Graphitic Carbon Nitride Functionalized with Polyethylenimine for

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Materials and Interfaces

Graphitic Carbon Nitride Functionalized with Polyethyleneimine for Highly Effective Capture of Carbon Dioxide Hailong Peng, Fu-Yu Zhong, Jian-Bo Zhang, Jia-Yin Zhang, Ping-Keng Wu, Kuan Huang, Jie-Ping Fan, and Lilong Jiang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b02275 • Publication Date (Web): 23 Jul 2018 Downloaded from http://pubs.acs.org on July 24, 2018

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Industrial & Engineering Chemistry Research

Graphitic

Carbon

Nitride

Functionalized

with

Polyethyleneimine for Highly Effective Capture of Carbon Dioxide Hai-Long Peng,† Fu-Yu Zhong,† Jian-Bo Zhang,† Jia-Yin Zhang,† Ping-Keng Wu,ǂ Kuan Huang,*† Jie-Ping Fan,† Li-Long Jiang*‡ †

Key Laboratory of Poyang Lake Environment and Resource Utilization of Ministry of

Education, School of Resources Environmental and Chemical Engineering, Nanchang University, Nanchang, Jiangxi 330031, China. ‡

National Engineering Research Center for Chemical Fertilizer Catalyst (NERC-CFC), School of

Chemical Engineering, Fuzhou University, Fuzhou, Fujian 350002, China. ǂ

Department of Chemical Engineering, Illinois Institute of Technology, Chicago, Illinois 60616,

United States. ABSTRACT Graphitic carbon nitride (g-C3N4) is a kind of 2D material with unique properties, and can be facilely prepared from low-cost precursors. It has promising application in CO2 capture. However, the low alkalinity of nitrogen species in g-C3N4 makes its interaction with CO2 very weak. Aiming to promote the use of g-C3N4 for CO2 capture, a class of amine-functionalized gC3N4 adsorbents were designed in the present work. The amine-functionalized g-C3N4 adsorbents were prepared by physical impregnation of polyethyleneimine (PEI), which contains many amine groups and exhibits extremely low volatility, into the stacked pores of g-C3N4. It is found that after the amine functionalization, the performance of g-C3N4 for CO2 capture (including capacities and selectivities) are significantly improved. The highest CO2 capacity of PEI—g-

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C3N4 composites is 3.77 mmol/g when determined at 100 °C and ambient pressure, being superior to most of other g-C3N4 materials and solid amines reported in the literature. Furthermore, the CO2 adsorbed by PEI—g-C3N4 composites can be facilely stripped out by concentration swing, and the loss in CO2 capacity is negligible after 10 cycles of adsorption and desorption. KEYWORDS graphitic carbon nitride; amine functionalization; physical impregnation; CO2 capture; recycling stability


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INTRODUCTION CO2 is the most common greenhouse gas. Its accumulation on the surface of earth has resulted in a number of problems, for examples, global warming and sea level rising. These problems may impose destructive impacts on the climate and environment on the earth.

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Agreements have been reached among the governments across the world to strictly control the increase in CO2 content on the surface of earth. The primary step is to eliminate CO2 from flue gas—the product of fossil fuels combustion, which is the major source of CO2 emission.

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A

most widely used method for removing gas is absorption in liquids. 3 Since flue gas is normally high in temperature (e.g., 75~120 °C), and low in CO2 content (e.g., 5~20 v/v%), the liquids used for capturing CO2 from flue gas should exhibit reversible chemical reactivity to CO2. 4 To meet this requirement, amine scrubbing process is developed. 4 In amine scrubbing process, aqueous organic amines are used as the liquid absorbents. However, organic amines are high in volatility and strong in corrosion. The use of large amount of water also contributes to the intensive energy penalty for the regeneration of liquid absorbents because water is high in heat capacity. Therefore, advanced materials are highly demanded for the capture of CO2. As an alternative to absorption in liquids, adsorption on solids is a very promising method for removing gas owing to the negligible volatility and corrosion of solid adsorbents.

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The

energy penalty required for the regeneration of solid adsorbents is also low because solid adsorbents are normally low in heat capacity. Until now, different kinds of solid adsorbents like zeolites,

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frameworks

porous carbon materials, 11, 12

7, 8

porous organic polymers,

9, 10

and metal organic

, have been proposed as the media for CO2 capture. Nonetheless, the practical

applications of these materials are limited by their high cost and/or low stability.


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In the past decade, graphitic carbon nitride (g-C3N4) has received tremendous attentions across academic and industrial communities.

13, 14

g-C3N4 is a kind of 2D material and has the

similar structure as graphene. It has many special features such as high stability and hardness, endowing them with promising applications in photocatalysis,

15-19

electrocatalysis,

20-22

electronic devices, 23 energy storage 15 and gas adsorption. 24-30 Most importantly, g-C3N4 can be synthesized very easily by the direct pyrolysis of precursors that are rich in nitrogen (e.g., urea, dicyandiamide and melamine) via polycondensation reaction, making it rather attractive from industrial perspective. 15 Regarding the use of g-C3N4 for CO2 capture, there are some pioneering work. 24-30 However, the performance of g-C3N4 for CO2 capture is not that satisfactory, with the CO2 capacity and selectivity being rather low under flue gas conditions. The major reason is that the nitrogen species in g-C3N4 are of low alkalinity because of the special electronic environment in g-C3N4, which makes the interaction of g-C3N4 with CO2 very weak. Functionalization with amines has been widely adopted to enhance the ability of solid adsorbents for capturing CO2. grafting

31

5

The amine functionalization can be realized by either chemical

or physical impregnation.

32, 33

In comparison with chemical grafting, physical

impregnation is more easily realized in large scale. The amine density in solid adsorbents can also reach much higher levels by physical impregnation. Aiming to increase the capacity and selectivity of g-C3N4 for CO2 adsorption, and thereby promote the use of g-C3N4 for CO2 capture, a class of amine-functionalized g-C3N4 adsorbents were designed in the present work. The amine-functionalized g-C3N4 adsorbents were prepared by physical impregnation of polyethyleneimine (PEI), which contains many amine groups and exhibits extremely low volatility, into the stacked pores of g-C3N4. As far as we know, this is the first time that the functionalization of g-C3N4 with amines to develop solid adsorbents for CO2 capture is reported.


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EXPERIMENTAL Chemicals Urea (99.0 wt.%) and polyethyleneimine (PEI, branched type, Mn=600) were provided by Sigma Aldrich Co. Ltd.. CO2 (99.99 v/v%) and N2 (99.99 v/v%) were provided by Airgas Co. Ltd.. Materials synthesis g-C3N4 was synthesized by heating urea to 550 °C at the rate of was 4 °C/min, and keeping at the final temperature for 4 h. PEI—g-C3N4 composites were prepared by physical impregnation. In brief, specific amount of PEI was dissolved in methanol, and g-C3N4 was added to the solution; the slurry was mixed thoroughly at ambient temperature for 24 h by magnetic stirring; most of the solvent was eliminated by rotary evaporation at 40 °C, and residual solvent was removed by drying at 60 °C and 0.1 kPa for 48 h. The prepared PEI—g-C3N4 composites were labled as nPEI@ g-C3N4, where n denotes the weight ratio of PEI to g-C3N4 in the samples. Characterizations Before characterizations, the samples were treated with N2 flow (150 mL/min) at 110 °C for 2 h. An Elementar Vario EL II system was used for elemental analysis, and the PEI loadings were calculated according to the measured N contents. A Micromeritics Gemini 2390a system was used for measuring N2 adsorption isotherms at -196 °C and calculating the porosity parameters. The BET equation was used to calculate the specific surface areas in the range of 0.05–0.20 relative pressure. The BJH model was used to calculate the pore size distributions. The t-plot method was used to calculate the micropore volumes. The quantity of N2 adsorbed at 0.97 relative pressure was used to calculate the total pore volumes. A Zeiss Auriga Crossbeam microscope was used for taking the SEM images. A Zeiss Libra200 microscope was used for


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taking the TEM images. A Bruker ICON2-SYS microscope was used for taking the AFM images and measuring the thickness of nanoflakes. An Excalibur HE 3100 spectrometer installed with a MVP-Pro ATR part was used for collecting the FTIR spectra. A Princeton MSL 532-50 spectrometer with λ=532 nm laser excitation was used for collecting the Raman spectra. A PANalytical powder diffractometer with Cu Kα radiation was used for collecting the XRD patterns. A Seiko 6300 TG/DTA system was used for determining the thermal stability under N2 flow (150 mL/min). CO2 adsorption Before the measurements of CO2 adsorption, the samples were treated with N2 flow (150 mL/min) at 110 °C for 2 h. The same Seiko 6300 TG/DTA system was used for measuring the CO2 capacities and recycling stability of solid adsorbents. The sample with the weight of ~5 mg was loaded in the heating cell of TG/DTA system. To measure the CO2 capacities under dry condition, the sample was treated with a CO2 flow (50 mL/min) for 3 h, and the increase in mass was taken as the amount of CO2 adsorbed. To measure the CO2 capacities under humidified condition, the sample was treated by a N2 flow (150 mL/min) with a relative humidity of 10% for 2 h, and the increase in mass was taken as the amount of water adsorbed; the N2 flow was then changed to a CO2 flow (50 mL/min) with the same relative humidity for 3 h, and the increase in mass was taken as the amount of CO2 adsorbed. To measure the recycling stability under dry condition, the sample was treated with a CO2 flow (50 mL/min) for 2 h, and the CO2 flow was then changed to a N2 flow (150 mL/min) for 2 h; the CO2 adsorption-desorption cycle was performed for 10 times. To measure the recycling stability under humidified condition, the sample was treated by a CO2 flow (50 mL/min) with a


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relative humidity of 10% for 3 h, and the CO2 flow was then changed to a N2 flow (150 mL/min) for 2 h; the CO2 adsorption-desorption cycle was also performed for 10 times. The same Micromeritics Gemini 2390a system was also used for measuring the CO2 and N2 adsorption isotherms at 25 °C. RESULTS AND DISCUSSION Characterization results The elemental compositions of synthesized g-C3N4 and PEI—g-C3N4 composites are summarized in the Supporting Information (see Table S1). According to the measured weight percentage of carbon (C) and nitrogen (N) in synthesized g-C3N4, the molar ratio of C to N was calculated to be 0.69, approaching to 0.75 which is the theoretical value. The PEI loadings of PEI—g-C3N4 composites were then calculated from the measured N contents, and results are presented in Table 1. The PEI loadings are ranging from 0.53 to 2.08 g/g. Figures S1 and S2 shows the N2 adsorption isotherms at -196 °C and BJH pore size distributions of g-C3N4 and PEI—g-C3N4 composites. The calculated porosity parameters are also presented in Table 1. It is within expectation that the surface areas and pore volumes of PEI—g-C3N4 composites decrease significantly as the PEI loadings increase, due to the occupation of stacked pores in g-C3N4 by PEI. It is interestingly found that g-C3N4 can accommodate up to 2.08 g/g of PEI, although the total pore volume of g-C3N4 is only 0.47 cm3/g, indicating that g-C3N4 is an effective support for amine compounds. This is attributed to the flexibility of the stacked pores in g-C3N4, which can be expanded by the filled liquids. Similar phenomenon was also observed in other 2D materials, for examples, graphene oxide (GO) 34 and hexagonal boron nitride (h-BN).

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However, the loading amounts of amine compounds in solid

materials with cylinder or slit pores are limited by their total pore volumes, and various


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complicated methods that are time-consuming and require expensive reagents were used to increase the porosity of those materials.

36-39

Therefore, 2D materials are believed to be

promising candidates for amine functionalization to prepared solid adsorbents for CO2 capture. The morphology of g-C3N4 was visualized by SEM, TEM and AFM. The microscope images are shown in Figure 1. In the SEM images, it is observed that abundant nanoscale flakes aggregate randomly to form the stacked pores in g-C3N4. In the TEM images, domains with layered architecture can be observed. In the AFM images, the thickness of g-C3N4 nanoflakes was determined to be 4.0~4.5 nm, corresponding to ~10 layers of g-C3N4 nanosheets. The SEM images of PEI—g-C3N4 composites were also collected, as shown in Figure 1. It is found that gC3N4 and PEI—g-C3N4 composites have similar morphology. Figure 2 displays the FTIR spectra of g-C3N4, PEI and PEI—g-C3N4 composites. In the FTIR spectra of g-C3N4, the band at 809 cm-1 is assigned to the vibration of triazine unit; the bands at 1245, 1318 and 1420 cm-1 are assigned to the stretching vibration of aromatic C—N bond; the bands at 1564 and 1629 cm-1 are assigned to the stretching vibration of C=N bond; the broad band at 3000~3500 cm-1 are assigned to the vibration of terminal amine groups (—NH2 or =NH). The FTIR spectra of g-C3N4 agrees well with those for bulk g-C3N4 reported in the literature.

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In the FTIR spectra of PEI—g-C3N4 composites, all the bands associated with g-

C3N4 can also be clearly identified, implying that the amine functionalization does not change the chemical structure of g-C3N4. However, it is difficult to identify the bands associated with PEI in the FTIR spectra of PEI—g-C3N4 composites. There are two reasons: firstly, the intensity of the bands associated with PEI are very weak because most of the PEI resides inside the stacked pores of g-C3N4, while not outside the surface; secondly, most of the bands associated with PEI overlap with those associated with g-C3N4. Even though, it can still be seen that the


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broad band at 3000~3500 cm-1 in PEI—g-C3N4 composites has much improved intensity in relative to that in g-C3N4, which agrees well with the fact that the amine functionalization of gC3N4 introduces more terminal amine groups (—NH2). The Raman spectra of g-C3N4 and PEI—g-C3N4 composites (see Figure S3) all display a broad band at 1200~1800 cm-1, which is formed by the overlap of different signals: sp2-C at 1380 cm-1; N in N=C=N at 1400 cm-1, N in C≡N at 1600 cm-1, and sp3-C at 1600 cm-1. The similar Raman spectra of g-C3N4 and PEI—g-C3N4 composites further suggests that the structure of g-C3N4 is well retained after amine functionalization. Figure 3 shows the XRD patterns of g-C3N4 and PEI—g-C3N4 composites. In the XRD pattern of g-C3N4, the peak at 27.3° is attributed to the (002) diffraction of interlayer stacking structure, and the peak at 13.0° is attributed to the (100) diffraction of interplanar packing structure. The XRD pattern of g-C3N4 agrees well with those for bulk g-C3N4 reported in the literature.

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In the XRD patterns of PEI—g-C3N4 composites, the (002) diffraction is very

similar to that of g-C3N4, implying that the amine functionalization does not affect the interlayer stacking structure of g-C3N4. However, obvious shifting of the (002) diffraction to lower angles was observed for other 2D materials (e.g., GO and h-BN) after amine functionalization, suggesting the insertion of amine compounds into the interlayer space to induce the extension of the interlayer distance.

34, 35

It was rationalized by the strong interaction of the acidic

hydroxyl/carboxyl groups in GO or boron atoms in h-BN with alkaline amine groups in PEI. In contrast, there are no acidic sites that enable strong interaction with PEI in g-C3N4. On the other hand, the (100) diffraction of g-C3N4 gradually disappears as the PEI loadings increase, implying that the amine functionalization breaks the interplanar packing structure of g-C3N4. This observation is consistent with the inference that the stacked pores in g-C3N4 can be expanded by


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the filled liquids, which contributes to the accommodation of large amount of amine compounds in g-C3N4. The thermal stability of g-C3N4, PEI and PEI—g-C3N4 composites were examined by thermogravimetric analysis (TGA). From the TGA curves (see Figure S4), it is observed that the PEI—g-C3N4 composites start to evaporate or decompose at around 200 °C. Therefore, the stability of PEI—g-C3N4 composites are high enough for the use in CO2 capture from flue gas. 41 Figure 4 shows the derivative thermogravimetry (DTG) curves of g-C3N4, PEI and PEI—g-C3N4 composites. The DTG peaks of PEI—g-C3N4 composites are at around 30.6 min (391 °C), being similar to that of pure PEI (30.9 min, 392 °C). Therefore, the thermal stability of PEI residing inside the stacked pores of g-C3N4 is almost the same as that of bulky PEI, further validating the weak interaction of g-C3N4 with PEI. Based on the characterization results, it can be concluded that the interaction of PEI with gC3N4 is rather weak, and PEI is encapsulated within the pores of g-C3N4 simply by capillary force. This is within expectation because there are no functional groups enabling strong interaction with PEI on the surface of g-C3N4. However, on the surface of GO and BN, there are abundant carboxylic groups and boron atoms, which enable acid-base interaction with PEI. Such acid-base interaction between PEI and GO or BN has been figured out in the literature.

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Another class of material frequently reported for amine impregnation and CO2 capture in the literature is mesoporous silica. On the surface of mesoporous silica, there are abundant hydroxyl groups, which enable hydrogen-bonding interaction with PEI. Such hydrogen-bonding interaction between PEI and mesoporous silica has also been figured out in the literature. 33, 42 As for the carbon materials reported for amine impregnation and CO2 capture in the literature, there are no particular information about the interaction between PEI and carbon materials. 43-45


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CO2 capacities The CO2 capacities of g-C3N4 and PEI—g-C3N4 composites at 25~100 °C were first measured by TGA under atmospheric pressure. The CO2 adsorption curves are presented in the Supporting Information (see Figures S5~S8). Obviously, the CO2 capacities of g-C3N4 are significantly improved after amine functionalization, and the improvements are more significant at higher temperatures. For example, the PEI—g-C3N4 composites can adsorb 0.66~1.71 mmol/g of CO2 at 25 °C, being 2.6~6.8 times as g-C3N4 (0.25 mmol/g); while the PEI—g-C3N4 composites can adsorb 2.20~3.77 mmol/g of CO2 at 100 °C, being 18.3~31.4 times as g-C3N4 (0.12 mmol/g). The significantly improved CO2 capacities of PEI—g-C3N4 composites is attributed to the strong reactivity of amine groups with CO2. The mechanism for reaction of amine-impregnated adsorbents with CO2 has been well established in the literature, that it is subjected to a carbamate route (see scheme 1). Therefore, the active sites for CO2 adsorption by amine-impregnated adsorbents are amine groups. 46 However, the pristine g-C3N4 has very weak interaction with CO2 due to its low alkalinity. Therefore, the amine functionalization is an effective method to enhance the performance of g-C3N4 for CO2 capture. The effect of PEI loadings on CO2 capacities at different temperatures is illustrated in Figure 5. The increase of PEI loadings provides more active sites for reacting with CO2, but unfavors the access of amine groups buried into deep channels to CO2. The first factor is beneficial for the adsorption of CO2, but the second one is not. If the PEI loadings are not high (e.g., 0~1.50 g/g), the first factor governs the adsorption of CO2; if the PEI loadings are high (e.g., 1.50~2.08 g/g), the second one governs the adsorption of CO2. Therefore, the CO2 capacities increase with PEI loadings in the loading range of 0~1.50 g/g, but decrease with PEI


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loadings in the loading range of 1.50~2.08 g/g. The optimized PEI loading is 1.50 g/g, at which the CO2 capacities of PEI—g-C3N4 composites are the highest. Figure 6 shows the effect of temperatures on CO2 capacities at different PEI loadings. The increase of temperatures improves the accessibility of amine groups buried in deep channels to CO2, but unfavors the reaction of amine groups with CO2. The first factor is favorable for the adsorption of CO2, but the second one is not. If the temperatures are not high (e.g., 25~50 °C), the second factor governs the adsorption of CO2; if the temperatures are high (e.g., 50~100 °C), the first one governs the adsorption of CO2. Therefore, the CO2 capacities decrease with temperatures in the temperature range of 25~50 °C, but increase with temperatures in the temperature range of 50~100 °C. The optimized PEI loading is 100 °C, at which the CO2 capacities of PEI—g-C3N4 composites are the highest. With the CO2 capacities and PEI loadings of PEI—g-C3N4 composites in hand, the amine efficiencies were calculated as the molar ratio of CO2 to amine groups. The calculated results are shown in Figure 7. The percentages of 1°, 2° and 3° amine groups in the PEI used in this work are 44%, 33% and 23%, respectively.

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Sine the tertiary amine groups are inactive for reaction

with CO2, and the reaction between primary/secondary amine groups and CO2 is subjected to 2:1 stoichiometry, the theoretical amine efficiency of PEI for CO2 adsorption is 0.38 mol/mol. The amine efficiencies of PEI—g-C3N4 composites are 0.11~0.27 mol/mol at 75 °C and 0.15~0.27 mol/mol at 100 °C, suggesting the effective consumption of amine groups during CO2 adsorption. Overall, the highest CO2 capacity of PEI—g-C3N4 composites is 3.77 mmol/g for 1.5PEI@ g-C3N4 when determined at 100 °C, being much advantageous over most of other g-C3N4 materials previously reported (see Table S2).

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Therefore, the amine functionalization of g-

C3N4 is proved to be effective to improve its performance for the capture of CO2 from flue gas.


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Even compared with most of other solid amines previously reported (see Table S3),

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the

PEI—g-C3N4 composites still show obvious superiority in CO2 capacities. The superiority of PEI—g-C3N4 composites for practical application in CO2 capture is more significant if the lowcost and facile synthesis of g-C3N4 are taken into account. Considering that there inevitably exists moisture in the flue gas, it is very important to investigate the impact of water on the adsorption of CO2 on solid adsorbents. Figure S9 shows the adsorption of CO2 with a relative humidity of 10%, which represents a relatively low activity of water vapor, on 1.5PEI@g-C3N4. As can be seen, the 1.5PEI@g-C3N4 can adsorb 3.27 mmol/g of CO2 at 75 °C under humidified condition. The CO2 capacity is similar to that under dry condition (3.37 mmol/g). Therefore, minor amount of moisture in flue gas has negligible impact on the adsorption of CO2 by PEI—g-C3N4 composites. CO2/N2 selectivities Besides CO2 capacities, the CO2/N2 selectivities are also critical as a parameter reflecting the ability for selective capture of CO2. Since the amounts of N2 adsorbed by g-C3N4 and PEI— g-C3N4 composites are hardly detected by TGA, the volumetric method was used to measure the CO2 and N2 adsorption isotherms of g-C3N4 and PEI—g-C3N4 composites at 25 °C, as shown in Figure 8. The CO2 capacities measured by this method are lower than those measured by TGA, because the adsorption of CO2 by PEI—g-C3N4 composites is diffusion-controlled. In the TGA method, the equilibrium time was set to be 3 h, while the equilibrium time was set to be 5 s in the volumetric method. It can be seen that the CO2 capacities of PEI—g-C3N4 composites increase dramatically with the CO2 pressures increasing first, but then keep almost unchanged with the CO2 pressures further increasing. The CO2 adsorption isotherms are typically chemical type. This observation is not surprising because the amine groups enable strong reactivity with CO2.


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Therefore, the PEI—g-C3N4 composites are also able to effectively capture CO2 from dilute sources. It is also found from Figure 8 that the PEI—g-C3N4 composites can adsorb only minor amount of N2. The CO2/N2 selectivities of g-C3N4 and PEI—g-C3N4 composites were then calculated according to the Ideal Adsorption Solution Theory (IAST).

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Figure 9 shows the

IAST selectivities for a mixture of CO2/N2 (0.15:0.85) at 25 °C and 1 bar. The CO2/N2 selectivities of PEI—g-C3N4 composites are 2.2~6.3 times as those of pristine g-C3N4, with the values of 2257~6588. Therefore, the amine functionalization of g-C3N4 is proved to be effective to improve its performance for the selective capture of CO2 from flue gas. Recycling of adsorbents The stability of solid adsorbents throughout consecutive adsorption-desorption cycles is critical for evaluating their long-term use in the industry. Therefore, the reusability of PEI—gC3N4 composites for CO2 adsorption was first examined with subject to concentration swing under dry condition by TGA, as shown in Figure 10. It is found that the CO2 adsorbed by PEI— g-C3N4 composites can be easily stripped out under the purge of N2, implying the high reversibility of CO2 adsorption. The CO2 capacities keep almost constant during the 10 adsorption-desorption cycles, with the decrease in CO2 capacities being smaller than 5%. Therefore, the PEI—g-C3N4 composites are highly stable for the long-term use in CO2 capture. However, the considerable sacrifice in CO2 capacities was observed for many other solid amines previously reported. For example, the PEI–SBA-15 composites, which are a class of most extensively studied solid amines, were reported to lose their CO2 capacities by 34% after 4 cycles. 49


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To examine the effect of moisture on the stability of PEI—g-C3N4 composites, the reusability of PEI—g-C3N4 composites for CO2 adsorption was also examined under humidified condition by TGA (see Figure S10). Obviously, the PEI—g-C3N4 composites are still very stable throughout consecutive adsorption-desorption cycles in the presence of minor water vapor. CONCLUSIONS In summary, a class of amine-functionalized g-C3N4 adsorbents were designed and prepared by physical impregnation of PEI into the stacked pores of in this work. g-C3N4 can accommodate large amount of PEI because of the flexibility of the stacked pores in g-C3N4. g-C3N4 is believed to be a promising candidate for amine functionalization to prepared solid adsorbents for CO2 capture. Significantly, the synthesized PEI—g-C3N4 composites exhibit much improved CO2 capacities and CO2/N2 selectivities in relative to the pristine g-C3N4. The CO2 adsorbed by PEI— g-C3N4 composites can be easily stripped out at a relatively moderate temperature, and the loss in CO2 capacity is negligible after 10 cycles of adsorption and desorption. According to the results obtained in the present work, it is concluded that the amine functionalization is an efficient method to improve the performance of g-C3N4 for selective CO2 capture from flue gas. ASSOCIATED CONTENT Supporting Information Structural characterizations, CO2 adsorption curves and summary of CO2 capacities. AUTHOR INFORMATION Corresponding Authors *E-mails: [email protected]; [email protected]. Notes The authors declare no conflicts of interest.


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ACKNOWLEDGMENT The authors acknowledge the Natural Science Foundation of Jiangxi Province (award number: 20171BAB203019) and the National Natural Science Foundation of China (award number: 31660482 and U1662108) for financial support. K. H. also thanks the support provided by Nanchang University. REFERENCES 1. Cox, P. M.; Betts, R. A.; Jones, C. D.; Spall, S. A.; Totterdell I. J. Acceleration of global warming due to carbon-cycle feedbacks in a coupled climate model. Nature 2000, 408 (6809), 184-187. 2. Figueroa, J. D.; Fout, T.; Plasynski, S.; McIlvried, H.; Srivastava, R. D. Advances in CO2 capture technology—The U.S. Department of Energy's Carbon Sequestration Program. Int. J. Greenh. Gas Con. 2008, 2 (1), 9-20. 3. Lei, Z.; Dai, C.; Chen, B. Gas solubility in ionic liquids. Chem. Rev. 2014, 114, (2), 12891326. 4. Rochelle, G. T. Amine scrubbing for CO2 capture. Science 2009, 325 (5948), 1652-1654. 5. Choi, S.; Drese, J. H.; Jones, C. W. Adsorbent materials for carbon dioxide capture from large anthropogenic point sources. ChemSusChem 2009, 2 (9), 796-854. 6. Gomez-Alvarez, P.; Calero, S. Highly selective zeolite topologies for flue gas separation. Chem. Eur. J. 2016, 22 (52), 18705-18708. 7. Geng, J. C.; Xue, D. M.; Liu, X. Q.; Shi, Y. Q.; Sun, L. B. N-doped porous carbons for CO2 capture: Rational choice of N-containing polymer with high phenyl density as precursor. AIChE J. 2017, 63 (5), 1648-1658.


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Graphic Abstract

Synopsis: The present work demonstrated an effective strategy to extend the application of graphitic carbon nitride in CO2 capture.


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Table 1. PEI loadings and porosity parameters of g-C3N4 and PEI—g-C3N4 composites Samples PEI loadings (g/g)a SBET (m2/g)b Vt (cm3/g)c Vm (cm3/g)d g-C3N4 0.5PEI@ g-C3N4

0 0.53

48.4 8.8

0.47 0.02

0 0

1.0PEI@ g-C3N4

1.09

4.3

0.01

0

1.5PEI@ g-C3N4

1.50

1.2

0

0

2.0PEI@ g-C3N4 2.08 0 0 0 a: mass ratio of PEI to g-C3N4; b: specific surface area; c: total pore volume; d: micropore volume.


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Scheme 1. Mechanism for reaction of amine-impregnated adsorbents with CO2.


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Figure 1. SEM (A, B), TEM (C, D) and AFM (E, F) images of g-C3N4; SEM images of 0.5PEI@g-C3N4 (G, H).


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Figure 2. FTIR spectra of g-C3N4 and PEI—g-C3N4 composites.


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Figure 3. XRD patterns of g-C3N4 and PEI—g-C3N4 composites.


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Figure 4. DTG curves of g-C3N4 and PEI—g-C3N4 composites.


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Figure 5. Effect of PEI loadings on CO2 capacities.


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Figure 6. Effect of temperatures on CO2 capacities.


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Figure 7. Amine efficiencies of PEI—g-C3N4 composites.


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Figure 8. CO2 and N2 adsorption isotherms of g-C3N4 and PEI—g-C3N4 composites at 25 °C.


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Figure 9. IAST CO2/N2 selectivities of g-C3N4 and PEI—g-C3N4 composites at 25 °C.


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Figure 10. Recycling of adsorbents under dry condition (adsorption: CO2, 50 mL/min, 75 °C, 2 h; desorption: N2, 150 mL/min, 75 °C, 2 h).


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Graphitic Carbon Nitride Functionalized with Polyethylenimine for

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