Selective scandium ion capture through coordination templating in a covalent organic framework

The use of coordination complexes within covalent organic frameworks can significantly diversify the structures and properties of this class of materials. Here we combined coordination chemistry and reticular chemistry by preparing frameworks that consist of a ditopic (p-phenylenediamine) and mixed tritopic moieties—an organic ligand and a scandium coordination complex of similar sizes and geometries, both bearing terminal phenylamine groups. Changing the ratio of organic ligand to scandium complex enabled the preparation of a series of crystalline covalent organic frameworks with tunable levels of scandium incorporation. Removal of scandium from the material with the highest metal content subsequently resulted in a ‘metal-imprinted’ covalent organic framework that exhibits a high affinity and capacity for Sc3+ ions in acidic environments and in the presence of competing metal ions. In particular, the selectivity of this framework for Sc3+ over common impurity ions such as La3+ and Fe3+ surpasses that of existing scandium adsorbents. Scandium is challenging and expensive to isolate in pure form using conventional solvent extraction. Now a covalent organic framework (COF) has been synthesized that can incorporate scandium coordination complexes; subsequent removal of the scandium ions generates open coordination sites, and the resulting ‘metal-imprinted’ COF can be used for highly selective, cyclable scandium capture.


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https://doi.org/10.1038/s41557-023-01273-3 Proton and 13 C NMR spectra and mass spectrometry data also support complex formation (Supplementary Figs. 2 and 3).Further, X-ray photoelectron spectra collected for Sc(C 8 H 8 NO 2 ) 3 and the compound Sc(O 2 CC 11 H 23 ) 3 (O 2 CC 11 H 23 − , laurate) 29 both feature a single Sc 2p peak at ~400 eV, confirming the presence of scandium(III) in similar coordination environments (Supplementary Fig. 4).Although the structure of Sc(C 8 H 8 NO 2 ) 3 has not been reported in the literature, analysis of FT-IR spectra obtained for that compound and several other scandium(III)carboxylate complexes supports bidentate coordination.In particular, the difference between the asymmetric and symmetric -COO stretches for those compounds were found to be much smaller than the corresponding difference for a free carboxylate 29,30 .Similarly, here we found that this difference for our scandium complex (Δ = 85 cm −1 ) is much smaller than that determined for Na(C 2 H 3 O 2 ) (Δ = 140 cm −1 ) 31 , which was used as a reference salt 29 .
Scandium-loaded COFs were prepared via the solvothermal reaction of Sc(C 8 H 8 NO 2 ) 3 with 1,3,5-triformylphloroglucinol and p-phenylenediamine in a mixture of mesitylene, N,N-dimethylacetamide and aqueous acetic acid; a reaction time of 72 h was deemed optimal for obtaining crystalline material (Methods for further details).Precise molar ratios were used to prepare the parent TpPa-1 structure and metal-covalent organic frameworks with scandium occupying 9% to 43% of the parent structure 'nodes' (Fig. 1a), referred to as Sc-COF-9 through Sc-COF-43.The occurrence of a Schiff base reaction in all cases and incorporation of Sc 3+ was verified by infrared, X-ray photoelectron and solid-state 13 C cross-polarization magic-angle-spinning NMR spectroscopies, as well as by elemental analysis (Supplementary Figs.4-6 and Supplementary Table 2).Simulations performed in Materials Studio suggest that the substituted nodes in the Sc-COFs consist of one scandium ion coordinated by three 4-aminophenylacetate ligands, wherein the scandium centre adopts a distorted octahedral geometry with an average Sc-O bond length of 2.18 Å 30,32 and a C-CH 2 -C angle of 128.5° (Fig. 1c).
In the Sc-COF infrared spectra, the characteristic -NH bands of p-phenylenediamine (3,200-3,500 cm −1 ) are absent, while new bands associated with -C=C and -C-N vibrations are present at 1,578 and 1,255 cm −1 (Supplementary Fig. 5).All spectra additionally feature bands at 518 and 1,616 cm −1 , assigned to a Sc-O vibration and the -COO stretch of a scandium-bound carboxylate, respectively.X-ray photoelectron spectroscopy characterization of Sc-COF-33 revealed a Sc 2p peak with an energy of ~400 eV, identical to that of Sc(C 8 H 8 NO 2 ) 3 (Supplementary Fig. 4), confirming the presence of six-coordinate scandium(III) in the extended material.Finally, all peaks in the solid-state 13 C NMR spectrum of Sc-COF-33 could be assigned to the carbon atoms of the keto-enamine salicylideneaniline units or the scandium carboxylate ligands (Supplementary Fig. 6).
Powder X-ray diffraction data for Sc-COF-9 through Sc-COF-33 feature a series of broad peaks centred at 2θ = 4.7, 8.3, 12.6 and 27.0° (associated with the (100), ( 200), ( 210) and (001) crystal planes, respectively; 2θ, the angle between the transmitted beam and reflected beam), consistent with the diffraction pattern reported for the parent TpPa-1 structure 28 (Supplementary Fig. 7a).Accordingly, we propose that the Sc-COFs crystallize in the hexagonal space group P6/m with eclipsed stacking of the COF sheets 28 , as previously established for TpPa-1.The π-π stacking distance between the layers in the Sc-COFs was determined to be ~3.4Å, based on the d-spacing between the (001) planes 28 .We note that the diffraction peaks for the Sc-COF frameworks are quite broad, which may indicate the presence of an amorphous phase or simply a sample composed of very small crystallites 33 .Based on our electron microscopy data (given in the following sections), the crystallite size is on the order of a few nanometres, and thus we hypothesize that the broadening is due in large part to crystalline domain size.
Under identical experimental conditions, the characteristic diffraction peak of the (100) plane at 4.7° was chosen to evaluate the relative crystallinity of the Sc-COFs relative to a sample of TpPa-1.materials, covalent organic frameworks (COFs) 17 have garnered particular interest, due to their low densities, high porosities and chemical diversity.For example, two-and three-dimensional COFs featuring carboxylate 18,19 , thioether 20,21 and ethylenediaminetetraacetic acid functional groups 22 have been shown to capture mercury, cobalt and the lanthanides with a high selectivity.In these materials, the capture functionality is installed post-synthetically at one of the organic building units or introduced as part of a primary building unit prior to framework synthesis.The same synthetic approaches have also been used in the design of so-called metal-COFs, which have attracted interest for applications such as catalysis, conductivity and molecular separations because they combine the electronic diversity accessible in metal-organic frameworks with the robustness of a structure buttressed purely by covalent bonds 23 .These hybrid materials have predominantly been generated via post-synthetic metalation, with the exception of two-dimensional variants prepared with metalated porphyrin or phthalocyanine monomers 23,24 .
Inspired by this emergent chemistry and the complementary concept of molecular imprinting in porous materials [25][26][27] , we envisioned a different approach for the preparation of metal-COFs that uses metal coordination complexes as secondary building units.Such an approach should enable fine control over the extent of the metal site incorporation, while also providing coordination sites selective for a given metal.Herein we describe the design and synthesis of a family of COFs based on the parent material TpPa-1 (ref.28) that feature varying quantities of Sc 3+ coordination units (Fig. 1).The scandium ions can be released from these Sc-COFs to yield organic structures with 'imprinted' metal coordination sites that, to our knowledge, have not previously been accessed in another porous material.The imprinted framework with the highest density of coordination sites is highly selective for the uptake of Sc 3+ over a number of other competing metal ions and is capable of extracting 98% of the scandium ions present in a nickel mineral sample with numerous competing metal ions at a pH of ~3.Notably, it is not necessary to use Sc 3+ as the templating ion, and frameworks synthesized using secondary building units based on abundant divalent transition metal ions also yield metal-imprinted COFs (MICOFs) that are highly selective for scandium(III).The approach presented here affords a powerful means of designing diverse metal-covalent organic frameworks and imprinted COFs for selective metal ion capture.

Synthesis and characterization of Sc-COFs and MICOFs
The robust, two-dimensional COF TpPa-1 served as our model structure for the design of a porous organic framework featuring Sc 3+ binding pockets 28 .The structure is built of repeating keto-enamine salicylideneaniline units and forms following the irreversible tautomerization of the enol-imine precursor synthesized from 1,3,5-triformylphloroglucinol and p-phenylenediamine (Fig. 1a).We identified 4-aminophenylacetate as a suitable ligand for forming a trigonally symmetric, six-coordinate scandium complex that would be used as a secondary building unit for the hybrid COF (Fig. 1b) 29,30 .Based on energy minimization calculations, the proposed complex Sc(C 8 H 8 NO 2 ) 3 features Sc 3+ in a distorted octahedral environment and has approximately the same size as the TpPa-1 repeat unit, with pendant amine groups suitable for the formation of an extended TpPa-1-type structure (Fig. 1c).
The complex was synthesized from the reaction of ScCl 3 •6H 2 O with 3 equiv.of 4-aminophenylacetic acid in a 4:1 (v/v) mixture of N,N′-dimethylformamide and water and isolated as a light yellow powder (Methods).The Fourier transform infrared (FT-IR) spectrum of the product features absorption bands at 1,540 and 1,455 cm −1 that are redshifted from the -C=O stretch of the free ligand (1,648 cm −1 ) and were assigned to the asymmetric and symmetric -COO stretches of a scandium-bound carboxylate 29 , respectively, while a new band at 508 cm −1 was assigned to a Sc-O vibration 29 , supporting coordination of the carboxylate ligand to scandium (Supplementary Fig. 1).

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https://doi.org/10.1038/s41557-023-01273-3 As the scandium complex content is increased from 0 to 33%, the relative intensity of the (100) peak decreases by only approximately 15%, while there is a clear peak shift and reduction in intensity when the scandium incorporation reaches 43%.The apparent structural degradation may be attributed in part to limited stability of the molecular scandium complex under the solvothermal conditions (Supplementary Fig. 9).However, it is also possible that the TpPa-1 structure is simply not stable to incorporation of scandium beyond ~33%, given that the inorganic building unit lacks the rigidity of the TpPa-1 repeating unit.Attempts to use tris(4-aminobenzoate) scandium(III) (without a methylene bridge), 4-aminophenylacetate in the absence of Sc 3+ , or larger 1,3,5-tris(4-aminobiphenyl) benzene as secondary building units in the construction of TpPa-1-type structures resulted in only amorphous materials, highlighting the importance of the size, tailored flexibility and complex stability for framework synthesis (Supplementary Fig. 7).
Brunauer-Emmett-Teller surface areas of 676, 629, 581, 516, 479 and 436 m 2 g -1 were determined for TpPa-1, Sc-COF-9, Sc-COF-17, Sc-COF-23, Sc-COF-29 and Sc-COF-33, respectively, from N 2 adsorption data collected at 77 K (Fig. 2a and Supplementary Figs. 10 and 11).Framework pore size distribution was determined using non-local density functional theory 34 , which revealed a peak at ~1.5 nm (Fig. 2b) in good agreement with the expected structure of TpPa-1.Scanning electron microscopy characterization of Sc-COF-33 revealed aggregated particles with sizes ranging from 1 to 5 μm (Supplementary Fig. 12), while transmission electron microscopy (TEM) revealed a porous structure with a pore size of 1.5 nm (Fig. 2e), consistent with the results of the pore size distribution analysis.The presence of scandium was confirmed by using aberration-corrected scanning TEM (STEM; Fig. 2f and Supplementary Fig. 13).The high-angle annular dark-field STEM image of Sc-COF-33 features a multitude of bright dots corresponding to scandium ions that are well dispersed in the covalent matrix.The diameter of the dots is in the range of ~3 Å, suggesting that each bright dot corresponds to one individual Sc 3+ ion.An expanded view of a portion of this image further suggests that each dot is embedded in the ordered framework structure and that there is no scandium cluster formation (Fig. 2f).
The Sc-COFs were treated with acid and base (Methods) to release the scandium ions 35 and generate framework materials featuring open coordination sites, referred to as MICOF-9 through MICOF-33.The Brunauer-Emmett-Teller surface areas of these materials are slightly greater than the parent COFs, as expected upon removal of the scandium ions, while the narrow pore size distributions (Fig. 2c,d) suggest

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https://doi.org/10.1038/s41557-023-01273-3 that they retain crystallinity and the TpPa-1 structure.Indeed, the powder X-ray diffraction pattern of MICOF-33 is indistinguishable from that of Sc-COF-33 (Supplementary Fig. 14), indicating that π-π interactions between adjacent layers are sufficient to stabilize the structure upon removal of the scandium ions.Elemental analysis of MICOF-33 revealed that less than 0.1% scandium remains in the structure after acid/base treatment (Supplementary Table 2), and thermogravimetric analysis revealed that the MICOFs are stable up to 250 °C (Supplementary Fig. 16).

Scandium uptake in MICOF-33
Scandium(III) adsorption data were collected at 298 K for MICOF-33 exposed to concentrations ranging from 2 to 500 ppm (pH, ~5.5).The resulting adsorption isotherm (Fig. 3a) features an initial steep rise, indicating a strong affinity between the framework and Sc 3+ ions, followed by a gradual plateau.At the highest examined Sc 3+ concentration (500 ppm), the framework equilibrium capacity is 52.7 mg g -1 .The uptake data were fit with a Langmuir model (Fig. 3a, inset, and Supplementary Section 3.1), yielding a saturation capacity of 52.7 mg g -1 , which surpasses that of a number of reported scandium(III) adsorbents 14,[36][37][38] (Supplementary Table 7).X-ray photoelectron spectroscopy characterization of MICOF-33 following scandium exposure revealed a Sc 2p peak with a binding energy identical to that of Sc-COF-33 and the scandium complex, confirming the successful uptake of Sc 3+ at the vacant coordination sites (Supplementary Fig. 17).For the lowest initial Sc 3+ concentration (2 ppm), 99.5% of the scandium was adsorbed after 48 h, corresponding to a large K d of 1.01 × 10 6 ml g -1 .MICOF-33 also exhibits rapid Sc 3+ adsorption kinetics, as seen in Fig. 3b.Rapid metal ion uptake occurs in the first 5 min before beginning to plateau at ~10 min, and the framework achieves 92% of its saturation capacity (48.6 mg g -1 ) after 180 min.Depending on source, composition and texture of a given mineral, many possible procedures may be required for extracting pure metals, including ore pretreating, leaching and solvent extraction 39 .Acidic leaching is a common process used to separate metal elements from mine tailings 39 , and therefore it is highly desirable to realize an adsorbent capable of extracting scandium during the leaching stage.The uptake of Sc 3+ in MICOF-33 and the framework stability were accordingly examined under varying concentrations of HCl (Fig. 3c).The capacity of Sc 3+ decreases by less than 50% upon increasing the  3 for details), yielding a saturation capacity of 52.7 mg g -1 .R 2 , correlation coefficient; C e , ion concentration at equilibrium; q e , ion sorption capacity at equilibrium.b, Scandium(III) uptake as a function of time (t) in MICOF-33 after exposure to a 20 ppm solution of aqueous scandium(III) chloride hexahydrate (pH, ~5.5).The inset show the fit of the timedependent uptake data to a pseudo-second-order model (k 2 = 0.0124 g mg -1 min -1 , R 2 = 0.9997; Supplementary Section 3.2 and Supplementary Table 4).q t , ion sorption capacity at time t.c, Time-dependent scandium(III) uptake in MICOF-33 as a function of HCl concentration.d, Cycling data for scandium(III) uptake in MICOF-33 from a 20 ppm aqueous solution (pH, ~5.5).Over the course of ten cycles, the capacity decreases by only 1.5%.Error bars in all panels represent the coefficient of variation obtained from three independent experiments.

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https://doi.org/10.1038/s41557-023-01273-3acid concentration by 30,000-fold, and even at the highest examined concentration of 10 M, the capacity remains moderate at 6.0 mg g -1 , exceeding the capacities of a number of scandium(III) adsorbents in the literature at higher pH values (Supplementary Table 7).Notably, MICOF-33 is also stable to repeated Sc 3+ adsorption/desorption cycling at pH ~5.5 and exhibits a drop in capacity of only 0.7 mg g -1 (1.5%) after ten cycles (Fig. 3d).

Scandium ion selectivity
The selectivity of MICOF-33 for scandium(III) was examined by exposing a sample of the framework to an aqueous solution of HCl (pH, ~3) containing 20 ppm of the Sc 3+ ion and 10 ppm of a number of competing metal ions (Na + , K + , Mg 2+ , Mn 2+ , Fe 2+ , Cu 2+ , Co 2+ , Ni 2+ , Zn 2+ , Cd 2+ , Al 3+ , Cr 3+ , Y 3+ and La 3+ ; Supplementary Section 3.6).Importantly, the framework exhibits excellent selectivity for Sc 3+ over all other examined metal ions (Fig. 4a), with selectivity coefficients (adsorbed mass ratios) ranging from 1.43 × 10 4 (Sc 3+ /K + ) to 1.10 × 10 2 (Sc 3+ /Fe 2+ ; Supplementary Table 6).The separation of iron(III) and scandium(III) is particularly challenging, given the similarity of their ionic radii and electronegativities 40 , and MICOF-33 was also found to selectively capture Sc 3+ over Fe 3+ with a selectivity coefficient of 43.Of note, a control material with Tp (1,3,5-triformylphloroglucinol), Pa (p-phenylenediamine) and 33% 4-aminophenylacetic acid as building units adsorbed only 4.1 mg g -1 scandium(III) and exhibited far lower scandium selectivities ranging from 41.0 (Sc 3+ /Na + ) to 1.20 (Sc 3+ /Al 3+ ; Supplementary Fig. 18 and Supplementary Section 1.2 for details), highlighting the critical role of ion imprinting to generate an adsorbent selective for Sc 3+ .To our knowledge, the selectivity of MICOF-33 for Sc 3+ over the common impurity ions La 3+ (Fig. 4b and Supplementary Table 7) and Fe 3+ (Supplementary Table 8) surpasses that of Sc 3+ adsorbents reported in the literature.Substantially, it is not necessary to use scandium(III) as the template ion in the formation of scandium-selective MICOF-33.Indeed, it was possible to synthesize the framework instead using Ni 2+ , Mg 2+ or Zn 2+ complexes with 4-aminophenylacetate as secondary building units following the same synthetic protocol (Supplementary Section 7).MICOF-33 samples prepared in this way exhibit similar scandium(III) uptake to the MICOF-33 sample prepared using Sc 3+ as a template and retain high selectivity for scandium over Ni 2+ , Mg 2+ or Zn 2+ .
To elucidate the ultra-high selectivity of MICOF-33 for Sc 3+ , density functional theory calculations were carried out to investigate the coordination environment of several different metal ions in neutral model complexes of the type M(C 8 H 7 O 2 ) 3 (Supplementary Section 5 for details).Calculations were implemented using Gaussian 16 code 41 with B3LYP/LANL2DZ+ empirical dispersion correction GD3 (ref.42).The calculated Sc(C 8 H 7 O 2 ) 3 bond energy of 1.30 × 10 3 kJ mol -1 (representing the adsorption energy at one site in the MICOF structure) is larger than that determined for various transition metal ions, Mg 2+ and Al 3+ (0.51 to 1.17 × 10 3 kJ mol -1 ; Supplementary Figs.22 and 23 and Supplementary Table 10), and likewise the approximated stability constant for the model complex with Sc 3+ is larger than for model compounds with the latter ions (Supplementary Section 5.1 and Supplementary Table 11), consistent with the selectivity of MICOF-33 for Sc 3+ over these metal ions.Interestingly, the calculated bond energies for the Y 3+ and La 3+ complexes are competitive with that calculated for Sc 3+ (and their estimated stability constants are larger), but uptake of these ions in MICOF-33 is far more sluggish: after 180 min, MICOF-33 achieves only 5.6% and 2.4% of its theoretical adsorption capacity for Y 3+ and La 3+ , respectively (Supplementary Table 12; compare with 91% in the case of Sc 3+ ).Time-dependent uptake data for Y 3+ and La 3+ were fit using a pseudo-second-order model (Supplementary Figs.24 and 25) to yield k 2 values of 0.00134 and 0.00132 g mg -1 min -1 , respectively, which are approximately an order of magnitude lower than that determined for Sc 3+ (0.0124 g mg -1 min -1 ).Based on these data, it is likely that the larger ionic radii of these ions relative to the hydrated Sc 3+ ion impede their facile access to the coordination cavities in the framework (also Supplementary Fig. 21).The results of our density functional theory calculations also serve to provide some rationale for the fact that samples of MICOF-33 prepared with different templating ions are still selective for scandium(III).In particular, in the calculated model compounds with Ni 2+ , Zn 2+ and Mg 2+ , the ligand C-CH 2 -C angles are 119.3°,118.7° and 118.5°, respectively, whereas the corresponding angle for the Sc 3+ compound was calculated to be 128.5°,which is much closer to the C-N-C angle of 127.5° in the optimized structure of the TpPa-1 molecular unit (Supplementary Figs.19 and 20 and Supplementary Table 10).Thus, while molecular precursors with other metal cations may enable synthesis of the metal-loaded COF and MICOF-33, when competing metal ions are reintroduced, those most likely to be taken up are those that will lead to the most stable COF structure, in this case, Sc 3+ .

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https://doi.org/10.1038/s41557-023-01273-3 a filtrate with a scandium(III) purity greater than 96%, as determined by ICP-MS (96.84%Sc 3+ , 2.55% Fe 3+ , 0.40% Al 3+ , 0.21% Mn 2+ ).Notably, by carrying out five successive metal ion extraction and scrubbing operations using a 10 mg sample of MICOF-33 (Methods), it was possible to achieve a total of 423.7 μg of Sc 3+ with a purity of 99.90%, suitable for applications in lighting and high-powered lasers 43,44 .From a preliminary sample calculation considering materials costs alone, based on these results, the cost to extract one kilogram of Sc 3+ with 99.90% purity using MICOF-33 could be as low as approximately US$116 (Supplementary Table 9).

Conclusions
We have shown that the use of a specifically tailored Sc 3+ coordination complex as a secondary building unit in the synthesis of the robust, two-dimensional framework TpPa-1 (ref.28) yields stable metal-COFs that can be further treated to generate metal-imprinted frameworks selective for scandium ion capture.The material with the highest number of metal ion binding pockets, MICOF-33, exhibits excellent Sc 3+ capacities, selectivities and cycling stability under acidic conditions, and it can be prepared from inexpensive, abundant transition metal ions, rendering it a promising candidate for practical use in scandium separation and purification from traditional mineral sources, as well as other scandium sources of interest such as electronics waste and bauxite residue 8 .More broadly, we envision the tunable synthetic approach presented here will serve as a powerful and generalizable route towards the synthesis of a class of hybrid metal-COFs and MICOFs tailored with coordination sites selective for different metal ions for diverse applications.

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Fig. 1 |
Fig. 1 | Design strategy and synthesis of the Sc-COFs and MICOFs.a, The parent COF structure chosen for this study, TpPa-1, is constructed from repeating keto-enamine salicylideneaniline units.b, To mimic the size, functionality and geometry of the TpPa-1 building units for the design of a hybrid Sc-COF, 4-aminophenylacetate was chosen as a ligand for the design of trigonal scandium complex secondary building units.DMF, N,N-dimethylformamide.c, Sc-COFs were prepared with varying degrees (X = 9, 17, 23, 29, 33%) of scandium ion incorporation.DMAC, N,N-dimethylacetamide.The Sc 3+ ions can subsequently be removed to generate MICOFs with open coordination sites (enlarged view).All illustrations of the COF fragments/networks are models generated using Materials Studio.Scandium, oxygen, nitrogen and carbon atoms are represented as dark yellow, red, blue and grey spheres, respectively.Hydrogen atoms are omitted for clarity.

Fig. 2 |
Fig. 2 | Characterization of Sc-COF-33.a-d, Experimental and theoretical Brunauer-Emmett-Teller (BET) surface areas (a,c) and pore size distributions (b,d) for the Sc-COFs (a,b) and MICOFs (c,d).The theoretical surface areas were determined starting from the Brunauer-Emmett-Teller surface area of TpPa-1 and calculating the change in mass and specific surface area upon replacing the COF structural unit with the varying quantities of the scandium(III) complex.On the right, illustrations of a portion of the Sc-COF (b) and MICOF (d) networks are shown as generated in Materials Studio.Scandium, oxygen, nitrogen and carbon atoms are represented as dark yellow, red, blue and grey spheres, respectively.Hydrogen atoms are omitted for clarity.e, TEM image of Sc-COF-33.f, High-angle annular dark-field STEM image of Sc-COF-33, where the scandium ions can be visualized as white dots; select scandium positions are highlighted with blue circles in the zoomed-in view.

1 )Fig. 3 |
Fig.3| Scandium(III) uptake in MICOF-33.a, Scandium(III) adsorption isotherm collected for MICOF-33 at 298 K upon exposure to solutions of scandium(III) chloride hexahydrate dissolved in aqueous HCl (pH, ~5.5) with initial concentrations ranging from 2 to 500 ppm.The inset shows equilibrium adsorption data and a fit using a Langmuir model (R 2 = 0.9998; Supplementary Section 3.1 and Supplementary Table3for details), yielding a saturation capacity of 52.7 mg g -1 .R 2 , correlation coefficient; C e , ion concentration at equilibrium; q e , ion sorption capacity at equilibrium.b, Scandium(III) uptake as a function of time (t) in MICOF-33 after exposure to a 20 ppm solution of aqueous

Fig. 4 |
Fig.4| Scandium(III) uptake selectivity.a, Comparison of the equilibrium uptake of Sc 3+ and various other metal ions in MICOF-33, highlighting the exceptional selectivity of the framework for scandium(III).b, Sc 3+ /La 3+ selectivity of MICOF-33 and reported adsorbents for scandium(III) capture.Supplementary Table7shows a full list of adsorbents by number, references and associated adsorption metrics.Error bars represent the standard deviation obtained from three independent experiments.