N-rich porous carbon with high CO₂ capture capacity derived from polyamine-incorporated metal–organic framework materials

Abstract

N-doped porous carbon has a wide range of applications in many fields, such as gas adsorption and super-capacitor, which has stimulated active research for developing efficient strategies to fabricate N-rich porous carbon. In the present study, a series of N-rich porous carbons are derived from polyamine-incorporated metal–organic framework materials (MOFs). Results show that the N content of as-prepared porous carbon is greatly increased by loading polyethyleneimine (PEI) into ZIF-70 frameworks because the loaded PEI can adsorb onto the pore walls of the ZIF-70 and simultaneously supply carbon and N sources during the carbonization process. As a result, the surface area of the as-prepared sample is also increased. In addition, the N content of the porous carbon can be tuned by PEI loadings and carbonization temperature. The as-prepared N-rich porous carbons exhibit greatly enhanced CO₂-selective adsorption capacity compared to ZIF-70 and porous carbon derived from ZIF-70. Here the CO₂ capture capacity of the as-prepared N-rich porous increases with increasing N content due to considerable interaction affinity between doped N and CO₂ molecules. Thus, the as-prepared porous carbon with N content of 11.08 wt% displays high CO₂ uptake of 4.86 mmol g⁻¹ at 0 °C and 1 bar, albeit it has a moderate surface area of 652 m² g⁻¹. Moreover, the N-rich porous carbon clearly shows an excellent separation performance for CO₂-over-N₂ and CO₂-over-CH₄. Overall, polyamine-incorporated MOFs are an efficient strategy for controllable fabrication of N-rich porous carbon. The resulting products display a high CO₂-selective capture performance. It should be noted that, to achieve the optimal CO₂ capture ability, a comprehensive optimization of the polyamine-MOFs-derived porous carbon should be performed.

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Introduction

Climate change associated with rapidly increasing concentration of CO₂ in the atmosphere is one of the greatest threats to humanity.¹ Developing an energy-efficient and economical manner to mitigate the CO₂ emission is greatly desired. Currently, capture and sequestration of CO₂ is deemed as a solution to deal with the global demand of CO₂ reduction. To date, aqueous solutions of amines are used widely in large scale to capture CO₂ from industrial streams due to their high acidic gas capture capacity.² However, amine-based solutions usually suffer from several drawbacks, including high energy requirements, equipment corrosion, and solvent degradation.³ Alternatively, solid porous adsorbents are emerging as more promising candidates for CO₂ capture because they exhibit several advantages, such as lower energy requirements and milder operating conditions.⁴ As a result, extensive research efforts have been dedicated to design and synthesize solid porous adsorbents for CO₂ capture.⁵ Nevertheless, capture of CO₂ from flue gas emissions using solid porous adsorbents still remains challenging due to absence of adsorbent with high capture capacity, rapid adsorption kinetics as well as high stability under practical conditions.⁶ Thus, porous materials with efficient CO₂ capture capacity are highly demanded.⁷

In the last years, a series of solid porous adsorbents such as amine-modified silicas,⁸ zeolites,⁹ porous carbons,¹⁰ and MOFs¹¹ have been developed for CO₂ capture. Among them, porous carbon is one of the most attractive and important materials,¹² which has a wide range of applications in many fields, such as adsorption and separation,¹³ super-capacitor and fuel cells.¹⁴ Thus, different strategies such as chemical vapor deposition, laser ablation, electric-arc technique and template method have been explored to generate porous carbon with controlled pore structure.¹⁵ To the best of our knowledge, the template method has been considered as one of the most effective approaches to synthesize porous carbon with controllable architecture and desirable physical and chemical properties.¹⁶ Therefore, a variety of template materials such as zeolites,¹⁷ mesoporous silicas,¹⁸ colloid crystals¹⁹ and aluminium oxide membranes have been developed for porous carbon materials synthesis. However, this traditional template method usually involves template removal, which limits its practical application, because the resulting porous carbon structure could be partially destroyed during the process of template removal using acid or alkali.²⁰ In addition, this process is unfriendly for the environmental development.

Recently, research on MOFs-derived porous carbon has received considerable attention because of their high porosity and the ability to control their pore textures.²¹ In addition, their organic ligands can serve as a precursor and contribute to the formation of high-quality porous carbon.²² More importantly, the textural properties and application performances of the resulting porous carbons can be tuned by properties of the parental MOFs.²³ For instance, Xu et al.²⁴ reported that nano-porous carbons derived from NH₄OH-incorporated ZIF-8 show high CO₂ uptakes. However, porous carbons obtained directly from carbonization of MOFs only show moderate CO₂ capture capacity as well as low selectivity.²⁵ Indeed, it has been found that the properties of porous carbons can be tuned by integrating heteroatom (i.e., N, B, P) to access a wide range of applications including heterogeneous catalysis, energy storage and gas adsorption and separation.²⁶ Thus, incorporation of heteroatoms with Lewis basic properties into porous carbons can simultaneously enhance their CO₂ capture capacity and selectivity due to considerable interaction between Lewis base and CO₂ molecules.²⁵

In a previous study, we have successfully fabricated N-rich polyamine-MOF porous materials by incorporation of polyethyleneimine (PEI) molecules into MOF frameworks.²⁷ The resulting composite materials exhibit a moderate porosity and very competitive CO₂ capture properties.²⁸ On the basis of this study, we propose that N-rich porous carbons with high CO₂ capture capacities can be facilely derived from PEI-incorporated MOF materials,²⁹ as shown in Fig. 1. Here linear PEI (with an average molecular-weight of 300) and ZIF-70 are chosen as the model system to fabricate polyamine-MOF composite materials. ZIF-70 is chosen because of its high porosity and large pore size (∼1.6 nm),³⁰ so that many PEI can be readily incorporated into its framework pores. Moreover, ZIF-70 framework has many N elements, which can be partly doped into carbon network during carbonization process.³¹

Fig. 1 Schematic illustration of the synthesis of N-rich porous carbons.

Therefore, both PEI and ZIF-70 can work as carbon and N precursors to generate N-rich porous carbon materials. The resulting porous carbons are expected to exhibit a very competitive CO₂ adsorption capacity owing to the high content of N doped into their framework. In this work, a series of N-rich porous carbons are fabricated by tuning the content of PEI loadings. In addition, the effect of carbonization conditions on the textural properties and the CO₂ capture capacities are also studied. The CO₂, N₂ and CH₄ adsorption properties of the as-prepared N-rich porous carbons are evaluated at 0 °C and 25 °C, respectively. Results show that porous carbons derived from PEI-loaded ZIF-70 display much higher CO₂ adsorption capacity than prepared directly by ZIF-70. Moreover, the CO₂ capture capacities of the as-prepared porous carbons increase with their N content increasing.³²

Experimental

Materials

Imidazole (TCI), zinc nitrate hexahydrate [Zn(NO₃)₂·6H₂O, TCI], 4-nitroimidazole (TCI), ethanol (EtOH, TCI), N,N-dimethylformamide (DMF, TCI) were used as received from vendors without further purification. Deionized water and alcohol were used as solvents. A linear polyethyleneimine (M_w = 300 Da) in the present study was purchased from Alfa Aesar.

Synthesis of ZIF-70

To obtain a well-crystallized ZIF-70 crystal, the solvothermal synthesis approach was adopted. The detailed process was as follows. First, zinc nitrate hexahydrate (5.354 g, 0.15 M), iminazole (1.634 g, 0.2 M) and nitroimidazole (2.713 g, 0.2 M) were dissolved in N,N-dimethylformamide (DMF) (120 ml, 0.2 M). Second, the mixed reactants were heated in a Teflon-lined autoclave at 110 °C for 72 h under autogenous pressure. The yielded solid was soaked in 20 ml methyl alcohol for three hours. This process was repeated three times. Finally, the crystals were evacuated at 110 °C under vacuum for 12 h.

Synthesis of PEI–ZIF-70 composite materials

The PEI–ZIF-70 composite materials were prepared by wet impregnation method. The detailed process was as follows. First, ZIF-70 crystals were heated at 110 °C under vacuum condition for 12 h, removing the adsorbed water. Second, 0.075–0.3 g PEI was dissolved in 1 mL anhydrous methanol under stirring for 10 min, and then 0.3 g ZIF-70 crystals were added step by step into the solution under stirring. Finally, the resulting product was dried overnight at room temperature. Then, the product was heated at 110 °C for 12 h under vacuum condition. After that, the PEI–ZIF-70 composite materials were obtained. Here three types of PEI–ZIF-70 samples with different PEI loadings (W_p = 25 wt%, 50 wt% and 100 wt%) were prepared. W_p is the weight ratio of PEI to ZIF-70. To confirm the accurate PEI loading, the weight of ZIF-70 was measured immediately after activation.

Synthesis of N-rich porous carbons

To prepare N-rich porous carbon, the PEI–ZIF-70 sample was transferred into a quartz boat and placed in a furnace, under Ar flowing for 5 h to exclude air. Subsequently, the sample was carbonized under Ar atmosphere at T °C for 3 h at a heating rate of 3 °C min⁻¹. The resulting porous carbon materials were denoted as C-W_p-T (T = 700 °C, 800 °C and 900 °C). For comparison, porous carbon was also prepared directly by carbonization of ZIF-70 using the same procedure at 900 °C, and the resulting product was denoted as C-0%-900. To increase the surface area of the carbon materials, sample C-100%-900 were further activated by KOH at 700 °C for 1 h under argon flow. The weight ratio of carbon to KOH was 1 : 4. After cooling down, the resulting sample was thoroughly washed with 2 mol L⁻¹ HCl for three times at room temperature, followed by further washing with distilled water until neutral pH value was achieved. Finally, the samples were dried in an oven at 120 °C for 5 h. The resulting sample was denoted as C-100%-900-KOH.

Characterizations

The as-prepared crystal morphology was examined using a field emission scanning electron microscope (Hitachi, S-4800). X-ray diffraction (XRD) data of the products were collected on a Bruker AXS D8 Advance diffractometer using CuKα at room temperature. The powder XRD pattern was scanned over the angular range of 5–50 (2θ) with a step size of 0.02 (2θ). The N adsorption/desorption isotherm was measured on an ASAP 2020M apparatus. The BET surface area was calculated over the range of relative pressures between 0.05 and 0.10 bar. Before measurement, the sample was outgassed under vacuum at 200 °C for 12 h. X-ray photoelectron spectroscopy (XPS) data were obtained using a Thermo Fisher ESCALAB 250 X-ray photoelectron spectrometer equipped with a twin anode MgKα X-ray source. Gas adsorption capacities were measured on an ASAP 2020M apparatus gas sorptometer via conventional static volumetric technique. Prior to the gas adsorption analysis, all samples were evacuated for 5 h at 300 °C under vacuum. The CO₂, N₂, CH₄ uptake capacities were performed at 0 and 25 °C. The breakthrough curve experiments for the CO₂/N₂, CO₂/CH₄ mixture were carried out using a column with a length of 20 cm and an internal diameter of 0.8 cm. The sample powder was packed in the middle part of the column. Here the used sample mass is 0.30 g. Breakthrough allows in situ activation of the sample under argon flow. The detailed experiment methodology can be found in ref. 33. The flow rates of all pure gases were controlled by mass flow controllers. Before the measurement, the sample was activated at 110 °C for 2 h. The gas stream from the column outlet was analyzed online with a mass spectrometer (TP-5080).

Adsorption

The porous carbons were immediately measured after moving from the furnace. Before each measurement, the sample was evacuated at 200 °C for 12 h. In the adsorption measurement, the apparatus first gave a desired pressure CO₂ gas to the sample holder, and we recorded the pressure values in the sample holder and calculated the amount of adsorbed CO₂.

The adsorption isotherms of CO₂, N₂ and CH₄ were measured using volumetric technique by an apparatus from Micromeritics America (ASAP-2020). The amount of gas adsorbed as a function of pressure was determined using the Langmuir–Freundlich fit for the isotherms:

Here, Q and Q_m are the uptake and the maximum uptake, respectively. P is the equilibrium pressure, B and t are the equation constants. To obtain the exact pressures P, corresponding to constant amount of gas adsorbed, the above equation can be rearranged to:

The CO₂/N₂ and CO₂/CH₄ adsorption selectivities (α) of all samples were calculated by the following equation, according to a previous report:³⁴

where Q_i is the adsorption capacity of component i, P_i is the partial pressure of component i. The adsorption capacities of the components are defined as the molar excess adsorption capacities determined without correction for absolute adsorption.

Results and discussion

Characterization of the as-prepared samples

As shown in Fig. 2, the morphologies of N-rich porous carbons are clearly consistent with that of ZIF-70. It indicates that PEI-loaded ZIF-70 is an excellent template for preparation of porous carbon materials.

Fig. 2 SEM images of ZIF-70 (A), C-25%-90 (B), C-50%-900 (C) and C-100%-900 (D).

Fig. 3 shows the XRD patterns of the as-prepared ZIF-70, PEI loaded ZIF-70 and porous carbon. Clearly, the structure of ZIF-70 synthesized in this work matches well with that reported in the literature.³⁵ The powder XRD patterns of ZIF-70 and PEI loaded ZIF-70 samples show that their Bragg diffraction angles indicating that ZIF-70 structure is well maintained after PEI loading. However, the peaks below 10° of PEI-loaded ZIF-70 almost disappear after PEI loading due to the filling of ZIF-70 pores of PEI. Apparently, the intensity of peaks above 10° of PEI-loaded ZIF-70 remains intact. As for the porous carbon, it can be seen that two broad diffraction peaks at 2θ values of about 25° and 44° corresponding to carbon (002) and (101) diffractions, respectively, can be found clearly from the wide-angle XRD patterns. Note that, results indicate that most of obtained carbon species are amorphous carbons because the peak (101) of graphitic crystallinity is very weak. In addition, no diffraction peaks of ZIF-70 can be observed in Fig. 3C, indicating that carbonization of ZIF-70 is complete during the carbonization process.

Fig. 3 XRD patterns of the as-prepared ZIF-70 (A), PEI-loaded ZIF-70 (B) and porous carbon (C) derived from PEI–ZIF-70 composite.

To evaluate pore structure of the resulting porous carbon, the N₂ adsorption–desorption isotherms of the samples are measured at 77.3 K. Here three PEI-loaded ZIF-70 samples, corresponding to 25 wt%, 50 wt% and 100 wt%, are prepared for porous carbon preparation. The carbonization temperatures are in the range of 700–900 °C. The resulting samples are denoted as C-0%-900, C-25%-900, C-50%-900, C-100%-900, C-100%-800 and C-100%-700, respectively. As shown in Fig. 4, all samples exhibit a type I mixed with type IV behaviours. As for KOH activated porous carbon, many microporous can be generated between the mesoporous pores during activation process, resulting in surface area increasing greately.³⁶ Generally, the isotherm at low relative pressure (P/P₀ < 0.3) is attributed to N₂ adsorption in micropores. The hysteresis of desorption isotherm at relative high pressure reveals the existence of mesopores in the sample. It should be noted that the presence of micropore and mesopore depends on the carbon gasification and high evaporation etching of the Zn.³⁷ Here the N₂ uptakes of samples prepared at 900 °C increase distinctly with PEI loading increasing. This is ascribed to the fact that loaded PEI could adsorb onto pore wall of the ZIF-70. Thus, it can supply carbon source during carbonization process, reducing the sacrifice of template framework (ZIF-70). Actually, a similar phenomenon has been observed in literatures.²⁴ The corresponding BET surface area and pore volume for all samples are calculated from the isotherms and summarized in Table 1. Clearly, the surface area and pore volume of porous carbon samples increase with PEI loading increasing. On the other hand, N₂ uptakes of the samples decrease distinctly with carbonization temperature decreasing. This could be explained because the loss of carbon source at lower temperature is lower. It has been found that KOH can clean the slag on the surface of the porous carbon to release some extra pores and expand some micropores. Therefore, KOH are further used to activate the C-100%-900 sample. The N₂ adsorption analysis indicates that both BET surface area and pore volume are greatly increased. Above results show that PEI loading, carbonization temperature and activation process greatly affect the pore structure of PEI–ZIF-70 derived carbon materials.

Fig. 4 N₂ adsorption–desorption isotherms of the as-prepared porous carbon materials at 77.3 K.

Table 1 Textural properties and composition content of materials derived from polyamine-incorporated metal-organic framework materials

Samples	Surface area^a (m^2 g^−1)	Pore volume^b (cm^3 g^−1)	N content^c (wt%)
a The specific surface area is calculated in the P/P0 range of 0.05–0.2.b Values at P/P0 = 0.99.c Obtained from XPS analysis.
C-0%-900	158.17	0.10	1.23%
C-25%-900	445.97	0.22	4.23%
C-50%-900	523.46	0.24	7.37%
C-100%-900	652.44	0.33	11.08%
C-100%-800	437.90	0.20	6.45%
C-100%-700	415.70	0.23	5.56%
C-100%-900-KOH	2107.50	1.09	2.65%

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The nature of N moieties on the as-prepared porous carbon surface is analyzed by XPS. Fig. 5 shows the N 1s XPS spectra of representative samples. For all samples, the XPS N 1s spectra can be deconvoluted into three peaks with the binding energies centered at 398.3, 400.1, and 401.5 eV, respectively. The peak at 398.3 eV is attributed to the pyridinic-N (N-6), while the peaks at 400.1 and 401.5 eV can be assigned to pyrrolic-/pyridonic-N (N-5) and quaternaryN (N-Q), respectively. Note that, the peaks (N-6, N-5, and N-Q) of N species is very weak after KOH activation, which is consistent with the reported ref. 38. Quantitative analysis reveals that N present in the form of N-5 is higher than that present in the form of N-6 and N-Q, in the order of N-5 > N-6 > N-Q for all N-doped samples. This result is beneficial to CO₂ capture, because it has been found that N-5 generally has a much greater contribution to CO₂ capture than N-6 and N-Q.³⁹ The N compositions of the as-prepared porous carbons are calculated and summarized in Table 1. Apparently, the N contents of the samples prepared by PEI-loaded ZIF-70 are much higher than that prepared directly by carbonization of ZIF-70. For instance, the N content of porous carbon prepared by carbonization of PEI-loaded ZIF-70 can reach 11.0 wt%. However, the N content of porous carbon prepared directly by carbonization of ZIF-70 is only 1.32 wt%. Results clearly indicate that PEI loading is an effective strategy for improving the N content of MOFs-derived porous carbons. In addition, the N contents of the samples increase with PEI loading increasing. In this work, the N contents of the samples increase with carbonization temperature increasing. It might be ascribed to the fact that much more carbon loss happens at higher temperature. However, the N contents of the sample activated by KOH is greatly decreased, because a large number of N species on the carbon surface would decompose during the activation process.³⁶

Fig. 5 N 1s XPS spectra of the as-prepared porous carbon materials.

Gas adsorption studies

As stated above, N doped porous carbons are excellent candidates for CO₂ adsorption. Therefore, the CO₂ adsorption properties of the as-prepared N-rich porous carbons are investigated at 0 °C and 25 °C, respectively. As shown in Fig. 6, clearly, the CO₂ uptakes of the samples increase with N content increasing. Porous carbons derived from PEI-loaded ZIF-70 distinctly show much higher CO₂ capture capacities than that derived directly from ZIF-70 due to their N contents increase greatly. The CO₂ uptakes of samples derived from PEI loaded ZIF-70 can reach 4.0 mmol g⁻¹ at 1 bar and 25 °C. However, the sample derived directly from ZIF-70 only displays the CO₂ uptakes of 2.0 mmol g⁻¹ at 1 bar and 25 °C.

Fig. 6 CO₂ uptakes of the as-prepared samples at 25 °C. (a) Porous carbons derived from ZIF-70 with different PEI loading at 900 °C, (b) porous carbons derived from ZIF-70 with 100 wt% PEI loadings at different temperatures.

It should be noted that the sample activated by KOH shows higher CO₂ capture capacities in high test pressure (>0.4 bar), whereas it shows a lower CO₂ capture capacities at low test pressure (<0.4 bar), compared to unactivated sample. It is ascribed to the lower N content and higher surface area after KOH activation. The CO₂ adsorption properties of the samples prepared at different temperatures are shown in Fig. 6b. Clearly, their CO₂ adsorption capacities increase with carbonization temperatures increasing because the higher N content and surface area can be achieved at higher temperature (see Table 1). For instance, the CO₂ capture capacity of C-100%-800 is distinctly higher than that of C-100%-700 due to its higher N content, albeit their surface areas are slightly different. It should be noted that, the highest CO₂ adsorption difference between C-100%-800 and C-100%-700 appears at around 0.4 bar. It might suggest that the affinity between doped N and CO₂ molecular are moderate. The CO₂ adsorption difference between C-100%-900 and C-100%-700 increases with the test pressure increasing because both N content and BET surface area of C-100%-900 are higher than those of C-100%-700. In brief, it indicates that the N contents of porous carbons greatly affect their CO₂ adsorption capacity. By the comparison of C-100%-900 and C-100%-900-KOH, apparently, C-100%-900 displays higher CO₂ adsorption capacities at low adsorption pressure (<0.6 bar) due to its much higher N content, albeit it has much smaller BET surface area than that of C-100%-900-KOH (see Table 1). This result further indicates that the affinity between doped N and CO₂ are moderate, which can greatly improve CO₂ adsorption capacity at low pressure. In high adsorption pressure, C-100%-900-KOH exhibits higher CO₂ adsorption capacity than that of C-100%-900 due to its much higher surface area. The CO₂ adsorption capacities of samples at low temperature (0 °C) are also investigated, as shown in Fig. 7. It can be seen that the CO₂ adsorption capacities of all samples increase greatly, compared with that of 25 °C. The highest CO₂ uptakes of sample can reach 5.4 mmol g⁻¹ at 1 bar and 0 °C. The CO₂ uptakes of samples at 0.15 bar (0 °C, 25 °C) and 1 bar (0 °C, 25 °C) are calculated and summarized in Table 2. As shown in Fig. 8 and 9, N₂ and CH₄ uptakes of all samples are further measured at 0 °C and 25 °C, respectively. Clearly, the N₂ and CH₄ uptakes are much lower than CO₂ at both temperatures. Since N-doped porous carbon can interact moderately with CO₂ molecules because doped N has Lewis basic property.

Fig. 7 CO₂ uptakes of the as-prepared samples at 0 °C. (a) Porous carbons derived from ZIF-70 with different PEI loading at 900 °C, (b) porous carbons derived from ZIF-70 with 100 wt% PEI loadings at different temperatures.

Table 2 CO₂ uptakes of the as-prepared samples at 0.15 bar (0 °C, 25 °C) and 1 bar (0 °C, 25 °C), respectively

Sample	CO2 uptakes (mmol g^−1)
	0.15 bar		1 bar
	0 °C	25 °C	0 °C	25 °C
C-0%-900	1.17	0.81	2.70	2.08
C-25%-900	1.60	1.04	3.97	2.51
C-50%-900	2.13	1.42	4.77	3.21
C-100%-900	2.20	1.36	4.86	3.56
C-100%-800	1.65	1.13	4.13	2.62
C-100%-700	1.49	0.99	3.53	2.44
C-100%-900-KOH	1.73	0.75	5.46	4.40

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Fig. 8 N₂ uptakes of the as-prepared samples at 25 °C (a) and 0 °C (b).

Fig. 9 CH₄ adsorption isotherms at 25 °C (a) and 0 °C (b). Lines are the fitted isotherms.

Isosteric adsorption enthalpies as a function of the quantity of gases adsorbed were calculated via variant of the Clausius–Clapeyron equation. As show in Table 3, we calculate the heats of adsorption of all samples. Here the increase of carbonization temperature, the heats adsorption of samples also will increase. In addition, the heats adsorption of C-100%-900 is much more than that of C-0%-900, which proves a point that the amount of CO₂ adsorbed correspondently increases to that of N. It is indicated that the CO₂ adsorption capacity of N-rich materials is superior to that of usual carbon materials, because of its quadrupole interactions with N in N-doped carbon material in addition to the van der Waals interactions. Hence, the heats adsorption of CO₂ supports the key role of the N-doping for stronger interactions with CO₂ molecules in the material.

Table 3 Heats of adsorption of all samples

Sample	Heats of adsorption (kJ mol^−1)
C-0%-900	19.97
C-25%-900	26.61
C-50%-900	26.10
C-100%-900	26.97
C-100%-800	25.93
C-100%-700	25.00
C-100%-900-KOH	15.22

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In addition, the quadrupole moment of CO₂ (−14 × 10⁻⁴⁰ C m²) is much higher than that of N₂ (−4.6 × 10⁻⁴⁰ C m²) and CH₄ (nonpolar). The synergetic effects of the two factors lead to higher uptakes of CO₂ than N₂ and CH₄. Here the samples show higher adsorption capacity for CH₄ over N₂, which is probably ascribed to the larger molecular size of CH₄. The adsorption results of CO₂, N₂ and CH₄ indicate that the as-prepared samples exhibit excellent CO₂-selective adsorption ability. In addition, separation performance of the as-prepared N-rich porous carbons increase with N content increasing. The separation performances of the as-prepared samples for CO₂-over-N₂ and CO₂-over-CH₄ mixtures are further corroborated by breakthrough experiments performed at 25 °C, as shown in Fig. 10. Results apparently show that N₂ and CH₄ elutes rapidly through the column, whereas CO₂ only starts to elute after a period of time. Since CO₂ molecules are selectively adsorbed by the as-prepared sample (C-100%-900), which elutes last in the sequence at Fig. 10. Results further indicate that the as-prepared N-rich porous carbon displays an excellent separation performance for CO₂-over-N₂ and CO₂-over-CH₄. We noted that there is a time interval that we can get pure CH₄ or N₂ using the N-rich carbon.

Fig. 10 Breakthrough curves of C-100%-900 with an equimolar CO₂/CH₄ mixture and CO₂/N₂ mixture at 25 °C.

Conclusions

In conclusion, we have successfully fabricated a series of N-rich porous carbon materials by carbonization of polyamine-incorporated MOFs. We show that the N content of the porous carbon can be readily tuned by the dose of polyamine loadings. The N content and surface area of the as-prepared porous carbon increase distinctly with PEI loading increasing, because PEI can be adsorbed onto the pore wall of ZIF-70 and simultaneously supply carbon and N sources during carbonization process. All the prepared N-rich porous carbon samples display greatly enhanced CO₂-selective adsorption capacity compared to ZIF-70 and porous carbon directly derived from ZIF-70. The C-100%-900 displays a high CO₂ adsorption capacity of 4.86 mmol g⁻¹ at 0 °C and 1 bar. The samples also exhibit excellent separation performance for CO₂-over-N₂ and CO₂-over-CH₄. Therefore, carbonization of polyamine-incorporated MOFs is an effective strategy for fabricating N-rich porous carbon, which is a promising candidate for CO₂ capture using the pressure-driven technique.

Acknowledgements

We acknowledge the financial support from NSFC (No. 51272260 and 51302278), natural science foundation of Zhejiang Province (No. LY15E020008 and LR14E020004), the aided program for science and technology innovative research team of Ningbo municipality (No. 2014B81004 and 2015B11002) and Ningbo natural science foundation (No. 2015A610045 and 2015A610053).

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