Carbon Dioxide Capture Using Poly(ethylenimine)-Impregnated Poly

May 13, 2014 - The PEI-impregnated PMMA-supported sorbent exhibited CO2 adsorption capacity up to 4.26 mmol/g with PMMA-55 at 75 °C in pure CO2 gas ...
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Carbon Dioxide Capture Using Poly(ethylenimine)-Impregnated Poly(methyl methacrylate)-Supported Sorbents Hyunchul Jung, Dong Hyun Jo, Chang Hun Lee, Wonkeun Chung, Dongkun Shin, and Sung Hyun Kim* Department of Chemical and Biological Engineering, Korea University, 1 Anam-Dong, Seongbuk-Gu, Seoul 136-713, Korea ABSTRACT: Solid amine sorbents having adequate particle size with high CO2 adsorption capacity were successfully prepared using poly(ethylenimine) (PEI) as the amine and mesoporous poly(methyl methacrylate) (PMMA) beads as the support. The PEI-impregnated PMMA-supported sorbent exhibited CO2 adsorption capacity up to 4.26 mmol/g with PMMA-55 at 75 °C in pure CO2 gas flow. The effect of the temperature on the adsorption capacity of PMMA-55 was investigated, and the maximum adsorption capacity was obtained at 50 °C with a CO2 exposure time of 180 min, different from the tendency of most silicasupported sorbents. The effect of surfactant addition on the adsorption performance of PMMA-supported sorbents differed from those of silica-supported sorbents because of the different surface characteristics between PMMA and silica. Adsorption/ desorption cycling was also performed to examine the suitability of the amine sorbent for potential application. generally fabricated by amines, such as diethanolamine,19 tetraethylenepentamine (TEPA),20 and poly(ethylenimine) (PEI).21 Apart from adsorption capacity, the other important parameters for good adsorbent are adsorption/desorption kinetics, heat of adsorption, working capacity, and durability. In this respect, PEI containing primary, secondary, and tertiary amines is the most suitable amine for amine-impregnated sorbents.1 Mesoporous silicas, including MCM-41, MCM-48, SBA-12, SBA-15, SBA-16, and KIT-6, have been used extensively as supports for amine-impregnated sorbents22,23 because of their unique properties, such as narrow pore size distribution, good thermal stability, and various structures that facilitate the control of surface area, pore volume, and pore size.24 According to previous reports, mesoporous silica supports having adequately large surface area, large pore volume, and large pore size are required to achieve high CO2 adsorption capacity.18 Solid sorbents using mesoporous silica possessing mesopores and hollow cores bearing exceptionally large pore volume capsules with PEI have exhibited a capacity up to 5.58 mmol/g in dry CO2 gas at 75 °C.25 In an attempt to use additives to increase the CO2 adsorption capacity in terms of enhancing the amine efficiency of PEI, the surfactant-promoted sorbent exhibits a capacity of 4.95 mmol/g in dry conditions at 75 °C.26 Mesoporous poly(methyl methacrylate) (PMMA) beads are cheaper than mesoporous silicas and show high surface area (∼500 m2/g, by manufacturer), pore volume, and pore size. The moving bed or fluidized bed, which is generally used for adsorption processes, requires a sorbent having adequate particle size (at least micrometer scale) and durability of abrasion. PMMA supports exhibit appropriate size (300−710 μm, by manufacturer) and sufficient robustness, while most mesoporous silicas for CO2 capture are in powder form and

1. INTRODUCTION Carbon dioxide (CO2) is one of the most influential greenhouse gases and is generally considered a prime suspect of global climate change.1 Carbon capture technology is required to reduce the emission of anthropogenic CO2 from power plants or other industrial sources. Post-combustion capture is the most appropriate near-term solution for CO2 capture, because the post-combustion CO2 capture process can be attached to existing plants without having to redesign the entire process, in contrast with other CO2 capture technologies, such as pre-combustion and oxy-combustion.2,3 The wet scrubbing process using aqueous solutions containing watersoluble amines is the state-of-the-art technology among the strategies to capture CO2 in flue gas.4 However, the monoethanolamine (MEA)-based solution process, the most favored amine-based solution process, requires high regeneration energy, consisting of the heat of sorption plus the sensible and latent heat because of the high heat capacity of H2O.5 Furthermore, the MEA-based process suffers critical problems, prohibiting it from wide application, such as corrosive fumes, solvent loss, and toxicity.6 Solid sorbents having low heat capacity have been proposed, instead of amine-based solution, to avoid these problems. Porous materials, including porous carbons, zeolites, polymers, and silicas, are usually used as solid sorbents.7,8 However, their low adsorption capacity and poor selectivity limit commercial applications; bare adsorbent adsorbs through physisorption originated from either ion−quadrupole interaction or van der Waals forces.9 A substantial number of studies have been performed about amine-functionalized sorbents, providing chemisorption using the porous materials, which are the same as bare solid sorbents as supports with amines.10−12 The two main approaches to functionalize the support with amines are chemical grafting10,13−15 and physical impregnation.12,16,17 Although amine-functionalized sorbents fabricated with the latter method can show leaching of amines in the regeneration process, they have attracted considerable attention as promising sorbents because of their high CO2 adsorption capacity and easy regeneration.18 The amine-impregnated sorbents are © 2014 American Chemical Society

Received: December 17, 2013 Revised: May 12, 2014 Published: May 13, 2014 3994

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Figure 1. (a) Schematic image of the experimental setup of TGA Q50 for CO2 adsorption and desorption. (b) Typical TGA profile for CO2 adsorption at 75 °C. (c) TGA profile for CO2 adsorption in isothermal conditions. nium bromide (CTAB, Aldrich, BioXtra, ≥99%), Span 80 (Span80, Sigma), and stearyltrimethylammonium bromide (STAB, Aldrich, 98%). 2.2. Preparation of PMMA-Supported Sorbents. The PMMAsupported sorbent without surfactant was prepared by a typical wet impregnation method. PEI (0.1−0.6 g) was dissolved in methanol (3 g) with sonication for 30 min and physically mixed with the PMMA bead support (Diaion HP-2MG, Aldrich, 0.9−0.4 g) by sonicating for 30 min. Then, the mixture was dried at 60 °C for 12 h to fabricate PMMA-x sorbent, where x denotes the weight percentage of PEI in the sorbent based on input mass. The PMMA-supported sorbent with surfactant was also prepared in the same manner. Mixing was followed by dissolving PEI (0.5−0.55 g) in methanol (3 g) and four surfactants, P123, CTAB, Span80, and STAB, individually, in methanol, with sonication for 30 min. After mixing the PEI solution and the surfactant solution, the PMMA supports were added to the above solution and sonicated for 30 min. Then, the mixture was dried at 60 °C for 12 h to fabricate PMMA-x + y sorbent, where x denotes the weight percentage of PEI in the sorbent and y denotes the weight percentage of the STAB. The weight percentages, x and y, were determined on the basis of input mass. 2.3. Measurement of CO2 Adsorption Capacity Using Thermogravimetric Analysis (TGA). A TA Instruments Q50 thermogravimetric analyzer was used for CO2 adsorption/desorption measurements, as shown schematically in Figure 1a. Pure CO2 (99.99%) was used for adsorption, and ultrahigh-purity N 2 (99.995%) was used as the purging gas for CO2 desorption. In a typical process, about 10−30 mg of the sorbent was placed in a platinum sample pan, pretreated first by heating to 100 °C in a N2 atmosphere at a flow rate of 100 mL/min, and then maintained at this temperature for 70 min to remove H2O and CO2 adsorbed from air until the weight was constant. Then, the sorbent was cooled to 75 °C for 60 min, and the gas was switched simultaneously to CO2 at the same flow rate, as shown in Figure 1b. The CO2 adsorption capacity

require a further forming process, which raises the cost of sorbents and depletes the durability. The forming process may also reduce the CO2 capacity because of the decreased surface area and pore volume during the process. Amine-impregnated solid sorbents using PMMA supports have already been used for CO2 capture in the space shuttle.16 However, few attempts have been made to develop amine-impregnated PMMAsupported sorbents for CO2 capture since then.7,27 The highest adsorption capacity obtained from the previous studies was 3.5 mmol/g with TEPA-impregnated PMMA-supported sorbent in 15% CO2/85% N2 at 40 °C, which is much smaller than that of the silica-supported sorbents.20 Further improvements in the adsorption performance of PMMA-supported sorbents are therefore possible because PMMA-supported sorbents have not been sufficiently investigated for CO2 capture, unlike silicasupported sorbents, as mentioned earlier. In this work, amine-impregnated sorbents were fabricated using adequately sized PMMA beads as the support and PEI as the amine for potential application to the CO2 adsorption process. We investigated the effect of PEI loading and temperature on the adsorption capacity and applied surfactants known to improve the adsorption capacity of silica-based sorbent to determine the influence on PMMA-supported sorbent. Adsorption/desorption cyclic testing was also performed to assess the long-term stability of the fabricated sorbent for potential application.

2. EXPERIMENTAL SECTION 2.1. Reagents. The following chemicals were used: PEI (Mw = 600, Alfa Aesar, 99%), methanol (J.T. Baker, ≥99.8%), poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol) (Pluronic P-123 or P123, Mn ∼ 5800, Aldrich), hexadecyltrimethylammo3995

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In a typical process, a known amount of sorbent was fixed in a reactor column by glass wool. Initially, a pretreatment step was performed using a pure N2 gas stream at 100 °C for 1 h. A gas stream consisting of a mixture of 15% CO2 in N2 at 1 bar was blown at 90 mL/min for adsorption to the column containing the sorbent at 75 °C for 20 min. Two kinds of conditions, including temperature and gas composition were used in the desorption stage. One condition is using pure N2 gas at 100 °C and 1 bar for 30 min as previous studies, and the other condition is using a mixture of 90% CO2 in N2 balance at 140 °C and 1 bar for 30 min. The regeneration stage in high-concentration CO2 at a high temperature is needed in the real process to produce high-purity CO2 product stream.20 The CO2 concentration was determined using a CO2 analyzer (S9610, Alpha Omega Instruments). 2.5. Surface Area and Pore Distribution Analysis. Nitrogen adsorption/desorption isotherm measurements were conducted at 77 K with a Micrometrics ASAP 2020. Prior to the measurements, the samples were degassed in a vacuum at 100 °C for 12 h. The specific surface area was calculated using the Brunauer−Emmett−Teller (BET) method, and the pore size distributions were derived from the adsorption branch using the Barrett−Joyner−Halenda (BJH) model. 2.6. Amine Efficiency Calculation. The amine efficiency was introduced to assess the impregnated PEI on how to adsorb efficiently the CO2 molecules with a certain amount of PEI. The efficiency was calculated by the number of moles of adsorbed CO2 divided by the moles of nitrogen in the impregnated PEI.1 The maximum amine efficiency of PEI was assumed to be 0.385 under dry conditions based on the reported ratio of primary/secondary/tertiary amines in lowmolecular-weight branched PEI (Mw = 800) of 44:33:23.14 Primary and secondary amines adsorb 0.5 mol of CO2/mol of N in dry conditions, according to following equations:

was calculated by the sample weight gain during the adsorption process. To evaluate the effect of the temperature on the CO2 adsorption capacity, the sorbent was pretreated by heating to 100 °C in a N2 atmosphere and the temperature was held for 180 min. The temperature of the sorbent was decreased to the desired point, such as 25, 50, and 75 °C, respectively, equilibrated at that temperature, and exposed to CO2 for 360 min in the adsorption process. For the cyclic adsorption/desorption tests using TGA, the samples were pretreated by heating to 100 °C in a N2 atmosphere and held at this temperature for 70 min. The sorbent was cooled to 75 °C while being exposed to CO2 for 60 min in the adsorption process and then heated to 105 °C in N2 for 90 min to desorb CO2 from the sorbent. The adsorption/desorption process was repeated to assess the cyclic capacity and stability of the sorbent, as shown in Figure 1c. 2.4. Measurement of CO2 Adsorption Capacity Using a Fixed-Bed System. A fixed-bed system was used to obtain the cyclic adsorption capacity in more realistic conditions, as shown in Figure 2.

CO2 + 2RNH 2 → RNH3+ + RNHCOO−

(1)

CO2 + 2R 2NH → R 2NH 2+ + R 2NCOO−

(2)

3. RESULTS AND DISCUSSION 3.1. Effect of PEI Loading. The effects of PEI loading (10−60 wt %, sorbent basis) on CO2 adsorption characteristics at 75 °C in 100% CO2 flow and the pore structure of asfabricated PEI-impregnated PMMA-supported sorbents were investigated. The dependence of the adsorption characteristics, such as adsorption capacity and kinetics, upon the PEI loading are depicted in Figure 3. Table 1 shows the surface area, pore volume, and pore size of these materials. The curves in Figure 3a show that the CO2 adsorption capacity increased with an

Figure 2. Schematic image of the experimental setup of the fixed-bed system for CO2 adsorption and desorption.

Figure 3. (a) Comparison of dynamic pure CO2 adsorption for PMMA-x with various PEI loadings under pure CO2 at 75 °C. (b) CO2 adsorption capacity of PMMA-x with various PEI loadings. 3996

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Table 1. Structural Properties of PMMA-Supported Sorbents with Different PEI Loadings and Silica Support PMMA-00 PMMA-10 PMMA-20 PMMA-30 PMMA-40 PMMA-45 PMMA-50 PMMA-55 PMMA-60 PMMA-54 + 2 PMMA-54 + 2 (72 h) SBA-PLT29

SBET (m2/g)

Vp (cm3/g)

Dp (nm)

670.8 334.0 138.0 80.4 44.7 31.0 13.1 9.1 3.2 8.5 1.8 590

1.26 0.92 0.65 0.48 0.35 0.23 0.17 0.09 0.02 0.07 0.14 1.40

12.0 12.9 18.6 24.3 33.0 32.8 59.7 60.0 66.5 86.1 159.1 13.6

increasing quantity of impregnated PEI in the range of 10−55% but then decreased as the PEI loading exceeded 55 wt %. The curves were similar for the PEI loading in the range of 10−55 wt %, whereas those of PMMA-58 and PMMA-60 required more time to reach the saturated adsorption capacity than the other sorbents. This implies that CO2 diffusion resistance increased with increasing PEI loading because of pore blocking of PMMA by excessive impregnation of PEI.28 The pore structure of PMMA-x in Table 1 shows the decrease in surface area (SBET) and pore volume (Vp) and the increase in pore size (Dp). This implies an increasing effect of pore blocking with increasing PEI loading; PMMA-60 exhibits an extremely small surface area and pore volume. Figure 3b shows a maximum CO2 adsorption capacity of 4.26 mmol/g at 55 wt % PEI loading. The capacity drastically decreased to 1.47 mmol/g as the PEI loading was further increased to 60 wt %. The amine efficiency of the sorbent with PEI loading below 30 wt % was lower than 0.3 because some amine groups in the PEI molecule consumed to cover the PMMA surface. The amine covering effect can be supported by a rapid decrease of the surface area in that PEI content range. In the range of PEI loading between 30 and 55 wt %, the amine efficiency was higher than 0.3, whereas the amine efficiency of the sorbent with PEI loading over 55 wt % drastically decreased. This suggests that excessive impregnation occurred with a decreasing number of adsorption sites of PEI themselves as well as pore blocking. 3.2. Effect of the Temperature. In an effort to determine the optimal adsorption temperature, the effect of the temperature on CO2 adsorption capacity and adsorption rate was investigated. Figure 4 shows the CO2 adsorption isotherm kinetics of the sorbent with PEI loading of 55 wt % at different adsorption temperatures of 25, 50, and 75 °C in pure CO2. The CO2 adsorption capacity decreased with increasing temperature, because the heat of adsorption and entropy change of the reaction are negative.1 Meanwhile, the CO2 adsorption rate apparently increased with increasing temperature, because diffusion is faster at a high temperature. The fitting parameters listed in Table 2 were obtained from the adsorption kinetics profile with different temperatures using the second-order model suggested by Ho according to the following equations,30 because the CO2 adsorption by chemisorption is a second-order reaction, as shown in eqs 1 and 2:28

dqt dt

= k(qe − qt )2

Figure 4. Comparison of dynamic pure CO2 adsorption in isothermal conditions for PMMA-55 at adsorption temperatures of 25, 50, and 75 °C after 600 min of exposure to reach near-equilibrium CO2 capacity.

Table 2. Fitting Parameters Obtained from the SecondOrder Model saturated adsorption capacity, qe (mmol/g) rate constant, k (g mmol−1 min−1) correlation coefficient, R

1 1 t = + t 2 qt q kqe e

25 °C

50 °C

75 °C

4.88 0.0059 0.9993

4.59 0.039 0.9999

4.20 0.084 0.9999

(4)

where k is the rate constant of adsorption (g mmol−1 min−1), qe denotes the adsorption capacity at equilibrium or saturated state (mmol/g), and qt is the adsorption capacity at any time, t (mmol/g). As listed in Table 2, the saturated adsorption capacity qe at the adsorption temperature of 25 °C is quite higher than the experimental maximum adsorption capacity, which indicates that the sorbent requires more time to be saturated. The rate constant k corresponding to the tendency of adsorption rates in Figure 4 increases with the temperature increase. The correlation coefficients R were obtained to assess the suitability of the second-order model. The correlation coefficients at the temperatures of 50 and 75 °C are close enough to 1, but a bit deviated value was obtained at 25 °C. The correspondence with the second-order model indicates that the CO2 adsorption mechanism requires two amine groups to adsorb CO2,28 as proposed in eqs 1 and 2, but a small deviation at 25 °C implies that another type of CO2 adsorption process, such as physisorption, could arise at a low temperature. The optimal adsorption temperature for PMMA-55 can be determined as 75 °C among the three tested temperatures, because fast adsorption at the initial stage is required for practical use in a fluidized-bed system.20 3.3. Comparison of PMMA and Silica Supports. To compare the adsorption characteristics of PMMA and silica supports, we surveyed the literature data of silica-supported sorbents (PMC,31 KIT-6,23 Monolith,17 and SBA-PLT29) with PEI as amine under the same adsorption temperature and impregnation ratio conditions. The adsorption capacity and amine efficiency of the PMMA-supported sorbent were not lower than those of the compared silica-supported sorbents

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maximum capacity at 75 °C.6,15,25,31 The result is contrary to the thermodynamic sorption trend of pure PEI: the sorption capacity of pure PEI decreases with increasing temperature like general adsorption. However, impregnated PEI in the silica support is in an entangled state at a low temperature, so that the kinetic barrier, which forms an inaccessible zone, could be developed in impregnated PEI. The increase in the adsorption temperature increases the dispersion of the PEI chains in the surface of the support material, and more adsorption sites are exposed. Thus, the adsorption capacity could be maximized at an adequate temperature by finding a suitable balance between thermodynamic sorption and kinetic mass transfer.25 Likewise, the balance effect between thermodynamic sorption and kinetic mass transfer exists in the PMMA-supported sorbent. There is, however, a difference in the temperature exhibiting an adsorption maximum. The PMMA-55 shows maximum adsorption capacity at 50 °C with exposure to CO2 flow for 180 min, which is the same exposure time of SBA-PLT.29 For PMMA-supported sorbent, the number of adsorption sites of the impregnated PEI in the pore structure of the support would be higher because CO2 may diffuse easily through the space between the support surface and PEI,32 even at a low temperature because of well-dispersed flexible PEI chains33 on the PMMA surface (−COOCH3 and −CH3). The affinity of PEI to PMMA is weaker than that of PEI to silica because the presence of acidic hydroxyl groups (−OH) on the silica surface strengthen the interaction between PEI and the silica surface,34 which lowers the configuration flexibility of PEI chains.32 3.4. Effect of Surfactants. Previous studies have examined the positive effect of the surfactant on the CO2 adsorption capacities of silica-supported sorbents and the promotion effect of surfactants, such as P123,28 CTAB,6 Span80,26 and STAB;26 Span80 and STAB were added in hierarchical porous silica (HPS)-supported sorbents. However, no adsorption data have been published for the effects of surfactant on PEI-impregnated PMMA-supported sorbents. As shown in Figure 6a, the effect of surfactant impregnation on the CO2 adsorption performance of the PMMA-supported sorbent at a PMMA/PEI/surfactant weight ratio of 45:50:5 was investigated with four surfactants (P123, CTAB, Span80, and STAB). Unlike the previous studies on silica supports, not all of the surfactant-impregnated PMMA-supported sorbents exhibited improved adsorption

reported in the literature. The comparison of these results revealed that the CO2 adsorption performance can be affected by the type of support even at the same PEI loading. The SBAPLT29 sorbent was chosen for a detailed comparison to the PMMA-supported sorbent, because it had the highest adsorption capacity among the silica-supported sorbents under the same conditions, with a PEI loading of 55% at 75 °C in pure CO2 flow.17,23,29,31 Although the PMMA support and the SBA-PLT support exhibited similar surface area, pore volume, and pore size as shown in Table 1, and the PMMAsupported sorbent showed a higher adsorption capacity than the SBA-PLT sorbent. This suggested that the pore structure of the support is not the only factor influencing the adsorption capacity. Figure 5 shows the difference of the temperature effect

Figure 5. Effect of the adsorption temperatures of 25, 50, and 75 °C on CO2 adsorption capacity of PMMA and SBA-15PLT29-supported sorbents with PEI loading of 55 wt % after 180 min of exposure to pure CO2.

on the adsorption capacity of the PMMA-supported and SBAPLT sorbents. Because most silica-supported PEI-impregnated sorbents exhibit a maximum adsorption capacity at a specific temperature of ca. 75 °C, the SBA-PLT sorbent shows a

Figure 6. (a) CO2 adsorption capacity of PMMA-50 and the sorbents consisting of 50 wt % PEI and 5 wt % surfactants. (b) Effect of the ratio of surfactant over PEI on the CO2 adsorption capacity at a fixed total loading of 55 wt %. 3998

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Figure 7. (a) CO2 adsorption capacity of adsorption/desorption cycling of PMMA-55. (b) CO2 adsorption capacity of adsorption/desorption cycling of PMMA-54 + 2.

Figure 8. (a) CO2 adsorption capacity of adsorption/desorption cycling of PMMA-55 (desorption in pure N2). (b) CO2 adsorption capacity of adsorption/desorption cycling of PMMA-55 (desorption in a high concentration of CO2).

STAB. In the result, the capacity was maximized at 4.27 mmol/ g for the sorbent consisting of 54 wt % PEI and 2 wt % STAB. 3.5. Cyclic Adsorption/Desorption Test Using TGA. A cyclic adsorption/desorption investigation of PMMA-supported sorbents is required to evaluate the long-term stability and cyclic capacity of the sorbent for potential applications. Figure 7 presents the cyclic capacities of PMMA-55 and PMMA-54 + 2 over 15 repetitive cycles with adsorption at 75 °C and desorption at 105 °C using TGA. The capacities of surfactant-free and surfactant-impregnated sorbents declined below 90%. The capacity reduction ratio of PMMA-55 was similar to that of surfactant-free HPS,26 but PMMA-54 + 2, a surfactant-impregnated PMMA sorbent, showed a different tendency from that of surfactant-impregnated HPS.26 The cyclic capacity of PMMA-54 + 2 also obviously decreased with an increasing cycle number. This revealed that the addition of surfactant may have no effect on improving the cyclic capacity in PMMA supports. The gradual decrease in CO2 adsorption cyclic capacity was attributed to the following three reasons. First, evaporation of amine can be considered as a culprit; evaporation of PEI gradually lowers the weight of sorbents at

capacity. Impregnation of Span80 and STAB promoted adsorption performance, whereas CTAB and P123 decreased the adsorption capacity. Moreover, the adsorption capacity of the sorbent using Span80 and STAB was lower than that of PMMA-55. This result is an extension of the hypothesis that we presented in reference to Figure 5. Although the surfactants were introduced to reduce the effect of CO2 diffusion limitation,26 no outstanding enhancement effect of the surfactant on mass transfer of CO2 was observed in the PMMA pore structure. The surfactants promoted the CO2 adsorption performance by lowering the diffusion limitation in relatively inflexible PEI chains in silica support, but they were uninfluential to flexible PEI chains in the PMMA support. Figure 6b compares the CO2 adsorption capacities of STABloaded sorbent to that of STAB-free sorbent at a fixed total loading amount of 55 wt %. The improvement decreased with an increasing STAB concentration in the range of 4−1 wt % at 75 °C. Therefore, the greatest improvement occurred at 9.3% from the sorbent consisting of 51 wt % PEI and 4 wt % STAB. We investigated the maximum adsorption capacity at 75 °C by changing the total loading amount and the ratio of PEI and 3999

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Energy & Fuels the end of desorption and cyclic capacity, increasing cycle number.35 Second, PEI migrated to small pores and blocked the pore mouths. It is supported by the average pore size of PMMA-54 + 2 increasing greatly, as shown in Table 1, after the cyclic test of 72 h because of the blocking of small pores by migrated PEI. Finally, the formation of urea may have consumed the amine groups of PEI. Sayari et al.35 reported that amine groups can be transformed into urea in dry conditions and that the provision of humidified gas prevents the disappearance of amine groups. 3.6. Cyclic Adsorption/Desorption Test Using a FixedBed Reactor. Cyclic adsorption/desorption tests of PMMA-55 were performed to measure cyclic capacity and evaluate thermal stability of the sorbent in more realistic conditions of the fixed bed, as shown in Figure 8. Figure 8a shows the cyclic capacity from the desorption condition in pure N2 stream. The maximum cyclic adsorption capacity was 3.71 mmol/g, and the result shows no sign of the decrease in the adsorption capacity for 10 cycles. The cyclic adsorption capacity when the regeneration stage was performed in high-concentration CO2 flow is shown in Figure 8b. The average value was 2.46 mmol/ g, which is good enough for potential industrial use. Although the temperature of 140 °C would be harmful for the sorbents in the desorption stage for the possibility of the degradation of PEI and PMMA, the adsorption performance of PMMA-55 did not show an obvious decreasing trend. Therefore, the number of adsorption/desorption cycles would be increased to evaluate the long-term stability.

ACKNOWLEDGMENTS

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REFERENCES

This work was supported by the Korea CCS R Center (KCRC) grant funded by the Korea government (Ministry of Science, ICT & Future Planning) (no. NRF-2011-0031986).

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4. CONCLUSION PMMA-supported PEI-impregnated sorbents were fabricated via a wet-impregnation method for potential industrial use. The effects of PEI loading, adsorption temperature, and surfactant addition on the adsorption performance of the sorbents were investigated. With various PEI loading at 75 °C under dry conditions, the highest adsorption capacity was obtained as 4.26 mmol/g from PMMA-55. Unlike most silica-supported sorbents exhibiting an adsorption capacity maximum at about 75 °C, the CO2 adsorption capacity of the PMMA-supported sorbent showed an adsorption capacity maximum at 50 °C with exposure to CO2 flow for 180 min and at 25 °C with exposure to CO2 flow for 600 min because of the existing balance effect between thermodynamic sorption and kinetic mass transfer. A comparison among the silica-supported sorbents and the PMMA-supported sorbent revealed the competitive adsorption characteristics of PMMA-supported sorbent. Although the addition of some surfactants with PEI in the PMMA support improved the CO2 adsorption capacity, the improvement was slight; this report is the first attempt to examine the effect of surfactant addition in PMMA supports. The adsorption capacity was maximized at 4.27 mmol/g at 75 °C, with the sorbent consisting of 54 wt % PEI and 2 wt % STAB. The cyclic adsorption capacities were obtained from the cyclic adsorption/ desorption tests of PMMA-55 conducted using TGA and a fixed-bed system.

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The authors declare no competing financial interest. 4000

dx.doi.org/10.1021/ef402485s | Energy Fuels 2014, 28, 3994−4001

Energy & Fuels

Article

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dx.doi.org/10.1021/ef402485s | Energy Fuels 2014, 28, 3994−4001