Tetraethylenepentamine-Modified Silica Nanotubes for Low

Nov 12, 2013 - Carbon Dioxide Absorption using Solid Sorbents Incorporating Purified Components of Tetraethylenepentamine. Ryohei Numaguchi , Firoz A...
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Tetraethylenepentamine-Modified Silica Nanotubes for LowTemperature CO2 Capture Manli Yao,† Yanyan Dong,† Xin Hu,‡ Xingxing Feng,† Aiping Jia,† Guanqun Xie,† Gengshen Hu,*,† Jiqing Lu,† Mengfei Luo,† and Maohong Fan*,§ †

Key Laboratory of the Ministry of Education for Advanced Catalysis Materials, Institute of Physical Chemistry, and ‡College of Chemistry and Life Sciences, Zhejiang Normal University, Jinhua Zhejiang, 321004, People’s Republic of China § Department of Chemical and Petroleum Engineering, University of Wyoming, Laramie, Wyoming 82071, United States S Supporting Information *

ABSTRACT: The objective of this research is to develop a new type of CO2 sorbent. The sorbents were synthesized with mesoporous ethane−silica nanotubes (E−SNTs) and tetraethylenepentamine (TEPA). They were characterized by nitrogen adsorption/desorption, thermogravimetric analysis, and infrared spectroscopy. A fixed-bed reactor equipped with an online mass spectrometer was used to test the CO2 capture performances of the sorbents. It was found that 75 °C is the optimal CO2 adsorption temperature for amine-impregnated E−SNT sorbents. The highest CO2 sorption capacities achieved with E−SNTs with 50 wt % TEPA loading (E−SNTs−50%) without and with uses of water vapor are 3.58 and 4.74 mmol/g, respectively, under the conditions of a 10.0% CO2/N2 mixture at 75 °C. Cyclic CO2 adsorption−desorption test results indicate that the new composite sorbents are stable and regenerable.

1. INTRODUCTION Because of the large-scale use of fossil fuels, such as coal, petroleum, and natural gas, the atmospheric CO2 concentration is gradually increasing. It has been reported recently that the CO2 concentration has exceeded 400 ppm in Hawaii, which is an important milestone to humans.1 The significant and continuous rise in the CO2 concentration leads to global climate change, which has been considered as a worldwide issue. Accordingly, developing technologies for efficient CO2 capturing and sequestration on a large scale is highly desired. At present, capturing CO2 from flue gas of power plants is considered to be the most effective way to reduce CO2 emissions.2 In post-combustion capture, the most mature technology for large-scale CO2 capture is to use amine solutions (such as methyldiethanolamine, diethanolamine, and monoethanolamine), which can selectively capture CO2 under ambient conditions.3 Despite the fact that their CO2 capture efficiency has yet to be proven, this technology still has many disadvantages, such as the corrosive properties of liquid amines and the energy consumption in the regeneration process.4,5 Therefore, many researchers have been devoted to developing alternative CO2 sorbents in recent years. Porous solids, such as zeolites,6,7 active carbon,8 or metal oxides,9 are good candidates for capturing CO2. However, zeolites or active carbon often show relatively low CO2 capture performance at low CO2 partial pressure and the degradation of CO2 capacity in the presence of water vapor in the flue gas.10 More recently, new metal organic frameworks (MOFs) were synthesized, and they show ultrahigh gas storage capacity at ambient temperature and high-pressure conditions.11 However, it has been realized that MOF materials with a large CO2 capacity at higher pressures often show worse performance at lower CO2 partial pressures,12 © 2013 American Chemical Society

and the presence of moisture usually decreases the CO2 adsorption capacity.13 To overcome the shortcomings associated with aqueous amine absorbents and porous materials based on CO2 capture technologies, scientists have tried various amine-functionalized sorbents,10,14 which have high selectivity and regenerability during CO2 capture separation processes. Birbara et al.15,16 first reported amines supported on alumina, polymeric resins, carbon molecular sieves, and zeolite as solid sorbents for CO2 capture.16 In 2002, Xu et al. used MCM-41-supported polyethyleneimine as a solid sorbent to capture CO2, and a CO2 capture capacity of 3.0 mmol/g in pure CO2 at 75 °C was achieved.17 After that, a series of mesoporous silica, such as SBA-15,18,19 SBA-16,20 SBA-12,21 MCM-48,22 MCM-41,23 mesocellular silica foams (MCFs),24−28 and hexagonal mesoporous silica (HMS),29,30 has been used as amine supports to improve the CO2 capture performance. It is increasingly recognized that the structures of the supports can significantly influence the CO2 capture performances of these sorbents. Generally speaking, large pore volume,3,25 large pore size,23,28 and good pore interconnection31 are beneficial to improving the CO2 capture capacity of sorbents. For example, Yan et al.28 found that MCF-based sorbents with a large pore size show higher CO2 capacity than those with a small pore size. They also found the CO2 capture performance increased linearly with the total pore volume of SBA-15.3 Chen et al.31 reported that polyethylenimine-impregnated hierarchical silica monolith exhibited significantly higher CO2 capturing capacity than other silica-supported amine sorbents. Received: October 2, 2013 Revised: November 11, 2013 Published: November 12, 2013 7673

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Figure 1. SEM and TEM images of E−SNT on (left panel) Hitachi S-4800 FE-SEM and (right panel) JEOL-2100F TEM operated at 200 kV. For instance, E−SNTs−50% means the adsorbent contains 50 wt % TEPA and 50 wt % E−SNT support. 2.4. Characterization of E−SNT-Based Sorbents. Nitrogen adsorption experiments were conducted at 77 K using a surface area and pore size analyzer (Quantachrome NOVA 4000e). Prior to each analysis, the E−SNT and TEPA-functionalized E−SNT sorbents were degassed under vacuum at 200 and 80 °C for at least 4 h, respectively. According to the Barrett−Joyner−Halenda (BJH) method, the size distribution of mesopores was derived from the desorption branches of isotherms, while the size distribution of micropores was analyzed using the Dubinin−Astakhov (DA) method. Their surface areas (SBET) were determined using the multi-point Brunauer−Emmett−Teller (BET) method in relative pressure range (P/P0) of 0.05−0.3. On the basis of the amount of adsorbed nitrogen at P/P0 = 0.99, the total pore volume (Vt) was determined. Transmission electron microscopy (TEM) images were obtained with a transmission electron microscope (JEOL-2100F) operated at 200 kV. Field emission scanning electron microscopy (FE-SEM, Hitachi S-4800) was used to obtain SEM images. The transmission infrared spectra of the support and sorbents were collected with a Nicolet 670 FTIR spectrometer. The infrared spectrum of liquid TEPA was acquired using an attenuated total reflection infrared (ATRIR) accessory. The thermal stability and TEPA loading of the sorbents were measured with a thermogravimetric analyzer (Netzsch STA 449C) under flowing nitrogen with a 10 °C/min heating rate in 30− 800 °C. Only the weight loss above 200 °C was accounted for the amine content.34 2.5. Adsorption and Recycle Measurements. The CO2 sorption performances of the prepared sorbents were tested using a home-built fixed-bed reactor, which is shown in Figure S1 of the Supporting Information. The adsorption process was operated at atmospheric pressure, and the gases at the outlet were analyzed by an online mass spectrometer (MS, Hiden QIC-20). In a typical measurement procedure, 0.30 g of dried sorbent was packed into the middle part of the quartz-tube reactor (6 mm inner diameter), which was heated by heating tapes. The sorbent was first dealt by exposing it to N2 stream with a flow rate of 20 mL/min at 100 °C for 1 h prior to each adsorption measurement. After the sorbent was cooled to the desired temperature, a 10% CO2/N2 mixture with a flow rate of 10 mL/min was flowed into the reactor. Sorption tests were stopped until sorption saturation was reached. Some tests were performed with the presence of moisture. The CO2 uptake was calculated on the basis of the carbon balance. First, the mean residence time was 0.8 min, which was determined by flowing the 10% CO2/N2 gas through the empty quartz-tube reactor (6 mm inner diameter). Then, all of the CO2 breakthrough curves were corrected by subtracting 0.8 min. The total CO2 uptakes under different conditions were directly proportional to the areas above the corrected CO2 breakthrough curves and calculated by integrating the CO2 sorption profiles. The cyclic CO2 adsorption−desorption operation processes were initiated by activating sorbents at 100 °C for 1 h using a heating ramp

Silica nanotubes (SNTs) are mesoporous materials with uniform pores and thin walls.32 The surface area and pore volume could reach 800 m2/g and 3.0 cm3/g, respectively. The mesochannels and pores on the walls make SNTs a promising candidate of amine supports.25,32 Therefore, it is expected that SNTs could load more amines by virtue of the larger pore volume than other mesoporous silicas and, thus, show better CO2 capture capacity. On the basis of the above judgment, tetraethylenepentamine (TEPA)-functionalized SNTs were prepared as solid sorbents for CO2 capture in this work. The results indicate that the TEPA-modified SNTs show better CO2 capture performance (3.58 mmol/g) than analogue TEPAmodified SBA-15 (3.38 mmol/g). More importantly, SNTbased sorbents exhibit lower transfer resistance than the SBA15-based sorbent, which may be due to the thinner walls and higher amine accessibility in the vertical direction of channels.

2. EXPERIMENTAL SECTION 2.1. Chemicals. Triblock copolymer P123 [EO20−PO70−EO20; molecular weight (MW), 5800] was obtained from Sigma-Aldrich. TEPA and 1,2-bis(trimethoxysilyl)ethane (BTME) were purchased from Chengdu Kelong and Adamas, respectively. HCl was obtained from Quzhou Reagent. KCl was supplied from Sinopharm. N2 and 10% CO2/N2 gases were obtained from Jinhua Gas. Distilled water was used in all experiments. 2.2. Synthesis of Ethane−Silica Nanotubes (E−SNTs). In a typical synthesis of E−SNTs,32 0.55 g of P123 and 3.49 g of KCl were dissolved in 180 mL of a 2 M HCl solution with stirring until the copolymer was fully dissolved at 38 °C, 3.5 mmol of BTME was added under vigorous stirring for 6 min, then the stirring was stopped, and the solution was kept quiescent for 24 h. Then, the mixture was transferred into an autoclave and kept at 100 °C for 24 h. After that, by filtration and drying at room temperature for 12 h, the as-synthesized samples were obtained. Finally, the P123 template was removed from as-synthesized E−SNTs by the extraction method. In a typical extraction process, 1.5 g of E−SNTs was packaged with a filter paper and refluxed with the mixture of 100 mL of ethanol and 1 mL of concentrated hydrochloric acid at 90 °C for 24 h using a Soxhlet extractor. For comparison, mesoporous SBA-15 with a pore volume of 1.1 cm3/g and a surface area of 729 m2/g was also synthesized, as described previously.33 2.3. Preparation of E−SNT-Based Sorbents. TEPA-modified E−SNTs were prepared by wet impregnation. In a typical preparation, the desired amount of TEPA was dissolved in 10 mL of ethanol under stirring for 10 min and then 0.5 g of E−SNTs was added to the TEPA solution under stirring for another 30 min. The mixture was refluxed at 80 °C for 2 h under stirring and then dried at 80 °C until the complete volatilization of solvent. The obtained solid sorbents were denoted as E−SNTs−x%, where x is the TEPA loading supported on E−SNTs. 7674

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of 10 °C/min in N2. Then, the sorbent was cooled to 75 °C, at which CO2 adsorption was performed under CO2/N2 flow (10 mL/min) for 80 min. To regenerate the sorbents, inert N2 (20 mL/min) was flowed through the sorbent bed at 100 °C for 60 min. To evaluate the cyclic stability of TEPA-impregnated E−SNTs, such an adsorption− desorption procedure was repeated 5 times.

Table 1. Textural Properties for E−SNT Materials before and after Impregnating TEPA average pore sizea

3. RESULTS AND DISCUSSION 3.1. Characterizations of Materials. The typical SEM and TEM images of the E−SNT sample are given in Figure 1. The SEM image indicates that there are a lot intergranular pores in the sample. The TEM image clearly shows that the sample was composed of nanotubes with an average diameter of ∼8 nm, while there are a lot intergranular pores between the nanotubes. Figure 2 shows the nitrogen adsorption−desorption isotherms of pure E−SNTs and E−SNTs with different

sample

surface area (m2/g)

E−SNTs E−SNTs−30% E−SNTs−40% E−SNTs−50% E−SNTs−60%

808 216 157 66 21

b

pore volume (cm3/g) 3.0 1.3 1.2 0.6 0.1

c

nanotube (nm)

void space (nm)

4.8 4.4 4.3 0 0

39.7 39.7 39.7 39.7 23.3

a

The average pore diameter was determined from the desorption branch according to the BJH method. bThe surface area was calculated using the BET method, P/P0 = 0.05−0.3. cTotal pore volume, P/P0 = 0.99.

of nanotubes. Figure 3 shows the pore size distributions of freshly made E−SNTs and composite sorbents based on the

Figure 2. N2 isotherms of E−SNTs, E−SNTs−30%, E−SNTs−40%, E−SNTs−50%, E−SNTs−60%, and E−SNTs−70% measured at 77 K.

Figure 3. Pore size distributions of E−SNTs and different contents of TEPA loading based on the N2 isotherms measured at 77 K.

BJH method. For E−SNTs, there are two maxima in pore size distribution. The first one (4.8 nm) is the average channel diameter of nanotubes, and the second one (39.7 nm) is the average pore diameter of the void space of the nanotubes. After loading 30% TEPA, the intensity of the first pore distribution is much smaller than that of unmodified E−SNTs and the average pore diameter decreased to 4.4 nm, indicating that TEPA was mainly loaded into the channels of E−SNTs. For E−SNTs− 50%, the first pore distribution was not detected and the intensity of the second pore distribution decreased dramatically in comparison to the unmodified E−SNTs, suggesting that the channels of E−SNTs had been almost filled with amines and some TEPA was dispersed in the voids of the nanotubes. With the further increase of the TEPA loading to 60%, the intensity of the second pore distribution decreased further and the average pore diameter decreased to 23.3 nm, revealing that more TEPA was dispersed in the void space of the nanotubes with higher TEPA loading. This agrees well with the results shown in Figure 2. Table 1 summarized the textural properties of the sorbents prepared in the current study. It can be seem that, with increasing amine loading, the surface area and pore size of composite sorbents decrease, indicating that TEPA molecules have been loaded onto the E−SNT surface. To confirm that amines have been loaded onto E−SNTs, the infrared spectra of the E−SNTs before and after loading TEPA

TEPA loadings. The samples exhibit type-IV isotherms. At the relative pressures P/P0 = 0.79−0.98 and 0.48−0.72, there are two hysteresis loops, which can be ascribed to the void space among the nanotubes and the hollow nanotube channel, respectively.32 The average pore diameter of E−SNTs calculated on the basis of the nitrogen desorption isotherm is about 4.8 nm, which agrees well with the TEM result shown in Figure 1. Therefore, the wall thickness of the nanotube is in the range of 1.2−2.3 nm because the TEM image indicates that the external diameter of the nanotube is in the range of 7.1−9.5 nm. Figure S2 of the Supporting Information also shows that there are micro-/mesopores on the walls of nanotubes. As listed in Table 1, the BET surface area of E−SNTs can reach as high as 808 m2/g and the total pore volume can reach 2.97 cm3/g. The nitrogen sorption isotherms of the sorbent with 30% TEPA loading have a sharp drop, and the hysteresis loop at P/ P0 = 0.48−0.72 is much smaller than that of unmodified E− SNTs, indicating that TEPA was mainly loaded into the channels of E−SNTs. For E−SNTs−50%, besides the disappearance of the hysteresis loop at P/P0 = 0.48−0.72, the hysteresis loops at P/P0 = 0.79−0.98 decreased dramatically in comparison to the unmodified E−SNTs, suggesting that TEPA was dispersed in the channels of E−SNTs as well as the void space among the nanotubes. When the TEPA loading was increased to 60%, more TEPA was dispersed in the void spaces 7675

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weight loss process can be divided into two stages. The first stage with a weight loss prior to 120 °C could be ascribed to the desorption of physically adsorbed water or other gases, such as pre-adsorbed CO2. The second stage with a weight loss of 9.95% starting at around 300 °C and ending at 600 °C is associated with the decomposition of organic species in E− SNTs. Then, the weight stays constant at temperatures over 600 °C. For E−SNTs−50%, the mass loss also has two stages, and the most significant weight loss (53.36%) occurring between 158 and 700 °C could be ascribed to both TEPA releasing and the decomposition of organic species. Excluding the mass loss of E−SNTs, the estimated TEPA amount in E− SNTs−50% is about 48.38%, which is very close to the nominal loading valves. 3.2. Temperature Effect on CO2 Uptake of E−SNTs− 50%. It is well-known that the sorption temperature is a crucial parameter for the sorption performance. To determine the optimal CO2 sorption temperature, the CO2 capture capacities at different sorption temperatures were tested. Figure 6 shows

as well as liquid TEPA were collected and are shown in Figure 4. For E−SNTs, the two strong infrared peaks at 1164 and

Figure 4. Transmission infrared spectra of E−SNTs and E−SNTs− 50% and ATR-IR spectrum of liquid TEPA measured on a Nicolet 670 FTIR spectrometer.

2894 cm−1 are attributed to Si−C and C−H stretching vibrations, respectively. While for pure silica SBA-15, only the strong peaks associated with Si−O and Si−O−Si vibrations were observed in Figure S3 of the Supporting Information. For TEPA, the two peaks at 1599 and 3282 cm−1 are ascribed to N−H bending and stretching modes, respectively. The peaks at 1456, 2814, and 2933 cm−1 are attributed to C−H bending and stretching vibrations.25 For E−SNTs−50%, some characteristics peaks of TEPA, such as the peaks at 1475, 1569, 2945, and 2983 cm−1, were observed. This reveals that TEPA molecules have been loaded onto E−SNTs. The C−H modes of TEPA shift to higher frequencies while N−H bands show a red shift in comparison to corresponding peaks of free TEPA. This reveals the chemical interaction between TEPA and E− SNTs.25 The real amount of TEPA loading on the E−SNT support can be evaluated from the weight loss of the sorbent determined by thermogravimetric analysis (TGA). The TGA curves of sorbents before and after loading 50% TEPA onto the E−SNTs are given in Figure 5. For the E−SNTs, the whole

Figure 6. CO2 breakthrough curves for E−SNTs−50% at different test temperatures under 10% CO2/N2 (10 cm3/min).

the CO2 breakthrough curves for 0.3 g of E−SNTs−50% measured at 30, 50, 75, and 90 °C, respectively. As shown in the figure, the breakthrough points (the point that CO2 was initially detected) are 18.4, 20.3, 22.3, and 21.4 min, respectively, and the corresponding total CO2 capture capacities are 3.08, 3.35, 3.58, and 3.32 mmol/g, respectively. These results show that, with the increase in the temperature from 30 to 75 °C, the CO2 uptake increases. As the temperature increases, the amine chains are thought to have higher mobility, which provides CO2 with better accessing probability to the interior of the solid adsorbents and leads to higher adsorption capacity.2,4,35−38 However, further increasing the temperature to 90 °C will weaken the interaction between CO2 and amine and, therefore, enhance the release of captured CO2.25 Therefore, in the following study, 75 °C was chosen as the adsorption temperature. 3.3. CO2 Adsorption Performances of TEPA-Loaded E−SNTs. Figure 7 shows the CO2 breakthrough curves of E− SNTs with different TEPA loadings. For E−SNTs, the CO2 uptake is 0.29 mmol/g. The quite low capacity results from the weak interaction between CO2 and silica. For E−SNTs−30%, the CO2 breakthrough time is about 16.4 min. The CO2 uptake before the breakthrough point and the total CO2 uptake are 2.44 and 2.86 mmol/g, respectively. With the increase of the

Figure 5. TGA curves for E−SNTs and E−SNTs−50% measured on a Netzsch STA 449C thermogravimetric analyzer with a ramp rate of 10 °C/min from 25 to 800 °C. 7676

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Figure 7. CO2 breakthrough curves for E−SNTs with different contents of TEPA (adsorption temperature, 75 °C; CO2/N2 gas flow rate, 10 cm3/min; and CO2 concentration, 10 vol %).

Figure 8. Effect of TEPA loading on the CO2 capture performance of E−SNTs and the corresponding TEPA efficiency (adsorption temperature, 75 °C; CO2/N2 gas flow rate, 10 cm3/min; and CO2 concentration, 10 vol %).

TEPA loading levels, the CO2 breakthrough points increase to 19.5 min for E−SNTs−40% and 22.3 min for E−SNTs−50%, indicating that increasing the TEPA loading could enhance the CO2 capture capacities of the sorbent. The total CO2 capture capacities of E−SNTs−40% and E−SNTs−50% were calculated to be 3.29 and 3.58 mmol/g, respectively. However, with the further increase of the TEPA content to 60 and 70%, the CO2 breakthrough points decreased to 20.7 and 16.4 min, respectively. The corresponding total CO2 capture capacity decreased to 3.49 and 3.00 mmol/g, respectively. This may be due to the increase in the mass-transfer resistance caused by the excessive TEPA loading on E−SNTs. The breakthrough curves in the region where the CO2 signal continuously rises can reflect the mass-transfer limitation because the sharp breakthrough curve reveals lower diffusion resistance, while the flat curve indicates the stronger diffusion resistance. Apparently, as shown in Figure 7, with the increase of TEPA loading, the breakthrough curve became flatter, suggesting the increase of diffusion resistance. This phenomenon is consistent with our previous results that more amine leading on the silica support would lead to higher mass-transfer resistance.25 Figure 8 shows the effect of TEPA loadings on the CO2 capture performance of E−SNTs and the corresponding TEPA efficiency. It can be found that the total CO2 uptake increases with TEPA loading but the amine efficiency continuously decreases. At low amine loading, the amine chains are more dispersible, making the adsorption sites easily available. However, when the amine loading increases, amine would begin to conglomerate within the pores, leading to poor distribution of amine sites, and once the pore space is nearly saturated with loaded amines, mass diffusion limits considerably affect the adsorption process; thus, CO2 molecules cannot easily access the available amine sites, which leads to the significant decrease in amine efficiency. SBA-15 has been widely used as an amine support for CO2 capture.4 For comparison purposes, SBA-15 with 50% TEPA loading was also prepared and tested for CO2 capture. Figure S4 of the Supporting Information shows the TGA curves of SBA-15−50% and E−SNTs−50%. It can be seen that, for SBA15−50%, the most significant weight loss occurring between 160 and 700 °C is around 49%, which is very close to the nominal loading valve (50%). Figure 9 shows the CO2 breakthrough curves of SBA-15−50% and E−SNTs−50%.

Figure 9. CO2 breakthrough curves for E−SNTs−50% and SBA-15− 50% sorbents (adsorption temperature, 75 °C; CO2/N2 gas flow rate, 10 cm3/min; and CO2 concentration, 10 vol %).

The breakthrough point of E−SNTs−50% is 22.3 min, while that of SBA-15−50% is only 19.6 min. The total CO2 uptakes for two sorbents are 3.58 and 3.38 mmol/g, respectively, indicating that E−SNTs−50% shows slightly better performance than SBA-15−50%. As shown in Table 2, the contributions in the periods before the breakthrough point (i.e., CO2 in the mixture was totally captured) are 84.6 and 92.9%, respectively, further suggesting that E−SNTs−50% has better performance than SBA-15−50%. As shown in Figure 9, the CO2 breakthrough curve of E−SNTs−50% after the breakthrough point is steeper than that of SBA-15−50%, indicating that, for the E−SNT-based sorbent, the mass-transfer resistance is much lower than that for the SBA-15-based sorbent. It has been mentioned above that there are pores on the walls of E−SNTs. The pores could facilitate the interaction between CO2 and amine groups. Although SBA-15 also has narrower pores in the pore walls, the parallel channels with thick walls lead to higher mass-transfer resistance. This could explain why E−SNTs−50% showed higher CO2 capture capacity and better capture efficiency than SBA-15−50%. 3.4. Influence of Moisture on CO2 Adsorption Performances. When CO2 sorbents are being developed, 7677

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Table 2. CO2 Sorption Capacity of E−SNTs with Various TEPA Loadings CO2 uptake before breakthrough point (mmol/g of adsorbent)

total CO2 uptake (mmol/g of adsorbent)

net CO2 uptake/total CO2 uptake (%)

CO2 uptake (mmol/g of TEPA)

2.44 2.91 3.32 3.08 2.44 2.86 4.74a

2.86 3.29 3.58 3.49 3.00 3.38 4.57a

85.31 88.45 92.90 88.25 81.33 84.62 96.41a

9.53 8.23 7.16 5.82 4.29 6.76 9.14a

E−SNTs−30% E−SNTs−40% E−SNTs−50% E−SNTs−60% E−SNTs−70% SBA-15−50% E−SNTs−50% a

In the presence of moisture.

people have to investigate the effect of moisture on the CO2 sorption capacity because water is almost always present in the gases that are dealt, including flue gases. The effect of moisture on the performance of E−SNTs−50% was investigated by flowing N2 or CO2/N2 through a water bottle and bubbling through a quartz bubbler to obtain a moist N2 or CO2−N2 28% relative humidity at 25 °C. The breakthrough curves for CO2 sorption on the E−SNTs−50% in the absence and presence of moisture are shown in Figure 10. In the absence of moisture,

Figure 11. CO2 signal curve (top) and the corresponding temperature profile (bottom) during the adsorption and desorption on E−SNTs− 50% [adsorption (temperature, 75 °C; CO2/N2 gas flow rate, 10 cm3/ min; and CO2 concentration, 10 vol %) and desorption (temperature, 100 °C; N2 flow rate, 20 cm3/min; and temperature ramp rate, 5 °C/ min)].

adsorption and desorption on E−SNTs−50%. It was found that, during the desorption process, the CO2 signal within the first ∼1.3 min was much stronger than that of simulated flue gas (10% CO2/N2) and the strongest signal was reached at ∼78.5 °C, indicating the occurrence of fast desorption of CO2. On the basis of the area under the desorption curve, it was estimated that around 59.6% adsorbed CO2 desorbed within the first ∼1.3 min, after which the desorption rate decreased and most of the adsorbed CO2 could be released within the first 10 min. To assess the stability of TEPA-modified SNTs in 10% CO2/ N2 flow, cyclic CO2 adsorption−desorption tests with E− SNTs−50% were carried out at 75 °C for five cycles, as presented in Figure 12. In cycle 1, E−SNTs−50% shows a CO2 capture capacity of 3.58 mmol/g. In cycle 2, the CO2 uptake is 3.49 mmol/g, a 7.8% reduction compared to the performance in cycle 1. The decrease in the CO2 uptake may be due to the strong chemical interaction between CO2 and TEPA, which leads to the incomplete release of pre−dsorbed CO2 at 100 °C.25 E−SNTs−50% has relatively stable CO2 sorption capacities in the last four cycles from cycle 2 to cycle 5. Figure S5 of the Supporting Information shows the TEM image of spent E−SNTs. In comparison to the TEM image of fresh E− SNTs, the nanotube structure of the sorbent was not changed, indicating that E−SNTs are very stable, even after the cyclic adsorption−desorption tests. In other words, the sorbent is regenerable.

Figure 10. CO2 breakthrough curves for E−SNTs−50% in the absence and present of moisture (adsorption temperature, 75 °C; CO2/N2 gas flow rate, 10 cm3/min; and CO2 concentration, 10 vol %).

the CO2 breakthrough time of E−SNTs−50% is 22.3 min, while the corresponding time increases to 30.7 min in the presence of moisture, indicating that moisture has a positive effect on CO2 capture performance of sorbents. The total CO2 uptake in the presence of moisture is 4.74 mmol/g, which is 32.4% higher than the value (3.58 mmol/g) achieved with the dry steam. The calculated molar ratios of CO2 adsorbed and accessible nitrogen in the absence and presence of moisture are 0.27 and 0.35, respectively. As suggested by other researchers,39,40 moisture can increase the CO2 capacity of the aminefunctioned material by allowing for the formation of bicarbonate ions, instead of carbamate ions. Therefore, the enhancement of the CO2 capture capacity in the presence of moisture could be explained by the generally accepted reaction mechanisms of CO2 with amines,41,42 with which 2 mol of −NH2 react with 1 mol of CO2 to form carbamate when H2O is absent from the reaction, whereas only 1 mol of NH2 is needed for removal of 1 mol of CO2 to form bicarbonate in the presence of moisture. 3.5. Cyclic Adsorption−Desorption Study of E−SNTs− 50%. Figure 11 shows the CO2 signal curve during the CO2 7678

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Changjiang Scholars and Innovative Research Team in Chinese Universities (IRT0980), and the Wyoming Clean Coal Program in the U.S.A. was greatly acknowledged.

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Figure 12. Cyclic adsorption−desorption performance of E−SNTs− 50% [adsorption (temperature, 75 °C; CO2/N2 gas flow rate, 10 cm3/ min; and CO2 concentration, 10 vol %) and desorption (temperature, 100 °C; N2 flow rate, 20 cm3/min; and temperature ramp rate, 5 °C/ min)].

4. CONCLUSION In summary, the synthesized mesoporous E−SNTs can be used as the supports of high-capacity solid CO2 sorbents, TEPA/E− SNTs. The CO2 adsorption testing results indicate that 75 °C is the suitable temperature and E−SNTs with 50 wt % TEPA loading exhibited the highest CO2 adsorption capacity of all of the sorbents. E−SNTs−50% also showed the highest CO2 uptake before the CO2 breakthrough point, which could be due to the suitable TEPA loading and the weak mass-transfer resistance. Besides the excellent CO2 adsorption capacity and amine efficiency, the E−SNTs−50% possessed good stability and regenerability after five adsorption−desorption cycles.

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ASSOCIATED CONTENT

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AUTHOR INFORMATION

REFERENCES

S Supporting Information *

Schematic diagram of the fixed-bed reactor system for CO2 breakthrough experiments (Figure S1), pore size distribution analyzed with the DA method (Figure S2), transmission infrared spectra of SBA-15, SBA-15−50%, and E−SNTs−50% measured on a Nicolet 670 FTIR spectrometer (Figure S3), TGA curves for SBA-15−50% and E−SNTs−50% measured on a Netzsch STA 449C thermogravimetric analyzer with a ramp rate of 10 °C/min from 25 to 800 °C (Figure S4), and TEM image of used E−SNTs on JEOL-2100F TEM operated at 200 kV (Figure S5). This material is available free of charge via the Internet at http://pubs.acs.org. Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The financial support of this work from the National Natural Science Foundation of China (21203167 and 21106136), the Personal Department (ZC304012002), Education Department (Y201121744), and Science and Technology Department (2013C31049) of Zhejiang Province, the Program for 7679

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