Impregnation of Polyethylenimine in Mesoporous Multilamellar Silica

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Impregnation of Polyethylenimine in Mesoporous Multilamellar Silica Vesicles for CO2 Capture: A Kinetic Study Lihuo Zhang, Ni Zhan, Qing Jin, Honglai Liu, and Jun Hu Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.5b04760 • Publication Date (Web): 27 Apr 2016 Downloaded from http://pubs.acs.org on May 16, 2016

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Industrial & Engineering Chemistry Research

Impregnation of Polyethylenimine in Mesoporous Multilamellar Silica Vesicles for CO2 Capture: A Kinetic Study Lihuo Zhang, Ni Zhan, Qing Jin, Honglai Liu, Jun Hu* Key Laboratory for Advanced Materials, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, China. KEYWORDS: Mesoporous multilamellar silica vesicles, CO2 Capture, Kinetic Study, Templates

ABSTRACT: Amine-impregnated solid sorbents have been approved as one of promising sorbents for CO2 capture. However, the low adsorption rate seriously limits their real application. Herein, we synthesized the mesoporous multilamellar silica vesicle (MMSV) and used them as a novel support for the impregnation of polyethylenimine (PEI). The mesoporous multilamellar structure, as well as the presence of surfactant templates, significantly improved the dispersion of PEI in sorbents, and hence improved both adsorption capacity and adsorption rate, simultaneously. Among various MMSV-PEIs samples, MMSV(a)-PEI-60% showed the best CO2 adsorption capacity, up to 4.73 mmol/g in pure and dry CO2 flow at 90 °C, and even exhibited a 22% enhancement in humid CO2 flow at 75 °C. Moreover, it also presented good

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adsorption/desorption cyclic performance. The kinetic study showed that the second-order kinetic model fitted quite well for the dynamic CO2 adsorption, and the adsorption rate constants revealed its faster adsorption rate and higher amine efficiency than most reported amine-impregnated sorbents.

1 Introduction Post-combustion CO2 capture technologies are promising to aid in controlling the emission level of CO2 in the atmosphere.1,

2

Although the liquid amine absorption technique is the

dominant technology for CO2 capture on industrial scale,3 its drawbacks, such as the toxic, flammable, corrosive, and the high regeneration energy make a bottleneck for real industrial applications.4,

5

Alternatively, solid-supported amine sorbents, which can overcome these

disadvantages, have caused particular interesting in recent years.6 In contrast to commercial sorbents, such as zeolites and activated carbons, solid-supported amine sorbents exhibit higher selectivity for CO2 adsorption due to the specific CO2–amine chemistry.7 Under humid conditions, CO2 capture capacity will be further improved because the adsorbed water has a positive effect to enhance the amine efficiency, thus eliminating the need for strict dehumidify control process prior to CO2 capture,8-13 which is very important for real industrial applications. Among various porous supports, mesoporous silica is considered as a promising type of support for amine immobilization. Following the first report of PEI supported on MCM-41,14 a series of mesoporous silica materials, such as MCM-41,9, 15-18 MCM-48,19 HMS,20 SBA-15,21-24 SBA-16,25 SBA-12,26 and KIT-618 have been proved as effective amines supports for CO2 capture. Recently, some new support materials with larger cavity, including silica monolith,10


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mesoporous silica capsules,7 and mesoporous silica foam (MCF),3,

27, 28

have been used to

immobilize more amine to achieve higher CO2 capacity. Among them, Qi et al.7 found that the amount of PEI loading in mesoporous silica capsules can be as high as 83 wt. %, resulting in a high CO2 adsorption capacity of 5.7 mmol/g under dry and pure CO2 at 75 °C. However, the amine efficiency was not good enough because the channels were blocked by overloading PEI chains. To overcome the kinetic barrier for CO2 diffusion, the effective way is to split the PEI aggregations into small portions.16, 29 Compared with capsules, we noticed that multilamellar vesicles,30-32 which could provide not only large cavities, but also good barriers for separating PEI aggregations apart, would be promising supports to fabricate amine-type sorbents. However, the mesoporous multilamellar vesicles has not been reported as the support for impregnation of PEI. Moreover, surfactants acted as templates23, 33, 34 or additives35 have been proved that they can effectively improve the CO2 adsorption performance. By introducing a CO2-neutral surfactant which had no direct contribution for CO2 adsorption capacity into PEI, extra diffusion channels can be created to facilitate CO2 diffusion into the deeper PEI films and improve the CO2 capture performance.29 In our previous work,36 we also demonstrated that without removing templates, the CO2 adsorption capacity can be significantly enhanced due to the synergetic adsorption effect of the surfactant. Therefore, the sorbents with both specific multilamellar structure and the presence of templates would facilitate high CO2 adsorption capacity and high CO2 adsorption rate at the same time. In this work, we synthesized mesoporous multilamellar silica vesicles (MMSVs) by using dual templates of dodecyltrimethylammonium bromide (DTAB)/dihexadecyldimethylammonium bromide (DHDAB) as the structure templates. MMSVs were used as the supports for the first time to fabricate PEI-impregnated sorbents, since we believed the advantages of MMSVs



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including (i) the large space of vesicles for loading enough PEI, (ii) the multilamellar structure for preventing the aggregation of PEI, (iii) mesopores on the layer for providing fast diffusion channels, and (iv) the presence of templates for constructing new CO2 diffusion pathway and improving the dispersion of PEI, would make great contributions for improving CO2 adsorption performance. By studying the kinetics of CO2 adsorption, the operation conditions of the amount of amine loading, the presence of surfactant template, the adsorption temperature, and the moisture were investigated and optimized, and the mechanism of improving CO2 adsorption by MMSV(a)-PEI was tentatively illustrated. Moreover, the adsorption stability was demonstrated by performing the cyclic adsorption/desorption tests. 2 Materials and Methods 2.1 Chemicals Tetraethylorthosilicate (TEOS, AR) was purchased from Shanghai Lingfeng Chemicals Co., Ltd. Dodecyltrimethylammonium bromide (DTAB, AR) and ammonia were purchased from Jiangsu Yonghua Chemicals Co., Ltd. Dihexadecyldimethylammonium bromide (DHDAB, 97%) was purchased from Aladdin Chemicals Co., Ltd. Anhydrous ethanol (99.7%) was purchased from Sinopharm Chemical Reagent Co., Ltd. Methanol (99.99%) was purchased from Shanghai Titanchem Co., Ltd. Polyethylenimine (PEI, Mw=600, 99%) was purchased from Alfa Aesar Chemicals Co., Ltd. All chemicals were used as purchased without further purification. 2.2 Preparation of MMSV-PEIs Sorbents In a typical synthetic procedure of MMSV, DTAB (0.06 g) was dissolved in 35.00 mL deionized water at 30 °C. Then DHDAB (with molar ratios of DTAB: DDHAB being 1:2) was


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added into the solution under stirring. After the surfactants were completely dissolved, 0.69 mL ammonia was added, and the mixture was stirred for another 2 h. Then, 2.0 g TEOS was added dropwise under vigorous stirring. The mixture was maintained at 30 °C under stirring for 24 h. Then, the obtained milky mixture was transferred into an autoclave and aged at 100 °C for 24 h under the static condition. The white solid product was recovered by the vacuum filtration, with the nominal pore size of 15-20 um filter, and washed with water and ethanol, dried at 60 °C overnight, which was denoted as MMSV(a). The resulting powder was further calcinated to remove the surfactant templates in a furnace (SX2-4-10) at 550 °C in atmosphere for 6 h, with a temperature ramp rate of 1.0 °C/min (TCW-32B Temperature Controller). The sample was denoted as MMSV(c). Amine-impregnated sorbents were prepared by the wet impregnation method. Typically, a desired quantity of PEI was dissolved in 1.0 g of methanol. Subsequently, the amine-methanol solution was added to 0.1 g of MMSV(a) or MMSV(c). The resultant mixture was stirred for 4 h. Finally, the obtained product was dried at 80 °C overnight to remove the solvent. PEI modiﬁed sorbents were denoted as MMSV(a)-PEI-x or MMSV(c)-PEI-x, where x denoted the weight percentage of PEI-impregnated in the sorbents. 2.3 Characterization Transmission electron microscopy (TEM) images were obtained using a JEM-1400 electron microscope. Field-emission scanning electron microscope (FESEM) images were conducted by using a Nova NanoSEM 450. Powder X-ray diffraction (PXRD) patterns were obtained on a D/Max2550 VB/PC spectrometer using Cu Kα radiation (40 kV and 200 mA). Nitrogen adsorption measurements were conducted at 77 K on a Micrometrics ASAP 2020 sorptionmeter.



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The surface area was determined by the BET (Brunauer-Emmett-Teller) model. The pore size distribution was determined by BJH (Barrett-Joyner-Halenda) model. Thermogravimetric analysis (TGA) measurement was conducted on a NETZSCH STA 449 F3 Jupiter. 2.4 CO2 Adsorption Measurement The thermogravimetric analysis (TGA) was used for CO2 adsorption/desorption measurement. The weight change of the adsorbents was monitored to show the adsorption and the desorption performance (Figure S1a). In a typical adsorption/desorption process under dry condition, the sample (about 10 mg) was placed in a flat alumina pan. It was degassed by heating to 110 °C in a pure N2 purge about 1 h until there was no weight change. Then the temperature was decreased to the testing adsorption temperature, such as 60, 75, 90 and 100 °C, respectively. The testing gas of the pure and dry CO2, or the simulated flue gas (15:85 v/v of CO2:N2) was then introduced at a flow rate of 20 mL/min for the adsorption test. After 1 h adsorption, the gas was switched into pure N2, and the temperature was raised again to 110 °C at a heating rate of 10 °C/min to perform desorption. The flow rate of N2 purge was set as 60 mL/min to make sure all the impurities or CO2 can be removed completely in a short time. The adsorption capacity was calculated by the sample weight change during this adsorption/desorption process. In order to examine the moisture effect on the CO2 adsorption capacity, we used a two-step adsorption, first, the sorbent was saturated by the moisture, and then, the CO2 adsorption capacity was determined (Figure S1b). Typically, after the same degas process as above, the temperature was decreased to the testing adsorption temperature of 90 or 75 °C under dry N2 purge. Then a humid N2 with a relative humidity of 30% was introduced at a flow rate of 20 mL/min. After the sample was saturated with H2O (about 1 h, no further weight increase), the


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testing gas was switched into humid CO2 with a relative humidity of 30% at a flow rate of 20 mL/min for the adsorption (about another 1 h). Following, the same desorption process with the dry N2 purge was conducted. The humid N2 and CO2 were obtained by passing the gases through a Modular Humidity Generator (MHG-32) at 25 oC, which can adjust and control the humidity automatically. 3 Results and Discussion The well-defined multilamellar structure with a central cavity of MMSV was clearly presented in its TEM image (Figure 1a). The average size of the vesicles was about 40-50 nm. From the insert enlargement, we can observe that the vesicle was consisted of an inner cavity with a size about 20-30 nm, and had a multilamellar structure with 3-5 layers, the distance between two neighboring layers was about 4.5 nm. SEM image (Figure S2) also showed an intact spherical nature of MMSV particles, and some of them were closely packed with each other. The small angle X-ray diffraction (SAXRD) pattern of MMSV(c) (Figure 1b) exhibited a broad reflection in the 2θ range of 1.0-2.50, which was an indication of relatively ordered vesicular framework.29 Moreover, the intensive peaks at 2θ of 1.90 and 2.13 o, corresponding to the d-spacing values of 4.14 and 4.66 nm, respectively, were characteristic peaks of mesoporous channels, in good agreement with the TEM results of the lamellar structure. Nitrogen adsorption/desorption isotherms and the corresponding pore size distributions of MMSVs were shown in Figure 1c. Without removing templates, MMSV(a) possessed a very low BET surface area and pore volume of 43.9 m2/g and 0.19 cm3/g. Because of the fully filled template in mesoporous channels, the week peak concentrated at 4 nm in the pore size distribution (Figure 1c, insert) would be attributed to the spacing between the layers. After the calcination, the



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isotherm of MMSV(c) exhibited two significant ascents at relative pressure of 0.25-0.4 and 0.9-1.0, respectively. In which, the former indicated the existence of mesoporous channels, while the latter referred to macrospores, probably attributed to the inner cavity of MMSV(c) and voids among the aggregated nanoparticles.37,

38

The BET surface area and pore volume were

significantly improved, as high as 797 m2/g and 0.857 cm3/g, respectively. The pore size distribution curve of MMSV(c) presented two peaks, one at 2.6 nm indicated the size of mespores in the multilayered shells, another week one at 3.8 nm was the spacing between two neighboring

layers,

revealing

a

little

shrink

after

the

calcination.


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Figure 1 (a) TEM image and (b) XRD pattern of MMSV(c), (c) Nitrogen adsorption/desorption isotherms and pore size distributions of MMSV(c) and MMSV(a), (d) TGA and DTG curves for MMSV(c)-PEI-60% and MMSV(a)-PEI-60% with a temperature ramp of 10 °C/min in pure N2

TGA and DTG profiles (Figure S3) revealed that MMSV(a) had a significant mass loss in the temperature range of 200-400 °C, attributed to the decomposition of DTAB and DHDAB templates, whereas, no weight loss was observed for MMSV(c) at this temperature range. The total amount of templates in MMSV(a) was then calculated as about 30%. After the impregnation of various quantities of PEI, all TGA curves of MMSV(c)-PEIs (Figure S4a) showed a sharp weight loss, and as shown in their DTG profiles (Figure S4b), all their decomposition temperatures were lower than that of pure PEI, the smaller the quantity of PEI impregnation, the lower the decomposition temperature, such as the decomposition peak of MMSV(c)-PEI-60% concentrated at 235 °C, much lower than that of pure PEI of 345 °C. Because the volatility of solid can be enhanced due to the smaller size of the aggregation,15, 18, 39 the reducing decomposition temperature indicated that the multilayer vesicle structure of MMSV(c) can successfully separate PEI aggregations apart into smaller particles. Whereas when we preserved templating agents in the vesicles, MMSV(a)-PEI-60% presented one broader intensive peak and a higher decomposition temperature of 335 °C than that of MMSV(c)-PEI-60% with the same quantity of PEI-impregnated (Figure 1d), but was still lower than that of pure PEI. Generally, strong interaction between two substances can usually increase the decomposition temperatures, so the interaction of PEI with the surfactant templates might result in this ascent, meanwhile, it might also enhance the dispersion of PEI. Moreover, both TGA curves showed two mass loss peaks at 62 and 100 °C, where the former was associated



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with the CO2 desorption, while the latter was attributed to the water loss. Much more intensive CO2 desorption peak in MMSV(a)-PEI-60% curve suggested its stronger CO2 affinity. After PEI impregnation, the porosity properties of all MMSV-PEI samples were also analyzed using N2 physisorption (Figure S5), and their calculated results were summarized in Table S1. The BET surface area and pore volume decreased with increasing PEI loading, confirming that PEI was successfully impregnated into MMSV. It is noted that after removing the templates, 70% PEI impregnations made samples almost lost the porosity, whereas, without removing the templates, 60% PEI impregnations made it fully lost the porosity. This difference inferred that most PEI were successfully impregnated into MMSV while the rest were deposited on the external surfaces. Because of the serious blockage due to the overloading, it was very difficult for N2 gas molecules to diffuse into MMSV-PEIs, which ensured the good adsorption selectivity of CO2 to N2. The CO2 dynamic adsorption performance on MMSV-PEIs in dry and pure CO2 flow at 90 °C (Figure 2a) showed that the CO2 adsorption capacity of both MMSV(a)-PEIs and MMSV(c)-PEIs increased with increasing the quantity of PEI-impregnated in the range of 40-60%, due to more active amino groups involved for CO2 adsorption (Figure 2b). Without removing the templates, the CO2 adsorption capacity of MMSV(a)-PEIs were 3.25, 3.79, and 4.73 mmol/g (with the standard deviation of ±0.058, Figure S6) at the PEI impregnation of 40, 50 and 60%, respectively; higher than that of the corresponding MMSV(c)-PEIs with the same quantity of PEI-impregnated but after removing the templates. Moreover, the amine efficiency, defined as the molar ratio of CO2 adsorbed to the amount of amino groups in the sorbent divided by two to account for the need of two amino groups for each CO2 under dry conditions, were 69.9%, 65.2%, 67.7% for MMSV(a)-PEIs, and 39.8%, 43.7%, 47.0% for MMSV(c)-PEIs,


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respectively. Therefore, the templating agents played an important role to enhance the CO2 adsorption performance. However, MMSV(a) itself exhibited a very poor CO2 adsorption capacity of 0.17 mmol/g (Figure S7) because of its low BET surface area, suggesting only the physical adsorption occurred and the surfactant templates had no direct contribution to the CO2 adsorption capacity. It is worthy to mention, although the PEI chains should be difficult to transfer into the vesicles due to the block of the templates, little change in the amine efficiency of each MMSV(a)-PEIs can be observed, suggesting the surfactant templates might efficiently interact with PEI chains to make them highly dispersed and also make extra pathways for CO2 diffusion. When PEI impregnation further increased to 70%, the CO2 adsorption capacity of MMSV(a)-PEI-70% drastically decreased to 2.62 mmol/g, and the amine efficiency decreased to 32%. Although a higher quantity of PEI-impregnated can supply more active sites, it can also block up the channels, resulting in higher diffusion resistance for CO2. So the quantity of PEI impregnation had a balance effect on the adsorption performance. Compared with the available reported data of the CO2 adsorption capacity and the amine efficiency of various sorbents of porous silica with PEI impregnated (Table S2), MMSV(a)-PEI-60% showed a higher CO2 adsorption capacity and better amine efficiency, which successfully confirmed the advantages of our approach by using the mesoporous multilamellar silica vesicle as the support material for the impregnation of PEI. Similar results were obtained when we changed the testing gas of pure CO2 into the simulated flue gas (with 15/85 (v/v) of CO2/N2). As listed in Table S3, MMSV(a)-PEI-60% also showed the highest CO2 capacity of 3.85 mmol/g, with the amine efficiency of 55.1%. Therefore, even at low CO2 partial pressure, MMSV(a)-PEI-60% still exhibited good CO2 adsorption capacity and



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selectivity, which ensured it to be a promising candidate for the potential application in CO2 capture. Table 1 CO2 adsorption properties of MMSV-PEIs adsorbents MMSV(a)-PEI

MSMV-PEI content (%) Experimental adsorption capacity*a

MMSV(c)-PEI

40

50

60

40

50

60

3.25

3.79

4.73

1.85

2.54

3.28

3.27

3.82

4.81

1.88

2.56

3.35

69.9

65.2

67.7

39.8

43.7

47.0

1.02

0.46

0.18

0.31

0.23

0.13

(mmol g-1) Saturated adsorption capacity qe *b (mmol g-1) Amine efficiency (%)*c -1

-1

Rate constant k (g mmol min ) *a

Experimental adsorption capacity was a result obtained by TGA at adsorption time of 60 min. Saturated adsorption capacity was a predicted result by the second-order rate law. *c The amine efficiency was defined as the ratio of mol CO2 captured per 2 mol of amino groups in the sorbent and expressed as a % of the maximum adsorption capacity. *b


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Figure 2 For MMSV(a)-PEIs and MMSV(c)-PEIs with different quantities of PEI-impregnated, (a) comparison of dynamic CO2 adsorption at 90 °C, (b) CO2 adsorption capacity (bar) and amine efficiency (curve), pure PEI is taken as the reference, (c) Fitting plots of t/qt against t as predicted by the second-order rate law, and (d) the rate constant k changes with the temperature

Amine-impregnated sorbents have shown an outstanding CO2 adsorption capacity, to overcome the CO2 diffusion resistance is a great challenge for significantly enhancing their overall CO2 adsorption performance. To clearly reveal the kinetics of CO2 adsorption performance on MMSV-PEIs, the second-order kinetics model (Eq. 1) was used to fit the dynamic adsorption data. dqt 1 1 t = k (qe − qt ) 2 or = 2 + t qt kq e qe dt

(1)

where qe and qt (mmol·g−1) are the CO2 adsorption capacities at saturated and at any time t, respectively; k (g·mmol−1·min−1) is the rate constant of a second-order kinetics model. Figure 2c showed a good linear relationship of the plots of (t/qt) against t for all MMSV(a)-PEIs, with the correlation coefficient R very close to 1. As listed in Table 1, the adsorption rate constants for both MMSV(a)-PEIs and MMSV(c)-PEIs decreased with increasing the quantity of impregnated PEI due to the blockage increasing in MMSV, and hence, corresponding the increases of CO2 diffusion resistance. It is worthy to mention, the rate constant k of MMSV(a)-PEIs was higher than that of MMSV(c)-PEIs with the same quantity of PEI impregnation, which further indicated that the surfactant templates could facilitate the diffusion of CO2 within the PEI chains, leading to a fast apparent CO2 adsorption rate. Alternatively, the ratio of the amount of adsorption at 5 min to the saturated capacity was chosen to roughly evaluate the adsorption rate. The values of



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the ratio were as high as 94.6%, 91.4% and 82.8% for MMSV(a)-PEIs at PEI impregnation of 40%, 50% and 60%, again higher than that of corresponding surfactant-free sorbent MMSV(c)-PEIs of 81.9%, 81.5% and 77.6%, respectively. It was a great advantage that the adsorption rates of MMSV(a)-PEIs were so fast that we only needed 5 min to achieve a good adsorption capacity. The presence of surfactant templates not only improved the CO2 adsorption performance but also omitted the process of removing surfactants, which can save energy and time, and increase the yield of the composite products. This will be particularly important for the future industrial application. The adsorption temperature is also an important factor for both CO2 chemical adsorption and CO2 diffusion, and hence on CO2 adsorption capacity and adsorption rate. The adsorption capacity of MMSV(a)-PEI-60 (Figure S8a) in pure and dry CO2 flow showed an increase with increasing adsorption temperature, and reached the maximum CO2 uptake at 90 °C. When the temperature further increased to 100 °C, the adsorption got saturated almost immediately but with a decreased saturated adsorption capacity. Higher temperature was benefit for accelerating the chemical reaction between CO2 and amino group as well as the diffusion of CO2, but also caused a reverse shift for desorption due to the exothermic CO2 adsorption. With different quantities of PEI-impregnated, the effect of temperature on the adsorption capacity of each MMSV(a)-PEIs (Figure S8b) showed a similar rule. However, at low amine loading, the porosity was relatively large (Table S1) and hence there was small resistance for CO2 diffusion and the CO2 adsorption capacity was less dependent on the adsorption temperature. Whereas at high amine loading, it showed much higher temperature-dependence. Again, fitted by the second-order models,

a significant linear relationships between t/qt against t on


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MMSV(a)-PEI-60% were obtained at different temperatures (Figure S8c). The adsorption rate constant significantly increased with increasing temperature (Figure 2d).

Figure 3 (a) Comparison of CO2 adsorption of MMSV(a)-PEI-60% under dry and humid pure CO2 at 75 °C and (b) Regeneration performance of MMSV(a)-PEI-60% and MMSV(c)-PEI-60%

One advantage of amine-impregnated sorbents is the promotion effect of moisture on CO2 adsorption. After saturated by water in a N2 flow with the relative humidity of 30% at 75 °C, the CO2 adsorption capacity of MMSV(a)-PEI-60% under humid CO2 with the relative humidity of 30% was as high as 4.97 mmol/g (Figure 3a), 22% improvements were achieved compared with that under pure dry CO2 adsorption. However, there was no enhancement when the temperature increases to 90 °C, the CO2 adsorption capacity was 4.69 mmol/g, since very little water was absorbed at this temperature. As we know the stoichiometry number ratio of CO2 to amino group changes from 1:2 under the dry condition to 1:1 under the humidity, it deserved much higher enhancement of CO2 adsorption performance under the humidity. However, the improvements were not as good as expected since the pre-saturated water could form a thin layer, forbidding CO2 molecules to contact with amino groups, and also could block the channels for CO2 transfer



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into the inner PEI in the cavity. In real CO2 captures, CO2 and H2O molecules will diffuse into the adsorbents simultaneously. Without preformed water thin layer, CO2 molecules can freely transfer and contact with amino groups, and then, better adsorption performance is anticipated. More importantly, the regeneration ability of MMSV-PEI-60% was also investigated in dry and humid conditions, respectively. As shown in Figure 3b and Figure S9, under 20 mL/min flow rate of pure CO2 at 90 °C, MMSV-PEI-60% demonstrated a good stability during 10 cyclic adsorption/desorption. Compared with the first cycle, the adsorption capacity showed a small reduction of about 3.7%. Whereas for the equally PEI loaded sorbents, the adsorption capacity MMSV(c)-PEI-60%, after removing templates, decreased by 10% of its initial value after 10 cycles. More importantly, we also found that regeneration stability of MMSV(a)-PEI-60% under humid environment (with the relative humidity of 30%) was even better than that under dry conditions, and the loss of stability was as low as 2%, which would be attributed to the formation of urea under the humid condition.11, 16 Therefore, surfactant templates played an important role not only in increasing CO2 adsorption capacity and adsorption rate, but also in improving the regeneration stability.

4 Conclusions PEI-impregnated mesoporous multilamellar silica vesicles (MMSV-PEIs) sorbents were designed and fabricated for CO2 capture. The second-order kinetic model was fitted well with their CO2 adsorption performance. Without removing the templates, MMSV(a)-PEI sorbents exhibited higher CO2 adsorption capacity, faster adsorption rate, and better thermal stability than MMSV(c)-PEIs with removing the templates. Combining the TGA results and the enhanced CO2


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adsorption performance, we proposed that the mesoporous multilamellar vesicle structure and the presence of surfactant templates might synergistically improve the dispersion of PEI chains in MMSV, and hence efficiently decreased the CO2 diffusion resistance. Among them, MMSV(a)-PEI-60% exhibited the best CO2 adsorption performance, with the CO2 adsorption capacity of 4.73 mmol/g at 90 °C under dry condition. It was further improved to 4.97 mmol/g under humid CO2 flow at 75 °C, and exhibited a good stability. Moreover, its amine efficiency was calculated as 67.8%, higher than most reported PEI-based CO2 sorbents under dry condition. The good CO2 adsorption performances in both thermodynamic and dynamic ways highlighted the potential application of MMSV(a)-PEI sorbents in CO2 capture. ASSOCIATED CONTENT Supporting Information Additional figures as described in the text. The Supporting Information is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]; Fax: 86-21-64252195; Tel: 86-21-64252195 ACKNOWLEDGMENT Financial support for this work is provided by the National Basic Research Program of China (2013CB733501), the National Natural Science Foundation of China (No. 91334203, 21376074), the 111 Project of China (No.B08021), the Fundamental Research Funds for the Central Universities of China and the project of FP7-PEOPLE-2013-IRSES(PIRSES-GA-2013-612230)..



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TOC

PEI-impregnated mesoporous multilamellar silica vesicles exhibited good capacity, high amine efficiency, high rate, and good regenerate stability for CO2 adsorption.


Impregnation of Polyethylenimine in Mesoporous Multilamellar Silica

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