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Synthesis of triptycene-based linear polyamide membrane for molecular siev-ing of N2 from the VOC mixture
Shanyin Dai, Ruoxing Liao, Haoli Zhou, Wanqin Jin
PII: S1383-5866(20)31829-3DOI: https://doi.org/10.1016/j.seppur.2020.117355Reference: SEPPUR 117355
To appear in: Separation and Purification Technology
Received Date: 26 March 2020Revised Date: 5 July 2020Accepted Date: 6 July 2020
Please cite this article as: S. Dai, R. Liao, H. Zhou, W. Jin, Synthesis of triptycene-based linear polyamidemembrane for molecular sieving of N2 from the VOC mixture, Separation and Purification Technology (2020),doi: https://doi.org/10.1016/j.seppur.2020.117355
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© 2020 Published by Elsevier B.V.
Synthesis of triptycene-based linear polyamide membrane for molecular sieving of
N2 from the VOC mixture
Shanyin Dai, Ruoxing Liao, Haoli Zhou*, Wanqin Jin*
State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemical
Engineering, Nanjing Tech University, 30 Puzhu Road(S), Nanjing 211816, PR China
﹡Corresponding author: E-mail: [email protected]; [email protected]
Tel: +86-25-83172272, Fax: +86-25-83172272
Abstract: In this study, triptycene-based carboxylic acid (triptycene-2,3,6,7-
tetracarboxylic acid) was synthesized for polycondensation with two amino compounds
to ultimately form linear polyamides with different backbone structures.
Characterizations, including nuclear magnetic resonance (NMR) spectroscopy, X-ray
photoelectron spectroscopy (XPS), and Fourier transform infrared (FTIR) spectroscopy,
were performed to confirm the polycondensation, and the structural properties were
determined. Solubility of the synthesized polyamides was also measured. Subsequently,
these polyamides were fabricated into membranes to investigate the separation of N2
over the VOCs, and their stability and separation performance under different operating
conditions were determined. Good separation performance, such as a rejection rate
above 95%, was obtained during the studied separation range for the N2/VOC mixture,
indicating that a linear polymer can also be used for the separation of a condensable
gas/vapor mixture and only the rotation of polymer chains can be restricted to a certain
extent by the side groups.
Keywords: triptycene carboxylic acid, linear polyamides, VOC recovery, polymer
membrane
1 Introduction
Volatile organic compound (VOC) recovery is important for industries because
they not only cause air pollution but also are a human health threat if they are not
recovered or treated [1]. Therefore, many governments have issued various laws and
regulations to limit the emission of VOCs. To meet the release limitation of VOCs,
many VOCs recovery technologies, such as absorption, condensation, and membrane
technology, have emerged [2]. Among them, membrane technology is regarded as one
of the most energy-efficient treatment technologies [3], and thus, different membrane
materials have been developed. Among them, polymer materials, which are roughly
classified into two groups of rubber polymer materials and glassy polymer materials,
are regarded as relatively suitable materials because of their easy film-forming
properties, low cost, easy fabrication, and scale-up abilities.
Rubber polymers, such as a polydimethylsiloxane (PDMS) membranes with VOC
permselectivity, have been developed for vapor permeation of VOCs waste streams [4].
However, the rubber membrane has a relatively low selectivity and exhibits a high
swelling because the adsorption of VOCs in the membrane also reduces the membrane
separation performance [5]. Therefore, besides developing VOCs permselective
membranes with high performance, glassy microporous polymer membranes with N2
permselectivity may be another good alternative because microporous polymers with a
specific pore size of less than 2 nm and a high porosity provide high permeability
combined with high selectivity, which has drawn widespread attention for their
excellent separation performance [6]. Different kinds of microporous polymers,
including polymers with intrinsic microporosity (PIMs) [7], thermally rearranged (TR)
polymers [8], and covalent organic frameworks (COFs) [9], were developed and
prepared as membranes for the separation of gas pairs of importance, such as CO2/CH4,
CO2/N2, and O2/N2 that show excellent separation performance [10]. However, few
studies have been carried out to separate N2/VOC mixtures. Accordingly, our group
previously reported a triptycene-based network microporous polyamide membrane
with N2 permselectivity for the separation of N2/VOC mixtures, and the rejection rate
was up to 99.2% [11], indicating excellent separation performance. However, like most
cross-linked aromatic network polyamides, their solubility in aprotic solvents is not
very good. Although the use of triptycene can maintain inter-chain distances to improve
the solubility of polymers [12], triptycene-based network microporous polyamides are
still not completely soluble in aprotic solvents, which may affect the success rate of
membrane fabrication with a high separation performance. Additionally, solubility,
especially that in certain solvents, is highly desirable for the preparation of next-
generation microporous membranes [13, 14]. In addition, it has been reported that linear
aromatic polyamides are more solution-processable compared to cross-linked network
polyamides [15]. Therefore, to explore whether membranes with a linear structure and
good solubility, especially in certain solvents, have the capability of molecular sieving
of N2 over VOCs, we designed a new triptycene-based aromatic linear polyamide
membrane. A rigid 3D-contorted triptycene was induced into the polymer chain to
generate intrinsic microporosity [16]. Moreover, triptycene can hinder the close
packing of polymer chains, which allows the solvent molecules to easily enter the
polymer chains, increasing the polymer solubility [17].
In this study, a triptycene derivative (triptycene-2,3,6,7-tetracarboxylic acid) was
first synthesized and characterized. Subsequently, it was reacted with diamines to
prepare two kinds of triptycene-based polyamides. A high-temperature solution
polymerization method was carried out to obtain a high molecular weight polymer.
Nuclear magnetic resonance (NMR) spectroscopy, X-ray photoelectron spectroscopy
(XPS), and Fourier transform infrared (FTIR) spectroscopy were utilized to confirm the
polymerization reaction. Then, their physico-chemical properties, such as the solubility
and Brunauer–Emmett–Teller (BET) surface area, were characterized. The polyamides
were then fabricated into the membranes for investigation of their separation
performances for a N2/cyclohexane mixture. The stability of the membranes was also
studied, and good stability was achieved for one of the polyamides. Finally, the
separation performances under different operating conditions were studied using the
stable polyamide membrane.
2 Experimental
2.1 Materials
N,N-dimethylformamide (DMF, purity: 99.8%), dimethyl sulfoxide (DMSO,
purity: 99.5%), and N-methyl pyrrolidone (NMP, purity: 99.5%) were purchased from
Aladdin Industrial Corporation. Anhydrous lithium chloride (purity: 99%), anhydrous
calcium chloride (purity: 99.9%), triphenyl phosphite (TPP, purity: 98%),
pyridine(purity: 99%), 2,2'-bis(trifluoromethyl)benzidine (TFDB, purity: 98%), and
tetramethylbenzidine (TMB, purity: 98%) were purchased from Shanghai Macklin
Biochemical Technology Corporation, China. All reagents were used as received.
Triptycene-2,3,6,7-tetracarboxylic acid was prepared according to a reported procedure
[18]. PA substrate with a pore size of 0.1 μm was purchased from Shanghai Xinya
Purification Equipment Co., Ltd, China.
2.2. Characterization
The chemical structures were analyzed using a Thermo Nicolet Fourier transform
infrared 8700 spectrometer in the range of 400–4000 cm−1 and using 1H (500 MHz)
nuclear magnetic resonance (NMR) spectroscopy (Bruker 400 AVANCE III HD
spectrometer). DMSO-d6 was chosen as the solvent. Carbon, oxygen, N2, and fluorine
contents in the compounds were determined by XPS (Thermo ESCALAB 250, USA)
with monochromatized Al K alpha radiation. Gel permeation chromatography (GPC)
was performed using a Waters GPC instrument (LC20, Shimadzu). NMP was used as
the eluent, and polystyrene was used as the standard sample. An RI detector was used
to record the signal. The thermal stability was investigated using a thermal analyzer
(Netzsch Corporation, Germany) under N2 atmosphere with a flow rate of 25 mL/min
and a heating rate of 10 °C /min from 30–800 °C. The polymer powder was pretreated
in a vacuum oven at 120 °C for 8 h. The crystal structures were investigated using XRD
(Bruker, D8 Advance) with Cu K-α radiation in the range of 5–60° at increments of
0.02° at 20 oC. The morphologies and structures were observed using field-emission
scanning electron microscopy (FE-SEM, Hitachi- 4800, Japan). The solubility in
different solvents was tested under the condition in which 0.2 g of polyamide was added
to 10 mL of solvent and stirred for 12 h at room temperature. Then, the change in the
color of the solution was visibly measured.
2.3. Polymer synthesis
Two triptycene-based polyamides (named Trip-TFDB, and Trip-TMB,
respectively) were prepared by the reaction of triptycene-2,3,6,7-tetracarboxylic acid
with two aromatic diamines in NMP in the presence of TPP and pyridine at 105 °C, as
illustrated in Scheme 1. Triptycene-2,3,6,7-tetracarboxylic acid (0.43 g, 1 mmol) and
an equimolar amine were dissolved in NMP (20 mL) in a 100-mL round-bottom flask
equipped with a stirrer. Anhydrous lithium chloride (0.45 g, 0.01 mol), anhydrous
calcium chloride (1.2 g, 0.01 mol), pyridine (2.5 mL), and TPP (2 mL) were added to
the flask. The reaction was carried out in a N2 atmosphere. The mixture was stirred at
105 °C for 12 h. Then, the polymer solution was poured into methanol (300 mL) during
constant stirring for precipitation. The polymer was filtered and washed with water and
methanol several times and dried at 150 °C in a vacuum oven for 24 h to produce a
yield of 85%. All of the polyamides were prepared using this general procedure.
HOOCHOOC
COOH
COOH
H2N NH2
NH
CF3
F3C
H3C
H3C
CH3
CH3
Trip-TFDB
Trip-TMB
HOOCCOOH
O nH2N NH2
CF3
F3C
HOOC
COOH
ONH
H3C
H3C
CH3
CH3
n
NMP solution.TPP,pridine
105oC,12h,LiCl
Scheme 1. Synthesis procedure for triptycene-based polyamides
2.4 Fabrication of composite membranes
The polymer powders were dissolved in DMF to form a membrane solution (3
w/v%), which was stirred for 8 h at room temperature. The solution was vacuum-
degassed for 20 min and coated on 0.1-μm PA substrate membranes using a coating
knife to obtain composite membranes. Then, the membranes were dried at 80 °C in a
vacuum oven for three days. The thickness of the selective layer of the composite
membrane was between 1.3 and 2 μm, as observed by scanning electron microscopy
(SEM).
2.5 Membrane performance tests
The N2/VOC mixed gas permeation properties were evaluated at different
temperatures, pressures, and feed concentrations to comprehensively study the gas
transport behavior of the membranes. The detailed method was reported previously [11],
and the effective membrane area was 12 cm2. The original flux (Ji, L·m−2·h−1) was
measured using a soap bubble flowmeter and calculated using Eq (1). The gas
permeability (Pi, Barrer) was calculated using Eq (2). The gas flux was normalized by
pressure to give the permeance (Pi/l, GPU), which was calculated using Eq (4).
(1)𝐽𝑖 = 36000 ×𝑉𝑖
𝐴•𝑡
The gas permeability(Pi) of component i was calculated using the following Eq:
(2)𝑃𝑖 = 1010 ×𝑉𝑖
𝐴•𝑡•∆𝑝 × 𝑙
The gas permeance(Pi/l) of component i was calculated using the following Eq:
(3)𝑃𝑖/𝑙 = 106 ×𝑉𝑖
𝐴•𝑡•∆𝑃
Where, is the standard volume (cm3 STP), A refers to the effective membrane area 𝑉𝑖
(cm2), t is the time(s), represents the transmembrane pressure (cmHg), and l is the ∆𝑃
thickness of the selective layer of the composite membrane (cm).
The rejection rate of VOCs was calculated using the following equation:
(4)R = (1 ―𝐶𝑝
𝐶𝑓) × 100%
Where and refer to the permeate VOC concentration and feed VOC 𝐶𝑃 𝐶𝑓
concentration, respectively.
2.6 Calculation of the fractional free volume (FFV)
The FFV of polyamides were obtained according to a previously reported method
[19], which can be calculated using the following equations:
(5) (𝐹𝐹𝑉)𝑛 = (𝑉 ― (𝑉0)𝑛)/𝑉
(6)(𝑉0)𝑛 = ∑𝑘𝑘 = 1𝛾𝑛𝑘(𝑉𝑤)𝑘
(7)𝑉 = ∑𝑘
𝑘 = 1𝛽𝑘(𝑉𝑤)𝑘
Where and refer to the gas and group, respectively. is the total volume 𝑛 𝑘 𝑉
of the polymer, and represents the modified occupied volume by the polymer (𝑉0)𝑛
chain of gas n. is the van der volume. refers to a set of empirical factors 𝑉𝑤 𝛾𝑛𝑘
determined based on gas and group .𝑛 𝑘
3. Results and discussion
3.1 Characterizations of triptycene-2,3,6,7-tetracarboxylic acid
Fig. 1 FTIR spectrum of monomer triptycene-2,3,6,7-tetracarboxylic acid
In this study, a monomer with carboxylic acid groups was required for the
following synthesis of the corresponding polymers. Therefore, the synthesized target
monomer, triptycene-2,3,6,7-tetracarboxylic acid, was first investigated using FTIR
characterization to check whether it is a carboxylic acid. The result is shown in Fig. 1,
which shows that the typical absorption peaks of the carboxylic acid group, such as the
C=O stretch bond and the C-O stretch bond, are distinctly visible at 1713 cm−1 and
1320–1210 cm−1. Simultaneously, a very broad band from 3400 to 2400 cm–1 can also
be observed, which is the O-H stretching vibration. A very broad O-H stretching
vibration along with a C=O peak and C-O peak can be observed, which indicates the
synthesized compound has a carboxylic acid group [20].
Fig. 2 1H NMR (a) and 13C NMR (b) for triptycene-2,3,6,7-tetracarboxylic acid
To further confirm the correct monomer, the monomer was characterized by NMR,
as shown in Fig. 2. It can be seen from Fig. 2(a), the number of hydrogens observed in
the 1H NMR spectrum agrees with the expected number in the monomer. The four
hydrogens at approximately 13.82 ppm represent four carboxylic acid groups.
Furthermore, 12C NMR also confirmed the position of the carbon atom in the monomer.
Therefore, NMR combined with FTIR confirmed that the target monomer was
synthesized.
3.1 Synthesis and characterization of the polymers
Scheme 1 illustrates the reaction of triptycene-2,3,6,7-tetracarboxylic acid with
different monomers to produce different polyamides, which were first characterized
using FTIR, as shown in Fig. 3.
Fig. 3. FTIR spectra of the synthesized polyamides.
The FTIR spectra of all polymers show a C=O stretch for amides at 1662 cm−1 and
N-H stretch for secondary amides at approximately 3300 cm−1 [20-22], indicating the
formation of amide linkages. This result was also further confirmed by the 1H NMR, as
shown in Fig. 4.
Fig. 4 1H NMR of Trip-TFDB (a) and Trip-TMB (b)
Fig. 4 shows that the 1H NMR spectra of all the different polymers possess the
amide hydrogen at approximately 10.4 ppm. The resonance peaks at 7–8 ppm and 6.3
ppm are ascribed to the aromatic hydrogen and the methylidyne bridge hydrogen in
triptycene [11]. The other aromatic hydrogen is shown at approximately 8.1 ppm. All
the results confirm the occurrence of the reaction between triptycene-2,3,6,7-
tetracarboxylic acid and the monomers. Additionally, the molecular weights of the Trip-
TFDB and Trip-TMB polymers, as shown in Table 1, are higher than 80,000, indicating
a high enough molecular weight for membrane preparation. However, upon comparing
Fig. 3 and Fig. 4, the 1H NMR spectra do not show any carboxylic hydrogen, whereas,
in the FTIR spectra, the absorption peaks at 1779 cm–1 and 1723 cm–1 (C=O asymmetric
and symmetric stretching) were assigned to the presence of a carboxyl acid group [23].
This was also verified by elemental analysis, as shown in Fig. 4, in which the core level
O1s and C1s spectra (Fig. 5) confirm an amide bond (at 531.4 eV (N-C=O) and 288 eV
(N-C=O)). Additionally, the carboxyl acid group (at 532.6 eV (O-C=O) and 289 eV (O-
C=O)) was also confirmed, which may be from the unreacted carboxyl acid groups in
triptycene-2,3,6,7-tetracarboxylic acid [24]. Therefore, these polymers theoretically
should have unreacted carboxyl acid groups because the molar ratio between two
monomers is 1:1, indicating that at least Trip-TFDB and Trip-TMB have unreacted
carboxyl acid groups. However, terminal groups on polymers should have unreacted
carboxyl acid groups and amino groups. Therefore, the hydrogen carboxylate of the
polyamides may be too reactive to be detected in NMR.
Table 1. Relative molecular weight of the synthesized Trip-polyamides
Polyamide Trip-TFDB Trip-TMB
Mw 577263 133949
Mn 330837 87306
Mw/Mn 1.74 1.53
Fig. 5 (a) X-ray photoelectron O1s spectra of Trip-TFDB; (b) X-ray photoelectron
C1s spectra of Trip-TFDB; (c) X-ray photoelectron O1s spectra of Trip-TMB; (d) X-
ray photoelectron C1s spectra of Trip-TMB
3.2 Physical properties of the polyamides
In microporous polymers, solution processability is highly desirable for easy
preparation of microporous membranes [13, 14, 25, 26]. Therefore, the solubility
performance was determined, as shown in Table 2, which shows that Trip-TMB and
Trip-TFDB polyamides are both soluble in aprotic organic solvents, such as NMP, at
room temperature. This is because the incorporation of side groups, such as methyl
groups or fluorinated groups, into the polyamide backbone helps to improve the
polymer solubility [27, 28]. Furthermore, insoluble properties of some common
solvents, such as chloroform, are also expected, which suggests that fabricated
membranes would have a good solvent tolerance and anti-swelling properties [29] and
a high rejection rate can thus be expected during gas/vapor separation.
Table 2. Solubility performance of polyamides
Solubility [a]Polyamides DMF NMP DMSO Chloroform THF Acetone Cyclohexane
Trip-TFDB + + +- - +- - -
Trip-TMB + + +- - +- - -
[a]: - Insoluble at room temperature; +- Partially soluble at room temperature; + Soluble at
room temperature.
Fig. 6 XRD curves (a) and CO2 sorption at 273 K (b) for the two polyamides
Fig. 6(a) shows the XRD patterns of the polyamides, and broad spectra are shown
for both polyamides, indicating that they possess amorphous structures. Simultaneously,
their d-space shows the following trend: Trip-TFDB (0.528 nm) < Trip-TMB (0.591
nm). This may be ascribed to effects by the side groups. It has been reported that two
ortho methyl substituents adjacent to the linkage site are expected to strongly restrict
the linkage bond rotation, resulting in a higher intrachain rigidity and poorer chain
packing [30, 31]. This leads to a higher d-space because peaks at 2θ≈16° are reportedly
regarded as the average chain–chain distance, whereas the shoulder peaks at 23° may
be due to the inter-chain distance of the polyamide chains because of hydrogen bonding
of carboxylic acid groups [32, 33]. Therefore, Trip-TMB has a higher d-space than that
of a polyamide with one fluoromethyl group. Additionally, Fig. 6(b) shows that Trip-
TMB has a higher surface area. This is possible because dimethyl groups have a smaller
size and thus only partially fill the inter-chain void [34], leading to more available space
in Trip-TMB than in Trip-TFDB; thus, a higher surface area can be expected for Trip-
TMB. This phenomenon was further verified by the FFV shown in Fig. 7, which shows
that the FFV of Trip-TMB is higher than that of Trip-TFDB. Therefore, the higher FFV
and d-space indicate more space will be available for N2 transportation through the
membrane, and a higher N2 permeability by the Trip-TMB membrane can thus be
expected, which is described in the following section.
Fig. 7 FFV of the Trip-TMB and Trip-TFDB polyamides
Fig. 8 SEM images of Trip-TFDB (a: surface image; c: sectional image) and
Trip-TMB (b: surface image; d: sectional image)
Fig. 8 shows SEM images of the polyamide membranes. A visible defect-free
surface was obtained, and the thickness of the selective layer was from 1.3 μm to 3.4
μm, suggesting good membrane-forming properties owing to its amorphous structure
and the good solubility in solvents.
Fig. 9 TGA curves for the three polyamides
Thermal stability is also an important factor that affects the operational conditions.
Therefore, TGA was used for this measurement, and the results are shown in Fig. 9.
The initial weight loss began at approximately 200 °C owing to the evaporation of NMP
and decomposition of carboxylic acid groups [33]. When the temperature exceeded
approximately 500 °C, the backbone began to decompose. This result indicates that the
thermal stability of this membrane is good for the separation of N2/VOC mixtures at
ambient temperature. To increase the usage temperature, a membrane with a higher
initial decomposition temperature is being synthesized by our group.
3.3 N2/VOC separation performance
Fig. 10 Permeability (a) and rejection rate (b) of the three polyamide membranes for
the separation of a N2/cyclohexane mixture (Feed pressures: 8 kPa and 36 kPa for
Trip-TMB and Trip-TFDB, respectively; cyclohexane concentration: 30000 + 1500
ppm; temperature: 25 °C)
According to the above characterizations, polyamides synthesized by using
different monomers show different structural properties. To correlate the structural
properties with the separation performance and to obtain information for future
investigations, these polyamides were fabricated into membranes, whose separation
performance were tested for the separation of N2/VOC mixtures, as shown in Fig. 10.
It is shown in Fig. 10(a) that the Trip-TMB membrane has a higher permeability than
Trip-TFDB. This is because the diffusion rate of separated species through a membrane
can be strongly influenced by its microporosity [35], which can be expressed by the
specific surface area, as shown in Fig. 6(b). The reason for using CO2 as the molecular
probe is that CO2 does not suffer from kinetic effects and is more suitable for analyzing
microporous materials [26, 36]. As shown in Fig. 6(b), the Trip-TMB polymer has a
higher surface area than that of Trip-TFDB, which is the same with the permeability
trend for the membranes, indicating that a high surface area will provide more space or
channels for the transportation of separated species through membranes, leading to a
higher permeability. Fig. 10(b) shows that both membranes showed rejection rates
above 97%, and the Trip-TMB membrane had lower rejection rate because of its higher
d-space owing to the function of the side groups, as shown in Fig. 6(a).
Fig. 11 Long-term stabilities of the Trip-TFDB (a, feed pressure: 36 kPa; cyclohexane
concentration: 30000 + 1500 ppm; temperature: 25 °C) and Trip-TMB membranes (b,
feed pressure: 14 kPa; cyclohexane concentration: 30000 + 1500 ppm; temperature:
25 °C) for the separation of a N2/cyclohexane mixture
Fig. 11 shows the long-term stability of the Trip-TFDB and Trip-TMB membranes
for the separation of a N2/cyclohexane mixture, and stability is one of the most
important factors that affect membrane industrial applications [25]. in Fig. 11 that the
stability of the Trip-TFDB membrane was not as good as that of Trip-TMB, and the N2
permeability of the Trip-TFDB membrane gradually increased with operating time,
whereas the rejection rate decreased accordingly. This may be because the bulky
fluorinated moieties in the Trip-TFDB membrane have a higher affinity to solvents [37,
38] and endow less rotational limitation on the backbone chain than that of Trip-TMB;
thus, VOCs may have a stronger solvating effect on the Trip-TFDB membrane,
disrupting interchain interactions and finally causing plasticization and unpredictable
loss of the rejection rate of Trip-TFDB [25]. Additionally, the Trip-TMB membrane
maintains a constant performance because the mobility of the backbone chain of the
Trip-TMB polymer is poor owing to the limited rotation of the amide bond by the
tetramethyl moieties [39]. Consequently, the Trip-TMB membrane demonstrated good
stability over 30 days. Therefore, the following separation performances of the
membrane under different conditions were investigated by using the Trip-TMB
polyamide membrane.
Fig. 12 shows the separation performance of the Trip-TMB membrane for the
separation of N2 over different VOC binary mixtures. Similar N2permeabilities were
observed because N2 accounted for most of the mixture. Simultaneously, the
permeabilities of VOCs are low compared with that of N2, and the effect of permeation
of VOCs on that of N2 can be ignored. Fig. 12(a) also shows that the lowest permeability
was for cyclohexane owing to its relative larger molecular dimensions; additionally,
hexane and heptane have similar molecular dimensions [11]; thus, the rejection rate of
cyclohexane was the highest, and the rejection rates of hexane and heptane were similar,
as shown in Fig. 12(b). Additionally, their rejection rates were all higher than 95%.
This also indicated that the membrane made in house followed a molecular sieving
mechanism.
Fig. 12 Permeability (a) and rejection rate (b) of the Trip-TMB composite membrane
for the separation of a N2/VOC mixture (Feed pressure: 10.8 kPa, cyclohexane
concentration: 30000 + 1500 ppm; temperature: 25 °C)
Membrane performance can be affected not only by the membrane materials but
also by the operating conditions. Therefore, this membrane was also investigated under
different operating conditions, as shown in Fig. 13. As illustrated in Fig. 13(a), the feed
temperature had a llittle impact on the membrane performance. This may be ascribed
to the relatively rigid backbone structure endowed by the tetramethyl moieties, which
restrict the mobility of the polymer chains; additionally, the rigid 3D triptycene in the
polymer chain may also interlock with itself, limiting the movement of the backbone
as the feed temperature increases [40]. Constant membrane performance with
temperature was thus observed. Fig. 13(b) shows that the feed pressure enhanced the
cyclohexane permeability and decreased the nitrogen permeability, resulting in a
decrease in the rejection rate. The reason for this may be that much more cyclohexane
molecules were pushed into the membrane pores with the help of the feed pressure,
which occupied transport channels in the membrane to block the transport of nitrogen
through the membrane, leading to an increased cyclohexane permeability and decreased
nitrogen permeability. As a result, the rejection rate of cyclohexane decreased with
increasing feed pressure. Fig. 13(c) shows the effect of the feed cyclohexane
concentration on the membrane performance. Increasing the feed cyclohexane
concentration increased the nitrogen permeability but decreased the cyclohexane
permeability. Two possible reasons may explain this phenomenon. One is that with
increasing feed cyclohexane concentration, more cyclohexane is adsorbed in the
membrane, which swells the membrane slightly. This leads to a higher permeability of
nitrogen owing to its lower molecular kinetic diameter than that of cyclohexane. The
other reason is that although a high feed cyclohexane concentration can cause more
cyclohexane to be adsorbed onto the membrane and be transported through the
membrane, the resistance in the membrane makes the increasing flux trend less than
that of the feed cyclohexane concentration. Thus, the cyclohexane permeability still
decreases. A higher rejection rate was thus observed with increasing feed cyclohexane
concentration.
Fig. 13 Effects of different operating conditions such as temperature (a, feed
pressure:14 kPa; cyclohexane concentration: 30000+1500 ppm), feed pressure (b,
cyclohexane concentration: 30000+1500 ppm; temperature: 25oC), and feed
cyclohexane concentration (c, feed pressure:14 kPa; temperature: 25oC) on the
separation performance of the Trip-TMB composite membrane.
Conclusions
Two triptycenc-based linear polyamides were synthesized by reacting triptycene-
2,3,6,7-tetracarboxylic acid with different amides. Different characterizations were
used to measure their properties. The results showed that side groups considerably
affected the properties, such as d-space. The separation performance of these polyamide
membranes for the separation of nitrogen over cyclohexane showed a greater than 97%
rejection rate, and the lower the d-space, the higher the rejection rate. Furthermore, two
ortho methyl substituents adjacent to the linkage site stabilized the polymer backbone
much more than that on the other site, leading to formation of the Trip-TMB membrane
with constant separation performance during the studied 30 days. Then, different
nitrogen/VOC binary mixtures were studied for the Trip-TMB membrane, and the
results showed a higher rejection rate for VOCs with larger molecular dimensions,
indicating molecular sieving followed by the membrane. Finally, the separation
performance under different operating conditions was also studied, and the results
showed that feed temperature had little impact on the membrane performance owing to
the rigid backbone and interlock of triptycene. The feed pressure weakened the rejection
rate and enhanced the permeability, whereas the feed VOC concentration favored the
permeation of nitrogen instead of that of the VOC.
Acknowledgments
This work was supported by the National Key Research and Development
Program of China (No. 2017YFC0210901), Six Talent Peaks Project in Jiangsu
Province (JNHB-041), the Qing Lan Project, the Fund of State Key Laboratory of
Materials-Oriented Chemical Engineering (ZK201715), the Top-notch Academic
Programs Project of Jiangsu Higher Education Institutions (TAPP), and the Innovative
Research Team Program by the Ministry of Education of China (No.IRT13070).
Reference
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