All-Dry Molecular Scale Processing for Shaping Open Pockets with CO₂ Affinity in a Covalent Organic Framework

Abstract

Carbon dioxide capture is key to achieving carbon neutrality and addressing climate change. Covalent organic frameworks (COFs) with tailored pore structures have emerged as potential candidates for CO₂ adsorption. However, previous postsynthetic modification methods often face an inevitable trade-off between increasing adsorption sites and maintaining pore accessibility. Herein, we report an all-dry molecular scale processing (MSP) approach that forms open pockets in COF mesopores by covalently grafting chains with Zn open metal sites and primary amine groups as dual CO₂-philic sites. The resulting COF not only maintains a BET surface area up to 1180 m² g^–1 but also enables an ideal selectivity for CO₂/N₂ of 153 at 100 kPa, which is almost 10-fold higher than the pristine COF. Mechanistic investigations reveal that micropore filling and strong binding sites contribute to enhanced CO₂ adsorption. The pore engineering strategy reported here provides essential insights for developing the next generation of adsorbent materials.

Introduction

Anthropogenic CO₂ emissions from fossil fuel combustion represent the dominant driver of global climate change, necessitating advanced materials and separation processes to achieve carbon neutrality. − Covalent organic frameworks (COFs) have emerged as potential candidates for CO₂ capture owing to their large porosity, resistance to moisture, and structural tunability. − Decorating COF pores with CO₂-philic adsorption sites via postsynthetic modification (PSM) could further boost their performance for CO₂ separation. However, these modification methods often face an inevitable trade-off between the increase of adsorption sites and the remaining porosity and surface area. ,

Various PSM strategies, including click chemistry, nucleophilic substitution, ring–opening reactions, and ion insertion, , have successfully incorporated various functional groups into COF pores to promote CO₂ adsorption. However, previous solution-based methods often have to sacrifice the porosity and specific surface area dramatically in order to introduce a large number of adsorption sites into the COF pores. Consequently, the limited mass transport poses difficulty in fully removing the unreacted agents, byproducts, and solvents from the modified pores and necessitates extensive pore activation procedures. − Additionally, the diffusion of CO₂ into these modified pores may also be severely hindered. These limitations underscore the urgent need for strategies to achieve the highly preferential adsorption of CO₂ while preserving a large porosity for COFs.

Herein, we report a unique strategy to create open pockets in COF mesopores using an all-dry molecular scale processing (MSP) method. By covalently grafting short and stiff molecular chains containing both Zn open metal sites and primary amine groups, we generated pentagonal open pockets in the COF pores (Figure ). The significant potential overlap in such small pockets could lead to micropore filling and determine the low-pressure CO₂ adsorption. While MOFs naturally possessing open pockets that exhibit micropore filling behaviors have been reported, − so far forming open pockets in as-synthesized COF pores has not yet been explored. In addition to the topological features of these pockets, the Zn open metal sites as Lewis acidic centers − and the primary amine end groups in the grafted ∥–Zn–O–C₂H₄NH₂ chains could also contribute to the enhanced interactions with CO₂ adsorbates.

1.

Schematic representation of all-dry MSP for pore engineering in COFs. Sequential self-limiting reactions with diethylzinc (DEZ) and ethanolamine (EA) precursors enable covalent grafting of −Zn–O–C₂H₄NH₂ chains in COF pores, forming open pockets (orange dashed lines) with Zn open metal sites and primary amine groups for selective CO₂ adsorption.

Results and Discussion

We chose a mesoporous imine-linked COF (COF_DT) as our model structure for pore engineering. We synthesized this COF via Schiff-base condensation between 1,3,5-tris­(4-aminophenyl)­benzene (TAPB) and 2,5-dihydroxyterephthalaldehyde (DHTA) (Figure S1). Powder X-ray diffraction (PXRD) reveals the formation of a highly crystalline COF_DT with an AA-stacking arrangement (Figure a and S3). , The characteristic peak at 2.8° corresponds to the (100) plane with a d-spacing of 3.2 nm, which agrees with the well-defined lattice fringes and aligned one-dimensional channels found in the transmission electron microscopic (TEM) images (Figure S4). − Additional X-ray diffraction peaks at 4.8°, 5.5°, 7.3°, and 9.6° were indexed to the (110), (200), (210), and (220) planes, respectively.

2.

(a) PXRD patterns. (b) HRTEM image of COF_DT-Zn-NH₂. O 1s and N 1s XPS spectra for (c, e) COF_DT and (d, f) COF_DT-Zn-NH₂.

After MSP functionalization, the resulting COF_DT-Zn-NH₂ retained high crystallinity (Figure a). Scanning electron microscopy (SEM) and TEM images show negligible morphological alteration after MSP functionalization (Figure b and S5), while EDS elemental mapping confirms the uniform distribution of Zn and N elements throughout the COF_DT-Zn-NH₂ (Figure S6).

We further used X-ray photoelectron spectroscopy (XPS) to analyze the structural changes due to MSP functionalization (Figure S7 and Table S1). The O 1s spectrum of pristine COF_DT exhibits a single peak at 529.8 eV, characteristic of the O–C bonds in the phenol groups of COF_DT (Figure c). , In comparison, COF_DT-Zn-NH₂ shows an additional O 1s component at 528.8 eV , attributed to Zn–O bonds in the ∥–Zn–O–C₂H₄NH₂ moieties (Figure d). Inductively coupled plasma mass spectrometry (ICP-MS) reveals a Zn content of 9.75 wt % in COF_DT-Zn-NH₂ (Table S2), which corresponds to approximately 5.60 Zn atoms per COF_DT pore unit, equivalent to a grafting efficiency of 93.3% with regard to the hydroxyl sites of COF_DT. Furthermore, N 1s spectra reveal that the peak area ratio between the N–C component (397.3 eV) and the N = C component (396.4 eV) increased dramatically from 0.18 to 0.63 after MSP functionalization (Figure e,f and Table S1). Fourier transform infrared (FTIR) spectrum shows N–H stretching at around 3350 cm^–1, C–H stretching at 2935 and 2866 cm^–1, C–C vibration at 1386 cm^–1, and C–O vibration at 1047 cm^–1 that are associated with the −O–C₂H₄NH₂ moieties in COF_DT-Zn-NH₂ (Figure S8). −

We characterized the COF_DT and COF_DT-Zn-NH₂ by using N₂ physisorption at 77 K (Figure a). The COF_DT exhibited a Brunauer–Emmett–Teller (BET) surface area of 2597 m² g^–1, substantially higher than the previously reported values. , Such a high BET surface area resulted from our optimized synthesis conditions and supercritical CO₂ activation protocols that ensured complete pore evacuation. After MSP functionalization, COF_DT-Zn-NH₂ maintained a BET surface area as high as 1180 m² g^–1.

3.

(a) N₂ adsorption isotherms measured at 77 K and (b) pore size distributions of COF_DT and COF_DT-Zn-NH₂.

We further analyzed the pore size distribution based on the N₂ isotherm using nonlocal density functional theory (NLDFT) (Figure b). Pristine COF_DT exhibited a sharp unimodal distribution centered at 3.2 nm, confirming its highly ordered mesoporous structure. After MSP modification, COF_DT-Zn-NH₂ showed a complete transformation of the original 3.2 nm mesopores into a bimodal distribution with distinct pore sizes centered at 2.7 and 1.3 nm. The rise of the small pores with a diameter of 1.3 nm is ascribed to the pentagonal open pockets formed by the ∥–Zn–O–C₂H₄NH₂ chains and the COF side walls.

We further investigated the adsorptive separation of CO₂/N₂ by COF_DT and COF_DT-Zn-NH₂. Adsorption isotherms for COF_DT at 298 K (Figure a) reveal a gradual increase of the levels of CO₂ and N₂ uptake with pressure. In contrast, with the open pockets formed in the COF pores, COF_DT-Zn-NH₂ exhibits a steep increase of CO₂ uptake at the pressure range below 2.54 kPa, which is a typical micropore filling behavior and indicates significantly enhanced CO₂ affinity. ,, We found that the CO₂ adsorption capacity of COF_DT-Zn-NH₂ at 100 kPa (32.5 cm³ g^–1) is 42% higher than the pristine COF_DT. Additionally, N₂ uptake of COF_DT-Zn-NH₂ at 100 kPa was suppressed to merely 0.14 cm³ g^–1, approximately 12-fold lower than the pristine COF_DT.

4.

(a) CO₂ and N₂ adsorption isotherms. (b) IAST selectivity of CO₂/N₂ (15:85, v/v) gas mixture for COF_DT and COF_DT-Zn-NH₂ at 298 K. (c) Comparison of BET surface area and IAST selectivity for COF_DT-Zn-NH₂ in this work with the COFs previously reported in literature. (d-g) Density distributions of N₂ and CO₂ molecules at 298 K and 100 kPa in COF_DT and COF_DT-Zn-NH₂. (Gray, white, blue, red, and green atoms represent C, H, N, O, and Zn, respectively.).

We evaluated CO₂/N₂ separation selectivity for simulated flue gas composition (CO₂/N₂ = 15:85, v/v) at 298 K using ideal adsorbed solution theory (IAST) (Figure b) . , COF_DT-Zn-NH₂ achieved a remarkable ideal selectivity of CO₂/N₂ over 1000 at pressures below 3.02 kPa. The ideal selectivity obtained by COF_DT-Zn-NH₂ at 100 kPa also reached 153, reflecting almost 10-fold enhancement over the pristine COF_DT (15.5). Compared with the COF materials reported previously, our COF_DT-Zn-NH₂ exhibits an unprecedented ideal selectivity of CO₂/N₂ while maintaining a BET surface area of over 1000 m² g^–1 (Figure c and Table S3). Such a high surface area facilitates the rapid diffusion of gas molecules within the pores, thereby enhancing the overall mass transfer efficiency.

We calculated the isosteric heat of adsorption (Q _st) using CO₂ adsorption data at 273 and 298 K (Figure S9). , COF_DT-Zn-NH₂ shows a higher initial Q _st (25.1 kJ mol^–1) than the pristine COF_DT (20.0 kJ mol^–1). This enhancement stems primarily from the Zn open metal sites and primary amine groups that we introduced into the COF pores. Density functional theory (DFT) simulation results (Figure S10) also reveal that the binding energy of CO₂ substantially changes from −4.90 kJ mol^–1 to −10.92 kJ mol^–1 due to the MSP functionalization, further confirming the stronger CO₂ affinity with COF_DT-Zn-NH₂.

To further understand the molecular-level adsorption mechanism, we conducted a grand canonical Monte Carlo (GCMC) simulations. Simulated adsorption isotherms at 298 K closely match the experimental results (Figure S11). CO₂ density distribution in COF_DT at 298 K and 100 kPa shows adsorption primarily near the imine bonds and phenol groups of the pristine COF_DT (Figure d). In comparison, densely adsorbed CO₂ molecules were found to accumulate in the pentagonal open pockets formed in COF_DT-Zn-NH₂ (Figure e), consistent with the micropore filling observed in our experiments. Within the open pockets, preferential CO₂ binding to the Zn open metal sites can be attributed to Lewis acid–base coordination. , Additionally, enhanced CO₂ density near the primary amine groups arises from hydrogen bonding and dipole–quadrupole interactions. , Figure f,g shows lower N₂ density in the COF_DT-Zn-NH₂ pores than in the COF_DT pores, which agrees with the N₂ isotherms measured at 298 K and can be explained by the weakened N₂ binding energy (Figure S10) found in our DFT simulation.

Finally, we performed breakthrough tests to further evaluate the separation of mixed CO₂/N₂ (15:85, v/v) at 298 K. Figure S12 shows that N₂, as a weakly adsorbed component, rapidly eluted from the column, whereas CO₂ required a prolonged period (29.3 min) to breakthrough. The dynamic adsorption capacity of CO₂ calculated from the breakthrough curve is 15.4 cm³ g^–1, which is comparable to the uptake of pure CO₂ at 15 kPa (16.9 cm³ g^–1) found in the adsorption isotherm.

Conclusion

In conclusion, we have developed an all-dry MSP approach to build open pockets within the COF pores. We used vapor-phase diethylzinc and ethanolamine precursors to covalently graft ∥–Zn–O–C₂H₄NH₂ chains to the COF pore walls. These chains together with the COF pore walls form pentagonal open pockets in the original 3.2 nm mesopores, leading to a bimodal distribution with distinct pore sizes of 2.7 and 1.3 nm. Both adsorption measurements and computational simulations confirm the micropore filling of CO₂ in the open pockets. We also found that the Zn open metal sites and amine groups of the grafted chains exhibit preferential CO₂ binding, while the modified pore structure suppresses N₂ adsorption. Consequently, the resulting COF structure achieves an exceptional ideal selectivity of CO₂/N₂ (15:85, v/v) of 153 at 100 kPa, which is an almost 10-fold increase compared to the pristine COF and surpasses any prior reports. Unlike previous solution-based pore modification methods, our all-dry MSP circumvents the inevitable diffusional constraints and allows the maintenance of the COF BET surface area as high as 1180 m² g^–1. Our pore engineering strategy offers insights for future development of high-performance materials for carbon capture and gas separation.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/cbe.6c00035.

• Experimental details, characterization, gas adsorption test, supplementary characterization results, synthesis route for COF_DT; pore functionalization in COF_DT via MSP; top and side views of the structure of COF_DT; TEM and HRTEM images of COF_DT; SEM images of pristine COF_DT and COF_DT-Zn-NH₂ after MSP functionalization; TEM image and EDS mapping images of COF_DT-Zn-NH₂; XPS survey spectra of COF_DT and COF_DT-Zn-NH₂; FTIR spectra of TAPB, DHTA, COF_DT, and COF_DT-Zn-NH₂; XPS binding energies and peak area ratios of COF_DT and COF_DT-Zn-NH₂; elemental composition of COF_DT and COF_DT-Zn-NH₂ determined by XPS and ICP-MS analyses; isosteric heat of CO₂ adsorption for COF_DT and COF_DT-Zn-NH₂; comparison of IAST selectivity between MSP-functionalized COF_DT-Zn-NH₂ in this work and COF materials reported previously in literature; DFT-optimized stable adsorption configurations and binding energies of CO₂ and N₂ molecules on representative structural units of COF_DT and COF_DT-Zn-NH₂; CO₂ and N₂ adsorption isotherms of COF_DT and COF_DT-Zn-NH₂ at 298 K obtained from GCMC simulations; and breakthrough curves of a CO₂/N₂ mixture at 298 K for COF_DT and COF_DT-Zn-NH₂ (PDF)

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Z.C. and Y.L. contributed equally. 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.