CO2 Adsorption Performance of Ionic Liquid [P66614][2-Op] Loaded

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CO2 adsorption performance of ionic liquid [P66614][2Op] loaded onto molecular sieve MCM-41 compared with pure ionic liquid in biohythane/pure CO2 atmosphere Jun Cheng, Yannan Li, Leiqing Hu, Junhu Zhou, and Kefa Cen Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.5b02857 • Publication Date (Web): 25 Feb 2016 Downloaded from http://pubs.acs.org on February 27, 2016

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CO2 adsorption performance of ionic liquid [P66614][2-Op] loaded onto molecular sieve MCM-41 compared with pure ionic liquid in biohythane/pure CO2 atmosphere Jun Cheng1∗, Yannan Li, Leiqing Hu, Junhu Zhou, Kefa Cen State key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou 310027, China ABSTRACT Upgrading fermentative biogas to produce gaseous transport fuel, the ionic liquid (IL) [P66614][2-Op], which adsorbed CO2 through multiple-site cooperative interactions, was loaded onto molecular sieve MCM-41 to adsorb CO2 in biohythane atmosphere (CH4+H2+CO2). The [P66614][2-Op] loaded onto the MCM-41 exhibited higher CO2 adsorption rate than pure IL during the initial stage because of its larger reaction surface area. However, the CO2 adsorption rate of [P66614][2-Op] loaded onto the MCM-41 became lower than that of pure IL as the reaction continued because CO2 was inaccessible to the IL blocked within the inner pores of the MCM-41. The CO2 adsorption rate of IL loaded onto the molecular sieve (MCM-41-50% IL) was (51.5 mg CO2/g-IL·min) 2.1 times higher than that of pure IL during the initial 1 min in pure CO2 atmosphere and was (24.6 mg CO2/g-IL·min) 2.2 times higher than that of pure IL during the initial 2 min in biohythane atmosphere. KEYWORDS: CO2 adsorption; Ionic liquid; Molecular sieve; Biohythane ∗

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].

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1. INTRODUCTION

With the worsening environmental pollution and energy shortages problems, studies on biogas upgrading are garnering considerable interest [1]. In recent years, biomethane (upgraded biogas) has been used as a reliable substitute for natural gas as vehicle fuel [2–4]. It has been investigated that adding H2 to natural gas could reduce NOx, CO, and CH4 emissions [5, 6]. Since CH4 is the main component of natural gas, adding H2 to CH4 can achieve the same effect. The main components of biohythane [7–11]

(fermented

gas

generated

from

biomass

or

waste

ferment)

are

methane-hydrogen-carbon dioxide (CH4+H2+CO2). Therefore, H2 and CH4 are obtained by removing CO2 from biohythane. Alkylamine solutions such as methyldiethanolamine, diethanolamine and monoethanolamine are widely used in CO2 absorption [12–20]. However, the regeneration of amine solvents results in considerable energy consumption, solvent loss, and equipment corrosion [21–25]. Ionic liquids (ILs) have exhibited special properties that make them promising candidates rivalling with amine absorbents. They are non-volatile, non-flammable, high thermo-chemical stability, recyclable and have high CO2 selectivity [26–30]. However, they still exhibit several disadvantages, such as high viscosity and high cost [31]. To overcome these drawbacks, ionic liquids (ILs) supported on porous molecular sieves have been studied. Immobilization of ILs on porous substrates was in order to enhance CO2 mass transfer rate [32]. Zhu et al. [33] modified active carbons (ACs) using imidazolium-based ILs and utilized these materials to capture CO2. The 2 / 26


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experimental results showed that the CO2 adsorption capacity of IL-modified ACs was better than that of amino-modified polymers; moreover, the smaller particle size (38-150 µm) of IL-modified ACs resulted in the highest adsorption capacity (187.26 mg/g-sorbent) at 298.15 K and 4.663 atm in pure CO2. Wan et al. [34] investigated how the ILs immobilized on mesoporous alumina or silica captured CO2 at high temperature. These supported ILs attained equimolar CO2 capture at 393 K for the first time; most adsorbed CO2 could be desorbed at 443 K. Ren et al. [35] synthesized and immobilized six amino acid salts ([apaeP444][AA])-type ILs on porous silica through the impregnation-vaporization method. They investigated the CO2 sorption and desorption behaviors of the supported sorbents. The sorption was carried out at 298 K and 1 atm with a gas (pure CO2) flow rate of 50 mL/min. SiO2-[apaeP444][Lys] and SiO2-[apaeP444][Gly] exhibited high CO2 uptakes of 72.6 mg/g-sorbent within 1 h. Erto et al. [36] investigated two commercial ACs impregnated with [Hmim][BF4] and [Emim][Gly]. They used AC-supported ILs as adsorbents to separate CO2 from synthetic flue-gas. The results proved that using [Hmim][BF4] resulted in the deterioration of the AC capture performance, whereas [Emim][Gly] was effective in increasing AC capture capacity at 353 K. Wang et al. [37] synthesized amino acid-functionalized ILs [EMIM][Lys] and immobilized them on porous poly(methylmethacrylate) (PMMA). The [EMIM][Lys]-PMMA sorbent achieved a CO2 capacity of 73.48 mg/g-sorbnet or 0.87 mol/mol-AAIL within 40 min at 298 K and 1 atm with a gas (pure CO2) flow rate of 200 mL/min.. Arellano et al. [38] developed zinc-functionalized IL sorbents supported on mesoporous silica. These hybrid 3 / 26


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sorbents exhibited an unprecedented high uptake capacity of up to 148.72 mg CO2/g-sorbent at 313 K and 1 atm (the gas was pure CO2 at 50 ml/min), which was a significant enhancement for low IL loading sorbents. These new sorbents exhibit dramatic performance enhancement by factors of 1.6–16.0 (cf. bare support) and 3.6–5.3 (cf. bulk EZT3) highlighting the synergy between the available contact surface and pore network of the support, and the high affinity for CO2 by the IL. However, the aforementioned bulk ILs presented a limited CO2 capacity (≤1 mol CO2/mol-IL). A new anion-functionalized IL [P66614][2-Op] with an extremely high CO2 capacity of up to 1.58 mol CO2/mol-IL (higher than the aforementioned ILs) through multiple-site cooperative interactions was synthesized in literatures [39]. This IL was loaded onto mesoporous molecular sieve MCM-41 with a high specific surface area to reduce CO2 diffusion limitation. It was found that CO2 adsorption rates of the hybrid sorbents were higher than that of pure IL during the initial reaction stage.

2. METHODS 2.1 Materials. MCM-41 was purchased from the catalyst plant of NanKai University, Tianjin, China. The IL [P66614][2-Op] was provided by the Department of Chemistry, Zhejiang University, China. Anhydrous ethanol was purchased from Sinopharm Chemical Reagent Co.,Ltd., China. N2 (≥99.99 %), CO2 (≥99.995 %), the simulated biohythane (with a volume ratio of CH4:H2:CO2=3:3:4) and biogas (with a volume ratio of CH4: CO2=6:4) were obtained from Hangzhou Jingong Special Gas Co., Ltd., China. 4 / 26


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2.2. Immobilization of [P66614][2-Op] on MCM-41. The immobilization of [P66614][2-Op] on MCM-41 was achieved using the wet impregnation method presented in [40]. Typically, 3 g [P66614][2-Op] was added into a round-bottom flask with 30 g anhydrous ethanol and then dissolved under magnetic stirring for 30 min at an ambient temperature. When the [P66614][2-Op] was completely dissolved, 3 g MCM-41 was added into the solution. The mixture was stirred and refluxed at 353 K for 3 h, and then dried in a blast oven at 353 K until the ethanol was volatilized. The remaining trace amount of ethanol was further removed in a vacuum oven for 1 h. The obtained sample was designated as MCM-41-50% IL, which meant that the weight percentage of IL in hybrid sorbent (composed of IL and MCM-41) was 50wt%. MCM-41-30% IL and MCM-41-40% IL were also obtained using the same method. 2.3. Characterization of MCM-41 based sorbents. (1) Fourier transform infrared spectroscopy (FTIR) was performed on a NicoletTM 5700 FTIR spectrometer (Thermo Fisher Scientific, USA). The spectra were recorded within the range of 4000-400 cm-1. (2) X-ray diffraction (XRD) was operated on an X'Pert Pro diffractometer (PANalytical, Holland). The measurements were obtained in a range of 2θ=1-10°(3) N2 adsorption-desorption analysis was conducted on an Autosorb® iQ-MP sorption analyser (Quantachrome Instruments, USA). Physical adsorption of N2 was conducted at 77 K. The samples were degassed under high vacuum at 353 K for 4 h. The surface areas of MCM-41 and hybrid sorbents were obtained using Brunauer-Emmett-Teller (BET) method. The relative pressure range (P/P0) was 0.03-0.3. The total pore volume was calculated from the amount of adsorbed nitrogen 5 / 26


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at P/P0 = 0.993. Pore size was derived from the desorption branch using Barrett-Joyner-Halenda (BJH) model. (4) Thermogravimetric analysis (TGA) was conducted on a Q500 thermogravimetric analyzer (TA Instruments, USA) to test the thermal stability of hybrid sorbents under N2 atmosphere (40 mL/min). The heating rate was 10 °C /min from 20 °C to 700 °C. 2.4. CO2 adsorption measurement. Schematic of experiment system for CO2 adsorption of ionic liquid [P66614][2-Op] loaded onto molecular sieve MCM-41 in biohythane/pure CO2 atmosphere was given in Fig. 1. CO2 capture experiments were performed using a homemade reactor. First, 0.5 g sorbent was packed into a stainless steel reactor (8 mm inner diameter and 8 cm height). Prior to each adsorption experiment, N2 (10 mL/min) was introduced through the pipeline to drive out the air. The flow rates of pure CO2, biohythane and biogas in experiments were controlled at 10 mL/min. The flow rate was controlled by a mass flow controller which was calibrated with N2 gas. The reactor was weighed every 1 min using an analytical balance (ML104, Mettle Toledo USA) to evaluate the adsorption amount of CO2. All the experiments were performed at an ambient temperature (19 ± 1°C) and atmospheric pressure.

3. RESULTS AND DISCUSSION 3.1. Characterizations of samples. FTIR patterns of molecular sieve MCM-41 loaded with various weight percentages of IL were shown in Fig. 2. For the MCM-41 supported absorbents, the peaks at 455, 791 and 1079 cm-1 were attributed to the bending vibration of Si-O, the symmetric and asymmetric stretching vibrations of 6 / 26


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Si-O-Si, respectively. The three peaks were characteristic absorption bands of mesoporous molecular sieve. The broad band at 3415 cm-1 was assigned to the stretching vibration of Si-OH and physically adsorbed H2O [22]. The absorption band at 1655 cm-1 was corresponded to a C-O stretch. The two peaks at 2859 cm-1 and 2927 cm-1 were assigned to C-H stretches [41]. As the weight percentage of loaded IL increased, the intensity of the characteristic peaks (C-O stretch and C-H stretches) were increased. Ionic liquids (ILs) were successfully impregnated onto MCM-41. XRD patterns of molecular sieve MCM-41 loaded with various weight percentages of ionic liquid (IL) were given in Fig. 3. The addition of IL provoked an apparent decrease of (100) diffraction peak and the absence of (110) and (200) reflections. The more IL loaded, the weaker the peak intensity, indicating that IL was filled into the inner pores of molecular sieve MCM-41. To further validate that supported ionic liquid cause pore blocking of MCM-41, N2 adsorption-desorption isotherms of molecular sieve MCM-41 loaded with various weight percentages of IL were presented in Fig. 4a. All curves exhibited typical type-IV isotherms. The curve of unmodified MCM-41 showed that H1 hysteresis loop was caused by the mesopores at pressures P/P0 = 0.45-0.95. The hysteresis loops of MCM-41-40% IL and MCM-41-50% IL disappeared, which were attributed to the IL filled into mesopores of molecular sieve MCM-41. For hybrid sorbents, the degree of pore blocking was confirmed by the data listed in Table 1. Surface areas of the sorbents were obtained using BET model at relative pressure P/P0 = 0.03-0.25. Pore size was calculated using desorption branch according 7 / 26


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to BJH model. Total pore volume was determined at P/P0=0.99. The surface area of MCM-41 was 975.57m2/g, which was basically consistent with that in literature [22]. Compared with a blank MCM-41, the surface area of MCM-41-30% IL decreased to 407.98 m2/g, [P66614][2-Op] was loaded into the pores of MCM-41. Pore volume decreased to 0.28 cm3/g, but pore size increased. The very small pores of MCM-41-30% IL were filled with IL and large pores were partly filled. Moreover, for MCM-41-40% IL and MCM-41-50% IL, surface area and pore volume were small, the vast majority of pores were already fulfilled. Schematic of IL (green color) filled in pores (red color) of molecular was supplemented in Fig. 4b. The amount of ILs successfully loaded onto MCM-41 was detected through TGA. The weight losses of hybrid sorbents were shown in Fig. 5. All samples were pretreated at 100°C for 5 min to remove moisture and remaining ethanol. Then temperature was cooled to 50°C, and the amount of IL was estimated within a range of 50°C to 700°C. For MCM-41, a slight weight loss of 0.38% was attributed to the residual moisture in pores. The weight losses of MCM-41-30% IL, MCM-41-40% IL and MCM-41-50% IL were 31.06%, 39.73% and 53.02%, respectively. The weight losses were basically consistent with designed IL loadings. 3.2 CO2 adsorption by the MCM-41-based sorbents. Data were collected for only 4 h because the main objective of experiment was to obtain the CO2 adsorption rates. CO2 capacity and the reaction principle of [P66614][2-Op] were already described in the literature [39]. Schematic of reaction process for CO2 adsorption of pure ionic liquid [P66614][2-Op] in biohythane/pure CO2 atmosphere was shown in Fig. 6. As 8 / 26


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shown in Stage 1, when biohythane/pure CO2 began to flow into the reactor, IL remained calm. Stage 2 showed that as biohythane/pure CO2 was introduced, IL reacted with CO2. Since [P66614][2-Op] had a large viscosity, the flow-gas was difficult to diffuse rapidly and the gas pressure of introduced biohythane/pure CO2 increased up to a penetration threshold. Stage 3 showed a current of biohythane/pure CO2 gas discharged from the reactor when the accumulated gas pressure was higher than a penetration threshold of pure ionic liquid. The CO2 absorption process caused the unreacted IL exposed to CO2 molecules. CO2 adsorption curves of ionic liquid (IL) loaded onto molecular sieve (MCM-41-50% IL) in pure CO2 atmosphere were shown in Fig. 7. (1)The amount of CO2 adsorbed by MCM-41-50% IL during the first 1 min was 51.5 mg CO2/g-IL, whereas only 24.6 mg CO2/g-IL was adsorbed by pure IL. CO2 adsorption rate of MCM-41-50% IL was 2.1 times that of pure IL. It was because MCM-41-50% IL had more exposed IL than pure IL. Once CO2 gas was introduced into the reactor, CO2 molecules rapidly spread throughout the reactor and contacted with the surfacial [P66614][2-Op], resulting in a higher CO2 adsorption rate than pure IL at the beginning. (2) As the reaction proceeded, for MCM-41-50% IL, the surfacial IL viscosity was increased and subsequent CO2 molecules were required to overcome the viscosity resistance of surfacial IL to diffuse into IL blocked in inner pores. As a result, the amount of CO2 adsorbed by pure IL drew closer to that of MCM-41-50% IL and then exceeded after 35 min. (The viscosities of pure IL [P66614][2-Op] before and after 9 / 26


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reaction with CO2 were 573 mPa·S and 2273 mPa·S.) CO2 adsorption curves of various-weight-percentage ionic liquid (IL) loaded onto molecular sieve in biohythane atmosphere were shown in Fig. 8. (1) During the first 2 min, the amount of CO2 adsorbed by MCM-41-50% IL was 24.65 mg CO2/g-IL, whereas 11.05 mg CO2/g-IL was adsorbed by pure IL. The CO2 adsorption rate of MCM-41-50% IL was 2.2 times that of pure IL. For sorbent MCM-41-50% IL, CO2 molecules rapidly spread and reacted with the surficial IL loaded onto MCM-41 once biohythane was introduced into the reactor. MCM-41-50% IL had more exposed IL than pure IL, so the CO2 adsorption rate of MCM-41-50% IL was higher than that of pure IL at the beginning. (2) After 25 min, the amount of CO2 adsorbed by pure IL drew closer to that of MCM-41-50% IL and then exceeded. Because for MCM-41-50% IL, the surfacial IL viscosity rose as the reaction proceeded to generate a protective film, which slowed down CO2 mass transfer into the IL blocked in inner pores. The CO2 adsorption curves of ionic liquid (IL) loaded onto molecular sieve (MCM-41-50% IL) in biohythane atmosphere with various flow rates were given in Fig. 9. The CO2 adsorption rate was enhanced 1.9 times when biohythane flow changed from 10 mL/min to 25 mL/min. However, the CO2 adsorption rate was enhanced 1.1 times when biohythane flow changed from 25 mL/min to 40 mL/min. The CO2 uptake of the supported ILs rapidly increased at low CO2 flow rates, but slowly increased at elevated CO2 flow rates. CO2 adsorption curves of ionic liquid (IL) loaded onto molecular sieve 10 / 26


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(MCM-41-50% IL) in various atmosphere were shown in Fig. 10. The adsorbed CO2 amounts in biohythane and biogas were 65.6 mg/g-IL and 62.80 mg/g-IL, respectively, and that in pure CO2 gas was 79.6 mg/g-IL. These results were attributed to that CO2 concentrations (40%) in biohythane and biogas were lower than that (100%) in pure CO2 gas. This implied that H2 and CH4 had little effects on CO2 adsorption of MCM-41-50% IL in biohythane and biogas atmospheres.

4. CONCLUSION Hybrid sorbents of ILs loaded on molecular sieves efficiently adsorbed CO2 from biohythane and biogas. The IL [P66614][2-Op] loaded onto molecular sieve MCM-41 exhibited higher CO2 adsorption rate than pure IL during the initial stage because of its larger reaction surface area. However, the CO2 adsorption rate of this hybrid sorbent became lower than that of pure IL as reaction continued, because CO2 was inaccessible to the IL blocked in inner pores of molecular sieve. The CO2 adsorption rate of IL loaded onto molecular sieve (MCM-41-50% IL) was (51.5 mg CO2/g-IL·min) 2.1 times higher than that of pure IL during the initial 1 min in pure CO2 atmosphere and was (24.6 mg CO2/g-IL·min) 2.2 times higher than that of pure IL during the initial 2 min in biohythane atmosphere. It is necessary to optimize porosities of molecular sieves loaded with IL and amine sorbents to further increase CO2 adsorption rate.

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Acknowledgements This study was supported by the National Natural Science Foundation-China (51476141); Zhejiang Provincial Natural Science Foundation-China (LR14E060002); National Key Technology R&D Program-China (2015BAD21B01). References [1]Xia, Z.M.; Li, X.S.; Chen, Z.Y.; Li, G.; Yan, K.F.; Xu, C.G.; Lv, Q.N.; Cai, J. Appl Energy, http://dx.doi.org/10.1016/j.apenergy.2015.02.016. [2]Bordelanne, O.; Montero, M.; Bravin, F.; Vernat, A.P.; Selmi, O.O.; Pierre, H.; Papadopoulo, M.; Muller, T. J Nat Gas Sci Eng, 2011, 3, 617-624. [3]Anderson, L.G. Renew Sust Energ Rev, 2015, 47, 162-172. [4]Subramanian, K.A.; Mathad, V.C.; Vijay, V.K.; Subbarao, P.M.V. Appl Energy, 2013, 105, 17-29. [5]Ma, F.; Wang, M.Y.; Jiang, L.; Chen, R.Z.; Deng, J.; Naeve, N. Int J Hydrogen Energ, 2010, 35, 6438-6447. [6]Ma, F.H.; Wang, Y.F.; Ding, S.F.; Jiang, L. Int J Hydrogen Energ, 2009, 34, 6523-6531. [7]Cheng, J.; Liu, Y.Q.; Lin, R.C.; Xia, A.; Zhou, J.H.; Cen, K.F. Int J Hydrogen Energ, 2014, 39, 18793-18802. [8]Xia, A.; Cheng, J.; Ding, L.K.; Lin, R.C.; Zhou, J.H.; Song, W.L.; Cen, K.F. Appl Energy, 2014, 120, 23-30. [9]Lin, R.C.; Cheng, J.; Song, W.L.; Ding, L.K.; Xie, B.F.; Zhou, J.H.; Cen, K.F. Bioresour Technol, 2015, 182, 1-7. [10]Nissila, M.E.; Li, Y.C.; Wu, S.Y.; Lin, C.Y.; Puhakka, J.A. Appl Energy 2012, 100, 58-65. 12 / 26


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[27]Hu, P.C.; Zhang, R.; Liu, Z.C.; Liu, H.Y.; Xu, C.M.; Meng, X.H.; Liang, M.; Liang, S.S. Energ Fuel, 2015, 29, 6019-6024. [28]Nonthanasin, T.; Henni, A.; Saiwan, C. RSC Adv, 2014, 4, 7566-7578. [29]Kortunov, P.V.; Baugh, L.S.; Siskin, M. Energ Fuel, 2015, 29, 5990-6007. [30]Xie, Y.J.; Zhang, Y.Y.; Lu, X.H.; Ji, X.Y. Appl Energy, 2014, 136, 325-335. [31]Wang, F.; Zhang, Z.Q.; Yang, J.; Wang, L.P.; Lin, Y.; Wei, Y. Fuel, 2013, 107, 394-399. [32]Arellano, I.H.; Madani, S.H.; Huang, J.H.; Pendleton, P. Chem Eng J, 2016, 283, 692-702. [33]Zhu, T.; Bi, W.T.; Row, K.H. Korean J Chem Eng, 2011, 28, 914-916. [34]Wan, M.M.; Zhu, H.Y.; Li, Y.Y.; Ma, J.; Liu, S.; Zhu, J.H. ACS Appl Mater Inter, 2014, 6, 12947-12955. [35]Ren, J.; Wu, L.B.; Li, B.G. Ind Eng Chem Res, 2012, 51, 7901-7909. [36]Erto, A.; Silvestre-Albero, A.; Silvestre-Albero, J.; Rodriguez-Reinoso, F.; Balsamo, M.; Lancia, A.; Montagnaro, M. J Colloid Interf Sci, 2015, 448, 41-50. [37]Wang, X.F.; Akhmedov, N.G.; Duan, Y.H.; Hopkinson, D.; Li, B.Y. Appl Mater Inter, 2013, 5, 8670−8677. [38]Arellano, I.H.; Huang, J.H.; Pendleton, P. RSC Adv, 2015, 5, 65074-65083. [39]Luo, X.Y.; Guo, Y.; Ding, F.; Zhao, H.Q.; Cui, G.K.; Li, H.R.; Wang, C.M. Angew Chem Int Ed, 2014, 53, 7053-7057. [40]Feng, X.X.; Hu, G.S.; Hu, X.; Xie, G.Q.; Xie, Y.L.; Lu, J.Q.; Luo, M. F. Ind Eng Chem Res, 2013, 52, 4221-4228. [41]Mello, M.R.; Phanon, D.; Silveira, G.Q.; Llewellyn, P.L.; Ronconi, C.M. Micropor Mesopor Mat, 2011, 143, 174-179. 14 / 26


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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

Energy & Fuels

List of Figures and Tables Fig. 1 Schematic of experiment system for CO2 adsorption of ionic liquid [P66614][2-Op] loaded onto molecular sieve MCM-41 in biohythane/pure CO2 atmosphere. Fig. 2 FTIR patterns of molecular sieve MCM-41 loaded with various weight percentages of ionic liquid (IL). Fig. 3 XRD patterns of molecular sieve MCM-41 loaded with various weight percentages of ionic liquid (IL). Fig. 4 Porosity properties of molecular sieve MCM-41 loaded with various weight percentages of ionic liquid (IL): (a) N2 adsorption-desorption isotherms. (b) Schematic of IL (green color) filled in pores (red color) of molecular sieve. Fig. 5 TGA curves of molecular sieve MCM-41 loaded with various weight percentages of ionic liquid (IL). Fig. 6 Schematic of reaction process for CO2 adsorption of pure ionic liquid [P66614][2-Op] in biohythane/pure CO2 atmosphere. Fig. 7 CO2 adsorption curves of ionic liquid (IL) loaded onto molecular sieve (MCM-41-50% IL) in pure CO2 atmosphere (flow rate = 10 mL/min). Fig. 8 CO2 adsorption curves of various-weight-percentage ionic liquid (IL) loaded onto molecular sieve in biohythane atmosphere (volume ratio of CH4:H2:CO2 = 3:3:4, flow rate = 10 mL/min). Fig. 9 CO2 adsorption curves of ionic liquid (IL) loaded onto molecular sieve (MCM-41-50% IL) in biohythane atmosphere (volume ratio of CH4:H2:CO2 = 3:3:4) with various flow rates. Fig. 10 CO2 adsorption curves of ionic liquid (IL) loaded onto molecular sieve (MCM-41-50% IL) in various atmospheres (pure CO2, biogas and biohythane). Table 1 Pore structures of molecular sieve MCM-41 loaded with various weight percentages of ionic liquid (IL).

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Energy & Fuels

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

Fig. 1 Schematic of experiment system for CO2 adsorption of ionic liquid [P66614][2-Op] loaded onto molecular sieve MCM-41 in biohythane/pure CO2 atmospheres.

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1079 791

2927

1655

2859

1238

IL 3415

455

Transmittance (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

Energy & Fuels

MCM-41

MCM-41-30% IL

MCM-41-40% IL

MCM-41-50% IL

0

1000

2000

3000

4000

-1

Wavenumber (cm )

Fig. 2 FTIR patterns of molecular sieve MCM-41 loaded with various weight percentages of ionic liquid (IL).

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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

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MCM-41

MCM-41-30% IL MCM-41-40% IL MCM-41-50% IL

0

2

4

6

8

10

2θ (°)

Fig. 3 XRD patterns of molecular sieve MCM-41 loaded with various weight percentages of ionic liquid (IL).

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Page 19 of 26

(a)

800

3

N2 volume absorbed (cm /g, STP)

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

Energy & Fuels

MCM-41 MCM-41-30% IL MCM-41-40% IL MCM-41-50% IL

700 600 500 400 300 200 100 0 0.0

0.2

0.4

0.6

0.8

1.0

Relative pressure(P/P0)

(b)

Fig. 4 Porosity properties of molecular sieve MCM-41 loaded with various weight percentages of ionic liquid (IL): (a) N2 adsorption-desorption isotherms. (b) Schematic of IL (green color) filled in pores (red color) of molecular sieve.

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Energy & Fuels

MCM-41

100

Weight percentage (%)

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

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80

MCM-41-30% IL MCM-41-40% IL

60

MCM-41-50% IL

40 20 IL

0 0

100

200

300

400

500

600

700

Temperature(°C)

Fig. 5 TGA curves of molecular sieve MCM-41 loaded with various weight percentages of ionic liquid (IL).

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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

Energy & Fuels

Fig. 6 Schematic of reaction process for CO2 adsorption of pure ionic liquid [P66614][2-Op] in biohythane/pure CO2 atmosphere. (Stage1: biohythane / pure CO2 began to flow into the reactor filled with pure ionic liquid; Stage 2: gas pressure of introduced biohythane / pure CO2 increased up to a penetration threshold; Stage 3: a current of biohythane / pure CO2 gas discharged from the reactor when the accumulated gas pressure was higher than a penetration threshold of pure ionic liquid.)

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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

CO2 adsorption per gram of IL (mgCO2/g-IL)

Energy & Fuels

100

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IL MCM-41-50% IL

80 60

60 50

40

40 30 20

20

10 0

0

0

0

100

5

10 15 20 25 30 35 40

200

300

400

Adsorption time(min)

Fig. 7 CO2 adsorption curves of ionic liquid (IL) loaded onto molecular sieve (MCM-41-50% IL) in pure CO2 atmosphere (flow rate = 10 mL/min).

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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

Energy & Fuels

CO2 adsorption per gram of IL (mgCO2/g-IL)

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90 IL MCM-41-50% IL MCM-41-40% IL MCM-41-30% IL

75 60 45

40

30

30 20

15

10

0

0 0

0

40

80

5

120

10

15

160

20

200

25

240


Fig. 8 CO2 adsorption curves of various-weight-percentage ionic liquid (IL) loaded onto molecular sieve in biohythane atmosphere (volume ratio of CH4:H2:CO2 = 3:3:4, flow rate = 10 mL/min).

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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

CO2 adsorption per gram IL(mgCO2 /g-IL)

Energy & Fuels

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75 60 45 Biohythane flow rate: 40ml/min 25ml/min 10ml/min

30 15 0 0

40

80

120

160

200

240


Fig. 9 CO2 adsorption curves of ionic liquid (IL) loaded onto molecular sieve (MCM-41-50% IL) in biohythane atmosphere (volume ratio of CH4:H2:CO2 = 3:3:4) with various flow rates.

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90 75

concentration(%)

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

Energy & Fuels

60 45 30 Pure CO2

15

Biogas (volume ratio of CH4:CO2=6:4) Biohythane (volume ratio of CH4:H2:CO2=3:3:4)

0 0

40

80

120

160

200

240


Fig. 10 CO2 adsorption curves of ionic liquid (IL) loaded onto molecular sieve (MCM-41-50% IL) in various atmospheres (pure CO2, biogas and biohythane).

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Energy & Fuels

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Table 1 Pore structures of molecular sieve MCM-41 loaded with various weight percentages of ionic liquid (IL). Samples MCM-41 MCM-41-30% IL MCM-41-40% IL MCM-41-50% IL

Specific surface area (m2/g)

Specific pore volume (cm3/g)

Mean pore diameter (nm)

975.57 407.98 14.61 3.48

1.10 0.28 0.10 0.01

2.75 3.81 3.82 3.82

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CO2 Adsorption Performance of Ionic Liquid [P66614][2-Op] Loaded

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