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Carbon dioxide adsorbent based on rich amines loaded nano-silica
Journal of Colloid and Interface Science 409 (2013) 123–128
Contents lists available at ScienceDirect
Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Carbon dioxide adsorbent based on rich amines loaded nano-silica Yanhui Du a, Zhongjie Du a, Wei Zou a, Hangquan Li a, Jianguo Mi b, Chen Zhang a,⇑ a b
Key Laboratory of Carbon Fiber and Functional Polymers, Ministry of Education, Beijing University of Chemical Technology, Beijing 100029, PR China State Key Laboratory of Inorganic and Organic Materials, Beijing University of Chemical Technology, Beijing 100029, PR China
a r t i c l e
i n f o
Article history: Received 17 May 2013 Accepted 31 July 2013 Available online 13 August 2013 Keywords: CO2 adsorption Nano-silica Polyacrylic acid Amine
a b s t r a c t An easy strategy to obtain an effective carbon dioxide adsorbent based on rich amines functionalized nano-silica was proposed. Polyacrylic acid (PAA), acted as a multi-functional bridge, was firstly immobilized onto the surface of silica nanoparticles. Each carboxylic acid group was subsequently reacted with an amine group of alkylamines, and plenty of remained amines groups could be coated onto silica nanoparticles. As a result, the rich amines loaded nano-silica was fabricated and applied as CO2 adsorbent. The structures and morphologies of amines modified nano-silica were characterized by FTIR, TGA, TEM, and CHNS elemental analysis. Moreover, the effect of molecular weight of PAA and that of alkylamine on CO2 absorption capacity was discussed. As expected, SiO2–PAA(3000)–PEI(10000) adsorbent possessed remarkably high CO2 uptake of approximately 3.8 mmol/g-adsorbent at 100 KPa CO2, 40 °C. Moreover, it was found that the adsorbent exhibited a high CO2 adsorption rate, a good selectivity for CO2–N2 separation, and could be easily regenerated. Ó 2013 Elsevier Inc. All rights reserved.
1. Introduction With the increasing concern for climate issues, carbon dioxide capture technologies, including amine-based aqueous solutions [1,2], cryogenic techniques [3], membranes [4], and solid sorbents [5–7] have been researched and developed extensively. Among these approaches, absorption by amine-based aqueous solution was the most common technology used to capture CO2 from industrial streams [8,9]. However, this process suffered from some drawbacks, such as high energy consumption due to high regeneration energy requirement, equipment corrosion, and the degradation of used amine solutions [10,11]. Therefore, the strategy of impregnating [5,12,13] or covalently grafting [5,10,14] amine groups onto the surfaces of porous solid materials, such as silica, mesoporous solids (MCM-41, MCM-48, SBA-15), and carbon fibers has attracted increasing attention. As a promising candidate of solid-supported amine adsorbent, nano-silica received considerable research interest due to its high surface area, large pore volume, and narrow pore size distribution. Yan et al. [15] prepared mesoporous silica SBA-15 samples with and without controlled framework microporosity and used them as adsorbent for CO2 directly or impregnated with polymer amine. It was found that the CO2 adsorption capacity increased with the increase in microporosity in pure SBA-15 samples but with the introduction of PEI in the pores, the role of micropores was not obvious. Gil et al. [16] synthesized mesoporous cubic MCM-48 ⇑ Corresponding author. Fax: +86 10 64428804. E-mail address: [email protected] (C. Zhang). 0021-9797/$ - see front matter Ó 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcis.2013.07.071
material functionalized with 3-aminopropyl triethoxysilane (APTES) and used for CO2 adsorption. With the amount of APTES increased, the CO2 adsorption capacity increased 120%, from 0.52 to 1.14 mmol/g. Liu et al. [17] described a simple method for the production of densely anchored amine groups on a solid adsorbent invoking direct incorporation of tetraethylenepentamine (TEPA) onto as-synthesized ordered mesoporous silica. It was concluded that CO2 adsorption capacity increased substantially from 0.1 to 4.6 mmol/g with the increase in N content in these samples. A significant conclusion from these studies was that the adsorption performance correlated strongly with the surface density of amine groups. In this work, polyacrylic acid acted as multi-functional bridge was firstly grafted onto the surface of nano-silica through esterification reaction, and a plenty of carboxyl acid groups could be introduced. Secondly, tetraethylenepentamine (TEPA) or polyethyleneimine (PEI) was reacted with carboxyl acid group and loaded on the surface of nano-silica. Since each carboxyl acid group on polyacrylic acid chain could act as a reactive point, an interface constituted by brush-like macromolecules with a great amount of amines were immobilized and covered on nano-silica. As a result, higher effective carbon dioxide adsorbent based on rich amine groups loaded nano-silica could be obtained (Scheme 1). The modified nano-silica could be suspended in water stably to capture CO2 and then easily separated by filter after use. Moreover, the adsorbent could not only be used in industry (fossil fuel and natural gas) [17] but also life support systems in confined space such as submarines, space vehicles, and other inhabited vessels for space exploration platforms [18].
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Scheme 1. Synthesis scheme of carbon dioxide adsorbent nanoparticles.
2. Experimental 2.1. Materials Hydrophilic nano-silica AerosilÒ 380 (nano-silica, average primary particle size = 7 nm) was obtained from Shenyang Chemical Co., China. Polyacrylic acid (PAA, Mw = 800 or 3000), tetraethylenepentamine (TEPA), polyethyleneimine (PEI, Mw = 1800 or 10,000), 4-(dimethylamino) pyridine (DMAP), and N,N-dicyclohexylcarbodiimide (DCC) were purchased from Aladdin Chemical Co. China. Ethanol (EtOH) and N,N-dimethylformamide (DMF) were provided from Vas Chemical Co., China. 2.2. Preparation of SiO2–PAA nanoparticles 3 g of nano-silica was dispersed in 100 ml of DMF and then mixed with the solution of 6 g of PAA dissolved in 100 ml of DMF. After the mixture was sonicated for 30 min, it was heated to 110 °C with vigorous stirring. Then, 0.1 g of DMAP (dissolved in 10 ml DMF) and 1 g of DCC (dissolved in 20 ml DMF) were added dropwise. The mixture was kept with stirring at 110 °C for 12 h. The precipitate (denoted as SiO2–PAA) was filtered and washed by ethanol 3 times, and finally dried in a vacuum oven at 60 °C. 2.3. Preparation of SiO2–PAA-amine adsorbents 200 ml of suspension of 3 g of SiO2–PAA nanoparticles in DMF was added dropwise into a stock solution of TEPA or PEI in 100 mL DMF with stirring and kept for 24 h at 0 °C. The obtained SiO2–PAA-amine adsorbent (denoted as SiO2–PAA–TEPA and SiO2–PAA–PEI, respectively) was filtered and washed by ethanol 3 times and finally dried in a vacuum oven at 60 °C.
method, and the experimental apparatus was illustrated in Fig. 1. The apparatus was equipped with online pressure transducer and temperature transducer, so that the pressure and temperature in sample holder and gas container can be monitored. The test temperature changed from 20 to 80 °C, and the pressure ranged from 0 to 100 KPa. The internal volume of the gas container is 400 ml. 10 g of nano-silica adsorbent sample dispersed in deionized water was injected into the sample holder. Carbon dioxide was firstly fed into gas container, and the pressure and temperature were recorded. According to the change of pressure and temperature in the gas container, the volume of CO2 introduced into the sample holder could be obtained. Finally, when the pressure and temperature in sample holder reached stable, according to the equation of P, T, V, CO2 capacity of the adsorbent was calculated.
3. Results and discussion 3.1. Characterization of nano-silica adsorbent The morphologies of silica nanoparticles and SiO2–PAA–amine nanoparticles were observed by TEM (Fig. 2). In Fig. 2a, attributed to nano-size effect caused by the nanoparticles with about 10 nm diameter, obvious agglomeration of silica nanoparticles was found (Fig. 2a). After the introduction of polyacrylic acid interface and then alkylamines, the dispersion of SiO2–PAA–amine nanoparticles (Fig. 2b) became better. The amine functionalized silica
2.4. Characterization FT-IR measurement was performed using a Nexus 670 infrared spectrometer. Thermal gravimetric analysis (TGA, TASC 414/4) was conducted from room temperature to 800 °C at 10 °C/min under nitrogen atmosphere. The morphology of the nanoparticles was studied by transmission electron microscope (TEM, JEOL JEM3010, operated at 200 kV). A few drops of sample suspension in EtOH were placed onto a carbon-coated copper grid. Carbon, hydrogen, and nitrogen (CHN) elemental analysis was performed using a varioELcube elemental analyzer under flowing oxygen. CO2 sorption analysis was performed using constant capacity
Fig. 1. The diagram of absorption apparatus. r Gas thermodetector, s liquid thermodetector, t mechanical stir, u heat transfer oil, and v heating jacket.
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Fig. 2. TEM images of silica nanoparticles (a) and SiO2–PAA–amine nanoparticles (b).
nanoparticles were not separated completely possibly because of the entanglement of polymer chains. The chemical structures of different silica nanoparticles were tested by FT-IR spectroscopy analysis (Fig. 3). In the FT-IR spectrum of silica nanoparticles (Fig. 3A), the absorption peak at 1020– 1110 cm 1 was assigned to Si–O–Si asymmetric stretching vibration, and the peaks at 960 and 800 cm 1 were ascribed to the asymmetric bending and stretching vibration of Si–OH, respectively. By contrast, FT-IR spectrum of SiO2–PAA nanoparticles (Fig. 3B) displayed the new peaks at 2850 cm 1 (symmetric stretching of –CH2–), 2925 cm 1 (antisymmetric stretching of – CH2–), and 1736 cm 1 (stretching vibration of –COO–), which indicated carbonyl groups were grafted onto SiO2–PAA nanoparticles. Finally, the emerged peak at 1562 cm 1 signed to the bending vibration of amino in the FT-IR spectrum of SiO2–PAA–amine (Fig. 3C) proved that amine groups were introduced successfully. Moreover, the peak of carbonyl groups shifted from 1736 cm 1 to 1665 cm 1 could be attributed to the presence of amino. The amount of PAA and TEPA grafted on the surface of nanosilica were measured by TGA. It could be estimated that silica nanoparticles (Fig. 4a) had a weight loss of around 4% due to the loss of water molecules adsorbed onto the surface and the release of the structural water resulted from the bonded hydroxyl groups. The thermal gravimetric curve of SiO2–PAA (Fig. 4b) showed a
Fig. 4. TGA curves of SiO2 (a), SiO2–PAA (b), and SiO2–PAA–TEPA (c) nanoparticles.
main decreasing weight located at around 200–300 °C and the weight loss was about 20%, which could be deduced that the content of PAA was about 16%. Furthermore, the TGA curve of SiO2–PAA–TEPA nanoparticles was presented in Fig. 4c and the introducing content of amines was calculated to be about 27% [19]. 3.2. CO2 adsorption of SiO2–PAA–amine adsorbent
Fig. 3. FT-IR spectra of silica (A), SiO2–PAA (B) and SiO2–PAA-amine (C) nanoparticles.
The CO2 adsorption isotherm curves of nano-silica and SiO2– PAA(800)–PEI(10000) adsorbent recorded at 40 °C were presented in Fig. 5. Both curves showed an increase in the amount of CO2 adsorbed with an increasing pressure, and there were no plateaus in adsorption isotherms in the tested pressure range. Moreover, the amine modified nano-silica exhibited obviously higher CO2 uptake capacities than pure nano-silica. At the CO2 pressure of 100 KPa, the adsorption capacity of nano-silica was only 1.3 mmol/g-adsorbent. When PEI was introduced onto the meso-porous materials, the CO2 adsorption of the hybrid adsorbents was a combination of both physisorption and chemisorption. Therefore, after modified with polyacrylic acid and PEI, the adsorption capacity was quite substantial (3.2 mmol/g-adsorbent). Lin [20] synthesized NOHM-I-PEI using colloidal silica and PEI via ionic bond for capturing CO2 and the capture capacity was 2.28 mmol/g at 30 °C. Son et al. [21] impregnated a series of silica materials with PEI in methanol, the PEI-loaded KIT-6 showed the highest CO2 capacity of 3.1 mmol/g. The comparison between the adsorption capacities
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Y. Du et al. / Journal of Colloid and Interface Science 409 (2013) 123–128 Table 1 Nitrogen contents and CO2 adsorption performances of different silica nanoparticles.
a b
Fig. 5. CO2 adsorption isotherm curves of nano-silica (a) and SiO2–PAA(800)– PEI(10000) adsorbent (b) at the temperature of 40 °C.
with other similar adsorbent materials indicated that a higher effective CO2 adsorbent was made, and the high efficiency came from the large amount of amines groups enlarged by PAA multifunctional bridges [22]. 3.3. SiO2–PAA–amine adsorbent with different molecular weight of PAA The adsorbent was fabricated by covalently bonding amine groups to nano-silica support, so the surface reactive group density of the support played an important role in the amine loading content. Since each carboxyl group would be an anchor point for alkylamine, the molecular weight of PAA would contribute to the grafted content of amines and then the CO2 adsorption performance of SiO2–PAA–amine adsorbent. The adsorption capacities of SiO2–PAA–amine adsorbent prepared from PAA with the molecular weight of 800 and 3000, respectively, were presented in Fig. 6. The CO2 absorption isotherm curves had a similar trend, but the absorption capacity of SiO2–PAA(800)–TEPA was 1.50 mmol/g-adsorbent at 100 KPa at the temperature of 40 °C, while that of SiO2–PAA(3000)–TEPA was 1.65 mmol/g-adsorbent. The similar result could be found in Fig. 6b. The CO2 capacities of the adsorbents were measured under 100 KPa CO2 at 40 °C, and the results were listed in Table 1. The higher molecular
Sample
N content (mmol g 1)a
Sorbent capacity (mmol g 1)b
SiO2 SiO2–PAA(800)–TEPA SiO2–PAA(3000)–TEPA SiO2–PAA(800)– PEI(1800) SiO2–PAA(3000)– PEI(1800) SiO2–PAA(800)– PEI(10000) SiO2–PAA(3000)– PEI(10000)
0 4.92 5.19 6.30
1.50 1.65 2.40
6.42
3.0
6.83
3.20
7.20
3.80
Nitrogen content measured by CHNS elemental analysis. Sorbent capacity measured at CO2 pressures of 100 KPa, 40 °C.
weight of PAA was added, the more reactive points were produced, and then the more alkylamine molecules were introduced. It was obvious to found that with the increase in N content, the CO2 capacity was improved. 3.4. SiO2–PAA–amine adsorbent with different molecular weight of alkylamines In this work, three kinds of alkylamines, including TEPA, PEI(1800), and PEI(10000), were grafted onto nano-silica, and the adsorption capacities were compared (Fig. 7). It could be seen that SiO2–PAA–PEI(10000) adsorbent had the highest adsorption capacity. TEPA is a kind of monomolecular compound, but PEI is polymer [12]. It could be easily deduced that the adsorbent grafted with PEI with higher molecular weight would exhibited a higher adsorption capacity, which was also presented in Table 1. 3.5. Temperature-dependence of CO2 adsorption The temperature-dependent CO2 adsorption isotherm for SiO2– PAA(3000)–PEI(10000) adsorbent at CO2 pressure of 100 KPa was shown in Fig. 8. With the increasing temperature, the adsorption capacity of SiO2–PAA(3000)–PEI(10000) adsorbent became larger and reached its maximum of 3.8 mmol/g-adsorbent at 40 °C. The low adsorption capacity at low temperature was caused by the kinetic limitation. When the temperature was increased to 80 °C, the adsorption capacity decreased to only 1.0 mmol/g-adsorbent, which due to that the reaction between CO2 and amines is an exothermic process (Van’t Hoff behavior) [23,24].
Fig. 6. CO2 absorption isotherm curves of SiO2–PAA–TEPA (a) and SiO2–PAA–PEI(1800) (b) adsorbent.
Y. Du et al. / Journal of Colloid and Interface Science 409 (2013) 123–128
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Fig. 7. CO2 absorption isotherm curves of different amine modified SiO2–PAA(800) (a) and SiO2–PAA(3000) (b) nanoparticles at the temperature of 40 °C.
but also fast adsorption kinetic and good selectivity toward CO2 in preference to other competing gases (i.e., N2) [25]. Fig. 9 showed the adsorption kinetics of CO2 and N2 at 100 KPa CO2, 40 °C for SiO2–PAA(3000)–PEI(10000) sample. It could be found that the capture of CO2 carried out at high adsorption rate, more than 95% of CO2 being adsorbed in 25 min while almost no uptake of nitrogen. As a result, CO2 and N2 can be successfully separated by the adsorbent.
3.7. Cycle study
Fig. 8. Effect of temperature on the adsorption of CO2 for the nano-silica adsorbent.
For potential practical applications, in addition to high CO2 capturing capacity, the adsorbent should possess long-term stability and regenerability with a minimum difference in adsorption/ desorption [17]. The cyclical adsorption (at 40 °C in pure CO2 atmosphere) and desorption were measured using SiO2–PAA(3000)– PEI(10000) sample, and the results were shown in Fig. 10. The adsorption capacities were almost constant after 10 cycles of adsorption and desorption, which indicated that the adsorption and desorption performance was stable under the condition investigated. The stable adsorption and desorption performance suggested that this adsorbent is promising for further study toward practical applications.
Fig. 9. Adsorption kinetics of CO2 and N2 for the nano-silica adsorbent.
3.6. Adsorption kinetics of CO2 adsorption Efficient CO2 capture from flue gases and other gas mixtures required that the adsorbent demonstrated not only large CO2 uptake
Fig. 10. Cyclic CO2 adsorption test of the nano-silica adsorbent.
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4. Conclusions A highly efficient CO2 adsorbent based on specially designed silica nanoparticles functionalized with polyacrylic acid layer and then polyethylenimine was fabricated. The molecular weight of polyacrylic acid and alkylamines remarkably determined the adsorption capacities of the adsorbent, and SiO2–PAA(3000)– PEI(10000) adsorbent presented remarkably high CO2 uptake of approximately 3.8 mmol/g-adsorbent at 100 KPa CO2, 40 °C. The CO2 adsorption properties, kinetics, and regeneration of the adsorbent were investigated. It was found that the adsorbent exhibited high CO2 adsorption rate and good selectivity for CO2–N2 separation, and they can be easily regenerated.
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[6] J.C. Hicks, J.H. Drese, D.J. Fauth, M.L. Gray, G.G. Qi, C.W. Jones, J. Am. Chem. Soc. 130 (2008) 2902–2903. [7] A.K. Mishra, S. Ramaprabhu, Energy Environ. Sci. 4 (2011) 889–895. [8] E. Rinker, S.S. Ashour, O.C. Sandall, Ind. Eng. Chem. Res. 39 (2000) 4346–4356. [9] S. Satyapal, T. Filburn, J. Trela, J. Strange, Energy Fuels 15 (2001) 250–255. [10] G. Zhao, B. Aziz, N. Hedin, Appl. Energy 87 (2010) 2907–2913. [11] E.D. Bates, R.D. Mayton, I. Ntai, J.H. Davis, J. Am. Chem. Soc. 124 (2002) 926– 933. [12] S. Choi, J.H. Drese, C.W. Jones, ChemSusChem 2 (2009) 796–854. [13] X. Xu, C. Song, B.G. Miller, A.W. Scaroni, Fuel Process. Technol. 86 (2005) 1457– 1472. [14] N. Hiyoshi, D.K. Yogo, T. Yashima, Chem. Lett. 33 (2004) 510–511. [15] X.L. Yan, S. Komarneni, Z.F. Yan, J. Colloid Interface Sci. (2013) 217–224. [16] M. Gil, I. Tiscornia, Óscar de la Iglesia, R. Mallada, J. Santamaría, J. Chem. Eng. 175 (2011) 291–297. [17] S.H. Liu, Y.C. Lin, Y.C. Chien, H.R. Hyu, J. Air Waste Manage. Assoc. 61 (2011) 226–233. [18] C. KnÖfel, C. Martin, V. Hornebecq, P.L. Llewellyn, J. Phys. Chem. C 113 (2009) 21726–21734. [19] H.F. Zhou, C. Zhang, H.Q. Li, Z.J. Du, Carbon 49 (2011) 126–132. [20] K.-Y.A. Lin, A.-H.A. Park, Environ. Sci. Technol. 45 (2011) 6633–6639. [21] W.J. Son, J.S. Choi, W.S. Ahn, Micropor. Mesopor. Mater. 113 (2008) 31–40. [22] J.C. Hicks, J.H. Drese, D.J. Fauth, M.L. Gray, G.G. Qi, C.W. Jones, J. Am. Chem. Soc. 130 (2008) 2902–2903. [23] O.M. Al-Azzawi, C.M. Hofmann, G.A. Baker, S.N. Baker, J. Colloid Interface Sci. 385 (2012) 154–159. [24] N. Gargiulo, F. Pepe, D. Caputo, J. Colloid Interface Sci. 367 (2012) 348–354. [25] M. Sevilla, P. Valle-Vigón, A.B. Fuertes, Adv. Funct. Mater. 21 (2011) 2781– 2787.
Contents lists available at ScienceDirect
Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Carbon dioxide adsorbent based on rich amines loaded nano-silica Yanhui Du a, Zhongjie Du a, Wei Zou a, Hangquan Li a, Jianguo Mi b, Chen Zhang a,⇑ a b
Key Laboratory of Carbon Fiber and Functional Polymers, Ministry of Education, Beijing University of Chemical Technology, Beijing 100029, PR China State Key Laboratory of Inorganic and Organic Materials, Beijing University of Chemical Technology, Beijing 100029, PR China
a r t i c l e
i n f o
Article history: Received 17 May 2013 Accepted 31 July 2013 Available online 13 August 2013 Keywords: CO2 adsorption Nano-silica Polyacrylic acid Amine
a b s t r a c t An easy strategy to obtain an effective carbon dioxide adsorbent based on rich amines functionalized nano-silica was proposed. Polyacrylic acid (PAA), acted as a multi-functional bridge, was firstly immobilized onto the surface of silica nanoparticles. Each carboxylic acid group was subsequently reacted with an amine group of alkylamines, and plenty of remained amines groups could be coated onto silica nanoparticles. As a result, the rich amines loaded nano-silica was fabricated and applied as CO2 adsorbent. The structures and morphologies of amines modified nano-silica were characterized by FTIR, TGA, TEM, and CHNS elemental analysis. Moreover, the effect of molecular weight of PAA and that of alkylamine on CO2 absorption capacity was discussed. As expected, SiO2–PAA(3000)–PEI(10000) adsorbent possessed remarkably high CO2 uptake of approximately 3.8 mmol/g-adsorbent at 100 KPa CO2, 40 °C. Moreover, it was found that the adsorbent exhibited a high CO2 adsorption rate, a good selectivity for CO2–N2 separation, and could be easily regenerated. Ó 2013 Elsevier Inc. All rights reserved.
1. Introduction With the increasing concern for climate issues, carbon dioxide capture technologies, including amine-based aqueous solutions [1,2], cryogenic techniques [3], membranes [4], and solid sorbents [5–7] have been researched and developed extensively. Among these approaches, absorption by amine-based aqueous solution was the most common technology used to capture CO2 from industrial streams [8,9]. However, this process suffered from some drawbacks, such as high energy consumption due to high regeneration energy requirement, equipment corrosion, and the degradation of used amine solutions [10,11]. Therefore, the strategy of impregnating [5,12,13] or covalently grafting [5,10,14] amine groups onto the surfaces of porous solid materials, such as silica, mesoporous solids (MCM-41, MCM-48, SBA-15), and carbon fibers has attracted increasing attention. As a promising candidate of solid-supported amine adsorbent, nano-silica received considerable research interest due to its high surface area, large pore volume, and narrow pore size distribution. Yan et al. [15] prepared mesoporous silica SBA-15 samples with and without controlled framework microporosity and used them as adsorbent for CO2 directly or impregnated with polymer amine. It was found that the CO2 adsorption capacity increased with the increase in microporosity in pure SBA-15 samples but with the introduction of PEI in the pores, the role of micropores was not obvious. Gil et al. [16] synthesized mesoporous cubic MCM-48 ⇑ Corresponding author. Fax: +86 10 64428804. E-mail address: [email protected] (C. Zhang). 0021-9797/$ - see front matter Ó 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcis.2013.07.071
material functionalized with 3-aminopropyl triethoxysilane (APTES) and used for CO2 adsorption. With the amount of APTES increased, the CO2 adsorption capacity increased 120%, from 0.52 to 1.14 mmol/g. Liu et al. [17] described a simple method for the production of densely anchored amine groups on a solid adsorbent invoking direct incorporation of tetraethylenepentamine (TEPA) onto as-synthesized ordered mesoporous silica. It was concluded that CO2 adsorption capacity increased substantially from 0.1 to 4.6 mmol/g with the increase in N content in these samples. A significant conclusion from these studies was that the adsorption performance correlated strongly with the surface density of amine groups. In this work, polyacrylic acid acted as multi-functional bridge was firstly grafted onto the surface of nano-silica through esterification reaction, and a plenty of carboxyl acid groups could be introduced. Secondly, tetraethylenepentamine (TEPA) or polyethyleneimine (PEI) was reacted with carboxyl acid group and loaded on the surface of nano-silica. Since each carboxyl acid group on polyacrylic acid chain could act as a reactive point, an interface constituted by brush-like macromolecules with a great amount of amines were immobilized and covered on nano-silica. As a result, higher effective carbon dioxide adsorbent based on rich amine groups loaded nano-silica could be obtained (Scheme 1). The modified nano-silica could be suspended in water stably to capture CO2 and then easily separated by filter after use. Moreover, the adsorbent could not only be used in industry (fossil fuel and natural gas) [17] but also life support systems in confined space such as submarines, space vehicles, and other inhabited vessels for space exploration platforms [18].
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Scheme 1. Synthesis scheme of carbon dioxide adsorbent nanoparticles.
2. Experimental 2.1. Materials Hydrophilic nano-silica AerosilÒ 380 (nano-silica, average primary particle size = 7 nm) was obtained from Shenyang Chemical Co., China. Polyacrylic acid (PAA, Mw = 800 or 3000), tetraethylenepentamine (TEPA), polyethyleneimine (PEI, Mw = 1800 or 10,000), 4-(dimethylamino) pyridine (DMAP), and N,N-dicyclohexylcarbodiimide (DCC) were purchased from Aladdin Chemical Co. China. Ethanol (EtOH) and N,N-dimethylformamide (DMF) were provided from Vas Chemical Co., China. 2.2. Preparation of SiO2–PAA nanoparticles 3 g of nano-silica was dispersed in 100 ml of DMF and then mixed with the solution of 6 g of PAA dissolved in 100 ml of DMF. After the mixture was sonicated for 30 min, it was heated to 110 °C with vigorous stirring. Then, 0.1 g of DMAP (dissolved in 10 ml DMF) and 1 g of DCC (dissolved in 20 ml DMF) were added dropwise. The mixture was kept with stirring at 110 °C for 12 h. The precipitate (denoted as SiO2–PAA) was filtered and washed by ethanol 3 times, and finally dried in a vacuum oven at 60 °C. 2.3. Preparation of SiO2–PAA-amine adsorbents 200 ml of suspension of 3 g of SiO2–PAA nanoparticles in DMF was added dropwise into a stock solution of TEPA or PEI in 100 mL DMF with stirring and kept for 24 h at 0 °C. The obtained SiO2–PAA-amine adsorbent (denoted as SiO2–PAA–TEPA and SiO2–PAA–PEI, respectively) was filtered and washed by ethanol 3 times and finally dried in a vacuum oven at 60 °C.
method, and the experimental apparatus was illustrated in Fig. 1. The apparatus was equipped with online pressure transducer and temperature transducer, so that the pressure and temperature in sample holder and gas container can be monitored. The test temperature changed from 20 to 80 °C, and the pressure ranged from 0 to 100 KPa. The internal volume of the gas container is 400 ml. 10 g of nano-silica adsorbent sample dispersed in deionized water was injected into the sample holder. Carbon dioxide was firstly fed into gas container, and the pressure and temperature were recorded. According to the change of pressure and temperature in the gas container, the volume of CO2 introduced into the sample holder could be obtained. Finally, when the pressure and temperature in sample holder reached stable, according to the equation of P, T, V, CO2 capacity of the adsorbent was calculated.
3. Results and discussion 3.1. Characterization of nano-silica adsorbent The morphologies of silica nanoparticles and SiO2–PAA–amine nanoparticles were observed by TEM (Fig. 2). In Fig. 2a, attributed to nano-size effect caused by the nanoparticles with about 10 nm diameter, obvious agglomeration of silica nanoparticles was found (Fig. 2a). After the introduction of polyacrylic acid interface and then alkylamines, the dispersion of SiO2–PAA–amine nanoparticles (Fig. 2b) became better. The amine functionalized silica
2.4. Characterization FT-IR measurement was performed using a Nexus 670 infrared spectrometer. Thermal gravimetric analysis (TGA, TASC 414/4) was conducted from room temperature to 800 °C at 10 °C/min under nitrogen atmosphere. The morphology of the nanoparticles was studied by transmission electron microscope (TEM, JEOL JEM3010, operated at 200 kV). A few drops of sample suspension in EtOH were placed onto a carbon-coated copper grid. Carbon, hydrogen, and nitrogen (CHN) elemental analysis was performed using a varioELcube elemental analyzer under flowing oxygen. CO2 sorption analysis was performed using constant capacity
Fig. 1. The diagram of absorption apparatus. r Gas thermodetector, s liquid thermodetector, t mechanical stir, u heat transfer oil, and v heating jacket.
Y. Du et al. / Journal of Colloid and Interface Science 409 (2013) 123–128
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Fig. 2. TEM images of silica nanoparticles (a) and SiO2–PAA–amine nanoparticles (b).
nanoparticles were not separated completely possibly because of the entanglement of polymer chains. The chemical structures of different silica nanoparticles were tested by FT-IR spectroscopy analysis (Fig. 3). In the FT-IR spectrum of silica nanoparticles (Fig. 3A), the absorption peak at 1020– 1110 cm 1 was assigned to Si–O–Si asymmetric stretching vibration, and the peaks at 960 and 800 cm 1 were ascribed to the asymmetric bending and stretching vibration of Si–OH, respectively. By contrast, FT-IR spectrum of SiO2–PAA nanoparticles (Fig. 3B) displayed the new peaks at 2850 cm 1 (symmetric stretching of –CH2–), 2925 cm 1 (antisymmetric stretching of – CH2–), and 1736 cm 1 (stretching vibration of –COO–), which indicated carbonyl groups were grafted onto SiO2–PAA nanoparticles. Finally, the emerged peak at 1562 cm 1 signed to the bending vibration of amino in the FT-IR spectrum of SiO2–PAA–amine (Fig. 3C) proved that amine groups were introduced successfully. Moreover, the peak of carbonyl groups shifted from 1736 cm 1 to 1665 cm 1 could be attributed to the presence of amino. The amount of PAA and TEPA grafted on the surface of nanosilica were measured by TGA. It could be estimated that silica nanoparticles (Fig. 4a) had a weight loss of around 4% due to the loss of water molecules adsorbed onto the surface and the release of the structural water resulted from the bonded hydroxyl groups. The thermal gravimetric curve of SiO2–PAA (Fig. 4b) showed a
Fig. 4. TGA curves of SiO2 (a), SiO2–PAA (b), and SiO2–PAA–TEPA (c) nanoparticles.
main decreasing weight located at around 200–300 °C and the weight loss was about 20%, which could be deduced that the content of PAA was about 16%. Furthermore, the TGA curve of SiO2–PAA–TEPA nanoparticles was presented in Fig. 4c and the introducing content of amines was calculated to be about 27% [19]. 3.2. CO2 adsorption of SiO2–PAA–amine adsorbent
Fig. 3. FT-IR spectra of silica (A), SiO2–PAA (B) and SiO2–PAA-amine (C) nanoparticles.
The CO2 adsorption isotherm curves of nano-silica and SiO2– PAA(800)–PEI(10000) adsorbent recorded at 40 °C were presented in Fig. 5. Both curves showed an increase in the amount of CO2 adsorbed with an increasing pressure, and there were no plateaus in adsorption isotherms in the tested pressure range. Moreover, the amine modified nano-silica exhibited obviously higher CO2 uptake capacities than pure nano-silica. At the CO2 pressure of 100 KPa, the adsorption capacity of nano-silica was only 1.3 mmol/g-adsorbent. When PEI was introduced onto the meso-porous materials, the CO2 adsorption of the hybrid adsorbents was a combination of both physisorption and chemisorption. Therefore, after modified with polyacrylic acid and PEI, the adsorption capacity was quite substantial (3.2 mmol/g-adsorbent). Lin [20] synthesized NOHM-I-PEI using colloidal silica and PEI via ionic bond for capturing CO2 and the capture capacity was 2.28 mmol/g at 30 °C. Son et al. [21] impregnated a series of silica materials with PEI in methanol, the PEI-loaded KIT-6 showed the highest CO2 capacity of 3.1 mmol/g. The comparison between the adsorption capacities
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Y. Du et al. / Journal of Colloid and Interface Science 409 (2013) 123–128 Table 1 Nitrogen contents and CO2 adsorption performances of different silica nanoparticles.
a b
Fig. 5. CO2 adsorption isotherm curves of nano-silica (a) and SiO2–PAA(800)– PEI(10000) adsorbent (b) at the temperature of 40 °C.
with other similar adsorbent materials indicated that a higher effective CO2 adsorbent was made, and the high efficiency came from the large amount of amines groups enlarged by PAA multifunctional bridges [22]. 3.3. SiO2–PAA–amine adsorbent with different molecular weight of PAA The adsorbent was fabricated by covalently bonding amine groups to nano-silica support, so the surface reactive group density of the support played an important role in the amine loading content. Since each carboxyl group would be an anchor point for alkylamine, the molecular weight of PAA would contribute to the grafted content of amines and then the CO2 adsorption performance of SiO2–PAA–amine adsorbent. The adsorption capacities of SiO2–PAA–amine adsorbent prepared from PAA with the molecular weight of 800 and 3000, respectively, were presented in Fig. 6. The CO2 absorption isotherm curves had a similar trend, but the absorption capacity of SiO2–PAA(800)–TEPA was 1.50 mmol/g-adsorbent at 100 KPa at the temperature of 40 °C, while that of SiO2–PAA(3000)–TEPA was 1.65 mmol/g-adsorbent. The similar result could be found in Fig. 6b. The CO2 capacities of the adsorbents were measured under 100 KPa CO2 at 40 °C, and the results were listed in Table 1. The higher molecular
Sample
N content (mmol g 1)a
Sorbent capacity (mmol g 1)b
SiO2 SiO2–PAA(800)–TEPA SiO2–PAA(3000)–TEPA SiO2–PAA(800)– PEI(1800) SiO2–PAA(3000)– PEI(1800) SiO2–PAA(800)– PEI(10000) SiO2–PAA(3000)– PEI(10000)
0 4.92 5.19 6.30
1.50 1.65 2.40
6.42
3.0
6.83
3.20
7.20
3.80
Nitrogen content measured by CHNS elemental analysis. Sorbent capacity measured at CO2 pressures of 100 KPa, 40 °C.
weight of PAA was added, the more reactive points were produced, and then the more alkylamine molecules were introduced. It was obvious to found that with the increase in N content, the CO2 capacity was improved. 3.4. SiO2–PAA–amine adsorbent with different molecular weight of alkylamines In this work, three kinds of alkylamines, including TEPA, PEI(1800), and PEI(10000), were grafted onto nano-silica, and the adsorption capacities were compared (Fig. 7). It could be seen that SiO2–PAA–PEI(10000) adsorbent had the highest adsorption capacity. TEPA is a kind of monomolecular compound, but PEI is polymer [12]. It could be easily deduced that the adsorbent grafted with PEI with higher molecular weight would exhibited a higher adsorption capacity, which was also presented in Table 1. 3.5. Temperature-dependence of CO2 adsorption The temperature-dependent CO2 adsorption isotherm for SiO2– PAA(3000)–PEI(10000) adsorbent at CO2 pressure of 100 KPa was shown in Fig. 8. With the increasing temperature, the adsorption capacity of SiO2–PAA(3000)–PEI(10000) adsorbent became larger and reached its maximum of 3.8 mmol/g-adsorbent at 40 °C. The low adsorption capacity at low temperature was caused by the kinetic limitation. When the temperature was increased to 80 °C, the adsorption capacity decreased to only 1.0 mmol/g-adsorbent, which due to that the reaction between CO2 and amines is an exothermic process (Van’t Hoff behavior) [23,24].
Fig. 6. CO2 absorption isotherm curves of SiO2–PAA–TEPA (a) and SiO2–PAA–PEI(1800) (b) adsorbent.
Y. Du et al. / Journal of Colloid and Interface Science 409 (2013) 123–128
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Fig. 7. CO2 absorption isotherm curves of different amine modified SiO2–PAA(800) (a) and SiO2–PAA(3000) (b) nanoparticles at the temperature of 40 °C.
but also fast adsorption kinetic and good selectivity toward CO2 in preference to other competing gases (i.e., N2) [25]. Fig. 9 showed the adsorption kinetics of CO2 and N2 at 100 KPa CO2, 40 °C for SiO2–PAA(3000)–PEI(10000) sample. It could be found that the capture of CO2 carried out at high adsorption rate, more than 95% of CO2 being adsorbed in 25 min while almost no uptake of nitrogen. As a result, CO2 and N2 can be successfully separated by the adsorbent.
3.7. Cycle study
Fig. 8. Effect of temperature on the adsorption of CO2 for the nano-silica adsorbent.
For potential practical applications, in addition to high CO2 capturing capacity, the adsorbent should possess long-term stability and regenerability with a minimum difference in adsorption/ desorption [17]. The cyclical adsorption (at 40 °C in pure CO2 atmosphere) and desorption were measured using SiO2–PAA(3000)– PEI(10000) sample, and the results were shown in Fig. 10. The adsorption capacities were almost constant after 10 cycles of adsorption and desorption, which indicated that the adsorption and desorption performance was stable under the condition investigated. The stable adsorption and desorption performance suggested that this adsorbent is promising for further study toward practical applications.
Fig. 9. Adsorption kinetics of CO2 and N2 for the nano-silica adsorbent.
3.6. Adsorption kinetics of CO2 adsorption Efficient CO2 capture from flue gases and other gas mixtures required that the adsorbent demonstrated not only large CO2 uptake
Fig. 10. Cyclic CO2 adsorption test of the nano-silica adsorbent.
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4. Conclusions A highly efficient CO2 adsorbent based on specially designed silica nanoparticles functionalized with polyacrylic acid layer and then polyethylenimine was fabricated. The molecular weight of polyacrylic acid and alkylamines remarkably determined the adsorption capacities of the adsorbent, and SiO2–PAA(3000)– PEI(10000) adsorbent presented remarkably high CO2 uptake of approximately 3.8 mmol/g-adsorbent at 100 KPa CO2, 40 °C. The CO2 adsorption properties, kinetics, and regeneration of the adsorbent were investigated. It was found that the adsorbent exhibited high CO2 adsorption rate and good selectivity for CO2–N2 separation, and they can be easily regenerated.
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