NHAPTS), 1.75 (m, 4H, CH2–CHA2PTS), 1.15 (t, 18H,
CHA3PTS), 0.67 (t, 4H, CH2–SiAPTS). 13C NMR (benzene-d6). d 169.0
(s, C]O), 71.6 (s, CH2–O), 58.5 (s, CH2–OSiAPTS), 41.8 (s, CH2–
NHAPTS), 23.6 (s, CH2–CH2APTS), 18.5 (s, CHA3PTS), 8.3 (s, CH2–
SiAPTS).
Synthesis of N,N-dioctyl-3,6-dioxaoctanediamide (TODOO-
DA).11b 315 mg (1.46 mmol) of DOODACl in 5 ml of benzene was
added dropwise to a solution N,N-dioctylamine (840 mg, 1.05
ml, 3.48 mmol) and triethylamine (2.21 g, 3.04 ml, 21.8 mmol)
in 30 ml of benzene at 0 ꢁC. The reaction was allowed to warm to
room temperature overnight. The reaction was evaporated and
then extracted with 3 ꢂ 10 ml of hexane. Evaporation of the
hexane ltrate gave 802 mg of a yellow oil (yield ¼ 87%). 1H
NMR (chloroform-d). d 4.20 (s, 4H, CH2–C]O), 3.76 (s, 4H,
CH2–O), 3.23 (dt, 8H, CH2–Noctyl), 1.52 (br, 8H, CH2–CHo3ctyl), 1.27
(S, 40H, CH2octyl), 0.87 (t, 12H, CHo3ctyl). 13C NMR (chloroform-d).
d 168.5 (s, C]O), 77.1 (s, CH2–C]O), 69.7 (s, CH2–O), 45.7
(d, CH2N), 31.7, 29.2, 28.8, 27.5, 26.9 (CH2), 22.5 (s, CH2–CH3),
14.0 (s, CH3).
Synthesis of N,N-dioctylfuran-2,4-diamide (TOFDGA). 300
mg (1.55 mmol) of FDGACl in 10 ml of THF was added dropwise
to a solution N,N-dioctylamine (1.17 ml, 0.937 g, 3.88 mmol)
and triethylamine (2.15 ml, 1.56 g, 15.4 mmol) in 30 ml of THF
at 0 ꢁC. The reaction was allowed to warm to room temperature
overnight. The reaction was evaporated and the residue was
dissolved in 30 ml of dichloromethane and extracted with 3 ꢂ
20 ml of 5% HCl in water. The organic fraction was dried on
MgSO4, ltered and evaporated to give 845 mg of a yellow oil
(yield ¼ 90%). 1H NMR (chloroform-d). d 6.93 (s, 2H, CH), 3.46
(br, 8H, CH2–Noctyl), 1.60 (br, 8H, CH2–CHo3ctyl), 1.26 (s, 40H,
CHo2ctyl), 0.88 (t, 12H, CHo3ctyl). 13C NMR (chloroform-d). d 159.6
(s, C]O), 148.6 (s, CH), 115.6 (s, CO), 46.7 (d, CH2N), 31.7, 29.2,
27.5, 27.0, 26.7 (CH2), 22.6 (s, CH2–CH3), 14.0 (s, CH3).
ꢁ
washed thoroughly with toluene and then dried at 60 C over-
night in air. Unreacted silane molecules were removed by
Soxhlet extraction in dichloromethane for 6 h. The resulting
products are noted as KIT-6-N-DGA, KIT-6-N-FDGA or KIT-6-N-
DOODA, respectively.
Characterization techniques
N2 adsorption–desorption isotherms were measured at ꢀ196 ꢁC
(77 K) using an Autosorb-iQ2 sorption analyzer (Quantachrome
Instruments, Boynton Beach, USA). Prior to the analysis, the
ꢁ
samples were outgassed for 10 h at 200 C (unmodied silica
support) or at 80 ꢁC for 12 h (functionalized sorbents). The
specic surface area (SBET) was determined using the Brunauer–
Emmett–Teller equation in the range 0.05 # P/P0 # 0.20 and the
total pore volume (Vpore) was measured at P/P0 ¼ 0.95. The pore
size distributions for all materials were calculated by using the
nonlocal density functional theory (NLDFT) method (Autosorb
ASiQWin soware) considering the sorption of nitrogen at
ꢀ196 ꢁC in silica with cylindrical pores.18 The pore widths were
determined by applying the NLDFT kernel of equilibrium
isotherms (desorption branch). 13C cross-polarization (CP/MAS)
and 29Si magic-angle-spinning (MAS) nuclear magnetic reso-
nance (NMR) analyses were carried out on a Bruker Avance 300
MHz spectrometer (Bruker Biospin Ltd, Milton, Canada) at 75.4
MHz for 13C and 59.6 MHz for 29Si. 29Si MAS NMR spectra were
recorded with a spin echo sequence to avoid instrument back-
Materials synthesis and functionalization
Following our previous success with actinide/lanthanide ground with a recycle delay of 30 seconds in a 4 mm rotor spin
adsorption, we selected high-surface area KIT-6 silica (with at 8 kHz. 13C CP/MAS NMR spectra were recorded with a 4 mm
a 3-D open pore structure) as a suitable support for the chelating MAS probe with a spinning rate of 8 kHz and contact time of 1
ligands.16 High quality KIT-6 silica materials were obtained ms. Chemical shis were referenced to tetramethylsilane (TMS)
following the procedure reported by Kleitz et al.17 Briey, 9.0 g of for 29Si and adamantane for 13C. Simultaneous thermogravi-
Pluronic P123 (EO20PO70EO20, Sigma-Aldrich) was dissolved in metric analysis-differential scanning calorimetry analysis (TGA-
325.0 g of distilled water and 17.4 g HCl (37%) was added under DTA/DSC) was performed using a Netzsch STA 449C thermog-
vigorous stirring. Aer complete dissolution, 9.0 g of n-butanol ravimetric analyzer, under air ow of 20 ml minꢀ1 and with
(BuOH, Aldrich, 99%) was added. The reaction mixture was le temperature-programmed heating rate of 10 ꢁC minꢀ1
.
under stirring at 35 ꢁC for 1 h, aer which 19.4 g of tetrae- Elemental analysis was performed by the combustion method
thoxysilane (TEOS, Sigma-Aldrich, 99%) was added at once to using a CHNS Analyzer Flash 2000, Thermo Scientic. X-ray
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photoelectron spectroscopy (XPS) measurements were con- europium was loaded and passed through the column, with the
ducted on a Kratos AXIS-ULTRA spectrometer with a mono- ow rate 1 ml minꢀ1
chromatic Al X-ray source operated at 300 W. High energy Sorbent reusability studies. For the reusability studies, 200
resolution spectra (N 1s) were recorded at 20 eV or 40 eV pass ml of a solution of 1 mg lꢀ1 of europium was passed through the
energy and step size of 0.05 eV or 0.1 eV. analytical setup presented in the previous section and the un-
.
Extraction methodology. Extraction behaviour of REEs and retained Eu analyzed by ICP-MS. Subsequently, the sorbent
other metals were investigated on DGA resin-normal (N,N,N0,N0- was washed with 0.1 M (NH4)2C2O4 (10 ml) and high purity
tetra-n-octyldiglycolamide resin; Eichrom; USA Lisle, IL), on water (20 ml). The material was then reconditioned with HNO3
unfunctionalized and DGA, DOODA and FDGA moieties teth- and used in the subsequent extraction tests by repeating the
ered on a mesoporous support (KIT-6, KIT-6-N-DGA, KIT-6-N- above mentioned procedure, to achieve 10 extraction–elution
DOODA and KIT-6-N-FDGA, respectively). Extraction experi- cycles.
ments were performed either as a batch or ow-through mode.
LLE experiments were also performed in order to determine the
effect of tethering the ligand on the extractive properties.
Results and discussion
Elemental quantication of the various elements was per- Recently, we have studied the effect of the chemical anchoring
formed by ICP-MS (Model 8800, Agilent Technologies). The of chelating ligands on mesoporous silica supports for the
initial and nal concentrations of the REEs and additional selective extraction of actinides and trivalent f-element ions.16 It
elements in solutions were determined and used to calculate was found that anchored phosphonate ligands were highly
either extraction capacity or distribution constant (Kd).
Batch extraction studies, SLE and SPE system. A 15 mg lꢀ1
efficient in extracting uranium from acidic media. Anchoring
DGA ligand (diglycol-2,4-diamido-propyltriethoxysilane,
a
solution of REEs containing additional metals (Al, Fe, U, Th) in Fig. 1A) on KIT-6 also demonstrated improved extractive prop-
HNO3 (pH ¼ 4) was prepared from the reference standard erties for all REEs in comparison to SLE resin while exhibiting
solutions (Plasma, Cal, SCP Science). The solution/solid ratio enhanced selectivity for some of them. In addition, the teth-
was xed to 500 (V mꢀ1). The samples (10 mg) were stirred in an ering of DGA on the silica surface overcomes the leaching of the
orbital shaker for 30 min and the supernatant was subsequently ligand observed in SLE providing a greater reusability of the
ltered through a 0.2 mm syringe lter. All experiments were resin. Our previous results also suggest that the mid-size ions
done in triplicates and only the average values are provided.
(e.g., Eu and Gd) have greater affinity for the tethered DGA than
Batch extraction studies, LLE system. A commercial Micro- smaller and larger ions. Therefore, there is a need for the
Kros hollow ber module was used (Spectrum Labs, USA) for the preparation of materials where the graed ligands would have
non-dispersive solvent extraction (NDSX) experiment. The total tailored bite angles for the selective extraction of larger or
surface area was 11 cm2 with 6 polysulfone bers. Both phases, smaller REEs. To achieve this goal, new hybrid mesoporous
aqueous and organic, were contacted in counter-current in the materials containing DOODA (3,6-dioxaoctanediamido-
hollow ber module and extraction occurs in the pores of the propyltriethoxysilane)
and
FDGA
(furan-2,4-diamido-
bers' wall. The aqueous solution was agitated continuously to propyltriethoxysilane) (Fig. 1B and C) ligands were developed
ensure uniform concentration while the organic solution was in order to explore the inuence of their bite angle and rigidity
circulated as a closed loop. 12 mmol of TODGA, TODOODA or on the selective extraction of REEs.
FDGA was added to 2.4 ml of dodecane, whereas 3 M nitric acid
The DOODA and FDGA ligands were synthesized using
containing 100 mg lꢀ1 of each REE was prepared from a standard a slightly modied reported protocol14a,15,19 and by modication
solution (100 mg lꢀ1; Plasma, Cal, SCP Science) by appropriate of the corresponding acid chlorides with 3-amino-
dilution, and used as aqueous phase. Aer the optimization of propyltriethoxysilane (APTS) (see ESI†). The modication of the
the ICP measurements, the 100 mg lꢀ1 solution of REEs was used ligand with APTS on both sides was conrmed by the appear-
in our experimental setup. Both phases (2.4 ml each) were re- ance of a broad peak at d (ppm) ¼ 6.61 for DOODA and 7.26 for
circulated for 90 minutes by a peristaltic pump through the FDGA in the 1H NMR spectrum, each integrating for two
lumen side (aqueous phase) and the shell side (organic phase) protons corresponding to the NH group. In 13C NMR, the
of the ber module. The organic phase ow rate was kept disappearance of the carbonyl peaks at d ¼ 171.7 (DOODACl),
between 600 and 1200 ml minꢀ1 whereas the aqueous phase ow 155.4 (FDGACl) and their appearance at d ¼ 168.6 (DOODA-
was set between 135 ml minꢀ1 and 185 ml minꢀ1
.
APTS), 158.1 (FDGA-APTS) also conrmed the formation of
Europium extraction studied in ow system. Extraction amide bond. These ligands were then graed on large pore
capacity studies of various SLE and SPE alternatives were per- mesoporous KIT-6 silica support.17,18a
formed using a solution of 100 mg lꢀ1 of europium standard
Aer graing, the resulting hybrid materials demonstrate
(Plasma, Cal, SCP Science). For the extraction tests, 150 mg of typical type IV isotherms with a steep capillary condensation
DGA-resin-normal and 30 mg of mesoporous sorbents were step and a hysteresis loop (type H1 adsorption–desorption
packed inside 2 ml SPE cartridges through a slurry-packing hysteresis) in the relative pressure range P/P0 of 0.6–0.8, which
method. Filled cartridges were washed with high purity water is characteristic of mesoporous solids with large cylindrical
(10 ml) and conditioned with HNO3 pH ¼ 4 (mesoporous mesopores (>4 nm)20 (Fig. 2). Aer anchoring of the organic
sorbents) or 3 M HNO3 (commercial resin), then the solution of ligands on the silica surface, the shape of the hysteresis loop
was well maintained and shied to the lower values of relative
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Fig. 2 N2 adsorption–desorption isotherms at ꢀ196 ꢁC for synthe-
sized materials (A) and respective pore size distributions calculated
from the desorption branch using the NLDFT method (silica with
cylindrical pore model) (B). KIT-6 (green); KIT-6-N-DGA (black); KIT-
6-N-DOODA (blue); KIT-6-N-FDGA (red).
Fig. 3 29Si MAS NMR and 13C CP/NMR spectra of the ligand-modified
KIT-6 materials (KIT-6-N-DGA black, KIT-6-N-DOODA blue, KIT-6-
N-FDGA red). The two additional bands (*) visible on the 13C NMR
spectra of KIT-6-DGA material are spinning side-bands of the carbonyl
carbon.
pressure (P/P0), indicative of a decrease in the pore size upon
surface modication. The physicochemical characteristics of
the hybrid sorbents are compiled in Table 1. The KIT-6-N-DGA
sample exhibits the highest BET specic surface area, 621 m2
gꢀ1, compared to the two other mesoporous sorbents, i.e., 588
m2 gꢀ1 for KIT-6-N-DOODA and 502 m2 gꢀ1 for KIT-6-N-FDGA
respectively. For all the materials, upon surface modication,
the pore widths of the synthetized sorbents become smaller and
similar gradual decrease is observed for the pore volume,
compared to the parent silica.
The structure of the anchored ligands was established by
solid-state NMR spectroscopy (Fig. 3A and B, Table S1, ESI†).
The peaks corresponding to the ligands were clearly observed in
the 13C CP/NMR spectra and were in accordance with those
observed in liquid 13C NMR of the APTS-modied ligands. The
covalent attachment of the ligands to the surface was conrmed
by 29Si MAS NMR revealing signals at about ꢀ50, ꢀ58, and ꢀ67
ppm corresponding to T1ꢀ3 species as (SiO)(OR)2Si–R,
(SiO)2(OR)Si–R, (SiO)3Si–R; indicating that the ligands are
covalently anchored to the surface through the silanol groups.21
The 29Si MAS NMR spectrum of KIT-6-N-FGDA showed only the
presence of T1 species, whereas the spectra of KIT-6-N-DOODA
and KIT-6-N-DGA materials show the presence of a mixture of
mono- and bipodal, and bi- and tripodal attachment for each
extremity of the ligand, respectively for these two materials.
Further, the composition of the organic moieties attached to the
silica surface was veried by thermogravimetry and by CHN
elemental analysis. The highest organic content, determined by
a total mass loss of 24%, was found for the KIT-6-N-DGA hybrid
material (Table 2). The KIT-6-N-DOODA and KIT-6-N-FDGA
sorbents showed weight loss proles being about 22% and
16%, respectively. The amounts of carbon, nitrogen and
hydrogen for all the modied samples were obtained by CHN
elemental analysis and the respective values are given in Table
2. Based on the data obtained from the low temperature N2
sorption and elemental analysis, it was possible to calculate the
apparent surface density of the ligands (Table 2). The highest
apparent surface density was estimated for KIT-6-N-DGA, while
the lowest surface density of ligand was found for the KIT-6-N-
FDGA sorbent, in agreement with the TGA mass loss observed
for these materials. Note that, these surface density values do
not take into account the accessibility of the ligand, but only
provide a relative coverage of the silica surface by the different
ligands.
First, the performance of the synthetized mesoporous
sorbents, in comparison to the commercial DGA resin, was
studied in the dynamic (ow) system for one selected element –
europium. Our results, similar to data published by others,22
showed that the DGA-normal resin has a capacity of 0.21 mmol
Eu gꢀ1 towards europium. Superior extraction abilities were
Table 2 Total amount of ligand introduced by grafting for the different
functionalized samples, based on TGA and CHN analysis and estimated
surface density of the ligands
Table 1 Physicochemical parameters derived from N2 physisorption
measurements at low temperature (ꢀ196 ꢁC)
CHN (%)
C
Density of
the ligandb
Mass
SBET (m2 gꢀ1
)
Pore sizea (nm)
Vporeb (cm3 gꢀ1
)
Sample
lossa (%)
N
H
(nmꢀ2
)
Material
KIT-6
910
621
588
502
8.1
7.6
7.4
7.3
1.21
0.82
0.78
0.78
KIT-6-N-DGA
KIT-6-N-DOODA
KIT-6-N-FDGA
24
22
16
13.60
14.74
6.53
2.58
1.92
0.89
2.53
2.35
1.19
0.83
0.73
0.46
KIT-6-N-DGA
KIT-6-N-DOODA
KIT-6-N-FDGA
a
Determined by TGA-DSC analysis in temperature range 140–650 ꢁC.
Calculated using N content (CHN analysis) and specic surface area
a
b
Values obtained from the NLDFT method, based on the desorption
branch. b Total pore volume determined at P/P0 ¼ 0.95.
(SBET).
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Table 3 Extraction capacity of different mesoporous sorbents and
commercial DGA resin in the dynamic system
preconcentration of these radioactive elements with the REEs.
This should translate into an easier management of contami-
nated wastes during the REEs separation process. This selec-
tivity to REEs reects the effect of introducing rigidity in the
ligand (through immobilisation onto a solid support) on its
chelating properties. While KIT-6-N-DGA exhibits selectivity for
Eu3+, Gd3+ and Tb3+, which are mid-size lanthanides with
atomic radii of 94.7, 93.8, and 92.3 pm, respectively, KIT-6-N-
DOODA shows selectivity towards smaller lanthanides.
Indeed, for the latter material, the Kd values are high for REEs
Material
Eu extraction (mmol gꢀ1
)
DGA-normal resin
KIT-6-N-DGA
KIT-6-N-DOODA
KIT-6-N-FDGA
0.21
1.10
0.99
0.49
from holmium to lutetium with values exceeding 5100 ml gꢀ1
.
calculated for all the synthesized mesoporous sorbents (Table
3). In comparison, extraction capacity ve times higher was
obtained for KIT-6-N-DGA (1.10 mmol Eu gꢀ1), which possesses
the highest amount of the ligand (24% mass loss from TGA
analysis). A slightly lower Eu uptake of 0.99 mmol Eu gꢀ1 was
observed for KIT-6-N-DOODA (22% mass loss from TGA), and
the lowest europium uptake was found for KIT-6-N-FDGA
material (0.49 mmol Eu gꢀ1). Nevertheless, the latter value
still is signicantly above the extraction capacity of the
commercial resin.
Although the same geometric parameters cannot be used to
compare a tridentate and a tetradentate ligand, it was observed
that for La-DOODA complexes the angle between adjacent
coordination sites ranges between 57.90–58.27ꢁ, while for the
DGA-La complexes larger angles are observed at 60.35–
60.93ꢁ.11b,12b The DOODA ligand has four coordinating oxygen
atoms and coordinates metal ions in a tetradentate geometry. It
is believed that, this small tetradentate pocket provides an
appropriate bite angle favouring coordination to the smaller
lanthanides.
The functionalized materials were further tested for extrac-
tion of REEs from a mixture of lanthanides, uranium, thorium
and additional metal ions (i.e., Al3+, Fe3+). The values of the
partition coefficients (Kd) for REEs and other relevant elements
obtained through batch extraction are plotted in Fig. 4 (see also
Fig. S19, ESI†). The tabulated data can be found in Table S2,
ESI.† The Kd values for KIT-6-N-DGA are also presented for
comparison purposes. From the results obtained, it is clear that
all functionalized KIT-6 materials show higher Kd values for
lanthanides in comparison to other competing ions commonly
present in environmental and mining wastes. We also observe
that the hybrid materials have generally lower Kd for U and Th
ions (with the exception of KIT-6-N-FGDA), thus limiting the
Considering the extraction properties of DOODA in liquid–
liquid extraction, it was observed that, under the conditions
tested, the Kd decreases from La to Lu (Fig. 4B), therefore
showing greater selectivity of DOODA for larger ions. This
selectivity was found to be sensitive to the concentration of
HNO3, water and the nature and concentration of the ligand.23
For example, tetraoctyl(TO)DGA in LLE is has higher Kd
(Fig. 4A), thus showing higher selectivity for smaller lanthani-
des.10b,24 In contrast, in our solid-state system, DGA becomes
more efficient for extracting middle lanthanides. These differ-
ences in our system and LLE can be attributed to the change in
the coordination sphere of the lanthanide ions, and the nature
of ligand binding them. In solution, the number of ligands
coordinated to lanthanide ions is sensitive to the concentration
of ligand, HNO3 and water. Three to four ligand molecules form
complexes with lanthanide ions along with water and nitrate
ions (NO3ꢀ) in the coordination sphere. Since the lighter
lanthanides have higher hydration energy, they tend to have
more water molecules in their coordination sphere and are thus
extracted less in non-polar solvents. The smaller/heavier
lanthanides have less hydration energy and small coordina-
tion sphere and are therefore extracted more.25 While as in our
system, the number of ligands coordinating to the metal cannot
change as they are immobilised on the pore surface, and hence,
only those metal ions which may bind stronger with the ligand
(i.e., have an appropriate radius for the bite angle of the ligand)
will be efficiently trapped; the others will easily be eluted. This
strikingly distinct behaviour of the DOODA and DGA ligands in
our SPE system is thus attributed to the chemical anchoring of
the ligand to the solid surface that decreases the exibility of the
coordinating carbonyl groups and further adds to the overall
rigidity of the ligand. This effect seems to provide a more xed
bite angle to the ligand yielding a more pronounced discrimi-
nating effect towards REE cations. Moreover, it is also seen that
the Kd value of TOFDGA in LLE is higher for larger lanthanide
ions and subsequently decreases along the series. Thus, as
Fig. 4 Distribution coefficient (Kd) values for functionalized hybrid
materials (SPE; left scale) compared to LLE and SLE counterparts (right
scale). The dashed line separates the REEs and other relevant
elements.
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expected for a ligand with a large bite angle, it is less efficient for stability of the organo-silica resins under the extraction condi-
extracting smaller lanthanides.
tions tested.
However aer graing on solid surface, the hybrid material
KIT-6-N-FDGA shows only a small perturbation in the Kd along
the lanthanide series with little selectivity when comparing with
the extraction of the other metals tested. Nevertheless, it is
interesting to note that the Kd value for Sc3+ is surprisingly high,
around 11 000 ml gꢀ1 (Fig. 4C), which is unexpected since Sc3+
has an ionic radius which is quite distinct from those of all of
the lanthanide ions, being much smaller (74.5 pm).6 This
selectivity for the smallest REE suggests that another factor
inuences the interaction of the REE ions with the FDGA ligand
upon graing, since the latter species has the largest bite angle
of the systems tested.
In order to get more insight into such discrepancy in the
extraction properties, a rst series of XPS analyses of pristine
KIT-6-N-FDGA along with those of materials saturated with Sc3+
and Gd3+ were carried out. It was observed that the N 1s shi for
Sc3+ (400.98 eV) compared to the pristine material (400.10 eV)
was much more pronounced than the one observed for Gd3+
(400.50 eV), suggesting that the interaction with Sc3+ is taking
place with the N–H rather than the C]O typically observed for
lanthanide coordination with DGA-type ligands, which would
explain the different selectivity.12 However, this coordination
mode is quite unusual and homogeneous models have yet to be
synthesized to learn more about this bonding type.
Conclusions
In summary, by a careful tuning of the bite angle of chelating
ligands and by imparting rigidity to the ligands on a surface,
a more selective extraction of lanthanides can be achieved. The
DOODA ligand graed on mesoporous silica (KIT-6) has
a smaller bite angle than the DGA ligand and shows preference
for extracting smaller lanthanides (Ho–Lu). Interestingly, aer
immobilizing FDGA, exceedingly high Kd values were obtained
for extracting scandium from a mixture of REEs. The critical
role of the support is discernible when comparing the extraction
behaviour of these ligands in LLE and the graed systems.
Obviously, it is certainly possible to further tune the bite angle
by changing the ligand structure, aiming at a selectivity for
other elements. Importantly, experiments under non-
equilibrium conditions such as ow-through system will also
be needed for mimicking extraction conditions and study
kinetic effects, which could affect performances. Finally, these
new materials will have to be tested in real practical settings for
the mining industry or to valorise wastes by “urban mining”.
Acknowledgements
Additionally, a comparison of the separation factors between
selected elements, i.e., Ce/La, Gd/La and Lu/La, for LLE and SPE
techniques, using DOODA or FDGA, is given in Table S3, ESI.† It
can clearly be seen that greater separation factors between these
lanthanides and La were obtained for the SPE systems. For
instance, Lu/La separation factor is only 0.3 for the LLE system,
while in case of the mesoporous sorbents this factor is
increased 6 times.
´ ´
The authors wish to acknowledge the Fonds Quebecois de
´
Recherche Nature et Technologies Quebec (FRQNT) and
National Sciences and Engineering Research Council of Canada
(NSERC) for nancial support.
Notes and references
¨
Finally, in order to study the stability of the hybrid organo-
silica sorbents, several loading-release cycles, in the ow/
dynamic system, were performed, as shown in Fig. 5. These
multiple adsorption–desorption cycles conrmed the high
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