Site-Enriched Quinoline Covalent Organic Frameworks for Selective Palladium Recovery under Acidic Radioactive Media

Abstract

Palladium, as a member of the precious platinum group metals and a strategic resource, exhibits uneven global distribution and price volatility. This has sparked significant interest in its efficient recovery technologies. To tackle the challenge of highly selective Pd²⁺ recovery and Pt/Pd separation under extreme conditions, we developed a covalent organic framework (COF) adsorbent enriched with pyridinic nitrogen and carboxyl groups, synthesized via one-pot Povarov and Doebner multicomponent reactions. Intriguingly, the incorporation of multiple binding sites within the COF framework significantly enhances Pd²⁺ interactions, enabling rapid recognition and efficient Pd²⁺ extraction under highly acidic and simulated high-level radioactive conditions. The as-synthesized site-enriched COF (PQCOF-2) exhibits the highest Pd²⁺ adsorption capacity of 376 mg g^–1 in 3 M HNO₃ and achieves a Pt/Pd separation factor of 4.10. Further, PQCOF-2 showed an exceptional Pd²⁺ distribution coefficient (K_d = 6.79 × 10⁵ mL g^–1) and removal efficiency exceeding 99% in simulated high-level radioactive waste solutions, together with notable recovery ability. In brief, this work provides a versatile example for developing advanced adsorbents aimed at capturing radioactive metal ions.

1. Introduction

Nuclear energy has become a cornerstone of the global transition to low-carbon power, − yet its rapid expansion generates large volumes of high-level radioactive waste (HLW), whose safe management remains a formidable challenge. − The international atomic energy agency (IAEA) underscores that reprocessing of HLW is essential to reduce, stabilize, and partially recycle radioactive constituents, thereby ensuring the long-term sustainability of nuclear power. Current strategies typically involve selective preadsorption of radionuclides followed by vitrification and geological disposal, but remain costly and vulnerable to radionuclide migration. − Alternatively, HLW contains critical metallic resources-including palladium, rhodium, cesium, and strontium-that risk being permanently discarded under the current approach. , This creates a strong impetus to convert high-level radioactive waste into recoverable resources through various approaches, such as selective adsorption. , Moreover, the challenging conditions associated with HLW-such as a highly acidic environment (3 to 6 M HNO₃), high concentrations of fission products, and intense radiation often lead to adsorbent instability, causing degradation of functional groups and structural collapse. − These challenges demand next-generation adsorbents combining abilities including high selectivity, acidic and radiative resistance, facile regeneration, as well as scalability to enable both nuclear safety and sustainable resource recycling. −

Covalent organic frameworks (COFs) have emerged as a promising platform for selective ion capture, owing to their ordered porosity, tunable binding sites, and chemical robustness. , Despite these advantages, their application in precious-metal recovery remains unevenly explored. Substantial progress has been made in the application of COFs to the immobilization of highly mobile radionuclides and the recovery of valuable metals from electronic waste. , Their well-defined nanochannels facilitate rapid ion diffusion and efficient mass transport, while enabling the precise integration of tailored coordination environments. , Therefore, well-defined coordination environments can be designed and incorporated into COFs, serving as molecular nanotraps that exhibit high affinity and selectivity toward target ions, while aromatic backbone structures further impart robust radiation resistance under HLW conditions. , However, although significant efforts have been dedicated to removing highly mobile radionuclides like technetium-99 , and iodine-129, − the selective capture of palladium (Pd)-one of the Fission Platinoids (FPs) generated during nuclear fission reaction remains an unresolved challenge. Efficient recovery of Pd is critical for two main reasons: its presence reduces the thermal stability of vitrified nuclear waste, while its extraction offers access to a high-value metal elements vital for catalysis, electronics, and energy applications. , Therefore, the development of porous adsorbents that combine site-specific selectivity and radiation resistance offers a promising approach supports the safe management of high-level radioactive waste.

To address the challenge of highly selective palladium recovery from HLW (Scheme a), a series of heteroatom-rich COFs were designed and synthesized through one-pot Povarov-Doebner reaction (Scheme b). For these materials, pyridinic nitrogen atoms act as strong chelating sites for Pd, while densely distributed carboxyl groups within the pores offer supplementary coordination centers. With the synergistically enhanced binding sites bearing within the crystalline framework, the aromatic backbones of these COF materials also allow for extreme conditions, leading to selective recognition and efficient adsorption even under the strong acidity and radiolytic conditions in HLW environments. Among four robust quinoline-linked COFs, PQCOF-2 demonstrated the highest Pd²⁺uptake capacity of 376 mg g^–1 in 3 M HNO₃ together with an exceptional Pd/Pt separation factor of 4.1. Further, under simulated HLW conditions, PQCOF-2 exhibited remarkable affinity for Pd²⁺, with a partition coefficient (K_d) of 6.79 × 10⁵ mL g^–1 and a removal efficiency exceeding 99%. Notably, PQCOF-2 showed excellent recyclability and its efficient regeneration via thiourea elution enabled stable performance over at least six consecutive cycles.

1. (a) Schematic Illustration of the COF Adsorbents Involved in the High-Level Radioactive Waste Treatment. (b) Chemical Structures and Synthetic Route toward PQCOF and DQCOF.

2. Results and Discussions

2.1. Structural Characterization

Typically, four structurally distinct multisite COFs were synthesized through two orthogonal, multicomponent one-pot strategies. To be specific, PQCOF-1 and PQCOF-2 were synthesized via a Povarov cycloaddition route, , whereas DQCOF-1 and DQCOF-2 were obtained through a Doebner-type condensation strategy. , The crystalline structures of the PQCOF and DQCOF series were elucidated using powder X-ray diffraction (Figure a). All four frameworks exhibited distinct low-angle reflections around 2θ ≈ 4.00°, indicative of long-range structural periodicity.

1.

(a) PXRD patterns of PQCOFs and DQCOFs. (b) PXRD patterns of PQCOF-2 and the simulated pattern for the eclipsed AA stacking mode. (c) Top and side view of PQCOF-2 AA stack mode. (d) FTIR spectra of PQCOF-2 and corresponding monomers. (e) N 1s spectra of PQCOF-2. (f) Nitrogen adsorption isotherms of PQCOF-2 at 77 K and pore-size distributions (inset). (g) ¹³C CP/MAS NMR spectra of PQCOF-2. (h) SEM images of PQCOF-2. (i) TEM image of PQCOF-2; the inset shows an enlarged view of the marked region, highlighting the local ordered porous structure and lattice features.

PQCOF-2 exhibited diffraction peaks at 3.99°, 6.98°, 7.90°, 10.50°, 13.75° and 25.62°, which can be assigned to the same crystallographic planes (Figure b). Similarly, PQCOF-1 showed peaks at 3.92°, 6.89°, 7.98°, 10.59°, 13.96° and 25.88°, corresponding to the (100), (210), (200), (310), (410) and (001) planes. DQCOF-1 and DQCOF-2 both exhibited reflections at ∼3.96°–3.98°, 6.83°–6.88°, 7.88°–7.90°, 10.49°–10.50° and 25.46°–25.61°, corresponding to the (100), (210), (200), (310) and (001) planes as well. To confirm the stacking model of PQCOF and DQCOF, structural models featuring two-dimensional monolayers with AA stacking were constructed and energy-minimized using Materials Studio. Simulated PXRD patterns derived from these models closely matched the experimental data, as demonstrated by top and side-view analyses, confirming an AA-stacked layered configuration for PQCOF-2 (Figure c). Pawley refinement of the PXRD data yielded unit cell parameters consistent with a hexagonal lattice. For PQCOF-1 and PQCOF-2, the refined unit cells were a = b = 25.81 Å, c = 3.49 Å, α = β = 90°, γ = 120°, with low residuals (PQCOF-1: Rwp = 5.53%, Rp = 4.33%; PQCOF-2: Rwp = 2.53%, Rp = 1.99%). Similarly, DQCOF-1 and DQCOF-2 exhibited nearly identical cell dimensions (a = b = 25.82 Å, c = 3.48 Å, α = β = 90°, γ = 120°), with refinement factors of Rwp = 2.86%, Rp = 2.32% and Rwp = 3.13%, Rp = 2.35%, respectively. The strong agreement between simulated and experimental patterns corroborates the high crystallinity and long-range structural order of both PQCOF and DQCOF frameworks (Figure S1). Further, the FTIR spectrum of PQCOF-2 shows the disappearance of the N–H stretching band (3150–3480 cm^–1) from 2,4,6-tris­(4-aminophenyl)-1,3,5-triazine (TAT) and the CO stretching vibration at 1693 cm^–1 from 1,3,5-tris­(2-formylpyridin-5-yl)­benzene (TFT), indicating complete monomer conversion. Concurrently, new absorption bands at 1700 cm^–1 and 1579 cm^–1 confirm the formation of quinoline rings and the successful integration of carboxylic acid groups into the framework (Figure d). Similar spectral features were also observed in the other COFs, with a characteristic band at ∼1700 cm^–1, confirming the retention of −COOH groups during polymerization. Additionally, a peak at 1597 cm^–1, corresponding to pyridine ring stretching within the quinoline moiety, further confirms the successful formation of quinoline-based COFs via the multicomponent one-pot strategy (Figure S2). ,

X-ray photoelectron spectroscopy (XPS) analysis affirmed the elemental composition of all four COFs, with survey spectra indicating the presence of carbon (C), nitrogen (N), and oxygen (O) (Figure S3). In specific, the high-resolution N 1s spectrum of PQCOF-2 exhibits two distinct peaks: the signal at 398.8 eV corresponds to triazine nitrogen from the TAT monomer, and the peak at 399.8 eV assigned to pyridinic nitrogen within the quinoline ring and pyridine nitrogen from the TFT monomer (Figure e). Similar N 1s signals were also observed in the remaining COFs, with binding energies indicative of pyridinic nitrogen species (Figure S4). Collectively, these data provide strong spectroscopic evidence for the successful formation of quinoline linkages within the COF frameworks. Nitrogen adsorption–desorption isotherms were measured at 77 K to evaluate the permanent porosity and specific surface area of the PQCOF series. Both samples exhibited type IV isotherms with pronounced hysteresis loops, indicative of mesoporous characteristics. PQCOF-2 exhibited a Brunauer–Emmett–Teller (BET) surface area of 1235 m² g^–1 and an average pore diameter of ∼1.18 nm (Figure f), while PQCOF-1 displayed a slightly higher surface area of 1270 m² g^–1 and a larger average pore size of ∼1.69 nm (Figure S5). For the DQCOFs, nitrogen adsorption–desorption isotherms measured at 77 K did not fully characterize their pore structures. Therefore, complementary carbon dioxide adsorption–desorption isotherms were measured at 298 K to gain further insight. BET analysis indicated that DQCOF-1 exhibits a specific surface area of 455 m² g^–1, consistent with previously reported values. In comparison, DQCOF-2 displayed a BET surface area of 202 m² g^–1 (Figure S6). The chemical stability of PQCOF-2 was assessed by immersing the material in 3 M NaOH, 3 M HNO₃ and 6 M HCl aqueous solutions. XRD and FTIR analyses confirmed that the framework structure and long-range order of PQCOF-2 were retained after immersion, with only a slight decrease in crystallinity (Figure S7). The surface wettability of the COFs was also analyzed by water contact angle measurements. PQCOF-2 exhibited a contact angle of 63.7°, while DQCOF-2 showed 72.3°, both values being markedly lower than those of PQCOF-1 (102.3°) and DQCOF-1 (125.9°) (Figure S8). These results indicate that PQCOF-2 has moderate hydrophilicity, rendering it suitable for adsorption applications in 3 M HNO₃. Further structural elucidation was achieved through solid-state ¹³C cross-polarization magic-angle spinning nuclear magnetic resonance (¹³C CP/MAS NMR) spectroscopy. In the PQCOF spectrum (Figures g and S9a), signals in the aromatic region (115–130 ppm) can be assigned to benzene rings. Notably, resonances at 156 and 149 ppm confirm the presence of quinoline moieties, strongly supporting the successful formation of the framework via the Povarov reaction. − For DQCOF (Figure S9b–c), the signal at 170 ppm is corresponds to carboxylic acid (−COOH) groups situated within the pore, while peaks at 156 and 149 ppm indicate the formation of quinoline linkages. These spectral features confirm the successful synthesis of quinoline-bridged COFs via the Doebner condensation route. ,

The morphologies of the four synthesized COFs were examined using scanning electron microscopy (SEM). As seen in Figure h, PQCOF-2 exhibited a well-defined honeycomb-like porous architecture, while PQCOF-1 and DQCOF-1 displayed rod-like structures consisting of aggregated spherical subunits. Notably, DQCOF-2 exhibited a morphology closely resembling that of PQCOF-2, indicating similar structures (Figure S10). Elemental mapping via energy-dispersive X-ray spectroscopy (EDS) confirmed the homogeneous distribution of carbon, nitrogen, and oxygen across the COF matrices, supporting the uniform incorporation of building units into the frameworks (Figure S11). High-resolution transmission electron microscopy (HRTEM) was employed to further characterize the nanostructure of PQCOF-2, revealing a well-ordered honeycomb-like porous morphology that aligns well with the findings from SEM analysis. Notably, distinct lattice fringes were clearly observed, demonstrating a high degree of crystallinity and long-range structural order in the framework (Figures i and S12).

2.2. Pd Adsorption Results

The adsorption performance of the four quinoline-linked COFs toward Pd²⁺ was systematically evaluated over a range of nitric acid concentrations (0.5–3.0 M) (Figure a). PQCOF-1 and DQCOF-1 exhibited insufficient adsorption efficiencies (<50%) with pronounced fluctuations under acidic conditions, suggesting structural instability. In contrast, PQCOF-2 and DQCOF-2 displayed good acid resistance, consistently achieving adsorption efficiencies >80% during test. Intriguingly, PQCOF-1 and DQCOF-1 exhibited relatively low Pd²⁺ adsorption efficiencies under strongly acidic conditions (<50%). This inferior performance is mainly attributed to their weaker effective binding affinity and less favorable pore microenvironment for Pd²⁺ recognition in concentrated HNO₃, rather than solely to framework instability. These findings affirm the inadequate correlation of acidity with adsorption efficiency for structurally robust PQCOF and DQCOF. Both PQCOF-2 and DQCOF-2 maintained excellent adsorption performance under highly corrosive conditions (3 M HNO₃), simulating the extreme environments typical of high-level radioactive waste. PQCOF-2 was chosen for further investigation given its exceptional acid stability and consistently high Pd²⁺ adsorption capacity. The adsorption performance of the COF materials toward Pd²⁺ was assessed via isothermal adsorption experiments (Figure b). At equilibrium, the maximum adsorption capacities of PQCOF-2, DQCOF-2, PQCOF-1 and DQCOF-1 were determined to be 376, 245, 145, and 127 mg g^–1, respectively. Among these, PQCOF-2 outperformed most reported adsorbents under comparable conditions (3.0 M HNO₃), highlighting its exceptional affinity for Pd²⁺ in strongly acidic media. The experimental isotherms were fitted using both Langmuir and Freundlich models to gain mechanistic insights into the adsorption behavior (Figures c and S13). As summarized in the accompanying table, the Langmuir model yielded superior correlation coefficients (R² > 0.99) for PQCOF-2 and DQCOF-2 (Table S1), suggesting that adsorption predominantly occurs via monolayer coverage on homogeneous surface sites.

2.

(a) Pd²⁺ removal by COFs at different concentrations of HNO₃. (b) Adsorption isotherms of four COFs toward Pd²⁺. (c) The adsorption isotherms of Pd²⁺ on PQCOF-2 were fitted by Langmuir models. (d) Time-dependent adsorption efficiencies of four COFs after the Pd²⁺ addition. (e) Pseudo-second-order Pd²⁺ adsorption kinetic plots of PQCOF-2. (f) The regeneration cycle of PQCOF-2 for the Pd capture at Pd²⁺ concentration of 50 ppm. (g) Comparison of the adsorption capacity of porous adsorbent materials for saturated Pd²⁺ at different nitric acid concentrations. (h) Selectivity of PQCOF-2 for different cations in simulated HLW solution. (i) Experimental column breakthrough curves for simulated high-level liquid waste solution by PQCOF-2.

Adsorption kinetics experiments demonstrated that PQCOF-2 achieved over 80% of its maximum Pd²⁺ uptake within the first 60 min and reached equilibrium after 12 h, with a final removal efficiency of 99.7%. In comparison, DQCOF-2 adsorbed over 50% of its capacity within 60 min and also reached equilibrium at 12 h, yielding a final removal efficiency of 88.9%. In contrast, PQCOF-1 and DQCOF-1 displayed significantly slower kinetics, with final Pd²⁺ removal efficiencies below 40% at equilibrium (Figure d).

The adsorption kinetic data are well-described by the pseudo-second-order model, as indicated by high correlation coefficients (R² > 0.99) (Figures e and S14), whereas the pseudo-first-order model yielded weaker correlations (R² < 0.9). The corresponding kinetic parameters are listed in Table S2, indicating that the better compatibility of the pseudo-second-order model with the adsorption kinetics of PQCOF-2 and DQCOF-2, which is in consistent with a chemisorption-controlled mechanism. These results underscore the superior adsorption kinetics of PQCOF-2, attributed to the synergistic interplay between its high surface area and densely distributed pyridinic nitrogen and carboxylic acid groups, which cooperatively facilitates rapid diffusion and binding of Pd²⁺ ions, supporting the material’s high-efficiency extraction capability. Alternatively, the recyclability of adsorption materials is crucial for cost-effectiveness and long-term sustainability in practical applications. To evaluate the regeneration performance of PQCOF-2, adsorbed Pd²⁺ was eluted using 0.1 M thiourea in 1 M HNO₃, enabling multiple adsorption–desorption cycles. PQCOF-2 retained nearly 100% of its initial adsorption efficiency after six consecutive cycles, demonstrating its reusability and structural stability under acidic regeneration conditions (Figure f). Compared to reported studies on the recovery of Pd²⁺ under highly acidic conditions, PQCOF-2 exhibits higher adsorption capacity than most examples (Figure g). ,,,−

Dynamic adsorption experiments were conducted using the setup shown in Figure S15, for evaluating the selective capture of Pd²⁺ from simulated high-level radioactive waste (3 M HNO₃) under flow conditions. The solution contained a low concentration of Pd²⁺ alongside excess competing ions, including Cs⁺, Ni²⁺, Sr²⁺, La³⁺, and UO₂ ²⁺ (Table S3). The calculated distribution coefficient (K_d) for Pd²⁺ was 6.79 × 10⁵ mL g^–1, with a removal efficiency of 99%, largely exceeding that of other coexisting metal ions (Figure h). Equilibrium was reached after processing 450 mL of solution (Figure i), yielding a dynamic adsorption capacity of 143.2 mg g^–1. The coexisting metal ions were rapidly eluted from the PQCOF-2-packed column, confirming the material’s preferential and highly selective binding of Pd²⁺. To assess the applicability of the quinoline-based COFs in high-level radioactive waste environments, the irradiation stability of PQCOF-2 and DQCOF-2 along with their postirradiation Pd²⁺ adsorption performance were both evaluated. After exposure to 400 kGy of high-dose β-ray irradiation for 12 h, PXRD analysis revealed a slight loss of crystallinity, while the overall crystal structures remained intact (Figure S16). FTIR spectra confirmed the preservation of quinoline linkages, indicating that the frameworks of both COFs are highly resistant to radiation-induced degradation. After irradiation, the maximum Pd²⁺ adsorption capacities of PQCOF-2 and DQCOF-2 were 347 mg g^–1 and 204 mg g^–1 (Figure S17), respectively, retaining over 80% of their preirradiation capacities. These results demonstrate that PQCOF-2 and DQCOF-2 maintained substantial adsorption performance under extreme irradiation conditions, highlighting their potential for Pd²⁺ recovery from high-level radioactive liquid waste. Given that practical applications typically involve dilute conditions, we further evaluated the adsorption performance of PQCOF-2 in 3 M HNO₃ at an initial Pd²⁺ concentration of 5 ppm. ICP-MS analysis shows that, after three adsorption cycles, the residual Pd concentration decreases to an average of 0.38 ppb, corresponding to >99% removal efficiency (Figure S18). These results demonstrate that PQCOF-2 maintains excellent adsorption performance under low-concentration conditions.

2.3. Pd–Pt Separation Results

In spite of the applicability to recover Pd from high-level radioactive waste, the primary sourcing of this metal element still relies on the ore mining, however, due to the extreme scarcity of natural Pd reserves, continued reliance on primary mineral sources is economically unsustainable. Therefore, the efficient separation and recovery of Pd from other secondary resources including spent catalysts, electronic waste, and nuclear waste streams is of critical economic and environmental importance. Nevertheless, palladium recovery from secondary waste sources continuously suffered from the coexistence of platinum and complicated by the nearly identical coordination chemistry of Pd and Pt, rendering their separation especially difficult.

To evaluate the Pt/Pd separation capability of PQCOF-2, adsorption selectivity experiments were performed using a binary solution containing equimolar concentrations (∼500 ppm) of Pd²⁺ and Pt⁴⁺. Specifically, 5 mg of PQCOF-2 was added to 10 mL of the mixed-metal solution and stirred for 24 h. After treatment, the Pd²⁺ concentration decreased from 500 to 343.8 ppm, while the Pt⁴⁺ concentration remained largely unchanged at 450.3 ppm (Table S4). The calculated distribution coefficients (K_d) were 909 mL g^–1 for Pd²⁺ and 221 mL g^–1 for Pt⁴⁺ (Figure a), indicating that PQCOF-2 exhibits more than 4-fold selectivity for palladium over platinum. This pronounced selectivity underscores the potential of PQCOF-2 for Pt/Pd separation from complex mixtures. Building on the batch adsorption results, the potential of PQCOF-2 for Pt/Pd separation was further validated through laboratory-scale dynamic adsorption experiments. The experimental setup is shown in Figure b, a binary mixture containing 50 ppm of both Pt⁴⁺ and Pd²⁺ was continuously passed through a 3 mL packed column loaded with 100 mg of PQCOF-2 at a flow rate of 0.5 mL min^–1 controlled by a syringe pump. PQCOF-2 exhibited efficient separation of the Pt/Pd mixture under flow conditions. ICP analysis confirmed that only Pt was detected in the eluted fractions collected during the initial 30 min, demonstrating complete retention of Pd throughout this period. High-purity Pt was recovered rapidly with no Pd breakthrough observed. Pd breakthrough occurred at approximately 40 min as the adsorbent approached saturation, leading to mixed fractions containing both metals. These dynamic breakthrough results further underscore the strong potential of PQCOF-2 for selective Pt/Pd separation, highlighting its promise for industrial-scale applications.

3.

(a) The selective adsorption performance of adsorbent evaluated in the binary solution of platinum and palladium with equal concentrations (500 ppm). (b) Experimental breakthrough curve of Pt/Pd (50/50 ppm) binary solution in the PQCOF-2 experimental column.

2.4. Adsorption Mechanism

To elucidate the mechanism underlying the selective adsorption of Pd²⁺ by PQCOF-2, the material was characterized by XPS and SEM before and after Pd²⁺ adsorption. XPS survey spectra confirmed the presence of Pd on PQCOF-2 after adsorption, indicating successful immobilization of Pd²⁺ within the framework (Figure a). N 1s spectra of PQCOF-2 prior to adsorption were deconvoluted into three components at 400.3, 399.7, and 398.8 eV (Figure b), corresponding to quinoline nitrogen, pyridine nitrogen, and triazine nitrogen, respectively. After Pd²⁺ adsorption, increased binding intensities associated with quinoline and pyridine nitrogen were observed, whereas the triazine nitrogen signal remained unchanged. These results suggest that quinoline and pyridine nitrogen sites serve as primary coordination sites for Pd²⁺, while triazine nitrogen remains uncoordinated. Additionally, the appearance of a peak at 406.2 eV indicates the possible involvement of nitrate anions in the Pd²⁺ coordination environment. The O 1s spectra revealed two distinct components assigned to C–O (532.6 eV) and CO (531.6 eV) groups (Figure c). After Pd²⁺ adsorption, both oxygen components exhibited increased binding energies, suggesting the involvement of carboxyl groups in Pd²⁺ complexation within the framework pores. Analysis of the Pd 3d region further confirmed the coordination of Pd²⁺ (Figure d). The doublet observed at 343.7 eV (3d_3/2) and 338.4 eV (3d_5/2), arising from spin–orbit splitting, is characteristic of divalent palladium, indicating that Pd²⁺ retains its original oxidation state after adsorption. To assess the morphological integrity of PQCOF-2 before and after Pd²⁺ adsorption, SEM was conducted. As shown in Figure e and f, the adsorbent maintained its honeycomb-like porous architecture without noticeable morphological alterations following Pd²⁺ uptake. Furthermore, elemental mapping by energy-dispersive X-ray spectroscopy (EDS) revealed a homogeneous distribution of Pd on the PQCOF-2 surface postadsorption (Figures g and S19), confirming uniform binding of Pd²⁺ throughout the framework.

4.

(a) XPS survey spectrum of PQCOF-2 before and after Pd²⁺ adsorption. (b) N 1s XPS spectra after Pd²⁺ adsorption. (c) O 1s XPS spectra after Pd²⁺ adsorption. (d) Pd 3d XPS spectra after Pd²⁺ adsorption. (e) Pristine SEM images of PQCOF-2. (f) SEM images of PQCOF-2 after adsorption of Pd²⁺. (g) SEM-EDS mapping images of PQCOF-2 after the capture of Pd²⁺.

To elucidate the molecular-level mechanism of Pd²⁺ adsorption and reduction, density functional theory (DFT) calculations were conducted using representative structural fragments of PQCOF-2. Electrostatic potential (ESP) mapping revealed a region of pronounced negative potential at the carboxylic acid moiety (Figure a), whereas the nitrogen atoms-particularly pyridinic nitrogen-exhibited high electron density, indicating their potential involvement in metal coordination. , Gibbs free energy (ΔG) calculations were performed to evaluate the thermodynamic favorability of Pd²⁺ coordination at different functional sites. As illustrated in Figure b, the coordination of Pd²⁺ with pyridinic nitrogen sites exhibits the most favorable Gibbs free energy change (ΔG = −197.8 kcal mol^–1), substantially lower than that associated with carboxyl groups (ΔG = −44.3 kcal mol^–1), indicating a strong thermodynamic preference for nitrogen-based coordination. In contrast, Ni²⁺ and Sr²⁺-divalent ions commonly coexisting with Pd²⁺ in high-level liquid waste-display significantly weaker interactions, with ΔG values of −66.8 and −63.3 kcal mol^–1, respectively. Cs⁺, the most abundant competing ion, exhibits the weakest interaction (ΔG = −34.5 kcal mol^–1) (Figure S20). These results collectively reveal that pyridinic nitrogen acts as the primary coordination site, while carboxyl groups serve as auxiliary binding motifs. The synergistic effect between these functionalities underpins the high binding affinity and remarkable selectivity of PQCOF-2 toward Pd²⁺ in complex radioactive environments. These findings are in good agreement with experimental observations. For platinum, a congenic metal to palladium, the calculated Gibbs free energy changes (ΔG) for coordination with the pyridinic nitrogen and carboxylic acid sites of the COF are −43.3 and −40.9 kcal mol^–1 (Figure S21), respectively. Notably, the binding affinity of the pyridinic nitrogen site for Pd²⁺ is significantly stronger than that for Pt⁴⁺, whereas the difference in interaction strength at the carboxyl site is relatively minor. These results underscore the site-specific selectivity of PQCOF-2, with pyridinic nitrogen serving as the dominant discriminatory site for Pd²⁺ over Pt⁴⁺. To further validate the selective separation capability of PQCOF-2 for Pd²⁺ over Pt⁴⁺, molecular dynamics (MD) simulations were applied for investigating the adsorption kinetics of both metal ions in a nitric acid environment (Figure c). The simulations employed a binary system with a defined Pd²⁺/Pt⁴⁺ molar ratio (1:1) to mimic competitive adsorption conditions. As shown in the calculating results, within the first 10 ps (ps), Pd²⁺ ions began to diffuse into and occupy the internal pores of PQCOF-2, whereas most Pt⁴⁺ ions traversed the framework without significant retention. By 50 ps, the number of adsorbed Pd²⁺ ions increased markedly, while Pt⁴⁺ adsorption remained minimal, indicating a pronounced kinetic preference for Pd²⁺ uptake under competitive conditions. At 2000 ps, Pd²⁺ adsorption significantly outpaced that of Pt⁴⁺, confirming the strong binding affinity of PQCOF-2 for Pd²⁺ in nitric acid solution. Figure d exhibits an enlarged snapshot of molecular dynamics at 2,000 ps, indicating that most Pd²⁺ ions are concentrated near carboxylate groups and pyridinic nitrogen sites, underscoring their crucial role in Pd²⁺ coordination within the COF framework. Figure e also presents the nonbonded interaction energy profiles of Pd²⁺/PQCOF-2 and Pt⁴⁺/PQCOF-2 systems, as obtained from molecular dynamics simulations. In the Pd²⁺/PQCOF-2 system, the van der Waals interaction energy was 18.43 kJ mol^–1, while the Coulombic component was −351.86 kJ mol^–1, yielding a total nonbonded interaction energy of −333.43 kJ mol^–1. By contrast, the total nonbonded interaction energy for Pt⁴⁺/PQCOF-2 was significantly weaker, being 1.38 kJ mol^–1. These findings highlight the stronger affinity of Pd²⁺ for PQCOF-2, primarily driven by electrostatic interactions with the densely distributed carboxylate and pyridinic nitrogen groups. The adsorption data, spectroscopic analyses, and theoretical modeling collectively confirm that PQCOF-2 exhibits excellent Pd²⁺ adsorption capacity and outstanding Pt/Pd selectivity, even under highly acidic and competitive ionic environments.

5.

(a) The electrostatic potential (ESP) distribution area diagram for PQCOF-2. (b) Models for Pd²⁺ binding to different sites in PQCOF-2 and the corresponding Gibbs free energy changes (ΔG). (c) Selected snapshots from the MD simulations show the Pd²⁺/Pt⁴⁺ adsorption process on PQCOF-2 in HNO₃ solution. (d) MD simulation snapshot 2000 ps local area enlarged diagram. (e) Time-dependent changes in the nonbonded interaction energy between COF and Pd²⁺and Pt⁴⁺ (Total-total energy of intermolecular interaction, vdw-van der Waals interaction, Coul–Coulomb interaction).

3. Conclusions

In summary, a series of quinoline-linked covalent organic frameworks (PQCOF and DQCOF) were designed and constructed through a one-pot multicomponent synthetic strategy. Among the synthesized COF materials, PQCOF-2, who retained a high density of pyridinic nitrogen and carboxylic acid groups, demonstrates the best selectivity for Pd²⁺ and exhibits exceptional structural stability both in 3 M HNO₃ and also under 400 kGy β-ray irradiation. Further, PQCOF-2 demonstrated a Pd²⁺ uptake capacity of 376 mg g^–1 and can be efficiently regenerated with thiourea eluents, this porous adsorbents also maintains high Pd²⁺ selectivity even in the presence of competing ions in complex waste matrices together with a high Pt/Pd separation factor. This work establishes a site-enrichment strategy within crystalline porous frameworks as a chemically and radiatively robust platform for efficient and selective Pd²⁺ capture, establishing theoretical and practical foundations to develop strong porous materials for radionuclide recovery from spent nuclear fuel reprocessing.

4. Experimental Section

4.1. Materials and Reagents

2,4,6-Tris­(4-aldehydephenyl)-1,3,5-triazine (TFT, 98%), 2,4,6-Tris­(4-aminophenyl)-1,3,5-triazine (TAT, 98%), 1,3,5-Tris­(2-formylpyridin-5-yl)­benzene (TT, 98%) were purchased from Shanghai Kylpharm Co., Ltd. 4-Vinylbenzoic acid (99%), 2,3-Dichloro-5,6-dicyanobenzoquinone (DDQ, 98%), Boron trifluoride diethyl ether (BF₃·OEt₂, 98%), Pyruvic acid (PA, 95%) were purchased from Macklin. Mesitylene (Mes, 99%), o-Dichlorobenzene (o-DCB, 99%), n-Butanol (n-BuOH, 99%), Tetrahydrofuran (THF, 99%), Thiourea (99%), N,N-Dimethylacetamide (DMAc, 99%), Methanol (MeOH, 99.8%), acetic acid (HAC, 99%), N,N-dimethylformamide (DMF, 99.5%) and Acetone (99%) used in this study were purchased from Sinopharm Chemical Reagent Co., Ltd. All of the chemicals were used as received.

4.2. Synthesis of Four COFs

PQCOF-1 was synthesized using a literature-reported procedure with slight modifications. ,, The framework was obtained through a one-pot multicomponent Povarov reaction. Specifically, TAT (2,4,6-tris­(4-aminophenyl)-1,3,5-triazine; 0.15 mmol, 56 mg), TFT (2,4,6-tris­(4-formylphenyl)-1,3,5-triazine; 0.15 mmol, 60 mg), and 4-vinylbenzoic acid (0.5 mmol, 75 mg) were added to a Pyrex tube containing a mixed solvent of n-butanol and o-dichlorobenzene (1:1, v/v; 3 mL) and 6 M acetic acid (0.2 mL). Subsequently, DDQ (2,3-dichloro-5,6-dicyanobenzoquinone; 5 mg) and BF₃·Et₂O (boron trifluoride diethyl etherate; 10 μL) were added as oxidant and Lewis acid catalysts, respectively. The reaction mixture was sonicated for 15 min, frozen in liquid nitrogen, and degassed through three freeze–pump–thaw cycles before being sealed under vacuum. The sealed tube was then heated at 120 °C for 3 days. The resulting solid was collected and sequentially washed with tetrahydrofuran, N,N-dimethylformamide, methanol, and acetone to remove residual monomers and oligomers, followed by vacuum drying at 100 °C to yield PQCOF-1. PQCOF-1 was obtained in 130 mg, corresponding to an isolated yield of 88%.

PQCOF-2 was synthesized using the same procedure as PQCOF-1, except that 2,4,6-tris­(4-formylphenyl)-1,3,5-triazine was replaced by 1,3,5-tris­(2-formylpyridin-5-yl)­benzene. PQCOF-2 was obtained in 121 mg, corresponding to an isolated yield of 82%.

DQCOF-1 was synthesized using a literature-reported procedure with slight modifications. ,, The framework was constructed through a one-pot multicomponent Doebner reaction. Specifically, 2,4,6-tris­(4-aminophenyl)-1,3,5-triazine (0.15 mmol, 56 mg), 2,4,6-tris­(4-formylphenyl)-1,3,5-triazine (0.15 mmol, 60 mg), and pyruvic acid (0.6 mmol, 60 μL) were added to a Pyrex tube containing a mixed solvent of n-butanol and o-dichlorobenzene (1:1, v/v; 3 mL), followed by the addition of 2,3-dichloro-5,6-dicyanobenzoquinone (DDQ; 10 mg) as the oxidant. The reaction mixture was sonicated for 15 min, frozen in liquid nitrogen, and degassed through three freeze–pump–thaw cycles before being sealed under vacuum. The sealed tube was then heated at 120 °C for 3 days. The resulting solid was collected and sequentially washed with tetrahydrofuran, N,N-dimethylformamide, methanol, and acetone to remove unreacted monomers and oligomers, followed by vacuum drying at 100 °C to yield DQCOF-1. DQCOF-1 was obtained in 112 mg, corresponding to an isolated yield of 90%.

DQCOF-2 was synthesized using the same procedure as DQCOF-1, except that 2,4,6-tris­(4-formylphenyl)-1,3,5-triazine was replaced by 1,3,5-tris­(2-formylpyridin-5-yl)­benzene. DQCOF-1 was obtained in 100 mg, corresponding to an isolated yield of 80%.

4.3. Characterizations

Powder X-ray diffraction (XRD) patterns were obtained on a Bruker D8 Advance diffractometer using Cu Kα radiation (2θ = 1.5–30°). Fourier-transform infrared Solid-state ¹³C NMR spectra were obtained using a Bruker Avance 400 spectrometer at 400 MHz. Fourier-transform infrared (FT-IR) spectra were recorded on a Thermo Scientific Nicolet iS5 spectrometer. Scanning electron microscopy (SEM) images were captured using a Hitachi SU8010 microscope, while transmission electron microscopy (TEM) images were acquired on a Talos F200S microscope. Scanning Transmission Electron Microscopy (STEM) images and energy-dispersive spectroscopy (EDS) data were collected using an FEI Tecnai Osiris and Titan3. The specific surface areas of the samples were determined via the Brunauer–Emmett–Teller (BET) method, and the pore size distribution was calculated using the nonlocal density functional theory (NLDFT) model, with all measurements performed on a Micromeritics ASAP 2046 surface area and porosity analyzer. XPS was conducted on a Thermo Fisher Scientific Escalab 250Xi system under high vacuum conditions (1 × 10^–9 Torr), with all binding energies calibrated to the C 1s peak at 284.8 eV. The concentrations of Pd²⁺ and other metal elements were measured using a NexION 350D inductively coupled plasma mass spectrometer (ICP-OES, PerkinElmer).

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/jacsau.6c00435.

• Experimental details; materials preparation; measurement methods; related characterizations of all COFs (FTIR, UV–Vis, BET, ¹³C NMR, XPS, SEM, TEM); palladium adsorption performance of all COFs; the radiation resistance of PQCOF-2 and its palladium adsorption performance before and after irradiation; details related to the DFT simulation (PDF)

F.C. and T.C. contributed equally to this work. F.C., W.Z., and Y.L. designed the study; F.C., T.C., X.H., and Z.L. performed the experiments; F.C., T.C., X.H., Z.L., Z.J., J.D., and X. L.conducted the device characterization; F.C. and T.C. wrote the paper with support from B.S., L.Q., W.Z., and Y.L. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

The authors declare no competing financial interest.