One-Pot Synthesis and CO2 Adsorption Properties of Ordered

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One-Pot Synthesis and CO2 Adsorption Properties of Ordered Mesoporous SBA-15 Materials Functionalized with APTMS Shiyou Hao, Hong Chang, Qiang Xiao, Yijun Zhong, and Weidong Zhu* Key Laboratory of the Ministry of Education for Advanced Catalysis Materials, Institute of Physical Chemistry, Zhejiang Normal University, 321004 Jinhua, People’s Republic of China ABSTRACT: Amino-functionalized SBA-15 materials were synthesized by cocondensation of tetraethoxysilane (TEOS) with organosilane aminopropyltrimethoxysilane (APTMS) in a wide range of molar ratios of APTMS to TEOS in the presence of triblock copolymer P123 under acidic synthesis conditions. The effects of F periodically added into the synthesis solution and of the APTMS concentration in the initial synthesis solution on the textural properties of the functionalized SBA15 were investigated in detail. The addition of APTMS into the initial synthesis solution can adversely affect the SBA-15 mesostructure, whereas the ordered mesostructure of the amino-functionalized SBA-15 materials can be reserved by the introduction of F into the synthesis solution. The results from the adsorption of CO2 on the synthesized materials show that the mesoporous SBA-15 functionalized with a high concentration of APTMS in the initial synthesis solution in the presence of F could be a potential adsorbent for CO2 capture and separation.

’ INTRODUCTION Because of the gradual increase in the atmospheric concentration of carbon dioxide (CO2) resulting from fossil fuel combustion and the link between an increase in the concentration of CO2 in the atmosphere and global climate changes, the capture and sequestration of CO2 have been considered as one of the options to reduce CO2 emissions. To reduce CO2 concentration significantly from the current level, various technologies, such as amine solutions for absorption,14 membrane separation,58 and adsorbents for adsorption,914 have been employed for the separation and recovery of CO2 emitted from power plants. Although a mature technology being applied extensively today, the major drawbacks of the use of amine solutions for CO2 absorption on an industrial scale include the large amount of energy required for the regeneration of the used amine solutions, equipment corrosion, and degradation of amine solutions in the presence of oxygen.1517 The advantages of membrane separation include high energy efficiency, ease of process scale-up, great operational flexibility, and environmental safety. However, the membrane-separation technology is suffering from some limitations, such as scale-up difficulty and high cost.18 Adsorption, in fact, has become the state of the art technology for the separation and recovery of CO2. Different kinds of adsorbents have been used for CO2 capture and separation. Activated carbons and zeolites can reversibly adsorb a remarkable amount of CO2 at low temperatures, However, their adsorption capacities decrease quickly at elevated temperatures, and the adsorption selectivity for CO2 in the presence of water vapor becomes very poor.19,20 Various basic metal oxides can chemically adsorb CO2 at high temperatures.21,22 However, these adsorbents are suffering from r 2011 American Chemical Society

severe energy penalties because of higher temperatures required for regeneration. Therefore, the development of efficient adsorbents is of utmost importance for CO2 capture and separation. Amino-functionalized mesoporous silicas, especially those synthesized from parent mesoporous SBA-15 materials, are potential adsorbents for capturing CO2 due to the reversible formation of ammonium carbamates and/or carbonates during CO2 adsorption, as well as due to their uniform, large pore, and high surface area.2327 Recently, several research groups have investigated the adsorption of CO2 on amino-functionalized SBA-15 materials. For example, Song and co-workers prepared the so-called molecular baskets consisting of polyethylenimine (PEI)-modified mesoporous SBA-15 molecular sieves and studied their CO2 adsorption properties.28,29 Chuang and co-workers synthesized amino-bearing SBA-15 adsorbents and investigated their thermal and chemical stability.26,30,31 To promote the adsorption capacity of CO2, SBA-15 silicas with high amino loadings were prepared by Zhu and co-workers.32,33 Up to now, most of the amino-functionalized SBA-15 mesoporous materials have been synthesized by a postgrafting method.2833 It is well known that the postgrafting method is commonly used in performing surface modification by covalently linking organosilane species with surface free and germinal silanol groups.34 However, there are several disadvantages during postgrafting. For example, the pore size of the resulting material will be reduced due to the attachment of multiple layers of Received: January 10, 2011 Revised: May 26, 2011 Published: June 01, 2011 12873

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The Journal of Physical Chemistry C functional moieties on the surface, which will cause a strong diffusion resistance to CO2 molecules.35 Additionally, the postgrafting method typically results in an inhomogeneous surface coverage due to organic moieties congregating near the entries of the mesopores and the exterior surfaces.36 A cocondensation method has the following advantages: (1) a short reaction time as it needs only a single step by copolymerization of organosilane with silica or organosilica precursors in the presence of surfactant,3740 (2) a higher and more homogeneous surface coverage of organosilane functionalities,36 and (3) a better chemical and thermal stability, compared with that of postmodified materials.41 Recently, in the preparation of aminopropyl-functionalized SBA-15 by the cocondensation method, Zhao and Cheng found that the amount of aminopropyltriethoxysilane (APTES) incorporated in the silica framework increased with increasing the APTES concentration in the initial synthesis solution, while the ordering of the mesoporous SBA-15 gradually decreased due to the adverse effect of the aminopropyl groups in the APTES on the SBA-15 mesostructure.42,43 To improve the mesoscopic order of SBA-15 amino-functionalized with APTMS via the co-condensation method, Wei and co-workers investigated several strategies, including allowing TEOS to prehydrolyze or varying the aging temperature and time of the reactive mixture.44 However, the molar ratio of APTMS to (APTMS þ TEOS) in the initial synthesis solution in their report was still low (1:10). Can the highly ordered mesoporous SBA-15 with a high amino loading be synthesized when the molar ratio of APTMS to (APTMS þ TEOS) in the initial synthesis solution is increased? In the present study, via introducing a required amount of F, amino-functionalized mesoporous SBA-15 materials were synthesized by a one-pot method with a high molar ratio of APTMS to (APTMS þ TEOS) in the initial synthesis solution. The synthesized amino-functionalized mesoporous materials were characterized by XRD, FT-IR, N2 adsorptiondesorption, TGDTA, SEM, TEM, and elemental analysis (EA) techniques, and the adsorption and desorption properties of CO2 on the synthesized materials were determined by volumetric and TPD techniques, respectively. Finally, the applicability of the synthesized amino-functionalized SBA-15 is anticipated for CO2 capture and separation.

’ EXPERIMENTAL SECTION Chemicals. Tetraethylorthosilicate (TEOS, 98%) was purchased from Acros, and 3-aminopropyltrimethoxysilane (APTMS, 95%) and the surfactant P123 (EO20PO70EO20, average molecular weight = 5800) were purchased from Aldrich. NH4F, 37% fuming hydrochloric acid, and absolute ethanol were purchased from Sinopharm Chemical Reagent Co. All the chemical reagents were used without further purification. The 2.0 M HCl solution was prepared from the 37% fuming hydrochloric acid. The deionized water was obtained from Millipore Milli-Q ultrapure water purification systems with the resistivity larger than 18.2 MΩ. Synthesis. Amino-functionalized SBA-15 materials were prepared by a one-pot synthesis method. The typical synthesis process was as follows: At room temperature, 1.2 g of P123 was dissolved in 36 g of 2.0 M HCl solution, followed by adding 9 g of deionized water. A calculated amount of TEOS was then added into the above-prepared solution. Afterward, the resulting solution was prehydrolyzed at 40 °C for 2 h, followed by slowly adding a calculated amount of APTMS under stirring. The molar

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composition was as follows: (1x) TEOS:x APTMS:6.1 HCl:0.017 P123:165 H2O, where x varied from 0 to 0.30, and when the effects of F on the mesostructure of SBA-15 were investigated, the required amount of NH4F was introduced at different times after adding APTMS into the solution, in which the APTMS molar composition x varied from 0.15 to 0.30. The resulting mixture was stirred at 40 °C for 24 h, then transferred into a Teflon autoclave, and the synthesis was carried out without agitation in an oven at 90 °C for 24 h. The product was filtered, and the solid was washed with deionized water. The washed solid was then dried at 60 °C overnight prior to a further analysis or use. The template was removed from the as-synthesized material by an extraction method. Typically, 1.0 g of the as-synthesized sample was dispersed in 100 mL of absolute ethanol at 25 °C with agitation, and then the mixture was refluxed at 90 °C for 24 h. Finally, the product was filtered and the solid was washed with deionized water and then with ethanol several times and dried at 80 °C overnight. The resulting samples were referred to as x-SBA-15-NH2, where x represents the molar composition of APTMS in the synthesis solution. To recover the basicity of the amine moiety, all the as-synthesized amino-fucntionalized samples were treated with 1 M NaOH solution for 10 min. Characterization. The FT-IR spectra were recorded by a Nicole Nexus 670 spectrometer with a resolution of 4 cm1 using a KBr compression method, and the in situ FT-IR spectra were obtained with an MCT detector. The powdered X-ray diffraction (XRD) patterns were performed on a Philips PW3040/60 powder diffractometer using Cu KR radiation (λ = 0.154 nm). The morphology and particle size of the synthesized samples were examined with scanning electron microscopy (SEM) and transmission electron microscopy (TEM) techniques. The SEM images were obtained using a Hitachi S-4800 instrument operating at 50 kV, and the TEM images were obtained by a 2100 JEOL working at 200 kV. The content of N in the amino-functionalized samples was measured by a Vario ELIII elemental analyzer. Thermogravimetric analysis (TGA) was performed on a NETZSCH STA 449C thermogravimetric analyzer at a temperature ramp of 10 °C min1 in air. The isotherms of N2 on the samples investigated were measured by a Micromeritics ASAP 2020 apparatus at 196 °C, and the specific surface areas of the investigated samples were calculated using the multiple-point BrunauerEmmettTeller (BET) method in the relative pressure range of p/p0 = 0.050.3. The pore size distribution curves were computed using the BarrettJoynerHalenda (BJH) method from the desorption branch, and the average pore sizes, DA, were obtained from the peak positions of the distribution curves. The total pore volumes, Vtotal, were derived from the desorption branch by the BJH model. CO2 and N2 Adsorption. The single-component adsorption isotherms of CO2 and N2 were measured by the Micromeritics ASAP 2020. The sample cell was loaded with ca. 250 mg of adsorbent. After the adsorbent was outgassed in vacuum at 120 °C for 12 h in order to remove any adsorbed impurities, the adsorption run was carried out using highly pure CO2 (99.999%) or N2 (99.99%) in a pressure range from 0.01 to 101 kPa at 25 °C. CO2 TPD. The temperature-programmed desorption (TPD) of CO2 was performed on a Micromeritics AutoChem II chemisorption analyzer. The sample cell was loaded with ca. 50 mg of adsorbent and then was heated in flowing helium at 120 °C for 2 h. After the temperature was cooled to 25 °C, a CO2 12874

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Figure 1. FT-IR spectra of 0.10-SBA-15-NH2 samples: (a) as-prepared and (b) ethanol-extracted.

Figure 2. TGA and DTA profiles of 0.10-SBA-15-NH2 samples: (a) as-prepared and (b) ethanol-extracted.

flow with a rate of 30 mL min1 was introduced to the sample cell for CO2 adsorption for 40 min. To remove the weakly adsorbed CO2, the sample was swept using flowing helium with a rate of 30 mL min1 at 25 °C for 1 h. Afterward, the TPD experiment was carried out in flowing helium with a rate of 30 mL min1 from 25 to 120 °C at a temperature ramp of 2 °C min1. After the temperature was cooled to 25 °C, the next run for CO2 adsorption and the TPD experiment were performed by following the same procedure as described above. The effects of water vapor on the adsorption of CO2 were investigated using a house-made TPD-MS analyzer (Balzer Oministar 200). The experimental procedure was the same as that for the CO2-TPD experiment described in the previous paragraph, except for a wet CO2 flow, which came from that the CO2 feed gas was saturated by water vapor when passed through a bottle filled with deionized water, the so-called saturator, instead of a dry CO2 flow for CO2 adsorption, and the mass spectrometer instead of the TCD detector for determining CO2 concentration from the desorption.

’ RESULTS AND DISCUSSION Template Removal. Figure 1 shows the FT-IR spectra of 0.10-SBA-15-NH2. For the as-prepared sample, the absorbance peaks corresponding to the CH stretching and bending vibrations appear in the range of 28503000 cm1 and at 1460 cm1, respectively. The amount of the surfactant in the sample can be

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Figure 3. XRD patterns of x-SBA-15-NH2 samples where x represents the molar composition of APTMS in the initial synthesis solution.

reflected from the intensity of bands around 1377 and 1350 cm1, which can be associated with the stretching vibrations of COC in P123.42,43 From Figure 1, it is easy to find that the intensity of these peaks in the ethanol-extracted sample is dramatically lower than that in the as-prepared sample. Similar observations present for other ethanol-extracted samples (as shown in Figure 8 later), indicating that the surfactant in the as-synthesized materials is effectively removed by extraction with ethanol. The TGA and differential thermal analysis (DTA) profiles of the as-prepared and ethanol-extracted samples of 0.10-SBA-15-NH2 are shown in Figure 2. From the DTA curves, one endothermic peak and one exothermic peak can be seen, corresponding to two weight loss regions in the TGA curves. The one observed at a temperature of ca. 100 °C is due to the weight loss of the physically adsorbed water, whereas the other one at a temperature of ca. 320 °C is primarily attributed to the thermal decomposition of the surfactant and aminopropyl groups incorporated in SBA-15-NH2.42,43 For the as-prepared sample, the intensity of the exothermic peak is much higher than that for the extracted sample, corresponding to a larger weight loss in the TGA curve. This confirms that the surfactant can be effectively removed from the as-synthesized sample by the ethanol-extraction method, in good agreement with the result from the FT-IR characterization (Figure 1). Effects of F. The XRD patterns of x-SBA-15-NH2 samples are shown in Figure 3. For the sample synthesized without the addition of APTMS, that is, x = 0, one intense diffraction peak indexed to the (100) plane and two well-resolved weak diffraction peaks corresponding to (110) and (200) planes can be seen, indicating that the crystallographic ordering of the mesopores is retained after the ethanol extraction. However, the peak intensity of the (100) plane decreases greatly and the higher-order (110) and (200) diffractions become less resolved when x varies from 0.05 to 0.10. All the diffraction peaks almost disappear when x is increased to 0.15, indicating that the addition of APTMS in the synthesis solution has a strongly adverse effect on the formation of SBA-15 with a ordered mesostructure. Similar phenomena were also observed by other investigators, when they synthesized aminopropyl-functionalized SBA-15 materials.42,43,45,46 This is probably due to the facts that aminopropyl groups would perturb the self-assembly of surfactant micelles and the silica precursor because of the formation of zwitterions (NH3þ 3 3 3 OSi) and the competition between protonated amino groups (NH2Hþ) and those positively charged silicate species (SiOH2þ) for the surfactant P123 to form (NH2Hþ)(XSiOH2þ).42,43 12875



Figure 4. XRD patterns of 0.15-SBA-15-NH2 synthesized by adding a required amount of NH4F (a F/Si molar ratio of 0.3) after an APTMS addition of 10, 30, and 60 min, respectively.

To investigate the introduction effects of F on the formation of 0.15-SBA-15-NH2, the XRD patterns of the samples synthesized by adding a required amount of NH4F after APTMS addition at different periods were recorded, shown in Figure 4. After an APTMS addition of 60 min, the sample synthesized by adding NH4F does not show well-resolved diffraction peaks. However, when F ions are introduced after an APTMS addition of 10 min, not only is the diffraction peak of the (100) plane intense but also the well-resolved diffraction peak of (200) plane is observed. Especially, the sample synthesized by adding F after an APTMS addition of 30 min exhibits one intense diffraction indexed to the (100) plane and two well-resolved weak diffraction peaks corresponding to (110) and (200) planes. These results clearly show that the ordered mesostructure of 0.15-SBA15-NH2 can be greatly reserved by introducing F into the synthesis solution and the optimized time for NH4F addition into the synthesis solution is 30 min after APTMS addition. The degree of the polymerization of silicates can be enhanced in the presence of F,47,48 and this enhancement can lead to a high charge density of the silicate network to improve the interactions among anions (e.g., Cl), charge-associated EO units [EO(H3Oþ)] of P123, and cationic silica species [e.g., Si(OEt)4n(OHþ 2 )n], resulting in a better formation of (S0Hþ)(XIþ).4951 In addition, when F ions are introduced at the early stage of hydrolysis and condensation of silicates, such as TEOS, APTMS, and their oligomerizations, the addition of F mainly has a direct effect on these silicates, and the introduced F ions probably play a catalytic role in favoring the polymerization of these silicate species.47 However, by introducing F after 60 min of the silicate polymerization, F may play other roles instead of improving the polymerization of the silicate species, for example, as balancing anions to combine with other cations, such as [EO(H3Oþ)], H3Oþ, and/or Si(OEt)4n(OH2þ)n. Furthermore, due to its small radius and high charge density, F can interact easily with a protonated amino group to form NH3þ 3 3 3 F, which could disturb and/or reduce the formation of the zwitterions (NH3þ 3 3 3 OSi) and (NH2Hþ)(XSiOH2þ). Consequently, the direct interactions of the surfactant P123 with the charged silicate species (SiOH2þ) will not be destroyed and an ordered mesostructure of SBA-15 is retained, when F ions are introduced at the early stage of hydrolysis and condensation of silicates. With the introduction of F, the effects of the APTMS content in the synthesis solution on the formation of the SBA-15

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Figure 5. XRD patterns of x-SBA-15-NH2 samples, where x represents the molar composition of APTMS in the initial synthesis solution, synthesized by adding a required amount of NH4F (a F/Si molar ratio of 0.3) after an APTMS addition of 30 min.

Figure 6. XRD patterns of 0.30-SBA-15-NH2 synthesized by adding different amounts of NH4F after an APTMS addition of 30 min.

mesostructure have been investigated. Figure 5 shows the XRD patterns of the samples synthesized by adding a required amount of NH4F (a F/Si molar ratio of 0.3) after an APTMS addition of 30 min. When the molar composition of APTMS, x, is equal to 0.15, 0.20, and 0.25, respectively, in the synthesis solution, the diffraction peaks of (100), (110), and (200) planes of these synthesized samples are clearly observed, but the intensity of these peaks decreases with increasing x value. When the value of x is increased to 0.30, no diffraction peak is observed. This is probably related to the basicity in the synthesis solution. It was observed that the pH value in the synthesis solution increases with increasing APTMS content because of the alkalescence of APTMS in nature and the mesostructure of SBA-15 would be decreased and/or disrupted when the pH value increases.43,52 Consequently, when the APTMS content is increased in the initial synthesis solution, the mesostructure ordering of the resulting sample decreases, in which F cannot compensate for the adverse effect resulting from amino groups in APTMS when the value of x is increased to 0.30. Figure 6 shows the XRD patterns of the 0.30-SBA-15-NH2 samples synthesized by adding different amounts of NH4F after an APTMS addition of 30 min. Apparently, when the molar ratio of F/Si is increased to 0.5, one intense and two well-resolved weak diffraction peaks are observed, indicating that the sample with a highly ordered SBA-15 mesostructure is obtained. This result implies that the adverse effects on the SBA-15 mesostructure due 12876



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to an increase of pH value resulting from APTMS addition into the initial synthesis solution can be compensated for by increasing the F content in the synthesis system. However, the synthesized sample will become disordered, when the molar composition of APTMS is further increased, although the molar ratio of F/Si is as high as 0.5. Textural and Physicochemical Properties. Figure 7 depicts the adsorptiondesorption isotherms of N2 on x-SBA-15-NH2 samples at 196 °C and their corresponding BJH pore size

distribution plots. The determined values of the BET surface area (SBET), average pore size (DA), and total pore volume (Vtotal) are summarized in Table 1. From Figure 7a, it can seen that the puresilica SBA-15 sample displays a type IV isotherm with H1 hysteresis and has a steep increase in the adsorbed amount in a relative pressure range of 0.650.83. When the values of x are equal to 0.05 and 0.10, both samples exhibit a type IV isotherm with H1 hysteresis and have a lower specific area and a smaller pore volume, compared with those of the pure-silica SBA-15 sample. However, their average pore sizes calculated from the desorption branches are only slightly lower than that of the puresilica SBA-15 (Figure 7b and Table 1). These results indicate that, in the presence of a relatively low concentration of APTMS in the initial synthesis solution, the textural properties of SBA-15 are substantially maintained, although a significantly mesostructural disordering is revealed by the XRD characterization shown in Figure 3. When the values of x are in the range from 0.15 to 0.30, the synthesized samples also exhibit a type IV isotherm with H1 hysteresis but their adsorbed amounts of N2 increase in company with the onset of the capillary condensation step shifting to a higher relative pressure range of 0.680.92, indicating that these samples possess a larger pore size and a higher pore volume. As shown in Figure 7b, the pore size distribution (PSD) of 0.05-SBA-15-NH2 and 0.10-SBA-15-NH2 is centered at about 6.2 nm, whereas it is at about 8.8 nm for 0.15-SBA-15-NH2 and at about 7.3 nm for other samples. These results imply that a larger amount of the surfactant P123 is connected to the charged silicate species (SiOH2þ) after F is introduced into the synthesis solution at the early stage. This could be due to the fact that F can displace part of the Cl in the acidic system to combine with the charge-associated EO units [EO(H3Oþ)] of P123 and the charged silicate species (SiOH2þ) by the electrostatic interactions, that is, (S0Hþ)(FIþ). Because of the small radius and high charge density of F, the electrostatic interactions between F and SiOH2þ are much stronger than those between Cl and SiOH2þ during the coordination sphere expansion around the silicon atom,49,50 resulting in more P123 assembling around the silicon atoms after the introduction of F. The FT-IR spectra of SBA-15 functionalized with different contents of APTMS in the initial synthesis solution are shown in Figure 8. In all the samples, the peaks around 1220, 1070, 801, and 473 cm1 are ascribed to the typical SiOSi bands of the condensed silica network and only a weak peak associated with

Figure 7. Adsorptiondesorption isotherms of N2 on x-SBA-15-NH2 samples where x represents the molar composition of APTMS in the initial synthesis solution at 196 °C (a) and their corresponding BJH pore size distribution plots from the desorption branches (b) (all the curves are setoff along the y axis for clarity).

Table 1. Textural and Physicochemical Properties of the Synthesized Mesoporous SBA-15 Materials elemental analysis of amine group N content (mmol 3 g1) theoretical

incorporation

SBET

Vtotal

amine surface

EA

value

ratiob (%)

(m2 3 g1)

DA (nm)

(cm3 3 g1)

densityc (group 3 nm2)

0.98

ratio of APTMS/ sample codea

(TEOS þ APTMS)

a

0

0

0

502

6.49

b

0.05

0.63

0.76

82.47

445

6.13

0.88

0.85

c

0.10

1.28

1.54

83.23

367

6.12

0.76

2.10

d

0.15

2.14

2.22

96.50

433

9.32

1.37

2.98

e

0.20

2.74

2.90

94.48

516

7.68

1.34

3.19

f

0.25

3.26

3.45

94.46

443

7.37

1.09

4.43

g

0.30

3.92

4.14

94.67

445

7.90

1.23

5.30

a The molar ratios of F to Si for synthesizing samples ag are 0, 0, 0, 0.3, 0.3, 0.3, and 0.5, respectively. b Incorporation ratio (%) = (N content of EA value/N content of theoretical value) 100%. c Amine surface density = N content of EA value/SBET.

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The Journal of Physical Chemistry C noncondensed SiOH groups is present at around 950 cm1,53 proving that the polymerization of TEOS, APTMS, and their oligomerizations is almost complete. The presence of the NH bending vibration around 687 cm1 and the NH2 scissor vibration around 1647 cm1 confirms the existence of amino groups.42,43 As shown in Figure 8, the intensity of these peaks increases with increasing the content of APTMS added in the synthesis solution, indicating the content of N in the samples

Figure 8. FT-IR spectra of x-SBA-15-NH2 samples where x = 0.05 (a), 0.10 (b), 0.15 (c), 0.20 (d), 0.25 (e), and 0.30 (f), respectively.

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corresponding to the aminopropyl amount of the added APTMS, in good agreement with the results from the elemental analysis shown in Table 1. The presence of aminopropyl groups in the functionalized SBA-15 is further corroborated by a broad band at 27003500 cm1, ascribed to the NH stretching around 3346 cm1 for free amine and around 3305 cm1 for terminal amino groups, respectively.54 The SEM images of x-SBA-15-NH2 samples are shown in Figure 9, from which it can be seen that all the samples are composed of uniform and ropelike domains, just like other SBA15 silica materials.5557 To further identify the mesostructure, especially the mesopore arrangement of all the samples, the TEM observations were performed and their images viewed from a perpendicular direction are presented in Figure 10, from which it can be seen that the well-ordered mesoporous SBA-15 materials were synthesized.. CO2 and N2 Adsorption. The adsorption isotherms of CO2 on the amino-functionalized SBA-15 and pure SBA-15 samples are plotted in Figure 11. It can be seen that the adsorbed amount of CO2 on the pure silica SBA-15 as a function of pressure shows a linear relationship in the whole pressure range investigated, indicating that only the physical adsorption occurs. However, all the isotherms of CO2 on the amino-functionalized SBA-15 adsorbents are of type I, according to the IUPAC classification, with a steep slope in a low pressure range from 0.01 to 15 kPa,

Figure 9. SEM images of x-SBA-15-NH2 samples where x = 0.05 (a), 0.10 (b), 0.15 (c), 0.20 (d), 0.25 (e), and 0.30 (f), respectively. 12878



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Figure 10. TEM images of x-SBA-15-NH2 samples where x = 0.05 (a), 0.10 (b), 0.15 (c), 0.20 (d), 0.25 (e), and 0.30 (f), respectively.

Figure 11. Adsorption isotherms of N2 on 0.30-SBA-15-NH2 (A) and of CO2 on x-SBA-15-NH2 samples where x = 0 (B), 0.05 (C), 0.10 (D), 0.15 (E), 0.20 (F), 0.25 (G), and 0.30 (H) at 25 °C.

reflecting the strong interactions between CO2 molecules and amino groups.58 In comparison of curve B with curves CH, it is clear that the adsorbed amounts of CO2 on the amino-functionalized adsorbents are much higher than that on the pure silica SBA15 in the whole pressure range investigated, accounting for the role of the presence of amino groups in the amino-functionalized adsorbents for CO2 adsorption. Additionally, the adsorbed amount of CO2 on the amino-functionalized adsorbents increases with the content of amino groups present in the adsorbents. According to Zelenak and co-workers’ results, amino-functionalized ordered mesoporous silicas with a large pore and a high amino surface density have a large CO2 adsorption capacity.59 As shown in

Table 1, the samples prepared with the aid of F possess a larger pore size and a higher amino surface density than those prepared without introduction of F, resulting in favor of CO2 adsorption. In comparison with the reported data at 101 kPa and 25 °C, the adsorbed amount of CO2 on 0.30-SBA-15-NH2 is 2.23 mmol g1, much higher than 1.25 mmol g1 on the amino-grafted MCM-41,60 0.40 mmol g1 on the PEI surface-modified MCM-48,61 0.80 mmol g1 on the APTMS surface-modified MCM-48,61 1.30 mmol g1 on the amino-functionalized silica nanospheres with centrosymmetric radial mesopores,62 and 1.12 mmol g1 on the APTMS surface-modified silica xerogel.24 Curve A in Figure 11 represents the adsorption isotherm of N2 on the 0.30-SBA-15-NH2 sample at 25 °C, indicating that N2 adsorption on this amino-functionalized material is negligible under the investigated conditions. The ideal adsorption selectivity for CO2 over N2 on 0.30-SBA-15-NH2 is 368 at 25 °C and 101 kPa, much higher than 18 on zeoliet-13X,9 8 on activated carbon,9 22 on MCM-41,63 and 308 on the triamino surfacemodified pore-expanded MCM-41 (TRI-PR-MCM-41),58 implying the potential applicability of the 0.30-SBA-15-NH2 adsorbent for CO2N2 separation. To elucidate the adsorption state of CO2 on the 0.30-SBA-15NH2 sample, the in situ FT-IR study was carried out, and these results are shown in Figure 12. The peaks at around 1632, 1563, and 1484 cm1 are assigned to NH3þ deformation, CdO stretch, and the ‘‘NCOO’’ skeletal vibration of alkylammonium carbamate (RNHCOOþH3NR; R = alkyl), respectively.64 Similar absorption bands were also found by Dreyfuss and coworkers,65,66 when they studied the reaction between liquid 12879



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Figure 12. In-situ FT-IR spectra of CO2 adsorption on the 0.30-SBA15-NH2 sample.

Figure 13. Multicycle TPD profiles of CO2 on the 0.30-SBA-15-NH2 sample.

APTMS and CO2 and concluded that these absorption bands were due to the formation of alkylammonium carbamate. In addition, the peak at around 3420 cm1 in Figure 12 could be ascribed to the NH stretch in RNHCOO, as assigned by Wang et al.,28 confirming that carbamate is formed after CO2 adsorption. Therefore, under dry conditions, the formation of carbamate-type zwitterions is believed to be the main reaction to account for CO2 adsorption, represented in reactions 13 (where R = alkyl). CO2 þ 2RNH2 f RNHCOO þ RNH3 þ

ð1Þ

CO2 þ 2R 2 NH f R 2 NCOO þ R 2 NH2 þ

ð2Þ

CO2 þ 2R 3 N f R 2 NCOO þ R 4 Nþ

ð3Þ

Figure 14. Effects of water vapor on the adsorption of CO2 on the 0.30SBA-15-NH2 sample.

In specific interactions that involved adsorption for CO2 separation, temperature swing adsorption (TSA) is a common process, in which raising the operating temperature can enhance the desorption of the adsorbed CO2. Therefore, the thermal stability of CO2 adsorbents is crucial in practical applications. To check the thermal stability of the synthesized amino-functionalized SBA-15, the multicycles of adsorption at 25 °C and the temperature-programmed desorption (TPD) were performed. Figure 13 shows the multicycle adsorption TPD profiles of CO2 on 0.30-SBA-15-NH2, indicating that, although the desorptionpeak area slightly decreases from the first run to the third run (Figure 13a), after the three cycles, the desorption-peak area maintains unchanged (Figure 13b). This result proves that the thermal stability of 0.30-SBA-15-NH2 allows it to be regenerated. By a conventional method to remove acidic gases, such as CO2 from natural gas, water vapor always plays an important role of a proton-transfer agent in the reaction of acidic gases and amine solutions.24 It is thus important to study the effects of water vapor on CO2 removal. The TPDMS technique was used to investigate the effects of water vapor on the adsorption of CO2 on 0.30-SBA-15-NH2. These results are shown in Figure 14, from which it can be seen that, when the feed of CO2 saturated with water vapor is used for CO2 adsorption at 25 °C, the desorptionpeak area is larger, compared with that after CO2 adsorption without the introduction of water vapor, clearly indicating that the presence of water vapor can enhance the adsorbed amount of CO2. This also implies that the synthesized amino-functionalized SBA-15 adsorbent has a potential applicability in practice for CO2 adsorption, in which water vapor is always present and can dramatically reduce the adsorption capacity of CO2 on most conventional adsorbents, such as zeolites. This promoting effect of water vapor can be explained on the basis of the mechanism of the reactions between CO2 and amino groups (i.e., the formation of carbamate under dry conditions and the partial formation of bicarbonate in the presence of moisture).24,67 Two moles of amino groups are required to remove 1 mol of CO2 to form 1 mol of carbamates, as shown in reactions 13 in the absence of water vapor, whereas 1 mol of amino groups is effective to remove 1 mol of CO2 to form 1 mol of bicarbonates, as shown in reactions 4 and 5 in the presence of water. Additionally, the amino groups themselves can also directly react with CO2 and H2O to form bicarbonates, as shown in reactions 68. Therefore, water vapor can enhance the adsorption of CO2 on the amino-functionalized materials. However, the molar ratio of CO2 12880



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to N is always between 0.5 and 1.0 in the presence of water vapor in reality.6870 This could be associated with the residence time of CO2, H2O, carbamates, and/or amino groups that are not long enough to reach equilibrium and that the faster carbamate production is the dominating reaction in the dynamic process.70 R 2 NCOO þ 2H2 O þ CO2 f R 2 NH2 þ þ 2HCO3

ð4Þ

RNHCOO þ 2H2 O þ CO2 f RNH3 þ þ 2HCO3

ð5Þ

CO2 þ RNH2 þ H2 O f RNH3 þ þ HCO3

ð6Þ

CO2 þ R 2 NH þ H2 O f R 2 NH2 þ þ HCO3

ð7Þ

CO2 þ R 3 N þ H2 O f R 3 NHþ þ HCO3

ð8Þ

’ CONCLUSIONS Well-ordered aminopropyl-functionalized SBA-15 mesoporous materials were synthesized by cocondensation of TEOS and APTMS using P123 as a pore-directing agent under acidic conditions. The long-range ordering of the mesoporous structure of the synthesized SBA-15 materials decreases with increasing the APTMS concentration in the initial synthesis solution. When APTMS with a high concentration in the initial synthesis solution is used, however, the long-range ordering of the SBA-15 mesoporous structure can be reserved by the aid of F. The synthesized amino-functionalized ordered mesoporous silica 0.30-SBA15-NH2 with a large pore and a high amino surface density has a larger CO2 adsorption capacity and a higher selectivity for CO2 over N2, compared with the amino-functionalized mesoporous adsorbents having been reported in the literature so far. Additionally, the synthesized amino-functionalized SBA-15 adsorbent shows a good thermal stability and the presence of water vapor can enhance CO2 adsorption, implying the applicability of this adsorbent in CO2 adsorption and separation. ’ AUTHOR INFORMATION Corresponding Author

*Tel/Fax: þ86 579 82282932. E-mail: [email protected].

’ ACKNOWLEDGMENT The financial support provided by the National Basic Research Program of China (2009CB626607), the National Natural Science Foundation of China (21036006), the Natural Science Foundation of Zhejiang Province, China (R4080084, Y4110289), and the Science and Technology Commission Foundation of Zhejiang Province, China (2011C31G2030050) is gratefully acknowledged. ’ REFERENCES (1) DeMontigny, D.; Tontiwachwuthikul, P.; Chakma, A. Ind. Eng. Chem. Res. 2005, 44, 5726–5732. (2) Veawab, A.; Tontiwachwuthikul, P.; Chakma, A. Ind. Eng. Chem. Res. 1999, 38, 3917–3924. (3) Blauwhoff, P. M. M.; Versteeg, G. F.; Van Swaaij, W. P. M. Chem. Eng. Sci. 1984, 39, 207–225. (4) Tontiwachwuthikul, P.; Meisen, A.; Lim, C. J. J. Chem. Eng. Data 1991, 36, 130–133. (5) Teramoto, M. Ind. Eng. Chem. Res. 1995, 34, 1267–1272.

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One-Pot Synthesis and CO2 Adsorption Properties of Ordered

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