Synthesis of Functionalized Mesoporous Silica for
Selective Extraction of Rare Earth Elements
Thèse
Yimu Hu
Doctorat en chimie
Philosophiæ doctor (Ph. D.)
Synthesis of Functionalized Mesoporous Silica for
Selective Extraction of Rare Earth Elements
Thèse
Yimu Hu
Sous la direction de :
Frédéric-Georges Fontaine, directeur de recherche
Dominic Larivière, codirecteur de recherche
Résumé
Les éléments terres rares (ETRs) sont un groupe de 17 métaux aux propriétés voisines comprenant le scandium, l'yttrium, et les 15 lanthanides. Ces éléments sont indispensables à la production de nombreux appareils de haute technologie et incontournables dans le développement des énergies durables. Contrairement à ce que suggère leur appellation, les ETRs sont assez répandus dans la croûte terrestre, alors que leur extraction, à savoir l’extraction liquide-liquide (ELL), est extrêmement difficile, coûteuse et surtout polluante. Afin de fournir une alternative aux procédés industriels, les matériaux mésoporeux à base de silice ont été sollicités à titre d’adsorbant dans l’extraction sur phase solide. Ces matériaux structurés sont intéressants pour l’adsorption d’ETRs puisqu’il est possible d’en modifier simplement la surface avec des ligands adaptés pour l’application voulue. Dans cette thèse, deux séries de ligands à base de structure phthaloyl diamide (PA) et phenylenedioxy diamide (PDDA) ont été synthétisés et greffés sur la surface de matériaux mésoporeux à base de silice KIT-6. Il s’avère, aux vues des résultats obtenus, que ces adsorbants permettent d’extraire sélectivement les éléments de taille différente selon le bite angle du ligand greffé, tandis que la sélectivité n’a pas été observée pour ses analogues homogènes sous la condition de l’extraction liquide-liquide.
En plus, les réseaux bimodaux monolithiques qui présentent simultanément des mésopores ainsi que des macropores (pores > 50 nm) sont avantageux, surtout pour l’extraction en conditions dynamiques (en colonne). Les mésopores permettent d’augmenter la surface spécifique du matériau ainsi la surface de contact entre l’agent actif et la solution. Les macropores, quant à eux, améliorent la capacité de transport des fluides, permettant d’éviter l’accumulation des produits et d’ainsi de bloquer les sites actifs. Dans cette étude, des monolithes à base de silice de taille de quelques centimètres ont été obtenus. Après la fonctionnalisation avec le ligand diglycolamide (DGA), les monolithes révèlent des profils de sélectivité exceptionnels envers le Th(IV) dans l’extraction en colonne.
La technique concernant les matériaux à empreinte ionique rend possible une sélectivité spécifique. Dans ce cas-ci, la synthèse de matériaux a été réalisée par la co-condensation entre l’organosilane et le précurseur siliceux en présence d’ion Dy3+
. Par la suite, la molécule empreinte est enlevée, laissant derrière elle les sites de liaisons et une cavité ayant la forme et la taille du Dy3+. Ainsi, les silices empreintes sont capables de séparer spécifiquement et sélectivement l’ion Dy3+
. Dans tous les trois systèmes présentés ici, ces matériaux peuvent aussi être considérés comme étant intéressants pour les applications industrielles, tant en terme de la stabilité sous les conditions
d’adsorption testées, que de la sélectivité et de la capacité d'adsorption envers les échantillons de déchets minéraux.
Abstract
Rare Earth Elements (REE) are a group of 17 chemically similar metals that have gained an increasing importance over the past decades, due to their unique properties and many applications in high-tech products. The term “rare” is rather deceptive, since they are quite abundant in the Earth’s crust. However, their extraction and purification can be challenging, and the industrial extraction processes of REEs are often costly and environmentally hazardous. This thesis aims at developing a competitive solid-phase extraction system, based on functional porous silica materials, for the selective extraction of REEs in solution that can be industrially applied. In particular, ordered mesoporous silica (OMS) is a versatile platform that can offer large specific surface area, variety in material structures and morphologies, and stability under applied extraction conditions. Furthermore, the surface properties of OMS allow for easy functionalization with a variety of organic ligands, which largely influence the extraction performance of the sorbents. In this work, the OMS KIT-6 were functionalized by grafting two series of chelating ligands on the silica surface, i.e., preorganized bidentate phthaloyl diamide (PA) ligands and tetradentate phenylenedioxy diamide (PDDA) ligands. By fine-tuning of the bite angles of these chelating ligands, we successfully separated REEs into three categories based on their ionic radius.
However, the use of small size particles as packing materials is often associated with a high backpressure of the column, thus limiting their industrial applicability in high flow-rate chromatography analysis. Therefore, the hierarchically structured silicas that contain both macropores (pore size > 50 nm) and mesopores are highly desirable. We report in this work a highly stable silica monolith exhibiting a bimodal, hierarchical macroporous-mesoporous structure for continuous column extraction. Upon grafting of diglycolyl amide (DGA) ligand, the applicability of the column was demonstrated by the removal of Th(IV) from two REE mineral leachates with largely enhanced kinetics and extraction capacity.
Finally, in order to further improve the selectivity of sorbents, molecular recognition approach was applied to synthesize highly ordered ion imprinted mesoporous silica (IIMS) through co-condensation using a combination of molecular imprinting technology and traditional OMS, in which dysprosium ion was used as the template. After template removal, the IIMS showed attractive recognition capacity toward Dy3+ from mild acidic solution. Beside the excellent selectivity, in all the three proposed systems, the sorbents were also proven robust and were able to be regenerated for multiple cycle uses, further demonstrating their potential for industrial applications.
Table of contents
Résumé ... ii
Abstract ... iv
Table of contents ... v
List of tables ... viii
List of figures ... x
List of schemes ... xvii
List of abbreviations ... xviii
Acknowledgements ... xxi
Inserted research articles and author contribution ... xxiii
Introduction ... 1
Chapter 1 Concepts of Importance and Literature Review ... 5
1.1 Rare earth elements: definition and nature occurrence ... 5
1.2 Characteristics and applications of rare earth elements ... 7
1.3 Industrial rare earth element extraction process: principle, methodology and challenges ... 9
1.4 Chemistry of mesoporous materials ... 13
1.5 Synthesis of hierarchically porous monolithic materials... 18
1.6 Recent advances in solid-phase extraction of rare earth elements using functionalized mesoporous materials ... 22
Chapter 2 Methodology ... 35
2.1 General consideration ... 35
2.2 Characterization techniques ... 36
2.3 Experimental methods ... 55
Chapter 3 Highly Efficient and Selective Recovery of Rare Earth Elements Using Mesoporous Silica Functionalized by Preorganized Chelating Ligands ... 66
3.1 Context of the research ... 66
3.2 Résumé ... 66
3.3 Abstract ... 67
3.4 Introduction ... 67
3.5 Results and discussion ... 70
3.5.1 Synthesis and characterization of the materials ... 70
3.5.2 Extraction studies ... 74
3.7 Experimental section ... 83
Chapter 4 Size-Selective Separation of Rare Earth Elements Using Functionalized Mesoporous Silica Materials ... 89
4.1 Context of the research ... 89
4.2 Résumé ... 89
4.3 Abstract ... 90
4.4 Introduction ... 90
4.5 Results and discussion ... 93
4.5.1 Synthesis and characterization of the materials ... 93
4.5.2 Extraction studies ... 97
4.5.3 Applications to real-world samples and stability assessment... 102
4.6 Conclusion and perspectives ... 105
4.7 Experimental section ... 105
Chapter 5 Selective Separation and Preconcentration of Th(IV) Using Organo-Functionalized, Hierarchically Porous Silica Monoliths ... 111
5.1 Context of the research ... 111
5.2 Résumé ... 111
5.3 Abstract ... 112
5.4 Introduction ... 113
5.5 Results and discussion ... 115
5.5.1 Synthesis and characterization of the materials ... 115
5.5.2 Continuous column studies ... 120
5.4 Mechanistic studies and comparison with mesoporous silica ... 123
5.5.4 Application to real-world samples and stability tests ... 128
5.6 Conclusion and perspectives ... 131
5.7 Experimental section ... 131
Chapitre 6 Highly Ordered Ion Imprinted Mesoporous Silica (IIMS) for Selective Extraction of Dysprosium ... 137
6.1 Context of the research ... 137
6.2 Résumé ... 137
6.3 Abstract ... 138
6.4 Introduction ... 138
6.5 Preliminary results ... 139
6.7 Experimental section ... 145 Conclusion... 148 General conclusions ... 148 Perspectives ... 151 Closing remarks ... 153 Bibliography ... 154
Annex 1 Supporting information for Chapter 3: Highly Efficient and Selective Recovery of Rare Earth Elements Using Mesoporous Silica Functionalized by Preorganized Chelating Ligands ... 171
Annex 2 Support information of Chapter 4: Size-Selective Separation of Rare Earth Elements Using Functionalized Mesoporous Silica Materials ... 188
Annex 3 Supporting information for Chapter 5: Selective Separation and Preconcentration of Th(IV) Using Organo-Functionalized Hierarchically Porous Silica Monoliths ... 207
Annex 4 Supporting information for Chapter 6: Highly Ordered Ion-Imprinted Mesoporous Silica (IIMS) for Selective Extraction of Dysprosium ... 223
List of tables
Table 1-1 : The rare earth elements (REEs) and some of their main applications ... 9 Table 2-1 : Characteristic FTIR absorption frequencies of organic functional groups ... 40 Table 3-1 : Physicochemical parameters derived from N2 physisorption measurements at -196 °C
……….. ... 72 Table 3-2 : Total amount of ligand introduced by grafting for the different functionalized materials, based on TGA and CHN analyses, and estimated surface density of the ligands ... 73 Table 3-3 : Kinetic constants for the pseudo-first-order and pseudo-second-order models ... 77 Table 4-1 : Physicochemical parameters of derived from N2 physisorption measurements at -196 °C,
and the total amount of ligand introduced and estimated surface density of the ligands based on TGA and CHN analysis... 94 Table 5-1 : Physicochemical parameters of the pristine silica M2 and the M-DGA monoliths derived from N2 physisorption measurements and mercury porosimetry ... 119
Table 5-2 : Thermodynamic parameters for Th(IV) adsorption on M2 and M-DGA ... 127 Table 6-1 : The physicochemical parameters of synthesized materials ... 143 Table 6-2 : Selective sorption of dysprosium on IIMS and NIMS. a β = Kd(Dy)/Kd(M), Kd(Dy) and
Kd(M) are the distribution coefficient of dysprosium and competing ions, respectively; b
βr =
βIIMS/βNIMS, βIIMS and βNIMS are the selectivity coefficients of IIMS and NIMS, respectively ... 143
Annex
Table A1-1 : Adsorption equilibrium constants for Langmuir and Freundlich isotherm models ... 185 Table A1-2 : Tabulated distribution coefficients (Kd) values for solid-phase extraction (SPE) ... 185
Table A1-3 : Tabulated distribution coefficients (Kd) values for liquid-liquid extraction (LLE) using
dichloromethane and dodecane as organic phase ……….. ... 186 Table A1-4 : Tabulated major elemental composition of diluted industrial mining wastes IS-1 and IS-2 ... 186 Table A1-5 : Tabulated distribution coefficients (Kd) values for of real-world samples ... 187
Table A2-1 :
Kinetic constants for the pseudo-first-order and pseudo-second-order models
... 205 Table A2-2 :
Adsorption equilibrium constants for Langmuir and Freundlich isotherm
models
... 205 Table A2-3 : Elemental composition of bauxite residue ……….. ... 206 Table A2-4 :Physicochemical parameters of KIT-6-1,2-PDDA and KIT-6-1,3-PDDA after
the reusability tests
... 206Table A3-1 : Synthesis conditions and physicochemical properties of the materials as measured by N2 physisorption and mercury porosimetry ... 220
Table A3-2 : Kinetic constants for the pseudo-first-order and pseudo-second-order models ... 220 Table A3-3 : Adsorption equilibrium constants for Langmuir and Freundlich isotherm models ... 220 Table A3-4 : Original elemental composition of OKA-2 and bauxite residue ... 221 Table A3-5 : Extraction percentages of M-DGA for OKA-2 and bauxite residue solutions ... 222
List of figures
Figure I-1 : Strategy used in this work for the design of novel solid phase extraction systems for the
selective and efficient extraction of rare earth elements ... 3
Figure 1-1 : Natural abundance of elements in Earth’s crust compared to Si ... 5
Figure 1-2 : Graphic representation of ionic radius of REE3+ ... 8
Figure 1-3 : General processing routes for REE ores ... 10
Figure 1-4 : Some of the ligands that are commonly used in industrial liquid-liquid extraction (LLE) of REEs ... 11
Figure 1-5 : TEM/SEM images of ordered mesoporous materials and the schematic representation of their mesostructure (insert) (from left to right: SBA-15 or MCM-41, KIT-6, and CMK-8) ... 15
Figure 1-6 : Formation of mesoporous materials by structure-directing agents: (a) true liquid-crystal template mechanism, (b) cooperative self-assembly templated mechanism ... 16
Figure 1-7 : Schematic illustration of the principles employed for functionalization of the mesopore surface ... 18
Figure 1-8 : Photograph of bimodal, mesoporous-macroporous monoliths of different shapes ... 19
Figure 1-9 : Schematic presentation of the synthesis route towards hierarchically organized silica monoliths by polymerization-induced phase separation ... 21
Figure 1-10 : Photograph and SEM/TEM images of a silica monolith exhibiting two separated pore size regimes ... 22
Figure 1-11 : (a) One-step and two-step modifications of the surface of KIT-6 silica to generate the mesoporous REE sorbents. (b) Extraction capacity for REEs in the presence of competitive ions (Al3+, Fe3+, Th4+ and UO2 2+ ) ... 25
Figure 1-12 : (a) DGA, (b) DOODA and (c) FDGA ligands grafted on KIT-6 silica, and the corresponding distribution coefficient (Kd) values for SPE (left scale) compared to LLE and SLE counterparts (right scale) ... 27
Figure 1-13 : Layer-by-layer synthesis route of MCM-41 silica functionalised titanium(IV) n-alkylphosphate materials (R = H, Et, n-Pr and n-Bu) ... 28
Figure 1-14 : Schematic representation for the synthesis of the ligand-functionalized mesoporous carbons ... 30
Figure 1-15 : (a) Schematic of the imprinted mesoporous silica (IMS) and adsorption mechanism of Dy3+. (b) Kd values of the IMS and NIMS for a mixture of Dy3+, Fe2+, Nd3+, Pr3+, and Tb3+ ... 31
Figure 1-16 : A 2D FJSM-SnS structure and schematic of the REE insertion process by FJSM-SnS through ion exchange process ... 33
Figure 2-1 : Structures of typical silica building units ... 38
Figure 2-3 : Infrared spectra of a sample of Aerosil 300 (a fumed silica sample characterized by a surface area of 300 m2 g-1) after sample pretreatment in vacuum at increasing temperatures ... 41 Figure 2-4 : IUPAC classification of physisorption isotherms ... 42 Figure 2-5 : Adsorption hysteresis type H1 and its correlation with pore structure coupled with underlying adsorption mechanism ... 44 Figure 2-6 : Schematic representation of typical mercury porosimetry curves obtained after two consecutive intrusion-extrusion cycles ... 46 Figure 2-7 : Standard powder XRD patterns obtained for the mesostructured KIT-6 silica materials synthesized with aging performed at 50, 60, 80, and 100 °C (KIT-6-50, -60, -80, and KIT-6-100) for 24 h ... 50 Figure 2-8 : Sc 2p XPS spectrum of Sc(NO3)3 (a), Sc(III)-loaded KIT-6 (b), and Sc(OH)3 (c) ... 51
Figure 2-9 : A specimen holder for a TEM. The insert shows the enlarged part of the holder end where the 3-mm disc (zoomed figure on the left) of the specimen is installed ... 53 Figure 2-10 : Schematic of ICP-MS major components: sample introduction system, plasma torch, and mass spectrometer ... 55 Figure 2-11 : Overall process of fabricating macroporous-mesoporous silica monolith ... 58 Figure 2-12 : Picture of bulk silica monoliths after hydrothermal treatment with different concentration of ammonia after 8h at 90 ˚C ... 60 Figure 2-13 : Picture of experimental set-up for the column used in chromatography extraction .... 61 Figure 2-14 : Schematic representation of a typical batch extraction procedure ... 62 Figure 2-15 : Schematic representation of REE extraction using column under dynamic condition .... ... 63 Figure 3-1 : N2 adsorption−desorption isotherms at −196 °C for the synthesized materials (a), and
respective pore size distributions calculated from the desorption branch using the NLDFT method (silica with cylindrical pore model) (b) ... 71 Figure 3-2 : Effect of pH on the REEs adsorption by functionalized KIT-6 sorbents ... 74 Figure 3-3 : Distribution coefficient (Kd) values for functionalized hybrid materials and pristine
KIT-6 silica ... 76 Figure 3-4 : Effect of the contact time on the lutetium sorption with kinetic model fitting of the pseudo-first- and -second-order model of the lutetium adsorption kinetics on KIT-6−1,2-PA and KIT-6 ... 77 Figure 3-5 : Experimental equilibrium isotherm data and modeling for the adsorption of Lu3+ on mesoporous silica KIT-6−1,2-PA (a) and KIT-6 (b) ... 79 Figure 3-6 : Reusability performance of KIT-6−1,2-PA sorbent over 10 loading-stripping-regeneration cycles in the dynamic system ... 80 Figure 3-7 : Selectivity of REEs adsorption for 6−1,2-PA (a), 6−1,3-PA (b), and KIT-6−1,4-PA (c) for industrial samples IS-1 and IS-2 ... 82
Figure 4-1 : (a) N2 adsorption-desorption isotherms at -196 ˚C for pristine KIT-6 and functionalized
sorbents, the isotherms for KIT-6-1,2-PDDA, KIT-6-1,3-PDDA, and KIT-6-1,4-PDDA are offset vertically by 250, 150, and 75 cm3 g-1, respectively. (b) Corresponding pore size distributions ... 94 Figure 4-2 : Solid state 13C CP/MAS NMR (a) and 29Si MAS NMR (b) for the different functionalized mesoporous materials ... 96 Figure 4-3 : Distribution coefficient (Kd) values for functionalized hybrid materials for the
lanthanide elements in the presence of competitive ions (Al3+, Fe3+, Th4+ and UO2 2+
) ... 98 Figure 4-4 : Extraction capacity values (Qe) for KIT-6-1,2-PDDA and KIT-6-1,3-PDDA at various
initial pH values ... 100 Figure 4-5 : Effect of the contact time on the ion sorption with kinetic model fitting of the pseudo-first- and -second-order model on KIT-6-1,2-PDDA (a) and KIT-6-1,3-PDDA (b) ... 101 Figure 4-6 : Extraction of REEs from a bauxite solution using KIT-6-1,2-PDDA (for Lu3+) and KIT-6-1,3-PDDA (for Ce3+) ... 103 Figure 4-7 : The average extraction efficiency using bauxite residue from 5 experiment runs for KIT-6-1,2-PDDA and KIT-6-1,3-PDDA ... 104 Figure 5-1 : Pictures of M2 after calcination (a) and DGA-functionalized monolith M-DGA (e) (the scale is in cm), HRSEM images of M2 (b, c) and M-DGA (f, g) showing the macroporous skeletons, and TEM images of M2 (d) and M-DGA (h) showing the hybrid macroporous-mesoporous structure ... 116 Figure 5-2 : N2 physisorption isotherms measured at - 196˚C (a) the respective NLDFT mesopore
size distributions of M2 and M-DGA (b), and pore size distribution (PSD) calculated from mercury intrusion porosimetry for M2 (c) and M-DGA (d) ... 118 Figure 5-3 : Extraction chromatograms of the feeding solution containing REEs, Al, Fe, Th and U (concentration of 300 µg L-1) with M-DGA (a) and M2 (c), and the corresponding recovery chromatograms using a 0.05 M ammonium oxalate solution (b and d). ... 121 Figure 5-4 : Effect of the contact time on the Th(IV) sorption with kinetic model fittings of pseudo-first- and pseudo-second-order models on M2 and M-DGA at room temperature ... 124 Figure 5-5 : Sorption isotherms of Th(IV) at 298 K for the M2 and M-DGA sorbents, and the corresponding Langmuir and Freundlich models fitting curves... 125 Figure 5-6 : FTIR spectra of M2 (a) and M-DGA (b and c) before and after binding with Th(IV)... ... 126 Figure 5-7 : Th(IV) adsorption by M2 and M-DGA at different temperatures (a), and the linear regression of LnKd vs. 1/T (b) ... 127
Figure 5-8 : Comparison of Th(IV) sorption capacity by various mesoporous silica materials (a), and comparison of the back pressure associated with columns using MCM-41-DGA powder and hierarchically structured monolithic silica M-DGA as stationary phase (b) ... 128 Figure 5-9 : Selective extraction of Th(IV) from real-world mineral samples by M-DGA (a) and reusability performance of the sorbent over 10 extraction−stripping−regeneration cycles in the dynamic system (b) ... 130
Figure 6-1 : N2 adsorption-desorption isotherms at -196 °C for the synthesized materials (a), and
respective pore size distributions (PSD) (b) ... 141
Figure 6-2 : FTIR spectra of all the pristine MCM-41, IIMS, and MCM-41-DGA-OH... 142
Figure 6-3 : Kd values of the IIMS and NIMS for a simulated mixture of Nd-Fe-B magnet leachate containing Dy and competitive ions ... 144
Annex
Figure A1-1 : 1H NMR of 1,2-PA-APTS (DMSO-d6, 400 MHz) ... 171Figure A1-2 : 13C{1H} NMR of 1,2-PA-APTS (DMSO-d6, 100 MHz). ... 172
Figure A1-3 : 1H NMR of 1,3-PA-APTS (DMSO-d6, 400 MHz) ... 172
Figure A1-4 : 13C{1H} NMR of 1,3-PA-APTS (DMSO-d6, 100 MHz). ... 173
Figure A1-5 : 1H NMR of 1,4-PA-APTS (DMSO-d6, 400 MHz) ... 173
Figure A1-6 : 13C{1H} NMR of 1,4-PA-APTS (DMSO-d6, 100 MHz). ... 174
Figure A1-7 : 1H NMR of 1,2-DOPA (chloroform-d, 400 MHz) ... 174
Figure A1-8 : 13C{1H} NMR of 1,2-DOPA (chloroform-d, 100 MHz). ... 175
Figure A1-9 : 1H NMR of 1,3-DOPA (chloroform-d, 400 MHz) ... 175
Figure A1-10 : 13C{1H} NMR of 1,3-DOPA (chloroform-d, 100 MHz). ... 176
Figure A1-11 : 1H NMR of 1,4-DOPA (chloroform-d, 400 MHz) ... 176
Figure A1-12 : 13C{1H} NMR of 1,4-DOPA (chloroform-d, 100 MHz). ... 177
Figure A1-13 : Low angle powder XRD patterns of the pristine KIT-6 and the functionalized materials. Patterns are shifted upwards for clarity. ... 177
Figure A1-14 : FTIR spectra of pristine mesoporous KIT-6 and all the functionalized KIT-6 samples (top) and enlarged zoom at 1400-1900 cm-1 (bottom). The spectra are normalized to the intensity of the SiO2 band at 1041 cm -1 . ... 178
Figure A1-15 : Solid state 13C CP/NMR (a) and 29Si MAS NMR (b) for the different functionalized mesoporous materials ... 179
Figure A1-16 : The thermogravimetric analysis (TGA, red) and differential thermal analysis (DTA, blue) curves of the ligand-modified KIT-6 silica samples (as indicated). ... 180
Figure A1-17 : Distribution coefficient (Kd) values for 1,2-DOPA, 1,3-DOPA, and 1,4-DOPA in liquid-liquid extraction (LLE) using dichloromethane (a) and dodecane (b) as organic phase ... 181
Figure A1-18 : Linear regression of the pseudo-second-order kinetic model for adsorption of Lu3+ on mesoporous KIT-6 and KIT-6-1,2-PA sorbents. ... 182
Figure A1-19 : Linear regression of the Langmuir isotherm model of KIT-6 and KIT-6-1,2-PA used for the adsorption isotherms experiments ... 182
Figure A1-20 : (a) N2 adsorption–desorption isotherm at -196 °C of KIT-6-1,2-PA sample after the
reusability test and (b) respective NLDFT pore size distribution. ... 183
Figure A1-21 : TGA (red) and DTA (blue) curves of KIT-6-1,2-PA sample after the reusability test ... 183
Figure A1-22 : 13C CP/MAS NMR spectrum of KIT-6-1,2-PA after the reusability test. ... 184
Figure A1-23 : Distribution coefficient (Kd) values of functionalized hybrid materials for competitive ions in industrial samples IS-1 (a) and IS-2 (b) ... 184
Figure A2-1 : 1H NMR of 1,2-PDDA-APTS (DMSO-d6, 400 MHz) ... 188
Figure A2-2 : 13C{1H} NMR of 1,2-PDDA-APTS (DMSO-d6, 100 MHz). ... 189
Figure A2-3 : 1H NMR of 1,3-PDDA-APTS (DMSO-d6, 400 MHz) ... 189
Figure A2-4 : 13C{1H} NMR of 1,3-PDDA-APTS (DMSO-d6, 100 MHz). ... 190
Figure A2-5 : 1H NMR of 1,4-PDDA-APTS (DMSO-d6, 400 MHz) ... 190
Figure A2-6 : 13C{1H} NMR of 1,4-PDDA-APTS (DMSO-d6, 100 MHz). ... 191
Figure A2-7 : 1H NMR of 2-methyl-1,3-PDDA-APTS (DMSO-d6, 400 MHz) ... 191
Figure A2-8 : 13C{1H} NMR of 2-methyl-1,3-PDDA-APTS (DMSO-d6, 100 MHz). ... 192
Figure A2-9 : 1H NMR of DO-1,2-PDDA (chloroform-d, 400 MHz) ... 192
Figure A2-10 : 13C{1H} NMR of DO-1,2-PDDA (chloroform-d, 100 MHz). ... 193
Figure A2-11 : 1H NMR of DO-1,3-PDDA (chloroform-d, 400 MHz) ... 193
Figure A2-12 : 13C{1H} NMR of DO-1,3-PDDA (chloroform-d, 100 MHz). ... 194
Figure A2-13 : 1H NMR of DO-1,4-PDDA (chloroform-d, 400 MHz) ... 194
Figure A2-14 : 13C{1H} NMR of DO-1,4-PDDA (chloroform-d, 100 MHz). ... 195
Figure A2-15 : 1H NMR of DO-2-methyl-1,3-PDDA (chloroform-d, 400 MHz) ... 195
Figure A2-16 : 13C{1H} NMR of DO-2-methyl-1,3-PDDA (chloroform-d, 100 MHz). ... 196
Figure A2-17 :
Low-angle powder XRD patterns of the pristine KIT-6 and the functionalized
materials
... 196Figure A2-18 : FTIR spectra of pristine mesoporous KIT-6 and all the functionalized KIT-6 samples in the enlarged zoom at 1400-1900 cm-1. ... 197
Figure A2-19 : The thermogravimetric analysis (TGA, red) and differential thermal analysis (DSC, blue) curves of the ligand-modified KIT-6 silica samples (as indicated). The arrow in the DSC graph indicates direction of exothermic process ... 199
Figure A2-20 : Distribution coefficient (Kd) values for DO-1,2-PDDA, DO-1,3-PDDA, DO-1,4-PDDA, and DO-2-methyl-1,3-PDDA in liquid-liquid extraction (LLE) using dichloromethane as organic solvent. ... 199
Figure A2-21 : Linear regression of the pseudo-second-order kinetic models for adsorption of Lu3+ on KIT-6-1,2-PDDA (a) and for adsorption of Ce3+ on KIT-6-1,3-PDDA (b) ... 200
Figure A2-22 : Experimental equilibrium isotherm data and modeling for Lu3+ sorption on KIT-6-1,2-PDDA (a) and Ce3+ sorption KIT-6-1,3-PDDA (b).. ... 201 Figure A2-23 : The average extraction efficiency using bauxite residue from 5 experiment runs for KIT-6-1,2-PDDA and KIT-6-1,3-PDDA ... 202 Figure A2-24 : N2 adsorption-desorption isotherms at -196 ºC (a) and respective NLDFT pore size
distributions (b) of KIT-6-1,2-PDDA (blue) and KIT-6-1,3-PDDA (red) after the reusability test. ... 203 Figure A2-25 : TGA (red) and DSC (blue) curves of KIT-6-1,2-PDDA (a) and KIT-6-1,3-PDDA (b) after the reusability test. The arrow in the DSC graph indicates direction of exothermic process . 201 Figure A2-26 : FTIR spectra of KIT-6-1,2-PDDA and KIT-6-1,3-PDDA after the reusability test. ... 205 Figure A3-1 : 1H NMR of DGA-APTS (chloroform-d, 400 MHz) ... 207 Figure A3-2 : N2 adsorption-desorption isotherms at -196 ˚C and the corresponding pore size
distributions (PSD) calculated from the adsorption branch using the NLDFT method for the synthesized materials M2_75, M1, M0.5, M0.1, and M0 (a, b) and the passivated monoliths M2-TMDS and M-DGA-M2-TMDS (c,d). ... 209 Figure A3-3 : N2 adsorption-desorption isotherms at -196 ˚C and the corresponding pore size
distributions calculated from the equilibrium model (desorption branch) using the NLDFT method for the mesoporous materials MCM-41 and MCM-41-DGA (a, b), and KIT-6 and KIT-6-DGA (c, d) ... 211 Figure A3-4 : Mercury porosimetry intrusion/extrusion curves and corresponding pore size distributions of M0 (a, b), M0.1 (c, d), M0.5 (e, f), M1 (g, h), M2_75 (i, j). ... 212 Figure A3-5 : The thermogravimetric analysis (TGA, red) and differential thermal analysis (DTA, blue) curves of the functionalized materials (M-DGA, MCM-41-DGA, KIT-6-DGA) and the monoliths after surface passivation (M2-TMDS and M-DGA-TMDS) ... 214 Figure A3-6 : Solid state 13C CP/MAS NMR spectrum for the DGA-functionalized monolith (M-DGA). ... 214 Figure A3-7 : Solid state 29Si MAS NMR spectra for the pristine monolith M2 and DGA-functionalized monolith M-DGA ... 215 Figure A3-8 : FTIR spectra of the pristine silica monolith M2 and the functionalized M-DGA. The spectra are normalized to the intensity of the SiO2 band at 1041 cm
-1
... 215 Figure A3-9 : Extraction chromatogram of rare earth elements (REEs, from Y to Lu) with M-DGA (a) and M2 (c), and the corresponding recovery chromatograms of using 0.05 M ammonium oxalate (b and d). ... 216 Figure A3-10 : Photograph of M2-TMDS and M-DGA-TMDS in water after surface passivation. M2-TMDS floats in water due to the strong surface hydrophobicity, while M-DGA-TMDS falls down to the bottom ... 217 Figure A3-11 : Extraction chromatogram of Al, Sc, Fe, Sc, Th, and U with passivated monoliths M2 -TMDS (a) and M-DGA-TMDS (c), and the corresponding recovery chromatograms of using 0.05
M ammonium oxalate (b and d). The initial feed solution contains REEs, Al, Fe, Th, and U
(concentration of 300 µg L-1, pH 3.0). ... 217
Figure A3-12 : Linear regression of the pseudo-second-order kinetic model for adsorption of Th(IV) on M2 and M-DGA under batch extraction conditions ... 217
Figure A3-13 : Linear regression of the Langmuir (a) and Freundlich (b) isotherm model of M2 and M-DGA used for the adsorption isotherms experiments ... 218
Figure A3-14 : Th 4f XPS spectra of Th(IV)-loaded M2 and M-DGA. ... 218
Figure A3-15 : N2 physisorption isotherm at -196 ºC of M-DGA after the reusability test (a) and the corresponding NLDFT pore size distribution (b) ... 219
Figure A3-16 : TGA and DTA curves of M-DGA after the reusability test. ... 219
Figure A3-17 : 13C CP/MAS NMR spectrum of M-DGA after the reusability test ... 219
Figure A4-1 : 1H NMR of DGA-APTS-OH (DMSO-d6, 400 MHz) ... 223
Figure A4-2 : 13C NMR of DGA-APTS-OH (DMSO-d6, 100 MHz). ... 224
Figure A4-3 : Distribution coefficient (Kd) values for MCM-41-DGA-OH for REEs with concentration of 1 mg L-1 at pH 4 ... 224
Figure A4-4 : Thermogravimetric analysis (TGA, red) and differential scanning calorimetry (DSC blue) curves of the IIMS (a), NIMS (b), and MCM-41-DGA-OH (c). ... 225
List of schemes
Scheme 2-1 : Synthesis procedure for organosilane ligands used in this work. (R = phthaloyl, diglycol, or phenylenedioxy) ... 56 Scheme 2-2 : Schematic representation of the synthesis of phthaloylamide-functionalized KIT-6 materials. Step 1: synthesis of organosilane. Step 2: functionalization of mesoporous silica surface by grafting ... 57 Scheme 3-1 : Schematic representation of the synthesis of phthaloylamide-functionalized KIT-6 materials ... 70 Scheme 4-1 : Synthesis route of ligands 1,2-PDDA-APTS, 1,3-PDDA-APTS, 1,4-PDDA-APTS, and 2-methyl-1,3-PDDA-APTS ... 92 Scheme 6-1 : Schematic presentation for the synthesis of Dy ion-imprinted mesoporous silica (IIMS) ... 140
List of abbreviations
Ac-phos Carboamolyphosphonate
APTS (3-Aminopropyl)triethoxysilane
ATR-FTIR Attenuated total reflection Fourier transform infrared spectroscopy
BET Brunauer-Emmett-Teller
BPG N,N-bisphosphono(methyl)glycine
CMK-x Carbon Molecular Sieves Korean Advanced Institute of
Science and Technology No. x
COF Colavent organic framwork
CP Cross polarization
CPG Controlled pore glass
CTAB Cetyltrimethylammonium bromide
DEHPA Di-(2-ethylhexyl)phosphoric acid
DETA Diethylenetriamine
DGA Diglycolamide
DSC Differential scanning calorimetry
DTPA Diethylenetriaminepentaacetic acid
DTPADA Diethylenetriaminepentaacetic dianhydride
EA Elemental analysis
EDTA Ethylenediaminetetraacetic acid
FSM-n Folded Sheet Mesoporous Materials-n
HREE Heavy rare earth element
ID Inner diameter
ILs Ionic liquids
IIMS Ion-imprinted mesoporous silica
IUPAC International union of pure and applied chemistry
LLE Liquid-liquid extraction
LREE Light rare earth element
MA Malonamide
MAS Magic-angle spinning
MCM-41 Mobil Composition of Matter No. 41
MMA Maleic acid amide
MOF Metal-organic framework
NIM Non-imprinted mesoporous material
NLDFT Non-local density functional theory
NMO Nanosized metal oxide
NMR Nuclear magnetic resonance
NPs Nanoparticles
OMC Ordered mesoporous carbon
OMS Ordered mesoporous silica
PA Phthaloyl diamide
PAA Phosphonoacetic acid
PAF Porous aromatic framework
PDDA Phenylenedioxy diamide
PEG Poly(ethylene glycol)
PEO Poly(ethylene oxide)
POP Porous organic polymer
ppm Part per million
PSD Pore size distributions
PVC Polyvinyl chloride
REE Rare earth element
REO Rare earth oxide
r.t. Room temperature
SAMMS Self-assembled monolayers on mesoporous support
SBA Santa Barbara Amorphous
SDA Structure-directing agent
SEM Scanning electron microscopy
SF Separation factor
SLE Solid-liquid extraction
SPE Solid-phase extraction
TBP Tributyl-phosphate
TEM Transmission electron microscopy
TEOS Tetraethyl orthosilicate
TGA Thermogravimetric analysis
TMOS Tetramethyl orthosilicate
TMS Tetramethylsilane
TTHA Triethylenetetraminehexaacetic acid
WWC Wrinkled mesoporous carbon
WWS Wrinkled mesoporous silica
XPS X-ray photoelectron spectroscopy
"The joy of discovery is certainly the liveliest that the mind
of man can ever feel." - Claude Bernard
Acknowledgements
First and foremost, I would like to thank my supervisor, Prof. Frédéric-Georges Fontaine for the great opportunity and a wonderful time in the Fontaine group. Thank you for the continuous support, for giving me the freedom of trying every crazy idea, and for helping me to become a better scientist every single day. Thank you for choosing me for this position and giving me the guidance to live one of my most challenging and fulfilling experiences through these years.
Further, I would like to thank my co-supervisor, Prof. Freddy Kleitz, with whom I may have not spent as much time as I would have liked to, but every time has always shown your willingness to help and to contribute to my thesis with his great knowledge and enthusiasm. Thank you for your support, encouragement, and invaluable advice, without which this work would never have been possible.
I would also like to thank my second co-supervisor, Prof. Dominic Larivière, for always showing an optimistic attitude, for sharing his knowledge and your passion for research. You really are the definition of mentorship. You have always helped me whenever I needed it, even if you had plenty of other things to do, there is always a space for everyone and that is something I truly appreciate. My experience would never have been so pleasant without my colleagues. I would like to thank Étienne Rochette, Nicolas Bouchard, Julien Légaré-Lavergne, Théo Rongère, Arumugam Jayaraman, Jonathan Gauvin-Audet, Vincent Desrosiers, and Thomas Bossé-Demers from Fontaine group; Claire Dalencour, Audrey Laberge-Carignan, Maxime Gagnon, Guillaume Blanchet-Chouinard, Mélodie Bonin, and Julie Rochette from Larivière group. I would also like to thank my co-authors, who have been so incredibly helpful by carrying out important experiments and by participating in scientific discussions: Elisabeth Drouin, Dr. Luis C. Misal Castro, Dr. Justyna Florek, Dr. Simon Giret, Jongho Han, and Dr. Rafael Meinusch. It has been a great pleasure to work with you. Special thanks to the staff of the Department of Chemistry at Laval, especially Serge Groleau for his help with ICP-MS, and Pierre Audet with solid-state NMR.
I would also like to thank the members of my thesis committee, Prof. Serge Kaliaguine, Prof. Jesse Greener, and Prof. Jennifer Shusterman for taking some of their precious time for reading my thesis and coming to my defense.
My sincere gratitude belongs to my dearest friends: Maria, Diana, Meike and Bardia. Thank you for being so supportive and for making my life in Quebec so full of joy and happiness. I would forever remember the laughter we had together.
I would like to express my deepest gratitude to my parents, 谢谢你们爸爸妈妈。感谢你们多年以 来的支持和鼓励，激励我追求自己的梦想，永不放弃。
Finally, I would like to thank my favorite person on Earth, Dr. Liang Zhong. For me, you are the combination of a husband, a friend, a lover, and a comrade: you are my partner. Thank you for your infinite love and support, for being with me through the brightest and darkest moments of my life, for believing in me more than I believed in myself, for making me laugh with joy, for bearing with me even when I became unbearable… I would never be afraid of anything, knowing that you will always have my back. You simply are the best thing that ever happened to me, and there are no words in this world good enough to describe how much I love you.
Inserted research articles and author contributions
The following section lists the published work that is included in every chapter as well as the contribution of authors for every publication. All of the publications included in this thesis have been published at the time this thesis is being deposited.
Chapter 3: Yimu Hu, Elisabeth Drouin, Dominic Larivière, Freddy Kleitz, and Frédéric-Georges
Fontaine, Highly Efficient and Selective Recovery of Rare Earth Elements Using Mesoporous Silica Functionalized by Preorganized Chelating Ligands, ACS Appl. Mater. Interfaces 2017, 9, 38584−38593.
Author contributions: Most of the experimental work was made by YH. ED performed the liquid-liquid extraction experiment. The manuscript writing was done by YH. The manuscript was reviewed and edited by FGF, FK, and DL.
Chapter 4: Yimu Hu, Luis C. Misal Castro, Elisabeth Drouin, Justyna Florek, Hanspeter Kählig,
Dominic Larivière, Freddy Kleitz, and Frédéric-Georges Fontaine, Size-selective Separation of Rare Earth Elements Using Functionalized Mesoporous Silica Materials, ACS Appl. Mater. Interfaces, 2019, 11, 23681−23691.
Author contributions: Most of the experimental work was made by YH. LCMC helped with the synthesis of compounds. ED performed the liquid-liquid extraction experiment. JF performed the XRD analysis. JF and HK helped with solid-state NMR measurements. The manuscript writing was done by YH. The manuscript was reviewed and edited by FGF, FK, and DL.
Chapter 5: Yimu Hu, Simon Giret, Rafael Meinusch, Jongho Han, Frédéric-Georges Fontaine,
Freddy Kleitz, and Dominic Larivière, Selective Separation and Preconcentration of Th(IV) Using Organo-Functionalized, Hierarchically Porous Silica Monoliths, J. Mater. Chem. A, 2019, 7, 289−302.
Author contributions: Most of the experimental work was made by YH. YH and SG designed the experiment set-ups. RM performed mercury porosimetry analysis. JH performed TEM and HRSEM experiments. The manuscript writing was done by YH. The manuscript was reviewed and edited by FGF, FK, and DL.
Introduction
The rare earth elements (REEs) are a group of 17 elements including 15 lanthanides (Ln), scandium (Sc) and yttrium (Y). The importance of REEs in modern technologies is booming owing to their unique electrical, magnetic, and optical properties and their wide applications in electronics, optics, metallurgy, and other advanced fields.1,2 Contrary to their name, REEs are moderately abundant in the earth’s crust; however, they are rarely found in sufficient abundance in a single location, and are often unevenly distributed in common ores/minerals.1 The recent reduction in the exportation quotas from China - which produces more than 90% of the world’s supply - has also increased the price of REEs, making mining prospects for these elements more compelling. However, most of the processes for the separation of REEs are not environmentally friendly, and the mining of REEs in order to meet the growing global demand has caused significant environmental damage.
The physicochemical similarity between the REEs is one of the biggest obstacles hampering the selective extraction of REEs. Hydrometallurgical approaches, including chemical precipitation, liquid-liquid extraction (LLE), resin-based supported-liquid extraction (SLE), and solid-phase extraction (SPE), are common chemical extraction methods of separating individual rare earth oxides (REOs) from the mineral concentrate.3 The LLE, which largely dominates the purification process nowadays, has limited selectivity among adjacent elements and therefore consumes a large amount of organic solvent during repetitive extraction cycles, thus generating undesired and harmful waste.4–6 In SLE, on the other hand, the leaching of the extracting functional groups into the aqueous phase is inevitable, since the ligands are only physically impregnated on the resin, which results in low reusability and high cost.7,8 In comparison, the emerging SPE systems require less solvent, provide high enrichment factors, and reduce the risk of cross contamination, and thus are a promising sample treatment technique. In SPE, the chemical anchoring of the functional groups on the support through covalent bonds ensures greater regenerative capacities, ultimately reducing the operating cost. The capacity and efficiency of the SPE systems are also determined by the characteristics of the solid supports.9,10 In particular, ordered mesoporous silica (OMS) possesses high specific surface area, high pore volume, and anchoring sites that are available to covalently bind various organic ligands to the surface, yielding materials with high functionality, stability, and enhanced regeneration abilities.11–13 However, the use of small size particles as packing materials in chromatography is often associated with high back pressure of the column, thus limiting their applicability.
Due to their similar physicochemical properties to REEs, uranium and thorium are often present in rare earth minerals via lattice substitution, resulting in radiation issues in rare earth processing.14
In particular, thorium is the main concern in rare earth production, mostly due to its radiation hazard and its limited market.15 Therefore, the implantation of appropriate methods to separate the substantial amount of thorium from rare earth minerals is another major obstacle to tackle with in order to avoid environmental pollution and contamination during REE production.
Overall, the thesis is dedicated to advancing the application of SPE systems based on functionalized nanoporous materials for the selective preconcentration, purification, and separation of REEs from mineral leachates. The extraction performance of the SPE systems is intimately related to three major factors: the nature of functional groups, characteristics of the nanoporous support, and functionalization pathways. Previous studies carried out by our group and other groups have shown that sorbents showing selectivity towards certain REEs can be achieved through grafting of organic chelating ligands on mesoporous silica such as SBA-15 and KIT-6. Based on the foundation of theses successes, we initiated a project to develop more efficient and recyclable REE sorbents using similar silica supports. The design of novel chelating ligands by tuning their coordination nature in order to achieve size-based selectivity is the focus of this project. Next, to tackle the problems encountered when utilizing silica powder as packing materials in flow systems, we proposed the use of hierarchically ordered macroporous-mesoporous silica monoliths. The macropores allow the easy movement of mobile phase through the support, while mesopores provide large surface area for functionalization by organic ligands. The monolithic materials exhibiting bimodal porosity would largely facilitate the preconcentration and purification of REE mineral leachates in flow-through systems. Finally, an alternative functionalization pathway other than surface grafting was attempted. That is, co-condensation of silica precursors, organosilane molecules, and rare earth ions as template leads to the framework ion-imprinted materials, which could potentially show specific ionic recognition towards the template REEs. By approaching from three different angles (the functional groups, nature of (meso)porous supports, and methods of functionalization), this thesis aims to explore the potential of novel nanomaterials for the extraction and separation of REEs, starting from the lab scale and the mechanisms behind adsorption, to the application in a scaled up process (Figure I-1).
Figure I-1. Strategy used in this work for the design of novel solid phase extraction systems for the
selective and efficient extraction of rare earth elements.
In Chapter 1, the fundamental concepts that are essential to the appreciation of the work are introduced, followed by a review on the recent research progress on the selective extraction of REEs, and how this work fits in the literature context. In Chapter 2, the methodology and techniques that made possible the development of the present work are briefly described.
In Chapter 3 and Chapter 4, we present a series of highly selective and robust adsorbents based on mesoporous KIT-6 silica. In particular, an efficient sized-based separation across the lanthanide family was achieved through elaborate design and fine tuning of bi-/tetradentate organic ligands grafted on the mesoporous KIT-6 silica support. Chapter 3 and 4 have been published in ACS Applied Materials and Interfaces16,17.
Chapter 5 demonstrates the potential for industrial application of functionalized silica-based hybrid materials using centimeter-size, porous silica monoliths exhibiting hierarchical macroporosity-mesoporosity as a column-type fixed bed sorbent for continuous flow extraction. Upon functionalization with diglycolamide (DGA), the monolithic hybrid material showed an exceptionally enhanced Th(IV) uptake in terms of capacity, selectivity and stability from rare earth mineral leachates, which would largely facilitate the individual separation of REEs in the purification process. This work has been published in Journal of Materials Chemistry A.18
In the final part of the work, described in Chapter 6, we introduced the ion-imprinting technique (IIT) into the REEs extraction/separation field in order to further improve the selectivity of sorbents. In this study, we explored the possibility of synthesizing highly ordered dysprosium (Dy3+)-imprinted mesoporous silica in a one-pot co-condensation synthesis, under either acidic (for
SBA-15-type structure) or basic (for MCM-41-type structure) conditions. After template removal, the extraction performance of mesoporous hybrid materials was tested under mild acidic conditions. For the sake of clarity, Chapters 3-6 start with a short introduction section describing the context of the topic covered in the chapter under which the research is conducted, in order to clarify to the readers the logic of the research strategies. Each Chapter completes with their own conclusion and perspectives. The supporting information of each chapter is compiled in Annexes 1-4. The thesis is ended by a general conclusion and perspectives of the entire project.
Chapter 1
Concepts of Importance and Literature Review
1.1 Rare earth elements: definition and nature occurrence
The rare earth elements (REEs), as defined by the International Union of Pure and Applied Chemistry (IUPAC), are a group of 17 metallic elements with similar chemical properties. The group includes the 15 lanthanide (Ln) elements (atomic number 57 – 71), from lanthanum (La) to lutetium (Lu) along with scandium (Sc, atomic number 21) and yttrium (Y, atomic number 39). The latter two are included as they share many similar physical and chemical properties with the lanthanides.
The REEs are usually subdivided into light rare earth elements (LREEs) and heavy rare earth elements (HREEs). The five elements from La to Sm belong to LREEs, whereas the elements from Eu to Lu belong to HREEs. Although Y is lighter than the LREEs, it is usually included in the HREE group because of its chemical and physical association with the latter in natural deposits. Sc differs from both the transition metals and the lanthanides in its chemistry, and is mainly obtained as a by-product from uranium extraction. According to some organizations, samarium, europium, and gadolinium are classified as medium REEs (MREEs) or abbreviated using the first letter of each element as SEG group.
Despite their name, REEs are indeed quite abundant in the earth’s crust; some of them are even more abundant than transition metals (Figure 1-1). For example, cerium (Ce), the most abundant lanthanide on earth, has a similar crustal concentration than Ni and Cu, whilst even Tm and Lu, the rarest REEs, are approximately 200 times more abundant than Au. A closer look into Figure 1-1 reveals two patterns: first, that the lighter lanthanides are more abundant than the heavier ones; secondly, that the elements with even atomic number are more abundant than those with odd atomic number. The abundances are a consequence of how the elements were synthesized by atomic fusion in the core of stars. Synthesis of heavier nuclei requires higher temperature and pressures and so gets progressively harder as the atomic number increases. The odd/even alternation (often referred to as the Oddo-Harkins rule) reflects the fact that elements with odd mass numbers have larger nuclear capture cross sections and are more likely to take up another neutron, and therefore elements with odd atomic number (and hence odd mass number) are less common than those with even mass number. Even-atomic-number nuclei are more stable when formed.19
Figure 1-1. Natural abundance of elements in Earth’s crust compared to Si. Reproduced with
permission from ref 20.
Among a large number of rare earth minerals, the major sources for rare earth (RE) production are bastnasite with the composition of (RE)(CO3)F, monazite of (RE)PO4, and xenotime of
YPO4, 14,15,21
with the first two the main resources of LREEs.22 Bastnasite and monazite have a content of approximately 70% (weight percentage after beneficiation process) of rare earth oxides (REO) that are primarily La, Ce, Pr and Nd.23 Xenotime has a REO content of approximately 67%, with much less LREEs (8.4%) compared to monazite or bastnasite. Despite its relative rarity, xenotime is one of the major sources of HREEs. Nowadays, the world’s largest REE mines are the Bayan Obo mine in China and the Mountain Pass mine in the United States.24 Besides targeted REEs, the presence of substantial uranium and/or thorium in the three primary rare earth minerals may cause considerable concern during the rare earth production. There is also a significant production of REEs from ion-adsorption clays in southern China, another very important source rich in yttrium and HREEs.15,25 Information about the size of the reserve outside of China and its extractability is only starting to become available in recent years.26–28 Bauxite residue (red mud), a waste product of the extraction of alumina from bauxite, is an untapped source of scandium.29,30 One emerging alternative mineral source for REEs are forms of aqueous feedstock where REEs are
present in solution. Geothermal brines, natural waters/lakes, and wastewater from fracking are known to contain elevated REE content due to long-term exposure to rare earth-rich rocks under geological settings.31,32 Although REE concentrations in geothermal brines are lower compared to solid feedstocks (ranging from 1.7 ppb to 3.2 ppm), they still represent an attractive mineral source for exploitation due to their abundance and availability, as well as their soluble form that is amenable to extraction without the leaching step.33
Considering the irreplaceable application of REEs in modern technologies, the environmental impact caused during REE production, as well as their supply vulnerability, it is crucial to seek for alternative sources of REEs. For instance, extraction of REEs from mining residues and recycling from end-of-life electronics are considered important strategies to both overcome supply limitations and reduce ecological footprints.
1.2 Characteristics and applications of rare earth elements
The abundance of an element or group of elements is, however, not necessarily the synonym to the ease of their exploitation. The pronounced similar characteristics of the lanthanides to each other, especially each to its neighbors, renders their classification and eventual separation extremely difficult. The lanthanides are those in which the 4f (and 5f) orbitals are gradually filled. Since the 5d subshell is lower in energy than 4f, La has the electron configuration [Xe] 6s2 5d1. With more protons adding to the nucleus, the 4f orbitals contract rapidly and become more stable than the 5d. The lanthanides adopt primarily the (+3) oxidation state in their compounds, with only few exceptions such as Eu2+ and Ce4+ (Table 1-1). The ionic radii of the lanthanide [M]3+ ions along with the ionic radius of Y3+ and Sc3+ are shown in Figure 1-2. Their similarity in ionic radii renders the REEs interchangeable in most minerals and consequently very difficult to separate individually. Figure 1-2 also illustrates why Y is considered to be one of the HREEs (on the basis of similar ionic radius and chemical properties) and why Sc, with its much smaller ionic radius, is often considered “misfit” and excluded from rare earth element groupings.
Figure 1-2. Graphic representation of ionic radius of REE3+. Adapted from ref 22.
The importance of REEs in advanced technologies is booming owing to their unique electrical, magnetic, and optical properties.1,2 Since the 4f orbitals in the Ln3+ ion are well shielded by the 5s2 and 5p6 orbitals, they do not participate directly in bonding. Their spectroscopic and magnetic properties are thus essentially independent of the environment (such as surrounding ligands), and remarkably different from transition metals. With the exception of La3+ and Lu3+, the Ln3+ ions all contain unpaired electrons and are paramagnetic. The magnetic moments in the second half of the series are greater than the moments in the first half; therefore, compounds of Dy3+ tend to be used in high-power magnets.34 The shielding of the 4f subshell from surrounding ligands by the filled “outer” 5s and 5p orbitals is also responsible for the specific properties of lanthanide luminescence, more particularly for the narrowband emission and for the long lifetimes of the excited states.19,35,36 Other sectors that use REEs include the oil industry (petroleum cracking catalysts, i.e., lanthanum in fluid-cracking catalyst), the sector of renewable energies (wind turbines with Nd-Fe-B magnets), speciality chemicals (polishing products, pigments for glass and ceramic industry), metallurgy (additives and alloys), nuclear (miniature magnets), electric and electronic equipment (superconductors and lasers), etc.37 To sum up, the main applications of REEs are compiled in Table 1-1.3,38
Table 1-1. The rare earth elements (REEs) and some of their main applications. Adapted from ref
3,38.
Element Symbol Atom +3 ion Applications
Scandium Sc [Ar] 3d1 4s2 [Ar] Metal alloys for the aerospace industry Yttrium Y [Kr] 4d1 5s2 [Kr] Capacitors, metal alloys, lasers, sensors,
superconductors
Lanthanum La [Xe] 5d1 6s2 [Xe] Ceramics, batteries, car catalysts, phosphors, pigments, X-ray Cerium Ce [Xe] 4f1 5d1
6s2
[Xe] 4f1 Catalysts, polishing, metal alloys, UV filters
Praseodymium Pr [Xe] 4f3 6s2 [Xe] 4f2 Pigments, lightning, lenses, glasses Neodymium Nd [Xe] 4f4 6s2 [Xe] 4f3 Permanent magnets, lasers, catalysts,
infrared filters
Promethium Pm [Xe] 4f5 6s2 [Xe] 4f4 Beta radiation source, fluid-fracking catalysts, phosphors
Samarium Sm [Xe] 4f6 6s2 [Xe] 4f5 High-temperature magnets; nuclear reactor control rods
Europium Eu [Xe] 4f7 6s2 [Xe] 4f6 Liquid crystal displays, fluorescent lighting, glass additives, phosphors Gadolinium Gd [Xe] 4f7 5d1
6s2
[Xe] 4f7 Magnetic resonance imaging contrast agent, glass additives
Terbium Tb [Xe] 4f9 6s2 [Xe] 4f8 Phosphors, electronics
Dysprosium Dy [Xe] 4f10 6s2 [Xe] 4f9 High-power magnets, lasers, guidance systems
Holmium Ho [Xe] 4f11 6s2 [Xe] 4f10 High-power magnets, nuclear industry Erbium Er [Xe] 4f12 6s2 [Xe] 4f11 Lasers, glass colorant, optical fibers,
ceramics
Thulium Tm [Xe] 4f13 6s2 [Xe] 4f12 High-power magnets
Ytterbium Yb [Xe] 4f14 6s2 [Xe] 4f13 Fiber-optic technology, solar panels, alloys, lasers, radiation source for portable X-ray units
Lutetium Lu [Xe] 4f14 5d1 6s2
[Xe] 4f14 X-ray phosphors, single crystal scintillators
1.3 Industrial rare earth element extraction process: principle,
methodology and challenges
While more abundant than many other elements, REEs are not concentrated enough to make them easily exploitable. Due to the complex matrices associated with the REE minerals, several processing steps are needed in order to physically and chemically break down the minerals. In general, the REE processing route includes the following major steps: mining, beneficiation, chemical treatment, separation, reduction, refining, and purification (Figure 1-3).39 The REE
mineral concentrates, which contain approximately 50-70% REOs in weight depending on the type of sources, are obtained from first mining the ores and are then physically separated from gauge minerals. The following physical beneficiation processes, each adapted to different types of minerals and ores, include grinding, sifting, gravitational separation, magnetic separation, and froth flotation. These treatments generate REEs in the form of fluorocarbonates and phosphates. The following chemical treatment step converts REEs minerals into carbonates or chlorides through hydrometallurgical processes, in which the REOs are exposed to a strong acid (HF, HCl, H2SO4) or
base (NaOH, Na2CO3). However, the leaching steps are seldom selective, and large amounts of
competing elements can be extracted.29 An additional difficulty arises from the unfavorable distribution (concentration) of distinct rare earth metals in common ores/minerals. Therefore, supplementary separation and purification steps are required in order to obtain the REEs with satisfying purity for further advanced applications (steps four and five).
Figure 1-3. General processing routes for REE ores. Reproduced and adapted with permission from
ref 39.
The conventional processes for the separation of REEs mainly include chemical precipitation,4 0 – 4 2 solvent extraction (or liquid-liquid extraction, LLE),43 ion exchange,44 and solid-liquid extraction, i.e., supported-liquid extraction (SLE) and solid-phase extraction (SPE) methods, which rely on the association between organic ligands and REEs.45 Ion exchange can be used to obtain REEs with purities higher than 99.9999%; however, the high operational cost limits the large-scale application of this approach.44 Nowadays, to retrieve individual REEs, multi-stage LLE is applied at the industrial level. The selection of extractant is limitless within the scope, but the right choice of extractant defines the separation efficiency and selectivity of REEs. Extractants are classified into cation exchanger, anion exchanger, chelating extractant, solvating (neutral)
extractant, and synergistic solvents; each has its own merits in REE the extraction process.46–48 The recent developments of the LLE process focus on the optimization of selective extractants and organic solvents in order to improve the separation efficiency and enrichment factors, as summarized by Verboom et al. in their recent reviews.43,49 It can be generalized that the high Lewis acidity of Ln3+ favors the coordination with hard nucleophilic organic ligands to form stable complexes. Currently, most of extractants used in industrial LLE of REEs contain oxygen, nitrogen, sulfur and/or phosphorous atoms, with the most effective ones being oxygen-donor chelating type ligands. Some examples are shown in the Figure 1-4.
Figure 1-4. Some of the ligands that are commonly used in industrial liquid-liquid extraction (LLE)
of REEs. Reproduced with permission from ref 38.
However, LLEs are plagued by practical problems such as slow extraction kinetics, the low solubility of some extractants in aliphatic diluents,50 and the formation of emulsions.6 Furthermore, a large number of extractants used in LLE suffer from poor selectivity among adjacent elements, thus consuming large volumes of high-purity solvents upon repetitive extraction cycles, and generating enormous amount of undesired and radioactive wastes. A life-cycle assessment of the production of REEs shows that mining, leaching and solvent extraction have the largest contribution to the overall environmental footprint.51 This adverse impact caused by LLE contradicts the green chemistry and energy principles and could overshadow the potential of technologies relying on
REEs. Ionic liquids (ILs) are currently being investigated as alternative extraction media to conventional organic solvents in LLE; however, this separation approach is restricted by the high viscosity and the low solubility of ILs, and by the difficulty of recovering the metal species due to the strong interactions between ILs and the targeted metals.52,53 Many attempts have been made by combining conventional solvent extractants and ILs to design new synergistic extractants that overcome their individual disadvantages.54–56 However, under the influence of LLE, there is still a possibility of reduction in selectivity and efficiency, formation of a third phase, loss of extractant into aqueous phase, and threat from high IL viscosity.
Radiation pollution is another great concern during the REE production. REEs are commonly accompanied by uranium and thorium via lattice substitution due to their similar chemical structures in the minerals. The concentration of thorium and uranium varies significantly depending on the type and origin of the ore. For instance, up to 5% of uranium is usually found in xenotime, and large amounts of thorium coexist with REOs in monazite, which can be up to 20 wt%. Bastnasite usually hosts a small amount of uranium and thorium.22 The concentration of radioactive elements, though being relatively benign for human health in the ore, rises significantly during beneficiation, and even larger amount of radioactive wastes are generated during repetitive cycles in LLE. Uranium, if it can be recovered economically as a by-product, is saleable as nuclear fuel. However, the current market of thorium is very limited. Most of thorium ends up either in long-term storage area or is permanently disposed. Therefore, thorium is usually the main concern in terms of radiation hazard in rare earth production.15,57 Past negligences of environmental impacts in REE mining countries in Asia have caused severe contamination of the surface and underground waters and soils with heavy metals, toxic chemicals, and radioactive elements.58
In comparison, solid-liquid extraction (SLE and SPE) is a greener technique for element extraction/separation than LLE, in part because it exploits the large surface area properties of highly porous materials. Both SLE and SPE use the affinity of a flowing liquid containing the dissolved or suspended analytes (known as the mobile phase) with a solid (known as the stationary phase), which leads to the separation of the mixture into desired and undesired fractions of the components. In industry, the most common supports used for the separation/purification steps are ion-exchange (IEC) and extraction (EXC) chromatographic-based resins.7 Typically, the backbone of most of the resins consists of an insoluble polymer matrix and the most classical ones are based on cross-linked polystyrene. A variety of functional groups can be introduced either by co-condensation of functional monomers or by physical mixture of different polymers. Compared to conventional LLE, these techniques feature a higher enrichment factor and a faster phase separation, and drastically reduce the consumption of solvent and the production of pollutants. In SLE, a REE-selective ligand
is dissolved in a hydrophobic organic phase which is physically impregnated or coated on the solid support, and the aqueous phase is then passed through the column (or cartridge) where the extraction takes place. Unfortunately, the wet impregnation strategies often result in the pronounced stripping/leaching of the stationary liquid phase, causing cross-contamination and limited lifetime, thus hampering their applicability.
To overcome these issues, SPE has been proposed as a cost-effective alternative, in which an extracting agent is chemically anchored on a solid support. In principle, the SPE procedure is based on the adsorption of the desired species onto the surface of a given adsorbent; therefore, the overall performance of the adsorbents can be optimized by tuning the characteristics of both the organic ligand and the solid support. In SPE, a good solid support should meet the following requirements: (1) it should have a large specific surface area in order to achieve high extraction capacity, (2) it should be easily modifiable with functional groups, (3) it should provide the possibility of shape control to adjust the materials to various applications, and (4) be robust and reusable. Earlier studies have focused on the use of traditional porous materials such as bare silica gels59 and activated carbon60 as solid supports. The development of advanced adsorbents bearing multi-functions and improved performance has become a continuing object of research, and the emerging nanoporous materials (pore size < 100 nm) have been widely tested as potential practical adsorbents because of their high intrinsic specific surface area. Based on their pore size, nanoporous materials can also be classified into four types: microporous (< 2 nm), mesoporous (2–50 nm), macroporous (> 50 nm), and hierarchically porous materials, which combine two or three of the above pore size ranges.12 In particular, ordered mesoporous silica (OMS) and carbon (OMC) materials have received great deal of attention as promising candidates for solid supports, because of their superior extraction capacity, stability, and possibility of functionalization.61 Advanced functional porous materials, such as ion-exchangers, metal-organic frameworks (MOF), nanosized metal oxides (NMO), amorphous porous organic polymers (POP), porous aromatic frameworks (PAF), and covalent organic frameworks (COF) have been designed to adapt to various conditions (e.g., pH, ionic strength, and presence of interfering metal ions) for REE extraction and radioactive waste removal, as will be elaborated in Section 1.6.4 of this Chapter.
1.4 Chemistry of mesoporous materials
The size and the volume of the pores in a given porous material have a dominant influence on the properties of the solid, such as adsorption, diffusion, storage capacity, exclusion, mechanical stability, confinement and exclusion effects, and reactivity.62 In 1992, the introduction of
