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Control of Uniform and Interconnected Macroporous Structure in PolyHIPE for Enhanced CO2 Adsorption/Desorption Kinetics Quanyong Wang,†,‡ Yao Liu,†,‡ Jian Chen,*,‡ Zhongjie Du,† and Jianguo Mi*,† †

State Key Laboratory of Organic−Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, China State Key Laboratory of Chemical Engineering, Department of Chemical Engineering, Tsinghua University, Beijing 100084, China

‡

S Supporting Information *

ABSTRACT: The highly uniform and interconnected macroporous polymer materials were prepared within the high internal phase hydrosol-in-oil emulsions (HIPEs). Impregnated with polyethylenimine (PEI), the polyHIPEs were then employed as solid adsorbents for CO2 capture. Thermodynamic and kinetic capture-and-release tests were performed with pure CO2, 10% CO2/N2, and moist CO2, respectively. It has shown that the polyHIPE with suitable surface area and PEI impregnation exhibits high CO2 adsorption capacity, remarkable CO2/N2 selectivity, excellent adsorption/desorption kinetics, enhanced efficiency in the presence of water, and admirable stability in capture and release cycles. The results demonstrate the superior comprehensive performance of the present PEI-impregnated polyHIPE for CO2 capture from the postcombustion flue gas. the key components in postcombustion flue gas, can compete with CO2 for adsorption sites and thus decrease the adsorption capacity. Moreover, with small pore size, the CO2 adsorption kinetics of microporous/mesoporous materials is hard to meet industrial requirements. An ideal solid sorbent for industrial application is expected to not only have large capacity and selectivity for CO2 capture but also have high rates of CO2 adsorption/desorption and to be durable, low cost, and easy for manufacture.23 In order to optimize the thermodynamic and kinetic performances of adsorbents, macroporous polymeric materials with high porosity, tunable interconnection of pores, and desired mechanical properties, have been considered as competitive candidates.24 Several synthesis methods, including direct templating,25 block copolymer self-assembly,26 and interfacial polymerization,27 have been employed to obtain special structure and framework of macroporous polymers. Among various strategies, high internal phase emulsion (HIPE) polymerization has been considered as one of the most efficient methods for preparing macroporous materials with well-defined porosity and low

1. INTRODUCTION Reduction of CO2 emission has already been an important issue in industries and hence there is ongoing search for improving the current technologies or developing new approaches for CO2 separation and capture.1−4 Different separation methodologies have been implemented for CO2 capture, such as solution absorption, solid adsorption, membrane diffusion, and cryogenic distillation.5,6 Of these technologies, solid adsorption has attracted great attention due to the relatively low energy consumption and low equipment cost. In this regard, porous materials, such as metal−organic frameworks,7−10 porous organic molecules,11−13 and microporous/mesoporous organic polymers,14−17 have been extensively investigated and validated with high selectivity for CO2 in the presence of other gases due to unsaturated metal centers or organic amines that chemically interact with CO2.18 The use of porous materials is a potential approach for CO2 separation. A multitude of previous investigations paid more attention to developing microporous/mesoporous materials with interconnected network of channels and extra-large surface to improve their CO2 adsorption capacity.19−22 Although extra-large surface area facilitates high gravimetric uptake of gases at low temperature and/or high pressure, it is not necessarily conducive to efficient separation for postcombustion CO2 capture.18 Meanwhile, the presence of water, which is one of © 2016 American Chemical Society

Received: Revised: Accepted: Published: 7879

February 3, 2016 June 18, 2016 June 20, 2016 June 20, 2016 DOI: 10.1021/acs.est.6b00579 Environ. Sci. Technol. 2016, 50, 7879−7888

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Environmental Science & Technology density.28 HIPEs are concentrated emulsion systems possessing a volume of internal phase more than 74%, and the spherical droplets of dispersed phase are deformed into polyhedra and separated by thin films of continuous phase. The polymerization of monomers in the continuous phase of a HIPE followed by the removal of the emulsion droplets in the internal phase yields a highly interconnected mesoporous/macroporous polymeric network, which is known as polyHIPE.29,30 With good moisture resistance, low cost, and simple preparation process, polyHIPEs could be a promising material for postcombustion CO2 capture.31 There are often highly significant differences between the structure of the original HIPEs and the structure of the resulting polyHIPEs. Typically, polyHIPEs were synthesized using conventional free radical polymerization32 in surfactantstabilized water-in-oil HIPEs. In recent years, other polymerization methods (e.g., atom transfer radical polymerization,33,34 reversible addition−fragmentation chain transfer polymerization35−37) and other HIPE stabilization techniques (e.g., particle-based stabilization in Pickering HIPEs24,38) were also investigated. The success achieved in the development of new polyHIPE synthesis, chemistries, wall materials, and porous structures have clearly established their structure−effect relationships, which are helpful for generating novel porous materials with predesigned porous structures and unique properties. In recent years, several polyHIPE materials have been synthesized and tested for CO2 adsorption.30,39−41 These tests have shown clearly that the specific interconnected macropores in polyHIPEs render them highly permeable, which should enhance mass transport and thus improve the CO2 capture performance. Synthesized from the traditional water-in-oil route, however, these materials did not exhibit remarkable adsorption capacity and kinetics. In this work, we present a new simple way to synthesize the polyHIPE adsorbent for CO2 capture. We focused on the precise control over pore size to optimize the thermodynamics and kinetics in CO2 adsorption/ desorption cycles. Inspired by the successful fabrication of inorganic oxides with narrow macropore size distribution via sol−gel reactions in the external phase of concentrated emulsions,42,43 we primarily considered adding hydrosol into the external phase to formulate hydrosol-in-oil emulsion, and employed double surfactants to obtain stable monodisperse system. The highly concentrated emulsion can be regarded as dual systems: micrometer-sized hydrosol droplets surrounded by nanometer-scale aggregates. The material possesses uniform pore size, three-dimensional interconnected pore channel, and short pore length. These unique features are of particular importance to improve the CO2 adsorption/desorption kinetics. The uniform and interconnected polyHIPE was then implanted with polyethylenimine (PEI) to substantially improve the CO2 adsorption capacity. Consequently, the contribution of PEI loading content on CO2 adsorption was characterized at different temperature to evaluate the adsorption thermodynamics. Finally, the adsorption/desorption kinetics and cycle stability were measured to evaluate the comprehensive performance of the new adsorbent for potential application.

zene and styrene. Before the experiment, divinylbenzene and styrene were purified by passing through a column of basic alumina to remove the inhibitor. 2.2. Preparation of Adsorbents. Ammonia was added into 25 mL 0.2 mol/L Fe(NO3)3 aqueous solution under stirring, and the pH was adjusted to 8.0 using NaOH aqueous solution. Deionized water was added to dilute the aqueous solution to 50 mL. The diluted aqueous solution was then transferred to autoclave under 150 °C for 2 h. After cooling to room temperature, the ferric hydroxide gel was obtained. The hydrosol-in-oil emulsions were formed through the slow addition of ferric hydroxide gel to the oil phase, consisting of styrene, divinylbenzene, Span80, Poloxamer 188, 2,2′-Azobis(2methylpropionitrile), and toluene, under constant stirring. The emulsions were polymerized at 65 °C for 24 h. The polyHIPEs from hydrosol-in-oil emulsions were fabricated after drying. After cut into pieces of desired dimensions, the polymers from hydrosol-in-oil emulsion were soaked in hydrochloric acid at 60 °C to remove Fe(OH)3, and washed with distilled water and ethanol for 24 h. The polyHIPE sorbents with PEI (MS = 600) loading were prepared by the wet impregnation. In typical preparation, a given amount of PEI was dissolved in methanol while stirring for about 30 min at room temperature, and then the solution was added to 0.2 g polyHIPE solid. After standing for 24 h, the methanol solvent was later removed by evaporation at 70 °C for 24 h under vacuum. The weight percentage of PEI can be calculated with η × m/(0.2 + η × m) × 100%, where η is the mass fraction of PEI in the PEI/methanol solution, and m is the mass of PEI/methanol solution added into the polyHIPE solid. 2.3. Characterization. The morphology and structure of macropores in the samples were characterized using scanning electron microscopy (SEM, TM3000, Hitachi, Japan). Transmission electron microscopy (TEM) analyses were conducted using a Hitachi H-7650 transmission electron microscope (Hitachi, Japan) operating at 75 kV. Their pore sizes were measured through SEM using ImageJ as a software tool providing the number distribution of various pores. In each measurement more than 100 pores were taken into account. For the in situ FT-IR spectroscopy, FT-IR spectroscopy (Tensor27, BRUKER, Gamerny) was used to measure IR spectrum. Nitrogen adsorption/desorption isotherms were determined on a Micromeritics QUDRASORB SI sorptometer (Quantachrome, America) at liquid nitrogen temperature. The specific surface areas of these samples were calculated by the Brunauer−Emmett−Teller (BET) method, and the pore volumes were evaluated from the desorption branch of the isotherm based on the Barrett−Joyner−Halenda (BJH) model. The amounts of PEI introduced to the samples were measured using thermogravimetric analysis (TGA, STA409PC, Netzsch, Germany), by heating the samples at 5 °C/min from room temperature to 600 °C under N2 flow (40 mL/min), and N elemental analysis (VARIO EL III, Elementar, Germany). All the characterization data except pore size distributions are given in Supporting Information. 2.4. CO2 Capture Test. Pure CO2 adsorption at different temperatures and adsorption isotherms were measured using a magnetic suspension balance (MSB, ISOSORP-Gas, Rubotherm, Germany). A consecutive CO2 adsorption and desorption cycle was performed using MSB: adsorption at 75 °C for 1 h and desorption at 110 °C under vacuum for 1 h. The moist CO2 adsorption performances were measured using the constant capacity method, and the experimental apparatus has

2. EXPERIMENT 2.1. Reagents. All the reagents were purchased from commercial sources (J&K Scientific, or Beijing Chemical Co.), and were used without further purification except divinylben7880

DOI: 10.1021/acs.est.6b00579 Environ. Sci. Technol. 2016, 50, 7879−7888

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Environmental Science & Technology

Figure 1. Synthetic procedure for PEI-impregnated polyHIPE.

been illustrated in the Supporting Information. The adsorption kinetics measurements were carried out with a thermogravimetric analyzer (STA409PC, Netzsch, Germany) with 10% CO2/N2 flue gas mixture.

3. RESULTS AND DISCUSSION 3.1. Adsorption Thermodynamics. Synthetic procedure for PEI-impregnated polyHIPE can be seen in Figure 1. Due to the insolubility of ferric hydroxide gel in oil phase, ferric hydroxide gel can be well dispersed in oil phase to form stable emulsion using the suitable emulsifier. Since the surface tension of ferric hydroxide gel is smaller than that of water, it is conductive to form smaller internal phase droplets in hydrosolin-oil emulsion. As a consequence, the coalescence of the internal phase droplets can be effectively delayed to formulate the uniform and interconnected macroporous polyHIPE. In order to screen an optimal structure, different polyHIPEs were impregnated with 70 wt % PEI for CO2 adsorption. The dry CO2 adsorption capacities of the original polyHIPEs and PEI-impregnated polyHIPEs were measured using a magnetic suspension balance (MSB). As can be seen from Figure 2, even though the CO2 uptake slightly increases as the surface area increases, the original polyHIPEs display very small adsorption capacities. After impregnation of 70% PEI, however, the samples show extremely enhanced adsorption uptakes. The results reveal that the CO2 adsorption is dependent on the chemical reaction between amine and CO2. When the specific surface area of polyHIPE is 154.2 m2/g, the sample after impregnation of PEI can adsorb 5.6 mmol/g CO2, showing a maximum adsorption capacity. The result could be attributed to the unique pore structure. Figure 3 displays the pore size distributions of polyHIPEs. It is clear that the polyHIPE with the surface area of 154.2 m2/g has single peak, showing a highly uniform macropore distribution. Such structure is beneficial for the uniform dispersion of PEI, thereby leads to effective contact with CO2 molecules. In contrast, if the surface area is too low or too large, the pore distribution displays two or more peaks, which means that the polyHIPEs have more large pores. PEI in large pores is easier to accumulate in the pores (as shown in Supporting Information (SI) Figure S2), leading to nonuniform dispersion. Hence the pore distribution with two or more peaks

Figure 2. CO2 adsorption uptakes of the 70% PEI-impregnated polyHIPE adsorbents with different specific surface areas. The measurements were performed using MSB with pure CO2 at 75 °C.

is not conductive to the uniform dispersion of PEI. The polyHIPE with suitable surface area corresponds to appropriate emulsion condition, which is crucial to formulate uniform pore size. In view of the highest CO2 adsorbed amount, such PEIimpregnated polyHIPE was selected to further optimize preparation of the PEI-modified adsorbent. Figure 4 shows the effect of temperature on CO2 adsorption uptake. The adsorption capacities of sorbents with different PEI impregnations display similar variation tendency. As the implanted PEI increases, the CO2 uptake increases, whereas the magnitude declines. As the temperature increases, the CO2 uptake at a given PEI content first increases, and then decreases. The highest CO2 uptake emerges at 75 °C, corresponding to the optimum temperature reported earlier.44,45 In a supported amine sorbent, the CO2 adsorption capacity is coherently related to the amount of loaded PEI. Here the effect of PEI loading on the CO2 loading was evaluated with dry and moist (18.38% H2O) CO2 streams, respectively. The results are shown in Figure 5. As can be seen from Figure 5a, the CO2 adsorption uptake increases with increasing PEI loading 7881

DOI: 10.1021/acs.est.6b00579 Environ. Sci. Technol. 2016, 50, 7879−7888

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Figure 3. Pore size distributions of the original polyHIPEs with different specific surface areas.

observed with increasing PEI content, as shown in Figure 5b. In the case of dry CO2 stream, the PEI utilization efficiency reaches the maximum of 9.2 mmol CO2/g PEI at 50% PEI impregnation. Under the condition of humid CO2, the maximum is 11.1 mmol CO2/g PEI at 45% PEI impregnation. In relatively low PEI loading, some PEI molecules fill both micropore and mesopore, and some amine groups in the PEI molecules have been consumed by covering the polyHIPE surface. With increasing PEI content, relatively more PEI molecules are exposed on the macropore surface, and the proportion of effective PEI molecules and amine groups increases; thus the PEI utilization efficiency rises. Once the PEI content is larger than 50%, however, the PEI bulk becomes thicker with increasing PEI content; hence CO2 molecules are difficult to diffuse into the inner PEI bulk. Moreover, the excess PEI molecules are easy to agglomerate. As a result, the PEI utilization efficiency decreases. Table 1 summarizes the performances of a number of PEIimpregnated adsorbents. It is shown that the CO2 adsorption uptake of the current polyHIPE adsorbent is higher than most mesoporous adsorbents. For the mesoporous materials, the pore channel can be easily blocked after impregnation of PEI, leading to enhanced CO2 diffusion resistance and relative low PEI adsorption equilibria. In contrast, the uniform and interconnected macroporous structure of polyHIPE can improve the dispersion of PEI and gas diffusion, resulting in excellent CO2 adsorption. Isosteric heat of adsorption (QST) is an important parameter to evaluate the thermodynamic and kinetic properties of

Figure 4. Effect of temperature on CO2 adsorption capacities of polyHIPE adsorbents with different PEI impregnations.

amount, and achieve the maximum adsorption uptakes of 5.6 and 6.4 mmol/g for the two streams. It is obvious that the adsorption capacity of the PEI-impregnated polyHIPE increases when water presents in the CO2 stream. Theoretically, two mole amine species can react with one mole CO2 under dry condition, whereas the reaction stoichiometry changes to 1:1 in the presence of water. The results confirm that the formation of bicarbonate in the moisture environment contributes to the enhancement of adsorption. For PEI efficiency (mmol CO2/g PEI), an initial increase, followed by a decrease, can be 7882

DOI: 10.1021/acs.est.6b00579 Environ. Sci. Technol. 2016, 50, 7879−7888

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Environmental Science & Technology

Figure 5. Effect of PEI impregnation on CO2 adsorption and PEI utilization efficiency.

from adsorption isotherms (Figure 6a) using the Clausius− Clapeyron equation

Table 1. Comparison of CO2 Uptakes by Different Adsorbents

⎛ ∂(lnP) ⎞ Q ST ⎜ ⎟ =− R ⎝ ∂(1/T ) ⎠q

CO2 adsorption uptake

porous materiala

PEI content (wt %)

a

methodb

48

65

TGA

HMS49

60

TGA

MCF50

50

fix bed

MCF51

50

MSB

mesoporous capsules52 U-MSF53

83

TGA

80

TGA

nonpolar resin47

50

TGA

PE-MCM-4154

70

TGA

SSF45

56.5

TGA

nano silica55

60

TGA

HSM

poly(DVB) polyHIPE/MPSO40 poly(VBC/DVB)EDA39 polyHIPE

adsorption condition

mmol-CO2/ g-adsorbent

100 vol % CO2; 75 °C 100 vol % CO2; 75 °C 15.1 vol % CO2; 75 °C 100 vol % CO2; 75 °C 20 vol % CO2; 75 °C 100 vol % CO2; 75 °C 100 vol % CO2; 25 °C 100 vol % CO2; 75 °C 100 vol % CO2; 75 °C 100 vol % CO2; 105 °C

4.77

PGU fix bed 70

MSB

where P is CO2 partial pressure, T is the adsorption temperature, and R is the universal gas constant. The values of QST for PEI-impregnated adsorbents at 40 °C, 50 °C, 60 °C, 70 °C, 75 °C, with constant qa of 3−4 mmol/g (in 0.2 mmol/g increments) were then determined from the slopes of the straight lines after plotting ln P against 1/T, as shown in Figure 6b. Results are given in Table 2. The values of QST are negative in the range of 313 and 348 K implying that the entropy is reduced in the adsorption process. This is consistent with the exothermic character of the adsorption process. The average isosteric heat of adsorption is 62.90 kJ/mol. Generally, isosteric heats for chemisorption lie in the range of 60−90 kJ/mol or even higher (strong adsorption) and 25−50 kJ/mol for physisorption (weak adsorption). The relative high QST is related to the favorable interactions between CO2 molecules and the Lewis basic amine. Adsorption selectivity is also important to evaluate a solid adsorbent in practical application. In order to test the CO2/N2 selectivity of the PEI-implanted polyHIPE, CO2 and N2 adsorption isotherms were measured and shown in Figure 7. It is obvious that there is very small adsorption amount of N2 at 1.0 bar and 75 °C. Here the CO2/N2 selectivity was evaluated under flue gas condition with single-gas isotherms, and the ideal adsorbed solution theory model of Myers and Prausnitz along with the pure component isotherm fits were applied to determine the molar loadings in the mixture for specified partial pressures in the bulk gas phase.46 The following formula was used to calculate CO2/N2 selectivity

4.18 3.45 4.11 4.91 5.80 4.11 4.61 4.28 4.23 3.46

4 vol % CO2; 25 °C 100 vol % CO2; 75 °C

(1)

2.18 5.60

a

HSM: hierarchical silica monolith. HMS: hexagonal mesoporous silica. MCF: mesocellular silica foam. U-MSF: ultra large mesopores silica foam. PE-MCM-41: pore-expanded MCM-41. SSF: millimetersized spherical silica foam. Poly(DVB)polyHIPE/M-PSO: poly(divinylbenzene)polyHIPE filled with maleimide-terminated poly(arylene ether sulfone) oligomers. Poly(VBC/DVB)-EDA: Poly(Vinylbenzyl chloride/Divinylbenzene)polyHIPE modified with ethylenediamine. b MSB: magnetic suspension balance. TGA: thermogravimetric analysis. PGU: Pilot gasification unit.

s=

q1 p1 / q2 p2

(2)

where p1 and p2 are the partial pressures of CO2 and N2, q1 is the adsorbed CO2 amount under the pressure p1, and q2 is the adsorbed N2 amount under the pressure p2. q1 and q2 can be taken from the adsorption isotherms. The CO2/N2 selectivity reaches 359 at CO2 partial pressure of 0.1 bar. High selectivity

adsorbents. QST is commonly defined as the enthalpy change on adsorption. The values of QST for the adsorption of CO2 onto PEI-impregnated polyHIPE adsorbents were calculated 7883

DOI: 10.1021/acs.est.6b00579 Environ. Sci. Technol. 2016, 50, 7879−7888

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Environmental Science & Technology

Figure 6. (a) CO2 adsorption isotherms at different temperatures and (b) regression lines of ln P versus 1/T under various qa.

are shown in Figure 8. As can be seen, the CO2 adsorption process consists of two stages. A sharp linear weight gain occurs

Table 2. Isosteric Heats of CO2 Adsorption with PolyHIPE at 70% PEI Loading qa (mmol/g)

−QST (kJ/mol)

r2

3.0 3.2 3.4 3.6 3.8 4.0 average

52.13 55.53 60.78 66.04 69.64 73.26 62.90

0.9973 0.9966 0.9965 0.9953 0.9972 0.9999

Figure 8. Complete adsorption and desorption curves of the PolyHIPE adsorbents with different PEI impregnations.

in the first stage, which is completed within 10 min, followed by a second, much slower adsorption process. After 1 h adsorption, the polyHIPEs with 40% and 50% PEI loadings achieve nearly adsorption equilibrium; however the adsorption curves of polyHIPEs with 60% and 70% PEI loadings still have a small rising trend. The reason is that, at high PEI loading, CO2 diffusion length in PEI bulk and diffusion resistance increase, resulting in slower adsorption rate and longer time to reach equilibrium. The polyHIPE with 70% PEI loading reaches a high CO2 adsorption capacity of 4.5 mmol/g under 10% CO2/N2 at 75 °C. Meanwhile, it shows fast desorption at 110 °C. Figure 9 presents the detailed adsorption and desorption kinetics for different polyHIPEs. A recent study has shown that the overloading of amines on adsorbent is conductive to the equilibrium capacity but sacrifices adsorption kinetics in a dynamic process.47 As can be seen from Figure 7a, as the implanted PEI increases, the adsorption capacity keeps

Figure 7. Adsorption isotherms of CO2 and N2 for the polyHIPE adsorbent with 70% PEI impregnation at 75 °C.

indicates huge potential of polyHIPE for CO2 capture from flue gas. 3.2. Adsorption/Desorption Kinetics. For CO2 capture, an adsorbent should possess not only a high adsorption capacity but also fast adsorption to be energy-efficient. Here a thermogravimetric analyzer was used to evaluate the kinetic performance of the PEI-impregnated polyHIPEs. Complete adsorption and desorption curves with different PEI loadings 7884

DOI: 10.1021/acs.est.6b00579 Environ. Sci. Technol. 2016, 50, 7879−7888

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Environmental Science & Technology

Figure 9. Adsorption (a) and desorption (b) kinetics of the PolyHIPE adsorbents with different PEI impregnations. Adsorption at 75 °C and desorption at 110 °C.

Figure 10. Effect of temperature on adsorption (a) and desorption (b) kinetics of the PEI-impregnated polyHIPE absorbent.

Table 3. Comparison of Adsorption and Desorption Kinetics of Different Adsorbents adsorbenta

PEI content (wt %)

methodb

HMS49 PAF-556

60 40

TGA TGA

HSM48 MAG57

65 25

TGA TPD

MWCNTs23

40

Fix bed

G-silica44

60

TGA

SSF45 polyHIPE

60 70

TGA TGA

adsorption condition

CO2 uptake (mmol·g−1) ( c)

pure CO2, 75 °C 15% CO2/N2, 75 °C pure CO2, 75 °C 5% CO2/He, 75 °C 10% CO2/N2, 70 °C 15% CO2/N2, 75 °C pure CO2, 75 °C 10% CO2/N2, 75 °C

4.16 (1.7 h) 2.61 (0.5 h)

time taken to reach 90% capacity (min)

desorption condition

desorption time (min)

10 11

N2, 75 °C N2, 70 °C

100 10

4.77 (1 h) 6.11 (3 h)

5

N2, 75 °C He,150 °C

130 15

2.56 (1 h)

21

N2,130 °C

25

3.89 (2 h)

20

N2,135 °C

25

4.28 (2.5 h) 4.5 (1 h)

35.4 6.5

N2, 100 °C N2, 110 °C

100 9.5

a

HMS: hexagonal mesoporous silica. MCF: siliceous mesocellular foam. PAF-5: porous aromatic framework. HSM: hierarchical silica monolith. MAG: hydrated layered silicate. MWCNTs: multiwalled carbon nanotubes. KIT-6: mesoporous molecular sieves. G-silica: graphene-based mesoporous silica. bTGA: thermogravimetric analyzer. TPD: temperature-programmed desorption. cAdsorption time.

increasing, but the adsorption rate is almost invariant in the first stage, which benefits from the uniform and interconnected

macroporous structure. The resulting time consumptions to reach 90% uptake of the polyHIPEs with 70%, 60%, 50%, and 7885

DOI: 10.1021/acs.est.6b00579 Environ. Sci. Technol. 2016, 50, 7879−7888

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nated with PEI for CO2 capture. Thermodynamic and kinetical tests reveal that the uniform and interconnected macroporous structure promotes the dispersion of PEI and CO2 diffusion, and the adsorbent has exhibited excellent adsorption/ desorption performances. The effects of temperature and PEI loading on CO2 adsorption capacity and adsorption/desorption kinetics have been clearly elucidated. At 75 °C, which is close to the temperature of the postcombustion flus gas, the adsorbent shows optimal thermodynamic and kinetic performances. The CO2 adsorption uptakes reach 5.6 and 4.5 mmol/g with pure CO2 and 10% CO2/N2, respectively. Under moisture environment, the CO2 adsorption amount can be further improved from 5.6 to 6.5 mmol/g. In other words, moisture has a growthpromoting effect on the adsorption separation of CO2 from simulated flue gas. At the same temperature, 90% CO2 adsorption amount from 10% CO2/N2 stream can be realized within 6.5 min. Meanwhile, the desorption can be finished in 9.5 min at 110 °C. In addition, the adsorbent has shown good stability after 50 adsorption/desorption cycles. As a result, the cost-efficient and easily prepared PEI-impregnated polyHIPE adsorbent is a very competitive and promising candidate for CO2 capture from postcombustion flue gas.

40% PEI impregnations are 6.5, 6.0, 5.0, and 4.5 min, respectively. Figure 7b reveals that these adsorbents can be quickly desorbed. The time to perform completely desorption are 9.5, 8.5, 7.5, and 7.5 min, respectively. The effect of temperature on the adsorption/desorption kinetics is illustrated in Figure 10. As the temperature goes up, the adsorption rate increases in the first stage. When the temperature reaches 80 °C, however, the adsorption uptake is smaller than it at 75 °C. High temperature can accelerate CO2 diffusion in the channels and PEI bulk. Once efficient contact between CO2 and adsorption sites has been established, the adsorption process is dictated by thermodynamics; above 75 °C the equilibrium shifts to the reverse direction and desorption is favored. Thus, the adsorption capacity declines at 80 °C. Figure 8b gives the effect of temperature on desorption kinetics. It is clear that high temperature can promote CO2 desorption. The total regeneration time consumptions are 9.5, 14.0, and 27.0 min at 110, 100, and 95 °C, respectively. Table 3 summarizes the adsorption/desorption kinetics date for the reported and current adsorbent. Compared with most adsorbents with mesoporous structure, the new polyHIPE adsorbent exhibits relatively higher adsorption capacity and faster adsorption and desorption rate. 3.3. Reversible Cycling Test. From the economic point of view, a stable CO2 adsorption/desorption capacity on a longterm basis and a rapid-cycle-time process is very important for industrial CO2 capture. To evaluate the possibility of the new adsorbent for CO2 capture from flue gas, the consecutive CO2 adsorption and desorption was conducted 50 times at 75 °C with pure CO2 gas. In Figure 11, the CO2 adsorption uptake

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.est.6b00579. Additional information as noted in the text (PDF)

■

AUTHOR INFORMATION

Corresponding Authors

*(J.C.) E-mail: [email protected]. *(J.M.) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■ ■

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Nos. 51134017 and 51373019). REFERENCES

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Figure 11. 50 adsorption/desorption circles for the PolyHIPE adsorbent with 70% PEI impregnation under pure CO2.

shows a slight reduction at the second adsorption cycle because of the evaporation of small PEI molecules. After 50 cycles of CO2 adsorption/desorption, the CO2 adsorption capacity has not been significantly reduced, with a reduction of about 6.5% compared with the first cycle. The adsorbent has achieved high CO2 adsorption capacity and again performed satisfactorily during 50 consecutive adsorption/desorption test cycles, showing no deterioration.

4. CONCLUSION In summary, a polyHIPE adsorbent with uniform and interconnected macrostructure was synthesized and impreg7886

DOI: 10.1021/acs.est.6b00579 Environ. Sci. Technol. 2016, 50, 7879−7888

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DOI: 10.1021/acs.est.6b00579 Environ. Sci. Technol. 2016, 50, 7879−7888