Amine Functionalization of Microsized and Nanosized Mesoporous

Jun 15, 2016 - Carbonaceous nanomaterials with uniform pore size are potential solid sorbents in various industrial applications, such as gas purifica...
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Amine Functionalization of Microsized and Nanosized Mesoporous Carbons for Carbon Dioxide Capture Song-Hai Chai,† Zhi-Ming Liu,† Kuan Huang,† Shuai Tan,‡ and Sheng Dai*,†,§ †

Department of Chemistry, University of Tennessee, Knoxville, Tennessee 37966, United States School of Chemical & Biomolecular Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, United States § Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States ‡

S Supporting Information *

ABSTRACT: Carbonaceous nanomaterials with uniform pore size are potential solid sorbents in various industrial applications, such as gas purification and water treatment, because of their easily tunable pore diameter and morphology. However, the carbon-based sorbents are greatly limited in CO2 capture, because of their weak interaction with CO2 (physical adsorption in nature). This work reports the amino functionalization of microsized and nanosized mesoporous carbons for CO2 capture. Two strategies, i.e., physical impregnation with branched polyethylenimine (PEI) and chemical grafting of ethylenediamine, are used to functionalize mesoporous carbon microparticles (MCMs) with a particle size of 100−200 μm. The amine-grafted MCMs (NH2-MCMs) show little advantage over PEI-impregnated MCMs (PEI/MCMs) in CO2 adsorption capacities, because of their similar surface functional groups and textural properties. In addition, mesoporous carbon nanospheres (MCNs) with a sphere size of 850−1000 nm are prepared by a silica-assisted self-assembly method for comparison with MCMs. The PEI-impregnated MCNs (PEI/MCNs) have higher CO2 adsorption capacities and amine efficiencies than PEI/MCMs at the same PEI loading, indicating a more efficient utilization of the incorporated PEI in the nanosized carbon spheres. The bestperforming PEI/MCNs adsorbent shows a CO2 capacity of 1.97 mmol-CO2 g−1 at 75 °C, which is more than three times that of PEI/MCMs.

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INTRODUCTION Developing efficient sorbent materials for carbon dioxide (CO2) capture is a promising way to alleviate the impact of greenhouse gases on climate and the environment.1−4 Chemical absorption process using aqueous solution of organic amines (e.g., monoethanolamine) has been widely practiced in industry for post-combustion capture of CO2. However, this process is highly energy-intensive and corrosive; thus, there is an urgent need to develop new solid adsorbents with superior performance and desired economics. Porous carbonaceous materials such as activated carbons and carbon molecular sieves are widely used as solid adsorbents in gas purification and water treatment, because of their wide availability, high surface area, excellent thermal and chemical stability, and tunable porosity and surface functionality.5,6 They have thus been studied as alternatives to organic amine-based liquid sorbents used industrially for post-combustion CO2 capture.7 Relatively weak interaction of CO2 with the carbon surface, resulting from physical adsorption, allows good regenerability for the carbon adsorbents, but causes a low adsorption capacity and selectivity of CO2 over N2 in the CO2 capture from flue gas. To enhance CO2 affinity and, consequently, its adsorption capacity and selectivity, nitrogen-containing functional groups are often © 2016 American Chemical Society

incorporated into the framework of porous carbons by various strategies, e.g., using nitrogen-containing compounds as carbon precursors,8,9 post-synthesis nitrogen doping with gaseous NH3 or strong base NaNH2,10,11 covalently grafting nitrogencontaining groups on the carbon surface,12 and physical impregnation with polyethylenimine (PEI).13,14 The former two methods do not have a fine control on the formed nitrogen groups, majorly producing pyridinic and pyrrolic nitrogen species that have a limited increase in the CO2 affinity and CO2/N2 selectivity, because of their relatively weak basicity. The other two methods, i.e., chemical grafting and physical impregnation of porous carbon with nitrogen-containing groups such as amino groups, allow precise control of the incorporated nitrogen groups at the molecular level, making it possible to investigate the fundamental relationship between the structure and function of the adsorption sites. Under dry conditions, two primary (R−NH2) or secondary (R1−NH−R2) amino groups react with one CO2 molecule to form one Received: Revised: Accepted: Published: 7355

February 29, 2016 May 6, 2016 June 15, 2016 June 15, 2016 DOI: 10.1021/acs.iecr.6b00823 Ind. Eng. Chem. Res. 2016, 55, 7355−7361

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Industrial & Engineering Chemistry Research carbamate and another protonated amino group (e.g., 2R−NH2 + CO2 = R−NHCOO− + R−NH2+), leading to the maximum amine efficiency of 0.5 mol CO2 per mol N.7 The presence of H2O in CO2 flow has been reported to improve the amine efficiency up to 1 mol CO2 per mol N, because H2O can act as a proton acceptor (R−NH2 + H2O + CO2 = R−NHCOO− + H3O+).15 Tertiary amines seem to react with CO2 only under humid conditions, resulting in the formation of bicarbonate anion and protonated amine.7 Generally, those amineimpregnated or amine-grafted solid adsorbents have shown great potential to meet the desired working capture capacity of 3−4 mmol g−1 under simulated flue gas conditions, along with good cyclic stability.4 Our group has developed different templating methods to synthesize mesoporous carbon microparticles (MCMs) and mesoporous carbon nanospheres (MCNs). The MCMs having a particle size of 100−200 μm are prepared by self-assembly of phenolic resins (acting as a carbon precursor) with triblock copolymer micelles (acting as a template) of poly(ethylene oxide) (PEO) and poly(propylene oxide) (PPO) in acidic aqueous solution, followed by a simultaneous template removal and carbonization at elevated temperatures to generate ordered mesoporosity.16,17 The MCNs with a sphere diameter of 180− 850 nm are synthesized via a silica-assisted self-assembly route, where uniform colloidal spheres consisting of phenolic resin (carbon precursor), silica oligomers, and hexadecyl trimethylammonium chloride (CTAC) are formed by base-catalyzed polymerization and condensation of resorcinol, formaldehyde, and tetraethylorthosilicate (TEOS) in an ethanol/ammonia solution.18 The final MCNs can be obtained by carbonization of the colloidal spheres at high temperatures and subsequent etching of silica with aqueous hydrofluoric acid. Silica plays a key role in the formation of spherical morphology, while CTAC as a template is removed during the carbonization process and responsible for the mesopore generation. The present work focuses on the functionalization of the templated mesoporous carbons with amino groups for CO2 capture, with the objective of studying the effects of functionalization methods and carbon supports (MCMs and MCNs) on the CO2 adsorption performance. The amino groups are incorporated into the carbon supports through two pathways, as shown in Scheme 1: (1) physical impregnation with branched PEI and (2) surface grafting of ethylenediamine via a covalent bond. Physical impregnation with branched PEI has been widely studied and proved to be an efficient method to functionalize mesoporous SiO2 in order to enhance the CO2 adsorption capacity and selectivity.13,14,19,20 Therefore, we selected the impregnation method for amino functionalization of the mesoporous carbons in the present work. Ethylenediamine was selected for the chemical grafting via an amide route (−CONH−CH2NH2), because (a) it is a well-established amino functionalization method for carbon materials such as carbon nanotubes21 and (b) ethylenediamine, which is similar to PEI, can provide a high content of amino groups that are active CO2-adsorption sites.

Scheme 1. Amino Functionalization of Mesoporous Carbons by Physical Impregnation of PEI and Chemical Grafting of Ethylenediamine

heated to reflux under stirring, followed by the addition of aqueous formaldehyde solution (37 wt %, 26.0 g). The reaction mixture was stirred for 2 h under refluxing and the formed yellow polymer particles (F127−phenolic resin composites) were then separated from the solution by filtration. The polymer particles were dried in an oven at 120 °C for 3 h and subsequently carbonized in a N2 flow at 850 °C for 2 h with a heating rate of 5 °C min−1. The MCNs were synthesized through a silica-assisted method with a minor modification.18 In a typical synthesis, resorcinol (5.0 g) was dissolved in a mixture of hexadecyl trimethylammonium chloride solution (25 wt % in H2O, 26 g), deionized H2O (475 mL), ethanol (200 mL), and ammonium hydroxide solution (28−30 wt % NH3 in H2O, 2.5 mL), followed by the addition of tetraethylorthosilicate (TEOS, 9.0 mL) and formaldehyde (37 wt % in H2O, 7 mL). After stirring the solution at 30 °C for 72 h, a solid precipitate was separated from the solution by centrifugation and then dried overnight at 80 °C. A carbon−silica nanocomposite was then obtained by carbonization of the precipitate in a N2 flow at 200, 350, 500, 600 °C for 2 h and 800 °C for 5 h, respectively. The heating rate was 1 °C min−1 below 600 and 5 °C min−1 between 600−800 °C. A subsequent etching of silica in the nanocomposite by aqueous hydrofluoric acid resulted in the final MCNs. PEI-impregnated mesoporous carbon samples (PEI/MCMs and PEI/MCNs) were prepared via impregnation of the carbon supports with a required amount of methanolic solution of branched PEI (Sigma−Aldrich, molecular weight of MW = 800) at room temperature, followed by drying at 110 °C for 3 h. According to the measured PEI loading (x, wt %) that is defined as the weight percentage of PEI in the adsorbent, the final adsorbents were denoted as xPEI/MCMs (x = 7, 16, and 29) and xPEI/MCNs (x = 7, 14, 24, and 45). The amine-grafted MCMs (NH2-MCMs) were prepared via a two-step method. First, the oxidation of MCMs was performed with a concentrated HNO3/H2SO4 (1/3 in volume) mixture for 3 h in a standard laboratory sonication bath to create carboxylic acid groups on the carbon surface.22 The

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EXPERIMENTAL SECTION Synthesis of Materials. The MCMs were synthesized by carbonization of nanostructured polymeric composites that were obtained by self-assembly of block copolymer (Pluronic F127) and phenolic resin (phloroglucinol−formaldehyde) under acidic conditions via a soft-template method.16 Briefly, a mixture of phloroglucinol (26.2 g), F127 (52.4 g), aqueous HCl solution (37 wt %, 10.0 g), and ethanol (1300 mL) was 7356

DOI: 10.1021/acs.iecr.6b00823 Ind. Eng. Chem. Res. 2016, 55, 7355−7361

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functional groups is easy to implement experimentally, and it can accommodate more guest molecules in the support pores. Therefore, the first part of our work is to compare the CO2 adsorption performance of MCMs functionalized by either chemical grafting of ethylenediamine (NH2-MCMs) or physical impregnation with polyethylenimine (PEI/MCMs), as shown earlier in Scheme 1. PEI/MCMs samples were prepared with various PEI loadings (7−29 wt %), for comparison with NH2MCMs. The MCMs have particle sizes in the range of 100−200 μm (Figure 1A) and a pore diameter of ∼9 nm (Figure 1B). The

NH2-MCMs were obtained by reaction of the oxidized MCMs (0.5 g) with ethylenediamine (10 mL) using N-[(dimethylamino)-1H-1,2,3-triazolo[4,5,6]pyridin-1-ylmethylene]-N-methylmethanaminium hexafluorophosphate N-oxide (HATU, 20 mg) as a coupling agent.21 The reaction was carried out for 4 h at 45 °C in the sonication bath. The final product was washed with methanol for several times to remove unreacted ethylenediamine. Characterization of Materials. N2 adsorption isotherms at −196 °C were measured on a surface area analyzer (Micromeritics, Model Gemini 2390a) to determine the textural properties of carbons. Each sample (100−200 mg) of PEI/ MCMs and PEI/MCNs was loaded in a sample tube, followed by treatment at 110 °C for 2 h on a flowing-N2 degassing unit (Micromeritics SmartPrep). After cooling in the N2 flow, the sample tube was transferred to the adsorption analyzer for testing. The total surface area was calculated from N2 adsorption isotherms, using the Brunauer−Emmett−Teller (BET) equation in the relative pressure (P/P0) range of 0.05−0.20. The total pore volume was determined from the amount of N2 adsorbed at the relative pressure of 0.97. Pore size distribution was derived from N2 adsorption isotherms using the Barrett−Joyner−Halenda (BJH) model. X-ray photoelectron spectra (XPS) were acquired using a spectrometer (PHI Instruments, Model 3056) with an AI anode source operated at 15 kV and an applied power of 350 W to determine surface elementary nature of the amino-functionalized samples. Scanning electron microscopy (SEM) was performed on a Zeiss Auriga Crossbeam SEM system at an acceleration voltage of 5 kV. Transmission electron microscopy (TEM) images were recorded on a TEM system (Zeiss, Model Libra 200) at 200 kV. Measurement of PEI Loading and CO2 Adsorption. The PEI loading of PEI/MCMs and PEI/MCNs was measured on a thermogravimetric analysis (TGA) device (Model Seiko 6300 TG/DTA, RT Instruments, Inc.). The weight loss between 110 and 550 °C was used to calculate the PEI loading. CO2 adsorption capacity at 1 bar and 30/75 °C was measured gravimetrically on the same TGA system. An ∼10 mg sample was placed in a ceramic crucible and transferred to the TGA chamber. Prior to CO2 adsorption measurement, the sample was pretreated in a dry N2 flow (Airgas, UHP grade, 99.999%, 150 mL min−1) at 110 °C for 3 h and then cooled in the same gas flow to the target adsorption temperature. After stabilization at the temperature for 1 h, the N2 flow was switched to a dry CO2 flow (Airgas, research grade, 99.999%, 50 mL min−1), and subsequently both the temperature and CO2 flow were maintained for 12 h. The weight gain during this period of time was taken as the CO2 adsorption capacity of the sample. Representative CO2 uptake curves as a function of time are shown in Figures S1 and S2 in the Supporting Information.

Figure 1. (A) Scanning electron microscopy (SEM) image and (B) transmission electron microscopy (TEM) image of mesoporous carbon microparticles.

textural properties of MCMs, NH2-MCMs, and PEI/MCMs were analyzed by N2-physisorption at −196 °C. Figure 2 shows the measured N2-adsorption isotherms and BJH pore size distributions of these samples. All the samples, independent of functionalization methods and PEI loadings, have a typical Type IV isotherm (Figure 2A) with a capillary condensation step within the P/P0 range of 0.6−0.8, but the N2 adsorption amount decreases as the PEI loading increases, as a result of the pore filling by PEI. The BET surface area of PEI/MCMs, as expected, decreases continuously from 357 m2 g−1 to 24 m2 g−1 as the PEI loading increases from 0 wt % to 29 wt %. The pore size distribution (Figure 2B) shows that the pore diameter of PEI/MCMs reduces slightly from 9 nm to 8 nm, when compared to the MCMs support. For NH2-MCMs, the grafting of ethylenediamine leads to a significant reduction in the BET surface area (from 357 m2 g−1 to 144 m2 g−1) and pore volume (from 0.52 cm3 g−1 to 0.23 cm3 g−1), along with the pore diameter decreasing by ∼1 nm (Figure 2B). The decrease of the surface area and pore volume after surface grafting of guest molecules has also been observed in our previous work, where amidoxime groups are covalently attached to the surface of mesoporous carbons for uranium extraction from seawater.22 This results from a partial occupancy of the support pores by incorporated guest molecules. To further compare the nature and content of incorporated amine groups, NH2-MCMs and PEI/MCMs were examined by XPS. As presented in Figure 3, the N 1s spectra of NH2-MCMs and PEI/MCMs have almost-identical peaks centered at 399.6−399.8 eV, which is characteristic of amine and imine species present in PEI and grafted ethylenediamine (−NH− C2H4−NH2).24 The nitrogen content of NH2-MCMs measured by XPS, ∼6.4 mmol-N g−1, is comparable to those of 7PEI/ MCMs (∼4.2 mmol-N g−1) and 16PEI/MCMs (∼9.2 mmol-N g−1). Therefore, the three samples are listed in Table 1 for comparison, in terms of their textural properties, nitrogen contents, and CO2 adsorption capacities. The CO2 adsorption capacity of NH2-MCMs at 75 °C and 1 bar CO2 is 0.37 mmol-

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RESULTS AND DISCUSSION Effect of Functionalization Methods. Surface functionalization of carbon materials, by covalent grafting of nitrogencontaining functional groups, can provide highly dispersed and stable adsorption sites, but the functionalization process usually involves tedious experimental procedures and hazardous chemicals, such as concentrated mineral acids for oxidation.12,21−23 The grafted functional groups are also limited in amount, because they are, at most, in a monolayer dispersion on the support surface. However, physical impregnation of porous carbons with guest compounds containing desired 7357

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Figure 2. (A) N2-adsorption isotherms and (B) Barrett−Joyner−Halenda (BJH) pore size distributions of MCMs, NH2-MCMs, and PEI/MCMs.

Figure 3. N 1s XPS of NH2-MCMs, 7PEI/MCMs, and 7PEI/MCNs. Figure 4. TEM images of MCNs.

CO2 g−1 that is comparable to those of 7PEI/MCMs and 16PEI/MCMs (0.37−0.40 mmol-CO2 g−1) under the same conditions. This seems to be reasonable, because NH2-MCMs have similar surface areas (144 m2 g−1 vs 112−200 m2 g−1) and nitrogen content (6.4 mmol-N g−1 vs 4.2−9.2 mmol-N g−1) with both PEI/MCMs samples. In summary, NH2-MCMs prepared by chemical grafting of ethylenediamine on the MCMs surface show little advantage over PEI/MCMs, with respect to CO2 adsorption performance. The preparation of PEI/MCMs by the impregnation method has a less complex experimental procedure. Therefore, the next part of this work investigates the PEI-impregnated adsorbents by using different mesoporous carbon supports. Effect of Carbon Supports. For comparison with the MCMs as the PEI support, MCNs with uniform sphere and pore sizes were synthesized at a gram scale via a silica-assisted method18 with minor modification. TEM images of the synthesized MCNs show a sphere diameter of 850−1000 nm (Figure 4A) and a pore width of 2−3 nm (Figure 4B). PEIimpregnated MCNs were prepared with the PEI loading of 7− 45 wt %, higher than that for PEI/MCMs (7−29 wt %),

because of higher pore volume (0.78 vs 0.52 cm3 g−1) and surface area (1073 vs 357 m2 g−1) of MCNs than MCMs. Figure 5 presents the N2-adsorption isotherms and BJH pore size distributions of PEI/MCNs with various PEI loadings. MCNs have a Type IV isotherm similar to MCMs, while the capillary condensation step is shifted down to P/P0 = 0.1−0.35 (vs 0.6−0.8 for MCMs), indicating a smaller pore size of MCNs (2−3 nm) than MCMs (9 nm), as confirmed by the pore size distributions (see Figures 5B and 2B). As the PEI loading increases, both BET surface area and pore volume of PEI/MCNs and PEI/MCMs decrease continuously to almost zero (Figure 6) when the support pores are completely filled by PEI. PEI/MCNs have higher surface area and pore volume than PEI/MCMs, when compared at the same PEI loading. The XPS measurement (Figure 3) shows that 7PEI/MCMs and 7PEI/ MCNs have almost the same N 1s XPS spectra, indicating no structural difference between the PEI loaded in the MCMs and MCNs. Figure 7 shows the CO2 adsorption capacities and amine efficiencies of PEI/MCNs and PEI/MCMs at 75 °C and 1 bar CO2, as a function of PEI loading. The amine efficiency,

Table 1. Textural Properties, Nitrogen Contents, and CO2 Adsorption Capacities of NH2-MCMs and PEI/MCMs

sample NH2-MCMs 7PEI/MCMs 16PEI/MCMs

PEI loadinga (wt %)

SBETb (m2 g−1)

Vtc (cm3 g−1)

6.8 15.8

144 200 112

0.23 0.41 0.25

Nitrogen Contentd (mmol-N g−1)

CO2 Adsorption Capacitye (mmol-CO2 g−1)

TGA

XPS

at 30 °C

at 75 °C

1.6 3.7

∼6.4 ∼4.2 ∼9.2

0.75 0.82 0.62

0.37 0.40 0.37

a Weight percentage of PEI in the sample measured by TGA. bBET surface area. cTotal pore volume measured at a relative pressure of 0.97. dThe PEI loading measured by TGA was used to calculate the nitrogen content for PEI/MCMs; XPS was also used to estimate the nitrogen content in NH2-MCMs and PEI/MCMs. eMeasured by TGA at 1 bar CO2.

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Figure 5. N2-adsorption isotherms and BJH pore size distributions of MCNs and PEI/MCNs.

defined as the molar ratio of adsorbed CO2 to amine group (mol CO2 (mol N)−1), is calculated from the data of CO2 capacities and PEI loadings (measured by TGA). Independent of the carbon support, the CO2 capacity decreases with increasing PEI loading in the low/intermediate range (up to 16 wt %), and then increases with PEI loading in the higher range (Figure 7A). This phenomenon seems to be inconsistent with mesoporous SiO2-supported PEI adsorbents,19,20 where the CO2 adsorption capacity increases with the PEI loading, even in the low loading range. This could be due to a weak CO2−SiO2 interaction that causes low CO2 adsorption capacity of bare SiO2 support. Thus, the CO2 capacity appeared to increase with the PEI loading, even when only a small amount of PEI was incorporated into the support. However, porous carbon materials, as well-known adsorbents of a wide range of gas and liquid molecules, have higher CO2 adsorption capacity than bare SiO2 support. It is likely that the CO2 capacities of PEI/ MCMs and PEI/MCNs decreased with the PEI loading in the low loading range, because the CO2 capacity contributed by the incorporated PEI may not be able to compensate for the loss of CO2 capacity from the bare surface of the carbon supports. Furthermore, some of the support pores could be clogged by PEI aggregate plugs (this has been proved in the case of mesoporous SiO2-supported PEI25), which would lead to more of the bare carbon surface being inaccessible to CO2 molecules and, consequently, lower the apparent CO2 capacity to a larger extent. The PEI/MCNs show higher CO2 capacities and amine efficiencies than PEI/MCMs at the same PEI loading, indicating that a higher percentage of PEI in MCNs is accessible to CO2 molecules. This could be due to the smaller

Figure 6. (A) BET surface area and (B) total pore volume of PEI/ MCMs and PEI/MCNs, as a function of PEI loading.

Figure 7. (A) CO2 adsorption capacities and (B) amine efficiencies of PEI/MCMs and PEI/MCNs under the conditions of 75 °C and 1 bar CO2. 7359

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can also increase the pore interconnectivity of the carbon support that may facilitate the CO2-adsorption kinetics. Further research on the amino functionalization of activated MCNs is ongoing in our group.

particle size of MCNs than MCMs, making more efficient use of the PEI located deep in the particle center for CO2 adsorption. The best-performing PEI/MCNs adsorbent (45PEI/MCNs) has a CO2 capacity of 1.97 mmol-CO2 g−1, which is more than three times that of 29PEI/MCMs (0.58 mmol-CO2 g−1). The amine efficiency of 45PEI/MCNs is ∼0.20 mol CO2 (mol-N)−1, which is more than twice that of 29PEI/MCMs (0.09 mol-CO2 (mol-N)−1). Therefore, the MCNs appear to be a promising supporting material of PEI for CO2 capture, and their textural properties could be further optimized by various chemical/physical activation techniques, allowing more PEI to be incorporated into the carbon support for CO2 adsorption. Because of its best CO2-adsorption performance, 45PEI/ MCNs was selected to do a cyclic stability test. Figure 8 shows

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CONCLUSIONS Mesoporous carbon microparticles with a particle size of 100− 200 μm are functionalized by physical impregnation with branched polyethylenimine and chemical grafting of ethylenediamine for CO2 adsorption. The PEI-impregnated MCMs have advantages over the amine-grafted MCMs by means of their convenient preparation and higher CO2 adsorption capacities. Mesoporous carbon nanospheres with a sphere size of 850−1000 nm, prepared using a silica-assisted selfassembly method, are compared with MCMs for use as the PEI supports. PEI/MCNs exhibit higher CO2 adsorption capacities and amine efficiencies than PEI/MCMs, when compared at the same PEI loading, indicating a more efficient utilization of the incorporated PEI in the nanosized carbon spheres. MCNs appear to be a preferable supporting material of PEI for CO2 capture.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.6b00823. Representative CO2 uptake curves as a function of time (PDF)

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Figure 8. Cyclic CO2 adsorption of 45PEI/MCNs in a pressure-swing mode (CO2 adsorption at 75 °C and 1 bar CO2 for 1 h and adsorbent regeneration at 75 °C and 1 bar N2 for 1 h).

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

the cyclic CO2 adsorption of 45PEI/MCNs in a pressure-swing mode, where the CO2 adsorption was carried out at 75 °C and 1 bar CO2 (50 mL min−1) for 1 h and the adsorbent regeneration was conducted at 75 °C and 1 bar N2 (100 mL min−1) for 1 h. The pressure-swing adsorption is easy to operate practically, with less energy consumption, when compared to temperature-swing adsorption, even though it is difficult to fully regenerate the adsorbent merely by reducing the partial pressure of CO2. This means that the only part of the CO2 capacity is reversible during the pressure-swing CO2 adsorption. As shown in Figure 8, 45PEI/MCNs exhibits a reversible CO2-adsorption capacity of ∼0.70 mmol-CO2 g−1, accounting for ∼47% of the total capacity (∼1.50 mmol-CO2 g−1). The reversible capacity is stable in 19 adsorptionregeneration cycles, suggesting the good cyclic stability of 45PEI/MCNs. In summary, the MCNs appear to be a promising supporting material of PEI for CO2 capture. However, those amineimpregnated or grafted solid adsorbents reported in the literature have the highest CO2 capacities, up to 4−5 mmolCO2 g−1 under similar adsorption conditions (75 °C and 1 bar CO2),4,7 which is much higher than the data of PEI/MCNs (∼2 mmol-CO2 g−1). This is due mainly to the relatively low pore volume of MCNs, which cannot accommodate more PEI in the solid adsorbent. Physical and chemical activation of MCNs by various agents such as steam, CO2, KOH, H3PO4, and ZnCl2 is a promising route to develop additional pores in the framework of MCNs for incorporation of more PEI.26,27 The activation

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ACKNOWLEDGMENTS This contribution was identified by Prof. Dr. De-en Jiang (University of California at Riverside) as the Best Presentation in the session “ENFL Porous Materials for Energy & Sustainability from Discovery to Application” of the 2015 ACS Fall National Meeting in Boston, MA. This work was supported as part of the Center for Understanding and Control of Acid Gas-Induced Evolution of Materials for Energy (UNCAGE-ME), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, at Oak Ridge National Laboratory and at Georgia Tech, under Contract No. DE-SC0012577.

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DOI: 10.1021/acs.iecr.6b00823 Ind. Eng. Chem. Res. 2016, 55, 7355−7361

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DOI: 10.1021/acs.iecr.6b00823 Ind. Eng. Chem. Res. 2016, 55, 7355−7361