N2 selectivity in novel hollow silica particles by modification with multi-walled carbon nanotubes containing amine groups

Polyhedron 166 (2019) 175–185

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Improved CO2 adsorption capacity and CO2/CH4 and CO2/N2 selectivity in novel hollow silica particles by modification with multi-walled carbon nanotubes containing amine groups Sara Zohdi a, Mansoor Anbia a,⇑, Samira Salehi b a b

Research Laboratory of Nanoporous Materials, Faculty of Chemistry, Iran University of Science and Technology, Farjam Street, Narmak, P.O. Box 16846-13114, Tehran, Iran Environment Research Department, Energy and Environment Research Center, Niroo Research Institute, End of Dadman Blvd, Shahrak-e-Ghods, P.O. Box 14665-517, Tehran, Iran

a r t i c l e

i n f o

Article history: Received 10 December 2018 Accepted 1 April 2019 Available online 6 April 2019 Keywords: Carbon dioxide Nanocomposite Hollow silica particles Carbon nanotubes Gas adsorption

a b s t r a c t Accumulation of greenhouse gases (GHGs) in the atmosphere is responsible for enhanced global warming of our planet. In the present study, hollow silica spherical particles (HSSP) and multi-walled carbon nanotubes containing amine groups composite (denoted NH2-CNT@HSSP) were synthesized as adsorbents in a single-step and used to investigate the adsorption capacity and selectivity of CO2, CH4 and N2. The crystal structures and morphology of HSSP modified with different amounts of NH2-CNTs were characterized by X-ray diffraction (XRD), Fourier transform infrared (FT-IR), Scanning electron microscopy (SEM), Transmission electron microscopy (TEM), thermogravimetric analysis (TGA) and N2 adsorption/desorption isotherms. The CO2 adsorption capacity of the NH2-CNTs/HSSP is better than HSSP. CO2 adsorption capacity for HSSP is about 7.71 mmol g
1 when HSSP were composited with the amine-modified carbon nanotubes, CO2 adsorption capacity is increased (from 7.71 to 13.80 mmol g
1) at 298 K and 20 bar. CO2 capture on the adsorption sites by amine loading is believed to occur via chemisorption mechanism by formation of ammonium carbamate. The optimum adsorbent showed its stability with the adsorption capacity remaining constant over three adsorption/desorption cycles. Ó 2019 Published by Elsevier Ltd.

1. Introduction Human activities result in the generation of GHGs into the atmosphere. The top three GHGs are; CO2 (72%), CH4 (18%), and N2O (9%) [1]. The global warming caused by the increased levels of these gases is one of the most serious environmental threats to the human race at present [2,3]. CO2 is recognized as a major component of GHG that contributes to global warming. On the other hand, the selective removal of CO2 from gaseous mixtures is important for natural gas upgrading and CO2 capture from flue gas. Separation needs for CO2 from N2 and hydrocarbons, mostly CH4, can be attributed to the minimization of global warming and enhancement of fuel value of CH4 [4]. The significant quantities of natural gas (biogas, coal-seam, and landfill gases) cannot be used efficiently due to the presence of CO2 and N2 in these gas sources, which lowers their energy content. Therefore, in CO2 sequestration and capture, a cost-effective and environmentally benign CO2 capture and storage medium is the key to its successful implementation [5,6]. CO2 capture methods ⇑ Corresponding author. Fax: +98 21 77491204. E-mail address: [email protected] (M. Anbia). https://doi.org/10.1016/j.poly.2019.04.001 0277-5387/Ó 2019 Published by Elsevier Ltd.

can be based on adsorption on solids, absorption in liquids, the differential permeability of gas molecules through membranes, cryogenic separation or a combination of these processes [7,8]. The choice among these methods depends on the operational cost and specifications of the feed gas and final product. Absorption technology using the aqueous solution of alkanolamines has been recognized as the most popular one [9]. The major drawbacks associated with liquid amine solutions for CO2 separation are large amount of energy required for regeneration of the solvent, extensive corrosion of the equipment, and the oxidative solvent degradation [10]. Cryogenic separation has the drawbacks of high energy requirement and not being suitable for low flow rates, while membrane separation does not have high selectivity and thus is not economical for bulk separation [11]. Among them, the adsorption process has always attracted much attention due to its relatively low operating and capital costs as well as abundant selection of adsorbents. Recently, various solid adsorbents have been examined worldwide for CO2 adsorption. In a number of excellent reviews on CO2 adsorption, it could be understood that to develop a suitable CO2 capture adsorbent should satisfy (1) high CO2 adsorption capacity, (2) high CO2 selectivity, (3) fast kinetics, (4) low heat

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capacity, (5) low-cost raw materials and (6) chemical, mechanical and thermal stabilities under extensive cycling [12]. Porous materials such as activated carbons, mesoporous silicas, metal–organic frameworks (MOFs), multiwall carbon nanotubes (MWCNTs), hollow spherical silica particles (HSSPs), zeolites and etc offer a wide variety of structures and compositions that are suitable for adsorption of various gases, including CO2 [13–15]. The HSSPs due to excellent CO2 capturing properties, high surface area, inexpensive precursors, and high pore volume in one hand and physical, chemical, and thermal stability in other hand is desirable to CO2 adsorption [16]. In the last decade, silica adsorbents have been prepared by complicated multi-steps methods using expensive precursors such as tetraethyl orthosilicate (TEOS), tetramethylorthosilicate (TMOS), phenyltrimethoxysilane (PTMS), and organic templates [17–19]. Monodisperse HSSP were prepared without using templates via a two-step method in an aqueous solution [20,21]. In the first step, the hydrolysis of silicate source was performed under acidic conditions. Hydrolysis time plays an important role in the formation of HSSPs. In the second step, the condensation of the silane progressed under basic conditions, resulting in production of monodisperse HSSPs. The adsorption performance of HSSPs, surface area, and pore volume can be increased by their modification. Common modifications of silica adsorbents, for improvement of CO2 adsorption capacity are incorporation of basic organic groups (amine) into their structure and employing different inorganic metal oxides (alkali metal or alkali-earth metal). Amine modification leads to an increase in CO2 adsorption capacity and selectivity due to chemical interactions between amine sites and CO2 molecules [22]. Modifying silicate surfaces with amino groups has been shown by some authors to lead to adsorbents with increased capacity as compared to the pristine materials [23]. Moreover, a number of studies have been conducted focusing on the preparation of mesoporous-based composite materials for the improvement of CO2 adsorption capacity [24]. MWCNTs applied as composite fillers, have been investigated for different applications and increased composite performance has been achieved [25]. We suggest a strategy to improve the adsorption capacity of the original material by adding MWCNTs. Intermixing MWCNTs with HSSPs lead to appearance of unique physical and chemical properties viz. good dispersibility and high pore volume, which is associated with increasing rapidity of adsorption. In this work, the adsorption capacity and selectivity of CO2, CH4 and N2 on amine-modified HSSP adsorbents have been obtained. Experiments were done through the application of volumetric technique at temperature range of 298–348 K and pressures up to 20 bars. The affordable, simple, and effective method was used to prepare of HSSPs through the co-precipitation technique. The CO2 adsorption capacity and selectivity of HSSP was enhanced using amine-modified MWCNTs. Presence of amine functional groups on the surface of MWCNT due to Lewis base properties can be increasing chemical CO2 adsorption that in this research was investigated. The crystal structures and morphology of synthesized adsorbents were characterized by XRD, FT-IR, SEM, TEM, TGA and N2 adsorption/desorption isotherms.

2.2. Synthesis of hollow silica spherical particle (HSSP) The preparation procedure of HSSPs was as follows: 30 mL of sodium silicate (28.5 wt%) was mixed with 750 mL of deionized water, and the mixture was vigorously stirred at 25 °C, then 225 mL of ethanol (95%) was added to this mixture with an addition rate of 15 mL/s. The resulting mixture was stirred for 85 min at room temperature. A white precipitate was recovered by filtration and washed several times with deionized water and dried in a vacuum oven at 90 °C for 7 h. 2.3. Synthesis of low amine modified MWCNT@HSSP nanocomposite (L/NH2-CNT@HSSP) Modification of the prepared NH2-CNT/HSSP was performed as follows: First, the MWCNTs were prepared by dispersing 0.15 g NH2-CNTs in 50 ml of deionized water and sonicated for 30 min to produce NH2-CNTs solution. After that, 30 mL of sodium silicate was mixed with 670 mL of deionized water, and the mixture was vigorously stirred for 10 min, then NH2-CNTs solution was added into the mixture, and at the same time, 225 mL of ethanol was added into the mixture. The resulting mixture was stirred for 120 min at room temperature. Finally, the obtained precipitate was filtered and washed repeatedly with 100 mL deionized water and dried in a vacuum oven at 90 °C for 4 h. 2.4. Synthesis of high amine modified MWCNT/HSSP nanocomposite (H/NH2-CNT@HSSP) For the preparation of H/NH2-CNT@HSSP, a similar process to that of the L/NH2-CNT@HSSP was used and 0.15 g NH2-CNTs was replaced by 0.3 g NH2-CNTs. 2.5. Characterization

2. Experimental

The features of HSSP and NH2-CNTs@HSSP were identified by Xray diffraction on a Philips 1830 diffractometer with Cu-Ka radiation. Adsorption-desorption isotherms of the modified and unmodified hollow silica materials were measured at 77 K on micromeritics model ASAP 2010 sorptometer to calculate the physical parameters. The BET specific surface area and the pore size distribution were determined by the BET (Brunauer, Emmett, and Teller) and BJH (Barrett–Joyner–Halenda) methods, respectively. The detection of functional groups of samples was considered by utilizing FT-IR analysis on a DIGILAB FTS 7000 instrument under attenuated total reflection (ATR) mode using a diamond module. SEM (PHILIPS XL30) is an instrument that applied to the characterization of organic and inorganic surface morphology samples. The prepared silica adsorbents were coated with gold before scanning to enhance their conductivity. The morphology of the samples was examined by a ZeissEM10C-100 KV transmission electron microscope (TEM) with a field emission gun operating at 200 kV. Thermogravimetric analysis was used to determine the thermal stability of the hollow silica materials and was carried out from room temperature to 800 °C using a Mettler Toledo 851 TGA/DTA analyzer at a heating rate of 20 °C/min under an air atmosphere.

2.1. Materials

2.6. Gas adsorption measurement

All the chemicals used were of analytical grade from Merck (Germany), except commercially available MWCNTs modified by an amine (NH2-CNTs, 0.45 wt %) that was supplied from Neutrino Co (Iran).

The gas (CO2, CH4, and N2) adsorption isotherms at different temperatures were determined by utilizing a laboratory setup according to a volumetric method that has been schematically shown in Fig. 1. The details of setup have been reported in previous

S. Zohdi et al. / Polyhedron 166 (2019) 175–185

Fig. 1. Setup for adsorption capacity test.

work [26]. At first adsorption cell (14) was filled with 0.5 g of adsorbent and attached to the system. After wards, to remove residual solvents trapped in nanopores during synthesis and washing, all the valves 8, 12, 13 and 16 were opened and other valves were closed Then, the vacuum pump was turned on the system was vacuumed and heated at 493 K for 1.5 h. After degassing the adsorption system, the temperature was decreased to the experiment temperature (273 K). Ultra-high purity gas was introduced into adsorption unit for the gas adsorption measurements. To perform an adsorption test, the valve of the gas cylinder opened the valves 1, 3, 5, 7, 8 were opened while other valves were closed and the gas pressure was regulated at the desired value, then valves 7 and 8 were opened to reach a pressure balance in the reference cell (9). Afterwards, valve 13 was immediately opened and the pressure dropwas recorded. 3. Results and discussion 3.1. Structural and textural properties of HSSP and NH2-CNT@HSSP nanocomposites The structural ordering and quality of hollow silica samples were assessed by powder X-ray diffraction. The XRD patterns for HSSP as well as modified hollow silica materials are shown in Fig. 2. It can be seen from Fig. 2 that the XRD patterns of NH2CNT@HSSP nanocomposites showed the same diffraction patterns as of HSSP. This confirms that the structure of HSSP was not

Fig. 2. XRD patterns of (a) HSSP, (b) L/NH2-CNT@HSSP and (c) H/NH2-CNT@HSSP.

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affected by the NH2-CNT incorporation. Furthermore, in the nanocomposites, the weak peak of CNT at 2H = 26° cannot be observed because of its overlapping with high-intensity peaks of HSSP. It was noticed that the intensity of the characteristic peaks of HSSP decreased with NH2-CNT loading, which may be ascribed to a fairly strong binding of NH2-CNT to HSSP. The FT-IR spectra of the modified and unmodified hollow silica materials are plotted in Fig. 3 in the 400–4000 cm
1 wavenumber region. The FT-IR spectrum of the HSSP exhibited a Si-O-Si stretching vibrational absorption peak at 1050 cm
1, and OAH stretching vibrational absorption peak was observed at 3435 cm
1. In the MWCNTs spectrum, the absorption band at 1200 cm
1 is assigned to CAC stretching vibration, and the band at 2850 to 3000 cm
1 is related to stretching vibration of CAH and the absorption bands in 1600–1650 cm
1 are attributed to bending vibration of the CAH bond. After modification with NH2-CNTs, prominent peaks appeared at 3425–3409 cm
1 which can be related to NAH symmetric and asymmetric stretching vibrations of primary amines. The IR results further confirmed that NH2-CNT had successfully been loaded onto the surfaces of HSSP during impregnation. SEM is employed to examine dispersion and confirm the particular presence of the CNTs in the synthesized nanocomposites. The SEM images of the HSSP revealing spherical nanoparticles with narrow particle size distribution. From Fig. 4 c and d, it was observed that NH2-CNTs with a tubular structure formed a threedimensional network. Fig. 5 shows the SEM images of NH2-CNT@HSSP nanocomposites with different NH2-CNT loadings. As can be seen, after MWCNT incorporation, the shape of HSSPs in the nanocomposite has not changed. The morphologies of NH2-CNT@HSSP nanocomposites reveal that the MWCNTs are indeed well attached with HSSP. The morphology of the HSSP and NH2-CNT@HSSP nanocomposites were obtained by TEM and is presented in Fig. 6. TEM images of the HSSP spheres (Fig. 6 a and b) showed that the silica spheres have the hollow structure. The particles appear translucent, and the shell thickness is given by the width of the dark rims. After HSSP was modified with NH2-CNT, the spherical structure was preserved and the hollow structure of HSSP did not change. N2 adsorption–desorption analysis is the best choice to gain reliable information about porous materials. Fig. 7 illustrates the nitrogen adsorption–desorption isotherms of HSSP and NH2CNT@HSSP nanocomposites. Nitrogen adsorption/desorption isotherm of HSSPs was Type IV, which is characteristic of mesoporous adsorbents [27]. This type of hysteresis loop is known to be associated with hollow particles with mesoporous shells. An adsorption at P/P0 values in the range

Fig. 3. FT-IR spectra of (a) HSSP, (b) L/NH2-CNT@HSSP and (c) H/NH2-CNT@HSSP.

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Fig. 4. SEM images of (a and b) HSSP and (c and d) NH2-CNT.

Fig. 5. SEM images of (a and b) L/NH2-CNT@HSSP and (c and d) H/NH2-CNT@HSSP.

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179

Fig. 6. TEM images of (a and b) HSSP, (c and d) L/NH2-CNT@HSSP and (e and f) H/NH2-CNT@HSSP.

of 0.75–0.85 could be related to the capillary condensation of nitrogen inside mesopores in the shells, and the second adsorption at P/P0  0.95 related to the filling of the huge hollow cores. Nanoporous nature is also confirmed from the steep rise at the high relative pressure in the isotherm. After NH2-CNTs loading, the Sshaped adsorption isotherm is still preserved [28]. This implies that the mesostructure of HSSPs has not been destroyed after the reaction between HSSP and NH2-CNT. The physical parameters obtained from the N2 isotherms, such as the Brunauer–Emmett–Teller surface area (SBET), total pore vol-

ume and average pore size, for the hollow silica nanocomposites, are shown in Table 1. Drastic changes both in isotherm and pore size distribution can be recognized after NH2-CNT loading. The BET surface area and pore volume of NH2-CNT@HSSP nanocomposites are higher than HSSP. The pore size distribution (PSD) curves in HSSP and NH2CNT@HSSP nanocomposites calculated by the BJH method from N2 adsorption data at 77 K are given in Fig. 8. The narrow pore size distributions centered at about 13.23 nm for HSSP and 7.52 nm for H/NH2-CNT@HSSP clearly demonstrate that during the preparation

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Fig. 7. Adsorption-desorption isotherms of nitrogen at 77 K on HSSP and NH2-CNT/HSSP nanocomposites.

Table 1 Textural properties determined from nitrogen adsorption–desorption experiments at 77 K. Adsorbent

SBET (m2 g
1)

VP (cm3 g
1)

Average pore size (nm)

HSSP L/NH2-CNTs@HSSP H/NH2-CNTs@HSSP

165 188 210

0.21 0.23 0.24

13.23 9.03 7.52

Fig. 9. TGA plots of (a) HSSP, (b) L/NH2-CNT@HSSP and (c) H/NH2-CNT@HSSP.

3.2. Adsorption capacity and selectivity of HSSP

Fig. 8. PSD of (a) HSSP, (b) L/NH2-CNT@HSSP and (c) H/NH2-CNT@HSSP.

of H/NH2-CNT@HSSP, the mesopore structure of HSSP did alter significantly. The pore size was reduced with the increasing of NH2CNT loading. Hence, it is assumed that upon loading NH2-CNTs, some filled pores are not accessible to N2 molecules so that the N2 uptake of NH2-CNT@HSSP nanocomposites is decreased. The TGA patterns of HSSP, L/NH2-CNT@HSSP, and H/NH2CNT@HSSP are shown in Fig. 9. For HSSP, only one weight loss was found to occur between 50 °C and 150 °C which is due to the evaporation of physisorbed water and other adsorbates. The TGA analyses confirmed that silica substrate is strongly resistant to thermal treatment as it lost a very low amount of its weight up to about 150 °C and then remained stable when temperature increased to 800 °C. After NH2-CNT loading in HSSP, except the first peak caused by the evaporation of physisorbed water and other adsorbates, TGA profile shows another mass loss peaks about 550 °C that is related to the decomposition of amine groups in CNT. Fig. 9 further indicates that both modified nanocomposites have almost similar thermal stability. For the NH2-CNT/HSSP nanocomposites, the weight loss increases with the NH2-CNT content.

To investigate the temperature-dependent adsorption of gas we applied the volumetric adsorption apparatus. With this device, gas adsorption experiments were conducted at pressures of 0–20 bar and temperatures of 298–348 K. The single-component adsorption isotherms of CO2, CH4 and N2 on HSSP are plotted in Fig. 10. It can be seen that the isotherms of CH4 and N2 are almost linear, unlike CO2, representing weak interactions between HSSP and those two gases. It is clear that at all temperatures, CO2 adsorption was the highest, followed by CH4 and N2. The quadrupole moment creates a strong attraction to the surface of adsorbent resulting in a higher adsorption. CO2 and N2 and molecules possess quadruple moments, but a CH4 molecule has no dipole or quadrupole moment. However, both CO2 and N2 molecules have quadruple moment, the quadruple moment of CO2 is about three times higher than that of N2 and that helps it to be preferentially adsorbed on the surface of HSSP. The properties of the adsorbate gases are reported in Table 2. From the table, it can be seen that the large polarizability and quadrupolar moment of CO2 are predominantly the cause of the difference in adsorption behavior. The effect of temperature on the adsorption of gas by HSSP was shown in Fig. 10. It has been shown that the gas adsorption decrease with increase in temperature (298–348 K). The temperature increases the kinetic energy of gas molecules, as a result, the reaction between the gas molecules and the active sites of the hollow silica is difficult. This is explained by the fact that the at higher temperature the desorption rate on the surface accelerate which leads to the increased tendency of the gas molecules to escape from the adsorbent surface and thus the decrease in adsorption

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Fig. 10. Adsorption isotherms of CO2, CH4 and N2 on HSSP at different temperatures (298 K, 323 K and 348 K).

3.3. Adsorption capacity and selectivity of NH2-CNT@HSSP nanocomposites

Table 2 The properties of adsorbate gases. Adsorbate

Polarizability (10
25 cm3)

Dipole moment (1018 esu.cm)

Quadrupole moment (10
26 esu.cm2)

CO2 CH4 N2

26.5 26.0 17.6

0.00 0.00 0.00

4.30 0.00 1.52

was obtained. The maximum adsorption capacity for HSSP is at 298 K. The selectivity of CO2/CH4 and CO2/N2 was estimated from the experimental single-component isotherms. Selective adsorption of HSSP for different gases (CO2, CH4, and N2) was calculated by using Eq. (1) [29]:

a1=2 ¼

q1 q2ðTPÞ

ð1Þ

where a1/2 is the adsorption selectivity of gas 1 over gas 2 and q is the adsorption capacity of gases in mmol g
1 at a certain temperature (T) and pressure (P). It can be seen from Fig. 11 that in the whole-pressure range, the selectivity of CO2/N2 was higher than that of CO2/CH4. Higher CO2 and lower N2 capacities which are always desirable to achieve a better CO2 separation from N2 were observed for HSSP. For all the above, HSSP shows moderate selectivities for CO2/CH4 and CO2/N2 separation.

Fig. 11. Adsorption selectivities of CO2/CH4 and CO2/N2 on HSSP at different pressures and 298 K.

To compare the adsorption capacity and selectivity between the pristine and amino-modified synthesized adsorbents, we performed pure CO2, CH4, and N2 adsorption on these samples. The gas adsorption isotherms of the of HSSP, L/NH2-CNT@HSSP and H/NH2-CNT@HSSP at 298 K are given in Fig. 12, where it can be seen that NH2-CNT@HSSP nanocomposites show higher amounts of adsorbed CO2 than HSSP. It can be seen from Fig. 12 that HSSP showed a physisorption-like behavior, with negligible CO2 adsorption at low pressures. The small amount of CO2 adsorbed on HSSP shows that the substrate does not interact very strongly with CO2 due to the surface OH groups are unable to induce adequately strong interactions and real adsorption sites are missing. It is also important to note that the type of functional group is an effective parameter on CO2 adsorption properties of nanoparticles. Several studies demonstrate that CO2 molecules are adsorbed more on amine-modified nanoparticles when compared to neat ones because the nucleophile reaction between gas molecules with NH2 functional groups is significantly stronger and more effective than with OH groups [30]. In addition, the performance of porous silica materials modified with amine groups could be improved by increasing amine loaded on the structure of porous silica substrate [31]. These silica materials have large pore volumes and high surface areas. By modifying the hollow silica surfaces with molecules with specific functionalities, the materials might become more useful as CO2 adsorbents. Investigation of adsorption isotherms shows that improvement in the surface chemistry of the hollow silica materials by introducing basic sites capable of interacting strongly with acidic CO2 in order to increase CO2 adsorption performance and to keep high selectivity for CO2 are considered very promising [32]. The mechanism of CO2 adsorption on amine- modified adsorbents has been reported in many publications [33]. Indeed, chemical adsorption happens through reactions between amino groups and CO2 molecules to form carbamate species. Many different porous materials loaded with basic nitrogen functionality has been synthesized and characterized to chemisorb CO2 from flue gas streams [34,35]. Chandra and coworkers obtained N-doped carbon produced by chemical activation of polypyrrole functionalized graphene sheets. The CO2 adsorption capacity at 25 °C reached 4.3 mmol/g [36]. On the other hand, Zhou and coworkers reported 2.3 mmol/g on N-doped nanoporous carbons in Y zeolite [37]. The

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mance was found to depend on the surface density of amine groups of the nanocomposite. The nanocomposite with the higher surface density of amines possesses higher adsorption capacity. It should also be noted that the chemical adsorption often occurs with a somewhat high rate at the initial stage, and then decreases due to less availability of active and exposed amine functional groups in addition to slow dispersion rate of CO2 molecules into the hollow silica adsorbent structure. Thus, in the later adsorption stage as well as the chemical adsorption, the physical adsorption of CO2 onto the adsorbent should be considered as effectively [40]. The results obtained indicate that with the increase of NH2-CNT loading amount, the adsorption capacity increase causes the CO2affinity sites increase. The adsorption capacities of L/NH2CNT@HSSP and H/NH2-CNT@HSSP are improved to 12.52 and 13.80 mmol g
1 respectively. The ideal selectivity of CO2/CH4 and CO2/N2, at ambient temperature and five equilibrium pressures was calculated, using Eq. (1), for L/NH2-CNT@HSSP, and H/NH2-CNT@HSSP and the results are illustrated in Fig. 13. As expected, the CO2/CH4 and CO2/N2 separation selectivity were affected by the incorporation of the amino group. The observed results presented in Fig. 13 indicate that for high loading NH2-CNT, the CO2/CH4 and CO2/N2 selectivity was higher, as compared to the low loading NH2-CNT. In addition, the selectivity of the H/NH2-CNT@HSSP for the CO2/CH4 is lower compared to the CO2/N2 because of higher adsorption capacity of CH4, as compared to N2. The NH2-CNT@HSSP nanocomposites showed higher equilibrium selectivities for CO2 over CH4 and N2, implying that they are potential adsorbents for selective separation of CO2 from landfill gases (CO2/CH4) and flue gases (CO2/N2).

3.4. Cyclic adsorption/desorption performance of adsorbent The performance of the adsorbent can be determined by regeneration process. Regeneration is a process of cyclic adsorption/desorption of adsorbate onto/from the adsorbent. Cyclic performance of the optimum adsorbent (H/NH2-CNT@HSSP) for CO2 is shown in Fig. 14 for three cycles. Regenerated adsorbent was exposed to CO2 environment for adsorption at 298 K and again regenerated at 393 K in helium atmosphere. The adsorption capacity was slightly lower after each cycle because the CO2 was incompletely desorbed, but the adsorption capacity remained good even after three cycles. The morphology and crystalline nature of H/NH2-CNT@H SSP after three CO2 adsorption/desorption cycles were investigated by SEM and XRD. SEM image (Fig. 15) and XRD pattern (Fig. 16) of the H/NH2-CNT@HSSP show that the crystal structures did not damage or destroy after three CO2 adsorption/desorption cycles.

Fig. 12. Adsorption isotherms of CO2, CH4 and N2 L/NH2-CNT@HSSP and H/NH2CNT@HSSP at different pressures and 298 K.

CO2 adsorption by Xing and coworkers on N-doped carbon is improved rather by hydrogen bonding interactions than by acid– base interactions [38]. The CO2 adsorption measured on their synthesized materials reached 4.24 mmol/g at 25 °C on carbon with the surface area of about 3000 m2/g. The high CO2 adsorption capacity (6.2 mmol/g) on polypyrrole derived carbons prepared by KOH activation proposed by Fuertes and coworkers [39]. In the NH2-CNT@HSSP nanocomposites, CO2 adsorption can occur by physisorption and chemisorption. The adsorption perfor-

Fig. 13. Adsorption selectivities of CO2/CH4 and CO2/N2 on amino-modified synthesized adsorbents at different pressures and 298 K.

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S. Zohdi et al. / Polyhedron 166 (2019) 175–185 Table 3 Comparison of adsorption capacity of CO2 onto various adsorbents.

Fig. 14. Cyclic CO2 adsorption/desorption performance of H/NH2-CNT@HSSP.

Adsorbent

Pressure (bar)

CO2 adsorption capacity (mmolco2 /gadsorbent)

References

MIL-101 PEHA-MIL-101 UMCM-1 MOF-200 IRMOF-1 IRMOF-14 PCN-6 ZSM-5 Silicate-1 MCM-41/Cu TRI-PE-MCM-41 Co-MO-5 Fe-MO-5 HTC MWCNT@MIL-101 MCM-48-PEHA-DEA CaX/PTh-15 LiX/PANI-40 CNT@MOF-199/30PZ CMK-3/PANI MIL-101/PPD HSSP L/NH2-CNTs@HSSP H/NH2-CNTs@HSSP

10 10 7 7 0–50 0–50 0–50 4 4 4 1 10 10 10 35 1 1 1 5 5 10 10 10 10

0.85 1.30 6.70 6.75 2.00 2.00 4.00 1.80 2.00 1.70 2.05 1.37 1.43 1.41 2.85 0.51 2.30 4.20 7.02 5.60 7.30 3.83 7.55 9.50

[41] [41] [42] [43] [44] [45] [46] [47] [48] [49] [50] [51] [51] [51] [13] [52] [53] [54] [55] [56] [57] This work This work This work

capacity and is useful in scale-up considerations. A comparison of adsorption capacity of CO2 on different adsorbents is shown in Table 3. The different competitive adsorption is probably due to the differences in the available adsorption sites on the surfaces of adsorbents. This outstanding ability of NH2-CNT@HSSP nanocomposites compared with other adsorbents is due to the unique structure of NH2-CNT@HSSP nanocomposites. 4. Conclusion Fig. 15. SEM image of H/NH2-CNT@HSSP after three CO2 adsorption/desorption cycles.

Fig. 16. XRD pattern of H/NH2-CNT@HSSP after three CO2 adsorption/desorption cycles.

3.5. Comparison with other adsorbents The value of CO2 adsorption capacity (mmolco2 /g adsorbent) is important to identify which adsorbent has the highest adsorption

The adsorption capacity and selectivity of CO2, CH4 and N2 on HSSP and NH2-CNT@HSSP nanocomposites were obtained by a volumetric adsorption setup in the pressure up to 20 bar and at the temperature ranging from 298 to 348 K. In this paper for synthesis HSSP used simple one-pot synthesis by sodium silicates. This method of synthesis is the economical, simple and safe. The morphology, surface area, and structure of the synthesized adsorbents were investigated by N2 adsorption-desorption isotherms, XRD, FT–IR, SEM, TEM and TGA. The novel nanocomposites prepared by incorporation of NH2-CNTs into HSSP. The NH2-CNT@HSSP nanocomposites were observed to possess higher gas adsorption performance compared to the performance of HSSP when used individually. The combination of HSSP with NH2-CNTs can be an approved way of making full use of the abundant pore structure of NH2-CNTs and high specific capacitance per unit area of HSSP. The NH2-CNT@HSSP nanocomposites exhibited improved adsorption capacity of CO2 by physisorption and chemisorption. The results of this study show that H/NH2-CNT@HSSP, with higher nitrogen content, has a higher CO2 adsorption capacity than L/ NH2-CNT@HSSP. The CO2 adsorption performance after three adsorption/desorption cycles indicated the high potential of H/ NH2-CNTs@HSSP for CO2 adsorption at third cycle without significant decrease in sorption capacity compared with first cycle. The H/NH2-CNT@HSSP has good adsorption performance of CO2 at 298 K as compared to many types of adsorbents documented in the literature, indicating that the H/NH2-CNT@HSSP is promising low-temperature adsorbent for CO2 adsorption. The results also indicated that the selectivity of CO2/CH4 and CO2/N2 is improved with increasing NH2-CNTs loading.

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Acknowledgments The authors are thankful to Research Council of Iran University of Science and Technology (Tehran) and Iran National Science Foundation (INSF) for the financial support of this study.

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