p-Nitro-tetradecyloxy-calix[4]arene as a highly selective stationary phase for gas chromatographic separations

Article abstract of DOI:10.1039/c9nj03813b

Here, we report the first example of the utilization of p-nitro-tetradecyloxy-calix[4]arene (C4A-NO₂) as a stationary phase for capillary gas chromatographic (GC) separations. The statically coated C4A-NO₂ column exhibited a high column efficiency of 3815 plates per m and moderate polarity. The C4A-NO₂ column was investigated for its separation performance and retention behaviours by utilizing a wide variety of isomer mixtures, covering alkylated benzenes and naphthalenes, alkenes, furans, alcohols, benzaldehydes, phenols and halogenated anilines. Importantly, the C4A-NO₂ column exhibited high resolving capability for both aliphatic and aromatic isomers. Particularly, it displayed advantageous resolving capability over the commercial DB-17 column for halogenated aniline isomers. This work provides a good reference for designing calixarene derivatives as GC stationary phases, which is important for developing a family of stationary phases with specific selectivity.

Full text of DOI:10.1039/c9nj03813b

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p-Nitro-tetradecyloxy-calix[4]arene as a
highly selective stationary phase for gas
chromatographic separations†
Cite this: New J. Chem., 2019,
43, 16960
Tao Sun, *a Xiaomin Shuai,b Li Bin,c Kaixin Ren,a Xingxing Jiang,a Haipeng Chen,a
Shaoqiang Hua and Zhiqiang Cai*b
Here, we report the first example of the utilization of p-nitro-tetradecyloxy-calix[4]arene (C4A-NO2) as a
stationary phase for capillary gas chromatographic (GC) separations. The statically coated C4A-NO2
column exhibited a high column efficiency of 3815 plates per m and moderate polarity. The C4A-NO2
column was investigated for its separation performance and retention behaviours by utilizing a wide
variety of isomer mixtures, covering alkylated benzenes and naphthalenes, alkenes, furans, alcohols,
benzaldehydes, phenols and halogenated anilines. Importantly, the C4A-NO2 column exhibited high
resolving capability for both aliphatic and aromatic isomers. Particularly, it displayed advantageous
resolving capability over the commercial DB-17 column for halogenated aniline isomers. This work
provides a good reference for designing calixarene derivatives as GC stationary phases, which is
important for developing a family of stationary phases with specific selectivity.
Received 23rd July 2019,
Accepted 8th October 2019
DOI: 10.1039/c9nj03813b
rsc.li/njc
GC has been widely used in petrochemical, environmental,
food and pharmaceutical fields because of its inherent advantages
1. Introduction
Calixarenes, the third generation of host supramolecules after of simplicity, selectivity, sensitivity, speed and low cost.13–16
crown ethers and cyclodextrins, are cavity-shaped cyclic oligomers Mangia et al. first used p-tert-butylcalix[8]arene as a stationary
consisting of phenol units linked via methylene bridges.1,2 phase in packed GC and studied its retention behaviour for
Calixarenes possess unique features such as a rigid electron- alkanols, chlorinated hydrocarbons and aromatic compounds.17
rich cavity, highly selective interactions with analytes via H-bonding, Mu˘nk et al. studied the inclusion properties of p-tert-butylcalix-
dipole–dipole, p–p and hydrophobic interactions, low toxicity and [4]arene by GC.18 However, their applications of unsubstituted
high chemical and thermal stability.3,4 By taking advantage of the calixarene as GC stationary phases are restricted due to their
introduction of functional groups into the aromatic skeleton (on high melting point and poor film-forming ability. Accordingly,
upper or lower rim), a wide variety of chemically modified calixarene calixarenes are usually used together with polysiloxanes either
derivatives have been synthesized.5–7 From the perspective of in physical mixtures or chemically grafted polymers in order to
separation science, there are a number of selective factors in the improve the column efficiency and separation performance.19–21
configuration of calixarenes such as 3D cavity, conformation So, the selectivity and retention behaviour of calixarene itself as
and substituents.8,9 Due to their unique structures and physi- a GC stationary phase are not clear.
cochemical properties, calixarenes have shown good potential
as separation materials for chromatographic separations.10–12
This work presents the investigation of utilizing p-nitro-
tetradecyloxy-calix[4]arene (C4A-NO2) as a stationary phase for
GC separations (Scheme 1). We introduced nonpolar long alkyl
chains and polar nitro functional groups at the lower and
upper rims of calix[4]arene, respectively, which can increase
its column efficiency and selectivity based on improving the
solubility and film-forming ability of the stationary phase. After
it was statically coated onto a fused-silica capillary column, the
C4A-NO2 stationary phase was investigated regarding its column
efficiency and polarity. Its separation performance was evaluated
by utilizing more than a dozen isomer mixtures of diverse types.
To our knowledge, this is the first report employing C4A-NO2 as
a College of Chemistry and Chemical Engineering, Henan Key Laboratory of
Function-Oriented Porous Materials, Luoyang Normal University, Luoyang 471934,
P. R. China. E-mail: suntao2226@163.com
b Liaoning Province Engineering Research Center for Fine Chemical Engineering of
Aromatics Downstream, School of Petrochemical Engineering,
Shenyang University of Technology, Liaoyang, 111003, Liaoning, P. R. China.
E-mail: kahongzqc@163.com
c Hebei Key Laboratory of Heterocyclic Compounds, Handan University,
Handan 056005, P. R. China
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c9nj03813b the stationary phase for GC separations.
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Scheme 1 The C4A-NO2 capillary column for GC separation.
2. Experimental
2.1 Materials and equipment
phenol (5.0 g, 0.03 mol) was dissolved in 40% formaldehyde
solution (30 mL, 0.04 mol), and sodium hydroxide (0.06 g,
1.5 mmol) was added to it. The mixture was stirred at 120 1C for
2 h and diphenyl ether (50 mL) was added dropwise. After
stirring for 5 h at the same temperature, the mixture was cooled
down to room temperature and then ethyl acetate (50 mL) was
added to it. There was a large amount of precipitation. After
removal of the precipitate by filtration, a white solid was
obtained. The white solid (1.5 g, 2.3 mmol) and 60% sodium
hydrogen (1.0 g, 25 mmol) were dissolved in anhydrous DMF
(15 mL). The mixture was stirred at room temperature for 0.5 h
and n-C10H21Br (20 mL) was added. The solution was heated to
85 1C for 6 h. A white precipitate began to precipitate out.
The excess sodium hydrogen was decomposed by water and
methanol. A solid (1.5 g, 1.2 mmol) was obtained by filtering,
which was dissolved in CH2Cl2 (90 mL) and ice acetic acid
(15 mL), then the solution was cooled to 0 1C, and 95% fuming
nitric acid (5 mL) was added to the mixture. After stirring for
0.5 h, the mixture was raised to room temperature until the
reaction was completed. The reaction mixture was diluted
with water, and the organic phase was dried over anhydrous
sodium sulfate, filtered, and concentrated in vacuo to give the
All the reagents and solvents were commercially available without
any further purification. All the analytes were of analytical grade and
dissolved in dichloromethane. Untreated fused-silica capillary
tubing (0.25 mm, i.d.) was purchased from Yongnian Ruifeng
Chromatogram Apparatus Co., Ltd (Hebei, China). The commercial
capillary column DB-17 (30 m ꢀ 0.25 mm, i.d., 0.25 mm film
thickness, 50% phenyl 50% dimethyl polysiloxane) was purchased
from Agilent Technologies and used as the reference column.
An Agilent 7890A gas chromatograph equipped with a split/
splitless injector, a flame ionization detector (FID) and an auto-
sampler was used for GC separations. All the separations were
performed under the following GC conditions: nitrogen of high
purity (99.999%) as carrier gas, injection port at 300 1C, split ratio at
60 : 1, and FID at 300 1C. Oven temperature programs for the GC
separations were individually provided in their figure captions.
1H NMR spectra were recorded using a Bruker Biospin 400 MHz
spectrometer with TMS as the internal standard. All chemical shifts
were reported in ppm. IR spectra were recorded using a Bruker
Platinum ART Tensor II FT-IR spectrometer. MALDI-7090 TOF-MS
was recorded using a Bruker BIFLEX III mass spectrometer. Thermo-
gravimetric analysis (TGA) was performed using a DTG-60AH instru-
ment (Shimadzu, Japan). Scanning electron microscopy (SEM) images
were recorded using a Zeiss Sigma 500 microscope (Zeiss, Germany).
1
crude product as a yellow powder. H NMR (CDCl3, 400 MHz):
d 8.29–7.64 (m, 8H), 4.29–4.08 (m, 8H), 4.01–3.75 (m, 8H), 1.76
(s, 8H), 1.24 (dd, J = 16.8, 11.6 Hz, 56H), 0.85 (t, J = 6.8 Hz, 12H);
IR (KBr, cmꢁ1): 2920 (C–H), 2850 (C–H), 1640 (CQC), 1580
(CQC), 1520 (CQC), 1450 (CQC), 1340 (N–O), 1260 (C–O–C),
1230 (C–O–C), 1090 (C–O–C), 748 (C–H); MALDI-TOF MS: m/z
calcd for C68H108N4O12: 1164.7338 (100%); found: 2352.4543
[2M + Na]+ (100%).
2.2 Synthesis of the C4A-NO2 stationary phase
C4A-NO2 was synthesized according to references and its syn-
thetic procedure is outlined in Fig. 1.22,23 para tertiary butyl
Fig. 1 Synthesis of C4A-NO2.
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Fig. 2 (a) TGA plot for the C4A-NO2 stationary phase from 40 1C to 600 1C at 10 1C minꢁ1; (b) Golay plot of the C4A-NO2 column determined by using
n-dodecane at 120 1C; (c) the cross-section SEM images on the inner wall surface and the coating of the C4A-NO2 column.
2.3 Fabrication of the C4A-NO2 capillary column
capillary column (10 m ꢀ 0.25 mm, i.d.) was pretreated with a
saturated solution of sodium chloride in methanol for the inner
surface roughening of the capillary column. Then, the solution
was removed and the column was conditioned up to 200 1C and
held for 3 h under a nitrogen atmosphere. After the pretreat-
ment, the column was statically coated with the solution of the
C4A-NO2 stationary phase in dichloromethane (0.15%, w/v) at
room temperature. After the column was filled with the coating
solution and sealed at one end, the solvent was evaporated at a
steady speed from the other end under vacuum. Finally, the
column was conditioned from 40 1C to 180 1C at 1 1C minꢁ1 and
The C4A-NO2 capillary column was fabricated by using the static
coating method.24,25 Before coating, one bare fused-silica
Table 1 McReynolds constants of the C4A-NO2 and commercial column
Stationary phases X0 Y0 Z0
U0 S0
General polarity Average
C4A-NO2
DB-17
92 179 155 241 214 881
154 134 176 266 218 948
176
190
X0, benzene; Y0, 1-butanol; Z0, 2-pentanone; U0, 1-nitropropane; S0,
pyridine. Temperature: 120 1C.
Fig. 3 Separations of the Grob mixture on the C4A-NO2 column in comparison to the C4A-2 column. Peaks: (1) 2,3-butanediol, (2) n-decane,
(3) n-undecane, (4) 1-octanol, (5) n-nonanal, (6) 2,6-dimethylphenol, (7) 2-ethylhexanoic acid, (8) 2,6-dimethylaniline, (9) methyl decanoate, (10) methyl
undecanoate, (11) dicyclohexylamine, (12) methyl dodecanoate. Temperature program on C4A-NO2 and C4A-2 columns: 40 1C for 1 min to 160 1C at
10 1C minꢁ1, and held at 160 1C for 3 min. Flow rate at 0.6 mL minꢁ1
.
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held at 180 1C for 7 h under nitrogen. The as-prepared C4A-NO2 (TGA). As shown in Fig. 2a, C4A-NO2 remains thermally stable
column was used for the following work. By using the same up to 295 1C, suggesting its good thermal stability as a sta-
procedure, the C4A-2 column was also obtained.
tionary phase for GC separations. For the fabricated C4A-NO2
column, the Golay plot relating the heights equivalent to a
theoretical plate (HETP) with flow rates was determined by
using n-dodecane at 120 1C and is shown in Fig. 2b. The Golay
3. Results and discussion
3.1 Column efficiency and Golay plot
plot showed the minimum HETP of 0.26 mm at 0.35 mL minꢁ1
,
The C4A-NO2 stationary phase was evaluated for its inherent corresponding to a column efficiency of 3815 plates per m. The
thermal stability and polarity by thermal gravimetric analysis high column efficiency can be attributed to the good solubility
Fig. 4 Separations of isomer mixtures of (a) propylbenzene and butylbenzene, (b) trimethylbenzene, (c) carvacrol/thymol, (d) xylenol, (e) methyl-
benzaldehyde, and (f) dichlorobenzaldehyde on the C4A-NO2 column. Temperature program: 40 1C for 1 min to 160 1C at 10 1C minꢁ1 for (a), (b), (c) and (e).
Temperature program: 40 1C for 1 min to 160 1C at 10 1C minꢁ1 for (d) and (f), and held at 160 1C for 3 min. Flow rate at 0.6 mL minꢁ1
.
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of the C4A-NO2 stationary phase in the solvent for column 3.3 Separation of the Grob test mixture
fabrication, facilitating its uniform coating on the capillary
The Grob test mixture containing 12 analytes is well-recognized
for comprehensive evaluation of separation performance of a GC
column and a chromatographic system. Some of the analytes are
quite tough to resolve from their adjacent analytes. Fig. 3 shows
the GC separation of the Grob mixture on the C4A-NO2 and C4A-2
columns. As shown, generally, the C4A-NO2 column achieved
better resolution and peak shape for the analytes in the mixture
than the C4A-2 column that coeluted the pair of n-undecane/
1-octanol/n-nonanal (peaks 3/4/5) with similar boiling points (b.p.
191–196 1C). As demonstrated, the polar groups were connected
to the upper rim of the C4A-NO2 stationary phase to improve
their selectivity and resolving capacity, due to the hydrogen
bonding and dipole–dipole interaction between stationary phase
and analytes. In addition, prolonged retention for 2-ethylhexanoic
acid (peak 7) and dicyclohexylamine (peak 11) occurred on
the C4A-2 phase probably due to their strong interaction with the
few exposed silanol groups on the C4A-2 column, leading to the
reversal in elution orders of 2-ethylhexanoic acid/2,6-dimethylaniline
wall. In addition, the SEM images of the cross section and
the inner coating of the C4A-NO2 capillary column are shown in
Fig. 2c, respectively, confirming its uniform coating with the
thickness of approximately 136 nm on the capillary column.
3.2 McReynolds constants and polarity
Polarity of a stationary phase provides important chromato-
graphic information for its selectivity and retention behaviours
in GC separations. It can be evaluated by using the McReynolds
constants of five probe compounds, i.e., benzene (X0), 1-butanol
(Y0), 2-pentanone (Z0), 1-nitropropane (U0) and pyridine (S0) at
120 1C.26,27 The general polarity and average polarity were
obtained by the sum and average of these five McReynolds
constants, respectively. Table 1 lists the McReynolds constants
of the C4A-NO2 stationary phase, suggesting its moderate polar-
ity close to that of the DB-17 phase. For this reason, we chose the
DB-17 phase as the reference for the following investigation.28
Fig. 5 Separations of cis-/trans-isomers of (a) 1,3-dimethylcyclohexane, (b) 1,4-dimethylcyclohexane, (c) 2,5-dimethoxytetrahydrofuran, (d) 1,2,3-trichloropropene,
(e) citral, (f) nerol/geraniol, (g) nerolidol, and (h) decahydronaphthalene on the C4A-NO2 column. Temperature program for (a), (b), (c), (d), (e), (f) and (h): 40 1C (1 min)
to 160 1C at 10 1C minꢁ1; temperature program for (g): 40 1C (1 min) to 160 1C at 10 1C minꢁ1, and held at 160 1C for 10 min. Flow rate at 0.6 mL minꢁ1
.
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(peaks 7/8) and dicyclohexylamine/methyl dodecanoate (peak 11/12). structural and positional isomers ranging from nonpolar to
The above results evidenced that the C4A-NO2 column has better polar nature on the C4A-NO2 column. As shown, the C4A-NO2
column inertness and separation performance for the tough analytes column achieved baseline resolution for all the isomer mixtures
than the C4A-2 column. For such acids and amines, derivatization with good peak shapes (R 4 1.5), demonstrating its extra-
with appropriate reagents is often required to improve their peak ordinary high resolving ability for the critical analytes. Fig. 4a
shapes and resolution prior to their GC analysis.
and b present the separation of propylbenzene, butylbenzene
and trimethylbenzene isomers on the C4A-NO2 column with
high resolution, and the analytes eluted in order of their boiling
3.4 Separation of structural, positional and cis-/trans-isomers
High-resolution separation of isomers is of vital importance in points. The advantageous separation performance may be
chemical industry and environmental analysis. It is also chal- attributed to the cooperation of p–p and dispersion interactions
lenging to separate compounds with close boiling points and between the 3D aromatic skeleton of calixarene and nonpolar
molecular masses in separation science. On the basis of its aromatic analytes. Also, we examined the separation capability
unique structural features, we undertook investigations on of the C4A-NO2 column for polar aromatic isomers, such as
the resolving capability of the C4A-NO2 stationary phase for a phenols and benzaldehydes, which are susceptible to severe
wide range of isomers. Fig. 4a–f shows the separations of six peak tailing and hard to resolve well. For carvacrol/thymol and
Fig. 6 Separations of halogenated aniline isomers on the C4A-NO2 column in comparison to the DB-17 column. Temperature program on the
C4A-NO2 column (10 m): 40 1C for 1 min to 160 1C at 10 1C minꢁ1, and held at 160 1C for 3 min; temperature program on the DB-17 column (30 m):
120 1C for 1 min to 160 1C at 10 1C minꢁ1, and held at 160 1C for 8 min. Flow rate at 0.6 mL minꢁ1
.
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xylenols isomers in Fig. 4c and d, the C4A-NO2 column exhib- it shows high resolving ability for halogenated aniline isomers,
ited baseline separation with symmetric peaks, suggesting its showing advantages over the commercial polysiloxane phase. This
high resolving ability for polar analytes. It is noteworthy that work demonstrates the potential of the C4A-NO2 stationary
thymol (b.p., 232 1C; 1.36 D) was eluted later due to its stronger phase in GC and provides the basis for further investigation
H-bonding and dipole–dipole interactions with the stationary of more calixarene derivatives in chromatographic analysis.
phase than carvacrol (b.p., 236 1C; 1.34 D). For the methyl-
benzaldehyde and dichlorobenzaldehyde isomers in Fig. 4e and f,
the C4A-NO2 column also achieved complete separations. Among
Conflicts of interest
them, the C4A-NO2 stationary phase achieved good resolution
for the critical pair of 2-methylbenzaldehyde (b.p., 200 1C) and
There are no conflicts to declare.
3-methylbenzaldehyde (b.p., 199 1C) with only a 1 1C difference in
their boiling points. As for the elution order, the methylbenz- Acknowledgements
aldehyde isomers follow the order of their extent of H-bonding
The work was supported by the National Natural Science
and dipole–dipole interactions (2-methylbenzaldehyde, 3.72 D;
Foundation of China (No. 21705072), Colleges and Universities
3-methylbenzaldehyde, 4.04 D; 4-methylbenzaldehyde, 4.12 D),
in Henan Province Key Science and Research Project (No.
similar to the case of carvacrol/thymol described above. Based
17A150039), and the Natural Science Foundation of Liaoning
on the above findings, we further explored the resolving
Province (20180550016).
capability of the C4A-NO2 column for eight groups of cis-/trans-
isomers, including 1,3-dimethylcyclohexane, 1,4-dimethylcyclo-
hexane, 2,5-dimethoxytetrahydrofuran, 1,2,3-trichloropropene, citral,
nerol/geraniol, nerolidol, and decahydronaphthalene. As displayed
References
in Fig. 5, the C4A-NO2 column also exhibited high resolution
for geometric cis-/trans-isomers and showed good peak shapes,
suggesting its high selectivity for both aliphatic and aromatic
isomers with varying polarities. The above results evidence the
high resolving performance of the C4A-NO2 column for diverse
types of isomers, which can be ascribed to its unique structure
and multiple molecular interactions involving H-bonding,
dipole–dipole, p–p and dispersion interactions.
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column in comparison to the DB-17 column. As shown, the
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