Three dimensional porous triptycene network with 12-connectivity of tröger’s base for iodine capture

Synthesis and general procedures

Reagents and conditions detailed: (i) Pd(PPh₃)₄, K₂CO₃, in THF and H₂O at 75 °C; (ii) TFA, (HCHO)_n, room temperature for 3 days. More detailed information, including the sources of the reagents and materials, the manufacturer, model, and country of origin for the experimental instruments were presented in the Supplementary Section 1.1 (Materials) and 1.2 (Characterization).

Synthesis of compound 1

The synthesis of compound 2 was detailed in the Supplementary Section 2. Compound 2 (930 mg, 2.41 mmol) was combined with 2,3,6,7,14,15-hexabromo-9,10-dihydro-9,10-[1,2]benzenoanthracene (217 mg, 0.301 mmol), K₂CO₃ (414 mg, 3.0 mmol), and Pd(PPh₃)₄ (120 mg, 0.104 mmol) in a mixture of tetrahydrofuran (THF, 60 mL) and H₂O (20 mL). The reaction was then heated to 75 °C for 3 days under an argon atmosphere. Once cooled to room temperature, the reaction was concentrated under vacuum, and the resulting residue was diluted with ethyl acetate (30 mL) three times. The combined organic phase was further washed with brine (100 mL), dried over MgSO₄ and evaporated. The residue was subsequently purified by column chromatography using a gradient of dichloromethane/methanol (initially 20:1, then 5:1) to yield compound 1 as a gray solid (400 mg, 74% yield). ¹H nuclear magnetic resonance (NMR) [600 MHz, dimethyl sulfoxide (DMSO)-d₆]: δ 7.82 (s, 6H), 7.45 (s, 6H), 7.22-7.19 (m, 36H), 6.55 (d, J = 6.0 Hz, 24H), 6.09 (s, 2H), 5.15 (s, 24H). ¹³C NMR (151 MHz, DMSO-d₆): δ 148.55, 144.93, 142.31, 141.43, 137.93, 128.42, 127.77, 125.26, 121.41, 114.63, 49.08. HRMS (ESI): m/z calculated for [(M+2H⁺)/2]²⁺ C₆₄H₅₀N₆₂⁺: 902.9036, found: 902.9116.

Synthesis of 12-TB-PTN

As illustrated in Scheme 1, 12-TB-PTN was synthesized according to our previously published procedure^[23]. Compound 1 (400 mg, 0.222 mmol) and paraformaldehyde (800 mg, 26.7 mmol) were mixed in trifluoroacetic acid (TFA, 30 mL), and stirred at 200 r·min^-1 under 20 °C for 4 days under an argon environment. The resulting mixture was gradually introduced to an ammonia solution and stirred vigorously for 4 h The solid product was collected via filtration, followed by washing with deionized water, methanol, dichloromethane, and dimethyl sulfoxide. The final product, 12-TB-PTN, was dried under a vacuum, acquiring 440 mg of a light brown solid with a 96% yield, and was observed to be insoluble in standard organic solvents.

Scheme 1. Synthesis of 12-TB-PTN. 12-TB-PTN: 12-Connected tröger’s base porous triptycene network.

Characterization of monomer 1

The synthesis of monomer 1 was carried out through a three-step palladium catalyzed coupling reaction in an overall yield of 42%, as verified by Fourier-transform infrared spectroscopy (FT-IR), ¹H and ¹³C NMR spectra. Furthermore, it was found that the electrospray ionization mass spectra (ESI-MS) of compound 1 revealed the peak at m/z 902.9116 for [(M+2H⁺)/2]²⁺, which was consistent with its structure [Figure 2A and Supplementary Figures 1-3]. Fortunately, we obtained suitable single crystals of compound 1 for X-ray diffraction (XRD) by gradually introducing isopropyl ether into an acetonitrile solution of 1. As shown in Figure 2B and C, compared to the 6-connected TB monomer’s triclinic crystal system and P-1 space group, monomer 1 was characterized by a monoclinic crystal system and P2₁/_n space group. As a result of the rigidity of the triptycene skeleton and the steric hindrance of the two adjacent 3,5-disubstituted aniline units, 1 exhibited a propeller-like and non-planar conformation [Supplementary Figure 4].

Figure 2. (A) ¹H NMR spectrum, top view (B) and side view (C) of X-ray crystal structures of 1. NMR: Nuclear magnetic resonance.

Characterization of 12-TB-PTN

Solid-state ¹³C cross-polarization magic-angle spinning (CP/MAS) NMR and FT-IR spectroscopic analyses disclosed the successful generation of 12-TB-PTN. As illustrated in Supplementary Figure 5, the signals at approximately 54, 59, 67, 116, 127 and 147 ppm were attributed to the varied carbon environments of the TB units formed within the 12-TB-PTN network, and then the peak at 142 ppm was ascribed to the carbon atoms within the substituted benzene rings. Within the FT-IR spectral analysis [Supplementary Figure 6], the new vibration bands (near 2,943, 2,895, 2,805, and 1,493 cm^-1) corresponded to the feature vibrations of −CH₂−, and 1,207 cm^-1 associated with the C−N stretching vibrations in TB. Thermogravimetric analysis (TGA, Supplementary Figure 7) showed that the nitrogen-rich 12-TB-PTN had high thermal stability. 12-TB-PTN retained more than 80% of its mass at 800 °C under nitrogen. As depicted in Supplementary Figure 8, field-emission scanning electron microscopy (FE-SEM) revealed that 12-TB-PTN predominantly exhibited a particulate morphology with different sizes. The powder X-ray diffraction (PXRD) pattern, as shown in Supplementary Figure 9, exhibited a notably broad peak, indicating that 12-TB-PTN was predominantly in an amorphous state.

Gas sorption analysis of 12-TB-PTN

The porosity characteristics of 12-TB-PTN were characterized through a standard typical N₂ adsorption-desorption isotherm at 77 K. Figure 3A illustrated that 12-TB-PTN displayed the characteristic type I adsorption behavior, with a steep increase in nitrogen gas uptake at a very low relative pressure (P/P₀ < 0.001), indicative of a permanent microporous structure. The observed hysteresis was attributed to the irreversible adsorption of gas molecules within the pores, which was presumably a result of the polymer network’s swelling in the presence of nitrogen at 77 K. The nonlocal density functional theory (NLDFT) was utilized to calculate the pore size distributions (PSD) of 12-TB-PTN. Figure 3B indicated that the primary pore size within the polymer was 0.81 nm, confirming the presence of a primary micropore structure along with some mesopores. The calculated micropore volume (V_micro) was determined to be 0.23 cm³·g^-1, and the total pore volume (V_total), estimated from the single point measurement at P/P₀ = 0.99, was 0.44 cm³·g^-1. The ratio of V_micro to V_total, approximately 0.50, further indicated the mesoporous characteristics of 12-TB-PTN. The mesoporous nature was attributed to the intrinsic porosity and the enhanced network expansion characteristic of the 3D rigid building blocks. The BET (Brunauer-Emmett-Teller) surface area for 12-TB-PTN was determined to be 862 ± 3 m²·g^-1, surpassing the 3-connected TB-MOP at 694 m²·g^-1, yet below the 6-connected TB-PTN with an area of 1,528 m²·g^-1. This observation could potentially be related to the crystalline structure of the monomer. In monomer 1, the significant steric hindrance between the two adjacent 3,5-disubstituted aniline groups on the triptycene skeleton may promote a certain degree of close packing during polymerization and assembly. This could lead to the formation of entangled pore channels within the 12-TB-PTN network, thereby affecting the polymer network’s porosity and BET surface area to some extent.

Figure 3. Structure characterization of 12-TB-PTN. (A) Nitrogen adsorption-desorption isotherms at 77 K; (B) Computed PSD; (C) CO₂ adsorption/desorption curves of 12-TB-PTN tested at 273 and 298 K; (D) Calculation of initial slopes from isotherms at 273 K. 12-TB-PTN: 12-Connected tröger’s base porous triptycene network; PSD: pore size distributions.

After establishing the pore property, the CO₂ adsorption capabilities of 12-TB-PTN were scrutinized at temperatures of 273 and 298 K. Figure 3C revealed that 12-TB-PTN exhibited a CO₂ uptake of 16.8 wt% at 1.0 bar and 273 K. This performance was nearly equivalent to the 3-connected TB-derived TB-MOP, which showed a 17.8 wt% uptake, but it was somewhat less than the 6-connected TB-derived TB-PTN, recording a 22.3 wt% uptake under the same conditions. In addition, 12-TB-PTN exhibited reversible CO₂ adsorption, with negligible hysteresis observed between the adsorption and desorption isotherms, suggesting that CO₂ was physisorbed in a fully reversible manner. The isosteric enthalpy of adsorption (Q_st) for 12-TB-PTN with respect to CO₂, derived from the adsorption isotherms using the Clausius-Clapeyron equation at 273 and 298 K as depicted in Supplementary Figure 10, was found to be 29 kJ·mol^-1. Compared with TB-MOP (29.5 kJ·mol^-1) and TB-PTN (30.2 kJ·mol^-1), this value was very similar, which was well below the values expected for a chemisorption process (> 40 kJ·mol^-1), but still higher than the reported values for many other MOPs. As selectivity is a crucial parameter for evaluating separation performance^[25], by analyzing the initial slopes of the CO₂ and N₂ adsorption isotherms at 273 K, the CO₂/N₂ selectivity for 12-TB-PTN was determined to be 30, as presented in Figure 3D.

Iodine vapor capture

Inspired by the outstanding porous property for 12-TB-PTN, we explored the potential of 12-TB-PTN to serve as an adsorbent for iodine vapor capture. In alignment with standard protocols for nuclear fuel reprocessing, the 12-TB-PTN powder was introduced to an ample supply of iodine vapor at 75 °C and under atmospheric pressure within a sealed glass vial. As shown in Figure 4A, the adsorption capacities rose almost linearly within the initial 3 h, accounting for about 80% of the total adsorption potential. Equilibrium was nearly attained after 10 h, with no additional changes detected beyond 48 h, indicating that the adsorption had reached its limit, the measured saturated adsorption capacity of 12-TB-PTN was 515 wt%, which was considered a high value among materials reported for iodine adsorption. At the same time, we conducted a comparative evaluation of the adsorption performance of various iodine adsorption materials based on K_80% (K_80% = 80% of the saturated adsorption capacity/adsorption time, in g·h^-1) more scientifically and reasonably^[6]. From Figure 4B, it can be seen that the K_80% value of 12-TB-PTN was relatively high, at 1.37 g·h^-1, which was better than some reported COFs and cages so far [Supplementary Table 2]. Meanwhile, as time went on, the hue of the material progressively intensified from a light yellow to a final dark black, as depicted in Supplementary Figure 11. The exceptional iodine sorption capacity of 12-TB-PTN was ascribed to the building blocks with higher connectivity, electron-rich TB units and excellent pore structures. It can be concluded that 12-TB-PTN was an effective adsorbent, demonstrating not only a notably high adsorption capacity but also a rapid adsorption rate. These remarkable characteristics position 12-TB-PTN as a compelling contender to traditional silver-based adsorbents, offering a potentially superior alternative.

Figure 4. (A) Gravimetric iodine vapor uptake by 12-TB-PTN over time at 75 °C and ambient pressure; (B) Comparison of saturated adsorption capacity and K_80% values for various porous materials, with “red star” denoting the 12-TB-PTN in this work; (C) Adsorption kinetics of I₃^- onto 12-TB-PTN, with accompanying solution images displayed as insets; (D) Langmuir isotherm analysis for I₃^- adsorption on 12-TB-PTN. 12-TB-PTN: 12-Connected tröger’s base porous triptycene network.

In order to comprehend the mechanism behind iodine adsorption, a suite of analytical methods, encompassing FT-IR, Raman spectroscopy (RM), and X-ray photoelectron spectroscopy (XPS), were meticulously applied to both the pristine 12-TB-PTN and the iodine-saturated I₂@12-TB-PTN samples. Supplementary Figure 12A demonstrated that the FT-IR spectra of 12-TB-PTN exhibited noticeable changes following iodine adsorption. The distinctive peaks around 1,207 cm^-1, corresponding to the C–N stretching vibration, and between 2,800 and 3,000 cm^-1, assigned to the –CH₂– bonds in the TB units, showed a reduction in intensity upon iodine adsorption, but recovered fully after desorption. In the RM spectra, it was observed that post-adsorption of iodine by 12-TB-PTN, the signature peaks corresponding to I₅^- manifested at around 109 and 168 cm^-1, markedly different from the 178 cm^-1 peak characteristic of unbound iodine molecules, as depicted in Supplementary Figure 12B. This discrepancy suggested a charge transfer interaction between the adsorbent and iodine^[6]. In addition, the binding energy shifts observed in the XPS spectra of I₂@12-TB-PTN indeed suggested the formation of I₅^-. As shown in Supplementary Figure 12C, after iodine adsorption, the binding energies of I3d_5/2 and I3d_3/2 for 12-TB-PTN post-iodine adsorption were observed at around 619.6 and 631.0 eV, respectively. These values exhibited a significant shift compared to the characteristic energies of 618.6 and 630.1 eV for elemental iodine, which can be ascribed to the formation of I₅^-.

The reversible iodine adsorption experiments, conducted in ethanol as presented in Supplementary Figure 13, revealed that the color of the solution intensified over time, signifying the release of iodine from the polymer networks. The iodine-loaded 12-TB-PTN underwent further purification through Soxhlet extraction with methanol for a duration of 2 days, followed by drying under vacuum. The regenerated 12-TB-PTN was subjected to subsequent cycles by re-exposure to iodine vapor. As depicted in Supplementary Figure 12D, it was observed that 12-TB-PTN retained 95% of its iodine uptake capacity after five consecutive cycles, thereby confirming its outstanding stability and recyclability for the enrichment of iodine. Furthermore, the FT-IR analysis depicted in Supplementary Figure 14 revealed no significant spectral alterations in the recovered 12-TB-PTN, thereby confirming the structural stability of 12-TB-PTN throughout the iodine enrichment process.

Iodine capture in aqueous solution

Nuclear wastewater discharge has always been a topic of great concern, as the radioactive iodine present in nuclear wastewater poses incalculable impacts on the ecological environment and human health. In the aqueous environment, iodine predominantly exists as I₃^- (I₂ is stabilized by an excess of KI via the triiodide equilibrium I₂ + I^- ⇋ I₃^-). Hence, it is of significant value to investigate the adsorptive capacity of 12-TB-PTN for capturing iodine from aqueous solutions. In practice, 12-TB-PTN powder was immersed in an I₃^- aqueous solution with a concentration of 0.2 mmol/L, and the ultraviolet-visible (UV-vis) absorption spectra were obtained by analyzing the supernatant at various time intervals. As shown in Figure 4C, the aqueous solution became nearly colorless within 5 min, indicating that over 95% of I₂ was rapidly removed from the water. To further explore adsorption behavior of 12-TB-PTN for I₃^-, the equilibrium data were subjected to analysis using the Langmuir and Freundlich isotherm models. Figure 4D demonstrated that the adsorption of 12-TB-PTN for I₃^- more closely aligns with the Langmuir isotherm model, as indicated by the high coefficient of determination (R_L² = 0.995). The adsorption isotherm revealed that the maximum adsorption capacity of 12-TB-PTN for I₃^- was 631 mg·g^-1, which was notably higher compared to certain porous organic polymers (POPs)^[26] and inorganic porous materials^[27] (Supplementary Table 3, based on Langmuir adsorption isotherm, not titration or gravimetric analysis).