Langmuir 2008, 24, 6567-6574
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CO2 Capture by Polyethylenimine-Modified Fibrous Adsorbent Peiyuan Li,† Bingqing Ge,† Sujuan Zhang,† Shuixia Chen,*,†,‡ Qikun Zhang,† and Yongning Zhao† PCFM Lab, OFCM Institute, School of Chemistry and Chemical Engineering, and Materials Science Institute, Sun Yat-Sen UniVersity, Guangzhou 510275, PR China, ReceiVed October 16, 2007. ReVised Manuscript ReceiVed March 27, 2008 This work focuses on developing a novel adsorbent for CO2 capture, by coating polyethylenimine (PEI) on glass fiber matrix and using epichlorohydrin (ECH) as cross-linking agent. The physicochemical properties of the fibrous adsorbent were characterized. The CO2 adsorption capacity was evaluated. Factors that affect the adsorption capacity of the fibrous adsorbent were studied. The experimental results show that this fibrous PEI adsorbent exhibits a much higher adsorption capacity for CO2 compared with another PEI fiber prepared in our previous work, which employed epoxy resin as the cross-linking agent. A CO2 adsorption capacity as high as 4.12 mmol CO2/g of adsorbent was obtained for this fibrous PEI adsorbent at 30 °C, equal to 13.56 mmol CO2/g of PEI, with a PEI/ECH ratio of 20:1. The adsorbent can be completely regenerated at 120 °C.
I. Introduction Global warming, caused by the excess emission of CO2, a product of combustion of fossil fuel and other human activities, has become a world issue.1–6 To reduce the amount of CO2 released into atmosphere, four main methods are being developed for CO2 separation and capture (CCS): solution absorption, adsorption, membrane diffusion, and cryogenic. Among them, adsorption is of great interest for its low energy consumption, low equipment cost, and easiness to be applied. A range of materials have been employed in the research of CO2 adsorption.7–9 Thereinto, materials with large surface area, such as zeolites and activated carbon,10–13 have been widely investigated. However, applications of these porous adsorbents are limited by the low CO2 selectivity and adsorption capacity at higher temperature. Recently, modification with amine groups on the surface of adsorption materials has attracted much attention, which is expected to offer the benefits of the liquid amines in the typical adsorption process.14 Research on amine-modified MCM-41,15–17 SBA-15,18 SBA-16,19 anthracites,20 or activated * To whom correspondence should be addressed. E-mail: cescsx@ mail.sysu.edu.cn. Fax: +86-20-84034027. † PCFM Lab, OFCM Institute, School of Chemistry and Chemical Engineering. ‡ Materials Science Institute.
(1) Song, C. Catal. Today 2006, 115, 2. (2) Chapman, L. J. Transp. Geogr. 2007, 15, 354. (3) Anwar, M. R.; O’Leary, G.; McNeil, D.; Hossain, H.; Nelson, R. Field Crops Res. 2007, 104, 139. (4) Rubin, E. S.; Chen, C.; Rao, A. B. Energy Policy 2007, 35, 4444. (5) Jean-Baptiste, P.; Ducroux, R. Energy Policy 2003, 31, 155. (6) Bachu, S. Energy ConVers. Manage. 2002, 43, 87. (7) Gomes, V. G.; Yee, K. W. K. Sep. Purif. Technol. 2002, 28, 161. (8) Gray, M. L.; Soong, Y.; Champagne, K. J. Sep. Purif. Technol. 2004, 35, 31. (9) Bertelle, S.; Vallie`res, C.; Roizard, D.; Favre, E. Desalination 2006, 200, 456. (10) Sun, Y.; Wang, Y.; Zhang, Y. Chem. Phys. Lett. 2007, 437, 14. (11) Guo, B.; Chang, L.; Xie, K. J. Nat. Gas Chem. 2006, 15, 223. (12) Martin-Martinez, J. M.; Mittelmeijer-Hazelegert, M. C. Langmuir 1993, 9, 3317. (13) Moreno-Castilla, C.; Rivera-Utrilla, J.; Carrasco-Marı´n, F.; Lo´pez-Ramo´n, M. V. Langmuir 1997, 13, 5208. (14) Filburn, T.; Helble, J. J.; Weiss, R. A. Ind. Eng. Chem. Res. 2005, 44, 1542. (15) Xu, X.; Song, C.; Miller, B. G.; Scaroni, A. W. Fuel Process. Technol. 2005, 86, 1457. (16) Xu, X.; Song, C.; Andresen, J. M.; Miller, B. G.; Scaroni, A. W. Energy Fuels 2002, 16, 1463.
carbon21 has been reported. An adsorbent prepared by impregnating MCM-41 in PEI, reported by Xu et al.,17 showed a CO2 adsorption capacity of 2.55 mmol/g. Zhao et al.18 investigated amine-modified SBA-15 with γ-(aminpropyl) triethoxysilane (APTS) and N-β-(aminoethyl)-γ-aminopropyl dimethoxy methylsilane (AEAPMDS), which exhibited CO2 capacities of 0.96 and 1.27 mmol/g, respectively. In a series of research, glass fiber is employed as the matrix for its high surface area, low price, and convenience to be used, using polyethylenimine (PEI) to offer the amine groups. It has been shown previously that the adsorbent exhibits a CO2 adsorption capacity of 2.03 mmol CO2/g of adsorbent, equal to 6.29 mmol CO2/g of PEI, with epoxy resin (EP) as the crosslinking agent.22 A basis for the assessment of the current costs of the CCS technology to be industrialized, provided by the Intergovernmental Panel on Climate Change (IPCC) special report, is estimated to be less than $10 per ton of the captured CO2. Therefore, the amount of adopted CO2 on adsorbent must be more than 2 mmol/g.18 For practical application, a higher CO2 adsorption capacity is needed. And a much higher adsorption capacity is expected to be obtained by making the diffusion of CO2 in the fibrous adsorbent more facile. In our previous work, a fibrous PEI adsorbent was prepared by using epoxy resin as cross-linking agent.22 However, higher molecular weight and larger steric obstruction led to the poor diffusion of CO2 in this adsorbent. Therefore, in this work, epichlorohydrin (ECH), with a lower molecular weight of 92.5, is chosen to act as the cross-linking agent, which has epoxy group that can react with amine groups to form a network structure. The objective of this paper is to investigate the CO2 adsorption capacity of a novel adsorbent coating with PEI on glass fiber, using ECH as the cross-linking agent. The physicochemical (17) Xu, X.; Song, C.; Andresen, J. M.; Miller, B. G.; Scaroni, A. W. Microporous Mesoporous Mater. 2003, 62, 29. (18) Zhao, H.; Hu, J.; Wang, J.; Zhou, L.; Liu, H. Acta Phys.-Chim. Sin. 2007, 23, 801. (19) Knofel, C.; Descarpentries, J.; Benzaouia, A. Microporous Mesoporous Mater. 2007, 99, 79. (20) Maroto-Valer, M. M.; Tang, Z.; Zhang, Y. Fuel Process. Technol. 2005, 86, 1487. (21) Plaza, M. G.; Pevida, C.; Arenillas, A.; Rubiera, F.; Pis, J. J. Fuel 2007, 86, 2204–2212. (22) Li, P.; Zhang, S.; Chen, S.; Zhang, Q.; Pan, J.; Ge, B. J. Appl. Polym. Sci. 2008, 6, 3851.
10.1021/la800791s CCC: $40.75 2008 American Chemical Society Published on Web 05/29/2008
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Figure 1. IR spectra of PG fiber: (1) PG-R5:1; (2) PG-R10:1; (3) PG-R15:1; (4) PG-R20:1; and (5) PG-R25:1.
Figure 2. Thermal stability of PG fibers with different PEI/ECH ratios: (1) PG-R10:1; (2) PG-R15:1; (3) PG-R20:1; and (4) PG-R25:1.
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Figure 3. Effect of PEI/ECH ratio on the water adsorption capacity.
Figure 4. Plot of water adsorption capacity vs coating weight of PGR20:1.
properties of the material are investigated, and the effects of the PEI/ECH ratio, coating weight, moisture, CO2 concentration of the simulated flue gas, and temperature on carbon dioxide adsorption is discussed.
II. Experimental Section 1. Preparation of the Fibrous Adsorbents. Glass fiber (GF), which has a high surface area, was selected as the matrix. The PEImodified glass (PG) fiber was obtained by coating polyethylenimine (PEI) on glass fibers (diameter 10 µm, Changzhou Changhai GERP Products Co., Ltd.) through a wet impregnation method, in which epichlorohydrin (ECH) was employed as cross-linking agent. Waterfree PEI (cat. no .40, 872-7, Aldrich Co.) was dissolved in methanol with a weight ratio of 1:2, and ECH was dissolved in dimethylformamide (DMF) to prepare a 50 wt % solution. The above PEI and ECH solutions were mixed at different ratios of PEI to ECH under stirring, and then 5 g of this solution mixture was added to 20 g of methanol and stirred for 10 min before 1 g of the glass fiber was immersed in. After 6 h, the glass fiber was taken out and dried at 80 °C for 6 h. For comparison, the weight ratios of PEI to ECH ranged from 5:1 to 30:1, and the coating layer weight was 45% to 312%; The PG fibers thus obtained are denoted as PG-Ra:b-Wc, where a and b are the mass ratios of PEI to ECH, and c is the weight percentage of the PEI and ECH coating layer (equal to the mass of PEI and ECH coating divided by the mass of glass fiber). 2. Characterization of the Fibrous Adsorbents. Infrared spectra were obtained in the 4000-800 cm-1 range using a Nicolet 670 Fourier transform infrared instrument with the materials placed on the microsample holder. A Netzch TG 209C thermogravimetric analyzer was employed for termogravimetric analysis (TGA). The experiments were carried out on heating the PG fiber from 50 to 700 °C at a constant rate (20 °C/min) with the flux of the protective gas (nitrogen) and sweeping nitrogen being 20 and 40 mL/min,
Figure 5. Breakthrough curves of CO2 adsorption on PG fibers with different PEI/ECH ratios: (1) PG-R5:1; (2) PG-R10:1; (3) PG-R15:1; (4) PG-R20:1; (5) PG-R25:1; and (6) PG-R30:1.
respectively. The morphology of the materials was observed on a LEITZ Orthoplan Pol large polarizing microscope (LPM) and ZOOM 645S series stereo microscope (SSM). 3. Water Adsorption Measurement. The adsorption capacity for water was calculated according to the differences in the weight before and after the adsorption of water. The PG fiber was outgassed at 80 °C under vaccum for 24 h before adsorption, and then it was placed in a tube, into which a water vapor flow was switched to pass until the weight of the PG fiber remained constant. 4. Carbon Dioxide Adsorption Measurement. The CO2 adsorption performance of the PG fiber was measured by using an Agilent 6820 gas chromatogram. In an adsorption/desorption process, about 1 g of PG fiber was outgassed at 80 °C under vaccum and placed in a sample tube, in which a dry nitrogen flow was introduced to pass at a flow rate of 20 mL/min for 10 min to remove the air
CO2 Capture by PEI-Modified Fibrous Adsorbent
Figure 6. Plot of CO2 adsorption capacities vs PEI/ECH ratios.
Figure 7. Breakthrough curves of CO2 adsorption on PG fibers with different cross-linking agents.
Figure 8. Comparison of CO2 breakthrough curves of PG fibers with different coating weights.
and moisture in the tube. The the dry or moist CO2/N2 mixture gas was then introduced through the tube at a flow rate of 25 mL/min. The flow rate of the gas was controlled by electronic flow control instruments. The moisture was produced by bubbling the gas through the water, and the relative humidity was measured by using a hygrometer. In the CO2 adsorption experiments, relative humidity is controlled to about 80%. The concentration of CO2 in the effluent gas was analyzed at regular intervals, using an Agilent 6820 gas chromatogram. After adsorption, the PG fiber was heated to 120 °C for 30 min to be regenerated.
III. Results and Discussion 1. Characterization of the Fibrous Adsorbents. Figure 1 shows the IR spectra of the PG fibers. The broad adsorption bands at 3700∼2980 cm-1 correspond to the stretching vibrations
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Figure 9. Plot of carbon dioxide adsorption capacity vs coating weight of PG fibers.
of O-H and N-H, which demonstrates that the amine groups have been coated on the glass fiber. The peaks near 2940 and 2828 cm-1 are due to the stretching vibration of methylene. With increasing PEI/ECH ratio, the peaks between 3700 and 2980 cm-1 become stronger, which reveals that the less cross-linked PG fibers contain more amine groups than more cross-linked PG fibers. It is expected that the less cross-linked PG fibers will have higher carbon dioxide capacity than more cross-linked PG fibers. TGA thermograms of the PG fibers with 45% coating weight (PG-W45) are presented in Figure 2. The mass loss profiles are similar for the PG-W45 fibers; all of them show about a 0.2% weight loss at 100 °C, owing to the desorption of CO2, moisture, and some other gases adsorbed. For PG-R20:1(curve 3 in Figure 2), the sharp loss of weight takes place at 230 °C, due to the decomposition of PEI. The temperature at 5% weight loss is 258.81 °C. In other words, PG-R20:1 can keep its thermal stability at 258.81 °C. From room temperature to 400 °C, the PG-R20:1 sample experienced a 12.35% weight loss, while the total weight loss of it at 700 °C is 44.09%. Similarly, PG-R10:1 (curve 1), PG-R15:1 (curve 2), and PG-R25:1 (curve 4) can keep their thermal stability at about 250 °C. 2. Water Adsorption Amount. As shown in Figure 3, the water adsorption capacity increases with increasing of PEI/ECH ratio. For PG-W45 (45% coating weight), with a PEI/ECH ratio ranging from 5:1 to 30:1, the water adsorption capacity increases from 4.50 to 7.96 mmol H2O/g of adsorbent; that is, the value increases from 15.66 to 18.49 mmol H20/g of PEI. The water adsorption capacity of PG fiber decreases with decreasing coating weight (Figure 4). For PG-R20:1-W312 (312% coating weight), the water adsorption capacity is 13.82 mmol H2O/g of adsorbent, equal to 20.98 mmol H20/g of PEI. And the water adsorption capacity of PG-R20:1-W45 (45% coating weight) is 6.69 mmol H2O/g of adsorbent, just 48.41% of PG-R20:1-W312. The adsorption capacity of water depends on the amount of amine groups which can chemically bond with water. With the decrease of the coating weight or the increase of the cross-linking agent dosage, the amount of uncross-linked amine groups deceases and, as a result, the water adsorption capacity decreases. 3. Carbon Dioxide Adsorption. 3.1. Effect of PEI/ECH Ratio on Carbon Dioxide Adsorption Capacity. The CO2 adsorption breakthrough curves of PG-W45 fiber with different PEI/ECH ratios are shown in Figure 5, where C and C0 are the concentration of influent and effluent gas, respectively. It is found that the adsorption rate is rapid at the beginning phase, leading to the approximately zero CO2 concentration of the effluent gas at the beginning stage. Among PG-W45 fibers, the durations of the
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Figure 10. Micrograph images of glass fiber (a) and PG fibers with different coating weights: PG-R20:1-W45 (b); PG-R20:1-W94 (c); PG-R20:1-W168 (d); and PG-R20:1-W312 (e).
breakthrough time of PG-R20:1 (curve 4 in Figure 5) before breakthrough is the longest. And the sharp breakthrough curves of PG-W45 fibers indicate the high utilization of the PG fibers. Their equilibrium CO2 adsorption capacities at dynamic conditions of the adsorbents as shown are all higher than 2mmol CO2/g of adsorbent with a PEI/EP ratio from 15:1 to 25:1 (Figure 6). And with an optimal ratio of 20:1, PG-R20:1 shows a 3.98 mmol CO2/ g of adsorbent adsorption capacity at 30 °C, equal to 13.08 mmol CO2/g of PEI.
As we know, PEI is a compound with plenty of amine groups. The structure of PEI is shown below. There is one amine group in one repeat unit, which has a small molecular weight of 43.
CO2 Capture by PEI-Modified Fibrous Adsorbent
Figure 11. CO2 adsorbed amount of PG adsorbents for different concentrations of CO2.
Figure 12. Plot of carbon dioxide adsorption capacity vs CO2 concentration in the simulated flue gas.
Assuming all of the amine groups can be used to react with CO2, the amount of CO2 adsorbed by 1 g of PEI calculated can achieve 23.6 mmol CO2/g of PEI. PG-R20:1, the optimal sample, exhibits a 54% utilization ratio of the amine compound. PEI is coated on the surface of glass fiber as a thin membrane, which provides a high efficiency of adsorption capacity for CO2. In our previous work, a PEI-coating fiber, using bisphenol-A epoxy resin (EP) as the cross-linking agent, achieved a highest CO2 adsorption capacity at a PEI/EP ratio of 10:1. The breakthrough curve of this PEI-coating fiber (FA-R10:1), compared with that of PG-R20:1 prepared in this paper with ECH acting as the cross-linking agent, is shown in Figure 7. It is obvious that CO2 is completely adsorbed by both of these PEI-coating fibers at the very beginning. And PG-R20:1 fiber (curve 1) showed much more effectiveness in the complete capture of CO2 from influent gas than FA-R10:1 fiber (curve 2). For FA-R10:1, the CO2 equilibrium adsorption capacity is 2.03 mmol CO2/g of adsorbent, equal to 6.29 mmol CO2/g of PEI, with 30% utilization of the amine compound. Whereas PG-R20:1 has an adsorption capacity of 3.98 mmol CO2/g adsorbent, nearly two times of that of FAR10:1. It is obvious that using ECH as the cross-linking agent, instead of epoxy resin, can prominently improve the CO2 adsorption capacity of the PEI-coating fiber. PEI is also used in other research on CO2 sorption. An adsorbent prepared by impregnating MCM-41 in PEI, reported by Xu et al.,17 showed a CO2 adsorption capacity of 2.55 mmol/g. The surface area, pore volume, and pore diameter of the MCM-41 used in their research were 1480 m2/g, 1.0 mL/g, and 2.75 nm, respectively. PEI is loaded in the pore of MCM-41. For MCM41-PEI-50 (loading of 50 wt % PEI), the mesoporous pores were
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Figure 13. Comparison of the adsorbed volume of CO2 from simulated dry and moist flue gas.
completely filled with PEI. The residual pore volume of MCM41-PEI-50 was only 0.011 mL/g, the surface area was estimated to be 4.2 m2/g, and the average pore diameter was smaller than 0.4 nm. The PEI filled in the pores is thick. Only when CO2 has diffused into the inner layer and contacted the inner PEI can the inner PEI be used. However, the diffusion is a slow process which needs a relatively long time. So, only the surface PEI layer (red part in the figure) can be utilized, and the inner PEI layer almost contributes nothing to the adsorption capacity. The high CO2 adsorption capacity of the adsorbent reported in this study is attributed to the high effective surface area of the fibrous adsorbent. In our research, glass fiber is employed as the matrix. The PEI is coated on the surface of glass fiber as a thin membrane. The time needed for diffusion is relatively short, so the PEI can be used more efficiently. The surface area of PG-R20:1-W70 is 3.03 m3/g, obtained by calculating the mass and radius of the glass fiber. At the same time, the nitrogen adsorption isotherm of the fiber has been measured using an ASAP 2020 apparatus. The pore parameters were calculated. The Brunauer-Emmet-Teller (BET) surface area of PG-R20:1-W70 based on the nitrogen adsorption isotherm is 3.65 m2/g, which is close to the value of glass fiber calculated on the radius of glass fiber. It appears that PEI coated on the surface of the glass fiber just forms a membrane with few microspores formed, which contributes little to the surface area. Besides, the PEI membrane may form between the filaments of the glass fiber. Thus, PEI can have more efficient contact with CO2, leading to a higher adsorption speed and a larger adsorption capacity. 3.2. Effect of the Coating Weight. The breakthrough curves of CO2 adsorption of PG-R20:1 with different coating weights are presented in Figure 8. The equilibrium CO2 adsorption capacities of the PG adsorbents with different coating weights are shown in Figure 9. The maximum CO2 adsorption capacity of 13.08 mmol CO2/g of PEI, which is of 54% utilization of amine compounds at 30 °C, is achieved at the lowest coating weight of 45%. The adsorption rate is rapid at the beginning phase, leading to a nearly zero concentration of CO2 in effluent gas for several minutes. And it takes a longer time for PG-R20:1-W45 to be saturated than PG-R20:1-W94. So, it is obvious that the CO2 adsorption capacity does not increase with increasing coating weight. Figure 10 shows the SSM images of PG-R20:1 with different coating weights. Compared to PG-R20:1-W45, the PEI polymer coated on a single glass fiber of PG-R20:1-W94 obviously increased. With further increasing coating weight, a thicker coating layer was formed. Besides, new coating materials were found at the interspace between the filaments of glass fibers. It is obvious that
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Figure 14. Schematic diagram of reactions between primary amine groups and carbon dioxide.
Figure 15. LPM images of fresh PG-R20:1-W45 (a) and the same PG adsorbent after water adsorption for 2 h (b).
Figure 16. Comparison of the adsorbed volume of CO2 from simulated dry and moist flue gas on PEI-modified fiber using epoxy resin as the cross-linking agent.
the benefit of the large surface area of glass fiber was weakened with increasing weight of coating material. The adsorption of CO2 in PG adsorbents is a kinetically controlled process. With increasing coating weight, the diffusion of CO2 became slower and a lower CO2 adsorption capacity was obtained. 3.3. Effect of CO2 Concentration of the Simulated Flue Gas. The influence of CO2 concentration on the adsorption performance was investigated at 30 °C. The CO2 adsorbed amounts of PGR20:1-W45 for CO2 are shown in Figure 11. The curves of Figure 11 show a steeper slope for the CO2 adsorbed amount, which
Figure 17. Plot of CO2 adsorbed amount vs adsorption time at different temperatures.
reveals the rapid rate of adsorption at the first stage. With increasing adsorption time, the slope of the curve decreased, and adsorption equilibrium was obtained in about 30-40 min. With a 24% concentration, the PG-R20:1-W45 sample reached the plateau of maximum adsorption capacity in 30 min. The time spent before adsorption equilibrium is longer with a lower CO2 concentration, indicating a faster kinetics of the adsorption process in a condition of higher CO2 concentration. PG-R20:1-W45 can effectively capture CO2 from gas steam with a CO2 concentration ranging from 8% to 24%. It can completely adsorb and remove CO2 from the simulated flue gas
CO2 Capture by PEI-Modified Fibrous Adsorbent
Figure 18. Comparison of CO2 adsorption capacity at different adsorption temperatures.
Figure 19. Regeneration of PG fiber.
Figure 20. IR spectra of (1) fresh PG-R20:1-W45; (2) spent PG-R20:1-W45; and (3) regenerated PG-R20:1-W45 fibers.
at the beginning; even the initial concentration of CO2 is as high as 24%. At an initial CO2 concentration of 24%, the adsorption capacity is 4.12 mmol CO2/g of adsorbent (13.56 mmol CO2/g of PEI), with a 57% utilization of amine groups. As shown in Figure 12, the adsorption capacity of PG-R20:1-W45 gradually decreases with decreasing CO2 concentration. The time spent to reach equilibrium adsorption is longer at lower CO2 concentrations, compared with that at higher concentration conditions. The absorbent shows a 2.78 mmol CO2/g of adsorbent adsorption capacity at an 8% initial concentration, which is about 32.5% lower than that at the 24% concentration. It reveals that the dynamic adsorption capacity of PG fibers for CO2 changes with the CO2 concentration of the influent stream. However, it is
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notable that PG-R20:1-W45 can keep a high CO2 adsorption capacity even when the CO2 concentration is very low. 3.4. Effect of Moisture. Moisture is an important factor that affects the CO2 adsorption performance. The adsorption breakthrough curves of PG-R20:1-R45 for CO2 from simulated dry and moist flue gas are shown in Figure 13. In the presence of moisture, PG-R20:1-R45 can effectively adsorb CO2. At the beginning of the adsorption process, CO2 was completely adsorbed and the CO2 concentration in effluent gas remained at zero for more than 20 min. However, in dry conditions, the value of C/C0 at the first point was higher than 70%. The dynamic adsorption capacity for CO2 is 3.98 mmol CO2/g of adsorbent in moist conditions, while it is only 0.26 mol CO2/g of adsorbent at dry conditions. The CO2 adsorption capacity for PG-R20:1-R45 in moist conditions is much higher than that in dry conditions, which indicates that the moisture has a promoting effect on the CO2 adsorption for PGR20:1-R45. In the presence of moisture, 1 mol of amine groups can adsorb 1 mol of CO2 in humid conditions to form a carbamate ion (eq 1), which will be hydrolyzed by water and regenerate an amine molecule (eq 2). A schematic diagram of the reaction process is present in Figure 14. However, in dry conditions, the carbamate formed cannot be hydrolyzed, so 2 mol of amine is consumed to adsorb 1 mol of CO2, which will therefore limit the CO2 adsorption capacity. Therefore, the CO2 adsorption capacity in dry conditions is much lower than that in moist conditions.
2RNH2 + CO2 T RNCHOO- + RNH3+
(1)
RNCHOO- + H2O T RNH2 + HCO3-
(2)
Besides, the PG fiber may be swollen in the presence of water, which will promote the adsorption of CO2, for the diffusion of CO2 can be more facile in a swollen PG fiber. However, the LPM images (Figure 15) showed that there is no distinct swollen phenomenon after water adsorption. The diameter of the PGR20:1-W45 is about 4 µm after keeping in saturated water conditions for 2 h, which is almost the same as that of the fresh PG-R20: 1-W45. It reveals that the swollen degree is very small. So, the different chemical reactions are the leading factor for the difference of the CO2 adsorption capacity in dry conditions and wet conditions. Besides, the adsorption in dry conditions is a gas-solid adsorption process, while it is a gas-liquid-solid process in the presence of moisture, which is much more propitious for the diffusion of CO2. We have also examined the improvement of moisture for the adsorption capacity of another PEI fiber that was prepared by using epoxy resin (EP, MW ) 370, Shell Chemical Co., England) as the cross-linking agent; the notable effect of moisture on adsorption capacity for CO2 also exists. The result is compared in the following figure. Figure 16 shows the CO2 breakthrough curve of PEI fiber with a PEI/EP ratio of 10:1, using simulated dry and moist flue gas. In moist conditions, CO2 is adsorbed completely for the first 6 min. With the slow increase of the concentration of the effluent gas, the adsorption reaches equilibrium at last. However, in dry conditions, the initial value of C/C0 is greater than 80%. In total, the CO2 adsorption capacity is 276.96 mg of CO2/g of PEI in moist conditions, while it is only 13.84 mg of CO2/g of PEI in dry conditions. 3.5. Effect of Adsorption Temperature. The CO2 adsorption performances of PG fiber were measured from 30 to 90 °C. The CO2 adsorbed amounts at different times are shown in Figure 17. With a higher temperature, it takes less time to achieve the equilibrium of adsorption, for the diffusion of CO2 can be
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accelerated. PG-R20:1-R45 can completely adsorb CO2 at the beginning with a temperature lower than 70 °C. However, with a higher temperature of 90 °C, it breaks out at the very beginning and about 20% of CO2 permeates. The most efficient adsorption occurs at room temperature, resulting in a 3.98 mmol CO2/g of adsorbent adsorption capacity. And with increasing temperature, the adsorption capacity of PGR20:1-R45 decreases (Figure 18). At a higher temperature of 90 °C, the adsorption capacity drops to 0.92 mmol CO2/g of adsorbent (2.18 mmol CO2/g of PEI), which is only 23.5% of the maximum adsorption capacity, with a 13% utilization of amine groups. It is obvious that a high temperature has a negative effect on the CO2 adsorption performance of the PG fiber, for the adsorption of CO2 into PEI is an exothermic process.23 Besides, with a high temperature, the CO2 molecules are inclined to be in the gas phase. The higher adsorption capacity is expected to be achieved at a lower temperature, for more CO2 molecules will stay in the surface of water, which is propitious for the adsorption of CO2. Even so, the CO2 adsorption capacity is still as high as 2.18 mmol CO2/g of PEI at 90 °C. 3.6. Regeneration of PG Fibers. Excellent regeneration performance of adsorbents is very important for practical applications. CO2 adsorption tests were performed on freshly prepared PG-R20:1-W45 and on spent PG-R20:1-W45 regenerated at 120 °C for 30 min. As shown in Figure 19, the breakthrough curves are similar to each other. Regenerated PG-R20:1-W45 exhibits a 3.68 mmol CO2/g of adsorbent (12.09 mmol CO2/g of PEI) adsorption capacity, which maintains 94% of that of the fresh PG-R20:1-W45. The IR spectra of PG-R20:1-W45 before and after CO2 adsorption, and after regeneration are presented in Figure 20. The broad adsorption band at 3700∼2980 cm-1 corresponds to the stretching vibrations of O-H and N-H, as mentioned above. It is obvious that this peak of the spent PG-R20:1-W45 (curve2), which has been saturated by CO2, is weaker than that of the fresh PG-R20:1-W45 (curve 1), for the amine groups have been consumed (23) Kohl, A.; Nielsen, R. Gas purification, 5th ed.; Gulf Publishing Co.: Houston, TX, 1997.
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in the CO2 adsorption process. However, no distinct difference has been found in the IR spectrum of the regenerated PG-R20: 1-W45 (curve 3) and fresh PG-R20:1-W45, indicating that the CO2 was completely released from the PG fiber after the desorption process. It reveals that the desorption process is complete and the adsorbent is stable in the adsorption and regeneration process.
IV. Conclusion A regenerable polyethylenimine (PEI)-modified fibrous adsorbent, using glass fiber as the matrix, has been successfully developed. Using epichlorohydrin (ECH) as the cross-linking agent in the preparation of the PEI fibrous adsorbent can effectively improve the thermal and adsorption properties of the adsorbent. The formation of a network structure between ECH and PEI can greatly enhance the thermal stability of the adsorbent. Besides, the adsorption performance of PEI can be improved because of the lower molecular weight and smaller steric obstruction of ECH, compared with epoxy (EP) employed in our previous study. Factors that affect the performance of the adsorbent include the thickness of the coating, ratio of PEI to ECH of the coating, moisture, CO2 concentration in the simulated flue gas, and adsorption temperature. The maximum adsorption capacity of this adsorbent for CO2 can reach a value as high as 4.12 mmol CO2/g of total adsorbent weight and 13.56 mmol CO2/g of the PEI mass on the adsorbent at 1 atm, 30 °C, and about 80% relative humidity, with 57% utilization of the amine compound. The advantages of this PEI-modified fibrous adsorbent involve high thermal stability (about 250 °C), high adsorption capacity for CO2, and low regeneration temperature of 120 °C. This fibrous PEI adsorbent is stable in the presence of moisture. Moreover, the adsorption capacity can be promoted by moisture, which is welcomed by most plants. Acknowledgment. This work is supported financially by the Natural Science Foundation of Guangdong Province (5003270) and by the Key Technologies R&D Program of Guangzhou (2006Z2-E0081). LA800791S
