Post‐Synthetic Modification of Dibromomethane Knitted Hyper‐Cross‐Linked Polymers With Sulfonic and Nitro Groups for Enhanced CO₂ Capture and Selectivity

ABSTRACT

Efficient and selective CO₂ capture represents a crucial technological challenge for carbon emission mitigation in post‐combustion processes. This study demonstrates a dual‐strategy approach combining high surface area engineering with post‐synthetic functionalization (sulfonation and nitration) that breaks the traditional trade‐off between adsorption capacity and selectivity in porous polymers for CO₂ capture, thereby simultaneously enhancing CO₂ adsorption capacity and CO₂/N₂ selectivity. We synthesized a hyper‐cross‐linked polymer (HCP‐TPB) using triphenylbenzene (TPB) as a rigid building block and dibromomethane as a cross‐linker, achieving exceptional textural properties (BET surface area: 2738 m² g⁻¹) and CO₂ uptake (21.3 wt% at 273 K). Through post‐synthetic sulfonation and nitration, the polymer framework was deliberately engineered to deliver three notable performance improvements: (1) increased CO₂ capacity to 23.7 wt% for HCP‐TPB‐SO₃H and 23.3 wt% for HCP‐TPB‐NO₂ at 273 K, (despite reduced surface area (1796 and 1564 m² g⁻¹) respectively); (2) enhanced CO₂/N₂ selectivity (from 16 for the pristine HCP‐TPB to 32 and 42 for HCP‐TPB‐SO₃H and HCP‐TPB‐NO₂ at 273 K), and (3) improved Ideal Adsorption Solution Theory (IAST)‐predicted selectivity (14→22→35) for 15:85 CO₂/N₂ mixtures at 298 K. These results establish an effective structure‐property relationship between sulfonic and nitro functionalities and gas separation performance.

This work presents a strategy that enhances CO₂ adsorption and CO₂/N₂ selectivity by engineering material properties through surface modifications and post‐synthetic functionalization. This approach successfully overcomes the capacity‐selectivity trade‐off, offering a promising pathway for the development of advanced CO₂ capture materials.

1. Introduction

The increasing concentration of atmospheric CO₂, primarily driven by fossil fuel combustion, is accelerating climate change at an unprecedented rate, posing serious threats to global ecosystems and human society [1]. While transitioning to clean energy sources offers a long‐term solution for reducing emissions, carbon capture and storage (CCS) technologies provide a more immediate approach to mitigate CO₂ emissions while maintaining fossil fuel utilization [2]. Current industrial post‐combustion CO₂ capture predominantly relies on aqueous amine scrubbing, yet this conventional method faces significant limitations, including the high energy demands for solvent regeneration, substantial water consumption, and corrosive effects on industrial equipment [3, 4]. These challenges underscore the urgent need to develop next‐generation CCS materials that simultaneously address cost‐effectiveness, energy efficiency, and environmental sustainability.

Solid adsorbents have emerged as an energy‐efficient alternative to liquid amine systems for CO₂ capture, with significantly lower regeneration energy requirements [5]. Among the various porous materials investigated, such as zeolites [6], activated carbons [7], and metal‐organic frameworks (MOFs) [8, 9], each exhibits fundamental limitations that hinder practical application. While porous carbons demonstrate high surface areas, their CO₂/N₂ selectivity remains insufficient for effective flue gas separation. Zeolites, despite their molecular sieving capabilities, suffer from three critical drawbacks: energy‐intensive thermal regeneration, limited pore volumes, and severe performance degradation under humid conditions, where water molecules preferentially occupy adsorption sites [10]. MOFs face similar challenges in moist environments, where their open metal sites exhibit a stronger affinity for water than CO₂, leading to dramatic reductions in CO₂ uptake capacity—a critical issue for real‐world flue gas scenarios that typically contain 5%–10% water vapor [11, 12]. These material‐specific limitations highlight the ongoing need for the development of cost‐effective adsorbents that exhibit robust performance under realistic post‐combustion conditions.

Porous organic polymers (POPs) have emerged as promising candidates for CO₂ capture due to their stable pore architectures, high surface areas, and exceptional physicochemical stability [13, 14]. These materials offer distinct advantages in terms of structural tunability, low skeletal density, and versatile functionality, making them suitable for diverse applications ranging from gas separation to energy storage and heterogeneous catalysis [15, 16]. Compared with MOFs and inorganic zeolites, which often require stringent synthesis conditions, POPs can be prepared using simpler, more scalable methodologies [17]. Particularly noteworthy are hyper‐cross‐linked polymers (HCPs), a subclass of POPs that combine high surface areas, exceptional thermal/chemical stability, tunable microporosity, and cost‐effective synthesis [18, 19, 20, 21, 22]. In addition, relative to other porous organic materials such as covalent organic frameworks (COFs) and conjugated microporous polymers (CMPs), HCPs offer further practical advantages, including facile synthesis, scalable production, and the use of inexpensive, commercially available precursors [15, 23, 24]. However, current HCP materials predominantly rely on physical adsorption mechanisms, which inherently limit their CO₂ capture efficiency and gas selectivity. Recent studies suggest that strategic incorporation of chemical adsorption sites (e.g., amine groups, that interact strongly and reversibly with acidic CO₂ molecules) could significantly enhance both CO₂ uptake capacity and selectivity [25, 26], yet amine‐functionalized HCPs often suffer from pore blockage or compromised stability under humid conditions [27, 28], and single‐functionalization strategies rarely achieve simultaneous improvements in capacity and selectivity representing a crucial direction for future material development.

Recent research has focused on the following strategies to overcome the limitations of porous polymers for CO₂ capture: (I) shape‐exclusion to maximize accessible adsorption surface area, (II) size‐exclusion for molecular sieving, (III) thermodynamic optimization of gas‐solid interactions, and (IV) electrostatic tuning based on molecular polarity [29]. For example, Wang et al. prepared carbonyl‐ rich HCPs using oxalyl chloride as a cross‐linker and further modified them by amination and KOH‐activated carbonization. Among these, the carbonized derivative HCP3‐C reaches a CO₂ uptake of 270 mg g⁻¹ at 273 K and 1 bar, while the amine‐functionalized HCP3‐N exhibits a CO₂/N₂ selectivity of 85 [27]. Ravi et al. synthesized an aromatic carbonyl‐based porous polymer (ACP) with a high BET surface area via a Friedel‐Crafts reaction, followed by post‐synthetic incorporation of melamine and ethylenediamine onto the carbonyl sites. The melamine‐functionalized sample (ACP‐MEL) retains a large BET surface area of 1082 m² g⁻¹, exhibits a CO₂ uptake of 206.9 mg g⁻¹ at 273 K and 1 bar, and shows a high CO₂/N₂ selectivity of 110 [30]. In a recent study, Moradi et al. reported a benzene‐based HCP that was further modified by grafting amine groups to enhance CO₂ capture performance. After functionalization, CO₂ uptake increased from 302 to 414 mg g⁻¹ at 298 K and 9 bar, and the CO₂/N₂ selectivity for a 15:85 mixture at 298 K was improved by over 1.4‐fold relative to the parent polymer [18]. Notably, these studies often exhibit a trade‐off: pore engineering enhances surface area but fails to improve selectivity, while functionalization boosts selectivity but may reduce adsorption capacity [31]—a key bottleneck in porous polymer design for CO₂ capture. These studies collectively demonstrate how strategic material design can optimize both CO₂ capacity and selectivity through controlled pore engineering and chemical functionalization.

Building upon our previous work, which demonstrated that the synergistic combination of size‐exclusion and electrostatic effects can deliver high CO₂/N₂ selectivity (up to 100) [32], a critical limitation remained: the inherent trade‐off between selectivity and CO₂ uptake in porous polymers. Although pore size regulation and the introduction of polar ‐NO₂, ‐NH₂ groups significantly enhanced selectivity, the achievable CO₂ uptake was restricted to approximately 14 wt%, underscoring a fundamental design bottleneck. In this study, we report a conecptually distinct dual‐optimization strategy that overcomes this limitation by simultaneously maximizing CO₂ uptake and selectivity (Scheme 1). First, high‐surface‐area is achieved through the delibrate selection of a rigid triphenylbenzene (TPB) monomer combined with dibromomethane as an efficient cross‐linker, enabling the formation of a hierarchical pore architecture optimized for CO₂ physisorption. Second, post‐synthetic modification enables the incorporation of strongly polar –SO₃H and –NO₂ functionalities while preserving the structural integrity and porosity of the framework, without inducing pore collapse. The –SO₃H groups promote CO₂ capture by enabling strong hydrogen‐bonding interactions between their acidic protons and the oxygen atoms of CO₂ [33], while the –NO₂ groups enhance adsorption through their high polarity and strong charge separation, facilitating additional electrostatic and quadrupole–dipole (CO_2–NO₂) interactions [34, 35]. This modification also improves CO₂/N₂ selectivity by favoring CO₂ molecules, while maintaining structural integrity despite functional group incorporation. By combining these two complementary strategies, we successfully break the traditional trade‐off between adsorption selectivity and CO₂ uptake in porous polymers, achieving both enhanced CO₂ uptake and superior CO₂/N₂ selectivity—critical for practical post‐combustion carbon capture. This integrated approach demonstrates how coordinated structure and porosity engineering can break the traditional adsorption‐selectivity trade‐off in porous polymer design.

SCHEME 1.

Synthesis and post‐synthetic functionalization of hyper‐cross‐linked polymers (HCP‐TPB‐SO₃H, HCP‐TPB‐NO₂).

2. Results and Discussion

HCP‐TPB was synthesized via a Friedel–Crafts reaction, following a procedure described in the literature [28]. The –SO₃H and –NO₂ polar functional groups were introduced into the preformed HCP‐TPB via post‐synthetic modification by treating the framework with concentrated sulfuric acid (sulfonation) and with concentrated nitric acid in the presence of concentrated sulfuric acid (nitration), respectively. The formation of the HCPs was verified using solid‐ state ¹³C NMR and Fourier‐transform infrared (FT‐IR) spectroscopy. As shown in Figure 1a, the FT‐IR spectra exhibit a peak around 2900 cm⁻¹, corresponding to the methylene linkage. The stretching vibration of the sulfonic group is observed at 1180 cm⁻¹, corresponding to the S = O bonds [31]. Additionally, the peaks at 850 cm⁻¹ and 755 cm⁻¹ are associated with the S‐O stretching vibration. These features confirm the incorporation of the sulfonic group into the polymer structure, consistent with the characteristic signals of –SO₃H groups in porous polymer [36, 37]. In HCP‐TPB‐NO₂, additional FT‐IR bands at 1530 and 1359 cm⁻¹, arising from the asymmetric and symmetric nitro group stretching vibrations, respectively, indicate successful incorporation of –NO₂ functionalities into the polymer network.

FIGURE 1.

(a) FT‐IR spectra of HCP‐TPB, HCP‐TPB‐SO₃H and HCP‐TPB‐NO₂; (b)¹³C CP/MAS NMR spectra of HCP‐TPB, HCP‐TPB‐SO₃H and HCP‐TPB‐NO₂; (c) SEM images of HCP‐TPB‐SO₃H; (d) SEM images of HCP‐TPB‐NO₂. Scalebars represent 5 µm.

Solid‐state ¹³C NMR was employed to further verify the structure of the as‐synthesized HCPs. As shown in Figure 1b, resonance peaks at approximately 137 and 130 ppm are attributed to the substituted and unsubstituted carbon atoms, respectively. Additionally, a peak at 37 ppm, corresponding to the methylene carbon, indicates cross‐linking of TPB units via methylene linkages. No obvious shifts in the aromatic carbon peaks are detected after functionalization, indicating that post‐synthetic sulfonation and nitration do not damage the backbone structure of HCP‐TPB—an essential feature for maintaining thermal and chemical stability.

The morphology of HCP‐TPB before and after post‐synthetic functionalization was analyzed using scanning electron microscopy (SEM) (Figure 1c,d; Figure S1 and S4). The pristine HCP‐TPB (Figures S1 and S2) exhibits a smooth, compact surface with well‐defined particle edges, indicating a dense and uniform polymer structure. After sulfonation, HCP‐TPB‐SO₃H (Figures 1c and S3) exhibits a rougher, more irregular surface, with a sponge‐like, loosely aggregated texture, reflecting the structural adjustment induced by the introduction of sulfonic groups and potentially creating additional accessible adsorption sites. The nitrated sample HCP‐TPB‐NO₂ (Figures 1d and S4), displays an irregular bulk morphology, likely resulting from the structural effect of nitro group incorporation.

Elemental analysis was employed to quantitatively verify the introduction and loading of functional groups introduced by post‐synthetic modification. As summarized in Table S1, pristine HCP‐TPB contains only carbon and hydrogen. In contrast, HCP‐TPB‐SO₃H exhibits a sulfur content of 3.74 wt%, corresponding to a ‐SO₃H loading of about 1.17 mmol g⁻¹. Likewise, HCP‐TPB‐NO₂ displays a nitrogen content of 1.51 wt%, equivalent to an ‐NO₂ loading of 1.08 mmol g⁻¹. The presence and quantification of sulfur and nitrogen provide direct evidence for the successful incorporation of ‐SO₃H and ‐NO₂ functional groups.

X‐ray photoelectron spectroscopy (XPS) was utilized to further confirm the successful incorporation of functional groups into the HCP framework. As shown in Figure 2a–c, the high‐resolution C1s spectrum of HCP‐TPB‐SO₃H shows a dominant peak at 284 eV corresponding to C‐C bonding, along with an additional component at 285 eV attributed to C‐S binding. The O1s spectrum centered at 532 eV confirms the presence of oxygen containing species. While the S2p spectrum displays two characteristic peaks at 166.2 and 167.4 eV, assigned to S‐C and S‐O bonds of ‐SO₃H functionalities [38, 39]. For HCP‐TPB‐NO₂ (Figure 2d–f), the C1s spectrum shows peaks at 284 eV (C‐C) and 286 eV (C‐N). The N1s spectrum can be deconvoluted into two components at 400 and 405 eV, corresponding to C‐N species and nitro (‐NO₂) groups, respectively [40, 41]. Additionally, the O1s peak located at 534 eV is consistent with oxygen atoms in nitro groups. These results collectively verify the successful introduction of functional groups into the HCP framework. Based on the XPS and elemental analysis results, the presence of characteristic bonding environments and heteroatom incorporation indicates that the functional groups are covalently attached to the polymer framework rather than physically adsorbed.

FIGURE 2.

XPS spectra of HCP‐TPB‐SO₃H and HCP‐TPB‐NO₂: (a) C 1s, (b) O 1s, and (c) S 2p for HCP‐TPB‐SO₃H;d) C1s, e) O 1s, and f) N 1s for HCP‐TPB‐NO₂.

Thermogravimetric analysis (TGA) was performed from 20 to 600 °C under a nitrogen atmosphere to evaluate the thermal stability of the materials (Figure S5). For HCP‐TPB and HCP‐TPB‐SO₃H, thermal decomposition initiates at approximately 120 °C. The weight loss observed below 200 °C is attributed to the low reactivity of dibromomethane, which results in polymers with a lower degree of polymerization [32]. A second decomposition stage emerges at around 220 °C. As illustrated in Figure S5, all three HCP samples retain considerable mass at 600 °C, indicating that –SO₃H and –NO₂ functionalization does not compromise the overall thermal robustness of the polymer framework [42].

To evaluate the porosity, nitrogen sorption isotherms were performed at 77 K for the pristine HCP‐TPB and the functionalized samples HCP‐TPB‐SO₃H and HCP‐TPB‐NO₂. The increased N₂ uptake at low relative pressure (P/P₀<0.05) indicates the presence of micropores within the structures. Furthermore, the appearance of a hysteresis loop at higher relative pressures (0.2<P/P₀<0.8) confirms the mesoporous nature of the materials (Figure 3a), which is beneficial for gas diffusion during adsorption and desorption processes. The Brunauer‐Emmett‐Teller (BET) surface area analysis of the as‐synthesized materials, conducted according to the method described in the Table S1, revealed that the HCP‐TPB exhibited a surface area of 2738 m² g⁻¹. While the sulfonated and nitrated samples HCP‐TPB‐SO₃H and HCP‐TPB‐NO₂ showed reduced surface area of 1796 and 1564 m² g⁻¹, respectively, this decrease in surface area for both functionalized materials is likely due to partial pore occupation by sulfonic and nitro groups, which could limit the overall accessible pore space [38]. Notably, the micropore volume of HCP‐TPB‐SO₃H and HCP‐TPB‐NO₂ increased from 0.43 cm³ g⁻¹ (pristine HCP‐TPB) to 0.51 and 0.48 cm³ g⁻¹, respectively (Table S2). This increase in micropore volume—favorable for CO₂ adsorption as micropores provide strong adsorption sites—compensates for the reduction in BET surface area and contributes to the enhanced CO₂ uptake capacity [43].

FIGURE 3.

(a) Adsorption and desorption isotherms of nitrogen at 77.3 K; (b) pore size distribution calculated from NLDFT analysis for HCP‐TPB, HCP‐TPB‐SO₃H and HCP‐TPB‐NO₂.

The pore size distributions (PSD) were determined using non‐local density functional theory (NLDFT). HCP‐TPB, HCP‐TPB‐SO₃H, and HCP‐TPB‐NO₂ all exhibit a hierarchical pore structure, with pore diameters ranging from 0.7 to 5 nm (Figure 3b). Importantly, the dominant pore size (2.0–2.3 nm) is compatible with the kinetic diameters of CO₂ (0.33 nm) and N₂ (0.36 nm), enabling size‐based adsorption discrimination while facilitating mass transfer. Furthermore, the total pore volumes of HCP‐TPB‐SO₃H and HCP‐TPB‐NO₂ (0.92 and 0.84 cm³ g⁻¹) are lower than that of HCP‐TPB (1.60 cm³ g⁻¹), which is consistent with pore filling by functional groups (Table S2).

2.1. Gas Uptake Ability

To assess the CO₂ capture performance of the HCP‐TPB and its counterparts functionalized with sulfonic and nitro groups for the adsorption of CO₂, adsorption isotherms for carbon dioxide and nitrogen were measured at 273 and 298 K. At 273 K and 1.13 bar, the CO₂ capture capacity of the HCP‐TPB was found to be 21.3 wt%, whereas HCP‐TPB‐SO₃H and HCP‐TPB‐NO₂ showed an improved CO₂ adsorption under the same conditions (Figure 4a). Despite the absence of heteroatoms in the HCP‐TPB structure, it demonstrated significant CO₂ adsorption performance, likely due to its high specific surface area, microporosity, and π‐electron‐rich framework. After the incorporation of sulfonic and nitro functional groups, CO₂ capture performance was further enhanced to 23.7 wt% and 23.3 wt%, respectively, despite the accompanying decrease in surface area.

FIGURE 4.

(a) CO₂ adsorption and desorption isotherms measured at 273 K, 1.13 bar; (b) CO₂ adsorption and desorption isotherms measured at 298 K and 1.13 bar; and (c) isosteric heat of adsorption for HCP‐TPB, HCP‐TPB‐SO₃H and HCP‐TPB‐NO₂.

The enhanced CO₂ uptake of HCP‐TPB‐SO₃H arises primarily from hydrogen‐bonding interactions introduced through post‐synthetic functionalization, whereas in HCP‐TPB‐NO₂, the strongly polar –NO₂ groups promote significant quadrupole–dipole interactions with CO₂, further strengthening adsorption. At 298 K and 1.13 bar (Figure 4b), a similar trend was observed, with the HCP‐TPB exhibiting a CO₂ capture capacity of 10.8 wt%, while HCP‐TPB‐SO₃H and HCP‐TPB‐NO₂ demonstrated an enhanced CO₂ capture performance of 14.2 and 13.9 wt%, respectively. The CO₂ adsorption performance of HCP‐TPB and its sulfonic‐ and nitro‐functionalized counterpart (HCP‐TPB‐SO₃H and HCP‐TPB‐NO₂) surpasses most reported polymers, demonstrating superior performance in comparison to other materials in the literature. As summarized in Table 1, for instance, DMOF‐2F (21.1 wt%) [44], MPOF‐ad‐3 (19.0 wt%) [45], CTF‐S6 (17.6 wt%) [46], P‐PhPh₃‐SO₃H (17.2 wt%) [47], PD‐COF (16.7 wt%) [48], show lower CO₂ uptake, while outperforming these materials and competing with top‐performing adsorbents such as BUGC (25.5 wt%) [49], and Ni_dmtz (28.0 wt%) [50], at 273 K and 1 bar.

TABLE 1.

CO₂ capture and selectivity performance compared with reported polymers.

Sample	CO2 uptake (wt%) a		CO2/N2 selectivity b	Qst c	References
	[273 K]	[298 K]		[kJ mol−1]
HCP‐TPB	21.3	10.8	16	24	This work
HCP‐TPB‐SO3H	23.7	14.2	32	28	This work
HCP‐TPB‐NO2	23.3	13.9	42	30	This work
TTPEPOP‐S	12.9	7.9	17	24	[16]
MPOF‐Ad‐3	19.0	6.2	38	19	[45]
DMOF‐2F	21.1	10.4	14	23	[44]
PD‐COF	16.7	10.7	16	31	[48]
HCP1	11.6	6.9	15	46	[59]
MPOF‐Ad‐1	13.6	8.7	37	30	[45]
Polymer‐1	9.8	6.2	20	28	[60]
CTF‐S6	17.6	12.3	25	22	[46]
PCP‐2A	7.8	4.6	25	26	[61]
A6CMP‐3	13.9	11.1	19	23	[62]
P‐PhPh3‐SO3H	17.2	10.5	16	25	[47]
P‐CMP‐1	10.6	5.8	—	26	[63]
P‐CMP‐2	13.5	8.9	—	32	[63]
TPB‐NH2	9.2	5.1	29	37	[64]
ACP‐EDA	23.3	13.2	—	43	[30]
BUGC	25.5	15.6	—	30	[49]
Ni_dmtz	28.0	23.5	—	59	[50]

^a CO₂ adsorption capacities measured up to 1 bar at 273 and 298 K.

^b CO₂/N₂ selectivity determined under low pressure conditions (P<0.3 bar) at 273 K.

^c CO₂ isosteric heat of adsorption (Q_st).

To evaluate the regeneration capability of the HCPs, cyclic adsorption‐desorption experiments were conducted up to five cycles (Figure S6a–c). All samples exhibit minimal loss in CO₂ uptake over consecutive cycles, indicating stable performance. The FT‐IR spectra (Figure S7) shows no significant changes after cycling, while XPS analysis of HCP‐TPB‐SO₃H (Figure S8) confirms preservation of S‐C and S‐O bonding environments. In addition, elemental analysis (Table S3) reveals minimal variation in sulfur and nitrogen contents, demonstrating the structural and chemical stability of the materials.

FIGURE 6.

CO₂/N₂ (15:85) dynamic breakthrough profiles recorded at 298 K with a total flow rate of 3 mL min⁻¹ for (a) HCP‐TPB, (b) HCP‐TPB‐SO₃H, and (c) HCP‐TPB‐NO₂.

To assess the adsorption mechanism, the CO₂ adsorption isotherms of HCP‐TPB, HCP‐TPB‐SO₃H, and HCP‐TPB‐NO₂ at 273 K were analyzed using Sips, Tóth, and Dubinin‐Astakhov (D‐A) models (Figure S9; Tables S4 and S6). The fitting quality was evaluated using the sum of squared errors (SSE), root mean square error (RMSE), and coefficient of determination (R²), where lower error values of (SSE) and (RMSE), along with higher values of (R²) indicate better agreement with experimental data [51]. The Tóth model provided the best fit, indicating adsorption on energetically heterogeneous surfaces, as it accounts for deviations from ideal Langmuir behavior [52]. This observation arises from the microporous structure of the HCP framework, while the incorporation of ‐SO₃H and ‐NO₂ groups increases adsorption affinity through enhanced interactions with CO₂ without changing the adsorption mechanism. The D‐A model yields adsorption energies of ∼ 10–11 kJ mol⁻¹, consistent with physisorption via micropore filling.

To further investigate the CO₂ adsorption properties, the isosteric heat of adsorption (Q_st) was calculated using CO₂ isotherms measured at 273 and 298 K. The Q_st values for the HCPs were obtained using the Clausius‐Clapeyron equation, providing a deeper insight into host‐guest interactions and the affinity of CO₂ molecules for the material's framework. The HCP‐TPB exhibited an isosteric heat of adsorption (Q_st) of 24 kJ mol⁻¹, whereas the sulfonic‐ and nitro‐functionalized HCPs showed an improved Q_st of 28 and 30 kJ mol⁻¹, respectively (Figure 4c). The gradual increase in Q_st with functionalization is consistent with the enhanced surface polarity induced by –SO₃H and –NO₂ groups, which strengthens the CO₂‐framework interaction [31, 53]. However, it is worth noting that the Q_st values are significantly lower than those of chemisorption, indicating that the adsorption is mainly physisorptive. This is advantageous for promoting the efficient release of CO₂.

2.2. Selectivity of CO₂ Over N₂

In post‐combustion carbon capture processes, CO₂ represents approximately 15% of the gas mixture, with nitrogen (N₂) accounting for the remaining 85%. Given the relatively low concentration of CO₂, it is essential for the sorbent material to have a stronger affinity for CO₂ than for N_2. To determine the CO₂/N₂ selectivity, the Henry's law initial slope method was employed, using the low‐pressure regions (< 0.3 bar) of the CO₂ and N₂ adsorption isotherms at 273 and 298 K, (N₂ isotherms, Figures S10–S12). At 273 K, the CO₂/N₂ selectivity was determined to be 16, 32, and 42 for HCP‐TPB, HCP‐TPB‐SO₃H and HCP‐TPB‐NO₂, respectively (Figure 5a–c). These results confirm that the incorporation of sulfonic and nitro functional groups significantly improvs the selectivity toward CO₂. The higher selectivity of HCP‐TPB‐NO₂ compared to HCP‐TPB‐SO₃H aligns with its higher Q_st value, reflecting stronger host‐guest interactions with CO₂ [54]. At 298 K, a similar trend was observed, with the selectivity improving from 7 for the HCP‐TPB to 22 and 25 for the HCP‐TPB‐SO₃H and HCP‐TPB‐NO₂, respectively (Figures S13–S15). The higher selectivity of the nitro‐functionalized polymer compared to its sulfonic analogue is consistent with its stronger host‐guest interactions with CO₂, as evidenced by its higher Q_st.

FIGURE 5.

(a‐c) CO₂/N₂ selectivity of HCP‐TPB, HCP‐TPB‐SO₃H and HCP‐TPB‐NO₂ calculated from Henry's law at 273 K (<0.3 bar) and (d) CO₂/N₂ selectivity at 298 K using IAST for 15:85 CO₂/N₂ gas mixture.

The CO₂/N₂ selectivity observed in this study is lower than that reported for certain other polymers. For example, pentiptycene–based MOP (PMOP) exhibits a selectivity of 47 at 273 [55]. While amine decorated MOF‐CAU‐1 showed a selectivity of 101 [56], and a triptycene–based porous network demonstrated a selectivity of 68 [57]. However, higher CO₂/N₂ selectivity is often accompanied by a reduction in overall CO₂ capacity due to the inherent adsorption‐selectivity trade‐off. For instance, a pyrrole‐based aromatic heterocyclic polymer (Py‐1) demonstrates a CO₂/N₂ selectivity of 117, but its CO₂ uptake is limited to 11.8 wt% at 273 K and 1 bar [58]. In contrast, the HCP‐TPB‐SO₃H in this study achieves a significantly higher CO₂ uptake of 23.7 wt% under the same conditions. The nitrated analogue HCP‐TPB‐NO₂ also shows an increased CO₂ uptake of 23.3 wt% together with an even higher CO₂/N₂ selectivity of 42. In this work, both CO₂ adsorption and selectivity are enhanced through post‐synthetic functionalization with sulfonic and nitro groups, demonstrating the advantages of this functionalization strategy.

To evaluate the selectivity performance of the HCP‐TPB and its post‐synthetically functionalized HCP‐TPB‐SO₃H and HCP‐TPB‐NO₂ analogues under a CO₂/N₂ (15:85) gas mixture at 298 K, the ideal adsorbed solution theory (IAST) was applied (Figure 5d). This method assesses the material's adsorptive separation efficiency and identified the optimal operating conditions [65]. The CO₂ and N₂ adsorption isotherms were modeled using the Dual‐site Langmuir‐Freundlich approach with high fitting reliability (R²> 0.99 for all samples. The IAST selectivity of the HCP‐TPB was measured as 14, increasing to 22 and 35 for HCP‐TPB‐SO₃H and HCP‐TPB‐NO₂, respectively. highlighting the improvement in selectivity following functionalization. This result is consistent with the values calculated using Henry's initial slope method.

To examine CO₂/N₂ separation performance, dynamic breakthrough measurements were carried out using a 15:85 CO₂/N₂ gas mixture at 298 K with a total flow rate of 3 mL min⁻¹ (Figure 6a–c). The parent HCP‐TPB shows N₂ elution at 61 s and CO₂ at 985 s, reflecting modest separation efficiency. In contrast, the modified frameworks show a clear improvement in separation behavior, where N₂ elutes much earlier (29 s for HCP‐TPB‐SO₃H and 24 s for HCP‐TPB‐NO₂) while CO₂ is retained for significantly longer durations (1369 and 1393 s, respectively). Among these materials, HCP‐TPB‐NO₂ demonstrates the most pronounced separation performance, combining rapid N₂ elution with extended CO₂ retention. This trend indicates stronger interactions between CO₂ molecules and the functionalized frameworks due to the presence of polar functional groups. Moreover, the experimental results follow the same order as the IAST selectivity values (HCP‐TPB < HCP‐TPB‐SO₃H < HCP‐TPB‐NO₂), supporting the reliability of theoretical predictions.

The effect of humidity on CO₂ adsorption was investigated by performing breakthrough experiments using a CO₂/N₂ (15:85) gas mixture at 298 K under both dry and humid conditions (50% RH) for HCP‐TPB‐NO₂, which exhibited the highest CO₂/N₂ selectivity. The CO₂ uptake shows only a slight decrease from 14.7 to 14.4 mL g⁻¹ at 50% relative humidity (Figure S16). In addition, the impact of humidity on separation performance was evaluated from breakthrough curves. N₂ elution is slightly delayed under humid conditions (24 s under dry conditions and 38 s at 50% RH), suggesting minor competitive adsorption, while the CO₂ breakthrough profiles remain largely unchanged. This behavior indicates that CO₂ retention is preserved in the presence of the moisture, demonstrating the good humidity tolerance of the material under conditions relevant to post‐combustion CO₂ capture.

3. Conclusion

This study demonstrates a synergistic dual‐optimization strategy that combines high surface area engineering with post‐synthetic functionalization to overcome the traditional trade‐off between adsorption capacity and gas selectivity in porous materials, thereby simultaneously enhancing CO₂ capture capacity and selectivity. Through sulfonation and nitration of an HCP‐TPB framework, we achieved significant performance improvements: (1) CO₂ uptake increased from 21.3 to 23.7 and 23.3 wt% at 273 K, even with a substantial reduction in BET surface area (from 2738 to 1796 and 1564 m² g⁻¹), and (2) CO₂/N₂ selectivity rose from 16 to 32 and 42 at 273 K, with IAST‐predicted selectivity for 15:85 CO₂/N₂ mixtures (simulating flue gas) further improving from 14 to 22 and 35 at 298 K. These enhancements are particularly notable given the challenging conditions of flue gas separation (15:85 CO₂/N₂ mixtures).

The –SO₃H functional groups form specific hydrogen bonding interactions with CO₂ molecules. In contrast, the –NO₂ groups introduce dipole‐quadrupole interactions—both of which strengthen CO₂‐framework affinity without compromising the HCP‐TPB matrix's structural integrity. This work establishes a clear and actionable structure‐property relationship: the combination of hierarchical porosity (facilitating mass transfer and physisorption) and targeted polar functionalization (enhancing specific CO₂ interactions) is the key to overcoming the capacity‐selectivity trade‐off.

Our findings not only validate the effectiveness of sulfonation and nitration as post‐synthetic modification strategies for HCPs but also provide a promising direction for developing advanced adsorbents for post‐combustion carbon capture applications, where both high capacity and selectivity are critical for energy‐efficient operation. This dual‐optimization design principle can be extended to other POPs, offering a generalizable approach to advancing gas separation materials. Looking forward, the dual‐optimization strategy demonstrated in this work provides a general design framework for the development of high‐performance porous polymers for gas capture. Future efforts may focus on further tuning functional group density and extending this coordinated porosity‐chemistry approach to other polymer backbones and CO₂‐philic functionalities.

Conflicts of Interest

The authors declare that they have no Conflicts of Interest.

Supporting information

Supporting file 1: chem71113‐sup‐0001‐SuppMat.docx.

Data Availability Statement

The data that supports the findings of this study are available in the supplementary material of this article