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Glow Discharge Plasma-Assisted Template Removal of SBA-15 at Ambient Temperature for High Surface Area, High Silanol Density, and Enhanced CO2 Adsorption Capacity Min-Hao Yuan, Lifeng Wang, and Ralph T. Yang* Department of Chemical Engineering, University of Michigan, 3074 H. H. Dow, 2300 Hayward Street, Ann Arbor, Michigan 48109, United States S Supporting Information *

ABSTRACT: Glow discharge plasma was successfully applied for effective removal of the organic template P-123 from SBA-15 ordered mesoporous silica at near-room-temperature (below 50 °C) and in a short operation time (2 h). The as-made SBA-15 treated with glow discharge exhibited a larger surface area of 1025 m2 g−1 with larger pores and microspore volume as compared with that of conventional calcination (550 °C and 5 h, 827 m2 g−1). In addition to less structural shrinkage, the plasma-prepared SBA-15 showed significantly increased silanol density from 5.4 to 6.6−7.6 mmol g−1, which led directly to higher amine loading from 1.8 to 3.0 mmol g−1. Consequently, the plasma-treated sample showed 77% more CO2 capacity and 60% higher CO2/N2 selectivity than the conventionally treated sample at 0.15 bar and 25 °C. The advantage of using glow discharge plasma for low-temperature template removal for achieving enhanced performance for CO2 adsorption is clearly demonstrated.

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INTRODUCTION Worldwide reduction of anthropogenic carbon dioxide (CO2) emissions caused by the combustion of fossil fuels is urgently needed to mitigate the global climate change and its potential impacts.1 Despite the extensive research conducted on renewable energy, fossil fuels are currently predicted to remain as the main source of energy for the next decades.2 Therefore, the development of advanced materials/technologies for efficient carbon capture, storage (sequestration), and utilization is a promising way to mitigate global climate change, at least as a short-term solution.3−27 Current pilot plants for CO2 capture from power plant flue gas usually employ aqueous solutions of alkanolamines to scrub the flue gases because amines can chemically bind the acidic CO2 molecules at low reaction temperature with high CO2 selectivity. However, amine scrubbing as a retrofit technology would increase the cost of electricity by nearly 80% and the demand of cooling water by 30% relative to the same plant without a CO2 capture system, resulting in a significant bottleneck in large-scale deployment of this technology.27,28 Besides liquid-phase systems, there is increasing interest in developing effective porous solid materials functionalized with amines (e.g., amine-grafted, amine-impregnated, or hyperbranched amine-containing materials) for CO2 capture from either ambient air (ca. 400 ppm) or stationary sources (containing ca. 13−15% CO2).3,4,6−17 The aminemodified materials offer a great opportunity for enhancing the adsorbate−adsorbent interactions (i.e. zwitterion mechanism) and the selectivity for CO2 adsorption as well as alleviating the energy penalty, equipment corrosion, and fouling associated with the conventional use of liquid amine absorption systems. © 2014 American Chemical Society

Among the variety of solid sorbents, mesoporous silicas (e.g., MCM-n, SBA-n, and HMS) and amine precursors with different amine contents (e.g., primary, secondary and tertiary amines) have been employed to synthesize amine-grafted silicas because of their large mesopores and high surface areas, which are advantageous for uniform grafting of amines.6−17 Primary amines exhibit significantly higher adsorption capacities and amine efficiencies (i.e., mmol-CO2 adsorbed per mmol-N) than the other amines with respect to CO2 adsorption and water affinity, while the amine efficiencies were about 0.30 and 0.24 to 0.28 for mmol-CO2 mmol-N−1 for primary and secondary/ tertiary amines, respectively.13 The silanol density (mmol g−1) or silanol number (OH groups nm−2) on the pore walls are of vital importance for increasing the loading of postsynthetic grafted amine by alkyl-silyl linkages of aminosilanes and intrachannel surface silanols,29 leading directly to increased CO2 capture capacity. However, the conventional thermal calcination for removing the templating surfactants that are occluded within the as-made mesoporous silicas always leads to reduced silanol density due to dehydroxylation at high temperatures and the long heating time (i.e., >540 °C for at least 5 h), and it also causes structure shrinkage. To alleviate the problem caused by high-temperature calcination, alternative methods for template removal under mild conditions were developed. Thus, template was removed through acid−ethanol extraction under low-temperature reflux Received: May 9, 2014 Revised: June 18, 2014 Published: June 19, 2014 8124

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(MCM-4115,30 and SBA-1515−17 usually at 78 °C and 8 h) or supercritical fluid (90 °C and 24 h),31 ozone oxidation,32 and microwave digestion.33 By comparing the alternative template removal methods and the traditional calcination method for amine grafting and CO2 uptake, Wang and Yang16 and Li et al.17 confirmed that more amines (3-aminopropyl) were grafted on the SBA-15 obtained via ethanol extraction due to increased silanol densities that were preserved during template removal. Subsequently, the CO2 capacity was increased by 42−52% (at 0.1 to 0.15 bar and 25 °C), even though a considerable amount of template might still remain within the frameworks via ethanol extraction, as reported by Tian et al.33 Because postsynthetic grafting routes are based on silanol groups as the anchoring site, more efforts are desirable for developing mild methods for effectively and completely removing organic templates from mesostructured materials at lower temperatures and shorter operation time to simultaneously preserve silanol groups and structural properties. Recently, nonthermal plasma techniques have been employed to remove the templates from zeolites and mesoporous silicas34−37 and mesoporous thin films.38,39 Nonthermal plasma, as referred to as nonequilibrium or cold plasma, is characterized by a relatively low gas temperature in contrast with a high electron temperature (e.g., 104 to 105 K or 1−10 eV).40 Maesen et al.34 reported the plasma-assisted template removal of MFIand TON-type zeolites by oxygen-forming radio frequency plasma at 97 °C and 50 Pa. However, the decomposition rate was quite low (46% carbon removal after 14 days) due to low input power and severe mass transfer limitation in the channel system. Similarly, the low decomposition efficiency was also observed in mesoporous silicate films for remove Pluronic F127 template by the oxygen-forming radio frequency.38 More recently, effective template removal from ZSM-535 and MCM-4136 has been reported by Liu’s group using dielectric barrier discharge (DBD) plasma under atmospheric-pressure ambient-air conditions. The relatively low temperature (below 130 °C) and short reaction time (3 min in each treatment and a total of 75−90 min) of the DBD plasma prevented the negative side effects of a thermal calcination treatment, such as less structure shrinkage and increased silanol density in MCM-41, and gave rise to a higher catalytic activity for the Suzuki reaction of p-bromophenol with phenylbornic acid over Pdsupported on the plasma prepared MCM-41.36 In this study, we employ a glow discharge method to remove the triblock copolymer template, Pluronic P123 (EO20PO70EO20), from the as-made SBA-15 under conditions of near room temperature (below 50 °C), short time (2 h), and low pressure (3 Torr). The glow discharge plasma method, typically at low pressure of 0.1−10 Torr, avoids a significant increase in bulk temperature by Joule heating of molecules in stream through energy transfer from the heated electrons to heavy neutrals in atmospheric-pressure plasma. The highest temperature of glow discharges, which is located in the region close to the cathode and usually below 50 °C,41 is lower than that of DBD plasma as well as that in ethanol and supercritical CO2 extraction methods, resulting in a larger amount of silanol groups. The glow discharge plasma method was compared directly to the traditional calcination method to investigate their influences on structure shrinkage, the retained silanol density, and subsequently the amine content, CO2 adsorption capacity, and CO2/N2 selectivities.

Article

EXPERIMENTAL SECTION Synthesis of As-Made SBA-15. As-made SBA-15 mesoporous silica was synthesized following the procedure reported by Zhao et al.42 using Pluronic P123 (EO20PO71EO20) as a polymeric template. 4.0 g of Pluronic P123 was dissolved under stirring in 125 mL of 2 M HCl at 40 °C. Next, 8.5 g of TEOS (tetraethylorthosilicate) was added to that solution and stirred at 40 °C for 20 h. The solution was then autoclaved and aged at 90 °C for 24 h under static conditions. The white solid product was filtered, washed with deionized water, and dried overnight at room temperature. Removal of Template (P123) from As-Made SBA-15. In removal by glow discharge plasma, the as-made SBA-15 (∼0.25 g) was loaded on a quartz boat that was placed in the positive column of plasma chamber, as shown in Figure 1a. The

Figure 1. Illustrations of (a) a systematic drawing of experimental setup and (b) moving striations in positive column for template removal of as-made SBA-15 by O2-forming glow discharge.

glow discharge plasma setup is similar to that described previously.43−45 A DC high-voltage generator (DL-200, Tianda Cutting and Welding Setup, China) supplied a voltage of 3.1 kV and a current of 5 mA with a sinusoidal waveform at a frequency of ∼60 Hz to generate glow discharge plasmas. The temperature of glow discharge plasma would be below 50 °C.41 A few drops of deionized water were added to the sample prior to each treatment with the intention of preventing entrainment of the fine particles to the vacuum pump. The chamber was evacuated to below 1 Torr and then fed with oxygen as the plasma-forming gas to a pressure of 3 Torr. The time of each plasma treatment was 10 min, and each sample was treated up to 12 times. The sample prepared this way was designated as SBA-15-P. For comparison, the template of as-made SBA-15 was also removed by traditional thermal calcination at 550 °C for 5 h at a heating rate of 1.75 °C min−1 (from 25 to 550 °C for 5 h) under an air flow. The SBA-15 obtained was designated as SBA-15-C. Grafting of 3-Aminopropyltrimethoxysilane on SBA15. The grafting of amine on SBA-15 was achieved by using a precursor of 3-aminopropyl-trimethoxysilane. A mixture of 0.5 g of SBA-15-C or SBA-15-P with 50 mL of toluene and 5.0 mL of 3-aminopropyltrimethoxysilane was stirred and refluxed at 80 °C for 12 h. The resulting 3-aminopropyl-grafted SBA-15 was washed repeatedly with toluene and then dried under a vacuum at 80 °C overnight. The obtained samples were designated as amine-SBA-15-C and amine-SBA-15-P. Characterization. IR spectra were obtained with a PerkinElmer Spectrum BX FT-IR spectrophotometer. All 8125

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compounds were analyzed in neat form with an ATR (attenuated total reflectance) accessory (ZnSe crystal). Chemical elemental analyses (carbon and nitrogen content) were performed by Carlo Erba 1108 elemental combustion analyzer by Atlantic Microlab. X-ray diffraction (XRD) patterns were recorded with Bruker Nanostar. Transmission electron microscope (TEM) photographs were obtained with a JEOL 3011 high-resolution electron microscope operated at 300 kV. Thermogravimetric analysis (TGA) was performed on a Shimadzu TGA-50 apparatus. For silanol density analysis, the sample was held to 100 °C for 1 h and heated to 800 °C at a heating rate of 5 °C min−1 under a helium flow. Nitrogen adsorption isotherms and CO 2 adsorption isotherms (0−760 Torr) were measured with a Micromeritics ASAP 2020 analyzer (i.e., volumetrically). Nitrogen adsorption isotherms were measured at −196 °C by liquid nitrogen. CO2 adsorption isotherms were measured at 25 °C maintained by circulating baths. Amine-grafted silica was degassed at 90 °C overnight.

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Figure 2. IR spectra of as-made SBA, SBA-15-P, and SBA-15-C.

RESULTS AND DISCUSSION Template Removal of SBA-15 by Glow Discharge. Moving Striation. It was visually appealing to observe the moving striations occurring in the positive column of glow discharge when the as-made SBA-15 powder was loaded, as shown in Figure 1b, suggesting the strong interaction of the flux of plasma-active species (O2+, O, electron) with mesoporous SBA-15 powder for facilitating the decomposition of templating surfactant molecules. The observation of moving striations was similar to that occurring with other porous materials such as zeolite, alumina, and nonporous materials such as TiO2 and ZrO2 during their treatments with glow discharge.41 A transition from the homogeneous column to one containing moving striations indicates the departure of active plasma species from their quasi-neutral values,46 suggesting the strong interaction between plasma active species and powder surface. Furthermore, it is of interest to point out that the presence of gaseous products could affect the performance of glow discharge, yet the quantity of plasma-forming gas works adequately to maintain stable discharge plasmas during the template decomposition, suggesting that those gaseous products did not disturb the performance of glow discharge plasma. FTIR and Carbon Analysis. Infrared spectra were collected for all solids after plasma treatment over 120 min and compared with as-made and traditionally air-calcined SBA-15. As illustrated in Figure 2, the C−H stretching (2850−3000 cm−1) and bending (1350−1500 cm−1) vibrations of the template P123 disappeared in both SBA-15-P and SBA-15-C. Moreover, a broad band around 3400 cm−1 due to O−H stretching was present for as-made and plasma-treated SBA-15, whose bending vibration mode was responsible for the band recorded at 1630 cm−1. A sharp Si−OH stretching band at 3740 cm−1 was not verified because of significant overlapping with the broad bands of O−H stretching from low temperature treatments. Similar results have been reported by the template removal of SBA-15 via microwave digestion.33 Nevertheless, the relative intensity of the Si−OH bending bands centered at 960 cm−1 was nearly the same as that for as-made and plasmatreated SBA-15, while it was much weaker for the calcined sample, suggesting that silanol groups were abundantly preserved in SBA-15-P.

The elemental analysis result showed that there was still 1.28 wt % carbon in SBA-15-P over 120 min, while the carbon content of SBA-15-C was below the detection limit (0.3 wt %). On the basis of the first-order rate equation for the template carbon removal, the reaction rate coefficient for the reaction time of 0−40 min (0.042 min−1) is 3.4 times that for the reaction time of 50−120 min (0.012 min−1, see Figure S1 in the Supporting Information). Significant change in the rate coefficient suggests that the residual template molecules within SBA-15 were more difficult to be decomposed via the glow discharge after 40 min. According to the estimated coefficients, the required reaction time for the residual carbon below the detection limit could be expected to be 244 min. Nevertheless, given that 20.3 wt % carbon within as-made SBA-15 was measured by our carbon analysis, these results indicate that glow discharge has effectively removed ∼94.8% of the template carbon over 120 min. Also, the removal efficiency was higher than that of mild template removal by ethanol (63% at 78 °C)33 and supercritical CO2 (79% at 90 °C)31 extraction. XRD and TEM. The structural characterization was carried out by the small-angle XRD patterns in both the plasmaprepared and thermally calcined samples. As shown in Figure 3,

Figure 3. XRD patterns of SBA-15-P and SBA-15-C. 8126

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both samples possessed three distinct Bragg diffraction peaks that could be indexed as (100), (110), and (200), suggesting the formation of highly ordered 2D hexagonal symmetry. The results also indicated less shrinkage of the silica framework of SBA-15-P because the XRD patterns of SBA-15-P exhibit a shift to smaller angles and narrower peaks when compared with those of SBA-15-C. For clarity of shrinkage by glow discharge, the XRD unit-cell parameter (a) and pore-wall thickness (hd) of the SBA-15-P and SBA-15-C samples were estimated. The lattice parameter was evaluated from the (100) interplanar spacing, d100, according to the following formula: a = 2 × 3−1/2 × d100. The pore wall thickness was assessed by subtracting Barrett−Joyner−Halenda (BJH) center pore size from the unitcell size. The results show that the unit-cell size of SBA-15-P (11.6 nm) is higher than that of SBA-15-C (10.9 nm), while the pore wall thickness of SBA-15-P (4.1 nm) is also higher than that of SBA-15-C (3.4 nm). These results consistently demonstrate that shrinkage upon glow discharge is less than conventional calcination due to low-temperature template removal. In good agreement with the XRD results, the highresolution TEM image of SBA-15-P (Figure 4) shows well-

Figure 5. N2 isotherms and BJH pore size distributions (inset) of SBA15-C (△) and SBA-15-P (□) at −196 °C. Isotherm of SBA-15-P was offset by 300 cm3/g STP for clarity.

thickness of the silica framework of SBA-15-P at near-roomtemperature, as also confirmed by the XRD results. Silanol Densities. The effectiveness of the plasma treatment was also evaluated by the preservation of active silanol groups formed on the pore surfaces. As shown in Figure 6, the TGA thermograms of SBA-15-C and SBA-15-P in helium show that the weight losses between 200−800 °C were 4.85 and 8.13% for SBA-15-C and SBA-15-P, respectively. It should be noted that multistep weight losses were observed on SBA-15 via TG experiments in helium. Physically adsorbed and chemically adsorbed water can be assigned to the mass loss below 100 °C and the loss between 100−200 °C, respectively.16,48 The weight loss above 200 °C would be attributed mainly to the dehydroxylation by condensation of silanols.49 On the basis of the weight loss above 200 °C, the estimated silanol densities were 5.4 mmol g−1 for SBA-15-C. For the case of SBA-15-P, while glow discharge removed ∼94.8% of the template carbon of SBA-15 over 120 min, a small amount of partially degraded template should be considered in the estimation of silanol density. Because it is difficult to verify the composition of partially degraded template, we assume two extreme cases for the estimation of silanol density. If the residual template was in the form of pure P-123 surfactant, or only carbon, the estimated silanol densities would be 6.6 or 7.6 mmol g−1, respectively, for SBA-15-P on the basis of the carbon analysis. The real value of silanol densities of SBA-15-P should be between 6.6 and 7.6 mmol g−1, which is higher than that of SBA-15-C. On the basis of the BET surface area, the silanol number (SOH) of SBA-15-P was 3.9 to 4.5 OH nm−2, which was close or higher than 3.9 OH nm−2 for SBA-15-C and 3.7 OH nm−2 for another calcined SBA-15 samples, as verified by Si NMR.50 It is obvious that glow discharge retains the high level of silanol density on the pore wall surfaces, in agreement with the FTIR results. These results unambiguously demonstrate that the glow discharge is able to remove organic template at near-room-temperature for postsynthesis of highly ordered inorganic frameworks with higher surface areas, larger pore volumes, lower structural shrinkage, and higher silanol densities compared with those obtained from conventional calcination. Amine-Grafted SBA-15 by Glow Discharge Template Removal. Textural Parameters. The grafting of amine (3aminopropyl) on the plasma-prepared and calcined samples

Figure 4. TEM image of SBA-15-P.

ordered hexagonal arrays of mesopores in large domains and wheat-like morphology similar to that of typical as-made SBA15. This further suggests that the glow discharge did not lead to a structural collapse of the as-made sample. Nitrogen Isotherms. The N2 adsorption−desorption isotherms of the plasma-prepared and calcined samples are shown in Figure 5. The results show typical adsorption curves of type IV with a hysteresis loop, which were characteristic of goodquality SBA-15 and similar to that previously reported.16,17,47 The Brunauer−Emmett−Teller (BET) surface area analysis shows that the surface area of SBA-15-P was 1025 m2 g−1. (See Table 1.) It is interesting that the sample treated with glow discharge shows a larger surface area, or a 24% increase, as compared with that of SBA-15-C (827 m2 g−1). The total pore volume, BJH pore volume, and the micropore volume of SBA15-P were 1.17, 0.84, and 0.07 cm3 g−1, respectively, while those of SBA-15-C were 0.96, 0.72, and 0.05 cm3 g−1, respectively. It is worth noting that the glow discharge treatment resulted in higher surface and pore volume without significant changes on the pore size. (See Figure 5.) The larger surface area and pore volume of SBA-15-P are attributed to less shrinkage and larger 8127

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Table 1. Textural and Structure Parameters of SBA-15 and Amine-Grafted SBA-15 by Thermal Calcination and Glow Dischargea sample

SBET (m2 g−1)

Vt (cm3 g−1)

VBJH (cm3 g−1)

Vmi (cm3 g−1)

a (nm)

hd (nm)

SOH (mmol g−1)

SBA-15-C SBA-15-P amine-SBA-15-C amine-SBA-15-P

827 1025 404 327

0.96 1.17 0.53 0.49

0.72 0.84 0.44 0.43

0.05 0.07

10.9 11.6

3.4 4.1

5.4 6.6−7.6

SN (mmol g−1)

QCO2 (mmol g−1)

QCO2/ QN 2

QCO2/ SN

1.8 3.0

0.64 1.13

76 120

0.36 0.38

a

SBET: BET specific surface area; Vt: total pore volume; Vmi, T-plot micropore volume; VBJH: BJH pore volume; a: XRD unit-cell parameter; hd: pore wall thickness; SOH: estimated silanol group; SN: amine content as N; QCO2 and QN2: adsorption capacity of CO2 and N2 at 0.15 bar and 25 °C; QCO2/ QN2: sorbent selectivity for CO2/N2; QCO2/SN: amine efficiency of CO2/N.

Figure 6. TGA thermogram of SBA-15-P and SBA-15-C in a helium atmosphere.

Figure 7. CO2 and N2 (solid) isotherms on amine-SBA-15-P (□) and amine-SBA-15-C (△) at 25 °C. At 0.15 bar, the CO2 capacity increased by 77% via glow discharge for template removal.

was performed and compared for CO2 adsorption. As shown in Table 1, the BET surface areas of amine-SBA-15-P and amineSBA-15-C were 404 and 327 m2 g−1, respectively. The total and BJH pore volumes were also reduced in both grafted samples, while the T-plot micropore volumes were below detection level, indicating that the microporosity in these samples was completely blocked after amine-grafting. The total pore volume of amine-SBA-15-P was reduced from 1.17 to 0.49 m3 g−1, while that of amine-SBA-15-C was reduced from 0.96 to 0.53 m3 g−1. The BJH pore size distributions (see Figure S2 in the Supporting Information) were centered at 5.3 nm for amineSBA-15-P and amine-SBA-15-C, which were smaller than those of SBA-15-P and SBA-15-C (7.5 nm). This result suggests that the grafted amine reduced the mesopore size in both samples. More importantly, the more severe reduction in BET surface area and pore volume of SBA-15-P sample after amine treatment is clear evidence for the significant pore clogging and larger weight gain from the grafted 3-aminopropyl groups than that of amine-SBA-15-C. On the basis of the elemental analyses, the amine contents as N were estimated to be 3.0 mmol g−1 for amine-SBA-15-P and 1.8 mmol g−1 for amine-SBA-15-C. These results confirm that more amine groups were grafted to amine-SBA-15-P than to amine-SBA-15-C. CO2 Isotherms and CO2/N2 Selectivity. The CO2 and N2 adsorption isotherms on amine-SBA-15-P and amine-SBA-15-C at 25 °C are compared in Figure 7. The very low-pressure portion of the CO2 isotherm is almost a vertical line, indicating strong CO2−amine interactions and the high rate of adsorption. At 1 bar, the amount of CO2 adsorbed was 1.60 and 1.19 mmol g−1 for amine-SAB-15-P and amine-SAB-15-C, respectively,

while it was ca. 0.04 mmol g−1 for N2 adsorbed on both sorbents. Considering the conditions relevant to flue gas (∼15% CO2 concentration), the CO2 capacity of amine-SBA15-P (1.13 mmol g−1 at 0.15 bar) was 77% more than that of amine-SBA-15-C (0.64 mmol g−1 at 0.15 bar). The CO2/N2 selectivity ratio for amine-SBA-15-P (120) is 1.6 times that for amine-SBA-15-C (76) at 0.15 bar. On the basis of the grafted amine loadings, the amine efficiencies of CO2/N near the knees of the isotherms were ∼0.5, in agreement with the formation of zwitterion carbamate. Also, the high amine efficiencies were 0.38 and 0.36 mmol-CO2 mmol-N−1 for amine-SBA-15-P and amine-SBA-15-C at 0.15 bar and 25 °C, respectively. It is apparent that amine-SBA-15-P can reach amine efficiencies that are close to the theoretical maximum (0.5 under dry conditions). The significant increase in CO2 adsorption capacity and CO2/N2 selectivity on amineSBA-15-P is attributed to higher amine loading.

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CONCLUSIONS Glow discharge plasma has been successfully applied for efficient template removal from mesoporous SBA-15 at nearroom-temperature (below 50 °C) and in a short time of 120 min. Compared with conventional thermal calcination, the plasma-treated sample has the advantageous features of having a higher surface area with a larger pore volume, lower structural shrinkage, and a higher silanol density. Because postsynthetic grafting routes are based on the presence of surface silanol groups as anchoring sites, the low-temperature template removal method using glow discharge leads to increased 8128

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loading of grafting amine and consequently enhanced performance for CO2 adsorption.

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ASSOCIATED CONTENT

S Supporting Information *

Carbon contents within SBA-15 from 0 to 120 min via glow discharge and N2 isotherms and BJH pore size distributions of amine-SBA-15 sample. 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]. Tel: +1 7349360771. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the MCube Project at the University of Michigan - Ann Arbor. M.-H.Y. thanks the postdoctoral research fellowship (102-2917-I-564-027) of National Science Council of Taiwan (ROC).

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REFERENCES

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