Adsorptive Removal of Carbon Dioxide Using Polyethyleneimine

Publication Date (Web): August 15, 2013 ... *Telephone/Fax: +86-571-87953088. E-mail: ... The resulted sorbents were characterized via a range of anal...
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Adsorptive Removal of Carbon Dioxide Using Polyethyleneimine Supported on Propanesulfonic-Acid-Functionalized Mesoporous SBA-15 Jie Liu,† Dandan Cheng,† Yue Liu,*,† and Zhongbiao Wu†,‡ †

Department of Environmental Engineering, Zhejiang University, Hangzhou 310058, People’s Republic of China Zhejiang Provincial Engineering Research Center of Industrial Boiler and Furnace Flue Gas Pollution Control, Hangzhou 310058, People’s Republic of China

‡

S Supporting Information *

ABSTRACT: One-step synthesized sulfonic-acid-functionalized SBA-15 (denoted as αSSBA-15) impregnated with polyethyleneimine (PEI) was used for CO2 capture in this study. The resulted sorbents were characterized via a range of analytical techniques, including transmission electron microscopy (TEM), 29Si magic angle spinning (MAS) nuclear magnetic resonance (NMR), infrared (IR), thermogravimetry−differential scanning calorimetry (TG−DSC), etc. Experimental results showed that the incorporation of propanesulfonic acid groups into the inner structure of the silica support had brought dramatic improvement in CO2 adsorption capacity, of which PEI/5SSBA-15 showed the highest CO2 adsorption amount. The main reason of this increased capacity could be attributed to the enhanced CO2 diffusion into bulk networks of PEI polymers because of its better dispersion in the pores of support, where the extended propanesulfonic acid groups on the inner surface could spatially disperse the subsequent loaded PEI molecules. Furthermore, the PEI/5SSBA-15 also exhibited superior stable cyclic adsorption/desorption performance compared to PEI/SBA-15, especially after 5 cycles. This was assumed because the enhanced surface acidity of PEI/5SSBA-15 anchored the NH2/NH groups through acid−base interaction, reducing the loss of active sites.

1. INTRODUCTION The substantive CO2 emission has motivated general public concern for its resulting global warming and climate change. Research activities about key technology options relating to CO2 capture, sequestration, and long-term storage have been extensively conducted.1,2 Chemical aqueous amine (e.g., monoethanolamine3 and diethamolamine4) absorption has already been commercialized for CO2 capture from gas streams,5 but the high energy consumption, large solvent makeup, and equipment corrosion significantly reduce its implementation for vast volumetric flue gas at low partial pressure.6,7 Thus far, predominant focus has been given to amine-functionalized solid sorbents, such as monomeric or polymeric amine-species-functionalized zeolites,8 resins,9 carbon nanotubes,10 glass fiber,11 and silica-supported materials.12−15 Polymeric amines could be introduced into the pores or onto the surface of porous materials through wet impregnation to achieve superior CO2 adsorption capacity. The adsorption capacity of CO2 would be enhanced with the increase of loaded amines.16,17 However, the loaded polyethyleneimine (PEI) and their CO2 sorption products (ammonium carbamates) could form a compact entity, which provided extremely strong diffusion limitation of CO2 molecules from the surface into the bulk of loaded amines, which would lower the amine efficiency and adsorption capacity.18 Therefore, a lot of efforts have been made to overcome this obstacle by additive usage and surface chemical modification.19 Recently, Wang et al.20 used CO2-neutral surfactants as a promoter to break the compact PEI films and create extra CO2 © 2013 American Chemical Society

transfer pathways, allowing more CO2 to diffusion into the deeper PEI networks. Hence, enhanced CO2 sorption capacity and more efficient use of amine groups were obtained. Moreover, other works18,21−24 had reported that tuning or modifying the surface properties of support could also enhance CO2 adsorption performances of polymeric amine-loaded materials. Yue et al.21 adopted both the as-synthesized (without removing the organic templates by calcination/extraction) and the corresponding calcined SBA-15 as support materials. They found that the former support material showed better performance than the latter after tetraethylenepentamine (TEPA) loading. The possible explanation was that the hydroxyl groups of the polymeric template in the pores of assynthesized supports could combine with the loaded TEPA, forming a more even distribution of functional groups, hence modifying the interactions between CO2 and amine. Fauth et al.18 had impregnated PEI onto a 3-(aminopropyl)-triethoxysilane (APTES)-grafted silica. They believed that the siteisolated 3-aminopropyl groups could facilitate CO2 diffusion into the bulk network of PEI. Kuwahara et al.23,24 had incorporated Zr as heteroatom species into the silicate support to alter the interactions between PEI and silicate surface. They argued that this amine-stabilizing effect had played a critical role in improving the CO2 adsorption performance. Inspired by these findings, novel composites of PEI supported on functionalized mesoporous silica prepared Received: June 16, 2013 Revised: August 15, 2013 Published: August 15, 2013 5416

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placed in a quartz cell equipped with KBr windows. The cell was connected to a vacuum line (experimental vacuumness = 10−2−10−3 Pa), temperature controller, and several ports for gases. IR spectra were recorded on a Brukers Optics Tensor 27 spectrometer within the range of 500−5600 cm−1 at a resolution of 4 cm−1. The acidity of the materials was studied using adsorbed pyridine as the probe molecule and was presented as differential spectra by subtracting the background recorded at the corresponding temperature. 2.3. Measurements of the CO2 Adsorption Performance. As described previously,16 the experimental tests of CO2 kinetic equilibrium adsorption were carried under atmospheric pressure on a quartz fixed bed (inner diameter = 8 mm and L = 200 mm) equipped with a controlled temperature programming furnace. The assynthesized powder sample was compressed and sized into 40−60 mesh. A total of 0.5 g of these adsorbent particles was put into the quartz chamber and preheated at 105 °C for a span of 1 h in a pure N2 atmosphere, followed by exposure to 7 vol % CO2/N2 mixture until pseudo-equilibrium when the temperature stabilized at 75 °C. The outlet CO2 concentration was recorded by an incubator CO2 analyzer (G100, Geotech, U.K.). The CO2 sorption amount (qe) was further calculated by integrating the breakthrough curve, which was expressed as eq 1

through wet impregnation were studied in this work. The ordered SBA-15 mesoporous silica anchored with sulfonic acid groups were directly synthesized through a co-condensation of tetraethoxysilane (TEOS) and 3-mercaptopropyltrimethoxysilane (MPTMS). We expected that the incorporation of propanesulfonic acid groups could bring two benefits: (i) The extended alkyl chains could divide loading PEI into small-sized particles, modify its dispersion, and enhance the CO 2 adsorption capacity. (ii) The terminal sulfonic groups and surface Si−OH could anchor the amino groups and, thus, improve its thermal stability, which would avoid the loss of active sites.

2. EXPERIMENTAL SECTION 2.1. Sample Preparation. Poly(ethylene oxide)−poly(propylene oxide)−poly(ethylene oxide) block copolymer species (Pluronic 123) was obtained from Sigma-Aldrich. All other used chemicals were purchased from Aladdin Chemistry Co., Ltd. and were used without further treatment. One-step preparation of sulfonic-acid-functionalized SBA-15 (denoted as αSSBA-15) was similar to previously reports,25,26 with minor modifications, which involved the co-condensation of TEOS and MPTMS in the presence of Pluronic 123 and H2O2 in HCl aqueous solutions. The optimal ratio of reactants was 0.0007P123, xTEOS, (0.041 − x)MPTMS, 20(0.041 − x)H2O2, 0.24HCl, and 6.67H2O. αSSBA-15 samples [α % = n(MPTMS)/(n(MPTMS + TEOS)) = (0.041 − x)/0.041] could be obtained according to the molar ratio of silicon supplied by MPTMS. Typically, when α = 5, 8.00 g of P123 was dissolved in deionized water under vigorous stirring at room temperature, followed by adding 50.15 g of concentrated HCl (35%) and 16.20 g of TEOS. Subsequently, 0.80 g of MPTMS and 9.28 g of H2O2 were suspended in the mixture after the pre-hydrolysis of TEOS. The resultant solution was stirred for 24 h at 40 °C and was aged in a hydrothermal synthesis reactor for another 24 h at 100 °C. The template was extracted from the as-synthesized solid product by 400 mL of 2.6% (v/v) HCl/alcohol solution 3 times under reflux and then dried in a vacuum oven at 60 °C overnight. PEI was incorporated into the pores of SBA-15 and αSSBA-15 through wet impregnation, which has been detailed in our previous report.16 A total of 2.00 g of support powders were added to PEI (1.33 g) methane solution (40 mL), then the mixture was stirred for 2 h in a sealed glass vessel at room temperature and another 6−10 h in a fume hood to allow for methane evaporation. After that, the residue was held at 100 °C overnight under reduced pressure. The resultant samples were denoted as PEI/SBA-15 or PEI/αSSBA-15. 2.2. Characterizations. The micromorphology characteristic was examined by transmission electron microscopy (TEM, Tecnai G2 F20, FEI Company, Hillsboro, OR). Nitrogen adsorption−desorption isotherms were obtained at 77 K under relative pressures ranging from 0.005 to 0.99 using the Tristar II 3020 (version 1.03) surface area and porosity measurement system (Micromeritics, Inc., Norcross, GA). The thermal stability of modified mesoporous silica was determined using thermogravimetry−differential scanning calorimetry (TG−DSC, Netzsch STA 409 Luxx, Selb/Bavaria, Germany). About 10 mg of samples were heated from room temperature to 600 °C at a heating rate of 10 K/min in an air atmosphere. Elemental analysis was performed on Flash EA1112 (Thermo Finnigan, Waltham, MA). The structural properties of SBA-15 and αSSBA-15 were characterized by 29Si nuclear magnetic resonance (NMR) performed on a Bruker AVANCE III 400 WB spectrometer operation at 9.4 T with a frequency of 79.48 MHz. 29Si magic angle spinning (MAS) and cross-polarization/magic angle spinning (CP/MAS) spectra were obtained using a recycle delay of 3 s, a contact time of 2 ms, and high power proton decoupling during detection. Typically, 4096 scans were acquired at a spinning rate of 6 kHz. Tetramethylsilane was used as a reference to obtain the chemical shifts. Infrared (IR) spectra of SBA-15 and αSSBA-15 samples were investigated using self-supported wafers (12 mg/cm2), which were

qe =

1 m

∫0

t

k(Q inC in − Q eff Ceff )dt

(1)

where m is the weight of the adsorbents (g), k is an apparent density (mg/mL), Qin is the inlet flow rate (mL/min), Qeff is the effluent flow rate (mL/min), Ceff is the effluent CO2 concentration (%), and t is the time (min) used until Ceff reaches Cin. After that, N2 flow was switched in again, and the temperature was held at 105 °C for another 1 h until no CO2 could be detected. The above-mentioned procedures were repeated 20 times to evaluate the extensive cyclic adsorption/ desorption performance of samples. CO2 adsorption capacity was also determined by a SDT Q600 simultaneous TGA instrument (TA Instruments, New Castle, DE). About 10 mg of sorbents were pretreated at 105 °C for 1 h with N2 purge flow at 120 mL/min. After cooling to 75 °C, a pure CO2 flow was switched in and passed through the crucible. The whole adsorption process lasted for about 1 h until a constant weight achieved. The final gained weight was regarded as the CO2 adsorption capacity.

3. RESULTS AND DISCUSSION 3.1. Structural and Textural Properties. The morphology of SBA-15 and αSSBA-15 samples was investigated by TEM images, and the results were presented in Figure 1. It could be observed that SBA-15 showed the periodic twodimensional (2D) hexagonal array of well-ordered channels (P6mm) with the typical honeycomb appearance, which was in accordance with the previous literature.25,27,28 The visible images of αSSBA-15 revealed that the ordered porous structure was retained until α values equaled 7, suggesting that no obvious damage occurred caused by incorporation of organic silica. However, a further increased concentration of the incorporated propanesulfonic acid groups might lead to the mesostructural disorder of functionalized samples. The incorporation of organotrialkoxysilane into the mesoporous materials was characterized by means of 29Si CP/MAS NMR (Figure 2), together with SBA-15 as a comparison. Distinct resonances could be observed for silicon atoms in siloxane [Qn = Si(OSi)n(OH)4−n (n = 1−4), where n is the number of siloxane bonds linking the Si site to the silica frameworks: Q4 at −109 ppm, Q3 at −100 ppm, and Q2 at −91 ppm] and organosiloxane [Tm = RSi(OSi)3(OH)3−m (m = 1− 3), where m is used for SiR groups with one covalent bond to alkyl chain and up to three to siloxane bonds: T3 at −66 ppm and T2 at −56 ppm] species.25,27,29 The relative intensities of 5417

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lattice deformation and structural collapse would occur after modification, which ensured the high surface area and pore volume of αSSBA-15. However, for 7SSBA-15, both the peaks of T2 and T3 emerged. This suggested that the trialkoxysilanes were anchored onto the silica frameworks generally through two and three siloxane bridges. It could be imagined that, with a further increase of the α value, the ratio of T2/T3 would ever increase, which might lead to structural collapse. Detailed information about the acid site types, distribution, strength, and density of the SBA-15 and αSSBA-15 samples was investigated by IR spectra using pyridine as the probe molecule (see Figure 3). The IR spectra of SBA-15 showed several peaks

Figure 1. TEM images of (a) extracted mesoporous silica SBA-15, (b) 5SSBA-15, (c) 7SSBA-15, and (d) 10SSBA-15.

Figure 3. IR spectra of pyridine adsorption on (upper) SBA-15 and (bottom) 5SSBA-15 at (a) room temperature, (b) 150 °C, and (c) 300 °C.

because of hydrogen-bonded pyridine (1597 and 1446 cm−1) and pyridine adsorbed on weak Lewis acid sites (1577 cm−1).32,33 However, these bands gradually disappeared after outgassing at elevated temperatures, indicating rather low acidity strength of pure siliceous materials. After incorporation of propanesulfonic groups, there emerged additional spectra of strong Lewis-acid-site-bound pyridine (1623 cm−1), Brønstedacid-site-bound pyridine (1547 and 1639 cm−1), and a band at 1492 cm−1 that could be assigned to pyridine associated with both Brønsted and Lewis acid sites.32,34,35 The intensities of these vibration peaks increased with the increase of the α value (see the IR spectra of 7SSBA-15 in Figure S1 of the Supporting Information). Even after evacuation at a high temperature of 300 °C, the pyridine adsorption peaks on these acid sites could not be completely removed. It was suggested that the enhanced acidity, especially pronounced in Brønsted acid sites, was originated from the involvement of propanesulfonic acid groups.

Figure 2. Solid-state 29Si NMR spectra of the extracted (a) SBA-15 and αSSBA-15: (b) 3SSBA-15, (c) 5SSBA-15, and (d) 7SSBA-15.

Tm signals to Qn were enhanced with the increase of the α value, suggesting the increased amounts of organo-functionalized moieties as a part of the silica wall structure. After the hydrolysis of Si−(MeO)3 in MPTMS, the silicon atom in −Si−(CH2)3−R connected with the oxygen atom in the O−Si bond derived from the hydrolysis of TEOS and, thus, produced a stable tetrahedral structure on the surface of mesoporous silica. For the 5SSBA-15 sample, only the T3 peak was observed, illustrating the dominant formation of threedimensional networks with three Si−O−Si linkages.30,31 It was suggested that within a certain range of α values, no significant 5418

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The textural properties, including specific surface areas (SBET), pore volumes (Vtotal), and pore diameters (dp), obtained from nitrogen adsorption isotherms of the extracted SBA-15 and αSSBA-15 materials were summarized in Table 1, and the Table 1. Textural Properties of SBA-15 and αSSBA-15 before and after PEI Loading without PEI adsorbent

SBETa 2

(m /g)

PEI/SBA-15 PEI/3SSBA-15 PEI/5SSBA-15 PEI/7SSBA-15 PEI/10SSBA-15

710.4 631.0 573.9 589.3 570.2

b

with PEI

Vtotal (cm3/g)

dpc

a

(nm)

SBET (m2/g)

Vtotalb (cm3/g)

dpc (nm)

1.07 1.05 1.04 1.01 0.73

5.4 6.3 6.3 6.1 3.5

113.1 85.2 76.5 66.9 28.3

0.21 0.16 0.15 0.12 0.04

4.4 5.0 5.1 4.4 3.4

a

Calculated from the adsorption branch of the N2 isotherm. bValues at P/P0 = 0.97. cObtained from the Barrett−Joyner−Halenda (BJH) desorption pore size distribution.

pore size distribution was showed in Figure S2 of the Supporting Information. SBA-15 exhibited extraordinary surface specific area as large as 710.4 m2/g, with a pore volume of 1.07 cm3/g. Only a minor loss of the surface area and pore volume occurred for αSSBA-15; e.g., high Brunauer−Emmett− Teller (BET) surface area of 589.3 m2/g and pore volume of 1.01 cm3/g remained for 7SSBA-15. It was inferred that monolayer-dispersed alkyl chains burgeoning out from the internal surface of the cylindrical mesoporous pores took up some accessible volume for nitrogen molecules, which was accountable for the slight loss of the BET surface area and pore volume. However, when the α value was greater than 7, the pore volume and pore size significantly declined. The pore volume of 10SSBA-15 decreased from 1.07 cm3/g of SBA-15 to 0.73 cm3/g, and its pore size distribution centered at 5.4 nm was also dramatically shifted to 3.5 nm. The possible reason was the mesostructural collapse because of the excessive incorporation of organic trimethoxysilane, which was in good agreement with the above-mentioned discussion about TEM and 29Si NMR results. Margolese et al.25 also had reported that a high concentration of MPTMS perturbed the self-assembly of the surfactant aggregates during the co-condensation process, which caused the decreased crystallinity. As discussed by Bollini et al.,7 the pore volume of the supporter played a critical role, determining the theoretical maximum loading content of amine-contained materials. The preserved structure provided sufficient space for subsequent amine loading. As seen in Table 1, the structural parameters of supporters fell subsequently after PEI impregnation, suggesting the successful introduction of PEI into the channels, which was in accordance with the previous studies.15,24 3.2. Thermal Stability Analysis. The TG−DSC study was carried out to determine the thermal stability of PEI-modified SBA-15 and αSSBA-15. Figure 4 presented that the PEI-loaded SBA-15 showed three obvious mass loss steps, which peaked at around 100, 265, and 380 °C in the synchronous DSC signal. The former endothermic DSC peak was attributed to the removal of physisorbed species (e.g., moisture), while the latter two exothermic peaks could be mainly associated with the decomposition of PEI according to the TG−DSC results of pure PEI (see Figure S3 of the Supporting Information).16,23 Although there were also two exothermic peaks at 263 and 436 °C because of the decomposition of Pluronic 123 and the

Figure 4. TG−DSC profiles of (A) PEI/SBA-15 and (B) PEI/5SSBA15.

dehydroxylation by silanol condensation on SBA-15 (see Figure S4 of the Supporting Information),36 this effect was limited because of the rather low mass ratio of residual Pluronic 123 to loaded PEI. As for PEI/5SSBA-15, three similar DCS peaks could be observed at 100, 210, and 380 °C, as showed in Figure 4B. The first sharp DSC exothermic peak of PEI-modified samples was also due to the partial decomposition of loaded PEI. However, different with PEI/SBA-15, the second endothermic peak at around 380 °C might be the combined result of both the decomposition of residual PEI and −CH2− CH2 −CH2 −SO3 H groups in the support,25 because a pronounced weight loss initiated at about 400 °C ascribed to the decomposition of propanesulfonic groups was observed over 5SSBA-15 (see Figure S5 of the Supporting Information). DSC plots of PEI/αSSBA-15 were listed in Figure 5 for further discussion. It was found that the first peak location of PEI decomposition shifted to the left from 265 °C for PEI/ SBA-15 to 210 °C for PEI/5SSBA-15 and then climbed up to 247 °C for PEI/10SSBA-15 again. The previous works15,17 have deduced that the better dispersion of PEI into smaller size particles with higher volatility in nanoporous supporters will lead to the decomposition of polymers at lower temperature. As such, the decrease of the PEI decomposition temperature could be ascribed to its well dispersion after the support was functionalized by propanesulfonic acid groups. The introduced PEI could be divided into small particles by the extended alkyl chains. Heydari-Gorji et al.37 also reported the improved PEI dispersion in the supports with the inner surface covered by a layer of long alkyl chains. With the further increase of the α value, the surface acidity increased greatly and the chemical interactions between surface acid sites and NH2/NH groups of PEI polymer became stronger, thereby enhancing the thermal stability of the composites. Our previous study also confirmed the improvement of the thermal stability of loaded amine with the increase of support acidity.16 3.3. CO2 Adsorption Properties. Breakthrough curves for PEI-loaded SBA-15 and αSSBA-15 materials under a 7 vol % CO2/N2 atmosphere were shown in Figure 6. The related CO2 5419

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adsorption capacity could be detected with the changing of the α value. CO2 adsorption capacity increased slightly from 96.1 mg/g for PEI/SBA-15 to 100.9 mg/g for PEI/5SSBA-15 and then declined to 29.7 mg/g for PEI/10SSBA-15. On the basis of these values, the corresponding amine efficiency, which was defined as the moles of CO2 molecules adsorbed for each mole atom in the amine functional group, was also presented in Table 2. The amine efficiency of the PEI/5SSBA-15 sample was calculated to be 0.281 mmolCO2/mmolN, which was comparable to the reported experimental results.13,18 It was worth noting that the enhancement of the CO2 kinetic adsorption amount on PEI/5SSBA-15 with respect to PEI/ SBA-15 was much higher than that obtained under a pure CO2 environment: 68 versus 5%. It was likely that the CO2 adsorption on PEI/5SSBA-15 was a diffusion-controlled process. The superior CO2 diffusion rate of PEI/5SSBA-15 to PEI/SBA-15 was revealed as the driven force of mass transfer that dramatically decreased for the dynamic gas-adsorption process under a 7 vol % CO2/N2 atmosphere compared to a pure CO2 environment. With the further increase of the α value, the limitation of CO2 diffusion became more obvious. For instance, the adsorbed amount of PEI/5SSBA-15 within the first 10 min accounted for above 80% of the equilibrium value under a CO2 environment, while that was only 58% for PEI/10SSBA-15 [see thermogravimetric analysis (TGA) profiles in Figure S6 of the Supporting Information]. The dramatic improvement in kinetic adsorption of CO2 might be related to the incorporated extended alkyl chains within the mesopores of support, which significantly improved the dispersion of loaded PEI. Better dispersion of the polymer in the pores would result in better accessibility of the amine active sites. Under the same loading amount, the well-dispersed amine on PEI/5SSBA-15 exposed more CO2 affinity groups and exhibited diminished diffusion resistance. On the contrary, PEI would like to agglomerate inside the channels of SBA-15, giving rise to a high diffusion resistance, thus lowering adsorption kinetics. The decreased CO2 adsorption capacities with the further increase of the α value could be ascribed to the following two points: (1) As the pore was progressively filled with impregnated polymeric amine, a kinetic limitation for CO2 diffusion would occur, which affected the subsequent CO2 adsorption because the incoming CO2 molecules could not easily access the inner amine sites of the aggregated amino polymer. As seen in Table 1, the pores of PEI/10SSBA-15 were essentially blocked completely, remaining only 0.04 cm3/g of the total pore volume and 3.4 nm of the pore diameter. According to the work by Son et al.,15 a reduced pore diameter would greatly inhibit the CO2 diffusion rate inside the pores, thereby lowering the adsorption capacity. (2) Part of amine moieties might be consumed by Brønsted acid sites generated

Figure 5. DSC profiles of PEI impregnated (a) SBA-15 and αSSBA15: (b) α = 3, (c) α = 5, (d) α = 7, and (e) α = 10.

Figure 6. Breakthrough curves of PEI/SBA-15 and PEI/αSSBA-15 sorbents at 75 °C.

adsorption capacities of different samples were calculated and listed in Table 2. The calculated CO2 adsorption capacity was enhanced from 30.7 mg/g for PEI/SBA-15 to 51.7 mg/g for PEI/5SSBA-15 and then dropped to 22.2 mg/g for PEI/ 10SSBA-15. The CO2 adsorption capacities under a pure CO2 atmosphere by TG tests were also determined as a comparison, and the results were given in Table 2. Similar trends in

Table 2. CO2 Adsorption Capacities and Amine Efficiencies of PEI/SBA-15 and PEI/αSSBA-15 Sorbents at 75 °C

a

adsorbent

CO2 adsorption capacitya (mg/g)

CO2 adsorption capacityb (mg/g)

loaded amine (mmolN/g)

N efficiencyc (mmolCO2/mmolN)

PEI/SBA-15 PEI/3SSBA-15 PEI/5SSBA-15 PEI/7SSBA-15 PEI/10SSBA-15

30.7 32.4 51.7 31.4 22.2

96.1 96.2 100.9 75.2 29.7

8.27 7.94 8.15 8.11 8.04

0.264 0.275 0.281 0.211 0.084

Caculated from breakthrough curves. bObtained through TG analysis. cDetermined by CO2 adsorption capacity (TG) divided by loaded amine. 5420

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cyclic adsorption performance of PEI/5SSBA-15 was more stable than that of PEI/SBA-15, especially after 5 cycles. It was assumed that the surface OH groups with enhanced acidity chemically bonded the NH2/NH groups through acid−base interaction, reducing the loss of active sites.

from the sulfonic acid groups through acid−base combination, which disabled their function in CO2 capturing. An extensive cyclic adsorption/desorption performance has been investigated on the quartz fixed bed. As demonstrated in Figure 7, PEI/SBA-15 showed a consecutive drop of adsorption

■

ASSOCIATED CONTENT

S Supporting Information *

IR spectra of pyridine adsorbed on 7SSBA-15 after desorption at different temperatures (Figure S1), pore size distribution of SBA-15 and αSSBA-15 before and after PEI loading (Figure S2), TG−DSC profiles of pure PEI (Figure S3), SBA-15 (Figure S4), and 5SSBA-15 (Figure S5), and TGA profiles of CO2 adsorption on PEI/SBA-15 and PEI/αSSBA-15 (Figure S6). This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*Telephone/Fax: +86-571-87953088. E-mail: [email protected]. cn.

Figure 7. Adsorption/desorption cyclic performance of PEI/SBA-15 and PEI/5SSBA-15.

Notes

The authors declare no competing financial interest.

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uptake and 38% of its original amount was lost after 20 cycles, which might be associated with the loss of active sites caused by PEI evaporation,13,16 amine degradation caused by urea formation,37,38 or gradual thermal conformational alteration of the PEI during repeated tests.24 However, the retention uptake in the 20th cycle for PEI/SSBA-15 was achieved high to 74%. Although it declined gradually in the initial 5 cycles, the adsorption amount tended toward stability in the subsequent adsorption cycles. It could be inferred that a small fraction of loaded amine in PEI/5SSBA-15 might leach from the pore system because of its relatively higher volatility than that in PEI/SBA-15, but the remaining part would be stable via the chemical bonding with surface acid sites. Tanthana et al.39,40 reported hydroxyl groups of the additive polyethylene glycol (PEG) could slow the degradation of amine-modified sorbents. We assumed herein that the Si−OH with enhance acidity and sulfonic groups would play the same role, which, however, need further investigation.

ACKNOWLEDGMENTS This work was financially supported by the Changjiang Scholar Incentive Program, Ministry of Education, People’s Republic of China, and the Natural Science Foundation of Zhejiang Province (Z5100116).

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4. CONCLUSION In this work, novel PEI-impregnated sulfonic-acid-functionalized SBA-15 sorbents (denoted as PEI/αSSBA-15) were synthesized for CO2 capture. It was found that αSSBA-15 could preserve a comparable large specific surface area and uniform mesoporous channels to that of SBA-15. However, a further increase in the amount of propanesulfonic acid would lead to the disorder of the mesostructure, thereby lowering its surface area and pore volume greatly. The kinetic CO2 adsorption results showed a dramatic improvement in adsorption capacity and amine efficiency of these amine/sulfonic-functionalized silica composites. For instance, the CO2 adsorption amount on PEI/5SSBA-15 was 68% higher than that of PEI/SBA-15. It was assumed that this enhancement was attributed to the increased CO2 diffusion into deeper polymer amine layers and the more exposed CO2 affinity sites with the improved dispersion of PEI polymer. TG−DSC results further confirmed this assumption because the decomposition temperature of PEI was reduced after the modification by propanesulfonic acid groups. Furthermore, the 5421

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