The increased CO₂ adsorption performance of chitosan-derived activated carbons with nitrogen-doping

Abstract

Highly porous nitrogen-doped activated carbons (NACs) were prepared by the chemical activation of chitosan using alkali carbonates. The NACs exhibited extremely high CO₂ capacities of 1.6 mmol g⁻¹ (15 kPa) and 4.9 mmol g⁻¹ (100 kPa) at 25 °C. Nitrogen atoms doped into carbon frameworks clearly enhanced CO₂ adsorption at low partial pressures.

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An increase in the atmospheric concentration of CO₂ is thought to contribute to global warming. In recent decades, many studies have investigated the reduction of CO₂ emissions by separation and recovery of CO₂ from large point sources such as coal-fired power plants. Absorption into aqueous amine solutions is the most promising technique for CO₂ recovery. However, it has limitations, including high regeneration energy, solvent evaporation/degradation, and equipment corrosion. An alternative technique that overcomes these drawbacks is adsorptive separation using porous materials such as zeolites, carbonaceous materials, siliceous materials, metal–organic frameworks, covalent-organic frameworks and composites.^1–4 Recently, amine-functionalized porous materials have received increasing attention for their properties such as high CO₂ adsorption capacity, good regenerability, low regeneration energy, and moisture tolerance.^1–8 Amine-functionalized materials can be classified into three types of materials, including amine-impregnated (Class 1), amine-grafted (Class 2) and in situ polymerized amine-grafted (Class 3) silica materials.⁹ Our group has focused on the development of Class 1 and 2 materials.^10–16 To date, we have developed highly regenerable amine-based solid sorbents with high CO₂ capacity¹⁶ and evaluated their performance in a laboratory-scale separation test.

More recently, a new class of materials – nitrogen atom incorporated porous carbon materials (N-doped carbons) – have been investigated as effective CO₂ adsorbents.^17–22 Generally, carbon materials have advantages over other materials for CO₂ capture, including low cost, low regeneration energy, high stability, and hydrophobicity. However, carbon materials, including carbon molecular sieves, exhibit low CO₂ adsorption capacity at low CO₂ partial pressure. Consequently, many studies have investigated the enhancement of CO₂ adsorption on carbon materials. The doping of a heteroatom into the carbon framework is an effective method for enhancing CO₂ adsorption. Many studies have reported that N- and O-doping into the carbon framework affects CO₂ adsorption.^17–24 Nitrogen atoms doped into carbon may increase the number of basic sites in the materials, which could enhance CO₂ adsorption. However, it is known that control of the porous structure (i.e., pore size, pore volume and surface area) is more important than N-doping into the carbon framework for enhancing CO₂ adsorption.²⁵ Sevilla et al. reported that N-doping did not affect the CO₂ adsorption capacity over a wide range of CO₂ pressures.²⁶ However, a distinct difference in the CO₂ adsorption capacities of carbon materials with and without heteroatom doping may appear at low CO₂ partial pressure because the interaction between CO₂ and the heteroatom atom doped into the carbon framework is only slightly stronger than CO₂-nondoped carbon interactions.²⁰ Accordingly, incorporating heteroatom atoms into porous carbon with well-controlled pore structures must be effective for increasing CO₂ adsorption.

Many studies have investigated the development of CO₂ adsorbents from waste materials, such as biomass and coal, because of the reduction in the cost of CO₂ capture.^25,27–29 The present study focused on a simple preparation method for nitrogen-doped activated carbons (NACs) from chitosan for CO₂ capture. In addition, the favorable physicochemical properties, such as N content and pore volume, for CO₂ adsorption were investigated.

NACs were prepared by the chemical activation of chitosan. Although hydroxide, particularly KOH, is a well-known activator for preparing high performance activated carbon, it is highly toxic and corrosive. Alternatively, in this study, alkali metal carbonates were employed as mild activators. Chitosan powder (5 g, low molecular weight, Sigma-Aldrich, St. Louis, MO) was dissolved in 95 g of 5% (mass fraction) lactic acid (Junsei Chemical Co., Ltd, Tokyo, Japan) aqueous solution. The chitosan solution was added dropwise into a 20% (mass fraction) alkali metal carbonate (Na₂CO₃, K₂CO₃, Rb₂CO₃ or Cs₂CO₃; Junsei Chemical Co., Ltd) aqueous solution to generate porous chitosan beads. After filtration without washing, the beads were dried overnight in an oven at 100 °C to obtain an alkali-metal-carbonate-doped carbon precursor. The dried beads were placed in a tube furnace, and heated to the target temperature (500–700 °C) at a rate of 10 °C min⁻¹ under a N₂ flow. This chemical activation was performed for 1 h. The product was washed with pure water until the filtrate was neutral. Then, the product was ground into a powder, and dried overnight in an oven at 120 °C. The obtained carbon materials are denoted by NAC-M(T), where M represents the alkali metal of alkali metal carbonate used to generate the carbon precursor and T is the activation temperature. For example, NAC-K(600) represents the product prepared from a chitosan precursor generated in 20% (mass fraction) K₂CO₃ with activation at 600 °C.

The prepared NACs were characterized by N₂ adsorption–desorption at −196 °C. The properties of the NACs are listed in Table 1. The N₂ adsorption–desorption isotherms and pore size distributions determined from the N₂ isotherms are shown in Fig. S1 and S2, respectively, in the ESI.† The N₂ isotherms of the NACs except NAC-Na were Type I isotherms, indicating the NACs had microporous structures. For NAC-Na, they showed Type III and Type IV isotherms, indicating that NAC-Na(500) was almost non-porous and NAC-Na(600) and NAC-Na(700) had micropores and mesopores. The pore size distributions also suggested the presence of micropores. The physicochemical properties indicated that the pore structure and N content could be controlled by the activation temperature and the alkali activator. Those NACs activated at higher temperatures had higher specific surface areas (S_BET) and pore volumes but lower N contents. An increase in the micropore volume with an increase in the activation temperature was also observed in the pore size distributions (Fig. S2, ESI†). NACs with high surface areas (>1000 m² g⁻¹) could be obtained by activation at a relatively low temperature (approximately 600 °C). At the same activation temperatures and activator loadings, the surface areas, pore volumes and pore sizes of the products increased in the following order: NAC-Na < NAC-K < NAC-Rb < NAC-Cs. This might be caused by the degree of reactivity of each alkali metal carbonate.^30–32

Table 1 Physicochemical properties of prepared NACs

	S BET [m^2 g^−1]	V total [cm^3 g^−1]	V micro [cm^3 g^−1]	N content [mmol g^−1]
NAC-Na(500)	10.4	0.04	0.00	5.68
NAC-K(500)	653.1	0.34	0.22	5.09
NAC-Rb(500)	873.9	0.36	0.30	4.02
NAC-Cs(500)	632.7	0.28	0.22	3.74
NAC-Na(600)	474.5	0.32	0.16	5.16
NAC-K(600)	1355.0	0.60	0.47	3.23
NAC-Rb(600)	1496.0	0.63	0.52	3.15
NAC-Cs(600)	1530.2	0.68	0.50	2.59
NAC-Na(700)	1328.3	0.78	0.43	2.24
NAC-K(700)	2044.5	0.96	0.62	1.55
NAC-Rb(700)	2145.3	0.99	0.55	1.61
NAC-Cs(700)	1274.4	0.60	0.26	1.39

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The powder X-ray diffraction patterns of the NACs showed a diffraction peak at around 2θ = 25° for (002), corresponding to a lamellar structure of carbon (Fig. S3 in the ESI†). This peak shifted to higher angle and gradually disappeared as the reactivity of the alkali metal carbonate increased, suggesting that the spacing between the carbon layers, the carbon content, and aromaticity increased. Transmission electron microscopy images of NAC-Rb(600) showed that the NACs had an amorphous layered graphene structure (Fig. 1).

Fig. 1 Low-magnification (a) and high-magnification (b) TEM images of NAC-Rb(600).

The CO₂ adsorption isotherms for NAC-M(600) measured at three different temperatures (0, 25 and 30 °C) are shown in Fig. 2 and Fig. S4 in the ESI.† All CO₂ isotherms showed almost no adsorption–desorption hysteresis, suggesting completely reversible physical adsorption. However, minor hysteresis was observed only in the CO₂ isotherms of NAC-Na(600). This might be because of the existence of narrower pores and the partial chemisorption of CO₂ onto the surface functional groups of NAC (e.g., remaining amino groups).

Fig. 2 CO₂ adsorption isotherms: (a) NAC-Na(600), (b) NAC-K(600), (c) NAC-Rb(600) and (d) NAC-Cs(600).

The CO₂ adsorption capacities of NAC-K(600) and NAC-Rb(600) at 25 °C and 100 kPa were 4.69 and 4.88 mmol g⁻¹, respectively. These values are extremely high compared with previously reported chitosan-derived carbons.^28,33 Therefore, the simple preparation procedure of activator-doped carbon precursor employed in this study gives a high-performance activated carbon. Moreover, a CO₂ capacity of 4.88 mmol g⁻¹ is close to the highest value (5.14 mmol g⁻¹)¹⁷ reported to date for all carbonaceous materials.

Generally, gas physisorption on porous materials is dependent on the porous structure, and materials with large micropore volumes and high surface areas are desirable for superior adsorption performance. As shown in Fig. 3, the CO₂ adsorption capacities of the NACs at 100 kPa depended on the specific surface areas and pore volumes, and particularly the micropore volumes. These results indicate that a physisorption process (i.e., pore-filling mechanism) governs the CO₂ adsorption on NACs. In contrast, the CO₂ capacities at low pressure (1 kPa) were dependent on the N contents of the carbon materials (Fig. 3), and independent of the surface areas and pore volumes. This is because the N contents of the NACs were inversely proportional to the specific surface areas and pore volumes of the NACs (Fig. S5 in the ESI†). In addition, it was obvious that the N content affected the CO₂ capacity at pressures less than 15 kPa (Fig. S6 in the ESI†). However, the lower correlation for the higher pressure suggested a decreased influence with the increase in the CO₂ partial pressure. Those NACs with high N contents exhibited large CO₂ adsorption capacities even with low micropore volumes, suggesting that the doping of N atoms into the carbon framework affects the CO₂ adsorption at low partial pressure. This is caused by acid–base³⁴ and/or hydrogen-bonding¹⁸ interactions between the N-containing functional groups and the CO₂ molecules. When considering CO₂ capture from coal-fired power plants, the CO₂ partial pressure in the flue gas is approximately 15 kPa. The CO₂ adsorption capacities of NAC-K(600) and NAC-Rb(600) at 25 °C and 15 kPa were 1.58 and 1.56 mmol g⁻¹, respectively. Because of the N-doping into their carbon frameworks, these NACs have CO₂ adsorption capacities that are high compared with previously reported carbon materials.³⁵ Further incorporation of N-containing functional groups may enhance CO₂ adsorption at low partial pressures.

Fig. 3 Dependence of CO₂ adsorption capacity of NACs (25 °C) at (a) high partial pressure and (b) low partial pressure.

Isosteric heats of adsorption were calculated from the isotherms at three different temperatures using the Clausius–Clapeyron equation. As shown in Fig. 4, NAC-Na(600) exhibited a relatively high heat of adsorption compared with the other NACs. In addition to the hysteresis loop observed in the CO₂ isotherm, this result suggests that some CO₂ molecules might be chemisorbed on amino groups on the carbon surface. As shown in Fig. 4, a slightly higher heat of adsorption was observed in the first stage of CO₂ adsorption (amount of CO₂ adsorbed <1 mmol g⁻¹) compared with the later stages. The experimentally obtained initial heats of adsorption for the NACs were almost identical to the computationally estimated value (33.57 kJ mol⁻¹) for N-doped carbon,²⁰ indicating that the primary CO₂ adsorption site on the NACs is N-containing functional groups.

Fig. 4 Isosteric heats of adsorption for NAC-M(600).

Therefore, CO₂ physisorption proceeds by a pore-filling mechanism, after CO₂ molecules are selectively adsorbed on the N-containing functional groups of the NACs in the first stage of adsorption. The interaction energy is low enough to allow easy regeneration because it is in the range of the heat of adsorption for physisorption (25–50 kJ mol⁻¹).² The porous NACs prepared in this study show potential for efficient CO₂ capture because of their CO₂ adsorption performance and easy fabrication.

In conclusion, a simple method for the preparation of NAC by the chemical activation of chitosan was developed, and the favorable physicochemical properties of the NACs for CO₂ adsorption were investigated. The preparation method provided high-performance CO₂ adsorbents. The NACs prepared using K₂CO₃ and Rb₂CO₃ exhibited extremely high CO₂ adsorption capacities. Furthermore, it was found that the N contents and pore structures (i.e., micropore volumes and surface areas) of the NACs clearly affected the CO₂ adsorption at low and high partial pressures, respectively. The prepared NACs are potential adsorbents for efficient CO₂ capture because of their high CO₂ capture performance and lower heats of adsorption than those of amine-modified materials.^5,13

This work was supported by the Ministry of Economy, Trade and Industry (METI), Japan.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Details of materials characterization and adsorption experiments, N₂ adsorption–desorption isotherms, pore size distributions and powder X-ray diffraction patterns. See DOI: 10.1039/c5cc06934c

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