CO2 Capture with Activated Carbon Grafted by Nitrogenous

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CO2 Capture with Activated Carbon Grafted by Nitrogenous Functional Groups Changming Zhang,†,‡ Wen Song,§ Guohua Sun,† Lijing Xie,†,‡ Jianlong Wang,*,† Kaixi Li,*,† Chenggong Sun,*,∥ Hao Liu,∥ Colin E. Snape,∥ and Trevor Drage∥ †

Key Laboratory of Carbon Materials, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, P. R. China Graduate University of Chinese Academy of Sciences, Beijing 100049, P. R. China § ShanXi XinHua Chemical Co., Ltd., Taiyuan 030008, P. R. China ∥ Energy and Sustainability Research Division, Faculty of Engineering, University of Nottingham, University Park, Nottingham NG7 2RD, United Kingdom ‡

ABSTRACT: Amino/nitro groups were introduced onto the surface of the activated carbon (AC) with nitration followed by reduction in order to improve its adsorption capacity toward CO2. These AC samples were characterized by N2 adsorption/ desorption, FTIR, and X-ray photoelectron spectroscopy (XPS). CO2 adsorption properties of the samples were investigated using a self-regulating high-pressure adsorption apparatus. Results showed that the contents of nitrogen on the treated samples’ surface increased from 0% to 1.38 after modification. The maximum CO2 adsorption capacity of the modified samples can reach 19.07 mmol/g at 298 K and 36.0 bar. The adsorbed amounts of CO2 on all samples decreased with an increase in the adsorption temperature, but the extent of the decrease with the modified samples was less than that of the raw AC sample. CO2 adsorption capacities of the modified sample for five cyclic adsorption−desorption runs were found to be nearly identical. performance.20−24 Several methods can be used to introduce basic nitrogen functionalities onto the carbon surface. One of the most common procedures is the reaction with nitrogen containing reagents (such as NH3 and amines), while another way is surface impregnation with amine-containing compounds. In addition, amino groups can be grafted on the surface of AC, using techniques such as nitration followed by reduction, anchoring diamines/polyamines, and surface-oriented polymerization of ethylene imine.25−30 Up to now, few have reported on the efficacy of nitrationreduction treatment in enhancing the performance of AC in CO2 capture. In the present work, a facile method of nitration/ reduction was investigated as a means to graft amino groups to the surface of AC. The surface of AC is first tailored with nitro groups via nitration reaction by sulfuric/nitric acid mixture which is one of the basic organic reactions. The surface nitro groups can then be efficiently transformed into amino groups via a simple reduction reaction by acetic acid and iron powder. The textural structures of the modified ACs were characterized by the N2 adsorption isotherm at 77 K. The surface nitrogencontaining groups and the contents of elements were analyzed by Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy analysis (XPS). The adsorption capacity of the modified activated carbons as CO2 adsorbents was investigated at different temperatures and pressures. Furthermore, the potential regeneration of the sorbents and their suitability for application in cyclic processes were also evaluated.

1. INTRODUCTION Carbon dioxide (CO2) is widely accepted as the most important greenhouse gas that causes global warming.1−4 To date, how to capture CO2 effectively has become a world issue. Absorption, adsorption, membrane separation, and cryogenic distillation are examples of the available methods for capturing CO2.5−7 Amine-based regenerative chemical absorption processes using aqueous solutions of amine, such as monoethanolamine (MEA), diethanolamine (DEA), diglycol-amine (DGA), N-methyldiethanolamine (MDEA), and 2-amino-2-methyl-1propanol (AMP) have been widely practiced for several years for CO2 capture from gas streams in natural gas, refinery offgases, and synthesis gas processing. Although absorption by means of amine solutions is the most mature CO2 capture technology in commercial applications, it has serious drawbacks, such as a high energy requirement and corrosion of process equipment.8−12 Adsorption is proposed as one of the most promising options for CO2 capture due to its potential of having a lower cost and less corrosion than amine scrubbing. Among all solid adsorbents, activated carbons (ACs) are well-known adsorbent materials, so it is no surprise that they have been studied in CO2 separation applications. These meso- or microporous carbonaceous structures have an advantage over other adsorbents, such as ease of regeneration, lesser sensitivity to moisture, and high CO2 adsorption capacity at ambient pressure.13−15 CO2 adsorption capacity of AC depends not only on its textural characteristics but also on its surface chemistry, which can be modified with different methodologies.16−19 CO2 is a weak acidic gas, so it has been recognized that the introduction of basic nitrogen functionalities onto the surface of activated carbon can enhance its CO2 capture © 2013 American Chemical Society

Received: March 22, 2013 Revised: June 19, 2013 Published: June 28, 2013 4818

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KBr was used as a reference sample for background measurements. The carbon-KBr mixtures at a ratio of 1:300 were ground in an agate mortar and then pressed under vacuum conditions in a hydraulic press. Before the spectrum of a sample was recorded, the background line obtained was arbitrarily and automatically subtracted. The spectra were recorded from 4000 to 400 cm−1 at a scan rate of 0.2 cm s−1, and the number of interferograms with a nominal resolution of 4 cm−1 was fixed at 100. The spectra were recorded in the range of 400−4000 cm−1 at a resolution of 4 cm−1. 2.5. CO2 Adsorption Measurements. The CO2 adsorption performance of the samples was assessed using a self-regulating highpressure adsorption apparatus as shown in Scheme 1, which was based

2. EXPERIMENTAL SECTION 2.1. Preparation of Activated Carbon. The AC was prepared following the method of Sun et al.31 Petroleum coke, obtained from Guangzhou (China), was crushed, sieved to the desired size (less than 200 μm), and dried at 378 K for 24 h. Then the petroleum coke was subjected to chemical activation with potassium hydroxide (KOH), at a coke/KOH mass ratio of 1:6 at 1103 K for 1 h in a vertical cylindrical furnace under a nitrogen flow. Finally, the obtained product was washed with deionized water, until the pH value of filtrate reached 7 before it was dried at 378 K for 24 h. 2.2. Modification of ACs with Amine Groups. The AC was functionalized with amine groups via a two-step process which involved the use of sulfuric/nitric acid mixture as the nitrating agent in the first step and iron powder as the reducing agent in the second step. To prepare the acid mixture, 60 mL of concentrated sulfuric acid (H2SO4, 98 wt %) was mixed with 54 mL of concentrated nitric acid (HNO3, 65 wt %) in a 500 mL glass beaker to give 114 mL of the acid mixture at a H2SO4/HNO3 volume ratio of 10:9. Then 114 mL of deionized water was slowly added into the concentrated acid mixture to yield 228 mL of diluted acid solution, which was then allowed to cool down to ambient temperature and used as the nitrating agent. However, for comparison purposes, 228 mL of a concentrated H2SO4/ HNO3 acid mixture was also used directly as the nitrating agent. Nitration modification of the selected carbon was performed at 323 K in a three-necked round-bottom flask containing 2 g of the carbon suspended in 80 mL of H2SO4/HNO3 with dropwise addition of the acid mixture via a filling funnel in 60 min. The reaction system was maintained at this temperature for 90 min under stirring conditions. The modified carbon was filtered and extensively washed with deionized water until the filtrate was neutral. Modified carbon thus obtained was then dried at 373 K overnight. The samples obtained from using concentrated and diluted acid mixtures are denoted as ACNO2 (1:1) and AC-NO2 (strong), respectively. Amination of the nitrated carbons was carried out as follows. A total of 250 mL of deionized water and 10 mL of acetic acid were mixed in a three-necked round-bottom flask, followed by the addition of 5 g of iron powder. The mixture was refluxed for 15 min with stirring to help activate elemental iron into ferrous acetate. A total of 1 g of ACs-NO2 was added into the reaction system with continuous refluxing for an additional hour before the system was allowed to cool down to room temperature. The removal of the superfluous iron was achieved by the use of a magnet and aqueous hydrochloric acid (0.01 N) solution for 30 min. Finally, the resultant mixtures were washed with deionized water and dried at 333 K overnight under vacuum conditions. The aminated AC samples are termed AC-NH2 (1:1) and AC-NH2 (strong). 2.3. Textural Properties of the Samples. The textural parameters of the as prepared ACs were investigated by N2 adsorption/desorption at 77 K using an ASAP 2020 instrument. The samples were separately degassed at 523 K in a vacuum environment for 4 h prior to measurements. Experimental adsorption data at a relative pressure (P/P0) less than 0.3 were used to calculate surface area values using the standard Brunauer, Emmett, and Teller (BET) equation. The pore size distribution (PSD) was determined by applying the density functional theory (DFT) method based on the nitrogen adsorption data. 2.4. Chemical Characterization of Samples. The obtained samples were characterized in terms of their surface chemical composition and functionalities due to the modifications. The X-ray photoelectron spectrum (XPS) was measured on a PHI5300 X-ray photoelectron spectrometer. A monochromatic Al Ka source (1486.6 eV) was used at a power of 210 W. The resolution of the instrument is 0.55 and 0.70 eV for Ag 3d and Au 4f peaks, respectively. Survey scans were collected for binding energy ranging from 1100 eV to 0 with an analyzer pass energy of 160 eV with a step of 0.6 eV for a dwell time of 150 ms. For the high-resolution spectra, the pass-energy was 20 eV with a step of 0.1 eV and a dwell time of 200 ms. FTIR spectra of the carbon samples were obtained utilizing a Perkin-Elmer Spectrum 100 FTIR spectrometer. A disk made of pure

Scheme 1. Schematic Apparatus for CO2 Adsorption Experiment

on a volumetric principle. The working principles of the rig and the methodologies used to measure the volumes of reference cell, adsorption cell, and pipeline were the same as previously described.32 The reference cell, which functions as a meter to measure the quantities of gas adsorbed, was placed in a thermostat in order to guarantee the constancy of temperature within 289.15 ± 0.05 K. Pressure in the reference cell was detected by a pressure transducer within a pressure range of 0−4 MPa (PAA-23/8465.1−100, Keller, Switzerland). The adsorption cell, where the adsorption was conducted for the loaded samples, was put in another thermostat maintained at different temperatures. Inching valves V7 and V9 were used to control the gas flow during the charging/discharging process, so as to control the equilibrium adsorption pressure. For a typical experiment, about 5 g of the sample was first dried at 383 K for 24 h before it was subjected to adsorption measurement. High purity helium (99.999%) was conduced to determine the void space of the adsorption cell. Systematic measurements of the adsorption capacity of high purity CO2 (99.999%) on the samples were conduced using the apparatus. CO2 adsorption capacity was evaluated after the adsorption equilibrium was reached. Then, the whole system was vacuumed, and the adsorption cell was heated to 353 K and held at this temperature for 1 h to regenerate the sample.

3. RESULTS AND DISCUSSION 3.1. Textural Properties. Table 1 presents the surface textural properties of the AC samples. It appears that the BET surface areas and total pore volumes of the AC-NO2 (1:1) and AC-NH2 (1:1) became considerably lower at varying degrees compared to those of the original AC, following an order of AC > AC-NO2 (1:1) > AC-NH2 (1:1) > AC-NH2 (strong). The largest change in surface area was observed for the AC-NH2 (strong) with its surface area reduced to only 507 m2 g−1 from 2994 m2 g−1 of the original carbon material used. This drastic decrease may be due to the oxidative ability of the concentrated HNO3/H2SO4 acid mixture used in nitration, which led to excessive oxidation and hence the collapse of the pore 4819

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Table 1. Porous Structure Parameters of the ACs sample AC AC-NO2 (1:1) AC-NH2 (1:1) AC-NH2 (strong)

SBET (m2 g−1)

Smicro (m2 g−1)

Vpore (cm3 g−1)

Vmicro (cm3 g−1)

DP (nm)

2994 2630

509 602

1.76 1.50

0.15 0.20

2.35 2.42

2413

810

1.33

0.24

2.59

507

329

0.30

0.18

3.28

structures of the AC. Similar results have also been reported by Tamai et al.33 Indeed, only slight decreases in surface area were obtained for the AC-NO2 (1:1) and AC-NH2 (1:1) samples where a diluted acid mixture was used as the nitrating agent. Although the formation of nitrogen-bearing functional groups may potentially lead to pore entrance blockage,34 the results presented in Table 1 indicate that the average pore diameter of the modified samples was increased, suggesting that pore widening, as opposed to pore entrance blockage, may play a more important role in determining the surface textural properties of the modified carbons via the nitration/reduction treatment. The nitrogen adsorption−desorption isotherms are shown in Figure 1 for the AC, AC-NO2 (1:1), AC-NH2 (1:1), and AC-

Figure 2. DFT pore size distribution for ACs.

Figure 3. FTIR spectra for the original AC and the modified ACs.

NO2 (1:1) were reduced to amino groups (NH2) by iron powder in aqueous acetic acid solution. For the aminated ACNH2 (1:1) sample, it can be seen that the vibrational peaks of NO2 almost totally disappeared while new bands appeared at 3357 cm−1, 883 cm−1, and 752 cm−1, which correspond to the C−N stretching mode and in-plane and out-of-plane deformation modes of NH2 groups,37−39 respectively. These results indicate that nitro groups can be efficiently converted to amino groups by a simple reduction process. In order to examine the surface functionalities induced by the nitration/reduction treatment, XPS characterization was also conducted for the modified carbons. The XPS spectra (Figure 4) show that besides the strong C(1s) and O(1s) peaks, the

Figure 1. N2 adsorption−desorption isotherms of ACs.

NH2 (strong) samples. It is evident that the amount of N2 adsorbed on the ACs decreases with increasing concentration of acid solutions, with the sample (AC-NH2 (strong)) prepared from using concentrated acid mixture having the lowest capacity for N2 adsorption. The shape of the adsorption isotherms of these samples are overall similar and can be considered as an intermediate to types I and IV, except ACNH2 (strong) where the amount of N2 adsorbed, after a sharp rise at very relative pressures, remained relatively constant with further increasing pressures. Figure 2 shows the DFT pore size distributions for the ACs. It can be seen that the ACs have mainly mesopores with some level of microposity. 3.2. FTIR and XPS Analysis. Figure 3 presents the FTIR spectra of the AC samples, including both the original and surface-modified carbons. The FTIR spectra demonstrate that, compared to the original carbon, the surface-modified carbon with the diluted HNO3/H2SO4 acid mixture at 323 K shows new vibrational bands at 1562 cm−1 and 700 cm−1, which can be assigned to the symmetric and asymmetric stretching of nitro functional groups (NO2).35,36 The nitro groups on AC-

Figure 4. XPS spectra of (a) the AC, (b) AC-NH2 (1:1), and (c) ACNO2 (1:1). 4820

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functionalities which are crucial for efficient CO2 capture with carbon-based adsorbent materials. 3.3. CO2 Capture Measurement. CO2 adsorption behaviors for the AC and modified AC samples have been studied, and the isotherms obtained at 298 K in the range of the pressures from 0 to 36.0 bar are shown in Figure 6. It can be

ones corresponding to N(1s) were clearly evidenced with considerable intensities for both the nitrated and aminated carbon samples. In addition, an Fe (2p) peak was also detected in the aminated carbon (AC-NH2 (1:1)), highlighting the presence of the residual Fe arising from the reduction treatment. Table 2 shows the surface elemental composition Table 2. Surface Composition from XPS Spectra atomic concentration (%) samples

C (%)

O (%)

N (%)

Fe (%)

AC AC-NO2 (1:1) AC-NH2 (1:1)

94.15% 82.69% 80.98%

5.85% 16.11% 15.75%

0.00% 1.20% 1.38%

0.00% 0.00% 1.89%

for these samples, which was obtained from calculations based on the peak areas of individual elements and their sensitivity factors (0.25 for C, 0.66 for O, 0.42 for N, and 2.0 for Fe). It was found that compared to the original carbon, the relative surface C content decreased to 82.69% for the AC-NO2 (1:1) and 80.98% for AC-NH2 (1:1) sample while the relative surface nitrogen content increased from virtually none in the original carbon to 1.20 atom % and 1.38 atom %, respectively. At the same time, the surface oxygen content in the modified carbons was also increased significantly, being indicative of the formation of oxygen functionalities during the nitration process. Although peak deconvolution must be viewed with caution, an attempt to further elucidate the nitrogen functionality was made by decomposing the envelope N(1s) peak into different bands corresponding to different nitrogen functionalities. It is evident from Figure 5(a,b) that a range of nitrogen functionalities of varying significance were present in the modified carbons, such as nitro-type N at 406.30 ± 0.2 eV, pyrrolic and pyridonic N at 400.35 ± 0.3 eV, and pyridine type N at 398.2 ± 0.1 eV. The very strong peak at 399.17 eV, which was only detected in the AC-NH2 (1:1) sample, is typical of amino-type nitrogen associated with a carbon matrix, such as imine, amine, and amide.40−42 It should be noted that the much lower peak intensities at 406.30 and 400.35 eV observed for the aminated sample, AC-NH2 (1:1), suggest that the amino nitrogen was mainly from the reduction of nitro groups followed by pyrrolic and pyridonic N. The characterization results reveal that the nitration/ reduction treatment can be an effective way to functionalize activated carbon materials with desirable basic nitrogen

Figure 6. CO2 capture capacities of the raw sample and modified samples at 298 K.

seen that the CO2 capture capacity of the carbon samples except AC-NH2 (strong) increased markedly with pressure increasing from 0 to 36.0 bar, particularly at the early stage of adsorption with pressures lower than 10.0 bar. The equilibrium adsorption capacities obtained at 298 K and 36 bar for AC-NH2 (1:1), AC-NO2 (1:1), and AC samples were 19.07 mmol/g, 17.34 mmol/g, and 16.67 mmol/g, respectively. The CO2 adsorption capacities of samples in the reported literature were 10 mmol/g at 293 K and 17 bar, 8.4 mmol/g at 298 K and 30 bar, and 10.3 mmol/g at 298 K and 30 bar.43−45 Under the same conditions, the modified AC samples were higher than those reported previously. Clearly, the CO2 adsorption capacities of AC-NO2 (1:1) and AC-NH2 (1:1) are both higher than those of the raw AC sample. Interestingly, however, the adsorbed amounts of CO2 by the aminated carbon (ACNH 2 (strong)), which was prepared from using the concentrated HNO3/H2SO4 acid mixture, were found to be much smaller than those of the raw AC sample. This drastically decreased CO2 capacity obtained for the AC-NH2 (strong)

Figure 5. N 1s peak of (a) AC-NO2 (1:1) and (b) AC-NH2 (1:1). 4821

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examined, being indicative of the higher affinity of the modified carbons for CO2. However, the similar adsorption capacities observed for the original and aminated carbons at the higher temperature of 323 K suggest that the affinity of the aminated carbon, AC-NH2 (1:1), for CO2 is of moderate strength, which is ideal for easy regeneration of the carbon adsorbent materials in practical applications (e.g., pressure or temperature swing adsorption cycles). As well as the adsorption capacity, a stable cyclic operation performance is also a critically important property when evaluating the ultimate performance of an adsorbent for CO2 capture. Figure 8 presents the test results from multiple

sample was believed to be due to the collapse of the porous structures of carbon due to excessive HNO3 oxidation, which led to a great reduction in surface area (Table 1). It is known that the CO2 capacity of carbon materials is not only determined by the pore volume but also by the basic functionalities present on the carbon surface (combination of physical and chemical adsorption).20,46 Indeed, the higher CO2 uptake obtained for the AC-NO2 (1:1) and AC-NH2 (1:1) samples, which had lower pore volumes and BET surface areas than the raw carbon, highlights the importance of the nitrogen functionalities on the surface of the modified carbons as revealed by above FTIR and the XPS characterizations, such as imine, amine, amide, pyrrolic, and pyridonic groups, which are the active sites for CO2 adsorption. Previous studies on CO2 adsorption with amine-impregnated activated carbons have shown that despite the strong basicity of amines, the CO2 retention performance of the impregnated carbons is not as satisfactory as expected and, in some cases, even lower than the original carbons, due to the drastic loss of carbon porosity.47 The results obtained in this work demonstrate that nitration followed by reduction can be an effective way of introducing desirable basic nitrogen functionalities to the carbon surface without compromising significantly the vital porosity of the carbon materials. To study the effect of adsorption temperature on CO2 capture, the CO2 adsorption characteristics of the original and aminated carbons were also examined at different temperatures in the range of pressures from 0 to 36.0 bar. As shown in Figure 7, the CO2 adsorption capacity of both the

Figure 8. Cyclic tests of CO2 adsorption on AC-NH2 (1:1) at 298 K.

adsorption−desorption cycles. It can be observed that the CO2 adsorption of the aminated carbons are fully reversible with superior cyclic stability, and the CO2 adsorption uptake was well maintained at 19.07 mmol/g for each adsorption− desorption cycle conducted at 298 K and 36 bar.

4. CONCLUSIONS A facile method of nitration followed by reduction was examined as an effective way to introduce basic nitrogen functionalities onto the surface of activated carbon. Characterizations of the nitrogen-enriched carbons by FTIR and XPS have confirmed that a range of desirable basic nitrogencontaining functional groups, such as pyrrolic, pyridonic, pyridinic, and, in particular, amino type nitrogen, could be effectively introduced via the nitration-reduction treatment onto the surface of activation carbon, which can greatly improve the affinity of activated carbon for CO2 capture. The modified carbons via the nitration-reduction method demonstrated very high CO2 capture capacities of up to 19.07 mmol/g at 298 K and 36 bar. Due to the moderate affinity of the modified carbons for CO2 retention, the adsorption was fully reversible, and the adsorbents could be easily regenerated with superior cyclic stability. Although further investigations are needed, the results obtained so far tend to suggest that the nonsophisticated nitration and reduction method can be used to effectively improve the performance of activated carbon materials as the adsorbents for CO2 capture, using a pressure or temperature adsorption process.

Figure 7. CO2 capture capacities of the AC and AC-NH2 (1:1) at different adsorption temperatures.

samples was found to be significantly affected by adsorption temperatures. The CO2 adsorption capacity of AC-NH2 (1:1) decreased from 19.07 mmol g−1 at 298 K to 15.05 mmol g−1 at 323 K and 7.28 mmol g−1 at 343 K, while the capacity of the original raw AC decreased from 16.67 mmol g−1 at 298 K to 12.05 mmol g−1 at 323 K and 6.79 mmol g−1 at 343 K, respectively. The observed decrease in the CO2 adsorption capacity with temperature is expected due to the exothermic character of physisorption where both the molecule diffusion rate and the surface adsorption energy increase with temperature.20 Similar behaviors have also been reported for other modified carbons prepared from using different precursors.46 However, the AC-NH2 (1:1) was found to have significantly higher CO2 capacities than the raw AC at all temperatures 4822

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 351 4250292. Fax: +86 351 4250292. E-mail: [email protected] (K.X.L.), [email protected] (J.L.W.), [email protected] (C.G.S.). Notes

Disclosure: The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is financially supported by National Nature Science Foundation of China (No. 51002166, 51172251, and 51061130536), Shanxi Province Science Foundation for Youths (No. 2010021023-3), Shanxi Province International Cooperation Found (No. 2013081016, 2010081031-1), International Cooperation Project of the Ministry of Science and Technology (No. 2010DFB90690), and by the U.K. Engineering and Physical Sciences Research Council (EPSRC, EP/I010955/1 and G063176).

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dx.doi.org/10.1021/ef400499k | Energy Fuels 2013, 27, 4818−4823