Influence of Silica Types on Synthesis and Performance of Amine

Article pubs.acs.org/JPCC

Influence of Silica Types on Synthesis and Performance of Amine−Silica Hybrid Materials Used for CO2 Capture Kai-Min Li,† Jian-Guo Jiang,*,†,‡ Si-Cong Tian,† Xue-Jing Chen,† and Feng Yan† †

School of Environment, Tsinghua University, Beijing 100084, P. R. China Key Laboratory for Solid Waste Management and Environment Safety, Ministry of Education of China, Beijing, P. R. China

‡

S Supporting Information *

ABSTRACT: Amine−silica hybrid materials have been investigated extensively in terms of their suitability for postcombustion CO2 capture. However, research on how the silica types affects the synthesis and performance of amine−silica hybrid materials is scarce. In this study, four types of commonly used and representative silica including precipitated silica, fumed silica, MCM-41, and silica gel are used to synthesize a series of comparable materials by grafting a silane onto them. We undertake a porosity analysis of plain silica and the amine−silica hybrid materials and determined the CO2 adsorption performance of amine−silica hybrid materials. The results suggest that precipitated silica is a superior and promising support material for amine−silica hybrid materials synthesis by grafting. The amine−silica hybrid material supporting with precipitated silica possesses relatively high amine content, exhibits good porosity, and obtains the highest CO2 adsorption capacity and amine efficiency compared to those of three other amine−silica hybrid materials.

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frameworks (MOFs),20,21 and amine−silica hybrid/composite materials.22−24 However, zeolites, activated carbons, and MOFs adsorb CO2 through a physical process, so they need large pressure and/or temperature gradient to achieve good adsorption−desorption performance.25 Moreover, their selectivity is low, and they are sensitive to temperature and moisture.6 To alkaline earth metal oxide adsorbents, they usually need to adsorb and desorb CO2 at a very high temperature, and this means more energy must be consumed to heat the adsorption and desorption equipment. Amine−silica hybrid/composite materials are comprised of amines and silica; they can exhibit relatively high CO2 adsorption capacity, good selectivity, fast adsorption and desorption rates, and low energy consumption.25 Moreover, amine−silica hybrid/ composite adsorbents are tolerant to moisture. Indeed, the adsorption capacity of CO2 of amine−silica hybrid/composite adsorbents can be enhanced under the condition of moisture. This is because 1 mol of primary amines or secondary amines can only react with 0.5 mol of CO2 under anhydrous conditions; however, under humidity condition 1 mol of amines can capture 1 mol of CO2.26−28 Therefore, amine−silica hybrid/composite materials can be a potential alternative to aqueous solution of amines technology. Generally, amine−silica hybrid/composite materials can be synthesized by grafting or impregnation. Grafting is a method of synthesizing amine−silica hybrid materials through the formation of a covalent bond between organic−amines and silica, while impregnation is a physical process (amine−silica composite materials), which does not involve formation of a covalent bond.29 Many forms of silica and many amines have been used to synthesize

INTRODUCTION Since the industrial era, human activities have led to global CO2 emissions increase about 80%, from 21 billion to 38 billion tons from 1970 to 2004.1 Because of the burning of fossil fuels, the CO2 concentration in the atmosphere has increased gradually, reaching 392 ppm in 2011.2 Indeed, by the year 2055, the CO2 concentration is predicted to reach ∼560 ppm,3 potentially resulting in enormous environmental change. In recent years, to decrease CO2 emission, the development of methods of CO2 capture has received much attention. During the past decade, many techniques for CO2 capture from flue gas have been investigated, such as aqueous solution systems,4,5 solid adsorption materials,6 cryogenic separation technology,7,8 and membrane separation technology.9 At present, the preferred industrial technology for CO2 capture involves use of aqueous solutions of alkanolamines. Aqueous solutions of primary amine (monoethanolamine (MEA)),10 secondary amine (diethanolamine (DEA)),5 and tertiary amine (N-methyldiethanolamine (MDEA))11 can capture CO2 in a molar ration of 1:1 by generating bicarbonate. However, this technology has several disadvantages. First, it has a high-energy consumption due to the large weight of water (70−90 wt % of the aqueous solution),12 which does not absorb CO2, must be heated during the regeneration step. Second, amines are readily chemically degraded because flue gas generally contains 3−10% oxygen.3 Third, this technology is easy to lead to corrosion of absorption equipment.13 Compared to aqueous solutions of amines system, solid adsorbents can save energy and are easy to handle and less likely to cause corrosion problems.13 Therefore, in recent years many solid adsorbents have been investigated in terms of their potential for CO2 capture; these include zeolites,14−16 activated carbon,17 alkaline earth metal oxides,6,18,19 metal−organic © 2014 American Chemical Society

Received: August 21, 2013 Revised: January 1, 2014 Published: January 14, 2014 2454

dx.doi.org/10.1021/jp408354r | J. Phys. Chem. C 2014, 118, 2454−2462

The Journal of Physical Chemistry C

Article

amine−silica materials. Goeppert et al.12 used several types of amines and silica to synthesize a series of amine−silica composite materials by wet impregnation, and they found precipitates silica is a superior support material for impregnation. At the same time, they found some amines would leach out from the amine−silica composites when temperature >70 °C. This is mainly due to the interaction between amine and silica is weak in amine−silica composites. Compared to wet impregnation methods, grafting can significantly improve the stability of amine−silica hybrid materials for the covalent bonds between silica and organic−amines. For CO2 capture, the thermal stability of amine−silica hybrid/composite materials is critical, which has a direct impact on the temperature range materials can be used and the frequency of replacement of materials (the temperature of real flue gas is usually 110−140 °C). Although Builes and Vega29 demonstrated that impregnation usually can result in a higher adsorption capacity than grafting due to the mobility of organic chains, we believe that grafting also is a promising method due to its good thermal stability. For grafting, a good support material is very important, it will influence the CO2 adsorption performance of amine−silica hybrid materials. So, finding a suitable support material for grafting to synthesize amine−silica hybrid materials with good CO2 adsorption performance is very important and meaningful. In this work, four commonly used forms of silica (precipitated silica, fumed silica, MCM-41, and silica gel) were identified. We grafted a silane onto them to synthesize four types of amine− silica hybrid materials. Through pore distribution analysis and CO2 adsorption performance investigation, we can find the best support which is most suitable to graft amines in the four types of silica.

performed using triplicate parallel samples, the weight of each sample was controlled at 2−3 mg. Fourier transform infrared spectroscopy (FTIR) analysis of silica and modified silica was performed using a Thermal NEXUS spectrometer with a resolution of 8 cm−1 and a scanning frequency of 32 min−1 at room temperature. Spectra were recorded in the 4000−400 cm−1 region. Thermogravimetric analysis was conducted using a Q500 thermogravimetric analyzer (TA Instruments) or a TGA/DSC 1 STARe system (METTLER TOLEDO). The four types of silica were heated from 50 to 1200 °C at a rate of 10 °C/min under argon at a flow rate of 90 mL/min in the TGA/DSC 1 STARe system. The four types of modified silica were heated from 30 to 800 °C at a rate of 10 °C/min under nitrogen at a flow rate 90 mL/min in the Q500 thermogravimetric analyzer. Sample weight was controlled at 5−10 mg. The TGA-MS analysis was performed in the temperature range from 30 to 200 °C with a TGA/DSC 1 STARe system (METTLER TOLEDO) and a Thermostar mass spectrometer (Pfeiffer). For the TGA part, N2 was used as reactive gas and Ar2 was used as protective gas. N2 adsorption−desorption analysis was performed using a ASAP2020 analyzer (Micrometrics) at −196 °C. Before N2 adsorption−desorption measurements, the samples were dried at 105 °C under vacuum condition (100 Å of DETA-M is slightly less than that of MCM-41 (0.12 vs 0.15 cm3 g−1). The results suggest that the pore size of DETA-M becomes smaller than that of MCM-41, and the pore volume of DETA-M is the result of pore ≤30 Å. Figure 3c shows that MCM-41 and DETA-M exhibit an obvious peak at pore diameter 35 and 26 Å, respectively. The pore of DETA-M is mainly consisted of pore ≤35 Å, and the pore of 26 Å accounts for a very large proportion. The pore volume of DETA-S (0.25 cm−1 g−1) decreases 48% compared to silica gel (0.48 cm−1 g−1). But, the pore distribution of DETA-S is very similar to that of silica gel. Figure 3d shows that silica gel and DETA-S exhibit a peak at pore diameter 56 and 66 Å, respectively. Figure 4d shows silica gel and DETA-S reach a plateau after 186 and 183 Å, respectively, suggesting that most of the pore volume of silica gel and DETA-S is the result of pore ≤186 and ≤183 Å, respectively.

Table 1. The N2 adsorption−desorption isotherms of MCM-41, DETA-M, silica gel, and DETA-S exhibit the typical type IV adsorption isotherms which are characteristic of mesoporous materials and display an approximate type H1 hysteresis loop, whereas the N2 adsorption−desorption isotherms of precipitated silica, DETA-P, fumed silica, and DETA-F exhibit the typical type II adsorption isotherm, and display type H3 hysteresis loop.33 The beginning of the capillary condensation step of DETA-M occurs at a lower pressure range compare to MCM-41, this suggests a systematic decrease in pore size.30,33 The pore volume of DEAT-M (0.66 cm3 g−1) decreases 45% compare to MCM-41 (1.19 cm3 g−1), but the cumulative pore volume of pore of 17−30 Å of DETA-M is significantly larger than that of MCM-41 (0.42 vs 0.13 cm3 g−1). Moreover, the cumulative pore volume of pore of 17−30 Å of DETA-M accounts for 64% of the total pore volume of DETA-M. The cumulative pore volume of pore of 30−100 Å of DETA-M is markedly less than that of MCM-41 2456



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Figure 3. Pore distribution analysis.

Figure 4. Cumulative pore volume as a function of pore diameter.

The pore volume of DETA-P (0.88 cm3 g−1) is 34% lower than that of precipitated silica (1.34 cm3 g−1), but the average pore diameter is slightly higher. Figure 3a shows that precipitated silica and DETA-P exhibit a peak at pore diameter 64 and 76 Å, respectively. The cumulative pore volume of pore located 76− 440 Å accounts for ∼60% of total pore volume.

The type H3 hysteresis loop of precipitated silica, DETA-P, fumed silica, and DETA-F appears in the high relative pressure region, in which the majority of N2 adsorption occurs. This phenomenon is most likely due to their plate-like aggregates, which form relative large slit-shape pores.30,33,34 2457



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where W is the amine content, NA is the Avogadro constant, and SBET is the surface area of modified silica. The results are summarized in Table 2. The amine contents of DETA-P and DETA-M are very close and higher than those of DETA-F and DETA-S; however, the grafting density of DETA-M is lower than that of DETA-P. For CO2 capture, grafting amount of amine is crucial to obtain a high adsorption capacity, but high grafting amount may result in a high grafting density and further affect the efficiency of amine because of the similar aligned preferential orientation of organic chains,29 so a balance is very important. For this perspective, MCM-41 seems to be better than precipitated silica. For grafting, a silane coupling agent was grafted on to silica through the reaction between methoxy groups or ethyoxyl groups of silane coupling agent and silanol groups of silica (Scheme 1). We hypothesize that both the grafting amount and grafting density will be affected by the number of hydroxyl groups and their density on the silica. Therefore, analysis of the hydroxyl groups on silica is necessary. Figure 5 shows the thermogravimetric analysis of precipitated silica, fumed silica, MCM-41, and silica gel. The weight loss at various temperature ranges can be determined using the thermogram (Table 3). Accordingly, the weight loss of silica below 100 °C and 100− 200 °C is attributed to the removal of physical and chemical adsorbed water,35 at 200−600 °C the weight loss is mainly due to the dehydroxylation following the condensation of geminal silanols and vicinal silanols,36 and above 600 °C the weight loss is mainly due to the dehydroxylation following the condensation of isolated silanols.36 According to the weight loss, the silanol content and density of silica can be calculated with formulas 2 and 3,37 respectively:

Table 1. Surface Area, Pore Volume, and Average Pore Diameter samples

BET (m2 g−1)

pore vol (cm3 g−1)

av pore diam (nm)

P-silica DETA-P F-silica DETA-F MCM-41 DETA-M Silica gel DETA-S

336.51 180.44 355.37 172.59 928.26 591.78 272.71 116.26

1.34 0.88 0.74 1.67 1.19 0.66 0.48 0.25

12.99 13.17 8.95 29.06 3.74 3.34 5.90 5.83

The total pore volume of DETA-F increases 126% compared to fumed silica due to the increased volume of pore of 300−1450 Å. The average pore diameter of DETA-F (290.6 Å) also increases significantly compared to fumed silica (89.5 Å). This phenomenon can be attributed to that one DETA molecule can react with hydroxyls located different silica particles; and therefore DETA can act as a cross-linker during the grafting process. This will lead to the aggregation of different silica particles and the particles become larger, and therefore some large pore can be produced in this process. Although pore of ≤ 50 Å of DETA-F accounts for a large proportion, the cumulative pore volume of pore of ≤ 50 Å only accounts for 3.3% of the total pore volume. By evaluating the surface area and pore distribution, we can find fumed silica and precipitated silica were more suitable support materials than MCM-41 and silica gel. DETA-P and DETA-F have larger pore volume and pore diameter than DETA-M and DETA-S. For CO2 capture, the larger pore volume and pore diameter were very important. Although DETA-M (591.78 m2 g−1) has a much larger surface area than DETA-P (180.44 m2 g−1) and DETA-F (172.59 m2 g−1), the smaller pore diameter easily leads to pore blockage.30 Amount of Grafted Amine and the Number of Hydroxyl Groups. The C, H, N elemental contents of silica and modified silica are shown in Table 2. The C, H, N content of Table 2. C, H, and N Content, Amine Content, and Grafted Density

C (%)

H (%)

N (%)

P-silica DETA-P F-silica DETA-F MCM-41 DETA-M silica gel DETA-S

0.02 10.23 0.40 8.76 0.27 10.89 0.01 6.00

0.09 2.20 0.10 1.64 0.13 2.22 0.28 1.16

0.00 4.59 0.03 3.81 0.01 4.68 0.00 2.64

amine content (mmol N/g)

CDETA (DETA/nm)

3.27

3.64

2.70

3.14

3.33

1.13

1.89

3.26

modified silica increases significantly compared to plain silica, and this can be an evidence that DETA is grafted onto silica successfully. According to the changes in nitrogen content, we can calculate the amine content of DETA-P, DETA-F, DETA-M, and DETA-S. The grafted density of DETA can be calculated with formula 1

C DETA =

W NA 3 SBET

2(W1 − W2) × 1000 M H 2O

(2)

COH =

2(W1 − W2) NA M H 2O SBET

(3)

where W1 and W2 are the weight of silica samples (wt %) at temperature 200 and 1200 °C, respectively, MH2O is the molecular weight of water, NA is the Avogadro constant, and SBET is the surface area of silica. The calculated results are summarized in Table 3. Comparing fumed silica, precipitated silica, and MCM-41, we find more hydroxyls could obtain more grafted amount of DETA. But to silica gel, although the silanol content of silica gel is similar to that of precipitated silica and higher than that of fumed silica, the grafted amount of amine of silica gel is markedly lower than that of precipitated silica and fumed silica. This is most likely because silica gel particles are spherical and their porosity is very low. Many hydroxyl groups are wrapped in the silica gel sphere. The hydroxyl content of MCM-41 is 15% higher than that of precipitated silica, but the amount of grafted amine on MCM-41 is only 1.8% higher than precipitated. This can be attributed to that the pore size decreasing as the reaction continued, leading to an excess of DETA that cannot react with OH inside the pores. If we define utilization factor of hydroxyl groups of silica as formula 4

elemental content samples

NOH =

θ= (1) 2458

1 amine content (mmol/g) × 100% 3 OH content (mmol/g)

(4)



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Scheme 1. Reaction Mode between DETA and Silica

pore volume of precipitated silica compared to those of MCM-41 and silica gel provide further potential for increases in the amount of grafted amine without leading to pore blockage. However, the small pore size decreases the utilization of hydroxyl groups inside MCM-41. Thermal Stability. Figure 6 shows the thermogravimetric analysis results for DETA-P, DETA-F, DETA-M, and DETA-S,

Figure 5. TGA analysis of plain silica.

Table 3. Weight Loss of Plain Silica, Silanol Content and Density, and Efficiency of OH weight loss (wt %) samples

50− 200 °C

200− 600 °C

600− 1200 °C

NOH (mmol/g)

COH (OH/nm2)

θ (%)

P-silica F-silica MCM-41 silica gel

2.77 0.42 1.71 2.74

2.30 1.18 1.60 2.11

0.92 0.83 2.06 1.09

3.58 2.23 4.13 3.56

6.40 3.78 2.68 7.86

30.3 40.3 27.0 17.7

Figure 6. TGA analysis of DETA-P, DETA-F, DETA-M, and DETA-S.

and with the dashed lines representing the differential curves of weight to temperature. The thermograms and differential curves can be divided into four typical steps; the first was from 30 to 100 °C. In this temperature range, the weight loss is mainly attributed to the release of physisorbed water.30,35 The second step is from 100 to 200 °C, where the weight loss is mainly attributed to the removal of chemisorbed water.35 The TGA-MS analysis (Figures S2−S6) also shows that there is no obvious decomposition of DETA between 30 and 200 °C. The third step is from 200 to 400 °C, where the weight loss is mainly due to the degradation of the DETA links grafted onto the silica.35 The fourth step is at 400−800 °C, and the weight loss is mainly due to the complete degradation of the diethylenetriamine links, particularly those deep into the irregular silica pores.38 Through above analysis, it is confirmed that the four kinds of DETA modified silica have a good thermal stability below 200 °C.

the utilization factor of OH can be calculated (Table 3). The results show fumed silica has the highest utilization factor and precipitated silica locates next. This is mainly due to the very large pore volume and pore size of precipitated silica and fumed silica; thus, the opportunity for reaction of silanol with DETA increases. And we think that the utilization factor of hydroxyl groups of precipitated silica and fumed silica can potentially be improved further. To MCM-41, the pore blocking effect impedes the diffusion of DETA toward the center of the pore of MCM-41 and leads to the hydroxyl groups located inside of the pore of MCM-41 cannot react with DETA. We determined that precipitated silica has a relatively high hydroxyl content, grafted amount of DETA, and utilization factor of hydroxyl groups of silica. Moreover, the larger pore size and 2459



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Figure 7. CO2 adsorption analysis: (a) 40, (b) 60, (c) 80, (d) 100, and (e) 120 °C. (f) Adsorption capacity comparison. Blue is DETA-P, red is DETA-F, purple is DETA-S, and black is DETA-M.

from 40 to 60 °C, 3.1% from 60 to 80 °C, 20.2% from 80 to 100 °C, and 30.7% from 100 to 120 °C; however, DETA-M decreased 31.6, 31.3, 50, and 47.8%, respectively. To this phenomenon, a possible reason is pore blocking. Because after grafting, the average pore diameter of DETA-M becomes smaller than MCM-41 and is 3.34 nm, this is only about 10 times the kinetic diameter of the CO2 molecule, which is 0.33 nm. With temperature rises, the reaction between amine and CO2 occurs more quickly. During the process CO2 diffused into the pore of DETA-M, CO2 is more likely captured by the amines located the front end of the pore, and this will lead to the diameter of the entrance of the pore further decrease and therefore affect the diffusion of CO2 into the center of pore. So the decrease of the equilibrium adsorption capacity of DETA-M is caused by thermal effect and pore blocking. The equilibrium adsorption capacity and efficiency of amine are summarized in Table 4. The efficiency of amine is defined as formula 5:

CO2 Adsorption Behavior. Figure 7a−e shows the CO2 adsorption process of four kind of DETA modified silica under 40, 60, 80, 100, and 120 °C, respectively. Each of the materials reaches the adsorption equilibrium in just under ∼5 min at each temperature, and 90% of equilibrium adsorption capacity was achieved in less than 2 min. Overall, the adsorption process is a relatively rapid process for each material. At each temperature, the equilibrium adsorption capacity of DETA-P is always the largest compared to that of another three materials. Although the equilibrium adsorption capacity of DETA-M (0.98 mmol/g) was similar to that of DETA-P (1.02 mmol/g) at 40 °C, it was much lower at 60, 80, 100, and 120 °C. The equilibrium adsorption capacity of each modified silica will decrease with increasing temperature (Figure 7f). This is because the reaction CO2 with amines is a reversible and exothermic process,23,39 so the higher the temperature is the more adverse to CO2 capture. Therefore, the adsorption capacity will decrease as the temperature rises.24 Also, we find the equilibrium adsorption capacity of DETA-M decreases much more rapidly than DETA-P, DET-F, and DETA-S. For example, the equilibrium adsorption capacity of DETA-P decreased 4.9%

η= 2460

CO2 uptake (mmol/g) × 100% N content (mmol/g)

(5)



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Table 4. Equilibrium Adsorption Capacity and Efficiency of Amine adsorption capacity (mmol/g) and efficiency of amine (%) 40 °C

samples DETA-P DETA-F DETA-M DETA-S a

a

1.02 (31) 0.77 (29) 0.98 (29) 0.63 (33)

b

60 °C

80 °C

100 °C

120 °C

0.97 (30) 0.69 (25) 0.67 (20) 0.54 (29)

0.94 (29) 0.68 (24) 0.46 (14) 0.52 (27)

0.75 (27) 0.53 (20) 0.23 (7) 0.40 (21)

0.52 (16) 0.35 (13) 0.12 (4) 0.27 (14)

Equilibrium adsorption capacity. bEfficiency of amine. (2) He, H. K.; et al. Reversible CO2 Capture with Porous Polymers Using the Humidity Swing. Energy Environ. Sci. 2013, 6, 488−493. (3) Bollini, P.; Choi, S.; Drese, J. H.; Jones, C. W. Oxidative Degradation of Aminosilica Adsorbents Relevant to Postcombustion CO2 Capture. Energy Fuels 2011, 25, 2416−2425. (4) Hoshino, Y.; Imamura, K.; Yue, M.; Inoue, G.; Miura, Y. Reversible Absorption of CO2 Triggered by Phase Transition of Amine-Containing Micro- and Nanogel Particles. J. Am. Chem. Soc. 2012, 134, 18177− 18180. (5) Conway, W.; Wang, X. G.; Fernandes, D.; Burns, R.; Lawrance, G. Toward Rational Design of Amine Solutions for PCC Applications: The Kinetics of the Reaction of CO2(aq) with Cyclic and Secondary Amines in Aqueous Solution. Environ. Sci. Technol. 2012, 46, 7422−7429. (6) Choi, S.; Drese, J. H.; Jones, C. W. Adsorbent Materials for Carbon Dioxide Capture from Large Anthropogenic Point Sources. ChemSusChem 2009, 2, 796−854. (7) Song, C. F.; Kitamura, Y.; Li, S. H.; Jiang, W. Z. Parametric Analysis of a Novel Cryogenic CO2 Capture System Based on Stirling Coolers. Environ. Sci. Technol. 2012, 46, 12735−12741. (8) Songa, C. F.; Kitamuraa, Y.; Li, S. H.; Jiang, W. Z. Analysis of CO2 Frost Formation Properties in Cryogenic Capture Process. Int. J. Greenhouse Gas Control 2013, 13, 26−33. (9) Shen, Y.; Lua, A. C. Preparation and Characterization of Mixed Matrix Membranes Based on PVDF and Three Inorganic Fillers (Fumed Nonporous Silica, Zeolite 4A and Mesoporous MCM-41) for Gas Separation. Chem. Eng. J. 2012, 192, 201−210. (10) Guido, C. A.; Pietrucci, F.; Gallet, G. A.; Andreoni, W. The Fate of a Zwitterion in Water from ab Initio Molecular Dynamics: Monoethanolamine (MEA)-CO2. J. Chem. Theory Comput. 2013, 9, 28−32. (11) Penders-van Elk, N. J. M. C.; Derks, P. W. J.; Fradette, S.; Versteeg, G. F. Kinetics of Absorption of Carbon Dioxide in Aqueous MDEA Solutions with Carbonic Anhydrase at 298K. Int. J. Greenhouse Gas Control 2012, 9, 385−392. (12) Goeppert, A.; Meth, S.; Prakash, G. K. S.; Olah, G. A. Nanostructured Silica as a Support for Regenerable High-Capacity Organoamine-Based CO2 Sorbents. Energy Environ. Sci. 2010, 3, 1949− 1960. (13) Sun, Z. Y.; Fan, M. H.; Argyle, M. Supported Monoethanolamine for CO2 Separation. Ind. Eng. Chem. Res. 2011, 50, 11343−11349. (14) Liu, Q. L.; Pham, T.; Porosoff, M. D.; Lobo, R. F. ZK-5: A CO2Selective Zeolite with High Working Capacity at Ambient Temperature and Pressure. ChemSusChem 2012, 5, 2237−2242. (15) Harlick, P. J. E.; Tezel, F. H. An Experimental Adsorbent Screening Study for CO2 Removal from N2. Microporous Mesoporous Mater. 2004, 76, 71−79. (16) Kim, J.; Lin, L. C.; Swisher, J. A.; Haranczyk, M.; Smit, B. Predicting Large CO2 Adsorption in Aluminosilicate Zeolites for Postcombustion Carbon Dioxide Capture. J. Am. Chem. Soc. 2012, 134, 18940−18943. (17) Wei, H. R.; Deng, S. B.; Hu, B. Y.; Chen, Z. H.; Wang, B.; Huang, J.; Yu, G. Granular Bamboo-Derived Activated Carbon for High CO2 Adsorption: The Dominant Role of Narrow Micropores. ChemSusChem 2012, 5, 2354−2360. (18) Valverde, J. M.; Perejon, A.; Perez-Maqueda, L. A. Enhancement of Fast CO2 Capture by a Nano-SiO2/CaO Composite at Ca-Looping Conditions. Environ. Sci. Technol. 2012, 46, 6401−6408.

The results show that the efficiency of amine of DETA-M is lower than that of DETA-P, DETA-F, and DETA-S. This also can be attributed to the pore blocking effect of DETA-M, which leads to the amines located at the center of the pore cannot be used by CO2.

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CONCLUSIONS Through analysis and comparison, we determine that precipitated silica has a relatively high hydroxyl content, albeit is slightly lower than that of MCM-41. After grafting, DETA-P has a much greater pore volume and diameter than DETA-M and DETA-S. Although DETA-F has a greater pore volume and average pore diameter than DETA-P, the hydroxyl content of fumed silica is less than that of precipitated silica; therefore, little amine is grafted. For CO2 adsorption, DETA-P has an obvious advantage in adsorption capacity. Although DETA-M contains more amine than DETA-P, its low pore volume and small pore size lead to a lower amine efficiency; therefore, the adsorption capacity is lower than that of DETA-P. Precipitated silica is a relatively inexpensive and easy-tomanufacture product. Its relatively high hydroxyl content and large pore volume and pore size provide advantages in terms find of support for grafting to prepare the CO2 adsorbent. We also find that the hydroxyl density of precipitated silica was relatively high, and hence it is preferable to use precipitated silica, which has a large surface area but no marked reduction in pore volume and size.

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ASSOCIATED CONTENT

S Supporting Information *

Additional data on Fourier transform infrared spectroscopy analysis of DETA-P, DETA-M, and DETA-S, and TGA-MS analysis of DETA-P, DETA-F, DETA-M, and DETA-S. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (J.-G.J.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the Ministry of Science and Technology of the People’s Republic of China (National 863 Program, Project NO. 2012AA06A116).

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REFERENCES

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Influence of Silica Types on Synthesis and Performance of Amine

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