Microporous Polyamide Membranes for Molecular Sieving of Nitrogen from Volatile Organic Compounds

Article abstract of DOI:10.1002/anie.201700176

Microporous polymer membranes continue to receive tremendous attention for energy-efficient gas separation processes owing to their high separation performances. A new network microporous polyamide membrane with good molecular-sieving performance for the separation of N₂ from a volatile organic compound (VOC) mixture is described. Triple-substituted triptycene was used as the main monomer to form a fisherman's net-shaped polymer, which readily forms a composite membrane by solution casting. This membrane exhibited outstanding separation performance and good stability for the molecular-sieving separation of N₂ over VOCs such as cyclohexane. The rejection rate of the membrane reached 99.2 % with 2098 Barrer N₂ permeability at 24 °C under 4 kPa. This approach promotes development of microporous membranes for separation of condensable gases.

Full text of DOI:10.1002/anie.201700176

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201700176
German Edition: DOI: 10.1002/ange.201700176
Membrane Gas Separation
Microporous Polyamide Membranes for Molecular Sieving of Nitrogen
from Volatile Organic Compounds
Haoli Zhou,* Fei Tao, Quan Liu, Chunxin Zong, Wenchao Yang, Xingzhong Cao, Wanqin Jin,*
and Nanping Xu
Abstract: Microporous polymer membranes continue to
receive tremendous attention for energy-efficient gas separa-
tion processes owing to their high separation performances. A
new network microporous polyamide membrane with good
molecular-sieving performance for the separation of N2 from
a volatile organic compound (VOC) mixture is described.
Triple-substituted triptycene was used as the main monomer to
form a fishermanꢀs net-shaped polymer, which readily forms
a composite membrane by solution casting. This membrane
exhibited outstanding separation performance and good
stability for the molecular-sieving separation of N2 over
VOCs such as cyclohexane. The rejection rate of the membrane
reached 99.2% with 2098 Barrer N2 permeability at 248C
under 4 kPa. This approach promotes development of micro-
porous membranes for separation of condensable gases.
excellent for gas separation (for example, CO2/N2 separation)
because of its molecular-sieving ability and preferential
adsorption,[7] their solubility in a range of solvents may
result in the formation of membranes with volatile organic
compound (VOC) permselectivity when these membranes
are used for the separation of N2/VOC mixtures.[8]
The recovery of VOCs is an important task in industry
because escaping VOCs are not only harmful to people and
the environment, but also cause a lot of waste.[9] Therefore,
many different technologies have been employed to treat this
problem.[9,10] Energy-efficient membrane technology is
regarded as more attractive than other traditional meth-
ods.[7,10,11] Many membrane materials with VOC permselec-
tivity have been developed, and significant progress has also
been made in their industrial applications.[12] However, the
requirement of a high driving force to obtain intrinsic
membrane performance would increase the operating cost,
and the plasticization phenomenon would weaken the
separation performance. Furthermore, this kind of membrane
technology is referred to as an end-of-the-pipe treatment.
In contrast to current VOC permselective membranes,
herein we report a new triptycene-based three-dimensional
(3D) network microporous polyamide membrane with N2
permselectivity for the separation of N2/VOC mixtures
(Scheme 1a). This kind of membrane can ensure the source
control of VOC emissions. It was reported that polymer chain
flexibility affects pore size, which could consequently impact
on size-selectivity.[7] Moreover, it is difficult for a polymer
with a backbone of fused rings to experience large-scale
conformational change without breakage of bonds.[13] In this
study, rigid 3D-contorted triptycene was selected as the
primary monomer, which has already been recognized as an
interesting building block for new microporous polymers.[14]
Its 3D structure could enhance the microporosity of the
membranes, leading to higher permeance,[14c,15] and materials
made of the 3D structure can form 3D-interconnected pores,
which are not easily blocked.[11] However, most studies using
triptycene note only two or four latent reaction sites, resulting
in the formation of linear polymers.[15,16] Some studies report
the synthesis of network polymers derived from triptycene
monomers.[17] Few previous works have employed triptycene
with three latent reaction sites in the synthesis of network
polymers. To the best of our knowledge, there are no reports
about using this kind of polymer to fabricate membranes for
I
n nature materials tend to avoid forming vacuums because
of their enthalpy effects, indicating that the formation of
pores is energetically unfavorable.[1] Synthesis of microporous
materials is thus a complicated process, while the existence of
micropores in materials has potential technological signifi-
cance for molecular separation (for example, catalysis, gas
storage, and so on).[2] Therefore, great effort have been
devoted to developing varied approaches for obtaining such
materials, including polymers of intrinsic microporosity
(PIMs),[3] thermally rearranged (TR) polymers,[4] and cova-
lent organic frameworks (COFs).[5] However, although many
microporous organic materials have been synthesized, one
significant challenge is the handful of microporous organic
polymers to be processed into membranes for chemical
separation, because most of the materials synthesized to date
have been produced as intractable solids, which greatly limits
their functionalization or post-processing into useful forms.[6]
Furthermore, even though the produced microporous materi-
als such as PIMs-1 are solution processable, and the separa-
tion performance of the prepared PIMs-1 membrane is
[*] Assoc. Prof. H. Zhou, F. Tao, Q. Liu, C. Zong, W. Yang,
Assoc. Prof. X. Cao, Prof. W. Jin, Prof. N. Xu
State Key Laboratory of Materials-Oriented Chemical Engineering
Jiangsu National Synergistic Innovation Center for Advanced Mate-
rials, College of Chemical Engineering, Nanjing Tech University
5 Xinmofan Road, Nanjing 210009 (P.R. China)
E-mail: zhouhl@njtech.edu.cn
wqjin@njtech.edu.cn
N2/VOC separation. Therefore,
a triptycene derivative
Assoc. Prof. X. Cao
(2,6,14-triaminotriptycene) was synthesized for the first time
in this study (see the Supporting Information for synthetic
details, Fourier transform infrared spectroscopy (FTIR; Fig-
ure S3), and 1H NMR spectroscopy (Figure S4)). Subse-
Institute of High Energy Physics, Chinese Academy of Sciences
Beijing 100049 (China)
Supporting information for this article can be found under:
https://doi.org/10.1002/anie.201700176.
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The solubility of synthesized
polyamides in common solvents
would effectively affect the mem-
brane preparation through solu-
tion casting. Thus, solubility of
synthesized polyamides in differ-
ent solvents was evaluated (Sup-
porting Information, Table S3);
the insolubility of the synthesized
polyamides in alkane solvents,
aromatic solvents, and alcohol
solvents was expected, which
could greatly enhance the plasti-
cization resistance induced by the
swelling of adsorbed solvents,
resulting in increased stability.[20]
The partial solubility of the syn-
thesized polyamides in the aprotic
organic solvents, such as dime-
thylformamide (DMF) (Support-
ing Information, Figure S16 and
Table S3) could enhance post-
Scheme 1. a) Diagram of a microporous polyamide composite membrane used as a N2 permselective
membrane for N2/VOC mixture separation. b) Synthesis of triptycene-based polyamides by solution
polymerization.
processing
into
membranes,
which readily form a composite
quently, a simple amidation reaction using triaminotriptycene
and acyl chlorides as monomers was employed to synthesize
polyamides; the method is quick and can be operated at low
temperature and pressure. Solution polymerization instead of
interfacial polymerization was used to synthesize polymers.
Co-condensation reactions of the contorted rigid monomer
(2,6,14-triaminotriptycene) and linear non-contorted mono-
mers (acyl chloride compounds, including glutaryl chloride,
adipoyl chloride, suberoyl chloride, and sebacoyl chloride)
provided polyamides (termed HT-PA1, HT-PA2, HT-PA3,
and HT-PA4, respectively), as illustrated in Scheme 1b.
Scanning electron microscopy (SEM) images (Supporting
Information, Figure S5) show the particle patterns of the
synthesized polyamides. Analyses of the FTIR (Supporting
membrane or a free-standing membrane with no visible
defects (Figure 1a; Supporting Information, Figures S17 and
S18), indicating good film-forming properties. The insolubility
of the free-standing polyamide membrane in the cyclohexane
solution is also expected (Figure 1b). Additionally, a constant
rejection rate (the reason for choosing rejection rate is given
in the Supporting Information) with decreasing membrane
thickness also suggests good film-forming properties of the
synthesized polyamides (Supporting Information, Fig-
ure S19).
Four membranes composed of different chloride com-
pounds were evaluated for separation of smaller N2 molecules
over larger cyclohexane molecules. A rejection rate of more
than 98.5% (Figure 2a) indicates an outstanding molecular-
sieving performance and the degree of network cross-linking
of the synthesized polyamide may not affect size-selectivity
significantly under these separation conditions.[1] This also
suggests that a significant proportion of micropores are of
a diameter that is dimensioned between N2 (0.364 nm)[19] and
cyclohexane (0.6 nm),[21] which has already been confirmed by
XRD. To further characterize the size and shape of the
micropores in these membranes, TEM (Figure 1c) and
positron annihilation lifetime spectra (PALS) analyses (Sup-
porting Information, Table S4) were used. Figure 1c presents
a “fishing net” structure in the membrane. Additionally, the
discrete ortho-positronium (o-Ps) lifetime parameters (Sup-
porting Information, Table S4) show the existence of two
types of pores: network pores in the center of each repeat unit
in membranes with radii ranging between 0.469–0.587 nm
from t3, and aggregate pores in the interstitial sites in
membranes with radii ranging between 0.742–1.048 nm from
t4,[6,22] which are schematically represented in Figure 1e. The
network pore is a small pore originating from the amide
linkage reaction and is cross-linked as previously reported.[22]
This verified the existence of a periodic fishermanꢀs net-
1
Information, Figure S6), H NMR (Supporting Information,
Figure S7) and 13C NMR (Supporting Information, Figure S8)
spectroscopies were used to confirm their amide structures
and the existence of triptycene. X-ray photoelectron spec-
troscopy (XPS) analysis (Supporting Information, Figur-
es S9–S12 and Table S1) was employed to predict their
structures[18] and both network structures and linear struc-
tures existed in the synthesized polyamides (Supporting
Information, Table S1) where the network structures resulted
from a repeat unit, as shown in Scheme 1b (termed M4).[5,6]
Powder X-ray diffraction (PXRD) images (Supporting Infor-
mation, Figure S13) show the amorphous structure of the
synthesized polyamides, and their d-spacing ranges from 0.438
to 0.527 nm. Thermal gravimetric analysis (TGA; Supporting
Information, Figure S14) indicates good thermal stability. The
Brunauer–Emmett–Teller areas (BET) for these network
polymers under N2 adsorption (Supporting Information,
Figure S15 and Table S2) are from approximately 20–
60 m2 gÀ1, and higher BET areas under CO2 adsorption are
observed. A similar phenomenon was also reported by
Livingston et al.[19]
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free volume holes.[22] Periodic microporosities
with correct sizes and continuity across the
membrane play an important role in gas separa-
tion.[6,11] Therefore, the PALS results clearly show
that intensity of aggregate pores is low, which
indicates the main portion of network pores exist
in the membranes, resulting in a high rejection
rate owing to the molecular-sieving mechanism.
The reason for the decreasing size of network
pores (Supporting Information, Table S4) with
increasing length of the acyl chloride chain is due
to enhanced chain flexibility (Supporting Infor-
mation, Figure S20), because higher chain flexi-
bility would strengthen the stacking density of the
polymer, reducing the pore size or free volume.[24]
Figure 2b shows a rejection rate for the
separation of different binary mixtures using
HT-PA1 and HT-PA2 membranes. Rejection
rates for cyclohexane, hexane, and heptane are
all around 99%, with the exception of acetone
(although the kinetic diameter of acetone is
a little higher than that of hexane).[21] Therefore,
excepting the effect of size selectivity, shape
selectivity must also be taken into consideration
in the separation. Molecular dimensions of these
organic molecules were compared (Supporting
Information, Table S5), and the lowest molecular
dimension and excluded area for the acetone
molecule led to its lowest rejection rate. Further-
more, molecular dynamic simulation also con-
firms these results because of the different
number and connectivity of accessible pathways
or pores, and the fractional accessible volume for
the permeation of VOC molecules through mem-
branes compared to that for N2 molecules (Sup-
porting Information, Figures S21 and S22).
Figure 1. a) Digital image of the four prepared polyamide composite membranes;
SEM surface image of the HT-PA4 composite membrane. Composite membranes
were prepared by coating a 5 wt% polyamides DMF solution on a 0.22 mm Nylon-6
support and cross-linking in a vacuum oven at 808C for three days. b) Solubility of
HT-PA2 free-standing HT-PA2 membrane in cyclohexane solution at room temper-
ature for 20 days. The free-standing membrane was prepared by coating 5 wt% HT-
PA2 polyamide DMF solution on a polytetrafluoroethylene (PTFE) plate and cross-
linking in a vacuum oven at 808C for three days. c) TEM images of the prepared
HT-PA2 membrane. d) Diagram of the periodic fisherman’s net-shaped structure of
the membrane. e) Representation of the possible molecular structure of the network
and aggregate pores in the membrane.
Good membranes should not only possess
a high rejection rate but also high permeability. A comparison
of single-gas permeation at 258C with different gas molecules,
including O2 (0.346 nm), N2 (0.364 nm), and CH4 (0.38 nm)[19]
was thus conducted (Supporting Information, Table S6). The
N2 permeabilities of the prepared membranes are higher than
most previously reported examples. This is because the
kinetic diameter of a N2 molecule is about 0.364 nm, which
is less than that of a network pore. Thus, N2 permeation can
take place in both types of pores, leading to high N2
permeation. Furthermore, the 3D fishermanꢀs net-shaped
structure across the membrane (formed by rigid 3D-contorted
triptycene) also promotes the permeation of N2. Similar
results were also obtained by Pinnau et al.,[15] stating that the
incorporation of triptycene would enhance membrane per-
meability. Livingston et al.[18] also reported that membranes
composed of network polymers will express ultrafast perme-
ance in contrast to those composed of linear polymers. Thus,
although a membrane is theoretically uneconomic for remov-
ing a major component, high permeability and the require-
ment of lower energy input offsets this weakness, suggesting
great potential in industrial applications.
Figure 2. a) Rejection rate of cyclohexane for the prepared polyamide
composite membranes coated on PA-6 support. b) Rejection rate of
different organic molecules using HT-PA1 and HT-PA2 composite
membranes. Separation was conducted in a crossflow mode at about
30000Æ1500 ppm feed VOC concentration (flux of 20 LmÀ2 hÀ1) and
temperature of 248C.
shaped structure in the membrane (Figure 1d,e), which is
formed from the repeat unit (M4). This unit also makes the
synthesis of 2D-polymer membranes possible in the near
future.[1,23] Furthermore, although it has been reported that
the chemical environment caused by polar atoms and groups
can affect the o-Ps lifetime, the similar backbone structures of
all the membranes in this work indicate that the intensity
component is correlated to the relative concentration of the
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A relatively low feed pressure is needed to drive this
separation owing to the 3D fishermanꢀs net-shaped structures
in the membranes. High pressure leads to low rejection rates
(Supporting Information, Figure S23a), because high feed
pressure pushes more organic molecules through the mem-
brane. More pores are therefore occupied by these sorbed
molecules, hindering the transportation of N2 molecules
through the membrane, and resulting in lower N2 perme-
abilities (Supporting Information, Figure S23b). Further-
more, higher rejection rates for the separation of the N2/
cyclohexane mixture than for the N2/hexane mixture (Sup-
porting Information, Figure S23a) further verified the impor-
tance of the molecule-sieving mechanism. Adsorption has
only a slight effect on the separation performance. This is
because, although the mass loading of cyclohexane is higher
than that of hexane (Supporting Information, Figure S24),
which means that cyclohexane has a higher affinity than
hexane to the membrane, the rejection rate of cyclohexane is
still higher than that of hexane because of cyclohexaneꢀs
higher kinetic diameter. A higher feed concentration of
condensable gas leads to a higher rejection rate, and
a rejection rate as high as 99.2% with N2 permeability up to
about 2098 Barrer (Supporting Information, Figure S25) can
be obtained. This is because the rigid 3D-contorted tripty-
cene, which is connected by strong covalent bonds, formed
Figure 3. Stability of the prepared HT-PA2 composite membranes for
separation of a N2/cyclohexane mixture. Separation was conducted in
a crossflow mode at 248C under 4 kPa.
demonstrated. Furthermore, good stability and higher rejec-
tion rates at higher feed concentrations suggest the impor-
tance of chain structure and conformation in the mitigation of
physical aging in organic vapors. Thus, the prepared mem-
branes offer outstanding separation performance, where
a rejection rate as high as 99.2% with high permeability can
be obtained for molecular-sieving separation of mixtures,
such as an N2/cyclohexane mixture. Furthermore, the low feed
operating pressure needed to drive this separation suggests
a new operation mode applicable for the membrane-based
separation of N2/VOC mixture and an economical operating
cost and therefore good commercial feasibility. This work
provides an important contribution to the preparation and
application of microporous polymer membranes and can
bring about a revolutionary change for the membrane-based
separation of N2/VOC mixtures.
a
rigid periodic fishermanꢀs net-shaped structure that
restricted the free movement of polymer chains in the
membrane and helped to prevent polymer swelling induced
by VOC molecules. Finally, the rejection rate increases with
higher feed concentrations of condensable gas. The highest
rejection rate, using the HT-PA2 composite membrane, was
compared to that of the reverse osmosis (RO), nanofiltration
(NF), and ultrafiltration (UF) membranes (Supporting Infor-
mation, Figure S26). The result further indicates the impor-
tance of a rigid 3D-contorted triptycene, which prevents
efficient packing of the polymer network and forms inter-
connected network pores (Supporting Information, Fig-
ure S21), leading to low feed pressures and higher rejection
rates.[14c,25]
Acknowledgements
Consequently, we further studied the evaluation of the
stability of the prepared composite membranes (Figure 3),
and good stability of the membraneꢀs rejection rate during
30 days of operation was observed. This is because, as
hypothesized by Pinnau et al.,[16b] chain architecture and
conformation play an important role in physical aging.
Herein, the synthesized polyamides have a more stable
fishermanꢀs net-shaped structure owing to the introduction
of rigid 3D-contorted triptycene, which greatly restricts the
mobility of the chains.[25] Furthermore, the built-in free
volume in triptycene is already in an equilibrium state,
preventing it from evolving towards equilibrium over time as
other microporous membranes do.[16b,26] This indicates the
hard reorganization of the “intrinsic” microporosity and good
stability. It also confirms the importance of chain architecture
and conformation in the mitigation of physical aging.
This work was supported by the National Natural Science
Foundation of China (Nos. 21306080 and 21490585), the
Research Subject of Environmental Protection Department
of Jiangsu Province of China (No. 2013018), 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).
Conflict of interest
The authors declare no conflict of interest.
Keywords: gas separation · microporous membranes ·
polyamides · triptycene · volatile organic compounds
In summary, novel membranes were successfully fabri-
cated using rigid 3D-contorted triptycene as a monomer and
have been used for N2/VOC separation. Molecular-sieving
ability and a periodic fishermanꢀs net-shaped structure were
4
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Manuscript received: January 7, 2017
Revised: March 29, 2017
Final Article published: && &&, &&&&
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Communications
Membrane Gas Separation
A triple-substituted triptycene micropo-
rous polyamide membrane, featuring
a fisherman’s net-shaped structure,
imparts the microporous material with
molecular-sieving network pores and
a low driving force. The membrane dis-
plays preferential nitrogen permeation
over volatile organic compounds such as
cyclohexane.
H. Zhou,* F. Tao, Q. Liu, C. Zong,
W. Yang, X. Cao, W. Jin,*
N. Xu
&&&& — &&&&
Microporous Polyamide Membranes for
Molecular Sieving of Nitrogen from
Volatile Organic Compounds
6
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