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Feb 24, 2013 - Mesocellular silica foam (MCF) materials with different pore volumes were prepared and modified with tetraethylenepentamine (TEPA) as ...
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Tetraethylenepentamine-Modified Siliceous Mesocellular Foam (MCF) for CO2 Capture Xingxing Feng,† Gengshen Hu,*,† Xin Hu,‡ Guanqun Xie,† Yunlong Xie,† Jiqing Lu,† and Mengfei Luo*,† †

Key Laboratory of the Ministry of Education for Advanced Catalysis Materials, Institute of Physical Chemistry, Zhejiang Normal University, Jinhua 321004, PR China ‡ College of Chemistry and Life Sciences, Zhejiang Normal University, Jinhua 321004, PR China ABSTRACT: Mesocellular silica foam (MCF) materials with different pore volumes were prepared and modified with tetraethylenepentamine (TEPA) as sorbents for CO2 capture. The as-prepared sorbents were characterized by XRD, TEM, SEM, nitrogen adsorption/desorption, and FTIR. CO2 capture performances of the adsorbents were tested in a fixed-bed reactor equipped with an online MS. The results indicated that the pore volume of supports has great effect on the CO2 capture performance. With the increasing of pore volume as well as the window and cell sizes of the MCF, more TEPA can be loaded into the pores of MCF. For MCF with larger pore volume, more unoccupied space is left after the same amount of TEPA was loaded into the pores. The unoccupied space is beneficial for higher CO2 uptake because the mass transfer limitation can be reduced to some extent and the interaction between CO2 and TEPA may be easier. MCF material with largest pore volume exhibited the largest CO2 uptake of 4.34 mmol/g of adsorbent with a 70 wt % TEPA loading tested by the fixed-bed reactor and at least 4.57 mmol/g tested by thermogravimetric analysis (TGA) under the conditions of 10.0% (v/v) CO2 in N2 at 75 °C. Repeated adsorption/desorption cycles revealed that its high CO2 capacity can be regenerated via temperature swing adsorption and so it may be useful for CO2 capture via TEPA functionalized MCFs. 48,20 SBA-15,21−23 SBA-12,24 SBA-16,25 and hexagonal mesoporous silica (HMS),26,27 to improve CO2 adsorption capacity and selectivity. It has been demonstrated that the structures of the supports play a crucial role in sorbent performance. In these solid-supported amine sorbents, the interaction between the sorbent and CO2 increased greatly due to the specific CO2−amine chemistry and thus resulted in high sorption capacity, high selectivity, and reversibility of these sorbents for CO2 capture. Because of these advantages, intensive efforts have been directed toward developing novel supports with optimal structure to further improve the CO2 adsorption capacity. Generally speaking, large pore size, large pore volume, and good pore interconnection tend to improve the sorbent’s CO2 capture capacity. Mesocellular siliceous foam (MCF) is a mesoporous material that consists of windows and cells.28 The windows interconnect the spherical cells and form a continuous three-dimensional (3D) pore system.29 The interconnected nature of the large uniform pores makes MCF a promising candidate of catalysts support and in separation involving large molecules.30 Thus, it was anticipated that MCF could accommodate a greater loading of amine relative to other mesoporous substrates by virtue of its larger pore volume and thus give better performance for CO2 adsorption.29,31−34 For example, Liu et al.34 reported a highly efficient and stable solid adsorbent for CO2 capture by direct incorporation of tetraethylenepentamine (TEPA) onto the assynthesized mesocelullar silica foam (MCF). These organic-

1. INTRODUCTION At present, atmospheric concentration of carbon dioxide (CO2) due to fossil combustion has drawn significant attention because of its anthropogenic contributors to climate change. As a consequence, more and more researchers are devoted to developing more efficient and improved processes for CO2 capture. A wide range of approaches, including amine solutions,1−4 membranes,5,6 and solid sorbents7−9 have been designed to capture CO2. The solutions of amines (such as monoethanolamine, methyldiethanolamine, and diethanolamine) are currently dominant technology for CO2 separation from flue gas. Unfortunately, the use of amine solutions results in large energy consumption, solvent loss, and corrosion of equipment.10 Therefore, porous solids such as zeolites11,12 or active carbon13 are good candidates for capturing CO2 through physical adsorption. However, the disadvantages associated with the use of zeolites or actives carbon are their relatively low CO2/N2 selectivity and significant degradation in CO2 capture capacity when water vapor is present in the flue gas.14 Moreover, high energy input required to regenerate the adsorbent is also a serious drawback. Amine-grafted silica was first proposed by Burwell and Leal for SO2 capture.15 Since then, amine-impregnated porous supports for CO2 capture have been widely reported.16,17 In 2002, Song and co-workers18 reported amine-impregnated mesoporous silica (polyethyleneimine supported on MCM-41) for CO2 capture, and they achieved a CO2 adsorption capacity of 3 mmol/g at 75 °C in pure CO2 gas. This type of new adsorbent is termed as “molecular basket” to capture CO2. In addition, recent studies have been conducted on various amine modified mesoporous materials, such as MCM-41,19 MCM© 2013 American Chemical Society

Received: Revised: Accepted: Published: 4221

July 23, 2012 February 23, 2013 February 24, 2013 February 24, 2013 dx.doi.org/10.1021/ie301946p | Ind. Eng. Chem. Res. 2013, 52, 4221−4228

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2.4. Characterization. Structural characteristics of the pristine silica support and the amine-functionalized adsorbents were collected via nitrogen physisorption analysis at 77 K using Quantachrome NOVA 4000e. The MCFs and TEPA-functionalized MCFs sorbents had been degassed prior to each measurement under high vacuum at 180 and 50 °C, respectively, for at least 4 h. The surface area was calculated using the multipoint Brunauer−Emmett−Teller (BET) method. The total pore volume (Vt) was determined as the volume of liquid nitrogen adsorbed at a relative pressure of 0.99. Cell sizes (Dc) and window sizes (Dw) of MCF were derived from the adsorption and desorption branches of the isotherms, respectively, according to the BJH method. Both Dc and Dw are defined as positions of the maxima on the pore size distribution curves. In addition, transmission electron microscopy (TEM) experiments were carried out on a JEOL-2100F transmission electron microscopy operated at 200 KV. Scanning electron microscopy (SEM) was performed using a Hitachi S-4800 field emission scanning electron microscope. Thermogravimetric analysis (TGA) was conducted on a NETZSCH STA 449C thermal graphic analyzer to estimate the TEPA loading on MCF. Attenuated total reflection infrared (ATR-IR) spectrum of liquid TEPA and transmission IR spectra of MCF and MCFTEPA were acquired on a Nicolet 670 FTIR spectrometer. To be close to the real conditions of flue-gas, the CO2 capture capacity of the adsorbent was determined by a fixedbed reactor system as shown in Figure 1. The adsorption

template occluded mesoporous silica−amine composites exhibited remarkably high CO2 uptake with the maximum value of 4.5 mmol/g at 348 K and 1 atm. Qi et al.33 developed silica foams with ultralarge mesopores for high-efficiency supported amine sorbents. The sorbents exhibited fast CO2 adsorption−desorption kinetics, and high adsorption capacity of up to 5.8 mmol g−1 under 1 atm of dry CO2. In this work, CO2 adsorbents based on MCF materials with different pore volumes and different TEPA loadings were developed and tested for CO2 capture. The results indicate that the pore volume of the support is one key factor to determine the CO2 adsorption capacity of the amine-impregnated composite sorbents. The support with larger pore volume leads to a higher amount of unoccupied space after the same amine loading, which can decrease the diffusion resistance of CO2 in pores and thus is beneficial for CO2 uptake.

2. EXPERIMENTAL SECTION 2.1. Materials. All materials were used as received from the chemical vendors without further purification. Distilled water was used in all experiments. Triblock copolymer P123 (EO20− PO70−EO20, Mv = 5800) were obtained form Sigma-Aldrich. 1,3,5-trimethylbenzene (TMB), ethyl silicate (TEOS), ammonium fluoride (NH4F), and ethanol were purchased form Sinopharm Chemical Reagent Co., Ltd. Hydrochloric acid (HCl) was supplied from Quzhou Reagent Juhua Co., Ltd. Tetraethylenepentamine (TEPA) was purchased from Chengdu Kelon Chemical Reagent Co., Ltd. 2.2. Preparation of Siliceous Mesocellular Foam (MCF). MCFs with different pore volumes, as well as different cell and window sizes, were synthesized based on the method reported by Schmidt-Winkel et al.28 In a typical synthesis, 2.0 g of P123 was dissolved in an acidic HCl solution (1.6 M, 75 mL) at room temperature and then swelling agent TMB was added. To control the pore structures of MCF materials, different amounts of TMB were added while the amount of P123 was kept constant. The resulting solution was stirred at 40 °C for 1 h, followed by the addition of 4.4 g of TEOS under vigorous stirring for an additional 5 min. The mixture was then kept quiescent for 20 h in an oven at 40 °C. If desired, 23 mg NH4F was added and the milky mixture was transferred into an autoclave and kept at 100 °C for 24 h under static conditions. The as-synthesized samples were recovered by filtration and dried at room temperature. The organic template was removed by calcination in air at 550 °C for 6 h before further use. The calcinated samples were denoted as MCF-x, where “x” indicates MCF with different pore volumes. In this study, the pore volumes of three MCF materials determined by N2 adsorption/ desorption experiments were 1.54, 1.82, and 2.04 cm3/g, respectively. 2.3. Preparation of TEPA-Impregnated MCF Adsorbents. TEPA-functionalized MCFs were prepared by wet impregnation. In a typical preparation, the desired amount of TEPA was dissolved in 10.0 g of ethanol under stirring for 30 min, and then 0.5 g of calcined MCF with different pore structures was added to the mixture. The resulting slurry was stirred and refluxed at 80 °C for 2 h, and then dried at 70 °C until complete volatilization of ethanol. The obtained samples were designated as MCF-x-y, with y being the weight percentage of TEPA introduced on MCF. For example, when MCF with a pore volume of 2.04 cm3/g was used as the support and loaded with 70 wt % TEPA, the obtained sample was named MCF-2.04-70.

Figure 1. Schematic diagram of the fixed-bed reactor system.

process was operated at atmospheric pressure and the outlet gases were analyzed by online mass spectrometry (MS, OminiStar 200). In the reactor, 0.50 g of dried sorbent was packed into the middle of the quartz-tube reactor (6 mm inner diameter) heated by heating tape. Prior to each adsorption measurement, the sorbent was first activated by heating to 100 °C and kept for 1 h in He stream at a flow rate of 20 mL/min. After cooling to the desired adsorption temperature (75 °C), 10% CO2 balanced with N2 stream at a total flow rate of 10 mL/min was introduced and passed through the adsorbent bed until adsorbent saturation was reached. For comparison, the CO2 uptake value was also investigated by thermogravimetric analysis method. The experimental conditions of thermogravimetric analysis were identical to those of the breakthrough experiments except the amount of sorbent used every time. Around 20 mg of dried sorbent was placed into the furnace of TGA. Prior to each adsorption measurement, the adsorbent was first activated by heating to 100 °C and kept for 1 h in He stream at a flow rate of 20 mL/ min. After being cooled to the desired adsorption temperature (75 °C), 10% CO2 balanced with N2 stream at a total flow rate 4222

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of 10 mL/min was introduced and passed through the adsorbent bed for 150 min. In the multiple adsorption−desorption cycles test, the sample was initially activated at 100 °C for 1 h in He. Then the temperature was cooled to 75 °C, and CO2 sorption was performed at this temperature under 10% CO2/N2 flow (10 mL/min) for 100 min. After that, the sorbent regeneration was carried out by flowing He (20 mL/min) through the bed at 100 °C for 100 min. The same sorption−desorption procedure was conducted for 8 cycles to test the cyclic stability of the silicasupported amines.

3. RESULTS AND DISCUSSION 3.1. Characterization of MCF. Figure 2 shows SEM and TEM images of MCF-2.04 before TEPA loading. The as-

Figure 3. Nitrogen adsorption−desorption isotherms of (a) MCF samples with different window size and (b) their functionalized products. For sorbents MCF-x-y, “x” are the pore volumes of MCFs and “y” are the TEPA loadings.

structure.28 As shown in Figure 3a, MCF-1.54 and MCF-2.04 possess the smallest and largest pore volume, respectively. After loading of 60 wt % TEPA, most of the mesopores were filled with TEPA, but the adsorption isotherms of all hybrid materials maintained the typical type IV isotherms (Figure 3b). Figure 4 shows pore size distribution for three MCF materials. The average cell and window sizes of MCF-1.54 are 18.6 and 9.1 nm, respectively. MCF-2.04 has the largest average cell and window sizes of 27.0 and 12.6 nm, respectively, in line with its largest pore volume of all the samples. Table 1 presents a summary of the corresponding textural properties of the samples prepared in this work. MCF-1.54-60 sorbent, which was prepared by loading 60% TEPA on MCF with a pore volume of 1.54 cm3/g, is almost filled up and only 0.01 cm3/g of pore volume is left. On the other hand, after loading of 60 wt % MCF-2.04-60 still has the pore volume of 0.05 cm3/g, which is consistent with the larger pore volume of MCF-2.04 than MCF-1.54. The larger pore volume may make gas diffusion easier for the amine-impregnated adsorbent. The amount of TEPA present in MCF by impregnation method can be estimated from the weight loss of the materials determined by TGA. Figure 5 shows the weigh losses of MCF2.04 and MCF-2.04-70 with increasing the furnace temperature. For MCF-2.04-70 which was prepared by loading 70% TEPA on MCF with a pore volume of 2.04 cm3/g, a small weight loss of about 9.5% was observed between 20 and 120 °C, which could be attributed to the removal of moisture and preadsorbed CO2 and ethanol which can not be completely removed during the preparation of sorbent. A more rapid decrease in mass was observed in the temperature range of 120−500 °C while no

Figure 2. SEM and TEM images of MCF with a pore volume of 2.04 cm3/g (MCF-2.04).

prepared MCF is in connected-ball shape with a diameter from 10 to 30 um. TEM image indicates that MCF consists of spherical voids interconnected by “windows” indicating the phase transition, which is different from ordered mesoporous silica SBA-15 comprising a hexagonally packed arrangement of cylindrical pores.35 Disordered pores in MCF are clearly visible, which is the characteristic morphological feature of MCF. Figure 3a and b show the nitrogen adsorption−desorption isotherms of MCFs with different window sizes and cell diameters before and after TEPA loading. The isotherms are of type IV and show steep hysteresis of type H1 at high relative pressures, which strongly support the proposed MCF pore 4223

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The weight loss in this range is around 67.2%, which is in accordance with the designed amine doping level. Above 500 °C no further mass change was observed indicating the outstanding thermal stability of the mesoporous silica support. To confirm the TEPA has been impregnated onto MCF or loaded into the pores of MCF, infrared spectra of MCF, TEPA, and MCF-2.04-70 were recorded. Figure 6 shows infrared

Figure 4. Pore size distributions of MCF-1.54, MCF-1.82, and MCF2.04. Herein, 1.54, 1.82, and 2.04 are the pore volume values of MCFs.

Figure 6. ATR-IR Spectra of TEPA and transmission IR spectra of MCF-2.04 and of MCF-2.04-70.

Table 1. Textural Properties for MCF Materials before and after TEPA Modification samplea

cell diameter (nm)b

window diameter (nm)c

pore volume (cm3/g)d

surface area (m2/g)e

MCF-1.54 MCF-1.82 MCF-2.04 MCF-1.54-60 MCF-1.82-60 MCF-2.04-60 MCF-2.04-70

18.6 23.7 27.0 3.2 25.3 27.3 25.1

9.1 11.4 12.6 3.0 9.5 14.5 12.1

1.54 1.82 2.04 0.01 0.03 0.05 0.01

844.0 788.3 658.6 2.7 5.1 8.9 1.8

spectra of composite sorbent together with those of TEPA and MCF for comparison. As shown in the figure, MCF shows strong IR bands at 1090 and 950 cm−1, which are assigned to Si−O−Si and Si−O vibrations, respectively. For pure TEPA, the bands at 3282 and 1599 cm−1 correspond to NH stretching and bending vibrations, while those at 2933, 2814, and 1456 cm−1 can be due to CH stretching and bending vibrations, respectively. As expected, after loading with TEPA, MCF− TEPA shows some characteristic peaks of TEPA, such as the peaks at 2945, 2983, 1568, and 1475 cm−1, indicating that TEPA has been impregnated onto support surface or into internal channel. More interestingly, the C−H bands of TEPA show a blue shift compared to that of free TEPA, while the N− H bands show a red shift. This reveals the interaction between TEPA and MCF for the amine-impregnated sorbent. It should be noted that when we tried to synthesize MCF-1.84-75, gellike instead of powder sample was obtained, which is because the amine loading exceeded the pore saturation and the additional guests were deposited on the external surface of the adsorbent. While for the serials of MCF-x-70, all the samples show the texture of well-separated powders indicting that TEPA is mainly located inside the pores of MCFs. 3.2. CO2 Adsorption Performances of TEPA-Loaded MCFs. To determine the optimal CO2 sorption temperature, the effect of sorption temperature on CO2 capture capacity was investigated and the results are shown in Figure 7. As can be observed in the figure, the breakthrough times for 0.5 g of MCF-2.04-70 at 25, 50, 75, and 90 °C are 28.6, 36.3, 44.6, and 38.2 min, respectively. And the total CO2 uptakes after sorption equilibrium are calculated to be 2.91, 3.60, 4.34, and 4.06 mmol/g, respectively. These results indicate that CO2 uptake increases with increasing temperature from 25 to 75 °C, further increasing temperature leads to lower CO2 adsorption capacity. As reported previously, 75 °C is the optimum adsorption temperature for most amine-functionalized sorbents.36−41 The effect of temperature on CO2 uptake is a compromise between

The samples are denoted MCF-x-y, where “x” are the pore volumes of MCFs, “y” are the TEPA loadings. bDiameter of cells, determined from adsorption branch according to BJH method. cDiameter of windows, determined from desorption branch according to BJH method. dTotal pore volume, p/p0 = 0.99. eSurface area was calculated using the BET method, p/p0 = 0.05−0.2. a

Figure 5. TG curves of MCF-2.04 and MCF-2.04-70 heated at a rate of 10 °C/min from 25 to 800 °C.

obvious weight loss was observed for MCF-2.04, indicating the removal of the impregnated TEPA molecules loaded on MCF. 4224

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breakthrough curves for these composite adsorbents decrease with increasing TEPA loadings. i.e., the order is MCF-2.04-60 > MCF-2.04-70 > MCF-2.04-75. This can be attributed to the mass transfer limitation caused by diffusion resistance when excessive TEPA was deposited on MCF-2.04. This phenomenon is consistent with the results from the previous studies.42 To study the effect of pore volume of the support on CO2 capture capacity, CO2 breakthrough curves of MCF-1.54-70, MCF-1.82-70, and MCF-2.04-70 were measured and are shown in Figure 9. It can be seen in Figure 9 that CO2 breakthrough

Figure 9. CO2 sorption capacity of MCF samples after TEPA loading of 70 wt % (a) MCF-1.54-70, (b) MCF-1.82-70, and (c) MCF-2.0470.

points for both MCF-1.54-70 and MCF-1.82-70 are at around 28 min and their total CO2 uptakes are 3.20 and 3.30 mmol/g, respectively, which is much lower than 4.34 mmol/g on MCF2.04-70 indicating that CO2 capture capacity of amine-modified MCF is directly related to the pore volume of the pristine MCF support. Table 2 summarizes the CO2 adsorption capacity of

Figure 7. CO2 uptakes of MCF-2.04-70 at different temperatures under 10% CO2/N2.

diffusion and thermal dynamics. Thus, 75 °C is selected as the adsorption temperature in the following study. Figure 8 shows the CO2 breakthrough curves of MCF-2.04 with various TEPA loadings. For MCF-2.04-60, the CO2

Table 2. Total CO2 Uptakes of MCF-1.54, MCF-1.82, and MCF-2.04 with Different TEPA Loadings CO2 uptake (mmol-CO2/g-adsorbent) samplea MCF1.54 MCF1.82 MCF2.04

theoretical maximum TEPA loadingb

60% TEPA loading

70% TEPA loading

60.6%

3.46

3.20

64.5%

3.63

3.30

67.1%

3.98

4.34

75% TEPA loading

3.64

The samples are denoted as MCF-x-y, where “x” are the pore volumes of MCFs and “y” are the TEPA loadings. bTheoretical maximum TEPA loading calculated based on the pore volume and TEPA density. a

Figure 8. CO2 adsorption performance of MCF-2.04-60, MCF-2.0470, and MCF-2.04-75 under 10%CO2/N2 at 75 °C.

MCFs with different pore structures and different TEPA loadings. As shown in the table, generally, the MCF based sorbents show high CO2 adsorption performance, larger than 3.20 mmol/g. For sorbents with 70% TEPA loading, when the pore volume of the substrate increased from 1.54 to 1.82 cm3/ g, there was only a slight increase in CO2 capture capacity from 3.20 to 3.30 mmol/g. However, with further increasing pore volume to 2.04 cm3/g, the CO2 capture capacity increased dramatically to 4.34 mmol/g. From these results, it can be concluded that the pore volume of support was a key factor in determining the CO2 capture capacity so that higher CO2 uptake is expected to be obtained by increasing the pore

breakthrough time is at around 42 min and the total CO2 uptake is 3.98 mmol/g. With increasing the TEPA loading to 70%, the CO2 breakthrough time increases to around 45 min and the total CO2 uptakes is 4.34 mmol/g, indicating that higher CO2 uptake can be obtained by increasing the TEPA loading. However, further increasing the TEPA loading to 75% leads to a decrease of breakthrough time and the total CO2 uptakes, which is only 3.64 mmol/g, suggesting that excessive amount of amine is detrimental to CO2 adsorption capacity. More interestingly, as shown in Figure 8, the slopes of 4225

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Table 3. Comparisons of the Performance of Amine-Impregnated MCFs amines

loading amount (wt %)

test conditions

method of testing

PEI

70

PEI PEI PEI

50 60 50

50% CO2/Ar; 105 °C; 70 mL/min; 20 min adsorption time 15.1% (v/v) of CO2 in N2; 10 mL/min; 75 °C 15.1% (v/v) of CO2 in N2; 10 mL/min; 75 °C Pure CO2 stream; 100 mL/min; 1 atm; 75 °C

PEI TEPA

80 70

pure CO2; 40 mL/min; 75 °C 10.0% CO2/N2; 10 mL/min; 75 °C

volume of MCF. It could be inferred that bulky TEPA can be more easily introduced to the interior pores when the pore volume of the mesoporous support increases and thus there are more active sites for CO2 adsorption. Furthermore, based on the data shown in Tables 1 and 2, enhanced CO2−TEPA interaction is achieved when more porosity is still retained after TEPA loading. This can be confirmed by the slopes of breakthrough curves as shown in Figure 8. Larger slope means less mass transfer resistance. Therefore, it can be concluded that less unoccupied space will lead to stronger mass transfer resistance and therefore lead to a lower CO2 capture efficiency. Table 3 summarizes the performances of a number of amines-impregnated MCFs reported previously.29,31−33 Subagyono et al.29 prepared PEI-modified MCF for CO2 capture. They found that the highest CO2 uptake was obtained on PEImodified MCF with the largest pore volume. In another study, Yan et al.31 also found larger pore volume is beneficial to higher CO2 uptake, which is consistent with our results. Zhao et al.32 found that MCF modified with lower molecular weight PEI has higher CO2 capture capacity, which can explain that our TEPAimpregnated MCFs has higher CO2 capture capacity43 than PEI-modified MCFs because the molecular weight of TEPA is smaller than that of PEI and has higher N amount per gram. In a more recent study, Qi et al.33 achieved a CO2 uptake of 5.8 mmol/g using a MCF with pore volume of around 1.17 cm3/g in the pure CO2 stream. They prepared MCF-based adsorbent with 80% PEI loading, while in our study MCF-1.54-75 and MCF-1.82-75 cannot be prepared due to the limited pore volumes of MCF-1.54 and MCF-1.82 as shown in Table 2. In most studies, thermogravimetric analysis (TGA) is used to determine the CO2 capture capacities. To compare the CO2 uptake results obtained by our fixed-bed reactor method and TGA method, TGA method is also used to determine the CO2 capture capacities. As shown in Figure 10, the CO2 adsorption capacity is 20.1 wt % per gram of adsorbent, which is equal to 4.57 mmol/g at the same testing conditions as the breakthrough experiments. Also, as shown in figure, the full adsorption capability has not been reached. Comparing the results obtained through two methods, it can be found that the CO2 capture capacities determined using TGA method are larger than that determined using fixed bed method. Three possible reasons may account for the fact that CO2 capacity determined by TGA is larger than that determined by the fixed bed reactor: first, the CO2 capacity determined by TGA method is calculated based on the mass of the sorbent after being treated at 100 °C for 1 h to remove the solvent which can not be completely removed during the preparation of sorbents and other gases adsorbed on the sorbent (as shown in Figure 5 in the temperature range of 25−100 °C), while the CO2

adsorption capacity (mmol/g)

ref

TGA

3.43

29

GC GC Rubotherm suspension balance TGA MS TGA

3.45 4.04 4.11

31 31 32

5.8 4.34 4.57

33 this work

Figure 10. Adsorption and desorption curves for MCF with 70% TEPA loading at 75 °C measured by thermogravimetric analysis (TGA).

capacity determined by fixed bed reactor method is calculated based on the original mass of the sorbent; second, the N2−CO2 mixture was fed into a compact space and N2 in the mixture may take out some CO2 that has been captured by the sorbent; third, the TGA results may include the adsorption of trace moisture or other gases in the mixture which leads to larger uptake value. 3.3. Cyclic Adsorption−Desorption Studies. The regenerability of the sorbent was also investigated. The MCF2.04-70 was selected to further study its cyclic CO2 sorption capacity due to its largest adsorption capacity. Figure 11 shows the cyclic adsorption measurement results of MCF-2.04-70 under the procedure described in the experimental section using the breakthrough system as shown in Figure 1. In cycle 1, MCF-2.04-70 showed CO2 capture capacity of 4.04 mmol/g, which is reduced by 7% compared with the aforementioned 4.34 mmol/g (Table 2). The decrease of CO2 uptake is due to

Figure 11. CO2 uptake of MCF-2.04-70 under 10% CO2/N2 at 75 °C as affected by the number of adsorption−desorption cycles. 4226

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the shortening of adsorption time from 150 to 100 min.39 In addition, in cycle 2, 3.90 mmol/g was obtained which is about 4% less with respect to cycle 1. This may be explained by the strong interaction between TEPA and CO2 and the incomplete release of adsorbed CO2 at 100 °C for 100 min. After this, MCF-2.04-70 keeps a constant CO2 uptake from cycle 2 to cycle 8, which further confirms this hypothesis that not all preadsorbed CO2 was released during desorption process. However, it is still apparent from the foregoing results that MCF-2.04-70 not only displays high adsorption capacity of CO2 but also shows a stable performance in the prolonged cyclic operation.

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4. CONCLUSIONS In summary, CO2 sorbents based on mesocellular siliceous foam (MCF) modified with TEPA were prepared and were tested for CO2 capture by using a fixed-bed reactor. It was found that the pore volume of pristine MCF has significant effect on CO2 capture performance of the sorbents. The larger pore volume may be beneficial for the gas transfer in the pores of MCF and therefore favor the higher CO2 uptake of the adsorbents. The largest CO2 uptake of 4.34 and 4.57 mmol/g determined by the fixed-bed reactor and TGA, respectively, was achieved on MCF-2.04 with 70 wt % TEPA loading using 10.0% (v/v) CO2 in N2 at 75 °C. It was also found that the sorbents exhibited good stability after 8 adsorption−desorption cycles.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected], [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the financial support of this work from Zhejiang Normal University (ZC304011016 and KYJ06Y11018), Education Department (Y201121744) and Personal Department (ZC304012002) of Zhejiang Province, and National Natural Science Foundation of China (21203167, 21173194 and 21106136).

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REFERENCES

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