Synthesis and characterization of Sc–COFs and metal-imprinted COFs. The robust, two-dimensional covalent organic framework TpPa-1 served as our model structure for the design of a porous organic framework featuring Sc3+ binding pockets28. 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 the scandium-containing secondary building units, given that Sc3+ readily forms trigonally-symmetric complexes with carboxylates29, and further that energy minimization calculations indicated Sc(C8H8NO2)3 to be 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 Sc(C8H8NO2)3 complex was synthesized from the reaction of ScCl3·6H2O with 3 equiv. of 4-aminophenylacetic in a 4:1 (v:v) mixture of N,N′-dimethylformamide and water (see Methods). The Fourier transform-infrared spectrum of Sc(C8H8NO2)3 features an absorption band at 1615 cm−1 that is redshifted from the –COOH stretch of the free ligand (1634 cm−1) and was assigned to the asymmetric –COO stretch of a scandium-bound carboxylate (Supplementary Fig. 1)29, while a new band at 520 cm−1 was assigned to a Sc–O vibration29. X-ray photoelectron spectra collected for Sc(C8H8NO2)3 and the known compound Sc(O2CC11H23)3 (O2CC11H23− = laurate)29 both feature a single Sc2p peak at ~400 eV, confirming the presence of scandium(III) in similar coordination environments (Supplementary Fig. 2).
Scandium-loaded covalent organic frameworks were prepared via the solvothermal reaction of Sc(C8H8NO2)3 with 1,3,5-triformylphloroglucinol and p-phenylenediamine in a mixture of mesitylene, dioxane, and aqueous acetic acid. 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; see Methods), referred to as Sc–COF-9 through Sc–COF-43. The occurrence of a Schiff base reaction in all cases and incorporation of Sc3+ was verified by infrared, X-ray photoelectron, and solid-state 13C CP/MAS NMR spectroscopies, as well as by elemental analysis (Supplementary Figs. 2–4 and Table 1). 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 center adopts a distorted octahedral geometry with an average Sc–O bond length of 2.18 Å30,31 and a C–CH2–C angle of 128.5° (Fig. 1c).
In the Sc–COF IR spectra, the characteristic –NH bands of p-phenylenediamine (3200–3500 cm−1) are absent, while new bands associated with –C=C and –C–N vibrations are present at 1578 and 1255 cm−1 (Supplementary Fig. 3). All spectra additionally feature bands at 520 and 1616 cm−1, assigned to a Sc–O vibration and the –COO stretch of a scandium-bound carboxylate, respectively. XPS characterization of Sc–COF-33 revealed a Sc2p peak with an energy of ~400 eV identical to that of Sc(C8H8NO2)3 (Supplementary Fig. 2), confirming the presence of six-coordinate scandium(III) in the extended material. Finally, all peaks in the solid-state 13C 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. 4).
Powder X-ray diffraction data for Sc–COF-9 through Sc–COF-33 revealed that the structures crystallize in the P6/m space group, retaining the symmetry of the parent TpPa-1 structure28 (Supplementary Fig. 5a). Diffraction peaks centered at 2q = 4.7, 8.3, 12.6, and 27.0° are associated with the (100), (110), (210), and (001) crystal planes, respectively. The π-π stacking distance between the layers in the Sc–COFs was determined to be ~3.4 Å, based on the d spacing between the (001) planes28. 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. As the scandium complex content is increased from 0 to 33%, the relative intensity of the (100) peak remains unchanged, but 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. 6). However, it is likely 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, 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. 5).
Brunauer–Emmett–Teller (BET) surface areas of 660, 616, 570, 522, 485, and 431 m2/g were determined for TpPa-1, and Sc–COF-9 through Sc–COF-33, respectively, from N2 adsorption data collected at 77 K (Fig. 2a; Supplementary Figs. 7 and 8). Framework pore size distribution was determined using nonlocal density functional theory,32 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. 9), while transmission electron microscopy images reveal a highly ordered structure with 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 transmission electron microscopy (STEM) (Fig. 2f). The high-angle annular dark field (HAADF)-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 2–3 Å, suggesting that each bright dot corresponds to one individual Sc3+ 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.
The Sc–COFs were next treated with acid and base (see Methods) to release the scandium ions33 and generate framework materials featuring open coordination sites, referred to as MICOF-9 through MICOF-33. The BET 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 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. 10), 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 1), and thermogravimetric analysis revealed that the MICOFs are stable up to 250 °C (Supplementary Fig. 12).
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 Sc3+ ions, followed by a gradual plateau. At the highest examined Sc3+ concentration (500 ppm), the framework equilibrium capacity is 58.5 mg/g, corresponding to occupation of ~92% of the expected adsorption sites. The uptake data were fit with a Langmuir model (Fig. 3a, inset; see the Supplementary Information), yielding a saturation capacity of 58.9 mg/g that surpasses a number of reported scandium(III) adsorbents14,34–36 (Supplementary Table 5). XPS characterization of MICOF-33 following scandium exposure revealed a Sc2p peak with a binding energy identical to that of Sc–COF-33 and the scandium complex, confirming the successful uptake of Sc3+ at the vacant coordination sites (Supplementary Fig. 12). For the lowest initial Sc3+ concentration (2 ppm), 99.5% of the scandium was adsorbed after 48 h, corresponding to a large Kd of 2.01×106 mL/g. MICOF-33 also exhibits rapid Sc3+ 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 82% of its saturation capacity (48.6 mg/g) after 180 min.
Depending on source, composition, and texture of a given mineral, there are many possible procedures that may be required for extracting pure metals, including ore pretreating, leaching, and solvent extraction37. Acidic leaching is a common process used to separate metal elements from mine tailings37, and therefore it is highly desirable to realize an adsorbent capable of extracting scandium during the leaching stage. The uptake of Sc3+ in MICOF-33 and framework stability were accordingly examined under varying concentrations of HCl (Fig. 3c). The capacity Sc3+ decreases by less than 50% upon increasing the acid concentration by 100,000-fold, and even at the highest examined concentration of 10 M, the capacity remains moderate at 6.1 mg/g, exceeding the capacities of a number of scandium(III) adsorbents in the literature at higher pH values (Supplementary Table 5). Notably, MICOF-33 is also stable to repeated Sc3+ adsorption/desorption cycling at pH ~ 5.5 and exhibits a drop in capacity of only 0.7 mg/g (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 Sc3+ ion and 10 ppm of a number of competing metal ions (Na+, K+, Mg2+, Mn2+, Fe2+, Co2+, Ni2+, Zn2+, Cd2+, Al3+, Cr3+, Fe3+, Y3+, and La3+; see the Supplementary Information). Importantly, the framework exhibits excellent selectivity for Sc3+ over all other examined metal ions (Fig. 4a), with selectivity coefficients (adsorbed mass ratios) ranging from 1.8×105 (Sc3+/K+) to 1.1×102 (Sc3+/Fe2+). The separation of iron(III) and scandium(III) is particularly challenging, given the similarity of their ionic radii and electronegativities38, and MICOF-33 was also found to selectively capture Sc3+ over Fe3+ with a selectivity coefficient of 42. Of note, a control material with Tp, Pa, and 33% 4-aminophenylacetic acid as building units absorbed only 4.1 mg/g scandium(III) and exhibited far lower scandium selectivities ranging from 10.3 (Sc3+/K+) to 1.2 (Sc3+/Al3+) (Supplementary Fig. 14), highlighting critical role of ion imprinting to generate an adsorbent selective for Sc3+. To our knowledge, the selectivity of MICOF-33 for Sc3+ over the common impurity ions La3+ (Fig. 4b and Supplementary Table 5) and Fe3+ (Supplementary Table 6) surpasses that of all Sc3+ adsorbents reported in the literature.
Significantly, 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 Ni2+, Mg2+, or Zn2+ complexes with 4-aminophenylacetate as secondary building units following the same synthetic protocol (see Supplementary Information). MICOF-33 samples prepared in this way exhibit similar scandium(III) uptake to the original MICOF-33 sample and retain high selectivity for scandium over Ni2+, Mg2+, or Zn2+.
In order to elucidate the exceptional selectivity of MICOF-33 for Sc3+, density functional theory (DFT) calculations were carried out to investigate the coordination environment of several different metal ions in a model complex featuring three 4-aminophenylacetate ligands. Calculations were implemented using Gaussian 16 code42 with B3LYP/LANL2DZ+ empirical dispersion correction GD343. The calculated Sc3+–acetate bond energy of 1.39×103 kJ/mol is larger than that determined for various transition metal ions, Mg2+, and Al3+ (0.54 to 1.21×103 kJ/mol; see Supplementary Fig. 19 and Supplementary Table 7), consistent with the selectivity of MICOF-33 for Sc3+ over these metal ions. The calculated bond energies for Y3+ and La3+ are competitive with that for Sc3+, but uptake of these ions in MICOF-33 is far more sluggish: after 180 min, MICOF-33 achieves only 5.7% and 2.5% of its theoretical adsorption capacity for Y3+ and La3+, respectively (Supplementary Table 8; c.f. 82% in the case of Sc3+). Time-dependent uptake data for Y3+ and La3+ were fit using a pseudo-second order model (Supplementary Figs. 15 and 16) to yield k2 values of 0.009 and 0.013 g/(mg∙min), respectively, which are over an order of magnitude lower than that determined for Sc3+ (0.27 g/(mg∙min)). It is likely that the larger ionic radii of these ions relative to Sc3+ impedes facile access to the coordination cavities in the framework.
Scandium extraction from minerals. A sample of nickel sulfide ore collected from Jilin, China was acidified (pH ~ 3) and treated with MICOF-33 to investigate the scandium(III) uptake properties of the framework from a real-world sample. After leaching, the initial metal ion solution was determined to contain Sc3+ (4.6 ppm), Al3+ (3.4 ppm), Fe3+ (1.2 ppm), Mg2+ (6.5 ppm), Ca2+ (222.1 ppm), Mn2+ (7.1 ppm), Co2+ (18.4 ppm), Ni2+ (3.5 ppm), Cu2+ (2.9 ppm), Zn2+ (1.7 ppm), Na+ (2528.9 ppm), and K+ (29.1 ppm) by ICP-MS. After 180 min, the MICOF-33 had adsorbed 98% of the Sc3+ present in the solution (corresponding to a capacity of 43.1 mg/g). Liberation of the captured scandium from the resulting sample using aqua regia yielded a filtrate with a scandium(III) purity greater than 96%, as determined by ICP-MS (96.84% Sc3+, 2.55% Fe3+, 0.40% Al3+, 0.21% Mn2+). Significantly, by carrying out five successive metal ion extraction and scrubbing operations (see Methods), it was possible to achieve a final Sc3+ purity as high as 99.90%, suitable for applications in lighting and high-powered lasers44,45.