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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: zhouhl@njtech.edu.cn; wqjin@njtech.edu.cn

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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