Efficient thiazole-based polyimines as selective and reversible chemical absorbents for CO2 capture and separation: Synthesis, characterization and application

Article abstract of DOI:10.1016/j.polymer.2019.121840

A new series of polyimines (PIMs-1-9) including ortho-linked thiazole units and flexible thioether linkages were synthesized from diamine monomers (DA-1-3) and some commercially available aromatic dialdehydes (terephthalaldehyde, isophthalaldehyde and 2,5

Full text of DOI:10.1016/j.polymer.2019.121840

Journal Pre-proof
Efficient thiazole-based polyimines as selective and reversible chemical absorbents
for CO capture and separation: Synthesis, characterization and application
2
Elaheh Akbarzadeh, Abbas Shockravi, Vahid Vatanpour
PII:
S0032-3861(19)30846-8
DOI:
https://doi.org/10.1016/j.polymer.2019.121840
JPOL 121840
Reference:
To appear in: Polymer
Received Date: 26 June 2019
Revised Date: 26 August 2019
Accepted Date: 24 September 2019
Please cite this article as: Akbarzadeh E, Shockravi A, Vatanpour V, Efficient thiazole-based
polyimines as selective and reversible chemical absorbents for CO capture and separation: Synthesis,
2
characterization and application, Polymer (2019), doi: https://doi.org/10.1016/j.polymer.2019.121840.
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition
of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of
record. This version will undergo additional copyediting, typesetting and review before it is published
in its final form, but we are providing this version to give early visibility of the article. Please note that,
during the production process, errors may be discovered which could affect the content, and all legal
disclaimers that apply to the journal pertain.
©
2019 Published by Elsevier Ltd.

Graphical abstract:

Efficient Thiazole-Based Polyimines as Selective and Reversible Chemical
Absorbents for CO Capture and Separation: Synthesis, Characterization
2
and Application
a,
a,
a
Elaheh Akbarzadeh, * Abbas Shockravi * and Vahid Vatanpour
a
Faculty of Chemistry, Kharazmi University, Mofatteh Avenue 49, 15719-14911 Tehran,
Iran
*
Corresponding authors e-mail: ela_akbarzadeh@khu.ac.ir;
abbas_shockravi@yahoo.co.uk; shockravi@khu.ac.ir
1

Abstract
A new series of polyimines (PIMs-1-9) including ortho-linked thiazole units and flexible
thioether linkages were synthesized from diamine monomers (DA-1-3) and some
commercially available aromatic dialdehydes (terephthalaldehyde, isophthalaldehyde and
2
,5-thiophenedicarboxaldehyde) via Schiff-base condensation reaction. The synthesized
polymers as amorphous solids were obtained with high efficiency (74-89%), inherent
-1
viscosities in the range of 0.98–1.33 dL g in DMF and high solubility in aprotic polar
solvents (DMSO, DMAc, DMF, NMP, and Py). The PIMs were characterized via
viscosimetry, elemental analysis, FT-IR spectroscopy, X-ray diffraction (XRD), Brunauer–
Emmett–Teller (BET), thermogravimetric analysis (TGA) and differential scanning
calorimetry (DSC). High thermal resistance revealed for PIMs as glass transition
temperatures (T s) ranging in 104-189 °C along with 10% weight loss temperatures
g
exceeding 268-390 °C in air and 310-430 °C in nitrogen atmosphere. The polymers were
examined for CO absorption at 298 K as well as 318 K and high absorption capacity
2
-1
-1
exhibited (maximum 3.72 mmol g or 163.68 mg g at 1 bar and 298 K for PIM-4) after 2h
and desorption at 373 K under vacuum conditions (100 mbar) in 20 min. More importantly,
remarkable ideal selectivity ratios of CO /N (77.3) and CO /CH (13.7) at 1 bar and 298 K were
2
2
2
4
obtained and recyclability of PIM-4 for CO capturing was determined without considerable
2
loss of gas absorption.
Keywords: Gas separation; CO capture; Selectivity; Polyimine; Thiazole, Heterocycle
2
2

1
. Introduction
Industrialization in terms of deforestation by human activities in the form of burning fossil
fuels for energy beside substantial land use changes has been increased CO emission which
2
is the world’s greatest environmental challenge in the atmosphere. Critical issues such as
global warming trends and shifting climate patterns are consequences of increasing CO2
levels along with other greenhouse gases [1, 2]. From other perspective, CO has harmful
2
effects in the form of corrosion in oil and gas industry on downstream process lines and
equipment which makes gas sweetening process as an inevitable step [3]. For these reasons,
more effective sorption materials and approaches in technologies with economic cost and
high-performance features are still demanding. In recent decades, great efforts have been
made to achieve this goal through solvents, sorbents and membranes [4]. Conventional
amines or ammonia bases dissolved in water solution, as a mostly routine method for
industrial CO capturing, suffers from multiple drawbacks including energy-intensive,
2
equipment corrosion, toxicity, solvent loss, and amine oxidative and thermal degradation [5].
To reduce amine loss and energy consumption, ionic liquids (ILs) have been applied as new
alternative [6, 7], but still critical properties remain including decomposition at temperatures
near the boiling point, water solubility and high viscosity which lead to gas diffusion
limitation. Recently, researchers have presented solid and physical adsorbents such as
zeolites [8], zeolitic imidazole frameworks (ZIFs) [9, 10], metal-organic frameworks (MOFs)
[
8, 11, 12], functionalized silicas [8, 13-15], silica with mixed-matrix [16-19] as CO2
separation membrane, activated carbons [8, 20] as attractive alternatives for aqueous amine
solution due to their non-corrosive and eco-friendly properties which is attributed to lower
energy consumption for regeneration. They are also privileged due to their smaller energy
requirements causing by lower adsorption enthalpy. However, adsorption on solid materials
(chemisorption) considered more desirable vs physical adsorption (physisorption) due to their
3

strong covalent bonds with CO and high heat of absorption. This means reduction in
2
required regeneration energy in contrast to weak van der Waals interactions and low heat of
physical adsorbents [21]. More importantly, many physical adsorbents (physisorbents) suffer
from insufficient selectivity particularly in gas mixture, which cause low separation
efficiency and additional CO purification and cost [21, 22]. Besides, regeneration of
2
chemical adsorbents (chemisorbents) such as solid amines, alkali metal ceramics, calcium-
based adsorbents and layered double hydroxides (LDHs), are energy-consumptive and water
sensitive [23]. Hence, sorbents that have high CO uptake/selectivity, low cost and sensitivity
2
towards water are still demanding. On the other hand, based on the high quadruple moment
and polarisability of CO , applying electronegative functionalities including nitrogen-rich
2
sites with high selectivity such as amines [5], amides [24], imines [25, 26], thiazole [27] and
thiadiazoles [28] seems more favorable in the case of CO absorption. In recent years,
2
applying imine-based covalent organic frameworks (COFs) [28-30], microporous organic
polymers (MOPs) [31-34], modified micro-/mesoporous polyimine with tris(2-
aminoethyl)amine [35], porous organic polymers (POPs) [36], benzimidazole-linked
polymers (BILPs) [37-40], ferrocene-linked porous organic polymers (FPOPs) [41] and
polymer of intrinsic microporosity [42-44] have become more prevalent alternatives for CO2
capturing. Additionally, it is determined that the porous polymers containing sulfur and
activated carbons exhibited moderate capacity for CO uptake under low pressure (1 bar)
2
conditions [45-47] and exceptionally high amounts at high pressure [48], which highlights
pivotal roles of sulfur presence in adsorbent structures. Actually, most of these sorbents
focused on gas storage by physisorption, due to their high surface area, free volume and heat
of adsorption or may be utilized complicated monomers for the synthesis of porous polymer
networks. Therefore, usage of rapid, vigorous, highly selective and reversible CO sorbents
2
with low-cost and available monomers, physicochemical stability, structural and physical
4

susceptibility (including flexibility, heat and water resistance) for various industry
applications is still demanding. Moreover, our group recently synthesized valuable thiazole-
based polyamides (PAs) [49, 50], polyimides (PIs) [51-53], and poly(amid-imide)s (PAIs)
[
53, 54] with thioether linkages resulted in thermal and thermo-oxidative stability, high
optical transparency and solution processability for polymers. Besides, novel polyoxadiazoles
were explored as highly CO permselective membranes [55] and novel symmetrical bis-Schiff
2
bases introduced [56, 57]. Therefore, we envisaged that polyimines (PIMs-1-9) containing
thiazole ring would be more susceptible units for CO capturing or separation due to thiazole
2
bonds (C-S or C-N) and nitrogen rich imine linkages which make polymers more active for
such purposes. To the best of our knowledge, this is the first time that three efficient
functional groups of thiazole, thioether and generated imine simultaneously have been used
together, as a chemical absorbent, in a polymer which can improve CO uptake and
2
selectivity (over N and CH ). However, thioether linkage can also increase flexibility and
2
4
thermal resistance of the polymer which subsequently provide opportunity for more
applications in industry (such as membranes). More importantly, a strategy to investigate
structural comparison of polyimine chains and effects of applied dialdehydes and functional
groups on monomers (NO , Br and naphthyl) were developed as well. In fact, the current
2
study aims to examine the performance of new PIMs for CO absorption capacity and their
2
selectivities under ambient conditions.
2
2
3
. Experimental section
.1. Materials
-Nitroacetophenone (MERCK), 4′-bromoacetophenone (MERCK), 2-acetylnaphthalene
(SIGMA-ALDRICH), iodine (SIGMA-ALDRICH), thiourea (SIGMA-ALDRICH) and
aromatic dialdehydes such as isophthalaldehyde (MERCK), terephthalaldehyde (MERCK)
5

and 2,5-thiophenedicarboxaldehyde (MERCK) were utilized as received. In the process of
purification, lithium Bromide (LiBr, MERCK) was dried at 200 °C for 24 h under vacuum
before use and N-methyl-2-pyrrolidone (NMP, MERCK) was distilled over calcium hydride
(
CaH ) under reduced pressure. Other commercially available reagents and solvents were also
2
used without further purification.
2
.2. Measurements
The elemental analysis was performed on a Perkin Elmer 2004 (II) CHN recorder (USA)
elemental analysis instrument and melting points of the diamines (DAs) by Electrothermal
engineering LTD 9200 apparatus. FT-IR spectra were measured with Perkin Elmer FT
-1
Spectrum RX1 (USA) over the range 400–4000 cm and inherent viscosities by an
-1
Ubbelohde suspended level viscometer with a 0.5 g dL dimethylformamide (DMF) solution
1
13
at 30 °C. H NMR (300 MHz) and C NMR (75 MHz) spectra of the diamines (DAs) were
obtained on a Bruker DRX 300 AVANCE spectrometer using DMSO-d as a solvent and
6
trimethylsilane (TMS) as the reference. The wide-angle X-ray diffraction (WAXD) patterns
of the polymers were recorded on a Philips PW 1800 diffractometer using the graphite
monochromatized Cu Kα radiation (wavelength: 0.15401 nm) with 2θ increments of 0.08 °/s
over a range of 2θ = 4–80° at room temperature. Glass transition temperatures (T ) were
g
recorded on a 2010 DSC thermal analysis (TA) instrument (METTLER TOLEDO,
Switzerland) and TGAs with a Du Pont 2000 thermal analysis system (METTLER TOLEDO,
Switzerland) under N and air atmospheres at a heating rate of 10 °C/min. Surface areas and
2
pore size of PIMs were undertaken using BELSORP-miniII from the N adsorption at relative
2
pressure P/P = 0.99 in the Brunauer–Emmett–Teller (BET) model.
0
6

2
.3. Synthesis of PIMs
The PIMs were synthesized via the Schiff-base condensation reactions of diamine monomers
DA-1-3) and some dialdehydes. In a typical experiment for preparation of PIM-1, a mixture
(
of 0.472 g (1 mmol) of 5,5'-Thiobis((2-amino-4-(3-nitrophenyl)thiazole) (DA-1), 0.134 g (1
mmol) of terephthaladehyde, 0.174 g (2 mmol) of LiBr, and 2 mL of NMP was heated and
stirred at 120 °C for 3 h, under N atmosphere. After 2.0 h, the viscosity of reaction solution
2
increased and additional amount of NMP (1 mL) was added to the reaction mixture. At the
end of the polycondensation, the PIM solution was slowly poured into vigorous stirring 300
mL of methanol and resulting dark colored precipitate was thoroughly filtered off with hot
water (2×100 mL) and hot methanol (100 mL). Subsequently, polymer was dried in a vacuum
oven at 100 °C overnight. The yield was almost quantitative and further purification was
accomplished through dissolving the PIM in NMP, filtering the polymer solution, and
reprecipitating into hot methanol. The same strategy was followed for the synthesis of other
PIMs.
2
.4. Gas absorption measurements
The gas (CO , CH and N ) absorptions were determined using a laboratory setup according
2
4
2
to a volumetric method that has been schematically shown in Figure 1. The purity of the
utilized gases was determined to be CO : 99.5%, N : 99.999%, CH : 99.5%. To ensure
2
2
4
removal of any moisture and any gas molecules absorbed through the air, the PIMs were
degassed at 423 K in a vacuum for 24 h prior to measurement. Afterwards, the samples were
cooled down to 298 K and gas was introduced into the system. During the experiment,
changes in the pressure of CO were measured over time. Regarding the recyclability of
2
polymers for CO capturing, they were recovered in a rotatory evaporator under 373 K and
2
vacuum condition (100 mbar) up to 20 min and then recycled in the CO absorption test. The
2
7

corresponding diagrams were plotted based on the general gas equation PV = nRT, wherein
the P, V and T represent pressure (atm), volume (L) and absolute temperature (K)
-1
-1
-1
respectively; n (g mol ) is the number of moles of gas and R = 0.0821 (L atm mol K ) is
ideal gas constant. The experiments were accomplished under 1 bar at 298 K and V = 12.82
3
cm , in which volume totally considered as sample volume of the exhaust valve and sample
tank.
Figure 1. Setup of CO uptake and selectivity tests.
2
3
3
. Results and discussion
.1. Monomer synthesis and characterization
Thiazole-containing aromatic diamines 5,5'-Thiobis((2-amino-4-(3-nitrophenyl)thiazole)
DA-1), 5,5'-Thiobis((2-amino-4-(4-bromophenyl)thiazole) (DA-2) and 5,5'-Thiobis((2-
(
amino-4-(naphthalen-2-yl)thiazole) (DA-3) were prepared according to our previous applied
method [50, 52]. These reactions proceeded by their relative thiazoles, iodine and thiourea in
presence of warm water (Scheme 1).
Scheme1. Synthesis of the diamine monomers (DA-1-3).
8

3
.2. Polymer synthesis and characterization
The aim of this research was introducing thiazole-based PIMs with high thermal and thermo-
oxidative stability and CO uptake capability. To achieve our goal, PIMs were synthesized by
2
direct polycondensation of prepared diamine monomers with some aromatic dialdehydes.
This reaction proceeds in the presence of LiBr as catalyst and freshly dried NMP as a solvent
in 120 °C for 3 h and production of water as the only side-product (Scheme 2).
Scheme 2. Synthesis of the thiazole-containing PIMs-1-9.
As summarized in Table 1, PIMs were obtained in high yields (74–89%) with inherent
-1
viscosities in the range of 0.98–1.33 dL g in DMF solution at 30 °C, indicating high
molecular weight of the synthesized PIMs. The highest efficiency of polymerization ascribed
to the more linear structure of terephthaladehyde and its less steric effect. Comparing other
dialdehydes, 2,5-thiophenedicarboaldehyde, as a heterocyclic dialdehyde, shows better
efficiency vs isophthalaldehyde causing by its more active structure. Furthermore, the
reaction is affected by the type of applied primary acetophenone which arranges as follow: 3-
nitroacetophenone˃2-acetylnaphthalene˃4′-bromoacetophenone.
Table 1. Synthetic conditions and inherent viscosities of PIMs.
a
b
c
Polymer Code
PIM-1
Ar
Ar′
(1)
(2)
(3)
(1)
(2)
NMP (mL) Yield (%)
ηinh (dL/g)
3-nitrophenyl
3-nitrophenyl
3-nitrophenyl
4-bromophenyl
4-bromophenyl
2+1
2
2
2+1
2
89
82
87
85
74
1.28
1.09
1.17
1.33
1.11
PIM-2
PIM-3
PIM-4
PIM-5
9

PIM-6
PIM-7
PIM-8
PIM-9
4-bromophenyl
2-naphthyl
(3)
(1)
(2)
(3)
2
2+1
2
80
87
80
85
1.25
1.20
0.98
1.02
2-naphthyl
2-naphthyl
2+1
a
Amount of each DA and dialdehyde monomer = 1.0 mmol; reaction temperature 120 °C; reaction
time 3 h. “2+1” means that an initial amount of 2 mL NMP was used and an additional 1 mL of NMP
was added when the reaction solution became too viscous.
b
Isolated yield of PIM from corresponding DA.
Measured at a concentration of 0.5 g dL-1 in DMF at 30 °C.
c
This order corresponds to the reaction mechanism that is largely dependent on the initial step
2
of monomer synthesis, in which thiourea attacks to acetophenone derivatives via SN
mechanism, and the more e-withdrawing substitution on acetophenone ultimately leads to
improved yields. The data of elemental analysis for the proposed structure of the resulting
PIMs were in good agreement with the calculated ones.
The FT-IR spectra of PIMs revealed the characteristic absorption bands for the imine C=N
-1
and aromatic C=C bonds stretching at ~ 1600-1675 cm , and N-H stretching of amine groups
-1
at ~ 3341-3436 cm . Absorption of aldehyde carbonyl stretching (C=O) end groups of
-1
polymers appears at 1699-1710 cm . The typical FT-IR spectra of PIM-1-3 are shown in
Figure 2.
1
0

Figure 2. FT-IR spectra of PIM-1-3.
3
.3. Polymer solubility
Solubility behavior was known as a key indicator to determine the morphology,
processability and performance of the polymer in different applications such as gas separation
membrane [58-60], printed electronics [61], solvent casting or immersion lithography [62,
6
3] and thin film for polymeric solar cells [64]. Furthermore, based on the literature,
templating roles of solvent directing the pore size of polymers with its close dimension were
reported [65]. On the other hand, solvent properties including its polarity, volatility and
specific interaction features with the polymer material plays a pivotal role on degree of
polymerization (DP), surface area, porosity and subsequently on CO uptake capacity and
2
1
1

selectivity of polymer. In this sense, high boiling point and polarity solvents such as DMSO
and DMF vs low polar solvents (dioxane, mesitylene) may result in high-polymerized and
porous polymer by reducing the amount of entanglements and improving fractional free
volume during the synthesis process [36]. It is also noteworthy that the mentioned solvent
effect is more influential on the highly porous polymers with high-surface area. Hence,
thiazole-containing PIMs were certified by dissolving 0.05 g of polymers in 1 mL of solvent
for 24 h at rt (25 °C) or upon heating in tightly closed vials. As shown in Table S1
(supporting information), polymers represented excellent solubility in polar solvents
including DMSO, DMF, DMAc, NMP, and pyridine at ambient temperature, and moderate
solubility in THF and acetone. These findings are in accordance with synergic effects of
flexible thioether units in the backbone with non-coplanar conformation and bulky pendants
segments, phenyl and naphthyl substituents, which result in weakening of H-bonding and
decreasing in close packing of polymer. However, the solubility of polymers was greatly
dependent to the type of aldehydes used in polymerization processes. From this perspective,
2
,5-thiophenedicarboaldehyde as an initial heteroaromatic dialdehyde in the synthesis of
PIM-3 and PIM -6, causes insolubility of these polymers in THF solvent. In contrast to other
aldehydes, it is completely compatible with the corresponding dialdehyde polar structure.
Moreover, in the case of naphthyl-related polymers (PIM-7, PIM-8 and PIM-9) relatively
better solubility was observed in less polar solvents including acetone and chloroform. This
simultaneously is associated with the intrinsic effect of the non-polar pendent naphthyl group
and its bulkiness through boosting detachment of polymer chains, and subsequently
disrupting regularity and dense chain packing efficiency.
1
2

3
.4. X-ray diffraction of the polymers
Wide-angle X-ray diffraction (WAXD) patterns were utilized for investigating the
morphological aspects of the PIMs in a spectral window ranging from 2θ = 4° to 80°.
In general, the behavior of polymers can better be understood in terms of crystallinity of
polymers which depend on the molecular and geometrical structure. On the other hand, bulk
polymers with branches or irregular pendant groups (such as phenyl and naphthyl) are not
completely crystalline as the value of entropy increases by more freely movement of the
chains while pushing or pulling the polymer, and subsequently have a random molecular
structure. This means amorphous polymers have no rigidity and sharp melting point, and
exhibit flexibility and elasticity with impact resistance which are penetrated more by solvents
than their crystalline counterparts. These features make amorphous polymers right choice for
many industrial applications which need flexibility.
As shown in Figure 3, the amorphous nature of PIMs were confirmed due to broad curves
without any obvious peak. This feature of polymers is mostly related to the presence of the
pendent groups in the polymer backbone as the symmetry and regularity of backbone
decreases by incorporation of bulky groups. It should be noted that these conditions occurs by
weakening intermolecular forces such as H-bonding between the polymer chains and
lowering chain packing efficiency which leads to decrease in crystallinity.
Figure 3. WAXD pattern of PIMs.
1
3

3
.5. Thermal properties
The thermal properties of PIMs were investigated by DSC and TGA methods. The DSC
analysis of the PIMs obviously confirmed full amorphous natures of polymers without any
tendency to crystallize, even in the cooling step, as no evidence of crystallization and melting
processes displayed. It is noteworthy that these data are fully consistent with the XRD results.
As shown in Table 2, T values of the PIMs were in the range of 104–189 °C and directly
g
affected by applied dialdehydes and initial diamines. Based on these data, increasing of T is
g
a function of decreasing in flexibility of the PIM structure. In this regard, PIM-1, PIM-4 and
PIM-7 exhibited higher T values, compared to respective homologous meta-substitution
g
(PIM-2, PIM-5 and PIM-8), due to the rigid structure of polymer caused by 1,4-phenylene
segments. While polymers PIM-7, PIM-8 and PIM-9 exhibited less T owing to their bulky
g
naphthyl groups on polymer backbone, which increases the free volume between the polymer
chains and greater flexibility. The main criteria to determine the thermal stability of polymers
is the temperatures for 10% weight loss (T ).
1
0
Table 2. Thermal characterizations of PIMs.
TGA
a
c
Polymer Code DSC T (°C) Decomposition Temperature (°C)
Char Yield
g
In Air
345
333
329
390
376
372
285
274
268
In Nitrogen
387
(%)
58
63
53
65
63
60
60
46
47
PIM-1
PIM-2
PIM-3
PIM-4
PIM-5
PIM-6
PIM-7
PIM-8
PIM-9
184
170
161
189
172
163
129
112
104
377
372
430
420
415
328
316
310
a
-1
b
T Measured by DSC at scanning rate of 10 °C min in flowing nitrogen. Temperature of 10%
g
c
weight loss determined in N and air atmospheres. Residual weight (%) at 700 °C in N .
2
2
The T values of the PIMs were in the range of 310–430 °C in N and 268–390 °C in air,
10
2
respectively. The TGA-curves of the PIMs in N atmosphere are shown in Figure 4 as well.
2
1
4

Accordingly, produced PIMs showed good thermal stability without notable weight loss
below 310 °C in N atmosphere. These evidences proved that PIMs do have thermal and
2
oxidative resistances despite of their flexible thioether linkages and pendant phenyl or
naphthyl groups.
-1
Figure 4. TGA-curves of the PIMs with a heating rate of 10 °C min in N atmosphere.
2
3
.6. BET analysis
In this study, non-coplanar conformation of PIMs and presence of bulky pendant segments
phenyl, naphthyl) in the polymer backbone resulted in decrease of intermolecular H-bonding
(
and close packing of polymer and this may provide a proper condition for creating small
pores among polymer chains to trap CO gas. In addition, the PIMs comprising both rigid
2
groups (benzene rings) and flexible linkages (thioether and C-C single bonds) lead to
irregular pores due to the random orientation of flexible linkages. Therefore, N adsorption-
2
desorption measurements at 77 K were performed to determine the porosity and specific
surface area of polymers. As illustrated in Figure 5 (a), the linear PIM-1 and PIM-4 exhibited
a type-III isotherm, according to the IUPAC classification, with slit-shaped pore
characteristic. Other PIMs revealed type-IV N isotherm with H3 and H4 hysteresis loops and
2
cylindrical and conical like pore shapes which was indicative of mesopores. The textural
properties of PIMs are presented in Table 3. The minor surface areas were obtained, for PIM-
1
5

2
2
1
(SBET = 50.3 m /g) and PIM-4 (S
= 56.1 m /g) by applying the BET model which
BET
correspond to compact linear polymer structure. On the other hand, PIM-7 has the highest
level of surface area (SBET = 507.9) which is consistent with presence of regular repetition of
bulky naphthyl groups and structure of polymer chains. The pore size distribution (PSD) was
estimated on the basis of BJH model, and determined most of the pores in mesoporous region
(2-50 nm, Figure 5 (b)).
Figure 5. (a) N adsorption/desorption isotherms and pore size distributions of PIMs at 77 K.
2
(b) pore size distributions of PIMs.
Table 3. Textural properties of the PIMs.
a
2
a,b
3
a,c
Polymer Code
PIM-1
SBET (m /g)
50.3
Vp (cm /g)
0.054
0.50
Dp, BJH (nm)
6.1
7.4
14.3
6.7
7.7
15.8
3.7
PIM-2
PIM-3
PIM-4
PIM-5
PIM-6
PIM-7
PIM-8
149.4
103.6
56.1
272.7
198.5
507.9
378.5
241.8
0.63
0.071
0.44
0.73
0.28
0.50
0.10
6.6
3.1
PIM-9
a
b
c
The parameters were measured by N adsorption at 77 K.
Total pore volume at P/P = 0.99.
Average pore diameter.
2
0
1
6

3
.7. CO uptake
2
The synthesized and characterized PIMs were subjected to CO absorption experiments.
2
Initially, all polymers were tested at rt (298 K and 1 bar) by using a laboratory setup for CO2
capturing. The CO absorptions of all polymers were determined in mmol or mg of CO per
2
2
-1
-1
gram of PIM value (mmol g
or mg g PIM). The results of absorption-desorption
PIM
experiments are shown in Figure 6.
Figure 6. CO absorption-desorption of PIMs; absorptions (at 298 K and 1 bar) and
2
desorptions (at 373 K under vacuum condition (100 m bar) for 15 min).
As can be observed, the absorption reaches to its maximum amount in 120 min and complete
desorption in 20 min. Furthermore, the CO absorption diagrams revealed that the amount of
2
polymers absorptions display a higher growth rate in the first 30 minutes of the experiment,
due to the active sites on polymers, and after occupying some of those active points, the CO2
absorption continues at a more balanced rate in the next few minutes. The prepared PIMs, as
a type of nitrogen-rich Schiff-base including dipolar thiazole-based heterocycle, would make
favorably interact with CO molecules through dipole-quadrupole interactions and H-bonds
2
(Scheme 3). As seen in Table 4, the PIMs prepared with incorporation of terephthaladehyde
exhibited more favorable CO capturing compared with their isophthalaldehyde derivative
2
ones. Despite low porosity and surface area of linear PIM-1 (3.42 mmol/g, 150.48 mg/g, SBET
2
2
=
50.3 m /g) and PIM-4 (3.72 mmol/g, 163.68 mg/g, S
= 56.1 m /g), their high gas
BET
1
7

absorption is associated with reduction of polymer agglomeration and steric effects, which
might resulted in more degree of polymerization (DP) and repetitive set of efficient
functional groups. However, a survey for a range of materials (including COFs, MOPs,
MOFs and BPL carbon) revealed that increasing in BET surface area is not determining
factor and a proper strategy for enhancing CO uptake at lower pressures (1 bar) [31].
2
Furthermore, CO sorption values for PIM-1 and PIM-4 are comparable to the previously
2
2
reported porous polymers at 1 bar such as TH-COF-1 (97 mg/g, 2.2 mmol/g, SBET = 684 m /g
2
at 298 K) [28]; [HO C]
-H P-COF (76 mg/g, S
= 364 m /g at 298 K) [30]; MOP-C (2.2
2
100%
2
BET
2
2
mmol/g, SBET = 1237 m /g at 298 K) [31]; DBF(2.2 mmol/g, S
= 661 m /g at 298 K) [33];
BET
2
CMOP-1 (1.85 mmol/g, SBET = 431 m /g at 273 K) [34]; FPOP-2 (1.31 mmol/g, S
= 954
BET
2
m /g at 298 K) [41]. The increased CO uptake in linear PIM-7 (121.44 mmol/g, S = 50.3
2
BET
2
m /g) could be related to the gap between the polymer chains via bulk naphthyl group and
subsequently increasing its specific surface area as well. It is also interesting that the PIMs
using 2,5-thiophenedicarboxaldehyde showed relatively more CO uptake, in spite of their
2
lower surface area than the PIMs synthesized with isophthalaldehyde. This is probably
related to sulfur atom on heterocyclic structure of dialdehyde and possibility of interaction
between the non-bonding electrons of sulfur with a CO molecule which can provide a stable
2
six-membered ring (as shown in Scheme 3). In addition, the bulky aromatic group of "2-
naphthyl" in PIM-7, PIM-8 and PIM-9 exhibited less CO absorption, compared to their
2
respective homologous ("3-nitrophenyl" and "4′-bromophenyl), due to detachment of
polymer chains which lead to disrupting regularity and dense chain packing efficiency,
increasing steric barrier for CO molecule approaching to these polymers, and absence of
2
influential -NO and -Br functional groups. These results highlight the positive and influential
2
effect of functionalization (chemisorption) vs the specific surface area (physisorption) of
polymers in terms of CO uptake. Interestingly, more CO capturing of PIM-4 (163.68 mg/g,
2
2
1
8

2
2
SBET = 56.1 m /g), PIM-5 (86.24 mg/g, S
= 272.7 m /g) and PIM-6 (141.24 mg/g, S
=
BET
BET
2
1
98.5 m /g) were observed compared to their homologues PIM-1 (150.48 mg/g, S
= 50.3
BET
2
2
2
m /g), PIM-2 (84.04 mg/g, S = 149.4 m /g) and PIM-3 (124.964 mg/g, S = 103.6 m /g).
BET
BET
These findings revealed that the -Br group with three non-bonding lone pairs, might
interacted more favorably with the CO molecule (as soft-soft interaction) compared with -
2
NO group, and provided more appropriate surface area for corresponding polymers.
2
However, PIM-1-3 have more nitrogen-rich groups which make them more susceptible for
2
CO absorption than PIM-8 (77.88 mg/g, S = 378.5 m /g) and PIM-9 (81.84 mg/g, S
=
2
BET
BET
2
2
41.8 m /g).
Scheme 3. Proposed mechanism for CO absorption of relevant PIMs.
2
We also measured CO absorption of initial diamines and compared them with their
2
respective polymers (Table 4).
Table 4. CO , uptakes at 298 and 318 K and ideal selectivity data for synthesized PIMs at 1
2
bar and 298 K.
Polymer
Code
CO Uptake
at 298 K
CO Uptake
at 318 K
Ideal
Selectivity
CO /N
Ideal
2
2
Selectivity
CO /CH
mg/g PIM
mmol/g PIM
mg/g PIM
mmol/g PIM
2
2
2
4
at 298 K
70.2
58.2
64
at 298 K
12.5
8
PIM-1
PIM-2
PIM-3
PIM-4
PIM-5
PIM-6
PIM-7
3.42
1.91
2.84
3.72
1.96
3.21
2.76
150.48
84.04
124.96
163.68
86.24
2.24
0.50
1.44
2.56
0.56
1.82
1.43
98.56
22
63.36
112.64
24.64
80.08
62.92
11.2
13.7
8.5
12
10.6
77.3
60.4
68.2
63.3
141.24
121.44
1
9

PIM-8
PIM-9
DA-1
DA-2
DA-3
1.77
1.86
1.40
1.48
1.37
77.88
81.84
61.6
65.12
60.28
0.35
0.44
-
-
-
15.5
19.36
55
57
-
-
-
6.2
7
-
-
-
-
-
-
In continuation, effects of temperature and pressure were investigated. As depicted in Figure
, increasing temperature from 298 K to 318 K, which means increase in kinetic energy of
7
CO molecule along with increasing polymer mobility especially by thiourea bridge, causes
2
weakening of CO interaction with amine and imine groups on polymers, and subsequently
2
reduction in the amount of absorbed CO molecules. In contrast, increase in pressure results
2
in gas density enhancement and consequently more gas absorption.
Figure 7. CO absorption of PIMs at (a) 298 K and (b) 318 K.
2
Since regeneration of polymers for CO experiment are valuable issue for gas separation, we
2
continued our studies with typical PIM-4 in order to evaluate the polymer recycling
capability for CO absorption. In this regard, we examined the polymer to another CO2
2
uptake to determine its reversibility after the first absorption-desorption analysis. It was
found that the PIM-4 can properly be recovered for at least seven cycles without noticeable
loss in efficiency (Figure 8). In addition to the CO absorption capacity and reversibility, CO2
2
selectivity of PIMs over N and CH was studied at 298 K up to 1 bar.
2
4
2
0

Figure 8. Recycling absorption of CO over PIM-4.
2
The diagrams are displayed in Figure 9, in which PIM-4 and PIM-1, respectively, exhibited
higher ideal selectivity ratios (CO /N : 77.3, 70.2 and CO /CH : 13.7, 12.5) among polymers
2
2
2
4
by virtue of their linear packed structure resulting greater affinity toward CO . Moreover, the
2
PIMs with naphthalene groups, particularly PIM-8 and PIM-9, revealed lowest gas selectivity
ratios (CO /N : 55, 57 and CO /CH : 6.2, 7 respectively). This is because of increasing the
2
2
2
4
polymer chains distance by presence of bulky naphthyl groups along with increasing steric
effects, which leads to a decrease in effective interaction of gas and polymer, and reducing
the selectivity of these polymers as a result. As shown in Table S2, a comparison of CO2
absorption and selectivity of PIMs with various polymers such as benzimidazole-linked
polymers (BILPs) [37-40], hyper-cross-linked heterocyclic MOPs [33], and imine-linked 2D-
COFs [28, 30] were investigated which introduces PIMs as promising candidates for CO2
uptake and selectivity.
The FT-IR spectroscopy was applied for comparison before and after CO absorption on
2
polymers. As illustrated in FT-IR spectra of PIMs-1-9 in Figures S10-27, the polymers
-1
exposed to CO exhibited characteristic broad and strong absorption band at 3331-3431 cm
2
for N-H and imine C-H stretching vibrations, which display a shift toward lower frequencies
due to the H-bonding with CO compared to corresponding not gas affected polymer. The
2
-1
aromatic C-H stretching vibrations were emerged in the range of 2929-3199 cm . The PIMs
2
1

-1
were also featured by strong absorption band at 1700-1708 cm for C=O stretching of
carbamate moiety, which was formed from the interaction of non-bonding electrons of imine
with CO molecule.
2
Figure 9. Selective gas (CO , CH , N ) absorptions for PIMs at 298 K.
2
4
2
However, the absorption of aldehyde carbonyl groups at two end sides of polymer chain was
-1
overlapped in carbonyl area. A broad stretching vibration band at ~ 1602-1627 cm related to
aromatic C=C and imine C=N bonds. The FT-IR spectra of typical PIM-7, before and after
CO absorption, have shown in Figure 10. These results clearly prove the formation of H-
2
2
2

bonding of polymer with CO molecules which is consistence with the proposed mechanism
2
for CO absorption on PIMs in Scheme 3.
2
Figure 10. Comparitive FT-IR spectra before and after CO uptake of PIM-7.
2
4
. Conclusion
In conclusion, nine non-coplanar ortho-linked thiazole-based PIMs with thioether linkage
were synthesized as efficient chemical absorbents for CO capturing with decent solubility,
2
processability, thermal stability, low cost and non-toxicity. The PIMs were characterized by
2
3

viscosity, FT-IR, XRD, BET, elemental and thermal analysis. Based on abundant and robust
polymeric imine groups and sulfur contents of thiazole heterocyclic ring in polymer structure,
they successfully exhibited very high CO absorption (particularly linear PIM-1 and PIM-4,
2
-1
3
.42 and 3.72 mmol g ) along with excellent ideal selectivity (up to CO /N : 77.3 and
2 2
CO /CH : 13.7) at ambient condition. These results revealed a correlation between high
2
4
selectivity and high CO uptake among utilized PIMs. The amount of CO absorption and
2
2
ideal selectivity explained by synergistic physically, chemically and structurally aspects of
PIMs and non-polarizable structures of N and CH compared with CO molecule. Recycling
2
4
2
absorption of PIM-4 proved no significant loss of CO uptake efficiencies after 7 cycles. Gas
2
absorption comparison of these polymers with their initial diamines was sufficient evidence
for significant influence of polymerization process on these materials. Resulted experiments
approved the consequences of increasing pressure and reducing temperature in increasing the
rate of polymers gas absorption. Impression of structure and substitution on polymers were
explored and it was found that the best results belongs to linear and packed polymers with
less steric barriers in terms of thermal resistance and CO absorption. These PIMs with high
2
chemical and thermochemical stabilities are considered as promising scaffolds by virtue of
cost-effective and readily available monomers, high selectivity and reversibility in CO2
capturing and usability in various industries (such as membranes) due to their non-corrosive,
environmentally compatibility and processing capability. Further efforts to prepare various
membranes of these PIMs are ongoing in our group, with the aim of efficient and selective
gas separations.
Supporting information
The characterization data of DA-1-3 and PIMs-1-9 and comparison table of CO uptake and
2
selectivity for polymers.
2
4

Conflicts of interest
There are no conflicts to declare.
Acknowledgements:
This work was supported by the Iran National Science Foundation (INSF) [grant No.
9
6007054] and Kharazmi University. The authors gratefully acknowledge for financial
support of researchers and elites of this project.
2
5

References
[
1] H. Kheshgi, H. de Coninck, J. Kessels, Carbon dioxide capture and storage: Seven years
after the IPCC special report, Mitig. Adapt. Strateg. Glob. Change. 17 (2012) 563–567.
2] J. D. Figueroa, T. Fout, S. Plasynski, H. McIlvried, R. D. Srivastava, Advances in CO2
capture technology—The U.S. Department of Energy's Carbon Sequestration Program,
Int. J. Greenh. Gas Control. 2 (2008) 9-20.
[
[
3] M. T. Ho, G. Leamon, G. W. Allinson, D. E. Wiley, Economics of CO and mixed gas
2
geosequestration of flue gas using gas separation membranes, Ind. Eng. Chem. Res. 45
(2006) 2546-2552.
[
[
4] B. Li, Y. Duan, D. Luebke, B. Morreale, Advances in CO capture technology: A patent
2
review, Appl. Energ. 102 (2013) 1439-1447.
5] B. Dutcher, M. Fan, A. G. Russell, Amine-Based CO Capture Technology Development
2
from the Beginning of 2013-A Review, ACS Appl. Mater. Interfaces. 7 (2015) 2137-
2
148.
[
6] Sh. Zeng, X. Zhang, L. Bai, X. Zhang, H. Wang, J. Wang, D. Bao, M. Li, X. Liu, S.
Zhang, Ionic-liquid-based CO capture systems: structure, interaction and process, Chem.
2
Rev. 117 (2017) 9625-9673.
[
[
7] M. Aghaie, N. Rezaei, S. Zendehboudi, A systematic review on CO capture with ionic
2
liquids: Current status and future prospects, Renew. Sust. Energ. Rev. 96 (2018) 502-525.
8] R. Ben-Mansour, M. A. Habib, O.E. Bamidele, M. Basha, N. A. A. Qasem, A.
Peedikakkal, T. Laoui, M. Ali, Carbon capture by physical adsorption: Materials,
experimental investigations and numerical modeling and simulations-A review, Appl.
Energ. 161 (2016) 225-255.
2
6

[
9] B. R. Pimentel, A. Parulkar, E-K. Zhou, N. A. Brunelli, R. P. Lively, Zeolitic imidazolate
frameworks: next-generation materials for energy-efficient gas separations,
ChemSusChem. 7 (2014) 3202-3240.
[
[
10] B. Chen, Zh. Yang, Y. Zhu, Y. J. Xia, Zeolitic imidazolate framework materials: recent
progress in synthesis and applications, Mater. Chem. A. 2 (2014) 16811-16831.
11] K. Sumida, D. L. Rogow, J. A. Mason, Th. M. McDonald, E. D. Bloch, Z. R. Herm, T-
H. Bae, J. R. Long, Carbon dioxide capture in metal-organic frameworks, Chem. Rev.
1
12 (2012) 724-781.
[
12] Ch. A. Trickett, A. Helal, B. A. Al-Maythalony, Z. H. Yamani, K. E. Cordova, O. M.
Yaghi, The chemistry of metal–organic frameworks for CO capture, regeneration and
2
conversion, Nat. Rev. Mater. 2 (2017) 17045.
[
[
13] Ch. Chen, S. Zhang, K. H. Row, W-S. Ahn, Amine-silica composites for CO capture: A
2
short review, J. Energy Chem. 26 (2017) 868-880.
14] K. S. Sánchez-Zambrano, L. L. Duarte, D. A. S. Maia, E. Vilarrasa-García, M. Bastos-
Neto, E. Rodríguez-Castellón, D. C. S. Azevedo, CO Capture with mesoporous silicas
2
modified with amines by double functionalization: assessment of adsorption/desorption
cycles, Materials. 11 (2018), 887.
[
[
15] Ch. Chen, S. Bhattacharjee, Mesoporous silica impregnated with organoamines for post-
combustion CO capture: a comparison of introduced amine types, Greenhouse Gas Sci.
2
Technol. 7 (2017) 1116-1125.
16] G-L. Zhuang, M-Y. Wey, H-H Tseng, The density and crystallinity properties of PPO-
silica mixed-matrix membranes produced via the in situ sol-gel method for H /CO2
2
separation II: Effect of thermal annealing treatment, Chem. Eng. Res. Des. 104 (2015)
3
19-332.
2
7

[
[
[
[
17] G-L. Zhuang, H-H. Tseng, M-Y. Wey, Preparation of PPO-silica mixed matrix
membranes by in-situ solegel method for H /CO separation, Int. J. Hydrogen Energy. 39
2
2
(2014) 17178-17190.
18] T-H. Weng, H-H. Tseng, M-Y. Wey, Effect of SBA-15 texture on the gas separation
characteristics of SBA-15/polymer multilayer mixed matrix membrane, J. Membr. Sci.
3
69 (2011) 550–559.
19] H-H. Tseng, P-T. Shiu, Y-Sh. Lin, Effect of mesoporous silica modification on the
structure of hybrid carbon membrane for hydrogen separation, Int. J. Hydrogen Energy.
3
6 (2011) 15352-15363.
20] M. S. Shafeeyan, W. M. A. W. Daud, A. Houshmand, A. Shamiri, A review on surface
modification of activated carbon for carbon dioxide adsorption, J. Anal. Appl. Pyrolysis.
8
9 (2010) 143–151.
[
[
21] Ch-H. Yu, Ch-H. Huang, Ch-S. Tan, A review of CO capture by absorption and
2
adsorption, Aerosol Air Qual. Res. 12 (2012) 745-769.
22] D. G. Madden, H. S. Scott, A. Kumar, K-J. Chen, R. Sanii, A. Bajpai, M. Lusi, T.
Curtin, J. J. Perry, M. J. Zaworotko, Flue-gas and direct-air capture of CO by porous
2
metal-organic materials, Philos. Trans. R. Soc. A. 375 (2017) 20160025.
[
[
23] Q. Wang, J. Luo, Z. Zhong, A. Borgna, CO capture by solid adsorbents and their
2
applications: current status and new trends, Energy Environ. Sci. 4 (2011) 42-55.
24] J. Ahmad, W. U. Rehman, K. Deshmukh, Sh. Kh. Basha, B. Ahamed, K. Chidambaram,
Recent advances in poly (amide-b-ethylene) based membranes for carbon dioxide (CO2)
capture: a review, Polym. Plast. Technol. Eng. 58 (2019) 366-383.
[
25] V. S. P. K. Neti, X. Wu, Sh. Deng, L. Echegoyen, Selective CO capture in an imine
2
linked porphyrinporous polymer, Polym. Chem. 4 (2013) 4566-4569.
2
8