Infrared Study of CO2 Sorption over “Molecular Basket” Sorbent

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J. Phys. Chem. C 2009, 113, 7260–7268

Infrared Study of CO2 Sorption over “Molecular Basket” Sorbent Consisting of Polyethylenimine-Modified Mesoporous Molecular Sieve Xiaoxing Wang,† Viviane Schwartz,‡ Jason C. Clark,‡,§ Xiaoliang Ma,† Steven H. Overbury,‡ Xiaochun Xu,†,| and Chunshan Song*,† Clean Fuels and Catalysis Program, EMS Energy Institute, and Department of Energy & Mineral Engineering, The PennsylVania State UniVersity, 209 Academic Projects Building, UniVersity Park, PennsylVania 16802, and Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831 ReceiVed: NoVember 11, 2008; ReVised Manuscript ReceiVed: January 13, 2009

An infrared study has been conducted on CO2 sorption into nanoporous CO2 “molecular basket” sorbents prepared by loading polyethylenimine (PEI) into mesoporous molecular sieve SBA-15. IR results from DRIFTS showed that a part of loaded PEI is anchored on the surface of SBA-15 through the interaction between amine groups and isolated surface silanol groups. Raising the temperature from 25 to 75 °C increased the molecular flexibility of PEI loaded in the mesopore channels, which may partly contribute to the increase of CO2 sorption capacity at higher temperatures. CO2 sorption/desorption behavior studied by in situ transmission FTIR showed that CO2 is sorbed on amine sites through the formation of alkylammonium carbamates and absorbed into the multiple layers of PEI located in mesopores of SBA-15. A new observation by in situ IR is that two broad IR bands emerged at 2450 and 2160 cm-1 with CO2 flowing over PEI(50)/SBA-15, which could be attributed to chemically sorbed CO2 species on PEI molecules inside the mesopores of SBA-15. The intensities of these two bands also increased with increasing CO2 exposure time and with raising CO2 sorption temperature. By comparison of the CO2 sorption rate at 25 and 75 °C in terms of differential IR intensities, it was found that CO2 sorption over molecular basket sorbent includes two rate regimes which suggest two distinct steps: rapid sorption on exposed outer surface layers of PEI (controlled by sorption affinity or thermodynamics) and the diffusion and sorption inside the bulk of multiple layers of PEI (controlled by diffusion). The sorption of CO2 is reversible at 75 °C. Comparative IR examination of the CO2 sorption/ desorption spectra on dry and prewetted PEI/SBA-15 sorbent revealed that presorbed water does not significantly affect the CO2-amine interaction patterns. 1. Introduction The rapid rise in atmospheric concentration of carbon dioxide (CO2), one of the major greenhouse gases, has caused worldwide concerns for the undesired global climate change. One of the major sources of CO2 emissions is the combustion of fossil fuels. Since the use of fossil fuel is anticipated to increase in the coming decades, CO2 emissions will further increase with the potential impact on the global climate change.1,2 Consequently, CO2 capture and sequestration (CCS) have become crucial for the reduction in greenhouse gas emission and are challenging tasks for researchers. Currently, the commercially available liquid amine processes, such as UCARSOL3 and Fluor Econamine, are used for the removal of acidic gases including CO2 from various gas streams.4-7 However, separation of chemically bonded CO2 in aqueous amine solution and the regeneration of amines and CO2 require an energy-intensive and expensive stripping process.8 Additionally, the liquid amine processes have several disadvantages, such as equipment corrosion, liquid loss during the regeneration, and degradation due to oxidation.9,10 Therefore, * Corresponding author, [email protected]. † The Pennsylvania State University. ‡ Oak Ridge National Laboratory. § Current address: Su¨d-Chemie Company, South Plant, PO Box 32370, Louisville, KY 40232. | Current address: ConocoPhillips Company, 342A PL, Bartlesville Technology Center, Bartlesville, OK 74004.

it is important to develop new environmentally benign approaches for CO2 capture with higher energy efficiency. Adsorption is one of the promising methods. In the last two decades, various adsorbents such as carbon materials,11-19 pillared clays,20,21 metal oxides22-26 and zeolites11,27-32 have been extensively investigated. However, these conventional adsorbents have not proven to be highly effective for the capture of CO2 from gas mixtures. Recently, organically functionalized mesoporous materials have attracted considerable attention due to their potential as adsorbent for CO2 separation and for catalyst applications.33-35 A CO2 adsorption capacity of 9-18 mg/g was obtained over γ-(aminopropyl)triethoxysilane grafted SBA-15 adsorbent.36 Khatri et al. observed an enhanced capacity of 44 mg/g over the diamine-grafted SBA-15 adsorbent.37 Hiyoshi et al. prepared aminopropylsilyl (AP)-, ethylenediamine[propyl(silyl)] (ED)-, and diethylenetriamine[propyl(silyl)] (DT)-functionalized SBA-15 adsorbents and found that the CO2 adsorption efficiency of amine could be improved by increasing surface density of the amine since CO2 adsorbed through formation of ammonium carbamate on the amine pairs.38 The CO2 adsorption capacity was around 45 mg/g.38 Separately, Knowles et al. used HMS as the mesoporous silica substrate to prepare AP-, ED-, and DT-functionalized materials and applied for CO2 capture, where about 70 mg/g of CO2 capacity was achieved.39,40 Zhou et al. developed triethanolamine (TEA) modified SBA-15 silica and CMK-3 carbon for methane storage and CO2 capture.19,41

10.1021/jp809946y CCC: $40.75  2009 American Chemical Society Published on Web 04/02/2009

CO2 Sorption Although these adsorbents can remove CO2 from various gas streams, the CO2 adsorption capacity is still relatively low, generally less than 70 mg/g. It seems to us that the low capacity is due to low amine density of the organics and low loading amount since the surface functional groups over the mesoporous materials are limited with the grafting method applied. Therefore, to overcome the limitations, our strategy is to load selected polymers with numerous CO2-affinity sites and high amine density such as polyethylenimine (PEI) into the pore channels of mesoporous materials with high surface area and large pore volume such as MCM-41 and SBA-15. The goal is to develop high-capacity, high-selectivity CO2 sorbents by increasing the loading amount of polymer, expanding the contacting interface, and improving the mass transfer in the sorption/desorption processes. The nanoporous organic-inorganic composite sorbents developed in our laboratory have been termed as “molecular basket” sorbents, which have exhibited outstanding performance in a series of studies on CO2 and H2S removal from various gas mixtures.42-47 Our previous results have shown that the CO2 sorption capacity of the molecular basket sorbents can reach as high as 133 mg/g.42,43 Recently, Zhu et al. have presented a similar approach by which they reported a high CO2 adsorption capacity of 173 mg/g over the 70 wt % of tetraethylenepentamine (TEPA) modified uncalcined-SBA-15 adsorbent.48 The objective of the present work is to identify the interactions between CO2-philic polymer PEI and SBA-15 and to gain an insight into the CO2 sorption/desorption mechanism and the nature of sorbed CO2 species on the nanoporous PEI/SBA-15 sorbent. SBA-15 was used instead of MCM-41 as the support because it possesses higher hydrothermal stability and larger pore size. The PEI-modified SBA-15 sorbents have been studied by diffuse-reflectance Fourier transform IR spectroscopy (DRIFTS). The CO2 sorption/desorption process over PEI/SBA15 has been monitored by in situ Fourier transform infrared (FT-IR) spectroscopy in transmission mode. 2. Experimental Section 2.1. Preparation of Sorbents. Mesoporous molecular sieve SBA-15 was synthesized by a hydrothermal method described in the literature.46,49,50 In a typical synthesis, a homogeneous mixture composed of triblock copolymer Pluronic P123 (EO20PO70EO20, MW ) 5800, Aldrich) and tetraethyl orthosilicate (TEOS) in hydrochloric acid was prepared and stirred at 40 °C for 20 h followed by hydrothermal treatment at 100 °C for 24 h. After synthesis, the resultant solid was recovered by filtration, washed, dried at 100 °C overnight, and finally calcined at 550 °C for 6 h. The structure of synthesized SBA-15 was confirmed by low-angle X-ray diffraction (XRD) analysis. The BET surface area, pore volume, and pore diameter of the synthesized SBA-15 were 950 m2/g, 1.31 cm3/g, and 6.6 nm, respectively, which were obtained from the physisorption of N2 at -196 °C on the Micromeritics ASAP 2010 surface area and porosity analyzer. The PEI-modified SBA-15 sorbents were prepared by a wet impregnation method. In a typical preparation, the desired amount of polyethylenimine (linear PEI with Mn of ∼423 from Aldrich) was dissolved in 32 g of methanol under stirring for ca. 30 min, then 4.0 g of calcined SBA-15 was added to the above solution and further stirred at room temperature for 8 h. The slurry was then dried with continuous stirring. The as-prepared sorbent is denoted as PEI(x)/SBA-15, where x represents the loading of PEI as weight percentage in the sample (PEI plus SBA-15).

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Figure 1. DRIFTS profiles of SBA-15 and PEI/SBA-15 samples at 30 °C under UHP N2 flowing with KBr as the background: (a) SBA15; (b) PEI(15)/SBA-15; (c) PEI(30)/SBA-15; (d) PEI(50)/SBA-15; (e) PEI(65)/SBA-15.

2.2. DRIFTS Characterization. A Nicolet NEXUS 470 FTIR spectrometer (Thermo Electron Corp.) was used to obtain the DRIFTS spectra of the SBA-15 and PEI(x)/SBA-15 samples. The powder of each sample (about 20 mg) was placed into the DRIFTS cell and pretreated in flowing ultrahigh purity (UHP) N2 at 75 °C for 2 h. Then the DRIFTS spectra were collected under N2 atmosphere at 30, 50, 75, and 100 °C, respectively. KBr was used as the background. The IR resolution was 4 cm-1. 2.3. In Situ Transmission IR Study of CO2 Sorption/ Desorption. In situ transmission IR measurements of the CO2 sorption/desorption process over PEI-modified SBA-15 sorbents were performed on the Bruker FT-IR spectrometer at the Center for Nanophase Materials Sciences facility of the Oak Ridge National Laboratory (ORNL), Oak Ridge, TN. About 20 mg of each sample was palletized as a self-support disk and was placed into a self-made in situ IR cell. Prior to CO2 sorption, the sorbent was pretreated by flowing 25 mL/min of He at 80 °C for 2 h in order to obtain a “clean” surface and infrared background free of presorbed moisture, CO2, and other compounds. When the desired sorption temperature was reached, CO2 sorption was carried out by switching the inlet flow from He to pure CO2 at 25 mL/min through a four-port valve. After 3 min of sorption, the inlet gas was switched back to He and the desorption process was monitored for 5 min. In previous studies the presence of water was reported to enhance the CO2 adsorption via introducing CO2 with moisture in feed gas according to the reaction shown below.34,36,44,51

CO2 + R1NHR2 + H2O T R1R2NH2+ + HCO3- (1) However, it does not exclude the possibility that water is first sorbed on the sorbent and then promotes CO2 sorption. To verify this, prewetted PEI/SBA-15 was also examined in the selected IR measurements, where the sorbent was first prewetted by a mixed stream of He/H2O vapor for 2 min at room temperature prior to the CO2 sorption using a dry CO2 gas stream. 3. Results and Discussion 3.1. DRIFTS Characterization of SBA-15 and PEI/SBA15. 3.1.1. Effect of PEI Loading. Figure 1 shows the DRIFTS spectra of SBA-15 and SBA-15-supported PEI sorbents at room

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Wang et al.

temperature in flowing N2 with KBr as the background. Two sharp bands at 3747 and 1634 cm-1, and a broad band at around 3500 cm-1 (Figure 1, curve a) can be assigned to hydrogen bonding in molecular H2O and the H-O-H bend on SBA15,52 which is similar to that observed on the silica materials in other studies.52,53 After PEI loading, these bands either disappeared completely or significantly reduced. It suggests that a part of PEI may be anchored on the surface of SBA-15 through the interaction with the isolated surface silanol groups, depicted as follows:

Si - OH + RNH2 f Si - O-N+H3R

(2)

Si - OH + R2NH f Si - O-N+H2R2

(3)

Palkovites et al. measured the surface silanol groups on SBA15, which was about 3.7 mmol/g.54 On the basis of eqs 2 and 3, this value means that the surface silanol groups on SBA-15 could consume up to about 13.5 wt % PEI assuming one surface silanol group reacts with one amine group. This part of PEI would not be able to capture CO2 or H2S. This is in good agreement with our previous work with 15 wt % PEI loading, where only about 1.5% PEI in the PEI(15)/SBA-15 sample contributed to H2S adsorption; the breakthrough and saturation capacities of H2S over PEI(15)/SBA-15 were similar to those of SBA-15 alone and were much lower than those over the samples with higher PEI loading.47 Apart from the disappearance of the IR signals for the silanol group, some other new vibration signals appeared upon loading PEI into SBA-15, as shown in Figure 1. The doublets at 3348 and 3290 cm-1 can be assigned to the amine N-H stretching vibrations, while the bands at 2935 and 2814 cm-1 are due to the CH2 asymmetric and symmetric stretching modes of the PEI chain.55 A broad band at 2700-3800 cm-1 is attributed to the NH3+ stretching vibration. C-N stretching vibration is normally observed at around 1000-1200 cm-1.55 However, the peak is unresolved due to overlapping of Si-O-Si stretching in the same range.56 In the spectra of PEI/SBA-15, the band at 1470 cm-1 can be attributed to the deformation of the primary amine groups (-NH2), which increased with increasing PEI loading. It was reported that NH3+ deformation of the protonated amine group (R-NH3+O-Si) gave an IR signal at about 1630 cm-1.57,58 Over the PEI(15)/SBA-15 sample, a band at around 1630 cm-1 could be identified, which may be due to the deformation vibration of N-H in Si-O-N+H2R2/Si-O-N+H3R formed from the interaction between PEI and the internal surface of SBA-15. At higher PEI loading (50 and 65 wt %), the band split into two peaks at around 1630 cm-1 (Figure 1, curves d and e) because of two types of N-H deformation vibration in Si-O-N+H2R2/Si-O-N+H3R. Although the bending of the secondary amine groups (-N(R)H2+) in PEI (which also gives an IR signal at about 1600-1700 cm-1)59 may cause difficulty in the assignment, it still gives a clue that the interactions shown by eqs 2 and 3 happened in PEI/SBA-15. To differentiate the IR profiles before and after PEI loading, the difference spectra recorded by subtracting the SBA-15 spectrum are shown in Figure 2. After PEI loading, all samples show a reversal band at 3747 cm-1, supporting that the isolated silanol groups were consumed by amine groups through the interaction as shown by eqs 2 and 3. A strong and broad band between 3300 and 2000 cm-1 can be observed over all the samples. Hydrogen bonding in adsorbed H2O molecules on SBA-15 gives a broad band at around 3500 cm-1 (Figure 1,

Figure 2. Difference DRIFTS profiles of PEI/SBA-15 samples by subtracting SBA-15 spectrum at 30 °C under UHP N2 flowing: (a) PEI(15)/SBA-15; (b) PEI(30)/SBA-15; (c) PEI(50)/SBA-15; (d) PEI(65)/ SBA-15.

curve a). After PEI loading, the adsorbed H2O is occupied or replaced by PEI molecules and the amine groups interact with the surface silanol groups to form Si-O-N+H3R and/or Si-O-N+H2R2, which also can result in hydrogen bonding. Thus, the new broad band in Figure 2 may be attributed to the hydrogen bonding in Si-O-N+H3R and/or Si-O-N+H2R2. However, the doublets at 3348 and 3290 cm-1 (Figure 1) assigned to the amine N-H stretching vibrations vanished in this mode. One possible reason is that these two bands are counteracted by the broad band of hydrogen bonding in adsorbed H2O because the spectra were obtained by subtracting the SBA15 spectrum. It may also be the reason for the absence of the bands below 1600 cm-1 over all the PEI/SBA-15 samples as shown in Figure 2. Another possible reason is that they may be overlapped by the broad band of hydrogen bonding in Si-O-N+H3R and/or Si-O-N+H2R2. For the PEI(15)/SBA-15 sample, the bands at 2935 and 2814 cm-1 due to the CH2 stretching of PEI chain are weak and may be enclosed in the broad band. As PEI loading increased, these two bands began to appear above the broad band and became stronger, especially the band at 2814 cm-1. Thus, the bands at 2935 and 2814 cm-1 may be regarded as an indicator of the PEI amount on SBA15. It should be mentioned that gas-CO2 vibration (the bands at 2340 and 2362 cm-1) was observed over all the samples except SBA-15 alone, as shown in Figures 1 and 2. By using the IR intensity of gas-CO2 vibration as the internal reference (the band at 2362 cm-1), all the PEI characteristic IR bands (3348, 3290, 2935, and 2814 cm-1) can be quantitatively calculated and the results are presented in Table 1. At 30 °C and with KBr as the background, the relative intensities of the bands at 2935 and 2814 cm-1 over the PEI(15)/SBA-15 are 1.53 and 1.42. As PEI loading increases, the relative intensities of these two bands increased to 1.67 and 1.56 for PEI(30)/SBA-15, 2.08 and 2.12 for PEI(50)/SBA-15, and 2.66 and 2.63 for PEI(65)/SBA-15. The same trend can be obtained when SBA-15 was used as the background, supporting that these two bands at 2935 and 2814 cm-1 are closely related to the PEI amount. Interestingly, as the PEI loading amount increased, the bands at 3348 and 3290 cm-1 exhibit different trends from those at 2935 and 2814 cm-1. The relative intensity of the band at 3348 cm-1 decreased from 0.73 to 0.62 with increasing PEI loading from 30% to 50%. Then it increased to 1.15 upon further

CO2 Sorption

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TABLE 1: Relative DRIFTS Intensities of the Characteristic Bands of the PEI/SBA-15 Samples at Different Temperature with KBr as the Background and after Subtraction of the SBA-15 Spectrum KBr as the background samples

T, °C

PEI(15)/SBA-15

30 50 75 100 30 75 30 50 75 100 30 50 75 100

PEI(30)/SBA-15 PEI(50)/SBA-15

PEI(65)/SBA-15

after subtraction of SBA-15 spectrum

I3348/I2362

I3290/I2362

I2935/I2362

I2814/I2362

I2935/I2362

I2814/I2362

0.73 0.76 0.62 0.66 0.57 0.71 1.15 1.27 1.14 1.22

1.14 1.03 1.14 1.06 0.95 0.99 0.91 0.91 0.85 0.97 1.63 1.73 1.61 1.65

1.53 1.68 1.56 1.69 1.67 1.78 2.08 2.22 2.17 2.40 2.66 2.86 3.07 2.85

1.42 1.58 1.46 1.58 1.56 1.62 2.12 2.26 2.17 2.41 2.63 2.74 3.07 2.80

0.75 1.24 1.42 1.62 0.81 1.61 0.95 1.64 2.01 2.26 1.35 2.03 2.68 2.67

1.00 1.41 1.49 1.66 1.16 1.66 1.65 2.25 2.32 2.56 2.06 2.57 2.98 2.89

increasing PEI loading to 65%. In addition, the relative intensity of the band at 3290 cm-1 first decreased from 1.14 for PEI(15)/ SBA-15 to 0.95 for PEI(30)/SBA-15 and to 0.91 for PEI(50)/ SBA-15, then it increased to 1.63 for PEI(65)/SBA-15. When PEI loading is below 50%, all PEI is dispersed and confined inside the mesopores of SBA-15.42,47 Consequently, the more PEI is loaded, the less space is left for PEI expanding and vibrating inside the pore. This consideration rationalizes why the relative intensities of these bands decreased when the PEI loading was increased to 50%. However, upon further increasing PEI loading to 65%, the space inside pore channels became crowded; thus some extra PEI was likely located near the pore mouth or on the external surface of SBA-15 particles,46,47 where there is more space for molecular expanding and vibrating. Thus, the relative intensities of these two bands increased again. These explanations of the new IR data in this work are consistent with our previous results based on XRD and N2-physisorption characterizations showing PEI is located inside the mesopores of SBA-15 when PEI loading is lower than 50%.46,47 3.1.2. Effect of Temperature. DRIFT spectra of SBA-15 and PEI(50)/SBA-15 were recorded at different temperatures by using KBr as the background, which can be found in Supporting Information (SI-1). Other samples with different PEI loading were also investigated by DRIFTS at different temperatures. The quantitative data generated from the IR spectra are listed in Table 1. For SBA-15 (SI-1, A), when the temperature was increased from 30 to 100 °C, the band at 3747 cm-1 changed little, while the band at 1634 cm-1 became smaller and the broad band around 3500 cm-1 became narrower and smaller. These results indicate that increasing temperature can reduce the adsorbed water but does not change the surface silanol groups. Khatri et al. also observed a similar phenomenon.37 It suggests that heating up to 100 °C will not change significantly the IR characteristics of SBA-15. For PEI(50)/SBA-15, IR profiles at different temperatures are very similar (SI-1, B) and the change is too subtle to enable reliable quantification and, as indicated in Table 1, no general trend was observed. Considering that the IR signal of SBA-15 may veil the effect of temperature, it should be reasonable to verify the effect by subtracting SBA-15 spectrum. It can be seen from Table 1 that when the SBA-15 spectrum is subtracted, the relative intensities of the bands at 2935 and 2814 cm-1 increase with the increasing temperature. At 30 °C, the relative intensities for 2935 and 2814 cm-1 bands were 0.95 and 1.65, respectively. When the temperature was increased to 50, 75, and then 100 °C, the relative intensities increased to 1.64, 2.01,

and 2.26 for the 2935 cm-1 band and to 2.25, 2.32, and 2.56 for the 2814 cm-1 band, respectively. The same trends were also observed over the PEI(15)/SBA-15, PEI(30)/SBA-15, and PEI(65)/SBA-15 samples. The bands due to vibrations of bonds in PEI became stronger with increasing temperature. These results indicate that PEI molecules become more flexible and can stretch their chains inside the mesopore channels of SBA15 at higher temperature. However, it should be pointed out that although the band intensities increased, the level of intensity change, especially that for the band at 2935 cm-1, decreased with increasing temperature. For the band at 2935 cm-1 over PEI(50)/SBA-15, the intensity increase was 0.69 from 30 to 50 °C, 0.37 from 50 to 75 °C, and 0.25 from 75 to 100 °C. In the case of PEI(65)/ SBA-15, the increase was 0.68 from 30 to 50 °C and 0.65 from 50 to 75 °C. When temperature was further increased from 75 to 100 °C, the change of intensity of the 2935 cm-1 band was negligible (-0.01). The decrease of the changing rate is probably because the expansion and vibration of PEI are spatially confined by the pores of SBA-15. In other words, most PEI is located inside the mesopores of SBA-15. In our previous studies, it was found that the CO2 sorption capacity first increased, then decreased when the sorption temperature was increased from room temperature to 100 °C.42,43 The highest capacity was achieved at 75 °C.42,43 On the basis of the present IR results, the change of PEI flexibility with temperature may be an important reason for the unique temperature dependence of the CO2 sorption capacity over molecular basket sorbents observed in our previous work.42,43 3.2. IR Study of CO2 Sorption/Desorption oWer PEI(x)/SBA15 Sorbents. 3.2.1. CO2 Sorption oWer PEI(50)/SBA-15 at 25 and 75 °C. Gaseous CO2 is a linear triatomic molecule with three fundamental vibrations

rO ) C ) Of ν1 (1285, 1388 cm-1) IR inactive

v

v

O)C)O V ν2 (667 cm-1) IR active

f)O r rO ) C ν3 (2349 cm-1) IR active

One is a stretching vibration ν1, which is IR inactive appearing as a doublet at 1285 and 1388 cm-1. Two others are IR active, doubly degenerate deformation ν2 at 667 cm-1 and the antisymmetric stretching ν3 at 2349 cm-1. The IR of adsorbed CO2 varies distinctly from the gas-phase CO2 spectra: physically

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Wang et al. SCHEME 1: Possible Alkylammonium Carbamate Species in CO2 Sorption over Linear-PEI Modified SBA-15 under Dry Conditions

Figure 3. In situ FTIR spectra of CO2 sorption over PEI(50)/SBA-15 at (A) 25 °C and (B) 75 °C. The spectra from bottom to top were recorded at 4, 14, 24, 34, 44, 54, 64, 74, 84, 94, 104, 124, 144, 164, and 184 s, respectively. The spectrum of PEI(50)/SBA-15 was subtracted.

TABLE 2: FT-IR Frequencies of the Possible Species in CO2 Sorption over PEI/SBA-1535,38 frequency (cm-1) 3420 3327 3203 3055 2450, 2160 1650 1520 1410 1320

assignment N-H stretch in RNHCOON-H stretch in R2NH+COON-H stretch in RNH2+COON-H stretch in RNH3+ chemically sorbed CO2 molecules N-H deformation in RNH3+ stretching CdO NCOO skeletal vibration NCOO skeletal vibration

of PEI(50)/SBA-15 as the background was subtracted. Before 14 s, the spectrum is essentially a straight line. New bands emerged at 24 s after switching on the value for CO2 flow. In the range of 4000-3000 cm-1, four bands can be observed: a strong band at 3420 cm-1, a middle band at 3327 cm-1, and a broad band at 3055 cm-1 with a shoulder band at 3203 cm-1. Two distinct and broad bands appeared at 2450 and 2160 cm-1. Another interesting IR region lies in the range of 1800-600 cm-1, which is often called the fingerprint region. Here, several strong bands at 1650, 1520, 1410, and 1320 cm-1 with a shoulder band at 1340 cm-1 could be easily observed in Figure 3. In related studies, Chuang et al. assigned the bands in this region as bicarbonate, carbonate, and carbamic species over the amine-grafted APTS-SBA-15 adsorbent.51,53 However, since dry carbon dioxide and dry sorbent were used in the present work, it is difficult to envision the formation of bicarbonate [HCO3-] or carbonate [CO3-/2-] species. Chaffee et al.,39,40 Leal et al.,35 and Hiyoshi et al.38 reported that the interaction between amine and CO2 resulted in the formation of ammonium carbamates under anhydrous conditions and the formation of ammonium bicarbonate and carbonate species in the presence of water. Therefore, the formation of carbamate species would be more reasonable in the present work, which involves the primary and secondary amines in PEI polymer chains through the following reactions

CO2 + 2RNH2 f RNHCOO- + RNH3+

(4)

CO2 + 2R2NH f R2NCOO- + R2NH2+

(5)

CO2 + R2NH + R′NH2 f R2NCOO- + R′NH3+ (or R′NHCOO- + R2NH2+)

(6)

-1

adsorbed CO2 shows mostly the ν3 vibration near 2349 cm . The adsorption of CO2 on reactive surfaces may give rise to several adsorbed species, such as carbonate, bicarbonate, and carbamate with characteristic adsorption bands.60 In order to elucidate the CO2 sorption process and the nature of CO2 sorbed on the PEI-modified SBA-15 sorbent, in situ transmission IR study was carried out. Figure 3 shows the IR spectra of CO2 sorption versus time on stream over the PEI(50)/ SBA-15 sorbent at 25 and 75 °C, where the number for every spectrum in the figure caption represents the time in seconds for exposure to constant CO2 flow. Before IR recording, the sample was pretreated in pure He at 80 °C for about 2 h to ensure that it was “clean” prior to the IR study. The spectrum

Consequently, the bands at 1650, 1520, 1410, and 1320 cm-1 in Figure 3 could be assigned to N-H deformation in RNH3+, CdO stretch, and “NCOO” skeletal vibration of alkylammonium carbamate species. The bands at 3420, 3327, 3203, and 3055 cm-1 may be attributed to N-H stretch in R-NHCOO-, RNH2+COO-, R2NH+COO-, and R-NH3+, respectively. The tentative assignments of these bands are summarized in Table 2. According to the IR spectra and the assignments, the possible alkylammonium carbamate species in CO2 sorption over linearPEI modified SBA-15 could be proposed as shown in Scheme 1 involving the (4)-(6) reactions. The IR results strongly indicate that CO2 is sorbed on the molecular basket sorbent likely through

CO2 Sorption formation of alkylammonium carbamate in which two nitrogen atoms in amine groups are mostly involved for one CO2 molecule. This also suggests that interchain alkylammonium carbamate species are likely formed between a CO2 molecule and two amine groups in neighboring PEI chains. It is very interesting to note the two broad IR bands emerged at 2450 and 2160 cm-1 with CO2 flowing. Since physically adsorbed CO2 shows mostly the ν3 vibration near 2349 cm-1, these two bands could be assigned to the chemically sorbed CO2 species. Such broad and strong bands indicate that the sorbed CO2 are chemically interacting with PEI molecules inside mesopore channels of SBA-15. These two IR bands may be the first observation on chemically sorbed CO2 in PEI(50)/SBA15, since no other reports were found in the literature search on IR of CO2 sorption exhibiting these two IR bands at 2450 and 2160 cm-1. At the current stage, it is difficult to determine the nature of such chemically sorbed CO2 species chemically interacting with PEI loaded in the pore channel of SBA-15. However, our recent work on the molecular simulation of CO2 sorption over a model PEI sorbent showed that CO2 can interact with primary and secondary amine groups via C atom rather than O atom of CO2 under dry conditions.61 The computational results provide a clue to the chemically sorbed CO2 species on PEI molecules. The facts that the intensities of the two bands increased significantly with increasing CO2 exposure time over PEI(50)/SBA-15 (Figure 3) indicate that CO2 sorption involves diffusion and sorption of CO2 molecules into the bulk of PEI multilayers located inside the mesopores of SBA-15. This finding can explain why the molecular basket sorbent can achieve much higher CO2 sorption capacity than many aminegrafted adsorbents where only limited numbers of surface-bound amine molecules can adsorb CO2. These IR results also explain why higher PEI loading in molecular basket sorbent can give higher CO2 sorption capacity. Figure 4A shows the intensities of the bands at 1410 and 2160 cm-1 which represent the carbamate species and chemically sorbed CO2 species, respectively, as a function of the CO2 flowing time over PEI(50)/SBA-15 at 25 and 75 °C. Because the same material is used for the study and the background is recorded at the same temperature as the one for spectra recording, the intensity of the same bands obtained here at different temperatures is comparable. It can be seen that at 25 °C, the intensities of the 1410 cm-1 band and the 2160 cm-1 band increased with CO2 exposure time up to about 120 s, but changed little with further increasing CO2 exposure time up to 180 s. In contrast, at 75 °C, the intensities of both bands increased monotonically with increasing CO2 exposure time up to 180 s. These results indicate that CO2 sorption and saturation on available amine groups at 25 °C is reached almost within about 2 min, while a longer time is needed to reach saturation at 75 °C. Furthermore, after 180 s at 75 °C, the intensities of the 1410 cm-1 band and the 2160 cm-1 band are even higher than those at 25 °C, indicating that the PEI(50)/SBA-15 sorbent has higher CO2 sorption capacity at 75 °C compared to 25 °C since the IR intensity is directly proportional to CO2 sorption capacity. Xu et al. also reported that the CO2 sorption capacity over the PEI/MCM-41 sorbent was higher at 75 °C than that at 25 °C.42,43 Figure 4B compares the CO2 sorption rates over the PEI(50)/ SBA-15 sorbent at 25 and 75 °C in terms of differential IR intensity (δI/δt) versus sorption time (t). The rates were derived by differentiating the plots in Figure 4A, where the differential IR intensity (δI/δt) versus CO2 exposure time at a constant CO2 flow represents the sorption rate. It can be seen that, either at

J. Phys. Chem. C, Vol. 113, No. 17, 2009 7265

Figure 4. (A) The IR intensities and (B) the differentiated IR intensities of the bands at 1410 and 2160 cm-1 as a function of sorption time during CO2 sorption over PEI(50)/SBA-15 at 25 and 75 °C.

25 or 75 °C, the CO2 sorption rate over the PEI(50)/SBA-15 sorbent first increases and then decreases with the sorption time. This trend may indicate that there are different rate-determining factors controlling CO2 sorption over the sorbent at different sorption time. Generally, the sorption rate is a function of temperature and heat of sorption (or sorption affinity). Sorption is typically exothermic and favored at lower temperature while desorption is endothermic and thus favored at higher temperature. This principle applies to the sorption on the given amount of accessible sites. But diffusion rate and concentration gradients can also affect the sorption rate when the amount of accessible sites is dependent on the sorption conditions. Since the molecular basket sorbents include a large amount of PEI, up to the level of filling up most of the pore channel in SBA-15, the accessibility of the available sites in the polymer chains also depends on the temperature. It should be recalled that CO2 sorption over PEI(50)/SBA-15 sorbent involves carbamate formation and chemically sorbed CO2 species in PEI multilayers. Surface carbamate formation is fast and thermodynamically determined. While the CO2 molecule sorption in PEI multilayers is mainly controlled by the diffusion of CO2 molecules, which is slow. The higher sorption rate at 25 °C than that at 75 °C within the first 40 s can be explained by the sorption occurring dominantly on the outer-surface or near-surface layers of the PEI multilayers within this time, in which the low temperature is thermodynamically preferred. When the sorption continues from the exposed surface layers into the bulk of the PEI multilayers, the diffusion plays a more and more important role in determining the sorption rate. In this case, higher temperature enhances the molecular flexibility of PEI and favors diffusion of CO2, thus resulting in the higher sorption rate at 75 °C than

7266 J. Phys. Chem. C, Vol. 113, No. 17, 2009

Wang et al.

Figure 5. The IR intensities of the bands at 1410 and 2160 cm-1 as a function of desorption time during CO2 desorption over PEI(50)/ SBA-15 in He flowing at 25 and 75 °C. Sorption was switched to desorption at 180 s. (a) 1410 cm-1 at 25 °C; (b) 1410 cm-1 at 75 °C; (c) 2160 cm-1 at 25 °C and (d) 2160 cm-1 at 75 °C.

Figure 6. In situ FTIR spectra of CO2 sorption at 75 °C over PEI(50)/ SBA-15 (solid) recorded at (a) 54, (b) 104, and (c) 184 s and over PEI(15)/SBA-15 (dot) recorded at (d) 54, (e) 104, and (f) 184 s, respectively.

that at 25 °C, as observed in Figure 4. Therefore, the results suggest that CO2 sorption over the molecular basket sorbent includes two steps: the first is the adsorption on exposed surface layers of the PEI multilayers inside the pore channels of SBA15 (controlled by sorption heat, or thermodynamics) and the second is the diffusion of CO2 molecules from surface layers into the bulk of PEI multilayers (controlled by diffusion-rate). This explanation is also supported by our computational simulations.61 On comparison of the sorption rate at different temperatures, it can be seen that the sorption rate at 25 °C is higher than that at 75 °C at early stages of the sorption. It indicates that CO2 sorption is thermodynamically favored at low temperature. However, as the time progresses, the sorption rate at 25 °C decreases much faster and becomes much lower than that at 75 °C at late stages of the sorption, which should be controlled by the diffusion of CO2 molecules in PEI multilayers. This observation suggests that a lower CO2 diffusion rate is the major obstacle for the CO2 sorption at low temperatures. Therefore, a smaller CO2 diffusion barrier at higher temperatures is probably the main reason why PEI(50)/SBA-15 achieves higher CO2 sorption capacity at 75 °C than that at 25 °C, as shown in Figure 4A. Xu et al.42 and Ma et al.61 reached the same conclusion on CO2 sorption over PEI/MCM-41 sorbent by experiments and computational simulation, respectively. 3.2.2. CO2 Desorption BehaWior oWer PEI(50)/SBA-15 at 25 and 75 °C. After CO2 sorption for 3 min, the gas was switched back to He for 5 min to perform desorption at the same temperature as that for sorption. The desorption process was recorded by the in situ transmission FT-IR every 10 s. The IR profiles show same peaks as those in Figure 3. In order to clearly showing the effect of temperature on the CO2 desorption behavior over PEI(50)/SBA-15, the intensities of the typical bands at 1410 and 2160 cm-1 in desorption versus the He flowing time over spent PEI(50)/SBA-15 at 25 and 75 °C are plotted in Figure 5 according to the desorption IR spectra (figure not shown). At 25 °C, the intensities of the 1410 and 2160 cm-1 bands are almost constant throughout the 5 min desorption period measured, indicating that the sorbed CO2 is difficult to desorb at 25 °C. However, at 75 °C, the intensities of these two bands decrease gradually with the desorption time. After about 4 min of desorption at 75 °C, the intensities of these bands are even lower than those at 25 °C. This trend indicates that at 75 °C,

the sorbed CO2 can be desorbed and may be completely removed if given a certain time, which is in good agreement with the results by Xu et al.42,43 Furthermore, the desorption behavior at different temperatures suggests that the CO2 sorption heat on PEI/SBA-15 could be overcome at 75 °C, but not at 25 °C. However, higher CO2 capacity was achieved at 75 °C. It further supports the above conclusion that higher CO2 sorption capacity at 75 °C can be attributed to a higher CO2 diffusion rate and larger number of more spatially accessible amine groups at higher temperatures when polymer molecules in the porefilled PEI phase become more flexible and expanded. 3.2.3. CO2 Sorption/Desorption oWer the PEI(15)/SBA-15 at 75 °C. The IR spectra of CO2 sorption and desorption over PEI(15)/SBA-15 at 75 °C were also recorded (Supporting Information, SI-2). CO2 sorption over PEI(15)/SBA-15 exhibits similar IR profiles as those on PEI(50)/SBA-15. After 14 s of sorption, bands at 3420, 3327, 3055, 3203, 2450, 2160, 1650, 1520, 1410, and 1340 cm-1 are also observed and the IR intensities of these bands increase with CO2 flowing time. It indicates that the CO2 sorption mechanism does not change with PEI loading. After switching back to He flow, the IR intensities of these bands decrease with desorption time, suggesting that CO2 sorption over PEI(15)/SBA-15 is also reversible at 75 °C. Figure 6 compares the IR spectra of CO2 sorption over PEI(50)/SBA-15 and PEI(15)/SBA-15 at specific sorption time (54, 104, and 184 s). Although most of the band positions are essentially identical, the intensities of these bands on PEI(15)/ SBA-15 are significantly lower, especially the bands in 2500-1800 cm-1. Almost no IR bands at 2450 and 2160 cm-1 could be observed over PEI(15)/SBA-15, which suggests that these two bands are more unique with the high-PEI loading sample such as PEI(50)/SBA-15. In addition, when the sorption time was increased from 54 to 104 s and further to 184 s, the intensities of the IR bands did not change on PEI(15)/SBA-15, while they are still increasing significantly over PEI(50)/SBA15. This indicates that CO2 sorption on PEI(15)/SBA-15 quickly reached saturation and its capacity for CO2 is much lower than that for PEI(50)/SBA-15. One possible reason is that PEI loading amount in PEI(15)/SBA-15 is lower. However, comparing the area of the bands at 2500-1800 cm-1, the difference (more than 10 times) is much bigger than the difference in PEI loading amount (only 3.3 times), implying that there is another reason contributing to this striking difference in CO2 sorption capacity between PEI(15)/SBA-15 and PEI(50)/SBA-15. Xu et al.

CO2 Sorption

J. Phys. Chem. C, Vol. 113, No. 17, 2009 7267 bands on prewetted PEI(50)/SBA-15 are slightly higher than those on dry PEI(50)/SBA-15, indicating that the CO2 sorption capacity over the prewetted sorbent is slightly higher than that of the dry one. Considering that the CO2 sorption capacity could be nearly doubled in the presence of water in CO2 sorption over amine-based adsorbents, the improvement in CO2 capacity due to prewetting observed in the present study is quite small. This result confirms that the positive effect of moisture on CO2 sorption over molecular basket sorbent could mainly be attributed to its coreacting with CO2 to amine groups, as shown by the eq 1. 4. Conclusions

Figure 7. The IR intensities of the bands at 1410 and 2160 cm-1 as a function of sorption/desorption time during CO2 sorption/desorption over dry and prewetted PEI(50)/SBA-15 at 75 °C.

reported that the CO2 sorption capacity over PEI(50)/MCM-41 is more than 5 times higher than that over PEI(15)/MCM-41.42 Wang et al. also found H2S sorption capacity over PEI(50)/ SBA-15 as being about 30 times larger than that over PEI(15)/ SBA-15.47 Recalling that loaded PEI is anchored on the surface of SBA-15 through the interaction with the isolated silanol groups on SBA-15 and up to about 13.5% PEI is consumed for this reaction as discussed in section 3.1.1, the PEI loaded in PEI(15)/SBA-15 sample contained only about 1.5 wt % PEI available for CO2 sorption. However, for PEI(50)/SBA-15, the formed PEI multilayers inside the mesopores of SBA-15 are estimated to be about 36.5 wt % (excluding the part of PEI interacting with surface silanols on SBA-15), significantly higher than that in the PEI(15)/SBA-15. Additionally, as suggested above, the CO2 sorption over PEI/SBA-15 sorbent includes chemical reaction (carbamates formation) and chemical sorption in the PEI multiple layers. Thus, the CO2 sorption over PEI(15)/ SBA-15 could be mainly attributed to formation of alkylammonium carbamates, while sorption of CO2 molecules in PEI multilayers of PEI(50)/SBA-15 is responsible for its higher capacity. These considerations rationalize the striking difference in CO2 capacity between PEI(15)/SBA-15 and PEI(50)/SBA15. It may also be the reason why the molecular basket sorbent exhibits much higher CO2 sorption capacity than other aminegrafted adsorbents. 3.2.4. CO2 Sorption/Desorption oWer the Prewetted PEI(50)/ SBA-15 at 75 °C. It is widely accepted that water can enhance the CO2 adsorption capacity of the amine-based sorbents34,36,44,51 and play an important role during CO2 sorption,53 as eq 1 depicts. However it was concluded on the basis of the case when CO2 and moisture were introduced into the sorption bed at the same time. It is not clear whether adsorbed water in PEI can promote the CO2 sorption. In order to answer this question, we first prewetted the PEI(50)/SBA-15 sorbent with steam and then performed the CO2 sorption/desorption, which was simultaneously recorded by the in situ transmission IR. The IR spectra of the prewetted PEI(50)/SBA-15 for CO2 sorption and desorption (not shown here) are almost identical to those of dry PEI(50)/SBA-15, indicating that presorbed water does not significantly affect the CO2-amine interaction. Figure 7 compares the intensities of the bands at 1410 and 2160 cm-1 during CO2 sorption and desorption over prewetted and dry PEI(50)/SBA-15 at 75 °C. Clearly, prewetted and dry PEI(50)/SBA-15 exhibit a similar trend for CO2 sorption and desorption. The only difference is that the intensities of these

The present IR study provided new insight into the interaction between CO2-philic polymer PEI and the support SBA-15 and the mechanism of CO2 sorption over PEI-modified SBA-15 sorbent by using DRIFTS and in situ transmission infrared spectroscopy. It was found that a part of loaded PEI may be anchored on the surface of SBA-15 through the interaction between amine groups and isolated surface silanol groups. When higher loadings are used, PEI multilayers may form inside mesopores of SBA-15. Increasing temperature results in the increased flexibility of loaded PEI polymer chains in addition to the enhanced CO2 diffusion, which contributes to the increase of CO2 sorption capacity at higher temperatures. CO2 is found to sorb on the amine groups in PEI through formation of alkylammonium carbamates, as represented by several IR bands including the 1410 cm-1 band whose intensity increased with increasing CO2 exposure time and also rose with CO2 sorption temperature. An important new observation by in situ IR is that two broad IR bands emerged at 2450 and 2160 cm-1 with CO2 flowing over PEI(50)/SBA-15, which is not likely a part of the alkylammonium carbamates and could be attributed to chemically sorbed CO2 species on PEI molecules inside the mesopores of SBA-15. The intensities of these two bands also increased with increasing CO2 exposure time and with raising CO2 sorption temperature. By comparison of the differential IR band intensities with time for CO2 sorption rate at 25 and 75 °C, it was found that CO2 sorption over molecular basket sorbent exhibits two rate regimes, which suggest that the sorption process involves two steps: rapid sorption of CO2 onto exposed amine groups in PEI located in outer surface layers of PEI multilayers located inside pore channels of SBA-15 (controlled by sorption affinity or thermodynamics) and slower diffusion of CO2 molecules from surface layers into the bulk of the PEI multilayers to access more amine groups (controlled by diffusion which depends on the spatial flexibility of polymer chains packed inside mesopores of SBA-15). The first step is favored at lower temperatures such as 25 °C, while the second step is faster at higher temperatures such as 75 °C. CO2 sorption over PEI/SBA-15 is reversible at 75 °C. These IR results account for why the molecular basket sorbents such as PEI/SBA-15 show higher sorption capacity with higher polymer loadings and at higher temperatures. These findings also rationalize why CO2 molecular basket sorbents exhibit much higher sorption capacity than many amine-grafted sorbents. The comparison of the CO2 sorption/desorption IR spectra on prewetted and dry PEI/SBA-15 sorbent revealed that presorbed water in PEI does not significantly affect the CO2amine interaction. Water may likely work simultaneously with

7268 J. Phys. Chem. C, Vol. 113, No. 17, 2009 CO2 and amine groups to enhance the CO2 sorption capacity over amine-based sorbents. Acknowledgment. The present research is supported in part by the Pennsylvania Energy Development Authority through PA Department of Environmental Protection and by the US Office of Naval Research based on our earlier study funded by US Department of Energy through National Energy Technology Laboratory. The in situ transmission FTIR study at Oak Ridge National Laboratory’s Center for Nanophase Materials Sciences was sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy. The authors wish to thank all of the above government agencies. Supporting Information Available: The DRIFTS spectra of SBA-15 (A) and PEI(50)/SBA-15 (B) at different temperatures and the in situ FTIR spectra of CO2 sorption (A) and desorption (B) over PEI(15)/SBA-15 at 75 °C. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Song, C. S. Catal. Today 2006, 115, 2. (2) Houghton, J. T.; Ding, Y.; Griggs, D. J.; Noguer, M.; van der Linden, P. J.; Dai, X.; Maskell, K.; Johnson, C. A. Climate Change 2001: The Scientific BasisCambridge University Press: Cambridge, England, 2001. (3) Itoh, J. Chemical & Physical Absorption of CO2. RITE International Seminar, January 14, 2005. (4) Eow, J. S. EnViron. Prog. 2004, 21, 143. (5) Hao, J.; Rice, P. A.; Stern, S. A. J. Membr. Sci. 2002, 209, 177. (6) Seader, J. D.; Henley, E. J. Separation Process Principles; John Wiley & Sons, Inc.: New York, 1998. (7) Skinner, F. D.; Mclntush, K. E.; Murff, M. C. Amine-based Gas Sweetening and Claus Suflur RecoVery Process Chemistry and Waste stream SurVey - Technical Report; Gas Research Institute, December 1995. (8) Chakma, A. Energy ConVers. Manage. 1997, 38, S51. (9) Tontiwachwuthikul, P.; Meisen, A.; Lim, C. J. J. Chem. Eng. Data 1991, 36, 130. (10) Veawab, A.; Tontiwachwuthikul, P.; Chakma, A. Ind. Eng. Chem. Res. 1999, 38, 3917. (11) Siriwardane, R. V.; Shen, M. S.; Fisher, E. P.; Poston, J. A. Energy Fuels 2001, 15, 279. (12) van der Vaart, R.; Huiskes, C.; Bosch, H.; Reith, T. Adsorption 2000, 6, 311. (13) Heuchel, M.; Davies, G. M.; Buss, E.; Seaton, N. A. Langmuir 1999, 15, 8695. (14) Chen, J. H.; Wong, D. S. H.; Tan, C. S.; Subramanian, R.; Lira, C. T.; Orth, M. Ind. Eng. Chem. Res. 1997, 36, 2805. (15) Dong, F.; Lou, H. M.; Kodama, A.; Goto, M.; Hirose, T. Sep. Purif. Technol. 1999, 16, 159. (16) Song, H.; Lee, K. Sep. Sci. Technol. 1998, 33, 2239. (17) Burchell, T. D.; Judkins, R. R.; Rogers, M. R.; Williams, A. M. Carbon 1997, 35, 1279. (18) Zinnen, H. A.; Oroskar, A. R.; Chang, C. H. U.S. Patent 4,810,266, 1989. (19) Sun, Y.; Liu, X.-W.; Su, W.; Zhou, Y. P.; Zhou, L. Appl. Surf. Sci. 2007, 253, 5650. (20) Ding, Y.; Alpay, E. Chem. Eng. Sci. 2000, 50, 3461. (21) Yong, Z.; Mata, V. G.; Rodrigues, A. E. Ind. Eng. Chem. Res. 2001, 40, 204. (22) Huang, H. P.; Shi, Y.; Li, W.; Chang, S. G. Energy Fuels 2001, 15, 263. (23) Pereira, P. R.; Pires, J.; Carvalho, M. B. Langmuir 1998, 14, 4584. (24) Yong, Z.; Mata, V. G.; Rodrigues, A. E. J. Chem. Eng. Data 2000, 45, 1093. (25) Iyer, M. V.; Gupta, H.; Sakadjian, B. B.; Fan, L.-S. Ind. Eng. Chem. Res. 2004, 43 (14), 3939.

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