Open and Hierarchical Carbon Framework with Ultra-Large Pore

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Energy, Environmental, and Catalysis Applications

Open and Hierarchical Carbon Framework with UltraLarge Pore Volume for Efficiently Capture of Carbon Dioxide Kuan Huang, Fujian Liu, Jie-Ping Fan, and Sheng Dai ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b12182 • Publication Date (Web): 26 Sep 2018 Downloaded from http://pubs.acs.org on September 27, 2018

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ACS Applied Materials & Interfaces

Open and Hierarchical Carbon Framework with Ultra Ultraltra-Large Pore Volume for Efficiently Efficiently Capture of Carbon Dioxide Kuan Huang,† Fujian Liu,‡*, Jie-Ping Fan† and Sheng Dai§ †

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 of Chemical Fertilizer Catalyst (NERC-CFC), School of Chemical Engineering, Fuzhou University, Fuzhou, Fujian 350016, China. § Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA. KEYWORDS Hierarchical carbon framework; Record high pore volume; Amine-impregnated adsorbents; Polyethyleneimine; Selective capture of CO2

ABSTRACT Amine-impregnated adsorbents are promising candidates for the selective capture of CO2 from flue gas. The key is to develop suitable supports possessing large pore sizes and very large pore volumes, and the material has to be facilely synthesized from readily available reagents. In this work, hierarchical carbon nanosheet (CNS) featuring large pore width (30–100 nm) and extraordinarily huge pore volume (8.41 cm3/g) was prepared through controlled carbonization of glucose and dicyandiamide. The CNS was physically impregnated with pentaethylenehexamine (PEHA) to act as adsorbents for selective capyure of CO2. Owing to the unique porosity of CNS, the amount of amine loading in CNS can be ultrahigh (6 g PEHA/g CNS) in comparison with those of known amine-impregnated adsorbents, and the CO2 capacity in a flow of 15 v/v% of CO2 balanced in N2 was up to 5.0 mmol/g at 75 ˚C. The synthesized PEHA–CNS composite materials perform well in the capture of CO2 under humid condition, and display good stability in a test of 10 adsorption-desorption cycles. It is believed that the CNS synthesized in this work has great potential to act as a support material for CO2 adsorption.

INTRODUCTION Recently, considerable attention has been paid on the capture of CO2 at source, such as removing CO2 from flue gas, which is the main byproduct of fossil fuels utilization.1,2 Within this regard, CO2 absorption in aqueous amines is widely adopted in the industry, owing to the low cost of organic amines and their reversible chemical reactivity to CO23,4. After CO2 adsorption, the amines loaded with CO2 are subject to temperature or pressure swing to release CO2 and to regenerate the adsorbents. However, amine scrubbing is both cost and energy demanding. There is the loss of volatile solvents, equipment damage due to the corrosive nature of amines, and intensive energy input for recovering the adsorbents. With large surface area and good stability, porous materials including zeolites5-7, carbons8-10, oxides11-13, covalent organic frameworks (COFs)14-18, and metal organic frameworks (MOFs)19-21 are suitable materials for selective capture of CO2. However, most of them still cannot compete with aqueous amines in terms of CO2 capacity and CO2/N2 selectivity in the industry (i.e., low CO2 concentrations and high temperatures).

Although some best-performing MOFs do in fact compete, but the capital costs are too high for implementation and replacing amine scrubbing technology. Most porous materials interact only weakly with CO2. A strategy to overcome this shortcoming is by means of amine functionalization, either by chemical grafting or physical impregnation22-25. Notably, the physical impregnation can be more facilely fulfilled, and can afford solid adsorbents with high amine density as compared with chemical grafting. The performance of an amine-impregnated adsorbent is directly related to the porous structure of support, especially in terms of pore width and pore volume. A large pore width is beneficial to amine dispersion as well as accessibility, while a large pore volume can enlarge amine accommodation. To be practical in industrial application, an adsorbent has to be easily synthesized from readily available reagents. So far, most of the support materials for the fabrication of adsorbents reported in the literature show limited porous structure and/or suffer from complicated synthetic processes26-39. For example, many zeolites, COFs and MOFs are microporous (pore width < 2 nm)26-33, although some are mesoporous34. And most mesoporous carbon and silica materials are usually moderate in pore

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volume (< 2 cm3/g). For the construction of materials with ordered mesoporosity, it is common to sacrifice large amount of high-cost templates35-40. Recently, a MOF-derived carbon monolith (MDCM) featured with 3D interconnected pores and pore volume as high as 5.35 cm3/g, was investigated for CO2 adsorption after impregnated with tetraethylenepentamine (TEPA)41. However, the synthesis of carbon precursor (MOF5) consumes large amount of organic solvents. Herein, we demonstrate the facile generation of hierarchical carbon nanosheet (denoted herein as CNS) that is enriched with nitrogen atoms and is large in pore width (30–100 nm) as well as in pore volume (8.41 cm3/g). It is worth pointing out that the pore volume is larger than any of the reported porous materials. Furthermore, the material can be easily synthesized through one-step carbonization of glucose and dicyandiamide42-45. It was demonstrated that after impregnated with pentaethylenehexamine (PEHA), CNS displays excellent ability for the capture of CO2. At least 6 g of PEHA can be loaded to 1 g of CNS, and the resulted PEHA–CNS composites can adsorb up to ~5.5 mmol/g of CO2 from a flow of pure CO2 or ~5.0 mmol/g of CO2 from a flow of 15 v/v % CO2 balanced in N2 at 75 ˚C. Such a performance is one of the best among the known solid adsorbents for selective capture of CO2. EXPERIMENTAL Chemicals Dicyandiamide (99.0 wt. %), D-(+)-glucose (99.5 wt. %) and pentaethylenehexamine (PEHA, technical grade) were supplied by Sigma Aldrich. CO2 (99.99 v/v %), CO2 (14.7 v/v % in N2) and H2 (99.99 v/v %) were purchased from Airgas. Synthesis Carbon nanosheet (CNS) was synthesized by direct heating a mixture of well-ground dicyandiamide (4.0 g) and glucose (0.1 g). The mixture was firstly heated to 600 ˚C with the heating rate of 2.5 ˚C/min. The carbonization temperature was then increased to 800 ˚C with the heating rate of 1.67 ˚C/min, and the temperature was kept at 800 ˚C for 1 h. The carbonization process was protected by flowing N2. PEHA–CNS composites were synthesized from physical impregnation. Typically, CNS was firstly dispersed into a solution containing methanol slovent and calculated content of PEHA. After stirring the mixture at room temperature for 24 h, the methanol was removed from rotary evaporation at 60 ˚C. The resultant sample was then dried under vacuum at 60 ˚C for 48 h. According to PEHA loadings, the prepared PEHA– CNS composites are denoted as xPEHA@CNS, where x indicates the PEHA to CNS mass ratio. Characterizations N2 adsorption-desorption isotherms at -196 ˚C were recorded on a Micrometrics ASAP 2020 instrument. The samples were pretreated at 110 ˚C for 2 h under flowing N2 (150 mL/min). Specific surface areas of samples were calculated using the BET method at the relative pressure of P/P0=0.05–0.20. Pore size distributions were estimated by using the BJH model. The micropore volumes were estimated by using the t-plot method. Meso-macropore volumes were calculated according to the BJH cummulative volumes of

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pores between 2 and 200 nm in diameter. Total pore volumes of samples were determined as the sum of micropore and meso-macropore volumes. H2 adsorption isotherm at -196 ˚C was also measured on the same Micrometrics ASAP 2020 instrument to evaluate whether there are ultramicropores for gas diffusion. Thermal stabilities of samples were evaluated on a Seiko 6300 TG/DTA thermogravimetric analyzer under the protection of N2 (150 mL/min), and the heating rate was 10 ˚C/min. The loss of weight in the temperature range of 110– 700 ˚C was taken to determine PEHA loadings. Elemental analysis (EA) was performed on an Elementar Vario EL II analyzer, which was used to determine the N contents of the loaded PEHA by the following equation: x=(b-a)/(0.3618-b) where a is the weight fraction of N in CNS, b is weight fraction of N in PEHA–CNS composite, and 0.3618 is the theoretical weight fraction of N in PEHA. SEM images were taken on a Zeiss Auriga Crossbeam electron microscope with the acceleration voltage of 5 kV. TEM images were taken on a JEOL2100F electron microscope with the acceleration voltage 200 kV. XPS spectra were collected on the Thermo ESCALAB250 spectrometer with Al Kα radiation, and the binding energies were calibrated against the C 1s peak of adventitious carbon at 284.9 eV. XRD patterns were collected on a PANalytical powder diffractometer using Cu Kα radiation. Raman spectra were performed on a Princeton MSL 532-50 spectrometer, and the wave length of excitation laser was 532 nm. FT-IR spectra were collected on an Excalibur Series instrument with MVP-Pro ATR accessory. CO2 adsorption Before the adsorption, the absorbents were pretreated at 110 ˚C for 120 min under the protection of N2 (150 mL/min). The capacities and recyclability of the adsorbents under different conditions were determined on the TGA instrument. Typically, specific amount of sample was introduced into a ceramic crucible, which was then transferred to the TGA chamber. For the adsorption of CO2 under dry conditions, the sample was firstly treated at a designated temperature (25–100 ˚C) for 60 min under the protection of N2 (150 mL/min) to make the baseline steady; then, the flow of N2 was changes to a flow of CO2 (pure or dilute in N2 with the concentration of 14.7 v/v%, 50 mL/min); the temperature and flow of CO2 were kept for 180 min. The mass increase during the adsorption period was regarded as the uptake of CO2. Three duplicate runs were performed for each sample to prove the reproducibility of measurements and estimate the standard errors. For the adsorption of CO2 under humid conditions, the sample was firstly treated at 75 ˚C for 60 min under the protection of N2 (150 mL/min); then the flow of N2 was changed to a flow of humidified N2 (150 mL/min, 10 % relative humidity), and kept for 180 min. The mass increase during the adsorption period was use to determine the uptake of H2O vapor. Finally, the humidified flow of N2 was changed to a humidified flow of CO2 (pure, 50 mL/min, 10 % relative humidity); and the adsorption process was kept for 180 min, and the mass increase during the adsorption period was recorded to calculate the uptake of CO2.


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To evaluate the cycling properties of samples in the adsorption of CO2 under dry conditions, the sample was firstly treated at 75 ˚C for 60 min under the protection of N2 (150 mL/min), the flow of N2 was then changed to a flow of CO2 (pure, 50 mL/min) for 120 min. Subsequently, the flow of CO2 was changed back to the flow of N2 for another 120 min. The recyclability of the adsorbent was tested for 10 repeated runs. To evaluate the cycling properties of samples in the adsorption of CO2 under humid conditions, after the sample was firstly treated at 75 ˚C for 60 min under the protection of N2 (150 mL/min), the flow of N2 was then changed to a humidified flow of CO2 (pure, 50 mL/min, 10 % relative humidity) for 180 min. Subsequently, the humidified flow of CO2 was changed back to the flow of N2 for another 120 min. The recyclability of the adsorbent was also tested for 10 repeated runs Adsorption isotherms of various samples for capture of CO2 were also measured on the Micrometrics ASAP 2020 instrument to evaluate the CO2 capture performance at low pressures. RESULTS AND DISCUSSION Figure 1 shows the synthetic process and morphology of CNS. The detailed procedures are available in the Experimental section and Supporting Information (Figures S1 and S2). Glucose acts as the carbon precursor, and dicyandiamide acts as the nitrogen precursor. First, glucose and dicyandiamide were thoroughly mixed through manual grinding. Second, the pyrolysis of dicyandiamide at 500–600 ˚C resulted in graphitic carbon nitride (g-C3N4)45,46, which binded with the carbon intermediate produced from the pyrolysis of glucose. Third, g-C3N4 acts as template, which decomposed completely at 800 ˚C to leave pure CNS (see Figure 1A). It is deduced that the release of gaseous products during g-C3N4 decomposition at high temperature results in the enlarged porous structure of CNS and incorporation of a considerable content of nitrogen sites in the CNS framework. Scanning electron microscopy (SEM) images of CNS (Figures 1B–D and S1) clearly reveal uniform graphene-like layers with highly interconnected and three-dimensional channels that are made up of micro-size flakes. The channels constitute the meso- macroporous architecture of CNS. Transmission electron microscopy (TEM) images (see Figures 1E–G and S2) illustrate that the carbon flakes are thin (3–4 nm) and flexible, equivalent to about 10 layers of graphene. Figure 2 depicts the textural properties and structural nature of CNS. Figure 2A shows the N2 adsorption-desorption isotherm at -196 ˚C. Notably, low N2 uptakes at low relative pressures and very high N2 uptakes at high relative pressures were observed. The N2 adsorption isotherm shows a steep increase at P/P0=0.900–0.995, suggesting the presence of abundant macropores in CNS. The CNS gives a BET specific surface area of 1410 m2/g. The BJH pore size distribution of CNS was cenetred at around 30, 70 and 120 nm (Figure 2B), indicating hierarchical meso-macroporosity, in accordance with the SEM and TEM results. According to the BJH cumulative volume of pores between pore width of 2 and 200 nm, the meso-macropore volume of CNS is estimated as 8.29 cm3/g (Figure 2B). The CNS shows a micropore volume of

0.12 cm3/g based on the t-plot method. Therefore, the total pore volume of CNS is estimated as 8.41 cm3/g, a record high value among porous materials to date (Table S1).

Figure 1. Illustration for the synthesis of CNS (A); SEM (B– D) and TEM (E–G) images of synthesized CNS.

Figure 2. N2 adsorption-desorption isotherms at -196 ˚C (A), BJH cumulative pore volume and pore size distribution (B), Raman spectrum (C), XRD pattern (D), XPS survey (E), and N 1s spectrum (F) of synthesized CNS. In the Raman spectrum, CNS shows four signals associated with D, G, 2D and S3 bands at 1346, 1580, 2639 and 2895 cm−1 (Figure 2C). The D band was assigned to sp3 carbon, and the G band was assigned to sp2 carbon. The strong intensity of D band in CNS indicates the presence of abundant defects and disorders at the edges of carbon layers. In X-ray diffraction (XRD) pattern (Figure 2D), a broad and weak peak at 26.2 ˚ associated with graphitic (002) planes can be observed, which



indicates the low crystallinity and few-layered structure of CNS. The above results confirm the graphene-like architecture of CNS, which is in consistence with the SEM and TEM observations. The XPS spectra (Figure 2E) demonstrates that CNS is mainly composed of C (73.0 at. %), N (19.2 at. %) and O (7.8 at. %). The deconvolution of N 1s profile (see Figure 2F) results in two components: pyridinic N at 398.4 eV (67.8 %) and graphitic N at 400.6 eV (32.2 %).

Figure 3. Photograph of PEHA–CNS composites (A); SEM images of 3.0PEHA@CNS (B and C), 4.0PEHA@CNS (D), 5.0PEHA@CNS (E) and 6.0 PEHA@CNS (F). To enhance the ability of CNS for CO2 adsorption, different amounts of PEHA, which is a kind of polyethyleneimine (PEI) with linear structure, were impregnated into the inner pores of CNS for the generation of a series of PEHA–CNS composites (denoted herein as xPEHA@CNS, where x is the PEHA to CNS mass ratio). In view of the large pore width and high pore volume of CNS, it is anticipated that CNS can function well as a support. Figure 3 shows the photographs and SEM images of the composites. After loading PEHA to give the PEHA–CNS composites, the related signals of PEHA can be observed in N 1s spectra (Figure S3), indicating the successful incorporation of PEHA in CNS. The PEHA loadings estimated by EA are similar to those estimated by TGA (Table S2). It is found that the PEHA loadings of PEHA–CNS composites can reach as high as 6 g/g, approaching the theoretical estimation based on the total meso-macropore volume of CNS. From the photographs, the four prepared PEHA–CNS composites are in the form of dry powders, showing morphology similar to that of CNS. Furthermore, the highly folded carbon flakes are also observed over the PEHA–CNS composites. Nonetheless, there is no severe stacking of carbon flakes, suggesting that PEHA was mainly dispersed into the pores of CNS rather than on the external surface. The well dispersion of PEHA of CNS can be attribute to two factors: First, the pores of large width make the filling of deep channels by PEHA molecules feasible; second, the pyridinic N species of CNS enable hydrogen bonding interaction with the amine groups in PEHA. The PEHA–CNS composites are compared with CNS in terms of textural property, structural nature and thermal stability (Figures S4–11 and Table S3). As expected, there is significant decrease in both BET specific surface area and pore volume after CNS is impregnated with PEHA (see Figures S4–6 and Table S3) due to the occupancy of PEHA in the inner pores of CNS. Notably, the decrease in pore volume with PEHA loading is larger than the theoretical value estimated

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from the PEHA density. On the other hand, the H2 uptakes at 196 ˚C of 3.0PEHA@CNS are much higher than N2 uptakes (Figure S12). This phenomenon implies that the distribution of PEHA in the inner pores of CNS forms ultramicropores that allow the penetration of H2, but exclude N2, because N2 is much larger than H2 in kinetic diameter. In Raman investigtion (Figure S7), the D and G bands of the PEHA–CNS composites are similar to those of CNS in position as well as in intensity ratio, indicating that the impregnation of PEHA has little effect on the CNS structure. The absolute intensities of D and G bands decrease only slightly with increasing PEHA, validating that PEHA mainly resides in the inner pores of CNS. In the XRD patterns (Figure S8), the peak of graphitic (002) plane at 26.2 ˚ shifts to 18.9 ˚ after the impregnation of PEHA, indicating extension of CNS interlayer distance. According to the Bragg equation, the interlayer distances of CNS and PEHA–CNS composites are calculated to be 3.17 and 4.38 Å, respectively. The extension of interlayer distance should be resulted from insertion of PEHA molecules into the interlayer of carbon flakes, which is induced by hydrogen-bond interaction between the pyridinic N species and amine groups of PEHA. Such a phenomenon was also observed over other layered materials47,48. However, the extension of interlayer distance should have little effect on the porosity of composite materials, because the porous structure of CNS is mainly attributed by the random packing and folding of carbon flakes, which is in good agreement with SEM images. Figure S9 shows the FTIR spectra of synthesized PEHA– CNS composites. T he peaks associated with PEHA can be clearly seen over the PEHA–CNS composites, confirming the successful incorporation of PEHA into CNS. There is no obvious shift of the characteristic peaks of PEHA, implying that hydrogen-bond interaction between pyridinic N species and PEHA is weak. Referring to TGA profiles (Figure S10), CNS is stable in the 110–700 ˚C range, while the PEHA–CNS composites start to lose weight at 160 ˚C due to the evaporation and/or decomposition of PEHA. With such thermal stability, the PEHA–CNS composites are suitable for the selective capture of CO2 from flue gas49. In the differential thermogravimetric (DTG) profiles (Figure S11), the peaks associated with the evaporation and/or decomposition of PEHA in the PEHA–CNS composites are at a position similar to that of pure PEHA. This result further validates that the hydrogen-bond interaction between pyridinic N species and PEHA is weak, and its effect on the stability of PEHA in the pores of CNS is negligible. The performance of the PEHA–CNS composites for CO2 capture was then investigated systematically. We first monitored CO2 adsorption under pure CO2 at different temperatures (25–100 ˚C) using TGA (Figures S13–17). PEHA–CNS composites show much higher capacities than CNS in the capture of CO2, and their differences are more enlarged at higher temperatures. For instance, PEHA–CNS composites show the CO2 capacities of 2.52–2.65 mmol/g, which are around 2 times higher than that of CNS (1.42 mmol/g) at 25 ˚C, while they (4.86–5.45 mmol/g) are around 7 times higher at 75 ˚C. The superiority of PEHA–CNS composites over CNS for CO2


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capture is strongly related to the strong chemical interaction of PEHA with CO2, however CNS exhibits weak physical interaction with CO2. Figure 4A depicts the variation of CO2 capacities with adsorption temperatures. Since the capture of CO2 on CNS is physical, CO2 capacity decreases with increasing temperature. As for the adsorption of CO2 on PEHA–CNS composites, temperature has two effects: a rise in temperature would enhance the diffusion of CO2 in PEHA but hinder CO2 adsorption on PEHA. At 25–75 °C, the transportation of CO2 in PEHA is a predominant factor, and CO2 capacity increases with increasing temperature. However, the adsorption of CO2 on PEHA was weakened as further increasing the temperature to 100 °C. The net result is that CO2 capacities of PEHA–CNS composites at 100 °C are lower than those at 75 °C, which suggests carbamate formation is not thermodynamically favorable at elevated temperature. Based on these results, the optimized temperature for CO2 adsorption on PEHA–CNS composites is 75 °C. Figure 4B shows the relationship between PEHA loadings and CO2 capacities. It is clear that too high a PEHA loading would result in blocking of channels while too low would affect the availability of acitve amine sites. At PEHA loadings of 0–4 g/g, the blocking of pores is not serious and CO2 capacities were enhanced as increasing of PEHA loadings. However, with the loading contents increasing to 4– 6 g/g, CO2 capacity decreases due to pore blockage. Based on these results, the PEHA loading for optimal CO2 adsorption on PEHA–CNS composites is 4 g/g. At 75 °C with PEHA loading of 4 g/g, CO2 capacity is 5.45 mmol/g (1.25 mmol/mL) which is among the highest of known amine-impregnated adsorbents (see Table S4).

Figure 4. CO2 capacities of CNS and PEHA–CNS composites (A and B); amine efficiencies of PEHA–CNS composites (C

and D); recycling of PEHA–CNS composites for CO2 adsorption under dry (E) and humidified (F) conditions. According to CO2 uptakes and PEHA loading amounts, utilization efficiencies of amine groups in PEHA–CNS composites defined as the molar ratio of adsorbed CO2 vs. amine groups were calculated. In general, amine efficiency increases with increasing temperature in the range of 25–75 °C, owing to enhanced CO2 diffusion. However, increasing the temperature to 100 °C causes decrease of amine efficiency due to weakened interaction between CO2 and PEHA (Figure 4C). On the other hand, amine efficiency decreases slightly with increasing PEHA loading due to blockage of channels and difficulty in CO2 diffusion (Figure 4D). The amine efficiencies of PEHA–CNS composites are around 0.25, which are similar or even superior to most of the known solid amines26-33,3541,47,48 . Overall, owing to the large pore width and high pore volume of CNS, the PEHA–CNS composites display competitive efficiency in amine utilization. It should be noted that the concentration of CO2 in flue gas is normally low, the ability of solid adsorbents for capturing CO2 from dilute sources is of relevance. Accordingly, we studied CO2 adsorption in a flow of 15 v/v % CO2 balanced in N2 at 75 °C by TGA (Figure S18), and reasonably high CO2 uptakes of 4.22 and 4.90 mmol/g were achieved over 4.0PEHA@CNS and 5.0PEHA@CNS, respectively. The CO2 adsorption isotherms of 4.0PEHA@CNS at different temperatures (25–75 ˚C) were also measured by a volumetric method (Figure S19), and results validate the strong ability of PEHA– CNS composites for adsorbing CO2 at low pressures. Furthermore, to clarify the effect of moisture, which is inevitably present in flue gas, the adsorption of CO2 was investigated under humid condition. Figure S20 shows CO2 adsorption in a flow of CO2, the relative humidity of which is 10 %, on the optimized PEHA–CNS composite at 75 °C. The relative humidities of pristine flue gas are different from case to case, but normally at low levels. Therefore, a relative humidity of 10 %, under which the activity of water vapor is low, was chosen as a representative for investigation. It is found that 1 g of 4.0PEHA@CNS can adsorb 0.245 g of CO2, corresponding to 5.57 mmol of CO2 per gram of adsorbent, from 10% humidified CO2. Actually, the CO2 capacity is slightly enhanced in comparison with that under dry condition. It is noted that the relative humidity of flue gas can reach 100 % after wet flue gas desulfurization (FGD) process, and the temperature of coal flue gas can be 40 °C. Therefore, the adsorption of CO2, which is saturated with water vapor, on 4.0PEHA@CNS at 40 °C was also investigated (Figure S21). It is found that 1 g of 4.0PEHA@CNS can adsorb 0.171 g of CO2, corresponding to 3.89 mmol of CO2 per gram of adsorbent, from 100 % humidified CO2. The CO2 capacity is enhanced by 22 % in comparison with that under dry condition (3.20 mmol/g). The favorable effect of moisture on CO2 capacity was observed before over amine-impregnated adsorbents26-33,35-41,47,48, which is attributed to the change of mechanism for reaction of CO2 with amine groups from 1:2 stoichiometry in the absence of H2O to 1:1 stoichiometry in the presence of H2O.



The stability of PEHA–CNS composites in consecutive adsorption-desorption cycles is relevant for long-term usage. The reusability of 4.0PEHA@CNS and 5.0PEHA@CNS under dry condition was tested by TGA (Figures S22 and S23). In both cases, there is complete release of adsorbed CO2 after N2 purging for 120 min at 75 °C, suggesting high reversibility of CO2 adsorption. The performance throughout the cycles are shown in Figure 4E. The CO2 uptake of 4.0PEHA@CNS and 5.0PEHA@CNS decreases by about 25 % (from ~5.3 mmol/g to ~4.0 mmol/g) after 10 times of recycling. Since the loss of PEHA from PEHA–CNS composites at the regeneration temperatuer is negligible (Figure S24), the reason for the decrease in CO2 capacity should be the transformation of carbamate to urea under elevated temperature, as suggested by Sayari et al.50 Despite the decrease, the recyclability of PEHA–CNS composites is still similar to or even better than that of many amine-impregnated adsorbents. For example, the PEI-SBA-15 composites, which is extensively studied as solid amines for CO2 capture, were reported to lose 34 % CO2 uptake after 4 times of recycling.51 It is deduced that the hydrogen-bond interaction of pyridinic N in CNS with PEHA can help prevent carbamate from evolving to urea. The cycling of CO2 adsorption on 4.0PEHA@CNS and 5.0PEHA@CNS under humidified condition was also tested, as shown in Figures S25 and S26. The CO2 capacities throughout the cycles are presented in Figure 4F. Obviously, the behavior of CO2 adsorption-desorption on 4.0PEHA@CNS and 5.0PEHA@CNS under humidified condition is similar to that under dry condition. Therefore, the presence of water vapor has negligible impact on the reusability of PEHA–CNS composites. The enthalpy changes for CO2 adsorption on PEHA–CNS composites are very important to evaluate the regeneration energy. However, we do not have a calorimetric apparatus to measure the adsorption enthalpies. On the other hand, the adsorption enthalpies can not be estimated from CO2 isotherms at different temperatures, because the adsorption of CO2 on solid amines is controlled by diffusion process, which results in the abnormally positive dependence of CO2 capacity on adsorption temperature (see Figure S19). However, it has been ever reported that the enthalpy changes for CO2 adsorption on amine-impregnated materials range from -60 to -80 kJ/mol, which are similar to aqueous MEA (-72 kJ/mol)40. Furthermore, since the CNS framework is highly porous and may not be architecturally robust even after the PEHA impregnation, it is useful to investigate the effect of palletization on CO2 adsorption behavior. Therefore, the prepared PEHA– CNS composites were pelletized (Figure S27) and cycling of CO2 adsorption was performed for 4.04.0PEHA@CNS and 5.0PEHA@CNS (Figure S28). It is found that pelletization has negligible effect on the CO2 capacities and long-term stability of PEHA–CNS composites. Based on this result, it is inferred that PEHA–CNS composites can be pelletized for practical application in CO2 capture. CONCLUSIONS Carbon nanosheet (CNS) of hierarchical architecture endowed with large pore width and huge pore volume was facilely synthesized through controlled pyrolysis of glucose

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and dicyandiamide. Then a class of PEHA–CNS composites were rationally designed by physical loading of CNS with PEHA at high loadings. The prepared composites show superior performance for the selective capture of CO2, with CO2 capacity among the highest of amine-impregnated adsorbents. The PEHA–CNS composites also show satisfied reusability in 10 adsorption-desorption cycles. It is envisaged that the CNS fabricated in this work is a promising “molecular basket” for amine compounds, which may find great applications in the supported CO2 capture from flue gas.

ASSOCIATED CONTENT Supporting Information Supporting Tables and Figures. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *[email protected] (F. L.). Notes The authors declare no conflicting interest.

ACKNOWLEDGMENTS ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21573150, 21203122), Natural Science Foundation of Zhejiang Province (LY15B030002), and Natural Science Foundation of Jiangxi Province (20171BAB203019). K. H. also acknowledges the sponsorship from Nanchang University. S. D. was sponsored by the U.S. Department of Energy, Office of Basic Energy Sciences, Chemical Sciences, Geoscience, and Bioscience Division.

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SYNOPSIS TOC Highly open and hierarchical carbon framework with very huge pore volume (8.41 cm3/g) is prepared in this work. The carbon is physically impregnated with ultrahigh content of pentaethylenehexamine (PEHA, 6 g PEHA/g carbon). Resultant PEHA–carbon composites exhibit extraordinary capacity of CO2 (~5.0 mmol/g) from 15 v/v % of CO2 balanced in N2 at 75 ˚C.

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Open and Hierarchical Carbon Framework with Ultra-Large Pore

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