Tuning ionic liquids for hydrate inhibition

Article abstract of DOI:10.1039/c0cc05676f

Pyrrolidinium cation-based ionic liquids were synthesized, and their inhibition effects on methane hydrate formation were investigated. It was found that the ionic liquids shifted the hydrate equilibrium line to a lower temperature at a specific pressure,

Full text of DOI:10.1039/c0cc05676f

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COMMUNICATION
Tuning ionic liquids for hydrate inhibitionw
a
b
c
Ki-Sub Kim, Jeong Won Kang and Seong-Pil Kang*
Received 20th December 2010, Accepted 18th April 2011
DOI: 10.1039/c0cc05676f
Pyrrolidinium cation-based ionic liquids were synthesized, and
their inhibition effects on methane hydrate formation were
investigated. It was found that the ionic liquids shifted the
hydrate equilibrium line to a lower temperature at a specific
pressure, while simultaneously delaying gas hydrate formation.
do not shift the equilibrium temperature at a specific pressure
and eventually allow hydrate formation, they do slow down
the rates of hydrate nucleation and growth. The other type is
anti-agglomerants (AAs), which allow gas hydrate formation
but prevent agglomeration of particulates. KHIs retard gas
hydrate formation and elongate induction time at low dosages
of 1–2 wt%. Thus, KHIs are expected to possess significant
economic and environmental advantages. All discovered KHIs
to date are water-soluble polymers. Poly(N-vinylpyrrolidone)
(PVP), poly(N-vinylcaprolactam) (PVCap), poly(N-methyl-N-
vinylacetamide) (VIMA), poly(N-vinylvalerolactam) (PVVam),
poly(acryoyl pyrrolidine) (PAPYD), and their copolymers are
Gas hydrates have received increased attention recently as
research on future green energy and carbon sequestration has
become more urgent. Natural gas obtained from methane
hydrates can be an alternative energy source, and carbon
dioxide can be captured in void space in hydrates in the form
1
,2
of ice-like solids. Recently, Lee et al. found a swapping
phenomenon between gaseous carbon dioxide and methane
2
all representative examples of KHIs. However, the economic
3
hydrate deposits. In a suggested process, carbon dioxide is
application of KHIs is still under development, especially
at a large degree of sub-cooling, meaning the temperature
difference between the equilibrium and operational temperatures.
Therefore, there is demand for new inhibitors or a combination
of inhibitors that are cost effective and efficient. Herein, we
present new hydrate inhibition methods and introduce ionic
liquids (ILs) as inhibitors.
directly sequestered into the methane hydrate layer while
methane is simultaneously produced with a high recovery rate
of higher than 90%. In addition, they opened up a new
research field of hydrogen storage based on hydrate materials.
4
The new storage medium captured hydrogen up to 4 wt%.
While gas hydrates have become a strategic material in
solving the crises relating to future energy needs and global
warming, the formation of hydrates in oil and gas industries
ILs are generally stable in the presence of air and water.
Their physical properties strongly depend either on the length
of the alkyl chain of the cation or on a specifically designed
cation and anion. Therefore, ILs can be designed and
synthesized for specific purposes. Adidharma and Xiao first
identified ILs as a novel methane hydrate inhibitor based
1
,2
has long been a severe problem. Gas hydrate formation
under the conditions typical of oil and gas production and
transport can cause pipeline blockage or explosion, resulting
in economic loss as well as ecological disasters. To deal with
these problems, research on the development of new and
effective inhibitors has been increased. Chemicals that possess
inhibition activities at comparatively small amounts have been
studied and named low-dosage hydrate inhibitors (LDHIs).
Until now, there have been two kinds of LDHIs. The first type
is a group of compounds called kinetic hydrate inhibitors
5
,6
on both thermodynamic and kinetic mechanisms.
Their
discovery of a new inhibitor system, so called ‘‘Ionic Liquids’’,
is expected to bring about progress in the area of hydrate
inhibition and help open up another phase in relevant
industries. However, they tested only imidazolium cation-
based ILs without optimizing the combination between the
cation and the anion. Whether or not there are other ILs that
can be used as inhibitors is an interesting research question,
the answer to which depends on how to design and synthesize
suitable ILs. In this context, we adopted a systematic
approach starting from IL design to synthesis, with the goal
of tailoring ILs to a single purpose: ‘‘Tuning Ionic Liquids for
Hydrate Inhibition’’.
(
KHIs), which retard gas hydrate formation. Although KHIs
a
Department of Chemical and Biological Engineering, Chungju
National University, 50 Daehak-ro, Chungju, Chungbuk 380-702,
Republic of Korea. E-mail: kks1114@cjnu.ac.kr;
Fax: +82 43 841 5220; Tel: +82 43 841 5222
Department of Computer Engineering, Chungju National University,
b
c
50 Daehak-ro, Chungju, Chungbuk 380-702, Republic of Korea.
E-mail: jwkang@cjnu.ac.kr; Fax: +82 43 841 5340;
Tel: +82 43 841 5466
Clean Fossil Energy Research Center, Korea Institute of Energy,
Currently, we are attempting to develop new and powerful
ILs as inhibitors of methane hydrates and evaluate their
inhibition activities. The design of ILs for hydrate inhibition
was based on two simple hypotheses. First, ILs must be
hydrophilic or hygroscopic. If the materials are hydrophobic,
then they will reside in a separate phase from water and will be
unable to access water molecules. Second, special functional
101 Gaejong-ro, Yuseong-gu, Daejeon 305-343, Republic of Korea.
E-mail: spkang@kier.re.kr; Fax: +82 42 860 3134;
Tel: +82 42 860 3475
w Electronic supplementary information (ESI) available: Synthetic
method of ILs and the experimental measurement of hydrate equilibria
and kinetic behavior. See DOI: 10.1039/c0cc05676f
This journal is c The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 6341–6343 6341

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4 4
Scheme 1 Synthesis of (a) [HEMP][BF ] and (b) [BMP][BF ].
groups such as oxygen or hydroxyl groups need to be introduced
in the IL structure since they would create intermolecular
hydrogen bonding with the hydroxyl groups of water molecules,
leading to disrupting the great number of hydrogen bonds
between water molecules, thus effectively preventing hydrate
Fig. 1 Induction times of methane hydrate formation.
7–11
formation.
According to the selected criteria, N-(2-hydroxyethyl)-N-
methylpyrrolidinium tetrafluoroborate ([HEMP][BF ]) and
N-butyl-N-methylpyrrolidinium tetrafluoroborate ([BMP][BF ])
The induction time for [HEMP][BF ] was found to be much
4
higher compared to those of the others. The differences in
induction time with 10 wt% of ILs were more distinguishable,
although typical polymer LDHIs are not available at high
concentrations. The induction times at 10 wt% for
4
4
were synthesized as shown in Scheme 1. The procedure was
carried out in two steps. The desired N-(2-hydroxyethyl)-N-
methylpyrrolidinium and N-butyl-N-methylpyrrolidinium
cations were formed, respectively, and they were then converted
[
EMIM][BF
7.3, 342.8 and 233.5 min, respectively. In the case of
EMIM][BF ], the induction time at 10 wt% was almost the
same as the time at 1 wt%, though the values of [EMIM][BF
at 0.1 and 1 wt% were between those of [HEMP][BF ] and
BMP][BF ] at the same concentrations. It is found that the
4 4 4
], [HEMP][BF ], and [BMP][BF ] were about
9
[
4
ꢀ
into the corresponding BF4 through an anion exchange. Details
of the synthesis and purification procedures are provided in
the ESI.wz
4
]
4
[
4
The pressure and temperature were recorded with time and
consequently the induction time was decided according to the
result.w The induction time is an important indicator to
characterize the kinetics of gas hydrate crystallization. Each
sample was prepared at concentrations of 0.1, 1, and 10 wt%
in water. For comparison with new samples, the induction
times for PVP, PVCap, and N-ethyl-N-methylimidazolium
inhibition effects of PVP and PVCap at 0.1 wt% were very
little. The times for PVP, PVCap, [EMIM][BF ], [HEMP][BF ]
and [BMP][BF ] were 0.4, 11.7, 35.3, 61.5, and 23.8 min,
4
4
4
respectively. Even at the very low concentration, the inhibition
effect of [HEMP][BF
4
] was outstanding.
The common characteristics between [HEMP][BF
other ILs are summarized as follows: (1) they each have BF
4
] and the
ꢀ
4
,
tetrafluoroborate ([EMIM][BF ]) were measured at 0.1 and
4
which is a well known as a representative hydrophilic group
that helps mix ILs and water together into one phase; (2) the
short alkyl chain of cation allows ILs to access water molecules
without any steric hindrance caused by a bulky group; (3) they
have strong electrostatic charges leading to interaction
between an ion and a dipole.
1
wt%. [EMIM][BF ] was the inhibitor that showed the best
4
performance among imidazolium ILs as Adidharma and Xiao
5
,6
suggested. The induction times on average for 1 wt% PVP
and PVCap were 1.8 and 43 min, respectively, whereas, as
shown in Table 1 and Fig. 1, the induction times for
[
4 4 4
EMIM][BF ], [HEMP][BF ] and [BMP][BF ] at the same
On the other hand, the presence of a hydroxyl group in
concentration were 88.4, 101.5, and 58.2 min, respectively.
4
[HEMP][BF ] yields major differences in how long hydrate
formation can be delayed although the ILs without a hydroxyl
group also showed reasonable induction times. The functional
group provides more chances for interrupting inter-molecular
forces between IL and water molecules, thus the selected
inhibitor can effectively delay hydrate formation. The introduction
of the functional group into the IL structure is the key point in
this research.
Table 1 Induction time of methane hydrate formation
Induction time/min
Inhibitor
PVP
0.1 wt%
0.5 wt%
1 wt%
10 wt%
—
0.4
1.6
(0.4–2.3)
19.9
(6–22)
—
1.8
(0.8–2.4)
43
(30–49)
88.4
(50–123)
101.5
(75–146)
58.2
(53–70)
(
0–1.1)
11.7
4–20)
35.3
24–56)
61.5
26–74)
23.8
15–46)
PVCap
—
(
It is also of interest to test whether or not ILs can function
as thermodynamic inhibitors (THIs). The concentration of ILs
used was 10 wt%. Thermodynamic inhibitors are usually used
at a concentration of 10 wt% or more while kinetic inhibitors
are usually used at a concentration of less than 2 wt%. All of
the IL samples tested in this work definitely showed inhibition
[EMIM][BF
[HEMP][BF
[BMP][BF
4
]
97.3
(65–133)
342.8
(262–413)
233.5
(180–303)
(
4
]
—
—
(
4
]
(
1
2
Numbers in parentheses: range of induction time. wt%: weight
percentage of inhibitors. T = 274.15 K, DT(sub-cooling) = 15 K,
P = 70 bar.
effects on methane hydrate formation.
Pyrrolidinium-
based ILs reduced the hydrate-aqueous liquid-vapor (HLV)
equilibrium dissociation temperature by 1.3–1.6 K while
6
342 Chem. Commun., 2011, 47, 6341–6343
This journal is c The Royal Society of Chemistry 2011

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(
No. 2010201030001A). K.-S. Kim acknowledges support
by Basic Science Research Program through the National
Research Foundation of Korea (NRF) funded by the Ministry
of Education, Science and Technology (2010-0004788). We
thank KBSI (Korea Basic Science Institute) for assistance
with NMR.
Notes and references
z Representative synthesis: 1-butyl-1-methylpyrrolidinium bromide
(
[BMP][Br]): 1-methylpyrrolidine (0.5 mol) in 200 mL of acetonitrile
was added dropwise to 0.5 mol of 1-bromobutane in a three-necked
round-bottom flask. The mixture was refluxed under nitrogen gas for
7
2 h at 343.15 K. The molten salt was then decanted from the hot
solution in a separatory funnel, washed three times with acetone, and
dried on a rotary evaporator for 5 h at 323.15 K under low pressure.
The solid product of [BMP][Br] was dried under vacuum conditions at
Fig. 2 Hydrate-aqueous liquid-vapor (HLV) equilibria of methane
1
3
23.15 K for more than 48 h: the H-NMR(DMSO) spectrum consisted
hydrate.
of the following peaks: 0.90–0.94(t, 3H), 1.26–1.35(m, 2H),
.64–1.72(m, 2H), 2.08(s, 4H), 3.02(s, 3H), 3.36–3.40(m, 2H), and
3.47–3.54(m, 4H).
1
[
EMIM][BF ] presented a weaker inhibition effect. Therefore,
4
the pyrrolidinium-based samples shifted the hydrate
equilibrium curves to a lower temperature, as shown in
Fig. 2. [HEMP][BF ] and [BMP][BF ] showed nearly the same
4
1-Butyl-1-methylpyrrolidine tetrafluoroborate ([BMP][BF ]):
BMP][Br] (0.2 mol) in acetone was reacted with 0.2 mol of sodium
[
tetrafluoroborate in a round-bottom flask. After 24 h of stirring, the
resulting NaBr precipitate was filtered through a plug of filter paper,
and the volatiles were removed using a rotary evaporator at 323.15 K.
The product was dissolved in dichloromethane, and the organic phase
was washed twice with water to ensure complete removal of the
bromide salt. Then, the product was dried for more than 24 h under
4
4
degree of HLV shift over the entire range of experiments. This
is a favorable outcome since pyrrolidinium-based IL inhibitors
play a homogeneous role over a wide range of pressures, even
though experiments over an expanded pressure range are still
necessary. In the case of conventional THIs such as MEG and
methanol, 10 wt% aqueous solution decreased the HLV
equilibrium curve by about 2.5–5 K, which was superior to
IL-based solutions at identical concentrations. Although ILs
showed no remarkable inhibition effects on the gas hydrate
equilibrium state, ILs, in theory, can also be classified as THIs.
This research has shown that specifically designed ILs for
hydrate inhibition can tremendously improve induction time
while shifting the original equilibrium line. We synthesized
1
vacuum conditions at 323.15 K: the H-NMR(DMSO) spectrum
consisted of the following peaks: 0.67–0.71(t, 3H), 1.03–1.12(m, 2H),
1
3
.40–1.48(m, 2H), 1.84(s, 4H), 2.73(s, 3H), 3.02–3.07(m, 2H), and
.18–3.23(m, 4H). The C-NMR(DMSO) spectrum consisted of the
1
following peaks: 13.3 (CH
butyl group), 47.5 (CH
pyrrolidinium). FAB MS (m/z) = [BMP] (C N H20) requires 142,
9 1
3
of butyl group), 21.0, 24.9, 62.9 (CH
of methyl group), 19.3, 63.4 (CH
2
of
of
3
2
+
exptl 142.
1 T. Kuznetsova, A. Sapronova, B. Kvamme, K. Johannsen and
J. Haug, Macromol. Symp., 2010, 287, 168.
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2
3
[
HEMP][BF
4
] and [BMP][BF
the hydroxyl groups in [HEMP][BF
4
] for this purpose and found that
] effectively interrupted
4
I. L. Moudrakovski, C. I. Ratcliffe and J. A. Ripmeester, Nature,
the hydrogen bonding between water molecules. This study
also showed the potential application of ILs as dual function
inhibitors that not only shift the equilibrium dissociation
conditions but also slow down the rates of nucleation and
2
005, 434, 743.
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K. N. Marsh, A. Deev, A. C. T. Wu, E. Tran and A. Klamt,
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growth. [HEMP][BF ] was used as an example to show that
4
inhibition performance strongly relies on the IL structure.
Therefore, [HEMP][BF ] can be a benchmark material for
4
future research. It is also expected that more effective thermo-
dynamic or kinetic inhibitors will be developed based on
molecular design.
5
56.
1
1
1
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This work was supported by the Energy Efficiency &
Resources Program of the Korea Institute of Energy Technology
Evaluation and Planning (KETEP) grant funded by the
Korea government Ministry of Knowledge Economy
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2, 2275.
2 W. M. Deaton and E. M. Frost, Gas Hydrates and Their Relation
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This journal is c The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 6341–6343 6343