Weiwei Shia,
Rongzhen Wanga,
Huili Liua,
Binbin Chang*a,
Baocheng Yang*a and
Zuling Zhangb
aHenan Key Laboratory of Nanocomposites and Applications, Institute of Nanostructured Functional Materials, Huanghe Science and Technology College, Zhengzhou, Henan 450006, China. E-mail: binbinchang@infm.hhstu.edu.cn; baochengyang@infm.hhstu.edu.cn
bHenan Provincial Chemi-Industries Research Station Co., Ltd, Zhengzhou 450000, China
First published on 26th July 2019
Considering the characteristics of abundant narrow micropores of <1 nm, appropriate proportion of mesopores/macropores and suitable surface functionalization for a highly-efficient carbon-based CO2 adsorbent, we proposed a facile and cost-effective strategy to prepare N and S dual-doped carbons with well-interconnected hierarchical pores. Benefiting from the unique structural features, the resultant optimal material showed a prominent CO2 uptake of up to 7.76 and 5.19 mmol g−1 at 273 and 298 K under 1 bar, and importantly, a superb CO2 uptake of 1.51 mmol g−1 at 298 K and 0.15 bar was achieved, which was greatly significant for CO2 capture from the post-combustion flue gases in practical application. A systematic study demonstrated that the synergetic effect of ultramicroporosity and surface functionalization determined the CO2 capture properties of porous carbons, and the synergistic influence mechanism of nitrogen/sulfur dual-doping on CO2 capture performance was also investigated in detail. Importantly, such as-prepared carbon-based CO2 adsorbents also showed an outstanding recyclability and CO2/N2 selectivity. In view of cost-effective fabrication, the excellent adsorption capacity, high selectivity and simple regeneration, our developed strategy was valid and convenient to design a novel and highly-efficient carbonaceous adsorbent for large-scale CO2 capture and separation from post-combustion flue gases.
In contrast, adsorption using porous solid materials as adsorbents for CO2 capture has been recognized as the most promising alternative technology owing to its low energy consumption, low cost, relatively high efficiency and easy handling.10–12 The key of this technology is to develop new porous solid sorbents with superior properties for CO2 capture. To this end, a number of porous solid materials, including zeolites, metal–organic frameworks, porous organic polymers, organic–inorganic hybrid adsorbents and porous carbon materials, have been extensively investigated.13–18 Among various types of adsorbent materials, porous carbons, as a special family of highly promising materials, have been extensively studied for high-efficiency CO2 capture by virtue of their unique structural advantages, including high accessible surface area, low-cost preparation, facile regeneration, easy-to-design surface functionality and porosity, inertness to both bases and acids and moderate heat of adsorption. Nevertheless, most traditional activated carbons exhibit a relatively low CO2 adsorption capacity of typically ca. 2–3 mmol g−1 under room temperature at 1 bar.19,20 In terms of CO2 capture by porous carbons, it has been widely reported that CO2 capture capacity at ambient pressure greatly depends on the proportion of narrow microporosity of smaller than 1 nm, and especially the ultramicroporosity of <0.7 nm, which plays a crucial role in determining CO2 uptakes of porous carbons.21–23 Thus, many efforts have been made to design microporous carbons with a large proportion of narrow micropores of <1 nm, especially to further tailor micropores to produce a more favorable ultramicroporous carbon for CO2 capture. For example, Sevilla et al. prepared microporous biomass-based carbon materials via KOH chemical activation of hydrothermal carbons derived from mixtures of algae and glucose, which possessed a large number of narrow micropores (<1 nm) and exhibited a superior CO2 adsorption capacity of 4.8 mmol g−1 at 298 K and 1 bar.24 Our group synthesized cross-linked microporous carbon beads by air-assisted activated method using glucose-derived carbon microspheres as precursor, which developed a large proportion of ultramicropores with primary pore size of 0.5–0.9 nm and showed a satisfactory CO2 uptake of 4.25 mmol g−1 at 298 K and 1 bar.25 Another valid strategy to further improve CO2 uptake of porous carbons is the incorporation of heteroatoms into carbon skeleton.26–28 Typically, nitrogen doping is the most attractive, which can provide more basic sites for enhanced interactions with acidic CO2 molecule. Specifically, it has been testified that the pyrrolic and amine nitrogen functionalities have the strongest interactions with CO2 molecules.29
In addition, it has been proposed that the enhancement of CO2 uptake on porous carbons can be related to the improvement in polar interaction and hydrogen bonding interaction.30,31 Since hydrogen bonding or polar interactions of CO2 within the carbon pore can more originate from other functional groups than nitrogen-based groups. Recently, CO2 capture over sulfur-doped porous carbons has attracted much attention. For sulfur-doped porous carbons, the lone pair of electrons in a sulfur atom induces polarizability and interactions with oxygen.32 Xia et al. reported an improved CO2 capture capacity of 2.4 mmol g−1 at 298 K and 1 bar on sulfur-doped porous carbon.33 Seema et al. obtained a reduced-graphene oxide/polythiophene complex via chemical activation route, which exhibited an excellent CO2 uptake of 4.5 mmol g−1 at 298 K and 1 bar.34 Bandosz et al. reported the chemical interactions between sulfur doped carbons and CO2.35 These investigations manifested sulfur-containing functional groups in carbon adsorbents could facilitate CO2 adsorption due to acid interactions of CO2 with neutral sulfur, polar interactions of CO2 with oxidized sulfur, and hydrogen bonding of CO2 with sulfonic acids. However, there are relatively few studies about the synergistic effect of nitrogen and sulfur dual-doping on hierarchically porous carbons for CO2 capture.36,37 Thus, it is significant for high-effective CO2 capture to design a novel porous carbon adsorbent comprising multiscale pores including abundant narrow micropores of <1 nm (which favor high CO2 uptake), appropriate proportion of mesopores/macropores (which facilitate efficient CO2 diffusion into and out of the adsorption sites) coupled with suitable surface nitrogen and sulfur functionalities (which further improve CO2 uptake).
Considering the characteristics described above, herein, nitrogen and sulfur dual-doped porous carbons with well-interconnected hierarchical pores were synthesized by a facile and cost-efficient strategy using bio-waste as carbon source and thiourea as nitriding and sulfurizing agent. Innovatively, the synergetic effect of ultramicropores (<0.7 nm) and nitrogen/sulfur dual-doping on CO2 capture was investigated in detail. Gratifyingly, the resultant optimal material exhibited a superior CO2 capture capacity of up to 7.76 and 5.19 mmol g−1 at 273 and 298 K under 1 bar, respectively. More importantly, such a CO2 adsorbent also showed an outstanding recyclability and CO2/N2 selectivity adsorption property. Hence, such results suggested that we proposed a valid strategy to exploit novel porous carbon sorbents for the removal of CO2 from post-combustion exhaust gases.
Scheme 1 Schematic illustrating the preparation of the waste paper-derived N and S dual-doped hierarchically porous carbons. |
To reveal the effect of carbonization and activation processes on the microcrystalline texture, the XRD patterns of all the resultant materials were shown in Fig. 1a. Two weak and broad peaks at approximately 22.1° and 43.6° can be observed in all samples, which are attributed to the (002) and (100) reflections of turbostratic carbon structure, suggesting an amorphous carbon texture with a low crystallinity. The diffraction intensity of the (002) and (100) peak in activated materials is lower than those in HPC under the same conditions of detection, resulting from the chemical activation to break down the hexagonal symmetry of the graphite lattice and lead to lattice defects in HPCK-1 and HPCZn-1 samples.38 In addition, the high intensity in the low angle region indicates the existence of abundant micropores in all samples.39 The evolution of the chemical compositions of the waste paper precursor, activated samples and N and S dual-doped samples were characterized by FTIR (Fig. 1b). For hydrothermally synthesized precursor, it was similar to the chemical composition of the other plant fibers,40 the main component of waste paper towel was lignocelluloses. The broad beak at about 3380 cm−1 was assigned to O–H stretching vibration. The band at 2900 cm−1 was attributed to the C–H stretching vibrations in methyl and methylene groups, and the bands at 1425 and 1370 cm−1 were ascribed to aromatic skeletal vibrations combined the C–H deformation vibrations, while the band at 1320 cm−1 was due to the CH2 rocking vibration. The bands at 1660 and 1635 cm−1 were ascribed to the CO stretching vibration and CC stretching vibration, respectively. The band at 1160 cm−1 was related to C–O–C asymmetric valence vibration, and the band at 1113 cm−1 was due to the C–C stretching or asymmetric in-phase ring stretching. The bands at 1060 and 1032 cm−1 were assigned to C–O valence vibration and C–O ether vibration, respectively. While, after carbonization and chemical activation, it was obvious that the bands in the resultant materials became broader and weaker, this could be related to the strong absorption of the carbon skeleton. Only the CC stretching vibration could be found in other activated samples, and the other characteristic absorption bands became weaker and even disappeared. Such results should be resulted from the carbonization and chemical activation, resulting in the elimination of surface functional groups.
Scanning electron microscopy was conducted to reveal the evaluation of microstructure and morphology from paper towel to activated carbons and N, S dual-doped carbons material. Paper towel showed a long fiber structure with a diameter of range from ∼5 to ∼20 μm (Fig. S1a†), and the hydrothermally synthesized precursors completely inherited the original fiber morphology of paper towel (Fig. S1b†). As displayed in Fig. 2a, the fibrous texture could be obviously observed in HPC sample, which indicated the original structure could be retained during the high-temperature carbonization treatment. After KOH activation, HPCK-1and HPCK-2 presented a three-dimensional interconnected honeycomb-like microstructure with numerous irregular holes (Fig. 2b and c), which was a unique structure that could effectively prevent the small and thin blocks from agglomerating on a large scale. Such prominent honeycomb-like porous structure was attributed to the KOH chemical activation and the followed CO/CO2 physical activation in situ induced from the reactions of KOH and C.41 With an increase in the addition amount of KOH, the density of pores presented a developing trend. Whereas, as the mass ratio of KOH/precursor increased to 3, a harsh activation reaction would occur in carbon skeleton, which brought the pore widening and the degradation of 3D honeycomb-like structure. As a result, carbon sheets was too thin to support the honeycomb-like structure, resulting in the collapse and restacking of partial sections to form the block with thick wall (Fig. 2d). Different from the morphology of HPCK-x samples, after ZnCl2 activation, all the resultant HPCZn-x samples exhibited a porous structure with some macropores (Fig. 2e–g). Such results could be ascribed to the activation mechanism of ZnCl2 chemical activation, and ZnCl2 often serves as a dehydrating agent to chemically activate carbonaceous precursors. Similarly, with the increasing of ZnCl2 dosage, the density of generated pores was decreased, which could be related to pore coalescence. After further nitrogen and sulfur dual-doped, the N,S-HPCK-1 retained the original honeycomb-like porous structure of HPCK-1 sample (Fig. 2h), and N,S-HPCZn-1 also kept the initial porous structure of HPCZn-1 (Fig. 2i), which testified that the further functionalized process at high temperature could not destroy the intrinsic morphology.
Fig. 2 SEM images. (a) HPC; (b) HPCK-1; (c) HPCK-2; (d) HPCK-3; (e) HPCZn-1; (f) HPCZn-2; (g) HPCZn-3; (h) N,S-HPCK-1; (i) N,S-HPCZn-1. |
The porous structure features of the resultant carbonaceous sorbents were analyzed by N2 adsorption–desorption at 77 K. In Fig. S2,† it can be clearly found that the hydrothermally synthesized precursors possess an only 1.79 m2 g−1 of BET total surface area and a 0.006 cm3 g−1 of pore volume, which indicated that such carbonaceous precursors obtained by sole hydrothermal treatment exhibited extremely poor porosity. After directly carbonized precursors at 800 °C, the HPC material depicted a typical type I curve (Fig. 3a), meaning a microporous characteristic pore structure, and these micropores could mainly originate from the decomposition of volatile matter and the elimination of surface O- and H-groups during the carbonization process. But, the relatively small N2 adsorbed amount manifested the low total surface area and pore volume (Table 1). With the KOH chemical activation, the HPCK-x still presented a type I isotherm of microporous structure, and the sharply increased and highly adsorbed quantity at a low relative pressure (P/P0 < 0.01) owed to the capillary filling of micropores, which testified the pore structure of HPCK-x mainly consisted of micropores. Comparatively, an abrupt increase of the isotherm occurred at the higher relative pressure (P/P0 > 0.9) for HPCK-2 and HPCK-3 samples, which was ascribed to the capillary condensation, implying that a larger pore size was generated when more KOH was employed as activating agent. Fig. 3b showed the pore size distribution of HPC and HPCK-x samples. HPC owned a micropore size distribution of 1.17/1.86 nm, and the proportion of 1.17 nm micropores was larger. Apparently, with the use of different dosage of KOH activating agent, the pore size gradually extended from bi-modal distribution to multi-modal distribution, and larger pore size and proportion stepwise enlarged with the enhancement of KOH dosage. The pore size distribution of HPCK-1 was 0.47–0.86 and 1.27 nm, and the overall pore sizes were still in the micropore range. The majority of the pore sizes centered at 0.47–0.86 nm, while the minority of the pore sizes was 1.27 nm. With the increase of KOH activating agent concentration, some new and larger pore size distributions of 1.61/2.02 nm and 2.10 nm were produced in HPCK-2 and HPCK-3 sample, respectively. More noteworthy, the majority of pore sizes distributed in 1.61 and 2.10 nm, and the minority of pore sizes was in micropores of 0.47–1.27 nm. When used ZnCl2 as activating agent, the HPCZn-1 sample exhibited a similar isotherm with HPCK-x, meaning a characteristic micropore structure of HPCZn-1. Whereas, HPCZn-2 and HPCZn-3 presented a transitional isotherm from type I to type IV (Fig. 3c), and even a marked H3 type hysteresis loop in the relative pressure region between 0.4 and 0.7 could be clearly observed in HPCZn-3. Such results manifested the coexistence of micropores and slit-shaped mesopores. The pore size distributions of HPCZn-x samples were shown in Fig. 3d. The pore size of HPCZn-1 centered at 0.68 and 1.27 nm, and overall pore size were in micropore, and the pore proportion was almost isometric. With the increase of ZnCl2 dosage, some new mesopores of 2.34 and 3.07 nm were generated in HPCZn-2 and HPCZn-3 samples, respectively, and these mesopores were dominant in porosity. By comparing the pore size distributions of HPCK-x and HPCZn-x samples, we could speculate such conclusions: (i) the hierarchical pore size distribution significantly depend on the activation of activating agents; (ii) KOH activating agent exhibits a more effective impact on tailoring multi-modal pore size distribution, especially in micropores; (iii) ZnCl2 activating agent plays a high-effective role in developing mesopores. In addition, the BET surface area and other textural properties of all the resultant materials are summarized in Table 1. It can be clearly found that the activating agents have a vital influence on developing porosity of materials. With the increase of activating agent dosage, both surface area and pore volume were greatly improved owing to the further etch of activating agent on carbonaceous skeleton. However, as the activating agent dosage further increase to 3, because of the excessive activation, numerous micropores coalesced to form a mesopore, which resulted in the continuous enhancement of mesoporous surface area and mesopore volume and decrease of total surface area and micropore volume. Therefore, varying the dosages of the activating agent is an effective strategy to engineer the proportion of mesopore/micropore in porosity. Moreover, after N and S dual-doping, the specific surface area and pore volume changed slightly, which testified that surface functionalization process could not destroy the original porosity of materials.
Fig. 3 N2 adsorption–desorption isotherms (a and b) and pore size distributions (c and d) of all the resultant materials. |
Sample | SBETa (m2 g−1) | Smicrob (m2 g−1) | Smesoc (m2 g−1) | Sultramicrod (m2 g−1) | Vtotale (cm3 g−1) | Vmicrof (cm3 g−1) | Vultramicrog (cm3 g−1) |
---|---|---|---|---|---|---|---|
a BET surface area.b Micropore surface area calculated using the V–t plot method.c Smeso = SBET − Smicro.d The cumulative ultramicropore <0.7 nm surface area measured by CO2 adsorption at 273 K using DFT model.e The total pore volume calculated by single point adsorption at P/P0 = 0.9945.f The micropore volume calculated using the V–t plot method.g The cumulative ultramicropore <0.7 nm volume measured by CO2 adsorption at 273 K using DFT model. | |||||||
HPC | 453.4 | 431.9 | 21.5 | 294.9 | 0.19 | 0.17 | 0.078 |
HPCK-1 | 1825.1 | 1698.7 | 126.4 | 499.9 | 0.81 | 0.68 | 0.122 |
HPCK-2 | 2016.2 | 1871.5 | 144.7 | 337.8 | 0.97 | 0.78 | 0.081 |
HPCK-3 | 1916.2 | 1676.7 | 239.5 | 237.7 | 0.95 | 0.73 | 0.071 |
HPCZn-1 | 1564.8 | 1488.4 | 76.4 | 202.1 | 0.68 | 0.62 | 0.058 |
HPCZn-2 | 2040.4 | 1572.8 | 467.6 | 156.7 | 1.05 | 0.73 | 0.050 |
HPCZn-3 | 1664.5 | 494.2 | 1170.3 | 143.9 | 1.08 | 0.23 | 0.027 |
N,S-HPCK-1 | 1770.7 | 1668.5 | 102.2 | 431.2 | 0.83 | 0.67 | 0.098 |
N,S-HPCZn-1 | 1262.1 | 1189.5 | 72.6 | 189.3 | 0.63 | 0.57 | 0.052 |
As is well known, the post-combustion flue gas streams produced in industrial processes (e.g., fossil fuel-fired power stations) contain a large proportion of CO2. The partial pressure of CO2 in the flue gas streams is typically around 0.15 bar, and it is still a challenge to capture CO2 from post-combustion flue gases. In order to examine the possibility of the resultant samples to act as the CO2 adsorbents for the flue gas streams, we also list the adsorbed CO2 amounts at 298 K under 0.15 bar in Fig. 5. The CO2 uptakes at 0.15 bar vary with the amount of the activating agent. The variation trend at 0.15 bar is slightly different from that at 1 bar. It can draw a conclusion that the CO2 capture amount more rely on the proportion of ultramicropores and pore size distribution of these ultramicropores, and thus the high CO2 uptake amount of 1.29 at 298 K at 0.15 bar for HPCK-1 arises from its prominent ultramicroporosity. More importantly, after modification with nitrogen and sulfur, N,S-HPCK-1 sample exhibits a more superior CO2 capture capacity of up to 1.51 mmol g−1 at 0.15 bar at 298 K, greatly surpassing the uptakes of previously reported carbon-based CO2 adsorbents (Table S1†). Consequently, the currently designed nitrogen and sulfur dual-doped hierarchically porous carbons have excellent potential for capturing CO2 from the post-combustion flue gases.
Fig. 5 The CO2 uptakes of all materials at 298 K under 0.15 bar and the correlation with ultramicropore volume. |
But, the CO2 capture capacities of the resultant materials are strongly correlated with their microporosity of pore size <1 nm, especially the ultramicropore volume between 0.3 and 0.7 nm. Especially, it can be clearly found that the CO2 uptakes at 0.15 bar greatly depend on the ultramicropore volume, and the CO2 capture capacity enhances with the increase of ultramicropore volume (Fig. 6c). Such result suggests that the micro and narrow mesopores were mainly responsible for the high CO2 sorption performance of the materials at low pressure, which is in agreement with previous findings.3,44 By analyzed the data in Fig. 6d, it can be concluded that the best CO2 capture capacity of HPCK-1 should be owed to the largest ultramicropore volume. In addition, HPCZn-x samples exhibit the lower CO2 uptakes compared to HPCK-x samples although they possess the comparative BET surface area, which should be correlated with the lower ultramicropore volumes of HPCZn-x. Thus, to achieve an outstanding CO2 capture capability, it is more important to tailor the ultramicropore size than to have a high surface area for porous carbon-based CO2 adsorbents. But, the CO2 uptakes at 1 bar are also still affected by the BET total surface area, as revealed by Fig. 6a, which should be related to the important impact large micropores and even mesopores on CO2 capture capacity at high pressure region. Therefore, the large proportion of microporosity and suitable ultramicroporosity combined with the largest surface area might be an essential factor for the outstanding CO2 uptake at 273 and 298 K under ambient pressure.
N 1s core spectrum contributes to a further understanding of the types and local environment of the nitrogen atoms modified on the surface of carbon matrix. The deconvolution of N 1s spectrum reveals the bonding of N with C (Fig. 7b), and the N 1s signal can be resolved into four individual peaks centered at binding energies of 398.5, 400.1, 400.9, 403.6 eV, which are related to four different types of nitrogen functional groups, corresponding to pyridinic-N (N–H), amine (–NH2), pyrrolic/pyridone-N (–N–H) and quaternary-N (N+–H), respectively. Apparently, the pyridinic-N and amine nitrogen groups are the most stable and abundant, which has been identified the pyridinic-N or amine nitrogen groups to have stronger interactions with CO2 molecules,29,45 resulting in bringing the more contribution for CO2 capture. In addition, it was also revealed that the introduction of nitrogen into carbon matrix facilitated hydrogen-bonding interactions between the carbon surface and CO2 molecules,30 favoring the improvement of CO2 uptake.
The deconvolution of S 2p core level peak of N,S-HPCK-1 sample displays the peaks located at 163.8 and 164.9 eV corresponding to neutral S, and the peak centered at 168.5 eV assigns to oxidized S (Fig. 7c). Sulfur-containing functional groups in carbon skeleton could facilitate CO2 adsorption owing to the acid interactions of CO2 with neutral sulfur (163.8 and 164.9 eV) and polar interactions of CO2 with oxidized sulfur (168.5 eV).37 As a result, N,S-HPCK-1 exhibits an enhanced CO2 capture capacity than that of HPCK-1. Additionally, to examine the synergistic effect of N and S dual-doping on CO2 capture capacity, a conjecture is made to ascertain whether there is an optimum N/S ratio, which furthest promoting the CO2 capture capacity. As presented in Fig. S6,† unexpectedly, there is no obvious correlation between N/S ratio and CO2 uptake, which could be related with the heterogeneous distribution of nitrogen and sulfur in the surface of carbon matrix and the limited sites for the doping of nitrogen and sulfur groups.
The surface oxygen functionalities on the carbon matrix was also been revealed to contribute to the enhancement of CO2 adsorption.46 The O 1s spectrum shown in Fig. 7d exposes the bonding of N with C, which can be fitted into three deconvoluted peaks located at ca. 531.4, 532.6 and 533.8 eV, respectively. The peak at about 531.4 eV ascribes to the contribution of oxygen in carboxyl groups and the peak at ca. 532.6 eV assigns to the –CO band, and the one centered at ca. 533.8 eV should be related to the band of –C–O–C. The oxygen content detected from XPS is correlated with the CO2 capture capacity of the as-obtained materials (Fig. S7†), and there is no obvious trend between the oxygen content and the CO2 uptake. Moreover, a guess is made to ascertain whether there is an optimum N/O ratio, which favors improving the CO2 uptake. As displayed in Fig. S8,† no correlation is observed with N/O ratio, which could be ascribed to the heterogeneous distribution of oxygen in the surface of carbon framework. Similarly, such no correlation relationship between CO2 capture capacity and the oxygen content was found in porous carbon sorbents.47
Fig. 8 Isosteric heat of CO2 adsorption on all the as-obtained samples calculated from the adsorption isotherms at 273 and 298 K. (a) HPC, HPCK-x and N,S-HPCK-1; (b) HPCZn-x and N,S-HPCZn-1. |
The ideal adsorbed solution theory (IAST) model was employed to evaluate the selectivity for CO2 adsorption from simulated post-combustion flue gas, which has been widely used to predict the selectivity of adsorbents for any two gases in a binary gas mixture by using the isotherms of pure gas. The calculated CO2/N2 selectivity of HPCK-1 and N,S-HPCK-1 samples with IAST model are displayed in Fig. 9c and d. The CO2/N2 ratio is 15/85 in the calculation, representing the typical composition of flue gas. Apparently, the selectivity of HPCK-1 and N,S-HPCK-1 samples significantly decrease in the low pressure region, finally reaching a plateau despite the increase in pressure. The highest selectivity of N,S-HPCK-1 in the low pressure are 72 and 73 at 273 and 298 K, respectively, and the corresponding values are 70 and 69 for HPCK-1 sample. Evidently, the selectivity of N,S-HPCK-1 is superior than that of HPCK-1 under the same conditions, manifesting that the introduction of nitrogen and sulfur groups into the HPCK-1 framework brings an improvement in CO2 capture and CO2/N2 selectivity. Such results should be attributed to the strengthened affinity of CO2 molecule and N,S-HPCK-1 skeleton owing to the more adsorption sites derived from the doping of nitrogen and sulfur. The same result can be found in N,S-HPCZn-1 and HPCZn-1 samples (Fig. S9†), which further confirms the improved effect of nitrogen/sulfur doping in carbon matrix for enhancing CO2/N2 selectivity. Importantly, these values are even greatly higher than most of reported porous carbons and nitrogen-rich carbon materials.55–58 Moreover, it is noticeable that the selectivity has no decrease with the increase of adsorption temperature.
Besides prominent separation performance, the recyclability of an adsorbent is a critical property determining the potential of practical utilization. After the adsorbent was saturated with CO2 up to 1 bar, the adsorbent was recycled. Then, the recycled adsorbent was degassed at the room temperature for 10 min to apply for the next adsorption. The regeneration test of N,S-HPCK-1 sample was conducted for ten consecutive cycles at 273 and 298 K. As presented in Fig. 10a and b, the CO2 uptakes are almost similar without noticeable loss, indicative of the outstanding recyclability of N,S-HPCK-1 with relatively low energy requirement for regeneration. Moreover, such recyclability test suggests CO2 can be completely desorbed from the adsorbent by only changing the pressure. It offers a chance for such adsorbents to be applied in the pressure swing adsorption technology.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c9ra03659h |
This journal is © The Royal Society of Chemistry 2019 |