Efficient Selective Capture of Carbon Dioxide from Nitrogen and Methane Using a Metal-Organic Framework-Based Nanotrap

Selective carbon capture from exhaust gas and biogas, which mainly involves the separation of CO2/N2 and CO2/CH4 mixtures, is of paramount importance for environmental and industrial requirements. Herein, we propose an interesting metal-organic framework-based nanotrap, namely ZnAtzCO3 (Atz− = 3-amino-1,2,4-triazolate, CO32− = carbonate), with a favorable ultramicroporous structure and electrostatic interactions that facilitate efficient capture of CO2. The structural composition and stability were verified by FTIR, TGA, and PXRD techniques. Particularly, ZnAtzCO3 demonstrated high CO2 capacity in a wide range of pressures, with values of 44.8 cm3/g at the typical CO2 fraction of the flue gas (15 kPa) and 56.0 cm3/g at the CO2 fraction of the biogas (50 kPa). Moreover, ultrahigh selectivities over CO2/N2 (15:85, v:v) and CO2/CH4 (50:50, v:v) of 3538 and 151 were achieved, respectively. Molecular simulations suggest that the carbon atom of CO2 can form strong electrostatic Cδ+···δ−O-C interactions with four oxygen atoms in the carbonate ligands, while the oxygen atom of CO2 can interact with the hydrogen atoms in the triazolate ligands through Oδ−···δ+H-C interactions, which makes ZnAtzCO3 an optimal nanotrap for CO2 fixation. Furthermore, breakthrough experiments confirmed excellent real-world separation toward CO2/N2 and CO2/CH4 mixtures on ZnAtzCO3, demonstrating its great potential for selective CO2 capture.


Introduction
Global warming, one of the biggest global issues, is causing various long-term disastrous environmental effects, including abnormal climate patterns, rising sea levels, accelerated extinction of species, and shifts in agricultural patterns, which pose severe threats to the survival and development of humanity [1][2][3][4].As the main source of greenhouse gases, carbon dioxide (CO 2 ), with a large annual emission (e.g., 36.8Gt in 2022), contributes to 69.4% of the anthropogenic greenhouse gases [5,6].Hence, CO 2 capture and utilization is of utmost importance to curb global warming [7].In particular, electricity generation from the combustion of fossil fuels, resulting in exhaust gas composed mainly of CO 2 and nitrogen (N 2 ), is the primary source of anthropogenic CO 2 emissions [8,9].Additionally, biogas, acknowledged as "renewable natural gas", is a green fuel with the efficient component of methane (CH 4 ) [10].Nevertheless, biogas contains a considerable amount of CO 2 that could significantly decrease the calorific value and lead to severe erosion in the equipment.Therefore, efficient selective capture of CO 2 from N 2 and CH 4 is of paramount significance for environmental protection and biogas upgrading.
Benefiting from a plausible gentle operation condition and reduced energy consumption for regeneration, adsorptive separation of CO 2 has been recognized as a promising alternative among the current CO 2 capture techniques [11][12][13][14].Fundamentally, an ideal adsorbent with high capacity and selectivity for the target molecule is the key to achieving satisfactory separation performance [15,16].As a class of innovative porous adsorbents, metal-organic frameworks (MOFs) are showing extraordinary versatility in pore tunability and chemical functionalities [17].Hence, adsorptive separation based on MOFs has been explored in miscellaneous separation circumstances, including carbon capture [8,9,11,[18][19][20].In general, there are several strategies that can enhance the CO 2 capture performance on MOF materials, including (i) incorporation of functionalities (such as open metal sites, Lewis basic sites, and polar functional groups) into the frameworks, (ii) utilization of ultramicropores to maximize the confinement effect, (iii) use of kinetic difference and even the molecular sieving effect, (iv) introduction of structural flexibility, (v) the combination thereof [16,21,22].Specifically, MOFs constructed by triazolate linkers represent one subclass of MOFs that are comprised of excellent CO 2 capture performance, affordable costs, and good stability under humid conditions [23,24].For example, by introducing amino groups into the triazolate linkers of the prototype MOF ZnF(TZ), the resultant ZnF(daTZ) demonstrates an appreciable volumetric CO 2 uptake of 75 cm 3 /cm 3  and high CO 2 /N 2 equilibrium selectivity of 120 based on the ideal adsorbed solution theory (IAST) as well as excellent CO 2 /H 2 O kinetic selectivity of 70 [25].This remarkable separation performance was probably originated from the main adsorptive site at the channel center for CO 2 to afford electrostatic interactions with the amino groups, while H 2 O was more likely to locate at the channel corner, according to GCMC simulations.Built from dual ligands of oxalate and 1,2,4-triazolate, zinc-based Calgary Framework 20 (CALF-20) exhibits a high CO 2 capacity of 87.36 cm 3 /g at atmospheric conditions, excellent selectivities over N 2 and H 2 O, and the facility for scalable production, making it the first practical MOF for industrial carbon capture [26,27].Molecular simulations suggest the CO 2 adsorption location in CALF-20 also lies in the channel center to form interactions with the zinc/nitrogen/carbon atoms in the framework.Likewise, the amine-appended zinc-oxalate-triazolate MOFs demonstrated enhanced CO 2 capacity at low CO 2 concentrations compared to CALF-20, due to higher-density interaction sites and more contracted pore sizes [28,29].Recently, we prepared a flexible MOF, namely ZnDatzBdc, that showed step-shaped CO 2 isotherm due to breakage/reformation of intra-framework hydrogen bonds and rotation of the phenyl rings, giving rise to an excellent CO 2 theoretical working capacity of 94.9 cm 3 /cm 3 if performed in typical pressure vacuum swing adsorption at 273 K [30].
By now, developing nanotraps with multiple host-guest interactions toward the target molecules offers a feasible strategy to accomplish high adsorption capacity and selectivity, which has been successfully applied in separation circumstances, such as C 2 H 2 /CO 2 separation [31,32], C 3 H 4 /C 3 H 6 separation [33,34], CH 4 /N 2 separation [35], and olefin/paraffin separation [36].For CO 2 adsorption, if a contracted pore exhibits opposite electrostatics on the adjacent positions and the same electrostatics on its opposite side, it can form strong electrostatic interactions with both the carbon and oxygen atoms in the CO 2 molecule, as shown in Figure 1.Hence, this type of pore can act as a suitable nanotrap for CO 2 fixation, from which a remarkable capacity and selectivity for CO 2 can be achieved.
Herein, we synthesized a novel MOF, namely ZnAtzCO 3 , with 3-amino-1,2,4-triazolate (Atz -) and carbonate (CO 3 2− ) as dual ligands.Inspiringly, ZnAtzCO 3 shows the desired ultramicropores due to the small-sized ligands and suitable crystal structure, which is desirable for the adsorption and separation of small molecules, such as CO 2 .Moreover, the favorable electrostatic environment of ZnAtzCO 3 makes it a feasible nanotrap to form multiple host-guest interactions with CO 2 , and hence, efficient CO 2 capture from N 2 and CH 4 could be achieved.In particular, at atmospheric temperature, equilibrium isotherms showed high CO 2 capacities of ZnAtzCO 3 in a wide pressure range, with values of 44.8 cm 3 /g (STP, standard temperature and pressure) at the typical fraction of the flue gas (15 kPa) and 56.0 cm 3 /g at the fraction of the biogas (50 kPa).Moreover, adsorptive selectivity based on the IAST model indicated that ultra-high CO 2 /N 2 and CO 2 /CH 4 selectivities of 3538 and 151 were realized at ambient conditions, respectively.The excellence in capacity and selectivity of this MOF-based nanotrap was illustrated by molecular simulations in terms of preferential adsorption sites, binding energy, and adsorption distributions.Furthermore, breakthrough experiments toward the binary mixtures of CO 2 /N 2 and CO 2 /CH 4 were conducted on ZnAtzCO 3 , which verified its efficient dynamic separation performance.Herein, we synthesized a novel MOF, namely ZnAtzCO3, with 3-amino-1,2,4-triazolate (Atz -) and carbonate (CO3 2-) as dual ligands.Inspiringly, ZnAtzCO3 shows the desired ultramicropores due to the small-sized ligands and suitable crystal structure, which is desirable for the adsorption and separation of small molecules, such as CO2.Moreover, the favorable electrostatic environment of ZnAtzCO3 makes it a feasible nanotrap to form multiple host-guest interactions with CO2, and hence, efficient CO2 capture from N2 and CH4 could be achieved.In particular, at atmospheric temperature, equilibrium isotherms showed high CO2 capacities of ZnAtzCO3 in a wide pressure range, with values of 44.8 cm 3 /g (STP, standard temperature and pressure) at the typical fraction of the flue gas (15 kPa) and 56.0 cm 3 /g at the fraction of the biogas (50 kPa).Moreover, adsorptive selectivity based on the IAST model indicated that ultra-high CO2/N2 and CO2/CH4 selectivities of 3538 and 151 were realized at ambient conditions, respectively.The excellence in capacity and selectivity of this MOF-based nanotrap was illustrated by molecular simulations in terms of preferential adsorption sites, binding energy, and adsorption distributions.Furthermore, breakthrough experiments toward the binary mixtures of CO2/N2 and CO2/CH4 were conducted on ZnAtzCO3, which verified its efficient dynamic separation performance.

Crystal Structure and Pore Properties
Reactions of ZnSO4 and 3-amino-1H-1,2,4-triazole (HAtz) in a binary solution of DMF/H2O afforded high-quality crystals of ZnAtzCO3.The single-crystal X-ray diffraction (SCXRD) measurement indicates that ZnAtzCO3 belongs to the triclinic crystal system (a = 9.6217 Å, b = 9.6316 Å, c = 16.3408Å, α = 81.355°,β = 86.938°,γ = 76.093°).Each asymmetric unit contains four zinc atoms, four 3-aminotriazolate ligands, and two triangular carbonate linkers (Figure S1).Because no carbonate was added to the reactants, it is assumed that the carbonate linker originated from the decomposition of the DMF molecule [37].In addition, the elemental analysis suggests no sulfur element in the framework, which further confirms the existence of the carbonate linker instead of the sulfate or sulfite linker in the framework.As shown in Figure 2a, the zinc atom coordinates with one oxygen atom from the carbonate linker and three nitrogen atoms from three different triazolate rings.Each carbonate linker contains one uncoordinated oxygen atom, which is fixed by the intra-framework hydrogen bonding with the amino group in the triazolate ligand.We tried to obtain isoreticular structures by replacing the HAtz ligand with 1H-   S1).Because no carbonate was added to the reactants, it is assumed that the carbonate linker originated from the decomposition of the DMF molecule [37].In addition, the elemental analysis suggests no sulfur element in the framework, which further confirms the existence of the carbonate linker instead of the sulfate or sulfite linker in the framework.As shown in Figure 2a, the zinc atom coordinates with one oxygen atom from the carbonate linker and three nitrogen atoms from three different triazolate rings.Each carbonate linker contains one uncoordinated oxygen atom, which is fixed by the intra-framework hydrogen bonding with the amino group in the triazolate ligand.We tried to obtain isoreticular structures by replacing the HAtz ligand with 1H-1,2,4-triazole (HTz) and 3,5-diamine-1H-1,2,4-triazole (HDatz), but the trial failed.Hence, we assume that these weak intra-framework hydrogen bonds are vital for the structural formation of ZnAtzCO 3 .ZnAtzCO 3 can be regarded as a pillared-layered structure by connecting the wavy and continuous zinc-triazolate layers by the carbonate pillars (Figure 2b).Inspiringly, the small size of the carbonate and triazolate ligands gives rise to an ultramicroporous structure desirable for adsorptive separation.Specifically, ZnAtzCO 3 contains two types of zig-zag channels with a cross-section area of 2.9 × 5.1 Å 2 and 3.5 × 5.1 Å 2 that interconnect with the adjacent channels through small apertures of 3.0 × 3.9 Å 2 , 2.2 × 2.8 Å 2 , and 2.6 × 3.3 Å 2 .We noticed that a similar form of Zn 2 (atz) 2 (CO 3 ) was previously reported by the reaction of Zn(NO 3 ) 2 , NaHCO 3 , and HAtz.Zn 2 (atz) 2 (CO 3 ) displayed the same connectivity but belonged to another different space group of a Pnma unit cell (a = 9.806 Å, b = 9.3353 Å, c = 16.194Å, α = β = γ = 90 • ) [38].The difference in structure is probably because the degrees of buckling for zinc-triazolate layers can vary significantly under different synthetic conditions.Because the specific crystal structure affects the pore systems and, subsequently, the sorption behavior, the following discussion was carried out with our obtained crystal data.
reported by the reaction of Zn(NO3)2, NaHCO3, and HAtz.Zn2(atz)2(CO3) displayed the same connectivity but belonged to another different space group of a Pnma unit cell (a = 9.806 Å, b = 9.3353 Å, c = 16.194Å, α = β = γ = 90°) [38].The difference in structure is probably because the degrees of buckling for zinc-triazolate layers can vary significantly under different synthetic conditions.Because the specific crystal structure affects the pore systems and, subsequently, the sorption behavior, the following discussion was carried out with our obtained crystal data.

Characterizations
The physicochemical behavior of ZnAtzCO3 was measured to investigate its textural characteristics.Fourier transform infrared reflection (FTIR) patterns were performed to

Characterizations
The physicochemical behavior of ZnAtzCO 3 was measured to investigate its textural characteristics.Fourier transform infrared reflection (FTIR) patterns were performed to further confirm the existence of the carbonate linker.Figure 3a suggests an intense broad band at 1408 cm −1 and an additional band at 1310 cm −1 , corresponding to the asymmetric stretching modes of carbonate [39].Besides, the medium band at 850 cm −1 was assigned to the bending mode of the carbonate.Figure 3b depicts the powder X-ray diffraction (PXRD) pattern of ZnAtzCO 3 , which is identical to that derived from SCXRD measurement, indicative of the high purity of the powder sample.In addition, the activation step did not lead to transformation or decomposition of the structure, as suggested by the well-maintained PXRD patterns.Thermogravimetric (TG) analysis in Figure 3c indicates that ZnAtzCO 3 is stable up to 500 K, and hence, it holds enough thermal stability for adsorptive separation, which usually requires moderate heating for regeneration.The porosity feature was derived from CO 2 sorption isotherms at 195 K, as shown in Figure 3d.The typical type-I CO 2 isotherms indicated the microporous nature of ZnAtzCO 3 , giving a Brunauer-Emmett-Teller (BET) surface area of 455.6 m 2 /g and a micropore volume of 0.196 cm 3 /g.to transformation or decomposition of the structure, as suggested by the well-maintained PXRD patterns.Thermogravimetric (TG) analysis in Figure 3c indicates that ZnAtzCO3 is stable up to 500 K, and hence, it holds enough thermal stability for adsorptive separation, which usually requires moderate heating for regeneration.The porosity feature was derived from CO2 sorption isotherms at 195 K, as shown in Figure 3d.The typical type-I CO2 isotherms indicated the microporous nature of ZnAtzCO3, giving a Brunauer-Emmett-Teller (BET) surface area of 455.6 m 2 /g and a micropore volume of 0.196 cm 3 /g.

Adsorption Equilibrium Behavior of CO2, N2, and CH4
Figure 4a depicts the pure adsorption isotherms of CO2, N2, and CH4 on ZnAtzCO3 at 298 K.It is noticed that CO2 capacity increased sharply at low pressure, while N2 and CH4 uptakes showed a slow increment as the pressure rose.Hence, ZnAtzCO3 showed a significantly higher CO2 capacity than N2 and CH4 between 0-100 kPa.At atmospheric pressure, CO2 capacity reached as high as 62.8 cm 3 /g, while this value for N2 and CH4 was 4.3 and 14.7 cm 3 /g, respectively.This distinction in adsorption capacity suggests excellent thermodynamic separation for CO2/N2 and CO2/CH4.Moreover, ZnAtzCO3 demonstrated

Adsorption Equilibrium Behavior of CO 2 , N 2 , and CH 4
Figure 4a depicts the pure adsorption isotherms of CO 2 , N 2 , and CH 4 on ZnAtzCO 3 at 298 K.It is noticed that CO 2 capacity increased sharply at low pressure, while N 2 and CH 4 uptakes showed a slow increment as the pressure rose.Hence, ZnAtzCO 3 showed a significantly higher CO 2 capacity than N 2 and CH 4 between 0-100 kPa.At atmospheric pressure, CO 2 capacity reached as high as 62.8 cm 3 /g, while this value for N 2 and CH 4 was 4.3 and 14.7 cm 3 /g, respectively.This distinction in adsorption capacity suggests excellent thermodynamic separation for CO 2 /N 2 and CO 2 /CH 4 .Moreover, ZnAtzCO 3 demonstrated an exceptional CO 2 adsorption capacity of 44.8 cm 3 /g at 15 kPa, highlighting its promising application prospects in low-concentration CO 2 capture, such as CO 2 elimination from the exhaust gas.Moreover, ZnAtzCO 3 exhibits a high CO 2 capacity of 56.0 cm 3 /g at 50 kPa, indicative of its potential in CO 2 separation with higher CO 2 concentrations, including CO 2 removal from the biogas.
To evaluate the competitive separation of CO 2 /CH 4 and CO 2 /N 2 on ZnAtzCO 3 , the IAST selectivities were calculated by means of the IAST model, taking into account the composition of CO 2 /N 2 (15:85, v:v) and CO 2 /CH 4 (50:50,v:v) in the exhaust gas and biogas, respectively [40,41].By incorporating the dual-site Langmuir-Freundlich (DSLF) parameter (Table S1) into the IAST model [42], the IAST selectivities were obtained and shown in Figure 4b.Significantly, CO 2 /CH 4 and CO 2 /N 2 selectivities are quite high in the whole pressure range of 0-100 kPa.At ambient pressure, the IAST selectivity for CO 2 /CH 4 and CO 2 /N 2 reached as high as 3538 and 151, respectively, which are comparable to those benchmark MOFs for selective CO 2 capture through thermodynamic separation [12].an exceptional CO2 adsorption capacity of 44.8 cm 3 /g at 15 kPa, highlighting its promising application prospects in low-concentration CO2 capture, such as CO2 elimination from the exhaust gas.Moreover, ZnAtzCO3 exhibits a high CO2 capacity of 56.0 cm 3 /g at 50 kPa, indicative of its potential in CO2 separation with higher CO2 concentrations, including CO2 removal from the biogas.To evaluate the competitive separation of CO2/CH4 and CO2/N2 on ZnAtzCO3, the IAST selectivities were calculated by means of the IAST model, taking into account the composition of CO2/N2 (15:85, v:v) and CO2/CH4 (50:50,v:v) in the exhaust gas and biogas, respectively [40,41].By incorporating the dual-site Langmuir-Freundlich (DSLF) parameter (Table S1) into the IAST model [42], the IAST selectivities were obtained and shown in Figure 4b.Significantly, CO2/CH4 and CO2/N2 selectivities are quite high in the whole pressure range of 0-100 kPa.At ambient pressure, the IAST selectivity for CO2/CH4 and CO2/N2 reached as high as 3538 and 151, respectively, which are comparable to those benchmark MOFs for selective CO2 capture through thermodynamic separation [12].Hence, compromised of excellent CO2 capacity and selectivity, ZnAtzCO3 reveals great potential for selective CO2 capture.
Furthermore, the isosteric heat (Qst) of CO2, N2, and CH4 on ZnAtzCO3, which can be derived from their pure adsorption isotherms at various temperatures (273, 288, and 298 K), is a crucial parameter for determining the adsorption interaction strengths.As shown in Figure 5b, the zero-coverage Qst for the three gases followed an order of CO2 (32.6 kJ/mol) > CH4 (22.4 kJ/mol) > N2 (18.1 kJ/mol), consistent with the dipole moment of the guest molecules (CO2: 29.1 × 10 −25 cm 3 , CH4: 25.9 × 10 −25 cm 3 , N2: 17.4 × 10 −25 cm 3 ) [43].Additionally, it is apparent that the Qst for all gases remained constant between 0-100 kPa, suggesting homogeneity on the pore surface.From the Qst result, we speculate that the remarkable CO2 selectivity is primarily attributed to its highest adsorption enthalpy.Furthermore, the isosteric heat (Q st ) of CO 2 , N 2 , and CH 4 on ZnAtzCO 3 , which can be derived from their pure adsorption isotherms at various temperatures (273, 288, and 298 K), is a crucial parameter for determining the adsorption interaction strengths.As shown in Figure 5b, the zero-coverage Q st for the three gases followed an order of CO 2 (32.6 kJ/mol) > CH 4 (22.4 kJ/mol) > N 2 (18.1 kJ/mol), consistent with the dipole moment of the guest molecules (CO 2 : 29.1 × 10 −25 cm 3 , CH 4 : 25.9 × 10 −25 cm 3 , N 2 : 17.4 × 10 −25 cm 3 ) [43].Additionally, it is apparent that the Q st for all gases remained constant between 0-100 kPa, suggesting homogeneity on the pore surface.From the Q st result, we speculate that the remarkable CO 2 selectivity is primarily attributed to its highest adsorption enthalpy.In addition, ZnAtzCO3 was compared with other MOFs constructed by triazolate linkers on their CO2 uptake at 15 kPa and 100 kPa, together with the isosteric heat [25,26,29,30,[44][45][46][47].As shown in Table 1, the CO2 uptakes on ZnAtzCO3 exceed those of the ZnF(Tz) series, ZnDatzBdc, Zn(FA)(datrz)2, and Zn2(TRZ)2(BDC), and are comparable to that of ZU-301, and slightly lower than those of ZnAtzOx and CALF-20.Hence, ZnAtzCO3 holds comparatively high CO2 capacity among these triazolate-based MOFs.Hence, ZnAtzCO3 can be regarded as a promising adsorbent in combination with good capacity and selectivity toward CO2.In addition, ZnAtzCO 3 was compared with other MOFs constructed by triazolate linkers on their CO 2 uptake at 15 kPa and 100 kPa, together with the isosteric heat [25,26,29,30,[44][45][46][47].As shown in Table 1, the CO 2 uptakes on ZnAtzCO 3 exceed those of the ZnF(Tz) series, ZnDatzBdc, Zn(FA)(datrz) 2 , and Zn 2 (TRZ) 2 (BDC), and are comparable to that of ZU-301, and slightly lower than those of ZnAtzOx and CALF-20.Hence, ZnAtzCO 3 holds comparatively high CO 2 capacity among these triazolate-based MOFs.Hence, ZnAtzCO 3 can be regarded as a promising adsorbent in combination with good capacity and selectivity toward CO 2 .The intrinsic mechanism for the excellent separation performance on ZnAtzCO 3 was illustrated by molecular simulations on the preferential adsorption sites, adsorption density distributions, and interaction energy with the aid of Material Studio 7.0 [48].To visualize the intrinsic host-guest interactions between ZnAtzCO 3 and the gas molecules, the preferential interaction site was calculated and depicted in Figure 6.Specifically, for the CO 2 molecule, the carbon atom can form four C δ+ ••• δ− O-C electrostatic interactions with the oxygen atoms in the carbonate linker, and each oxygen atom can form O δ− ••• δ+ H-C electrostatic interactions with the hydrogen atoms in the aminotriazolate rings.The multiple host-guest interactions verify that the favorable electrostatic environment of ZnAtzCO 3 can form an efficient nanotrap for CO 2 .It is noticed that the amine group did not form electrostatic interactions with CO 2 , which might originate from insufficient contact with CO 2 .For CH 4 , CH 4 interacts with three oxygen atoms in the carbonate linker, two nitrogen atoms in the amino groups, and one adjacent triazolate ring through dispersion forces.Considering its significantly smaller polarizability and more inert nature, the preferential site displayed weaker affinity for CH 4 than CO 2 .Likewise, being the weakest adsorbate, N 2 forms dispersion interactions with three hydrogen atoms in the aminotriazolate linkers and one oxygen atom in the carbonate linker.From the result above, ZnAtzCO 3 shows stronger host-guest interactions with CO 2 compared to N 2 and CH 4 .
Molecules 2023, 28, 7908 8 of 15 can form an efficient nanotrap for CO2.It is noticed that the amine group did not form electrostatic interactions with CO2, which might originate from insufficient contact with CO2.For CH4, CH4 interacts with three oxygen atoms in the carbonate linker, two nitrogen atoms in the amino groups, and one adjacent triazolate ring through dispersion forces.
Considering its significantly smaller polarizability and more inert nature, the preferential site displayed weaker affinity for CH4 than CO2.Likewise, being the weakest adsorbate, N2 forms dispersion interactions with three hydrogen atoms in the aminotriazolate linkers and one oxygen atom in the carbonate linker.From the result above, ZnAtzCO3 shows stronger host-guest interactions with CO2 compared to N2 and CH4. Figure 7 presents the ambient-temperature adsorption density distributions of the three gases on ZnAtzCO3 at low (15 kPa) and ambient pressure (100 kPa).At both 15 kPa and 100 kPa, the density distribution for CO2 was the highest, followed by CH4 and N2, which is a valid proof of the significantly higher capacity of CO2 than N2 and CH4.As the pressure rose from 15 kPa to 100 kPa, the adsorption density increased for all gases because, generally, the increment in pressure can provide an increasing driving force for gas adsorption.Figure 7 presents the ambient-temperature adsorption density distributions of the three gases on ZnAtzCO 3 at low (15 kPa) and ambient pressure (100 kPa).At both 15 kPa and 100 kPa, the density distribution for CO 2 was the highest, followed by CH 4 and N 2 , which is a valid proof of the significantly higher capacity of CO 2 than N 2 and CH 4 .As the pressure rose from 15 kPa to 100 kPa, the adsorption density increased for all gases because, generally, the increment in pressure can provide an increasing driving force for gas adsorption.Figure 7 presents the ambient-temperature adsorption density distributions of the three gases on ZnAtzCO3 at low (15 kPa) and ambient pressure (100 kPa).At both 15 kPa and 100 kPa, the density distribution for CO2 was the highest, followed by CH4 and N2, which is a valid proof of the significantly higher capacity of CO2 than N2 and CH4.As the pressure rose from 15 kPa to 100 kPa, the adsorption density increased for all gases because, generally, the increment in pressure can provide an increasing driving force for gas adsorption.In addition, the distinction of the simulated interaction energy was calculated to further confirm the difference in the adsorption enthalpy.As shown in Figure 8, the average energies between ZnAtzCO 3 and the gas molecules were −6.85, −6.06, and −3.50 kcal/mol for CO 2 , CH 4 , and N 2 , respectively, which shows a consistent trend with the Q st result.Hence, the stronger electrostatic host-guest interactions, apparently higher adsorption density distributions, and larger adsorption energy, comprehensively explain the selective CO 2 adsorption over N 2 and CH 4 on ZnAtzCO 3 .
In addition, the distinction of the simulated interaction energy was calculated to further confirm the difference in the adsorption enthalpy.As shown in Figure 8, the average energies between ZnAtzCO3 and the gas molecules were −6.85, −6.06, and −3.50 kcal/mol for CO2, CH4, and N2, respectively, which shows a consistent trend with the Qst result.Hence, the stronger electrostatic host-guest interactions, apparently higher adsorption density distributions, and larger adsorption energy, comprehensively explain the selective CO2 adsorption over N2 and CH4 on ZnAtzCO3.

Dynamic Breakthrough Experiments
For evaluation of the capability for selective CO2 capture from the exhaust gas and biogas on ZnAtzCO3, the breakthrough experiments were performed to simulate the dynamic separation performance toward CO2/N2 (15:85, v:v) and CO2/CH4 (50:50, v:v) at ambient conditions (Figure 9).It is noticed that CH4 and N2 were detected shortly after induction of the gas mixtures and reached equilibrium rapidly, confirming their uptake was inappreciable on ZnAtzCO3.In contrast, CO2 broke through at 81 min/g and reached equilibrium at 134 min/g for the CO2/N2 mixture, and the breakthrough time and equilibrium time for CO2/CH4 mixture were 37 and 53 min/g, respectively.In addition, the CO2 breakthrough time remained constant for five cycles for both mixtures, suggesting their excellent cyclicity in real-world CO2 capture conditions.

Dynamic Breakthrough Experiments
For evaluation of the capability for selective CO 2 capture from the exhaust gas and biogas on ZnAtzCO 3 , the breakthrough experiments were performed to simulate the dynamic separation performance toward CO 2 /N 2 (15:85, v:v) and CO 2 /CH 4 (50:50, v:v) at ambient conditions (Figure 9).It is noticed that CH 4 and N 2 were detected shortly after induction of the gas mixtures and reached equilibrium rapidly, confirming their uptake was inappreciable on ZnAtzCO 3 .In contrast, CO 2 broke through at 81 min/g and reached equilibrium at 134 min/g for the CO 2 /N 2 mixture, and the breakthrough time and equilibrium time for CO 2 /CH 4 mixture were 37 and 53 min/g, respectively.In addition, the CO 2 breakthrough time remained constant for five cycles for both mixtures, suggesting their excellent cyclicity in real-world CO 2 capture conditions.

Characterizations
SCXRD analysis was carried out on a Rigaku Oxford Diffraction (Rigaku, Tokyo, Japan) with a hybrid pixel array detector.Reflections combined with SHELXL corresponding to the crystal class were employed for the calculation of statistics and refinement to solve the non-hydrogen atoms, while the locations and numbers of all hydrogen atoms were calculated theoretically.Elemental analysis was performed on a Vario EL elemental analyzer (Elementar, Langenselbold, Germany) in the CHNS mode.FTIR spectroscopy in the range of 1800-400 cm −1 was recorded on a Thermo Scientific iN10 (Thermo Fisher Scientific, Waltham, MA, USA) microscope with potassium bromide as the matrix.PXRD patterns were collected on a Bruker D8 Advance diffractometer (Bruker, Mannheim, Germany).TG measurements of the as-synthesized ZnAtzCO 3 were performed on a TGA 550 thermal gravimetric analyzer (Thermo Fisher Scientific, Waltham, MA, USA), and the sample was heated from 303 K to 973 K at a ramping rate of 10 K/min under flowing nitrogen.

Single-Component Gas Sorption Isotherm Measurements
Single-component sorption isotherm measurements between 0-100 kPa were carried out on 3Flex (Micromeritics, Norcross, GA, USA) at various temperatures.In the preparation process, approximately 100 mg of ZnAtzCO 3 was activated under dynamic vacuum for 6 h at 393 K to afford a guest-free sample.During the test, the sample tube was placed in a thermostatic environment by using the ice-acetone bath (195 K) or circulating water bath (288 K, 298 K, and 313 K) to maintain a constant operational temperature.

Adsorption Selectivity Based on IAST Model
Before the IAST selectivity calculation, the experimental isotherms of CO 2 , N 2 , and CH 4 require accurate fitting to a mathematical model.In this work, the DSLF equation, based on the assumption that two types of adsorption sites are present in the structure, was selected and described to describe the adsorption equilibrium of the single-component gases Equation (1) [30,42].
where p is the specific pressure when the gas phase and adsorbed phase reach a steady state; q ei is the saturated uptake of site i; k i is the affinity coefficients of site i; t i represents the divergence from an absolute homogeneous surface on site i.
where x i and y i refer to the volume fractions of component i in the adsorbed phase and the gas phase, separately.

Isosteric Heat (Qst) Calculation
The experimental adsorption isotherms of CO 2 , N 2 , and CH 4 on ZnAtzCO 3 at various temperatures were fitted to the Virial equation, Equation (3) [49], and the parameters are shown in Table S2.
where p is the pressure, N is the gas capacity, T is the absolute temperature, a i and b j refer to the corresponding parameter in the Virial equation, while m and n refer to the number required for the accurate fitting of the Virial equation.Subsequently, the corresponding Q st was figured out by substituting the parameter in the Virial equation into the Clausius-Clapeyron equation, Equation (4) [50].
where R is short for the ideal gas constant, 8.314 J/mol/K.

Breakthrough Experiments
Dynamic separation experiments of CO 2 /N 2 (15:85, v:v) and CO 2 /CH 4 (50:50, v:v) mixtures were performed on self-assembly breakthrough equipment (Figure S3).A smallscale adsorption column was prepared by loading approximately 600 mg of the activated ZnAtzCO 3 in a stainless-steel column (Φ 50 × 150 mm).For activation, the column packed with the sample was heated at 393 K for two hours to eliminate the adsorbed contaminants.Subsequently, the column was inserted into the breakthrough equipment and purged by He flow (10 mL/min) at ambient conditions until the baseline was flattened.Finally, the gas was shifted to CO 2 /N 2 or CO 2 /CH 4 at a flow rate of 3 mL/min.The outlet component was monitored on a thermal conductivity detector (TCD) until the outlet composition reached that of the feed gas, which suggested the breakthrough column reached equilibrium.The adsorption column was recovered by purging He flow at 373 K to liberate the adsorbed gas molecules in the cyclability test.

Simulation Details
The molecular simulations on the adsorption mechanism were calculated by utilizing the Materials Studio 7.0 software [50].First, the structures of ZnAtzCO 3 and the adsorbates were optimized with the aid of the Forcite and Dmol3 modules.The adsorption characteristics, including the optimal adsorption sites, adsorption density distribution, and stabilized adsorption energy, were simulated in the Sorption module with the Metropolis Monte Carlo method.The adsorption behavior of the guest molecules on ZnAtzCO 3 was described by several motion types, including exchange, conformation, rotation, translation, and regeneration.The Ewald and atom-based methods were employed to depict the electrostatic interaction and Van der Waals interactions between the structure and the guest molecules, respectively.The cutoff for the Metropolis Monte Carlo simulation was set as 12.5 Å.One gas molecule was randomly inserted into the framework in the Location task in the Sorption module, with 1 × 10 5 steps for equilibrium and production, separately.

Conclusions
In conclusion, we propose an interesting type of MOF-based nanotrap, namely ZnAtzCO 3 , for efficient selective capture of CO 2 from N 2 and CH 4 .The favorable electrostatic environment and narrow pore geometry of ZnAtzCO 3 show stronger interaction with CO 2 than N 2 and CH 4 .Specifically, ZnAtzCO 3 accomplished high CO 2 capacities with values of 74.0 cm 3 /cm 3 at the fraction of the flue gas (15 kPa) and 91.4 cm 3 /cm 3 at the fraction of the biogas (50 kPa), together with ultra-high CO 2 /N 2 and CO 2 /CH 4 selectivities of 3538 and 151 at ambient conditions, respectively.This excellent separation performance was comprehensively explained by molecular simulations, which suggests that the carbon atom of CO 2 can form strong electrostatic C δ+ ••• δ− O-C interactions with the oxygen atoms in the carbonate ligand and the oxygen atom of CO 2 can interact with the hydrogen atoms in the triazolate ligand through O δ− ••• δ+ H-C interactions, enabling ZnAtzCO 3 as an optimal nanotrap for CO 2 fixation.Moreover, breakthrough experiments confirm excellent dynamic separation toward CO 2 /N 2 and CO 2 /CH 4 on ZnAtzCO 3 , highlighting its potential for selective CO 2 capture.Furthermore, constructing suitable nanotraps with optimal electrostatic environment and pore geometry is worthy of further exploration in other separation circumstances.

Molecules 2023, 28 , 7908 3 of 15 Figure 1 .
Figure 1.Selective CO2 capture from N2 and CH4 on a nanotrap with a suitable electrostatic environment via multiple host-guest interactions.The blue dotted lines represent the electrostatic interactions between the framework and the CO2 molecule.

Figure 1 .
Figure 1.Selective CO 2 capture from N 2 and CH 4 on a nanotrap with a suitable electrostatic environment via multiple host-guest interactions.The blue dotted lines represent the electrostatic interactions between the framework and the CO 2 molecule.

Figure 2 .
Figure 2. The crystal structure and pore property of ZnAtzCO3: (a) coordination mode, (b) crystal structure shown in the b-axis, and the Connolly surface in the b-axis (c) and a-axis (d) by using a spherical probe exhibiting a radius of 1 Å.The intraframework N-H•••O hydrogen bonds are marked by the golden dotted lines.Ⅰ and Ⅱ in Figure 2c represent the two types of cavities on the structure.

Figure 2 .
Figure 2. The crystal structure and pore property of ZnAtzCO 3 : (a) coordination mode, (b) crystal structure shown in the b-axis, and the Connolly surface in the b-axis (c) and a-axis (d) by using a spherical probe exhibiting a radius of 1 Å.The intraframework N-H•••O hydrogen bonds are marked by the golden dotted lines.I and II in Figure 2c represent the two types of cavities on the structure.

Figure 6 .
Figure 6.Preferential adsorption sites on the ZnAtzCO3 structure for CO2 (a), CH4 (b), and N2 (c) on ZnAtzCO3.The dashed line represents the host-guest interactions between the ZnAtzCO3 and the gas molecules.The unit of the distance between the gas molecules and the adsorption site is Å.

Figure 6 .
Figure 6.Preferential adsorption sites on the ZnAtzCO 3 structure for CO 2 (a), CH 4 (b), and N 2 (c) on ZnAtzCO 3 .The dashed line represents the host-guest interactions between the ZnAtzCO 3 and the gas molecules.The unit of the distance between the gas molecules and the adsorption site is Å.

Figure 6 .
Figure 6.Preferential adsorption sites on the ZnAtzCO3 structure for CO2 (a), CH4 (b), and N2 (c) on ZnAtzCO3.The dashed line represents the host-guest interactions between the ZnAtzCO3 and the gas molecules.The unit of the distance between the gas molecules and the adsorption site is Å.

Figure 7 .
Figure 7. (a-c) The simulated adsorption density distribution of CO 2 , CH 4 , and N 2 on ZnAtzCO 3 crystal framework at 15 kPa.(d-f) The simulated adsorption density distribution of CO 2 , CH 4 and N 2 on ZnAtzCO 3 crystal framework at 100 kPa.

Table 1 .
Comparisons of CO2 uptakes at 15 kPa and 50 kPa on typical MOFs constructed by triazolate

Table 1 .
Comparisons of CO 2 uptakes at 15 kPa and 50 kPa on typical MOFs constructed by triazolate linkers.