(572e) A Study of Porous Support Amine-IL Binary System for CO2 Capture

A
study of porous support amine-IL binary system for CO₂ capture

Min Xiao ^a, Helei Liu ^a, Raphael Idem ^a,b, Paitoon Tontiwachwuthikul ^a,b, and Zhiwu Liang ^a,b*

^a Joint
International Center for CO₂ Capture and Storage (iCCS), Provincial Hunan
Key Laboratory for Cost-effective Utilization of Fossil Fuel Aimed at Reducing
CO₂ Emissions, College of Chemistry and Chemical Engineering, Hunan
University, Changsha 410082, PR China

^b Clean
Energy Technologies Research Institute (CETRI), Faculty of Engineering and
Applied Science

University of Regina, Regina,
Saskatchewan, S4S 0A2, Canada

Global
warming, which brings extreme weather, has become a serious issue and draws people's
attention. As a kind of greenhouse gas, carbon dioxide is mainly blamed for the
rise of global temperature. Large amount of CO₂ is straight released
to atmosphere through burning of fossil fuel to meet the demand for energy.
There is urgent need to reduce CO₂ emission to protect the
environment. Meanwhile, CO₂ is a kind of potential resources which
can be applied to Enhanced Oil Recovery (EOR), reform with methane to synthesis
gas etc. Thus many research have been done on separating CO₂ from tail
gas. Although absorption CO₂ with alcohol amine has been widely studied for
years and  proved to
capture CO₂ efficiently, there reamins some
problem to limit its usage such as corrosivity, volatility, degradation,
especialy high energy consumption. The amount of energy required for regenerating rich solvent accounts
for roughly 70-80% of the total operating cost of the system.

In this case, room
temperature ionic liquid (RTIL), exists as liquid state at room temperature and
shows unique properties, arise people's interesting in application for CO₂
capture. RTIL has low vapor pressure, commending for environmental friendly. Besides,
RTIL shows good thermal/chemical stability and tunable physicochemical
characteristics, which has been successfully used as green solvent for reaction
or catalyst for organic reactions¹. However, the
viscosity of ionic liquid is commonly high and will show dramatically increase
after CO₂ absorption.²

 
In present work, a kind of low viscosity amine-IL binary system is proposed for
CO₂ capture. Imidazole based ILs 1-butyl-3-methylimidazolium
tetrafluoroborate ([BMIM]BF₄) and 1-butyl-3-ethylimidazolium
tetrafluoroborate ([BEIM]BF₄) were chosen for relative low viscosity
compared with other ILs. Conventional amines, Monoethanolamine (MEA) and Methyldiethanolamine
(MDEA) were selected as blended amine. The structure and abbreviation of amines
and cation of ILs were shown in Figure 1. During the experiment on
studying amine-IL binary system, MEA was found precipitate out after reacting
with CO₂ while MDEA could be well dissolved into both ILs.

 [BMIM]BF₄ and [BEIM]BF₄were prepared on basis of imidazole. Completely anion exchange was
confirmed using silver nitrate until no bromide can be detected. After
evaporating unreacted component and water, ILs were stored into vacuum drier. The
Nuclear Magnetic Resonance (NMR) technology was employed to characterize cation
of ILs and outcome of absorbent after CO₂ absorption. Thermogravimetric
Analysis (TGA) was introduced to evaluate the water content and thermal
stability of ILs. Then the dynamic viscosity of pure ILs and amine-ILs binary
system was measured. The CO₂ absorption experiment process is
briefly interpreted as below. The pure CO₂ flow rate was fixed at 50
mL•min^-1 by mass flow controller, then CO₂ was deal with concentrated
sulfuric acid to get rid of trace amount of water. Approximate 0.8 mL absorbent
was stabilized into porosity filter candle of gas washing bottle and CO₂
continuous flow through the bottle. The gas-washing bottle was immersed into
water bath at 293.15K. The amount of absorption CO₂ was measured by
detecting weight changes using electron analytical balance.

 
TGA result of pure [BMIM]BF₄ and [BEIM]BF₄ was shown in Figure
2. Water content of ILs was confirmed less than 1% and thermal decomposition
temperature (10% mass loss) of [BMIM]BF₄ and [BEIM]BF₄
was 635.65K and 646.96K, respectively. Result shows that ILs behaved good
thermal stability. The viscosity of [BMIM]BF₄
and [BEIM]BF₄ were measured at temperature rang from 298.15K to
353.15K. The viscosity of [BMIM]BF₄ at 293.15K is 101.2cP ,which fit
well with Harris's work with deviation of 2.6%.³ [BEIM]BF₄
is found more viscous than [BMIM]BF₄ due to longer carbon chain. An
extended Arrhenius equation (1) is applied to correct viscosity to temperature:

The
R-square of viscosity fitting for [BMIM]BF₄ and [BEIM]BF₄
is 0.997 and 0.998, respectively. The summaries of E_a and ƞ₀ are
listed in Table 1. The CO₂ absorption curves of pure ILs were
shown in Figure 3(a). CO₂ equilibrium solubility of [BMIM]BF₄
and [BEIM]BF₄ at 298.15 K was 0.039 and 0.049 mol CO₂•mol
amine^-1, respectively. The equilibrium could be reached within 10
minutes and [BEIM]BF₄ showed higher CO₂ solubility than
[BMIM]BF₄. However, both ILs revealed low physical CO₂
absorption without additional amines. To intensify CO₂ absorption, 30%
mass fraction of MEA and MDEA were blended with ILs respectively. Four CO₂
absorption curves were shown in Figure 3(b). It is clear that CO₂
capacity of ILs is greatly intensified by additional amines and the CO₂
equilibrium could achieved within 120 minutes. The equilibrium CO₂
loadings of MEA-[BMIM]BF₄ and MEA-[BEIM]BF₄ at 298.15K
were 0.494 and 0.515 mol CO₂•mol amine^-1, respectively. Chemical
reaction between MEA and CO₂ was verified by new resonance at
165.08ppm after CO₂ absorption through ¹³C NMR technology.
Two MEA molecules could combine one molecule CO₂ to form carbamate
without existence of water. The CO₂ absorption of MDEA-[BMIM]BF₄
and MDEA-[BEIM]BF₄ could be accomplished within 80 minutes and the
equilibrium CO₂ loadings at 298.15K was 0.214 and 0.334 mol CO₂•mol
amine^-1, respectively, which is relatively lower than MEA-ILs system
at atmosphere pressure. However, no additional peak was detected in absorbent
which means the binary system only revealed physical CO₂ absorption.
To further study viscosity of binary MDEA-ILs system, approximately 1mL ionic
liquids was exposed to pure CO₂ atmosphere for 180 minutes. The
viscosity of absorbents before and after CO₂ absorption at 298.15K was
shown in Table 2. The viscosity of MDEA-[BMIM]BF₄ increase
from 49.46cP to 63.40cP after CO₂ absorption while MDEA-[BEIM]BF₄
increase from 49.79cP to 77.77cP. It could be concluded that both absorbents
viscosity increase slightly. It is interesting that amine-[BEIM]BF₄
performs better than amine-[BMIM]BF₄ on CO₂ absorption.
ILs with higher CO₂ loadings was found to intensify amine to react
with CO₂ as well. In our current work, MDEA-[BEIM]BF₄ shows
to be a potential binary system for CO₂ capture. The binary system
is stable, able to capture CO₂ fast and shows relative low viscosity.
Although MEA-ILs shows higher CO₂ capacity than MDEA-ILs, it has to
be pointed out that in MDEA-[BEIM]BF₄ system, only physical absorption
occurs which means the binary system can readily be regenerated by heating to
strip out CO₂.

Figure 1 the structure and abbreviation of conventional amines,
[BMIM]⁺ and [BEIM]⁺

Figure 2 Thermogravimetric
Analysis on [BMIM]BF₄ and [BEIM]BF₄

Figure 3 the
CO₂ loading of pure ILsversus time at 298.15K (a) and CO₂
loading of binary system with amine mass fraction of 30%at 298.15K
(b)

Table 1 Summaries
of E_a and ƞ₀for
pure [BMIM]BF₄ and [BEIM]BF₄

Table 2 The
viscosity of binary MDEA-ILs system before and after CO₂ absorption
at 298.15K

Acknowledgment: The
financial supports from the National Natural Science Foundation of China (Nos.
21476064, 21376067 and U1362112), National Key Technology R&D Program (Nos.
2012BAC26B01 and2014BAC18B04), Innovative Research Team Development
Plan-Ministry of Education of China (No. IRT1238), Shanxi Yanchang Petroleum
(Group) Co., LTD, Specialized Research Fund for the Doctoral Program of Higher
Education (No. 20130161110025),and China's State 'Project 985' in Hunan University-Novel Technology
Research and Development for CO₂ Capture as well as Hunan University
to the Joint International Center for CO­₂ Capture and Storage (iCCS)
is gratefully acknowledged.

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