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Enhancing Sorption Performance of Solid Amine Sorbents for CO2 Capture by Additives
Available online at www.sciencedirect.com
Energy Procedia 37 (2013) 205 – 210
GHGT-11
Enhancing Sorption Performance of Solid Amine Sorbents for CO2 Capture by Additives Zhonghua Zhanga, Baodong Wanga*, Qi Suna , Xiaoliang Mab,c a
Nationcal Institute of Clean-and-Low-Carbon Energy, P.O. Box 001 Shenhua NICE, Future Science & Technology city, Changping, Beijing, 102209, China b Petrolem Reserch & Studies Center, Kuwait Institue for Scientific Reserch, P.O. Box 24885 Safat, 13109, Kuwait c EMS Energy Institute Pennsylvania State University, 209 Academic Projects Building, University Park, PA 16802, USA
Abstract
In order to improve the CO 2 sorption performance of solid amine sorbent, the additives has been introduced into polyethylenimine (PEI) for reducing the diffusion resistance for CO 2 molecular. Several additives have been added into the PEI in the sorbent to examine their effects on the sorption capacity and the sorption rate. The sorption performance of the sorbents with different additive in PEI was evaluated by using TGA, and the results show that only some additives can significant increase the sorption capacity, but not all of them. In order to understand how the additives improved the sorption performance, the viscosity of PEI with and without the additives was measured in the presence and absence of CO2. It was found that the decrease in viscosity after adding the additives may be one of the reasons for facilitating the diffusion of the sorbed CO2 molecules from the surface of PEI into the bulk of PEI, probably due to weakening the interaction of the CO2 molecule with the amine group. The additives may also provide some channels for CO2 moving into deeper PEI films, and thus increase the total accessible sites and improve the efficiency of amine. And this may be another reason for additives enhance sorption performance.
© 2013 The TheAuthors. Authors.Published PublishedbybyElsevier Elsevier Ltd.Ltd. Selection and/orpeer-review peer-reviewunder underresponsibility responsibility GHGT Selection and/or of of GHGT
Keywords: Additives; CO2 capture; Solid Amine Sorbent;
* Baodong Wang. Tel.: +86-01-5733-9633; fax: +86-01-5733-9649-4107. E-mail address: [email protected]. * Qi Sun. Tel.: +86-5733-9628; fax:+86-5733-9649-4210. E-mail address: [email protected].
1876-6102 © 2013 The Authors. Published by Elsevier Ltd.
Selection and/or peer-review under responsibility of GHGT doi:10.1016/j.egypro.2013.05.103
206
Zhonghua Zhang et al. / Energy Procedia 37 (2013) 205 – 210
1. Introduction CO2 capture and storage from flue gas have been considered as a key option for mitigation of greenhouse gas impact [1~3]. Among various approaches reported for CO 2 capture, the most mature technology on the industrial scale is the amine scrubbing process currently [4]. However, this process has several problems, including high-energy consumption, low-absorption/desorption rate, corrosion of the amine solution, and degradation of the amine solution in cycles. Compare to the liquid amine scrubbing process, the sorption on the solid amine sorbents, especially the supported-PEI sorbents, has deemed to be a promising alternative technology for CO2 capture due to its high capacity, no or less corrosion, higher sorption/desorption rate and lower energy consumption. Currently, there are two types of the solidsupported amine sorbents according to their preparation methods. The sorbents of type 1 are prepared through the formation of the covalent bonds between the amine-containing compounds and the surface of the solid support [5~10]. The sorbents of type 2 are prepared by loading the amine containing polymer or oligomer, such as polyethylenimine (PEI) or oligoethylenimine, on the nanoporous materials using a simple wet-impregnation [11~15]. Although the supported-PEI sorbents give the high capacity, the efficiency of amine is only about 35%. This is because of the high kinetic barrier for diffusion of the sorbed CO 2 molecules from surface into the bulk of PEI. In order to further increase the CO 2 sorption capacity of the sorbents and enhance the sorption performance, it is necessary to reduce the CO2 diffusion resistance in the PEI bulk in the sorbent. In the present study, several additives with different amounts have been introduced into the PEI in the sorbent to examine their effects on the sorption capacity and the sorption rate. The additive may reduce the viscosity of PEI, and the decrease in viscosity after adding the additives may facilitate the diffusion of the sorbed CO2 molecules from the surface of PEI into the bulk of PEI, probably due to weakening the interaction of the CO2 molecule with the amine group. The additives may also provide some channels for CO2 moving into deeper PEI films, and thus increase the total accessible sites and improve the efficiency of amine to enhance sorption performance. 2. Experimental 2.1. Preparation of sorbent samples The sorbent samples were prepared by a wet-impregnation method. In a typical preparation, the desired amount of PEI and additives were dissolved in 20 mL of ethanol under stirring for ~60 min to make a mixture solution, and then desired amount of the silica gel after vacuum drying at 108 oC was added to the mixture solution. The slurry was continuously stirred at room temperature for about 5 h, allowing the ethanol in the slurry to be evaporated. After ethanol was mainly evaporated, the sample was further dried in a vacuum oven at 40 oC, 1 kPa for 12 h, and then was sealed in a bottle before use. The sorbent samples with different PEI and additive loading amount were denoted as PEI(x)/SG, where x is the weight percentage of PEI-loading amount in the sorbent. 2.2. Evaluation of CO2 sorption/desorption performance The CO2 sorption/desorption performance of the sorbent was evaluated by using TGA (NETZSCH). A typical analysis is described as below: About 50 mg of the sorbent was placed at the TGA sample pan. The sample as first heated from room temperature to 100 °C at a rate of 10 °C/min under a pore N 2 flow with a flow rate of 60 ml/min, and was kept at 100 °C for 60 min to remove the remained solvent, the sorbed CO2 and moisture from the sample. The sample was cooled down to the desired sorption
Zhonghua Zhang et al. / Energy Procedia 37 (2013) 205 – 210
temperature and kept at this temperature for 30 min. The sorption was stated by switching the flow as from pore N2 to 10 v% CO2 at the same flow rate and hold at the sorption temperature for 30 min for CO 2 sorption. After the sorption, the flow gas was switched back to the pore N 2 and the temperature was increased to 100 °C for desorption. The sorption capacity of the sample was calculated according to the mass change during sorption. 2.3. Measurement of viscosity and density of PEII additive mixture The measurement of the mixture of PEI and additive viscosity were using the capillary viscometer at different temperature. The liquid density of mixture of PEI and additive was measured by using liquid density meter (METTLER TOLEDO DM40) at different temperature. 3. Results and Discussion 3.1. Effect of sorption temperature on sorption capacity of PEI(50)/SG Effect of the sorption temperature on the sorption capacity of PEI(50)/SG was examined using TGA with 10 v% CO2 gas at 30, 50, 65, 75, 85 and 100 oC, respectively. The results are shown in Figure 1. At 30 oC, the CO2 sorption capacity of PEI(50)/SG was 81.2 mg-CO2/g-sorbent. With increasing temperature, the CO2 sorption capacity became higher and reached the highest capacity of 127.2 mgCO2/g-sorbent at 75 oC. Continuously increasing temperature from 75 to 100 oC, the sorption capacity decreased to 118.3 mg-CO2/g-sorbent. The best capacity was obtained at 75 oC, similar to the results observed for PEI(50)/MCM-41[14] and PEI(50)/SBA-15 [12]. The results indicate that at the temperature range from 30 to 75 oC, the diffusion of CO2 in the PEI may be a sorption rate-control step. Consequently, elevating temperature results in increase of the number of accessible amine sites in the sorbent, and, thus, increases the CO2 sorption capacity. When the temperature is higher than 75 oC, the thermodynamics may play a more important role, thus, resulting in decrease of the sorption capacity with increasing temperature.
Fig. 1. The effect of sorption temperature on CO2 sorption capacity of PEI/SG
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3.2. Effect of additives on sorption capacity of PEII additive/SG Figure 2 showed the sorption capacity and sorption rate of additive-plus sorbents. Six additives were introduce into sorbents to investigate the promotion of the sorbents, including piperazine (PPZ), NFormylmorpholine (NFM), Poly(melamine-co-formaldehyde) acrylated (PMA), triethanolamine (TEA), Poly(melamine-co-formaldehyde) methylated (PMG) and N-methyl pyrrolidinone(NMP). The sorption capacity of sorbent was tested by TGA under 10 v % CO2 flow gas at 75 oC, and the PEI and additive amount in sorbents were 45% and 5% respectively. The results were shown that PPZ and NFM improve the CO2 sorption capacity of sorbent by 12.4% and 11.5% respectively. Both PPZ and NFM contained six-membered ring structure, and this structure seemed to provide a channel for CO2 diffusion from the surface of PEI into the deeper films. In other words, these additives promote the CO2 diffusion process in the bulk of PEI film, leading the higher capacity of the sorbents. However, other additives reduced the CO2 sorption capacity due to its structure may not provide a pathway for CO2 diffusion and reduce the accessible site of amine group. But all of additive promoted the CO2 sorption rate of solid amine sorbent.
Fig. 2. The effect of additives on CO2 sorption capacity and rate of PEI-additive/SG
3.3. Density and viscosity of PEII additive mixture
Fig. 3. The viscosity of PEI-additive mixture before and after sorbing CO2
Zhonghua Zhang et al. / Energy Procedia 37 (2013) 205 – 210
The viscosity of PEI before and after reacting with CO 2 was measured by the capillary viscometer at 40, 60 and 75 oC, and the results were shown in Figure 3. The dramatic viscosity increases after sorbing CO2 of PEI, by 44.9%, 35.0% and 17.7% at 40, 60 and 75 oC respectively, has been seen, which increases the diffusion resistance for CO2 and reduces the accessible sorption site of PEI film due to the CO2 molecular harder getting into the deeper PEI film. The density and viscosity of PEI-additive mixture were measured at 40, 60 and 75 oC, and the results were shown in Figure 4. The density and viscosity of PEI-additive mixture decreased at all temperature, favoring the CO2 diffusion into the bulk of PEI. That may be the reason why the additive can enhance the sorption performance of solid amine sorbents.
(a)
(b)
Fig. 4. The viscosity (a) and the density (b) of PEI-additive mixture
4. Conclusion A new direction of enhancing sorption performance of solid amine sorbents through improving the utilization efficiency of amine groups for CO2 capture was provided. The additive was introduced into the polymer amines to increasing the accessible CO2 sorption site, and the deeper polymer amine films f could be used due to the diffusion resistance decreasing. It is leading to enhance CO2 sorption performance of solid amine sorbents. However, further investigation in fundamental understanding of diffusion mechanism of the sorbed CO2 in PEI bulk at the molecular level is desired for improving the sorption performance of the solid amine sorbents. Acknowledgements This work was supported by 863-project of China , and partly support by U.S. Department of Energy, National Energy Technology Laboratory (NETL) through DOE Grants: DEFE0000458. References [1] Sheppard MC, Socolow RH. Carbon-Constrained World by Rapid Commercialization of Carbon Capture and Sequestration. AIChE J 2007;53:3022 3028.
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[2] Song CS. Global challenges and strategies for control, conversion and utilization of CO2 for sustainable development involving energy, catalysis, adsorption and chemical processing. Catalysis Today 2006;115:2 32. [3] Haszeldine RS. Carbon capture and storage: How green can black be? Science 2009;325:1647 1652. [4] Rochelle GT. Amine Scrubbing for CO2 Capture. Science 2009;325:1652 1654. [5]Zheng F, Tran DN, Busche BJ, Fryxell GE, Addleman RS, Zemanian TS, Aardahl CL. Ethylenediamine-modified SBA-15 as regenerable CO2 sorbent. Ind Eng Chem Res 2005;44:3099 3105. [6]Hiyoshi N, Yogo K, Yashima T. Adsorption characteristics of carbon dioxide on organically functionalized SBA-15. Micropor Mesopor Mater 2005;84:357 365. [7]Liang Z, Fadhel B, Schneider CJ, Chaffee AL. Stepwise growth of melamine-based dendrimers into mesopores and their CO2 adsorption properties. Micropor Mesopor Mater 2008;111:536 543. [8]Knofel C, Descarpentries J, Benzaouia A, Zelenak V, Mornet S, Llewellyn PL, Hornebecq V. Functionalised micro/mesoporous silica for the adsorption of carbon dioxide. Micropor Mesopor Mater 2007;99:79 85. [9] Wei J, Shi J, Pan H, Zhao W, Ye Q, Shi Y. Adsorption of carbon dioxide on organically functionalized SBA-16. Microporous Mesoporous Mater 2008;116:394 399. [10]Serna-Guerrero R, Dana E; Sayari A. New Insights into the Interactions of CO2 with Amine-Functionalized Silica. Ind Eng Chem Res 2008;47:9406 9412. [11]Son WJ, Choi JS, Ahn WS. Adsorptive removal of carbon dioxide using polyethyleneimine-loaded mesoporous silica materials. Micropor Mesopor Mater 2008;113:31 40. [12]Ma XL, Wang XX, Song CS. Molecular basket sorbents for separation of CO2 and H2S from various gas streams. J Am Chem Soc 2009;131:5777 5783. [13]Xu XC, Song CS, Andresen JM, Miller BG, Scaroni AW. Novel polyethylenimine-modified mesoporous molecular sieve of MCM-41type as high-capacity adsorbent for CO2 capture. Energy Fuels 2002;16:1463 1469. [14]Xu XC, Song CS, Miller BG, Scaroni AW. Adsorption separation of carbon dioxide from flue gas of natural gas-fired boiler by a novel nanoporous molecular basket Fuel Process Technol 2005;86:1457 1472. [15] Zhang ZH, Ma XL, Wang DX, Song CS, Wang YG. Development of Silica-Gel-Supported Polyethylenimine Sorbents for CO2 Capture from Flue Gas. AIChE J 2012;58:2495 2502.
Energy Procedia 37 (2013) 205 – 210
GHGT-11
Enhancing Sorption Performance of Solid Amine Sorbents for CO2 Capture by Additives Zhonghua Zhanga, Baodong Wanga*, Qi Suna , Xiaoliang Mab,c a
Nationcal Institute of Clean-and-Low-Carbon Energy, P.O. Box 001 Shenhua NICE, Future Science & Technology city, Changping, Beijing, 102209, China b Petrolem Reserch & Studies Center, Kuwait Institue for Scientific Reserch, P.O. Box 24885 Safat, 13109, Kuwait c EMS Energy Institute Pennsylvania State University, 209 Academic Projects Building, University Park, PA 16802, USA
Abstract
In order to improve the CO 2 sorption performance of solid amine sorbent, the additives has been introduced into polyethylenimine (PEI) for reducing the diffusion resistance for CO 2 molecular. Several additives have been added into the PEI in the sorbent to examine their effects on the sorption capacity and the sorption rate. The sorption performance of the sorbents with different additive in PEI was evaluated by using TGA, and the results show that only some additives can significant increase the sorption capacity, but not all of them. In order to understand how the additives improved the sorption performance, the viscosity of PEI with and without the additives was measured in the presence and absence of CO2. It was found that the decrease in viscosity after adding the additives may be one of the reasons for facilitating the diffusion of the sorbed CO2 molecules from the surface of PEI into the bulk of PEI, probably due to weakening the interaction of the CO2 molecule with the amine group. The additives may also provide some channels for CO2 moving into deeper PEI films, and thus increase the total accessible sites and improve the efficiency of amine. And this may be another reason for additives enhance sorption performance.
© 2013 The TheAuthors. Authors.Published PublishedbybyElsevier Elsevier Ltd.Ltd. Selection and/orpeer-review peer-reviewunder underresponsibility responsibility GHGT Selection and/or of of GHGT
Keywords: Additives; CO2 capture; Solid Amine Sorbent;
* Baodong Wang. Tel.: +86-01-5733-9633; fax: +86-01-5733-9649-4107. E-mail address: [email protected]. * Qi Sun. Tel.: +86-5733-9628; fax:+86-5733-9649-4210. E-mail address: [email protected].
1876-6102 © 2013 The Authors. Published by Elsevier Ltd.
Selection and/or peer-review under responsibility of GHGT doi:10.1016/j.egypro.2013.05.103
206
Zhonghua Zhang et al. / Energy Procedia 37 (2013) 205 – 210
1. Introduction CO2 capture and storage from flue gas have been considered as a key option for mitigation of greenhouse gas impact [1~3]. Among various approaches reported for CO 2 capture, the most mature technology on the industrial scale is the amine scrubbing process currently [4]. However, this process has several problems, including high-energy consumption, low-absorption/desorption rate, corrosion of the amine solution, and degradation of the amine solution in cycles. Compare to the liquid amine scrubbing process, the sorption on the solid amine sorbents, especially the supported-PEI sorbents, has deemed to be a promising alternative technology for CO2 capture due to its high capacity, no or less corrosion, higher sorption/desorption rate and lower energy consumption. Currently, there are two types of the solidsupported amine sorbents according to their preparation methods. The sorbents of type 1 are prepared through the formation of the covalent bonds between the amine-containing compounds and the surface of the solid support [5~10]. The sorbents of type 2 are prepared by loading the amine containing polymer or oligomer, such as polyethylenimine (PEI) or oligoethylenimine, on the nanoporous materials using a simple wet-impregnation [11~15]. Although the supported-PEI sorbents give the high capacity, the efficiency of amine is only about 35%. This is because of the high kinetic barrier for diffusion of the sorbed CO 2 molecules from surface into the bulk of PEI. In order to further increase the CO 2 sorption capacity of the sorbents and enhance the sorption performance, it is necessary to reduce the CO2 diffusion resistance in the PEI bulk in the sorbent. In the present study, several additives with different amounts have been introduced into the PEI in the sorbent to examine their effects on the sorption capacity and the sorption rate. The additive may reduce the viscosity of PEI, and the decrease in viscosity after adding the additives may facilitate the diffusion of the sorbed CO2 molecules from the surface of PEI into the bulk of PEI, probably due to weakening the interaction of the CO2 molecule with the amine group. The additives may also provide some channels for CO2 moving into deeper PEI films, and thus increase the total accessible sites and improve the efficiency of amine to enhance sorption performance. 2. Experimental 2.1. Preparation of sorbent samples The sorbent samples were prepared by a wet-impregnation method. In a typical preparation, the desired amount of PEI and additives were dissolved in 20 mL of ethanol under stirring for ~60 min to make a mixture solution, and then desired amount of the silica gel after vacuum drying at 108 oC was added to the mixture solution. The slurry was continuously stirred at room temperature for about 5 h, allowing the ethanol in the slurry to be evaporated. After ethanol was mainly evaporated, the sample was further dried in a vacuum oven at 40 oC, 1 kPa for 12 h, and then was sealed in a bottle before use. The sorbent samples with different PEI and additive loading amount were denoted as PEI(x)/SG, where x is the weight percentage of PEI-loading amount in the sorbent. 2.2. Evaluation of CO2 sorption/desorption performance The CO2 sorption/desorption performance of the sorbent was evaluated by using TGA (NETZSCH). A typical analysis is described as below: About 50 mg of the sorbent was placed at the TGA sample pan. The sample as first heated from room temperature to 100 °C at a rate of 10 °C/min under a pore N 2 flow with a flow rate of 60 ml/min, and was kept at 100 °C for 60 min to remove the remained solvent, the sorbed CO2 and moisture from the sample. The sample was cooled down to the desired sorption
Zhonghua Zhang et al. / Energy Procedia 37 (2013) 205 – 210
temperature and kept at this temperature for 30 min. The sorption was stated by switching the flow as from pore N2 to 10 v% CO2 at the same flow rate and hold at the sorption temperature for 30 min for CO 2 sorption. After the sorption, the flow gas was switched back to the pore N 2 and the temperature was increased to 100 °C for desorption. The sorption capacity of the sample was calculated according to the mass change during sorption. 2.3. Measurement of viscosity and density of PEII additive mixture The measurement of the mixture of PEI and additive viscosity were using the capillary viscometer at different temperature. The liquid density of mixture of PEI and additive was measured by using liquid density meter (METTLER TOLEDO DM40) at different temperature. 3. Results and Discussion 3.1. Effect of sorption temperature on sorption capacity of PEI(50)/SG Effect of the sorption temperature on the sorption capacity of PEI(50)/SG was examined using TGA with 10 v% CO2 gas at 30, 50, 65, 75, 85 and 100 oC, respectively. The results are shown in Figure 1. At 30 oC, the CO2 sorption capacity of PEI(50)/SG was 81.2 mg-CO2/g-sorbent. With increasing temperature, the CO2 sorption capacity became higher and reached the highest capacity of 127.2 mgCO2/g-sorbent at 75 oC. Continuously increasing temperature from 75 to 100 oC, the sorption capacity decreased to 118.3 mg-CO2/g-sorbent. The best capacity was obtained at 75 oC, similar to the results observed for PEI(50)/MCM-41[14] and PEI(50)/SBA-15 [12]. The results indicate that at the temperature range from 30 to 75 oC, the diffusion of CO2 in the PEI may be a sorption rate-control step. Consequently, elevating temperature results in increase of the number of accessible amine sites in the sorbent, and, thus, increases the CO2 sorption capacity. When the temperature is higher than 75 oC, the thermodynamics may play a more important role, thus, resulting in decrease of the sorption capacity with increasing temperature.
Fig. 1. The effect of sorption temperature on CO2 sorption capacity of PEI/SG
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3.2. Effect of additives on sorption capacity of PEII additive/SG Figure 2 showed the sorption capacity and sorption rate of additive-plus sorbents. Six additives were introduce into sorbents to investigate the promotion of the sorbents, including piperazine (PPZ), NFormylmorpholine (NFM), Poly(melamine-co-formaldehyde) acrylated (PMA), triethanolamine (TEA), Poly(melamine-co-formaldehyde) methylated (PMG) and N-methyl pyrrolidinone(NMP). The sorption capacity of sorbent was tested by TGA under 10 v % CO2 flow gas at 75 oC, and the PEI and additive amount in sorbents were 45% and 5% respectively. The results were shown that PPZ and NFM improve the CO2 sorption capacity of sorbent by 12.4% and 11.5% respectively. Both PPZ and NFM contained six-membered ring structure, and this structure seemed to provide a channel for CO2 diffusion from the surface of PEI into the deeper films. In other words, these additives promote the CO2 diffusion process in the bulk of PEI film, leading the higher capacity of the sorbents. However, other additives reduced the CO2 sorption capacity due to its structure may not provide a pathway for CO2 diffusion and reduce the accessible site of amine group. But all of additive promoted the CO2 sorption rate of solid amine sorbent.
Fig. 2. The effect of additives on CO2 sorption capacity and rate of PEI-additive/SG
3.3. Density and viscosity of PEII additive mixture
Fig. 3. The viscosity of PEI-additive mixture before and after sorbing CO2
Zhonghua Zhang et al. / Energy Procedia 37 (2013) 205 – 210
The viscosity of PEI before and after reacting with CO 2 was measured by the capillary viscometer at 40, 60 and 75 oC, and the results were shown in Figure 3. The dramatic viscosity increases after sorbing CO2 of PEI, by 44.9%, 35.0% and 17.7% at 40, 60 and 75 oC respectively, has been seen, which increases the diffusion resistance for CO2 and reduces the accessible sorption site of PEI film due to the CO2 molecular harder getting into the deeper PEI film. The density and viscosity of PEI-additive mixture were measured at 40, 60 and 75 oC, and the results were shown in Figure 4. The density and viscosity of PEI-additive mixture decreased at all temperature, favoring the CO2 diffusion into the bulk of PEI. That may be the reason why the additive can enhance the sorption performance of solid amine sorbents.
(a)
(b)
Fig. 4. The viscosity (a) and the density (b) of PEI-additive mixture
4. Conclusion A new direction of enhancing sorption performance of solid amine sorbents through improving the utilization efficiency of amine groups for CO2 capture was provided. The additive was introduced into the polymer amines to increasing the accessible CO2 sorption site, and the deeper polymer amine films f could be used due to the diffusion resistance decreasing. It is leading to enhance CO2 sorption performance of solid amine sorbents. However, further investigation in fundamental understanding of diffusion mechanism of the sorbed CO2 in PEI bulk at the molecular level is desired for improving the sorption performance of the solid amine sorbents. Acknowledgements This work was supported by 863-project of China , and partly support by U.S. Department of Energy, National Energy Technology Laboratory (NETL) through DOE Grants: DEFE0000458. References [1] Sheppard MC, Socolow RH. Carbon-Constrained World by Rapid Commercialization of Carbon Capture and Sequestration. AIChE J 2007;53:3022 3028.
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[2] Song CS. Global challenges and strategies for control, conversion and utilization of CO2 for sustainable development involving energy, catalysis, adsorption and chemical processing. Catalysis Today 2006;115:2 32. [3] Haszeldine RS. Carbon capture and storage: How green can black be? Science 2009;325:1647 1652. [4] Rochelle GT. Amine Scrubbing for CO2 Capture. Science 2009;325:1652 1654. [5]Zheng F, Tran DN, Busche BJ, Fryxell GE, Addleman RS, Zemanian TS, Aardahl CL. Ethylenediamine-modified SBA-15 as regenerable CO2 sorbent. Ind Eng Chem Res 2005;44:3099 3105. [6]Hiyoshi N, Yogo K, Yashima T. Adsorption characteristics of carbon dioxide on organically functionalized SBA-15. Micropor Mesopor Mater 2005;84:357 365. [7]Liang Z, Fadhel B, Schneider CJ, Chaffee AL. Stepwise growth of melamine-based dendrimers into mesopores and their CO2 adsorption properties. Micropor Mesopor Mater 2008;111:536 543. [8]Knofel C, Descarpentries J, Benzaouia A, Zelenak V, Mornet S, Llewellyn PL, Hornebecq V. Functionalised micro/mesoporous silica for the adsorption of carbon dioxide. Micropor Mesopor Mater 2007;99:79 85. [9] Wei J, Shi J, Pan H, Zhao W, Ye Q, Shi Y. Adsorption of carbon dioxide on organically functionalized SBA-16. Microporous Mesoporous Mater 2008;116:394 399. [10]Serna-Guerrero R, Dana E; Sayari A. New Insights into the Interactions of CO2 with Amine-Functionalized Silica. Ind Eng Chem Res 2008;47:9406 9412. [11]Son WJ, Choi JS, Ahn WS. Adsorptive removal of carbon dioxide using polyethyleneimine-loaded mesoporous silica materials. Micropor Mesopor Mater 2008;113:31 40. [12]Ma XL, Wang XX, Song CS. Molecular basket sorbents for separation of CO2 and H2S from various gas streams. J Am Chem Soc 2009;131:5777 5783. [13]Xu XC, Song CS, Andresen JM, Miller BG, Scaroni AW. Novel polyethylenimine-modified mesoporous molecular sieve of MCM-41type as high-capacity adsorbent for CO2 capture. Energy Fuels 2002;16:1463 1469. [14]Xu XC, Song CS, Miller BG, Scaroni AW. Adsorption separation of carbon dioxide from flue gas of natural gas-fired boiler by a novel nanoporous molecular basket Fuel Process Technol 2005;86:1457 1472. [15] Zhang ZH, Ma XL, Wang DX, Song CS, Wang YG. Development of Silica-Gel-Supported Polyethylenimine Sorbents for CO2 Capture from Flue Gas. AIChE J 2012;58:2495 2502.