Amine-Impregnated Mesoporous Silica Nanotube as an Emerging

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Amine-impregnated mesoporous silica nanotube as an emerging nanocomposite for CO2 capture Mengya Niu, Huaming Yang, Xiangchao Zhang, Yutang Wang, and Aidong Tang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b05044 • Publication Date (Web): 17 Jun 2016 Downloaded from http://pubs.acs.org on June 19, 2016

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ACS Applied Materials & Interfaces

Amine-Impregnated Mesoporous Silica Nanotube as an Emerging Nanocomposite for CO2 Capture

Mengya Niua,b, Huaming Yanga,b,c,*, Xiangchao Zhangd, Yutang Wangd, Aidong Tange,*

a

Centre for Mineral Materials, School of Minerals Processing and Bioengineering, Central South University, Changsha 410083, China

b

Key Lab for Mineral Materials and Application of Hunan Province, Central South University, Changsha 410083, China c

d

State Key Lab of Powder Metallurgy, Central South University, Changsha 410083, China

Hunan Key Lab of Applied Environmental Photocatalysis, Changsha University, Changsha 410022, China e

School of Chemistry and Chemical Engineering, Central South University, Changsha 410083, China

*

Corresponding author, Email: [email protected], [email protected], Fax: +86-731-88830549, Tel.: +86-731-88710804

ABSTRACT：Pristine halloysite nanotubes (HNTs) were pre-treated to produce mesoporous silica nanotubes (MSiNTs), which was further impregnated with polyethenimine (PEI) to prepare an emerging nanocomposite MSiNTs/PEI (MP) for CO2 capture. Thermogravimetric analysis (TGA) was employed to analyze the influences of PEI loading amount and adsorption temperature on CO2 adsorption capacity of the nanocomposite. The SBET value of MSiNTs was 6 times and corresponding pore volume was more than 2 times higher than that of HNTs. The well dispersion of PEI within the nanotubes of MSiNTs benefits more CO2 gas 1 ACS Paragon Plus Environment


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adsorption, and the adsorption capacity of the nanocomposite could reach 2.75 mmol/g at 85°C for 2 h. The CO2 adsorption on the nanocomposite was demonstrated to take place in two stages process, there was first a sharp liner weight increase at beginning, and then a relatively slow adsorption step. The adsorption capacity could reach as high as 70% within 2 min. Also the nanocomposite exhibited good stability on CO2 adsorption/desorption performance, indicating that the as-prepared emerging nanocomposite show an interesting application potential in the field of CO2 capture. Keywords: CO2 capture, clay, halloysite nanotubes, mesoporous silica nanotube, adsorption kinetics.

1. INTRODUCTION The increasing concentrations of carbon dioxide (CO2), one of the main greenhouse gases, have led to several environmental issues like global warming and climate change1,2, thus, it is essential to develop more researches about the efficient capture and subsequent utilization of CO2. Technologies for CO2 capture and sequestration (CCS) were proved to be an important measure to reduce carbon dioxide emissions and quickly developed recently3,4. Liquid amine based solution such as monoethanolamine (MEA)5,6, diethanolamine (DEA)7 has been efficiently applied for CO2 capture in the industrial fields. However, solvent based adsorbent 2 ACS Paragon Plus Environment

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have many disadvantages, amine degradation, high energy requirement, foaming problems, equipment corrosion, and many others8-10. Amine-based solid adsorbent is gradually attracting more attention due to less energy requirement, higher CO2 capacity, higher resistance for contaminants, and higher stability compare of aqueous amines2. Amine-based solid adsorbent is prepared by immobilizing11-13 or grafting organic molecules containing amine groups on support materials which generally have high surface areas and porous structure14,15. In the previous studies, most of researches for amine-based solid sorbents were about the choice of the supporting materials, for example, N-doped porous carbons showed satisfactory CO2 uptake capacity16. Many other supporting materials, including metalorganic frameworks (MOF)17,18, mesoporous carbon19, resins20, mesoporous silica21, aerogel22, macroporous silica23, MCM-4124,25, TiO226, SBA-1527, also have been studied in great detail. However, the adsorption capacity of materials were reduced to some degree in presence of moisture and became unstable sometimes28. Clay usually contains several clay minerals and small amounts of impurities, and has been moderately modified to prepare advanced functional materials29-39. Acting as the supporting material for the sorbent, clay possesses obvious advantages in the low cost, high mechanical and chemical stability, etc., which has attracted specific research attention in recent years40,41. Wang et al. developed new clays (kaolinite and montmorillonite) supported polyethylenimine composite for CO2 capture8, the obtained CO2 sorption capacity could reach 2.54 mmol/g under dry environment and 3.23 mmol/g under the moisture addition. Irani et al. prepared an inorganic-organic CO2 sorbent by immobilizing TEPA onto acid modified nanosepiolite and obtained capacity of 3.8 mmol/g for 1 vol. % CO2 in N2 along with ~1 3 ACS Paragon Plus Environment


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vol. % H2O at 60°C42. Additionally, halloysite (Al2Si2O5(OH)4·nH2O), a dioctahedral 1:1 clay mineral with natural nanotubular structure43, is composed of hollow cylinders resulted from the multiple rolled layers. The outer surface of halloysite nanotubes (HNTs) has tetrahedral SiO4 group, whereas the inner surface consists of octahedral aluminium hydroxide group. As an abundantly natural nanomaterial, halloysite also has the low cost (~$50/t), which is attractive and convenient for technological applications compared wiht other support materials for amine-based CO2 absorbent. This work aims to develop an emerging nanocomposite for capturing CO2. Natural clay mineral should be pretreated to modify its poor textural properties, while thermal treatment could activate the aluminosilicate network of HNTs, and subsequent acid treatment selectively removed the alumina component to produce mesoporous silica nanotubes (MSiNTs). Then a series of nanocomposites MSiNTs/PEI with different PEI contents were synthesized. The textural characteristics, changes in morphology and thermal stability of the nanocomposite were investigated. The influence of PEI loading content and adsorption temperature on the CO2 adsorption properties were studied by thermogravimetric analyzer (TGA). Finally, we demonstrated the adsorption-desorption properties to make this emerging nanocomposite as an attractive one.

2. EXPERIMENTAL 2.1 Materials synthesis Pristine HNTs used herein was obtained from Shanxi, China. Ethylenediamine branched (average MW~800) were purchased from Aladdin, and analytical reagents were used in all 4 ACS Paragon Plus Environment

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experiments. HNTs were calcined for 4 h at 850°C in air. Then the calcined HNTs (5g) were added into 150 mL 6M hydrochloric acid solution, subsequently constantly stirred in a conical flask at 80°C for 6 h. The suspension was then filtered, water-washed until no acid could be detected, and put in a drying oven at 80°C overnight to finally produce mesoporous silica nanotubes (MSiNTs). MSiNTs/PEI nanocomposite was synthesized via wet impregnation route. PEI was added into 30 g of methanol to form a mixture solution, MSiNTs were then put into the solution and stirred for 24 h under ambient conditions. The final materials were maintained in a vacuum drying oven at 55°C to produce a white solid powder. MSiNTs/PEI nanocomposites with different PEI loadings at 30, 40, 50, and 60% were named MP-30, MP-40, MP-50, and MP-60, respectively. 2.2 Characterization X-ray diffraction (XRD) analysis of the samples were carried out with a RIGAKU D/max-2550VBR+ 18 kW powder diffractometer (Cu Kα-radiation, λ=1.5418 Å) with 2θ of 5~80°. The Fourier transform infrared spectroscopy (FTIR) spectra were recorded on a Shimadzu Iraffinity-1 FTIR spectrometer from 4000 to 400 cm-1 with pressed disks of the sample and KBr mixture. Scanning electron microscopy (SEM) was employed to observe the sample morphology by using TESCAN MIRA3 LMU field emission scanning electron microscope, the sample surface should be first gold-sprayed. The elemental analysis of samples was also performed using the attached energy dispersive X-ray spectroscopy (EDS) detector of TESCAN SEM. Transmission electron microscopy (TEM) images were collected with JEM-2100F operating at 200 KV. The BET specific surface area and pore characteristics 5 ACS Paragon Plus Environment


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of the samples were characterized by Quantachrome Instruments N2 adsorption/desorption analyzer. The thermal stability of the samples was evaluated using TGA in the range of 25~850°C under N2 flow, which was obtained with STA449 F5 Netzsch Germany. 2.3 CO2 adsorption CO2 adsorption/desorption measurements of MSiNTs/PEI nanocomposite were characterized by TGA (STA449 F5 Netzsch Germany). 10 mg of the nanocomposite was put into alumina pan, sealed in a cylindrical container with N2 as protective gas and CO2 as purge gas. The temperature was increased from 25 to 100°C at a heating rate of 10°C/min, kept at 100°C for 1 h under pure N2 (40 mL/min) to remove the moisture and adsorbed CO2. When the temperature dropped to a certain value (25, 50, 75, 85°C), the nanocomposite was measured in the flow of 40 mL/min N2 and 60 mL/min CO2 for 2 h. Reproducibility for CO2 adsorption was measured using TGA. The reproducibility was obtained five times in a mixture of gases (40 mL/min N2, 60 mL/min CO2) for 2 h at desired sorption temperature and N2 atmosphere (40 mL/min) for 40 min at 110°C.

3. RESULTS AND DISCUSSION XRD patterns of HNTs, MSiNTs, and MSiNTs/PEI nanocomposite with different PEI loading were shown in Fig. 1a. HNTs exhibits (001) diffraction reflection at 2θ = 12.0°attributed to the basal spacing of 0.73 nm decided by Bragg's law, indicating that this sample is halloysite-7Å. After thermal and acid treatment, the diffraction reflections of HNTs were not observed, only a broad reflection at 2θ of 22° appeared, demonstrating the amorphous structure of the resultant silica. There shows a broader amorphous reflection in the XRD 6 ACS Paragon Plus Environment

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pattern of the nanocomposite after PEI impregnation, and amorphous reflection became broader with the increase of PEI contents because MSiNTs/PEI has the amorphous reflection in the same site of amorphous silica, and these two reflections overlap42. Fig.1b shows FTIR spectra of these samples, and Table S1 gives the characteristic vibrational positions and corresponding assignments of pristine HNTs. Compared with the pristine HNTs, after heat and acid treatment, MSiNTs exhibit some change in FTIR bands. The Al-O-Si deformation band at 532 cm-1, and the Al-OH stretching bands at 3693 and 3623 cm-1 disappeared, indicating the gibbsite octahedral sheet of HNTs was destroyed. The broad Si-O stretching band at 1037 cm-1 shifted to 1088 cm-1, which may be associated with the substitution of H atoms for Al atoms with difference in electronegativity. The band at 960 cm-1 was attributed to the terminal Si-OH deformation band44. The Si-O-Si band at 470 cm-1 increased in intensity after heat and acid treatment, which might be related to the leaching of Al3+ and the formation of some amorphous silica. After PEI loading in MSiNTs, there appeared some new bands. Bands at 1569 and 1477 cm-1 resulted from the symmetric and asymmetric bending vibration of –NH244. The stretching vibration of –CH2 in the PEI chain were observed at 2972 and 2845 cm-1, demonstrating the successful loading of PEI within the MSiNTs. The disappearance of band at 960 cm-1 for MSiNTs was mainly associated with the interaction between terminal Si-OH and NH groups. Band at 2360 cm-1 was attributed to the C=O vibration possibly caused by adsorbed CO2, therefore, a pretreatment to remove the adsorbed CO2 is necessary before we tested the CO2 uptake capacity. One thing should be noticed that the intensity of –NH2 was different at the different PEI loadings. The nitrogen adsorption–desorption isotherms and porous parameters of HNTs, MSiNTs, 7 ACS Paragon Plus Environment


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and MSiNTs/PEI nanocomposites (MP-30, MP-40, MP-50, MP-60) are given in Fig.1c&d and Table 1, respectively. HNTs and MSiNTs showed a typical type-IV adsorption isotherm (Fig. 1c). The obvious hysteretic loops at 0.2 < P/P0< 1.0 indicated a mesoporous structure. With the introduction of 30, 40, 50, and 60% PEI, the pores were gradually filled and the hysteretic loops became slimmer. As shown in Fig.1d for HNTs, the mesopores were centered at 1.7~7.3 nm, and the pore size of MSiNTs was centred in the range of 2~30 nm. After loading the PEI, the decrease of the pore size with increasing the PEI loading amount was due to the modification of amine groups on HNTs. The SBET, pore volume of MSiNTs were estimated to be 366.4 m2/g, 0.55 cm3/g, respectively. The SBET value was 6 times and pore volume was more than 2 times larger than that of HNTs, which can provide more accessible sites. The average pore size of MSiNTs reduced from 15.3 nm to 6.0 nm, implied that some newly formed smaller mesopore/micropore could exist in MSiNTs. The increased BET specific surface area of MSiNTs was mainly associated with the mesopore/micropore formed by the space from the removed Al3+. After PEI loading in MSiNTs, the SBET and pore distribution were altered. The total pore volume of mesopores and SBET decreased with increasing the amount of loaded PEI. However, the average pore size increased after impregnation, suggesting that the newly formed pores were reduced and PEI was loaded within the outer surface instead of the inside of the nanotubes. When the content of PEI increased, the pore size decreased. The pore size of the nanocomposite was centered in the range of 2~10 nm (Fig. 1d), which is larger than the Kinematics diameter of CO2 molecules (0.33 nm). Fig. 2a shows the cylindrical hollow HNTs with average length in the range of 0.7~1.4 μm, with the outer diameter around 60~80 nm and the inner diameter about 20~30 nm (Fig. 8 ACS Paragon Plus Environment

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2d). HNTs contain main elements such as O, Al, Si, as authenticated by the EDS results. After heat and acid treatment, MSiNTs retained the characteristic tube morphology, however the length is obviously reduced compared with HNTs having the length of 0.3~0.7 μm. The morphology of MSiNTs like HNTs was broken into smaller tubes. From the inside image of Fig.2e, MSiNTs contains two-level pores, the original tubes and mesoporous walls resulted from the leaching of alumina, and the pore size distributions of MSiNTs also validate this results from inset in Fig.1d, the newly formed mesopores were centered in the range of 2~5 nm, this results agree well with the Zhu’s work46. In addition, MSiNTs mainly contains Si and O, and there is little Al content retained as expected. MP-50 sample shows that the tubes were wrapped in the gelatinous PEI, like these tubes sunk into glue (Fig. 2c) and the chain of PEI seems filled into the inner tubes or twined around tubes (Fig. 2f). The EDS spectrum further demonstrated the presence of N with three main constituents of O, Al and Si in MP-50 due to the interaction between PEI and HNTs. The TG curves for CO2 adsorption capacity measurement for HNTs, MSiNTs and MP-50 were shown in Fig. 3. All samples were pre-treated at 100°C for 60 min under N2, and subsequently cooled to 50°C for CO2 adsorption test. The weight of HNTs was gained at first due to the absorption of N2, when the temperature rose to 100°C, there was a slight weight loss resulted from the removal of litter moisture. HNTs showed an obvious increase in absorption capacity in N2 flow or N2/CO2 flow, and then a subsequent plateau due to the saturation of absorption capacity. After heat and acid treatment, MSiNTs showed the different phenomenon. The weight of MSiNTs reduced at first step due to its higher BET value and porous characteristics, which could supply the absorbed water with enough space. Meanwhile 9 ACS Paragon Plus Environment


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the loss of moisture adsorbed by MSiNTs was more than the amount of absorbed gas. When the temperature cooled to 50°C and feed gas increased, the behavior of gas adsorption dominated, so there shows a slight weight increase. As for MP-50 sample, the weight decrease at the initial stage was possibly due to the removal of the absorbed H2O and CO2, subsequently showed a balance of weight in the flowing of N2 gas. When the feed gas changed to the N2/CO2, the sample weight began to increase (the weight gain in Fig. 3), which indicated the CO2 adsorption in this stage, and the sample also showed the selectivity of CO2 adsorption from a mixture gas flow when the feed gas changed. Fig. 4a presents the comparison of CO2 adsorption on the nanocomposites with different PEI loading measured by TG at 50°C for 2 h under 40 mL/min N2 and 60 mL/min CO2. The results showed that the amount of PEI loading could affect the CO2 adsorption capacity of the nanocomposite, which had an increase in the range of 1.29~1.83 mmol/g at the PEI loading from 30% to 50%. When the PEI content reached 60%, the CO2 adsorption capacity decreased. As for the amine-based nanocomposites, the CO2 adsorption capacity mainly depends on the accessibility of amine groups on the polymer which interacts with CO2, the more loading will provide more sites for CO2 adsorption. On the other hand, the amount of loaded amine is not the sole factor measuring the CO2 uptake47, the growth of the polyamine chains needs enough space, because too much amine loading agglomerated together will lead to the pores becoming plugged and hindering CO2 adsorption. Therefore, 50% could be the optimum amine loading amount suitable for the high-performance adsorbents. The CO2 isotherm of MP-50 at different temperatures (25~85°C) showed that the adsorption capacity increased with increasing the temperature (Fig. 4b), while the capacity decreased when the 10 ACS Paragon Plus Environment

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adsorption temperature reached up to 95°C. Increasing temperature could accelerate the PEI molecules motion in these nanocomposites, simultaneously led to the enhanced adsorption capacity. A more detailed CO2 capture analysis of MP-50 in 8 min (Fig. S1) showed that the exothermic peak (within 4 min) was present in the DSC curves, indicating that CO2 adsorption was exothermic, and the temperature influenced the equilibrium adsorption capacity. When the temperature reached up to 85°C, the equilibrium shifted to desorption, and showed a decrease of capacity. Furthermore, the exothermic is deeper with the increasing temperature expect at 95°C. The maximum CO2 uptake reached 2.75 mmol/g at 85°C in a mixture gases (40 mL/min N2, 60 mL/min CO2), which is higher than the others’ work about clay-based adsorbents, for example, PEI/HCl-modifed montmorillonite had a capacity of 112 mg CO2/g for CO2 capture even in a pure CO2 flow8. These curves in Fig. 4 also indicate that the CO2 uptake takes place in two stage process. The beginning stage is a sharp liner weight gain due to the surface chemical reaction, the second is a comparatively slow diffusion stage23. The chemical reaction between amines and CO2 on the sample surface decides the CO2 uptake rate at the beginning stage. Thereafter, the slow diffusion of the CO2 into the sorbents makes the rate slow down. The similar two-step adsorption process had also been seen in the other amine-based adsorbents48. Table 2 was used to estimate how fast the CO2 adsorption. From the analysis of CO2 isotherm for the nanocomosites with different amine loading amount, a peak adsorption rate arises at the initial 1~3 min, the adsorption capacity is 70% in 2 min, data for MP-50 shows that it takes 37 min to attain 90% of the peak adsorbed amount at 50°C. Additionally, the CO2 isotherm shows that MP-50 only needs 2 min to attain 70% of maximum adsorption amount when the 11 ACS Paragon Plus Environment


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temperature increased to 75°C, and remains stable kinetic performance at 85°C. From Fig. S1, the results also indicated CO2 adsorption was mainly reacted within 4 min, and the exothermic peaks of MP-50 at 75, 85, 95°C were all located in the range of 0~2 min, which indicated the rapid reaction process of MP-50. And the increasing temperature only decreases the CO2 adsorption capacity instead of slowing down the absorption rate. Three adsorption kinetic models have been considered for the adsorption kinetics of CO2 on the nanocomposite, including pseudo-first-order, pseudo-second order and fractional-order kinetic model. Pseudo-first-order indicated the reversible adsorption between gas and solid surface as an equilibrium, pseudo-second order was on the basis of a hypothesis that the chemical adsorption was the rate-controlling step48, and fractional-order kinetic model is used to describe the rate of CO2 chemical adsorption on the active sites of the nanocomposites, and the adsorption rate is hypothetically in proportion to the nth power of the driving force and mth power of the adsorption time. Fig. 5 shows the experimental CO2 adsorption capacity and their relevant fits to the following kinetic models: Qt =Qe -Qe e-k1t

Pseudo-first-order

(1)

Pseudo-second order

(2)

Fractional-order kinetic model

(3)

Q

Qt = Qe - 1+k etQ 2

Qt =Qe -(𝑄𝑒1−𝑛 +

𝑛−1 𝑚

e

1

𝑘𝑛 𝑡 𝑚 )1−𝑛

where, Qe (mmol/g) and Qt (mmol/g) is the adsorption capacity at equilibrium and a given time t, respectively. k1(1/min) and k2 (g/mmol min) is the adsorption rate constant for Pseudo-first-order, Pseudo-second order model, respectively. kn, m, and n is the constant of fractional-order model. Obviously, the Pseudo-first and Pseudo-second order model did not apply to the description for the adsorption kinetics of MP-50, suggesting that the CO2 12 ACS Paragon Plus Environment

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adsorption of MP-50 is neither physisorption nor chemisorption. The fractional-order kinetic model agreed well with the experimental results than two others (Fig. S2). Fig. 5 represents the adsorption capacity of MP-50 and correspondingly fits to fractional-order kinetic model, and the calculated parameters are listed in Table S2. The R2 coefficient of determination value is in the range of 0.950 to 0.996, and MP-50 fit very well at 85°C which R2 reached up to 0.996. Chemical stability of the amine-based adsorbents is an important issue during their regeneration process. In this work, the regenerability test of the nanocomposite was performed (Fig. 6), showing the reversibility test for CO2 adsorption on the MP-50 sample up to 10 cycles. 5 cycles test of MP-50 at 75°C indicated that the saturated adsorption capacity slightly decreased in the adsorption efficiency with the cycle number (Fig. S3). The adsorption capacity reduced to 2.66 mmol/g from 2.75 mmol/g after 10 cycles, the decrease may be due to the degradation of PEI; and MP-50 also remained stable during the entire 5 cycles tests (Fig. S3). The regenerability tests demonstrated that MP-50 nanocomposite could meet the practical industrial requirements (2~3 mmol/g after 10 cycles) 48. In addition, Table 3 provides a comparison of the reported CO2 adsorbents with our MSiNTs/PEI nanocomposite. The top half of Table 3, from HP20/PEI-50 to S14-50PEI, shows all kinds of PEI–based CO2 adsorbents. The supporting materials of these samples generally need a complex synthesis process or a high cost. The application of clays in this field has also attracted attention recently. The bottom half of Table 3 shows some works on clay used for supporting materials, such as montmorillonite and bentonite. Compared with these work, our MSiNTs/PEI nanocomposite shows a better adsorption properties. 13 ACS Paragon Plus Environment


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Furthermore, the thermal stabilities of MSiNTs and nanocomposites were investigated (Fig. 7). MSiNTs and nanocomposites with different PEI impregnation amount were tested from 25 to 850°C in N2, all curves decrease below 100°C was possibly related to the removed moisture and solvent and/or others. MSiNTs showed a whole weight decrease of 18.6% at 25~850°C. Therefore, the samples should be pretreated in N2 before adsorbing CO2. MSiNTs/PEI nanocomposites tended to be stable with temperature increasing, and a sharp weight loss between 200 and 370°C was clear in all samples, which was associated with the degradation/decomposition of PEI chains11, thermally decomposed completely when the temperature increased above 400°C. The results showed that these nanocomposites were thermally stable until 200°C, therefore, the evaluation on their CO2 adsorption performance below 200°C was correspondingly necessary. Through the above results, an overall schematic presentation of the synthesis procedure of MSiNTs/PEI nanocomposite is shown in Fig. 8. Thermal treatment activated the aluminosilicate network of HNTs, and subsequent acid treatment selectively removed the alumina component, therefore produced mesoporous silica nanotubes (MSiNTs). After PEI impregnated in MSiNTs, a view of MSiNTs with PEI loaded was obtained from following figure, MSiNTs assemble a structure with PEI. The nanotubes played as a dispersing agent, which helped to form a higher surface area and pore volume solid sorbent, therefore is beneficial to more CO2 gas adsorption. PEI existed in two sites in the adsorbent, partial PEI was confined within the nanotubes of MSiNTs, and most of PEI was wrapped within the outside of the nanotubes.

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4. CONCLUSIONS In summary, we developed the polyethenimine (PEI) impregnated MSiNTs (MP) as an emerging nanocomposite for CO2 capture. Heat and acid pretreatment effectively promoted the specific surface area and pore volume of MSiNTs, these nanotubes assembled a whole structure with PEI to form an emerging nanocomposite. Well dispersion of PEI within MSiNTs could provide more space for CO2 diffusion. The nanocomposite with PEI loading of 50 % had a peak adsorption capacity of 2.75 mmol/g at proper conditions. Moreover, they showed quick kinetics and better stability on 10 cycles of CO2 adsorption/desorption behavior. The as-prepared nanocomposite is believed to be promising for CO2 capture due to environmentally friendly, low-price, thermally stable and high efficiency characteristics.

Supporting Information Supporting Information is available, including FTIR assignments of HNTs and the parameters of kinetic models for CO2 adsorption over samples.

Author Information Corresponding Authors *

E-mail:

[email protected],

[email protected].

Tel:

+86-731-88710804.

Notes The authors declare no competing financial interest.


+86-731-88830549,

Fax:


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Acknowledgements This work was supported by National Science Fund for Distinguished Young Scholars (51225403), the National Natural Science Foundation of China (41572036), State Key Lab of Powder Metallurgy, Central South University (2015-19) and the Hunan Provincial Co-Innovation Centre for Clean and Efficient Utilization of Strategic Metal Mineral Resources (2014-405).

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Table 1

Porous parameters of HNTs, MSiNTs and nanocomposites

Sample

SBET (m2/g)

Total pore volume (cm3/g)

Average pore size (nm)

HNTs

63.4

0.24

15.3

MSiNTs

366.4

0.55

6.0

MP-30

76.1

0.35

18.2

MP-40

52.5

0.17

12.9

MP-50

18.3

0.04

8.4

MP-60

23.5

0.03

4.5



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Table 2 CO2 capture kinetics of nanocomposites Time elapsed for interim adsorption amount (min) C2h(mmol/g) Samples

t70%

t80%

t90%

25°C

50°C

75°C

85°C

95°C

25°C

50°C

75°C

85°C

95°C

25°C

50°C

75°C

85°C

95°C

25°C

50°C

75°C

85°C

95°C

MP-30

-

1.25

-

-

-

-

4

-

-

-

-

14

-

-

-

-

29

-

-

-

MP-40

-

1.75

-

-

-

-

4

-

-

-

-

18

-

-

-

-

46

-

-

-

MP-50

1.23

1.83

2.32

2.75

2.18

11

4

2

2

3

29

16

7

11

10

63

37

32

40

33

MP-60

-

1.47

-

-

-

-

4

-

-

-

-

41

-

-

-

-

65

-

-

-


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Table 3

Comparison of different samples for CO2 capture under dry conditions

Samples

Support

Amine

Capacity (mmol/g)

CO2 partial pressure (Bar)

T(°C)

Ref.

HP20/PEI-50

Resin

PEI

1.92

1

75

20

PEI (40 wt%)/ PAF-5

Porous aromatic framework

PEI

2.52

0.15

40

49

50%-PEI–silica

Precipitated silica

PEI

3.14

1

75

12

Silica gel-PEI-50

Silica gel

PEI

1.775

1

40

50

SBA-15-PEI (50)

SBA-15

PEI

2.05

1

75

51

S14-50PEI

Mesocellular silica foams

PEI

2.3

1

25

52

30PEI/bentonite

Bentonite

PEI

1.1

1

75

53

MMT CTAB N2

Montmorillonite

AEAPTS

2.4

1

100

54

C-APTMS-PEI

Montmorillonite nanoclay

APTMS+ PEI

1.7

1

85

55

PEI/Mon_HCl_6M

Acid-montmorillonite

PEI

2.54

1

75

8

HNTs-NH2

Halloysite

APTES

0.13

1

25

28

MP-50

Mesoporous silica

PEI

2.75

0.6

85

This work

23

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Figures captions

Fig. 1 (a) XRD patterns of HNTs, MSiNTs, and nanocomposites, (b) FTIR spectra of HNTs, MSiNTs, and nanocomposites, (c) Nitrogen adsorption/desorption isotherms of HNTs, MSiNTs, and nanocomposites and (d) corresponding pore size distributions. Fig. 2 SEM images of (a) HNTs, (b) MSiNTs, (c) MP-50; TEM images of (d) HNTs, (e) MSiNTs, (f) MP-50; and EDS patterns of (g) HNTs, (h) MSiNTs, (i) MP-50. Fig. 3 TG curves for HNTs, MSiNTs and MSiNTs/PEI nanocomposite (MP-50) Fig. 4

(a) CO2 isotherms of nanocomposites with different PEI content at 50°C for 2 h in

flowing N2/CO2 flow, (b) effect of sorption temperature on CO2 sorption capacity of MP-50 Fig. 5 Adsorption capacity of MP-50 and corresponding fits to fractional-order kinetic model. Fig. 6

Reversibility test for CO2 adsorption on MP-50 under sorption conditions: 40

mL/min N2, 60 mL/min CO2 at 85°C for 2 h, and desorption conditions: 40 mL/min N2 at 110°C for 40 min. Fig. 7 Thermal stabilities of MSiNTs, and nanocomposites Fig. 8 Proposed synthesis illustration of an emerging nanocomposite for CO2 capture


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Fig. 1


(a) XRD patterns of HNTs, MSiNTs, and nanocomposites; (b) FTIR spectra of HNTs,

MSiNTs, and nanocomposites; (c) Nitrogen adsorption/desorption isotherms of HNTs, MSiNTs, and nanocomposites and (d) corresponding pore size distributions.



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Fig. 2 SEM images of (a) HNTs, (b) MSiNTs, (c) MP-50; TEM images of (d) HNTs, (e) MSiNTs, (f) MP-50; and EDS patterns of (g) HNTs, (h) MSiNTs, (i) MP-50.


Page 27 of 33

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Fig. 3

TG curves for HNTs, MSiNTs and MSiNTs/PEI nanocomposite (MP-50)



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Fig. 4

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(a) CO2 isotherms of nanocomposites with different PEI content at 50°C for 2 h in

flowing N2/CO2 flow, (b) effect of sorption temperature on CO2 sorption capacity of MP-50


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Fig. 5 Adsorption capacity of MP-50 and corresponding fits to fractional-order kinetic model.



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Fig. 6 Reversibility test for CO2 adsorption on M-50 under sorption conditions: 40 mL/min N2, 60 mL/min CO2 at 85°C for 2 h, and desorption conditions: 40 mL/min N2 at 110°C for 40 min.


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Fig. 7

Thermal stabilities of MSiNTs, and nanocomposites



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Fig. 8 Proposed synthesis illustration of an emerging nanocomposite for CO2 capture


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