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Reaction mechanism and kinetics study of CO absorption into [COHmim][Lys] Sujing Li, Chongjian Zhao, Cheng Sun, Yun Shi, and Wei Li Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b01773 • Publication Date (Web): 31 Aug 2016 Downloaded from http://pubs.acs.org on September 6, 2016
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Energy & Fuels
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Reaction mechanism and kinetics study of CO2 absorption into [C2OHmim][Lys]
2 3
Sujing Li, Chongjian Zhao, Cheng Sun, Yun Shi, Wei Li*
4
Key Laboratory of Biomass Chemical Engineering of Ministry of Education, Institute
5
of Industrial Ecology and Environment, College of Chemical and Biological
6
Engineering, Zhejiang University, Yuquan Campus, Hangzhou 310027, China
7
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Highlights
9
1.
CO2/mol IL.
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2.
3.
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The mechanism of CO2 absorption into pure [C2OHmim][Lys] and aqueous solution of [C2OHmim][Lys] were completely different.
14 15
Three different reaction products between CO2 and pure [C2OHmim][Lys] were separated and detected by 13C NMR.
12 13
The capacity of [C2OHmim][Lys] without and with water were 1.68 and 1.26 mol
4.
The reaction mechanism and kinetics between CO2 and aqueous solution of [C2OHmim][Lys] in low CO2 loading and high CO2 loading were different.
17
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Energy & Fuels
In
this
work,
a
novel
hydrophilic
ionic
liquid
(IL)
18
ABSTRACT:
19
1-hydroxyethyl-3-methylimidazolium lysine ([C2OHmim][Lys]) with a considerable
20
CO2 absorption capacity of 1.68 mol CO2/mol IL was synthesized. Three reaction
21
products between CO2 and neat [C2OHmim][Lys] were successfully separated and
22
detected by
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aqueous solutions with an initial conentration of 1mol/L was 1.2604 mol CO2/mol IL.
24
The reaction mechanism and kinetic study of CO2 adsorption into aqueous solution of
25
[C2OHmim][Lys] were also investigated by using 13C NMR and wetted wall column.
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In the early stage of absorption, the dominant chemical reaction was the formation of
27
carbamate and belonged to the fast reaction. The reaction order was found to be an
28
average value of 1.862 with respect to [C2OHmim][Lys]. At high CO2 loading (more
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than 0.95 mol CO2/mol IL), the hydrolysis of carbamate was the main reaction and it
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was between the fast reaction and the medium-speed reaction. Also, the two amino
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groups on the lysine anion have different reaction behaviors in the hydrolysis stage，
32
suggesting that the carbamate linked to carboxyl in lysine anion participates in
33
hydration reaction while the other carbamate does not.
13
C NMR. Meanwhile, the absorption capacity of [C2OHmim][Lys]
34 35
1. INTRODUCTION
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CO2 emitted from the combustion of fossil fuels is one of the major courses to
37
global warming. The chemical absorption of CO2 is the most practical technology to
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reduce the emission of CO2 in fuel-fired power plants.1 Aqueous solutions of
39
alkanolamines are widely used as absorbent because of their rapid absorption rate at
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low CO2 partial pressure.2 Unfortunately, this technology has some obvious
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disadvantages, such as high energy costs and poor cyclic capacity.3 So it is highly
42
desirable to develop more efficient technologies to capture CO2.
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In recent years, capturing CO2 with ionic liquids has been paid much attention
44
because of their negligible vapor pressure, high thermal stability and easy assembly.4
45
Compared with alkanolamines, IL for CO2 capture has no contamination of gas stream,
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negligible losses, and lower energy consumption. Many research groups have focused
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on the development of IL for CO2 capture. The absorption capacity of
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1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][PF6]) reached 0.72 mol CO2/
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mol IL at 93 bar.5 Gurau et al. reported that the theoretical maximum absorption
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capacity of imidazolium cation at atmospheric pressure was 0.33 mol CO2/ mol IL.6
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Maginn
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1-butyl-3-methylimidazolium acetate by using NMR.7
et
al.
interpreted
the
absorption
mechanism
of
CO2
in
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By introducing suitable functional groups (like amine) into the conventional ionic
54
liquids, functionalized ionic liquids (also called task-specific ionic liquids, TSIL) can
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improve the CO2 absorption properties, such as absorption capacity and absorption
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rate.
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([NH2p-bim][BF4]) approached 0.5 mol CO2/ mol IL (called 1:2 mechanism) similar
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to the alkanolamine.8 Zhang et al. synthesized a new type of TSIL which absorbed
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CO2 in nearly 1:1 stoichiometry,9 and the results from FTIR spectrum verified the 1:1
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reaction mechanism.10 To further improving the absorption capacity, Shunmugavel et
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al. synthesized several amino-ammonium-based IL containing two or multi amino
The
capacity
of
1-aminopropyl-3-butylimidazolium
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tetrafluoroborate
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groups and their maximum CO2 absorption capacity was 2 mol CO2/ mol IL.11
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However, current research on the ionic liquids is still on the laboratory scale
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although there is great potential for industrial applications. Shiflett et al. estimated by
65
simulation that CO2 capture with 1-butyl-3-methylimidazolium acetate ([bmim][Ac])
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can reduce the energy losses by 16% compared to a commercial MEA process12.
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Wappel et al. selected 60% IL-water to capture CO2 from post-combustion flue gas.
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They found the enthalpy of absorption was calculated to be 41.1±3.2 kJ mol-1 IL
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which is about half of the 82 kJ mol-1 for MEA, and total energy demand of the
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stripping process of the IL gives possible energy savings between 12 and 16%13. Xue
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et al. synthesized 1-aminoethyl-2,3-dimethylimidazolium taurine ([aemmim][Tau])
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with
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[aemmim][Tau] can be reused at high temperature or under vacuum and there was no
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significant mass loss after six recycles. Lv et al. investigated the regeneration
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performance
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([Apmim][Gly])-water.15 They found that the regeneration efficiency was higher than
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97.5%.
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The
amino-functionalized
biggest
of
imidazolium
30%
barrier
is
cation
and
taurine
anion14.
1-aminopropyl-3-methylimidazolium
the
high
viscosity
of
pure
IL.
The
glycine
Though
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Trihexyl(tetradecyl)-phosphonium sarcosine
([P66614][Me-Gly]) in dry condition has
80
a low viscosity of 312 mPa·s at 30 °C.16 It is still much higher than industrial
81
absorbents. Research has found that the viscosity of IL could be significantly
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decreased if adding water to it.17 This prompts a possible way to large-scale industrial
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application of IL. Selected ionic liquids and water can considerably reduce the
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enthalpy of absorption and increase the kinetics of the mixture.18 But the absorption
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capacity of CO2 in IL-water solvents is lower than that in neat ionic liquid.19 It is
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showed that the absorption mechanism in IL-water solvent may be different from that
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in neat IL. Even though, it is widely common for the researchers to apply zwitterion
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mechanism to kinetics study of the aqueous solution of IL.15,20 To date, there is few
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research focus on the reaction mechanism of CO2 absorption into aqueous solution of
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ILs. And the direct difference in reaction mechanism between neat ILs and aqueous
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solution of ILs have rarely been studied. Hence, to full understand its absorption
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performance, reaction mechanism of aqueous solution of [C2OHmim][Lys] is required
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to be carried out for accurate kinetics study, rather than just applying the zwitterion
94
mechanism. Also, kinetic data were necessary to thoroughly assess the reaction rates
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of the aqueous solution of [C2OHmim][Lys] to evaluate the solvent performance.
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In this study, we report the synthesis of new amine-functionalized ionic liquid
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([C2OHmim][Lys]) and the performances of [C2OHmim][Lys] with and without water
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for capturing CO2. The reaction mechanism of neat and aqueous solution of
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[C2OHmim][Lys] were also studied via
13
C NMR analysis. Meanwhile, according to
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the reaction mechanism of CO2 adsorption into aqueous solution of [C2OHmim][Lys],
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the kinetics of CO2 into aqueous solution of [C2OHmim][Lys] were investigate at
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different CO2 loading (0-1.1 mol CO2/mol IL) and temperatures (303.15-333.15 K)
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using a wetted wall column.
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2. MATERIALS AND METHODS
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Energy & Fuels
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2.1. Chemicals. 1-Methylimidazole (99% purity) and l-lysine (98% purity) were
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purchased from Aladdin Chemical Co., Ltd., China. 2-Chlorine-1-ethanol (99.5%
108
purity) was purchased from Xiya Chemical Reagent Co., Ltd., China. The
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anion-exchange resin (Dowex Monosphere 550A (OH)) was purchased from Dow
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Chemical Company. CO2 (99.999% purity) and N2 (99.999% purity) was supplied by
111
Zhejiang jin-gong Gas Co, China. Analytical reagents of ethyl acetate was purchased
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from the Sinopharm Chemical Reagent Co., Ltd., China. All aqueous solution were
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prepared with ultrapure water (≥18.2 MΩ·cm) and no further purification was
114
performed on the materials used.
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2.2. Synthesis of [C2OHmim][Lys]. The synthesis process of [C2OHmim][Lys]
116
was
carried
out
as
follows:
the
synthesis
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Chlorinated-1-hydroxyethyl-3-methylimidazolium ([C2OHmim]Cl) was reported in
118
our previous work.21 2-Chlorine-1-ethanol (0.96 mol) and 1-Methylimidazole (0.8
119
mol) reacted at 80 °C under stirring for 24 h. The product was purified by extraction
120
using ethyl acetate to remove the unreacted 2-chlorine-1-ethanol and then the product
121
was dried at 80 °C under vacuum for 48 h. [C2OHmim][OH] was prepared from
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[C2OHmim]Cl using an anion-exchange resin. The anion-exchange resin was washed
123
with ultrapure water until reaching a neutral pH before use. The concentration of OH-
124
was determined by titration with aqueous solution of HCl. Equimolar l-lysine was
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added to the aqueous solution of [C2OHmim][OH] and was stirred for 12 h at 25 °C,
126
and then the product was dried at 80 °C under vacuum for 48 h to obtain
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[C2OHmim][Lys]. The water content (9 and α
