Chiral 3D open-framework material Ni(D-cam)(H2O)2 used as GC stationary phase

Article abstract of DOI:10.1002/chir.22260

Metal-organic frameworks (MOFs) have been explored for analytical applications because of their outstanding properties such as high surface areas, flexibility and specific structure features, especially for chromatography application in recent years. In this work, a chiral MOF Ni(D-cam)(H ₂O)₂ with unusual integration of molecular chirality, absolute helicity, and 3-D intrinsic chiral net was chosen as stationary phase to prepare Ni(D-cam)(H₂O)₂-coated open tubular columns for high-resolution gas chromatographic (GC) separation. Two fused-silica open tubular columns with different inner diameters and lengths, including column A (30 m × 250 μm i.d.) and column B (2 m × 75 μm i.d.), were prepared via a dynamic coating method. The chromatographic properties of the two columns were investigated using n-dodecane as the analyte at 120 °C. The number of theoretical plates (plates/m) of the two metal-organic framework (MOF) columns was 1300 and 2750, respectively. The racemates, isomer and linear alkanes mixture were used as analytes for evaluating the separation properties of Ni(D-cam)(H₂O)₂-coated open tubular columns. The results showed that the columns offered good separations of isomer and linear alkanes mixture, especially racemates. Chirality 26:27-32, 2013. 2013 Wiley Periodicals, Inc.

Full text of DOI:10.1002/chir.22260

CHIRALITY 26:27–32 (2014)
Chiral 3D Open-Framework Material Ni(D-cam)(H2O)2 Used as GC
Stationary Phase
1
SHENGMING XIE,1 BANGJIN WANG,1 XINHUAN ZHANG,1 JUNHUI ZHANG,2 MEI ZHANG,2 AND LIMING YUAN
*
1Department of Chemistry, Yunnan Normal University, Kunming 650500, P.R. China
2Department of Chemistry, East China Normal University, Shanghai 200241, P.R. China
ABSTRACT
Metal-organic frameworks (MOFs) have been explored for analytical applications
because of their outstanding properties such as high surface areas, flexibility and specific structure
features, especially for chromatography application in recent years. In this work, a chiral MOF Ni
(D-cam)(H2O)2 with unusual integration of molecular chirality, absolute helicity, and 3-D intrinsic
chiral net was chosen as stationary phase to prepare Ni(D-cam)(H2O)2-coated open tubular
columns for high-resolution gas chromatographic (GC) separation. Two fused-silica open tubular
columns with different inner diameters and lengths, including column A (30 m × 250 μm i.d.) and
column B (2 m × 75 μm i.d.), were prepared via a dynamic coating method. The chromatographic
properties of the two columns were investigated using n-dodecane as the analyte at 120 °C. The
number of theoretical plates (plates/m) of the two metal–organic framework (MOF) columns
was 1300 and 2750, respectively. The racemates, isomer and linear alkanes mixture were used as
analytes for evaluating the separation properties of Ni(D-cam)(H2O)2-coated open tubular columns.
The results showed that the columns offered good separations of isomer and linear alkanes mixture,
especially racemates. Chirality 26:27–32, 2013. © 2013 Wiley Periodicals, Inc.
KEY WORDS: metal–organic frameworks; open tubular columns; stationary phase; gas
chromatography; separation
INTRODUCTION
MOFs have been synthesized,34–40 and used as seaparation ma-
terials and catalysis. To date, there are only a few studies on
Chiral MOFs that are used as chiral stationary phases (CSPs)
for separation of racemic compounds in liquid chromatography
(LC)41–44 and high-resolution gas chromatography (GC).45–47
MOFs with an intrinsically chiral topology such as quartz,
srs, and zeolite beta have attracted interest for decades due
to their potential applications in chiral recognition. Chiral
Nickel(II) complex containing (+)-camphor has been used
as the chiral stationary phase in GC, which exhibited excel-
lent chiral recognition ability for racemic alkyl-substituted
cyclic ethers.48 Here we report a chiral MOF Ni(D-cam)
(H2O)2 with unusual integration of molecular chirality, abso-
lute helicity, and 3-D intrinsic chiral net (Figure 1) as chiral
stationary phase for high-resolution gas chromatographic
(GC), which was synthesized by using D-(+)-camphoric acid
as ligand according to the method of Zhang et al.49 In this
work, we fabricated a Ni(D-cam)(H2O)2-coated open tubular
column for separation of organic compounds, including
racemates, aromatic isomers and linear alkanes. The chiral
column has excellent selectivity and also possesses good rec-
ognition ability toward these organic compounds, especially
chiral compounds.
Recently, metal-organic frameworks (MOFs) represent a
relatively novel class of hybrid inorganic–organic porous crys-
talline materials, which have received increasing attention be-
cause of their fascinating coordination structures and unusual
properties, such as uniform structured nanoscale cavities,
high surface area, chemical tunability, as well as excellent
chemical and thermal stability. The crystal lattice of the MOFs
is constructed by coordination bonds between metal ions or
clusters and organic building blocks (multidentate organic
ligands). Up to now, a large number of MOFs have been
synthesized and obtained diverse applications in many fields
including gas storage, catalysis, drug delivery, sensing and
separation, which have been covered in some significant re-
views.1–9 Particularly, a series of MOFs have been explored
for analytical applications because of their diverse structures
and unusual properties.10 For instance, MOFs have been used
as novel multi-functional materials for direct application to
in-field sampling,11,12 solid-phase extraction (SPE),13–17
solid-phase microextraction (SPME),18–21 high-resolution gas
chromatography (GC),19,22–26 high performance liquid chroma-
tography (HPLC)27–32 and capillary electrophoresis (CE).33
Chirality plays important roles in chemistry and biology,
and biological evolution is absolutely dependent on the occur-
rence of chiral recognition and catalysis. Considering the im-
portance of chiral recognition, tremendous research efforts
have been invested into the development of chiral recognition
materials. Therefore, the design and synthesis of chiral
MOFs have attracted great attention because of their poten-
tial applications in chiral separation and asymmetric catalysis.
The main advantages for chiral MOFs are that their frame-
work structures, chiral pore environment and functionality
can be finely controlled by the choice of metal ions or clusters
and chiral ligands. In recent years, a large number of chiral
EXPERIMENTAL
Materials and Chemicals
All chemicals were of at least analytical grade and used without further
treatment. D-(+)-camphoric acid (99%) was purchased from Adamas. Ni
*Correspondence to: Li-Ming Yuan, Department of Chemistry, Yunnan
Normal University, Kunming 650500, P.R. China. E-mail: yuan_limingpd@aliyun.
com
Received for publication 21 June 2013; Accepted 18 September 2013
DOI: 10.1002/chir.22260
Published online 8 November 2013 in Wiley Online Library
(wileyonlinelibrary.com).
© 2013 Wiley Periodicals, Inc.

28
XIE ET AL.
racemate was 1 μg. The n-alkanes were n-C10 to n-C15 mixture by 3 μg in-
jection mass. The α,β-ionone isomer was 500 ng injection mass. Ethanol
(HPLC grade) was given from TEDIA.
Instrumention
The scanning electron microscopy (SEM) micrographs were recorded
on a Philip model XL30ESEMTMP scanning electron microscope at 30.0
kV. The X-ray diffraction (XRD) patterns were recorded with a D/max-3B
diffractometer (Rigaku, Japan) using Cu Kα radiation. The
thermogravimetric analysis (TGA) experiment was performed on a ZRY-
1P simultaneous thermal analyzer (Shanghai, P. R. China) from room
temperature to 800 °C at a ramp rate of 10 °C minÀ1
.
All GC separations were performed on a Shimadzu GC-2014C (Kyoto,
Japan) system with a flame ionization detector (FID), a split injection port,
and a capillary control unit. The instrument control and data acquisition
were carried out by the N-2000 software (Zhida Information Engineering
Co., Ltd., Zhejiang University, P. R. China). Nitrogen (99.999%) was used
as the carrier gas. The inlet temperature of gas chromatograph was set to
200 °C, and the temperature of FID was set to 250 °C.
Synthesis of Ni(D-cam)(H2O)2
Ni(D-cam)(H2O)2 crystals were synthesized according to Zhang et al.41
Typically, D-camphoric acid (0.1021 g), Na2CO3(0.0503 g), and Ni(NO3)
6H2O (0.3098 g) were dissolved in 7 mL ultrapure water in a 25 mL Teflon
cup under stirring for 20 min at room temperature. The vessel was then
sealed and heated at 120 °C for 5 days. The autoclave was subsequently
allowed to cool to room temperature. The green crystals were obtained.
Samples for XRD and TGA were dried at 120 °C.
Fig. 1. (a) A view of 1-D left-handed M-helix (dashed blue line) in MOF Ni
(D-cam)(H2O)2;(b)3-D framework in MOF Ni(D-cam)(H2O)2.
Fabrication of the Ni(D-cam)(H2O)2-coated Open Tubular
Column
Untreated fused silica capillary columns with a polyimide outer coating
were manufactured by Yongnian Optical Fiber Factory (Hebei province,
P. R. China). The fused silica open tubular column was treated according
to the following method before dynamic coating: the open tubular column
(NO3)6H2O (98%) was purchased from Alfa Aesar. Na2CO3 was from
Tianjin Fengchuan Chemical Reagent Science and Technology Co. Ltd.
(Tianjin, China). Racemates of citronellal, alanine, methionine, leucine,
proline, 1-phenyl-1,2-ethandiol, 2-amino-1-butanol, glutamic acid and phe-
nyl-succinic acid were obtained from Sigma, their injection mass of each
Fig. 2. (a) Simulated and Synthesized XRD patterns of Ni(D-cam)(H2O)2; (b) TGA curve of the prepared Ni(D-cam)(H2O)2; (c) SEM images of Ni(D-cam)(H2O)2
deposited on the inner wall of the open tubular column A; (d) SEM images of the cross section of Ni(D-cam)(H2O)2 coating in part of the open tubular column A.
Chirality DOI 10.1002/chir

CHIRAL MOF USED AS STATIONARY PHASE IN GC
29
was filled with 1 M NaOH, sealed at both ends, and maintained for 2 h.
Thereafter, the open tubular column was washed successively with
ultrapure water for 1 h, 0.1 M HCl for 2 h, and ultrapure water again until
the outflow reached neutrality. The column was then purged with nitro-
gen for 6 h at 120 °C.
The Ni(D-cam)(H2O)2-coated open tubular column was fabricated by a
dynamic coating method. Briefly, a 2 mL ethanol suspension of the Ni
(D-cam)(H2O)2 ( 1 mg mLÀ1 ) was introduced into the open tubular column
under gas pressure, and then pushed through the column at a rate of 50 cm
minÀ1 to leave a wet coating layer on the inner wall of the open tubular
column. A 2 m long buffer tube was attached to the end of the open tubular
column as a restrictor to avoid acceleration of the solution plug near the end
of the column. Finally, the coated open tubular column was flushed for 6 h
with nitrogen and then conditioned from 30 to 250 °C, increasing its temper-
ature at a rate of 1 °C minÀ1 and was held at 250 °C for 6 h.
1-nitropropane, and pyridine as test solutes. The respective
constants of the five selected test solutes are thought to mea-
sure various interactions between the stationary phase and
the analytes, including weak dispersion forces, polarizability,
hydrogen-bonding ability, dipolar and acidic characters of the
stationary phase. Squalane is used as a standard nonpolar sta-
tionary phase, then the McReynolds constants of the chiral
MOF column were compared to that of squalane. The
obtained McReynolds constants of five reference analytes
were 153, 254, 192, 166, 391 for benzene, 1-butanol,
2-pentanone, 1-nitropropane and pyridine on column B at
140 °C, respectively. The average of the five McReynolds con-
stants was 231, which indicates that the polarity of the Ni
(D-cam)(H2O)2 is moderate. The elution sequence of the probes
was benzene, nitropropane, 2-pentanone, 1-butanoland pyridine.
Figure 3 gives the Van Deemter plot of column B for
n-dodecane under 120 °C. The optimal linear velocity value
of this open tubular column was 11.5 cm sÀ1
RESULTS AND DISCUSSION
Characterization of the Synthesized Ni(D-cam)(H2O)2
and Ni(D-cam)(H2O)2-coated Open Tubular Columns
In this study, two Ni(D-cam)(H2O)2-coated open tubular
columns with different inner diameters and lengths, including
column A (30 m × 250 μm i.d.) and column B (2 m × 75 μm
i.d.), were prepared by a dynamic coating method using Ni
(D-cam)(H2O)2 as the stationary phase. The column efficiency
of the two Ni(D-cam)(H2O)2-coated open tubular columns
was measured by using n-dodecane as the test compound at
120 °C. The number of theoretical plates (plates/m) of column
A and column B were 1300 and 2750, respectively.
Gas Chromatographic Separation Properties of Ni(D-cam)
(H2O)2-coated Open Tubular Column (column B)
In this work, we chose linear alkanes mixture and position
isomer as analytes to investigate the separation ability and
The prepared Ni(D-cam)(H2O)2 crystals were dried at 120 °C
under reduced pressure for XRD characterization. The suc-
cessful synthesis of Ni(D-cam)(H2O)2 crystals was con-
firmed by XRD analysis (Figure 2a). Figure 2b shows that
the TGA curve of the Ni(D-cam)(H2O)2 crystals, which is sta-
ble up to 250 °C and therefore suitable for GC usage. SEM
analysis was used to investigate the coating properties of
the MOF crystals on the inner wall of the column. The
SEM images showed that the column A had an approxi-
mately 2 μm thick Ni(D-cam)(H2O)2 coating on the inner
wall (Figure 2c and d).
To evaluate the classification of novel chromatographic
stationary phases, the McReynolds constants were used to
describe the polarity of Ni(D-cam)(H2O)2-coated open
tubular column by choosing benzene, 1-butanol, 2-pentanone,
Fig. 4. GC chromatograms on the open tubular column B (a) for the sepa-
ration of alkanes using a temperature program: 50 °C for 0.5min, 40 °C/min to
180 °C, and 3 μg of injection mass of each alkane; (b) for the separation of
ionone isomer at 110 °C, and 500 ng of injection mass of α,β-ionone isomer.
TABLE 1. Separation of racemates on column B
Temperature
(°C)
Retention
factor (k1)
Separation
factor (α)
Racemates
citronellal
100
100
97
105
100
120
120
135
95
1.21
0.96
1.15
0.46
0.49
1.64
1.13
1.67
1.13
1.14
1.56
1.32
2.42
1.99
1.26
1.15
1.22
1.13
alaninea
methioninea
leucinea
prolinea
glutamic acida
2-amino-1-butanolb
1-phenyl-1,2-ethandiolb
phenyl-succinic acidc
aTrifluoroacetyl isopropyl ester derivate; btrifluoroacetyl derivate; cisopropyl
ester derivate.
Fig. 3. Van Deemter plot for n-dodecane on capillary column B(2 m long × 75 μm
i.d.)under 120 °C.
Chirality DOI 10.1002/chir

30
XIE ET AL.
Fig. 5. GC chromatograms on the MOF-coated open tubular column B(2 m long × 75 μm i.d.)for the separation of racemates. a) citronellal at 100 °C under a N2
linear velocity of 11.9 cm s-1; b) alanine derivative at 100 °C under a N2 linear velocity of 12.5 cm s-1; c)methionine derivative at 97 °C under a N2 linear velocity of 12.3
cm s-1; d) leucine derivative at 105 °C under a N2 linear velocity of 13.0 cm s-1; e) proline derivative at 100 °C under a N2 linear velocity of 13.2 cm s-1; f ) glutamic acid
derivative at 120 °C under a N2 linear velocity of 11.0 cm s-1; g)1-phenyl-1,2-ethandiol derivative at 120 °C under a N2 linear velocity of 11.5 cm s-1; h) 2-amino-1-butanol
derivative at 135 °C under a N2 linear velocity of 10.0 cm s-1; i) phenyl-succinic acid derivative at 95 °C under a N2 linear velocity of 12.1 cm s-1. 1 μg of injection mass of
each racemate. a Trifluoroacetyl isopropyl ester derivate; b trifluoroacetyl derivate; c isopropyl ester derivate.
selectivity of Ni(D-cam)(H2O)2-coated column. Figure 4a
shows that all the seven n-alkanes (n-C10 to n-C16) were base-
line separated on the column B with sharp peaks and in a
short time by temperature-programmed control. The elution
sequence of the seven n-alkanes on the column B follows the
order of boiling points because longer linear alkane has stron-
ger Van der Waal’s force with MOF.24 Except for the normal
alkanes mixture, the column can separate isomer, and the sep-
aration chromatogram of α,β- ionone is shown in Figure 4b. As
can be seen from Figure 4b, the α,β- ionone obtained baseline
separation on column B. The experimental results show that
the Ni(D-cam)(H2O)2 possesses high selectivity and excellent
separation performance for alkanes mixture and isomers.
In Ni(D-cam)(H2O)2, the enantiopure ligand D-cam coexist
with the 3-D intrinsically chiral topology net. In addition,
the MOF Ni(D-cam)(H2O)2 has three homochiral features: a
3-D intrinsically homochiral net, homohelicity, and
enantiopure molecular chirality. 3-D framework materials
with integrated homochiral features have the potential for en-
hanced chiral recognition and enantioselectivity. Therefore,
the chiral 3-D framework Ni(D-cam)(H2O)2 was mainly used
for the separation of racemates. Nine racemates were separated
on column B, including citronellal, alanine, methionine, leucine,
proline, glutamic acid, 2-amino-1-butanol, 1-phenyl-1,2-ethandiol
and phenyl-succinic acid. Table 1 showed the retention factors
(k1) for the first eluted enantiomer and separation factors (α).
Their enantiomeric resolutions on column B are exhibited in
Figure 5. As can be seen from the separation chromatograms,
the column B gave high resolution with short retention times
for the enantioseparation of nine racemates, which show
baseline separation for all enantiomer pairs except for proline,
1-phenyl-1,2-ethandiol and 2-amino-1-butanol. There are the
same elution orders for five amino acid derivates, in which L
enantiomers are eluted after D.
Ni(D-cam)(H2O)2-coated column has excellent chiral rec-
ognition ability and selectivity toward racemates due to the in-
fluence of the chiral microenviroment of Ni(D-cam)(H2O)2
with its unusual integration of molecular chirality, left-handed
M-helix, and 3-D intrinsic chiral net. Consequently, the chiral
recognition ability of the Ni(D-cam)(H2O)2-coated column for
the separation of racemates primarily arises from the surface
of the crystals in which the steric fit between the homochiral
framework and conformation of the solute molecule is the
main interactive force.50 Except for these interactions,
many other interactions between racemates and chiral MOF
Ni(D-cam)(H2O)2 have also enhanced the chiral recognition
and enantioselectivity in GC, including hydrogen bonding,
dipole-dipole, dispersion, and van der Waals forces. In addi-
tion, Ni(D-cam)(H2O)2-coated column (column B) possesses
good stability, which can withstand over 500 injections
without significant decline of selectivity and separation ability
under 250 °C.
CONCLUSIONS
In conclusion, we have demonstrated a homochiral MOF
Ni(D-cam)(H2O)2 as a promising chiral stationary phase
for GC separation. The Ni(D-cam)(H2O)2-coated open tubu-
lar column showed excellent selectivity. The results
presented that chiral MOF Ni(D-cam)(H2O)2 has excellent
recognition ability toward alkanes and isomer, especially
for racemates.
ACKNOWLEDGMENTS
This work was supported by the National Science Foundation
(No. 21275126, No. 21127013).
Chirality DOI 10.1002/chir

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