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Adsorption characteristics of carbon dioxide on organically functionalized SBA-15
Microporous and Mesoporous Materials 84 (2005) 357–365 www.elsevier.com/locate/micromeso
Adsorption characteristics of carbon dioxide on organically functionalized SBA-15 Norihito Hiyoshi 1, Katsunori Yogo *, Tatsuaki Yashima Research Institute of Innovative Technology for the Earth, 9-2 Kizugawadai, Kizu-cho Soraku-gun, Kyoto 619-0292, Japan Received 28 December 2004; received in revised form 21 May 2005; accepted 13 June 2005 Available online 27 July 2005
Abstract Aminosilane-modified SBA-15 was prepared by grafting various aminosilanes on mesoporous silica SBA-15, and its adsorption characteristics towards carbon dioxide were examined. The amount of carbon dioxide adsorbed was almost the same for both in the presence and in the absence of water vapor. It was found that the efficiency of adsorption, defined as the number of adsorbed carbon dioxide molecules per nitrogen atom of aminosilane-modified SBA-15, increased with increasing the surface density of amine. Infrared spectroscopy revealed that carbon dioxide was adsorbed on aminosilane-modified SBA-15 through the formation of alkylammonium carbamate in the presence and in the absence of water vapor. It was suggested that amine pairs, on which carbon dioxide was adsorbed through formation of alkylammonium carbamate, increased with increasing surface density of amine. In addition, influence of amine structure on the adsorption capacity has also been discussed. 2005 Elsevier Inc. All rights reserved. Keywords: Adsorption; Carbon dioxide; Organically functionalized SBA-15; Water vapor; Aminosilane
1. Introduction The gradual increase in the atmospheric concentration of carbon dioxide (CO2) due to fossil fuel combustion is becoming a serious environmental problem. Capture and sequestration of CO2 has been considered as one of the options to reduce CO2 emission. Various processes, such as liquid solvent absorption [1], membrane separation [2], and pressure (and/or temperature) swing adsorption (P(T)SA) [1,3], have been proposed for
* Corresponding author. Tel.: +81 774 75 2306; fax: +81 774 75 2319. E-mail addresses: [email protected] (N. Hiyoshi), yogo@rite. or.jp (K. Yogo). 1 Research Center for Compact Chemical Process, National Institute of Advanced Industrial Science and Technology (AIST), 4-2-1, Nigatake, Miyagino, Sendai 983-8551, Japan. Tel.: +81 222372028; fax: +81 222375224.
1387-1811/$ - see front matter 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2005.06.010
the separation and recovery of CO2 emitted by power plants. However, the costs of CO2 separation from flue gases account for approximately 70–80% of the total energy cost for CO2 sequestration. Therefore, it is important to develop new energy-efficient techniques for CO2 separation. The conventional PSA or PTSA process using zeolite requires a dehumidification step which consumes about 30% of the total energy, because water vapor is adsorbed more strongly than CO2 on a zeolite surface. Therefore, a new adsorbent [4–13] that preferably adsorbs CO2 in the presence of water vapor is desirable for the development of a simple and energy-efficient CO2 removal process that will eliminate the dehumidification step. Recently, organically functionalized mesoporous silica has attracted considerable attention because of the wide range applications as adsorbents and catalysts [8– 21]. Since mesoporous silica has uniform and large pores as well as high surface area, a large number of active
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sites or adsorption sites can be introduced uniformly on pore walls of mesoporous silica by its surface modification with organosilane molecules. SBA-15 is a suitable mesoporous silica for applications in removal/separation of gases containing water vapor, due to its higher hydrothermal stability [22]. In this study, aminosilanemodified mesoporous molecular sieve was prepared by grafting various aminosilanes on SBA-15, and its adsorption characteristics were studied. Surface density of amine on SBA-15 was found to be an important factor for the reaction of CO2 with amine immobilized on the mesoporous support.
ent aminosilanes, N-methylaminopropyltrimethoxysilane (H3CNHCH2CH2CH2Si(OCH3)3, MAPS, Gelest Inc.), (N,N-dimethyl-3-aminopropyl)trimethoxysilane ((H3C)2NCH2CH2CH2Si(OCH3)3, DMAPS, Gelest Inc.), and N-(2-aminoethyl)-3-aminopropylmethyldiethoxysilane (H2NCH2CH2NHCH2CH2CH2Si(CH3)(OCH3)2, AEAPMS, Chisso co.) were also used as grafting agents. (iii) SBA-15 was boiled in water for 2 h followed by grafting of aminosilanes. Boiled SBA-15 was dried at 398 K for 6 h in air, and then modified with 17 vol% toluene solutions of aminosilanes. The adsorbent was named as e.g. APS/SBA(I), which was prepared using APS under a set of conditions given in (i).
2. Experimental
2.3. Characterization
2.1. Preparation of SBA-15 support
Elemental analysis was carried out with PE-2400 (Perkin Elmer). N2 adsorption–desorption isotherms were measured at 77 K by an automatic adsorption system (Autosorb, Quantachrome). Before measurements, the samples were kept under a vacuum at 473 K for 3 h. The surface areas and the pore size distributions were calculated by the BET and BJH method, respectively. The total pore volume was calculated from the amount of adsorbed N2 at P/P0 = 0.97. Infrared spectra of SBA-15 and aminosilane-modified SBA-15 were obtained using FT/IR-610 (Jasco Co.) equipped with an MCT detector. In order to remove adsorbed CO2 and H2O, a sample pellet of 20 mm in diameter was placed in a flow cell with ZnSe windows, and then heated in a He flow at 423 K for 1 h. The IR spectra were recorded at 333 K in a He flow (resolution: 4 cm1, 64 scans).
The SBA-15 support was prepared using tetraethoxyorthosilicate (TEOS, Kishida Chem. Co.) as a silica source and polyethyleneglycol-block-polypropyleneglycol-block-polyethyleneglycol with an average molecular weight of 5800 (PEO20PPO70PEO20, Aldrich) as a structure directing agent [22]. The reaction mixture of PEO20PPO70PEO20 (50 g), TEOS (0.53 mol), hydrochloric acid (2.0 mol) and distilled water (80 mol) was heated under stirring at 303 K for 20 h, and then at 368 K for 24 h. The resulting precipitate was filtered and washed with 4000 cm3 of distilled water, and then dried at 343 K over night. The dried precipitate was calcined in air at 823 K for 8 h to remove the structure directing agent. The 2-D hexagonal structure of calcined SBA15 thus prepared was confirmed by X-ray diffraction (XRD) from which the lattice constant of SBA-15 was estimated to be 11.3 nm. After calcination, a part of SBA-15 was boiled in water at 368 K for 2 h. 2.2. Preparation of aminosilane-modified SBA-15 Various aminosilanes used as grafting agents were 3-aminopropyltriethoxysilane (H2NCH2CH2CH2Si(OCH2CH3)3, abbreviated as APS, Aldrich), N-(2aminoethyl)-3-aminopropyltrimethoxysilane (H2NCH2CH2NHCH2CH2CH2Si(OCH3)3, AEAPS, Chisso co.) and (3-trimethoxysilylpropyl)diethylenetriamine (H2NCH 2 CH 2 NHCH 2 CH 2 NHCH 2 CH 2 CH 2 Si(OCH 3 ) 3 , TA, Gelest Inc.). Samples of aminosilane-modified SBA-15 were prepared under three different grafting conditions: (i) Calcined SBA-15 (5.0 g), which was previously dried at 398 K for 6 h in air, was refluxed in the toluene solution of aminosilane (1.7 vol%, 250 cm3) at 383 K for 24 h under an Ar flow. The product was washed with toluene (200 cm3) and dried at 333 K over night. (ii) The same procedure as given in (i) was followed except that 17 vol% of aminosilane solutions were used. Here, differ-
2.4. Adsorption experiments The adsorption capacity of the adsorbent was determined by a flow method using CO2–H2O–N2 or CO2–N2 mixtures. A typical procedure using a CO2– H2O–N2 mixture was as follows. Aminosilane-modified SBA-15 (1.5 g, on dry basis) was placed in a Pyrex tube (13 mm inner diameter) and heated at 423 K for 1 h in a He flow (30 cm3 min1), and then cooled to 333 K. A mixture of 12 kPa H2O with He balance (total flow rate: 60 cm3 min1) was passed over the adsorbent at 333 K until the adsorbent was saturated with water vapor, and then the gas flow was switched to a mixture of 15 kPa CO2 and 12 kPa H2O with N2 balance (total flow rate: 30 cm3 min1). The breakthrough curve of CO2 was obtained by analysing the effluent gases using a gas chromatograph (GC-332, GL science Inc.) equipped with a gaskuropack 54 column (2 m) and a TCD detector. In the case of measurement with a CO2–N2 mixture, a mixture of 15 kPa CO2 with N2 balance (total flow rate: 30 cm3 min1) was passed over the dried adsorbent without pre-adsorption of water vapor.
N. Hiyoshi et al. / Microporous and Mesoporous Materials 84 (2005) 357–365
359
3. Results and discussion 3.1. SBA-15 support
800
V/cm3(STP).g-1
600
(dV/dDp)/cm3 nm-1 g-1
Fig. 1 shows the N2 adsorption–desorption isotherms of SBA-15 before and after boiling in water for 2 h. Both the isotherms were classified as type IV characteristic of mesoporous silica. Although, the amount adsorbed on SBA-15 boiled in water was slightly smaller than that on SBA-15 before boiling, the difference in the shapes of the isotherms was not significant. The surface area and pore volume of SBA-15 before boiling were 910 m2 g1 and 1.11 cm3 g1, and those after boiling
2
3742
4000
3500
3000
2500
1970 1860
a
2000
1500
Wavenumber/cm-1 Fig. 2. Infrared spectra of SBA-15 (a) before and (b) after boiling for 2 h.
were 820 m2 g1 and 1.07 cm3 g1. The pore size distributions of SBA-15 were almost identical before and after boiling, showing sharp peaks at 6.0 nm (Fig. 1). These results suggest that the pore structure of SBA15 partially collapsed after the boiling treatment for 2 h due to the hydration of Si–O–Si bonds, which results in the decrease in the surface area and pore volume; however, the collapse of the pore structure is not remarkable. The IR spectra of SBA-15 supports (Fig. 2) revealed that the isolated hydroxyl group (3742 cm1) decreased after boiling treatment for 2 h. On the other hand, the integrated peak area of overlapping peaks from 3000 to 3740 cm1 due to internal hydroxyl group and hydrogen bonded hydroxyl group, etc. [23,24] increased by approximately 20%. This change in hydroxyl stretch region would result from hydration of surface Si–O–Si bonds by boiling treatment.
1
3.2. Characterization of aminosilane-modified SBA-15 0
400
4 5 6 7 8 Pore diameter/nm
200
Before boiling treatment After boiling for 2 h 0 0
b
3640 3560
Infrared spectra of adsorbents in CO2–He and CO2– H2O–He mixtures were obtained using FT/IR-610 (Jasco Co.) equipped with an MCT detector. The measurement with a CO2–H2O–He mixture was performed as follows. A sample pellet placed in a flow cell was heated in a He flow at 423 K for 1 h. A mixture of 2.3 kPa H2O with He balance was fed to the flow cell at 333 K for 30 min so that the water vapor could be adsorbed on the sample surface, and then a background spectrum was recorded. After this, a mixture of 3 kPa CO2 and 2.3 kPa H2O with He balance was introduced at 333 K for 5 min, and then a spectrum was obtained. In the case of measurement with a mixture of 3 kPa CO2 with He balance, a spectrum was obtained without pre-adsorption of water vapor.
Absorbance (a.u.)
2.5. Infrared spectra in CO2–He and CO2–H2O–He mixtures
0.1
0.2
0.3
0.4
0.5
0.6
0.7 0.8
0.9 1.0
P/Po Fig. 1. Nitrogen adsorption–desorption isotherms for SBA-15 before and after boiling in water for 2 h, and pore size distributions calculated from desorption branches.
The amount of aminosilane anchored on SBA-15 surface was determined by the elemental analysis. Amine contents and surface densities of amine on various adsorbents are summarized in Table 1. The amine content was defined as the number of nitrogen atoms per gram of the adsorbent, and the surface density of amine was defined as the number of nitrogen atoms per nm2 of SBA-15 surface. The amine contents and the surface densities of amine for various adsorbents were found to be in the following order: APS/SBA(I) < APS/ SBA(II) < APS/SBA(III) for APS-modified SBA-15, AEAPS/SBA(I) < AEAPS/SBA(II) < AEAPS/SBA(III) for AEAPS-modified SBA-15 and TA/SBA(I) < TA/ SBA(II) < TA/SBA(III) for TA-modified SBA-15. Thus APS, AEAPS and TA were more densely anchored on
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N. Hiyoshi et al. / Microporous and Mesoporous Materials 84 (2005) 357–365
Table 1 Physicochemical properties of adsorbents Adsorbent
Surface area (m2 g1)
Pore volume (Vcal.a) (cm3 g1)
Amine contentb (N mmol g1)
Surface densityc (N atom nm2)
Surface coverage (%)
APS/SBA(I) APS/SBA(II) APS/SBA(III) AEAPS/SBA(I) AEAPS/SBA(II) AEAPS/SBA(III) TA/SBA(I) TA/SBA(II) TA/SBA(III) MAPS/SBA(II) DMAPS/SBA(II) AEAPMS/SBA(II)
616 364 374 480 312 250 473 240 183 414 400 297
0.76 0.53 0.54 0.67 0.47 0.40 0.65 0.37 0.29 0.57 0.54 0.47
1.11 2.57 2.61 2.26 3.76 4.61 2.75 4.85 5.80 1.88 1.79 3.52
0.83 2.37 2.68 1.79 3.49 5.21 2.19 4.69 6.85 1.62 1.56 3.23
25 70 79 35 69 103 38 80 117 57 –d –d
c d
Pore volume predicted from aminosilane loadings. The amount of nitrogen atom per 1 g of adsorbent. The amount of nitrogen atom per 1 nm2 of SBA-15 support. Not calculated.
the surface of SBA-15 boiled in water than that of nonboiled SBA-15. This suggests that not only the isolated hydroxyl groups but also the hydrogen bonded hydroxyl groups are suitable sites for anchoring aminosilanes. The surface coverage of aminosilane is also summarized in Table 1. The calculation of surface coverage is based on cross-section areas of aminosilanes. The cross-section areas of APS, AEAPS, TA and MAPS were estimated to be 0.30, 0.39, 0.51 and 0.35 nm2, respectively [13]. The surface coverage of adsorbents prepared according to the condition (i) was in the range of 25–38%. The surface coverage of adsorbents prepared according to the conditions (ii) and (iii) were found to be in the range of 57–80% and 79–117%, respectively. There were small multilayer portions on the surfaces of AEAPS/SBA(III) and TA/SBA(III). The N2 adsorption–desorption isotherms of aminosilane-modified SBA-15 showed that the uniform mesoporous structure and high surface area were still retained after grafting. In Table 1, the pore volume measured by N2 adsorption is compared with the predicted value (Vcal.) [13]. Although the predicted value was estimated roughly, the experimentally measured pore volume was close to the predicted value for each adsorbent. This suggests that pore plugging by aminosilane grafting is negligible. Fig. 3 shows pore size distributions of aminosilane-modified SBA-15. The uniform pore size distribution was retained for aminosilane-modified SBA-15. The mean pore diameters of aminosilanemodified SBA-15 were lower than those of the SBA-15 supports (6.0 nm), indicating that aminosilanes were anchored on pore walls of SBA-15. The mean pore diameters of APS-, AEAPS- and TA-modified SBA-15 decreased in the following order, APS > AEAPS > TA. This order was in accordance with the order of molecular sizes of the respective aminosilanes. Also, the pore diameters were influenced by the amine content; the
1.5
APS/SBA(I) AEAPS/SBA(I) TA/SBA(I)
1.0 0.5 0 1.5 (dV/dDp)/cm3.nm-1.g-1
a b
(0.87) (0.54) (0.54) (0.76) (0.51) (0.39) (0.73) (0.40) (0.29) (0.60) (–d) (–d)
APS/SBA(II) AEAPS/SBA(II) TA/SBA(II)
1.0 0.5 0 1.5
APS/SBA(III) AEAPS/SBA(III) TA/SBA(III)
1.0 0.5 0 2
3
4
5
6
7
8
Pore diameter/nm Fig. 3. Pore size distributions of aminosilane-modified SBA-15.
mean pore diameters of (I) samples were larger than corresponding (II) and (III) samples. Fig. 4 shows the IR spectra of aminosilane-modified SBA-15. In the IR spectrum of APS/SBA(II) (Fig. 4a), absorption bands at 3368, 3302 and 1595 cm1 were observed. These were assigned to asymmetric NH2 stretch (masNH2), symmetric NH2 stretch (msNH2) and
N. Hiyoshi et al. / Microporous and Mesoporous Materials 84 (2005) 357–365
c
1667 1602 1457
2933 2868
b
Wavenumber (cm1) masNH2
msNH2
dNH2
APA/SBA(I) APA/SBA(II) APA/SBA(III)
3371 3368 3367
3308 3302 3301
1595 1595 1594
masNH2
msNH2 and mNH
AEAPA/SBA(I) AEAPA/SBA(II) AEAPA/SBA(III)
3367 3362 3361
3306 3302 3299
masNH2
msNH2 and mNH
TA/SBA(I) TA/SBA(II) TA/SBA(III)
3364 3359 3360
3302 3297 3296
1600 1602 1600
1601 1602 1603
3000
2500
2000
1472 1448
1984 1850
1595
3368 3302
3648
1458
1651 1602
2932 2881 2819
Adsorbent
3362 3302
Absorbance (a.u.)
3359 3297
2932 2883 2821
Table 2 Wavenumber of N–H vibrations for APS-, AEAPS- and TA-modified SBA-15
a 3500
361
1500
Wavenumber/cm-1 Fig. 4. Infrared spectra of (a) APS/SBA(II), (b) AEAPS/SBA(II) and (c) TA/SBA(II).
NH2 deformation (dNH2) of hydrogen bonded amino group, respectively [25–27]. Besides these, absorption bands due to (Si–)OH stretch (3648 cm1), overtone of Si–O–Si lattice vibration (1984 and 1850 cm1) [23,24], CH2 stretch (2868 and 2933 cm1) and CH2 deformations (1472 and 1448 cm1) were also observed in this spectrum [28]. The spectra of AEAPS/SBA(II) and TA/SBA(II) were similar (Fig. 4b and c) to that of APS/SBA(II). The absorption bands around 3360 and 1602 cm1 were assigned to asymmetric NH2 stretch (masNH2) and NH2 deformation (dNH2) [21], and that around 3300 cm1 would be due to overlap of NH stretch of secondary amine (mNH) and symmetric NH2 stretch (msNH2). It has been reported that there are various structures of amine on APS-modified silica, e.g. free, hydrogen bonded and protonated amino groups ðR NHþ 3 O SiÞ, etc. [25–27,29–31]. According to IR studies by several groups, the absorption bands due to masNH2 and msNH2 of APS-modified silica shift to lower frequency by hydrogen bonding [25–27]. Okabayashi et al. reported that the absorption bands due to masNH2, msNH2 and dNH2 of the hydrogen bonded amino group on APS-modified silica were observed at 3372, 3305 and 1597 cm1, respectively, while those of non-hydrogen bonded NH2 of APS molecules in toluene solution were observed at 3384, 3324 and 1605 cm1 [27]. In addition, Culler et al. and Okabayashi et al. reported that an IR absorption band due to NHþ 3 deformation of the pro-
tonated amino group ðR NHþ 3 O SiÞ was ob1 served around 1630 cm , and that the protonated amino group species were dominant at low APS loading [27,29]. In this study, in order to examine whether or not there is a difference in the amine structure among aminosilane-modified SBA-15 prepared under different conditions, position and intensity of the absorption bands due to N–H were studied. The IR peak positions of N–H vibrations for APS/SBA(I), APS/SBA(II) and APS/SBA(III) observed in this study are summarized in Table 2. The IR spectra of APS/SBA(I), APS/SBA(II) and APS/SBA(III) showed no distinct absorption band of the protonated amino group (1630 cm1) or free amino group (3384 cm1). The absorption bands due to masNH2 (3367–3371 cm1), msNH2 (3301–3308 cm1) and dNH2 (1594–1595 cm1) of the hydrogen bonding amino group were clearly observed for these APSmodified SBA-15. It is interesting to note that the absorption bands due to the NH2 stretching vibrations (masNH2 and msNH2) of APS-modified SBA-15 shifted slightly to lower frequency with increasing the surface density of amine. The IR peak positions of NH vibrations of AEAPS- and TA-modified SBA-15 are also summarized in Table 2. The IR peak positions of N–H stretch (masNH2 and msNH2 + mNH) of these adsorbents also shifted to lower frequency with increasing the surface density of amine. These shifts suggest that densely anchored amine forms stronger hydrogen bonding than isolated amine. It is probable that the contribution of amine–amine hydrogen bonding increases with increasing the surface density of amine. Fig. 5 shows the peak areas of N–H vibrations of aminosilane-modified SBA-15 as a function of surface density of amine, which were obtained by integration of the respective absorption peaks and taking the absorption band due to Si–O–Si lattice vibration as a reference band. These were almost in proportion to the surface density of amine. Thus, the amount of
N. Hiyoshi et al. / Microporous and Mesoporous Materials 84 (2005) 357–365 : νsNH2 : δNH2
: νsNH2+νNH : δNH2
: νsNH2+νNH : δNH2
A
B
C
A 1.0
1.5
0.8
1.0
C/C 0
Peak area (a.u.)
362
0.6 APS/SBA(I) APS/SBA(II) APS/SBA(III)
0.5 0.4
0
0
2
4
6
0
2
4
6
0
2
4
6
0.2
8
Surface density/N-atom.nm-2
0
Fig. 5. Integrated IR peak area for (A) APS-, (B) AEAPS- and (C) TA-modified SBA-15.
10 Time/min
15
20
1.0
C/C 0
0.8 0.6 AEAPS/SBA(I) AEAPS/SBA(II) AEAPS/SBA(III)
0.4 0.2 0
0
5
10 Time/min
15
20
C 1.0 0.8 C/C 0
Fig. 6 shows the breakthrough curves of CO2 adsorption in the presence of water vapor. Breakthrough times of CO2 were in the following order: APS/SBA(I) < APS/ SBA(II) < APS/SBA(III) for APS-modified SBA-15, AEAPS/SBA(I) < AEAPS/SBA(II) < AEAPS/SBA(III) for AEAPS-modified SBA-15 and TASBA(I) < TA/ SBA(II) < TA/SBA(III) for TA-modified SBA-15. Thus the condition (iii) was found to be the best of all the three preparation conditions to obtain an adsorbent with high adsorption capacity. These aminosilane-modified SBA-15 could be regenerated by heating in a He flow after adsorption at 333 K. For example, CO2 adsorbed on TA/SBA(III) was completely desorbed by heating in a He flow at 373 K for 60 min or at 423 K for 8 min. Furthermore, the adsorption capacity of TA/SBA(III) was not changed after 50 cycles of adsorption at 333 K and desorption at 373 K. The CO2 adsorption capacities of the various adsorbents in the absence and in the presence of water vapor are summarized in Table 3. The adsorption capacities in the presence as well as in the absence of water vapor were comparable over aminosilane-modified SBA-15, though the adsorption capacities, in particular for AEAPS/SBA(III) and TA/SBA(III), increased slightly by the presence of water vapor. Water vapor would cause the swelling and/or hydrolysis of multilayer aminosilane on the surfaces, which would enable the interior amino groups to react with CO2 [13]. These results demonstrate that aminosilane-modified SBA-15 is effective both in the absence and presence of water vapor. In particular, TA/SBA(III) had the highest adsorption capacity among other aminosilane-modified SBA-15. The adsorption capacity of TA/SBA(III) reached 1.8 mmol g1 under 15 kPa CO2 and 12 kPa H2O at 333 K. On the other hand, the adsorption capacity of DMAPS/SBA(II) in the presence and in the absence of
5
B
hydrogen bonded amine corresponded to the surface density of amine. 3.3. Adsorption properties
0
0.6 TA/SBA(I) TA/SBA(II) TA/SBA(III)
0.4 0.2 0
0
5
10 Time/min
15
20
Fig. 6. Breakthrough curves of CO2 over (A) APS-modified SBA-15, (B) AEAPS-modified SBA-15 and (C) TA-modified SBA-15 under 15 kPa CO2 and 12 kPa H2O with N2 balance at 333 K. Total flow rate: 30 cm3 min1; adsorbent weight: 1.5 g.
water vapor were very low, indicating that tertiary amine was not effective for CO2 adsorption. Fig. 7 shows temperature and pressure dependence of the adsorption capacity for TA/SBA(III). The partial pressure of water vapor did not affect the amount of adsorbed CO2 at temperature >333 K. The amount of adsorbed CO2 decreased as the partial pressure of CO2 decreased, and as the temperature increased. At 373 K, the adsorption capacity of TA/SBA(III) at 1 kPa of
N. Hiyoshi et al. / Microporous and Mesoporous Materials 84 (2005) 357–365 Table 3 Adsorption capacities of the adsorbents at 333 K under 15 kPa of CO2 Adsorbed CO2 (mmol g1) 0 kPa-H2O
12 kPa-H2O
SBA-15 APS/SBA(I) APS/SBA(II) APS/SBA(III) AEAPS/SBA(I) AEAPS/SBA(II) AEAPS/SBA(III) TA/SBA(I) TA/SBA(II) TA/SBA(III) MAPS/SBA(II) DMAPS/SBA(II) AEAPMS/SBA(II)
0.05 0.15 0.52 0.66 0.26 0.87 1.36 0.35 1.10 1.58 0.25 0.05 0.91
0.04 0.10 0.50 0.65 0.34 0.90 1.51 0.39 1.21 1.80 0.25 0.05 0.93
2.0 A
Adsorbed CO2/mmol.g-1
Adsorbent
363
1.5
1.0
0.5
0
0
2
4
6
Amine content/N-mmol.g-1 2.0
0.3 1.5
Amine efficiency/-
Adsorbed CO2/mmol.g-1
B
1.0
0.5
0 320
340
360
380
0.2
0.1
0 0
Temperature/K Fig. 7. Temperature and pressure dependence of adsorption capacity for TA/SBA(III). (d) 15 kPa CO2 and 12 kPa H2O with N2 balance, (·) 15 kPa CO2 and 3 kPa H2O with N2 balance, (n) 15 kPa CO2 with N2 balance, () 5 kPa CO2 with N2 balance, (s) 1 kPa CO2 with N2 balance.
CO2 was very low, indicating that the interaction between amine and CO2 may not be so strong. Fig. 8A shows the relationship between amine content of the adsorbents and the CO2 adsorption capacities. The amount of adsorbed carbon dioxide for APS-, AEAPS- and TA-modified SBA-15 increased with increase in amine contents. It should be noted that the adsorption capacities were not in proportion to their amine content, but increased exponentially. Fig. 8 B shows the amine efficiency defined by Eq. (1) for APS-, AEAPS- and TA-modified SBA-15 as a function of the surface density of amine. Amine efficiency ðÞ ¼ adsorbed CO2 ðmmol g1 Þ= amine content ðmmol g1 Þ ð1Þ
2
4
6
8
Surface density/N-atom.nm-2 Fig. 8. Relationship between amine content and CO2 adsorption capacity (A) and that between surface density of amine and amine efficiency (B) at 333 K with 15 kPa CO2 and 12 kPa H2O with N2 balance. (d) APS-, (n) AEAPS- and () TA-modified SBA-15.
The amine efficiency for these adsorbents increased with increase in their surface densities. Thus, adsorption site for CO2 seems to be the densely anchored amine, and not the isolated amine. Based on our IR results, the idea that amine interacting strongly with silanols particularly at low surface density of amine is not effective for CO2 adsorption would be excluded. Fig. 8B also shows the comparison of amine efficiencies of various aminosilanes at identical surface density of amine, and were found to be in the following order: APS > AEAPS > TA. The possible reasons for this order could be: (i) primary amine is more effective than secondary amine. However, the amine efficiency for MAPS/SBA(II) (100% secondary amine) was 0.13 at surface density of 1.56 N atom nm2 and comparable
1566 1619
1484
1566 1614
1529 1492
3304
1623 1563
3312 3301
1626
b
1488
3366 3365
c
3.4. Infrared spectroscopy In order to elucidate the adsorption state of CO2 on aminosilane-modified SBA-15, in situ IR study was carried out. Fig. 9 shows the IR spectra of aminosilanemodified SBA-15 in CO2–He or CO2–H2O–He mixtures at 333 K. Fig. 9a shows the spectra of APS/SBA(II) in CO2–H2O–He and CO2–He mixtures. These spectra were almost identical, indicating that the presence of water does not affect the amine–CO2 reaction. Negative peaks observed at 3365 cm1 and 3301 cm1 were consistent with masNH2 and msNH2 of hydrogen bonded amino group of APS/SBA(II). Thus hydrogen bonded amino group was involved in the adsorption of CO2. Positive peaks at 3439, 1628, 1563 and 1488 cm1 were assigned to N–H stretch, NHþ 3 deformation, C@O stretch and ‘‘NCOO’’ skeletal vibration of alkylammonium carbamate (R-NHCOO +H3N-R, R: alkyl), respectively [32]. Dreyfuss and co-workers also reported similar absorption bands for solid products formed by the reaction between liquid APS and CO2, and concluded that these are due to alkylammonium carbamate [33,34]. They also showed that absorption due to N–H stretch of carbamate (–NHCOO) was observed around 3300–3370 cm1, and this absorption band was an evidence for formation of alkylammonium carbamate. The frequency of carbamate N–H stretch observed in this study for APS/SBA(II) (3439 cm1) was higher than that reported by Dreyfuss and co-workers. This could be due to the fact that the sample analysed by Dreyfuss and co-workers was in the solid form in which carbamates would form a stronger hydrogen bonding than that formed on a high surface area support as in our work. Fig. 9b–d shows IR spectra of MAPS/SBA(II), AEAPS/SBA(II) and TA/SBA(II) in the CO2–H2O– He and CO2–He mixtures. The spectra in the CO2– H2O–He and CO2–He were almost identical for all of these adsorbents, indicating that water does not affect
3303
3366
3420
d
3428
to that for AEAPS-modified SBA-15 (50% secondary amine) for the identical surface density. Hence, this reason may be ruled out. (ii) Another possibility is the steric hindrance due to long organic chains that cover a part of the nitrogen atom of aminosilane and hinder the access of CO2 molecule. The result on AEAPSM/SBA(II) strongly suggests this possibility. Methyl group bonded to silicon atom of AEAPSM is expected to afford space that facilitates adsorption of CO2 on the ‘‘covered’’ nitrogen atom. In fact, the amine efficiency of AEAPSM/SBA(II) was 0.26 at surface density of 3.23 N atom nm2 and was slightly higher than that of AEAPS-modified SBA-15 at identical surface density. Therefore, the difference in the amine efficiency of different amines would be due to the steric hindrance caused by the bulky groups attached to the nitrogen atom rather than the type of amine as primary or secondary.
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3437
364
a 3500
3300
1700
1500
Wavenumber/cm
1300
-1
Fig. 9. Infrared spectra in a CO2–He mixture (solid line) and a CO2– H2O–He mixture (dashed line) at 333 K. (a) APS/SBA(II), (b) MAPS/ SBA(II), (c) AEAPS/SBA(II) and (d) TA/SBA(II).
amine–CO2 reaction for these adsorbents. The absorption bands which could be assigned to N–H stretch of –NHCOO were observed for AEAPS/SBA(II) and TA/SBA(II). In the case of MAPS/SBA(II), N–H vibration of carbamate was not observed because of the absence of N–H bond in the carbamate (–N(CH3)COO) derived from the secondary amine.
CO2 NH2
Si O O O Isolated amine
Alkylammonium carbamate H2N
Si O O
NH2
O
Si O O
HNCOO-
Si O O
O
+H N 3
Si O O
Densely anchored amine SBA-15
Scheme 1. Adsorption of CO2 on aminosilane-modified SBA-15.
N. Hiyoshi et al. / Microporous and Mesoporous Materials 84 (2005) 357–365
As described above, CO2 is adsorbed on aminosilanemodified SBA-15 through formation of alkylammonium carbamate, in which two nitrogen atoms of amine are involved. Thereby the densely anchored aminosilane which affords an amine pair is an effective adsorption site for CO2, as shown in Scheme 1. 4. Conclusion The adsorbents prepared by grafting aminosilanes on SBA-15 support exhibited high adsorption capacities for CO2 under moist as well as dry conditions. The efficiency of amine was found to be improved by increasing surface density of the amine. It was suggested that the amine pairs, on which CO2 was adsorbed through formation of ammonium carbamate, were increased with increasing the surface density of amine. Acknowledgment This work was supported by the New Energy and Industrial Technology Development Organization (NEDO), Japan.
References [1] H. Ohta, S. Umeda, M. Tajika, M. Nishimura, M. Yamada, A. Yasutake, J. Izumi, Int. J. Global Energy Issues 11 (1998) 203. [2] S. Kazama, T. Teramoto, K. Haraya, J. Membr. Sci. 207 (2002) 91. [3] T. Inui, Y. Okugawa, M. Yasuda, Ind. Eng. Chem. Res. 27 (1988) 1103. [4] H. Hayashi, J. Taniuchi, N. Furuyashiki, S. Sugiyama, S. Hirano, N. Shigemoto, S. Hirano, Ind. Eng. Chem. Res. 37 (1998) 185. [5] O. Leal, C. Bolı´var, C. Ovalles, J.J. Garcı´a, Y. Espidel, Inorg. Chim. Acta 240 (1995) 183. [6] S. Satyapal, T. Filburn, J. Trela, J. Strange, Energy Fuels 15 (2001) 250. [7] H. Yoshida, S. Oehlenschlaeger, Y. Minami, M. Terashima, J. Chem. Eng. Jpn. 35 (2002) 32. [8] A.C.C. Chang, S.S.C. Chuang, M. Gray, Y. Soong, Energy Fuels 17 (2003) 468.
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[9] H.Y. Huang, R.T. Yang, D. Chinn, C.L. Munson, Ind. Eng. Chem. Res. 42 (2003) 2427. [10] X. Xu, C. Song, J.M. Andresen, B.G. Miller, A.W. Scaroni, Energy Fuels 16 (2002) 1463. [11] X. Xu, C. Song, J.M. Andresen, B.G. Miller, A.W. Scaroni, Micropor. Mesopor. Mater. 62 (2003) 29. [12] N. Hiyoshi, K. Yogo, T. Yashima, Chem. Lett. 33 (2004) 510. [13] N. Hiyoshi, K. Yogo, T. Yashima, J. Jpn. Petrol. Inst. 48 (2005) 29. [14] X. Feng, G.E. Fryxell, L.-Q. Wang, A.Y. Kim, J. Liu, K.M. Kemner, Science 276 (1997) 923. [15] S. Inagaki, S. Guan, Y. Fukusima, T. Ohsuna, O. Terasaki, J. Am. Chem. Soc. 121 (1999) 9611. [16] T. Kimura, S. Saeki, Y. Sugahara, K. Kuroda, Langmuir 15 (1999) 2794. [17] A. Matsumoto, K. Tsutsumi, K. Schumacher, K.K. Unger, Langmuir 18 (2002) 4014. [18] Y. Kubota, Y. Nishizaki, H. Ikeya, M. Saeki, T. Hida, S. Kawazu, M. Yoshida, H. Fujii, Y. Sugi, Micropor. Mesopor. Mater. 70 (2004) 135. [19] Y.-J. Han, G.D. Stucky, A. Butler, J. Am. Chem. Soc. 121 (1999) 9897. [20] H. Yoshitake, T. Yokoi, T. Tatsumi, Chem. Mater. 15 (2003) 1713. [21] H. Yoshitake, T. Yokoi, T. Tatsumi, Bull. Chem. Soc. Jpn. 76 (2003) 847. [22] D. Zhao, J. Feng, Q. Huo, N. Melosh, G.H. Fredrickson, B.F. Chmelka, G.D. Stucky, Science 279 (1998) 548. [23] E.F. Vansant, P. van der Voort, K.C. Vrancken, Stud. Surf. Sci. Catal. 93 (1995) 193. [24] C. Setzer, G. van Essche, N. Pryor, in: F. Schu¨th, K.S.W. Sing, J. Weitkamp (Eds.), Handbook of Porous Solids, vol. 3, WeilyVCH, Germany, 2002. [25] L.D. White, C.P. Tripp, J. Coll. Interf. Sci. 232 (2000) 400. [26] K.C. Vranchen, P. van der Voort, I.G. DÕHamers, E.F. Vansant, J. Chem Soc. Faraday Trans. 88 (1992) 3197. [27] H. Okabayashi, I. Shimizu, E. Nishio, C.J. OÕConnor, Coll. Polym. Sci. 275 (1997) 744. [28] C.-H. Chang, H. Ishida, J.L. Koenig, J. Coll. Interf. Sci. 74 (1979) 396. [29] S.R. Culler, H. Ishida, J.L. Koenig, J. Coll. Interf. Sci. 106 (1985) 334. [30] K.M.R. Kallury, P.M. Macdonald, M. Thompson, Langmuir 10 (1994) 492. [31] A.A. Golub, A.I. Zubenko, B.V. Zhmud, J. Coll. Interf. Sci. 179 (1996) 482. [32] M. Aresta, E. Quaranta, Tetrahedron 48 (1992) 1515. [33] K.P. Battjes, A.M. Barolo, P. Dreyfuss, J. Adhes. Sci. Technol. 5 (1991) 785. [34] Y. Eckstein, P. Dreyfuss, J. Adhes. 15 (1983) 163.
Adsorption characteristics of carbon dioxide on organically functionalized SBA-15 Norihito Hiyoshi 1, Katsunori Yogo *, Tatsuaki Yashima Research Institute of Innovative Technology for the Earth, 9-2 Kizugawadai, Kizu-cho Soraku-gun, Kyoto 619-0292, Japan Received 28 December 2004; received in revised form 21 May 2005; accepted 13 June 2005 Available online 27 July 2005
Abstract Aminosilane-modified SBA-15 was prepared by grafting various aminosilanes on mesoporous silica SBA-15, and its adsorption characteristics towards carbon dioxide were examined. The amount of carbon dioxide adsorbed was almost the same for both in the presence and in the absence of water vapor. It was found that the efficiency of adsorption, defined as the number of adsorbed carbon dioxide molecules per nitrogen atom of aminosilane-modified SBA-15, increased with increasing the surface density of amine. Infrared spectroscopy revealed that carbon dioxide was adsorbed on aminosilane-modified SBA-15 through the formation of alkylammonium carbamate in the presence and in the absence of water vapor. It was suggested that amine pairs, on which carbon dioxide was adsorbed through formation of alkylammonium carbamate, increased with increasing surface density of amine. In addition, influence of amine structure on the adsorption capacity has also been discussed. 2005 Elsevier Inc. All rights reserved. Keywords: Adsorption; Carbon dioxide; Organically functionalized SBA-15; Water vapor; Aminosilane
1. Introduction The gradual increase in the atmospheric concentration of carbon dioxide (CO2) due to fossil fuel combustion is becoming a serious environmental problem. Capture and sequestration of CO2 has been considered as one of the options to reduce CO2 emission. Various processes, such as liquid solvent absorption [1], membrane separation [2], and pressure (and/or temperature) swing adsorption (P(T)SA) [1,3], have been proposed for
* Corresponding author. Tel.: +81 774 75 2306; fax: +81 774 75 2319. E-mail addresses: [email protected] (N. Hiyoshi), yogo@rite. or.jp (K. Yogo). 1 Research Center for Compact Chemical Process, National Institute of Advanced Industrial Science and Technology (AIST), 4-2-1, Nigatake, Miyagino, Sendai 983-8551, Japan. Tel.: +81 222372028; fax: +81 222375224.
1387-1811/$ - see front matter 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2005.06.010
the separation and recovery of CO2 emitted by power plants. However, the costs of CO2 separation from flue gases account for approximately 70–80% of the total energy cost for CO2 sequestration. Therefore, it is important to develop new energy-efficient techniques for CO2 separation. The conventional PSA or PTSA process using zeolite requires a dehumidification step which consumes about 30% of the total energy, because water vapor is adsorbed more strongly than CO2 on a zeolite surface. Therefore, a new adsorbent [4–13] that preferably adsorbs CO2 in the presence of water vapor is desirable for the development of a simple and energy-efficient CO2 removal process that will eliminate the dehumidification step. Recently, organically functionalized mesoporous silica has attracted considerable attention because of the wide range applications as adsorbents and catalysts [8– 21]. Since mesoporous silica has uniform and large pores as well as high surface area, a large number of active
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sites or adsorption sites can be introduced uniformly on pore walls of mesoporous silica by its surface modification with organosilane molecules. SBA-15 is a suitable mesoporous silica for applications in removal/separation of gases containing water vapor, due to its higher hydrothermal stability [22]. In this study, aminosilanemodified mesoporous molecular sieve was prepared by grafting various aminosilanes on SBA-15, and its adsorption characteristics were studied. Surface density of amine on SBA-15 was found to be an important factor for the reaction of CO2 with amine immobilized on the mesoporous support.
ent aminosilanes, N-methylaminopropyltrimethoxysilane (H3CNHCH2CH2CH2Si(OCH3)3, MAPS, Gelest Inc.), (N,N-dimethyl-3-aminopropyl)trimethoxysilane ((H3C)2NCH2CH2CH2Si(OCH3)3, DMAPS, Gelest Inc.), and N-(2-aminoethyl)-3-aminopropylmethyldiethoxysilane (H2NCH2CH2NHCH2CH2CH2Si(CH3)(OCH3)2, AEAPMS, Chisso co.) were also used as grafting agents. (iii) SBA-15 was boiled in water for 2 h followed by grafting of aminosilanes. Boiled SBA-15 was dried at 398 K for 6 h in air, and then modified with 17 vol% toluene solutions of aminosilanes. The adsorbent was named as e.g. APS/SBA(I), which was prepared using APS under a set of conditions given in (i).
2. Experimental
2.3. Characterization
2.1. Preparation of SBA-15 support
Elemental analysis was carried out with PE-2400 (Perkin Elmer). N2 adsorption–desorption isotherms were measured at 77 K by an automatic adsorption system (Autosorb, Quantachrome). Before measurements, the samples were kept under a vacuum at 473 K for 3 h. The surface areas and the pore size distributions were calculated by the BET and BJH method, respectively. The total pore volume was calculated from the amount of adsorbed N2 at P/P0 = 0.97. Infrared spectra of SBA-15 and aminosilane-modified SBA-15 were obtained using FT/IR-610 (Jasco Co.) equipped with an MCT detector. In order to remove adsorbed CO2 and H2O, a sample pellet of 20 mm in diameter was placed in a flow cell with ZnSe windows, and then heated in a He flow at 423 K for 1 h. The IR spectra were recorded at 333 K in a He flow (resolution: 4 cm1, 64 scans).
The SBA-15 support was prepared using tetraethoxyorthosilicate (TEOS, Kishida Chem. Co.) as a silica source and polyethyleneglycol-block-polypropyleneglycol-block-polyethyleneglycol with an average molecular weight of 5800 (PEO20PPO70PEO20, Aldrich) as a structure directing agent [22]. The reaction mixture of PEO20PPO70PEO20 (50 g), TEOS (0.53 mol), hydrochloric acid (2.0 mol) and distilled water (80 mol) was heated under stirring at 303 K for 20 h, and then at 368 K for 24 h. The resulting precipitate was filtered and washed with 4000 cm3 of distilled water, and then dried at 343 K over night. The dried precipitate was calcined in air at 823 K for 8 h to remove the structure directing agent. The 2-D hexagonal structure of calcined SBA15 thus prepared was confirmed by X-ray diffraction (XRD) from which the lattice constant of SBA-15 was estimated to be 11.3 nm. After calcination, a part of SBA-15 was boiled in water at 368 K for 2 h. 2.2. Preparation of aminosilane-modified SBA-15 Various aminosilanes used as grafting agents were 3-aminopropyltriethoxysilane (H2NCH2CH2CH2Si(OCH2CH3)3, abbreviated as APS, Aldrich), N-(2aminoethyl)-3-aminopropyltrimethoxysilane (H2NCH2CH2NHCH2CH2CH2Si(OCH3)3, AEAPS, Chisso co.) and (3-trimethoxysilylpropyl)diethylenetriamine (H2NCH 2 CH 2 NHCH 2 CH 2 NHCH 2 CH 2 CH 2 Si(OCH 3 ) 3 , TA, Gelest Inc.). Samples of aminosilane-modified SBA-15 were prepared under three different grafting conditions: (i) Calcined SBA-15 (5.0 g), which was previously dried at 398 K for 6 h in air, was refluxed in the toluene solution of aminosilane (1.7 vol%, 250 cm3) at 383 K for 24 h under an Ar flow. The product was washed with toluene (200 cm3) and dried at 333 K over night. (ii) The same procedure as given in (i) was followed except that 17 vol% of aminosilane solutions were used. Here, differ-
2.4. Adsorption experiments The adsorption capacity of the adsorbent was determined by a flow method using CO2–H2O–N2 or CO2–N2 mixtures. A typical procedure using a CO2– H2O–N2 mixture was as follows. Aminosilane-modified SBA-15 (1.5 g, on dry basis) was placed in a Pyrex tube (13 mm inner diameter) and heated at 423 K for 1 h in a He flow (30 cm3 min1), and then cooled to 333 K. A mixture of 12 kPa H2O with He balance (total flow rate: 60 cm3 min1) was passed over the adsorbent at 333 K until the adsorbent was saturated with water vapor, and then the gas flow was switched to a mixture of 15 kPa CO2 and 12 kPa H2O with N2 balance (total flow rate: 30 cm3 min1). The breakthrough curve of CO2 was obtained by analysing the effluent gases using a gas chromatograph (GC-332, GL science Inc.) equipped with a gaskuropack 54 column (2 m) and a TCD detector. In the case of measurement with a CO2–N2 mixture, a mixture of 15 kPa CO2 with N2 balance (total flow rate: 30 cm3 min1) was passed over the dried adsorbent without pre-adsorption of water vapor.
N. Hiyoshi et al. / Microporous and Mesoporous Materials 84 (2005) 357–365
359
3. Results and discussion 3.1. SBA-15 support
800
V/cm3(STP).g-1
600
(dV/dDp)/cm3 nm-1 g-1
Fig. 1 shows the N2 adsorption–desorption isotherms of SBA-15 before and after boiling in water for 2 h. Both the isotherms were classified as type IV characteristic of mesoporous silica. Although, the amount adsorbed on SBA-15 boiled in water was slightly smaller than that on SBA-15 before boiling, the difference in the shapes of the isotherms was not significant. The surface area and pore volume of SBA-15 before boiling were 910 m2 g1 and 1.11 cm3 g1, and those after boiling
2
3742
4000
3500
3000
2500
1970 1860
a
2000
1500
Wavenumber/cm-1 Fig. 2. Infrared spectra of SBA-15 (a) before and (b) after boiling for 2 h.
were 820 m2 g1 and 1.07 cm3 g1. The pore size distributions of SBA-15 were almost identical before and after boiling, showing sharp peaks at 6.0 nm (Fig. 1). These results suggest that the pore structure of SBA15 partially collapsed after the boiling treatment for 2 h due to the hydration of Si–O–Si bonds, which results in the decrease in the surface area and pore volume; however, the collapse of the pore structure is not remarkable. The IR spectra of SBA-15 supports (Fig. 2) revealed that the isolated hydroxyl group (3742 cm1) decreased after boiling treatment for 2 h. On the other hand, the integrated peak area of overlapping peaks from 3000 to 3740 cm1 due to internal hydroxyl group and hydrogen bonded hydroxyl group, etc. [23,24] increased by approximately 20%. This change in hydroxyl stretch region would result from hydration of surface Si–O–Si bonds by boiling treatment.
1
3.2. Characterization of aminosilane-modified SBA-15 0
400
4 5 6 7 8 Pore diameter/nm
200
Before boiling treatment After boiling for 2 h 0 0
b
3640 3560
Infrared spectra of adsorbents in CO2–He and CO2– H2O–He mixtures were obtained using FT/IR-610 (Jasco Co.) equipped with an MCT detector. The measurement with a CO2–H2O–He mixture was performed as follows. A sample pellet placed in a flow cell was heated in a He flow at 423 K for 1 h. A mixture of 2.3 kPa H2O with He balance was fed to the flow cell at 333 K for 30 min so that the water vapor could be adsorbed on the sample surface, and then a background spectrum was recorded. After this, a mixture of 3 kPa CO2 and 2.3 kPa H2O with He balance was introduced at 333 K for 5 min, and then a spectrum was obtained. In the case of measurement with a mixture of 3 kPa CO2 with He balance, a spectrum was obtained without pre-adsorption of water vapor.
Absorbance (a.u.)
2.5. Infrared spectra in CO2–He and CO2–H2O–He mixtures
0.1
0.2
0.3
0.4
0.5
0.6
0.7 0.8
0.9 1.0
P/Po Fig. 1. Nitrogen adsorption–desorption isotherms for SBA-15 before and after boiling in water for 2 h, and pore size distributions calculated from desorption branches.
The amount of aminosilane anchored on SBA-15 surface was determined by the elemental analysis. Amine contents and surface densities of amine on various adsorbents are summarized in Table 1. The amine content was defined as the number of nitrogen atoms per gram of the adsorbent, and the surface density of amine was defined as the number of nitrogen atoms per nm2 of SBA-15 surface. The amine contents and the surface densities of amine for various adsorbents were found to be in the following order: APS/SBA(I) < APS/ SBA(II) < APS/SBA(III) for APS-modified SBA-15, AEAPS/SBA(I) < AEAPS/SBA(II) < AEAPS/SBA(III) for AEAPS-modified SBA-15 and TA/SBA(I) < TA/ SBA(II) < TA/SBA(III) for TA-modified SBA-15. Thus APS, AEAPS and TA were more densely anchored on
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Table 1 Physicochemical properties of adsorbents Adsorbent
Surface area (m2 g1)
Pore volume (Vcal.a) (cm3 g1)
Amine contentb (N mmol g1)
Surface densityc (N atom nm2)
Surface coverage (%)
APS/SBA(I) APS/SBA(II) APS/SBA(III) AEAPS/SBA(I) AEAPS/SBA(II) AEAPS/SBA(III) TA/SBA(I) TA/SBA(II) TA/SBA(III) MAPS/SBA(II) DMAPS/SBA(II) AEAPMS/SBA(II)
616 364 374 480 312 250 473 240 183 414 400 297
0.76 0.53 0.54 0.67 0.47 0.40 0.65 0.37 0.29 0.57 0.54 0.47
1.11 2.57 2.61 2.26 3.76 4.61 2.75 4.85 5.80 1.88 1.79 3.52
0.83 2.37 2.68 1.79 3.49 5.21 2.19 4.69 6.85 1.62 1.56 3.23
25 70 79 35 69 103 38 80 117 57 –d –d
c d
Pore volume predicted from aminosilane loadings. The amount of nitrogen atom per 1 g of adsorbent. The amount of nitrogen atom per 1 nm2 of SBA-15 support. Not calculated.
the surface of SBA-15 boiled in water than that of nonboiled SBA-15. This suggests that not only the isolated hydroxyl groups but also the hydrogen bonded hydroxyl groups are suitable sites for anchoring aminosilanes. The surface coverage of aminosilane is also summarized in Table 1. The calculation of surface coverage is based on cross-section areas of aminosilanes. The cross-section areas of APS, AEAPS, TA and MAPS were estimated to be 0.30, 0.39, 0.51 and 0.35 nm2, respectively [13]. The surface coverage of adsorbents prepared according to the condition (i) was in the range of 25–38%. The surface coverage of adsorbents prepared according to the conditions (ii) and (iii) were found to be in the range of 57–80% and 79–117%, respectively. There were small multilayer portions on the surfaces of AEAPS/SBA(III) and TA/SBA(III). The N2 adsorption–desorption isotherms of aminosilane-modified SBA-15 showed that the uniform mesoporous structure and high surface area were still retained after grafting. In Table 1, the pore volume measured by N2 adsorption is compared with the predicted value (Vcal.) [13]. Although the predicted value was estimated roughly, the experimentally measured pore volume was close to the predicted value for each adsorbent. This suggests that pore plugging by aminosilane grafting is negligible. Fig. 3 shows pore size distributions of aminosilane-modified SBA-15. The uniform pore size distribution was retained for aminosilane-modified SBA-15. The mean pore diameters of aminosilanemodified SBA-15 were lower than those of the SBA-15 supports (6.0 nm), indicating that aminosilanes were anchored on pore walls of SBA-15. The mean pore diameters of APS-, AEAPS- and TA-modified SBA-15 decreased in the following order, APS > AEAPS > TA. This order was in accordance with the order of molecular sizes of the respective aminosilanes. Also, the pore diameters were influenced by the amine content; the
1.5
APS/SBA(I) AEAPS/SBA(I) TA/SBA(I)
1.0 0.5 0 1.5 (dV/dDp)/cm3.nm-1.g-1
a b
(0.87) (0.54) (0.54) (0.76) (0.51) (0.39) (0.73) (0.40) (0.29) (0.60) (–d) (–d)
APS/SBA(II) AEAPS/SBA(II) TA/SBA(II)
1.0 0.5 0 1.5
APS/SBA(III) AEAPS/SBA(III) TA/SBA(III)
1.0 0.5 0 2
3
4
5
6
7
8
Pore diameter/nm Fig. 3. Pore size distributions of aminosilane-modified SBA-15.
mean pore diameters of (I) samples were larger than corresponding (II) and (III) samples. Fig. 4 shows the IR spectra of aminosilane-modified SBA-15. In the IR spectrum of APS/SBA(II) (Fig. 4a), absorption bands at 3368, 3302 and 1595 cm1 were observed. These were assigned to asymmetric NH2 stretch (masNH2), symmetric NH2 stretch (msNH2) and
N. Hiyoshi et al. / Microporous and Mesoporous Materials 84 (2005) 357–365
c
1667 1602 1457
2933 2868
b
Wavenumber (cm1) masNH2
msNH2
dNH2
APA/SBA(I) APA/SBA(II) APA/SBA(III)
3371 3368 3367
3308 3302 3301
1595 1595 1594
masNH2
msNH2 and mNH
AEAPA/SBA(I) AEAPA/SBA(II) AEAPA/SBA(III)
3367 3362 3361
3306 3302 3299
masNH2
msNH2 and mNH
TA/SBA(I) TA/SBA(II) TA/SBA(III)
3364 3359 3360
3302 3297 3296
1600 1602 1600
1601 1602 1603
3000
2500
2000
1472 1448
1984 1850
1595
3368 3302
3648
1458
1651 1602
2932 2881 2819
Adsorbent
3362 3302
Absorbance (a.u.)
3359 3297
2932 2883 2821
Table 2 Wavenumber of N–H vibrations for APS-, AEAPS- and TA-modified SBA-15
a 3500
361
1500
Wavenumber/cm-1 Fig. 4. Infrared spectra of (a) APS/SBA(II), (b) AEAPS/SBA(II) and (c) TA/SBA(II).
NH2 deformation (dNH2) of hydrogen bonded amino group, respectively [25–27]. Besides these, absorption bands due to (Si–)OH stretch (3648 cm1), overtone of Si–O–Si lattice vibration (1984 and 1850 cm1) [23,24], CH2 stretch (2868 and 2933 cm1) and CH2 deformations (1472 and 1448 cm1) were also observed in this spectrum [28]. The spectra of AEAPS/SBA(II) and TA/SBA(II) were similar (Fig. 4b and c) to that of APS/SBA(II). The absorption bands around 3360 and 1602 cm1 were assigned to asymmetric NH2 stretch (masNH2) and NH2 deformation (dNH2) [21], and that around 3300 cm1 would be due to overlap of NH stretch of secondary amine (mNH) and symmetric NH2 stretch (msNH2). It has been reported that there are various structures of amine on APS-modified silica, e.g. free, hydrogen bonded and protonated amino groups ðR NHþ 3 O SiÞ, etc. [25–27,29–31]. According to IR studies by several groups, the absorption bands due to masNH2 and msNH2 of APS-modified silica shift to lower frequency by hydrogen bonding [25–27]. Okabayashi et al. reported that the absorption bands due to masNH2, msNH2 and dNH2 of the hydrogen bonded amino group on APS-modified silica were observed at 3372, 3305 and 1597 cm1, respectively, while those of non-hydrogen bonded NH2 of APS molecules in toluene solution were observed at 3384, 3324 and 1605 cm1 [27]. In addition, Culler et al. and Okabayashi et al. reported that an IR absorption band due to NHþ 3 deformation of the pro-
tonated amino group ðR NHþ 3 O SiÞ was ob1 served around 1630 cm , and that the protonated amino group species were dominant at low APS loading [27,29]. In this study, in order to examine whether or not there is a difference in the amine structure among aminosilane-modified SBA-15 prepared under different conditions, position and intensity of the absorption bands due to N–H were studied. The IR peak positions of N–H vibrations for APS/SBA(I), APS/SBA(II) and APS/SBA(III) observed in this study are summarized in Table 2. The IR spectra of APS/SBA(I), APS/SBA(II) and APS/SBA(III) showed no distinct absorption band of the protonated amino group (1630 cm1) or free amino group (3384 cm1). The absorption bands due to masNH2 (3367–3371 cm1), msNH2 (3301–3308 cm1) and dNH2 (1594–1595 cm1) of the hydrogen bonding amino group were clearly observed for these APSmodified SBA-15. It is interesting to note that the absorption bands due to the NH2 stretching vibrations (masNH2 and msNH2) of APS-modified SBA-15 shifted slightly to lower frequency with increasing the surface density of amine. The IR peak positions of NH vibrations of AEAPS- and TA-modified SBA-15 are also summarized in Table 2. The IR peak positions of N–H stretch (masNH2 and msNH2 + mNH) of these adsorbents also shifted to lower frequency with increasing the surface density of amine. These shifts suggest that densely anchored amine forms stronger hydrogen bonding than isolated amine. It is probable that the contribution of amine–amine hydrogen bonding increases with increasing the surface density of amine. Fig. 5 shows the peak areas of N–H vibrations of aminosilane-modified SBA-15 as a function of surface density of amine, which were obtained by integration of the respective absorption peaks and taking the absorption band due to Si–O–Si lattice vibration as a reference band. These were almost in proportion to the surface density of amine. Thus, the amount of
N. Hiyoshi et al. / Microporous and Mesoporous Materials 84 (2005) 357–365 : νsNH2 : δNH2
: νsNH2+νNH : δNH2
: νsNH2+νNH : δNH2
A
B
C
A 1.0
1.5
0.8
1.0
C/C 0
Peak area (a.u.)
362
0.6 APS/SBA(I) APS/SBA(II) APS/SBA(III)
0.5 0.4
0
0
2
4
6
0
2
4
6
0
2
4
6
0.2
8
Surface density/N-atom.nm-2
0
Fig. 5. Integrated IR peak area for (A) APS-, (B) AEAPS- and (C) TA-modified SBA-15.
10 Time/min
15
20
1.0
C/C 0
0.8 0.6 AEAPS/SBA(I) AEAPS/SBA(II) AEAPS/SBA(III)
0.4 0.2 0
0
5
10 Time/min
15
20
C 1.0 0.8 C/C 0
Fig. 6 shows the breakthrough curves of CO2 adsorption in the presence of water vapor. Breakthrough times of CO2 were in the following order: APS/SBA(I) < APS/ SBA(II) < APS/SBA(III) for APS-modified SBA-15, AEAPS/SBA(I) < AEAPS/SBA(II) < AEAPS/SBA(III) for AEAPS-modified SBA-15 and TASBA(I) < TA/ SBA(II) < TA/SBA(III) for TA-modified SBA-15. Thus the condition (iii) was found to be the best of all the three preparation conditions to obtain an adsorbent with high adsorption capacity. These aminosilane-modified SBA-15 could be regenerated by heating in a He flow after adsorption at 333 K. For example, CO2 adsorbed on TA/SBA(III) was completely desorbed by heating in a He flow at 373 K for 60 min or at 423 K for 8 min. Furthermore, the adsorption capacity of TA/SBA(III) was not changed after 50 cycles of adsorption at 333 K and desorption at 373 K. The CO2 adsorption capacities of the various adsorbents in the absence and in the presence of water vapor are summarized in Table 3. The adsorption capacities in the presence as well as in the absence of water vapor were comparable over aminosilane-modified SBA-15, though the adsorption capacities, in particular for AEAPS/SBA(III) and TA/SBA(III), increased slightly by the presence of water vapor. Water vapor would cause the swelling and/or hydrolysis of multilayer aminosilane on the surfaces, which would enable the interior amino groups to react with CO2 [13]. These results demonstrate that aminosilane-modified SBA-15 is effective both in the absence and presence of water vapor. In particular, TA/SBA(III) had the highest adsorption capacity among other aminosilane-modified SBA-15. The adsorption capacity of TA/SBA(III) reached 1.8 mmol g1 under 15 kPa CO2 and 12 kPa H2O at 333 K. On the other hand, the adsorption capacity of DMAPS/SBA(II) in the presence and in the absence of
5
B
hydrogen bonded amine corresponded to the surface density of amine. 3.3. Adsorption properties
0
0.6 TA/SBA(I) TA/SBA(II) TA/SBA(III)
0.4 0.2 0
0
5
10 Time/min
15
20
Fig. 6. Breakthrough curves of CO2 over (A) APS-modified SBA-15, (B) AEAPS-modified SBA-15 and (C) TA-modified SBA-15 under 15 kPa CO2 and 12 kPa H2O with N2 balance at 333 K. Total flow rate: 30 cm3 min1; adsorbent weight: 1.5 g.
water vapor were very low, indicating that tertiary amine was not effective for CO2 adsorption. Fig. 7 shows temperature and pressure dependence of the adsorption capacity for TA/SBA(III). The partial pressure of water vapor did not affect the amount of adsorbed CO2 at temperature >333 K. The amount of adsorbed CO2 decreased as the partial pressure of CO2 decreased, and as the temperature increased. At 373 K, the adsorption capacity of TA/SBA(III) at 1 kPa of
N. Hiyoshi et al. / Microporous and Mesoporous Materials 84 (2005) 357–365 Table 3 Adsorption capacities of the adsorbents at 333 K under 15 kPa of CO2 Adsorbed CO2 (mmol g1) 0 kPa-H2O
12 kPa-H2O
SBA-15 APS/SBA(I) APS/SBA(II) APS/SBA(III) AEAPS/SBA(I) AEAPS/SBA(II) AEAPS/SBA(III) TA/SBA(I) TA/SBA(II) TA/SBA(III) MAPS/SBA(II) DMAPS/SBA(II) AEAPMS/SBA(II)
0.05 0.15 0.52 0.66 0.26 0.87 1.36 0.35 1.10 1.58 0.25 0.05 0.91
0.04 0.10 0.50 0.65 0.34 0.90 1.51 0.39 1.21 1.80 0.25 0.05 0.93
2.0 A
Adsorbed CO2/mmol.g-1
Adsorbent
363
1.5
1.0
0.5
0
0
2
4
6
Amine content/N-mmol.g-1 2.0
0.3 1.5
Amine efficiency/-
Adsorbed CO2/mmol.g-1
B
1.0
0.5
0 320
340
360
380
0.2
0.1
0 0
Temperature/K Fig. 7. Temperature and pressure dependence of adsorption capacity for TA/SBA(III). (d) 15 kPa CO2 and 12 kPa H2O with N2 balance, (·) 15 kPa CO2 and 3 kPa H2O with N2 balance, (n) 15 kPa CO2 with N2 balance, () 5 kPa CO2 with N2 balance, (s) 1 kPa CO2 with N2 balance.
CO2 was very low, indicating that the interaction between amine and CO2 may not be so strong. Fig. 8A shows the relationship between amine content of the adsorbents and the CO2 adsorption capacities. The amount of adsorbed carbon dioxide for APS-, AEAPS- and TA-modified SBA-15 increased with increase in amine contents. It should be noted that the adsorption capacities were not in proportion to their amine content, but increased exponentially. Fig. 8 B shows the amine efficiency defined by Eq. (1) for APS-, AEAPS- and TA-modified SBA-15 as a function of the surface density of amine. Amine efficiency ðÞ ¼ adsorbed CO2 ðmmol g1 Þ= amine content ðmmol g1 Þ ð1Þ
2
4
6
8
Surface density/N-atom.nm-2 Fig. 8. Relationship between amine content and CO2 adsorption capacity (A) and that between surface density of amine and amine efficiency (B) at 333 K with 15 kPa CO2 and 12 kPa H2O with N2 balance. (d) APS-, (n) AEAPS- and () TA-modified SBA-15.
The amine efficiency for these adsorbents increased with increase in their surface densities. Thus, adsorption site for CO2 seems to be the densely anchored amine, and not the isolated amine. Based on our IR results, the idea that amine interacting strongly with silanols particularly at low surface density of amine is not effective for CO2 adsorption would be excluded. Fig. 8B also shows the comparison of amine efficiencies of various aminosilanes at identical surface density of amine, and were found to be in the following order: APS > AEAPS > TA. The possible reasons for this order could be: (i) primary amine is more effective than secondary amine. However, the amine efficiency for MAPS/SBA(II) (100% secondary amine) was 0.13 at surface density of 1.56 N atom nm2 and comparable
1566 1619
1484
1566 1614
1529 1492
3304
1623 1563
3312 3301
1626
b
1488
3366 3365
c
3.4. Infrared spectroscopy In order to elucidate the adsorption state of CO2 on aminosilane-modified SBA-15, in situ IR study was carried out. Fig. 9 shows the IR spectra of aminosilanemodified SBA-15 in CO2–He or CO2–H2O–He mixtures at 333 K. Fig. 9a shows the spectra of APS/SBA(II) in CO2–H2O–He and CO2–He mixtures. These spectra were almost identical, indicating that the presence of water does not affect the amine–CO2 reaction. Negative peaks observed at 3365 cm1 and 3301 cm1 were consistent with masNH2 and msNH2 of hydrogen bonded amino group of APS/SBA(II). Thus hydrogen bonded amino group was involved in the adsorption of CO2. Positive peaks at 3439, 1628, 1563 and 1488 cm1 were assigned to N–H stretch, NHþ 3 deformation, C@O stretch and ‘‘NCOO’’ skeletal vibration of alkylammonium carbamate (R-NHCOO +H3N-R, R: alkyl), respectively [32]. Dreyfuss and co-workers also reported similar absorption bands for solid products formed by the reaction between liquid APS and CO2, and concluded that these are due to alkylammonium carbamate [33,34]. They also showed that absorption due to N–H stretch of carbamate (–NHCOO) was observed around 3300–3370 cm1, and this absorption band was an evidence for formation of alkylammonium carbamate. The frequency of carbamate N–H stretch observed in this study for APS/SBA(II) (3439 cm1) was higher than that reported by Dreyfuss and co-workers. This could be due to the fact that the sample analysed by Dreyfuss and co-workers was in the solid form in which carbamates would form a stronger hydrogen bonding than that formed on a high surface area support as in our work. Fig. 9b–d shows IR spectra of MAPS/SBA(II), AEAPS/SBA(II) and TA/SBA(II) in the CO2–H2O– He and CO2–He mixtures. The spectra in the CO2– H2O–He and CO2–He were almost identical for all of these adsorbents, indicating that water does not affect
3303
3366
3420
d
3428
to that for AEAPS-modified SBA-15 (50% secondary amine) for the identical surface density. Hence, this reason may be ruled out. (ii) Another possibility is the steric hindrance due to long organic chains that cover a part of the nitrogen atom of aminosilane and hinder the access of CO2 molecule. The result on AEAPSM/SBA(II) strongly suggests this possibility. Methyl group bonded to silicon atom of AEAPSM is expected to afford space that facilitates adsorption of CO2 on the ‘‘covered’’ nitrogen atom. In fact, the amine efficiency of AEAPSM/SBA(II) was 0.26 at surface density of 3.23 N atom nm2 and was slightly higher than that of AEAPS-modified SBA-15 at identical surface density. Therefore, the difference in the amine efficiency of different amines would be due to the steric hindrance caused by the bulky groups attached to the nitrogen atom rather than the type of amine as primary or secondary.
1479
N. Hiyoshi et al. / Microporous and Mesoporous Materials 84 (2005) 357–365
3437
364
a 3500
3300
1700
1500
Wavenumber/cm
1300
-1
Fig. 9. Infrared spectra in a CO2–He mixture (solid line) and a CO2– H2O–He mixture (dashed line) at 333 K. (a) APS/SBA(II), (b) MAPS/ SBA(II), (c) AEAPS/SBA(II) and (d) TA/SBA(II).
amine–CO2 reaction for these adsorbents. The absorption bands which could be assigned to N–H stretch of –NHCOO were observed for AEAPS/SBA(II) and TA/SBA(II). In the case of MAPS/SBA(II), N–H vibration of carbamate was not observed because of the absence of N–H bond in the carbamate (–N(CH3)COO) derived from the secondary amine.
CO2 NH2
Si O O O Isolated amine
Alkylammonium carbamate H2N
Si O O
NH2
O
Si O O
HNCOO-
Si O O
O
+H N 3
Si O O
Densely anchored amine SBA-15
Scheme 1. Adsorption of CO2 on aminosilane-modified SBA-15.
N. Hiyoshi et al. / Microporous and Mesoporous Materials 84 (2005) 357–365
As described above, CO2 is adsorbed on aminosilanemodified SBA-15 through formation of alkylammonium carbamate, in which two nitrogen atoms of amine are involved. Thereby the densely anchored aminosilane which affords an amine pair is an effective adsorption site for CO2, as shown in Scheme 1. 4. Conclusion The adsorbents prepared by grafting aminosilanes on SBA-15 support exhibited high adsorption capacities for CO2 under moist as well as dry conditions. The efficiency of amine was found to be improved by increasing surface density of the amine. It was suggested that the amine pairs, on which CO2 was adsorbed through formation of ammonium carbamate, were increased with increasing the surface density of amine. Acknowledgment This work was supported by the New Energy and Industrial Technology Development Organization (NEDO), Japan.
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