Carbon Dioxide Capture by Amine-Impregnated Mesocellular-Foam

Article pubs.acs.org/IECR

Carbon Dioxide Capture by Amine-Impregnated MesocellularFoam-Containing Template Wei Yan, Jing Tang, Zijun Bian, Jun Hu,* and Honglai Liu State Key Laboratory of Chemical Engineering and Department of Chemistry, East China University of Science and Technology, Shanghai, 200237, China S Supporting Information *

ABSTRACT: By impregnating polyethylenimine (PEI) into silica mesocellular foam with the template remaining (MCF(a)), a novel sorbent with both high CO2 adsorption capacity and high thermal stability was obtained. The remaining P123 template in the MCF played a great role in promoting the CO2 adsorption capacity, which could be 4.5 mmol·g−1 (adsorbent) when the amount of amine loading and the adsorption temperature were optimized as 60% and at 70 °C for the sample MCF(a)/PEI. Meanwhile, MCF(a)/PEI had a high thermal stability and selectivity, after 10 adsorption−desorption cycles, MCF(a)/PEI almost held a constant adsorption capacity; for different compositions of CO2 and N2 mixed gases, it always kept a high adsorption selectivity of CO2/N2. The mechanism of the template synergistic effect was elucidated by the result of a secondorder rate law through CO2 adsorption kinetic studies. Moreover, as predicted by the Langmuir adsorption model with n = 2 (two active adsorption sites for one CO2 molecule), the adsorption enthalpy was calculated as about −85 kJ·mol−1, a value which belonged to typical chemical adsorption.

1. INTRODUCTION The increasing atmospheric CO2 concentration is considered as a main contributor to global climate change in the past century.1,2 CO2 emissions are mainly caused by fossil fuel combustion, and coal accounts for roughly 25% of the world energy supply and 40% of the carbon emissions, so the CO2 capture, usage, and storage (CCUS) from fossil fuel power plants must be aggressively pursued.3,4 Whether for secondary use or the geologic burial, CO2 capture is a necessary step.5,6 The costs of separation and capture are estimated to be about three-fourths of the total costs of CCUS in the economics of the process.7 Many efforts have been devoted to developing technologies for efficient CO2 capture. The current commercial technologies for CO2 capture from flue gas are based on absorption using liquid amines, most commonly monoethanolamine and diethanolamine (MEA and DEA).8−10 However, the problems of high regeneration energy, large equipment size, solvent degradation, and equipment corrosion make the process impractical for further applications.11,12 In an effort to overcome the disadvantage of the liquid amine-based process, research as focused on adsorption because of its low energy consumption and low equipment cost. A large number of adsorbents have been investigated for CO2 removal, including zeolites,13−15 activated carbons,16,17 and periodic mesoporous silicas,18 as well as metal organic frameworks (MOFs).19−22 In recent years, amine-functionalized adsorbents have also received increasing attention for their properties, such as high CO2 adsorption capacity, moisture tolerance,23 and easy regeneration. A wide variety of mesoporous silica materials such as MCM41,24−29 MCM-48,30−33 SBA-12,34 SBA-15,35−47 SBA-16,48 HMS,49,50 MCF,51−54 MUS-1,55 and KIT-656 have been investigated as the substrates of amine-functionalized adsorbents. The amine functionalizations can be carried out generally © 2012 American Chemical Society

in two approaches, chemical grafting and physical impregnation. With the latter, amine-functionalized adsorbents can load more amino groups, hence a higher adsorption capacity of CO2. But the impregnated amine would increase the diffusion resistance for CO2 molecules in the mesoporous materials, and also there is a serious leaching problem in the sorbents regeneration process.56 Song et al. prepared “molecular basket” adsorbents by modifying the mesoporous molecular sieve of MCM-41-type with polyethylenimine (PEI); the CO 2 adsorption capacity was 3.02 mmol·g−1(adsorbent) at 75 °C.24 Zhu et al. used the as-prepared mesoporous silica SBA-1538 and MCM-4129 as substrates and modified them with tetraethylenepentamine (TEPA). They found that the presence of surfactant increased the adsorption capacity of the materials; the pure CO2 adsorption capacity was up to 3.93 mmol·g−1 and 5.39 mmol·g−1, respectively. Mesocellular siliceous foam (MCF) is a kind of mesoporous material that consists of uniform cells and windows.57 The continuous three-dimensional (3D) pore system with the uniform windows interconnected to the spherical cells can enhance the amine loading amount and reduce the CO2 diffusion resistance. Liang et al. prepared a dendrimer-grafting MCF to capture CO2.51 Christopher et al. prepared three types of amino silica materials based on MCF supports and treated them under accelerated steaming conditions of steam/air and steam/nitrogen at various temperatures.54 Liu et al. investigated directly incorporating tetraethylenepentamine (TEPA) onto the assynthesized MCF without removing the organic templates, and the CO2 adsorption capacity was as high as 4.5 mmol·g−1.52 Alan et al. reported the sorbents by SBA-15 and MCF loaded with Received: Revised: Accepted: Published: 3653

September 13, 2011 December 22, 2011 February 9, 2012 February 9, 2012 dx.doi.org/10.1021/ie202093h | Ind. Eng. Chem. Res. 2012, 51, 3653−3662

Industrial & Engineering Chemistry Research

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(TEM) were taken on a JEOL JEM-2010. The thermal stability was investigated by using a thermogravimetric analysis (TGA) unit on NETZSCH STA 499 F3. About 5 mg of sample was heated from 20 to 600 °C at a heating rate of 5 °C·min−1 in nitrogen with a flow rate of 40 mL·min−1. 2.4. CO2 Adsorption Measurement. The thermogravimetric analysis (TGA, NETZSCH STA 499 F3) was used for CO2 adsorption/desorption measurement. The weight change of the adsorbent was monitored to show the adsorption and the desorption performance. As shown in Figure 1, in a typical

linear polyethyleneimine (PEI) or branched PEI, separately, and the CO2 adsorption capacity for MCF loaded with branched PEI at 105 °C was as high as 3.43 mmol·g−1 (in 50% CO2/Ar, 20 min adsorption time).53 However, all of them seldom focused on the investigation of the stability or reproducibility, which is particularly important for further industrial applications. Meanwhile, the systematical investigations of adsorption thermodynamics and kinetics of CO2 adsorption on the multiple-amine functionalized MCF, which can reveal the CO2 adsorption process intrinsically, are still limited. In this work, we have prepared the sorbent by incorporating polyethylenimine (PEI) and tetraethylenepentamine (TEPA) into MCF with or without a surfactant template. The effect of a surfactant template on the adsorption capacity of materials was investigated. The adsorption process was discussed in both kinetic and thermodynamic ways. More importantly, the selectivity and stability of the sorbent were also investigated.

2. MATERIALS AND METHODS 2.1. Chemicals. The following materials were used: Pluronic P123 EO-PO-EO triblock copolymer(Aldrich), 1,3,5trimethylbenzene (TMB, 97%), tetraethyl orthosilicate (TEOS, ≥ 98%), poly(ethyleneimine) (PEI, Mn = 600, Alfa Aesar), methanol (≥99.5%), ethanol (≥99.7%), hydrochloric acid (36.0−38.0%), ammonium fluoride (NH4F, ≥ 96%), tetraethylenepentamine (TEPA, ≥ 90%), and polyethylene glycol (PEG, Mn = 300). 2.2. Preparation of Adsorbents. The mesoporous material MCF was synthesized based on the method reported by Winkel et al.57 Briefly, P123 (8.0 g) was dissolved in HCl (1.6 mol·L−1, 250 mL) at room temperature. After TMB (26.4 g) and NH4F (72 mg) were added, the mixture was heated to 40 ◦C, and stirred for 1 h, then, TEOS (14.52 g) was added. After being stirred at 40 ◦C for 24 h, the whole slurry was transferred into an autoclave and aged at 110 °C for 24 h under static conditions. The mixture was then filtered, washed with deionized water and ethanol, and then dried at 60 °C overnight to get as-synthesized MCF, which was denoted as MCF(a). The MCF(a) was calcined at 550 °C for 6 h to completely remove the template, which was denoted as MCF(c). A 3.0 g portion of MCF(a) was added into 150 mL of ethanol, and stirred for 2 h at 40 °C. The mixture was filtered and dried to prepare MCF(p), in which the surfactant was partially removal. Amine-modified MCF was prepared by the impregnation method. PEI (0.3−0.7 g) was dissolved in methanol (5 g) under stirring for 10 min, and then MCF(a) powder (0.7−0.3 g) was added into the solution. After being stirred for 2 h, the mixture was dried at 70 °C for 12 h to prepare MCF(a)/PEI-X, where X is the weight percentage of PEI in the sample, respectively. Similar to the above method, PEI was replaced by TEPA, then MCF(a)/TEPA-X was also prepared, respectively. PEI (0.5 g) was dissolved in methanol (5 g) under stirring for 10 min, and then MCF(c) and MCF(p) powders (0.5 g) were added into solution. After being stirred for 2 h, the mixtures were dried at 70 °C for 12 h to prepare MCF(c)/PEI50% and MCF(p)/PEI-50%, respectively. PEI (0.5 g) and PEG (0.2 g) were dissolved in 5 g of methanol under stirring for 10 min, and then MCF(c) (0.5 g) was added into the solution. After being stirred for 2 h, the mixture was dried at 70 °C for 12 h to get MCF(c)/PEI+PEG. 2.3. Characterizations of Adsorbents. The field emission scanning electron micrographs (FE-SEM) were taken on FEI Nova Nano SEM 30 series, while the transmission electron micrographs

Figure 1. () Weight and (---) temperature changes of a sample (MCF(a)/PEI-60%) in the adsorption/desorption cyclic processes. The four cycles were performed at four different adsorption temperatures of 90, 70, 50, and 30 °C, respectively.

adsorption/desorption process, a sample (about 15 mg) was placed in an alumina pan, was degassed first by heating to 100 °C in a N2 atmosphere at a flow rate of 60 mL·min−1, and held at that temperature until there was no weight change (about 30 min). Then the temperature was decreased to the specified testing temperature, such as 30, 50, 70, 90 °C, respectively. The testing gas mixture of CO2 and N2 at the flow rate of 40 mL·min−1 and 20 mL·min−1, respectively (CO2:N2 2:1), or that of CO2 of 10 mL·min−1 and N2 of 50 mL·min−1, respectively (CO2:N2 = 1:5), was introduced. After 1 h adsorption, the testing gas was switched into pure N2 at a flow rate of 60 mL·min−1, and the temperature was raised again to 100 °C at a heating rate of 5 °C·min−1 to perform desorption. The adsorption capacity was calculated by the sample weight change in this adsorption/desorption process. The adsorption apparatus (Micrometrics ASAP 2020) was also used for CO2 adsorption/desorption isotherm measurement at different CO2 pressures. The sample was degassed by vacuum for 10 h at 100 °C before measurement. The measuring temperature was set at 30, 50, and 70 °C, respectively.

3. RESULTS AND DISCUSSION 3.1. Characteristics of MCF/PEIs. The FE-SEM image of MCF(a)/PEI-50, as shown in Figure 2A, reveals that PEI has been impregnated into MCF(a) particles, which have not fully covered the surface. Meanwhile, MCF(a)/PEI-50% has the characteristics of an open-pore structure. As shown in the TEM image of Figure 2B, the average pore size is about 20−30 nm and the wall is relatively thin. The other materials, such as MCF(c)/PEI-50% and MCF(a)/TEPA-50% have similar pore structures and particle morphologies, and their SEM and TEM images are shown in the Supporting Information Figures S1 and S2. A comparison of the substrates of MCF(a) and MCF(c), 3654

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Figure 2. Images of MCF(c)/PEI-50%: (A) FESEM, (B) TEM.

Figure 3. The TG and DTG curves for (A) MCF(a), MCF(p) and (B) MCF(c)/PEI-50% and MCF(a)/PEI-50% with a temperature ramp of 5 °C/min in pure N2.

hinder the decomposition of the inside surfactants. From the TGA analysis, we can see the MCF/PEI-50% type adsorbents are thermally stable below 250 °C, which is valuable for future industrial application. 3.2. Template Contribution to the CO2 Adsorption Capacity. Figure 4 shows the CO2 adsorption capacity of

after loading amines, shows that the powders of MCF(c)s are much stickier than that of MCF(a). It is a contrast to the general rule that the calcination can provide more space for loading, suggesting the remaining surfactant template P123 can make amine transfer into the pores more easily because of the interaction between the amine and P123. The TGA analysis profiles of MCF(a) and MCF(p) are shown in Figure 3A. Each of them has a significant mass loss in the temperature range of 250−400 °C, with the loss peak at about 330 and 315 °C in their DTG curves, respectively. This mass loss was attributed to P123 surfactant decomposition, and the partial extraction of the template made the P123 decomposition temperature in MCF(p) be a little lower than that in MCF(a). Besides, the quantity of the mass loss in MCF(p) is quite smaller than that in MCF(a). Calculated by TGA curves, the surfactant content was estimated as about 40% and 18% in MCF (a) and MCF (p), respectively. After PEI was impregnated into MCF(c) and MCF(a), as shown in Figure 3B, the DTG curves of both MCF(c)/PEI50% and MCF(a)/PEI-50% show two mass loss peaks at 55 and 95 °C, respectively. The former was associated with the CO2 desorption, while the latter was attributed to the water loss. This gives clear evidence that MCFs/PEI is a type of adsorbents with good adsorption ability for CO2 and H2O even at room temperature. In each DTG curve of MCF(c)/PEI and MCF(a)/PEI, the onset of a significant mass loss is at about 250 °C, while the peak concentrates at about 310 °C, corresponding to the thermal decomposition of PEI. In addition, MCF(a)/PEI has an extra mass loss peak at about 390 °C, corresponding to the thermal decomposition of P123 surfactant. The higher surfactant decomposition temperature in MCF(a)/PEI-50% suggests that the impregnated PEI could

Figure 4. CO2 adsorption capacity of MCF(c)/PEI-50%, MCF(p)/ PEI-50%, and MCF(a)/PEI-50% MCF(c)/PEI+PEG at atmospheric pressure and different adsorption temperatures for CO2/N2 (2:1, v/v) mixture separation.

MCFs/PEI-50% and MCF(c)/PEI+PEG at different adsorption temperatures for CO2/N2 (2:1, v/v) mixture. When the surfactant template totally remains, the adsorption capacity of MCF(a)/PEI is much higher than that of the others. The template has a positive influence on the adsorption performance. 3655



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For the sorbent MCF(c)/PEI-50% where the amine group is the only functional group existing, the CO2 adsorption mechanism follows reaction 1, in which two mole of amines are required for the adsorption of one mole of CO2, − CO2 + 2R2NH → R2NH+ 2 + R2NCOO

(1)

However, for the sorbent MCF(a or p)/PEI, besides the amine functional groups, there are many coexisting ether groups in P123 that can act as other hydrogen ion receptors due to their strong H+ affinity. Then, the CO2 adsorption mechanism follows reaction 2, in which one mole of amine is need for the adsorption of one mole of CO2 CO2 + R′O‐ + R2NH → R′OH+‐ + R2NCOO−

(2)

So the CO2 adsorption capacity of the MCF(a or p)/PEI is obviously increased compared with that of MCF(c)/PEI-50%. The more the surfactant remains, the higher the adsorption capacity is. PEG is also a type of hydrogen ion acceptor which has a similar structure as P123. When PEG was added together with PEI to modify MCF(c), the CO2 adsorption capacity of MCF(c)/PEI+PEG was markedly improved compared to that of the MCF(c)/PEI. As shown in Figure 4, the CO2 adsorption capacity of MCF(c)/PEI+PEG at different adsorption temperatures of 30, 50, and 70 °C are 1.52, 2.42, and 2.90 mmol·g−1, respectively, which are almost twice that of MCF(c)/PEI-50% at 0.79, 1.26, and 1.86 mmol·g−1. This result confirms the above proposal of the synergistic contribution of the P123 template to the CO2 adsorption. Furthermore, the contribution of a similar amount of PEG is not as much as that of P123. As shown in Figure 4, without removing P123, corresponding to 0.2 g of P123 in 0.5 g of MCF(a), the CO2 adsorption capacity of MCF(a)/PEI-50% is 3.84 mmol·g−1 at 70 °C, much higher than that of MCF(c)/PEI+PEG, since the P123 template in the channels might still preserve the micelle structure which can act as a cage to enhance the CO2 adsorption. Mmeanwhile, the synergistic interaction between P123 and PEI made the PEI chains more dispersed, and hence more adsorption sites were exposed. When the amounts of impregnated PEI were about 50%, the CO2 adsorption capacity of all the samples increased with the adsorption temperature due to the similar reason of more active sites exposed. The fact that the surfactant template can play a synergism role in the adsorption is significant because the presence of surfactants not only improves the adsorption capacity but also omits the process of removing surfactants, which can save energy and time, and increase the yield and characteristics of the composite products. This will be particularly important for industrial application. 3.3. CO2 Adsorption Thermodynamics of MCF(a)/PEI. The effect of the CO2 pressure on the adsorption was investigated by CO2 adsorption isotherms of MCF(a)/PEI60% at various adsorption temperatures of 30, 50, and 70 °C, respectively. As shown in Figure 5, when the CO2 pressure is lower than 10 kPa, the adsorption capacity qp increases sharply with the pressure, then it approaches saturation with further increasing pressure. Take the isotherm at the adsorption temperature of 70 °C, for an example. When the CO2 pressure is only 10 kPa, the qp can be about 4 mmol·g−1. The content of CO2 in the real flue gases from power plants is usually about 10%, corresponding to the CO2 partial pressure of 10 kPa. So the high CO2 adsorption capacity at this low pressure is a really encouraging result.

Figure 5. CO2 adsorption isotherms of MCF(a)/PEI-60% at various adsorption temperatures of 30, 50, and 70 °C and the fitting lines by the Langmuir model, in which the dots are the experimental results and the lines are the fitting results by the Langmuir model with n equals 3, 2, and 1 (·-·-·, n = 3; , n = 2; ---, n = 1), respectively.

Sayari et al.58 proposed a successful semiempirical dual model to describe the CO2 adsorption on amine-functionalized mesoporous silica, in which the total amount of CO 2 adsorption can be divided into two contributions of the chemisorption and physisorption. However, for MCF(a)/PEIs adsorbents, such as MCF(a)/PEI- 60%, with P123 remaining and PEI loaded, the surface area was only 2.94 m2·g−1 left. Compared the CO2 adsorption capacity of MCF(a) and MCF(a)/PEI-60%, (as shown in S3 in the Supporting Information), the contribution of physisorption was so little that it can be ignored. So we simply adopted the Langmuir model to describe the chemical adsorption. Figure 5 shows the corresponding profiles predicted by the Langmuir model as expressed in the eq 3. θ=

qp qe

=

(bp)1/ n 1 + (bp)1/ n

or

qp =

(bp)1/ n 1 + (bp)1/ n

qe (3)

where θ is the surface coverage; qp is the adsorption capacity at a specific pressure p, b is a ratio of the rate constant of adsorption to that of desorption, which is related to the adsorption temperature; n is a parameter related to the adsorption mechanism, that n functional groups are necessary for adsorption of one CO2 molecule. When n equals 1, 2, 3, and so on, the adsorption follows first-order, second-order, and third-order rate law, respectively. The solid fitting lines with n = 2 are closer to the experimental points than the dash lines with n = 1, moreover, we can obtain a better fitting result at higher temperature than at lower temperature. This suggests that the chemical adsorption is dominant at higher temperature, and the adsorption is more likely a second-order dynamic process that two functional groups are necessary for the adsorption of one CO2 molecule; at lower temperature, the physical adsorption may also make some contributions, multipoints adsorption occurs, the model will fit the data at higher values of n, such as n = 3 at 50 °C, n = 4 at 30 °C. The CO2 adsorption at MCF(a)/PEI-60% is mainly monolayer chemical adsorption, the equilibrium adsorption 3656



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capacity depends more on the number of effective adsorption sites. Both the experimental and the fitting results show that the saturated qe increases with the temperature obviously. As listed in Table 1, for n = 2, the saturated qe increases from Table 1. Fitting Parameters of the CO2 Adsorption on MCF(a)/PEI-60% by the Langmuir Model n=1 n=2

qe (mmol·g−1) b qe (mmol·g−1) b

30 °C

50 °C

70 °C

3.22 250.00 3.27 142.56

3.71 33.83 3.86 22.18

4.34 2.89 4.85 1.56

3.27 mmol·g−1 at 30 °C to 4.85 mmol·g−1 at 70 °C. The reason may lie in that the impregnated PEI chains and P123 surfactant chains can move or rotate themselves more freely at higher temperature, which can provide more active sites for adsorption. Since adsorption is an exothermic process, the high temperature is more favorable for desorption. From Table 1, we can see that the parameter b decreases obviously with the increasing temperature. The amount of CO2 adsorption on pure PEI significantly decreases with the adsorption temperature (CO2 adsorption on pure PEI at 90, 70, 50, and 30 °C are shown in Figure S4 in the Supporting Information). Compared with the CO2 adsorption on MCF(a)/PEI-60% (Figure 1) at the same operation conditions, both the CO2 adsorption rate and capacity are much lower. Take the adsorption temperature at 70 °C for an example. The CO2 adsorption capacity of pure PEI is only 1.6 mmol·g−1. Plotting t/qt against t as predicted by the second-order rate law of eq 6, gives a curve that dparts a little from a straight line, and then the CO2 adsorption rate constant of PEI is estimated as about 0.081, much smaller than that of MCF(a)/PEIs. This comparison shows further evidence that P123 in MCF(a) played an important role in the dispersion of PEI, which increased the CO2 adsorption rate and capacity obviously. As we mentioned above, MCF(a) itself has only a very small adsorption capacity, so the excess of CO2 adsorption capacity comes from the synergic effect of P123. Moreover, the desorption of CO2 on pure PEI is more difficult, at the desorption temperature about 110 °C, CO2 can not be desorpted completely. The adsorption enthalpy ΔHq at a fixed adsorption capacity of q can be calculated by the Clausius−Clapeyron equation: ln pq =

ΔHq RT

+C

Figure 6. Variation of ln p with 1/T at different q, and the fitting plots by Clausius−Clapeyron equation when q = 1, 2, and 3 mmol·g−1 of MCF(a)/PEI-60%.

Scheme 1. The CO2 Adsorption Mechanism at the Different Adsorption Quantity States

MCF(a), while a few PEI chains also insert into the micelles. At the low adsorption quantity stage (A), the chemical adsorption mainly comes from the reaction between CO2 molecules and −NH2 groups of PEI at the top layer of modified surface (eq 1). With increasing adsorption quantity to stage B, the adsorptive CO2 molecules swell the space among PEI chains, and more active sites are exposed; moreover, CO2 molecules can transfer more deeply into the P123 micelles, Then, −O− groups of P123 in the inner layer also take part in the adsorption; hence reaction 2 dominates the chemical adsorption, which makes the adsorption enthalpy a little more negative. However, when the adsorption quantity increases to 3 mmol·g−1, the linear relationship between ln p and 1/T no longer exists. Because the easily accessible adsorption sites may have been occupied at that moment, the adsorption process became more complex, not only depending on the CO2 adsorption itself, but also depending on the interaction and arrangement between PEI and P123 chains. Since the latter is very dependent on the temperature, the overall adsorption enthalpy will be a function of temperature. When the adsorption temperature is higher, corresponding to lower 1/T, there are enough active sites for higher adsorption capacity and the plot of ln p against 1/T has a similar trend. On the contrary, at higher 1/T, to obtain the same high adsorption quantity, additional energy may need to be supplied, and the overall adsorption enthalpy can then change into positive.

(4)

where pq is the pressure at the adsorption capacity q, C is a constant. As shown in Figure 6, the plots of ln p against 1/T are straight lines at the adsorption quantity of 1 and 2 mmol·g−1, and the corresponding adsorption enthalpies calculated by the slopes are −82.64 and −88.45 kJ·mol−1, respectively, which belong to the scope of the chemical adsorption. Generally, the absolute value of the adsorption enthalpy decreases with the increasing adsorption quantity due to the surface heterogeneity; in other words, the adsorption prefers those sites with higher activity, which results in larger absolute value of adsorption enthalpy. But for the adsorbent of MCF(a)/PEI-60%, the abnormal increase of the absolute value of adsorption enthalpy just confirms the adsorption mechanism appropriately. As shown in the Scheme 1, for MCF(a) with P123 template remaining, most of the impregnated PEI chains are usually at the outside surface of P123 micelles in the windows of 3657



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Figure 7. (A) Breakthrough curves of CO2 adsorption on MCF(a)/PEIs containing various amounts of PEI from 30% to 70% at 70 °C and atmospheric pressure with CO2/N2 (2:1, v/v) mixture. (B) Fitting plots of t/qt against t as predicted by the second-order rate law of eq 6.

Table 2. The Fitting Parameters of the Equilibrium Adsorption Capacity qe and the Rate Constants k by eq 6 PEI content (%)

30

40

50

60

70

saturated adsorption capacity qe (mmol·g−1) rate constant k ((mmol·g−1)−1 min−1) correlation coefficient R

2.77 1.32 0.99999

3.31 0.89 0.99999

4.14 0.33 0.99993

4.42 0.27 0.99989

3.15 0.17 0.99954

3.4. CO2 Adsorption Dynamics of MCF(a)/PEI. The breakthrough curves of the CO2 adsorption on MCF(a)/PEIs with different quantities of impregnated PEI are shown in Figure 7A, the adsorption capacity q60 min increases with the quantity of impregnated PEI; they are 2.77, 3.31, 4.12, and 4.38 mmol·g−1 with the impregnated PEI of 30, 40, 50, and 60%, respectively. From the insert image of the partial enlargement, when the quantity of impregnated PEI is in the range of 30−50%, the breakthrough time are similar with each other, about 2 min. As the quantity of impregnated PEI increases to 60%, the breakthrough time decreases to 0.8 min, which shows the fastest adsorption rate. However, the further increase of the quantity of impregnated PEI to 70% makes the breakthrough time extremely long, more than 60 min, and the adsorption capacity q60 min also decreases to 3.10 mmol·g−1. There are two control steps in an adsorption process, the gas diffusion and the surface adsorption. The left free space (or the porosity) in the MCFs after amine impregnated is the transfer channel for gas diffusion. Although a higher quantity of impregnated PEI can supply more active sites, it can also block up the channels at the same time. So the quantity of impregnated PEI has a balance effect on the adsorption rate. As a result, MCF(a)/PEI-60% possesses both high saturated adsorption capacity qe and low breakthrough time which will be good characteristics for the adsorption performance and for saving operation time in future industrial applications. Figure 7B shows the corresponding profiles as predicted by the second-order models with the expressions in either of the following eqs 5 and 6:59

dqt

= k(qe − qt)2

(5)

t 1 1 = + t 2 qt qe kqe

(6)

dt

The plots of (t/qt) against t for MCF(a)/PEIs have a significant linear relationship, with the correlation coefficient R very close to 1, so the CO2 adsorption rate in these materials follows the second-order rate law very well, and the adsorption rate is proportional to the square of the free adsorption sites. This conclusion perfectly coincides with the proposal of the CO2 adsorption mechanism, that for one CO2 molecule, it needs two active sites, such as two −NH2 groups, or one −NH2 group and one −OH group, to complete the adsorption. The saturated adsorption capacity qe and the rate constants k can be calculated by the slope and the intercept of the fitting lines. As listed in Table 2, the adsorption rate constant obviously decreases with an increase in the quantity of impregnated PEI due to the plug of PEI in the pores of MCF, and the corresponding CO2 diffusion resistance in sorbents increases. Meanwhile, the predicted results of the effect of the quantity of impregnated PEI to the saturated adsorption capacity qe coincide with the experimental results in Figure 7A quite well. The temperature effect on the adsorption rate in MCF(a)/ PEI-60% is further illustrated in Figure 8. Since the diffusion of CO2 molecules is very dependent on the temperature, the CO2 adsorption rate obviously increases with the adsorption temperature. When the temperature reaches 90 °C, the adsorption gets saturated almost immediately. However, the saturated adsorption capacity is no more than that at 70 °C. Because the chemical adsorption of CO2 is an exothermic process, there will be a reverse shift of desorption in the saturated equilibrium state at high temperature. Therefore, the adsorption temperature also shows a balance effect between the diffusion and the adsorption. 3.5. Comparisons between MCF(a)/PEI and MCF(a)/ TEPA. Like PEI, TEPA is also a type of multiamine with a high amine group content, which can also effectively improve the CO2 adsorption capacity of the mesoporous materials by impregnation.28,37,51 Figure 9 shows the CO2 adsorption capacity comparison between MCF(a)/PEI and MCF(a)/ TEPA with different quantities of impregnated amine. The CO2 adsorption capacity of MCF(a)/PEI and MCF(a)/TEPA both

where qe and qt (mmol·g−1) are the CO2 adsorption capacities at saturated and at time t, respectively; k (g·mmol−1·min−1) is the rate constant of a second-order adsorption. 3658



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are much larger than that of MCF(a)/PEI, which suggests that MCF(a)/TEPA is more sensitive in adsorption temperature than MCF(a)/PEI. The further stability study of 10 cyclic CO2 adsorption and desorption processes on MCF(a)/PEI-60% and MCF(a)/ TEPA-50% is shown in Figure 10. The baseline of MCF(a)/ TEPA-50% (red solid line) declines almost linearly with time due to its mass loss in the desorption process, suggesting MCF(a)/TEPA-50% is not stable at the desorption temperature of 100 °C. In contrast, the baseline of MCF(a)/PEI-60% (black solid line) is a stable horizontal line, since PEI is a relatively stable reagent with a decomposition temperature higher than 250 °C. The data of TGA analysis, as shown in Figure 3B, gave clear evidence that MCF/PEI adsorbents were thermally stable below 250 °C. As compared to that of PEI, the molecular chain length of TEPA is much shorter. The TEPA affinity to the P123 template or the silica surface in the open windows of MCF(a) is not strong enough to make it stable in the desorption process. Moreover, the decrease of the CO2 adsorption capacity is not absolutely proportional to the TEPA weight loss. As shown in Figure 10A, the slope of the decrease of CO2 adsorption capacity with time (blue dash line) is not the same as the baseline of MCF(a)/TEPA-50%, suggesting that the CO2 adsorption capacity declines not so much as the TEPA loss, which gives a further evidence that the CO2 adsorption not only comes from the amine contribution, but also from the P123 template. Figure 10B shows analysis results of the CO2 adsorption capacity on two materials for 10 adsorption/desorption cycles at 70 °C and a CO2/N2 (2:1, v/v) mixture. MCF(a)/PEI-60% shows high stability with the CO2 adsorption capacity holding almost the same 4.35 mmol·g−1 for 10 cycles. However, the CO2 adsorption capacity of MCF(a)/TEPA-50% is not only smaller than that of MCF(a)/PEI-60% initially, but also decreases significantly with the increase of the number of adsorption/desorption cycles. Even more, the CO2 purity in the desorption gases from MCF(a)/TEPA adsorbents will not be suitable for reuse in many industrial application areas. Therefore, MCF(a)/PEI is a much more effective adsorbent for the CO2 adsorption/desorption process. For future industrial application, we have to be aware that there will be a molecular weight distribution of the PEI reagent, and the lower molecular weight components may have a mass loss in a desorption cyclic operation. When the simulated flue gas with CO2/N2 (1:5, v/v) is applied, as shown in Figure 11, the CO2 adsorption capacity of MCF(a)/PEI-60% at 70 °C also shows a high stability in

Figure 8. Breakthrough curves of CO2 adsorption on MCF(a)/PEI60% at 30, 50, 70, 90 °C and atmospheric pressure.

increase obviously as the quantity of impregnated amine increases initially; after a maximum, they decrease with more amine impregnated. At 70 °C, the maximum CO2 adsorption capacity of MCF(a)/PEI is 4.49 mmol·g−1, whereas that of MCF(a)/TEPA reaches 4.44 mmol·g−1 when the quantity of impregnated amine reaches about 60%, respectively. At low amine loading, the porosity is relatively large and hence there is small resistance for CO2 transfer. As shown in Figure 9A, for MCF(a)/PEI-30%, at various adsorption temperatures of 30, 50, and 70 °C, the CO2 adsorption capacities are almost the same. So the surface adsorption is the determine step in this stage; the CO2 adsorption capacity is less dependent on the adsorption temperature. However, with the increase of the content of amine loading, the adsorption temperature becomes the dominant factor of the CO2 adsorption capacity. Take the MCF(a)/TEPA-50% for an example, as shown in Figure 9B, the CO2 adsorption capacity at 70 °C is 4.43 mmol·g−1, much higher than 2.41 mmol·g−1 at 30 °C. Coincided with the above thermodynamic discussion of MCF(a)/PEI, at higher temperature the more active adsorption sites are exposed; in addition, the amine molecules mix well with P123 molecules at a high temperature and the template can make the most of the synergic effect. In a word, when the amount of organic amine is high, the CO2 adsorption capacity increases with the temperature. Comparing Figure 9 panels A and B, the intervals between two different adsorption temperature curves of MCF(a)/TEPA

Figure 9. Influence of the amount of amine loading on the CO2 adsorption capacity with a CO2/N2 (2:1, v/v) mixture at atmospheric pressure in (A) MCF(a)/PEI and (B) MCF(a)/TEPA at various adsorption temperatures. 3659



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Figure 10. (A) Weight changes and (B) CO2 adsorption capacity of cyclic CO2 adsorption and desorption of MCF(a)/PEI-60% and MCF(a)/ TEPA-50% at 70 °C and atmospheric pressure with CO2/N2 (2:1, v/v) mixture.

trated in that, at lower quantity of adsorption, −NH2 groups in the surface layer in windows of MCF made the dominant contribution to the adsorption. With increasing quantity of adsorption, P123 micelles in the inner layer also took part in the adsorption. (3) Dynamic studies showed that the CO2 adsorption on MCF(a)/PEIs followed the second-order kinetics, in which one CO2 molecule needed two active sites, such as two −NH2 groups, or one −NH2 group and one −OH group, to complete the adsorption. There were two control steps in an adsorption process, the gas diffusion and the surface adsorption. The quantity of impregnated PEI had a balance effect on the adsorption, supplying more active sites in one way and blocking up the channels in another opposite way. (4) PEI was a more suitable amine for the synthesis of CO2 capture sorbents, since MCF(a)/PEI was much more stable than MCF(a)/TEPA.

Figure 11. CO2 adsorption capacity of MCF(a)/PEI-60% with different gas mixtures of CO2/N2 (2:1, v/v) and CO2/N2 (1:5, v/v) at 70 °C and atmospheric pressure in 10 adsorption and desorption cycles.

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10 adsorption−desorption cycles at about 4.15 mmol·g−1. In a comparison to that of the gas mixture of CO2/N2 (2:1, v/v), the CO2 concentration dramatically changes from 66.7% to 16.7%, but the adsorption capacity has only a little reduction of 4.6%. This gives clear evidence that MCF(a)/PEI-60% has a particular high selectivity for CO2 adsorption. Compared with the others in the literature,23 MCF(a)/PEI-60% possessed better properties, such as high CO2 adsorption capacity, fast adsorption rate, good stability, easy regeneration, and high selectivity.

ASSOCIATED CONTENT

S Supporting Information *

Additional figures as described in the text. 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].

ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (Grant Nos. 20736002, 2011766) and the National High Technology Research and Development Program of China (Grant No. 2008AA062302).

4. CONCLUSIONS Amine impregnated MCFs with template remaining are effective sorbents for CO2 capture. Among them, MCF(a)/ PEI-60% exhibited particularly good characteristics, such as a high CO2 adsorption capacity of 4.49 mmol·g−1 at 70 °C, fast adsorption rate, good stability, easy regeneration, and high selectivity. Some general conclusions are as follows: (1) Surfactant template can play a synergistic role in the adsorption process. It not only improved the CO2 adsorption capacity and stability, but also simplified the adsorbent synthesis process of removing surfactants, which can save energy and time. . (2) Thermodynamic studies showed that CO2 adsorption on PEI impregnated MCF(a) was a chemical adsorption with the adsorption enthalpy about −85 kJ·mol−1, which can be described by the Langmuir isotherm quite well. Meanwhile the adsorption mechanism was also illus-

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