Constructing Hierarchically Porous N-Doped Carbons Derived from Poly(ionic liquids) with the Multifunctional Fe-Based Template for CO2 Adsorption.

Nitrogen-doped hierarchical porous carbons with a rich pore structure were prepared via direct carbonization of the poly(ionic liquid) (PIL)/potassium ferricyanide compound. Thereinto, the bisvinylimidazolium-based PIL was a desirable carbon source, and potassium ferricyanide as a multifunctional Fe-based template, could not only serve as the pore-forming agent, including metallic components (Fe and Fe3C), potassium ions (etching carbon framework during carbonization), and gas generated during the pyrolysis process, but also introduce the N atoms to porous carbons, which were in favor of CO2 capture. Moreover, the hierarchically porous carbon NDPC-1-800 (NDPC, nitrogen-doped porous carbon) had taken advantage of the highest specific surface area, exhibiting an excellent CO2 adsorption capacity and selectivity compared with NDC-800 (NDC, nitrogen-doped carbon) directly carbonized from the pure PIL. Furthermore, its hierarchical porous architectures played an important part in the process of CO2 capture, which was described briefly as follows: the synergistic effect of mesopores and micropores could accelerate the CO2 molecules' transportation and storage. Meanwhile, the appropriate microporous size distribution of NDPC-1-800 was conducive to enhancing CO2/N2 selectivity. This study was intended to open up a new pathway for designing N-doped porous carbons combining both PILs and the multifunctional Fe-based template potassium ferricyanide with wonderful gas adsorption and separation performance.

Introduction

Nitrogen-doped porous carbon (NDPC) has attracted great interest and is still an increasingly extending topic due to its tailorable pore texture, exceptionally larger surface area, lower density, as well as higher chemical and thermal stability.[1−5] Therefore, it has a wide application scope in the fields of adsorption, energy, catalysis, environment and separation, and so forth.[6−9] The general strategy to prepare porous carbon materials with large surface areas and the abundant pore structure can be summarized thus: hard or soft templates are applied in the carbonization process of a nitrogen-containing precursor.[10,11] However, this method still faces challenges in some degree. In simple terms, the removal of hard templates such as mesoporous silica suffers from the shortages of time- and energy-consuming procedures; besides, the soft templates may lose their structure at the high carbonization temperature; hence, this strategy goes against practical applications. Therefore, designing a sort of NDPC material with larger specific surface areas and high porosity together with simple and lower energy- and time-consuming approaches is still an essential issue to be researched.

Traditionally, nitrogen-doped carbon (NDC) materials were typically constructed by direct pyrolysis of nitrogen-containing organic compounds.[12−15] However, this approach usually suffers from either completely evaporating or decomposing into gaseous products of most organic precursors during the process of carbonization; thus, these traditional precursors are limited to natural or synthetic organics with low vapor pressures. Considering these issues of both low vapor pressures and complicated syntheses associated with organic precursors, a series of novel precursors have been reported by research workers as potential precursors of porous carbon materials, such as ionic liquids (ILs)[16,17] and metal organic frameworks.[18,19] ILs, as a novel kind of green materials together with their inappreciable vapor pressure and higher thermal stability, have received a substantial amount of attention as precursors of carbon materials.[16,20,21] These superior properties of ILs are beneficial for carbonization processes without any applied pressure, leading to minimizing mass loss before the start of the decomposition process; thus, we can obtain carbon material from direct carbonization. Moreover, the composition, structure, and property of carbon materials can be tailored by the structural designability and compositional diversity of ILs.[21,22] Poly(ILs) (PILs), a kind of polymer solidified from IL monomers, exhibit outstanding properties of both ILs and macromolecule polymers.[23,24] The main advantages of using PILs as a unique class of carbon precursors are showed below. First, PILs, the same as ILs, have high thermal stability in order to minimize the mass loss and maximize the yields of carbon materials. Second, most PILs contain heteroatoms in their molecular structure such as nitrogen or sulfur, which can be preserved in a carbon matrix after carbonization, thus enhancing CO2 uptake due to the improved affinity interaction between basic nitrogen active sites and acidic CO2 molecules. Third, PILs, as solid materials, overcome the defects of ILs such as high viscosity and can be used conveniently in the process of carbonization. Consequently, PILs can be selected as the ideal carbon resources to prepare NDPCs, which are appropriate for CO2 capture.

CO2, as a predominant greenhouse gas and a renewable carbon resource, is supposed to be captured and stored effectively owing to global climate change and unreasonable usage of energy.[25−27] Alkaline amine-based solutions are currently utilized for CO2 sorption by means of the chemical interaction between basic amino functional groups and acidic CO2 molecules.[3,28] However, the shortages of this traditional method, such as equipment corrosion, chemical instability, and the high energy consumption of regeneration, worry the researchers.[29,30] In view of this, various adsorbents have been applied for CO2 capture with high adsorption capacity.[31−33] Among them, NDPC is expected to be a promising CO2 adsorbent because of its wide accessibility and a variety of advantages mentioned before.[34,35] As a result, developing a kind of porous carbons, which are suitable for CO2 capture, is an attractive goal to be pursued.

In this work, we proposed a facile approach for preparing N-doped hierarchical porous carbon derived from a bisvinylimidazolium-based PIL with potassium ferricyanide, subsequently removing the template with hydrochloric acid. The multifunctional Fe-based template potassium ferricyanide could provide not only the ferrous compounds, gas, and potassium ions for pore-forming during the pyrolysis process but also the N atoms to porous carbons, which were beneficial to CO2 adsorption. The hierarchical porous carbon prepared at 800 °C had taken advantage of higher specific surface areas and well-developed porosity, exhibiting excellent adsorption performance compared with the carbon material directly carbonized from the pure PIL. Briefly, the mesopores could provide continuous channels for transporting CO2 molecules; simultaneously, that of the microporous architecture would adsorb CO2 molecules massively.

Results and Discussion

The preparation route of our NDC materials is shown in Scheme . The hierarchical porous NDCs with abundant pore channels were obtained via a facile two-step process including the carbonization of the homogenized mixture (the pure PIL as an ideal carbon source and K3[Fe(CN)6] as a multifunctional template) and the removal of the Fe-based template with hydrochloric acid subsequently. The diagrammatic drawing of an amplified portion on the porous carbon (NDPC) illustrated that the hierarchical structure of micropores and mesopores existed in this material, which further connected to those macropores finally. Furthermore, the NDC with hardly any pore structure was pyrolyzed directly from P[C4DVIM]Br2 in order to show the superiorities of K3[Fe(CN)6].

Scheme 1

Preparation of the N-Doped Hierarchically Porous Carbon Derived from PIL and Potassium Ferricyanide

The surface morphologies of NDC-800 and NDPC-1-800 were observed by SEM. As shown in Figure a, scarcely any pore structure could be visualized from NDC-800, which was directly carbonized from the pure PIL without potassium ferricyanide, and it corresponded precisely with the N2 adsorption–desorption isotherms and pore size distribution of NDC-800 (Figure a,c). However, the shape of NDPC-1-800 was evidently distinguished from that of NDC-800. It could be seen from Figure b–d that the obtained NDPC-1-800 presented an interconnected framework with a rich pore structure. It was mainly owing to the gas and ferrous compounds originating from potassium ferricyanide during the process of carbonization and then acid-etching treatment to form the abundant pore channels. Moreover, the existence of K+ during the carbonization process could also etch the carbon skeleton to construct more well-interconnected pore channels.[36,37] EDS mapping images of carbon and nitrogen (Figure e,f) in the selected NDPC-1-800 sample validated the homogeneous distribution of the doped N atoms on the surface of the porous carbon. The pore structure of NDPC-1-800 could be further confirmed from the TEM images. In Figure g,h, a large number of disordered macropores built up from carbonization and etching were observed clearly, and these porous channels provided CO2 molecules more passageways to transport fast and conveniently. Moreover, the quite transparent nanosheets proved their highly porous textures. The high-resolution TEM (HRTEM) image (Figure i) revealed a curved nanosheet in NDPC-1-800, and that was in fact a graphite-like structure.

Figure 1

SEM images of NDC-800 (a) and NDPC-1-800 (b–d). EDS mapping images of C (e) and N (f) elements for NDPC-1-800 according to the corresponding SEM image (d). TEM images of NDPC-1-800 (g,h) and HRTEM image of NDPC-1-800 (i).

Figure 3

N2 adsorption–desorption isotherms (a,b), mesoporous size distributions (c,d), and microporous size distributions (e,f) of NDC-800 and NDPC-x-y.

The ordered degree of carbon materials after thermal treatment was obtained by XRD patterns and is shown in Figure a,b. The appearance of two diffraction peaks at approximately 26 and 43° corresponded to the (0 0 2) and (1 0 0) planes of the graphitic carbons, respectively.[38,39] Meanwhile, it could be seen that a higher carbonization temperature or the existence of more potassium ferricyanide template during the process of carbonization resulted in a sharper and stronger (0 0 2) peak, indicating a higher graphitization degree.[40,41] Interestingly, the broad (0 0 2) peak became sharper and stronger obviously in Figure a once the temperature reached 700 °C, suggesting that a rapid graphitization process began at this temperature. In addition, the crystalline phases of metallic Fe and Fe3C (JCPDS no. 06-0696 and 35-0772 respectively) could be tested in the XRD pattern of FNDPC-1-800 (FNDPC, Fe-containing nitrogen-doped porous carbon), the one without the removal of template (Figure S2), indicating that the major constituents of the template were both Fe and Fe3C.

Figure 2

XRD patterns (a,b) and Raman spectra (c,d) of NDC-800 and NDPC-x-y.

Raman spectroscopy was applied to further confirm the graphitic degree of the carbon materials, which were prepared at different pyrolysis temperatures (Figure c) or with different amounts of potassium ferricyanide (Figure d). The spectra showed that two intensive bands at about 1350 cm–1 (D band) and 1580 cm–1 (G band) corresponded to the defective carbon structure and the ordered structure of carbon with the sp2 electronic configuration, respectively.[42,43] Usually, the degree of graphitization was estimated via the intensity ratio of the G/D band (IG/ID). Meanwhile, the value of IG/ID could also be influenced by the nitrogen content. Therefore, the IG/ID values of NDPC-x-y suggested that the higher pyrolysis temperature (IG/ID = 0.94–1.11) or the more potassium ferricyanide template amount (IG/ID = 0.94–1.21) would increase the graphitization degree gradually, which might be due to the change of nitrogen content at different conditions (Table ).

Table 1

Textural Properties and Chemical Compositions of NDC-800 and NDPC-x-y

	textural property		chemical composition
sample	SBETa (m2·g–1)	Vtotalb (cm3·g–1)	Cc(wt %)	Nd(wt %)
NDC-800	14	0.01	84.75	8.81
NDPC-1-500	82	0.21	62.32	16.84
NDPC-1-600	257	0.39	65.48	15.78
NDPC-1-700	752	1.13	75.26	6.09
NDPC-1-800	1189	1.35	83.89	2.62
NDPC-1-900	447	0.46	89.18	1.87
NDPC-0.5-800	383	0.41	87.35	1.55
NDPC-2-800	640	0.96	83.32	2.77

Specific surface areas were calculated by the BET method.

Total pore volumes were determined at P/P0 = 0.99.

Carbon content was measured by elemental analysis.

Nitrogen content was measured by elemental analysis.

The N2 adsorption–desorption measurements were performed at 77 K to analyze the textural properties. The adsorption–desorption isotherms and corresponding pore size distributions of the carbon materials in this work are exhibited in Figure , and the results of Brunauer–Emmett–Teller (BET) specific surface areas (SBET) and total pore volumes (Vtotal) are listed in Table . NDC-800, the carbon material carbonizing directly from the pure PIL at 800 °C, showed a nonporous structure that could be estimated from its SBET and pore distribution, bearing a small SBET value of 14 m2·g–1 and a low Vtotal (0.01 cm3·g–1). The isotherms of carbon samples carbonized at different temperatures with the mass ratio of the PIL/potassium ferricyanide = 1 are shown in Figure a. NDPC-1-500, 600 presented type-IV isotherms, suggesting the existence of mesopores with a few-micropore structure. Interestingly, when the temperature was at or higher than 700 °C, N2 adsorption–desorption isotherms of these carbons transformed into type-I/IV with high adsorption capacities at low relative pressure (P/P0 < 0.1) and exhibited more pronounced hysteresis loops, index of the structure involving micro-/mesopores.[44,45] The pore-forming of these hierarchically porous carbons was mainly attributed to the gas and Fe/Fe3C templates originating from potassium ferricyanide during the process of carbonization and then acid-etching treatment with hydrochloric acid, while potassium ions could also etch the carbon framework during the process of pyrolysis. This result was in accordance with that in Table ; the SBET and Vtotal increased abruptly with 700 °C as the dividing line. The hierarchically porous structure of these carbon materials was conducive to CO2 diffusion and adsorption that mesopores were used for transporting CO2 molecules, while the micropores could offer plenty of space for accommodating the CO2 molecules. This hierarchically porous structure could be further verified by pore size distributions.[23] The microporous size distribution curves were calculated by the Horvath–Kawazoe method, while the mesoporous size distribution curves were obtained through the Barrett–Joyner–Halenda method. It is clear that the peak values of these mesopores were located at approximately 3.7 nm, and the micropores were at the range of 0.54–0.81 nm, which was conducive to the CO2/N2 selectivity.[46] Meanwhile, the highest SBET of the carbons above was achieved on the NDPC-1-800 sample (1189 m2·g–1). To investigate the influence of the different added amounts of potassium ferricyanide on the textural properties, the isotherms and pore size distributions of our carbons with different mass ratios of the PIL/potassium ferricyanide at 800 °C are depicted in Figure b,d,f, and the textural properties are listed in Table .

Elemental analyses were performed to access the element compositions of the carbon materials in this work, especially N content, which was beneficial to CO2 capture.[47−49] As shown in Table , a common tendency of the decreasing N content from 16.84 to 1.87 wt % with the increasing pyrolysis temperature for carbon samples NDPC-1-y was observed. Among them, the N content of NDPC-1-800 was much lower than that of NDC-800, mainly due to the utilization of hydrochloric acid when removing templates. Notably, the N content of NDPC-1-700 decreased suddenly to 6.09 wt %, and the situation of this was similar to the corresponding values of textural properties, which might be related to the graphitic degree. Furthermore, the use of more potassium ferricyanide caused more nitrogen during preparation and it might be because potassium ferricyanide, as a multifunctional template, could provide not only the Fe-based templates Fe/Fe3C for pore-forming but also the N atoms to porous carbons, which had interactions with acidic CO2 molecules.

For better understanding the advantages of potassium ferricyanide templates, the porous carbon with the FeCl3 template was synthesized at the same preparation conditions of NDPC-1-800 (shown in the Supporting Information), which was written as NDPC-FeCl3. The textural properties and chemical compositions of these carbons are exhibited in Figure S3a and Table S1. It was clear that NDPC-FeCl3 presented a micro-/mesoporous structure with a smaller SBET of 539 m2·g–1, which was much smaller than that of NDPC-1-800. Moreover, the N content of NDPC-1-800 was higher than that of NDPC-FeCl3, indicating that potassium ferricyanide could provide both Fe-based templates Fe/Fe3C for pore-forming and N atoms. Meanwhile, the N content of composite FNDPC-1-800 (10.28 wt %) without the removal of the template could further confirm that potassium ferricyanide offered N atoms to carbon skeleton during pyrolysis.

XPS was carried out to investigate the binding environment of the nitrogen dopant in our carbon materials. Figure exhibits the high-resolution N 1s spectra of NDC-800 (a) and NDPC-1-800 (b). The asymmetric N 1s peaks could be decomposed into three peaks mainly made up of pyridinic N (397.8 ± 0.2 eV), pyrrolic N (399.5 ± 0.3 eV), and graphitic-type quaternary N (400.7 ± 0.1 eV).[50,51] The N contents on the surfaces of NDC-800 and NDPC-1-800 tested by XPS were 6.60 and 2.78 wt %, respectively, which were almost the same as those (8.81 and 2.62 wt %) in the bulk materials detected by elemental analyses, indicating a uniform dispersion of N atoms, and the result was well coincident with the EDS elemental mapping analysis (see Figure f). Moreover, relatively more pyridinic and pyrrolic nitrogen inferred from the N 1s XPS spectra existed in these two carbons, which were in favor of CO2 capture.[52] Meanwhile, it could be seen obviously from Figure S4 that the content of graphitic N increased during the pyrolysis process with potassium ferricyanide, which meant the increasing graphitic degree, and it was well in line with the result of the Raman spectrum.

Figure 4

High-resolution N 1s XPS spectra of NDC-800 (a) and NDPC-1-800 (b).

The CO2 adsorption isotherms of as-prepared carbon materials are depicted in Figure with values between 10.32 and 34.65 cm3·g–1 measured at 25 °C and 1.0 bar. The CO2 uptakes of as-prepared carbons increased steadily following increasing CO2 pressure up to 1.0 bar, indicating that more CO2 adsorption capacity could be achieved at higher pressures. In Figure a, the carbon derived from the pure PIL at 800 °C exhibited a lower CO2 uptake of 10.32 cm3·g–1 at 25 °C and 1.0 bar despite its high N content (8.81 wt %). Although acidic CO2 molecules could interact with the nitrogen atoms on the outer surface of NDC-800, hardly any pore channels existed. After thermal treatment and template removal of potassium ferricyanide, the amount of CO2 sorption increased along with the temperature used for carbonization until 800 °C (34.65 cm3·g–1), and the tendency of increasing CO2 uptake seemed good in relation to those of the BET specific surface areas. However, at a higher temperature of 900 °C, the CO2 adsorption capacity declined sharply, even lower than that of NDPC-1-500, and the reason was not only the decreasing BET specific surface area but also the much lower nitrogen content (1.87 wt %). Usually, the doped nitrogen interacted with the acidic CO2 molecules readily. Meanwhile, the CO2 sorption isotherms of carbons with different amounts of potassium ferricyanide at 800 °C are exhibited in Figure b. It could be observed that the adsorption capacity of NDPC-1-800 was higher than those of both NDPC-0.5-800 and NDPC-2-800, suggesting that the biggest CO2 adsorption capacity was presented when the mass ratio of K3[Fe(CN)6]/PIL = 1 due to its highest SBET and hierarchically porous structure. The carbon materials derived from the bisvinylimidazolium-based PILs with the multifunctional template potassium ferricyanide exhibited higher CO2 adsorption capacities than the nonporous carbon NDC-800, principally because its hierarchical porous architecture played an important part in the process of CO2 adsorption, briefly describing that CO2 molecules could transport and diffuse effectively in their mesoporous channels and simultaneously be adsorbed in micropores massively. Therefore, we could conclude that the CO2 uptake of NDPC was determined by two significant factors: pore structure and nitrogen content.

Figure 5

CO2 adsorption isotherms of NDPC-1-y (a) and NDPC-x-800 (b) at 25 °C. The DSLF equation fitting of CO2 and N2 adsorption on NDC-800 (c) and NDPC-1-800 (d) at 25 °C (points, experimental data; lines, fitting curves). The CO2/N2 selectivities (e) on NDC-800 and NDPC-1-800 at 25 °C. Five consecutive cycles (f) of CO2 adsorption–desorption on NDPC-1-800 at 25 °C and 1.0 bar.

The ideal adsorption solution theory (IAST) was applied to determine the CO2/N2 selectivity, which was crucial for practical applications.[53,54] In Figure d, the N2 adsorption capacity of NDPC-1-800 was much lower than its CO2 uptake rather than that of NDPC-1-800 in Figure c, indicating the high CO2/N2 selectivity of NDPC-1-800. Moreover, the dual-site Langmuir–Freundlich (DSLF) model was taken in Figure c,d for fitting the adsorption isotherms of CO2 and N2, which could describe these adsorptions more accurately.[55] The DSLF model (eq ) is defined as follows.q (mmol/g) herein was the equilibrium adsorption capacity at the corresponding pressure of p (kPa); q (mmol/g) and b (1/kPa) were the saturation capacity and the correlative coefficients for site i, respectively; and n was the deviation from the ideal surface. All the experimental data of NDC-800 and NDPC-1-800 were matched well with the fitting curves, indicative of the appropriate utilization of this model. The fitting parameters of DSLF are listed in Table . The CO2/N2 (15/85 v/v) selectivity was simulated from the flue gas mixture and the IAST as follows was picked to calculate the CO2/N2 selectivity (Sads, eq ).[54,56]where Sads represented the adsorption selectivity calculated by IAST, while q (mmol·g–1) was the adsorption amount of the given gas in the equilibrium partial pressures of P (kPa). It is clear in Figure e that the selectivity of both NDC-800 and NDPC-1-800 decreased following the increasing pressure, and the CO2/N2 selectivity of NDPC-1-800 was always higher than that of NDC-800. At 0.1 bar, the CO2/N2 selectivity of NDPC-1-800 could reach 43.69, as much as around 3.1 times higher than that of NDC-800. Meanwhile, the CO2/N2 selectivity of NDPC-1-800 was still up to 8.23, whereas that of NDC-800 was only 2.32. It was mainly due to the advantages of both pore structure and N active sites of NDPC-1-800. In brief, the high selectivity of CO2/N2 was dependent on its high specific surface area; the tailored microporous structure peaked at 0.54 nm and the strong interaction with acidic CO2 molecules.[46] However, NDC-800, which owned a nonporous structure, could be only effected by its N active sites. The high CO2 uptake and CO2/N2 selectivity made NDPC-1-800 a potential candidate adsorbent for solving environmental issues.

Table 2

Fitting Parameters for the DSLF Isotherm Model

	NDC-800		NDPC-1-800
	CO2	N2	CO2	N2
q1 (mmol/g)	0.6142	3.5725	2.1881	0.7969
b1 (1/kPa)	0.5915	0.0939	0.5522	0.1350
n1	1.5886	0.4904	1.1574	0.8916
q2 (mmol/g)	0.6142	0.1058	2.1881	5.0466
b2 (1/kPa)	0.5915	0.0696	0.5522	0.0802
n2	1.5886	1.4045	1.1574	0.4444
R2	0.99912	0.99998	0.99996	0.99996

In consideration of practical applications, the selectivities of CO2 adsorption from complicated conditions were further discussed. The CO2/N2 selectivities with different temperatures and CO2 volume ratios were further investigated, and these related results are shown in Figure S5 and Table S2. In Figure S5b, it was clear that the CO2/N2 (15/85 v/v) selectivity of NDPC-1-800 at 25 °C was always higher than that of NDPC-1-800 at 35 °C (3.92), briefly due to the lower adsorption performance following the higher temperature. Subsequently, the CO2/N2 selectivities with different CO2 volume ratios (10/15/20%) at the temperature of 25 °C were studied and are shown in Figure S5c. The DSLF model and corresponding fitting parameters are also exhibited in Figure d and Table . It could be observed that the CO2/N2 selectivity decreased following the increasing CO2 volume ratio (from 11.18 to 5.26).

The regeneration capability of a CO2 adsorbent was important for practical applications as well, and the reversibility of CO2 adsorption–desorption on NDPC-1-800 was measured over five cycles at 25 °C and 1.0 bar. The sample was activated at 120 °C for 6 h under vacuum in order to make sure that the adsorbed CO2 molecules could be removed thoroughly. In Figure f, no evident decrease in CO2 uptake could be observed after five cycles, indicative of the good regenerability of our carbon material NDPC-1-800 with maintained stability.

To investigate the strength of interaction between the adsorbents and CO2 molecules, the isosteric heat of adsorption (Qst) was calculated by the CO2 adsorption isotherms at 25 and 35 °C with the Clausius–Clapeyron equation (eq ).[57,58] In Figure a, CO2 isotherms of NDC-800 and NDPC-1-800 at 25 and 35 °C were presented to research the effect of CO2 uptakes with temperature. Obviously, the CO2 uptake of NDPC-1-800 decreased following the increasing temperature, indicating its physisorption process.[59] However, variation of temperature had little effect on the CO2 adsorption capacity of NDC-800, mainly because most of the adsorbed CO2 molecules on NDC-800 depended on the abundant N atoms of its outer surface with hardly any pore structure. Eq was as follows.

Figure 6

CO2 adsorption isotherms (a) of NDC-800 and NDPC-1-800 at 25 and 35 °C. Isosteric heats of CO2 adsorption (b) on NDC-800 and NDPC-1-800.

Equation could be rewritten as shown below (eq ) for convenient application.

Qst (kJ·mol–1) herein was the isosteric heat of CO2 adsorption, R was the universal gas constant of 8.314 kJ·mol–1·K–1, and P (Pa) was the pressure at the temperature of T (K). In Figure b, the Qst value of NDC-800 was about 31 kJ·mol–1 at a low surface coverage and decreased sharply to 18 kJ·mol–1 at a high surface coverage. The reason for this situation was that NDC-800 would have a strong interaction between basic N active sites and acidic CO2 molecules at low CO2 loading without any pore structure. Meanwhile, the favorable binding sites were occupied at a higher CO2 coverage. However, the Qst values on NDPC-1-800 dropped gradually from 34 to 28 kJ·mol–1 and was always higher than that of NDC-800, suggesting a stable physisorption process and strong interaction with CO2 molecules on NDPC-1-800. What is more, the high Qst value of NDPC-1-800 contributed to enhance its CO2/N2 selectivity.

In Figure , we proposed a possible CO2 adsorptive mechanism of NDC and NDPC based on the characterization analysis above and previous related studies,[23,60,61] together with the advantages of NDPC. The adsorptive behavior of NDC is illustrated in Figure a, wherein CO2 molecules could only attach on the outer surfaces of NDC just owing to the nonporous structure and the interaction between acidic CO2 molecules and basic nitrogen atoms on the surfaces of NDC. In other words, the CO2 uptake of NDC with hardly any pore structure just depended on the N content outside the surface. Nevertheless, potassium ferricyanide, as a multifunctional template, was so important for the hierarchically porous structure and chemical compositions of these carbons. It could provide not only the gas and Fe-based templates Fe/Fe3C for pore-forming but also the N atoms to porous carbons. The excellent CO2 adsorption capacity of NDPC was determined by both N content and pore structure, especially its hierarchically porous structure. In Figure b, we could find that the existence of both mesoporous and microporous structures provided a convenient way to CO2 diffusion and adsorption; briefly, mesopores were just like many tubes to transport CO2 molecules and simultaneously micropores offered plenty of space for accommodating CO2 molecules. Furthermore, CO2 molecules could also adhere to the passageway walls because of the interaction between the acidic CO2 molecules and the doped nitrogen atoms on the surfaces of NDPC. The diagram of CO2/N2 selectivity on NDC-800 and NDPC-1-800 is depicted in Figure c. The higher CO2/N2 selectivity of NDPC-1-800 was mainly owing to its high SBET, appropriate microporous size distribution, and hierarchically porous structure. The large SBET could provide more contact areas for the interaction between basic N active sites and acidic CO2 molecules, while the micropores peaking at 0.54 nm was attributed to the enhanced selectivity of CO2 over N2. Meanwhile, its hierarchical porous structure was able to accelerate the CO2 molecule transportation. Therefore, this hierarchical porous carbon could combine many superiorities of CO2 capture and would be a promising candidate adsorption material for practical applications.

Figure 7

Mechanism diagram of CO2 adsorption on NDC (a) and NDPC (b) adsorbents. Mechanism diagram of CO2/N2 selectivity (c) on NDC-800 and NDPC-1-800.

Conclusions

In conclusion, we proposed a facile approach for preparing N-doped hierarchical porous carbons with abundant pore channels. It had been known that the bisvinylimidazolium-based PILs were ideal carbon sources, while potassium ferricyanide, as a multifunctional template, could provide not only the gas and Fe-based compounds (metallic Fe and Fe3C) for pore-forming during the pyrolysis process but also the N atoms to porous carbons. The use of more potassium ferricyanide during preparation caused a higher nitrogen content, which had interaction with acidic CO2 molecules. Moreover, the existence of K+ ions during the carbonization process could also etch the carbon skeleton to construct more well-interconnected pore channels. Interestingly, the phase structure of these NDPCs would be changed during carbonization of the PIL with this multifunctional template; generally, a higher pyrolysis temperature or more potassium ferricyanide amount increased the graphitization degree gradually. Meanwhile, the hierarchical porous carbon prepared at 800 °C (NDPC-1-800) had taken advantage of the highest specific surface area (1189 m2·g–1) and the biggest pore volume (1.35 cm3·g–1), exhibiting an excellent CO2 adsorption capacity (34.65 cm3·g–1) compared with the carbon material directly carbonized from the pure PIL, around 3.36 times as much as that of NDC-800. The existence of a hierarchical porous structure was in favor of the process of CO2 capture, wherein the mesopores could provide a continuous channel for transporting CO2 molecules; simultaneously, the microporous structure would offer plenty of space for accommodating CO2 molecules massively. For practical applications, this carbon could be regenerated and reused easily with almost no drop in CO2 uptake after five cycles. We demonstrated a novel and facile method to prepare N-doped hierarchical porous carbons and made sure that this kind of carbon materials could solve not only the threat of environment but also the shortage of energy.

Experimental Section

Materials

1,4-Dibromobutane, 1-vinylimidazole, methanol, azobis(isobutyronitrile) (AIBN), ethanol, dichloromethane, and potassium ferricyanide (K3[Fe(CN)6]) were bought from Aladdin Chemical Co. Toluene, diethyl ether, acetone, and hydrochloric acid were purchased from Shanghai Lingfeng Chemical Reagent Co. All analytical-grade chemicals and solvents were obtained from commercial sources and used without further purification.

Synthesis of IL [C4DVIM]Br2

The bisvinylimidazolium salt [C4DVIM]Br2 was conveniently obtained by reaction between 1,4-dibromobutane and 1-vinylimidazole.[62,63] In brief, 1,4-dibromobutane (4.32 g, 20 mmol) and 1-vinylimidazole (3.96 g, 42 mmol) were added in toluene (20 mL), and the solution was stirred at 90 °C for 24 h. After cooling to normal temperature, the crude product was washed five times with diethyl ether. Subsequently, the obtained product was dissolved in methanol with activated carbon and stirred overnight for further purification. The final [C4DVIM]Br2 could be gained by centrifugation, rotary evaporation, and following vacuum drying at 40 °C. The Scheme for the synthesis of IL [C4DVIM]Br2 is shown in Figure S1.

Synthesis of the PIL Material

The PIL was prepared via the free radical polymerization of [C4DVIM]Br2.[62,63] Typically, [C4DVIM]Br2 (1.11 g, 2.75 mmol), AIBN (0.04 g), and ethanol (25 mL) were placed in a three-necked flask. Subsequently, the above mixture was reacted at 78 °C for 20 h under an N2 atmosphere. After cooling down to normal temperature, the suspension was washed three times with methanol and dichloromethane. The PIL material P[C4DVIM]Br2 was finally obtained via vacuum drying for 2 h at 40 °C.

Synthesis of Nitrogen-Doped Hierarchically Porous Carbon Materials

First, 5 g of PIL and a defined amount of K3[Fe(CN)6] were dispersed in 100 mL of acetone through magnetic stirring for 48 h at normal temperature in order to gain the uniform mixture of K3[Fe(CN)6]/PIL. The organic solid could be achieved by washing with acetone and following vacuum drying overnight at 40 °C. The K3[Fe(CN)6] and PIL were mixed completely together in a K3[Fe(CN)6]/PIL mass ratio of 0.5, 1, or 2, and donated as PF-x (x = 0.5, 1, 2). Afterward, these composites PF-x were heated at a certain temperature for 4 h with a nitrogen flow rate of 50 mL/min under the heating rate of 5 °C/min and then cooled to normal temperature. The carbonized samples were donated as FNDPC-x-y, which represented iron-containing NDPC. The solid carbon product FNDPC-x-y was then stirred in the concentrated hydrochloric acid (37 wt %) overnight. Finally, NDPC NDPC-x-y could be obtained by washing with water several times until pH = 7 and vacuum drying overnight at 80 °C. For comparison, P[C4DVIM]Br2 was pyrolyzed with the same conditions as FNDPC-x-y and donated as NDC-y, where x represented the K3[Fe(CN)6]/PIL mass ratio and y was the pyrolysis temperature.

Characterization

SEM images were obtained from a HITACHI S-4800 field emission scanning electron microscope, and EDS was also provided with this equipment. TEM images were collected on a JEM-2100 (JEOL) electron microscope. Raman spectra were obtained from a Jobin Yvon (Laboratory RAM HR1800) confocal micro-Raman spectrometer. XRD measurements were performed with a SmartLab 9kW. N2 adsorption–desorption measurements were performed on the BELSORP-MINI analyzer, and these samples would be pretreated at 120 °C for 6 h under vacuum. Element analyses were carried out with a CHNS elemental analyzer Vario EL cube to determine the N contents of the samples. The XPS spectrum was recorded on a PHI-5000 Versa Probe system.

CO2 and N2 Adsorption Measurement

CO2 adsorption isotherms were obtained on the MicrotracBEL BELSORP-max at 25 °C and 35 °C under the low pressure of 0–1.0 bar, and a 0.1 g sample was pretreated at 120 °C for 6 h under vacuum before measurements. N2 adsorption isotherms were obtained on the BELSORP-MINI analyzer at 25 and 35 °C under the low pressure of 0–1.0 bar; a 0.05 g sample was pretreated at 120 °C for 6 h under vacuum before measurements. A CO2 adsorption cyclic performance test was carried out multiple times at 25 °C to research the reusability of these adsorbents. These samples were vacuumized at 120 °C for 6 h after each adsorption process to ensure that the adsorbed CO2 could be removed fully.