Separation of olefin/paraffin mixtures using zwitterionic silver complexes as transport carriers

Article abstract of DOI:10.1002/chem.200600869

Zwitterionic silver nitrate salts of 1-(1-melhyl-3-imidazolio)pro-pane-3- sulfonate, 1-(1-methyl-1-pyrrolidinio)propane-3-sulfonate, and 1-(4methyl-4-morpholio)propane-3-sulfonate have been prepared and tested as carriers for facilitated olefin transport membranes in the separation of ethylene/ethane, propylene/propane, and C₄ mixtures. The interactions of olefins with silver ions hound to sulfonate groups were investigated by FTIR spectroscopy as well as the correlation between the binding affinity of olefins and facilitated transport.

Full text of DOI:10.1002/chem.200600869

FULL PAPER
DOI: 10.1002/chem.200600869
Separation of Olefin/Paraffin Mixtures Using Zwitterionic Silver Complexes
as Transport Carriers
Hoon Sik Kim,*[a] Jin Yong Bae,[b, c] Sang Joon Park,[a, b] Hyunjoo Lee,[b]
Hyun Woo Bae,[a] Sang Ook Kang,[c] Sang Deuk Lee,[b] and Dai Ki Choi*[b]
Abstract: Zwitterionic silver nitrate
salts of 1-(1-methyl-3-imidazolio)pro-
pane-3-sulfonate, 1-(1-methyl-1-pyrroli-
dinio)propane-3-sulfonate, and 1-(4-
methyl-4-morpholio)propane-3-sulfo-
nate have been prepared and tested as
carriers for facilitated olefin transport
membranes in the separation of ethyl-
ene/ethane, propylene/propane, and C4
mixtures. The interactions of olefins
with silver ions bound to sulfonate
groups were investigated by FTIR
spectroscopy as well as the correlation
between the binding affinity of olefins
and facilitated transport.
Keywords: ionic liquids · ionomer ·
membranes · olefins · zwitterions
Introduction
branes often require costly, complicated, and environmental-
ly unfriendly preparation.[15–20] Therefore, the search for new
membrane materials that overcome the disadvantages of
ion-exchange membranes still remains a challenging prob-
lem.
Recently, ionic liquids have attracted much interest as en-
vironmentally benign media in many processes, such as or-
ganic transformations, electrolytes for batteries and capaci-
tors, extraction, adsorption, nanoparticle formation, and sep-
aration processes.[21–28] Accordingly, various types of ionic
liquids have been developed for specific purposes.[29–35]
In a previous communication, we demonstrated that zwit-
terionic imidazolium salts with a covalently bound sulfonate
group can be used as an alternative to Nafion in the separa-
tion of isoprene from n-pentane mixtures.[36]
Olefin/paraffin separation by facilitated transport mem-
branes, using silver salts as carriers, is considered as a prom-
ising alternative to the conventional energy intensive distil-
lation process.[1–7] The basis for the separation is the ability
of silver ions to react reversibly with olefins forming silver–
olefin complexes. There have been many reports on the fa-
cilitated transport of olefins using supported liquid mem-
branes or dense polymer membranes containing copper or
silver ions as carriers.[8–14]
Silver ion confined facilitated membranes, such as dense
polymer membranes, however, often suffer from a lack of
long-term stability owing to the reduction of silver ions con-
fined in the membrane matrix. In this context, perfluorosul-
fonated ionomer membranes like Nafion have been inten-
sively studied because the sulfonate groups in Nafion are
able to bind silver ions, thereby preventing the reduction of
silver ions. However, perfluorosulfonated ionomer mem-
Continuing this line of study, we report herein on the syn-
thesis and characterization of a series of zwitterionic silver
complexes and their performance as carriers for facilitated
transport membranes in separating ethylene/ethane and pro-
pylene/propane mixtures.
[a] Prof. H. S. Kim, S. J. Park, H. W. Bae
Department of Chemistry and Research Institute of Basic Sciences
Kyung Hee University, Seoul 130-701 (Korea)
E-mail: khs2004@khu.ac.kr
Results and Discussion
1-(1-Methyl-3-imidazolio)propane-3-sulfonate (1a), 1-(1-
methyl-1-pyrrolidinio)propane-3-sulfonate (1b), and 1-(4-
methyl-4-morpholinio)propane-3-sulfonate (1c) were pre-
pared by the reactions of 1,3-propane sultone with 1-methyl-
imidazole, 1-methylpyrrolidine, or 4-methylmorpholine, as
shown in Scheme 1.[30] The formation of a sulfonatopropylat-
ed zwitterionic compound was confirmed unambiguously by
[b] J. Y. Bae, S. J. Park, Dr. H. Lee, Dr. S. D. Lee, Dr. D. K. Choi
Division of Environment and Process Technology
Korea Institute of Science and Technology, 39-1, Hawolgokdong
Seongbukgu, Seoul 136-791 (Korea)
E-mail: dkchoi@kist.re.kr
[c] J. Y. Bae, Prof. S. O. Kang
Department of Advanced Material Chemistry, Korea University
208 Seochang, Jochiwon, Choong-nam, 339-700 (Korea)
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crystal X-ray crystallographic
analysis of the 1:1 adducts was
unsuccessful because of the fail-
ure to obtain suitable crystals.
However, the bonding between
the silver ion and the sulfonate
group was supported by the X-
ray structural characterization
of the product obtained from 1-
(1-phenyl-3-imidazolio)pro-
pane-3-sulfonate and AgBF4,
which clearly demonstrates that
a silver ion is tightly bonded to
the sulfonate groups of two sul-
fonatopropylimidazolium
ions.[36]
Scheme 1. Synthesis of zwitterionic silver complexes 2a–c.
The prepared zwitterionic
silver complexes were tested as
olefin transport carriers for the
X-ray crystallographic analysis of 1-(4-methyl-4-morpholi-
nio)propane-3-sulfonate. The molecular structure of 1-(4-
methyl-4-morpholinio)propane-3-sulfonate (1c) in Figure 1
clearly shows that the molecule is a zwitterionic salt with a
sulfonatopropyl group bonded to a nitrogen atom.
separation of ethylene/ethane, propylene/propane, and C4
olefinic mixtures through composite membranes. The mem-
branes were prepared by casting an aqueous solution of a
zwitterionic silver complex onto porous polyester supports.
Only defect-free membranes were used for the separation
measurements. It was assumed that the membranes were
defect-free when the permeability of N2 was too low to be
determined in the permeation module.
Figure 2 and Figure 3 showthe changes in selectivity for
ethylene over ethane and the flux of permeated gases with
time at room temperature through the membranes. The se-
lectivities for ethylene over ethane ranged between 50 and
58 depending on the zwitterionic silver complex employed.
The imidazolium-based silver complex 2a showed the high-
est selectivity, whereas the pyrrolidinium-based complex 2b
exhibited the lowest selectivity. The fluxes were measured
after 5 h of separation measurements using a bubble flow
Figure 1. Molecular structure of 1c with atom labeling. Ellipsoids are
shown at the 30% probability level and the hydrogen atoms have been
À
omitted for clarity. Selected bond lengths [Š] and angles [8]: N1 C1
À
À
À
À
1.508(4), N1 C6 1.514(4), N1 C4 1.508(4), N1 C5 1.509(4), O1 C2
À
À
À
À
1.420(4), O1 C3 1.428(4), S1 O2 1.439(3), S1 O3 1.441(3), S1 O4
À
1.454(3), S1 C8 1.762(3); C1–N1–C6 106.0(2), C1-N1-C4 108.3(2), C1-
N1-C5 111.4(3), C1-N1-C6 106.0(2), C5-N1-C6 109.7(2), C4-N1-C5
110.4(2), C2-O1-C3 111.2(2), O2-S1-O3 113.0(2), O2-S1-O4 112.53(17),
O2-S1-C8 106.52(18), O3-S1-O4 112.19(19), O3-S1-C8 107.26(18), O4-S1-
C8 104.64(16).
Zwitterionic silver complexes were prepared by the reac-
tion of silver nitrate with the corresponding zwitterionic salt
containing a covalently bound sulfonate group, as shown in
Scheme 1.[36]
Elemental and spectroscopic analysis of the zwitterionic
silver complexes indicated that all the silver complexes are
1:1 adducts of AgNO3 and a zwitterionic salt. The single-
Figure 2. Change in selectivity for ethylene over ethane with time for var-
ious membranes containing zwitterionic silver complexes.
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Separation of Olefin/Paraffin Mixtures
FULL PAPER
Figure 5. Change in flux of a permeated propylene/propane mixture with
time for various membranes containing zwitterionic silver complexes.
Figure 3. Change in flux of a permeated ethylene/ethane mixture with
time for various membranes containing zwitterionic silver complexes.
meter. In contrast with the observed selectivity, the highest
and lowest fluxes were achieved with 2b and 2c, respective-
ly (see Figure 2). The high flux through the 2b-containing
membrane seems to be attributable to the greater solubility
of the ethylene/ethane mixture in 2b than in 2a and 2c.
The zwitterionic silver complexes were also tested as car-
riers for the separation of a propylene/propane mixture.
However, unlike in the separation of the ethylene/ethane
(50/50) mixture, the highest selectivity and flux were ach-
ieved with membranes containing 2c and 2a, respectively
(Figure 4 and Figure 5). The reasons for the different behav-
ior of each zwitterionic complex in the separation of ethyl-
ene/ethane and propylene/propane mixtures is not clear at
the moment, but it is likely that the difference is somewhat
influenced by the different p–p complexation ability of each
of the zwitterionic silver complexes with ethylene and pro-
pylene.
The interaction of olefins with silver ions confined to a
carbonyl-containing polymer electrolyte membrane has
been demonstrated clearly by FTIR spectroscopy,[3,4] but the
interaction of olefins with silver ions bound to sulfonate
groups has never been investigated. For this reason, we con-
ducted a FTIR spectroscopic investigation of the interaction
of sulfonate-bound silver ions with propylene using 2b. For
simplicity, spectra were recorded using 2b as a background.
Figure 6a and 6b showthe IR spectra of free propylene and
1,3-butadiene, respectively. When the IR cell containing a
film of 2b was pressurized with 30 psig of propylene, a new
peak associated with the C=C stretching frequency of the
coordinated propylene appeared at 1587.7 cmÀ1, which is
about 77 cmÀ1 lower than the C=C stretching frequency of
free propylene at 1664 cmÀ1 (see Figure 6c). The newpeak
remained even after a long period of N2 flushing (Fig-
ure 6d). Interestingly, the peak at 1587.7 cmÀ1 was replaced
À1
by a newpeak at 1548.5 cm
corresponding to coordinated
1,3-butadiene upon exposure of the propylene-coordinated
2b to 2 atm of 1,3-butadiene for 2 min followed by purging
with N2 (Figure 6e). This result is a clear indication that an
olefin coordinated to silver ions can be easily exchanged by
an incoming olefin, as shown in Scheme 2. With such a rapid
olefin exchange process, olefins can diffuse from the feed
stream across the membrane to the permeate side, thereby
resulting in selective separation of olefin from a paraffin
mixture.
Figure 7 demonstrates the binding affinity of C4 olefins to
silver ions in 2b. When the film of 2b was exposed to
20 psig of a 1-butene/isobutylene (v/v=50/50) mixture for
2 min and then purged with N2 for 3 min, the IR spectrum
(Figure 7d) showed a peak centered at around 1589.1 cmÀ1.
This peak most likely corresponds to a mixture of coordi-
nated 1-butene and isobutylene. Deconvolution of the peak
shows that the 1-butene-coordinated species is predominant.
On the other hand, exposure of the membrane to 20 psig of
Figure 4. Change in selectivity for propylene over propane with time for
various membranes containing zwitterionic silver complexes.
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2657

H. S. Kim, D. K. Choi et al.
Figure 7. FTIR spectra showing the interactions of 2b with mixtures of 1-
butene/isobutylene and 1-butene/1,3-butadiene: a) 2b+1-butene fol-
lowed by N2 purge; b) 2b+isobutylene followed by N2 purge; c) 2b+
1,3-butadiene followed by N2 purge; d) 2b+1-butene/isobutylene fol-
lowed by N2 purge; e) 2b+1-butene/1,3-butadiene followed by N2 purge.
Figure 6. FTIR spectra showing the interactions of 2b with propylene
and 1,3-butadiene: a) free propylene; b) free 1,3-butadiene; c) 2b+pro-
pylene; d) sample in (c) after N2 purge; e) sample in (d)+1,3-butadiene
followed by N2 purge.
a 1-butene/1,3-butadiene (v/v=50/50) mixture for 2 min and
subsequent treatment with N2 for 3 min led to peaks corre-
sponding to coordinated 1-butene and 1,3-butadiene, ap-
proximately in a ratio of 1:2 (Figure 7e), indicating that 1-
butene has a lower binding affinity than 1,3-butadiene.
From the IR spectroscopic results, it is likely that 1,3-buta-
diene has the highest binding affinity to silver ions of 1-
butene, isobutylene, and 1,3-butadiene.
To correlate the binding affinity of C4 olefins to silver
ions in the membrane with olefin transport, 1-butene/1,3-bu-
tadiene (v/v=50/50), 1-butene/isobutylene (v/v=50/50), and
isobutylene/1,3-butadiene (v/v=50/50) were separated using
2b at 258C and 20 psig of feed pressure. As shown in
Figure 8, the selectivities were found to be around 2.3 for
1,3-butadiene/1-butene, 1.7 for 1-butene/isobutylene, and 4.1
for 1,3-butadiene/isobutylene, respectively. The order of fa-
cilitated olefin transport is 1,3-butadiene>1-butene>isobu-
tylene, which is in good agreement with the order of olefin
binding affinity to silver ions.
Experiments to separate cis-2-butene/trans-2-butene and
1-butene/trans-2-butene were also conducted, but, as shown
in Figure 9, the separation factors were as low as 1.6 and
2.1, respectively.
Even though the selectivity of one olefinic isomer over
another is not high, it is possible that fine-tuning of the fa-
cilitated carriers could improve the selectivity.
The selectivities for ethylene over ethane and for propyl-
ene over propane increased rapidly to 40–60 in 2–5 h, de-
pending on the type of zwitterionic silver complex em-
ployed, and then remained nearly constant throughout the
experiments, demonstrating the stability of zwitterionic
silver complexes towards reduction of the silver ions. In fact,
the initial white color of the membranes was retained even
after 100 h of use. Such a high stability of zwitterionic com-
plexes can be ascribed to the
strong interaction of silver ions
with the sulfonatopropyl groups
of the zwitterionic compounds.
Note that ionomer mem-
branes containing zwitterionic
silver complexes as carriers are
much easier to prepare and
Scheme 2. Reversible olefin exchange on zwitterionic silver complex 2b.
more cost effective than con-
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Separation of Olefin/Paraffin Mixtures
FULL PAPER
to form propylene-coordinated 2b, which in turn transforms
into 1,3-butadiene-coordinated 2b upon contact with 1,3-bu-
tadiene. This behavior indicates that an olefin coordinated
to silver ions can be easily exchanged with an incoming
olefin. Such a rapid olefin exchange process could be the
driving force for olefins to diffuse from the feed stream
across the membrane to the permeate side, resulting in se-
lective separation of an olefin from a paraffin mixture.
Experimental Section
Method and materials: All manipulations were carried out under argon
unless otherwise stated. The solvents were freshly distilled before use ac-
cording to literature procedures. 1-Methylimidazole, 1-methylpyrrolidine,
4-methylmorpholine, 1,3-propane sultone, and silver nitrate were pur-
chased from Aldrich Chemical Co. and used as received. Ethylene,
ethane, propylene, propane, 1-butene, isobutylene, and 1,3-butadiene
were obtained from Sinyang Gas Co. Gas chromatographic analyses of
feed mixtures and permeates were conducted on a Gaw-Mac gas chroma-
tograph equipped with a TCD and Unibead 2S 60/80 column. 1H NMR
spectra were recorded on a Varian Unity 300 spectrometer. Elemental
analysis was carried out by using a Perkin-Elmer 2400 CHNS analyzer.
Figure 8. Separation factors for 1-butene/isobutylene, 1,3-butadiene/1-
butene, and 1,3-butadiene/isobutylene mixtures through a 2b-containing
membrane.
Synthesis of zwitterionic salts: Zwitterionic salt 1a was prepared by re-
acting 1,3-propane sultone with 1-methylimidazole, similarly to the litera-
ture procedure employed in preparing the 1-ethyl-3-sulfonatopropylimi-
dazolium compound.[30] 1-(1-Methyl-1-pyrrolidinio)propane-3-sulfonate
(1b) was prepared as follows. In a 100-mL flask, 1-methylpyrrolidine
(8.52 g, 0.1 mol) was refluxed with 1,3-propane sultone (13.44 g, 0.11 mol)
in acetone (30 mL) for 6 h. After the reaction was complete, the precipi-
tates were filtered, washed with acetone to remove any remaining 1,3-
propane sultone, and dried under reduced pressure to give an air-stable
1
3
white solid. Yield: 96%. H NMR (300 MHz, D2O, 25 8C): d=2.24 (m, J-
(H,H)=9.6 Hz, 4H; CH2), 2.28 (t, 3J
(H,H)=6.0 Hz, 2H; CH2), 3.00 (t,
3J(H,H)=7.5 Hz, 2H; CH2), 3.10 (s, 3H; CH3), 3.52 (t, 3J
(H,H)=3.9 Hz,
2H; CH2), 3.57 (m, 3J(H,H)=6.0 Hz, 2H; CH2), 3.61 ppm (t, 3J
(H,H)=
A
ACHTREUNG
A
ACHTREUNG
A
ACHTREUNG
1.2 Hz, 2H; CH2); Elemental analysis calcd (%) for C8H17NO3S: C 46.35,
H 8.27, N 6.76; found: C 46.37, H 8.30, N 6.73.
Compound 1c was prepared in a similar manner to that employed in syn-
thesizing 1b, by refluxing 4-methylmorpholine (10.12 g, 0.1 mol) with 1,3-
propane sultone (13.44 g 0.11 mol) in acetone (30 mL) for 6 h. Yield:
96%. 1H NMR (300 MHz, D2O, 25 8C): d=2.27 (m, 3J
A
3
2H; CH2), 3.01 (t, J
ACHTREUNG
Figure 9. Separation factors for cis-2-butene/trans-2-butene and 1-butene/
trans-2-butene mixtures through a 2b-containing membrane.
3J
G
ACHTREUNG
4.07 ppm (t, 3J
AHCTREUNG
for C8H17NO4S: C 43.03, H 7.67, N 6.27; found: C 42.97, H 7.70, N 6.28.
Synthesis of zwitterionic silver complex, 2a: AgNO3 (1.693 g, 11 mmol)
was treated with 1a (2.043 g, 10 mmol) in MeOH (30 mL) at room tem-
perature for 3 h. The solution was filtered to remove excess AgNO3 and
the filtrate was evaporated under vacuum to give 2a as an air-stable
white solid.
ventional fluorinated ionomer membranes. More important-
ly, membranes containing zwitterionic silver complexes are
highly effective for separating olefins from paraffin mix-
tures.
Yield: 97%. 1H NMR (300 MHz, D2O, 25 8C): d=2.32 (m, 3J
(H,H)=
3
3.8 Hz, 2H; CH2), 2.92 (t, J
C
C
Conclusion
8.61 ppm (s, 1H; CH); elemental analysis calcd (%) for C8H17NO4S: C
22.47, H 3.23, N 11.23; found: C 22.63, H 3.21, N 11.20.
Silver nitrate complexes of 1-(1-methyl-3-imidazolio)pro-
pane-3-sulfonate, 1-(1-methyl-1-pyrrolidinio)propane-3-sul-
fonate, and 1-(4-methyl-4-morpholinio)propane-3-sulfonate
were found to be highly effective carriers for facilitated
transport ionomer membranes in the separation of ethylene/
ethane, propylene/propane, and olefinic C4 mixtures.
Zwitterionic silver complexes 2b and 2c were prepared similarly to 2a.
1
3
2b: Yield: 93%. H NMR (300 MHz, D2O, 25 8C): d=2.22 (m, J(H,H)=
ACHTREUNG
3
3
ACHTREUNG
3.2 Hz, 4H; CH2), 2.24 (t, J
7.2 Hz, 2H; CH2), 3.10 (s, 3H; CH3), 3.50 (t, J
3.53 (m, 3J(H,H)=4.5 Hz, 2H; CH2), 3.55 ppm (t, 3J
(H,H)=2.7 Hz, 2H; CH2), 3.00 (t, J
3
AHCTREUNG
A
ACHTREUNG
CH2); elemental analysis calcd (%) for C8H17N2O6AgS: C 25.48, H 4.54,
N 7.43; found: C 25.45, H 4.57, N 7.42.
2c: Yield: 94%. 1H NMR (300 MHz, [D2]H2O, 25 8C): d=2.27 (m, 3J-
An FTIR study has demonstrated that silver ions bound
to the sulfonatopropyl groups of 2b interact with propylene
A
U
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H. S. Kim, D. K. Choi et al.
3H; CH3), 3.52 (t, 3J
2H; CH2), 4.07 ppm (t, 3J
A
(H,H)=3.5 Hz,
[10] R. Rabago, D. L. Bryant, C. A. Koval, R. D. Noble, Ind. Eng. Chem.
Res. 1996, 35, 1090.
AHCTREUNG
calcd (%) for C8H17N2O7AgS: C 24.44, H 4.36, N 7.13; found: C 24.45, H
4.38, N 7.10.
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FTIR experiments: The samples for the FTIR experiments were pre-
pared by coating a 25”3 mm CaF2 window with an aqueous solution of
10 wt% 2b. The coated window was then vacuum-dried for 2 h at room
temperature. The coated and uncoated CaF2 windows were placed in a
specially designed gas cell because the silver–olefin interaction is sensi-
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finity spectrophotometer.
X-ray crystallographic study: Single crystals of 1c suitable for X-ray dif-
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graphite-monochromated MoKa radiation. The ORTEP structure and
crystal data are presented in the Supporting Information.
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Performance of membranes: Composite membranes were prepared by
casting an aqueous solution of a zwitterionic silver complex (2.0 mmol in
0.5 mL H2O) onto a polyester microporous membrane support (0.1 mm,
47 mm, Whatman Industries Inc.) using a coater knife. The coated mem-
brane was dried in an oven at room temperature for 12 h under a stream
of nitrogen and then further dried in a vacuum oven at 408C for 24 h.
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To perform the separation measurements the dense membranes were
placed in a stainless steel separation module with an olefin/paraffin (50/
50) gas mixture, as described elsewhere.[39] The flowrates of the feed
mixtures were controlled by using mass flow controllers. The total feed
pressure of the mixed gas was set at 20 psig using back-pressure regula-
tors. The permeated gases were analyzed using a Gaw-Mac gas chroma-
tograph equipped with a TCD and Unibead 2S 60/80 column.
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Acknowledgements
We acknowledge financial support from Carbon Dioxide Reduction &
Sequestration Research Center (grant CF2-101-1), one of the 21st Centu-
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Received: June 21, 2006
Revised: October 6, 2006
Published online: December 15, 2006
2660
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