Chiral metal-organic frameworks for high-resolution gas chromatographic separations

Article abstract of DOI:10.1021/ja2044453

Chiral metal-organic framework coated open tubular columns are used in the high-resolution gas chromatographic separation of chiral compounds. The columns have excellent selectivity and also possess good recognition ability toward a wide range of organic

Full text of DOI:10.1021/ja2044453

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pubs.acs.org/JACS
Chiral MetalꢀOrganic Frameworks for High-Resolution Gas
Chromatographic Separations
Sheng-Ming Xie, Ze-Jun Zhang, Zhi-Yu Wang, and Li-Ming Yuan*
†
,‡
†
†
,†
†
Department of Chemistry, Yunnan Normal University, Kunming 650092, P.R. China
‡
Department of Chemistry, East China Normal University, Shanghai 200241, P.R. China
S Supporting Information
b
Table 1. Separation of Racemates on MOF Column D
ABSTRACT: Chiral metalꢀorganic framework coated open
tubular columns are used in the high-resolution gas chro-
matographic separation of chiral compounds. The columns
have excellent selectivity and also possess good recognition
ability toward a wide range of organic compounds such as
alkanes, alcohols, and isomers.
Temp
Retention
Separation
Racemates
(°C)
factor (k
1
)
factor (R)
Citronellal
Camphor
90
105
115
100
90
1.13
1.25
1.72
2.04
2.07
1.49
2.32
1.29
1.26
1.02
1.31
1.12
1.33
1.30
1.17
1.01
a
Alanine
a
Leucine
a
Valine
etalꢀorganic frameworks (MOFs), a subclass of the broad-
a
Isoleucine
115
105
120
M
er family of coordination polymers, consist of an extended
a
Proline
network of metal ions (or clusters) coordinated to multidentate
1
2-Methyl-1-
organic molecules. In recent years, MOFs have been shown to
b
2
3
butanol
hold great promise for applications in gas storage, separation,
4
5
1-Phenyl-1,2-
125
100
135
1.19
1.12
1.28
1.32
1.29
catalysis, and many other areas due to their fascinating struc-
tures. The selective adsorption and high thermal and chemical
stability of MOFs have also made these materials useful for gas
b
ethandiol
Phenyl-
6
7
c
chromatography (GC) and liquid chromatography. A large
succinic acid
8
number of chiral MOF materials have been synthesized, and the
1-Phenyl-
1.18
b
application of chiral MOFs in the separation of racemic com-
ethanol
9
pounds has been reported. In order to overcome the poor reso-
a
c
b
Trifluoroacetyl isopropyl ester derivate. Trifluoroacetyl derivate.
lution on MOF packed columns, which arises from the consider-
able diffusion resistance of bulky packing, MOF-coated capillary
columns have been reported to offer improved separation
Isopropyl ester derivate.
10
helices lose water molecules and cross-link to produce a chiral
open framework [{Cu(sala)} ]. Rehydration experiments have
efficiency in thin coatings. The first example reported of a chiral
chromatographic column in high performance liquid chromato-
graphy was when the enantiopure three-dimensional porous
Znꢀorganic framework was used as the stationary phase for
n
1
5
been unsuccessful, suggesting that this process is irreversible.
We chose the chiral open framework [{Cu(sala)} ] as the chiral
n
1
1
stationary phase because of its excellent thermal stability and
three-dimensional framework with chiral channels. Table S1
summarizes the chromatographic properties of four columns with
different inner diameters or lengths.
Figure S3 shows scanning electron microscopy (SEM) images
of the cross section of the fabricated open tubular column A and
the coating on its inner wall. The fabricated column had an
approximately 1 μm thick MOF coating on the inner wall. As can
be seen from the thermogravimetric analysis curve (Figure S2),
the column was thermally stable below 220 °C and therefore
suitable for GC usage.
the separation of a racemic mixture of chiral alkyl aryl sulfoxides.
-
There are many GC chiral stationary phases, such as cyclodextrin
1
2
13
and its derivatives, chiral ionic liquids, especially the chiral metal-
14
complex stationary phases, that have been investigated. However,
to the best of our knowledge, there has been no attempt to utilize
chiral MOFs as stationary phases in GC.
Herein we report a chiral MOF-coated open tubular column
for high-resolution GC separation of compounds. The stationary
phase possesses a three-dimensional chiral channel framework which
has the formula [{Cu(sala)} ] (H sala = N-(2-hydroxybenzyl)-
n
2
L-alanine). Some racemates, isomers, alkanes, alcohols, and
Grob’s test mixture were used as the targets for separation.
The [{[Cu(sala)] (H O)} ] was synthesized by the addition
Table S2 gives the McReynolds constants of the five reference
analytes on column A. The average of the five McReynolds
constants is sometimes used as an approximate polarity scale.
The respective constants are thought to measure various
2
2
n
of cupric acetate to H sala according to the method of Ranford
2
1
5
et al. (see the Supporting Information), and the coated fused-
1
6
interactions between the stationary phase and the analytes. The
silica open tubular column with [{Cu(sala)} ] was prepared by a
n
dynamic coating method using an ethanol suspension of the chiral
framework material. [{[Cu(sala)] (H O)} ] is a single-handed
Received: May 15, 2011
Published: July 13, 2011
2
2
n
helical coordination polymer. On heating, the single-stranded
r 2011 American Chemical Society
11892
dx.doi.org/10.1021/ja2044453
|J. Am. Chem. Soc. 2011, 133, 11892–11895

Journal of the American Chemical Society
COMMUNICATION
Figure 1. GC chromatograms on the MOF-coated open tubular column D (2 m long ꢁ 75 μm i.d.) for the separation of racemates. (a) Isoleucine
ꢀ
1
ꢀ1
derivative at 115 °C under a N
ethandiol at 125 °C under a N
succinic acid at 100 °C under a N
2
linear velocity of 12.5 cm s ; (b) citronellal at 90 °C under a N
2
linear velocity of 13.6 cm s ; (c) 1-phenyl-1,2-
ꢀ
1
ꢀ1
2
linear velocity of 13.5 cm s ; (d) leucine derivative at 100 °C under a N
2
linear velocity of 13.7 cm s ; (e) phenyl-
linear velocity of 13.8 cm s ; (g) valine
ꢀ
1
ꢀ1
ꢀ1
2
linear velocity of 14.0 cm s ; (f) 1-phenyl-ethanol at 135 °C under a N
2
ꢀ
1
derivative at 90 °C under a N linear velocity of 13.5 cm s ; (h) alanine derivative at 115 °C under a N linear velocity of 13.1 cm s ; (i) proline
2
2
ꢀ
1
ꢀ1
derivative at 105 °C under a N
derivative at 120 °C under a N
2
2
linear velocity of 13.0 cm s ; (j) camphor at 105 °C under a N linear velocity of 12.5 cm s ; (k) 2-methyl-1-butanol
ꢀ
1
2
linear velocity of 11.0 cm s .
elution sequence of the probes was benzene, nitropropane,
-pentanone, pyridine, and 1-butanol. The McReynolds con-
stants show that the polarity of the MOFs is moderate.
Figure S5b represents a chromatogram illustrating the GC
2
separation of a mixture of alcohols including n-C to n-C ; on
2
6
column C excellent resolutions were obtained for these adsorp-
In order to evaluate the column, the performance of the column
was tested by using Grob’s test mixture, which contains six com-
pounds with a variety of functional groups, imparting a wide range of
polarity properties. Unexpectedly, the complicated mixture was
separated efficiently with sharp peaks and in a short time (Figure S4).
Figure S5a shows a temperature-programmed chromatogram
of an n-C10 to n-C15 normal alkane mixture at high resolution.
tive analytes.
The MOF column also showed good selectivity for positional
isomers of organic compounds. The results of the separation are
expressed as separation factors (R) in Table S3. The separation
factor is the ratio of adjusted retention time (R = t /t ). One
typical chromatogram is shown in Figure S6. From the chroma-
togram, it is evident that the isomer was well resolved.
0
0
2
1
1
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dx.doi.org/10.1021/ja2044453 |J. Am. Chem. Soc. 2011, 133, 11892–11895

Journal of the American Chemical Society
COMMUNICATION
The most important advantage of this stationary phase in GC
is its enantioselective and resolving abilities. The following 11
racemates were separated on column D: citronellal, camphor,
alanine, leucine, valine, isoleucine, 1-phenyl-1,2-ethandiol, phenyl-
succinic acid, and 1-phenyl-ethanol; the details are provided in
Table 1. Their enantiomeric resolutions on the new stationary
phase are exhibited in Figure 1.
’ ACKNOWLEDGMENT
We thank Dr. A. X. Zhu and Mr. J. Jia for their help in the X-ray
data analysis. We also acknowledge Miss M. Zhang for her
experimental support. The work is supported by the National
Natural Science Foundation (No. 21075109) and the National
Basic Research Program (No. 2011CB612312) of China.
Those racemates include aldehyde, ketone, organic acid,
amino acid, and alcohol. The chromatograms show baseline or
at least 85% valley separation for all enantiomer pairs except for
camphor and 2-methyl-1-butanol. There are the same elution
orders for four amino acid derivates, in which L enantiomers are
eluted after D. All enantioseparations have short retention times,
and it is suitable to perform a quick analysis of the enantiomers.
Van Deemter plots of capillary column D (2 m long ꢁ 75 μmi.d.)
for n-dodecane under 120 °C (Figure S7) and for (ꢀ)- and
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yuan_limingpd@yahoo.com.cn
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