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Synthesis and characterization of nitrogen-containing hydrothermal carbon with ordered mesostructure
Chemical Physics Letters 716 (2019) 237–246
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Research paper
Synthesis and characterization of nitrogen-containing hydrothermal carbon with ordered mesostructure
T
⁎
JuHyon Yu, JuHyok So
Department of Chemistry, University of Science, Pyongyang 950003, Democratic People’s Republic of Korea
H I GH L IG H T S
carbons were synthesized via a hydrothermal approach. • Nitrogen-doped contributes a lot in the assembly of nitrogen-containing polymer species. • Ethylenediamine • Obtained materials possess an ordered 2-D hexagonal (p6m) mesostructure and good CO /N 2
2
selectivity.
A R T I C LE I N FO
A B S T R A C T
Keywords: Mesoporous carbon Nitrogen group Hydrothermal CO2 capture
One-pot hydrothermal synthetic approach for preparing nitrogen-containing mesoporous carbons with high surface area and rich nitrogen content through organic-organic self-assembly of ampliphilic triblock copolymer (Pluronic F127), phenolic resins and ethylenediamine as nitrogen source has been achieved. Ethylenediamine molecules participate in cross-linking of resorcinol and aldehyde, whereas the formed resin molecules bridge with Pluronic F127 via hydrogen bonding during hydrothermal process, leading to the formation of nitrogencontaining carbons with ordered mesostructure during pyrolysis at 800 °C. The obtained nitrogen-containing carbon had a large surface area and pore volume, nitrogen incorporated into carbon frameworks was capable for enhancing the CO2 adsorption capacity.
1. Introduction Among the various porous carbon materials, high-ordered mesoporous carbons have gained great interest in the fields of catalysis, adsorption, separation, fuel cell, energy storage and drug delivery, mainly due to their considerable pore channels, uniform structure regularities, high specific surface areas, thermal and chemical stabilities [1–4]. Especially, the functional groups on the carbon surface containing foreign atoms (N, P, Br or S) can significantly increase the surface and/or framework activity to enhance the practical utility of ordered mesoporous carbons (OMCs) [5–10]. For instance, recent studies have demonstrated that nitrogen containing can effectively improve electrochemical and adsorption/separation performance of OMCs owing to high hydrophilicity, polarity and electric conductivity of the carbon surface [5,11,12]. Therefore, the effective introduction of nitrogen in mesocarbon framework can be an alternative strategy to enhance their application in CO2 capture. The most widespread approach to prepare nitrogen-containing
ordered mesoporous carbons (NOMCs) consists in hard-templating method (i.e., nanocasting route), in which the suitable nitrogen sources were introduced into the mesopores of hard templates (e.g., MCM-41, MCM-48, SBA-15, etc) by impregnation [13–19]. This hard-templating synthetic route often requires multistep procedure (such as polymerization, carbonization and etching step) and is time consuming, leading to low economic efficiency in an industrial application. In recent years, several research groups have demonstrated that soft-templating method via self-assembly of amphiphilic block copolymers (e.g., pluronic F127, polyethylene oxide-polypropylene oxide-polyethylene oxide, EO106-PPO70-PEO106) and phenolic resins was a versatile route for the preparation of nitrogen-containing porous carbons with wellordered structure, which escapes from a fussy nanocasting process [20–23]. Phenolic resins and polyethylene oxide (PEO)-containing copolymers is an appropriate pair to synthesize ordered mesoporous structure by direct self-assembly method, because of hydrogen-bonding interactions between phenolic resins and PEO segments of block copolymer. Based on this soft-templating strategy, a series of nitrogen
⁎ Corresponding author at: Basic Chemical Research Institute, Department of Chemistry, University of Science, Pyongyang 950003, Democratic People’s Republic of Korea. E-mail address: [email protected] (J. So).
https://doi.org/10.1016/j.cplett.2018.12.014 Received 26 September 2018; Received in revised form 19 November 2018; Accepted 6 December 2018 Available online 25 December 2018 0009-2614/ © 2018 Elsevier B.V. All rights reserved.
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hexagonal mesostructure (p6m), high specific surface area and uniform pore size were obtained through hydrothermal treatment, whereas the incorporated nitrogen groups in the mesoporous carbon frameworks significantly improved the CO2 adsorption capacity.
containing mesoporous carbon with ordering channels have been successfully prepared by using phenol/formaldehyde resins as the carbon precursors, whereas urea [24], dicyandiamide [25], 1, 6-diaminohexane [26] and lysine [27] have been used as nitrogen sources. Especially, evaporation-induced self-assembly (EISA) pathway, which is widely used for synthesizing most of reported ordered mesoporous carbons, is a powerful tool for the preparation of NOMCs; nevertheless, this step-by-step EISA route needs the evaporation of large volumes of organic solvents and prepolymerization procedure [24,25,28]. On the other hand, the facile syntheses of preparing NOMCs through the posttreatment [13,29,30] and ionic liquids procedure [31,32] have also been reported; however, the resulting mesoporous carbons showed corrosion of carbon matrix and obvious collapse or degradation of the mesostructure, leading to the limitation of their applications. To overcome these limitations, an aqueous route via the cooperative self-assembly in a dilute aqueous solution, has recently been emerged as a versatile route for the preparation of NOMCs [33,34]. This method has been developed to directly prepare OMCs and heteroatom-doped OMCs by common dilute aqueous reaction route under the milder conditions, when compared with the pathways mentioned above. Although the aqueous self-assembly is very interesting, it always involves about 2–7 days to complete the reaction due to the slow self-polymerization rate, which is time- and energy-consuming. In recent years, we have developed an aqueous self-assembly to prepare NOMCs from resorcinol-urea-formaldehyde [35] or resorcinol-melamine-formaldehyde resins [36] and copolymer F127 ingredients without using additional prepolymerization and hydrothermal solidification steps, whereas the hydrogen bonding between multiple hydroxyl groups of organic polymer and ethylene oxide (EO) repeating units in F127 template and further assembly is involved for the construction of ordered mesoporous framework during polymerization and carbonization steps. However, this one-pot synthesis also takes reaction time for 24 h. The hydrothermal synthesis is a faster and more energy efficient approach to prepare nitrogen-doped carbons with an ordered structure than the EISA route and dilute aqueous approach [26,27]. In general, the polymerization of phenol-formaldehyde system is very fast during hydrothermal process under acidic and/or basic condition, leading to collapsed mesostructures of the resulting carbons and low content of impregnated nitrogen. Additionally, in the synthesis of NOMCs with hydrothermal treatment, assembly of nitrogen containing organic precursors with amphipilic surfactant usually could lead to disordered mesoporous carbon frameworks, therefore, to control the rate kinetics of the polymerization reaction for the formation of NOMCs through direct self-assembly under hydrothermal condition is challenging task. Herein, we report in detail a one-step method to prepare NOMCs through hydrothermal method by using resorcinol and hexamethylenetetramine (HMT) as carbon sources and ethylenediamine (EDA) as nitrogen precursor, while copolymer of Pluronic F127 was employed as template with 1, 3, 5-trimethylbenzene (TMB) added as an organic swelling agent. The use of HMT instead of formaldehyde can control the rate of polymerization reaction to prepare NOMCs during self-assembly. Cosolvent organic molecules such as TMB, which is usually useful in the pore size expansion of mesoporous powders, can be effective agents in controlling pore size of carbon mesoshapes [4,34]. As result of the interaction TMB with PPO segment of triblock copolymers, the lattice can be enlarged or phase transition can occur. On the other hand, the nature of the organic amines seems to be a crucial factor in determining the mesostructure assembly between the polymer species and the copolymer template. In molecular design of nitrogen containing porous polymers and their carbonaceous materials with phenols, aldehydes and diamines (nitrogen sources), EDA is an applicable amine to form co-polymer with resorcinol and formaldehyde, while multiple hydrophilic segments of polymer species could interact with poly ethylene oxide (PEO) segments of triblock copolymer by hydrogen bonding, resulting in the formation of NOMCs after calcination process. Our results showed that the high-quality of NOMCs with 2D
2. Experimental 2.1. Materials Triblock poly (ethylene oxide)-b-poly (propylene oxide)-b-poly (ethylene oxide) copolymer Pluronic F127 (PEO106-PPO70-PEO106, Mw = 12600) was purchased from Sigma-Aldrich Corp. Other chemicals resorcinol, hexamethylenetetramine (HMT), ethylenediamine (EDA) and 1, 3, 5-trimethylbenzene (TMB) were supplied by Co., Ltd, Shanghai, China. All chemicals were used as received without further purification. 2.2. Synthesis In a typical synthesis procedure of nitrogen-containing mesoporous carbon sample with 2-D hexagonal mesostructure, 0.55 g resorcinol(R), 0.35 g HMT, 0.23 g TMB and 1.0 g Pluronic F127 were dissolved in 18 g distilled water with magnetic stirring at room temperature for 2 h. Afterwards, 0.15 ml of EDA was added to the above solution and stirred for about 0.5 h. After the reaction system turned to pale yellow during stirring, this homogeneous solution was poured into an autoclave and cured at 100 °C for 10 h. The reactant molar ratio of F127/R/HMT/ TMB/EDA/H2O was 0.0159/1.0/0.5/0.38/0.45/200. The resulting asmade polymer was collected by filtration and washed with water and ethanol several times and dried at 60 °C for 24 h. Finally, the sample was calcined in a tubular furnace at 800 °C for 3 h, with a heating rate 1 °C min−1 under nitrogen atmosphere, to obtain nitrogen-containing carbon material. In order to activate with KOH, 0.4 g of mesoporous carbon was impregnated with KOH solution (1.6 g KOH in 4 g of water) for 1 h, followed by water evaporation at 100 °C. The activation process was carried out in a tube furnace under nitrogen flow, while heating the sample at a rate of 10 °C min−1 up to 800 °C and held at the temperature for 60 min. After activation, the sample was thoroughly washed three times with 10 wt% HCl solution and then washed with distilled water and ethanol for several times until the wash had a constant pH of 7.0. Finally, the sample was dried in an oven at 100 °C for 12 h, obtaining the activated carbon. 2.3. Characterization The powder of X-ray Diffraction (XRD) patterns of mesoporous carbons were recorded on Riguku D/MAX2550 diffractometer using CuKα radiation (λ = 0.15418 nm). The unit cell parameters of mesostructures were calculated from the formula a 0 = (2/ 3 ) d100. Thermo gravimetric analysis (TGA) was conducted on a TGA G500 thermal analyzer system. Transmission electron microscopy (TEM) images were obtained with FEI Tecnai F20 electron microscope operating at 200 kV. Nitrogen absorption/desorption isotherms were measured at 77 K using an Autosorb iQ2 adsorptometer (Quantachrome Instruments). Before measurements, the dried samples were outgassed at 180 °C for at least 12 h under vacuum in the degas port of the adsorption instruments. Brunauar-Emmett-Teller (BET) method was utilized to calculate the specific surface areas (SBET). The pore volumes and pore size distributions (PSDs) were derived from the adsorption branches of isotherms using the Barrett-Joyner-Halenda (BJH) model, whereas the total pore volumes (Vt) were estimated from the amount adsorbed at a relative pressure P/P0 of 0.99. Micropore volumes (Vmi) were obtained via tplot analysis. The t values were estimated as a function of the relative pressure (P/P0) ranging from 0.08 to 0.25. Elemental analysis was done on a CHN elemental analyzer (Vario Micro, Elementar). Fourier 238
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transform infrared (FT-IR) spectra were collected on IFS 66V/S Fourier transform infrared spectrometer at room temperature in the range of 400–4000 cm−1, with KBr pellets. SEM images of samples were obtained using field-emission scanning electron microscopy (FE-SEM: JEOL JSM6700F). The XPS spectra were carried out on an ESCALAB250 spectrometer. 2.4. CO2 sorption measurement CO2 adsorption isotherms of the samples were measured through a gravimetric determination method by using an Autosorb iQ2 adsorptometer (Quantachrome Instruments). Each sample was degassed at 150 °C for 18 h to remove the guest molecules from pores. After cooling down to the required adsorption temperature (0 or 25 °C), the introduction of CO2 into the system was followed. The CO2 adsorption capacity in terms of adsorbed volume was recorded until equilibrium was achieved. 3. Result and discussion 3.1. Thermo gravimetric analysis The poly resin or polybenzoxazine formed from an appropriate amount of EDA, resorcinol and formaldehyde via self-assembly forms hydrogen-bonds with the PEO segments of F127 templating agent to a significant extent, while the micelles evolving from block copolymer molecules are fixed in the polymer framework, which would attain an alignment of ordered channels during co-condensation. We first analyzed TGA profiles to investigate the pyrolysis behavior of copolymer F127 and mesoporous polymer matrix. As shown Fig. 1, the weight loss of as-synthesized sample without F127, in the temperature range of 300–400 °C, is only 7.91%, which is due to partial condensation and decomposition of nitrogen-containing polymer; However, the mass reduction of the mesoporous polymer prepared by using the template agent is 43.35%, which can be considered to be derived from the complete decomposition of the self-organizing casting agent in an inert atmosphere. This result is in agreement with the previous literatures [35,36]. On the other hand, a gentle and gradual weight loss of 6.02% from room temperature to 300 °C results from desorption of moisture and carbon dioxide and the cleavage of small molecules from the polymerization. 24.88% weight loss takes place in the temperature range of 400–700 °C, suggesting that the hydrocarbons and oxygen containing groups can be considerably decomposed and eliminated, which indicates that framework condensation and carbonization are improved at temperature of above 400 °C. Above 700 °C, carbon
Fig. 2. XRD patterns of CN-0.38 (a), CN-0.45 (b), CN-0.52 (c), CN-0.59 (d) and CN-0.66 (e).
frameworks show comparable thermal stability with higher pyrolysis temperature .The weight loss from 700 to 800 °C becomes 1.41%, indicating the occurrence of cleavage/rearrangement of nitrogen-containing functionalities and carbonization of the material, while a residue of approximate 24.34% is obtained at 800 °C. It can thus be concluded that the thermally stabilized carbon frameworks can be achieved at 800 °C. 3.2. Structure and morphology As mentioned above, EDA is a good option for the synthesis of nitrogen-doped carbon materials under the hydrothermal condition. In order to investigate the influence of EDA on mesoporous structure, a series of samples was synthesized by fixing the molar ratio of F127 to resorcinol at 0.0159:1 and varying the molar ratio of EDA. The obtained samples were denoted as CN-x, where × is the molar ratio of EDA to resorcinol. As can be seen in Fig. 2, low angle XRD patterns reveal that the samples CN-0.38, and CN-0.45 have four resolved diffraction peaks at 2θ = 0.9–2.5°. The corresponding reciprocal d-spacing value ratios are 1: 3 : 4 : 7 , which are indexed as 10, 11, 20 and 21 reflections of a 2D hexagonal space group (p6m), consistent with FDU-15 mesoporous carbons [37]. With further increasing amount of EDA, the XRD diffractions yield less resolved peaks, indicating that the ordered mesostructures are slightly degenerated. Furthermore, the corresponding reflections are slightly shifted to lower 2θ angles with increased amounts of EDA, suggesting that higher amounts of EDA can dramatically cause
Fig. 1. TG analysis of as-prepared samples without (a) and with F127 (b), heating rate 10 K min−1 to 800 °C under nitrogen. 239
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3.2 to 3.8 nm (Table 1). Table 1 summarizes textural characteristics of the NOMCs synthesized from different molar ratios of EDA/R. As listed in Table 1. The BET surface area and total pore volume are in the range of 517–611 m2 g−1 and 0.35–0.40 cm3 g−1, respectively, indicating the high specific surface area and pore volumes of resulting material. Especially, CN-0.45 sample with the alignment of mesopore channels possesses the highest specific surface area and pore volumes. The microporosity of carbons, prepared by soft-templating route, is mainly originated from the removal of PEO segments from the pore walls and carbonization of the organic polymer. 3.3. Composition and nitrogen-containing functionality The chemical composition of CN-x samples was determined, elemental analysis results shows nitrogen contents of various samples to be 2.32, 2.97, 3.42, and 3.73%, respectively (Table 1). The evolution of chemical composition of the obtained carbon matrix was confirmed by FT-IR analysis (Fig. S1, Supporting Information). For CN-0.45 sample, the band at ∼3408 cm−1 is attributed to NeH and/or eOH stretching vibration [30]. The band at 1560 cm−1 can be identified as originating from aromatic CeN stretching vibration [38,39]. Meanwhile, the weak peaks at 1364, 1100 and 804 cm−1 can be caused by the CeN stretching vibration and the existing of s-triazine rings in the carbon matrix, respectively [40,41]. Hence, these results prove the existence of NeH and CeN species in the carbon product. The successful incorporation of nitrogen into carbon materials networks could be also confirmed by X-ray photoelectron spectroscopy (XPS) measurement. As can be seen in Fig. 7a, XPS spectrum of the nitrogen-containing mesoporous carbon (CN-0.45) shows three peaks from C, N and O elements at ∼285 eV (C1s), ∼400 eV (N1s), and ∼532 eV (O1s) respectively, which is in agreement with the previous results about NOMCs [25,30]. The surface nitrogen contents obtained from XPS measurements (inset in Fig. 7a) are 2.02% for CN-0.45, which is close to the amount of bulk nitrogen obtained from elemental analysis. This implies that that the nitrogen species are uniformly distributed in the carbon framework. The XPS N1s spectrum of CN-0.45 (Fig. 7b) is split into four peaks at binding energies of ∼398.37, 400.71, 401.41 and 402.51 eV, attributed to pyridinic-type nitrogen (N-6), pyrrolic and/or pyridone-type nitrogen (N-5), quaternary nitrogen (NQ) and pyridine nitrogen-oxide (N-X), respectively [38,42,43]. The nitrogen contents of four peaks from deconvolution of the N1s spectrum are listed in Table 2. N-6, N-5 and N-Q groups for CN-0.45 sample account for up to 85.86%, of total nitrogen content, suggesting that these nitrogen species are the dominant functional groups. XPS N1s spectrum shows that four nitrogen groups in the carbon sample are incorporated into framework, which is believed to be highly promising for CO2 capture. Meanwhile, C1s spectrum of the mesoporous carbon CN-0.45 can be deconvoluted into three peaks centred at 284.73, 285.64 and 286.91 eV, respectively (Fig. 7c). The strongest peak at 284.73 eV is indicative of saturated carbon, which is assigned to the sp2 graphitic carbon having the same chemical shift as benzene has at 285 eV [30]. The second signal can be associated with several possible structures including CeN and CeO functionalities [30,44]. The relatively weak peak centered at 286.91 eV can be mainly attributed to the carbon atoms in quinine and/or pyridine groups. The XPS data provides the evidence that CN-0.45 sample possesses graphitic pore walls.
Fig. 3. Wide angle XRD patterns of CN-0.45.
the expansion of mesostructures. The unit parameters calculated from XRD results are 11.20, 11.32, 11.84 and 11.94 nm for the sample with the molar ratio EDA/R of 0.38, 0.45, 0.52 and 0.59, respectively, whereas the sample with the molar ratio EDA/R of 0.66 shows no obvious XRD peak, which suggests that no organic-organic self-assembly transpires by excess EDA. On the other hand, XRD results reveal more information about graphitic character of the carbon materials. It can be clearly seen from the wide angle XRD patterns (Fig. 3) that the sample CN-0.45 exhibits one obvious broad characteristic peak around 23.8° and another relatively weak peak around 44.5°, which are assigned to diffractions from the (0 0 2) and (1 0 0) planes of disordered graphitic pore walls [19]. The structural regularity of the nitrogen-doped mesoporous carbons can be also verified by TEM, as shown in Fig. 4. TEM images indicate that CN-0.45 possesses a high degree of periodicity viewed from [1 1 0] and [0 0 1] directions of 2D hexagonal mesostructures over large domains (Fig. 4a, b). TEM images further reveal that the mesostructural ordering of the carbons are slightly degenerated with an increase in the amount of EDA, probably due to the decomposition of the excess EDA, which coincides with XRD diffraction results (Fig. 4c–e). The amount of EDA is very important in determining the ordered mesostructure in carbon framework. Of course, the higher nitrogen content might coincide with increasing EDA concentration and the proper amount of EDA contributes a lot in the assembly of nitrogen-containing polymer species around F127 micelles. However, in the case of the larger amount of EDA, the polymerization and cocondensation becomes faster than the assembly of pure resorcinol-formaldehyde resin with amphipilic surfactant. This fast and uncontrollable reaction rate eventually leads to the loss of mesostructure. On the other hand, excess amount of EDA induces small-scale production with disordered mesostructure. Therefore, the formation of nitrogen-containing polymer with desired ordered mesostructure requires the hydrogen-bonding interaction between polymers, formed from an appropriate amount of EDA, resorcinol and formaldehyde and PEO segments of copolymer F127. The SEM image of CN-0.45 in Fig. 5 shows that it is an irregularly polyhedral particle with many fine holes. The N2 adsorption-desorption curves and the corresponding PSDs pore size distribution plots of mesoporous carbons, prepared with adding different amounts of EDA, are plotted in Fig. 6. N2 sorption isotherms of NOMCs (sample CN-0.38, CN-0.45, CN-0.52 and CN-0.59) show typical type Ⅳ curves with H1-type hysteresis loops, indicative of uniform cylindrical pores. In addition, the pore size distributions of the CN-x samples, calculated using the BJH theory, exhibit that, as x increases from 0.38 to 0.59, the uniform pore size slightly increases from
3.4. Influence of the amount of F127 template used F127 plays a crucial role in the synthesis of ordered mesoporous carbons through self-assembly route. The self-organized arrays of noncovalently associated amphiphiles may exist as various structural phases, depending on the changes in concentration of templating agent. To investigate the effects of templating agent (F127) on the formation of mesostructures in carbon matrix, XRD was used to characterize the 240
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Fig. 4. TEM images of CN-0.45 (a, b), CN-0.52 (c), CN-0.59 (d) and. CN-0.66 (e).
sample with a certain mesoporous feature with less ordered mesostructure was formed as indicated by its weak XRD diffraction peaks. This indicates that the order degree of carbon frameworks is dependent on F127 concentration. 3.5. Influence of the TMB contents In general, TMB as an organic additive plays an important role in the synthesis of the ordered hexagonal mesoporous materials [45,46]. When the ordered mesoporous materials via direct self-assembly are synthesized, TMB can serve as a pore size swelling agent and TMB/ surfactant ratio determines the template structure. The addition of an appropriate amount of TMB can be assumed to enhance the self-organization of copolymer (e.g. P123, F127), leading to the formation of ordering mesostructure. But, the excessive amounts of TMB induce the phase transformation from ordering mesostructure to the mesostructured cellular foams with disordered structure. To investigate the influence of the TMB concentrations on the mesoscopic ordering of the carbon matrix, the products were prepared by varying the amounts of TMB added to the reaction system, while F127/R ratio keeps at 0.0159(Fig. 9). No XRD peaks (data not shown) could be obtained when the sample was synthesized without adding TMB. The sample with TMB/R ratio of 0.19 only possesses one broad XRD peak, reflecting a comparatively disordered mesostructure. With increasing TMB concentration, the XRD diffractions yield more resolved peaks, which
Fig. 5. SEM image of CN-0.45.
carbon frameworks obtained under different molar ratios of F127/ R = 0.0079, 0.0159 and 0.0239, respectively, while EDA/R ratio was fixed at 0.45 (Fig. 8). XRD patterns of sample with F127/R molar ratio lower than 0.0079 shows less resolved intensity as compared with the product with 0.0159 ratios, implying poor ordering of carbon framework. Additionally, when F127/R ratio is increased up to 0.0239, the 241
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Fig. 6. Nitrogen sorption isotherms of CN-0.38 (a), CN-0.45 (b), CN-0.52 (c) and CN-0.59 (d). Insert shows the corresponding pore distributions.
assembly of block copolymer could be disturbed, leading to the less ordered mesoporous structure. On the other hand, XRD peak positions are slightly shifted to lower angle and the value of the d-spacing of the 10 peaks becomes larger with higher TMB concentrations, indicating higher amounts of TMB causing a dramatic increase of unit-cell parameter. The unit parameters calculated from XRD results is 10.62, 11.32, 12.01 and 13.07 nm for the sample with the molar ratio TMB/R of 0.29, 0.38, 0.48 and 0.57.
Table 1 Textural parameters of the nitrogen-containing mesoporous carbons. Sample
a0 (nm)
SBET (m2/g)
Vt (cm3/g)
Vmi (cm3/g)
Dme (nm)
Nitrogen compositiona (wt %)
CN-0.38 CN-0.45 CN-0.52 CN-0.59
11.20 11.32 11.84 11.94
597 611 576 517
0.386 0.402 0.357 0.348
0.19 0.22 0.21 0.18
3.21 3.35 3.43 3.82
2.32 2.97 3.42 3.73
a0 : Unit-cell parameter, was calculated using the formula a0 = (2/ 3 ) d100, where d110 represent the d-spacing values of the (1 0 0) diffractions; SBET: BET surface area; Vt: total pore volume; Vmi : micropore volume; Dme: mesopore diameters at the maxima of PSDs curves; a: Elemental analysis.
3.6. Influence of reaction temperatures on the structural ordering The hydrothermal temperatures have been varied in order to examine its effects on the formation of ordered structures in carbon frameworks. As seen in Fig. 10, the obtained sample with the reaction temperature of 70 °C only shows a broad XRD peaks with week intensity and low precipitate was observed. At below 70 °C, small amounts of the lowly cross-linked polymers were obtained, because the hydrolysis of HMT can be disturbed under these conditions, leading to the poor pore regularity of the corresponding carbonaceous products. Meanwhile, the sample synthesized at 130 °C could possesses well-defined XRD reflections as same as the product obtained at 100 °C, which is assigned to 2D hexagonal mesostructure. This reveals that the mesostructure
suggest that better ordered materials are produced. In contrast, the XRD reflections of the samples synthesized by adding excessive TMB with the molar ratio up to TMB/R of 0.48, gradually becomes week, which suggests that the degradation of ordered mesostructure occurred. As mentioned above, the appropriate amounts of TMB increase the hydrophobicity of the copolymer F127/TMB templates to improve the mesostructure assembly ability, resulting in the formation of ordered mesophase. In the case of extra addition of TMB, however, the self-
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Fig. 7. XPS survey-scan spectra (a), N1s (b) and C1s XPS spectra (c) of CN-0.45.
Fig. 11 shows the CO2 adsorption isotherms of CN-0.45 and mesocarbon activated with KOH at 0 and 25 °C. CN-0.45 exhibited a CO2 uptake of 3.43 mmol g−1 and 2.71 mmol g−1, respectively, at 0.95 bar (Fig. 11a), which are higher than those of commercial activated carbons [25,26]. By contrast, N2 adsorption capacity at 25 °C and 0.95 bar is only 0.28 mmol g−1. The initial slopes of the CO2 and N2 adsorption isotherms of this sample are calculated to be 10.77 and 0.38, whereas the ratios of these slopes, that is, the CO2/N2 selectivity is estimated to be 28.34, indicating that the selectivity of CN-0.45 for CO2 over N2 is competitively high with the selectivities of the known nitrogen-doped micro-/mesoporous carbons [25,41]. To further increase CO2 capacity, the CN-0.45 sample is activated by KOH at 800 °C for 1 h (denoted as ACN). After activation, the surface area and total pore volume of mesocarbon increase up to 1457 m2 g−1 and 0.70 cm3 g−1, respectively (Fig. S2 and Table S1, Supporting Information), the mesoporous carbon activated with KOH still preserves ordered structure as was of inactivated mesoporous carbon (Fig. 12). N1s XPS spectra of ACN sample showed three fitting peaks, assigned to the pyridinic-type nitrogen (N6), pyrrolic and/or pyridone nitrogen (N-5), and quaternary nitrogen
assembly is improved at the temperature up to 100 °C, resulting in the enhancement of cross-linking degree of frameworks with well-organized mesostructure. 3.7. CO2 capture by the mesoporous carbon The incorporation of basic nitrogen groups in ordered mesoporous carbon frameworks with high specific area and pore volume ensures an improved CO2 adsorption and separation for CO2 acidic gas owing to the significant enhanced interaction between their surface and CO2 gas. CO2 molecule, which has a large electric quadruple moment, can easily interact with the basic sites of porous carbon surface by weak covalent bonds [41,47]. This implies that ordered mesoporous carbons with high surface area and pore system with exposed basic sites can possess excellent performance as an absorbent for CO2 capture. Commercial activated carbons, which possess comparatively high surface area (3000 m2 g−1) with abundant microporosity, have been widely used as CO2 sorbent, but they exhibit a lower adsorption capacity for CO2 gas, limiting their widespread usefulness [26,29].
Table 2 Binding energies and relative surface concentrations of nitrogen species obtained by fitting the N1s core level XPS spectra. Sample
CN-0.45
N-6
N-5
N-Q
N-X
B.E. (eV)
%
B.E. (eV)
%
B.E. (eV)
%
B.E. (eV)
%
398.37
27.68
400.71
30.84
401.41
27.34
402.51
14.14
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Fig. 10. XRD patterns of the mesocarbons prepared under different hydrothermal temperature (a: 70 °C, b: 100 °C, c: 130 °C).
Fig. 8. XRD patterns of the mesocarbons with different molar ratio F127/R: 0.0079 (a) 0.0159 (b) and 0.0239 (c).
capacities reported for different activated carbons and several other mesoporous carbon prepared through soft-templating approach, under the measurement conditions [25,30]. This impressive result exhibits that, with an increasing surface area further modified pore system with exposed basic sites upon KOH activation can promote CO2 capture. Meanwhile, the CO2/N2 selectivity calculated from the ratio of the initial slopes of the CO2 and N2 adsorption for ACN sample are calculated to be 18.06. As a result above, ACN also has high selectivity, but the selectivity of CN-0.45 toward CO2 is higher than that of ACN at low pressures. CO2 adsorption capacity for two porous carbon samples gradually decreases as the temperature is increased, indicating the CO2 adsorption process on the carbon samples is exothermic process. By fitting the CO2 adsorption isotherms measured at 0 and 25 °C and using the Clausius-Clapeyron equation, the isosteric heats of CO2 adsorption of CN-0.45 and ACN samples were calculated to lie from 33.61 and 33.95 kJ mol−1 to 26.58 and 25.30 kJ mol−1, respectively, as the CO2 adsorption amount increases from 0.26 to 2.56 mmol g−1 (Fig. S4, Supporting Information). The higher isosteric heat of adsorption in the initial stage is mainly due to the strong quadrupolar interactions of acidic CO2 molecules with nitrogen containing groups; serve as Lewis bases in the carbon framework at low pressures. These results reveal that our samples seem promising for CO2 capture with high capacity well retained. We also conducted the dynamic breakthrough experiments to determine the gas separation properties of our carbon absorbents, using CO2/N2 mixture at ambient conditions. After injection of CO2/N2 mixture gas with CN-0.45 and ACN samples under continuous flow conditions at 25 °C and 1 bar, the separation profiles measured by gas chromatography (GC) were shown in Fig. 13. As expected from of Fig. 13, CO2 is selectively adsorbed on the mesocarbons, giving rise to a retention time of 10 min for CN-0.45 and about 8 min for ACN for this adsorbate, meanwhile, the concentrations of N2 gas detected through column first were 100% in this return time. After this time, the curve reached the breakpoint and then smoothly went back to the original gas mixture. These resulting breakthrough curves provide clear evidence that two samples can completely separate CO2 from the N2 gas under the ambient condition. The CO2 profiles on two samples are almost straight showing the absence of diffusional limitations, which are favorable for industrial applications. As mentioned above, a retention time of ACN for separation of CO2 over N2 than that of CN-0.45 sample
Fig. 9. XRD patterns of the mesocarbons with different molar ratio of TMB/R: 0.19 (a), 0.29 (b), 0.38 (c), 0.48 (d) and 0.57 (e).
(N-Q), respectively (Fig. S3, Supporting Information). CO2 uptake curves (0 and 25 °C) of ACN indicate an increased CO2 capacity of 4.53 mmol g−1 and 3.16 mmol g−1, respectively, at 0.95 bar (Fig. 11b), suggesting the suitability of this material for enhancing the adsorption capacity of CO2. These CO2 capacities are compared with the highest 244
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Fig. 11. CO2 and N2 adsorption isotherms for CN-0.45 (a) and ACN (b): adsorption isotherms for CO2 at 0 °C and 25 °C and for N2 at 25 °C.
4. Conclusions In conclusion, we have demonstrated a simple one-step hydrothermal method for the synthesis of nitrogen-containing mesoporous carbons, by employing F127 triblock copolymer as structural template agent and phenolic resins as carbon precursors and ethylenediamine as nitrogen source. The polymer formed from resorcinol, aldehyde and ethylenediamine was easily connected to Pluronic F127 via hydrogen bonding, which makes the reaction components co-assembly into ordered mesostructures during the hydrothermal process. Besides high surface area and pore volume, the obtained carbons possess an ordered 2-D hexagonal mesostructure with narrow pore size and rich nitrogen content, while enriched nitrogen species incorporated into the surface and framework transformed mesoporous carbons into more hydrophilic and thus notably enhance CO2 adsorption and selectivity. As a result, nitrogen-containing mesoporous carbons show excellent performance as absorbents for CO2 capture (2.71–3.16 mmol g−1, 298 K, 0.95 bar) and selectivity (18.06–28.34, 298 K). This approach may be useful for the large-scale industrial production of nitrogen-containing ordered mesoporous carbonaceous materials in adsorption, separation, catalysis and electrochemistry brunches.
Fig. 12. XRD patterns of CN-0.45 (a) and ACN (b).
even though it possesses highly specific area and well-developed microporosity, indicating that CN-0.45 represents more higher selectivity and improved separation behavior for adsorbing CO2 over N2, which is in agreement with the pure CO2 and N2 adsorption capacities obtained from Fig. 11 at low pressures.
Acknowledgements The work was supported by the Department of Chemistry, University of Science, Pyongyang (D.P. R. Korea).
Fig. 13. Breakthrough curves of N2/CO2 mixture (2:1 vol) for CN-0.45 (a) and ACN (b) at 25 °C. Filled circle is CO2 and opened circle is N2. 245
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Appendix A. Supplementary material
Mesopor. Mater. 159 (2012) 81–86. [23] Y.R. Liang, R.W. Fu, D.C. Wu, ACS Nano 7 (2013) 1748–1754. [24] M.J. Xie, Y.F. Xia, J.Y. Liang, L.H. Chen, X.F. Guo, Micropor. Mesopor. Mater. 197 (2014) 237–243. [25] J. Wei, D.D. Zhou, Z.K. Sun, Y.H. Deng, Y.Y. Xia, D.Y. Zhao, Adv. Funct. Mater. 23 (2013) 2322–2328. [26] G.P. Hao, W.C. Li, D. Qian, G.H. Wang, W.P. Zhang, T. Zhang, A.Q. Wang, F. Schüth, H.J. Bongard, A.H. Lu, J. Am. Chem. Soc. 133 (2011) 11378–11388. [27] G.P. Hao, W.C. Li, S. Wang, G.H. Wang, L. Qi, A.H. Lu, Carbon 49 (2011) 3762–3772. [28] D.D. Zhou, W.Y. Li, X.L. Dong, Y.G. Wang, C.X. Wang, Y.Y. Xia, J. Mater. Chem. A 1 (2013) 8488–8496. [29] X.Q. Wang, J.S. Lee, Q. Zhu, J. Liu, Y. Wang, S. Dai, Chem. Mater. 22 (2010) 2178–2180. [30] Z.X. Wu, P.A. Webley, D.Y. Zhao, J. Mater. Chem. 22 (2012) 11378–113898. [31] M. Zhou, X.W. Li, T.T. Liu, T.W. Cai, H.C. Zhang, S.Y. Guan, Int. J. Electrochem. Sci. 7 (2012) 9984–9996. [32] W. Yang, T.P. Fellinger, M. Antonietti, J. Am. Chem. Soc. 133 (2011) 206–209. [33] Y. Meng, D. Gu, F.Q. Zhang, D. Gu, Y. Yan, Z.X. Chen, B. Tu, D.Y. Zhao, Chem. Mater. 18 (2006) 5279–5288. [34] D. Liu, J.H. Lei, L.P. Guo, D. Qu, Y. Li, Carbon 50 (2012) 476–487. [35] J.H. Yu, M.Y. Guo, F. Muhammad, A.F. Wang, F. Zhang, Q. Li, G.S. Zhu, Carbon 69 (2014) 502–514. [36] J.H. Yu, M.Y. Guo, F. Muhammad, A.F. Wang, G.L. Yu, H.P. Ma, G.S. Zhu, Micropor. Mesopor. Mater. 190 (2014) 117–127. [37] Y. Meng, D. Gu, F.Q. Zhang, Y.F. Shi, L. Cheng, D. Feng, Z.X. Wu, Z.X. Chen, Y. Wan, A. Stein, D.Y. Zhao, Chem. Mater. 18 (2006) 4447–4464. [38] L. Liu, Q.F. Deng, T.Y. Ma, X.Z. Lin, X.X. Hou, Y.P. Liu, Z.Y. Yuan, J. Mater. Chem. 21 (2011) 16001–16009. [39] M. Sevilla, P. Valle-Vigón, A.B. Fuertes, Adv. Funct. Mater. 21 (2011) 2781–2787. [40] G.P. Hao, W.C. Li, A.H. Lu, Adv. Mater. 22 (2010) 853–857. [41] Q.F. Deng, L. Liu, X.Z. Lin, G.H. Du, Y.P. Liu, Z.Y. Yuan, Chem. Eng. J. 203 (2012) 63–70. [42] H.H. Zhou, S. Xu, H.P. Su, M. Wang, W.M. Qiao, L.C. Ling, D.H. Long, Chem. Commun. 49 (2013) 3763–3765. [43] Y. Wang, Y.Y. Shao, D.W. Matson, J.H. Li, Y.H. Lin, ACS Nano 4 (2010) 1790–1798. [44] H.F. Gorgulho, F. Gonçalves, M.F.R. Pereira, J.L. Figueiredo, Carbon 47 (2009) 2032–2039. [45] F.Q. Zhao, Y. Meng, D. Gu, Y. Yan, Z.X. Chen, B. Tu, D.Y. Zhao, Chem. Mater. 18 (2006) 5279–5288. [46] T. Amatani, K. Nakanishi, K. Hirao, T. Kodaira, Chem. Mater. 17 (2005) 2114–2119. [47] C. Chen, J. Kim, W.S. Ahn, Fuel 95 (2012) 360–364.
Supplementary data to this article can be found online at https:// doi.org/10.1016/j.cplett.2018.12.014. References [1] Y.H. Deng, Y. Cai, Z.K. Sun, G. Gu, J. Wei, W. Li, X.H. Guo, J.P. Yang, D.Y. Zhao, Adv. Funct. Mater. 20 (2010) 3658–3665. [2] J.M. Xu, A.Q. Wang, T. Zhang, Carbon 50 (2012) 1807–1816. [3] T.Y. Ma, L. Liu, Z.Y. Yuan, Chem. Soc. Rev. 42 (2013) 3977–4003. [4] A.F. Leonard, C.J. Gommens, M.L. Piedbouef, J.P. Pirard, N. Job, Micropor. Mesopor. Mater. 195 (2014) 92–101. [5] W.Z. Shen, W.B. Fan, J. Mater. Chem. A 1 (2013) 999–1013. [6] G.P. Mane, S.N. Talapaneni, C. Anand, S. Varghese, H. Iwai, Q.M. Ji, K. Ariga, T. Mori, A. Vinu, Adv. Func. Mater. 22 (2012) 3596–3604. [7] H.Q. Wang, C.F. Zhang, Z.X. Chen, H.K. Liu, Z.P. Guo, Carbon 81 (2015) 782–787. [8] X.C. Zhao, Q. Zhang, B.S. Zhang, C.M. Chen, A.Q. Wang, T. Zhang, D.S. Su, J. Mater. Chem. 22 (2012) 4963–4969. [9] V. Schwartz, H. Xie, H.M. Meyer III, S.H. Overbury, C.D. Liang, Carbon 49 (2011) 659–668. [10] H. Wang, X.J. Bo, Y.F. Zhang, L.P. Guo, Electrochim. Acta 108 (2013) 404–411. [11] Z.X. Wu, D.Y. Zhao, Chem. Commun. 47 (2011) 3332–3338. [12] C. He, X.J. Hu, Adsorption 18 (2012) 337–348. [13] G. Yang, L. Tang, G.M. Zeng, Y. Cai, J. Tang, Y. Pang, Y.Y. Zhou, Y.Y. Liu, J.J. Wang, S. Zhang, W.P. Xiong, Chem. Engin. J. 259 (2015) 854–864. [14] Y.H. Qu, Z.A. Zhang, X.H. Zhang, G.D. Ren, Y.Q. Lai, Y.X. Liu, J. Li, Carbon 84 (2015) 399–408. [15] Y.F. Zhao, L. Zhao, K.X. Yao, Y. Yang, Q. Zhang, Y. Han, J. Mater. Chem. 22 (2012) 19726–19731. [16] Y.F. Song, D.D. Zhou, Y.G. Wang, C.X. Wang, Y.Y. Xia, New J. Chem. 37 (2013) 1768–1775. [17] N. Liu, L. Yin, C. Wang, L. Zhang, N. Lun, D. Xiang, Y.X. Qi, R. Gao, Carbon 48 (2010) 3579–3591. [18] E.Z. Lee, Y.S. Jun, W.H. Hong, A. Thomas, M.M. Jin, Angew. Chem. Int. Ed. 49 (2010) 1–6. [19] B. Wang, T.P. Ang, A. Borgna, Micropor. Mesopor. Mater. 158 (2012) 99–107. [20] Y.H. Deng, T. Yu, Y. Wan, Y.F. Shi, Y. Meng, D. Gu, L.J. Zhang, Y. Huang, C. Liu, X.J. Wu, D.Y. Zhao, J. Am. Chem. Soc. 129 (2007) 1690–1697. [21] L. Chuenchom, R. Kraehnert, B.M. Smarsly, Soft Matter 8 (2012) 10801–10812. [22] P. Li, Y. Song, Q.Y. Lin, J.L. Shi, L. Liu, L.L. He, H.Q. Ye, Q.G. Guo, Micropor.
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Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Research paper
Synthesis and characterization of nitrogen-containing hydrothermal carbon with ordered mesostructure
T
⁎
JuHyon Yu, JuHyok So
Department of Chemistry, University of Science, Pyongyang 950003, Democratic People’s Republic of Korea
H I GH L IG H T S
carbons were synthesized via a hydrothermal approach. • Nitrogen-doped contributes a lot in the assembly of nitrogen-containing polymer species. • Ethylenediamine • Obtained materials possess an ordered 2-D hexagonal (p6m) mesostructure and good CO /N 2
2
selectivity.
A R T I C LE I N FO
A B S T R A C T
Keywords: Mesoporous carbon Nitrogen group Hydrothermal CO2 capture
One-pot hydrothermal synthetic approach for preparing nitrogen-containing mesoporous carbons with high surface area and rich nitrogen content through organic-organic self-assembly of ampliphilic triblock copolymer (Pluronic F127), phenolic resins and ethylenediamine as nitrogen source has been achieved. Ethylenediamine molecules participate in cross-linking of resorcinol and aldehyde, whereas the formed resin molecules bridge with Pluronic F127 via hydrogen bonding during hydrothermal process, leading to the formation of nitrogencontaining carbons with ordered mesostructure during pyrolysis at 800 °C. The obtained nitrogen-containing carbon had a large surface area and pore volume, nitrogen incorporated into carbon frameworks was capable for enhancing the CO2 adsorption capacity.
1. Introduction Among the various porous carbon materials, high-ordered mesoporous carbons have gained great interest in the fields of catalysis, adsorption, separation, fuel cell, energy storage and drug delivery, mainly due to their considerable pore channels, uniform structure regularities, high specific surface areas, thermal and chemical stabilities [1–4]. Especially, the functional groups on the carbon surface containing foreign atoms (N, P, Br or S) can significantly increase the surface and/or framework activity to enhance the practical utility of ordered mesoporous carbons (OMCs) [5–10]. For instance, recent studies have demonstrated that nitrogen containing can effectively improve electrochemical and adsorption/separation performance of OMCs owing to high hydrophilicity, polarity and electric conductivity of the carbon surface [5,11,12]. Therefore, the effective introduction of nitrogen in mesocarbon framework can be an alternative strategy to enhance their application in CO2 capture. The most widespread approach to prepare nitrogen-containing
ordered mesoporous carbons (NOMCs) consists in hard-templating method (i.e., nanocasting route), in which the suitable nitrogen sources were introduced into the mesopores of hard templates (e.g., MCM-41, MCM-48, SBA-15, etc) by impregnation [13–19]. This hard-templating synthetic route often requires multistep procedure (such as polymerization, carbonization and etching step) and is time consuming, leading to low economic efficiency in an industrial application. In recent years, several research groups have demonstrated that soft-templating method via self-assembly of amphiphilic block copolymers (e.g., pluronic F127, polyethylene oxide-polypropylene oxide-polyethylene oxide, EO106-PPO70-PEO106) and phenolic resins was a versatile route for the preparation of nitrogen-containing porous carbons with wellordered structure, which escapes from a fussy nanocasting process [20–23]. Phenolic resins and polyethylene oxide (PEO)-containing copolymers is an appropriate pair to synthesize ordered mesoporous structure by direct self-assembly method, because of hydrogen-bonding interactions between phenolic resins and PEO segments of block copolymer. Based on this soft-templating strategy, a series of nitrogen
⁎ Corresponding author at: Basic Chemical Research Institute, Department of Chemistry, University of Science, Pyongyang 950003, Democratic People’s Republic of Korea. E-mail address: [email protected] (J. So).
https://doi.org/10.1016/j.cplett.2018.12.014 Received 26 September 2018; Received in revised form 19 November 2018; Accepted 6 December 2018 Available online 25 December 2018 0009-2614/ © 2018 Elsevier B.V. All rights reserved.
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hexagonal mesostructure (p6m), high specific surface area and uniform pore size were obtained through hydrothermal treatment, whereas the incorporated nitrogen groups in the mesoporous carbon frameworks significantly improved the CO2 adsorption capacity.
containing mesoporous carbon with ordering channels have been successfully prepared by using phenol/formaldehyde resins as the carbon precursors, whereas urea [24], dicyandiamide [25], 1, 6-diaminohexane [26] and lysine [27] have been used as nitrogen sources. Especially, evaporation-induced self-assembly (EISA) pathway, which is widely used for synthesizing most of reported ordered mesoporous carbons, is a powerful tool for the preparation of NOMCs; nevertheless, this step-by-step EISA route needs the evaporation of large volumes of organic solvents and prepolymerization procedure [24,25,28]. On the other hand, the facile syntheses of preparing NOMCs through the posttreatment [13,29,30] and ionic liquids procedure [31,32] have also been reported; however, the resulting mesoporous carbons showed corrosion of carbon matrix and obvious collapse or degradation of the mesostructure, leading to the limitation of their applications. To overcome these limitations, an aqueous route via the cooperative self-assembly in a dilute aqueous solution, has recently been emerged as a versatile route for the preparation of NOMCs [33,34]. This method has been developed to directly prepare OMCs and heteroatom-doped OMCs by common dilute aqueous reaction route under the milder conditions, when compared with the pathways mentioned above. Although the aqueous self-assembly is very interesting, it always involves about 2–7 days to complete the reaction due to the slow self-polymerization rate, which is time- and energy-consuming. In recent years, we have developed an aqueous self-assembly to prepare NOMCs from resorcinol-urea-formaldehyde [35] or resorcinol-melamine-formaldehyde resins [36] and copolymer F127 ingredients without using additional prepolymerization and hydrothermal solidification steps, whereas the hydrogen bonding between multiple hydroxyl groups of organic polymer and ethylene oxide (EO) repeating units in F127 template and further assembly is involved for the construction of ordered mesoporous framework during polymerization and carbonization steps. However, this one-pot synthesis also takes reaction time for 24 h. The hydrothermal synthesis is a faster and more energy efficient approach to prepare nitrogen-doped carbons with an ordered structure than the EISA route and dilute aqueous approach [26,27]. In general, the polymerization of phenol-formaldehyde system is very fast during hydrothermal process under acidic and/or basic condition, leading to collapsed mesostructures of the resulting carbons and low content of impregnated nitrogen. Additionally, in the synthesis of NOMCs with hydrothermal treatment, assembly of nitrogen containing organic precursors with amphipilic surfactant usually could lead to disordered mesoporous carbon frameworks, therefore, to control the rate kinetics of the polymerization reaction for the formation of NOMCs through direct self-assembly under hydrothermal condition is challenging task. Herein, we report in detail a one-step method to prepare NOMCs through hydrothermal method by using resorcinol and hexamethylenetetramine (HMT) as carbon sources and ethylenediamine (EDA) as nitrogen precursor, while copolymer of Pluronic F127 was employed as template with 1, 3, 5-trimethylbenzene (TMB) added as an organic swelling agent. The use of HMT instead of formaldehyde can control the rate of polymerization reaction to prepare NOMCs during self-assembly. Cosolvent organic molecules such as TMB, which is usually useful in the pore size expansion of mesoporous powders, can be effective agents in controlling pore size of carbon mesoshapes [4,34]. As result of the interaction TMB with PPO segment of triblock copolymers, the lattice can be enlarged or phase transition can occur. On the other hand, the nature of the organic amines seems to be a crucial factor in determining the mesostructure assembly between the polymer species and the copolymer template. In molecular design of nitrogen containing porous polymers and their carbonaceous materials with phenols, aldehydes and diamines (nitrogen sources), EDA is an applicable amine to form co-polymer with resorcinol and formaldehyde, while multiple hydrophilic segments of polymer species could interact with poly ethylene oxide (PEO) segments of triblock copolymer by hydrogen bonding, resulting in the formation of NOMCs after calcination process. Our results showed that the high-quality of NOMCs with 2D
2. Experimental 2.1. Materials Triblock poly (ethylene oxide)-b-poly (propylene oxide)-b-poly (ethylene oxide) copolymer Pluronic F127 (PEO106-PPO70-PEO106, Mw = 12600) was purchased from Sigma-Aldrich Corp. Other chemicals resorcinol, hexamethylenetetramine (HMT), ethylenediamine (EDA) and 1, 3, 5-trimethylbenzene (TMB) were supplied by Co., Ltd, Shanghai, China. All chemicals were used as received without further purification. 2.2. Synthesis In a typical synthesis procedure of nitrogen-containing mesoporous carbon sample with 2-D hexagonal mesostructure, 0.55 g resorcinol(R), 0.35 g HMT, 0.23 g TMB and 1.0 g Pluronic F127 were dissolved in 18 g distilled water with magnetic stirring at room temperature for 2 h. Afterwards, 0.15 ml of EDA was added to the above solution and stirred for about 0.5 h. After the reaction system turned to pale yellow during stirring, this homogeneous solution was poured into an autoclave and cured at 100 °C for 10 h. The reactant molar ratio of F127/R/HMT/ TMB/EDA/H2O was 0.0159/1.0/0.5/0.38/0.45/200. The resulting asmade polymer was collected by filtration and washed with water and ethanol several times and dried at 60 °C for 24 h. Finally, the sample was calcined in a tubular furnace at 800 °C for 3 h, with a heating rate 1 °C min−1 under nitrogen atmosphere, to obtain nitrogen-containing carbon material. In order to activate with KOH, 0.4 g of mesoporous carbon was impregnated with KOH solution (1.6 g KOH in 4 g of water) for 1 h, followed by water evaporation at 100 °C. The activation process was carried out in a tube furnace under nitrogen flow, while heating the sample at a rate of 10 °C min−1 up to 800 °C and held at the temperature for 60 min. After activation, the sample was thoroughly washed three times with 10 wt% HCl solution and then washed with distilled water and ethanol for several times until the wash had a constant pH of 7.0. Finally, the sample was dried in an oven at 100 °C for 12 h, obtaining the activated carbon. 2.3. Characterization The powder of X-ray Diffraction (XRD) patterns of mesoporous carbons were recorded on Riguku D/MAX2550 diffractometer using CuKα radiation (λ = 0.15418 nm). The unit cell parameters of mesostructures were calculated from the formula a 0 = (2/ 3 ) d100. Thermo gravimetric analysis (TGA) was conducted on a TGA G500 thermal analyzer system. Transmission electron microscopy (TEM) images were obtained with FEI Tecnai F20 electron microscope operating at 200 kV. Nitrogen absorption/desorption isotherms were measured at 77 K using an Autosorb iQ2 adsorptometer (Quantachrome Instruments). Before measurements, the dried samples were outgassed at 180 °C for at least 12 h under vacuum in the degas port of the adsorption instruments. Brunauar-Emmett-Teller (BET) method was utilized to calculate the specific surface areas (SBET). The pore volumes and pore size distributions (PSDs) were derived from the adsorption branches of isotherms using the Barrett-Joyner-Halenda (BJH) model, whereas the total pore volumes (Vt) were estimated from the amount adsorbed at a relative pressure P/P0 of 0.99. Micropore volumes (Vmi) were obtained via tplot analysis. The t values were estimated as a function of the relative pressure (P/P0) ranging from 0.08 to 0.25. Elemental analysis was done on a CHN elemental analyzer (Vario Micro, Elementar). Fourier 238
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transform infrared (FT-IR) spectra were collected on IFS 66V/S Fourier transform infrared spectrometer at room temperature in the range of 400–4000 cm−1, with KBr pellets. SEM images of samples were obtained using field-emission scanning electron microscopy (FE-SEM: JEOL JSM6700F). The XPS spectra were carried out on an ESCALAB250 spectrometer. 2.4. CO2 sorption measurement CO2 adsorption isotherms of the samples were measured through a gravimetric determination method by using an Autosorb iQ2 adsorptometer (Quantachrome Instruments). Each sample was degassed at 150 °C for 18 h to remove the guest molecules from pores. After cooling down to the required adsorption temperature (0 or 25 °C), the introduction of CO2 into the system was followed. The CO2 adsorption capacity in terms of adsorbed volume was recorded until equilibrium was achieved. 3. Result and discussion 3.1. Thermo gravimetric analysis The poly resin or polybenzoxazine formed from an appropriate amount of EDA, resorcinol and formaldehyde via self-assembly forms hydrogen-bonds with the PEO segments of F127 templating agent to a significant extent, while the micelles evolving from block copolymer molecules are fixed in the polymer framework, which would attain an alignment of ordered channels during co-condensation. We first analyzed TGA profiles to investigate the pyrolysis behavior of copolymer F127 and mesoporous polymer matrix. As shown Fig. 1, the weight loss of as-synthesized sample without F127, in the temperature range of 300–400 °C, is only 7.91%, which is due to partial condensation and decomposition of nitrogen-containing polymer; However, the mass reduction of the mesoporous polymer prepared by using the template agent is 43.35%, which can be considered to be derived from the complete decomposition of the self-organizing casting agent in an inert atmosphere. This result is in agreement with the previous literatures [35,36]. On the other hand, a gentle and gradual weight loss of 6.02% from room temperature to 300 °C results from desorption of moisture and carbon dioxide and the cleavage of small molecules from the polymerization. 24.88% weight loss takes place in the temperature range of 400–700 °C, suggesting that the hydrocarbons and oxygen containing groups can be considerably decomposed and eliminated, which indicates that framework condensation and carbonization are improved at temperature of above 400 °C. Above 700 °C, carbon
Fig. 2. XRD patterns of CN-0.38 (a), CN-0.45 (b), CN-0.52 (c), CN-0.59 (d) and CN-0.66 (e).
frameworks show comparable thermal stability with higher pyrolysis temperature .The weight loss from 700 to 800 °C becomes 1.41%, indicating the occurrence of cleavage/rearrangement of nitrogen-containing functionalities and carbonization of the material, while a residue of approximate 24.34% is obtained at 800 °C. It can thus be concluded that the thermally stabilized carbon frameworks can be achieved at 800 °C. 3.2. Structure and morphology As mentioned above, EDA is a good option for the synthesis of nitrogen-doped carbon materials under the hydrothermal condition. In order to investigate the influence of EDA on mesoporous structure, a series of samples was synthesized by fixing the molar ratio of F127 to resorcinol at 0.0159:1 and varying the molar ratio of EDA. The obtained samples were denoted as CN-x, where × is the molar ratio of EDA to resorcinol. As can be seen in Fig. 2, low angle XRD patterns reveal that the samples CN-0.38, and CN-0.45 have four resolved diffraction peaks at 2θ = 0.9–2.5°. The corresponding reciprocal d-spacing value ratios are 1: 3 : 4 : 7 , which are indexed as 10, 11, 20 and 21 reflections of a 2D hexagonal space group (p6m), consistent with FDU-15 mesoporous carbons [37]. With further increasing amount of EDA, the XRD diffractions yield less resolved peaks, indicating that the ordered mesostructures are slightly degenerated. Furthermore, the corresponding reflections are slightly shifted to lower 2θ angles with increased amounts of EDA, suggesting that higher amounts of EDA can dramatically cause
Fig. 1. TG analysis of as-prepared samples without (a) and with F127 (b), heating rate 10 K min−1 to 800 °C under nitrogen. 239
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3.2 to 3.8 nm (Table 1). Table 1 summarizes textural characteristics of the NOMCs synthesized from different molar ratios of EDA/R. As listed in Table 1. The BET surface area and total pore volume are in the range of 517–611 m2 g−1 and 0.35–0.40 cm3 g−1, respectively, indicating the high specific surface area and pore volumes of resulting material. Especially, CN-0.45 sample with the alignment of mesopore channels possesses the highest specific surface area and pore volumes. The microporosity of carbons, prepared by soft-templating route, is mainly originated from the removal of PEO segments from the pore walls and carbonization of the organic polymer. 3.3. Composition and nitrogen-containing functionality The chemical composition of CN-x samples was determined, elemental analysis results shows nitrogen contents of various samples to be 2.32, 2.97, 3.42, and 3.73%, respectively (Table 1). The evolution of chemical composition of the obtained carbon matrix was confirmed by FT-IR analysis (Fig. S1, Supporting Information). For CN-0.45 sample, the band at ∼3408 cm−1 is attributed to NeH and/or eOH stretching vibration [30]. The band at 1560 cm−1 can be identified as originating from aromatic CeN stretching vibration [38,39]. Meanwhile, the weak peaks at 1364, 1100 and 804 cm−1 can be caused by the CeN stretching vibration and the existing of s-triazine rings in the carbon matrix, respectively [40,41]. Hence, these results prove the existence of NeH and CeN species in the carbon product. The successful incorporation of nitrogen into carbon materials networks could be also confirmed by X-ray photoelectron spectroscopy (XPS) measurement. As can be seen in Fig. 7a, XPS spectrum of the nitrogen-containing mesoporous carbon (CN-0.45) shows three peaks from C, N and O elements at ∼285 eV (C1s), ∼400 eV (N1s), and ∼532 eV (O1s) respectively, which is in agreement with the previous results about NOMCs [25,30]. The surface nitrogen contents obtained from XPS measurements (inset in Fig. 7a) are 2.02% for CN-0.45, which is close to the amount of bulk nitrogen obtained from elemental analysis. This implies that that the nitrogen species are uniformly distributed in the carbon framework. The XPS N1s spectrum of CN-0.45 (Fig. 7b) is split into four peaks at binding energies of ∼398.37, 400.71, 401.41 and 402.51 eV, attributed to pyridinic-type nitrogen (N-6), pyrrolic and/or pyridone-type nitrogen (N-5), quaternary nitrogen (NQ) and pyridine nitrogen-oxide (N-X), respectively [38,42,43]. The nitrogen contents of four peaks from deconvolution of the N1s spectrum are listed in Table 2. N-6, N-5 and N-Q groups for CN-0.45 sample account for up to 85.86%, of total nitrogen content, suggesting that these nitrogen species are the dominant functional groups. XPS N1s spectrum shows that four nitrogen groups in the carbon sample are incorporated into framework, which is believed to be highly promising for CO2 capture. Meanwhile, C1s spectrum of the mesoporous carbon CN-0.45 can be deconvoluted into three peaks centred at 284.73, 285.64 and 286.91 eV, respectively (Fig. 7c). The strongest peak at 284.73 eV is indicative of saturated carbon, which is assigned to the sp2 graphitic carbon having the same chemical shift as benzene has at 285 eV [30]. The second signal can be associated with several possible structures including CeN and CeO functionalities [30,44]. The relatively weak peak centered at 286.91 eV can be mainly attributed to the carbon atoms in quinine and/or pyridine groups. The XPS data provides the evidence that CN-0.45 sample possesses graphitic pore walls.
Fig. 3. Wide angle XRD patterns of CN-0.45.
the expansion of mesostructures. The unit parameters calculated from XRD results are 11.20, 11.32, 11.84 and 11.94 nm for the sample with the molar ratio EDA/R of 0.38, 0.45, 0.52 and 0.59, respectively, whereas the sample with the molar ratio EDA/R of 0.66 shows no obvious XRD peak, which suggests that no organic-organic self-assembly transpires by excess EDA. On the other hand, XRD results reveal more information about graphitic character of the carbon materials. It can be clearly seen from the wide angle XRD patterns (Fig. 3) that the sample CN-0.45 exhibits one obvious broad characteristic peak around 23.8° and another relatively weak peak around 44.5°, which are assigned to diffractions from the (0 0 2) and (1 0 0) planes of disordered graphitic pore walls [19]. The structural regularity of the nitrogen-doped mesoporous carbons can be also verified by TEM, as shown in Fig. 4. TEM images indicate that CN-0.45 possesses a high degree of periodicity viewed from [1 1 0] and [0 0 1] directions of 2D hexagonal mesostructures over large domains (Fig. 4a, b). TEM images further reveal that the mesostructural ordering of the carbons are slightly degenerated with an increase in the amount of EDA, probably due to the decomposition of the excess EDA, which coincides with XRD diffraction results (Fig. 4c–e). The amount of EDA is very important in determining the ordered mesostructure in carbon framework. Of course, the higher nitrogen content might coincide with increasing EDA concentration and the proper amount of EDA contributes a lot in the assembly of nitrogen-containing polymer species around F127 micelles. However, in the case of the larger amount of EDA, the polymerization and cocondensation becomes faster than the assembly of pure resorcinol-formaldehyde resin with amphipilic surfactant. This fast and uncontrollable reaction rate eventually leads to the loss of mesostructure. On the other hand, excess amount of EDA induces small-scale production with disordered mesostructure. Therefore, the formation of nitrogen-containing polymer with desired ordered mesostructure requires the hydrogen-bonding interaction between polymers, formed from an appropriate amount of EDA, resorcinol and formaldehyde and PEO segments of copolymer F127. The SEM image of CN-0.45 in Fig. 5 shows that it is an irregularly polyhedral particle with many fine holes. The N2 adsorption-desorption curves and the corresponding PSDs pore size distribution plots of mesoporous carbons, prepared with adding different amounts of EDA, are plotted in Fig. 6. N2 sorption isotherms of NOMCs (sample CN-0.38, CN-0.45, CN-0.52 and CN-0.59) show typical type Ⅳ curves with H1-type hysteresis loops, indicative of uniform cylindrical pores. In addition, the pore size distributions of the CN-x samples, calculated using the BJH theory, exhibit that, as x increases from 0.38 to 0.59, the uniform pore size slightly increases from
3.4. Influence of the amount of F127 template used F127 plays a crucial role in the synthesis of ordered mesoporous carbons through self-assembly route. The self-organized arrays of noncovalently associated amphiphiles may exist as various structural phases, depending on the changes in concentration of templating agent. To investigate the effects of templating agent (F127) on the formation of mesostructures in carbon matrix, XRD was used to characterize the 240
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Fig. 4. TEM images of CN-0.45 (a, b), CN-0.52 (c), CN-0.59 (d) and. CN-0.66 (e).
sample with a certain mesoporous feature with less ordered mesostructure was formed as indicated by its weak XRD diffraction peaks. This indicates that the order degree of carbon frameworks is dependent on F127 concentration. 3.5. Influence of the TMB contents In general, TMB as an organic additive plays an important role in the synthesis of the ordered hexagonal mesoporous materials [45,46]. When the ordered mesoporous materials via direct self-assembly are synthesized, TMB can serve as a pore size swelling agent and TMB/ surfactant ratio determines the template structure. The addition of an appropriate amount of TMB can be assumed to enhance the self-organization of copolymer (e.g. P123, F127), leading to the formation of ordering mesostructure. But, the excessive amounts of TMB induce the phase transformation from ordering mesostructure to the mesostructured cellular foams with disordered structure. To investigate the influence of the TMB concentrations on the mesoscopic ordering of the carbon matrix, the products were prepared by varying the amounts of TMB added to the reaction system, while F127/R ratio keeps at 0.0159(Fig. 9). No XRD peaks (data not shown) could be obtained when the sample was synthesized without adding TMB. The sample with TMB/R ratio of 0.19 only possesses one broad XRD peak, reflecting a comparatively disordered mesostructure. With increasing TMB concentration, the XRD diffractions yield more resolved peaks, which
Fig. 5. SEM image of CN-0.45.
carbon frameworks obtained under different molar ratios of F127/ R = 0.0079, 0.0159 and 0.0239, respectively, while EDA/R ratio was fixed at 0.45 (Fig. 8). XRD patterns of sample with F127/R molar ratio lower than 0.0079 shows less resolved intensity as compared with the product with 0.0159 ratios, implying poor ordering of carbon framework. Additionally, when F127/R ratio is increased up to 0.0239, the 241
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Fig. 6. Nitrogen sorption isotherms of CN-0.38 (a), CN-0.45 (b), CN-0.52 (c) and CN-0.59 (d). Insert shows the corresponding pore distributions.
assembly of block copolymer could be disturbed, leading to the less ordered mesoporous structure. On the other hand, XRD peak positions are slightly shifted to lower angle and the value of the d-spacing of the 10 peaks becomes larger with higher TMB concentrations, indicating higher amounts of TMB causing a dramatic increase of unit-cell parameter. The unit parameters calculated from XRD results is 10.62, 11.32, 12.01 and 13.07 nm for the sample with the molar ratio TMB/R of 0.29, 0.38, 0.48 and 0.57.
Table 1 Textural parameters of the nitrogen-containing mesoporous carbons. Sample
a0 (nm)
SBET (m2/g)
Vt (cm3/g)
Vmi (cm3/g)
Dme (nm)
Nitrogen compositiona (wt %)
CN-0.38 CN-0.45 CN-0.52 CN-0.59
11.20 11.32 11.84 11.94
597 611 576 517
0.386 0.402 0.357 0.348
0.19 0.22 0.21 0.18
3.21 3.35 3.43 3.82
2.32 2.97 3.42 3.73
a0 : Unit-cell parameter, was calculated using the formula a0 = (2/ 3 ) d100, where d110 represent the d-spacing values of the (1 0 0) diffractions; SBET: BET surface area; Vt: total pore volume; Vmi : micropore volume; Dme: mesopore diameters at the maxima of PSDs curves; a: Elemental analysis.
3.6. Influence of reaction temperatures on the structural ordering The hydrothermal temperatures have been varied in order to examine its effects on the formation of ordered structures in carbon frameworks. As seen in Fig. 10, the obtained sample with the reaction temperature of 70 °C only shows a broad XRD peaks with week intensity and low precipitate was observed. At below 70 °C, small amounts of the lowly cross-linked polymers were obtained, because the hydrolysis of HMT can be disturbed under these conditions, leading to the poor pore regularity of the corresponding carbonaceous products. Meanwhile, the sample synthesized at 130 °C could possesses well-defined XRD reflections as same as the product obtained at 100 °C, which is assigned to 2D hexagonal mesostructure. This reveals that the mesostructure
suggest that better ordered materials are produced. In contrast, the XRD reflections of the samples synthesized by adding excessive TMB with the molar ratio up to TMB/R of 0.48, gradually becomes week, which suggests that the degradation of ordered mesostructure occurred. As mentioned above, the appropriate amounts of TMB increase the hydrophobicity of the copolymer F127/TMB templates to improve the mesostructure assembly ability, resulting in the formation of ordered mesophase. In the case of extra addition of TMB, however, the self-
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Fig. 7. XPS survey-scan spectra (a), N1s (b) and C1s XPS spectra (c) of CN-0.45.
Fig. 11 shows the CO2 adsorption isotherms of CN-0.45 and mesocarbon activated with KOH at 0 and 25 °C. CN-0.45 exhibited a CO2 uptake of 3.43 mmol g−1 and 2.71 mmol g−1, respectively, at 0.95 bar (Fig. 11a), which are higher than those of commercial activated carbons [25,26]. By contrast, N2 adsorption capacity at 25 °C and 0.95 bar is only 0.28 mmol g−1. The initial slopes of the CO2 and N2 adsorption isotherms of this sample are calculated to be 10.77 and 0.38, whereas the ratios of these slopes, that is, the CO2/N2 selectivity is estimated to be 28.34, indicating that the selectivity of CN-0.45 for CO2 over N2 is competitively high with the selectivities of the known nitrogen-doped micro-/mesoporous carbons [25,41]. To further increase CO2 capacity, the CN-0.45 sample is activated by KOH at 800 °C for 1 h (denoted as ACN). After activation, the surface area and total pore volume of mesocarbon increase up to 1457 m2 g−1 and 0.70 cm3 g−1, respectively (Fig. S2 and Table S1, Supporting Information), the mesoporous carbon activated with KOH still preserves ordered structure as was of inactivated mesoporous carbon (Fig. 12). N1s XPS spectra of ACN sample showed three fitting peaks, assigned to the pyridinic-type nitrogen (N6), pyrrolic and/or pyridone nitrogen (N-5), and quaternary nitrogen
assembly is improved at the temperature up to 100 °C, resulting in the enhancement of cross-linking degree of frameworks with well-organized mesostructure. 3.7. CO2 capture by the mesoporous carbon The incorporation of basic nitrogen groups in ordered mesoporous carbon frameworks with high specific area and pore volume ensures an improved CO2 adsorption and separation for CO2 acidic gas owing to the significant enhanced interaction between their surface and CO2 gas. CO2 molecule, which has a large electric quadruple moment, can easily interact with the basic sites of porous carbon surface by weak covalent bonds [41,47]. This implies that ordered mesoporous carbons with high surface area and pore system with exposed basic sites can possess excellent performance as an absorbent for CO2 capture. Commercial activated carbons, which possess comparatively high surface area (3000 m2 g−1) with abundant microporosity, have been widely used as CO2 sorbent, but they exhibit a lower adsorption capacity for CO2 gas, limiting their widespread usefulness [26,29].
Table 2 Binding energies and relative surface concentrations of nitrogen species obtained by fitting the N1s core level XPS spectra. Sample
CN-0.45
N-6
N-5
N-Q
N-X
B.E. (eV)
%
B.E. (eV)
%
B.E. (eV)
%
B.E. (eV)
%
398.37
27.68
400.71
30.84
401.41
27.34
402.51
14.14
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Fig. 10. XRD patterns of the mesocarbons prepared under different hydrothermal temperature (a: 70 °C, b: 100 °C, c: 130 °C).
Fig. 8. XRD patterns of the mesocarbons with different molar ratio F127/R: 0.0079 (a) 0.0159 (b) and 0.0239 (c).
capacities reported for different activated carbons and several other mesoporous carbon prepared through soft-templating approach, under the measurement conditions [25,30]. This impressive result exhibits that, with an increasing surface area further modified pore system with exposed basic sites upon KOH activation can promote CO2 capture. Meanwhile, the CO2/N2 selectivity calculated from the ratio of the initial slopes of the CO2 and N2 adsorption for ACN sample are calculated to be 18.06. As a result above, ACN also has high selectivity, but the selectivity of CN-0.45 toward CO2 is higher than that of ACN at low pressures. CO2 adsorption capacity for two porous carbon samples gradually decreases as the temperature is increased, indicating the CO2 adsorption process on the carbon samples is exothermic process. By fitting the CO2 adsorption isotherms measured at 0 and 25 °C and using the Clausius-Clapeyron equation, the isosteric heats of CO2 adsorption of CN-0.45 and ACN samples were calculated to lie from 33.61 and 33.95 kJ mol−1 to 26.58 and 25.30 kJ mol−1, respectively, as the CO2 adsorption amount increases from 0.26 to 2.56 mmol g−1 (Fig. S4, Supporting Information). The higher isosteric heat of adsorption in the initial stage is mainly due to the strong quadrupolar interactions of acidic CO2 molecules with nitrogen containing groups; serve as Lewis bases in the carbon framework at low pressures. These results reveal that our samples seem promising for CO2 capture with high capacity well retained. We also conducted the dynamic breakthrough experiments to determine the gas separation properties of our carbon absorbents, using CO2/N2 mixture at ambient conditions. After injection of CO2/N2 mixture gas with CN-0.45 and ACN samples under continuous flow conditions at 25 °C and 1 bar, the separation profiles measured by gas chromatography (GC) were shown in Fig. 13. As expected from of Fig. 13, CO2 is selectively adsorbed on the mesocarbons, giving rise to a retention time of 10 min for CN-0.45 and about 8 min for ACN for this adsorbate, meanwhile, the concentrations of N2 gas detected through column first were 100% in this return time. After this time, the curve reached the breakpoint and then smoothly went back to the original gas mixture. These resulting breakthrough curves provide clear evidence that two samples can completely separate CO2 from the N2 gas under the ambient condition. The CO2 profiles on two samples are almost straight showing the absence of diffusional limitations, which are favorable for industrial applications. As mentioned above, a retention time of ACN for separation of CO2 over N2 than that of CN-0.45 sample
Fig. 9. XRD patterns of the mesocarbons with different molar ratio of TMB/R: 0.19 (a), 0.29 (b), 0.38 (c), 0.48 (d) and 0.57 (e).
(N-Q), respectively (Fig. S3, Supporting Information). CO2 uptake curves (0 and 25 °C) of ACN indicate an increased CO2 capacity of 4.53 mmol g−1 and 3.16 mmol g−1, respectively, at 0.95 bar (Fig. 11b), suggesting the suitability of this material for enhancing the adsorption capacity of CO2. These CO2 capacities are compared with the highest 244
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Fig. 11. CO2 and N2 adsorption isotherms for CN-0.45 (a) and ACN (b): adsorption isotherms for CO2 at 0 °C and 25 °C and for N2 at 25 °C.
4. Conclusions In conclusion, we have demonstrated a simple one-step hydrothermal method for the synthesis of nitrogen-containing mesoporous carbons, by employing F127 triblock copolymer as structural template agent and phenolic resins as carbon precursors and ethylenediamine as nitrogen source. The polymer formed from resorcinol, aldehyde and ethylenediamine was easily connected to Pluronic F127 via hydrogen bonding, which makes the reaction components co-assembly into ordered mesostructures during the hydrothermal process. Besides high surface area and pore volume, the obtained carbons possess an ordered 2-D hexagonal mesostructure with narrow pore size and rich nitrogen content, while enriched nitrogen species incorporated into the surface and framework transformed mesoporous carbons into more hydrophilic and thus notably enhance CO2 adsorption and selectivity. As a result, nitrogen-containing mesoporous carbons show excellent performance as absorbents for CO2 capture (2.71–3.16 mmol g−1, 298 K, 0.95 bar) and selectivity (18.06–28.34, 298 K). This approach may be useful for the large-scale industrial production of nitrogen-containing ordered mesoporous carbonaceous materials in adsorption, separation, catalysis and electrochemistry brunches.
Fig. 12. XRD patterns of CN-0.45 (a) and ACN (b).
even though it possesses highly specific area and well-developed microporosity, indicating that CN-0.45 represents more higher selectivity and improved separation behavior for adsorbing CO2 over N2, which is in agreement with the pure CO2 and N2 adsorption capacities obtained from Fig. 11 at low pressures.
Acknowledgements The work was supported by the Department of Chemistry, University of Science, Pyongyang (D.P. R. Korea).
Fig. 13. Breakthrough curves of N2/CO2 mixture (2:1 vol) for CN-0.45 (a) and ACN (b) at 25 °C. Filled circle is CO2 and opened circle is N2. 245
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Appendix A. Supplementary material
Mesopor. Mater. 159 (2012) 81–86. [23] Y.R. Liang, R.W. Fu, D.C. Wu, ACS Nano 7 (2013) 1748–1754. [24] M.J. Xie, Y.F. Xia, J.Y. Liang, L.H. Chen, X.F. Guo, Micropor. Mesopor. Mater. 197 (2014) 237–243. [25] J. Wei, D.D. Zhou, Z.K. Sun, Y.H. Deng, Y.Y. Xia, D.Y. Zhao, Adv. Funct. Mater. 23 (2013) 2322–2328. [26] G.P. Hao, W.C. Li, D. Qian, G.H. Wang, W.P. Zhang, T. Zhang, A.Q. Wang, F. Schüth, H.J. Bongard, A.H. Lu, J. Am. Chem. Soc. 133 (2011) 11378–11388. [27] G.P. Hao, W.C. Li, S. Wang, G.H. Wang, L. Qi, A.H. Lu, Carbon 49 (2011) 3762–3772. [28] D.D. Zhou, W.Y. Li, X.L. Dong, Y.G. Wang, C.X. Wang, Y.Y. Xia, J. Mater. Chem. A 1 (2013) 8488–8496. [29] X.Q. Wang, J.S. Lee, Q. Zhu, J. Liu, Y. Wang, S. Dai, Chem. Mater. 22 (2010) 2178–2180. [30] Z.X. Wu, P.A. Webley, D.Y. Zhao, J. Mater. Chem. 22 (2012) 11378–113898. [31] M. Zhou, X.W. Li, T.T. Liu, T.W. Cai, H.C. Zhang, S.Y. Guan, Int. J. Electrochem. Sci. 7 (2012) 9984–9996. [32] W. Yang, T.P. Fellinger, M. Antonietti, J. Am. Chem. Soc. 133 (2011) 206–209. [33] Y. Meng, D. Gu, F.Q. Zhang, D. Gu, Y. Yan, Z.X. Chen, B. Tu, D.Y. Zhao, Chem. Mater. 18 (2006) 5279–5288. [34] D. Liu, J.H. Lei, L.P. Guo, D. Qu, Y. Li, Carbon 50 (2012) 476–487. [35] J.H. Yu, M.Y. Guo, F. Muhammad, A.F. Wang, F. Zhang, Q. Li, G.S. Zhu, Carbon 69 (2014) 502–514. [36] J.H. Yu, M.Y. Guo, F. Muhammad, A.F. Wang, G.L. Yu, H.P. Ma, G.S. Zhu, Micropor. Mesopor. Mater. 190 (2014) 117–127. [37] Y. Meng, D. Gu, F.Q. Zhang, Y.F. Shi, L. Cheng, D. Feng, Z.X. Wu, Z.X. Chen, Y. Wan, A. Stein, D.Y. Zhao, Chem. Mater. 18 (2006) 4447–4464. [38] L. Liu, Q.F. Deng, T.Y. Ma, X.Z. Lin, X.X. Hou, Y.P. Liu, Z.Y. Yuan, J. Mater. Chem. 21 (2011) 16001–16009. [39] M. Sevilla, P. Valle-Vigón, A.B. Fuertes, Adv. Funct. Mater. 21 (2011) 2781–2787. [40] G.P. Hao, W.C. Li, A.H. Lu, Adv. Mater. 22 (2010) 853–857. [41] Q.F. Deng, L. Liu, X.Z. Lin, G.H. Du, Y.P. Liu, Z.Y. Yuan, Chem. Eng. J. 203 (2012) 63–70. [42] H.H. Zhou, S. Xu, H.P. Su, M. Wang, W.M. Qiao, L.C. Ling, D.H. Long, Chem. Commun. 49 (2013) 3763–3765. [43] Y. Wang, Y.Y. Shao, D.W. Matson, J.H. Li, Y.H. Lin, ACS Nano 4 (2010) 1790–1798. [44] H.F. Gorgulho, F. Gonçalves, M.F.R. Pereira, J.L. Figueiredo, Carbon 47 (2009) 2032–2039. [45] F.Q. Zhao, Y. Meng, D. Gu, Y. Yan, Z.X. Chen, B. Tu, D.Y. Zhao, Chem. Mater. 18 (2006) 5279–5288. [46] T. Amatani, K. Nakanishi, K. Hirao, T. Kodaira, Chem. Mater. 17 (2005) 2114–2119. [47] C. Chen, J. Kim, W.S. Ahn, Fuel 95 (2012) 360–364.
Supplementary data to this article can be found online at https:// doi.org/10.1016/j.cplett.2018.12.014. References [1] Y.H. Deng, Y. Cai, Z.K. Sun, G. Gu, J. Wei, W. Li, X.H. Guo, J.P. Yang, D.Y. Zhao, Adv. Funct. Mater. 20 (2010) 3658–3665. [2] J.M. Xu, A.Q. Wang, T. Zhang, Carbon 50 (2012) 1807–1816. [3] T.Y. Ma, L. Liu, Z.Y. Yuan, Chem. Soc. Rev. 42 (2013) 3977–4003. [4] A.F. Leonard, C.J. Gommens, M.L. Piedbouef, J.P. Pirard, N. Job, Micropor. Mesopor. Mater. 195 (2014) 92–101. [5] W.Z. Shen, W.B. Fan, J. Mater. Chem. A 1 (2013) 999–1013. [6] G.P. Mane, S.N. Talapaneni, C. Anand, S. Varghese, H. Iwai, Q.M. Ji, K. Ariga, T. Mori, A. Vinu, Adv. Func. Mater. 22 (2012) 3596–3604. [7] H.Q. Wang, C.F. Zhang, Z.X. Chen, H.K. Liu, Z.P. Guo, Carbon 81 (2015) 782–787. [8] X.C. Zhao, Q. Zhang, B.S. Zhang, C.M. Chen, A.Q. Wang, T. Zhang, D.S. Su, J. Mater. Chem. 22 (2012) 4963–4969. [9] V. Schwartz, H. Xie, H.M. Meyer III, S.H. Overbury, C.D. Liang, Carbon 49 (2011) 659–668. [10] H. Wang, X.J. Bo, Y.F. Zhang, L.P. Guo, Electrochim. Acta 108 (2013) 404–411. [11] Z.X. Wu, D.Y. Zhao, Chem. Commun. 47 (2011) 3332–3338. [12] C. He, X.J. Hu, Adsorption 18 (2012) 337–348. [13] G. Yang, L. Tang, G.M. Zeng, Y. Cai, J. Tang, Y. Pang, Y.Y. Zhou, Y.Y. Liu, J.J. Wang, S. Zhang, W.P. Xiong, Chem. Engin. J. 259 (2015) 854–864. [14] Y.H. Qu, Z.A. Zhang, X.H. Zhang, G.D. Ren, Y.Q. Lai, Y.X. Liu, J. Li, Carbon 84 (2015) 399–408. [15] Y.F. Zhao, L. Zhao, K.X. Yao, Y. Yang, Q. Zhang, Y. Han, J. Mater. Chem. 22 (2012) 19726–19731. [16] Y.F. Song, D.D. Zhou, Y.G. Wang, C.X. Wang, Y.Y. Xia, New J. Chem. 37 (2013) 1768–1775. [17] N. Liu, L. Yin, C. Wang, L. Zhang, N. Lun, D. Xiang, Y.X. Qi, R. Gao, Carbon 48 (2010) 3579–3591. [18] E.Z. Lee, Y.S. Jun, W.H. Hong, A. Thomas, M.M. Jin, Angew. Chem. Int. Ed. 49 (2010) 1–6. [19] B. Wang, T.P. Ang, A. Borgna, Micropor. Mesopor. Mater. 158 (2012) 99–107. [20] Y.H. Deng, T. Yu, Y. Wan, Y.F. Shi, Y. Meng, D. Gu, L.J. Zhang, Y. Huang, C. Liu, X.J. Wu, D.Y. Zhao, J. Am. Chem. Soc. 129 (2007) 1690–1697. [21] L. Chuenchom, R. Kraehnert, B.M. Smarsly, Soft Matter 8 (2012) 10801–10812. [22] P. Li, Y. Song, Q.Y. Lin, J.L. Shi, L. Liu, L.L. He, H.Q. Ye, Q.G. Guo, Micropor.
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