Nitrogen-Rich Porous Organic Polymers with Supported Ag Nanoparticles for Efficient CO2 Conversion

As CO2 emissions increase and the global climate deteriorates, converting CO2 into valuable chemicals has become a topic of wide concern. The development of multifunctional catalysts for efficient CO2 conversion remains a major challenge. Herein, two porous organic polymers (NPOPs) functionalized with covalent triazine and triazole N-heterocycles are synthesized through the copper(I)-catalyzed azide–alkyne cycloaddition (CuAAC) reaction. The NPOPs have an abundant microporous content and high specific surface area, which confer them excellent CO2 affinities with a CO2 adsorption capacity of 84.0 mg g−1 and 63.7 mg g−1, respectively, at 273 K and 0.1 MPa. After wet impregnation and in situ reductions, Ag nanoparticles were supported in the NPOPs to obtain Ag@NPOPs with high dispersion and small particle size. The Ag@NPOPs were applied to high-value conversion reactions of CO2 with propargylic amines and terminal alkynes under mild reaction conditions. The carboxylative cyclization transformation of propargylic amine into 2-oxazolidinone and the carboxylation transformation of terminal alkynes into phenylpropiolic acid had the highest TOF values of 1125.1 and 90.9 h−1, respectively. The Ag@NPOP-1 was recycled and used five times without any significant decrease in catalytic activity, showing excellent catalytic stability and durability.


Introduction
The massive consumption of fossil energy has led to an increase in CO 2 emissions in recent years, and the accompanying environmental problems are becoming progressively rigorous. Reducing CO 2 emissions has been an imperative and urgent measure at the present moment [1,2]. To address this issue, the conversion of CO 2 into high-valueadded chemical products at atmospheric pressure is a promising approach, since CO 2 is a sustainable and accessible C 1 feedstock [3]. There have been reports on the conversion of CO 2 into various valuable chemicals, including CO [4,5], CH 4 [6,7], formic acid [8,9], methanol [10,11], cyclic carbonate [12,13], oxazolidinones [14,15], propargylic acid [16,17], etc. Considering the thermodynamic stability and kinetic inertia of CO 2 [18], it is both challenging and groundbreaking to explore catalysts with efficient catalytic activity. Up to now, many catalytic systems have been applied to the high-value conversion of CO 2 , including zeolites [19,20], ionic liquids [21,22], inorganic salts [23,24], metal-organic frameworks (MOFs) [25,26], covalent organic frameworks (COFs) [27,28], porous organic polymers (POPs) [29,30], etc. Despite the previous efforts of many researchers, there are some deficiencies in the catalytic efficiency and catalytic scope of these systems; therefore, it is of great importance to develop catalysts with high efficiency and stability that can be applied to multiple scopes of high value-added conversions of CO 2 .
Covalent triazine frameworks (CTFs) are a novel catalogue of porous organic polymers. Due to their controllable functional framework, adjustable pore structure, high specific The structures of NPOP-1 and NPOP-2 were identified by Fourier transform infrared (FT-IR) spectra ( Figure 1). After the reaction, the characteristic peaks of azide groups at 2114 cm −1 and 2143 cm −1 for TAM and DAB attenuated significantly, and the terminal alkynyl-based characteristic absorption peak of TET at 3290 cm −1 disappeared [55]. In addition, the characteristic peak of a triazole ring was observed at 1609 cm −1 and 1615 cm −1 for NPOP-1 and NPOP-2, respectively, demonstrating the conducting of click reactions and the formation of a triazole ring [57]. Moreover, the characteristic absorption peaks of Ag@NPOP-1 and Ag@NPOP-2 have no obvious difference, which confirms that the chemical environment has no change after the embedding of Ag NPs. In particular, the FT-IR spectra are consistent with the original spectrum, and the characteristic peaks are retained after 24 h of immersion in the 6 mol L −1 HCl aqueous solution or 6 mol L −1 NaOH aqueous solution ( Figure S6), proving the excellent chemical stability of NPOP-1 and NPOP-2. The chemical structure of the two NPOPs were further investigated using solid-state NMR solid-state 13 C cross-polarized/magic-angle-spinning nuclear magnetic resonance ( 13 C CP/MAS NMR) spectroscopy ( Figure 2). The characteristic resonance peak at 170.7  The chemical structure of the two NPOPs were further investigated using solidstate NMR solid-state 13 C cross-polarized/magic-angle-spinning nuclear magnetic resonance ( 13 C CP/MAS NMR) spectroscopy ( Figure 2). The characteristic resonance peak at 170.7 ppm can be attributed to the sp 2 -hybridized carbon atom in the triazine ring [58]. The peaks at 147.6 ppm and 133.9 ppm originate from the two carbon atoms on the triazole ring, proving that the click reaction was conducted, which is further evidenced by the absence of the characteristic signal of alkyne carbon at 80 ppm. Additionally, the peak at 66.3 ppm in NPOP-1 corresponds to the alkane quaternary carbon in the precursor TAM, and the remaining peaks, in the range from 143 ppm to 110 ppm, are attributed to the phenyl carbon atoms [59]. The chemical structure of the two NPOPs were further investigated using solid-state NMR solid-state 13 C cross-polarized/magic-angle-spinning nuclear magnetic resonance ( 13 C CP/MAS NMR) spectroscopy ( Figure 2). The characteristic resonance peak at 170.7 ppm can be attributed to the sp 2 -hybridized carbon atom in the triazine ring [58]. The peaks at 147.6 ppm and 133.9 ppm originate from the two carbon atoms on the triazole ring, proving that the click reaction was conducted, which is further evidenced by the absence of the characteristic signal of alkyne carbon at 80 ppm. Additionally, the peak at 66.3 ppm in NPOP-1 corresponds to the alkane quaternary carbon in the precursor TAM, and the remaining peaks, in the range from 143 ppm to 110 ppm, are attributed to the phenyl carbon atoms [59]. Scanner electron microscopy (SEM) and a transmission electron microscope (TEM) were employed to characterize the morphologies of NPOPs. The SEM images of NPOP-1 and NPOP-2 revealed randomly aggregated porous structures by tiny particles with disordered pores ( Figure 3). NPOP-1 shows a fluffy and porous three-dimensional network Scanner electron microscopy (SEM) and a transmission electron microscope (TEM) were employed to characterize the morphologies of NPOPs. The SEM images of NPOP-1 and NPOP-2 revealed randomly aggregated porous structures by tiny particles with disordered pores (Figure 3). NPOP-1 shows a fluffy and porous three-dimensional network structure, while NPOP-2 is made of tight randomly packed rod-shaped units. There is no significant change in the morphology of Ag@NPOPs, which proves that the anchoring of Ag nanoparticles does not alter the structure of NPOPs. Furthermore, EDS elemental distribution mappings of Ag@NPOP-1 manifest the uniform distribution of Ag, C, and N elements ( Figure 3 and Figure S7). TEM images further present the porous structure of NPOPs; both NPOP-1 and NPOP-2 exhibit a highly cross-linked network-like structure ( Figure 4). HR-TEM demonstrates that the Ag nanoparticles are uniformly distributed on the NPOPs' substrates, the diameters of the Ag nanoparticles are 5.28 ± 1.64 nm and 9.72 ± 1.16 nm for Ag@NPOP-1 and Ag@NPOP-2, respectively. Apparently, Ag@NPOP-1 has much smaller Ag NPs. elements (Figures 3 and S7). TEM images further present the porous structure of NPOPs; both NPOP-1 and NPOP-2 exhibit a highly cross-linked network-like structure ( Figure 4). HR-TEM demonstrates that the Ag nanoparticles are uniformly distributed on the NPOPs' substrates, the diameters of the Ag nanoparticles are 5.28 ± 1.64 nm and 9.72 ± 1.16 nm for Ag@NPOP-1 and Ag@NPOP-2, respectively. Apparently, Ag@NPOP-1 has much smaller Ag NPs.  Powder X-ray diffraction (PXRD) patterns show that NPOP-1 and NPOP-2 both exhibit broad diffraction peaks at around 20°, indicating the amorphous structure of NPOPs ( Figure S8). In particular, the PXRD pattern of Ag@NPOP-1 shows no diffraction peaks of the Ag NPs, which may be attributed to the high dispersion and small particle size of the Ag NPs [60]. In comparison, the PXRD pattern of Ag@NPOP-2 shows a weak peak at 38°, corresponding to the Ag (111) crystal planes [61]. These results are consistent with the HR-TEM, indicating the smaller Ag NPs of Ag@NPOP-1. The thermogravimetric analysis displays that the weight loss below 100 °C is attributed to the volatilization of the remaining solvents. NPOP-1 has comparatively better thermal stability than NPOP-2 ( Figure S9); both NPOP-1 and NPOP-2 exhibit a highly cross-linked network-like structure ( Figure 4). HR-TEM demonstrates that the Ag nanoparticles are uniformly distributed on the NPOPs' substrates, the diameters of the Ag nanoparticles are 5.28 ± 1.64 nm and 9.72 ± 1.16 nm for Ag@NPOP-1 and Ag@NPOP-2, respectively. Apparently, Ag@NPOP-1 has much smaller Ag NPs.  Powder X-ray diffraction (PXRD) patterns show that NPOP-1 and NPOP-2 both exhibit broad diffraction peaks at around 20°, indicating the amorphous structure of NPOPs ( Figure S8). In particular, the PXRD pattern of Ag@NPOP-1 shows no diffraction peaks of the Ag NPs, which may be attributed to the high dispersion and small particle size of the Ag NPs [60]. In comparison, the PXRD pattern of Ag@NPOP-2 shows a weak peak at 38°, corresponding to the Ag (111) crystal planes [61]. These results are consistent with the HR-TEM, indicating the smaller Ag NPs of Ag@NPOP-1. The thermogravimetric analysis displays that the weight loss below 100 °C is attributed to the volatilization of the remaining solvents. NPOP-1 has comparatively better thermal stability than NPOP-2 ( Figure S9); Powder X-ray diffraction (PXRD) patterns show that NPOP-1 and NPOP-2 both exhibit broad diffraction peaks at around 20 • , indicating the amorphous structure of NPOPs ( Figure S8). In particular, the PXRD pattern of Ag@NPOP-1 shows no diffraction peaks of the Ag NPs, which may be attributed to the high dispersion and small particle size of the Ag NPs [60]. In comparison, the PXRD pattern of Ag@NPOP-2 shows a weak peak at 38 • , corresponding to the Ag (111) crystal planes [61]. These results are consistent with the HR-TEM, indicating the smaller Ag NPs of Ag@NPOP-1. The thermogravimetric analysis displays that the weight loss below 100 • C is attributed to the volatilization of the remaining solvents. NPOP-1 has comparatively better thermal stability than NPOP-2 ( Figure S9); NPOP-1 maintains thermal stability up to 230 • C, and NPOP-2 gradually decomposes above 100 • C.
The chemical states and interaction of Ag with N in the NPOPs were determined by XPS measurements ( Figure 5 and Figure S10). The N 1s spectra of NPOPs can be deconvoluted into three peaks, appearing at: 401.28 eV, 400.22 eV, and 399.13 eV for NPOP-1, and 401.29 eV, 400.05 eV, and 398.90 eV for NPOP-2, which correspond to trizolic N=N, trizolicpyrrolic N, and triazine C=N, respectively [62]. After loading Ag nanoparticles, due to the coordination between N and Ag, the N 1s peaks have a positive shift. The trizolic N=N and trizolicpyrrolic N in Ag@NPOP-1 have a 0.22 eV shift toward the higher BE filed in comparison with a 0.12 eV shift of triazine C=N. Similarly, Ag@NPOP-2, trizolic N=N, and trizolicpyrrolic N have a 0.19 eV shift toward the higher BE filed in comparison with a 0.10 eV shift of triazine C=N. This indicates that stronger coordination interactions exist between Ag and the triazole ring. The XPS spectra of the Ag 3d region of Ag@NPOPs further revealed the presence of Ag species. The double binding energy signals, at 374.62 eV and 368.61 eV for Ag@NPOP-1, and at 375.23 eV and 369.31 eV for Ag@NPOP-2, are attributed to the 3d 3/2 and 3d 5/2 binding energies of the Ag(0) peaks, respectively [63].In addition, the signals existing at 375.70 eV and 369.63 eV for Ag@NPOP-1 are 3d 3/2 and 3d 5/2 of Ag(δ+) species, which may be due to partial oxidation of the material during preparation and testing. eV, 400.05 eV, and 398.90 eV for NPOP-2, which correspond to trizolic N=N, trizolicpyrrolic N, and triazine C=N, respectively [62]. After loading Ag nanoparticles, due to the coordination between N and Ag, the N 1s peaks have a positive shift. The trizolic N=N and trizolicpyrrolic N in Ag@NPOP-1 have a 0.22 eV shift toward the higher BE filed in comparison with a 0.12 eV shift of triazine C=N. Similarly, Ag@NPOP-2, trizolic N=N, and trizolicpyrrolic N have a 0.19 eV shift toward the higher BE filed in comparison with a 0.10 eV shift of triazine C=N. This indicates that stronger coordination interactions exist between Ag and the triazole ring. The XPS spectra of the Ag 3d region of Ag@NPOPs further revealed the presence of Ag species. The double binding energy signals, at 374.62 eV and 368.61 eV for Ag@NPOP-1, and at 375.23 eV and 369.31 eV for Ag@NPOP-2, are attributed to the 3d3/2 and 3d5/2 binding energies of the Ag(0) peaks, respectively [63].In addition, the signals existing at 375.70 eV and 369.63 eV for Ag@NPOP-1 are 3d3/2 and 3d5/2 of Ag(δ+) species, which may be due to partial oxidation of the material during preparation and testing.    The porosity of the NPOP-1 and NPOP-2 were investigated by N 2 isothermal adsorptiondesorption measurements at 77 K. As shown in Figure 6, NPOP-1 displays a representative type IV adsorption isotherm, and NPOP-2 exhibits a typical type I isotherm. The N 2 adsorption-desorption isotherms of both the NPOPs manifest a rapid growth of N 2 uptake in a low relative pressure range P/P o < 0.01, demonstrating the existence of micropores. The presence of mesopores is evidenced by the hysteresis loop that accompanies the desorption curve. Moreover, the sharp increase of N 2 uptake after P/P o > 0.9 in the adsorption isotherm of NPOP-2 indicates the presence of macropores. The Brunauer-Emmett-Teller (BET) specific surface areas of NPOP-1 and NPOP-2 are calculated to be 481 and 233 m 2 g −1 , respectively, as shown in Table S2. The total pore volumes of NPOP-1 and NPOP-2 measured at P/P o = 0.99 are 0.39 and 0.55 cm 3 g −1 , respectively. The micropore volumes are 0.23 cm 3 g −1 for NPOP-1 and 0.11 cm 3 g −1 for NPOP-2, accounting for 59.0% and 20.0% of the total pore volumes, respectively. The higher specific surface area of NPOP-1 probably results from the rigid steric structure of the precursor TAM, which qualifies the permanent pore structure and high porosity seen in NPOP-1. Nevertheless, the higher total pore volume of NPOP-2 may be attributed to its partial macropores. Furthermore, the BJH and Horvath-Kawazoe models are used to evaluate the pore size distribution of NPOPs. The BJH model indicates that no macropores exist in NPOP-1, while a wide distribution range of macropores and mesopores from 20-80 nm are shown in NPOP-2 ( Figure S11). The Horvath-Kawazoe model is used to explore the micropore distribution of NPOPs. The results show that the micropore distribution plot of NPOP-1 shows a predominant peak at 0.82 nm, and the micropores of NPOP-2 are mainly concentrated in the range of 1.19-3 nm. It is worth mentioning that the specific surface areas of the NPOPs are lower than the related materials, which may be attributed to the high catalytic activity of Cu(PPh 3 ) 3 Br [64][65][66]. This leads to the rapid generation of a low-polymerized framework with few overlapping parts, and the remaining precursors connect randomly resulting in the formation of an unregulated framework structure. mett-Teller (BET) specific surface areas of NPOP-1 and NPOP-2 are calculated to be 481 and 233 m 2 g −1 , respectively, as shown in Table S2. The total pore volumes of NPOP-1 and NPOP-2 measured at P/Po = 0.99 are 0.39 and 0.55 cm 3 g −1 , respectively. The micropore volumes are 0.23 cm 3 g −1 for NPOP-1 and 0.11 cm 3 g −1 for NPOP-2, accounting for 59.0% and 20.0% of the total pore volumes, respectively. The higher specific surface area of NPOP-1 probably results from the rigid steric structure of the precursor TAM, which qualifies the permanent pore structure and high porosity seen in NPOP-1. Nevertheless, the higher total pore volume of NPOP-2 may be attributed to its partial macropores. Furthermore, the BJH and Horvath-Kawazoe models are used to evaluate the pore size distribution of NPOPs. The BJH model indicates that no macropores exist in NPOP-1, while a wide distribution range of macropores and mesopores from 20-80 nm are shown in NPOP-2 ( Figure S11). The Horvath-Kawazoe model is used to explore the micropore distribution of NPOPs. The results show that the micropore distribution plot of NPOP-1 shows a predominant peak at 0.82 nm, and the micropores of NPOP-2 are mainly concentrated in the range of 1.19-3 nm. It is worth mentioning that the specific surface areas of the NPOPs are lower than the related materials, which may be attributed to the high catalytic activity of Cu(PPh3)3Br [64][65][66]. This leads to the rapid generation of a low-polymerized framework with few overlapping parts, and the remaining precursors connect randomly resulting in the formation of an unregulated framework structure.  Considering the high porosity and abundant N content of NPOPs, we further investigated their CO 2 adsorption performance. As shown in Figure 6, at 273 K and 0.1 MPa, the CO 2 adsorption capacity is 84.0 mg g −1 for NPOP-1 and 63.7 mg g −1 for NPOP-2. At 298 K and 0.1 MPa the CO 2 adsorption capacities of NPOP-1 and NPOP-2 are 51.6 and 39.8 mg g −1 , respectively. The higher CO 2 uptake value of NPOP-1 may be attributed to its higher specific surface area and abundant microporous content. Both the NPOPs exhibit competitive CO 2 adsorption values compared to related triazine-/triazole-based porous organic polymers reported in the literature (Table S3) [65][66][67][68][69][70][71][72][73][74][75][76][77][78][79][80][81][82][83]. To further understand the interaction between CO 2 and NPOPs, the CO 2 isosteric adsorption heats (Q st ) are calculated based on the Clausius-Clapeyron equation using the CO 2 adsorption isotherms at 273 K and 298 K (Figures 6, S12 and S13). The Q st values of NPOPs both decrease with the increase in CO 2 adsorption amounts. At zero coverage, the Q st values of NPOP-1 and NPOP-2 are calculated to be 32.56 and 32.51 kJ mol −1 , respectively. The Q st values are higher than other porous adsorbents and triazine-/triazole-based porous organic polymers (Table S3) [84,85]. Higher Q st values suggest stronger interactions between NPOPs and CO 2 ; moreover, the NPOP-1 shows a larger Q st than NPOP-2 at the same absorption value, implying a stronger affinity between CO 2 and NPOP-1. The high CO 2 uptake may originate from the large specific surface area and abundance of the electron-rich N element of NPOPs [86]. The conversion of CO 2 into high-value chemicals under mild conditions (atmospheric pressure) is a promising approach to reducing the greenhouse effect and saving valuable fossil energy. Among these promising reaction pathways, the carboxylative cyclization reaction of CO 2 with propargylamine to 2-oxazolidinone is representative; moreover, oxazolidinones have been widely used as chemical intermediates and pharmaceuticals [87,88]. It is of vital importance to explore the mild reaction conditions and the higher reaction efficiency. Considering that the NPOPs have a high adsorption capacity for CO 2 , Ag@NPOPs were used as the catalysts in the carboxylative cyclization reaction of CO 2 with propargylamine. The N-benzylprop-2-yn-1-amine [89] was used as a model substrate to study optimal reaction conditions (Figures S14 and S15), and the results of various controlled experiments are shown in Table 1. When CH 3 CN was used as a solvent and DBU as a base, the yield of 2-oxazolidinone was 97.0% for Ag@NPOP-1 with a TOF value of 1125.1 h −1 , and 93.0% for Ag@NPOP-2 with a TOF value of 777.7 h −1 (Table 1, Entry 2, 3). The Ag@NPOP-1 shows a higher catalytic efficiency because of its larger specific surface area, higher CO 2 adsorption capability, and smaller Ag NPs size. No product was detected when NPOP-1 or NPOP-2 were used as catalysts, which proves the vital role of Ag NPs in the catalysis reaction (Table 1, Entry 4, 5). Similarly, no product was detected in the absence of DBU, proving the essential role of DBU as a base. Additionally, the catalytic efficiency in different solvents was investigated, and the product yields in DMSO, DMF, and EtOH were 58%, 42%, and 15%, respectively (Table 1, Entry 7-9). Therefore, CH 3 CN is the optimal solvent. In addition to DBU, other different common bases were also adopted to explore the catalytic effect, when we replaced DBU with Cs 2 CO 3 and K 2 CO 3 , the 2-oxazolidinone yields were only 4.0% and 2.0%, respectively, in CH 3 CN. When NaOH was used as a base, no product was detected. The influence of temperature in the reaction was also investigated. At lower temperatures, the product yields were significantly reduced as a result of inadequate reactivity. When the reaction temperatures were 40 • C and 30 • C, the product yields of 2-oxazolidinone were 86.0% and 40.0%, respectively. Propiolic acid is a valuable intermediate utilized in pharmaceuticals and fine chemicals [90,91]. We chose 1-ethynylbenzene as a model substrate ( Figure S16), and the results of various controlled experiment are shown in Table 2. When Ag@NPOP-1 was used as the catalyst and the reaction time was 12 h at 60 °C with 3 eq. Cs2CO3, the yield of 3-phenylproparylic acid reached 94.0%, slightly higher than that of Ag@NPOP-2 (92.1%) ( Table  2, Entry 1, 2). Surprisingly, the reaction also proceeded with metal-free NPOP-1 and NPOP-2, but the product yield was only 55.4% and 51.2%, respectively (Table 2, Entry 3, 4), which suggests that the nitrogen atoms in the triazole and triazine rings act as active sites to promote CO2 conversion. By increasing the amount of catalyst from 2.0 mg to 5.0 mg, the yield of 3-phenylpropargylic acid was 94.2%, proving that a larger amount of catalyst could not significantly facilitate the catalytic efficiency. Subsequently, several solvents were screened: DMF gave a medium yield of 73.7%, and the product yields in MeCN and EtOH were much lower at 11.9% and 3.4%, respectively ( Table 2, Entry 6-8). Accordingly, DMSO is the best solvent for this reaction because Cs2CO3 has a higher solubility in DMSO and it also works as a good solvent for CO2 due to its higher polarity. In addition to Cs2CO3, other bases were tested. When K2CO3, DBU, or NaOH were used as a base, the yield of 3-phenylproparylic acid was 11.5%, 51.0%, and 4.3%, respectively ( Table 2, Entry 9-11), suggesting that Cs2CO3 is the preferable base. Apart from that, the influence of different base dosages in the reaction was explored. When Cs2CO3 was 2 eq. and 1 eq., the corresponding product yield was 60.8% and 24.5%, respectively. The reduction in the amount of base was followed by a reduction in the product yield. It shows the base's essential role in the conversion of CO2, as it helps the deprotonation process of phenylacetylene to facilitate the formation of reaction intermediates. Additionally, the impact of temperature on the reaction was also investigated, and reducing temperature led to a notably decreased yield, when the reaction was conducted at 50 °C and 40 °C, the product yield was 65.2% and 18.0%, respectively. amount of base was followed by a reduction in the product yield. It shows the base's essential role in the conversion of CO2, as it helps the deprotonation process of phenylacetylene to facilitate the formation of reaction intermediates. Additionally, the impact of temperature on the reaction was also investigated, and reducing temperature led to a notably decreased yield, when the reaction was conducted at 50 °C and 40 °C, the product yield was 65.2% and 18.0%, respectively. The carboxylation of terminal alkynes to phenylpropiolic acid is another feasible way to realize the high-value transformation of CO 2 . Furthermore, it accords with the atomic economy concept and is more environmentally friendly compared to traditional methods. Propiolic acid is a valuable intermediate utilized in pharmaceuticals and fine chemicals [90,91]. We chose 1-ethynylbenzene as a model substrate ( Figure S16), and the results of various controlled experiment are shown in Table 2. When Ag@NPOP-1 was used as the catalyst and the reaction time was 12 h at 60 • C with 3 eq. Cs 2 CO 3 , the yield of 3-phenylproparylic acid reached 94.0%, slightly higher than that of Ag@NPOP-2 (92.1%) ( Table 2, Entry 1, 2). Surprisingly, the reaction also proceeded with metal-free NPOP-1 and NPOP-2, but the product yield was only 55.4% and 51.2%, respectively (Table 2, Entry 3, 4), which suggests that the nitrogen atoms in the triazole and triazine rings act as active sites to promote CO 2 conversion. By increasing the amount of catalyst from 2.0 mg to 5.0 mg, the yield of 3-phenylpropargylic acid was 94.2%, proving that a larger amount of catalyst could not significantly facilitate the catalytic efficiency. Subsequently, several solvents were screened: DMF gave a medium yield of 73.7%, and the product yields in MeCN and EtOH were much lower at 11.9% and 3.4%, respectively (Table 2, Entry 6-8). Accordingly, DMSO is the best solvent for this reaction because Cs 2 CO 3 has a higher solubility in DMSO and it also works as a good solvent for CO 2 due to its higher polarity. In addition to Cs 2 CO 3 , other bases were tested. When K 2 CO 3 , DBU, or NaOH were used as a base, the yield of 3-phenylproparylic acid was 11.5%, 51.0%, and 4.3%, respectively (Table 2, Entry 9-11), suggesting that Cs 2 CO 3 is the preferable base. Apart from that, the influence of different base dosages in the reaction was explored. When Cs 2 CO 3 was 2 eq. and 1 eq., the corresponding product yield was 60.8% and 24.5%, respectively. The reduction in the amount of base was followed by a reduction in the product yield. It shows the base's essential role in the conversion of CO 2 , as it helps the deprotonation process of phenylacetylene to facilitate the formation of reaction intermediates. Additionally, the impact of temperature on the reaction was also investigated, and reducing temperature led to a notably decreased yield, when the reaction was conducted at 50 • C and 40 • C, the product yield was 65.2% and 18.0%, respectively.

Catalyst Stability
Recycling tests were conducted to confirm the reusability and endurance of Ag@NPOP-1 toward the carboxylative cyclization of propargylic amines and the carboxylation of phenylacetylene with CO2 (Figure 7a,b). After being used for five times, the yield of 2-oxazolidinone and phenylpropionic acid were 93.0% and 90.4%, respectively, with no distinct loss of catalytic efficiency. TEM of reused Ag@NPOP-1 (Figure 7c) shows that the Ag NPs remain uniformly dispersed on the NPOP substrate with slight agglomeration, with an average particle diameter of 5.7 nm. The FT-IR and PXRD (Figures S17 and S18) of Ag@NPOP-1 show that there is no significant change after it is used five times, proving the excellent reusability of Ag@NPOP-1.

Catalyst Stability
Recycling tests were conducted to confirm the reusability and endurance of Ag@NPOP-1 toward the carboxylative cyclization of propargylic amines and the carboxylation of phenylacetylene with CO 2 (Figure 7a,b). After being used for five times, the yield of 2-oxazolidinone and phenylpropionic acid were 93.0% and 90.4%, respectively, with no distinct loss of catalytic efficiency. TEM of reused Ag@NPOP-1 (Figure 7c) shows that the Ag NPs remain uniformly dispersed on the NPOP substrate with slight agglomeration, with an average particle diameter of 5.7 nm. The FT-IR and PXRD (Figures S17 and S18) of Ag@NPOP-1 show that there is no significant change after it is used five times, proving the excellent reusability of Ag@NPOP-1.
Based on the above observations and previous reports [100,111], a possible catalytic mechanism of carboxylative cyclization of propargylic amines with CO 2 is proposed (Scheme 2). When propargylamine enters the pores of NPOP-1, the amino and carboncarbon triple bonds in propargylamine interact with Ag NPs, further leading to the activation of hydrogen protons on the amino group, and, with the assistance of DBU, the CO 2 molecule adsorbed by NPOP-1 attacks the amino group to produce a carbamate intermediate (III). Subsequently, the negatively charged oxygen attacks the carbon-carbon triple bond, resulting in an intramolecular cyclization of propargylamine with the formation of a negatively charged carbon-carbon double bond (IV). After the proton from DBUH + is seized, the final 2-oxazolidinone product is generated.
Apart from that, the mechanism of the carboxylation of terminal alkynes is postulated (Scheme 3) [112,113]. First, the terminal alkynes enter the pores of NPOP-1 and are deprotonated by the immobilized Ag NPs with the assistance of Cs 2 CO 3 , forming the Ph-C≡C-Ag@NPOP-1 intermediate (II). Subsequently, the CO 2 molecule enriched by adsorption of abundant nitrogen atoms in the triazine and triazole rings attacks the adjacent nucleophilic active alkyne carbon and further inserts into the C-Ag bond to form a cesium carboxylate species (IV). In the presence of Cs 2 CO 3 and nearby terminal alkynes, the cesium carboxylate detaches from the Ag NPs and enters the solvent, where Ag@NPOP-1 is regenerated and adsorbs new terminal alkynes for the next round of catalysis. Eventually, the carboxylate is acidified by hydrochloric acid to obtain propargylic acid. Nanomaterials 2022, 12, x FOR PEER REVIEW 13 of 18 regenerated and adsorbs new terminal alkynes for the next round of catalysis. Eventually, the carboxylate is acidified by hydrochloric acid to obtain propargylic acid.

Conclusions
In summary, we have synthesized organic porous polymers NPOP-1 and NPOP-2 functionalized with triazine and triazole nitrogen heterocycles by the click reaction. The NPOPs are porous with specific surface areas of 481 m 2 g −1 for NPOP-1 and 223 m 2 g −1 for NPOP-2. The abundant nitrogen content confers high CO 2 affinity and adsorption capacity on the NPOPs with the CO 2 adsorption capacity of NPOP-1 and NPOP-2 of 84.0 and 63.7 mg g −1 , respectively, at 273 K and 1 atm. The Ag@NPOPs are obtained by simple wet impregnation and in situ reductions with highly dispersed small-size Ag nanoparticles. The Ag@NPOPs show excellent catalytic activity in the catalysis of the high-value conversion of CO 2 with propargylamine and terminal alkynes, with the highest TOF values reaching 1125.1 h −1 and 90.9 h −1 , respectively. The Ag@NPOP-1 shows higher catalytic efficiency because of its larger specific surface area, higher CO 2 adsorption capability, and smaller Ag NPs size. The Ag@NPOPs show excellent catalytic stability and durability with no significant decrease in catalytic activity after five consecutive cycles. More than demonstrating a dual-functional CO 2 conversion catalyst, this work provides some new inspirations for the design and construction of novel multifunctional catalysts.

Author Contributions:
The manuscript was written with the contributions of all authors. All authors have read and agreed to the published version of the manuscript.