Energy & Fuels 2007, 21, 3555–3559
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Monoethanol Amine Modified Zeolite 13X for CO2 Adsorption at Different Temperatures P. D. Jadhav, R. V. Chatti, R. B. Biniwale, N. K. Labhsetwar, S. Devotta, and S. S. Rayalu* National EnVironmental Engineering Research Institute, Nehru Marg, Nagpur 440 020, India ReceiVed January 23, 2007. ReVised Manuscript ReceiVed July 30, 2007
Zeolite 13X has been modified with monoethanol amine (MEA). MEA loadings of 0.5–25 wt % have been achieved using the impregnation method in different solvents. The mode of incorporation based on methanol with stirring at room temperature appears to be the most feasible. The adsorbent has been characterized for crystallinity, surface area, pore volume, and pore size. The thermal stability of the adsorbent is studied using a thermal analyzer. The CO2 adsorption capacity of adsorbents is evaluated using the breakthrough adsorption method with a packed column on a 10 g scale. The adsorption capacities of adsorbents are estimated in the temperature range 30–120 °C. The adsorbents show improvement in CO2 adsorption capacity over the unmodified zeolite by a factor of ca. 1.6 at 30 °C, whereas at 120 °C the efficiency improved by a factor of 3.5. For adsorption at these temperatures, different MEA loading levels were found to be suitable as per the governing adsorption phenomena, that is, physical or chemical. The adsorbent is also studied for CO2 selectivity over N2 at 75 °C. The MEA-modified adsorbent shows better CO2 selectivity, which was improved further in the presence of moisture.
1. Introduction Increasing CO2 emissions leading to global warming and related climate changes is an established link. The Intergovernmental Panel on Climate Change (IPCC) reported that by the end of this century there could be a global average temperature in the range 1.4–5.8 °C and sea level rise of 9–88 cm.1 To avoid these catastrophic climate changes, stabilization of atmospheric CO2 concentration through postcombustion capture is a potential option. Zeolites, carbon molecular sieves, and activated carbon have been conventionally used for the CO2 separation and capture by selective adsorption. However, the CO2 adsorption capacity of these materials decreases drastically with the increasing temperature.2 In addition, zeolites have very low CO2 adsorption capacity in the presence of moisture that is essentially present in flue gas.3 Amine-treated adsorbents have been successfully used for CO2 adsorption from moist gas at low and moderate temperatures.3–6 The mesoporous materials are reported to have the highest CO2 adsorption capacities of 184 mg/g (4.18 mmol/g) at 25 °C for reformulated immobilized amine sorbent and 151 * Corresponding author. Telefax: +91-712-2247828. E-mail ID:
[email protected]. (1) IPCC Climate Change 2001: The Scientific Basis, IntergoVernmental Panel on Climate Change; Houghton, J. T., Ding, Y., Griggs, D. J., Noguer, M., van der Linden, P. J., Dai, X., Johnson, C. A., Maskell, K., Eds.; Cambridge University Press: Cambridge, U.K., 2001. (2) Siriwardane, R. V.; Shen, M.-S.; Fisher, E. P. Energy Fuels 2005, 19, 1153–1159. (3) Hiyoshi, N.; Yogo, K.; Yashima, T. Chem. Lett. 2004, 33 (5), 510– 511. (4) Gray, M. L.; Soong, Y.; Champagne, K. J.; Pennline, H.; Baltrus, J. P.; Stevens, R. W., Jr.; Khatri, R.; Chuang, S. S. C.; Filburn, T. Fuel Process. Technol. 2005, 86, 1449–1455. (5) Xu, X.; Song, C.; Miller, B. G.; Scaroni, A. W. Fuel Process. Technol. 2005, 86, 1457–1472. (6) Xu, X.; Song, C.; Andresen, J. M.; Miller, B. G.; Scaroni, A. W. Microporous Mesoporous Mater. 2003, 62, 29–45.
mg/g at 100 °C for tetraethylenepentamine (TEPA)-loaded SBA-15.4,7 Also, CO2/N2 selectivity of >1000 is reported for polyethyleneimine (PEI) impregnated MCM-41 at 75 °C.6 However, these materials are not commercially available yet and targeted CO2 sequestration costs may make their application uneconomical in the near future. In this connection, the need for a low-cost CO2 adsorbent has been realized, and a few attempts on aminated fly ash carbon sorbents were made.8,9 These materials owing to their low surface area showed very low CO2 adsorption capacities. Birbara et al. have mentioned zeolites as one of the possible supports for amines.10 Siriwardane et al. have used aminated bentonite and other clays as an alternative to high surface area supports and polymeric materials.11 The effect of NaX zeolite exchanged with polyvalent cations of transition metals on CO2 sorption was also studied.12,13 The metals studied were Zn2+, Cu2+, Ni2+, and Cr3+ at different percentages of exchange. They observed a decrease in CO2 sorption with an increasing number of metal ions. However, all these efforts were restricted to adsorption at room temperature. Zeolites with polar, cationic frameworks and significant surface area and pore volume present a potential option for CO2 capture. Zeolite contains Na+ cations, which impart polarity (7) Yue, M. B.; Chun, Y.; Cao, Y.; Ding, X.; Zhu, J. H. AdV. Funct. Mater. 2006, 16, 1717–1722. (8) Gray, M. L.; Soong, Y.; Champagne, K. J.; Baltrus, J.; Stevens, R. W.; Toochinda, P.; Chuang, S. S. C. Sep. Purif. Technol. 2004, 35, 31– 36. (9) Arenillas, A.; Smith, K. M.; Drage, T. C.; Snape, C. E. Fuel 2005, 84 (17), 2204–2210. (10) Birbara, P. J.; Filburn, T. P.; Nalette, T. A. Regenerable solid amine sorbent., United States Patent 5,876,488, 1999. (11) Siriwardane, R. V. Solid sorbents for removal of carbon dioxide from gas streams at low temperature. United States Patent 6,908,497 B1, 2005. (12) Khelifa, A.; Benchehida, L.; Derriche, Z. J. Colloid Interface Sci. 2004, 278, 9–17. (13) Khelifa, A.; Derriche, Z.; Bengueddach, A. Microporous Mesoporous Mater. 1999, 32, 199–209.
10.1021/ef070038y CCC: $37.00 2007 American Chemical Society Published on Web 09/28/2007
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(charge) on the zeolite surface.2 Zeolites 13X and NaX have been studied for CO2 capture.14,15 This paper addresses synthesis of aminated zeolite adsorbents and their potential for CO2 adsorption on monoethanol amine (MEA)-modified zeolite 13X at high temperature. The adsorbents are studied for CO2 adsorption in the range 30–120 °C. The temperature of 120 °C is important in the targeted application of CO2 removal in postcombustion treatment of flue gas from coal-based boilers of thermal power plants. The typical temperature of thermal power plant flue gas is in the range of 120–160 °C, and it would be desirable to avoid the energy requirement for cooling the gas. Depending on the adsorbent performance and process optimization, adsorption is likely to be carried out in this temperature range in field applications. 2. Materials and Methods a. Materials. Zeolite 13X and monoethanolamine were procured from Merck India, Mumbai. The amine loading method used was impregnation, because the desired levels of loading were not obtained with reflux method. The optimal amine loading level reported in recent literature is equivalent to the pore filling of base material.6,16,17 When pore volume of zeolite 13X is taken as 0.3 mL/g and the density of MEA as 1.0117 g/mL, the weight percent loading of MEA equivalent to pore filling is 23.28%. b. Preparation and Characterization of Aminated Zeolite 13X. Preparation. Zeolite 13X was added to MEA solution of desired MEA concentration (2–50 wt %) in methanol. The solid to liquid ratio was 1:2. This was then kept shaking for 15 min, and the solution was filtered. Filtrate along with the initial MEA solution was analyzed on a gas chromatograph with flame ionization detector (GC-FID) using a Carbowax packed column. The difference in the MEA concentration was expressed as the weight of MEA loaded per unit weight of adsorbent. The adsorbent was then dried at 110 °C for 3 h to remove the solvent. Earlier aqueous solutions of MEA with the shaking time of 4 h were used for the synthesis to achieve targeted loading levels. Because the synthesis time was significantly reduced using MEA solution in methanol, this route was used for further synthesis. Characterization. The characterization was done for zeolite 13XMEA-50 in comparison with unmodified zeolite 13X. The XRD analysis was conducted using Philips Xpert diffractometer with monochromated Cu KR radiation in the 2θ range 5°–60°. The bare (unmodified) and amine-treated 13X zeolite samples were characterized using thermogravimetric (TG) analysis to study their thermal stability and dehydration characteristics. About 20 mg of adsorbent was kept in the TG pan and was heated in air atmosphere from room temperature to 500 °C at a heating rate 5 °C/min. The weight loss and the rate of weight loss (dTG ) dW/dT) were recorded with temperature (°C) on the X-axis. The surface area (SA) and pore volume (PV) were determined using a Quantachrome Autosorb automated gas sorption system. The samples were evacuated at 90 °C (to avoid possible amine degradation), and N2 adsorption was measured at liquid N2 temperature (-196 °C). The results of Brunauer–Emmett–Teller (BET) surface area and pore volumes were obtained by single point adsorption method. c. Evaluation of Adsorbents. CO2 Adsorption Studies. CO2 adsorption studies were carried out using breakthrough curves in a packed column on a 10 g scale. The method offers advantages of determination of dynamic adsorp(14) Siriwardane, R. V.; Shen, M.-S.; Fisher, E. P.; Poston, J. Energy Fuels 2001, 15, 279–284. (15) Ishibashi, M.; Ota, H.; Akutsu, N.; Umeda, S.; Takija, M.; Izumi, J.; Yasutake, A.; Kabata, T.; Kageyama, Y. Energy ConVers. Manage. 1996, 37, 6–8, 929–933. (16) Filburn, T.; Helble, J. J.; Weiss, R. A. Ind. Eng. Chem. Res. 2005, 44, 1242–1546. (17) Franchi, R. S.; Harlick, P. J. E.; Sayari, A. Ind. Eng. Chem. Res. 2005, 44, 8007–8013.
JadhaV et al. tion capacities and evaluation in a practical manner, that is, packed bed, flow system, etc. In a typical experimental protocol, 10 g of moisture-free adsorbent (oven-dried at 120 °C for 3 h) was packed in a glass column (diameter ) 9 mm, height ) 30 cm). The adsorbent was held in place with ceramic wool at the top and bottom of the adsorbent bed. The experimental set up, described elsewhere, had a feed section with mass flow controllers to regulate the gas flows and a mixing chamber. Adsorption column was held in a PID temperature controller equipped furnace.18 Upstream of the column was a sample selector valve and gas chromatograph for the analysis. The analysis was done on a thermal conductivity detector (TCD) using Porapak-Q packed column. The adsorbent was first pretreated at 140 °C for 6 h under He gas at a flow rate of 20 mL/min. It was then cooled to the desired adsorption temperature. The flow of gas was then changed to feed gas containing 15 vol % CO2 in He balance. The feed flow rate was kept at 52 mL/min (CO2 ) 8 mL/min and He ) 44 mL/min). The adsorption was continued until saturation, that is, CO2 concentration in column effluent gas equals that in the feed. Adsorption capacities are expressed in milliliters of CO2 adsorbed per gram of adsorbent at standard temperature and pressure (STP) conditions. CO2 SelectiVity of the Adsorbents. CO2 selectivity of the adsorbents was studied using a binary mixture of 15 vol % CO2 in N2 prepared by setting the flows from mass flow controllers. Prior to the adsorption study, the preweighed adsorbent was pretreated in a He flow of 20 mL/min for 4 h at 140 °C. Then it was cooled to adsorption temperature, 75 °C. The binary mixture was then passed over the pretreated adsorbent, and the outlet was continuously monitored by GC-TCD using packed column Porapak-Q. In the results, since N2 breakthrough occurred very early, only the saturation capacities are compared, that is, till the total adsorption capacity of adsorbents was used up. To study the effect of moisture on adsorption capacity of modified adsorbent, in that particular experiment, the binary feed was passed through distilled water. This is a reported method to introduce moisture in the feed, because the feed gas stream takes up moisture equivalent to its saturation capacity at the experimental conditions.
3. Results and Discussion a. Synthesis and Characterization of Aminated Zeolite 13X. The actual loading levels for the initial MEA concentration 2, 10, and 50 wt % from aqueous solution were 0.5, 2.9, and 25.0 wt %. These are designated as zeolite 13X-MEA-2, 13XMEA-10, and 13X-MEA-50, respectively. From the methanol solution, only the optimal loading level adsorbent, that is, 13XMEA-50 was synthesized and the loading level was 18.7 wt %. This particular adsorbent was used for characterization and adsorption and selectivity studies at 75 °C. It was concluded from the XRD analysis that the major peaks are retained in the MEA-modified zeolite 13X (Supporting Information). As can be seen from Figure 1 depicting TGA data, both the adsorbents show weight loss till 400 °C. As reported earlier, zeolites show dehydration till 350 °C.2 However, this temperature is also subject to the heating rate applied in the experimental run. Because the heating rate used in this experiment was higher than the above reported study, the completion of dehydration observed is at comparatively higher temperatures. The pre-adsorbed moisture along with other volatiles (including methanol that was used for wetting of the zeolite before amine loading) and other atmospheric contaminants were desorbed initially in all the samples. The bare zeolite 13X was also wetted with methanol (for comparison with MEA-modified zeolite from MEA solution in methanol) and both the samples were studied (18) Jadhav, P. D.; Rayalu, S. S.; Biniwale, R. B.; Devotta, S. Curr. Sci., 2007, in press.
MEA-Modified Zeolite 13X for CO2 Adsorption
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Figure 2. Adsorption capacities at 30 °C.
Figure 1. TG of zeolites 13X and 13X-MEA-50. Table 1. BET Surface Area, Pore Size, and Pore Volume of Adsorbents sr. no.
adsorbent
surface area (m2/g)
pore volume (cm3/g)
avg pore size (Å)
1 2
13X-commercial 13X-MEA-50
615.5 9.15
0.34 0.059
11 11
using TG without predrying. Because all the zeolites have affinity for moisture, it slowly desorbs even after 250 °C, which seemed to be complete at about 450 °C. As can be seen in the case of 13X, there is only one “step” with continuous weight loss from room temperature to 450 °C. Then the weight stabilized after a total weight loss of 18.73%. The maximum weight loss rate is observed at 130 °C. The entire weight loss can be attributed to desorption of methanol and the moisture. In the case of 13X-MEA-50, a total of three distinct weight loss steps are observed with distinct weight loss at 70, 150, and 240 °C. Because MEA has a boiling point of 170.8 °C, the second weight loss between 120 and 200 °C is probably due to volatilization and degradation of MEA. The adsorbent shows a weight loss of 5.63% in this region and a total of 22.56%, which is 4% higher than the unmodified adsorbent. The results of BET surface area and pore volumes obtained by single point adsorption method are presented in Table 1. Zeolite 13X shows a specific surface area (SA) of 615.5 m2/g and a pore volume (PV) of 0.34 cm3/g. This surface area is on the lower side of the reported values for commercial samples (Zeolite 13X, manufacturer Zeochem, Inc., SA ) 710 m2/g, PV not reported, and pore diameter ) 10 Å).2 This is due to the difference in pretreatment/degassing conditions. To avoid volatilization of the impregnated MEA, degassing temperature of both the samples was restricted to 90 °C. After modification by MEA, SA reduced, as expected, to 9.15 m2/g and PV to 0.059 cm3/g. These results correlate with the pore filing effect of MEA and also confirm that the MEA was loaded into the pores of zeolite 13X. The trend matches with the published results.5 Good correlation is observed between SA and PV for the two adsorbents analyzed. Both SA and PV for the modified adsorbent decrease by the same fraction as compared to the base material. This confirms the incorporation of MEA in the zeolite pores. The pore size of the zeolite 13X and 13X-MEA-50 was determined by using nonlinear density functional theory (NLDFT). The pore size of bare 13X had the highest peak at about 11 Å indicating its ordered pore structure. This has not changed after MEA modification, suggesting that the “pore openings/ mouths” are mostly intact. However, as expected, the pores are partly filled showing a drop in the pore volume of material in
each pore size range. When the cumulative pore volume is plotted as a function of pore size, the theory that there is no change in pore openings is proved, because there are no points below 11 Å. In addition, the drop in the pore volume is about 82% of the initial value. b. Evaluation of Aminated Zeolite 13X. CO2 Adsorption Studies. Figure 2 presents the comparison of results of CO2 adsorption at 30 °C for various adsorbents, namely, zeolite 13XMEA-2, 13X-MEA-10, 13X-MEA-50, and unmodified zeolite 13X. Here, 10 g of each zeolite was pretreated at 140 °C in He flow (20 mL/min) and exposed to feed gas (15 vol % CO2 in He balance at 52 mL/min). As can be seen from Figure 2, for adsorption at 30 °C, 13XMEA-10 has shown the highest breakthrough (BT) adsorption capacity of 44 mL/g compared with 28 mL/g of unmodified zeolite 13X. This proves enhancement in adsorption of acidic gas CO2 by introduction of basicity on various matrices such as mesoporous silicate based materials like MCM-41 and SBA15 and carbon molecular sieves with amine loading as reported by many researchers.5,10,19,20 A small MEA loading (0.5 wt %) has CO2 adsorption capacity almost unchanged. But even with a very much higher loading (25 wt %), capacity has decreased to 17.5 mL/g. This can be attributed to reduced surface area and pore volume and restricted access to adsorption sites for CO2 at higher loadings. For adsorption at room temperature, where adsorption is mostly physical in nature, higher surface area and pore volumes are essential. However, as observed in the characterization study, both the surface area and pore volume of the modified zeolite were reduced significantly. For adsorption at 120 °C, 10 g of each zeolite was pretreated at 140 °C in He flow (20 mL/min) and exposed to feed gas (15 vol % CO2 in He balance at 52 mL/min). The results of CO2 adsorption studies at 120 °C are presented in Figure 3. At 120 °C, higher loading (13X-MEA-50) has resulted in the highest adsorption capacity of 14 mL/g compared with 4 mL/g of unmodified zeolite and 6.6 mL/g for 13X-MEA-10. The higher adsorption capacity at 50% loading despite reduced pore volume and lower surface area indicates the role of the hybrid mechanism at higher temperatures, unlike at 30 °C, where physisorption is dominant. At 120 °C, combined adsorption–absorption may be playing role in sorption of CO2 wherein the MEA molecule in the pore of the zeolite functions as a (19) Birbara, P. J.; Filburn, T. P.; Michels, H. H.; Nalette, T. A. Sorbent system and method for absorbing carbon dioxide (CO2) from the atmosphere of a closed habitable environment. United States Patent No. 6,364,938, 2002. (20) Chang, A. C. C.; Chuang, S. S. C.; Gray, M.; Soong, Y. Energy Fuels 2003, 17, 468–473.
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JadhaV et al. Table 3. Comparative Breakthrough Adsorption Capacities adsorption capacity (mL/g) adsorption temperature (°C)
zeolite 13X
zeolite 13X-MEA-50
30 50 75 120
28 18 8.1a 4
17.5 8.1a 10.1a 14
a Values estimated after pressure drop correction to CO and He flow 2 rates.
Figure 3. Adsorption capacities of modified 13X at 120 °C. Table 2. C, H, and N Content of the Adsorbent 13X-MEA-50 ser. no. 1 2 3
cycle of adsorption for 13X-MEA-50 first cycle second cycle third cycle
C content (mg/g)
H content (mg/g)
N content (mg/g)
37.1 48.8 47.3
27.5 24.1 23.4
18.8 10.0 11.5
solvent contained in a reactor providing better contact between CO2 and MEA. Thereby it overcomes typical problems of absorption of CO2 in MEA like lower heat and mass transfer rates, liquid handling, corrosion, etc. This finding is consistent with the published results.5,6 Reusability of the Adsorbent at 120 °C. Out of all the adsorbents studied, 13X-MEA-50 showed the best adsorption capacity at 120 °C. Therefore, it was studied for reusability with desorption up to 140 °C in the same run followed by pretreatment at 140 °C. It was seen that the adsorbent successfully retained adsorption capacity in three consecutive reuse cycles. There is slight loss of capacity after the first cycle probably owing to some irreversible adsorption in the first cycle that could not have been desorbed at the selected pretreatment temperature. C, H, and N Content of the Adsorbent after Repeated Cycles of Adsorption. In order to confirm the stability of 13XMEA, particularly the stability of amines on the zeolite matrix, adsorption cycles were repeated three times. C, H, N analysis was carried out after each adsorption cycle, and the results are given in Table 2. It was observed that the carbon content initially increased after the first cycle and then stabilized at 47.3 mg/g. This may be due to a some irreversible adsorption of CO2, resulting in the possible formation of carbonates. A decline was observed in the hydrogen and nitrogen content of the adsorbents after the first cycle. The hydrogen content was largely the same after the second and the third cycle, at about 23–24 mg/g, whereas for nitrogen it was about 10–11.5 mg/g. This indicates that some loss of MEA has taken place in the first cycle and no further loss was observed in the subsequent cycles. Comparison of CO2 Adsorption at Different Temperatures. CO2 adsorption was studied at temperatures of 30, 50, 75, and 120 °C on 13X and 13X-MEA-50. Here, 10 g of each zeolite was pretreated at 140 °C in He flow (20 mL/min) and exposed to feed gas (15 vol % CO2 in He balance at 52 mL/min). As can be seen from Table 3, CO2 breakthrough adsorption capacity of zeolite 13X drops drastically with the increasing temperature. For zeolite 13X-MEA-50 at 30 and 50 °C, the adsorption capacities are lower than that of unmodified zeolite. At these temperatures, physisorption is a dominant process and adsorption capacity is directly proportional to surface area
of adsorbent. With MEA loadings, surface area of zeolite was reduced significantly as discussed earlier. This explains the decreased adsorption capacity of 13X-MEA-50 at 30 °C despite amine loading. As the adsorption temperature is increased, chemisorption becomes dominant and aminated zeolite adsorbs more CO2 than unmodified zeolite. For increase in temperature from 30 to 50 °C, the adsorption capacity decreased by a factor ca. 0.5. For increase in temperature from 50 to 75 °C, adsorption increased by a factor of ca. 1.2, and for further increase from 75 to 120 °C, adsorption increased by a factor of ca. 1.4. At elevated temperatures, the kinetics of the reaction becomes significant increasing the adsorption capacity with the temperature. The results are in agreement with earlier reports.6,21 13XMEA-50 adsorbs more CO2 at 75 than at 50 °C. At 50 °C, there was no sharp breakthrough indicating slow physisorption followed chemisorption at active sites. Whereas at 75 °C, more sites may be active because chemisorption is an activated process.22,23 Optimal adsorption at 75 °C is also reported earlier.6 According to the study, the affinity sites inside the particles can react with CO2 only when CO2 is diffused into the particles. This is termed as a (kinetically) diffusion-controlled process. Different optimal adsorption temperatures for different amines on aminated adsorbents are also observed earlier.21 In the case of CO2 capture from flue gas, which itself is at higher temperature, it is important to have higher adsorption capacity even at higher temperatures, and amine-loaded 13X shows promising results. CO2 Selectivity Studies. CO2 SelectiVity of Bare 13X. Here, 5 g of zeolite 13X bare was pretreated at 140 °C in He flow (20 mL/min) and exposed to feed gas (15 vol % CO2 in N2 balance at 60 mL/min). As reported by many studies, the adsorption capacity of zeolite 13X is found to be higher for CO2 than for N2.2,14 The saturation adsorption capacities of zeolite 13X for CO2 and N2 are 18.6 and 8.1 mL/g, respectively (Figure 4). As discussed earlier, this is due to the different electrical properties of the adsorbate. The CO2 selectivity (of an adsorbent defined as the ratio of number of moles of CO2 by N2 in adsorbed phase to that in the feed) is also calculated for the adsorbents in present study. For zeolite 13X bare, the CO2 selectivity over N2 is calculated to be 13.18. CO2 SelectiVity of 13X-MEA-50. Here, 5 g of zeolite 13XMEA-50 was pretreated at 140 °C in He flow (20 mL/min) and exposed to feed gas (15 vol % CO2 in N2 balance at 60 mL/ min). The CO2 and N2 adsorption capacity of zeolite 13X-MEA50 is observed to be 18.1 and 5.5 mL/g, respectively (Figure 5). Under similar conditions, zeolite 13X bare had adsorption (21) Maroto-Valer, M. M.; Andresen, J. M.; Zhang, Y.; Lu, Z. DeVelopment of fly ash deriVed sorbents to capture CO2 from flue gas of power plants; Final Technical Progress Report; The Energy Institute, The Pennsylvania State University, University Park, PA, 2005. (22) Klepel, O.; Hunger, B. J. Therm. Anal. Calorim. 2005, 80, 201– 206. (23) Bulow, M. Adsorption 2002, 8, 9–14.
MEA-Modified Zeolite 13X for CO2 Adsorption
Figure 4. Breakthrough curve of CO2 and N2 over zeolite 13X.
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Figure 6. Breakthrough curve of CO2 and N2 over zeolite 13X-MEA50 in moist stream.
dry stream) to 15.3 mL/g. The adsorption capacity for N2 is also substantially lower at 0.4 mL/g, which means further increase in the CO2 selectivity of the adsorbent. The CO2 selectivity over N2 is estimated to have increased to 188.4. 4. CO2 Adsorption Mechanism
Figure 5. Breakthrough curve of CO2 and N2 over zeolite 13X-MEA50.
capacities of 18.6 and 8.1 mL/g, respectively. This means, after amine modification, the CO2 adsorption capacity of the zeolite was very similar to that of bare zeolite from binary mixture. However, the N2 adsorption capacity of the modified adsorbent had decreased; in other words, the CO2 selectivity increased over N2. The CO2 selectivity of zeolite 13X-MEA-50 over N2 is 18.46, which is slightly higher than that of bare zeolite 13X. The breakthrough for N2 shows a hump (C/C0 > 1). This is the typical nature of a breakthrough curve where competitive adsorption is taking place. Initially, N2 being in higher concentration occupies more sites. With time, kinetic selectivity for CO2 plays a role, and the adsorbed N2 is displaced making space for CO2. Thus, the outlet concentration of N2 shows a hump of concentration over that in the feed. Effect of Moisture on CO2 SelectiVity of 13X-MEA-50. Here, 5 g of zeolite 13X-MEA-50 was pretreated at 140 °C in He flow (20 mL/min) and exposed to moisture-saturated feed gas (15 vol % CO2 in N2 balance at 60 mL/min). Because the flue gas from thermal power plants also contains 3–4 vol % moisture, it can interfere with the CO2 adsorption over the zeolite-based adsorbents. The decrease in the CO2 adsorption capacities of zeolites is also observed because zeolites have good affinity for moisture too.3,17,24 Therefore, aminemodified zeolite 13X-MEA-50 was subjected to a watersaturated feed stream with composition as earlier experiments. As can be seen from Figure 6, the CO2 adsorption capacity in moist stream decreases slightly (compared with that in the (24) Brandani, F.; Ruthven, D. M. Ind. Eng. Chem. Res. 2004, 43, 8339. (25) Khatri, R. A.; Chuang, S. S. C.; Soong, Y.; Gray, M. Energy Fuels 2006, 20, 1514–1520.
The mechanism of CO2 adsorption on amine-modified adsorbents has been reported in many recent publications. Gray et al. have proposed reaction sequences for primary, secondary, and tertiary alkanolamines reacting with dissolved CO2 by formation of amine carbamates and carbonates.8 Solid amine CO2 sorbents were also expected to have similar reactions with airborne CO2, water vapor, and the amine functional group on its surface. Also, they have done DRIFTS studies to support the proposed adsorption mechanism. This is further supported by in situ infrared (IR) studies during CO2 adsorption on aminated adsorbents.8,25 IR peaks corresponding to carbamate and bicarbonate species formed by reaction of CO2 with amine functional groups were observed. Further studies are in progress to elucidate the mechanism. 5. Conclusion Monoethanol amine modified 13X zeolite adsorbents have been synthesized, and they have been evaluated for CO2 adsorption. The adsorbents show improvement in CO2 adsorption capacity over the unmodified zeolite by a factor of ca. 1.6 at 30 °C, whereas at 120 °C, the efficiency improved by a factor of 3.5, thus emerging as suitable adsorbents for capture of CO2. The MEA-modified adsorbent also emerged as a potential adsorbent for CO2 in the presence of moisture. The performance of the adsorbent was also satisfactory in repeated cycles of adsorption. Acknowledgment. This work has been undertaken in the National Thermal Power Corporation (NTPC) sponsored project S-3-1392 and CSIR Network Project No. CORE-08 (1.1). The authors also take this opportunity to sincerely acknowledge JNARDDC, Nagpur, and Blue Star India Ltd, Mumbai, for providing their valuable assistance in characterization studies conducted in the course of this work. Supporting Information Available: Details of XRD analysis for bare and modified zeolites. This material is available free of charge via the Internet at http://pubs.acs.org. EF070038Y
