Amine-functionalized low-cost industrial grade multi-walled carbon nanotubes for the capture of carbon dioxide

Journal of Energy Chemistry 23(2014)111–118

Amine-functionalized low-cost industrial grade multi-walled carbon nanotubes for the capture of carbon dioxide Qing Liua , Yao Shia ,

Shudong Zhenga , Liqi Ningb ,

Qing Yec ,

Mengna Taoa , Yi Hea∗

a. Key Laboratory of Biomass Chemical Engineering of Ministry of Education, Department of Chemical and Biological Engineering, Zhejiang University, Hangzhou 310027, Zhejiang, China; b. Department of Earth and Planetary Sciences, Washington University in St. Louis, MO 63130, St. Louis, USA; c. Department of Life Sciences, Shaoxing University Yuanpei College, Shaoxing 312000, Zhejiang, China [ Manuscript received August 7, 2013; revised September 12, 2013 ]

Abstract Industrial grade multi-walled carbon nanotubes (IG-MWCNTs) are a low-cost substitute for commercially purified multi-walled carbon nanotubes (P-MWCNTs). In this work, IG-MWCNTs were functionalized with tetraethylenepentamine (TEPA) for CO2 capture. The TEPA impregnated IG-MWCNTs were characterized with various experimental methods including N2 adsorption/desorption isotherms, elemental analysis, X-ray diffraction, Fourier transform infrared spectroscopy and thermogravimetric analysis. Both the adsorption isotherms of IGMWCNTs-n and the isosteric heats of different adsorption capacities were obtained from experiments. TEPA impregnated IG-MWCNTs were also shown to have high CO2 adsorption capacity comparable to that of TEPA impregnated P-MWCNTs. The adsorption capacity of IG-MWCNTs based adsorbents was in the range of 2.145 to 3.088 mmol/g, depending on adsorption temperatures. Having the advantages of low-cost and high adsorption capacity, TEPA impregnated IG-MWCNTs seem to be a promising adsorbent for CO2 capture from flue gas. Key words CO2 adsorption; multi-walled carbon nanotubes (MWCNTs); tetraethylenepentamine (TEPA); heat of adsorption; amine modification

1. Introduction Growing concerns for global warming and climate change in recent years have motivated research activities toward developing more efficient and improved processes for carbon dioxide (CO2 ) capture and sequestration (CCS) from large point sources, such as coal-fired power plants, natural gas processing plants, and cement plants [1]. The conventional technology for CCS with aqueous solutions of amine such as monoethanolamine (MEA), diethanolamine (DEA), diglycol-amine (DGA), N-methyldiethanolamine (MDEA), and 2-amino-2-methyl-1-propanol (AMP) not only requires a great amount of energy but also has serious corrosion issues with its equipment [2]. Therefore, as an alternative approach, CO2 capture with solid sorbents has attracted much attention in recent years [3−7]. The solid sorbents for CO2 capture include activated carbons [8], carbon molecular sieves [9,10], carbon nanotubes [11,12], zeolites [13−16], alkali-based sorbents [17], polymers [18,19] and metal-organic frameworks (MOFs) [20−24]. Among them, carbon nanotube-based solid sorbents ∗

have shown some promising results for the separation of CO2 from gas mixtures over the last several years. One viable method of synthesizing solid sorbents designed for CO2 capture involves the grafting of molecules with amino groups onto carbon nanotubes (CNTs). A paper by Lu et al. [11] compared the CO2 capture capabilities of CNTs, granulated activated carbons (GAC), and zeolites with 3-aminopropyltriethoxysilane (APTES). The modified CNTs demonstrated the greatest improvement in adsorption capacity amoung the three. Su et al. [25] also investigated the CO2 adsorption capacity of APTES-functionalized CNTs at various temperatures and discovered that adsorption capacity increased with water content while decreased with temperature, which reflected the exothermic process of adsorption. Their experimental CO2 adsorption capacity of ∼2.59 mmol/g at 293 K for CNT (APTES) is the evidence for the potential of CNTs as low-temperature adsorbents. Single-walled nanotubes (SWNTs) synthesized with another amine functional group, PEI, by Dillon et al. [26] reached a maximum adsorption capacity of 2.1 mmol/g at 300 K. In order to trim down the regeneration time, a combination of vacuum and thermal

Corresponding author. Tel/Fax: +86-571-88273591; E-mail: [email protected] This work was supported by Zhejiang Provincial Natural Science Foundation of China (Grant No. LZ12E08002).

Copyright©2014, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. All rights reserved. doi: 10.1016/S2095-4956(14)60124-8

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desorption was proposed by Hsu et al. [12]. They were able to sustain adsorption/regeneration of CNT (APTES) for twenty cycles at 493 K while maintaining the CNTs’ physiochemical properties and adsorption capacity. The other method of preparing CO2 capture CNTs is impregnation. It carries the benefits of high adsorption capacity, more convenient synthesis and less corrosiveness than grafting [27]. Impregnation incorporates amine non-covalently onto the surfaces and into the pores of CNTs [28]. CO2 adsorption capacity was found to vary with the loadings of amine. Measured against other amine-impregnated adsorbents, tetraethylenepentamine (TEPA)-functionalized sorbents possess one of the highest adsorption capacities [29,30]. In our previous work [31], a solid adsorbent for CO2 capture was prepared by impregnating P-MWCNTs with tetraethylenepentamine (TEPA) for confined spaces, such as submarines and space capsules. An adsorption capacity of approximately 3.87 mmol/g was achieved at 313 K, CO2 2.0 vol%, H2 O 2.1%, and 50 cm3 /min. One of the major obstacles for potential industrial applications of P-MWCNTs is its high cost [31]. Therefore, it is important to find a lowcost substitute. IG-MWCNTs, at one-tenth the price of PMWCNTs, are one such substitute that has not been previously studied. Similar to P-MWCNTs, IG-MWCNTs possess unique, topological, hollow, tubular structures and offer many advantages such as large pores, reduced mass and volume, good thermal conductivity and chemical stability [32,33]. More importantly, IG-MWCNTs are much less costly than the P-MWCNTs, which make them a promising material for capturing CO2 from large point sources. The major difference, however, is that it is still not clear what the impact of impurities on CO2 adsorption and desorption of solid sorbents made from IG-MWCNTs. Therefore, we systematically examined the performance of solid sorbents made from IG-MWCNTs. The IG-MWCNTs were impregnated with TEPA and characterized with various experimental methods including N2 adsorption/desorption isotherms, elemental analysis (EA), X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy and thermogravimetric analysis (TGA). The optimal working temperature of the sorbent was investigated. In addition, we measured the performance of the sorbents made from IG-MWCNTs by examining the adsorption capacities, dualcomponent isotherms, and enthalpy of adsorption. 2. Experimental 2.1. Preparation of adsorbents Solid sorbents for CO2 capture were prepared by impregnating TEPA in solution onto MWCNTs. Commercially available P-MWCNTs (TNM8, Organic Chemical Co., Ltd., China) and IG-MWCNTs (TNIM8, Organic Chemical Co., Ltd., China) were selected as the supports. The purity of PMWCNTs is over 95% while the purity of IG-MWCNTs is only 90% or more. TEPA [NH2 (CH2 CH2 NH)3 CH2 CH2 NH2 ] (90%, Sinopharm Chemical Reagent Co., Ltd., China) was in-

corporated into the P-MWCNTs and IG-MWCNTs supports by wet impregnation. The impregnation was performed in a sealed vessel under atmospheric pressure. Specifically, a desired amount of TEPA was dissolved in 50 g of ethanol (99.7%, Sinopharm Chemical Reagent Co., Ltd., China) and stirred for 30 min at room temperature before the addition of 2 g of MWCNTs supports. After stirring for 3 h, the mixture was evaporated at 353 K and subsequently dried at 373 K in open air for 1 h. Finally, the samples were grinded into powder and sealed in a vial. These samples were denoted as P-MWCNTs-n and IG-MWCNTs-n, where n represented the weight percentage of TEPA in the composites. 2.2. Characterizations of adsorbents The morphology of sorbents was analyzed by a highresolution transmission electron microscope (HR-TEM, JEM2010, JEOL, Tokyo, Japan). The surface area and pore volume were measured with static volume adsorption system (ModelASAP 2020, Micromeritics Inc., USA) by obtaining the N2 adsorption/desorption isotherms at 77.4 K. Prior to the adsorption measurement, the samples were out-gassed at 473 K for 24 h. The N2 adsorption/desorption data were recorded at the liquid nitrogen temperature (77 K) and then used to determine the surface areas with the Brunuer-Emmett-Teller (BET) equation. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were carried out with a thermogravimetric analyzer (SDT Q600, TA Instruments, Inc., New Castle, DE) under a dynamic N2 atmosphere from 303 to 873 K, with a heating rate of 10 K/min. The elemental analysis (C, N, H) was conducted on an elemental analyzer (Flash EA 1112, ThermoFinnigan, Italy). The crystal phase and the surface functional groups of sorbents were characterized by a powder X-ray diffractometer (XRD, Rigaku D/Max 2550/PC, Rigaku Co., Ltd., Japan) using Cu Kα radiation (40 kV, 30 mA) and by an Fourier transform infrared spectrometer (FTIR, NICOLET 6700, thermal scientific, USA), respectively. 2.3. Adsorption experiment The experimental setup for CO2 adsorption was carried out using the protocol reported by Liu et al. [34]. The adsorption column was made of quartz glass with a length of 14 cm and an inner diameter of 1.5 cm. The adsorption column was placed into a temperature-controlled heating jacket and filled with 2.0 g of adsorbents. CO2 was mixed with N2 (99.99%) at a predetermined composition. The concentration of CO2 was measured by a gas chromatograph (GC), connected to the outlet of the adsorption column. In a typical experimental setting, adsorbents were treated under a nitrogen flow of 50 cm3 /min at 423 K for 1.5 h and then cooled to test temperatures. The nitrogen flow was then switched to a CO2 -containing simulated flue gas at a flow rate of 50 cm3 /min. The mole fraction of CO2 was kept at 10 vol% at atmospheric pressure. The adsorption capacity of CO2 on adsorbents at a given

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3. Results and discussion

time is calculated by Equation 1: 1 q= M

Z

t

0

 c0 − c T0 1 Q dt 1−c T Vm

(1)

where, q is the adsorption capacity of CO2 (mmol/g), M is the mass of adsorbent (g), Q is the gas flow rate (cm3 /min), c0 and c are influent and effluent CO2 concentrations (vol%), t denotes time (min); T0 is 273 K, T is the gas temperature (K), and Vm is 22.4 mL/mmol.

3.1. Characterizations of adsorbents Figure 1 exhibits the TEM images of IG-MWCNTs and P-MWCNTs. The P-MWCNTs and IG-MWCNTs both have out diameters of 50−60 nm and inner diameters of 5−15 nm. Because IG-MWCNTs are of a lower purity than PMWCNTs, the IG-MWCNTs in the image exhibit amorphous carbon and structural flaws on their surfaces.

Figure 1. TEM images of (a) P-MWCNTs and (b) IG-MWCNTs

In order to calculate surface areas and pore volumes of IG-MWCNTs after impregnation with TEPA, the N2 adsorption/desorption isotherms of IG-MWCNTs, IG-MWCNTs-50 were measured and shown in Figure 2. Both nitrogen adsorption isotherm curves exhibit a type II behavior according to the IUPAC classification. IG-MWCNTs show a larger adsorption capacity of N2 than IG-MWCNTs-50, indicating IG-MWCNTs have a higher porosity than the modified form, which is consistent with the properties in Table 1. The decrease in porosity is due to the obstruction of pores by the impregnated TEPA. The surface area and pore volume of IG-MWCNTs decrease with an increase in TEPA loading

amount, which agrees with the observations in other studies on amine-loaded CNTs [11,25]. At 60 wt% TEPA loading, the surface area and pore volume reached minimums of 2 m2 /g and 0.028 mL/g, respectively. However, the average pore diameter increased to 53.81 nm. It is probably because TEPA occupies the micro- and meso-pores of IG-MWCNTs, which changes the pore size distribution. Table 1. Physical properties of IG-MWCNTs and IG-MWCNTs-n Samples IG-MWCNTs IG-MWCNTs-10 IG-MWCNTs-30 IG-MWCNTs-50 IG-MWCNTs-60 a

SA = surface area; diameter

Figure 2. N2 adsorption (solid symbols) and desorption (hollow symbols) isotherms of IG-MWCNTs and IG-MWCNTs-50

SAa (m2 /g) 62 52 32 17 2 b

PVb (mL/g) 0.367 0.323 0.181 0.108 0.028

PV = pore volume;

c

APDc (nm) 23.44 24.95 22.82 25.47 53.81

APD = average pore

The elemental analysis results of IG-MWCNTs, IGMWCNTs-50 and IG-MWCNTs-50-CO2 (IG-MWCNTs-50 with adsorbed CO2 ) are listed in Table 2. The calculated TEPA content is less than 50% due to the readily adsorbed water of TEPA. Because of the adsorption of CO2 , the contents of nitrogen and hydrogen of IG-MWCNTs-50-CO2 are much lower than those of IG-MWCNTs-50, as shown in Table 2. The calculated H2 O content of IG-MWCNTs-50-CO2 is also significantly lower than that of IG-MWCNTs-50. This may be the result of the preadsorption of CO2 , diminishing the capacity of IG-MWCNTs-50-CO2 to adsorb water onto TEPA.

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Table 2. Elemental analysis results of IG-MWCNTs, IG-MWCNTs-50 and IG-MWCNTs-50-CO2 Samples IG-MWCNTs IG-MWCNTs-50 IG-MWCNTs-50-CO2 a

N 0 16.949 14.388

Elemental content (%) C 97.733 69.594 68.984

H 0 6.246 4.846

Calculated TEPA (%)a

Calculated H2 O (%)b

N/Ac 45.815 38.891

N/Ac 5.358 0.740

According to the nitrogen content; b According to the TEPA and hydrogen content; c Not Applicable

The XRD patterns of IG-MWCNTs and IG-MWCNTs-n are given in Figure 3. The strong diffraction peak at 2θ = 26o and weak diffraction peaks at 2θ = 44o, 57o , and 78o of IGMWCNTs correspond to the graphite (002), (100), (004) and (110) lattice planes, respectively. After TEPA impregnation, the peak locations do not change, indicating that the structure of IG-MWCNTs is retained. However, the peak intensity decreases with increasing loading capacity, suggesting that TEPA had been impregnated into IG-MWCNTs. Figure 4 shows FTIR patterns of IG-MWCNTs, TEPA, IG-MWCNTs-50 and IG-MWCNTs-50-CO2. All of the adsorbents exhibit bands at 3300−3500 cm−1 , which indicates the formation of hydroxyl groups from the adsorbed water.

The bands at 2939 and 2833 cm−1 correspond to C–H asymmetric and symmetric vibration peaks, respectively. –NH2 and –NH bending vibration peaks can be associated with the bands at 1570 and 1470 cm−1 , respectively. The band at 1120 cm−1 is ascribed to the C–N stretching vibration peak, while the bands at 900−650 cm−1 can be attributed to the outside rocking peak of C–N. The intensity of peaks decreases in the following order: TEPA>IG-MWCNTs-50>IG-MWCNTs-50CO2 . The carbonyl stretching frequency at 1635 cm−1 suggests the existence of carbamate moiety and thus a chemical adsorption of CO2 [35].

Figure 3. XRD patterns of IG-MWCNTs and IG-MWCNTs-n. MWCNTs-60 was too viscous to test

Figure 4. FTIR spectra of (1) TEPA, (2) IG-MWCNTs-50, (3) IGMWCNTs-50-CO2 and (4) IG-MWCNTs

IG-

Figure 5. TGA and DSC profiles of IG-MWCNTs-50 and IG-MWCNTs-50-CO2

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TGA and DSC profiles of IG-MWCNTs-50 and IGMWCNTs-50-CO2 are shown in Figure 5. For IG-MWCNTs50, the first weight loss region (<393 K) exhibits a weight loss of 12.36% with a maximum weight loss rate at 347.67 K, which could be attributed to the evaporation of adsorbed water. The second region (393−700 K) shows a weight loss of 43.66% with a maximum weight loss rate at 509.31 K due to the evaporation of impregnated TEPA. As for IGMWCNTs-50-CO2 , the first weight loss region (<353 K) exhibits a weight loss of 11.01% with a maximum weight loss rate at 345 K, which could be ascribed to the evaporation of physically adsorbed water and CO2 . The second region (353−403 K) displays a weight loss of 8.084% with a maximum weight loss rate at 378.61 K due to the evaporation of water and desorption of chemically adsorbed CO2 . The third region (403−700 K) shows a weight loss of 36.96% due to the evaporation of impregnated TEPA.

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Figure 7. The adsorption capacities of IG-MWCNTs-n and P-MWCNTs-n

3.2. Adsorption behavior of adsorbents

3.2.2. Ef fects of temperature on adsorption

3.2.1. Ef fects of amine loading

The effects of adsorption temperature on TEPA impregnated IG-MWCNTs were evaluated by analyzing breakthrough curves at different temperatures. Figure 8 shows the adsorbent reaches its maximum adsorption capacity at 343 K. The adsorption capacities of P-MWCNTs-50 and IG-MWCNTs-50 at varying temperatures are given in Figure 9. The adsorption capacity of IG-MWCNTs-50 increases with increasing temperature from 2.145 mmol/g at 293 K to 3.088 mmol/g at 343 K, which were slightly lower than those of P-MWCNTs-50 in the same temperature range. The reasons may be as follows. The activity of TEPA and CO2 increases with increasing temperature (up to 343 K), leading to the easy adsorption and reaction of CO2 with amino groups and thus increased adsorption capacity. However, when the temperature rises above 343 K, the activity of TEPA and CO2 increases to the point where part of amino-bound CO2 desorbs from the IG-MWCNTs. These adsorption values are all greater than 2.5 mmol/g in the flue gas environment of 313∼343 K, which suggests that IG-MWCNTs-50 are an efficient CO2 adsorbent in post-combustion.

Figure 6 shows the breakthrough curves of CO2 adsorption on IG-MWCNTs and IG-MWCNTs-n at 313 K. The adsorption capacities of IG-MWCNTs-n were 0.442, 1.181, 1.730, 2.386 and 2.593 mmol/g, corresponding to TEPA loadings of 10, 20, 30, 40 and 50 wt%, respectively (Figure 7). However, at 60 wt%, the adsorption capacity of IGMWCNTs-60 slightly decreased to 2.044 mmol/g, and the breakthrough time became shorter. This behavior can be attributed to the blockage of the pore entrances and interior by an excessive amount of TEPA, as the pore volume decreased sharply from 0.108 mL/g for IG-MWCNTs-50 to 0.028 mL/g for IG-MWCNTs-60. Similar results were also observed in other studies [29,36,37]. Although IG-MWCNTs are of a lower purity than P-MWCNTs, as well as containing amorphous carbon on its surface, it was found that IG-MWCNTs-n exhibit similar CO2 adsorption capacity as P-MWCNTs-n. It is possible that the plenty structural flaws of IG-MWCNTs-n facilitate TEPA impregnation and thus generate more active adsorption sites for CO2 .

Figure 6. Breakthrough curves of 10 vol% CO2 adsorption on IG-MWCNTs-n at 313K

Figure 8. Breakthrough curves of 10 vol% CO2 adsorption on IG-MWCNTs50 at different temperatures

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Table 3. CNTs-related materials used for CO2 adsorption from the literature and this work Support CNTs CNTs IG-MWCNTs

Functionalization style grafting grafting impregnation

Adsorption capacity (mmol/g) 1.32 0.93 3.29

Amine APTES APTS TEPA

Experimental conditions pCO2 (atm) T (K) 0.15 293 0.1 298 0.1 343

Ref. [12] [11] this work

TEPA-related and CNT-related materials used for CO2 adsorption taken from the literature are given in Table 3. It is clear that under similar conditions, IG-MWCNTs-50 have an excellent adsorption capacity at low CO2 partial pressures.

Figure 10. CO2 adsorption isotherms for IG-MWCNTs-50 at various temperatures in a binary mixture of CO2 and N2 Table 4. Constants of Freundlich model for CO2 adsorption on IG-MWCNTs-50 Figure 9. The adsorption capacity of P-MWCNTs-50 and IG-MWCNTs-50

3.2.3. Adsorption isotherms and adsorption enthalpy Results in Figure 9 show characteristics of chemical adsorption. The amount of CO2 adsorption increases as temperature increases. The Freundlich equation, which is a classic model used for describing adsorption behavior, was used to fit the isotherm data of IG-MWCNTs-50-CO2. The equation is defined as: q = kcn0

Temperature (K) 293 303 313 323 333 343

k 0.807 0.866 1.089 1.267 1.475 1.593

n 0.365 0.374 0.332 0.297 0.269 0.260

R2 0.960 0.975 0.966 0.964 0.954 0.973

Isosteric heats of adsorption for CO2 were given in Figure 11. The isosteric heat of adsorption for CO2 decreased with the increase in CO2 adsorption capacity. The high heat of adsorption in the lower adsorption region was due to the reaction of CO2 with the active sites of TEPA. The heat of

(2)

where, k and n are Freundlich constants. Calculated parameters for the Freundlich equation and the correlation coefficients are presented in Table 4. The predicted values are shown as lines in Figure 10. Isosteric heat of adsorption (Q) is commonly defined as the enthalpy change of adsorption. Values of Q for the adsorption of CO2 onto IG-MWCNTs-50 were calculated from adsorption data obtained at different temperatures using the Clausius-Clapeyron equation:   ∂(lnP ) Q =− (3) ∂(1/T q R where, P is CO2 partial pressure (Pa), T is the absolute temperature (K), and R denotes the universal gas constant, 8.3145 J/(mol·K).

Figure 11. Isosteric heats of adsorption of CO2 for IG-MWCNTs-50 in a binary mixture of CO2 and N2

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chemical adsorption is generally greater than 60 kJ/mol, while the heat of physical adsorption is about 20 kJ/mol. Therefore the process of IG-MWCNTs-50 adsorption of CO2 was partly physical and partly chemical adsorption. 3.2.4. Adsorbent regenerability A durable cyclic adsorption/regeneration behavior of the adsorbents is essential for long-term operation [34]. Figure 12 depicts the adsorption capacity of IG-MWCNTs-50 during five cycles of CO2 adsorption at 323 K, with regeneration under flowing N2 at a temperature of 403 K. A regeneration test was run for 30 min for complete desorption of CO2 . The cyclical data reveals that the adsorption performance of IGMWCNTs-50 is fairly stable, with only a 1.58% drop in adsorption capacity after five adsorption/regeneration cycles.

Figure 12. Cycling adsorption/regeneration runs of IG-MWCNTs-50 (adsorption at 323 K; CO2 , 10 vol%; gas flow rate, 50 cm3 /min; regeneration at 403 K; N2 flow rate, 50 cm3 /min)

4. Conclusions In this work, IG-MWCNTs-n produced by impregnating IG-MWCNTs with TEPA were prepared for CO2 capture. The IG-MWCNTs-n displayed a CO2 adsorption capacity as large as that of P-MWCNTs-n. The IG-MWCNTs-based sorbent with 50% TEPA loading has a maximum CO2 adsorption capacity of 3.088 mmol/g at 343 K. Areas of future research might be focused on exploring the effects of contaminants such as SOx , NOx , and H2 O on CO2 adsorption capacity of IG-MWCNTs-n, repeated adsorption/desorption cycles on sorbent durability in the presence of contaminants in order to manage the cost of capture, and switching from powder to pellet form to facilitate transportation and storage. Nonetheless, IG-MWCNTs are much cheaper than P-MWCNTs in price, making IG-MWCNTs a feasible low-cost substitute for P-MWCNTs, and thus a promising material for CO2 capture from large point sources. Acknowledgements We would like to thank the Zhejiang Provincial Natural Science Foundation of China (Grant No. LZ12E08002) and the Fundamental Research Funds for the Central Universities for financial support.

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