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Sulfuric Acid Modified Bentonite as the Support of Tetraethylenepentamine for CO2 Capture Weilong Wang,†,‡ Xiaoxing Wang,‡ Chunshan Song,‡ Xiaolan Wei,§ Jing Ding,*,† and Jing Xiao*,§ †

Center for Energy Conservation Technology, School of Engineering, Sun Yat-sen University, Guangzhou, 510006, China Clean Fuels and Catalysis Program, EMS Energy Institute, and Department of Energy & Mineral Engineering, Pennsylvania State University, University Park, Pennsylvania 16802, United States § Department of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, China ‡

S Supporting Information *

ABSTRACT: In this work, an inexpensive and commercially available bentonite was modified by sulfuric acid and explored as the new type of support to immobilize tetraethylenepentamine (TEPA) for CO2 capture from flue gas. By applying sulfuric acid treatment, the textural properties, in particular, pore volume and surface area of bentonite, were significantly improved. Bentonite treated with 6 M sulfuric acid (Ben_H2SO4_6M) can reach a pore volume of 0.77 cc/g from that of the parent bentonite of 0.15 cc/g. With the maximum TEPA loading of 50 wt % onto the Ben_H2SO4_6M sorbent, the maximum CO2 breakthrough sorption capacity reached 130 mg of CO2/g of sorbent at 75 °C under a dry condition. With an addition of moisture to the simulated flue gas, the CO2 sorption capacity can be further improved to 190 mg of CO2 at 18 vol% of moisture addition sorbent due to the bicarbonate formation under a wet condition. The TEPA/Ben_H2SO4_6M sorbents show a good regenerability and thermal stability below 130 °C. The high CO2 sorption capacity, positive effect of moisture addition, and low capital cost of the raw bentonite materials imply that TEPA/Ben_H2SO4_6M could be a promising sorbent for cost-efficient CO2 capture from flue gas. The sulfuric acid treatment was demonstrated as an effective method for bentonite modification to immobilize TEPA for CO2 capture.

1. INTRODUCTION Reducing anthropogenic CO2 emission and lowering the concentration of greenhouse gases in the atmosphere have quickly become one of the most urgent environmental issues,1 as the increasing CO2 emissions are thought to be one contributor to global warming.2 Carbon capture and storage (CCS) is a viable strategy for mitigating CO2 emissions while retaining the continuous use of fossil-fuel-based energy. In the industrial scale, amine scrubbing, using aqueous solutions of amine, that is, monoethanolamine (MEA) and diethanolamine (DEA), to selectively absorb CO2 from flue gases, has been applied in conventional absorber/stripper systems in power plants for effective CO2 capture.3 However, one major concern of amine scrubbing technology is the high energy penalty, as well as equipment corrosion,2,4−6 and the amine degradation especially in the presence of oxygen and/or sulfur dioxide,3 etc. Therefore, less energy-intensive technologies should be developed for postcombustion CO2 capture. A new concept of “molecular basket” sorbents (MBS),4,7,8 which selectively capture CO2 molecules onto a functional “basket”, has been proposed for CO2 capture. The MBS-type of sorbents are prepared by immobilizing an amine-functional polymer, that is, polyethyleneamine (PEI), onto a porous supporting material. MBS has shown great advantages for CO2 capture, including superior sorption−desorption characteristics (i.e., sorption capacity, selectivity, regenerability, stability, and kinetics, etc.), no or less corrosion, and a lower energy consumption9 compared to the conventional amine scrubbing. Various mesoporous silica molecular sieves, that is, MCM41,10,11 MCM-48,12 and SBA-15,7,13,14 have been studied as the © 2013 American Chemical Society

supporting materials for PEI for CO2 capture. However, most of them are not commercially available and also expensive to prepare in an industrial scale. It was pointed out that the support material accounts for over 70% of the total capital cost for sorbent preparation.15 Moreover, mesoporous silica molecular sieves can be degradable due to their poor hydrothermal stability. Therefore, developing MBS with lowcost and/or rich natural abundance supporting materials and good hydrothermal stability guide a new direction for CO2 capture. So far, studied supporting materials include mesoporous silica gel,16 and activated carbon.9 Bentonite is one of the most common raw clay materials, consisting mainly of the montmorillonite group. The inner layer is composed of one octahedral alumina sheet placed between two tetrahedral silica sheets. Due to the isomorphous substitutions within the layers of Al3+ for Si4+, the surface of bentonite is negatively charged.17 The sorption features over clay are believed to be related to the nature of the parent clay, and an attempt18 to rationalize the fact was made considering the Si/Al ratio, together with textural characteristics. Taking the advantages of low cost (∼$50/ton), easy availability, and high mechanical and chemical stability, bentonite has been widely used for industrial separation processes, that is, sorption materials for the adsorption of ethofumesate19 and dibenzofuran20 in water treatment processes. However, the microstructure, in particular, the micropore size distribution, limits Received: December 17, 2012 Revised: February 17, 2013 Published: March 12, 2013 1538

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TEPA/Ben sorbent, or 11, 25, 43, 67, 100, and 150 wt % of TEPA loading referring to the weight of the bentonite support, were prepared and further studied. 2.4. Characterization of TEPA/Ben Sorbents. 2.4.1. N2 Adsorption Test. The textural structure of the adsorbents were characterized using a Micromeritics ASAP2020 surface area and porosimetry analyzer. BET surface area was calculated from adsorption isotherms using the standard Brunauer−Emmett−Teller (BET) equation. The t-plot method was applied to derive the micropore surface area. Pore size distributions (PSDs) were determined using density functional theory (DFT) based on statistical mechanics.22 2.4.2. SEM. To investigate the physical and chemical compatibility of the TEPA/Ben sorbents, the morphology was investigated using SEM (model S-520/ISIS-300, Hitachi/Oxford). The samples were prepared by fracturing the specimen in liquid nitrogen and then casting it with gold (AU) powder for SEM imaging. 2.4.3. FT-IR. The functionalities on the TEPA/Ben sorbents were characterized using FT-IR (model Nicolet/Nexus 670) in the frequency range of 4000−400 cm−1. 2.4.4. TGA. TGA (model of Q600SDT) was used for the thermal stability study of the TEPA/Ben sorbents in comparison to the TEPA. All the samples were ground to powder and scanned within a temperature range of 50−800 °C. 2.5. Evaluation of Sorption/Desorption Performance. 2.5.1. TGA Tests. The sorption−desorption performance of prepared MBS samples was evaluated using a thermogravimetric analyzer (TGA Q600SDT) on the basis of the weight change during the sorption and desorption. The analysis is described as below: About 10 mg of the sample was placed into the sample pan, and the temperature was increased at a rate of 10 °C/min from 30 to 100 °C and equilibrated at 100 °C for 40 min under N2 (99.999%) with a flow rate of 100 mL/ min to remove the possible moisture, solvent, or other adsorbates from the samples. The temperature was then cooled down to the desired temperature of 75 °C, and the gas was switched from N2 to pure CO2 (99.99%) and maintained at this temperature for 40 min for CO2 sorption. After that, the temperature was increased to 110 °C, and the gas flow was switched from CO2 to N2 for desorption. The mass-based CO2 sorption capacity (mg of CO2/g of sorb) was calculated according to the weight change of the sample measured by TGA in the sorption/desorption process. Here, it should be pointed out that the sorption capacity measured by TGA in this study is not an equilibrium sorption capacity, as the weight of the sample still showed an increasing trend after 40 min sorption due likely to the slow sorption rate.8,23 2.5.2. Fixed-Bed Flow Sorption Tests. The CO2 sorption performance of some samples from a simulated flue gas was also investigated in a fixed-bed flow sorption system. Scheme 1 shows the

its potential use in many catalytic and separation processes. To extend the application ranges of bentonite, the textural and chemical properties, that is, Si/Al ratio, can be tuned by different activation methods, that is, mechanochemical activation, intercalation, thermochemical activation, chemical activation, etc., which may possibly contribute to the generation of a new porous structure and the creation of new/more active sites on the surface, resulting in improved adsorptive or catalytic behaviors of the modified bentonite. Among them, acid activation has been studied as a chemical treatment method for the alternation of the textural (i.e., surface area, pore volume) and chemical properties (i.e., catalytic, adsorptive) of clays. The activation process includes the following steps: (i) leaching of the clays with inorganic acids, (ii) causing disaggregation of clay particles, (iii) elimination of mineral impurities, and (iv) dissolution of the external layers.21 The acid treatment is beneficial in terms of increased surface area, porosity, and number of acid sites with respect to the parent bentonite, which could be potentially a good supporting material for the immobilization of a basic amine-functionalized polymer (macromolecule) as a new MBS for CO2 capture. To the best of our knowledge, no work has been carried out in this area. The objective of this study is to explore the use of inexpensive and commercially available bentonite (∼$50/ton) modified by sulfuric acid as a support to prepare the bentonitesupported PETA (TEPA/Ben) MBS for CO2 capture. The textural properties and morphology of the bentonite-supported MBS were determined by N2 adsorption tests, scanning electron microscope (SEM), and Fourier transfer-infrared spectroscopy (FT-IR). The CO2 sorption/desorption performance was evaluated in a thermogravimetric analyzer (TGA) and a fix-bed flow sorption system. The effects of TEPA loading, sorption temperature, and H2O addition on CO2 sorption were examined and discussed. A 10-cycle sorption−desorption test was carried out to study the regenerability of the TEPA/Ben, and its thermal stability was investigated in a TG experiment.

2. EXPERIMENTAL SECTION 2.1. Materials. Bentonite was provided by JiangXi Yushan Bentonite Company, China. Tetraethylenepentamine (TEPA) was purchased from Shanghai Wuhua Company. Ethanol with a purity of 99.8% was purchased from Guangzhou Guanghua Chemicals for using as a solvent in the preparation of TEPA/Ben sorbents. Sulfuric acid (concentrated) was provided by Guangzhou Chemicals Company, which was further diluted in the lab for different concentrations. 2.2. Bentonite Modification by Acid Treatment. To increase the porosity of the supporting materials, bentonite was modified by sulfuric acid at different concentrations (3, 6, and 9 M), which were noted as Ben_H2SO4_3M, Ben_H2SO4_6M, and Ben_H2SO4_9M. The desired amount of bentonite was added into the sulfuric acid solution at a fixed ratio of 10 cc of sulfuric acid to 1 g of bentonite. The mixture was then heated at 95 °C in an oil bath, stirring at 600 rpm for 4 h. After that, the suspension was filtered and washed with a large amount of distilled water and dried in an oven at 100 °C overnight. Finally, the powder was heated continuously under vacuum for 24 h. 2.3. Preparation of TEPA/Ben Sorbents. The modified bentonite-supported MBS were prepared by the wet impregnation method. About 3 g of TEPA was dissolved in 30 g of ethanol under stirring for 30 min. The sulfuric acid modified bentonite was then added into the above solution and further stirred for 3 h at room temperature. After that, the slurry was dried in a vacuum oven at 45 °C for 12 h. To optimize the TEPA loading onto the sulfuric acid modified bentonite, different amounts, including 10, 20, 30, 40, 50, and 60 wt % of TEPA loading referring to the weight of the modified

Scheme 1. Schematic Diagram of the Fixed-Bed Flow System

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Table 1. Textural Properties (Vtotal, SBET, dave), Mass Loss Percentage (Mi/Mo), and Maximal CO2 Sorption Capacity of the Parent Bentonite and the Modified Bentonites by Sulfuric Acid at Different Concentrations of 3, 6, and 9 M samples

Vtotal (cc/g)

SBET (m2/g)

dave (nm)

Mi/Mo (%)

max CO2 sorption cap (mg of CO2/g of sorb)

parent bentonite Ben_H2SO4_3M Ben_H2SO4_6M Ben_H2SO4_9M

0.15 0.53 0.77 0.41

118 307 316 265

4.2 5.7 6.9 6.5

0 2.7 4.9 8.7

26 (15 wt % of TEPA) 80 (38 wt % of TEPA) 135 (50 wt % of TEPA) 62 (30 wt % of TEPA)

schematic diagram of the fixed-bed flow sorption system for CO2 capture. Three gas lines connect to the nitrogen gas, simulated flue gas, and water bubbler, respectively. In the system, there is also a furnace with a temperature controller, flow controllers, and an online SRI 8610C gas chromatography (GC). The gas flow controllers were calibrated by a soap-film flow meter. The sorbent sample was packed into the stainless column with a 80 mm length and 5 mm inner diameter. The column was placed in the furnace and heated at 100 °C for 5 h under an ultra-high-purity (UHP) nitrogen flow of 50 mL/min to remove the existing moisture and/or carbon dioxide in the samples. After the column was cooled down to the desired sorption temperature of 75 °C under the nitrogen flow, the simulated flue gas (15 vol % CO2, 4.5 vol % O2 in N2) was introduced at the desired flow rate of 20 mL/min. The concentration of CO2 in the effluent was monitored by online GC at an interval of 2.5 min. After the sorbent was saturated, the simulated flue gas was changed back to the UHP nitrogen gas at a flow rate of 50 mL/min, and simultaneously the sorbent bed temperature was increased to 100 °C and held at this temperature for 1 h to perform the desorption. The sorption capacity of CO2 was calculated based on the breakthrough curve, which is described in our previous study.8,24

modification of textural properties by sulfuric acid may vary with the nature of the bentonite. According to the literature,29 the reaction between bentonite and sulfuric acid can be described by the following chemical equation: Al2O3·2SiO2·2H2O + 3H2SO4 = Al2(SO4)3 + 2SiO2 + 5H2O. As the content of the acid increases, the Al2O3, MgO, CaO, and K2O contents presented in the parent bentonite may be dissolved from the octahedral layer, resulting in the creation of new pores on modified bentonites, as shown in Scheme 2. Scheme 2. Simplified Illustration of Bentonite Modification by Sulfuric Acid

3. RESULTS AND DISCUSSION 3.1. Effect of Acid Modification on the Textural Properties of Bentonite. Textural properties, including pore volume, pore size, and surface area, of the supporting material play a critical role for the immobilization of tetraethylenepentamine (TEPA) for CO2 capture. It was found out that large pore volume of supporting materials can load more amine-functionalized polymer, resulting in a higher CO2 sorption capacity. It was also reported that the pore volume and pore size are key factors for MBS performance for CO2 capture.23 In the case of bentonite, even though the parent bentonite may have little surface area and pore volume,19,25 its textural properties can be improved by acid treatment. As sulfuric acid was reported as an effective acid to enhance the textural properties of bentonites,21 it was chosen as the modification agent of bentonites in this work. Table 1 lists the textural properties of modified bentonite with sulfuric acid at different concentrations (3, 6, and 9 M), where less than 5% of variance in textural parameters (as shown in the Supporting Information, Table 1), including pore volume, surface area, and pore size, suggested that the modification method and results can be reproduced. In comparison to the parent bentonite, the acid-modified bentonites show larger pore volumes and surface areas, suggesting that new pores may be generated during sulfuric acid treatment. In addition, all of the acid-modified bentonites show increased pore sizes, which can be ascribed to the leaching/dissolving of metal ions located on the surface of the smaller pores, and/or the generation of relatively larger new pores during acid treatment. It should be addressed that the commercial bentonite has a range of surface areas and pore volumes, that is, a surface area of 20−250 m2/g and pore volume of 0.1−0.3 cc/g,26−28 due to its nature, as well as the element compositions, that is, the Si/Al ratio and the ratios of other metals, such as Mg, Ca, K, etc. The effectiveness of the

Besides the changes in the textural properties, the composition of the modified bentonite, that is, the Si/Al ratio, would also change considerably due to the leaching of the Al3+ ions and other basic mental ions. Table 1 listed the weight loss of bentonite modified by sulfuric acid at different concentrations of 3, 6, and 9 M, which were 2.7, 4.9, and 8.7%, respectively, suggesting more weight loss or metal leaching by a more concentrated sulfuric acid. It should also be mentioned here that the surface area and pore volume of Ben_H2SO4_9M are smaller than those of Ben_H2SO4_6M, which can be ascribed to the pore collapse during strong acid treatment when the concentration of sulfuric acid was 9 M. Table 1 also lists the maximal CO2 sorption capacity at the optimized loading of PEI. It can be noted that, for the parent bentonite, the maximal CO2 sorption capacity only reached 26 mg of CO2/g of sorbent at 15 wt % of TEPA loading. In contrast, the maximal sorption capacity of sulfuric acid modified bentonite reached 135 mg of CO2/g of sorbent at 50 wt % of TEPA loading, which should mainly be ascribed to its maximal pore volume of 0.77 cc/g. On the basis of the screening experiments, Ben_H2SO4_6M was selected as the supporting material for MBS preparation and further studied for CO2 capture in this work. 3.2. Morphology of the Bentonite-Supported MBS Samples. Figure 1 shows the SEM images of the bentonitesupported MBS samples, including Ben_H 2 SO 4 _6M, TEPA(50)/Ben_H2SO4_6M, and TEPA(60)/ Ben_H2SO4_6M. Almost no TEPA was observed in the SEM image of TEPA(50)/Ben_H2SO4_6M, indicating that all the immobilized TEPA went into the bentonite pores. The result further suggested that TEPA prefers to fill in the pores of the modified bentonite support, which may be ascribed to the capillary action of pores, or the amine-philic functional groups 1540

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peak position in FT-IR spectra between the TEPA and the acidmodified bentonite-supported TEPA was observed, indicating an interaction between TEPA and the internal surface of bentonite,24,35 which may be ascribed to the interaction between the basic −NH2 groups on TEPA and the acidic sites on the acid-modified bentonite support, that is, Al3+ ions, Si-OH, and others. 3.4. Effect of TEPA Loading over the Acid-Modified Bentonite on CO2 Sorption. In the acid-modified bentonitesupported MBS, the accessible amine sites on TEPA play a key role for CO2 sorption as the sorption goes through an interaction between amine groups and CO2. Therefore, the TEPA loading can greatly affect the CO2 sorption capacity of the prepared MBS. It was observed during the sorbent preparation that all the acid-bentonite-supported TEPA sorbents were fine powders, except the one with the TEPA loading as high as 60 wt %, which showed a sticky appearance. Therefore, semiempirically, the Ben_H2SO4_6M support can accommodate a maximum TEPA amount of around 50 wt %, where the largest amount of TEPA entered the channels and/or pores of the supporting material. To further confirm the assumption, the CO2 sorption performance of the TEPA/ Ben_H2SO4_6M sorbents at different TEPA loadings was evaluated by TGA, with the adsorption capacity as shown in Figure 3. It can be seen that, with the TEPA loading lower than

Figure 1. SEM images of (a) Ben_H2SO4_6M support, (b) TEPA(50)/Ben_H2SO4_6M, and (c) TEPA(60)/Ben_H2SO4_6M.

inside the pores. By increasing the TEPA loading, more TEPA can be observed in the SEM images of bentonite-supported TEPAs. It can be noted that some TEPA was present on the external surface of the TEPA(60)/Ben_H2SO4_6M particles, which may result in the partial glomeration of TEPA. The SEM results further suggested the presence of amine-philic sorption sites inside the pores of the Ben_H2SO4_6M support, which attracted TEPA to fill in the pores at a lower TEPA loading (60 wt %), which was further confirmed by the N2 adsorption test of TEPA(60)/Ben_H2SO4_6M; almost no pore volume (0.02 cc/ g) and surface area (10 m2/g) were measurable. 3.3. FT-IR Study of the Bentonite-Supported MBS Samples. The FT-IR spectra of the Ben_H2SO4_6M and the Ben_H2SO4_6M supported TEPA sorbents at different TEPA loadings are shown in Figure 2. In the spectrum of the

Figure 3. Effect of TEPA loading onto Ben_H2SO4_6M on CO2 sorption capacity and amine efficiency at 75 °C.

50 wt % on the Ben_H2SO4_6M support, the CO2 sorption capacity increases with the increase of the TEPA loading amounts. The TEPA/Ben_H2SO4_6M sorbents reached the maximal CO2 sorption capacity of 135 mg of CO2/g of sorbent at the 50 wt % of TEPA loading. It was noticeable that, when further increasing the TEPA loading to 60 wt %, the CO2 sorption capacity dropped significantly, suggesting a decreased amount of accessible −NH2 sites for CO2 capture. This can be due to the TEPA agglomeration when in an excess loading amount. The result further suggested the presence of hard-toaccess amine sites in TEPA/Ben_H2SO4_6M for CO2 due to the great diffusion barrier. It can be noted in Figure 3 that the amine efficiency (mole of CO2 absorbed per mole of amine functional group on the amine sorbent (TEPA/Ben) of TEPA/ Ben_H2SO4_6M) follows the same trend as the CO2 sorption capacity, further suggesting that the optimal TEPA loading amount over the Ben_H2SO4_6M support was 50 wt %. It should be mentioned here that, theoretically, the maximal

Figure 2. FT-IR spectra of TEPA/Ben_H2SO4_6M sorbents at different TEPA loadings: (a) 0, (b) 10, (c) 30, (d) 50, (e) 60 wt %.

Ben_H2SO4_6M support, the peak at 3433 cm−1 was assigned to Si-OH,30 while peaks appeared at 789 cm−1 due to the bending vibration of the Si-O-Si group. Another peak at 1094 was caused by the C−O stretching. In the spectra of the Ben_H2SO4_6M supported TEPA sorbents, the bands at 2819 and 2943 cm−1 can be ascribed to the stretching vibration of −CH2 in the TEPA chain.31−33 Two bands at 1566 and 1450 cm−1 were caused by symmetric and asymmetric bending vibrations of −NH2, while another band at 1660 cm−1 was attributed to the bending vibration of −N(R)H in TEPA. With the increase in the TEPA loading,34 the bands for these specific functional groups of TEPA became greater, which suggests that more TEPA molecules are dispersed inside the pores and/or over the inner surface of Ben_H2SO4_6M. A slight shift of the 1541

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also shown in Figure 4. It can be seen that the maximal amine efficiency of 0.23 can be reached at a sorption temperature of 75 °C. It should be mentioned that it is most energy efficient to adsorb CO2 at or close to the real flue gas temperature. In a typical coal-fired power plant, the temperature of the outlet flue gas is around 120−160 °C. After an installed denitrogenation and desulfurization process, the flue gas temperature is reduced to around 50−80 °C,38 where the optimized CO2 sorption temperature of the TEPA(50)/Ben_H2SO4_6M sorbent fits in well as the CO2 desorption only requires 25 °C higher than the sorption temperature. Therefore, TEPA(50)/Ben_H2SO4_6M can be an energy-efficient sorbent for CO2 capture after the denitrogenation/desulfurization process from flue gas. 3.6. Effect of Moisture Addition on CO2 Sorption from a Simulated Flue Gas. When considering the removal of CO2 from the real flue gas, it is of great importance to clarify the effect of moisture on the CO2 sorption capacity because water is always present in the flue gas. The effect of moisture on the CO2 sorption over the TEPA(50)/Ben_H2SO4_6M sorbent was evaluated in the fixed-bed flow sorption system using a simulated flue gas containing 15 vol % CO2 and 4.5 vol % O2 in N2. The breakthrough curves for CO2 sorption on the TEPA(50)/Ben_H2SO4_6M sorbent in the presence and absence of moisture in the simulated flue gas are shown in Figure 5. In the dry condition, the breakthrough capacity of the

amine efficiency is 0.5 mol of CO2/mol of amine. However, due to the presence of inaccessible amine sites on the TEPA/ Ben_H2SO4_6M sorbents, the practical maximal amine efficiency (0.23 mol of CO2/mol of amine) was lower than 0.5. The result also hinted that the CO2 sorption performance of MBS may be further improved by facilitating the CO2 diffusion in MBS by various methods, that is, addition of CO2neutral surfactant to amine.36 3.5. Effect of Sorption Temperature on CO2 Sorption. It is well-known that sorption temperature is a crucial parameter for the sorption performance, that is, capacity and kinetics, of various sorbents. In this study, TEPA(50)/ Ben_H2SO4_6M with the maximal CO2 sorption capacity, was selected to investigate the effect of the sorption temperature on the CO2 sorption capacity. Figure 4 shows

Figure 4. Effect of sorption temperature on CO2 sorption capacity and amine efficiency of the TEPA/Ben_H2SO4_6M sorbent.

the effect of sorption temperature, 30, 50, 60, 75, 85, and 100 °C, on CO2 sorption capacity of TEPA/Ben_H2SO4_6M. At 30 °C, the CO2 sorption capacity was 79.5 mg of CO2/g of sorbent. The CO2 sorption capacity increases with an increase in sorption temperature. The MBS sample gave the highest sorption capacity of 135.0 mg of CO2/g of sorbent at 75 °C. By further increasing the sorption temperature from 75 to 100 °C, the sorption capacity decreased to 110 mg of CO2/g of sorbent. Compared to some other MBS-type sorbents, such as PEI(50)/ SG16 and PEI(50)/SBA-1535 in our group, a similar trend can be observed on the sorption temperature effect, which may suggest different dominancy on CO2 sorption at the two temperature ranges. At a sorption temperature from 30 to 75 °C, the diffusion of CO2 to reach a greater amount of amine sites in the sorbent dominates the sorption. Therefore, a higher diffusion rate at a higher temperature results in a higher CO2 sorption capacity. It should be mentioned that the amine−CO2 chemisorption occurs from almost room temperature (30 °C) to 100 ◦C, suggesting that the activation barrier of the reaction is quite low, which is different from some chemisorption cases, that is, the adsorption of organic thiophenic compounds over Ni0-based sorbent occurs at around 200 °C37 as it is required to overcome a high activation barrier. When the sorption temperature is higher than 75 °C, even with a higher diffusion rate of CO2, the CO2 sorption is unfavorable thermodynamically as the CO2 desorption dominates; therefore, the CO2 capacity decreased. The amine efficiency of the TEPA(50)/ Ben_H2SO4_6M sorbent at different sorption temperatures is

Figure 5. CO2 sorption breakthrough curves over the TEPA(50%)/ Ben_H2SO4_6M sorbent under (a) dry and (b) wet conditions from a simulated flue gas containing 15 vol % CO2 and 4.5 vol % O2 in N2 (a) with and (b) without an addition of 3 vol % of moisture. (Inset: effect of moisture concentration in the simulated flue gas on CO2 sorption capacity.)

TEPA(50)/Ben_H2SO4_6M sorbent was 130 mg of CO2/g of sorbent, whereas the breakthrough capacity of the TEPA(50)/ Ben_H2SO4_6M sorbent increased to 169 mg of CO2/g of sorbent in the wet condition, corresponding to an increased amine efficiency of 0.29 mol of CO2/mol of amine from 0.23 mol of CO2/mol of amine. The results suggested that moisture addition in the flue gas promoted the CO2 sorption over the TEPA(50)/Ben_H2SO4_6M sorbents, which can be due to the formation of bicarbonate with an ideal amine efficiency of 1 under a wet condition, rather than carbamate with an ideal amine efficiency of 0.5 under a dry condition.39 Generally, the water content in the flue gas is 8−20%.40 Different water amounts were introduced into the simulated flue gas by controlling the water bubbling temperature as the saturated 1542

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vapor pressure varies with temperature. The inset in Figure 5 shows the effect of different water contents in the simulated flue gas on the CO2 sorption performance of the prepared MBS from the flow system tests. For 0, 3, 6, 10, and 18 vol % of water content in the simulated flue gas (15 vol % CO2), the CO2 sorption capacities were 130, 169, 182, 189, and 190 mg of CO2/g of sorbent, respectively, indicating an increase in the CO2 sorption capacity with the increase in the water content in the simulated flue gas until the ratio of CO2 to H2O reached a desirable value, here, in between 0.83 and 1.5, in good agreement with the theoretical value of 1 for complete bicarbonate formation. It was noted that the sorption capacities were 130 mg of CO2/g of sorbent for 15 vol % CO2 (flow system test) and 135 mg of CO2/g of sorbent for 100 vol % CO2 (TGA test), respectively. The quite close CO2 sorption capacity at different CO2% suggested that the CO2% on the sorption capacity is not significant, which is probably due to the strong amine−CO2 interaction, that is, heat of sorption of 70−90 kJ/mol-CO241 for the amine adsorbents. The CO2 sorption capacity of the TEPA(50)/Ben_H2SO4_6M sorbent is close to that of mesoporous molecular sieve supported PEI sorbents, that is, PEI/MCM-41 (133 mg of CO2/g of sorb),39 PEI/SBA-15 (140 mg of CO2/g of sorb),7 and other supported MBS, that is, PEI/ silica gel (138 mg of CO2/g of sorb)16 and PEI/carbon black (154 mg of CO2/g of sorb),9 suggesting the inexpensive acidmodified bentonite (The cost of parent bentonite was ∼$50/ ton; the cost of sulfuric acid is ∼$7/ton,42 where the desired amount of the sulfuric acid required was 18.4 g (10 cc) of concentrated sulfuric acid per gram of bentonite. Hence, the total cost of acid-modified supporting materials was ∼$180/ ton) can be a potential supporting material of MBS for CO2 capture. Reported other types of sorbents include MgO (8.8 mg of CO2/g of sorbent, 400 °C),43 activated carbon (135 mg of CO2/g of sorbent, 25 °C),44 nitrogen-doped template carbon (176 mg of CO2/g of sorbent, 25 °C),45 etc. It should be also mentioned that real flue gas is a complex mixture; besides CO2, O2, N2, and moisture, trace amounts of some coexisting acidic gases, that is, NOx and SOx, may even adsorb strongly over amine-based sorbents due to their strong acidity. Generally, less than 400 ppm of NOx is present in flue gas after the denitrogenation process, where 80−90% is NO40 (barely affects the CO2 sorption capacity over the amine sorbents). Meanwhile, hundreds of parts per million of SO2 is present in real flue gas. It was reported that the coadsorbed acidic NO2 (rather than NO) and SO2 can cause the degradation of MBS due to the formation of the heat-stable and irreversible amine salts,7 that is, sulfite/sulfate and nitrate amine salts (O2 in real flue gas may contribute to the oxidation of SOx/NOx), while orders of 10 ppm of SOx and NOx are desirable46 to avoid more severe degradation of amine sorbents for CO2 capture. Therefore, the strong acidic gases should be removed prior to CO2 capture by amine-based sorbents from flue gas. 3.7. Regenerability and Thermal Stability of MBS Samples. For the practical application, the sorbents should possess not only high sorption capacity and selectivity but also good regeneration and stability. In this study, the regenerability of the TEPA/Ben_H2SO4_6M sorbent was investigated in 10 sorption−desorption cycles using TGA. The sorption temperature was set at 75 °C, and the desorption temperature was 100 °C, respectively. The sorption capacities in the 10 sorption− desorption cycles are shown in Figure 6. It can be noted that

Figure 6. Recovery percentage of the CO2 sorption capacity of the TEPA/Ben_H2SO4_6M sorbent as a function of the cycle number during the multiple sorption−desorption cycles.

the TEPA/Ben_H2SO4_6M sorbent can be regenerated at 100 °C, suggesting a reversible CO2 sorption−desorption cycle. However, the sorption capacity dropped by around 4.4% from 112 to 107 mg of CO2/g of sorbent after 10 cycles of regeneration, indicating a loss of accessible −NH2 sites for CO2 sorption while increasing of the sorption−desorption cycle numbers. The loss of accessible −NH2 sites may be due to the loss (evaporation) of ethyleneamine oligomers with a lower molecular weight,16 which may be improved by arching TEPA onto the surface of the supporting material using adhesives, that is, a silane coupling agent.47 It is essential to have fast adsorption/desorption kinetics for CO2 capture under the operating conditions, as it controls the cycle time of a fix-bed adsorption system.48 Even a material with a high sorption capacity will have little applicability if it sorbs the carbon dioxide too slowly. Figure 7 shows the CO2

Figure 7. Rate of CO2 sorption/desorption on the TEPA/ Ben_H2SO4_6M sorbent.

sorption/desorption rate of the TEPA/Ben_H2SO4_6M sorbent in a CO2 sorption/desorption cycle from the TGA test. Assuming diffusion into porous spheres, the transient fraction uptakes can be described by the following equation in the region with fractional uptake qt/qe of less than 70%49 qt qe

≅

6 rc

DM t π

(1)

2

where DM (cm /s) is the intracrystalline diffusion coefficient, and rc (cm) is the crystal radius. In this case, the diffusion 1543

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constant (DM/rc2, s−1) was obtained from the slope of qt/qe versus √t. By regression of the uptake curves with eq 1, the diffusion constant (DM/rc2, s−1) for CO2 adsorption is calculated to be ∼7.1 × 10−4 s−1 for the initial 90% of CO2 sorption, and ∼7.9 × 10−5 s−1 for the initial 80−90% of CO2 desorption. In the literature, the diffusion constant of some sorbents for CO2 sorption at 25 °C were reported, that is, 1.3 × 10−3 s−1 for MOF-5,50 and 3 × 10−3 s−1 for chromium terephthalate MIL101,51 and 8.1 × 10−3 s−1 for a N-doped template carbon,45 etc. It should be mentioned that the diffusion rate for CO2 sorption/desorption decreased significantly at a later close-toequilibrium stage, that is, ∼2.3 × 10−6 s−1 for CO2 sorption, and ∼1.2 × 10−5 s−1 for CO2 desorption, which can be attributed to the great diffusion barrier to reach certain amine sites in the TEPA(50)/Ben_H2SO4_6M sorbent. The thermal stabilities of Ben_H2SO4_6M and TEPA(50)/ Ben_H2SO4_6M were also investigated in this work. Figure 8

improved (to as high as 0.77 cc/g from 0.15 cc/g). With a maximal TEPA loading of 50 wt %, the CO2 breakthrough sorption capacity can reach as high as 130 mg of CO2/g of sorbent with the amine efficiency of 0.23 mmol of CO2/mmol of amine at 75 °C under a dry condition. With moisture addition in the fuel gas, the CO2 breakthrough sorption capacity can be further improved to 190 mg of CO2 at 18 vol% of moisture addition sorbent with the amine efficiency of 0.29 mmol of CO2/mmol of amine. Moreover, the TEPA/ Ben_H2SO4_6M sorbents show a good regenerability in 10 sorption−desorption cycles, and a good thermal stability below 130 °C. The high CO2 sorption capacity, positive effect of moisture, and low capital cost of the raw bentonite materials suggest that TEPA/Ben_H2SO4_6M could be a promising cost-effective sorbent for CO2 capture from flue gas. The sulfuric acid treatment was demonstrated as an effective method for bentonite modification to immobilize TEPA for CO2 capture.

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ASSOCIATED CONTENT

* Supporting Information S

Textural properties with variance of the parent bentonite and the modified bentonites. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (J.D.), [email protected] (J.X.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are pleased to acknowledge the support by the NSFCGuangdong Joint Fund Project (U1034005), the National Natural Science Foundation of China (51106185), and the National Basic Research Program of China (2012CB720404).

Figure 8. Thermal stability of the Bentonite_H2SO4_6M, TEPA, and TEPA/Ben_H2SO4_6M.

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shows the weight loss of the samples from room temperature to 800 °C. It can be observed that the Ben_H2SO4_6M sample was quite stable at the temperature less than 800 °C. The sorbent weight only decreased by 3.5 wt %, possibly due to the water evaporation in the network. A weight loss on unsupported TEPA was observed at 120 °C, and it was further decomposed completely at 250 °C. For TEPA (50)/ Ben_H2SO4_6M, a sharp weight loss was observed between 130 and 300 °C, which should be also due to the decomposition of TEPA. It should be mentioned here that, compared to unsupported TEPA, the Ben_H2SO4_6M supported one showed an increased initial decomposition temperature by 10 °C, indicating that the Ben_H2SO4_6M support had a positive effect on the thermal stability of TEPA. The results further suggested the presence of the amine-philic sites on the Ben_H2SO4_6M support to stabilize the supported TEPA. Overall, the TEPA (50)/Ben_H2SO4_6M sorbent should be thermally stable below 120 °C.

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