Commercially available reagents were used in all reactions without further purification. Tris (4-amino phenyl) amine (TAPA) was purchased from Sigma-Aldrich Chemical Co. and used without further purification. All epoxides and the internal standard used for catalytic reactions were purchased from TCI chemicals and used without further purification. Trichloro heptazine (TCH) was synthesized using a previously reported procedure (Kailasam et al., 2013; Kailasam et al., 2016).

FT-IR (Fourier transform infrared) spectra of the samples were measured by using a Bruker Vertex FT-IR 70/80 spectrometer in the spectral range of 4,000–650 cm⁻¹. An Elementar Vario MACRO cube elemental analyzer was used to carry out elemental analysis (i.e., C, H, and N). Thermogravimetric analysis (TGA) was carried out by a SHIMADZU DTG-60H analyzer under the N₂ environment (flow rate of 30 ml/min) with the temperature ranging from 50 to 500°C (ramping rate of 10°C/min). A solid-state NMR experiment was performed on a JNM-ECA/ECX series using a 5-mm FG NMR tunable probe. The products of the catalytic reactions were identified, and the catalytic conversions were determined by ¹H-NMR spectra using 1, 1′, 2, 2′-tetrachloroethane as an internal standard and recorded in CDCl₃ on the JEOL JNM-ECS-400 spectrometer operating at a frequency of 400 MHz. X-ray diffraction (XRD) patterns were examined using a Bruker D8 Advance diffractometer equipped with a scintillation counter detector, with a Cu-K_α radiation (λ = 0.15418 nm, 2θ = 5─80^o) source operating at 40 kV and 40 mA.

The N₂ physisorption measurements were measured at 77 K on an Autosorb iQ3 instrument (Quantachrome). The CO₂ physisorption measurements were carried out at 273, 281, and 298 K. Ultrapure (99.995%) N₂, He, and CO₂ gases were used for the adsorption–desorption measurements. Prior to adsorption measurements, the sample (∼100 mg) was evacuated at 393 K under vacuum (20 mTorr) for 10 h. The specific surface area of the sample was recorded from the N₂ adsorption isotherm via the Brunauer–Emmett–Teller (BET) micropore assistant method (p/p₀ < 0.1). Pore size distributions were obtained from N₂ desorption isotherms, using the NLDFT method. However, the total pore volume was calculated by the amount of N₂ adsorbed at p/p₀ ∼ 0.9.

A temperature-programmed desorption (TPD) study was carried out by BELCAT II (BELSORB) equipped with a thermal conductivity detector to estimate the volume of absorbed CO₂. Initially, the sample was pretreated at 250°C to remove any trapped solvents and moisture. The TPD profile of CO₂ was recorded at the rate of 10 C min⁻¹ ranging from 50 to 350°C under He flow.

Tris-(4-aminophenyl)amine (0.344 mmol) in 1,4-dioaxne (40 ml) solution was added to a dry round-bottom (RB) flask accompanied by the dropwise addition of diisopropylamine (DIPEA; 2.064 mmol). Furthermore, this homogeneous solution mixture was allowed to cool at 0°C. In another RB flask, a solution of trichloroheptazine (Supplementary Figure S1; 0.344 mmol) in 1,4-dioaxne (10 ml) was prepared and added to the aforementioned homogeneous solution mixture with constant stirring. The temperature of the reaction mixture was maintained at 0°C during dropwise addition and then for another 30 min. After that, it was allowed to stir at room temperature, followed by refluxing at 100°C under nitrogen for 72 h with constant stirring.

Thereafter, the solid residue was collected by filtration and washed with THF, methanol, and acetone, followed by 48-h Soxhlet purification with THF:methanol mixture. After 48 h, the vacuum-dried product was obtained with 73% yield. The elemental analysis of solvent-free samples, calculated%, C:H:N, is found to be C: 55.84, H: 5.066, and N: 27.81 wt%. FTIR (cm⁻¹): 3350, 1641, 1510, 1480, 1360, and 800.

A 25 ml high-pressure glass reactor was used for the cycloaddition reactions of CO₂ and various epoxides. The purified and activated HMP-TAPA catalyst was measured (10 mg) and transferred to the reactor. Then the epoxides were added at room temperature, and the reactor was slowly pressurized with CO₂, flushed twice. Then the required pressure was obtained (0.1–0.6 MPa) for 12 h. After that, the mixture was stirred at a temperature range of 60–80°C for the required time. After catalytic reactions, the reactor was allowed to cool down to RT, the leftover CO₂ gas was released slowly, and the catalyst was separated by centrifugation. The recovered catalyst after each catalytic reaction was collected and washed with DCM–methanol mixture three times and then finally with pure MeOH. This is followed by drying at RT, and the catalyst is reused for successive catalytic cycles. The catalytic conversions were determined using 1,1′,2,2′-tetra-chloroethane as an internal standard. Percentage conversion can be calculated by using the formula.

To construct a heptazine-based polymeric network, TCH (a heptazine precursor) and Tris-(4-aminopenyl) amine were taken into an RB flask, which further underwent a substitution nucleophilic reaction in the presence of DIPEA (as the base) for 72 h. Scheme 1 shows the synthesis procedure of HMP-TAPA. Furthermore, purification was done by Soxhlet extraction using the THF:MeOH (1:1 ratio) mixture for 72 h, and after Soxhlet extraction, HMP-TAPA was vacuum-dried at 120°C before being used for further studies.

In order to explicate the structure of HMP-TAPA, FTIR and, ¹³C CP/MAS, NMR spectroscopy was performed. As shown in Figure 1A, FTIR spectra of HMP-TAPA display an intense peak at 800 cm⁻¹, which corresponds to the breathing mode of the heptazine moiety. The absorption band at 1641 cm⁻¹ harmonized to the C=N stretching vibration of the heptazine moiety, which could be further validated by vanishing of C-Cl stretching vibration at 942 cm⁻¹ in HMP-TAPA (Ma et al., 2016).

Additionally, ¹³C CP/MAS NMR spectroscopy also confirmed the structure of HMP-TAPA. In Figure 1B, HMP-TAPA exhibits an NMR signal at 162 and 156 ppm corresponding to the sp²-hybridized carbon of the heptazine moiety. Furthermore, the disappearance of the NMR peak at 176 ppm suggests the complete substitution of three chlorine atoms. The NMR signal in the range of 110–150 ppm corresponds to the sp² carbon of the TAPA precursor. Furthermore, the PXRD pattern reveals the amorphous nature of the material (Supplementary Figure S2). This might be due to the irreversible kinetic control during the polymerization process of HMP-TAPA, which resulted in the disorderly growth of the framework (Ma et al., 2016).

After probing the chemical structure of the framework, the porous nature of the catalyst was investigated by measuring N₂ adsorption–desorption isotherms at 77 K (Figure 2A). HMP-TAPA shows type-I isotherm indicating the presence of a micropore region with precipitous nitrogen uptake at the low-pressure region. The hysteresis loop indicates the presence of mesopores in HMP-TAPA. The Brunauer−Emmett−Teller (BET) surface area was calculated to be 424 m²/g. The majority of pores lies in the region of 0.7–1.2 nm, and some of the pores are of 2–4 nm, as suggested by pore size distribution (PSD) obtained by using the NLDFT method from the desorption branch of the N₂ isotherm (Figure 2B). In Figure 2A, the observed hysteresis loop is not closed but open. It could also be explained by the swelling effects and softness of the porous organic polymeric network (Weber et al., 2008).

The nitrogen (N) content of HMP-TAPA was estimated around 27.81% (C, H, and residue) by CHN analysis. This high N content, which also corresponds to the Lewis basic nature of framework, may be considered to increase its efficiency toward CO₂ sorption and also its selectivity for CO₂ molecules. It has been well-documented that N-rich frameworks with increased Lewis basic sites render high selectivity.

The selectivity for CO₂ over N₂ is an important requirement for CO₂ adsorbence since the burning of fossils emits fuel gases that contain 15–16% CO₂ having 0.15 bar partial pressure^. Thus, to investigate the separation behavior of heptazine-based porous polymers, binary mixture selectivity at two different temperatures was estimated by the IAST method developed by Myers and Prausnitz (Myers and Prausnitz, 1965). It shows higher CO₂ uptake than N₂, which is about 106.7 mg/g and 11 mg/g at 273 K, (Figure 3B), respectively, (1 bar), and thus, it shows high selectivity of the heptazine-based framework toward CO₂ molecules than N₂. Furthermore, the Langmuir–Freundlich isotherm model was used for fitting the isolated sorption isotherms for CO₂ and N₂ (Supplementary Figures S3–S5). The observed selectivity for HMP-TAPA is 26.27 and 30.97 at 273 and 298 K, respectively, which is the highest achieved value among the heptazine-based porous polymers reported so far (Dang et al., 2015). Moreover, the strong affinity of heptazine-based porous frameworks toward CO₂ has been explained on the basis of the dipole–quadrupole interaction between the N atoms in the pores and the adsorbed CO₂ molecules (Oschatz and Antonietti, 2018). Thus, the selective CO₂ sorption could be easily attributed to a high surface area and rich N content of the porous framework.

To gain further insights into CO₂ adsorption efficiency, isosteric heat of adsorption (Q_st) was calculated from the CO₂ adsorption isotherm at 273, 281, and 298 K, according to the Clausius–Clapeyron equation (Figure 4). The Q_st value measures the initial interaction strength at low gas loading and also establishes the physisorption/chemisorption interaction nature between the adsorbent gas (CO₂) and the porous framework (HMP-TAPA). The Q_st value was calculated to be 32.8 kJ/mol, which suggested a moderate interaction between CO₂ molecules and free -N-, N–H, and -NH₂ groups over the surface of the HMP-TAPA polymeric network.

Interestingly, the as-synthesized porous organic polymer (HMP-TAPA) was found to exhibit high chemical and thermal stability. To test the chemical stability, HMP-TAPA was soaked in different organic solvents (ranging from highly protic to aprotic) with vigorous stirring for 7 days and then the solids were recovered by filtration and washed thoroughly. Water stability was tested at various pH conditions in which HMP-TAPA was stirred in 6 and 9 M NaOH, 18 M H₂SO₄, and 12 M HCl solutions for 10 days (Figure 5A). HMP-TAPA was also found to be insoluble in boiling water, and this proved the remarkable stability of HMP-TAPA, as shown using FTIR (Figure 5A). This unusual behavior could be originating due to the heptazine core of the framework and the close stacking of aromatic rings, making the surface more hydrophobic.

The thermal stability of the synthesized HMP-TAPA was determined by TGA. The TGA profile showed stability of HMP-TAPA up to 420°C, suggesting high thermal stability. The weight loss between 30°C and 100°C could be assigned to the removal of any organic solvent/moisture present in the framework (Figure 5B).

A temperature-programmed desorption (TPD) measurement was performed to detect the concentration of basic sites under gaseous CO₂ flow. TPD analysis showed two peaks at 92°C and 352°C, corresponding to the presence of weak and strong basic sites, respectively, as shown in Supplementary Figure S6. Mainly, the large number of N–H groups on the surface of HMP-TAPA accounted for the strong basic sites. However, the entire concentration of the basic sites was estimated to be 278 μmol/g. Motivated by the high thermal stability and chemical stability along with selective CO₂ capture properties and high basic nature of HMP-TAPA, we envisioned that it can act as an efficient heterogeneous catalyst for the cycloaddition of CO_2, as shown in Scheme 2. Therefore, the catalytic activity of HMP-TAPA was investigated for the cycloaddition reaction using styrene oxide as a model substrate (epoxide) at different conditions of CO₂ pressure and tetra-butyl ammonium bromide (TBAB) as the co-catalyst.

To our surprise, the catalytic results showed the formation of the corresponding cyclic carbonate with about 98% conversion in 12 h (Table 1, entry nos. 1-9). Furthermore, controlled experiments revealed that both HMP-TAPA and TBAB are essential for the catalytic activity (Table 1, entry nos. 1–9). It is known that TBAB acts as a nucleophilic co-catalyst and facilitates the ring opening of the epoxides.

With no additional by-products, 100% selectivity for cyclic carbonates was obtained (Figure 6). The progress of the catalytic reaction can be easily monitored by ¹H NMR spectra of the aliquots taken at regular time intervals with reference to an internal standard (1,1′,2,2′-tetra-chloroethane). The time-dependent ¹H NMR of styrene oxide is been shown in Figure 6.

The optimum conditions required for the cycloaddition reaction were investigated from the controlled experiments which revealed that a temperature of 80°C with catalyst:TBAB loading of (0.1 mol%) and CO₂ pressure of 0.6 MPa are needed (Table 1, Entry no. 9). Having confirmed the optimum conditions for HMP-TAPA, the catalytic activity was further extended to other epoxides having different lengths of alkyl and aromatic units. Interestingly, the catalytic conversion of alkyl epoxides such as 1, 2-epoxybutane, 1, 2-epoxyhexane, and 1, 2-epoxydecane, were found to be lower than that of 1,2-epoxypropane, 2-methyl-1,2-epoxy propane, and epichlorohydrin, which can be attributed to their reduced activity owing to the presence of an alkyl chain which is shorter in case of the first three compounds (Table 2, entries 1 and 3 and Supplementary Figures S7–S8). Thus, the catalytic conversion can be correlated to the confinement in the pores of HMP-TAPA and restricted diffusion of the compounds with larger chain lengths having much slower activity (Table 2, entry 4 to 6 and Supplementary Figures S8–S10). The higher catalytic activity of HMP-TAPA can be attributed to the presence of high density of basic nitrogen sites, resulting in enhanced activity for selective CO₂ capture and conversion. Also, butyl glycidyl ether gives relatively higher catalytic conversion over allyl glycidyl ether which can be ascribed to the electron-donating nature of the former than the latter (Table 2, entry 7-8 and Supplementary Figures S11–S12). Furthermore, aromatic epoxides, such as styrene oxide and phenyl glycidyl ether, were investigated for cycloaddition reaction, which showed a conversion of 81 and 57%, respectively. With the extended time for aromatic epoxides, it was found that styrene oxide takes 12 h and the latter takes 15 h for the complete conversion. (Table 2, entries 9–10 and Supplementary Figures S13–S14). It should be noted that the reaction conditions used in this study for the cycloaddition reaction are relatively milder than the conditions normally employed in the literature for triazine-based frameworks, as shown in Supplementary Table S1.

To rule out the possibility of homogeneous catalysis by HMP-TAPA precursors or monomers leached into the solution, the reaction was stopped at 4 h, and the conversion of styrene oxide was found to be ∼63%. Then the HMP-TAPA catalyst was removed by filtration, and the filtrate was allowed to stir for an additional 8 h in the presence of 6 bar pressure of CO₂. (Figure 7A). The analysis of the aliquot taken at 12 h revealed a slight increase (∼2%) in the conversion of styrene oxide, which is considerably lower than the reaction carried out in the presence of HMP-TAPA. Furthermore, HMP-TAPA was easily separated from the reaction mixture by simple filtration and reused for subsequent cycles after washing with DCM/MeOH. Remarkably, the catalytic activity of HMP-TAPA was retained even after five consecutive cycles (Figure 7B). Also, the FT-IR spectra of the recovered sample were recorded, and it was found that the structure integrity and functional groups present were intact after catalysis (Supplementary Figure S15).

The proposed mechanism for the HMP-TAPA–catalyzed cycloaddition of CO₂ with epoxides is shown in Scheme 3. The first step involves the polarization and subsequent coordination of CO₂ at the basic -N-, -NH-, and -NH₂- moieties present in TAPA and heptazine of HMP-TAPA, resulting in the formation of carbamate species which acts as a nucleophile. Furthermore, these carbamate nucleophiles attack epoxides present on the surface and assist in the ring opening of the epoxide. Also, TBAB helps in faster ring opening of epoxides, which further facilitates the selectivity of the cyclic carbonate in the reaction. The subsequent ring closure reaction by an intramolecular nucleophilic attack of the oxyanion with CO₂ leads to the formation of cyclic carbonate. Then the reductive elimination of the cyclic carbonate regenerates the catalyst for incoming molecules of CO₂ and epoxides, making the catalytic cycle as a continuous process.

PXRD plots of the compounds, FTIR, additional figures, gas adsorption–desorption isotherms, ¹H NMR spectra, ¹³C NMR of heptazine chloride, CO₂ TPD data, IAST selectivity, Langmuir–Freundlich, reaction in different solvents, and IR data to check chemical stability.