Graphene nanoplatelet and reduced graphene oxide functionalized by

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Environmental and Carbon Dioxide Issues

Graphene nanoplatelet and reduced graphene oxide functionalized by ionic liquid for CO2 capture Yannan Li, Jun Cheng, Leiqing Hu, Jianzhong Liu, Junhu Zhou, and Kefa Cen Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b00889 • Publication Date (Web): 16 May 2018 Downloaded from http://pubs.acs.org on May 17, 2018

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Graphene nanoplatelet and reduced graphene oxide functionalized by ionic liquid for CO2 capture Yannan Li, Jun Cheng*, Leiqing Hu, Jianzhong Liu, Junhu Zhou, Kefa Cen State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou 310027, China

Abstract: An ionic liquid (IL) [P66614][Triz] with low regeneration temperature was loaded on graphene nanoplatelet (GNP) and reduced graphene oxide (RGO) to accelerate CO2 absorption rate. High-resolution transmission electron microscopy (HRTEM) patterns showed that GNP was composed of regular “honeycomb” lattice, but regular lattice structure was not observed for RGO due to the functional group on it. CO2 absorption capacity (63.6 mg CO2/g IL) and absorption peak rate (22.4 mg CO2/(g IL·min) of IL loaded on GNP were increased by 8.2% and 72.3% respectively, compared with those of neat IL. The supported IL performed better because [P66614][Triz] was oriented in a favorable dispersion due to the negative zeta potential of GNP surface. By contrast, CO2 absorption rate of RGO-20% IL (The mass ratio of RGO: IL = 4: 1) was lower than that of IL, which could be attributed to the hydrogen bond between surface oxygen functional groups and IL.

13

C NMR and Mulliken atomic charge calculated by

Gaussian were used to support the CO2 absorption mechanism. Keywords: biohythane, CO2 absorption, ionic liquid, graphene nanoplatelets 1. Introduction Raw biohythane (mixture of raw biogas and H2) is fermented from organic waste [1, ∗Corresponding author: Prof. Dr. Jun Cheng, State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou 310027, China. Tel.: +86 571 87952889; fax: +86 571 87951616. E-mail: [email protected]. 1

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2]. After purification, biohythane is a substitute for hydrogen-enriched natural gas as vehicle fuel [3, 4]. In process of raw biohythane upgrading, removing of CO2 is an important step. Recently, ionic liquids (ILs) attained immense interest in CO2 adsorption/absorption applications, because selected ILs had large and reversible CO2 uptakes [5-7]. Besides, ILs were regarded as green solvents due to their low regeneration temperature, reasonable thermal stability and non-corrosion [8-11]. Boot-Handford [12], Bara [13] et al. reviewed a lot of research using IL as alternative solvents for Carbon Capture and Storage (Sequestration) (CCS). However, high viscosity of ILs limited the diffusion of CO2 [14, 15]. By contrast, the diffusion of CO2 can be enhanced by supporting IL on porous molecular sieves. Ruckart et al. [16] impregnated IL on SBA-15 and the composites showed significantly increased CO2 uptakes relative to the parent SBA-15. Zhu et al. [17] loaded [P8883][BF4] and [P4443][BF4] on silica and the hybrid sorbents (SiO2–Si–P8883BF4 and SiO2–Si–P4443BF4) expressed higher diffusion coefficients of CO2 adsorption than neat IL, which was attributed to the significant CO2 adsorption diffusion coefficient of SiO2. Arellano et al. [18] impregnated zinc-functionalized ILs (ET and ET3) into mesoporous silica beads (SALG) and found sorption kinetic of SALG-EZT3 was higher than neat EZT3 and SALG because the thin layer of EZT3 increased availability of adsorption sites and promoted faster kinetics. Zhang et al. [19] synthesized ILs and loaded ILs on porous silica to capture CO2, finding that CO2 absorption was improved 2


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than in bulk ILs. Zhu et al. [20] explored task-specific IL-derived nitrogen-doped carbon sorbents for CO2 uptake instead of comparison with bulk IL. In addition to porous molecular sieve, graphene oxide (GO) and graphene have become immensely important supporting materials to load IL for capturing CO2 because of its unique properties, such as large active specific surface area and microporous structure on surface. Hong [22] synthesized 3-aminopropyltriethoxysilane-modified graphite oxide (oxidizing graphite powder with NaClO3 in fuming nitric acid according to the Brodie’s method) to capture CO2, while Tamilarasan [23, 24] prepared amine-rich IL ([DAMT][BF4]) and IL ([BMIM][BF4]) grafted graphene (reduced and exfoliated by hydrogen from graphene oxide, specific surface area=344 m2/g) for sub-ambient CO2 adsorption. Furthermore, Zhao [25, 26] initially synthesized aminated graphite oxide (AGO) for CO2 adsorption and subsequently prepared metal–organic frameworks (MOF-5) and AGO composites for CO2 capture. Mishra [27] synthesized polyaniline– graphene (reduced and exfoliated by hydrogen from graphene oxide) nanocomposite as a CO2

capture

candidate,

and

Huang

[28]

used

several

poly(ionic

liquid)s

(P[MATMA][BF4]) to functionalize GO (specific surface area=1.27 m2/g) for CO2 capture. Xin et al [29] fabricated composite membranes by incorporating amino acid-functionalized graphene oxide to separate CO2. However, graphenes prepared by different methods were quite different (such as C/O atomic ratio, specific surface area, etc). It is interesting to find out the effect of graphenes prepared by different methods (loading with same IL) on CO2 absorption, but 3


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very few literatures reported about this. So, in this work, graphene nanoplatelets (GNP, prepared by thermal annealing of chemically reduced graphene oxide sheets, higher C/O atomic ratio with negligible functional groups) and reduced graphene oxide (prepared by Hummers' method, lower C/O atomic ratio with oxygen functional groups) were chosen as supporting materials and loaded with IL [P66614][Triz] (CO2 absorption capacity was 0.95 mol CO2/mol IL) to capture CO2 from biohythane. CO2 absorption rates of loaded IL on GNP and RGO were compared and found [P66614][Triz] loaded on RGO had lower CO2 uptake rate. In addition, 13C NMR and Mulliken atomic charge calculated by Gaussian were used to support the CO2 absorption mechanism. As described in the literature [31], the use of ILs to capture CO2 at scale was similar to CO2 absorption process by amine aqueous solution, the CO2 separation system based on IL was generally consisted of absorption tower and stripper. The raw gas was injected into the absorption tower (IL was also added in this tower) to remove CO2, then the CO2-enriched-IL was pumped to the stripper. CO2 was collected after it was released (by heating method) from the stripper. The collected CO2 could be used to synthesize chemicals or cultivate microalgae. 2.

Materials and methods

2.1. Materials IL [P66614][Triz] ( ≥ 98.8%, in which, H2O ≤ 0.1%, [P66614][Br] ≤ 1.1%) was synthesized according to the method listed in Ref [30], by the Department of Chemistry, Zhejiang University, Hangzhou, Zhejiang, China. Anhydrous methanol (≥99.5%) and 4


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Energy & Fuels

anhydrous ethanol (≥99.7%) were purchased from Sinopharm Chemical Reagent Co., Ltd., Shanghai, China. Simulated raw biohythane (with a volume ratio of CH4: H2: CO2 = 12: 3: 10) was obtained from Hangzhou Jingong Special Gas Co., Ltd., Hangzhou, Zhejiang, China. Graphene nanoplatelets (GNP, ≥99.5%) was purchased from Shanghai Aladdin Bio-Chem Technology Co., Ltd., Shanghai, China. Graphene oxide (GO) was synthesized using a modified Hummer’s method and was reduced by hydrazine hydrate to obtain reduced graphene oxide (RGO) according to the method in literature [32]). 2.2. Preparation of hybrid sorbents Hybrid sorbents ([P66614][Triz] loaded on GNP (or RGO) were synthetized through the dipping method according to literature [33]. The mass ratio of GNP (or RGO): IL = 4: 1, as a result, the obtained hybrid sorbent was designated as GNP-20% IL (or RGO-20% IL). 3.

Results and discussion

3.1. Characterization of hybrid sorbents X-ray diffraction (XRD) was operated on an X'Pert Pro diffractometer (PANalytical, Holland). The measurements were obtained in a range of 2θ=5-40° with a scanning step of 0.02°. The sample was ground into powder (the particle size was less than 48 µm) and then the powder was made into a test piece before putting on XRD test-bed. Fig. 1 showed the XRD patterns of GNP and RGO. In the XRD pattern of GNP, a sharp peak was observed at 2θ= 26.5°, which regard to (002) reflection and the corresponding d-spacing was 0.34 nm. The XRD pattern of RGO showed a broad diffraction peak (002) 5


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centered at 2θ= 24°, corresponding to a d-spacing of 0.37 nm. The inter-layer distance of RGO was slightly larger than that of GNP (0.34 nm) due to the residual oxygen functional group after rapid reduction of GO. The broadening and weak intensity of the (002) peak of RGO confirmed the high degree of disorder in the graphitic structure. In order to intuitively reflect the degree of disorder in the graphitic structure, a high-resolution transmission electron microscopy (HRTEM; JEOL JEM-2100F, Japan) with an acceleration voltage of 200 kV was used to obtain TEM images. GNP or RGO was dispersed in ethanol (oscillated by an oscillating device at 50 kHz) and then the mixture was dripped onto a copper grid to wait for the ethanol volatilization before HRTEM test. The HRTEM patterns of GNP and RGO were shown in Fig. 2. As described in the literature [34], graphene (GNP) consisted of an extended two dimensional sp2 bonded carbon “honeycomb” lattice. For RGO, “honeycomb” lattice was hard to be observed and this due to either oxygen functionalities from the initial oxidation or nitrogen functionalities from the hydrazine reduction.The crystallization degree of RGO was far below GNP, which was consistent with the results of XRD. Energy dispersive spectrometer (EDS, X-max80, UK) was conducted to analyze the elements of GNP and RGO qualitatively. It was collocated with scanning electron microscopy (SEM) and the results were shown in Fig. 3. To ensure reliability, the analytical data was collected from various regions. GNP had negligible element of O while RGO had a relatively higher oxygen components, which proved there were functional groups on RGO. 6


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To observe the surface characteristics of GNP, RGO, GNP-20% IL and RGO-20% IL, scanning electron microscopy (SEM; Hitachi SU8010, Japan) was operated at an energy beam of 30.0 kV. Before observation, the sample was fixed on the conductive adhesive. Fig. 4 showed the SEM images of GNP, RGO, hybrid sorbent GNP-20% IL and RGO-20% IL. GNP exhibited agglomeration of several thick sheets and formation of large lamellas and RGO was scattered and covered with wrinkles. The morphology of hybrid sorbent GNP-20% IL and RGO-20% IL were unspoiled, but coated with a layer of liquid film. In order to obtain the specific surface areas of supporting materials and hybrid sorbents, the N2 adsorption–desorption analysis was conducted using an Autosorb® iQ-MP sorption analyzer (Quantachrome Instruments, USA). The samples were degassed at 353 K in a high vacuum, and the adsorption–desorption isotherms were obtained from the physical adsorption of N2 at 77 K. Brunauer–Emmett–Teller (BET) method was used to calculate the specific area according to multiple points of relative pressure (P/P0) data from 0.2 to 0.5. P/P0 of 0.99 was used to calculate the total pore volumes. Fig. 5 showed the adsorption–desorption isotherms of GNP, RGO, GNP-20% IL and RGO-20% IL. Specific areas of GNP and RGO calculated by Brunauer–Emmett–Teller (BET) method were 15.77 m2/g and 393.98 m2/g respectively. But after the loading of IL, the BET surface areas of GNP-20% IL and RGO-20% IL were decreased to 1.06 m2/g and 5.45 m2/g, respectively. Frenkel–Halsey–Hill (FHH) method (adsorption) was used to calculate the fractal dimension of GNP and GNP-20% IL with relative pressure P/P0 = 7


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0.02-0.5. The slits between plates of graphene nanoplatelets were filled with loaded IL, resulting in a decrease of fractal dimension from 2.6 to 2.0, which was consistent with the decreased BET surface area. 3.2. CO2 uptakes of hybrid sorbents All the CO2 uptake experiments were carried out at 20±1°C and atmospheric pressure. CO2 absorption uptake by hybrid sorbents was measured through the weighing method and the details were given in literature [33]. Fig. 6 showed CO2 uptakes comparison of neat IL, IL loaded on GNP and RGO. The CO2 absorption equilibrium time of neat IL was 10 min, whereas that of IL loaded on GNP was 6 min, which was 40% shorter than that of neat IL. CO2 absorption capacity (63.6 mg CO2/g IL) and absorption peak rate (22.4 mg CO2/(g IL·min) of IL loaded on GNP were increased by 8.2% and 72.3% respectively, compared with those of neat IL. There were two reasons: firstly，the area of exposed IL was extended on GNP compared with neat IL; secondly, the surface electrostatic property of GNP was negative (-32.47 mV, which was characterized on a zeta potential analyzer), which was propitious to the reaction between CO2 and the anion because the cation of the IL was arranged near the GNP surface while the anion upturned due to the repulsive force [35]. However, CO2 absorption rate of IL loaded on RGO (RGO-20% IL) was lower than that of neat IL. This might due to RGO had relatively higher oxygen functional groups (GNP had negligible oxygen functional groups). The evenly distributed O elements on RGO surface might be existed in the form of –COO– and –OH functional groups. 8


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According to literature [36], we speculated that there were hydrogen bonds between the active

site

of

N

in

anions

and

the

hydroxyl

groups

on

the

RGO.

A schematic representation of hydrogen bonds was given in Fig. 7. The CO2 needed to overcome the block of hydrogen bond before it reacted with the active site of N in anion, which reduced the absorption rate of CO2. A simple comparison with previously published work about IL loaded on graphene was shown in Table 2. [P66614][Triz] before and after CO2 absorption were investigated by

13

C NMR

(AVANCE III 500 MHz, Bruker, Switzerland) to support the CO2 absorption mechanism given in literature [30], which was shown in Fig. 8. The ILs before and after reaction with CO2 were dissolved in dimethylsulfoxide-d6 (DMSO-d6) and then were filled in NMR tubes. Besides, Mulliken atomic charge of the atoms in the anion before and after absorption CO2 were calculated using the Gaussian 03 program (carried out at the B3LYP/6-311G(p,d) level) to support the 13C NMR data in this work, which was shown in Table 1. For fresh [P66614][Triz], the peak at 148.56 ppm was attributed to NO.3 atom C and NO.5 atom C and similar Mulliken charges (0.010184 and 0.010146, respectively) between them might explain their same displacement. A new signal in the

13

C NMR

spectra at 158.33 ppm produced after the absorption of CO2, which was attributed to carbamate carbonyl carbon. The shift of NO.5 atom C moved to 155.8 ppm because Mulliken charge changed (turn into 0.152548) after IL reacted with CO2. 3.3. CO2 absorption capacity of IL loaded on GNP at various temperatures Fig. 9 showed the CO2 absorption capacity of IL loaded on GNP at various 9


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temperatures. When temperature was increased from 293 K to 303 K, the CO2 absorption capacity of IL loaded on GNP was substantially unchanged. The result was under the jointly effect of two reasons: (1) the diffusion of CO2 was facilitated to react with loaded [P66614][Triz] and the viscosity of IL slightly decreased, which were advantageous for absorption; (2) but the increase of temperature promoted the reverse reaction (CO2 desorption). When temperature was further increased, the absorption capacity decreased, because CO2 desorption was dominant. The low desorption temperature indicated that the absorbed CO2 was easily desorbed from the IL sites. This process emphasized low regeneration energy consumption. 3.4. Regenerability The CO2 desorption was performed at 90 °C under N2 for 20 min. A total of 10 absorption–regeneration cycles were conducted to test the regeneration ability of GNP-20% IL (Fig. 10). The absorption capacity of GNP-20% IL remained stable after 10 regenerations (≥ 92% CO2 absorption capacity ) because of high stability of IL, indicating that the CO2 capture process by GNP-20% IL is reversible. 3.5. Other influencing factors CO2 uptake pressure. As shown in literatures [16, 17, 20, 22, 23, 24, 26, 27, 28], CO2 absorption capacity of hybrid sorbents (IL loaded on supporting materials) were increased as pressure increased. H2S in raw biohythane. According to reference [30], the acidity coefficient pKa (in dimethyl sulfoxide) of [P66614][Triz] was 13.9, which meant [P66614][Triz] presented weak 10


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alkalinity. [P66614][Triz] would react with H2S before CO2 absorption. H2O in raw biohythane. CO2 absorption balance time of moist CO2 (containing ~7.4% water) was accelerated by 30.7% compared with dry CO2 in reference [6]. CO2 absorption balance time would also be accelerated in this work because the viscosity of IL was decreased after mixing with water. 4.

Conclusions IL [P66614][Triz] with low regeneration temperature was supported on GNP and

RGO. The IL loaded on GNP (GNP-20% IL) exhibited relatively higher CO2 capacity and absorption rate than neat IL, attributing to a larger reaction area caused by GNP. For RGO-20% IL, hydrogen bonds were formed between the functional group on RGO and the active site of N in anion, resulting in a reduction of CO2 absorption rate.

13

C NMR

and Gaussian 03 program were used to explore the reaction mechanism of [P66614][Triz] and CO2. The effect of absorption temperature was also investigated, and the results showed that a low temperature was favorable to CO2 absorption. Acknowledgements This work was supported by the National key research and development program-China (2016YFE0117900), National Natural Science Foundation-China (51676171), Zhejiang Provincial Key Research and Development Program-China (2017C04001). References [1] Cheng, J.; Ding, L. K.; Lin, R. C.; et al. Fermentative biohydrogen and biomethane co-production from mixture of food waste and sewage sludge: Effects of 11


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4918-4922. [31] Zhang, Y. Y.; Ji, X. Y.; Xie, Y. J.; et al. Screening of conventional ionic liquids for carbon dioxide capture and separation. Applied Energy, 2016, 162: 1160–1170. [32] Bo, Z.; Zhang, X. Y.; Yu, K. H.; Wang, Z. H.; et al. Metal-free and Pt-decorated graphene-based catalysts for hydrogen production in a sulfur−iodine thermochemical cycle. Ind. Eng. Chem. Res. 2014, 53, 11920−11928. [33] Cheng, J.; Li, Y. N.; Hu, L. Q.; et al. CO2 absorption and diffusion in ionic liquid [P66614] [Triz] modified molecular sieves SBA-15 with various pore lengths. Fuel Processing Technology, 2018, 172: 216-224. [34] Erickson, K.; Erni, R.; Lee, Z.; et al. Determination of the local chemical structure of graphene oxide and reduced graphene oxide. Adv. Mater. 2010, 22, 4467–4472. [35] Wan, M.M.; Zhu, H.Y.; Li, Y.Y.; Ma, J.; Liu, S.; Zhu, J.H. Novel CO2‑capture derived from the basic ionic liquids orientated on mesoporous materials. ACS Appl Mater Inter, 2014, 6, 12947-12955. [36] Shin, G. J.; Rhee, K. Y.; Park, S. J. Improvement of CO2 capture by graphite oxide in presence of polyethylenimine. Int. J. Hydrogen Energ. 2016, 41: 14351-14359.

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List of Figures and Table Fig. 1 X-ray diffraction (XRD) patterns of graphene nanoplatelets (GNP) and reduced graphene oxide (RGO). Fig. 2 High-resolution transmission electron microscopy (HRTEM) patterns of graphene nanoplatelets (GNP) and reduced graphene oxide (RGO). Fig. 3 Elemental analysis of (a) graphene nanoplatelets (GNP) and (b) reduced graphene oxide (RGO). Fig. 4 SEM images of (a) graphene nanoplatelets (GNP); (b) hybrid sorbent composed of 80wt% GNP and 20wt% IL (GNP-20% IL); (c) reduced graphene oxide (RGO); (d) hybrid sorbent composed of 80wt% RGO and 20wt% IL. Fig. 5 N2 adsorption-desorption isotherms of reduced graphene oxide (RGO), graphene nanoplatelets (GNP), hybrid sorbent RGO-20% IL(composed of 80wt% RGO and 20wt% IL) and hybrid sorbent GNP-20% IL. Fig. 6 Comparison in CO2 uptakes between IL loaded on GNP (GNP-20% IL) and neat IL: (a) CO2 capacity (b) CO2 absorption rate. Fig. 7 Anion [Triz] before and after the capture of CO2: (a)CO2 absorption mechanism by anion [Triz] in literature [30];(b) 13C NMR spectra of [P66614][Triz] before and after the capture of CO2. Fig. 8 Schematic representation of hydrogen bonds between active N in anions and hydroxyl groups on the RGO. Fig. 9 Effects of temperatures on CO2 capacity of IL loaded on GNP (GNP-20% IL) Table 1 Mulliken atomic charges of atoms in anion [Triz] and [Triz-CO2].

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Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Intensity (a.u.)

Energy & Fuels

RGO GNP 10

15

20

25

30 2θ (°)

35

40

45

10

15

20

25 30 2θ (°)

35

40

Fig. 1 X-ray diffraction (XRD) patterns of graphene nanoplatelets (GNP) and reduced graphene oxide (RGO).

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45

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Fig. 2 High-resolution transmission electron microscopy (HRTEM) patterns of graphene nanoplatelets (GNP) and reduced graphene oxide (RGO).

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Fig. 3 Elemental analysis of (a) graphene nanoplatelets (GNP) and (b) reduced graphene oxide (RGO).

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Energy & Fuels

Fig. 4 SEM images of (a) graphene nanoplatelets (GNP); (b) hybrid sorbent composed of 80wt% GNP and 20wt% IL (GNP-20% IL); (c) reduced graphene oxide (RGO); (d) hybrid sorbent composed of 80wt% RGO and 20wt% IL.

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Energy & Fuels

1200 3 -1 N2 volume @ STP(cm •g )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1000

RGO GNP RGO-20% IL GNP-20% IL

800 600 400 200 0 0.0

0.2

0.4 0.6 0.8 Relative pressure (P/P0)

1.0

Fig. 5 N2 adsorption-desorption isotherms of reduced graphene oxide (RGO), graphene nanoplatelets (GNP), hybrid sorbent RGO-20% IL(composed of 80wt% RGO and 20wt% IL) and hybrid sorbent GNP-20% IL.

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(a) 70

(b)

60 6 50 5 GNP-20% IL

40

Neat IL

4

RGO-20% IL

30

3

GNP/RGO

20

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10

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CO2 uptakes (mg CO2/g GNP)

7 CO2 uptakes (mg CO2/g IL)

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0

0 0

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4

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8

10 12 Time(min)

14

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CO2 absorption rate (mg CO2/(gIL min)

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GNP-20% IL Neat IL RGO-20% IL

20

15

10

5

0 0

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2

3

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5

6

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8

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Time(min)

Fig. 6 Comparison in CO2 uptakes between neat IL, IL loaded on GNP and RGO (GNP-20% IL and RGO-20% IL): (a) CO2 capacity (b) CO2 absorption rate.

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10

11

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Fig. 7 Schematic representation of hydrogen bonds between active N in anions and hydroxyl groups on the RGO.

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Energy & Fuels

Fig. 8 Anion [Triz] before and after the capture of CO2: (a)CO2 absorption mechanism by anion [Triz] in literature [30];(b) 13C NMR spectra of [P66614][Triz] before and after the capture of CO2.

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Energy & Fuels

70 60 CO2 capacity (mg/g IL)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

50 40 30 20 10 0

293 K

303 K

313 K

323 K

333 K

Temperature (K)

Fig. 9 Effects of temperatures on CO2 capacity of IL loaded on GNP (GNP-20% IL)

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70 60

CO2 uptake (mg/g IL)

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Energy & Fuels

50 40 30 20 10 0

0

1

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3

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6

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Regeneration cycle

Fig. 10 Regeneration properties of IL loaded on GNP (GNP-20% IL) for CO2 absorption

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Table 1 Mulliken atomic charges of atoms in anion [Triz] and [Triz-CO2] Number 1 2 3 3 4 5 5 6 7 8

Atoms in anion [Triz] N N C H N C H ‫ܥ‬஼ைమ ܱ஼ைమ ܱ஼ைమ

Milliken charges -0.312625 -0.312759 0.010184 0.002420 -0.399779 0.010146 0.002413 0.496726 -0.248363 -0.248363

Atoms of anion [Triz-CO2] N N C H N C H C O O

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Milliken charges -0.281506 -0.198584 0.042342 0.054270 -0.366308 0.152548 0.083032 0.416104 -0.470153 -0.431745

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Energy & Fuels

Table 2 Comparison in CO2 uptake by IL loaded on graphene supports between this work and literatures Support IL Hybrid sorbents Temperature CO2 uptake Reference (mg CO2/g (K) IL) Graphene-n [P66614][Triz] GNP-20%IL 293 K 63.6 This anoplatelet work Graphene [BMIM][BF4] HEG-50%IL 293 K 52.0 [23] Graphene [DAMT][BF4] HEG-50%IL 293 K 59.8 [24] Graphene-o P[MATMA][BF4] Graphene oxide 273K 39.1mg CO2/g [28] xide -P[MATMA][BF4] sorbent

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Graphene nanoplatelet and reduced graphene oxide functionalized by

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