Size-selective separation of rare earth elements using functionalized

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Size-selective separation of rare earth elements using functionalized mesoporous silica materials Yimu Hu, Luis Carlos Misal Castro, Elisabeth Drouin, Justyna Florek, Hanspeter Kählig, Dominic Larivière, Freddy Kleitz, and Frédéric-Georges Fontaine ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 22 May 2019 Downloaded from http://pubs.acs.org on May 30, 2019

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ACS Applied Materials & Interfaces

Size-Selective Separation of Rare Earth Elements Using Functionalized Mesoporous Silica Materials Yimu Hu,a,b Luis C. Misal Castro,a,b Elisabeth Drouin,a,b Justyna Florek,c Hanspeter Kählig,d Dominic Larivière,*a,b Freddy Kleitz,*c and Frédéric-Georges Fontaine*a,b,e a Department

of Chemistry, Université Laval, Québec, G1V 0A6, QC, Canada E-mail: [email protected]; [email protected] b Centre en Catalyse et Chimie Verte (C3V), Université Laval, Québec, G1V 0A6, QC, Canada c Department of Inorganic Chemistry – Functional Materials, Faculty of Chemistry, University of Vienna, 1090 Vienna, Austria E-mail: [email protected] d Department of Organic Chemistry, Faculty of Chemistry, University of Vienna, 1090 Vienna, Austria e Canada Research Chair in Green Catalysis and Metal-Free Processes ABSTRACT: The separation and preconcentration of rare earth elements (REEs) from mineral concentrates in an economically and environmentally sustainable manner is a difficult task due to their similar physicochemical properties. Herein, a series of tetradentate phenylenedioxy diamide (PDDA) ligands was synthesized and grafted on large-pore 3-dimensional (3-D) KIT-6 mesoporous silica. In solid-phase extraction, the hybrid sorbents enable a size-selective separation of REEs on the basis of the bite angles of the ligands. In particular, smaller REE3+ ions are preferentially extracted by KIT-6-1,2-PDDA, whereas light REEs with larger ionic radius are favored by KIT-6-1,3-PDDA. The exposure of bauxite residue digestion solution containing REEs as well as a number of types of competitive ions (including Th and U) to the sorbents results in selective recovery of target REEs. The possibility of regenerating the mesoporous sorbents through simple loading-stripping-regeneration process is demonstrated over up to 5 cycles with no significant loss in REEs extraction capacity, suggesting adequate chemical and structural stability of the new sorbent materials. KEYWORDS: mesoporous silica; rare earth elements; selective extraction; bauxite residue; red mud

1. Introduction Rare earth elements (REEs) are a group of 17 elements comprising the lanthanide series (Ln) together with scandium (Sc) and yttrium (Y). Due to their unique properties and their use in a large range of advanced technologies such as catalysis, optical materials, and high performance magnets,1–3 REEs have gained considerable attention, and the demand for these critical materials is likely to increase exponentially in the following decades. Although REEs are actually moderately abundant in the earth’s crust, they are often found in a low concentration and unfavorable distribution in common mineable ores.1 Moreover, they possess subtle differences in their physicochemical properties, which renders the separation and preconcentration of individual REEs notoriously difficult.4 Industrial recovery of REEs is well established nowadays using continuous liquid-liquid-extraction (LLE). However, due to the limited separation efficiency, repetitive extractions steps are required in order to obtain the desired purity for each individual element. This process thus generates large amount of hazardous and radioactive waste, and the negative environmental impacts caused by the production of REEs cast a shadow over the potential “clean” technologies that rely on

these elements. Alternative chemical extraction methods including resin-based supported-liquid extraction (SLE),5,6 solid-phase extraction (SPE),7,8 ion-exchange,9 selective crystallization,10–13 bio-adsorption,14–16 and supramolecular self-assembly17–19 have been under the spotlight as more environmentally friendly techniques for separating individual REEs from the mineral concentrate. Among them, the emerging SPE systems offer a facile strategy for a high level of recyclability of sorbents by significantly prohibiting ligand loss, thus avoiding cross-contamination and high cost in industrial level applications.5,20 Numerous studies on novel solid state absorbents using porous silica and carbon,7,8 covalent organic frameworks (COF),21 and metal-organic framework (MOF)22–27 have been reported recently. Ordered mesoporous silica materials (OMS) are particularly interesting candidate due to their large specific surface area, well-defined pore structures, and can be easily functionalized by organic ligands. In this regard, the three-dimensional (3-D) cubic mesoporous silica KIT-6 possesses large and interconnected pores, which would facilitate the mass transport during sorption and reduce the risk of pore blocking during

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functionalization, and thus is considered as a promising solid support material in metal ion adsorption.28–32

Scheme 1. Synthesis route of ligands 1,2-/1,3-/1,4-/2-methyl-1,3-PDDA-APTS. The lanthanides have similar ionic radii with an average 1 pm difference between adjacent elements, as well as physicochemical properties, which render the separation of individual element extremely difficult. To date, several sizebased separation systems have been investigated. For instance, we previously described the hybrid sorbents based on mesoporous KIT-6 silica functionalized by diglycolamide (DGA) and its derivatives, in which middle/small-sized REEs were separated by tuning the bite angle of chelating ligands grafted on the silica support.33 However, the rigidity of hybrid materials was hampered by the rotation of the σ bonds in DGA and its derivatives. In a more recent study, we synthesized a series of preorganized bidentate ligands based on phthaloyl diamide (PA), in which a rigid structure is provided by the conjugated aromatic ring.28 The bite angles of these ligands were tuned by varying the position of the carbonyl group on the aromatic ring, and upon grafting on KIT-6, a clear sizeselective affinity was observed based on the ions radii of the REEs. These results represented an opportunity to correlate molecular design principles with the performance (especially the selectivity) of SPE systems. We report herein a novel series of hybrid mesoporous KIT-6 silica sorbents

functionalized by tetradentate polyoxo ligands based on phenylenedioxy diamide (PDDA) structure (Scheme 1), whose coordination angles enabled distinctive early/late REEs separation by distinguishing their ionic radii. The adsorption mechanism was studied in detail under batch conditions, and the SPE system showed fast adsorption kinetics and high extraction capacities. In comparison, no selectivity was observed with the PDDA analogues in homogeneous extraction systems (LLE) in the conditions tested. Finally, the applicability and reusability of the sorbents was demonstrated in dynamic extraction tests by selective recovery of REEs from real-life bauxite residues (red mud). 2. Materials and methods Materials. Chemicals and solutions of REEs were purchased and prepared as previously described,28 except for the following materials: Catechol (99%), resorcinol (99%), hydroquinone (99%), and chloroacetic acid (99%) were purchased from Alfa Aesar and used as received. Raw bauxite residues (red mud) were collected from Rio Tinto Alcan (QC, Canada).


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Synthesis of 1,2-phenylenedioxy diamidopropyltriethoxysilane (1,2-PDDA-APTS). The 1,2phenylenedioxy diacetic acid (1,2-PDDA) and 1,2phenylenedioxy diacetyl chloride (1,2-PDDACl) were synthesized as previously reported in the literature (steps I and II, Scheme 1).34 The 1,2-PDDA-APTS was synthesized according to previously reported procedure (step III),28,33 by one-step condensation reaction between 1,2-PDDACl (1.35 g, 5 mmol) and APTS (2.5 mL, 10.2 mmol), using triethylamine (7 mL, 50 mmol) as a catalyst. The final product was obtained as a yellow solid. Yield: 2.87 g (91%). 1H NMR (DMSO-d6) (Figure S1): δ 8.07 (br, 2H, NHAPTS), 6.97-6.93 (m, 4H, aromatics), 4.50 (s, 4H, CH2–O), 3.73 (q, 12H, CH2–OSiAPTS), 3.10 (q, 4H, CH2–NHAPTS), 1.48 (m, 4H, CH2–CH2APTS ), 1.13 (t, 18H, CH3APTS), 0.50 (t, 4H, CH2–SiAPTS). 13C{1H} NMR (DMSO-d6) (Figure S2): δ 167.6 (C꞊O), 147.9 (aromatic C-O), 122.0 (4,5-aromatic), 115.1 (3,6-aromatic), 68.4 (CH2–C꞊O), 58.5 (CH2–O–SiAPTS), 41.1 (CH2–NHAPTS), 22.7 (CH2– CH2APTS), 18.2 (CH3APTS), 7.3 (CH2–SiAPTS). Elemental Anal. Calc. for C28H52N2O8Si2: C, 53.14; H, 8.28; N, 4.43%. Found: C, 53.12; H, 8.22; N, 4.44%. Synthesis of 1,3-phenylenedioxy diamidopropyltriethoxysilane (1,3-PDDA-APTS). The synthesis of 1,3-PDDA-APTS was similar to that of 1,2-PDDA-APTS instead using resorcinol as starting material. The final product was obtained as a yellow solid. Yield: 2.75 g (87 %). 1H NMR (DMSO-d6) (Figure S3): δ 8.09 (br, 2H, NHAPTS), 7.18 (t, 1H, 5-aromatic), 6.59-6.55 (m, 3H, 2,4,6-aromatics), 4.44 (s, 4H, CH2–O), 3.73 (q, 12H, CH2–OSiAPTS), 3.10 (q, 4H, CH2– NHAPTS), 1.48 (m, 4H, CH2–CH2APTS ), 1.14 (t, 18H, CH3APTS), 0.52 (t, 4H, CH2–SiAPTS). 13C{1H} NMR (DMSO-d6) (Figure S4): δ 167.3 (C꞊O), 158.9 (1,3-aromatic C-O), 129.9 (5aromatic), 107.5 (4,6-aromatic), 101.9 (2-aromatic), 67.1 (CH2–C꞊O), 57.7 (CH2–O–SiAPTS), 41.1 (CH2–NHAPTS), 22.7 (CH2–CH2APTS), 18.2 (CH3APTS), 7.3 (CH2–SiAPTS). Elemental Anal. Calc. for C28H52N2O8Si2: C, 53.14; H, 8.28; N, 4.43%. Found: C, 53.12; H, 8.30; N, 4.41%. Synthesis of 1,4-phenylenedioxy diamidopropyltriethoxysilane (1,4-PDDA-APTS). The synthesis of 1,4-PDDA-APTS was similar to that of 1,2-PDDA-APTS instead using hydroquinone as starting material. The final product was obtained as a yellow solid. Yield: 2.55 g (81%). 1H NMR (DMSO-d ) (Figure S5): δ 8.05 (br, 2H, NHAPTS), 6 6.89 (s, 4H, aromatics), 4.38 (s, 4H, CH2–O), 3.73 (q, 12H, CH2–OSiAPTS), 3.10 (q, 4H, CH2–NHAPTS), 1.47 (m, 4H, CH2– CH2APTS ), 1.13 (t, 18H, CH3APTS), 0.50 (t, 4H, CH2–SiAPTS). 13C{1H} NMR (DMSO-d ) (Figure S6): δ 167.6 (C꞊O), 152.2 6 (1,4-aromatic C-O), 115.6 (2,3,5,6-aromatic), 67.6 (CH2– C꞊O), 57.7 (CH2–O–SiAPTS), 41.0 (CH2–NHAPTS), 22.7 (CH2– CH2APTS), 18.2 (CH3APTS), 7.3 (CH2–SiAPTS). Elemental Anal. Calc. for C28H52N2O8Si2: C, 53.14; H, 8.28; N, 4.43%. Found: C, 53.01; H, 8.26; N, 4.44%. Synthesis of 2-methyl-1,3-phenylenedioxy diamidopropyltriethoxysilane (2-methyl-1,3-PDDA-APTS). The synthesis of 2-methyl-1,3-PDDA-APTS was similar to that of 1,2-PDDA-APTS instead using 2-methylresorcinol as starting material. The final product was obtained as a yellow oil. Yield: 2.30 g (71%). 1H NMR (DMSO-d6) (Figure S7): δ 7.92 (br, 2H, NHAPTS), 7.04 (t, 1H, 5-aromatic), 6.52 (d, 2H, 4,6aromatics), 4.45 (s, 4H, CH2–O), 3.73 (q, 12H, CH2–OSiAPTS), 3.10 (q, 4H, CH2–NHAPTS), 2.14 (s, 3H, 2-CH3-aromatic), 1.48 (m, 4H, CH2–CH2APTS ), 1.14 (t, 18H, CH3APTS), 0.52 (t, 4H,

CH2–SiAPTS). 13C{1H} NMR (DMSO-d6) (Figure S8): δ 167.6 (C꞊O), 156.6 (1,3-aromatic C-O), 126.3 (5-aromatic), 114.4 (2-aromatic), 105.3 (4,6-aromatic), 67.7 (CH2–C꞊O), 57.7 (CH2–O–SiAPTS), 41.0 (CH2–NHAPTS), 22.7 (CH2–CH2APTS), 18.2 (CH3APTS), 8.7 (2-methyl-aromatic), 7.3 (CH2–SiAPTS). Elemental Anal. Calc. for C29H54N2O8Si2: C, 53.84; H, 8.41; N, 4.33%. Found: C, 53.92; H, 8.42; N, 4.32%. Synthesis of N,N-dioctyl-1,2-phenylenedioxy diamido (DO-1,2-PDDA). A solution of 1,2-PDDACl (1.31 g, 5 mmol) in dry THF (15 mL) was added dropwise to a solution of N,Ndioctylamine (3.3 mL, 10.5mmol) and Et3N (10.5 mL, 75 mmol) in dry THF (15 mL) at 0 ˚C. The reaction was allowed to warm to room temperature with stirring overnight. The solvent was evaporated and the product was extracted with hexane. The solvent was removed under reduced pressure, and the residue was subjected to column chromatography (SiO2 EtOAc/hexane). A light yellow oil was obtained as final product. Yield: 2.29 (68%). 1H NMR (chloroform-d) (Figure S9): δ 6.91 (m, 4H, aromatics), 4.72 (s, 4H, CH2–O), 3.29 (t, 4H, CH2–Noctyl), 3.27 (t, 4H, CH2–Noctyl), 1.52 (m, 8H, CH2– CH3octyl), 1.23 (m, 40H, CH2octyl), 0.85 (t, 12H, CH3octyl). 13C{1H} NMR (chloroform-d) (Figure S10): δ 167.4 (C=O), 148.4 (1,2-aromatics), 122.2 (4,5-aromatics), 115.3 (3,6aromatics), 68.5 (CH2-O), 47.2 (CH2-N), 45.9 (CH2-N), 31.9, 29.4, 29.3, 29.0, 27.6, 27.1, 27.0 (CH2), 22.7 ( CH2–CH3), 14.1 (CH3). Elemental Anal. Calc. for C42H76N2O4: C, 74.95; H, 11.38; N, 4.16%. Found: C, 74.75; H, 11.36; N, 4.14%. Synthesis of N,N-dioctyl-1,3-phenylenedioxy diamido (DO-1,3-PDDA). The synthesis of DO-1,2-PDDA was similar to that of DO-1,2-PDDA, except using 1,2-PDDACl as starting material. The final product was obtained as clear oil. Yield: 2.05 g (61%). 1H NMR (chloroform-d) (Figure S11) δ 7.14 (t, 1H, 5-aromatic), 6.54 (m, 3H, 2,4,6-aromatics), 4.62 (s, 4H, CH2–O), 3.30 (t, 4H, CH2–Noctyl), 3.24 (t, 4H, CH2– Noctyl), 1.53 (m, 8H, CH2–CH3octyl), 1.24 (m, 40H, CH2octyl), 0.85 (t, 12H, CH3octyl). 13C NMR (chloroform-d) (Figure S12): δ 167.2 (C=O), 159.4 (1,3-aromatics), 130.1 (5-aromatic), 107.6 (4,6-aromatics), 102.1 (2-aromatic), 67.4 ( CH2-O), 47.4 (CH2-N), 46.0 (CH2-N), 31.8, 29.4, 29.3, 29.1, 27.5, 27.1, 27.0 (CH2), 22.7 (CH2–CH3), 14.6 (s, CH3). Elemental Anal. Calc. for C42H76N2O4: C, 74.95; H, 11.38; N, 4.16%. Found: C, 74.88; H, 11.35; N, 4.16%. Synthesis of N,N-dioctyl-1,4-phenylenedioxy diamido (DO-1,4-PDDA). The synthesis of DO-1,4-PDDA was similar to that of DO-1,4-PDDA, except using 1,4-PDDACl as starting material. The final product was obtained as a clear oil. Yield: 2.19 g (65%). 1H NMR (chloroform-d) (Figure S13): δ 6.85 (s, 4H, aromatics), 4.59 (s, 4H, CH2–O), 3.30 (t, 4H, CH2–Noctyl), 3.25 (t, 4H, CH2–Noctyl), 1.55 (d, 8H, CH2– CH3octyl), 1.25 (m, 40H, CH2octyl), 0.86 (t, 12H, CH3octyl). 13C{1H} NMR (chloroform-d) (Figure S14): δ 167.5 (C=O), 152.9 (1,4-aromatics), 115.7 (2,3,5,6-aromatics), 68.1 (CH2O), 47.4 (CH2-N), 46.0 (CH2-N), 31.9, 29.4, 29.3, 29.1, 27.4, 27.1, 27.0 (CH2), 22.7 (CH2–CH3), 14.2 (CH3). Elemental Anal. Calc. for C42H76N2O4: C, 74.95; H, 11.38; N, 4.16%. Found: C, 75.20; H, 11.36; N, 4.15%. Synthesis of N,N-dioctyl-2-methyl-1,3-phenylenedioxy diamido (DO-2-methyl-1,3-PDDA). The synthesis of DO-2methyl-1,3-PDDA was similar to that of DO-1,2-PDDA, except using 2-methyl-1,3-PDDACl as starting material. The final product was obtained as a yellow oil. Yield: 1.82 g (54%). 1H NMR (chloroform-d) (Figure S15): δ 7.03 (t, 1H, 5-



aromatic), 6.52 (d, 2H, 4,6-aromatics), 4.65 (s, 4H, CH2–O), 3.30 (m, 8H, CH2–Noctyl), 2.17 (s, 3H, CH3-aromatic), 1.53 (m, 8H, CH2–CH3octyl), 1.25 (m, 40H, CH2octyl), 0.86 (t, 12H, CH3octyl). 13C{1H} NMR (chloroform-d) (Figure S16): δ 167.6 (C=O), 157.2 (1,3-aromatics), 126.4 (5-aromatic), 115.3 (2aromatic), 105.3 (4,6-aromatics), 68.2 (CH2-O), 47.3 (CH2N), 46.0 (CH2N), 31.9, 29.4, 29.3, 29.0, 27.5, 27.0 (CH2), 22.7 (CH2–CH3), 14.2 (CH3), 8.59 (CH3-aromatic). Elemental Anal. Calc. for C43H78N2O4: C, 75.17; H, 11.44; N, 4.08%. Found: C, 79.38; H, 11.53; N, 4.28%. Synthesis and functionalization of the mesoporous materials. Ordered mesoporous KIT-6 silica was obtained following the procedure reported by Kleitz et al.35 The pristine KIT-6 silica was activated overnight at 150 °C under vacuum before ligand grafting procedure. The functionalization of the materials was carried out using procedure previously reported in the literature.28,29,33 Typically, a mixture containing predispersed KIT-6 with appropriate amount of chosen silanemodified ligand (i.e., 0.76 g of 1,2-/1,3-/1,4-PDDA-APTS and 0.78 g of 2-methyl-1,3-PDDA-APTS for every 1.0 g of KIT-6) in dry toluene was refluxed under inert condition for 24 h. The functionalized silica was obtained after filtration and Soxhlet extraction in CH2Cl2 in order to remove unreacted silane molecules. The resulting products are noted as KIT-6-1,2-/1,3/1,4-PDDA and KIT-6-2-methyl-1,3-PDDA, respectively. Materials characterization. N2 adsorption-desorption isotherm measurements were carried out at -196 °C (77 K) using an Autosorb-1 sorption analyzer (Quantachrome Instruments, Boynton Beach, USA). The samples were outgassed for 12 h (200 °C for pristine KIT-6 silica and 80 °C for hybrid sorbents) prior to the analysis. The specific surface area (SBET) was calculated using the Brunauer-Emmett-Teller equation in the range 0.05 ≤ P/P0 ≤ 0.20. The pore size distributions were calculated by applying the nonlocal density functional theory (NLDFT) method considering the equilibrium isotherm sorption of N2 at -196 °C in cylindrical silica pores. The total pore volume (Vpore) was measured at P/P0 = 0.95. FTIR spectra were recorded using a Nicolet Magna FTIR spectrometer with a narrow band MCT detector (Specac Ltd., London). Spectra were obtained from 64 scans with a 4 cm-1 resolution. Simultaneous thermogravimetric analysis-differential scanning calorimetry (TGA-DSC) was performed using a Netzsch STA 449C thermogravimetric analyzer under air flow of 20 mL min-1, with a heating rate of 10 ˚C min-1 from 35 to 700 °C. The low-angle powder X-ray diffraction (XRD) patterns were recorded on a PANalytical EMPYREAN diffractometer (Malvern PANalytical, United Kingdom) operated at a voltage of 45 kV and a tube current of 40 mA, using Cu Kα1+2 radiations. XRD scanning was performed in transmission geometry and in continuous mode with a step size 2θ of 0.013°, a time per step of 300 sec and with a fixed divergence slit of 0.76 mm. Elemental analysis was performed by the combustion method using a CHNS Analyzer Flash 2000, Thermo Scientific. The liquid NMR spectra were obtained on a Varian Inova NMR AS400 spectrometer at 400.00 (1H) and 100.58 MHz (13C). The NMR chemical shifts are referenced to the residual solvent signal (DMSO-d6 δ = 2.50 ppm and CDCl3-d δ = 7.26 ppm for 1H), or to carbons of the deuterated solvent (DMSO-d6 δ = 39.52 ppm and CDCl3-d δ = 77.16 ppm for 13C{1H}). The solid state NMR spectra were obtained on a Bruker Avance NEO 500 wide bore system (Bruker BioSpin, Rheinstetten, Germany) using a 4 mm triple resonance magic angle spinning (MAS)

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probe operating in the dual mode. The resonance frequency was 125.78 MHz for 13C and 99.38 MHz for 29Si. The MAS rotor spinning was set to 14 kHz for 13C and to 8 kHz for 29Si. Cross polarization (CP) was achieved for both nuclei by a ramped contact pulse from 50 % to 100 % with a contact time of 2 ms for 13C and 5 ms for 29Si. During acquisition 1H was high power decoupled using SPINAL with 64 phase permutations. 29Si was measured also without cross polarization using a relaxation delay of 60 s. The chemical shifts are reported in ppm and are referenced external to adamantane for 13C by setting the low field signal to 38.48 ppm, and to 4,4-dimethyl-4-silapentane-1-sulfonic acid (DSS) for 29Si, respectively. Batch extraction methodology. Typical batch solid-phase extraction (SPE) experiments of REE solution with functionalized mesoporous KIT-6 were carried out as previously described,28 where a solution of REE with competitive ions (Al, Fe, U, and Th) in nanopure water was prepared in desired concentration (30 ppb). Unless mentioned otherwise, the extraction experiments were carried out at room temperature, the pH of the REE solution was adjusted at 4 using diluted NH4OH or HNO3, and the equilibrium time for the extraction was 60 min. The experiments were carried out in triplicate and the average values are given. After adsorption reached equilibrium (typically after 2 h), the supernatant was filtered through a 0.2 µm PVDF syringe filter. Elemental quantification of the various elements after extraction was performed by ICP-MS/MS (Model 8800, Agilent Technologies). The distribution coefficient Kd, used for the determination of the affinity and selectivity of modified sorbents for REEs is given by the equation: 𝑉

K𝑑 = 𝑚 ×

(𝐶0 ― 𝐶𝑓) 𝐶𝑓

(1)

where V, m, C0 and Cf represent the volume of the solution, the amount of the sorbents, the initial and final concentration of the REEs, respectively. In this work, the solution/solid ratio was fixed to 500 mL g-1. Adsorption mechanism studies. The kinetic and isotherm studies were performed for KIT-6-1,2-PDDA and KIT-6-1,3PDDA materials. For KIT-6-1,2-PDDA, a solution of Lu3+ (60 mg L−1, pH 4) was used for kinetic studies, with the contact time varying from 1 min to 2 h. Solutions of Lu3+ with different concentrations (5 to 100 mg L−1, pH 4) were used for isotherm studies, with a contact time of 2 h. For KIT-6-1,3PDDA, the same extraction conditions were applied, using solutions of Ce3+ at pH 4. The amount of adsorbed ions at equilibrium was calculated by equation (2): 𝑉

Q𝑒 = 𝑚 × (𝐶0 ― 𝐶𝑓)

(2)

For the isotherm studies, solutions of Lu3+ (for KIT-6-1,2PDDA) and Ce3+ (for KIT-6-1,3-PDDA) at pH 4 with concentrations ranging from 5 to 80 mg L−1 were used. The contact time was 2 h. Liquid-liquid extraction. The liquid-liquid extraction (LLE) was carried out using a procedure described elsewhere.28 Typically, the organic phase contains a solution of DO-1,2-PDDA, DO-1,3-PDDA, DO-1,4-PDDA, or DO-2-


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methyl-1,3-PDDA (5 mmol L-1) in 5 mL of dichloromethane, whereas 5 mL of REEs (100 μg L-1, dissolved in 3 M nitric acid) was used as the aqueous phase. The two phases were mixed mechanically for 90 min before being separated. The distribution coefficient (Kd) for LLE is calculated by the following equation: K𝑑 =

𝐶𝑜𝑟𝑔 𝐶𝑎𝑞

NMR spectroscopy were performed. For the modified materials, the presence of the amide bond was confirmed by the bands visible at 1660 and 1540 cm−1 in the FTIR spectra, which are characteristic for C=O stretching and N-H deformation in amine I, respectively (Figure S18). This observation is consistent with previously reported phtaloyl diamide (PA)-modified KIT-6 mesoporous materials, where these bonds were observed at 1670 and 1520 cm-1.28

(3)

where Corg and Caq represent the concentration of REEs in organic and aqueous phase after extraction, respectively. Application to mineral leachates and reusability tests. Inside a 5 mL cartridge (Eichrom Technologies, USA), 15 mg of KIT-6-1,2-PDDA or 30 mg of KIT-6-1,3-PDDA sorbent was packed, washed, and conditioned using the slurry-packing technique described elsewhere.36 If tightly packed, the volume of 15 mg of KIT-6-1,2-PDDA is approximately 0.25 mL whereas 30 mg of KIT-6-1,3-PDDA is approximately 0.5 mL. The column was loaded with an acidic digestion solution of bauxite diluted in HNO3 (5mL, pH 4) using a peristaltic pump (Minipuls 3, Gilson, USA). The volumetric flow rate was fixed at 0.5 mL min-1 (For elemental composition of the bauxite residue solution, see Table S2).37 The elements retained in the column were recovered using a solution of (NH4)2C2O4 (0.1 M, 5 mL). The columns were regenerated by washing with nanopure water (10 mL) and diluted HNO3 (10 mL, pH 4) before being used for the second extraction. This procedure was repeated 5 times. 3. Results and discussion Synthesis and characterization of the materials. The carboxylic diacids 1,2-PDDA, 1,3-PDDA, 1,4-PDDA, and 2methyl-1,3-PDDA were synthesized using a slightly modified reported protocol34 and the corresponding acyl chlorides were modified using 3-aminopropyltriethoxysilane (APTS). The successful modification of the ligands with APTS was confirmed by the appearance of a broad peak at δ (ppm) = 8.07, 8.09, 8.05, 8.05, and 7.92 in 1H NMR spectra for 1,2/1,3-/1,4-PDDA-APTS and 2-methyl-1,3-PDDA-APTS, respectively, each integrating for two protons corresponding to the two NH groups (Figure S1-S8). These ligands were then grafted on mesoporous KIT-6 silica using a one-step modification procedure as previously reported.29 For all functionalized materials, typical type IV isotherms were obtained from the N2 physisorption analyses. In the relative pressure range P/P0 of 0.6–0.8, a steep capillary condensation step with type H1 adsorption–desorption hysteresis loop were observed (Figure 1). After grafting, the shape of the adsorption-desorption isotherms were well-maintained, whereas the hysteresis loop shifted to the lower values of relative pressure, indicating a decrease in the pore size.29,36,38 According to the N2 sorption analysis, the size distribution and the pore volumes are also consistent with modified mesoporous supports (Table 1). As expected for KIT-6derived materials, the low-angle powder XRD patterns of the hybrid materials also demonstrate well resolved diffraction peaks corresponding to the Ia3d symmetry (Figure S17).35 To further demonstrate the covalent attachment of the organic species on the KIT-6 materials, FTIR, thermogravimetric analyses (TGA), 13C and 29Si solid state

Figure 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-6-1,4PDDA are offset vertically by 250, 150, and 75 cm3 g-1, respectively; and corresponding pore size distributions (b). The efficiency of the grafting procedure was evaluated using TGA, and the mass losses of the various ligands grafted on the modified supports are presented in the Table 1. The decomposition process begins at temperatures higher than 140 °C, as shown on the TGA and associated differential scanning calorimetry (DSC) curves (Figure S19), which suggests that the organic moieties are covalently tethered on the silica surface. A ligand loading between 21% and 25% was achieved for all functionalized materials. Among all four functionalized sorbents, the highest ligand loading was observed for KIT-61,4-PDDA. Indeed, similar correlation between the positioning of the anchoring groups on a ligand and its loading efficiency was observed for 1,2-/1,3-/1,4-PA modified mesoporous silicas, in which the KIT-6-1,4-PA sorbent exhibits the highest mass loss in TGA and the lowest specific surface area and pore volume, while maintaining a similar pore size as its KIT6-1,2-PA and -1,3-PA counterparts.28 The density of ligand grafted was calculated based on the amount of nitrogen obtained from the elemental analyses combined with the specific surface area and TGA values (Table 1). However, the ligand density on the silica surface might be overestimated since the solid state 13C CP/MAS NMR spectra confirm the



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presence of untethered ethoxyl groups remaining in the silanes (vide infra) which also contribute to the weight loss in TGA.28 Table 1. Physicochemical parameters obtained from N2 physisorption analysis at -196 °C, and the total amount of ligand introduced and estimated ligand density on the surface based on TGA and CHN analysis. Materials SBET Vpore Pore size Mass loss N C Ligand density (m2 g-1) (cm3 g-1) (nm) (%) (%) (%) (nm-2) KIT-6 956 1.32 8.1 KIT-6-1,2-PDDA 572 0.86 7.3 21 1.70 11.8 0.51 KIT-6-1,3-PDDA 564 0.83 7.3 22 1.58 12.7 0.50 KIT-6-1,4-PDDA 522 0.76 7.3 24 1.47 13.3 0.57 KIT-6-2-methyl-1,3-PDDA 545 0.74 7.3 23 1.51 13.8 0.54


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Figure 2. Solid state 13C CP/MAS NMR (a) and 29Si MAS NMR (b) for the different functionalized mesoporous materials. Solid state NMR spectroscopy was then used to further characterize the structure of the anchored ligands. In the 13C CP/MAS NMR spectra, the well-defined peaks are in good agreement with those observed in the liquid 13C{1H} NMR spectra of the silane ligands (Figure 2a). For instance, the peaks in the range of δ 120-150 are characteristic of the aromatic carbon atoms, while the band appearing at 170 ppm represents the carbonyl group. Three intense peaks at around 8, 22, and 42 ppm can be assigned to the methylene carbon atoms in the -Si-CH2-CH2-CH2-NH- chain. Besides, the two additional peaks representing the ethoxy groups in (-Si-OCH2-CH3) at δ = 59 and 17 suggest an uncomplete functionalization, leaving a small portion of the ethoxysilane species unreacted, as has been previously observed.28 The 29Si MAS NMR spectra show that for KIT-6-1,2-PDDA, the ligand has been grafted mostly through T2 ((SiO)2(OR)Si-R) at -58 ppm and partially through T3((SiO)3Si-R) at -65 ppm, whereas for KIT-6-1,4-PDDA, the ligand was almost equally grafted through T1, T2 and T3 species (Figure 2b). For ligands with silanes positioned in para (KIT-6-1,3-PDDA and KIT-6-2methyl-1,3-PDDA), the majority of them were grafted through T1 species at -52 ppm (especially for KIT-6-1,3-PDDA) and partially through T2 species. The 29Si MAS spectra not only further confirm the covalent anchoring of the organic moieties on the surface, but also are consistent with the 13C CP/MAS spectra. For instance, the peaks corresponding to carbons on untethered ethoxyl groups in KIT-6-1,4-PDDA (carbon No. 5 and No. 8, Figure 2a) are significantly less intense than those in KIT-6-1,3-PDDA (carbon No. 7 and No. 10), which is in accordance with the higher amount of T2 and T3 species for KIT-6-1,4-PDDA in 29Si MAS spectra than for KIT-6-1.3PDDA. In addition to increase the stability of the ligands, the grafting also increased their rigidity, thus improving the selectivity of the hybrid materials compared to their

counterparts in homogeneous systems in the extraction of REEs (vide infra). Extraction studies. Selectivity. Extraction properties for the functionalized KIT-6 materials across the lanthanide series in the presence of competitive ions (e.g., Al, Fe, Th, and U) were evaluated using the value of distribution coefficient (Kd, mL g1) calculated according to equation (1). Depending on the bite angle of organic ligands, the modified materials clearly express significantly distinct selectivity toward lanthanides, according to their atomic radii (Figure 3). To be more precise, 1,2-PDDA-APTS exhibits the smallest bite angle, and the corresponding functionalized material KIT-6-1,2-PDDA shows high affinity toward heavy lanthanides with the smallest ionic radii (namely, Tm3+, Yb3+, and Lu3+), with Kd values higher than 30,000 mL g-1, which are approximately 3 to 6 times higher compared to the Kd for the rest of lanthanides. For KIT-6-1,3-PDDA, a selectivity toward light lanthanides (i.e., La3+, Ce3+, and Pr3+) was well-defined. Since the bite angle of 1,3-PDDA-APTS is larger compared to that of 1,2PDDA-APTS, the observed selectivity is in good accordance with our theory. After grafting, the chelating angle (bite angle) of organic moieties and the Si-O-Si or Si-OH moieties of the silica surface form a multidentate coordination cavity for REEs.28,36,39 Only the lanthanides with appropriate ionic radius will be “trapped” in the cavity and coordinate with the binding sites of the ligand (oxygen atoms in this case). In our previous work, we established that for KIT-6-1,3-PA, the presence of the hydrogen atom in 2 position of the aromatic ring could hamper the complexation of the carbonyl groups with REE ions, which reduces the Kd values by a factor of 3.28 However, no such significant decrease in Kd values was observed for KIT-6-1,3-PDDA despite the presence of an hydrogen atom in the 2 position. This is probably due to the larger cavity formed between the grafted ligand and silica surface that mitigates the



steric effect of a single hydrogen atom. Therefore, the introduction of more sterically hindered groups at the same position would lead to a smaller cavity, rendering the coordination sites less accessible to the lanthanide ions. Indeed, the Kd values were reduced by an order of magnitude and a loss in selectivity was observed upon addition of a methyl group to the 2 position of the aromatic ring (KIT-6-2methyl-1,3-PDDA), just as hypothesized (Figure 3). For KIT-6-1,4-PDDA, although the Kd values were largely enhanced compared to the unmodified KIT-6 (in general, the average Kd values for Ln3+ and Y3+ are less than 100 mL g-1 for KIT-6, as previously reported),28,30 the functionalized material shows limited selectivity through the lanthanide series. Similarly to the previously reported 1,4-PA-APTS modified mesoporous KIT-6 sorbent, the two carboxylate groups in 1,4-PDDA-APTS in the para position prevent any synergistic action of the moieties that would lead to a selective uptake of REEs by the sorbent. The extraction tests for the mixture of REEs using hybrid sorbents were carried out in the presence of additional competitive ions (i.e., Al, Fe, Th, and U) that are commonly present in environmental and mining wastes. All functionalized KIT-6 materials show higher Kd values for REEs in comparison to elements of similar valence (Al3+ and Fe3+) or size (UO22+). However, a high Kd values for REEs in comparison to elements of similar valence (Al3+ and Fe3+) or size (UO22+). However, a higher Kd value was found for the extraction of thorium (in the form of Th(OH)22+ under present condition) with all functionalized KIT-6 sorbents. Indeed, it

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has been suggested that extractants containing soft sulfur or nitrogen atoms combined with hard donors such as oxygen are preferred to recognize light actinides (e.g., U, Th, and Pu) over lanthanides.32,40–43 The interaction with Th(OH)22+ is thus taking place with the C=O typically observed for lanthanides. The higher uptake for thorium over uranium could be explained by the higher valence of Th(OH)22+ over UO22+ under acidic condition (pH 4 in this case).39 Typically, UO22+ adsorption is more favorable at higher pH.44 The liquid-liquid extraction (LLE) experiments were also performed for DO-1,2-PDDA, DO-1,3-PDDA, DO-1,4PDDA, and DO-2-methyl-1,3-PDDA. Unsurprisingly, considering the nonoptimal coordination environment of these ligands, all ligands tested for LLE exhibit a poor extraction capacity (Kd values generally smaller than 0.5), as well as limited selectivity across the lanthanide series with little variation in Kd values (Figure S20). In SPE, the chemical anchoring of the ligands to the solid surface increases the hydrophilicity of the ligands (thus the high extraction capacity) as well as their overall rigidity (thus the distinctive selectivity).32 That way, a cavity is formed where ligands and silica surface act synergistically to capture the ions having the right volume to occupy the coordination environment. In the homogeneous system (LLE), however, the rotation of σ-σ bonds in these ligands may be an adverse effect on the sorption performance. These observations further demonstrate the necessity of ligand immobilization to enhance the overall extraction performance of the ligand.

Figure 3. Lanthanide elements extraction (Kd) of functionalized hybrid materials in the presence of competitive ions (Al, Fe, Th, and U) at pH 4. The initial concentration of each element is 30 ppb. Adsorption of REEs at Different pH Values. Since KIT-61,2-PDDA and KIT-6-1,3-PDDA show exceptionally high extraction performance in terms of selectivity and Kd values, the two materials were chosen as model materials for further studies. The optimization of the pH values of the solution is crucial to maximize the extraction of REEs using silica-based sorbents, as the pH of the solution can significantly affect the nature of the metal ions, the surface charge and the degree of

ionization of the functional groups. The REE extraction capacity Qe for KIT-6-1,2-PDDA and KIT-6-1,3-PDDA using equation (2) was measured in the pH range of 2-7 using a solution of REEs at 500 µg L-1. For clarity, only the ions with the highest Qe values (i.e., Lu3+ for KIT-6-1,2-PDDA and Ce3+ for KIT-6-1,3-PDDA) are shown. It can be seen from Figure 4 that the REE extraction capacities of KIT-6-1,2-PDDA and KIT-6-1,3-PDDA increases rapidly and reaches to maximum


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when the pH goes from 2 to 5, after which the Q e declines slightly. At the pH range chosen for these experiments, the lanthanides remain mainly as Ln3+. However, at lower pH, the acidity of the medium (H+) can compete with the metal ions

Figure 4. Extraction capacity values (Qe) of Lu3+ by KIT-61,2-PDDA and Ce3+ by KIT-6-1,3-PDDA at various initial pH values. for the chelation while positively charges the sorbent surface, resulting in the electrostatic repulsion between the target ions and the silica surface. At higher pH (pH ≥ 7), the REEs tend to form hydroxide complexes with OH- and precipitate out of solution. We thus choose pH 4 as the optimal condition for the following studies. Adsorption mechanism studies. The effect of contact time on REE adsorption for KIT-6-1,2-PDDA and KIT-6-1,3PDDA was investigated using Ln3+ containing solution (60 mg L-1, Lu3+ and Ce3+ for KIT-6-1,2-PDDA and KIT-6-1,3PDDA, respectively) over a range of 2-120 min. As shown on Figure 5, an abrupt increase of adsorption was observed in the first 20 min, and the system reached equilibrium after 60 min. The equilibrium time of the sorbents is close to that of functionalized mesoporous silica- and carbon-based sorbents under similar conditions,28,45,46 and graphene oxide composite (typically 1-2 h),47 and higher than MOF (2-4 h) systems23,48 and TiO2 nanoparticles (more than 12 h).49,50 The kinetic fitting curves of KIT-6-1,2-PDDA and KIT-6-1,3-PDDA with pseudo-first-order and pseudo-second-order kinetic models were are shown in Figure 5 and Table S1. The Qe values calculated from both the pseudo-first and second-order models are in good agreement with the experimental data Qe,exp. However, based on the correlation coefficient R2, the pseudosecond-order model is a better fit for the adsorption data of both functionalized sorbents (Figure S21). This model is also frequently associated with the chemical sorption mechanism.28

Figure 5. Effect of the contact time on the Lu3+ sorption on KIT-6-1,2-PDDA (a) and Ce3+ sorption on KIT-6-1,3-PDDA (b) with kinetic model fitting. Based on the results obtained, a contact time of 60 min and pH 4 were chosen as the optimal conditions, under which the effect of initial Ln3+ concentration (isotherm studies) was investigated. The relationship between equilibrium adsorption capacity Qe and residual ion concentration Ce (Lu3+ and Ce3+ for KIT-6-1,2-PDDA and KIT-6-1,3-PDDA, respectively) is shown in Figure S22, and the corresponding sorption profiles were simulated with Langmuir and Freundlich models (Figure S23). According to the curve fitting of experimental data using the two proposed models (Figure S23), the Langmuir model gives a better fit (R2 > 0.99), suggesting monolayer adsorption on the surface. Also, the calculated adsorption capacity (Qm) in Langmuir isotherm model is consistent with experimental data (Table 2). Compared to our previously reported bidentate PAmodified system (with maximum Lu3+ extraction capacity of 8.57 mg g-1 for KIT-6-1,2-PA),28 the adsorption capacity of KIT-6-1,2-PDDA for the same element was enhanced by a factor of 2.3 (19.8 mg g-1). Herein, the novel sorbents exhibit high ligand density upon grafting as well as a tetradentate coordination mode, which provide a stronger affinity toward Ln3+ ions. The grafting also increases the overall rigidity of the ligand through decreasing the flexibility of the chelating sites (oxygen atoms), thus also enhancing extraction capacity. Applications to real-world samples and stability assessment. To test the efficacy of KIT-6-PDDA systems for REE extraction from an industrially relevant source, dynamic extraction of the acid leachates of raw bauxite residues (red mud) collected from Rio Tinto Alcan (QC, Canada) was



performed with sorbents KIT-6-1,2-PDDA and KIT-6-1,3PDDA. The choice of bauxite residues is based on their abundance as aluminum ore mining waste, which is a targeted raw material for potential alternative sources for recovery of REEs.51,52 After acidic digestion using H2SO4,37 the leachate was diluted with 4% HNO3, and the elemental composition was determined by ICP-MS/MS. As shown in Table S2, the aqueous solution contains several REEs with concentrations that are potentially economically exploitable, namely, La3+ (13.7 mg L-1), Ce3+ (23.3 mg L-1), and Nd3+ (8.27 mg L-1), whereas the concentration of late lanthanides are much lower (e.g., 0.013 mg L-1 for Lu3+). Further, the bauxite solution contains various competitive ions whose concentrations are several orders of magnitude higher than that of REEs: Al3+ at 17251 mg L-1 and Fe3+ at 263 mg L-1, for instance.

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Two columns containing KIT-6-1,2-PDDA (15 mg) and KIT-6-1,3-PDDA (30 mg) were prepared for dynamic extraction tests. After a pre-conditioning with HNO3 at pH 4, 5 mL of bauxite leachate was passed through each column at a volumetric rate of 0.5 mL min-1 (loading), followed by stripping the adsorbed REE ions using a 0.1 M (NH4)2C2O4 solution. The columns were then regenerated using diluted HNO3 at pH 4. For the reusability assessment of the sorbents, the same loading-stripping-regenerating process was repeated 5 times. The efficiency of the dynamic extraction by these columns is represented in Figure 6, with focus on Lu3+ (by KIT-6-1,2-PDDA) and Ce3+ (by KIT-6-1,3-PDDA). Only a minor deterioration in the column performance was recorded after 5 runs, with the average extraction efficiency > 97% for Lu3+ and > 81% for Ce3+. The difference in extraction efficiency between the two sorbents stems from the much higher concentration of early lanthanides compared to late lanthanides in the initial solution (Table S2). Despite the complex elemental composition and the presence of ions that could potentially compete with the targeted REEs (such as Na+, Fe3+, Al3+, and Ca2+), the overall selectivity of the sorbents was relatively well preserved, as shown by the average extraction efficiency for all the elements during the 5 experiment cycles (Figure 7). Furthermore, most of the Lu3+ and Ce3+ extracted by the columns from the initial solution were recovered by the oxalate solution, as shown in Figure 6.

Figure 6. Extraction of REEs from a bauxite solution using KIT-6-1,2-PDDA (for Lu3+) and KIT-6-1,3-PDDA (for Ce3+). Table 2. Adsorption equilibrium constants for Langmuir and Freundlich isotherm models Langmuir Freundlich Material Qm,exp (mg g-1) Qm, cal (mg g-1) KL (L mg-1) R2 KF (mg g-1) 1/n KIT-6-1,2-PDDA 19.8 20.0 1.07 >0.99 12.4 0.13 KIT-6-1,3-PDDA 12.5 12.2 3.77 >0.99 11.7 0.027

Figure 7. The average extraction efficiency using bauxite residue from 5 experiment runs for KIT-6-1,2-PDDA and KIT-6-1,3-PDDA. After 5 experimental runs, KIT-6-1,2-PDDA and KIT-61,3-PDDA were recovered from the column, and their physicochemical properties were evaluated and compared to those of as-made hybrid materials (Table S3). From N2 physisorption analysis, a slight increase in both surface area (from 572 to 588 m2 g-1 for KIT-6-1,2-PDDA and 564 to 589

R2 0.93 0.94

m2 g-1 for KIT-6-1,3-PDDA) and pore volume (from 0.86 to 0.87 cm3 g-1 for KIT-6-1,2-PDDA and 0.83 to 0.84 cm3 g-1 for KIT-6-1,3-PDDA, Figure S24) were observed for both materials, which can be attributed to the partial loss of organic moieties. This assumption was then confirmed by TGA, which shows a small reduction in mass loss compared to freshly synthesized hybrid materials (from 20% to 18% for KIT-61,2-PDDA and 22% to 19% for KIT-6-1,3-PDDA, Figure S25). Finally, the FTIR spectra of the used sorbents showed that the characteristic peaks at 1660 and 1540 cm-1 for C=O stretching and N-H deformation in amine I were well maintained, indicating the overall preservation of the ligand structure after the reusability tests (Figure S26). It is important to mention that the covalent grafting applied in this case represents a principal advantage of SPE compared to the physical impregnation of ligands onto a polymer resin in SLE, which thus largely reduces the risk of cross-contamination and the cost. 4. Conclusion and perspectives Process development for the recovery of REEs from various production wastes is a strategy to address potential supply shortages for these critical materials. Herein, we reported a series of selective and efficient mesoporous sorbents


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functionalized by phenylenedioxy diamide (PDDA) ligands for the recovery of REEs from bauxite residue (red mud). The sorbents were easily prepared by the one-step grafting of the ligands on KIT-6 silica, and the obtained hybrid materials demonstrated significant size-based selectivity for REEs depending on the geometry of organic moieties, especially for the separation between early and late lanthanides. Under optimal conditions (at pH 4), the KIT-6-1,2-PDDA and KIT-61,3-PDDA sorbents showed fast rapid adsorption kinetics and high extraction capacities. Under dynamic conditions, the selective extraction of REEs from real-world samples (bauxite residue) and the reusability of materials were demonstrated. The sorbents recovered after 5 loading-stripping-regeneration cycles showed no significant degradation in terms of ligand structure and material mesoporosity. Overall, this work suggests that the evaluated sorbents have a potential for industrial REE separation process as an environmentally sustainable alternative to conventional liquid−liquid extraction. Future work will focus on exploring novel SPE sorbents with higher level of individual element recognition and separation, such as ion-imprinting techniques.45,53–55

acknowledge the funding support of the University of Vienna (Austria). Dr. Remy Guillet-Nicolas is acknowledged for his help with the XRD measurements.

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Associated Content Supporting Information Supplementary Information (SI) available: Low angle XRD patterns of the materials, 1H and 13C NMR spectra, FT-IR spectra, graphics of TGA-DSC, linear regression and the corresponding parameters of the kinetics and adsorption isotherm experiments, N2 sorption isotherms, TGA-DSC profiles, and FT-IR spectra of KIT-6-1,2-PDDA and KIT-61,3-PDDA after reusability tests, liquid-liquid extraction performance, and elemental composition of bauxite residue solution.

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Author Information Corresponding Authors *E-mail: [email protected] *E-mail: [email protected] *E-mail: [email protected]

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ORCID Justyna Florek: 0000-0001-8891-2474 Dominic Larivière: 0000-0003-1860-1181 Freddy Kleitz: 0000-0001-6769-4180 Frédéric-Georges Fontaine: 0000-0003-3385-0258 Conflict of Interests The authors declare no competing financial interest.

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Acknowledgement The authors acknowledge the Natural Sciences and Engineering Research Council of Canada (NSERC) and Fonds de recherche du Québec - Nature et technologies (FRQNT) for financial support. NSERC supported this work through a Strategic Project Grant (Grant no. STPGP 463032-14). This research was undertaken, in part, thanks to the funding from the Canada Research Chairs program. F.K. and J.F. also

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