Research Article pubs.acs.org/journal/ascecg
Enhanced CO2 Capture Capacity of Nitrogen-Doped Biomass-Derived Porous Carbons Jie Chen,† Jie Yang,† Gengshen Hu,‡ Xin Hu,*,† Zhiming Li,† Siwei Shen,† Maciej Radosz,§ and Maohong Fan§ †
College of Chemistry and Life Sciences and ‡Zhejiang Key Laboratory for Reactive Chemistry on Solid Surfaces, Institute of Physical Chemistry, Zhejiang Normal University, 688 Yingbin Avenue, Jinhua, Zhejiang 321004, PR China § Department of Chemical and Petroleum Engineering, University of Wyoming, 1000 East University Avenue, Laramie, Wyoming 82071, United States S Supporting Information *
ABSTRACT: Nitrogen-doped porous carbons obtained from coconut shell by urea modification and KOH activation are found to exhibit very high CO2 uptake at 1 bar, almost 5 mmol g−1 at 25 °C and over 7 mmol g−1 at 0 °C, respectively. The high CO2 uptake of the sorbent can be ascribed to its high microporosity and nitrogen content. In addition, these sorbents possess high CO2/N2 selectivity, stable cyclic ability, high initial heat of CO2 adsorption, fast adsorption kinetics, and high dynamic CO2 capture capacity under simulated flue gas conditions. When combined with the low cost of the coconut-shell precursor, these properties make them exceptionally attractive sorbent candidates for CO2 capture. KEYWORDS: CO2 adsorption, Nitrogen-doped porous carbons, Urea modification, KOH activation, Renewable biomass resource
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INTRODUCTION Recently, increasing concerns about climate change and environmental protection are motivating new research in CO2 capture technologies.1 The state-of art technology among the strategies to capture CO2 from flue gas is amine scrubbing.2 The disadvantages of this technique, however, include large energy expenditure, corrosion issues with the equipment, solvent degradation, and potentially harmful waste. To circumvent such problems, solid sorbents that selectively adsorb CO2 under ambient conditions (25 °C, 1 bar) have been investigated.3 The challenge is to develop sorbents with high CO2 uptake capacity, high CO2/N2 selectivity, and high sorption/desorption rates. Examples of new sorbents that attract research attention are carbons,4,5 zeolites,6 metal− organic frameworks (MOFs),7 porous polymers,8,9 and ionic liquids.10 The porous carbonaceous materials are especially promising because they are inexpensive and easy to prepare. They have superior chemical, thermal, and mechanical stability, their pore structure is easily designed, they are easy to regenerate, and particularly worth mentioning is their relative insensitivity to water vapor.11 Among precursors used for synthesis of porous carbons, biomass feedstocks, such as coconut shell,12 are competitive because they are not only abundant, sustainably renewable, and cost-effective but also happen to have excellent CO2-sorption capacities. For example, even at pressures as low as atmospheric, coconut shell-based porous carbons were found to adsorb as much as 4 mmol/g of CO2 at 25 °C and 6 mmol/g of CO2 at 0 °C,12 which is attributable to small pores, say less than 1 nm in diameter.13−17 © XXXX American Chemical Society
However, promising, even higher capacities of porous carbons are needed to reduce the CO2 capture cost. One clue, originally inspired by the exceptional amine selectivity for CO2, is to modify the solid sorbent surface with nitrogen functional groups.18−20 Therefore, the goal of this work is to demonstrate that the CO2 capacity of microporous-carbon sorbents derived from coconut shell can be substantially improved by modifying their surface with nitrogen-containing material, such as urea. Toward this end, a simple urea nitrogenation and KOH activation of carbonized coconut shell are used to develop novel, nitrogen-doped porous carbon adsorbents for CO2 capture. The effect of sorbent porous texture and surface properties on their CO2 sorption properties is characterized.
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EXPERIMENTAL SECTION
Carbonization. Coconut shells to be used as a precursor have been cleaned, powdered, and sieved into the particles ranging in size between 100 and 140 mesh (105−150 μm). Carbonization was performed in a horizontal quartz tubular reactor in a flow of 200 mL/ min nitrogen. The precursors were heated to 500 °C at a rate of 5 °C/ min and then held for 2 h. Afterward, the samples were cooled down in the nitrogen atmosphere. To simplify, carbonized coconut shell was denoted as C. Modification of Carbonized Coconut Shell by Urea. A mixture of carbonized coconut shell (C) and urea (weight ratio 1:1) was heated in air at 350 °C for 2 h. The collected samples were rinsed with Received: November 4, 2015 Revised: January 2, 2016
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DOI: 10.1021/acssuschemeng.5b01425 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
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hot distilled water a few times to get rid of any unreacted urea. Finally, the resulting products were put into the oven to dry at 120 °C overnight. The resulting sample was denoted as NC. KOH Activation. In a typical preparation, 2 g of NC were combined with a solution that contained 6 g of KOH. After having been stirred vigorously for 6 h, the mixture was left overnight to dry at 120 °C in an oven. Afterward, the samples were activated to 650 °C for 1 h. During the activation process, the heating rate is 5 °C/min, and the nitrogen flow rate is 400 mL/min. Following activation, all the sorbents were rinsed with distilled water until the pH value of the filtrate was roughly 7. The wet samples were then dried at 150 °C under vacuum for 24 h. The as-synthesized samples were designated as NC-X-Y, where X represents the activation temperature and Y represents for the ratio of KOH to NC. For example, the abovedescribed sample can be denoted as NC-650-3. For comparison, a control sample was synthesized by direct KOH activation of carbonized coconut shell and denoted as C-650-3. This sample was further modified by urea as described above, and the resulting sorbent was designated as C-650-3-N. Characterization. X-ray diffraction (XRD; PHILIPS PW3040/60) was performed using CuKα radiation (λ = 0.15406 nm). Scanning electron microscopy (SEM, Hitachi S-4800) was employed to examine the morphology of the carbon samples. Transmission electron microscopy (TEM, JEOL-2100F) was operated at 200 kV. Fourier transformed infrared (FTIR) spectra were collected on a Nicolet 670 FTIR spectrometer with potassium bromide pellets. Nitrogen adsorption/desorption isotherms were determined by physisorption at −196 °C in a Beishide 3H-2000PS2 sorption analyzer. The specific surface area (SBET) was calculated using the Brunnauer-Emmett-Teller (BET) method, using adsorption data in the relative pressure range between 0.005 and 0.05. The total pore volume (V0) was determined from the amount of nitrogen adsorbed at a relative pressure of P/P0 = 0.99, and the micropore volume (Vt) was determined according to the t-plot method. The pore size distributions (PSDs) were estimated via the Harvath−Kawazoe (H−K) method using N2 adsorption data. To avoid diffusional problems of N2 molecules at 77 K inside the narrow micropores, CO2 adsorption isotherms at 0 °C were used to evaluate narrow microporosity (size 0.1, characteristic of microporous materials. In addition, no hysteresis loops are found in the isotherms in the entire P/P0 range. The plot poresize distribution of representative carbons achieved by the Harvath−Kawazoe (H−K) method (Figure 3d) suggests that D
DOI: 10.1021/acssuschemeng.5b01425 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
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Figure 4. CO2 adsorption isotherms at 25 °C (empty symbols) and 0 °C (filled symbols) for nitrogen-doped carbons prepared under different conditions (a) 600 °C, (b) 650 °C, and (C) 700 °C, and (d) isosteric heat of CO2 adsorption on NC-600-4, NC-650-3, NC-700-3, and NC-700-4 calculated from the experimental adsorption isotherms at 0 and 25 °C.
capacity of 5.4 mmol/g at 25 °C and 1 bar.32 In addition, a table (Table S2), which summarizes the carbon materials with similar or higher CO2 uptakes than our samples, is given in the Supporting Information. When we further explore the effect of porous texture and nitrogen content of the adsorbents on their CO2 adsorption capacity, we find that the CO2 uptake at 25 °C and 1 bar did not have any direct correlation to any of the porous properties characteristics (i.e., SBET, V0, Vt, or Vn) or nitrogen content. For example, NC-650-3, which has intermediate SBET, V0, Vt, and Vn, as well as nitrogen content, exhibits the highest CO2 uptake among all the samples. By contrast, NC-700-4, which has the most developed microporosity but the lowest nitrogen content, and NC-600-2, which owns the highest nitrogen amount but the least developed microporosity, both show lower CO2 uptake than NC-650-3. These results suggest that both porous properties and nitrogen amount work together in determining the superior CO2 adsorption for these carbons. It should be noted that NC-650-3 shows higher CO2 capture capacity than the control sample i.e. C-650-3, which could possibly be ascribed to the nitrogen-containing framework and highly developed narrow microporosity of NC-650-3. To investigate the effect of urea modification before and after KOH activation
CO2 Adsorption Properties. CO2 adsorption experiments are carried out using a Beishide 3H-2000PS2 sorption instrument under ambient pressure at 25 and 0 °C (Figure 4). The sorbent materials developed in this work are found to exhibit excellent CO2 capture capacities ranging from 4.0 to 4.8 mmol/g at 25 °C and 5.6 to 7.0 mmol/g at 0 °C under atmospheric pressure. Among all the samples, NC-650-3 has the highest CO2 capture capacity of 4.8 mmol/g at 25 °C. This capacity is higher than most of the known nitrogen-decorated porous carbons,30,31 MOFs,7 COFs,8 and PAFs9 at 25 °C and 1 bar. For instance, Wang and co-workers synthesized N-doped templated carbons, that exhibited a maximum CO2 adsorption capacity of 4.0 mmol/g.30 Sevilla et al. reported that their nitrogen-doped activated carbons synthesized from polypyrrole achieved a CO2 uptake of 3.9 mmol/g.31 The MOF-177, MOF210, and IRMOF-1 capacities were found to be around 1.0 mmol/g.7 Under the same conditions, a series of COFs8 synthesized by Yaghi et al. were found to have the CO2 capacities of less than 3 mmol/g. PAFs9 developed by Qiu’s group were found to have the CO2 capacities of 1.1−1.8 mmol/ g. However, our results are still lower than those of recent high N-containing porous carbon materials developed by Sayari and co-workers, which possess the maximum CO2 adsorption E
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carbonized coconut shell are found to exhibit enhanced CO2 capacity, reasonable selectivity relative to nitrogen, easy regeneration, and excellent kinetic properties. When combined with the low cost of the coconut-shell precursor, these properties suggest promising sorbent materials for industrial applications.
on CO2 adsorption, we compared the CO2 adsorption capacity between NC-650-3 and C-650-3-N. It is found that NC-650-3 shows much higher CO2 capture capacity than C-650-3-N. Although C-650-3-N has a higher nitrogen content than NC650-3, it possesses much lower values in each porous properties characteristics (i.e., SBET, V0, Vt, or Vn). The descent of the porous properties parameters of sample C-650-3-N is due to the demolition of walls between pores or pore-blocking by the abundant nitrogen species introduced on urea modification. This seems to suggest that porous properties play a more important role than the content of nitrogen in determining the CO2 uptake of these sorbents. For complete characterization, the Clausius−Clapeyron equation was applied to the adsorption isotherms at 25 and 0 °C to calculate isosteric heat of adsorption (Qst)33 (Figure 4d). The Qst value in the initial adsorption stage (low CO2 uptake) for the studied carbons is found between 29−33 kJ mol−1, which is higher than previously reported values for typical carbonaceous adsorbents.4,27,34,35 These values are far below the energy required to break covalent bonds, which suggests an easily reversible, low-energy desorption. We also note that Qst decreases with increasing CO2 loading and reaches a near plateau, which suggests the heterogeneity of CO2 binding energies in the pores. In addition, high initial Qst values suggest strong adsorbate−adsorbent interactions between CO2 molecules and the nitrogen-containing carbon framework, which favors their enhanced adsorption. For large scale CO2 capture applications, the adsorbent should also have high CO2/N2 selectivity, good stability during adsorption−desorption cycles, as well as fast adsorption kinetics. N2 adsorption on NC-650-3 shows a very low uptake of 0.65 mmol g−1 at 25 °C and 1 bar, indicating that NC-650-3 can be used for selective capture of CO2 from the combustion flue gas (Figure S4). The selectivity of CO2 over N2 gas, calculated from the ratio of the initial slopes of N2 and CO2 adsorption isotherms, is found to be 19 for NC-650-3. This is better than or comparable to those reported in the literature.36−39 The CO2/N2 selectivity for NC-650-3 is also calculated using the ideal adsorption solution theory (IAST)40 for the gas mixtures (CO2: N2 = 0.10:0.90) at 25 °C. The corresponding value is calculated to be 15, indicating its potential in realistic postcombustion capture of CO2. However, it should be mentioned that this CO2/N2 selectivity is still lower compared with some previous reported results.41 CO2 adsorption−desorption cycles conducted at 25 °C show only a 2.5% drop in CO2 adsorption capacity after 5 cycles indicating that this material has promising regenerability and stability (Figure S5). As for adsorption kinetics of CO2 on NC-650-3 at 25 °C, the CO2 capture rate is found to be very high, with around 95% of the CO2 being adsorbed within 6 min (Figure S6). Finally, the most promising sorbent material, NC-650-3, is characterized in continuous-flow breakthrough experiments to determine its dynamic CO2 capture capacity from a CO2/N2 (10:90 v/v) gas mixture. Its dynamic CO2 capture capacity is found from the CO2 breakthrough curve to be 1.1 mmol/g (Figure S7), confirming its potential in capturing CO2 from flue gas. In the practical application of flue gas treatment, the lowpressure CO2 capture capacity (0.1−0.2 bar) is more important than that at 1 bar.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.5b01425. Schematic diagram of the carbon dioxide capture system, X-ray diffraction (XRD) patterns, FT-IR spectrum, N2 adsorption isotherm, CO 2 adsorption kinetic, CO2 breakthrough curve under simulated flue gas condition, and CO2 cyclic adsorption study of NC-650-3 (PDF)
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AUTHOR INFORMATION
Corresponding Author
*Phone: 86-151-0579-0257. Fax: 86-579-8228-8269. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors acknowledge the support of the Zhejiang Provincial Natural Science Foundation (LY15B060004), the NSF of China (21106136), and the National Undergraduate Training Program for Innovation and Entrepreneurship of China (201510345015), as well as collaboration with the University of Wyoming.
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CONCLUSION To summarize, the porous nitrogen-doped carbons developed in this work by urea modification and KOH activation of F
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